MXene quantum lands: emerging trends and breakthroughs

Abstract

Chemically stable two-dimensional MXene quantum dots (MQDs) have gained significant attention owing to their exceptional optical properties, tunable surface chemistry, and promising biocompatibility. Leveraging these properties, MQDs have found broad applicability across diverse domains, including optoelectronics (LEDs, lasers, detectors, and solar cells), energy storage (batteries and supercapacitors) and energy conversion (CO₂ reduction and hydrogen evolution), sensing, and biomedicine. This review provides a comprehensive overview of recent advancements in eco-friendly synthesis and surface modification strategies aimed at enhancing the radiative recombination efficiency of fluorescent MQDs. Furthermore, we critically assess the wide-ranging practical applications of MQDs and evaluate the progress achieved through both experimental and computational approaches. Special emphasis is placed on the most promising avenues for improving their optical performance and integration into high-efficiency devices. Finally, we outline key challenges and offer insights into future research directions. This review bridges fundamental understanding with technological development, reinforcing the transformative potential of MQDs in next-generation applications.

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Mahdi Hasanzadeh Azar

Mahdi Hasanzadeh Azar received his M.A.Sc. degree (B.Sc. degree) in Engineering Physics (Materials Science and Engineering) from McMaster University, Canada, with research experience in synthesizing, characterizing, and applying fluorescent quantum dots and wells into various optoelectronic devices. He is currently a Ph.D. student in the Functional Nanomaterials Group (FNG) at the University of Waterloo, holding Ontario Graduate Scholarship in Canada. His research focuses on developing scalable spatial atomic layer deposition method for the large-scale fabrication of perovskite thin film solar cells modified by transition metal dichalcogenides quantum dots.

Fatemeh Etehadi

Fatemeh Etehadi received her M.Sc. in Materials Engineering and Nanotechnology from Politecnico di Milano, Italy. She worked at the Leibniz Institute for Composite Materials on sustainable wood-based composites and is currently pursuing a Ph.D. at the Karlsruhe Institute of Technology, focusing on printed eco-friendly sensors. Her research interest is in developing sustainable and stable solutions by integrating different aspects of sensor technology.

Nima Mohamadbeigi

Nima Mohamadbeigi received his Ph.D. degree in Nanotechnology, focusig on design and fabrication of self-powered gas and optical sensors based on triboelectric and thermoelectric nanogenerators for real-time health monitoring. He has substantial experience in developing multifunctional nanomaterials and integrating them into flexible, energy-harvesting sensing architectures. His current research interests include the advancement of self-powered wearable sensor technologies through the coupling of energy conversion and sensing mechanisms, with the overarching goal of realizing next-generation, battery-free health monitoring platforms.

Hessam Shahbazi

Hessam Shahbazi is a researcher at University of Illinois Chicago working at the interface of quantum materials and energy storage, where he investigates how electronic structure and chemical potentials govern material behavior in energy storage and conversion systems. His works expands to in situ and ex situ characterization of solid–solid and solid–liquid interfaces with first-principles calculations to probe reaction energetics and quantum phenomena at atomic scale, strengthened by hands-on R&D experience in lithium battery manufacturing sector at Tesla and early-stage hardtech startups in the field of quantum.

Sara Salehi Siouki

Sara Salehi Siouki received her M.Sc. degree in Materials Engineering and Nanotechnology from the Polytechnic University of Milan, Italy. She is currently pursuing a Ph.D. focused on nucleic acid immobilization strategies on photopolymer surfaces at the Technical University of Leoben, Austria. Her technical experience includes surface analysis and functionalization, antibacterial hydrogels, 3D bioprinting, DNA-based biosensors, and polymer synthesis.

Abdolreza Simchi

Dr Abdolreza (Arash) Simchi is an esteemed Professor of Materials Science and Engineering (MSE) at Fraunhofer Institute IFAM and an elected Fellow of the World Academy of Sciences (TWAS-UNESCO). Prof. Simchi's research focuses on advanced materials, particularly nanomaterials, additive manufacturing, and sustainable materials for biomedical and environmental applications. His contributions have been recognized with several prestigious awards, including the George Foster Research Award from the Alexander von Humboldt Foundation in 2020. He has also been ranked among the top 1% of world scientists based on citations by the ISI Web of Science since 2012.

1. Introduction

MXenes, a groundbreaking class of two-dimensional transition metal carbides (TMCs) and nitrides, have gained remarkable attention for their chemical stability, tuneable surface functionalities, and exceptional electrical conductivity.^1–3 These materials have been extensively studied and successfully integrated into diverse applications, including energy storage,^4–6 catalysis, optoelectronics,^7–9 and biomedical devices.^8,10 Despite the wealth of research on MXenes, their versatility continues to unveil new opportunities with new areas, such as nanoscale and quantum-sized materials.^1,2,11 Therefore, there is yet untapped potential for scientific exploration and technological advancement.

Among emerging frontiers, MXene Quantum Dots (MQDs) stand out as promising yet underexplored materials. By downsizing MXenes to quantum scales, MQDs retain the advantageous properties of their parent material, such as structural stability and chemical tunability, while gaining unique features like enhanced photoluminescence (PL), superior dispersibility, and improved biocompatibility.^1,8,9,12,13 These characteristics make MQDs well-suited for a wide range of applications, from biosensing and bioimaging to energy and optoelectronic and conversion/storage devices (Fig. 1), with the latter including thousands of studies on MQDs and other nanostructured materials in less than two decades (Fig. S1).^14–17 However, their synthesis, properties, and full application potential remain in the early stages of investigation, leaving ample room for breakthroughs in both fundamental science and practical implementations. Although some review papers have covered specific aspects of MQDs,^3,18–21 there is a noticeable lack of comprehensive review covering the recent advances in the synthesis, properties, and applications of MQDs.

Fig. 1 A schematic diagram illustrating the innovative synthesis methods and applications of MQDs.

This review presents a comprehensive and conceptually unified framework that systematically integrates the rapidly expanding body of knowledge on MQDs. It aims to elucidate how the electronic structure and surface chemistry of MQDs govern their functional performance across diverse applications, while identifying key challenges and outlining prospective research opportunities. The discussion is organized into three interrelated domains, synthesis methodologies, structure–property relationships, and emerging applications, to provide a coherent connection between fundamental understanding and technological relevance. This article first introduces the structural diversity and compositional tunability of MXene materials, highlighting their vast potential as a versatile family for next-generation technologies. It then explores various synthesis techniques, including both top-down and bottom-up approaches, to emphasize how synthesis parameters critically influence particle size, surface states, and the resulting physicochemical properties.¹ The subsequent sections delve into the optical, electronic, and biocompatible characteristics of MQDs, followed by an in-depth examination of their implementation in key application areas such as sensing,^2,5–7,11 optoelectronics,⁸ energy conversion and storage,²² and biomedicine.^10,13,23,24 Finally, through the introduction of a challenges–solutions–outlook framework, this review integrates the discussed aspects into a single analytical perspective, linking nanoscale design strategies to macroscopic device performance. This structured and comparative approach provides a comprehensive and forward-looking overview of the MQDs landscape, offering valuable insights into future research directions and underscoring the transformative potential of MQDs in advanced technologies.

2. Structural features

MAX phases, consisting of transition metal carbides or nitrides, serve as three-dimensional precursors to two-dimensional MXenes. They are represented by the formula M_n+1AX_n (n = 1 to 4), where ‘M’ refers to an early transition metal (mostly from groups 3–6 (Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W)), ‘A’ denotes an element from groups 13–15 (such as Al, Ga, In, Si, Ge, Sn, P, As, etc.), and ‘X’ is either carbon or nitrogen.^25,26 The distinct bonding structure of MAX phases features relatively weaker and more chemically reactive metallic M–A bonds compared to mixed covalent/ionic M–X bonds. Therefore, the A-layer in the MAX phase is selectively removed by a controlled etching process to form MXenes with the formula of M_n+1X_nT_x (n = 1 to 4). Here, T_x denotes the type and number of surface terminations (–F, –OH, –Cl, or –O).^27,28 Typically, n + 1 layers of transition metal enclose n layers of carbon or nitrogen. When n = 1, the resulting structure, M₂X, adopts a hexagonal close-packed structure with the ABABAB stacking sequence. For n = 2 and n = 3, the crystal structures of M₃X₂ and M₄X₃ are face-centered cubic (FCC) with the ABCABC stacking arrangement. It is distinct that the properties of MXene are profoundly influenced by its crystal structure and surface terminations.^27,28

Over the last decade, computational simulations have been instrumental in understanding the MXenes crystal structures and identifying novel, stable compounds, offering critical insights to experimental advancements. These simulations suggest that MXenes can manifest in five distinct structures (Fig. 2a): (i) mono-transition metal: structures like Ti₂C, which consist of a single type of transition metal uniformly distributed within the M-layers (first row); (ii) solid solutions, like (Ti,V)₃C₂, where two different metals are randomly arranged within the M-layers (second row); (iii) ordered double-transition metals, including Mo₂TiC₂, with a well-organized arrangement of two distinct metals within the M-layers (third row); (iv) ordered divacancy, such as Cr_4/3C, featuring systematically arranged two vacancy sites within the M-layers (fourth row); (v) high-entropy, like TiVNbMoC₃, an analogous to the high-entropy oxide of similar chemistry with multiple transition metals randomly distributed within the M-layers (fifth row).^29,30 Density functional theory (DFT) calculations further reveal that certain ordered MXenes are energetically more stable than their solid-solution counterparts. These findings have led to predictions of over 25 different ordered MXenes, highlighting the structural diversity and stability potential within this material class (third row).²¹

Fig. 2 (a) MXene structures and compositions: experimentally studied compositions are marked in orange and theoretical compositions in gray. Surface terminations are excluded. The schemes also include phases synthesized through bottom-up methods or phase transformations, such as W₂N, V₂N, and Mo₂N.^31,32 (b) Cross-sectional and (c) planar view of the available low-symmetry sites on an M₃X₂-type MXene surface. The four primary sites include HCP (purple), bridge (red), FCC (green), and atop (orange), with the vertical positioning of the terminating species being virtual and variable depending on the species (reprinted with permission from Elsevier, copyright © 2019).²⁷

Regarding functional groups, the reactive M-element surfaces are rapidly functionalized by terminating species introduced during the etching process. These surface terminations occur at various symmetry sites, including FCC, HCP, atop positions directly above surface M elements, bridging sites between two M elements (Fig. 2b and c). Experimentally, synthesized MXenes predominantly exhibit –O, –OH, and –F terminations due to their greater thermodynamic stability compared to pristine MXenes.¹⁹ However, the hierarchy of functional group stability remains compound-specific and highly dependent on the synthesis method. Theoretical studies suggest that the stability of MXenes increases in the order of –OH < –F < –O for Ti₄N₃T_x, Ti₃C₂T_x, and Nb₄C₃T_x.³⁰ The lower stability of OH terminations is attributed to hydrogen atom replacement or its conversion to –O at elevated temperatures. In practice, however, the stability order is also influenced by the etching method. For instance, in HF-based methods, the stability follows the order of –F > –O > –OH, whereas thermal treatments or HCl–LiF methods result in the order of –O > –F > –OH.^27,29 However, most simulations overlook the influence of the etching solution, causing significant discrepancies in predictions regarding the functionalization order.^27,29

When the lateral size of MXenes is reduced below 10 nm, quantum confinement effect may come into play, resulting in the formation of MQDs.¹⁹ While MQDs retain the essential structural attributes of 2D MXenes, they benefit from abundant, tunable functional groups, enhanced optical properties, and greater dispersibility. As a result, MQDs are exceptionally well-suited for sensing, energy storage, optoelectronics, and biomedical applications. The superior properties of MQDs provide a significant edge over their 2D counterparts, making them a versatile platform for cutting-edge research and technological advancements.²⁰

3. Synthesis of MQDs

The synthesis of MQDs can be broadly categorized into two main strategies: top-down and bottom-up approaches. This section provides a general discussion of these methods. Table 1 presents details on the methods used for synthesizing various types of MQDs.

Table 1 Synthesis of MQDs by various methods

MQD	Method	Chemicals	Morphology		Size		Mechanism	Ref.
			NS	QD	l (nm)	t (nm)
Average lateral size (l), thickness (t), 2,3-diaminophenazine (DAP), 2-methylimidazole (MIM), 2,2′-diethanolamine (DETA), 3-aminopropyltriethoxysilane (APTES), bovine serum albumin (BSA), carbon nanotube (CNT), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), glutathione (GSH), methanol quantum dots (MQDs), nanosheet (NS), polyethyleneimine (PEI), poly(vinylpyrrolidone) (PVP), quantum dot (QD), tetramethylammonium hydroxide (TMAOH), tetrapropylammonium hydroxide (TPAOH), uric acid (UA), zeolitic imidazolate frameworks (ZIF).a MXene QDs functionalized with the thiol group.
Ti2N	Sonication	NSs: Ti2AlN, HCl, KF	(1) 12 h etching of Ti2AlN	(1) 24 h ice bath sonication	4.83	1.81	—	19
		QDs: NMP, ice bath, PBS, Ti2N, ethanol, soybean phospholipid, chloroform	(2) 2 h sonication at 45 °C	(2) 20 h tip sonication
			(3) Centrifugations at 3500 rpm	(3) Centrifugation at 9000 rpm
			(4) Decantation	(4) Centrifugation at 12 000 rpm
				(5) Rinse by PBS or DI water
				(6) 10 min sonication
				(7) Rotary evaporator at 60 °C
Nb2C	Sonication	NSs: Nb2AlC, HF, ethanol	(1) 2 days etching of Nb2AlC	(1) Stirring in TPAOH	1–5	1.2	—	33
		QDs: TPAOH	(2) 15 min centrifugation	(2) 10 h pulsed ultra-sonication in ice bath
			(3) Wash with ethanol	(3) Filtration
				(4) Freeze-drying
Ti3C2	Sonication	NSs: Ti3AlC2, HF	(1) 20 h etching of Ti3AlC2	(1) 24 h stirring	3.4	1.65	—	31
		QDs: Ti3C2Tx, TMAOH	(2) Centrifugation	(2) Centrifugation
			(3) Wash (pH = 5–6)	(3) 20 h sonication
				(4) 10 min centrifugation
Ti3C2OH	Sonication	NSs: Ti3AlC2, HF	(1) Ti3AlC2 + HF	(1) Sonication	2.4–3.2	1.48–1.99	—	34
		QDs: Ti3C2Tx, NaOH	(2) Washing	(2) Agitation
			(3) Freeze-drying	(3) Centrifugation
				(4) Dialysis
				(5) Freeze-drying
Ti3C2Tx	Sonication	NSs: Ti3AlC2, LiF	(1) 48 h at 40 °C in oil bath	(1) Ultrasonication	10	—	—	35
		QDs:	(2) Washing	(2) Centrifugation
				(3) Vacuum freeze-drying
Ti3C2Tx	Centrifugation + stirring	NSs: Ti3AlC2, LiF, HCl, Ti3C2(OH)2, Ti3C2F2	(1) 24 h mixing	(1) 1 h stirring	2–6	5	—	36
		QDs: —	(2) Washing	(2) 1 h centrifugation
			(3) Filter membrane
			(4) Drying
N-doped Ti3C2	Solvothermal + sonication	NSs: —	—	(1) 1 h sonication	3.09	—	—	37
		QDs: Ti3C2, DMF, ammonium hydroxide		(2) Ammonium hydroxide → pH ∼ 9
				(3) 6 h heating
				(4) Centrifugation
Ti3C2Tx	Sonication	NSs: Ti3AlC2, Ag wire, Pt wire, [EMIM][PF6], MeCN	(1) Electrochemical cell, V = 3–7 V, 5 h	(1) 10 h ultrasonication	5.34	4	Strong binding capability of F^− with Al → etching	38
		QDs: MeCN	(2) Centrifugation	(2) Centrifugation
Ti3C2 QDs-AuNPs	Sonication	NSs: Ti3AlC2, LiF	(1) 24 h stirring	(1) 10 h sonication	6.5	—	—	39
		QDs: —	(2) 3 times centrifugation	(2) Centrifugation
				(3) Filtration
Ti3C2	Sonication	NSs: Ti3AlC2, HF	(1) 24 h stirring at 45 °C	(1) 24 h stirring	∼1.75	1.2	—	40
		QDs: Ti3C2	(2) Centrifugation	(2) Centrifugation
			(3) Washing (pH = 6)	(3) Washing
			(4) Drying	(4) 10 h sonication
				(5) Centrifugation
				(6) Filtration
Ti3C2/watermelon peel aerogels	Sonication	NSs: Ti3AlC2, LiF, HCl, H2SO4	(1) 48 h stirring at 30 °C	(1) 10 min purging by N2	<10	—	Immersing freeze-drying-dried fresh watermelon peel into QD dispersion	41
		QDs: —	(2) Washing by H2SO4	(2) 5 h ultrasonic
			(3) 5–10 min stirring	(3) Centrifugation
			(4) Centrifugation	(4) Filtration
			(5) Washing
			(6) Stirring
			(7) Centrifugation
Mo2C@NG	Sonication	NSs: —	—	(1) 5 min stirring	1.8 (MQD)	11.5 µm (MQD@NG)	—	42
		QDs: (NH4)6Mo7O24·4H2O (AHM), graphene oxide, poly(oxypropylene)		(2) 1 min heating at 90 °C
		Diamines		(3) Freeze-drying
				(4) 1 h heating at 500 °C in quartz tube
				(5) 4 h heating at 700 °C under Ar.
Ti3C2	Sonication + hydrothermal	NSs: Ti3AlC2, NaF, HCl, DMSO	(1) 12 h etching of 1 g Ti3AlC2	(1) 10 h sonication	Diameter: 3.96	—	—	32
		QDs:	(2) Centrifugation in HCl	(2) 12 h heating
			(3) Washing	(3) Filtration
			(4) Drying	(4) Dialysis
			(5) 24 h stirring of Ti3C2
			(6) Centrifugation
			(7) Sonication
Ti3C2	Hydrothermal	NSs: Ti3AlC2, HF, DMSO	(1) 64 h etching of 2 g Ti3AlC2	(1) 12 h heating	—	—	—	43
		QDs: PEI	(2) Washing	(2) Filtration
			(3) Under vacuum	(3) Dialysis
			(4) 24 h stirring
			(5) Centrifugation
			(6) Sonication
V2C	Hydrothermal	NSs: V2AlC, HF	(1) V2AlC 24 h stirring	(1) 6 h at 120 °C	4.13	2–3	—	44
		QDs: ammonium hydroxide	(2) 48 h stirring	(2) Filtration
			(3) Freeze-drying	(3) Dialysis
			(4) Sonication
Ti3C2	Hydrothermal	NSs: HF, Ti3AlC2, TMAOH	(1) HF etching of Ti3AlC2	(1) 6 h heating at 100 °C at autoclave	4.2	1.2–2.0	—	45
		QDs: Ti3C2, ammonia	(2) TMAOH intercalating	(2) Cooling
				(3) Filtration
				(4) Freeze-drying
Ti3C2	Hydrothermal	NSs: —	Chemical exfoliation	(1) 5 h sonication	4	—	—	1
		QDs: Ti3C2 NSs, ethylenediamine		(2) 12 h heating at 160 °C
				(3) 220 nm mesh filter
				(4) 12 h dialysis
BSA@Ti3C2	Hydrothermal	NSs: Ti3AlC2, HF	(1) 20 h etching of 0.5 g Ti3AlC2	(1) 50 mg Ti3C2 NS 6 h heating at 120 °C	2	—	—	30
		QDs: Ti3C2, BSA	(2) 15 min centrifugation	(2) 15 min sonication
			(3) Drying under vacuum	(3) pH = 9 by ammonia
				(4) Centrifugation
				(5) Filter
Ta4C3Tx	Hydrothermal	NSs: Ta4AlC3, HCl, NaF	(1) 48 h etching	(1) 12 h heating of Ta4C3Tx NSs	Diameter: 3.5	—	(1) HCl/NaF etchant: fewer surface defects compared to HF	20
		QDs: Ta4C3Tx	(2) Centrifugation				(2) ToxicityNaF < toxicityLiF
			(3) Washing				(3) Mechanical vibration and/or sonication: intercalation and interlayer spacing increment
			(4) Freeze-drying
			(5) Drying at 60 °C
			(6) Sonication
			(7) Homogenizer
Nitrogen-doped-Ti3C2	Hydrothermal	NSs: Ti3AlC2, HF	(1) 48 h etching of 1.5 g Ti3AlC2	(1) 36 h mixing	3.8	1.24	—	46
		QDs: Ti3C2, HNO3, H2SO4, NaOH, ethylenediamine	(2) Centrifugation	(2) Cooling to 25 °C
			(3) pH tuning (∼7)	(3) pH tuning (∼7)
			(4) Vacuum-drying	(4) 25 min ultrasonication
				(5) 12 h heating at 180 °C
				(6) Filtration
				(7) Dialysis
S and N doped-Ti3C2Tx	Hydrothermal	NSs: Ti2AlC, TiC, HF	(1) 16 h ball milling	(1) 2 h heating at 100 °C	N-MQDs: 50	14.15	—	9
		QDs: HNO3, H2SO4, Ti3C2, NaOH, Na2S2O3, NH3·H2O	(2) 2 h at 1350 °C	(2) NaOH → pH = 7	S-MQDs: 15–35	20.41
			(3) Crushing	(3) 12 h heating
			(4) Washing	(4) Dialysis
			(5) Centrifugation
			(6) Drying
Ti3C2Tx	Hydrothermal	NSs: Ti3AlC2, HF	(1) 72 h stirring	(1) 6 h heating	<10 nm	1.98	—	47
		QDs: ammonia	(2) Washing	(2) Centrifugation
			(3) Drying
			(4) Sonication
Ti3C2 QDs decorated TiO2/Nb2O5 composite	Hydrothermal	NSs: Ti3AlC2, HF	(1) 18 h etch	(1) Ammonia → pH = 9	3.8	—	—	48
		QDs: ammonia	(2) Rinsing	(2) 6 h heating
			(3) Centrifugation	(3) Filtration
			(4) Drying	(4) Drying
			(5) Ultrasonication
Na2Ti3O7/Ti3C2Tx QDs	Hydrothermal	NSs: Ti3AlC2, HF	(1) 24 h stirring	(1) 4 h vibration	—	—	—	49
		QDs: Ti3C2Tx, dimethyl sulfoxide, NH3·H2O	(2) Washing	(2) Washing
			(3) Drying	(3) 6 h heating
Ni–Co LDH@Ti3C2 QDs	Hydrothermal	NSs: Ti3AlC2, LiF, HCl	(1) 5 min stirring	(1) +Ammonia → pH = 9	3.06 ± 0.78	—	—	50
		QDs: —	(2) 24 h stirring	(2) 6 h heating at 100 °C
			(3) Washing	(3) Centrifugation
			(4) 1 h sonication
N-doped Ti3C2	Hydrothermal	NSs: Ti3AlC2, HF	(1) 24 h stirring	(1) 6 h heating at 100 °C	7.5	—	—	51
		QDs: Ti3C2, H2SO4, HNO3, NaOH, methanol	(2) Centrifugation	(2) Decant to ice water
			(3) Washing	(3) +NaOH (pH = 7)
			(4) Drying	(4) Centrifugation
				(5) 6 h drying
				(6) 5 h heating at 160 °C
				(7) Centrifugation
				(8) Evaporating
AgPt@N-Ti3C2 QDs	Hydrothermal	NSs: Ti3C2, nitric acid	(1) 24 h in oil bath at 100 °C	(1) 12 h heating at 160 °C	2–5	—	—	52
		QDs: ethanediamine	(2) +Ice water + NaOH → pH = 7	(2) Filtration
			(3) Centrifugation
N-Ta4C3	Hydrothermal	—	(1) Etch	(2) Hydrothermal	1.9–3.3 (avg: 2.6)	0.8–3.2 (avg: 1.9)	—	53
Eu doped-Ti3C2	Hydrothermal	NSs: Ti3AlC2, HF, NH3·H2O	(1) 20 h stirring	(1) 6 h heating at 120 °C	2.81	4.83	—	54
		QDs: —	(2) Washing	(2) 2 h heating at 90 °C
			(3) Centrifugation	(3) Centrifugation
			(4) 48 h drying
			(5) 30 min sonication
TiCN (titanium carbonitride, MXene-like)	Hydrothermal	NSs: —	—	(1) 5 min stirring	2.7 ± 0.2	2.7–4 (avg: 3.2 ± 0.3)	—	55
		QDs: TiCN		(2) 10 h heating at 120 °C
				(3) Centrifugation
N, P-Ti3C2	Hydrothermal	NSs: Ti3C2, HCl, nitric acid	(1) 12 h at 100 °C	(1) 12 h at 120 °C	N,P-MQDs: 1–8 (2.73 ± 0.50)	0.85 ± 0.02	—	56
		QDs: diammonium phosphate (DAP)	(2) 100 mL ice + NaOH → pH = 7	(2) Filtration	N-MQDs: 3.14	0.93
				(3) 2 days dialysis	P-MQDs: 2.97	0.71
				(4) Freeze-drying
S, N-Nb2C	Hydrothermal	NSs: Nb2C, sulphuric acid, nitric acid	(1) 12 h at 100 °C	(1) 12 h heating at 160 °C	S, N-Nb2C: 2.6–4.7 (avg: 3.56)	S, N-Nb2C: 1.74	—	57
		QDs: L-cysteine	(2) +NaOH → pH = 7	(2) Filtration	Nb2C: 2.4	Nb2C: 5.01
				(3) 2 days dialysis	N-Nb2C: 2.66	N-Nb2C: 2.80
N-doped Ti3C2 QDs@DAP	Hydrothermal	NSs: Ti3C2, NH3·H2O	(1) 30 min ultrasonic	(1) 6 h heating at 120 °C	3.4	0.5–1.3	—	58
		QDs: —	(2) +NH3·H2O (under N2) → pH = 9	(2) Centrifugation
				(3) Filtration
Amino-functionalized Ti3C2	Hydrothermal	NSs: Ti3AlC2, HF, NH3·H2O	(1) 24 h stirring at 60 °C	(1) 6 h heating at 120 °C	2.73	2.76	—	59
		QDs: —	(2) Rinse by DI	(2) 1 h dialysis
			(3) Centrifugation	(3) Centrifugation
			(4) 20 h drying	(4) Filtration
			(5) 30 min sonication	(5) Freeze-drying
N-doped Ti3C2	Hydrothermal	NSs: Ti3AlC2, LiF, HCl, APTES, ethanol	(1) 10 min stirring	(1) 12 h heating at 120 °C	7–10 (avg: 8.63)	—	—	60
		QDs: —	(2) 24 h stirring at 35 °C	(2) Filter
			(3) Centrifugal washing	(3) Dialysis
			(4) Vacuum-drying	(4) Freeze-drying
			(5) 1 h ultrasonication
			(6) Centrifugation
			(7) Ultrasonication
			(8) 24 h stirring
			(9) Centrifugation
TiO2/Ti3C2	Hydrothermal	NSs: Ti3AlC2, LiF, HCl	(1) Etch	(1) Heating	Diameter: ∼8.2	1.02	—	61
		QDs: —	(2) Sonication
GSH-Ti3C2 QDs	Hydrothermal	NSs: Ti3AlC2, HF	(1) 24 h stirring at 40 °C	(1) 12 h heating at 120 °C	2.5	—	Sonication: Ti3C2 film cutting + hydrothermal	62
		QDs: Ti3C2 NP, GSH, NaOH	(2) Washing	(2) Filtration
			(3) Centrifugation	(3) 12 h vacuum drying
			(4) 12 h drying
			(5) 5 h sonication
N, B-Ti3C2	Hydrothermal	NSs: —	—	(1) 12 h heating at 100 °C	2.25	—	—	63
		QDs: Ti3C2, boric acid, ammonia hydroxide		(2) Filtration
				(3) Freeze-drying
N-Ti3C2	Hydrothermal	NSs: —	—	(1) 24 h at 100 °C in oil bath	Hydrothermal: 120 °C: 3.93	Hydrothermal: 120 °C: 0.7	—	64
		QDs: Ti3C2, nitric acid, ethanediamine		(2) In beaker containing 100 mL ice, +NaOH → pH = 7.	160 °C: 3.7	160 °C: 1.4
				(3) 12 h heating at 160 °C	200 °C: 5.76	200 °C: 3.8
				(4) Filtration
				(5) 2 days dialysis
Nb2C	Solvothermal	NSs: —	—	(1) 0.5 h stirring of 15 mg Nb2C	—	—	—	65
		QDs: Nb2C, ammonia		(2) pH = 6 by ammonia
				(3) 6 h heating
				(4) Filtration
				(5) Drying
Nb2C-SH^a	Sonication	NSs: —	—	(1) 30 min sonication	2.3–5.4	—	—
		QDs: Nb2C, n-octadecyl mercaptan		(2) Stirring overnight
				(3) Filtration
Carbide-derived graphene QDs	Solvothermal	NSs: Ti3C2Tx, HF	(1) 5 h etching	(1) Centrifugation	4–10	1.36	(1) Solvothermal → C–Ti bonds break	66
		QDs: DMF	(2) Wash	(2) 20 h heating			(2) C atoms self-assemble into GQDs
			(3) Centrifugation	(3) Centrifugation
			(4) 3 h sonication	(4) Dialysis
Nb2C	Solvothermal	NSs: HF, Nb2AlC, TPAOH	(1) HF etching of Nb2AlC for 5 days	(1) 30 min sonication	5 nm	—	—	29
		QDs: 20 mg Nb2C NS, 20 mL TPAOH	(2) Stirring TPAOH + Nb2AlC for 5 days	(2) 24 h solvothermal at 110 °C
			(3) Sonication	(3) 120 h dialysis in water
				(4) Freeze-drying
Ti3C2	Solvothermal	NSs: Ti3AlC2, HF	(1) 24 h etching of Ti3AlC2	(1) Centrifuging	3	—	—	67
		QDs: DMSO, ammonia	(2) Drying	(2) Washing
			(3) Sonication	(3) pH tuning (∼9) by ammonia
				(4) Sonication
				(5) 6 h heating
				(6) Filtration
Ti3C2Tx	Solvothermal	NSs: Ti3AlC2, HF	(1) 12 h etching of Ti3AlC2	(1) 24 h shaking of as-prepared Ti3C2Tx	4.99	—	—	68
		QDs: Ti3C2Tx, TMAOH	(2) Thermal annealing	(2) Centrifugation
				(3) Sonication
				(4) Centrifugation
				(5) Filtration
				(6) Dialysis
Ti3C2Tx	Solvothermal	NSs: HF	(1) Etch	(1) Disperse	s-MQDs: 1.8	1–2.5	(1) Higher polarity solvent → stronger interaction → easier exfoliation of NSs to QDs	69
		QDs: DMF, ethanol, DMSO		(2) 6 h heating	e-MQDs: 2.5		(2) Lower boiling point solvent → higher pressure → stronger exfoliation efficiency
				(3) Centrifugation	f-MQDs: 3.3
N-doped Ti3C2	Solvothermal	NSs: Ti3C2, KOH	(1) 3 h sonication	(1) 12 h heating at 140 °C	2.3	—	—	2
		QDs: Ti3C2, DMF	(2) Centrifugation	(2) Centrifugation
N-Ti3C2	Solvothermal	NSs: DMF	(1) Sonication (12 h at 750 W)	(1) 8 h heating at 150 °C	6.2	1	—	70
		QDs: DETA, DMF	(2) Centrifugation	(2) 5 centrifugation re-suspension cycles
			(3) Drying	(3) Drying
Ti3C2	Solvothermal	NSs: Ti3AlC2, HCl, LiF	(1) 5 min stirring	(1) 24 h heating at 120 °C under N2	10.2	∼1.0		71
		QDs: PEI	(2) 24 h stirring at 40 °C	(2) Filtration
			(3) Wash by DI	(3) Dialysis
			(4) Centrifugation
			(5) Decanting → pH = 6
			(6) 10 min purging with N2
			(7) 5 h ultrasonication
			(8) Centrifugation
PLL-protected Ti3C2	Solvothermal	NSs: Ti3AlC2, HF	(1) 24 h stirring at 40 °C	(1) 24 h heating at 100 °C under N2	3	—	—	72
		QDs: Ti3C2 NP, ε-poly-L-lysine (PLL), NaOH	(2) Wash	(2) Filtration
			(3) Centrifugation	(3) Dialysis
			(4) Purging with N2 → film	(4) Drying
			(5) 5 h sonication	(5) 12 h vacuum drying at 60 °C
			(6) Centrifugation
Ti3C2	Reflux	NSs: Ti3AlC2, HF, TMAOH	(1) 5 h etching Ti3AlC2	(1) 1 day reflux	8	—	—	73
		QDs: TMAOH	(2) Wash	(2) Centrifugation
			(3) Centrifugation	(3) Vacuum drying.
			(4) Vacuum-dry
			(5) Intercalation by TMAOH
			(6) Centrifugation
			(7) Vacuum filtration
			(8) Vacuum drying
N-Ti3C2Tx	Reflux	NSs: Ti3AlC2	(1) 24 h stirring at 60 °C	(1) 3 days oil bath refluxing at 120 °C	2.7	—	—	74
		QDs: Ti3C2Tx, TMAOH	(2) Centrifugation	(2) Centrifugation
			(3) Wash → pH = 7	(3) Overnight vacuum drying at 200 °C
			(4) Overnight desiccation under vacuum at 80 °C
Ti3C2Tx QDs/TiO2/FTO	Reflux	NSs: Ti3AlC2, HF, TMAOH	(1) 12 h stirring	(1) 1 day reflux at 110 °C	—	Avg: ∼1.0	—	75
		QDs: TMAOH	(2) Centrifugation	(2) Centrifugation
			(3) Drying at 200 °C under vacuum	(3) Vacuum drying at 200 °C.
			(4) 12 h stirring
			(5) Centrifugation
			(6) Vacuum drying
Ti3C2Tx	Microwave + reflux	NSs: Ti3AlC2, HF	(1) 24 h stirring	(1) 30 min microwave irradiation	3.3	2.5	—	76
		QDs: Ti3C2Tx	(2) Washing	(2) Filtration
			(3) 12 h drying	(3) Reflux
UA@Ti3C2 QDs	Microwave	NSs: Ti3C2, HNO3, NaOH, HCl	(1) 24 h heating at 100 °C	(1) Microwave irradiation	6.4 ± 0.5	—	—	77
		QDs: UA	(2) In beaker → pH = 7	(2) Centrifugation
			(3) Centrifugation
N, P-Ti3C2 QDs	Microwave	NSs: HF	(1) Etch	(1) Microwave	3.11 ± 0.86	—	—	78
		QDs: Ti3C2, formamide, ammonia		(2) Centrifugation
				(3) Filtration
				(4) Vacuum drying
Ti3C2	Microwave	NSs: Ti3AlC2, ammonia	(1) 24 h stirring at 40 °C	(1) Microwave (800 W, 5 min, 90 °C)	Diameter: 2 ± 0.33	—	—	63
		QDs: —	(2) Washing	(2) Filtration
			(3) Centrifugation	(3) 12 h vacuum drying at 60 °C
			(4) Drying at 60 °C under vacuum
			(5) 3 min ultrasonication
Cl, N doped-Ti3C2	Potential static	NSs: —	—	(1) V = 0.1 V, 1 h	3.45	—	(1) Cl^− promotes N doping in graphene	79
		QDs: Ti3AlC2, Pt wire, TMAOH, NH4Cl		(2) Centrifugation			(2) Cl^− breaks Ti–Al bonds
				(3) Filtration			(3) NH4OH is intercalated
				(4) Dialysis
Ti3C2Tz	Acoustomicrofluidic	NSs: Ti3AlC2, HF	(1) 24 h stirring	(1) Cycles of centrifugation	7.8 ± 5.2	1.1 ± 0.2	Nebulization of sample driven by high frequency acoustic waves	80
		QDs: —	(2) Washing	(2) Membrane vacuum-filtering
				(3) 24 h drying
Ti3C2Tx	Micro-explosion	NSs: Ti3AlC2, HF	(1) 3days stirring at 40 °C	(1) +L-N2 → evaporation	7.23	5	Temperature difference of L-N2 and hot deionized water	81
		QDs: Ti3C2Tx	(2) Centrifugation	(2) 24 h stirring
			(3) Washing	(3) Filtration
			(4) 24 h drying	(4) Centrifugation
				(5) Freeze-drying
WO3/Ti3C2 QDs/In2S3	Freezing-and-thawing + sonication	NSs: Ti3AlC2, LiF	(1) 24 h stirring	(1) 5 min centrifugation	1.66 ± 0.04	0.5–2	—	82
		QDs: —	(2) Centrifugation	(2) 3 h at 4 °C
				(3) 3 h at −38 °C
				(4) 4 times freezing-and-thawing
				(5) 2 h sonication
				(6) Filtration
Mo2C QDs embedded in carbon NSs	Molten salt	NSs: —	—	(1) 15 min stirring at 70 °C	2–3	3.5 (Mo2C/C NS)	—	83
		QDs: molybdenum acetylacetonate, sucrose, ethanol, NaCl		(2) +NaCl
				(3) 2 h calcination at 800 °C under Ar
				(4) 3 times washing
				(5) Centrifugation
Mo2C nanoparticle-decorated carbon polyhedrons	Solvothermal + pyrolysis	NSs: —	—	(1) 48 h heating at 160 °C	<5	—	—	84
		QDs: Zn(CH3COO)2·2H2O, MIM, PVP, DMF, Mo/ZIF-8, H2SO4		(2) Centrifugation
				(3) Washing
				(4) Drying at RT
				(5) 2 h pyrolysis
				(6) Treating in 0.5 M H2SO4 aqueous
Mo2C QDs-decorated CNT networks	Spray-drying	NSs: —	—	(1) 1 h ultrasonication	<3	—	Mo2C QDs: polar and conductive → provide active sites to absorb + Mo2C QDs welding spots for CNT	85
		QDs: ammonium molybdate tetrahydrate, CNT		(2) Spray drying
				(3) 1 h carbonization at 500 °C
				(4) 4 h at 700 °C under Ar
Mo2C/C nanoflowers (ultrathin carbon NSs decorated with Mo2C QDs)	In situ synthesis + calcination	NSs: —	—	(1) 30 min stirring	2–4	—	—	86
		QDs: dopamine hydrochloride, ammonium molybdate, PEG, ethanol, NH3·H2O		(2) Injecting of NH3·H2O
				(3) 6 h stirring
				(4) Centrifugation
				(5) Washing
				(6) 2 h stabilizing at 450 °C
				(7) 3 h calcining at 900 °C under Ar

---

3.1. Top-down approaches

The top-down procedure is generally comprised of three main steps, including (i) preparation of MXene by etching layer “A” from the MAX phase, (ii) intercalation and exfoliation of layers to produce MXene nanosheets, and (iii) treatment to convert the nanosheets to QDs.

In the first step, the layer “A” is removed with either a chemical etching or electrochemical etching. Etchants like HF are commonly used to target MAX powders, for example, Nb₂AlC and Ti₃AlC₂, to produce MXene phases.^39,50,82 However, HF is a hazardous material, and even time-consuming multiple centrifugation processes have failed to remove HF completely.²⁴ Numerous attempts have been performed to use alternative chemical etchants. For instance, mixtures of HCl/KF, HCl/sodium fluoride (NaF), and LiF/HCl have proven effective for removing Al from Ti₂AlN,²³ Ta₄AlC₃ or Ti₃AlC₂,^{35,38,41,43,60,64,71} and Ti₃AlC₂.⁸⁷ The acid (HCl) and fluorine components react with the MAX phases to remove Al by the formation of AlCl₃ and other products, such as metal hexafluoroaluminate, resulting in the formation of multilayered MXene. Alternatively, electrochemical etching can be conducted in a 3-electrode cell (Fig. 3a), consisting of a working electrode (e.g., Ti₃AlC₂), an Ag quasi-reference electrode, and a Pt counter-electrode. An electrolyte (e.g., [EMIM][PF₆]/MeCN) is used to remove the A-layer (Al) to prepare the MXene phase (Ti₃C₂T_x).⁷⁷

Fig. 3 (a) Electrochemical etching of Ti₃AlC₂ to produce Ti₃C₂T_x MQDs (reprinted with permission from American Chemical Society, copyright © 2020).⁷⁷ (b–d) Size distribution, (e–g) AFM images, and (h–j) thickness of QDs depending on the processing temperature: (b, e and h) 120 °C; (c, f and i); 160 °C; (d, g and j) 200 °C (reprinted with permission from Royal Society of Chemistry, copyright © 2022).³⁶

For the preparation of MXene nanosheets, studies have determined that stirring multilayer MXenes in certain solvents like tetramethylammonium hydroxide (TMAOH) and sonication in tetrapropylammonium hydroxide (TPAOH),⁵⁰ dimethyl sulfoxide (DMSO),³³ and potassium hydroxide (KOH)² enable synthesizing MXene nanosheets.³⁴ While converting MXene multilayers to MXene nanosheets can be accompanied by various methods, a following procedure is required to synthesis MQDs.⁸⁸ A broad spectrum of approaches to synthesize MQDs are briefly presented below.

3.1.1. Stirring in intercalating solutions and sonication. Several studies have reported that facile procedures of stirring of a mixture of water and MXene nanosheets, followed by centrifugation, can effectively produce MQDs.⁸⁹ However, the sonication of MXene in certain solutions, such as N-methylpyrrolidone (NMP),^23,82 TPAOH,⁴⁵ TMAOH,⁸² NaOH,⁶⁹ and ethanol,⁴⁶ has been the most widely adopted methods to convert MXene to MQDs. For example, Ti₂N QDs with lateral sizes of 2–6 nm and an average thickness of 1.8 nm have been prepared by sonicating the solution of Ti₂N nanosheets/NMP in an ice bath, followed by centrifugation.^69,89 Physical shearing forces caused by ultrasound as well as chemical reactions between MXene and NMP are outstanding driving forces inducing cleavage and fragmentation.⁴⁵

3.1.2. Hydrothermal methods. Hydrothermal processing of MQDs is one of the most effective methods to process various types of QDs from nanosheets. This technique utilizes high pressure and temperature in a heated sealed autoclave containing an aqueous solution of MXene nanosheets to synthesize QDs in a reactive environment.^57,66 For instance, hydrothermal treatment of Ta₄C₃T_x nanosheets, prepared through HCl/NaF etching and sonication processes, at 180 °C for 12 h in an ammonia solution results Ta₄C₃T_x QDs.⁶⁶ A combination of hydrothermal and sonication methods can also be utilized to enhance the product yield of Ti₃C₂ QDs.¹

Although various parameters influence the hydrothermal processing of MQDs, temperature manifests the most determining effect on their properties. For instance, Xu et al. synthesized MQDs with different sizes by varying the hydrothermal temperature of MXene nanosheets from 120 to 200 °C. After heating the solution of Ti₃C₂ nanosheets in nitric acid at 100 °C for 24 h and adjusting the pH to 7, 2 mL ethanediamine was added to 20 mL Ti₃C₂ nanosheets in a stainless-steel autoclave, followed by heating at 120, 160, and 200 °C for 12 h to synthesize nitrogen-doped Ti₃C₂ QDs. Taking the atomic force microscopy (AFM) and transmission electron microscopy (TEM) results into account, the lateral size/thickness of the N-Ti₃C₂ QDs changed from 3.93/0.7 nm at 120 °C to 3.7/1.4 nm at 160 °C and 5.76/3.8 nm at 200 °C, respectively (Fig. 3b–j). These results showed that the QDs treated at 120 °C, 160 °C, and 200 °C correspond to monolayer, bilayer, and multilayer structures, respectively, revealing the effect of temperature on the number of layers. Despite revealing the effect, no mechanism was proposed to explain this trend, which should be addressed in future investigations. The XPS characterization of N-MQDs shows that C=N bonds was dominant at 200 °C stating that overcoming the energy barrier requires higher temperature. The life-time investigation demonstrates that temperature increment up to 160 °C increases life-time to a maximum, however, at higher temperature life-time decreases possibly owing to chemical state change.³⁶

Both doping and surface functionalization of MQDs can be performed during hydrothermal synthesis. In the former case, studies have shown that adding HNO₃, H₂SO₄, and ethylenediamine results in nitrogen (N)-doping of Ti₃C₂ QDs.⁸¹ As a result of doping, the size of MQDs can be altered. For example, during N-doping of Nb₂C, the thickness decreased from 5.0 to 2.8 nm with a marginal effect on the lateral size.⁹⁰ Regarding the latter case, Al-Duais et al.³⁹ have indicated that sonication-assisted hydrothermal treatment of aqueous solution of Ti₃C₂ and bovine serum albumin (BSA) forms BSA-functionalized Ti₃C₂ QDs (BSA@Ti₃C₂), serving as sensitive fluorescent probes.³⁹

An alternative benefit of hydrothermal treatments is the fabrication of MQD/semiconductor heterostructures. Ding et al.⁸¹ have prepared N-Ti₃C₂ QDs/CdS nanorods heterostructure through a self-assembly strategy by mixing the CdS nanorods (length = 1–2 µm and D = 30 nm) with N-Ti₃C₂ (D = 3.8 nm and t = 1.24 nm) in DI water for two days. Using a similar strategy, g-C₃N₄@Ti₃C₂ QD composites were prepared by mixing the g-C₃N₄ NSs and Ti₃C₂ QDs for 24 h. Ti₃C₂ QDs assisted g-C₃N₄ in: (i) boosting the density of the active sites, (ii) increasing the specific surface area, and (iii) improving carrier transfer efficiency due to excellent electronic conductivity in view of metallic characteristic.³³

3.1.3. Solvothermal methods. Solvothermal treatments of MXene nanosheets, similar to the hydrothermal method but differing in the type of solvent used, have garnered considerable attention. In this process, organic solvents, such as TPAOH,⁵⁰ DMF,^2,91 DMSO,⁷⁹ and ethanol,⁶⁷ are used instead of an aqueous solution to synthesize MQDs. For instance, the solvothermal processing of Nb₂C NSs in TPAOH at 110 °C for a day yields spherical Nb₂C QDs with a diameter of 5 nm.⁵⁰

The type of the solvent plays an important role in altering the properties of MQDs. Fig. 4a–f clearly shows that solvothermal treatment of MXene nanosheets in DMF, DMSO, and ethanol provides MQDs with a different lateral size. The higher polarity of the solvent increases the interaction between solvent and nanosheets, leading to exfoliation facilitation and further size reduction. Therefore, DMSO with the highest polarity and strongest oxidation ability formed the smallest particles. In contrast, DMF with higher polarity than ethanol shows larger particles, corresponding to the effect of boiling point. The lower the boiling point, the higher the vessel pressure, paving the way for preparing smaller particles. In this case, the boiling point of ethanol (74.8 °C) is lower than DMF and the solvothermal temperature (120 °C), thereby producing more evaporated gas molecules and higher pressure, enhancing the exfoliation and size reduction.⁶⁷

Fig. 4 TEM images and size distribution of (a and d) DMSO-MQDs, (b and e) ethanol-MQDs and (c and f) DMF-MQDs (reprinted with permission from Wiley, copyright © 2022).⁶⁷

3.1.4. Micro-explosion and freezing-and-thawing methods. MQDs have also been synthesized using an efficient and innovative method called micro-explosion. In this approach, Ti₃C₂T_x is intercalated by liquid nitrogen (L-N₂) and hot deionized water.⁹² Constantly evaporating L-N₂ at room temperature, owing to its low boiling point (−196 °C), intercalates Ti₃C₂T_x layers. By adding deionized water at high temperatures (100 °C), rapid gasification of the interlayer L-N₂ in a confined space occurs, resulting in micro-explosion due to temperature difference, forming Ti₃C₂T_x QDs with a lateral size and diameter of 7.23 nm with 5 nm, respectively (Fig. 5a).⁹²

Fig. 5 (a) Synthesis of Ti₃C₂T_x MQDs by the micro-explosion method (reprinted with permission from Wiley, copyright © 2020).⁹² (b) Synthesis of Ti₃C₂T_x and N-Ti₃C₂T_x MQDs by reflux methods (reprinted with permission from The Royal Society of Chemistry, copyright © 2022).⁹³

The freezing-and-thawing method is one of the newest methods that is commonly applied in the exfoliation of boron nitride nanosheets,⁹⁴ metal–organic nanosheets,⁹⁴ graphene oxide (GO),⁹⁴ and antimony triselenide (Sb₂Se₃).⁹⁴ To prepare Ti₃C₂ QDs with 1.7 nm average lateral size, the solution of Ti₃C₂ experiences freezing (at −38 °C for 3 hours) and thawing (at room temperature) cycles, followed by sonication to convert MXene flakes to QDs. By repeating freezing and thawing cycles, water is solidified and expanded multiple times, creating a honeycomb structure. This structure boosts van der Waals breakage by exerting extrusion force, thus promoting nanosheets peeling and QDs formation.³⁵

3.1.5. Other methods. Apart from the abovementioned methods, some new approaches have been introduced to prepare MQDs. Potential static is one of the them that enables synthesizing Cl, N doped-Ti₃C₂ QDs with a lateral size of 3.4 nm. This was done by applying a specific amount of voltage to the electrochemical cell, comprising Ti₃AlC₂ as a working electrode, Pt wire as a counter electrode, and mixture of TMAOH and NH₄Cl as the electrolyte.⁷⁵ Reflux is another method that was used to prepare a broad spectrum of QDs, including nitrogen-doped carbon (NDC),⁷³ CdS,⁷⁶ CA-capped CdTe,⁷⁶ TiO₂,⁷⁸ and Cu₃BiS₃ QDs.⁸⁰ The key feature of this method is that the condenser prevents solvent vapors from escaping, while a circulating coolant, such as water, condenses the vapor and returns it into the reaction vessel.^83,95 This method has been employed to synthesize Ti₃C₂T_x QDs by processing of Ti₃C₂ and TMAOH at 110 °C for a day (Fig. 5b).⁸³ The microwave method, which has been used for the synthesis of a wide range of QDs, has also been found effective in forming bare and doped MQDs. For instance, Ti₃C₂T_x nanosheets turned into QDs by microwave heating at 560 W for 3 min and subsequent refluxing.^84,96 Acoustomicrofluidic is a chemical-free method that can be used to synthesize MQDs with a size of 7.8 nm and thickness of 1.1 nm.⁹⁷ This approach utilizes surface-reflected bulk waves to minimize oxidation, causing the material to nebulize and form aerosol droplets, which then condensed within the glass enclosure. The wave has a surface acceleration of 10⁸ m s⁻² and strong evanescent electric field of 10⁷ V m⁻¹, facilitating exfoliation through protonation of lithium fluoride into hydrofluoric acid. The influence of nebulization cycles was examined by performing 1, 5, and 10 cycles, which transformed nanosheets to ultrathin QDs by reducing thickness and lateral size. The average thicknesses decreased from 8.5 nm to 4.8 nm and 1.1 nm, while the mean lateral sizes shrank from 68.4 nm to 27.1 nm and 10.7 nm, respectively. Although these results demonstrate that the size and morphology of the material can be precisely tuned by controlling the number of nebulization cycles, no mechanism was proposed to explain these observations.⁹⁷

3.2. Bottom-up approaches

Compared to top-down methods, which offer a wide range of synthesis designs for MQDs preparation, only a few bottom-up processes have been proposed for producing MQDs, primarily Mo₂C QDs. Various studies have demonstrated the great potential of molten salts to synthesis various QDs, such as polymeric carbon nitride,⁸⁵ In_1−xGa_xP, In_1−xGa_xAs, and Mo₂C. In the case of the latter, NaCl is mixed with Mo-containing precursor and sucrose in ethanol/H₂O (4 : 1), followed by continuous NaCl nanocubic crystal precipitation and stacking growth. Subsequently, sucrose and the Mo-containing precursor are confined within the interstices of the NaCl crystals. Finally, heat treatment at 800 °C for 2 hours in argon results in the formation of molybdenum carbide nanodots embedded in ultrathin carbon nanosheets (Mo₂C/C NSs with an average size of 2–3 nm) (Fig. 6a).³⁷

Fig. 6 (a) Molten salt synthesis of Mo₂C/C nanosheets (reprinted with permission from Wiley, copyright © 2018).³⁷ (b) Pyrolysis synthesis of Mo₂C-decorated carbon polyhedrons (reprinted with permission from American Chemical Society, copyright © 2018).⁴² (c) Spray-drying synthesis of Mo₂C QDs-decorated CNT networks (reprinted with permission from Wiley, copyright © 2021).⁹⁸

Pyrolysis is a temperature-induced method, which creates QDs by chemical decomposition of organic materials in the absence of oxygen.⁴⁰ To prepare Mo₂C nanoparticle-decorated carbon polyhedrons, Mo/ZIF-8, formed by the hydrothermal treatment of the mixture of precursors, is heated for 2 hours in an argon-filled tube furnace at 700 °C. As a result of pyrolysis, Mo/ZIF-8 framework is transformed into a carbon matrix, simultaneously forming Mo₂C NPs less than 5 nm (Fig. 6b).⁴²

Spray-drying is another process for preparing particles, in which a solution of raw materials is pressurized to form droplets and then dried at high temperatures to form particles.⁴⁴ This method was used to prepare Mo₂C QDs-decorated CNT networks.⁹⁸ As illustrated in Fig. 6c, an ultrasonication-treated ammonium molybdate tetrahydrate in CNT solution is spray-dried at a flow rate of 1 L h⁻¹ at 170 °C, followed by carbonization in the quartz tube to complete the procedure.⁹⁸

4. Properties

MQDs possess key properties, including high colloidal, chemical, and photostability, making them promising candidates for various applications, including biomedicine, sensing, and optoelectronics. We aim to highlight two interesting properties biocompatibility/cytotoxicity and optical behavior which highly influence the performance of biomedical and optoelectronic devices.

4.1. Biocompatibility and cytotoxicity

MQD with excellent in vitro and in vivo biocompatibility have unlocked new opportunities in biomedical engineering across various fields, including transplantation medicine, cancer photo-thermal therapy, and bio-imaging.⁴⁷ This section addresses recent advancements in bioinertness, bioactivity, biodistribution, cytotoxicity, and cellular dysregulation caused by oxidative stress, collectively referred to under the broad term biocompatibility.

4.1.1. In vitro assays. A crucial step in evaluating the biomedical potential of MQDs is assessing their compatibility with biological systems. This evaluation involves identifying challenges and advancements in understanding the biocompatibility of MQDs, emphasizing the importance of prioritizing human health and safety. For example, Ti₃C₂ QDs exhibit good biocompatibility with multiple therapeutic cell types, such as Mesenchymal Stem Cells (MSCs), induced Pluripotent Stem Cells (iPSCs)-derived Fibroblasts (iPS-Fibs), and human immune cells.^48,49 Some studies have shown the potential of MQDs to enhance cell survival and function.⁵¹ MSCs and iPS-Fibs are particularly promising due to their inherent capacity to repair and regenerate damaged tissues through paracrine actions and versatile differentiation.^48,49 To investigate the in vitro biocompatibility of MQDs, different methods, including calcein AM/EthD-1 cell viability assay, flow cytometry, fluorescence microscopy using calcein AM and 7-AAD, LDH cytotoxicity assay, and cell proliferation assay have been employed. It is also important to evaluate the cytocompatibility of MQDs at different concentrations over both short-term (24 h) and long-term (1 week) periods.⁵¹

Materials selection is a crucial factor influencing the biocompatibility of MQDs. For instance, Ti₃C₂T_x can elevate intracellular reactive oxygen species (ROS), leading to oxidative stress with both beneficial and detrimental effects. While it can be used to treat tumors and bacterial infections,⁵² it also has the potential to cause cellular damage.⁵³ In contrast, Ti₂N QDs demonstrate no cytotoxic effects on human embryonic kidney cells, U87 human malignant glioma cells, or 4T1 murine mammary carcinoma cells (in vitro) in a standard 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) cytotoxicity assay.²³ The destruction of cancer cells using Ti₂N QDs only occurs when they are exposed to near-infrared (NIR) laser irradiation for a short duration, a result that could not be achieved with either MQDs or NIR irradiation alone. This phenomenon positions Ti₂N as a promising candidate for photothermal cancer therapy. Fig. 7a shows the confocal fluorescence images after the treatment with calcein AM (green) and PI (red) for live and dead cells, respectively.²³

Fig. 7 (a) Confocal images showing calcein-AM staining for live cells (green) and PI staining for dead cells (red). The scale bar is 100 µm. Injection (reprinted with permission from Elsevier, copyright © 2020).²³ (b) H&E-stained images of major organs of the Ti₂N QDs treated mice at 20 days post-injection (reprinted with permission from Elsevier, copyright © 2020).²³

In addition to materials selection, surface functional groups significantly influence the biocompatibility and bioactivity of MQDs. For instance, the surface of Ta₄C₃T_x MQDs, enriched with negatively charged functional groups, enhanced bioactivity by facilitating interactions between the QDs and cells.^24,54 In another study, it has been demonstrated that the surface charge of Ta₄C₃T_x MQDs varies with pH, enabling these QDs to escape endosomes shortly after cellular uptake, and localize near the nucleus.^24,55

The concentration of MQD colloidal solution is another important factor that greatly influences its biocompatibility. In one study, various concentrations of Ta₄C₃T_x MQDs, ranging from 2 μg mL⁻¹ to 100 μg mL⁻¹ in PBS, were applied in cultures of human umbilical vein endothelial cells (HUVECs). Using CellROX green fluorescent dye and the CellEvent fluorescence-based apoptosis detection kit, the researchers found that these concentrations neither induced intracellular oxidative stress nor activated caspase-3 and caspase-7, which are key mediators of the cell death.²⁴ No significant changes in cellular cytotoxicity or cell growth were observed over a seven-day period.²⁴ It is noteworthy that most of the aforementioned findings are predominantly based on laboratory in vitro experiments; hence, in vivo investigations using diverse disease models and real clinical conditions are needed to fully understand the performance of MQDs in living organisms.

4.1.2. In vivo assays. Researchers actively explore the in vivo performance of MQDs to address concerns about their potential toxicological effects. For instance, the long-term toxicity of Ti₂N QDs used as nanoagents in photothermal therapy (PTT), was systematically evaluated by monitoring mice at a dose of 20 mg kg⁻¹ over 20 days. Detailed blood biochemistry assays and histological examinations revealed no signs of organ damage in the treated group compared to the control group (Fig. 7b). Therefore, within the observed timeframe, the investigated concentrations of Ti₂N QDs did not elicit a significant toxic response.²³

Biodistribution is also crucial for MQD applications in living organisms. Encapsulation of MQDs within stable substances like albumin has been explored,⁵⁶ along with other techniques; for example, Ti₃C₂ MQDs, used as photothermal agents for cancer treatment, gain improved dispersion and biocompatibility by ultrasonic oxidation, introducing hydrophilic groups without surface modification.⁵⁸ Biodistribution studies on Ti₂N QDs for photothermal cancer therapy determined accumulation of the MQDs in tumors, the liver, and kidneys. The enhanced permeability and retention effect considerably promoted the uptake of MQDs in cancerous areas, improving the PTT efficacy. The high content of Ti₂N within the liver primarily resulted from the clearance activity of the reticuloendothelial system, whereas the kidney absorption might be associated with potential renal elimination. The amounts of Ti₂N QDs detected in the feces and urine of mice were similar to those accumulated in the kidney that were metabolized, showing a high clearance efficiency. Therefore, it was concluded that the long-term toxicity concerns for Ti₂N QDs, as inorganic PTT nanoagents, are less significant.²³

4.2. Optical properties

MXenes have garnered significant attention for their exceptional electrical conductivity (up to 20 000–24 000 S cm⁻¹ (ref. 59)). However, their PL quantum yield remains extremely low.⁶¹ This raises the question: Can the transition from a 2D structure to a QD morphology enhance the optical properties of MXenes? Through what mechanisms?

The answer is yes. The 2D to QD transformation induces various fascinating optical characteristics. First, QDs exhibit a strong quantum confinement effect, especially when their size falls below the Bohr radius.⁶² In this case, the photogenerated excitons, i.e., a pair of electron and hole, are tightly confined within the QD, increasing the probability of radiative recombination and enhancing photoluminescence quantum yield (PLQY). Second, QDs feature tunable bandgap energy.⁶² As the lateral size of QDs decreases, the highest occupied molecular orbital (HOMO)–the lowest unoccupied molecular orbital (HOMO–LUMO) gap gradually widens, the absorption edge is blue-shifted, and the continuous spectral bands change to sharp spectral peaks, resulting in a stronger quantum confinement effect.⁶² Beyond the quantum confinement effect, surface states and edge effects with various F–, OH–, and O– functional groups play an important role in tuning the optical properties of QDs.⁶³

To quantitatively assess the optical properties of MQDs, ultraviolet-visible spectroscopy (UV-Vis) and PL spectrometry have widely been employed, providing valuable insights into the absorption edge, emission peak, and bandgap energy (E_g). Regarding absorbance, Ti₃C₂ MQDs have an absorption band between 245–275 nm related to π–π* electronic transitions.⁶⁵ In some cases, an absorption peak between 300–320 nm was also observed, corresponded to the n–π* transition.⁶⁵ In terms of PL, the strongest emission peak typically appears at ∼460 nm when QDs are optically excited at 365 nm (Fig. 8a).⁶⁵ This emission may be controlled by the quantum confinement effect or edge defects,⁶⁵ leading to excitation-dependent or excitation-independent PL, respectively. While increasing the excitation wavelength shifts the emission peak to longer wavelengths in the former case, there is no shift in the emission peak in the latter case. Although the underlying mechanisms of this phenomenon have thoroughly been explored in other QDs like SiC, there is a lack of in-depth investigation into this behavior for MQDs. In terms of bandgap energy, studies have revealed that by changing the lateral size of MQD, their bandgap energy can be tuned between 0.1–3.52 eV (Table 2).⁴⁶

Fig. 8 (a) UV–vis absorption spectrum of Ti₃C₂ MQDs. The inset shows the emission and photographs of fluorescence spectra (reprinted with permission from Elsevier, copyright © 2022).⁹⁹ (b) UV-Vis absorption spectrum and PL spectrum of N-doped MQDs (reprinted with permission from Elsevier, copyright © 2021).¹⁰⁰ (c) The emission spectra of Ti₃C₂ MQDs prepared in different solvents (reprinted with permission from Elsevier, copyright © 2022).² (d) Normalized ECL emission spectrum (red curve) of N-Ti₃C₂ QDs in 10 mM S₂O₈²⁻ solution (reprinted with permission from Elsevier, copyright © 2022).¹⁰¹ The charge density difference of (e) Ti₃C₂ MQDs, (f) N-functionalized Ti₃C₂ MQDs, (g) P-functionalized Ti₃C₂ MQDs, and (h) N, P functionalized MQDs. (i) PL decay spectra of doped MQDs (reprinted with permission from Royal Society of Chemistry, copyright © 2019).¹⁰² (j) UV-vis and PL spectra of SN-MQDs (reprinted with permission from Elsevier, copyright © 2019).⁹

Table 2 Effects of functionalization on the optical properties of MQDs

MXene QDs	Size*	λabs (nm)	λexc (nm)	λem (nm)	PLQY (%)	τ (ns)	Ref.
	l (nm) and t (nm)
* Average lateral size (l), thickness (t), quantum dot (QD), absorption wavelength (λabs), excitation wavelength (λexc), emission wavelength (λem), excitation wavelength (λexc), carrier lifetime (τ), photoluminescence quantum yield (PLQY), glutathione (GSH), ε-poly-L-lysine (PLL).
TiCN	l = 2.7	232	310	400	—	—	103
	t = 3.2
Ti3C2	l = 2.75	—	365	445	4.5	—	68
Ti3C2	l = 1.75	—	320	410	7.7	—	70
N-Ti3C2	l = 1.75	239, 293, 427	420	580	5.42	—	100
(1) N-Ti3C2	(1) l = 3.14, t = 0.93	234, 295	—	—	(1) 10.3	(1) 6.02	102
(2) P-Ti3C2	(2) l = 2.97, t = 0.85		—	—	(2) 2.4	(2) 4.99
(3) N, P-Ti3C2	(3) l = 2.73, t = 0.71		480	560	(3) 20.1	(3) 8.54
N-Ti3C2 QDs	l = 3.4	229, 290	369	448	DMF = 11.3, ethanol = 1.09, H2O = 0.34	—	104
	t = 0.5–1.3
N-Ti3C2	l = 2.3	—	335	420	14.46	5.24	61
N-Ti3C2	l = 8.63	—	330	427	N-Ti3C2 (APTES) = 15.4	6.86	64
					Ti3C2 = 6.7
S, N co-doped Nb2C	l = 3.8	—	390	520	17.25	7.12	2
	t = 1.2
N-doped Ti3C2	l = 3.4	—	360	470	18.7	7.06	36
N, B-Ti3C2	l = 2.25	220, 280	335	448	18.9	4.39	101
GSH-Ti3C2	l = 2.5	300	337	430	21	—	105
PLL-protected Ti3C2	l = 3	—	330	415	22	—	64
N-doped Ta4C3	l = 2.6	234, 293	380	450	23.4	7.92	106

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There are several factors that play a crucial role in optoelectronic properties of MQDs, with the synthesis method being one of the most important. For example, the ultrasonication-assisted HF process in various organic solvents was among the earliest approaches used for synthesizing fluorescent MQDs. While MQDs synthesized in dimethylformamide (DMF) had an average size of 2.75 nm with a PLQY of 4.5%,⁶⁸ using DMSO reduced the MQD size to less than 2 nm and enhanced the PLQY to 7.7%.^70,72 Compared with the ultrasonication method, the solvothermal process in organic solvents exhibited different results: a brownish solution of Ti₃C₂T_x QDs in DMSO (D = 1.8 nm) exhibited an emission wavelength and PLQY of 570 nm and 4.1%, respectively; a yellowish QDs in DMF (D = 3.3 nm) and colorless QDs in ethanol (D = 2.5 nm) indicated lower emission wavelengths at 436 and 370 nm but higher PLQYs at 10.7% and 6.9%, respectively.⁶⁷ These size and optical property differences were attributed to the polarity, boiling temperature, and oxidation capability of the solvents. Solvents with higher polarity and lower boiling temperature normally paves the way for 2D to QD transformation, due to the strong 2D/solvent interaction and higher gas pressure, leading to smaller QDs with higher PLQY. However, Ti₃C₂T_x QDs showed a reverse trend in terms of PLQY since DMSO formed the smallest QDs with the lowest PLQY because the solvent had the highest intensity of oxygen-containing functional groups (e.g., C–O–C, C=O, and Ti–O). This oxidation degraded Ti₃C₂T_x into anatase or rutile TiO₂ NCs, leading to a decrease in PLQY.⁶⁷

Beyond the type of solvents, the processing time and temperature significantly influence the optical properties of MQDs. Studies have shown that extending the solvothermal duration from 1 hour to 8 hours results in a blue shift of the emission peak and an increase in emission intensity of nitrogen-doped Ti₃C₂, attributed to the quantum size effect.⁷⁴ Another study indicated that decreasing the hydrothermal temperature from 150 °C to 100 could increase the PLQY of the Ti₃C₂ MQDs from 7.9% to 9.9%, showing the optimized temperature value is dependent on type of MQDs.⁸⁶ However, no mechanism was provided for this trend. Anyway, despite the capability of synthesis methods in preparing fluorescent MQDs, the PLQY yet remained quite low.

Doping has been introduced as an efficient way to modify the electronic structure and properties of MQDs. Here, nitrogen is the commonly used dopant, found remarkably effective in improving various properties.¹⁰⁰ It has revealed that N-doping of Ti₃C₂ QDs changes the absorption spectrum by adding a new absorption peak at 427 nm which corresponds to the n–π* transition of C=N (Fig. 8b). Consequently, a new yellow fluorescence peak at 580 nm appears. Besides, the fluorescence intensity of N-Ti₃C₂ QDs is highly stable in the presence of various compounds (e.g., 1.0 M NaCl)¹⁰⁷ due to the abundant hydrophilic groups, providing excellent water solubility.¹⁰⁷

More importantly, nitrogen doping can remarkably boost the PLQY which is dependent on many factors. First of all, not all dopants are capable of improving the PLQY. For instance, by N-doping of MQDs, the PLQY and carrier lifetime could reach 10.3% and 6.02 ns, respectively, significantly higher than the phosphorous (P)-doped MQDs (2.40% and 4.99 ns).^2,90,102,108 Regarding N-doping, the selected solvent plays an important role. Using DMF as both a nitrogen doping source and a medium for the solvothermal process, the PLQY of N-Ti₃C₂ QDs reached 11.13%.¹⁰⁴ This improvement was related to the reaction of dimethylamine, derived from DMF decomposition, and carboxylic groups of QDs. In another study, a combination of DMF and KOH was used to synthesize N-Ti₃C₂ QDs with a PLQY of 14.46%.² This value is significantly higher than that of QDs prepared by HNO₃ and KOH (less than ∼2% for both) (Fig. 8c), owing to the nitrogen doping confirmed by the detection of C–N–C and N–H bonds in X-ray photoelectron spectroscopy (XPS). 3-Aminopropyltriethoxysilane (APTES) was another N-containing source that succeeded in enhancing the PLQY to 15.4%, attributed to the surface-passivating amino groups, which exert a strong electron-donating effect at the edges of the MQDs.⁶⁴ To further enhance the PLQY of nitrogen-doped Ti₃C₂ QDs to 16.9%, MQDs were solvothermally synthesized by adding diethylenetriamine to DMF.⁷⁴ Compared to diethylenetriamine, ethylenediamine was more effective in enhancing PLQY to 18.7% by prolonging the carrier lifetime to 7.06 ns due to the gap states formed close to LUMO.³⁶ High-resolution N 1s XPS spectrum determined four new peaks at 397.9, 399.5, 400.2, and 401.2 eV, corresponding to the Ti–N band, pyrrole-like nitrogen (C=N), N–H, and graphitic nitrogen C–N, respectively. Ethylenediamine also demonstrated electro-chemiluminescent (ECL) properties, exhibiting a ECL emission peak at 612 nm with a quantum yield of 1.58%. This emission was attributed to electronic injection into surface traps within the QDs’ band gap (Fig. 8d).¹⁰¹ Similar to Ti₃C₂, ethylenediamine enhanced the optical properties of Ta₄C₃ by forming –NH₂ and C–N bands, resulting in a PLQY of 23.4%.¹⁰⁶ Additionally, the formation of –NH groups on the surface of N-MQDs improved pH resistance.¹⁰⁶

Co-doping offers a greater potential for enhancing the optical properties of Ti₃C₂ QDs. For instance, Q. Guan et al.¹⁰² have shown that simultaneous doping of monolayer Ti₃C₂ QDs by nitrogen and phosphorus enhance PLQY to 20.1%. High-resolution N 1s and P 2p XPS spectra have indicated the characteristic peaks of NO₃⁻ (407.2 eV), N–Ti (396.1 eV), C–N (398.2 eV), N–H (400.7 eV), P–O (133.9 eV), and PO-C (134.8 eV). The electron density difference (EDD) distribution underscores the electron transfer between bonding atoms (Fig. 8e–h). In this image, the blue and red isosurfaces represent electron-depleted and electron-accumulated regions, respectively. Oxygen sites, with high electronegativity, appear red, while phosphorus atoms show blue electron-depleted regions. The enhanced blue intensity at P atoms in P, N-doped Ti₃C₂ QDs indicates electron transfer from P to N, leading to improved radiative recombination and optical performance.¹⁰² The trend of carrier lifetime in N, P-Ti₃C₂ QDs, N-Ti₃C₂ QDs, and P-Ti₃C₂ QDs (8.54 ns, 6.02 ns, and 4.99 ns, respectively) also confirm the photoluminescent enhancement (Fig. 8i).

Besides the P and N dopant pair, other dopants have also been explored to improve the optical properties of Ti₃C₂ QDs. For instance, nitrogen and boron co-doped Ti₃C₂ MQDs (N, B-Ti₃C₂ MQDs) were hydrothermally prepared from ammonia and boric acid, exhibiting good water solubility, strong stability, and high optical characteristics.¹⁰⁹ Co-doped S, N Ti₃C₂ MQDs showed the most redshifted PL emission of 580 nm as well as the highest PLQY of 28.12%.⁹ Notably, the PLQY value was higher than that of N-Ti₃C₂ MQDs (8.33%) and S-Ti₃C₂ MQDs (7.78%), which correlated with the carrier lifetime enhancement from 4.67 ns for S-doped MQDs and 5.81 ns for N-doped MQDs to 7.74 ns for S, N-doped MQDs (Fig. 8j). Co-doping was also found effective in tuning the optical properties of other MQDs. For instance, co-doped Nb₂C QDs (S, N-doped Nb₂C MQDs) with green excitation-dependent emission at 520 nm indicated a high PLQY of 17.25%.² Based on the S 2p XPS spectra, the peaks located at 168.2 eV and 169.3 eV, correspond to different oxidized sulfur forms of SO_x (x = 2–4) bond, typically associated with the surface of the Nb₂C layers.

Co-doped MQDs also offer higher PLQY combined with improved pH resistivity. Studies indicated that the fluorescence intensity of N, B-Ti₃C₂ QDs remained constant when pH was in the range of 3–13, showing high pH-resistive behavior of co-doped QDs.¹⁰⁹ In contrast, the N-doped MQDs exhibited emission stability only within the pH range of 4–9, attributed to the presence of both amino and hydroxyl groups on the surface of QDs.¹⁰⁸ Beyond this range, the intensity quenched out in both acidic and basic solutions. In alkaline solutions, deprotonation of hydroxyl groups forms O⁻ groups on the MQDs surface. Consequently, oxidation of MQDs is accelerated and the PL intensity is reduced.¹⁰⁹

Surface functionalization of MQDs with various molecules was also found effective in enhancing the PLQY of MQDs. This improvement is highly dependent on the synthesis procedure and type of molecules. In terms of synthesis methods, the hydrothermal reaction plays a critical role in the synthesis of fluorescent MQDs, as simple stirring of a mixture of molecules with Ti₃C₂ has been shown to be ineffective.³⁹ Regarding the type of molecules, while the PLQY of BSA@MQDs and polyethyleneimine (PEI)-MQDs were 8% and 7.13%, respectively, an aqueous solution of GSH-Ti₃C₂ MQDs demonstrated a much higher PLQY of 21%. Although PEI successfully functionalized the surface of QDs by forming amide and N–H bonds,⁴³ GSH increased the surface defect sites and enhanced the PLQY more effectively.¹⁰⁵

It is noteworthy that molecule-passivated MQDs exhibit intriguing differences in their optical responses to pH tuning. Studies have revealed that the optical properties of BSA@MQDs and PEI@MQDs are highly sensitive to pH changes.³⁹ The highest emission of BSA@MQDs is attained at pH = 2, and increasing the pH drastically reduces the PL intensity. At the low pH value, the full protonation of BSA amine groups enhances vibrational coupling with hydroxyl functional groups, while deprotonation at higher pH levels leads to the formation of nonradiative recombination centers associated with increased surface defects.³⁹ A similar trend has been observed in PEI-functionalized QDs, as confirmed by time-resolved photoluminescence (TRPL) analysis.⁴³ While the slow decay time constant (τ₁), associated with intrinsic transitions of Ti₃C₂ MQDs, remains unchanged across different pH levels, the fast decay time constant (τ₂), related to emissions from surface defect sites, decreases with increasing pH. This reduction is explained by the deprotonation of surface defects, which transforms luminescent surface defects into non-luminescent ones, thereby diminishing the emission intensity of Ti₃C₂ MQDs. In contrast, GSH-Ti₃C₂ MQDs,¹⁰⁵ UA@Ti₃C₂ MQDs,⁸⁸ and PLL-protected Ti₃C₂ MQDs exhibit stable PL intensity across a wide pH range of 1–13. This stability is attributed to the superior surface passivation provided by the functional molecules, effectively shielding the QDs from pH-induced surface changes.^105,110

5. Applications

Leveraging the exceptional optical properties and inherent biocompatibility of MQDs, their utility has been demonstrated across a diverse range of applications. In this section, we will cover a wide range of applications, commencing with sensing technologies and highlighting the effective detection of various analytes. Subsequently, we will evaluate the significant impact of MQDs on the performance enhancement of optoelectronic devices, energy conversion systems, and biomedical applications.

5.1. Sensing

• Mechanisms

QDs have revolutionized nanotechnology, offering tunable optical and electronic properties, including broad excitation spectra, adjustable emission properties, and intense fluorescence with long-term photostability driven by quantum confinement effects. This versatility has propelled advancements in various fields, such as optoelectronics, bioimaging, and sensing.^6,107,111 Despite these merits, QDs face several limitations, particularly in sensing applications. First, their applicability in fluorescence detection is hindered by inherent biological toxicity, particularly in Pb- and Cd-based QDs.^2,5,6,11 Second, many QDs are soluble only in toxic, nonpolar organic solvents, restricting their direct use in aqueous systems. Additionally, certain QDs, such as CdS, degrade under acidic conditions (pH < 7.0), resulting in the loss of their fluorescence properties over time.⁵ Therefore, MQDs with strong pH resistance in certain wavelengths and stable optical properties in aqueous solutions have gained remarkable attention in recent years.³⁵ By taking advantage of various optical, electrochemical, and electrochemiluminescence (ECL) sensing techniques, MQDs have manifested great potential for use in sensors.⁸⁸

So far, optical sensing by fluorescence quenching has been the one of the most used and efficient techniques.⁷ Fluorescence quenching refers to the fluorescence intensity reduction of a fluorophore due to different intermolecular interactions, such as excited state reactions, molecular rearrangements, Förster resonance energy transfer (FRET), photoinduced electron transfer (PET), inner filter effect (IFE), ground state complex formation, and collisional quenching.⁷ IFE, FRET, and PET are the mechanisms that have been widely utilized for sensing. The IFE is a non-specific mechanism (no probe-analyte interactions) that occurs when there is an overlap between the absorption spectrum of the quencher (analyte) and the excitation or emission spectra of the luminescent probe. This phenomenon reduces the amount of light available to excite the probe or diminishes the intensity of the emitted light, resulting in decreased luminescence.⁷ FRET is a non-radiative energy transfer process that occurs when a donor molecule (the luminescent probe) in an excited state transfers energy to an acceptor molecule. This phenomenon occurs when there is an overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor. Therefore, the efficiency of FRET highly depends on the donor–acceptor distance.⁷ PET, which occurs over relatively longer distances than FRET, involves the transfer of an electron from the donor to the acceptor, forming a non-emissive complex and effectively quenching the probe's luminescence.⁷ Therefore, the PET efficiency is influenced by the redox properties as well as the HOMO and LUMO of the donor and acceptor.⁷

Compared to single-emission, dual-emission reverse change ratio PL offers higher sensitivity and accuracy due to intrinsic signal amplification.⁸⁸ In dual-emission, the fluorescence of one component decreases while another's fluorescence significantly increases in the presence of the analyte. The ratio of the fluorescence intensities of the two components is then determined for quantitative analysis. For instance, curcumin (CUR) caused the fluorescence emission of Ti₃C₂ MQDs at 430 nm to be suppressed by the FRET, whereas CUR's fluorescence emission at 540 nm increased.⁸⁸

Colorimetric sensors are another group of optical sensors that exhibit a distinct color shift in response to the analyte's reaction.¹¹² This color transition is detected by the naked eye or specific instrumentation to determine the change in intensity at a certain wavelength within the visible (400–800 mm) range.¹¹² For example, N, P-Ti₃C₂ MQDs showed a transparent-to-orange color change upon Fe²⁺ detection, demonstrating potential for portable sensors.⁸⁴ ECL has been widely used in analytical chemistry and bioassays due to its high sensitivity, low background noise, and compatibility with miniaturized devices.⁷² In ECL, a voltage is applied to an electrode, which causes a redox reaction to occur and generates excited-state species. Then, these species emit light after returning to their ground state.¹⁰¹ To assess the redox activity, cyclic voltammetry (CV) is used by sweeping the potential of the working electrode linearly with time and measuring the resulting current, providing information about redox potentials, electron transfer kinetics, and surface area.¹¹³ By leveraging advanced sensing techniques and MQDs, researchers have successfully detected various types of targets. Table 3 provides a summary of the most recent advances in this field. The following sections offer a critical review on the application of MQDs in sensing.

Table 3 Categorized applications for various MQD platforms and their features

Application	Platform	Features	Mechanisms	Ref.
MXene quantum dots (MQDs), electrochemiluminescence (ECL), fluorescence (IFE), photoelectrochemical (PEC), ascorbic acid (AA), glucose (GLU), uric acid (UA), diaminopyridine (DAP), photoinduced electron transfer (PET), limit of detection (LOD), dopamine (DA), glucose oxidase (GOD), alkaline phosphatase (ALP), transferrin (Trf), lysozyme (Lyz), ovalbumin (Ova), chymotrypsin (Chy), immunoglobulin G (IgG), colony-forming unit (Cfu), kanamycin (KAN), cefixime (CFX), chloramphenicol (CHL), norfloxacin (NOR), oxytetracycline (OTC), doxorubicin (DOX), aspartic acid (Asp), serine (Ser), lysine (Lys), glycine (Gly), glutathione (GSH), nitrobenzoic acid (NBA), benzoic acid (BA), benzidine (BD), nitrobenzene (NB), 2,4-dinitrophenylhydrazine (DNPH), p-nitrophenyl phosphate (PNPP), tryptophan (Try), cysteine (Cys), Förster resonance energy transfer (FRET), electron spin resonance (ESR), hydrogen evolution reaction (HER), nitrate oxidation reduction reaction (NORR), nitrogen reduction reaction (NRR), covalent organic framework based on pyridine and dihydroxybenzidine (PY-DHBD-COF).
miRNA sensing	Ti3C2 MQDs@Au nanobones heterostructure	• Target: miRNA-26a	ECL (short-distance electron transfer)	72
		• LOD: 1.7 fM
		• Linear range: 5 fM–10 nM
		• Selectivity: miRNA-126, miRNA-155, miRNA-221
		• Sensitivity in real conditions: human serum samples
Proteins sensing	Amino-functionalized Ti3C2 MQDs	• Target: histidine	FL (IFE)	114
		• LOD: 2.1 nM
		• Linear range: 100–1000 nM
		• Selectivity (Ni^+): Li^+, K^+, Ag^+, Na^+, Mn^2+, Zn^2+, Mg^2+, Ca^2+, and Al^3+
		• Sensitivity in real conditions: human serum samples
Organic acid sensing	Ti3C2 MQDs/TiO2 inverse opal heterojunction	• Target: glutathione	PEC	115
		• LOD: 9.0 nM
		• Linear range: 0.1–1000 μM and 1 μM to 200 μM
		• Selectivity: AA, GLU, UA, Fe^3+
Other targets sensing	N-doped Ti3C2 MQDs@DAP	• Target: H2O2	Sensing: ratiometric fluorescence	92
		• LOD: 0.57 μM	Quenching: PET
		• Linear range: 2–50 μM
		• Selectivity: DA, AA, Phe, Glu, Lys, L-Cys, Tyr, GSH, GSSG, K^+, Mg^2+, Ca^2+, Na^+, Zn^2+, Cd^2+, Fe^2+, Fe^3+, Ag^+, I^−, and Br^−
		• Target: xanthine
		• LOD: 0.24 μM
		• Linear range: 1–50 μM
		• Selectivity: adenosine, BSA, DA, AA, UA, cytidine, urea, Glu, K^+, and Ca^2+
		• Sensitivity in real conditions: human serum
Proteins sensing	Ti3C2 MQDs-AuNPs	• Target: polynucleotide kinase	ECL	71
		• LOD: 2.7 × 10^−5 U mL^−1
		• Linear range: 0.0001 to 10 U mL^−1
		• Selectivity: glucose oxidase (GOD), alkaline phosphatase (ALP), thrombin and lysozyme
		• Sensitivity in real conditions: HeLa cell lysate
miRNA sensing sensing	TiO2 nanosheet arrays/Ti3C2 MQDs	• Target: MicroRNA-155	PEC	71
		• LOD: 0.025 pM
		• Linear range: 0.1 pM–10 nM
		• Selectivity: miRNA-122, miRNA-141, miRNA-21
Proteins and amino acids sensing	ε-Poly-L-lysine-protected Ti3C2 MQDs	• Target: cytochrome c	Sensing: FL	110
		• LOD: 20.5 nM	Quenching: IFE
		• Linear range: 0.2 to 40 μM
		• Selectivity: Trf, Lyz, Ova, Chy and BSA
		• Target: trypsin
		• LOD: 0.1 μg mL^−1
		• Linear range: 0.5 to 80 μg mL^−1
		• Selectivity: ALP, lysozyme, BSA, pepsin, thrombin and IgG
		• Sensitivity in real conditions: 50-fold diluted serum samples
Environmental pollutants sensing	Aptamer modified polyhedral oligomeric silsesquioxane-perovskite quantum dots (POSS-PQDs-Apt)/Ti3C2	• Target: Vibrio parahaemolyticus (VP)	Sensing: FL	116
		• LOD: 30 cfu mL^−1	Quenching: FRET
		• Linear range: 10^2 to 10^6 cfu mL^−1
		• Selectivity: common bacteria, including Listeria monocytogenes, Salmonella typhimurium, Escherichia coli, and Staphylococcus aureus (all concentrations were at 10^5 cfu mL^−1) common ions and pesticide residues in water on the test results, including Na^+, K^+, Mg^2+, Ca^2+, and dimethoate.
		• Sensitivity in real conditions: real seawater samples
Environmental pollutants sensing	N, B-doped Ti3C2 MQDs	• Target: tetracycline	Sensing: ratiometric fluorescence	109
		• LOD: 20 nM (using the smartphone: 45 nM)	Quenching: IFE
		• Linear range: 0.2–20 μM (using the smartphone: 0.4–20 μM)
		• Selectivity: other antibiotics (KAN, CFX, CHL, NOR, OTC, and DOX) and some potentially coexisting substances (Na^+, Ba^2+, Ca^2+, Zn^2+, Fe^3+, BSA, Asp, Ser, Lys, Gys, His, GSH, GLU)
		• Sensitivity in real conditions: milk
Environmental pollutants sensing	Eu-doped Ti3C2 MQDs	• Target: tetracycline	Sensing: ratiometric fluorescence	65
		• LOD: 48.79 nM (using colorimetric method: 11.36 nM)	Quenching: FRET
		• Linear range: 0–1000 μM
		• Selectivity: K^+, Ag^+, Li^+, Na^+, Mn^2+, Zn^2+, Ba^2+, Mg^2+, Ca^2+, Al^3+ and antibiotics (Kana, Azi, Chl)
		• Sensitivity in real conditions: milk, and soil
		• Target: oxytetracycline (OTc)
		• LOD: 3.86 nM
		• Linear range: 100–1000 μM
		• Target: chlortetracycline (CTc)
		• LOD: 64.73 nM
		• Linear range: 100–1000 μM
Environmental pollutants sensing	N-doped Ti3C2 MQDs	• Target: Alizarin red	Sensing: FL	100
		• LOD: 1.21 μM	Quenching: IFE
		• Linear range: 0–80 μM
		• Sensitivity in real conditions: actual water samples
Environmental pollutants sensing	Uric acid-capped Ti3C2 MQDs	• Target: 2,4,6-trinitrophenol	Sensing: FL	88
		• LOD: 9.58 nM	Quenching: IFE
		• Linear range: 0.01–40 μM
		• Selectivity: NBA, BA, BD, NB, DNPH; PNPP, K^+, Cl^−, Ca^2+, Ba^2+, Cr^3+, Al^3+, NH^4+, S^2−, SO3^2−, PO4^3−, Mg^2+, Zn^2+, Mn^2+, Fe^2+, and NO^3−, Cu^2+, Pb^2+, and Fe^3+, Co^2+ and Cd^2+
		• Sensitivity in real conditions: in lake and tap water
Other targets sensing	Ti3C2 MQDs	• Target: dopamine	—	113
		• LOD: 3 nM
		• Linear range: 0.01–20 μM
		• Selectivity: ascorbic acid, uric acid, glucose, NaCl, KCl and urea
		• Sensitivity in real conditions: DA hydrochloride injection purchased from a drugstore
Proteins and amino acids sensing	N-doped Ti3C2 MQDs	• Target: mucin 1	Sensing: ECL	101
		• LOD: 0.31 fg mL^−1
		• Selectivity: carcinoem-bryonic (CEA), α-1-fetoprotein (AFP), human serum albumin (HSA) and hemoglobin (Hb)
		• Sensitivity in real conditions: serum samples
Ions/organic acids sensing	N-doped Ti3C2 MQDs	• Target: chromium(VI)	Sensing: FL	107
		• LOD: 0.012 μM	Quenching: IFE
		• Linear range: 0.1–500.0 μM
		• Selectivity: NH^4+, K^+, Na^+, Ag^+, Mg^2+, Ca^2+, Mn^2+, Zn^2+, Cu^2+, Cd^2+, Co^2+, Hg^2+, Fe^2+, Fe^3+, MnO4
		• Sensitivity in real conditions: in tap and Yudai Lake water
		• Target: ascorbic acid
		• LOD: 0.02 μM
		• Linear range: 0.1–500.0 μM
		• Selectivity: common amino acids (L-phenylalanine; L-tyrosine; L-/D-tryptophan; L-arginine; L-glutamate) and other potentially biomolecules (glutathione; glucose; uric acid; urea)
		• Sensitivity in real conditions: vitamin C tablets and lemon juice samples
Organic acids sensing	GSH-Ti3C2 MQDs	• Target: uric acid	Sensing: ratiometric fluorescence colorimetric (absorption)	105
		• LOD: 125 nM (ratiometric fluorescence), 200 nM (absorption)	Quenching: FRET
		• Linear range: 1.2–75 μM (ratiometric fluorescence), 1.2–100 μM (absorption)
		• Selectivity: Try, Cys, urea, Nap, Kp, GSH, DA, GLU, NO^3−, SO^4−, Cl^−, UA
		• Sensitivity in real conditions: serum and saliva
Other targets/ions sensing	Ti3C2 MQDs	• Target: curcumin	Sensing: ratiometric fluorescence	117
		• LOD: 20 nM	Quenching: FRET
		• Linear range: 0.05–10 μM
		• Selectivity: interfering ions (K^+, Na^+, Fe^3+, Al^3+, Ca^2+, Mg^2+, SO4^−, Fe^2+, Zn^2+, Cl^−, and HPO4^−), various amino acids, GSH, folic acid, and L-ascorbic acid, two carbohydrates (fructose and glucose).
		• Target: hypochlorite
		• LOD: 5 μM
		• Linear range: 25 to 150 μM and 150 to 275 μM
		• Selectivity: ClO^−: various oxidized anions and free radicals
Other targets sensing	N-doped Ti3C2 MQDs	• Target: H2O2	Sensing: FL	118
		• LOD: 1.2 nM
		• Linear range: 5 to 100 nM
		• Selectivity: Na^+, K^+, Mg^2+, Ca^2+, Fe^2+, Zn^2+, and Al^3+ ions
Other targets sensing	Ti3C2 MQDs	• Target: H2O2	—	97
		• LOD: 5 nM
Ions sensing	N-doped Ti3C2 MQDs	• Target: Fe^3+	Sensing: FL	119
		• LOD: 2 μM
		• Linear range: 2 to 5000 μM
		• Selectivity: Na^+, Mg^2+, Cu^2+, K^+, Fe^3+, Mn^2+, Zn^2+, Ca^2+, Al^3+, Ce^3+, Cu^+ and Ni^2+
Ions sensing	N-doped Ti3C2 MQDs	• Target: Cu^2+	Sensing: FL	74
		• Selectivity: Na^+, K^+, Mg^2+, Ca^2+, Fe^2+, Fe^3+, Co^2+, Ni^2+, Zn^2+, Cd^2+, Pb^2+, Hg^2+, and Ag^+
Ions sensing	Ti3C2 N, P-MQDs	• Target: Cu^2+	Sensing: FL	102
		• LOD: 2 μM	Quenching: IFE
		• Linear range: 2–100 μM and 250–5000 μM
		• Selectivity: Zn^2+, Pb^2+, Na^+, La^3+, Mn^2+, Ni^2+, Ce^3+, Fe^3+, Ca^2+, K^+, Ag^+
Ions sensing	N-doped Ti3C2 MQDs	• Target: Cu^2+	Sensing: FL	120
		• LOD: 0.15 μM	Quenching: IFE
		• Linear range: 0.5–100 μM
		• Selectivity: Hg^2+, Mn^2+, Ag^+, Cd^2+, Al^3+, Zn^2+, Mg^2+, Ca^2+, K^+, and Na^+
		• Sensitivity in real conditions: water sample
		• Target: Fe^3+
		• LOD: 0.17 μM
		• Linear range: 0.5–100 μM
		• Selectivity: Hg^2+, Mn^2+, Ag^+, Cd^2+, Al^3+, Zn^2+, Mg^2+, Ca^2+, K^+, Na^+
		• Sensitivity in real conditions: water sample
Ions sensing	TiCN MQDs	• Target: Fe^3+	Sensing: FL	103
		• LOD: 1.0 μM, 2.4 ± 0.1 μM (tap water)	Quenching: IFE
		• Linear range: 2–400 μM and 500–800 μM
		• Selectivity: K^+, Cd^2+, Cu^2+, Fe^2+, Hg^2+, Mg^2+, Ni^2+, Pb^2+, Ce^3+, Mn^2+, Co^2+, Al^3+, and Zn^2+
		• Sensitivity in real conditions: tap water
Ions sensing	Ti3C2 MQDs	• Target: Fe^3+	Sensing: FL	70
		• LOD: 310 nM	Quenching: IFE
		• Selectivity: cysteine, serine, arginine, ascorbic acid, dopamine, H2O2, Mn^2+, Fe^2+, Cu^2+, Ca^2+, Al^3+, Cd^2+, Co^2+, Cr^3+, Mg^2+, Na^+, Ni^2+, Pb^2+, Sn^2+, and Zn^2+
		• Sensitivity in real conditions: serum and seawater samples
Ions/physical parameters sensing	N-doped Ta4C3 MQDs	• Target: Fe^3+	Sensing: FL	106
		• LOD: 2 μM
		• Selectivity: NO3^−, SO4^−, Cl^−, Hg^+, Mg^+, Ce^+, Na^+, Ca^+, K^+, Fe^+, Cu^+, Ni^+, Ba^2+
Ions sensing	BSA@Ti3C2 MQDs	• Target: Fe^3+	Sensing: FL	121
		• LOD: 1.25 nM	Quenching: IFE
		• Linear range: 0–150 μM
		• Selectivity: ascorbic acid, Sn^2+, Ca^2+, Ni^2+, Cd^2+, Pb^2+, Al^3+, Zn^2+, Ag^+, Cr^3+, Cr^6+, Cu^2+, Na^+, and K^+
		• Sensitivity in real conditions: tap water samples
Ions/pH sensing	Amino-functionalized Ti3C2Tx MQDs	• Target: Fe^3+/pH	Sensing: ratiometric	108
		• LOD: 2 nM	Fluorescence
		• Selectivity: K^+, Ca^2+, Na^+, Al^3+, Zn^2+, La^3+, Co^2+, Ni^2+, Cu^2+, Cd^2+, Ce^4+, Fe^2+
		• Sensitivity in real conditions: tap water samples
Ions sensing	N-doped Ti3C2 MQDs	• Target: Co^2+	Sensing: FL	2
		• LOD: 0.21 μM	Quenching: IFE
		• Linear range: 0.5–100 μM
		• Selectivity: K^+, Ca^2+, Mg^2+, Al^3+, Zn^2+, Fe^3+, Cr^3+, CO3^2−, SO4^2−, SO3^2−, PO4^3−, NH^4+, Fe^2+, Pb^2+ and Cu^2+
		• Sensitivity in real conditions: tap water, river water
		• Target: Ag^+
		• LOD: 0.10 μM
		• Linear range: 0.5–150 μM
		• Selectivity: K^+, Ca^2+, Mg^2+, Al^3+, Zn^2+, Fe^3+, Cr^3+, CO3^2−, SO4^2−, SO3^2−, PO4^3−, S2O3^2−, NH^4+, Pb^2+, and Cu^2+
		• Sensitivity in real conditions: tap water, river water
Ions sensing	Ti3C2 MQDs	• Target: Cr^3+	Sensing: FL	5
		• LOD: 30 mM
		• Selectivity: Cu^2+, Na^+, Ca^2+, Fe^3+
Ions sensing	N, P-doped Ti3C2 MQDs	• Target: NO2^−	Sensing: colorimetry (absorption) fluorometric dual-modal	84
		• LOD: FL intensity ratio: 0.25 μM absorbance ratio: 0.71 μM	Quenching: IFE
		• Linear range: FL intensity ratio: 1.5–80 μM, absorbance ratio: 4–85 μM
		• Selectivity: anions (F^−, Cl^−, Br^−, I^−, NO3^−, CO3^2−, SO3^2−, SO4^2−, PO4^3−, HPO4^2−), cations (Ca^2+, Mg^2+, Fe^3+, K^+, Zn^2+, Ba^2+) and coexisting substances (Arg, Try, Gly, Lys, Asp, Ser, His, Cys, Ala)
		• Sensitivity in real conditions: six types of sausage samples
Ions sensing	Fe3O4/Ti3C2 MQDs	• Target: Cr(VI)	Sensing: colorimetry	122
		• LOD: 0.26 μM
		• Linear range: 0–60 μM
		• Selectivity: Na^+, Ni^2+, K^+, Ca^2+, Mg^2+, Co^2+, Cu^2+, Mn^2+, Al^3+, Fe^3+, Pb^2+ and Cr^3+
		• Sensitivity in real conditions: tap water samples
pH sensing	Ti3C2 MQDs	• Target: intracellular pH	Sensing: ratiometric fluorescence	43
		• Linear range: 6–8/5–9
Physical parameters sensing	MQD within natural 3D watermelon peel matrix	• Target: pressure sensor	—	38
		• Sensitivity: 323 kPa^−1 for 0–0.4 kPa and 51.38 kPa^−1 for 0.4–20 kPa
Others sensing	Ti3C2Tx MQD thin film on quartz bulk acoustic wave	• Target: infrared radiation	Sensing: electromechanical (admittance and frequency shift of the sensor throughout infrared radiation absorption)	106
Ions sensing	Ti3C2 MQDs	• Target: Ag^+	Sensing: FL	123
		• LOD: 0.45 μM
		• Linear range: 0.5–40 μM
		• Selectivity: Na^+, Au^3+, Mg^2+, Ca^2+, Zn^2+, Cu^2+, Ce^3+, Mn^2+, Cd^2+, Ni^2+, Al^3+, Fe^3+, Cr^3+, and Hg^2+
		• Sensitivity in real conditions: tap and river water samples
Others sensing	Ti3C2 MQDs–EDTA–Eu^3+	• Target: 2,6-dipicolinic acid (DPA)	Sensing: colorimetry (absorption) fluorometric dual-modal	124
		• LOD: 0.26 nM	Quenching: absorbance energy-transfer effect
		• Linear range: 0–11 μM
		• Selectivity: BA, PH, PA, NA, ISA, Cys, Leu, Arg, UA, Phe
		• Sensitivity in real conditions: river water samples
Environmental pollutants sensing	Cu@Ti3C2 MQDs-aerogel	• Target: pirimiphos-methyl	Sensing: colorimetry (absorption)	125
		• LOD: 0.65 nM
		• Linear range: 100–3200 nM
		• Sensitivity in real conditions: seawater, tap water, pear juice, orange juice, milk, tomato juice and corn juice
Others sensing	N-doped Ti3C2 MQDs	• Target: quercetin (QC)	Sensing: FL	126
		• LOD: 1.35 nM	Quenching: static quenching/radiation-free complex formation and IFE
		• Linear range: 25–600 nM
		• Selectivity: rutin, gallic acid, chrysin, morin, kaempferol, phloroglucinol, myricetin, galangin, dopamine, uric acid, serotonin, cysteine, methionine, glutathione, resorcinol, catechol, hydroquinone, lysine, Na^+, Cu^2+, Mg^2+, K^+
		• Sensitivity in real conditions: onions, oranges, and red wine
WLED	S-doped, N-doped, and S–N-doped Ti3C2 MQDs	• CIE coordinates of (0.31, 0.35)	Yellow, blue, and orange emissions to make white LED	9
		• High stability under UV irradiation for a month
Single-component WLED	PDMS + Ti3C2 MQDs nanocomposite	• Tuning the white emission temperature from cool to warm	Strong two-photon white fluorescence, originating from both carbon core and OLA-passivated surface states.	127
		• Piezochromic effect
White laser	V2C MQDs	• Enhancement in the PL intensity, covering the entire visible range	—	128
Solar cell	Ti3C2Clx MQDs modified-perovskite	• PCE: 21.31%	(1) Interaction between the Cl^− and Pb^2+ ions induce the preferred grain orientation with smaller residual tensile strain.	129
		• Increase in Jsc and Voc	(2) Ti3C2Clx QDs near the bottom substrate improve the perovskite film crystallinity.
		• Decrease in normal hysteresis
		• Maintaining over 84% of PCE after 1000 h
Solar cell	Modified perovskite and TiO2 with Ti3C2 MQDs and nanosheets.	• PCE improvement from 12.0% to 17.1%	(1) Positive effect of nanosheets on electron mobility improvement of TiO2	34
		• Reduced hysteresis effect	(2) Reduced defects concentration via perovskite film passivation and increased perovskite crystallinity
Solar cell	Introduction of Nb2C MQDs into SnO2	• PCE improvement by 3%, reaching high value of 22.86%	(1) Grains enlargement	130
		• Maintaining 98% of PCE after 40 days	(2) Modifying the roughness and surface energy
			(3) Lowering defects content
			(4) Enhanced perovskite crystallinity and effective carrier transport
Solar cell	Ti3C2Tx MQDs-modified SnO2	• Enhanced absorption in visible range	Improving the crystal quality and phase stability of the perovskite	131
		• PCE: 23.3%
		• Outstanding stability against humidity and light soaking
Solar cell	Nb2O5 and Ti3C2 MQDs into the TiO2	• Increase in both photocurrent and efficiency (7.24%)	(1) Tuning the energy level alignment of the photoanode through the introduction of Nb2O5 and Ti3C2 QDs into the TiO2 photoanode	132
		• Higher and wider absorption range for photoanodes in the visible region and better charge transfer	(2) Suppressing electron–hole recombination at the photoanode interface
UV detector	Ti3C2Tx MQDs into perovskite	• UV detector −1100% increase in spectral responsivity (264 A W^−1 at 310 nm)	Enhancement of charge carriers and light absorption	8
Ultrafast photonics	Ytterbium-doped fiber laser/Ti3C2Tx MQDs	• Broadband saturable absorption from 540 to 1550 nm	Switching the polarization state in the cavity resulted in tunable mode-locked optical spectra	77
		• Stable 357 ps mode
		• Locked pulses centered at 1069.17 nm under a low threshold power of 54 mW
Ultrafast photonics	The photodeposition method to transfer Ti3C2Tx MQDs to the tapered area to fabricate the SA	• Ultrafast photonics – tapered fiber saturable absorber (SA) device for an Er^3+-doped fiber laser (EDFL)	—	46
		• Low saturation intensity (1.983 GW cm^−2) and high modulation depth (11.6%)
		• Ultrashort pulses of 466 fs at a wavelength of 1566.57 nm with a fundamental frequency of 22.78 MHz
Nonvolatile memory device	PVP + MQDs nanocomposite	• Stable operation during a retention test (1.2 × 10^4 s), with an on/off current ratio of up to 100	(1) MQD charge trapping, originated from quantum confinement and the dissolvability of memristive components	133
			(2) Tuning the electrical conductance of an ITO/MQD-PVP/Au structure from insulator behavior to irreversible resistive switching, reversible resistive switching, and conductor behavior
Li–O2 battery	Ti3C2 MQDs/N-doped carbon nanosheets (E'lyte: 1 M LiTFSI/TEGDME)	• Discharge capacity: 16 000 mAh g^−1	Enhanced electrocatalytic activity, through adsorption of Li^+ to C side of the edge-rich of MQDs, and charge accumulation for Li2O2 production on cathode	134
		• Cycling stability: 240 cycles at 200 mA g^−1
		• Voltage gap: 0.62 V (O2 consumption/evolution overpotential)
		• Coulombic efficiency: 93.8%
Zn–air battery	Ti3C2 MQDs/Ni, Fe-layered dihydrate (Ni, Fe-LDH)/NG (E'lyte: 0.1 M KOH)	• Higher OER activity than pristine LDH/NG	Improved electrical conductivity associated with MQDs (metallic behavior), and provided a higher carrier density compared to LDH sample	135
		• Faster OER kinetics with smaller Tafel slope of 57 mV dec^−1 compared to 120 mV dec^−1 for Pt/C
		• 98.8% current retention after 10 h of operation, exceeding 84.3% for the commercial Pt/C
Li–S battery	g-C3N4@MQDs (E'lyte: 1 M LiTFSI@DME : DOL v/v 1 : 1 + 2 wt% LiNO3)	• Confinement of polysulfide shuttle	Improved capture–confinement–conversion through electrostatic adsorption of protonated g-C3N4 and MQDs to improve capturability of polysulfides	136
		• Capacity = 1433 mA h g^−1 at 0.1C
		• Capacity decay of 0.024% per cycle after 1000 cycles at 2C
		• Retention rate of nearly 70%, higher than Li–S batteries with GC separator (58%)
Supercapacitor	Integrated Ti3C2Tx MQDs onto MXene nanosheets (PVA/H2SO4 gel electrolyte)	• Areal capacitance: 2202 mF cm^−2 at 3 mA cm^−2	(1) Heterodimensional structure: 0D TCQDs + 2D TCNs → improved electron transport & ion accessibility	137
		• Areal energy density: 90.33 μWh cm^−2 at 450 mW cm^−2	(2) Strong interface interactions: polar functional groups enhance structural integrity & stability
		• Capacitance retention: 84% after 10 000 charge–discharge cycles
Supercapacitor	MQD-Ni(OH)2 composite (1 M KOH)	• Specific capacitance: 1660 F g^−1 at 1 A g^−1	(1) Quantum confinement of QDs → enhanced conductivity → faster electron transport, and boosted electrochemical reactivity	138
		• 98 F g^−1 for asymmetric supercapacitor	(2) Thin Ni(OH)2 nanosheets → increased active sites
		• Energy density: 30.6 Wh kg^−1 (at 750 W kg^−1 power density)
		• Capacitance retention: 84% after 10 000 cycles
		• Rct: 6.7 Ω (lower than Ni(OH)2-only)
Supercapacitor	MQDs/Laser Reduced Graphene Oxide (LRGO) (PVA/H2SO4 gel electrolyte)	• Areal capacitance: 10.42 mF cm^−2 (91% transmittance)	Enhanced surface area & edge states → improved charge storage synergistic MQD/LRGO interface → higher electron/ion transport	12
		• 64.6 mF cm^−2 (53% transmittance)
		• Energy density: 2.04 × 10^−3 mWh cm^−2
		• Power density: 129.4 μWh cm^−2
		• Cycle life: 97.6% retention after 12 000 cycles
		• Transparency: over 90%
		• Conductivity: 760.4 S m^−1
Supercapacitor	Ti3C2 MQDs/large Ti3C2Tx (L-Ti3C2Tx) fiber (PVA/H2SO4 gel electrolyte)	• Electrode capacitance: 1560 F cm^−3 at 1 A cm^−3	(1) Large L-Ti3C2Tx NSs → improved mechanical strength	139
		• Capacity retention: 79% at 20 A cm^−3	(2) Synergistic interaction of Ti3C2Tx QDs & L-Ti3C2Tx → higher efficiency
		• Mechanical strength: 130 MPa
		• Supercapacitor capacitance: 413 F cm^−3 at 0.5 A cm^−3
		• Cycle stability: 97% retention after 10 000 cycles
		• Energy density: 36.7 mWh cm^−3 at 311 mW cm^−3
		• Temperature range: −40 to 60 °C
Photocatalytic HER	Ti3C2 MQDs/g-C3N4 (0.5 M Na2SO4)	• Co-catalysts for photocatalytic H2 evolution	Increased H2 reduction rate due to high aqueous solubility of MQDs and more abundant active edge sites	140
Photocatalytic HER	N-doped Ti3C2/1D-CdS (0.1 M Na2SO4)	• HER rate: 17 094 μmol g^−1 h^−1 (14.79× higher than pure CdS)	Facilitated separation and charge carriers transfer from photoexcitation of CdS QDs, and supply Lewis acid sites for adsorption of 1,2-BM to accelerate H2 evolution	138
		• Rct: 3.57 Ω (significantly reduced)
		• Electron lifetime: 6.65× longer than pure CdS
		• Photostability: no significant loss after 4 cycles
Photocatalytic HER	MQDs/Co2(OH)3Cl/CF (0.1 M KOH)	• Overpotential: 91 mV@10 mA cm^−2	(1) Interface charge transfer: induces Co^2+δ high-valence sites for better H2O activation	141
		• Tafel slope: 67 mV dec^−1	(2) Lattice strain effect: improves structural integrity
		• Low charge transfer resistance (Rct): 4.514 Ω	(3) Lowering the kinetic barrier for H2O dissociation (Volmer step): tuning the H* adsorption Gibbs free energy toward the thermoneutral ideal.
		• Electrochemical Surface Area (ECSA): 240 cm^−2
		• Stability: 82.6% retention after 30 h
Photocatalytic HER	Ti3C2 MQDs modified 3D/2D TiO2/g-C3N4 (0.1 M Na2SO4)	• Hydrogen production rate of 5540.2 μmol g^−1 h^−1, 1.6 times higher than that of TiO2/g-C3N4 and 33 times higher than that of pure g-C3N4	(1) Promoting strong interaction between cocatalysts due to MQDs’ functional groups	142
			(2) Stronger performance due to reducing electron–hole recombination
			(3) Accelerating electron migration from the conduction band of g-C3N4
Nitrogen Reduction Reaction (NRR)	Ti3C2 MQDs/2D Ni-MOF (0.5 M Na2SO4)	• Ammonia yield: 88.79 μmol g^−1 h^−1 (4× higher than pristine Ni-MOF)	(1) MQD excitation → electrons transfer to Ni-MOF	138
		• Band gap: 3.6 eV (Ni-MOF), 4.92 eV (Ti3C2-QD)	(2) Ni sites act as active centers → adsorption & activation of N2 MQDs with abundant active sites enable efficient NRR
NRR	Ti3C2 MQDs decorated on the 2D Ni-MOF (3 M KOH)	• Photo-reduction N2 activity of 88.79 μmol g^−1 h^−1, which was 4 times higher than that of the pristine 2D Ni-MOF	(1) Interfacial area of the Ti3C2 MQD/Ni-MOF composite to facilitate the photocarrier's direction migration	4
			(2) Reducing the free-energy corresponding N–N bond and length of the N2 fixation pathways
NORR	Ti3C2 MQDs/Cu NWs (0.1 M K2SO4)	• Electrochemical reduction of NO into NH3 high NH3 yield of 5346.3 μg h^−1 mg^−1	NO into NH3 reaction pathway (*NO → *HNO → *N → *NH → *NH2 → *NH3) and potential determination step (*N → *NH) for the NORR on Ti3C2 QDs/Cu NWs	143
		• Faradaic efficiency of 95.5% in 0.1 M K2SO4 solution at a much lower potential of −0.4 V vs. RHE
		• Zn–NO battery with Ti3C2 QDs/Cu NWs as the cathode showed power density of 3.03 mW cm^−2 and an NH3 yield of 925.2 μg h^−1 mg^−1
NOx oxidation	Ti3C2Tx MQDs/g-C3N4/bismuth oxyiodide (BiOI) (0.2 M Na2SO4)	• NO removal rate: 42.23% (highest among tested catalysts)	p–n heterojunction led to enhanced charge separation and transfer, as well as promoting gas absorption and activization, further achieving an excellent photocatalytic performance.	138
		• NO2 production: 31.52 ppb (lowest among tested catalysts)
		• NO2 selectivity: 0.17 (indicates efficient NO oxidation)
		• Stability: 5-cycle test shows minimal degradation
CO2 photoreduction	Ti3C2Tx MQDs/g-C3N4/BiOI 0.5 M Na2SO4	• CO production rate: 57.8 μmol g^−1 h^−1 (highest among tested catalysts)	Electron-withdrawing ability of MQDs, further promotes the generation of more active species involved in reactions	144
		• CH4 production rate: 3.6 μmol g^−1 h^−1
		• Adsorption capacity: 4.53 cm^3 g^−1 for CO2
		• CN/MQDs/BOI showed CO production rate of 57.8 μmol g^−1 h^−1 and CH4 production rate of 3.6 μmol g^−1 h^−1
CO2RR	Ti3C2 MQDs decorated Cu2O NWs on Cu mesh (0.5 M Na2SO4)	• Methanol production	(1) Improved the stability of Cu2O NWs, enhanced charge transfer, carrier density, light adsorption, and decreasing band bending edge and charge recombination	145
		• Photocatalytic reduction of CO2 by Ti3C2 QDs/Cu2O NWs/Cu is 8.25 times and 2.15 times of that from Cu2O NWs/Cu and Ti3C2 sheets/Cu2O NWs/Cu	(2) Ti3C2 QDs favorably chemisorbed CO2 instead of H2O and further facilitated CO2 reduction by promoting H^+ attachment and donating electrons
Photocatalysis	Ti3C2 MQDs/Cu2O NWs/Cu (0.5 M Na2SO4)	• Photoelectrochemical CO2 reduction	Light absorption improvement, facilitated charge separation, and transport	146
H2O2 production	Nb2C MQDs@PY-DHBD-COF (pyrene-based)	• Achieved a H2O2 yield record of 3560 μmol g^−1 h^−1, and AQY of 12.8% at 400 nm together with SCC efficiency of 0.27%	• Adsorption of O2 on the MQDs to form *O2	147
			• Transfer of e^− to the *O2 to be reduced to *OOH
			• Combination of H^+ with *OOH to form *HOOH
			• Hydrogenation of *HOOH to form H2O2
Nucleus-target low-temperature photothermal therapy	V2C MQDs modified with the cell nucleus-target TAT peptides and cell target Arg-Gly-Asp	• Tumor clearance at low temperature	Processed proteins facilitated drug nuclear-targeting of cancer cells. Under NIR-II laser irradiation, V2C-PEG-TAT's temperature raised, leading to genetic material damage.	148
		• Without side effects non-toxic
Antitumor activity in vivo	Non-oxidizing N-MQDs-Ti3C2Tx	• Tumor inhibition rate of 91.9%	(1) By intratumorally MQDs administering in HeLa tumor xenografts, the Ti^3+ bonded to cancer cells in H2O2, catalyzing its production of highly toxic species led to cancer cell death.	92
		• No significant toxicity during the therapeutic treatment	(2) Tumor microvascular permeability caused hemorrhage.
		• Excreted by the kidneys of animals 48 h after administration
Photothermal cancer therapy	Fluorine-free Ti3C2 MQDs	• Photothermal conversion efficiency of 52.2%	Al(OH)4 on the surface of MQDs caused strong absorption in the NIR region. Irradiation of 808 nm with a power of 0.5 W cm^−2 led to a tumor destruction rate of 100% with no significant change in the bodyweight of mice.	149
		• Extinction coefficient of 52.8 L g^−1 cm^−1
		• High photostability
		• No biotoxicity at a concentration of 100 PTT (in vitro) on HeLa, MCF-7, U251, and HEK 293 cells.
		• No toxic effects and side effects of the QDs on mouse cells
PA imaging guided photothermal therapy	Ti2N MQDs	• Applicable in NIR-I and NIR-II region	Mice injected with Ti2N QDs. After, 10 min laser irradiation at 1.0 W cm^−2 power density in NIR-I and NIR-II, the temperature increased from 37 °C to more than 60 °C and 69 °C.	148
		• High absorption
		• High photothermal conversion efficiency
		• Photothermal stability
PA/photothermal imaging-guided PTT/intracellular microRNA imaging	Mo2C MQDs	• Photothermal conversion efficiency of 42.9% at low concentration (100 µg mL^−1)	After intravenous administration of Mo2C QDs in mouse bearing B16-10F tumor, an 808 nm laser (0.64 W cm^−2, 10 min) irradiated. This increased the tumor temperatures from 27.9 °C to 63 °C.	92
		• Biocompatibility
		• Photothermal stability
		• Efficient tumor accumulation through EPR
		• Providing photoacoustic imaging signal
Photothermal immunoassay/protein point-of-care (POC) testing and diagnostics	Ti3C2 MQDs-encapsulated liposomes	• High photothermal efficiency	PSA detected at 808 nm with 1.5 W cm^−2. NIR laser energy was converted into heat using Ti3C2 QDs and the temperature shift was assessed.	149
		• Ability to detect infinitesimal concentrations of prostate-specific antigen
Multicolor cellular imaging	Different morphologies of Ti3C2 MQDs with RAW264.7 cells	• Strong photoluminescence	Within 4 h incubation, the MQDs were incorporated into the cells by endocytosis. Then the images were taken with confocal fluorescence microscopy.	82
		• High biocompatibility
		• Multicolor excitations at 405, 488, and 543 nm
		• Penetrate easily into the cell without damaging the nucleus
Imaging and biolabeling abilities	Ratiometric fluorescent pH sensors based on Ti3C2 MQDs	• Excellent performance for living cells	Images taken with confocal fluorescence microscopy of MCF-7 cells after endocytosis of Ti3C2 QDs. As pH decreases from 8 to 6, fluorescence from the Ti3C2 QDs increases.	150
Fluorescence probes applicable for the cellular study	Nb2C MQDs	• Highly chemically stable	MQDs were cocultured with cells for 4 h. The cytoplasm of the cells was selectively stained.	82
		• Biocompatible
		• Photobleach resistant
		• Selectively stained the cytoplasm of the cells
Cell imaging/detecting Cu^2+ ions/diagnosis of cytophagy-related diseases	N, P-MQDs	• The tests are performed at nontoxic concentrations	The N, P-MQDs were taken up by the THP-1 macrophages in 24 h and emitted green colors when exposed to 488 nm light source.	151
		• Photoluminescence quantum yield (PLQY) of 20.01% (by doping)
		• Constant fluorescence intensity in a wide pH and temperature range
		• Good stability and stable expression in complex organisms

---

5.1.1. Ion detection. MQDs have frequently been used to detect a broad spectrum of ions, particularly transition metals. For instance, Cu²⁺ is an essential element that serves as a catalytic co-factor for numerous enzymes, participating in redox reactions.¹⁵² However, excessive intake causes severe liver and kidney damage, and it is associated with diseases like Wilson's and Alzheimer's.¹⁵² Fluorescence quenching of MQDs has widely been utilized to detect copper. Feng et al.⁷⁴ used N-Ti₃C₂ MQDs to detect Cu²⁺, observing a gradual decrease in emission intensity as the copper concentration increased up to 2000 μM. This fluorescence quenching was highly selective for Cu²⁺, showing a significant response (∼56%) compared to negligible interference from other metal cations. This selectivity was attributed to the strong interaction between Cu²⁺ and N-Ti₃C₂, which altered the energy states of the MQDs.⁷⁴ Feng et al.⁷⁴ demonstrated that the fluorescence quenching mechanism of N-MQDs was primarily static, by which the quencher forms a stable complex with the fluorophore. Other investigations underscored the role of surface passivating amino groups on S, N-Nb₂C MQDs and N, P-Ti₃C₂ MQDs in capturing Cu²⁺.¹⁰² These groups formed absorbent complexes on the surface of the MQDs, inducing electron–hole recombination annihilation and aggregation of the MQDs into larger complexes. These complexes resulted in strong fluorescence quenching with a robust linear relationship across different concentrations (Fig. 9a). The reported LOD for both S, N-Nb₂C MQDs and N, P-Ti₃C₂ QDs were at as low as 2 μM, revealing superior sensitivity relative to previous studies on Cu²⁺.¹⁰²

Fig. 9 (a) PL emission spectra of Cu²⁺-(N, P-MQD) depending on the Cu²⁺ concentration (reprinted with permission from The Royal Society of Chemistry, copyright © 2019).¹⁰² (b) Fluorescence-based detection of Fe³⁺ ions by BSA@MQDs (reprinted with permission from Wiley, copyright © 2022).³⁹ (c) Fluorescence quenching and recovery of Ti₃C₂ MQDs by metal ions and EDTA addition, respectively (reprinted with permission from Elsevier, copyright © 2022).⁵ (d) UV-Vis spectra of Fe₃O₄@Ti₃C₂ MQDs depending on the Cr(VI) concentration (reprinted with permission from Elsevier, copyright © 2023).¹²² (e) ECL curves of MUC1 immunosensor (reprinted with permission from Elsevier, copyright © 2022).¹⁰¹ (f) ECL detection of miRNA-26a with various concentrations by Ti₃C₂ MQD@Au nanobones (reprinted with permission from the American Chemical Society, copyright © 2021).⁷² (g) Changes in the fluorescence spectra of N-Ti₃C₂ MQDs with increasing AA concentrations (reprinted with permission from Elsevier, copyright © 2021).¹⁰⁷ (h) Variations in the fluorescence spectra of N-MQDs with ARS concentrations. The inset shows the colloidal samples under daylight (left) and 365 nm UV (right) (reprinted with permission from Elsevier, copyright © 2021) images.¹⁰⁰ (i) The florescence spectra of UA@Ti₃C₂ MQDs fluorescence depending on the TNP concentrations (reprinted with permission from The Royal Society of Chemistry, copyright © 2020).⁸⁸

Fe³⁺ is another critical element that has widely been detected by MQDs via fluorescence technique because iron deficiency can lead to anemia, while excessive iron levels are associated with conditions such as hyperferremia and cancer.¹²¹ Gao et al.⁶⁸ used fluorescence quenching of Ti₃C₂ MQDs to detect Fe³⁺ with a high sensitivity (0.6377 mM⁻¹) and low LOD (1.4 μM) through the electrostatic-induced aggregation mechanism.⁶⁸ Amino-rich Ti₃C₂ MQDs demonstrated even greater sensitivity, achieving a LOD of 0.17 μM with excellent linearity for Fe³⁺ detection, through binding of the metal ions with the amino groups.⁶⁴ The lowest reported LOD for Fe³⁺ detection (1.25 nM) was attained when BSA@Ti₃C₂ MQDs were used.^70,121 The detection mechanism involved electron and energy transfer processes where the metal ions facilitated electron–hole recombination annihilation and altered the electronic state of the MQDs (Fig. 9b).

Cobalt and silver ions are of particular concern due to their environmental risks¹⁵³ and potential to cause cellular diseases,² making their sensitive and simultaneous detection highly significant. Cr³⁺ plays an essential role in biological processes and metabolism; however, elevated concentrations of Cr³⁺ can adversely affect cellular structure scan.² N-Ti₃C₂ MQDs enabled real-time and simultaneous detection of Co²⁺ and Ag⁺ with a LOD of 0.21 μM and 0.10 μM, respectively.² The primary detection mechanism was attributed to a combination of the IFE and the static quenching effect, resulting from the overlap between the absorption spectra of Co²⁺/Ag⁺ ions and the emission and excitation spectra of the N-Ti₃C₂ QDs.² In another study, by taking advantage of Ti₃C₂ MQDs, Cr³⁺ was detected with a low limit of detection of 30 mM with high selectivity in the presence of interfering ions.⁵ Although all Cr³⁺, Fe²⁺, Co³⁺, and Cu²⁺ quenched the emission of QDs, the addition of ethylenediaminetetraacetic acid recovered the PL response for all interfering elements except Cr³⁺, indicating selective detection of the target ion (Fig. 9c).⁵ Another recent progress is colorimetric detection of Cr(VI) ions by Fe₃O₄@Ti₃C₂ MQDs since an exceeding concentration of Cr(VI) in drinking water causes various diseases.¹²² This colorimetric method includes a catalytic oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) by Fe₃O₄@Ti₃C₂ MQDs in the presence of H₂O₂, resulting in the formation of a blue-colored product. Then, introducing 8-Hydroxyquinoline (8-HQ) reduces the blue color intensity of oxidized TMB. Introducing Cr(VI) with various concentrations up to 60 μM leads to a linear increase in the absorbance and recovery of the blue color (Fig. 9d), allowing sensitive and selective detection of Cr(VI) with a LOD of 0.26 μM.¹²² Ti₃C₂ MQDs have also been utilized for the detection of manganese (Mn(VII)) with a low LOD of 5.2 nM, which is superior to carbon dots with a LOD of 230 nM. This exceptional sensitivity was associated to the inherent reducibility of the MQDs, which facilitated a redox reaction with Mn(VII), resulting in pronounced fluorescence quenching. In contrast, the fluorescence quenching mechanism in carbon dots involved a synergistic IFE and static quenching due to the overlap between the absorption peaks of Mn(VII) and the excitation/emission spectra of the CDs. Further studies underscored the potential of these MQDs to serve as a dual-functional platform for simultaneous detection and scavenging of Mn(VII) in plant leaves, showcasing their impeding application in environmental safety and biological monitoring.¹⁵⁴

5.1.2. Detection of amino acids and proteins. MQDs have been utilized to detect various amino acids and proteins.¹¹⁴ Histidine, a fundamental amino acid for human tissue growth and repair, was detected with the LOD of 2.1 nM using the nonfluorescent complex of Ni²⁺-Ti₃C₂ MQDs via the IFE mechanism. Due to its strong affinity for Ni²⁺ ions, histidine competes with Ti₃C₂ MQDs for binding, thereby reversing fluorescence quenching. The sensor demonstrated high accuracy in detecting histidine in human samples, with a recovery rate ranging from 6.75% to 103.24%.¹¹⁴ Another example is the application of Ti₃C₂ MQDs for the detection of cytochrome (cyt-c) and trypsin.¹¹⁰ Introducing cyt-c to fluorescent Ti₃C₂ MQDs quenches the emission due to electron transfer from the QDs to the analyte. The degree of fluorescence quenching provides a quantitative measure of cyt-c concentration. Upon adding trypsin, the fluorescence is restored as trypsin hydrolyzes cyt-c into smaller peptides and removes them from the Ti₃C₂ QDs solution.¹¹⁰

Researchers have developed a novel MXene-based biosensor for detecting polynucleotide kinase (PNK).⁷¹ This enzyme plays a crucial role in phosphorylation-related DNA repair and nucleic acid metabolism. This biosensor, which operates via an ECL mechanism (eqn (1)–(5)) utilizes a hybrid Ti₃C₂ MQD/Au NPs in the presence of a co-reactant (K₂S₂O₈). The hybrid NPs offer significant advantages over Ti₃C₂ MQDs, including a threefold enhancement in the ECL signal due to surface plasmon resonance (SPR), superior electrocatalytic performance, and charge transfer capabilities. PNK interacts with the DNA probe immobilized on the modified electrode surface, triggering a series of reactions that amplify the ECL signal. Under optimal conditions, a linear relationship between ECL intensity and PNK concentration is attained, making this biosensor a powerful tool for PNK analysis in comparison to other sensors.⁷¹

S₂O₈²⁻ + e⁻ → SO₄²⁻ + SO₄˙⁻ (1)

Ti₃C₂ QDs–AuNPs + e⁻ → Ti₃C₂ QDs˙⁻–AuNPs (3)

Ti₃C₂ QDs*–AuNPs → Ti₃C₂ QDs–AuNPs + hv (5)

Jiang et al.¹⁰¹ detected Mucin 1 (MUC1), a key molecule involved in cell signal transduction,¹⁵⁵ by an ECL biosensor based on N-Ti₃C₂ MQDs.¹⁰¹ The MQDs exhibited fluorescence and ECL emissions at 455 nm and 612 nm, respectively. To fabricate the biosensor, Au NPs were electrodeposited onto a glassy carbon electrode (GCE) and functionalized with anti-MUC1 antibodies, followed by BSA treatment to prevent nonspecific binding. Next, the MUC1 antigen was introduced to this surface. The biosensor platform was then promoted with adding an AgPt@N-Ti₃C₂ QDs-anti-MUC1-BSA bioconjugate to enhance ECL sensitivity. The AgPt@N-Ti₃C₂ QDs nanocomposites played a crucial role in developing the ECL immunosensor for MUC1 detection through interacting with the co-reactant S₂O₈²⁻ in phosphate-buffered saline (PBS). As the concentration of MUC1 increased from 1 fg mL⁻¹ to 1 ng mL⁻¹, the ECL signal was linearly enhanced, providing a low LOD of 0.31 fg mL⁻¹ (Fig. 9e).¹⁰¹

5.1.3. miRNAs detection. Identifying specific organic molecules as biological indicators is crucial for diagnosing health problems in the early stages. For instance, triple-negative breast cancers are diagnosed by miRNA-26a identification, which plays a critical role in preventing cell proliferation, invasion, and migration in breast cancer cells.⁷² Recently, MQDs have engaged considerable attention to be used for miRNAs detection. By taking advantage of Ti₃C₂ MQD@Au nanobones (NBs), miRNA-26a was detected up to a concentration of 10 nM with the a LOD of 1.7 fM (Fig. 9f).⁷² The biosensor was also applied to detect clinical patient serum samples. The sensing mechanism involved a series of interactions that led to measurable changes in ECL. The heterostructure was modified with capture DNA that bound to miRNA-26a. Upon hybridization, T7 exonuclease cleaved the DNA-miRNA complex and released miRNA-26a, which served as an RNA walker, interacting with more capture DNA. Simultaneously, amino-functionalized MnO₂ nanosheets (MnO₂ NSs@probe DNA) were introduced to quench the ECL signal of MQD@Au NBs. As the miRNA-26a concentration increased, more capture DNA was cleaved, thereby reducing the interaction between the capture DNA and the probe DNA on the MnO₂ nanosheets that ultimately increased the ECL signal.⁷² In another study, Ti₃C₂T MQDs were used to enable sensitive detection of microRNA-155, an important indicator of B-cell lymphoma and breast cancer diseases.⁷¹ To this end, a photoelectrochemical (PEC) platform, comprising TiO₂ nanosheets with exposed (001) facets, antibody, RNA–DNA, Ti₃C₂T_x QDs, and (001) TiO₂/FTO, was fabricated. To achieve this, AuNPs were deposited on the electrode to immobilize thiol-modified DNA, which bound to microRNA-155 selectively. The electrode was then treated with 6-mercapto-1-hexanol (MCH) to block nonspecific sites and to enhance specificity. Afterwards, the hybridized microRNA–DNA structure was recognized by an antibody (S9.6 antibody), followed by the introduction of IgG-ALP, which generated an electron donor, increasing the photocurrent for quantitative analysis. The PEC platform demonstrated high sensitivity with a detection limit of 0.025 pM for microRNA-155.⁷¹ The high surface energy of nanosheets as well as the Ti₃C₂T_x QDs/TiO₂ type II heterostructure minimized electron transfer loss.

5.1.4. Sensing of organic acids. MQD-based sensors have frequently been utilized for detecting organic acids, such as ascorbic acid (AA), which is an essential antioxidant regulating internal redox homeostasis during various life processes.¹⁰⁷ The fluorescent N-Ti₃C₂ MQD nanoprobe has been effective in the determination of Cr(VI) and AA with a LOD of 0.02 μM and 0.012 μM, respectively.¹⁰⁷ In the absence of AA, the fluorescence of N-Ti₃C₂ MQDs is significantly impeded by Cr(VI) due to the IFE and static quenching. However, upon the addition of AA, the redox reaction between Cr(VI) and AA weakens the impact of the IFE, resulting in the restoration of the fluorescence signal of N-Ti₃C₂ MQDs (Fig. 9g). The nanoprobe demonstrates high sensitivity, selectivity, and reproducibility for the detection of Cr(VI) and AA in real samples.¹⁰⁷ Ti₃C₂ MQDs also allow monitoring of uric acid (UA) with the a LOD of 125 nM.¹⁰⁵ UA is a key biomarker for various medical conditions, such as obesity, fatty liver, kidney stones, and diabetes.¹⁰⁵ To achieve this, o-phenylenediamine (OPD) is oxidized to produce yellow-colored 2,3-diaminophenazine (oxOPD) in the presence of horseradish peroxidase (HRP) and hydrogen peroxide. Using an overlap between fluorescence emission of glutathione (GSH)-functionalized Ti₃C₂ MQDs at 430 nm and the absorption of oxOPD at 425 nm, FRET is facilitated. As the concentration of UA increases, the GSH-Ti₃C₂ MQDs emission is gradually diminished, while the 568 nm emission from oxOPD increases progressively, enabling precise detection of UA.¹⁰⁵

5.1.5. Environmental pollutants. Environmental pollutants pose serious risks to human and public health, thus requiring advanced detection methods.^111,156 MQDs-based platforms have manifested great potential for sensitive and efficient detection of different contaminants, including Vibrio parahaemolyticus (VP),¹¹⁶ Alizarin Red (ARS),¹⁰⁰ 2,4,6-trinitrophenol (TNP),⁸⁸ pirimiphos-methyl,¹²⁵ and tetracycline (TC).¹⁰⁹

To detect VP with a LOD of 30 cfu mL⁻¹, a composite probe comprised of modified polyhedral oligomeric silsesquioxane-perovskite quantum dots (POSS-PQDs) with aptamers (as the signal probe) and Ti₃C₂ MQDs (as the quencher) was used. The fluorescence emission of the probe was quenched through FRET in the absence of the VP. Introducing VP recovered the PL signal and amplified it in a concentration-dependent manner.¹¹⁶

ARS is a commonly used dyeing agent for textiles; however, its toxicity poses a significant risk to human health and ecological stability.¹⁰⁰ Recently, N-Ti₃C₂ MQDs were found effective in the successful detection of ARS, achieving a LOD of 1.21 μM in the linear range of <80 (Fig. 9h).¹⁰⁰ The quenching mechanism was identified as IFE due to three key observations. First, the absorption spectra of ARS overlapped with the excited emission spectra of N-Ti₃C₂ MQDs, thereby reducing the emission of N-Ti₃C₂ QDs. Second, the fluorescence lifetimes of N-Ti₃C₂ MQDs remained nearly unchanged before and after the addition of ARS, suggesting that the quenching is not due to static quenching but rather to IFE. Third, no new absorption peaks were observed in the absorption curves after adding ARS, indicating that there are no chemical interactions.¹⁰⁰

TNP is a versatile chemical compound widely used in various industrial applications, including explosives, matches, dyes, and leather products.⁸⁸ TNP is highly toxic, and prolonged exposure can cause adverse symptoms such as headaches, dizziness, and nausea. Wang et al.⁸⁸ successfully detected TNP by UA-modified Ti₃C₂ (UA@Ti₃C₂ MQDs). A significant overlap was observed between the absorption spectrum of TNP and the emission spectrum of UA@Ti₃C₂ MQDs, suggesting the potential involvement of FRET or IFE. Further analysis using UV absorption and the average fluorescence lifetime of UA@Ti₃C₂ MQDs revealed that the quenching occurred via the IFE mechanism. The fluorescence intensity decreased with increasing TNP concentration, enabling the detection of TNP within a range of 0.01 to 300 μM, with a LOD of 9.58 nM (Fig. 9i). The detection limit was lower than the maximum residue limit of TNP in drinking water, as set by the US Environmental Protection Agency.⁸⁸ In another study, Ding et al.¹²⁵ used Cu@Ti₃C₂ MQDs-aerogel to detect pirimiphos-methyl, a kind of organophosphorus pesticides. The detection mechanism was based on the peroxidase-like activity of Cu@Ti₃C₂ MQDs-aerogel, which catalyzed the decomposition of hydrogen peroxide (H₂O₂) into reactive hydroxyl radicals (˙OH). This process enabled an effective colorimetric assay by measuring the peak intensity at 652 nm, corresponding to the oxidation of TMB. As the concentration of pirimiphos-methyl increased, the catalytic activity decreased, allowing for its precise detection with a LOD of 0.65 nM.¹²⁵

Tc is a versatile antibiotic in animal care, but its misuse poses health risks, including allergic reactions and ecological accumulation.¹⁰⁹ Therefore, there is an urgent need for efficient detection of Tc residues in food and water.¹⁰⁹ A ratiometric fluorescent sensor based on N, B co-doped Ti₃C₂ MQDs and Eu³⁺ was found effective in Tc detection with a LOD of 20 nM. When Tc was added to a N, B co-doped Ti₃C₂ MQDs solution, the emission intensity at 448 nm was slightly reduced due to the IFE mechanism. As the Tc concentration increased, the 448 nm emission peak was gradually weakened, while another peak that appeared at 616 nm appeared, underscoring the prominent effect of Eu³⁺ in Tc detection. The successful performance of Eu³⁺/N, B-Ti₃C₂ MQDs in the detection of Tc in the milk samples demonstrated their strong potential for practical use.¹⁰⁹

5.1.6. Other biological targets. GSH is a strong antioxidant that plays a crucial role in the detoxification, the metabolism of chemotherapeutic drugs, and the support of cellular functions.¹¹⁵ It is well known that deviations in GSH levels cause severe cell apoptosis and cancer. Chen et al.¹¹⁵ employed the PEC technique and a Ti₃C₂ MQD/TiO₂ heterojunction electrode to detect GSH with a LOD of 9 nM. The electrode was simultaneously biased at 0.1 V and excited by 150 W xenon light. The results revealed that the photocurrent of TiO₂/Ti₃C₂/Nafion electrode gradually increased with the addition GSH, enabling its detection at a low applied bias with strong surface adsorption.¹¹⁵

There is also an urgent need to advance sensitive and non-toxic detection methods for H₂O₂.¹¹⁸ As an example of potential application, the tumor microenvironment in cancer cells often exhibits an overproduction of H₂O₂, leading to the formation of highly reactive and toxic hydroxyl radicals (˙OH). These hydroxyl radicals, in turn, induce oxidative stress and damage to cancer cells, prompting processes such as apoptosis (programmed cell death) and inhibition of tumor growth.⁹² Hence, detecting H₂O₂ levels accurately is critical for understanding its role in diseases and for the development of sensitive, non-toxic detection methods.¹¹⁸ Recently, N-Ti₃C₂ MQDs were utilized for H₂O₂ sensing with a LOD of 1.2 nM which is the lowest reported value to date.¹¹⁸ In another study, it was shown that Ti₃C₂ MQDs synthesized via the acoustic method exhibit more H₂O₂ sensing capability compared to those prepared by the hydrothermal method.⁹⁷ The sensor platform was based on Ti₃C₂ MQDs/glassy carbon (Naf/MQD/GCE).⁹⁷ The square wave voltammetry revealed a LOD of 5 nM (Fig. 10a and b), with a higher linear regression slope of the current response plots.⁹⁷ H₂O₂ was also detected by N-Ti₃C₂T_x MQDs via a colorimetric approach using TMB.⁹² Here, the breakdown of the O–O bond in H₂O₂ into ˙OH, which oxidized TMB, resulting in two distinctive absorbances at 370 and 652 nm. This enabled the detection and quantification of hydroxyl radicals and H₂O₂.⁹²

Fig. 10 (a and b) Square wave voltammetry curves of Naf/MQD/GCE and Naf/HMQD/GCE at varying H₂O₂ concentrations (reprinted with permission from the American Chemical Society, copyright © 2021).⁹⁷ (c) CV curves of Ti₃C₂-MQDs@3DE in PBS for DA detection (reprinted with permission from Elsevier, copyright © 2023).¹¹³ (d) Normalized PL spectra of [Ru(dpp)₃]Cl₂ and Ti₃C₂ MQDs at different pH levels (reprinted with permission from The Royal Society of Chemistry, copyright © 2019).⁴³ (e) Quartz BAW infrared detector response to NIR irradiation before/after coating (reprinted with permission from Optica Publishing Group, copyright © 2022).¹⁵⁷

N-Ti₃C₂ MQDs functionalized with oxOPD (N-Ti₃C₂ QDs@DAP) have proven highly effective for detecting substances like xanthine, which generates H₂O₂ through specific enzymatic reactions.⁹² The operation of this nanoprobe involves a multi-step enzymatic process. Xanthine is oxidized by xanthine oxidase (XOD) to produce H₂O₂, which subsequently reacts with OPD in the presence of HRP. This reaction results in the formation of DAP. The presence of DAP not only quenches the fluorescence of N-Ti₃C₂ MQDs but also provides a new emission peak at 560 nm, establishing a dual-emission sensor. Here, N-Ti₃C₂ MQDs serve as electron donors, while DAP acts as an electron acceptor. Consequently, a complex is formed on the functionalized surface of the QDs, quenching the fluorescence intensity. This dual-emission system provides a sensitive and reliable method for detecting xanthine and H₂O₂, showcasing the versatility and efficacy of N-MQDs@DAP in diagnostic applications.⁹²

Ti₃C₂ MQDs have also been utilized for the ratiometric fluorescence detection of a Bacillus anthracis biomarker, 2,6-dipicolinic acid (DPA).¹²⁴ Ti₃C₂ MQDs are endowed with abundant hydroxyl and amino groups, which are utilized for coordination with EDTA and Eu³⁺ to form a composite probe, named Ti₃C₂ QDs–EDTA–Eu³⁺. While Ti₃C₂ QDs emit stable blue color as a reference signal, the fluorescence of Eu³⁺ is quenched by surrounding water molecules. The introduction of DPA replaces the water molecules and enhances the Eu³⁺ fluorescence intensity at 616 nm due to the absorbance energy-transfer effect (AETE) from DPA to Eu³⁺. A visible fluorescence shift from blue to red with the DPA concentration enables quantitative detection by a smartphone equipped with color recognition software.¹²⁴

In a recent study, Ti₃C₂-MQDs have been utilized to detect CUR, which has antioxidant, anti-inflammatory, antibacterial, and anticancer properties.⁶² It is known that excessive amounts of CUR may boost DNA oxidative activity, lower Adenosine triphosphate (ATP) levels, and trigger necrotic processes.⁶² Detection of CUR was achieved by leveraging the overlap between the emission peak of Ti₃C₂ MQDs at 430 nm and the absorption spectrum of CUR. When CUR is introduced, the fluorescence emission of Ti₃C₂ MQDs is quenched via FRET to CUR, while CUR fluorescence emission increases. Following the addition of ClO⁻ to this solution, the fluorescence emission of Ti₃C₂ MQDs is recovered, as CUR's phenol and methoxy groups are oxidized to quinones in the presence of ClO⁻. The linear detection range of ClO⁻ has been found to be 25–150 μM and 150–275 μM, with a detection limit of 5 μM. For CUR, the linear detection range is 0.05–10 μM, with a LOD of 20 nM. The MQDs also exhibit a colorimetric response to CUR, which causes a visible color change from colorless to yellow.⁹⁹

In addition to CUR, quercetin, a well-known antioxidant bioflavonoid, was detected by bright bluish-green emissive N-doped Ti₃C₂ MQDs.¹²⁶ The introduction of quercetin resulted in a significant fluorescence quenching due to static quenching/radiative-free complex formation and IFE. Consequently, the quantification of quercetin over a linear range of 25–600 nM with a low LOD of 1.35 nM was performed. The sensor demonstrated exceptional specificity for quercetin in real food samples, such as orange, onion, and red wine, with a quenching efficiency exceeding 95% compared to less than 10% for potential interferents. This finding demonstrates the significant potential of MQDs in various applications, including food safety, pharmacological assays, and the detection of various biological markers.¹²⁶

Ti₃C₂ MQDs was also utilized to detect dopamine (DA), a catecholamine released by the adrenal medulla.¹¹³ A Ti₃C₂ QDs@3D-printed electrode sensor enabled DA monitoring with a LOD of 3 nM over a linear range of 0.01 to 20 μM. CV determined an oxidative potential of 0.29 V for DA with an increasing trend in the current density with its concentration (Fig. 10c).¹¹³

Recently, a nanosurface molecularly imprinted polymer (Ti₃C₂ MQD@MIP) resonance Rayleigh scattering (RRS) spectral probe has been developed for the sensitive and selective detection of thiocyanate (SCN⁻).¹⁵⁸ In this platform, Ti₃C₂ MQDs serve as the matrix, while SCN⁻ act as the template during the imprinting process. The functional monomer, (3-aminopropyl) triethoxysilane, and the cross-linker (tetraethoxysilane) facilitate the formation of the polymer network, with ammonia serving as the polymerization initiator. The detection mechanism is based on changes in RRS intensity, where the interaction between SCN⁻ and the Ti₃C₂ MQD@MIP probe results in a concentration-dependent reduction in scattering intensity. This allows for accurate quantification of SCN⁻ with a linear detection range of 0.87–5.22 μg L⁻¹ and a LOD of 0.37 μg L⁻¹.¹⁵⁸ In another study, NO₂⁻ (nitrite), a crucial additive in food preservation, was successfully detected by the dual colorimetric/fluorometric method using N, P-Ti₃C₂ MQDs and the Phen-Fe²⁺ complex.⁸⁴ The fluorescence emission of N, P-Ti₃C₂ QDs was quenched by the Phen-Fe²⁺ complex via the IFE, accompanied by the intensified orange color. Upon the addition of NO₂⁻, the redox reaction between NO₂⁻ and Fe²⁺ generated Fe³⁺, reducing IFE by fluorescence recovery as well as a gradual change from orange to colorless. Using a smartphone-assisted colorimetric filter paper based on the N, P-Ti₃C₂ QDs/Phen-Fe²⁺ system, a linear relationship between the red, green, blue (RGB) ratio and NO₂⁻ concentration (up to 80 μM) was attained. More recently, N, B-doped Ti₃C₂ MQDs incorporated onto a functionalized paper were developed for the efficient adsorption and detection of dichromate ions (Cr₂O₇²⁻).¹⁵⁹ The adsorption mechanism was attributed to a combination of electrostatic interactions and chemical bonding. The sensing mechanism involved fluorescence quenching via IFE with an efficiency of 99.9%, a response time of 10 s, and a LOD of 1.2 μM. The incorporation of green and renewable wood pulp fibers as raw materials further emphasized the potential of this innovative material for large-scale production and industrial application in water pollution management.¹⁵⁹

5.1.7. pH sensors. N-Ti₃C₂T_x MQDs have been introduced as one of the promising and efficient candidates for pH sensors. The pH dependent-fluorescence properties of the MQDs are attributed to surface functionalization, surface defects, protonation/deprotonation processes, and change in absorption and emission characteristics.¹⁰⁸ Measuring the fluorescence intensity of Ti₃C₂T_x MQDs at various pH values under 365 nm excitation revealed a linear relationship between the ratio of 514 nm to 464 nm. The pH-responsive fluorescence was visually confirmed with blue emission at pH = 12.1 and green emission at pH = 2.0.¹⁰⁸

The pH sensing potential of Ti₃C₂ MQDs, as effective ratiometric pH sensors, was also demonstrated when combined with the pH-insensitive [Ru(dpp)₃]Cl₂ dye.⁴³ This setup enabled precise intracellular pH monitoring via fluorescence intensity ratios. The ratiometric fluorescence change due to pH variations was characterized by a decrease in the intensity of the Ti₃C₂ MQD emission at 460 nm, while the emission of reference dye remained stable. A linear decrease in the fluorescence intensity ratio (I₄₆₀/I₆₁₅) with the pH in the range of 5 to 9 was noticed (Fig. 10d). This linear calibration curve for the fluorescence ratio (I₄₆₀/I₆₁₅) enabled accurate pH differentiation in the physiological range of 6.0 to 8.0. The sensors also demonstrated profound imaging and bio-labeling capabilities for reliable, long-term pH monitoring in biological environments.⁴³

5.1.8. Physical parameters. Rehabilitation training plays a crucial role in aiding patients-such as those recovering from strokes or car accidents-to regain motor functions after an injury. Personalized guidance from professionals is often necessary, demanding a significant allocation of human resources. Hence, there's an urgent need for a sensor that can precisely monitor patients’ movement in real-time. Flexible pressure sensors demonstrate the ability to monitor and interpret diverse human motion signals like walking, pulse/heart rate, sound, hand, and knee movements.³⁸ These devices turn this physiological information into visualized, real-time impulse signals. Recently, Sun et al.³⁸ have evaluated, the sensitivity of MQDs within natural 3D watermelon peel matrix aerogels using a dynamic positioning system, collecting pressure-sensitive data via an electrochemical workstation. The resulting current–voltage curve demonstrates an increase in the current with the applied forces ranging from 0.02 to 20 kPa. The sensor's sensitivity also reveals distinct regions, including 323 kPa⁻¹ for 0–0.4 kPa and 51.38 kPa⁻¹ for 0.4–20 kPa. Thus, this sensor accurately detects various pressures, generating a recordable and analyzable current signal. In the design implementation, speech recognition controls sensors attached to the throat to differentiate phrases and vocal behaviors like “Hello” versus “Hi” and detect laryngeal swallowing patterns. The sensors can be fixed on different joints, such as the knee, elbow, and finger joints, to carry out corresponding tests. The signal waveforms are recorded as the sensors are fixed on the sole during fast walking, slow walking, running, and jumping. The sensor can accurately and quickly record the signals of different movements, which provides the possibility of the new-generation real-time remote monitor for sports rehabilitation. Measurement of the resonant frequency of a Y-cut quartz bulk acoustic wave (BAW) sensitive unit, fabricated by microelectromechanical systems, is employed for infrared sensing. Coating the surface of the quartz BAW resonator with the MQD film via spin coating technology significantly enhances the infrared absorption efficiency of the infrared detector, demonstrating promising potential for achieving high-performance infrared detection through the amalgamation of these two components. Test outcomes reveal a nearly 41% increase in the sensor's responsivity at an 830 nm wavelength following film coating, rising from 10.88 MHz W⁻¹ to 15.28 MHz W⁻¹. This observation underscores substantial enhancement in the sensor's efficiency in absorbing infrared radiation (Fig. 10e).¹⁵⁷

5.2. Optoelectronics

5.2.1. White light emitting diodes (WLEDs) and lasers. Multicolor and white light diodes hold significant potential for diverse applications, including bioimaging, full-color displays, optoelectronic devices, and PTT.¹⁶⁰ A common approach to producing white light is the combination of a blue LED with a yellow phosphor because most of the fluorescent materials are incapable of directly emitting pure white light.¹⁶⁰ However, this method lacks sufficient red (or other colors) spectral components, resulting in a lower color rendering index (CRI). Recently, the fabrication of novel LEDs has focused on leveraging strong quantum confinement in QDs to achieve tunable band gaps, enabling the emission of light at different wavelengths.^161,162

One of the earliest MQDs-based WLED with Commission Internationale de l’Éclairage (CIE) coordinates of (0.31, 0.35) was fabricated by Xu Quan et al.⁹ in 2019. To fabricate this device, which was stable at a voltage of 3 V after a month (Fig. 11a), Ti₃C₂T_x-MQDs were hydrothermally co-processed with sodium thiosulfate (S-doped), an ammonia solution (N-doped), and a combination of these two (SN-doped) to emit blue (440 nm), yellow (540 nm), and orange (580 nm), respectively, followed by mixing with polyvinylpyrrolidone (PVP) and casting on a blue chip. Similar to lead-free double perovskites and SiC QDs, Ti₃C₂ MQDs also manifested great potential to be used for single-component white LEDs (Fig. 11b).^127,163 In this case, a hybrid nanocomposite was prepared by polymerizing Ti₃C₂ MQDs, having strong two-photon white fluorescence with the full width at half maximum (FWHM) and PLQY of 220 nm and 9.36%, respectively, in a polydimethylsiloxane solution. This working LED could emit white light with the color coordinates of (0.30, 0.34). Moreover, the fluorescence of the Ti₃C₂ MQDs under high pressure (2.53 GPa) enabled tuning the white emission temperature from cool to warm, (Fig. 11c) which is a fascinating phenomenon for the next generation of single-component warm WLEDs.¹²⁷

Fig. 11 (a) CIE coordinates of WLED based on MQDs/PVP under 360 nm excitation. The inset showing fluorescence images of the W-MQDs/PVP composite under 365 nm emission (reprinted with permission from Elsevier, copyright © 2019).⁹ (b) PL spectra of the visible-light-emitting MQDs under 360 nm emission (reprinted with permission from Elsevier, copyright © 2019).⁹ (c) PL spectra of Ti₃C₂ QDs depending on the applied pressure (reprinted with permission from Wiley, copyright © 2019).¹²⁷ (d) Images showing different excitation wavelengths for V₂C MQDs (xenon lamp) (reprinted with permission from Wiley, copyright © 2019).¹²⁸

White lasers are increasingly important in fields such as laser display technology, communication technology, biomedicine, and environmental detection.^164–167 To fabricate white lasers, a wide range of materials, including dye mixtures¹⁶⁸ semiconductors,¹⁶⁹ laser or nonlinear optical crystals,¹⁶⁶ and QDs¹²⁸ have been used.^170–172 However, constrained by compactness, difficulty in growth or wavelength control, and environmentally unfriendly chemicals, white lasers are still underdeveloped, and the available materials and approaches are still being extensively sought. One of the most promising white lasers was fabricated by Huang et al.¹²⁸ using V₂C MQDs. By employing passivation treatments, the PL intensity of V₂C MQDs, spanning the entire visible spectrum, was significantly enhanced. Fig. 11d illustrates the light emission images of passivated V₂C QDs under various excitation wavelengths. The localized nonlinear random scattering of the passivated V₂C MQDs has been achieved through the generation of excitation-power-dependent solvent bubbles. This phenomenon plays a critical role in producing a white laser with multiple color outputs. With optimized excitation conditions, the amplification and simultaneous lasing of blue (490 nm), green (545 nm), yellow (587 nm), and red (613 nm) light has been demonstrated.

5.2.2. Solar cells. With a high-power conversion efficiency (PCE) of 25.7%,¹⁷³ perovskite solar cells (PSCs) have become a strong candidate for the next generation of energy harvesters.^174,175 However, preparing a perovskite thin film with high crystallinity and long-term stability against severe humidity is an important issue for commercialization.^176–178 Among various strategies, the modification of perovskite thin films by appropriate materials has become an effective approach to meet both requirements.¹⁷⁹ To this end, MXenes could be a promising material due to their metallic conductivity, high carrier mobility, profound optical transparency, tunable work function, and superior mechanical properties.¹⁸⁰ Their diverse transition elements and surface-terminating groups also allow for tuning band energy alignment between absorber and charge transport layers, improving charge carrier extraction.¹⁸¹ It has been demonstrated that incorporating MXenes in PSCs reduces interfacial recombination resistance and facilitates smooth charge transfer paths, enhancing the conductivity and stability of PSCs.¹⁸² However, MXene fails to address the challenge of the small crystal size in the perovskite layer, which can negatively impact the PCE.¹⁸³ This limitation has led researchers to explore the use of MQDs, offering quantum confinement effect and fruitful surface terminations, high surface area, and improved charge transport, making them a promising alternative for enhancing the performance of PSCs.^184,185

For instance, incorporation of Cl-terminated Ti₃C₂ MQDs into the FA_xMA_1−xPbI₃ perovskite could increase the PCE from 19.53% to 21.31%. Not only short-circuit current density (J_sc) and open-circuit voltage (V_oc) of the cells increased, but also the difference between the forward and reverse scan curves, called normal hysteresis, decreased (Fig. 12a and b). This performance improvement can be attributed to two key factors. First, the strong interactions between the Cl terminations of Ti₃C₂Cl_x and Pb²⁺ ions promote a preferred grain orientation with reduced residual tensile strain by slowing the crystallization rate, thereby improving the crystallinity of the perovskite film. Second, the natural formation of a top-down gradient distribution leads to a higher concentration of Ti₃C₂Cl_x MQD additives near the bottom substrate. These additives serve as nucleation sites for the growth of high-quality perovskite crystals to facilitate efficient charge extraction between the SnO₂ electron transport layer (ETL) and the perovskite layer. Besides, Cl-terminated Ti₃C₂T_x MQDs avoid possible deprotonation of protonated organic amine in perovskite. Consequently, the solar cell exhibits outstanding long-term stability by maintaining over 84% of its initial PCE after being aged for 1000 h under 40% relative humidity.¹²⁹ In another study, the MAPbI₃ perovskite and TiO₂ layers were modified by Ti₃C₂ MQDs and MXene nanosheets, respectively. Apart from the positive effect of nanosheets on electron mobility improvement of the ETL, 0.8 mg MQDs reduced defects concentration via successful perovskite film passivation and increased perovskite crystallinity. These claims could be supported by an enhancement in the PL emission intensity and carrier lifetime, as shown in Fig. 12c–e. As a result of this modification, the PCE improved from 12.0% to 17.1%, along with the reduced hysteresis effect.³⁴

Fig. 12 (a) Schematic illustration of the Ti₃C₂Cl_x MQDs-treated FA_xMA_1−xPbI₃ PSCs with colored cross-sectional scanning electron microscopy (reprinted with permission from Elsevier, copyright © 2022).¹²⁹ (b) Reverse and forward scans for the pristine and 0.2 mg mL⁻¹ Ti₃C₂Cl_x MQDs-treated solar cells (reprinted with permission from Elsevier, copyright © 2022).¹²⁹ (c) UV-Vis absorption spectra, (d) PL spectra, and (e) J–V curve of perovskite films with different content of Ti₃C₂ MQDs (reprinted with permission from Elsevier, copyright © 2020).³⁴

Similar to MXene nanosheets, the ETL modification by MQDs manifested great potential to boost both PCE and stability. For instance, it was demonstrated that the introduction of Nb₂C MQDs into SnO₂ (as the ETL) was effective in enlarging the grains, modifying the roughness and surface energy, and reducing defect sites.¹³⁰ Subsequent deposition of Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃ perovskite films on the SnO₂–Nb₂C ETL enhanced perovskite crystallinity and effective carrier transport, leading to a PCE of 22.86%. The device maintained 98% of its as-prepared efficiency after 40 days holding at 25 °C under 40–60% humidity (Fig. 13a and b).¹³⁰ In another survey, an in situ synchrotron-based two-dimensional grazing-incidence X-ray diffraction (GIXRD) technique was used to explore the effect of Ti₃C₂T_x MQDs-modified SnO₂ (MQDs-SnO₂) on the perovskite crystallization kinetics.¹³¹ It was shown that the modified structure facilitated perovskite nucleation from the precursor solution, forming an intermediate perovskite phase upon anti-solvent treatment. As a result, a substantial improvement in the crystal quality and phase stability of the perovskite film was achieved. As shown in Fig. 13c–e, the more uniform and larger grain sizes, the more light absorption capacity, the shorter lifetime in TRPL, and the much brighter scattered rings in GIXRD patterns underscore the better crystallinity and charge extraction in the target layer compared to the reference cells. The steady-state PCE of up to 23.3% as well as outstanding stability against humidity and light soaking was achieved for the corresponding PSCs due to the superior charge extraction properties of the MQDs-SnO₂ layer.¹³¹ Xu Chen et al.¹⁸⁶ prepared high-performance Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃ PSCs with simultaneously improved performance and stability via incorporating Ti₃C₂T_x MQDs into the ETL and active layer. The incorporation of MQDs in the mesoporous TiO₂ reduced the interfacial defects and improved the electron extraction and injection from the perovskite absorber into the mesoporous TiO₂. On the other hand, introducing MQDs into the perovskite active layer increased the crystallization size and conductivity of the perovskite, while reducing the grain boundaries and the intrinsic defect density, by passivating the charge recombination centers. The MQD-modified PSCs exhibited a hysteresis-free response with a PCE of 21.64%, which was remarkably higher than that of the reference device (18.31%).¹⁸⁶

Fig. 13 High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM) images of (a) SnO₂ and (b) SnO₂–Nb₂C MQDs (reprinted with permission from Elsevier, copyright © 2021).¹³⁰ The SEM images of the perovskite films deposited on (c) SnO₂ and (d) SnO₂–Nb₂C MQDs ETL basement.¹³⁰ (e) UV-vis absorption and PL spectra of the reference and target perovskite films (reprinted with permission from Royal Society of Chemistry, copyright © 2021).¹³¹

MQDs have also been used for structural modification of dye-sensitized solar cells (DSSCs) towards higher efficiencies.¹³² The PCE of DSSCs still requires further improvement despite their apparent benefits such as low-cost preparation, multicolor transparency, and a wide range of practical applications.¹³² Recently, substantial increase in the photocurrent and efficiency of DSSCs has been achieved by successfully altering the energy level alignment of TiO₂ photoanode through the introduction of niobium oxide (Nb₂O₅) and Ti₃C₂ MQDs. The modified photoanode exhibits a distinct mesoporous structure, offering high dye adsorption and with a broader absorption range in the visible region. An improved charge transfer suppressed electron–hole recombination at the photoanode interface, and enhanced PCE (7.24%) relative to the reference group (4.60%) has been demonstrated.¹³²

5.2.3. UV detectors. Excessive exposure to ultraviolet (UV) radiation can have harmful effects on the skin, causing premature aging, sunburn, and an increased risk of skin cancer.¹⁸⁷ Both UVA (315–400 nm) and UVB (280–315 nm) rays can cause damage. Although the latter is the main cause of sunburns and is thought to contribute to most skin cancers, the former is associated with skin aging. Given the potential health risks associated with UV exposure, there is a need for efficient UV detectors to help individuals monitor their exposure levels and take necessary precautions to protect their skin.^188,189 Several categories of QDs have been widely explored for UV photodetection, including II–VI semiconductor QDs (e.g., CdSe), metal oxide QDs (e.g., ZnO), carbon-based QDs (such as nitrogen-doped carbon QDs), perovskite QDs (e.g., CsPbCl₃, CsPbBr₃), and graphitic carbon nitride (g-C₃N₄) QDs. These materials exhibit strong UV absorption, tunable bandgaps, and favorable photoelectric conversion efficiency, making them suitable for diverse photodetector architectures. In comparison, MQDs provide several unique advantages for UV detection. Owing to their intrinsically high dielectric constants and extinction coefficients, MQDs enable superior light absorption and facilitate efficient separation of photogenerated charge carriers, resulting in enhanced responsivity and quantum efficiency. Their rich surface chemistry allows versatile functionalization and heteroatom doping, offering additional control over sensitivity and electronic coupling at device interfaces. Furthermore, MQDs display remarkable chemical stability and biocompatibility, with significantly reduced cytotoxicity relative to conventional QDs such as CdSe and even their 2D MXene parent structures. Benefiting from pronounced quantum confinement and edge effects, MQDs also exhibit excellent photostability and strong light–matter interactions, enabling efficient conversion of optical energy into electrical or thermal signals. These collective attributes render MQDs a promising class of materials for next-generation, flexible, and bio-integrated UV photodetectors.^190–192

Similar to MQD-modification of ETL in solar cells, MQDs have been used to enhance UV detector performance. For instance, Yiqiang Zheng et al.⁸ used Ti₃C₂T_x MQDs to enhance charge carriers and light absorption of 2D perovskite Ca₂Nb₃O₁₀. The results showed a 1100% increase in spectral responsivity (264 A W⁻¹ at 310 nm) compared to the reference photodetector without MQDs. The flexible MQDs/perovskite photodetector was then integrated with a data collector to simulate a wearable visible-blind UV monitoring system for skin and plant protection. The ambient UV radiation information in various weather conditions could be trained and recognized by an artificial neural network to alert users.⁸ Similar to MQD-modification of ETL in solar cells, MQDs have been used to enhance UV detector performance. For instance, Yiqiang Zheng et al.⁸ used Ti₃C₂T_x MQDs to enhance charge carriers and light absorption of 2D perovskite Ca₂Nb₃O₁₀. The results showed a 1100% increase in spectral responsivity (264 A W⁻¹ at 310 nm) compared to the reference photodetector without MQDs. The flexible MQDs/perovskite photodetector was then integrated with a data collector to simulate a wearable visible-blind UV monitoring system for skin and plant protection. The ambient UV radiation information in various weather conditions could be trained and recognized by an artificial neural network to alert users.⁸

5.2.4. Ultrafast photonics. Ultrafast lasers, which show short pulse duration from picosecond to femtosecond regions and high pulse energy, has garnered tremendous attention. Materials with nonlinear optical properties are commonly used as a saturable absorber, which is one of the most important parts of an ultrafast laser system.¹⁹³ Other types of QDs such as CdSe, graphene, MoS₂, NbSe₂ and phosphorene are extensively used in ultrafast photonics and ultrafast laser systems as saturable absorbers due to their strong nonlinear optical properties and ability to generate ultrashort laser pulses. These QDs enable the generation of femtosecond to picosecond laser pulses and are exploited for high-speed optical communications, medical imaging, and advanced sensing technologies. MQDs offer several advantages over traditional QDs in ultrafast photonics. MQDs show a much larger nonlinear absorption coefficient and ultrafast carrier dynamics, with transient absorption signals. When used as saturable absorbers, MQDs enable the generation of femtosecond laser pulses with high signal-to-noise ratios and ultra-narrow linewidths, surpassing many conventional QD materials. Their tunable surface terminations and high modulation depth allow for low-threshold mode-locking and efficient ultrafast pulse generation, making them ideal for fiber laser applications. MQDs also exhibit exceptional stability and photodurability, further supporting their advantage for robust ultrafast photonic devices.^194–196 Among 2D materials, MXenes have attracted huge attention due to their outstanding photoelectric characteristics and adjustable performance caused by flexible compositional or surface functional group regulation. For instance, Fumei Yang et al.⁷⁷ used Ti₃C₂T_x-MQDs with various functional groups to fabricate a tapered fiber saturable absorber device for an Er³⁺-doped fiber laser. The single-mode fiber was stretched into a tapered fiber using a fiber TEC machine. The photo deposition method was then employed to stably transfer Ti₃C₂T_x QDs to the tapered area to fabricate the saturable absorber. The process involved dispersing the material in a solution, immersing the substrate, and exposing it to light to initiate the deposition. The size and abundance of the deposited nanosheets could be controlled by regulating the immersion time. Compared to other QDs such as PbS QDs, the saturable absorber device exhibited a low saturation intensity (1.983 GW cm⁻²) and high modulation depth (11.6%), enabling easier mode-locked pulse generation. The device produced ultrashort pulses of 466 fs at a wavelength of 1566.57 nm with a fundamental frequency of 22.78 MHz in the communication band.⁴⁶

5.2.5. Nonvolatile memory devices. Nonvolatile memory refers to computer memory that can retain stored information even when power is removed and this is in contrast to volatile memory, which requires constant power to maintain data. Some key characteristics of nonvolatile memory devices include the ability to retain data without power, re-writability, and faster access times than traditional storage.¹⁹⁷ New nonvolatile memory technologies aim to provide even faster speeds, higher densities, and improved endurance compared to flash memory.^197,198 Incorporating hydrothermally prepared Ti₃C₂T_x-MQD in PVP films enables precise tunning of the electrical conductance of an indium tin oxide (ITO)/MQD-PVP/structure from insulator behavior to irreversible resistive switching, reversible resistive switching, and conductor behavior.¹³³ Schematic images and I–V diagrams for different concentrations of Ti₃C₂T_x-MQD (C_a = 3.42 mg mL⁻¹, C_b = 1.14 mg mL⁻¹, C_c = 380 µg mL⁻¹, C_d = 20 µg mL⁻¹) are shown in Fig. 14a–h. As seen, the currents for voltage sweep in positive and negative directions are completely different. The hybrid films with irreversible and reversible resistive switches demonstrate write-once-read-many times and flash memory effects, respectively. These devices also exhibit stable operation during a long retention test (1.2 × 10⁴ s), with a high on/off current ratio of up to 100. The tunable memory and transient features of these hybrid films are likely due to MQD charge trapping, which is a result of their quantum confinement and the dissolvability of memristive components. Overall, MQDs hold promise as resistive switching triggers for emerging nonvolatile memories for data storage, particularly in data storage security applications.¹²⁹

Fig. 14 Memory unit and I–V characteristics of MQDs-PVP memory devices containing (a and b) C_d, (c and d) C_c, (e and f) C_b, and (g and h) C_a (reprinted with permission from Wiley, Copyright © 2019).¹³³

5.3. Energy

Incorporation of MQDs can play a key role in enhancing energy storage efficiency in batteries and supercapacitors by improving electrochemical reaction kinetics and thermodynamics. This results in higher capacitance and energy density, facilitated by improved ion transport, species adsorption, and surface reactions to develop more efficient, high-capacity, and long-lasting energy storage devices. In the following section, we will explore various types of energy storage devices, highlighting the mechanisms behind the performance improvements when MQDs are incorporated.

5.3.1. Energy storage devices. There are several reasons why MQDs are considered promising candidates for enhancing the performance of energy storage devices, such as batteries and supercapacitors; (i) the presence of abundant surface terminations enhances electrochemical performance, and their small lateral size allows molecule and ions accessibility,^199,200 (ii) they benefit from superior electron transport efficiency, (iii) their layered structure could avoid mechanical degradation during charge–discharge cycles, accommodating the crystallographic strain from ion intercalation due to volume expansion, (iv) dopants in QDs can enhance redox reaction kinetics by their localized mid-gap electronic states as highly active catalytic sites, ultimately leading to longer lasting and more efficient batteries.^201–203 Despite these advantages, MQDs face challenges for large-scale commercialization.²⁰⁴ Recent advancements aim to address the long-lasting issue of aggregation of QDs over time.²⁰⁵ Strategies for enhancing the surface-active sites through surface functionalization,²⁰⁶ and introducing them into heterostructure settings²⁰⁷ are crucial ongoing step forward to overcome limitation of energy storage applications of MQDs.^199,200

To fully understand the role of MQDs in energy storage and conversion devices, several key parameters must be considered. Specific capacitance (F g⁻¹) measures the ability of the material to store charge per unit mass, which is a crucial parameter for energy storage systems. Cycling stability refers to the number of charge–discharge cycles an electrode material can undergo while maintaining performance. Overpotential (V) is the extra voltage required for electrocatalytic reactions, such as redox reactions, which should be minimized to improve energy efficiency.²⁰⁸ Power density (W kg⁻¹) and energy density (Wh kg⁻¹) define how much power and energy a material can deliver, particularly for supercapacitors.^208–212 Charge carrier lifetime represents the duration of photogenerated charge carriers remaining separated before recombination, a critical factor in photocatalysis. Lastly, coulombic efficiency (CE) and faradaic efficiency (FE), and apparent quantum yield (AQY) are quantifying indicators of the cycle-to-cycle efficiency and ratio of the actual amount of product formed to the theoretical amount, and performance metric of photocatalysis that measures the fraction of absorbed photons, respectively.^213,214 In the following sections we will delve into each application and explain recent achievements in each area to have a deeper understanding of energy storage applications of MQDs.

• Lithium–oxygen batteries

Li–O₂ batteries, leveraging oxygen redox reactions, offer a high theoretical energy density of 3000 Wh kg⁻¹.²¹⁵ Electrocatalysts play a crucial role in these systems by accelerating reaction kinetics, minimizing energy losses, and enhancing battery stability, making them key to improving performance and cyclability.^12,216 Among various electrocatalysts for electrochemical applications, the unique properties of MXenes such as good electrical conductivity,¹³³ edge active catalytic sites,¹³⁵ tunable structure,¹³⁶ and dispersibility¹⁴⁶ make them highly suitable for oxygen reduction/evolution reactions.²¹⁷ However, literature reports on 2D materials, particularly MXenes, still lacks a systematic framework for the rational design of highly active electrocatalysts for such an application.

To address this challenge, various strategies, ranging from defect engineering to doping, have been explored.^218–220 Defect-rich structure of MQDs, owing to higher edge-to-basal plane surface area ratio, can effectively improve electrochemical performance of carbon nanosheets (CNSs), while maintaining the same chemistry as their MXene counterparts.²²¹ Ti₃C₂ MQD/CNSs catalysts demonstrated a remarkably low voltage gap of 0.62 V, which is substantially lower than 1.29 V for Ti₃C₂ nanosheets/CNSs (Fig. 15a). Besides, Ti₃C₂ QDs/CNSs cathode delivered a high discharge capacity of ∼16 000 mAh g⁻¹ with a CE of 93.8%, outperforming Ti₃C₂ nanosheets/CNSs (∼11 000 mAh g⁻¹, 70.9%). In terms of stability, the former was stable during 240 cycles at 200 mA g⁻¹ compared to 65 cycles for the latter. DFT calculations unveiled the relation between electron rearrangement over the grain boundaries and edge sites, helping charge transfer between intermediates and active sites during redox reactions. While for Ti₃C₂ sheets, charge accumulation was restricted to oxygen-containing clusters, confirming the inactivity of basal planes, MQDs showed stronger chemical affinity to LiO₂ intermediates. This finding was attributed to the fact that Li⁺ and oxygen atoms could attach to edge rich C-side and functional groups of the edge-rich MQDs as catalytically active sites. Edge defects altered local charge distribution, modulated intermediate adsorption behavior of oxygen species needed for storage of products, and reduced redox energy barriers during Li₂O₂ formation/decomposition.²²²

Fig. 15 MQDs as a key component of different type of batteries. (a) Li–O₂ battery: the discharge–charge curves of electrodes at a fixed capacity of 1000 mAh g⁻¹ and current density of 200 mA g⁻¹. QDs as cathode material showed lowest charge overpotential compared to nanosheets (0.62 V) (reprinted with permission from Wiley, copyright © 2021).²²² (b) Zn–air battery: the energy density of rechargeable ZABs with LDH/Ti₃C₂-MQDs/NG hybrids, LDH/NG hybrids, LDH/NG + MQDs, bare NG, commercial Pt/C, and IrO₂ + Pt/C air cathodes at a discharge current density of 5 mA cm⁻² demonstrating a higher energy density for QDs containing sample (∼700 Wh kg⁻¹) (reprinted with permission from Wiley, copyright © 2021).²²³ (c) Li–S battery: long-term cycling of the battery with various separators (red: GMC, blue: GC, green: Celgard) at 0.2C (reprinted with permission from Royal Society of Chemistry, copyright © 2023).²²⁴

• Zinc–air batteries

Zinc–air battery (ZAB) is a promising energy storage system which benefits from vast utilization of aqueous electrolytes and high theoretical energy density of 1086 Wh kg⁻¹ (five times higher than that of Li-ion).²²⁵ Although traditional noble metal-based electrocatalysts, such as Pt- and Ir/Ru-based materials perform well in electrocatalytic reactions, they still suffer from high costs and poor long-term stability. A major constraint to novel materials in zinc–air batteries is their sensitivity to the CO₂ concentration in air. Among different methods to overcome this issue, such as electrolyte optimization and electrode engineering,²²⁶ developing highly active electrocatalysts for oxygen redox reactions has gained significant attention.^227,228 Considering that layered double hydroxide (LDH) nanosheets have limited conductivity and sluggish catalytic kinetics,²²⁹ the introduction of MQDs to LDH/N-doped graphene (NG) is effective in boosting kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) during cycling.²²⁹ Confirmed by Mott–Schottky analysis and DFT calculations, LDH/Ti₃C₂ MQD/NG hybrid exhibits higher carrier density and a deeper Fermi level while pristine LDH suffers from poor charge transport. The LDH/Ti₃C₂-MQD hybrid also shows a lower Tafel slope (57 mV dec⁻¹) than Pt/C (120 mV dec⁻¹), indicating faster reaction kinetics. Furthermore, the MQD-enhanced hybrid offers a high energy density of 700 Wh kg⁻¹, matching commercial Pt/C and far exceeding IrO₂ (580 Wh kg⁻¹) (Fig. 15b), and low potential difference of 0.81 V, significantly outperformed LDH/NG (0.92 V), commercial Pt/C (1.01 V), and IrO₂ + Pt/C (0.91 V). The material's structural integrity during prolonged operation (150 h at 5 mA cm⁻²) and its mechanical flexibility, even in a bent state, highlights its potential for applications beyond batteries, such as wearable electronics.²²³

• Lithium–sulfur batteries

Li–sulfur (Li–S) batteries stand out among many candidates as the next generation of energy storage devices due to their high theoretical capacity (1675 mAh g⁻¹) and the abundance of sulfur.²³⁰ However, industrialization of this type of battery is hindered by the shuttle effect of lithium polysulfides (LiPSs) and irreversible energy loss, leading to poor performance and cyclability.²³¹ Strategies to mitigate this include engineering the sulfur host, such as using free-standing N-doped carbon/Ti₃C₂T_x MQDs (NC-S/Ti₃C₂T_x) films; this design uses polar N-doped carbon to anchor LiPSs while the Ti₃C₂T_x network provides conductivity and a physical barrier, enabling high areal capacities up to 3.41 mAh cm⁻².^215,232

A game-changing solution emerges with the introduction of an ion selective layer in electrolyte to capture LiPS. Through local confinement enabled by the small size of MQDs and their abundant active sites for polysulfide adsorption, the g-C₃N₄@Ti₃C₂T_x MQDs-modified Celgard (GMC) separator forms a highly functional modification layer.²²⁴ This approach leverages the strong interactions of polar materials to gradually trap and accumulate polysulfides, effectively creating a shuttle barrier between the cathode and anode. As a result, a significant improvement in the efficiency of polysulfide conversion is attained. A long-term stability with only 0.024% capacity decay per cycle after 1000 cycles at 2C and the retention rate of nearly 70% at 0.2C after 100 cycles are also obtained. The retention rate is higher than that of the Li–S batteries assembled with unmodified g-C₃N₄ Celgard separator (58%), ensuring significantly extended lifespan compared to traditional designs (Fig. 15c). The modified separator also enhances the stability of Li₂S deposition (final discharge product), as evidenced by longer and more stable plateau regions in discharge curves.²²⁴ By efficiently capturing, confining, and catalyzing polysulfide conversion, MQDs eliminate the shuttle effect bottleneck in Li–S batteries.

• Supercapacitors

A flexible power supply is an essential component of portable and wearable electronic devices. Supercapacitors with high power density, long cycle life, and fast charging capabilities have garnered considerable attention in this field. They are lightweight, compact, and operate efficiently across a wide temperature range, making them suitable for portable, energy-efficient devices. The key to providing such engineering products to the market is how to increase their volume energy densities without sacrificing their volume power densities, life cycles, and other performance parameters.²³³ To achieve high volumetric energy density, recent research has focused on new strategies, such as fabrication of hybrid capacitors,²³⁴ utilizing high-voltage ionic liquid electrolytes,²³⁵ preparing high surface area electrodes,^221,236 and all-solid-state systems.²³³

Among other flexible electrodes, integrating 2–4 nm Ti₃C₂T_x MQDs into RGO/Ti₃C₂T_x fiber electrodes (Q3M7) overcame a long-standing challenge in solid-state electrolyte supercapacitors by providing a trade-off between high capacitance and mechanical flexibility.²³⁷ Using MQDs as pillar agents, a highly conductive, structurally stable fiber was obtained. An exceptional volumetric capacitance of 1560 F cm⁻³ at 1 A cm⁻³ in 1 M H₂SO₄ (solution-based electrolyte) was also achieved, which is far surpassing other MXene-based fibers such as Ti₃C₂T_x/PEDOT:PSS (poly(3,4-ethylenedioxythiophene)polystyrene sulfonate) (614 F cm⁻³) and Ti₃C₂T_x/RGO (542 F cm⁻³). The Nyquist plots revealed a lower charge transfer resistance, confirming MQD pillars create expanded interlayer channels, which minimize ion transport tortuosity and facilitate rapid pseudocapacitive intercalation and ion diffusion and electrolyte accessibility by adding MQDs.^215,238 Even a 20× higher current densities (20 A cm⁻³) gave rise to a capacity retention of 79%, compared to the 61% in Ti₃C₂T_x/PEDOT:PSS fibers.²³⁹ When assembled into an all-solid-state supercapacitor, the MQD-enhanced electrode demonstrated a volumetric capacitance of 413 F cm⁻³ at 0.5 A cm⁻³, an energy density of 36.7 mWh cm⁻³, and an impressive 97% capacity retention after 10 000 cycles, which hardly changed over time. Additionally, the Q3M7 fibers boast a remarkable mechanical strength of 130 MPa, ensuring their suitability for wearable and flexible electronics. Moreover, no change observed to the capacitive behavior of the fibers after bending, confirming the flexibility of this system under bending test (Fig. 16a).²³⁷ The volumetric energy density exceeded that of commercial supercapacitors (e.g., 2.75 V/44 mF, and 5.5 V/100 mF), but also outperformed previously reported MXene- and RGO-based designs (e.g., MnO₂/Ti₃C₂T_x/RGO, Ti₃C₂T_x/PET fiber, Ti₃C₂T_x/CNT fiber, V₂O₅/single-walled carbon nanotube) (Fig. 16b).²³⁷

Fig. 16 (a) Mechanical bending test showing the flexibility of solid-state supercapacitor system based on Ti₃C₂T_x MQDs/L-Ti₃C₂T_x fibers made with MQDs confirming flexibility after multiple bending steps (reprinted with permission from Wiley, Copyright © 2023).²³⁷ (b) Ragone plot indicating the potential of the MQD containing fibers at top right corner of energy density vs. power density plot (highest compared to other works) (reprinted with permission from Wiley, copyright © 2022).²³⁶ (c) Capacitance retention and CE of the 0D/2D MQD/MXene symmetric solid-state pseudocapacitor after 10 000 cycles showing a high retention of ∼84%. The first cycle and the last cycle are shown as an inset confirming minimal changes (reprinted with permission from Wiley, copyright © 2023).²³⁷ (d) Ti₃C₂T_x MQD/LRG laser fabrication process, image of exfoliated materials, SEM (scale bar, 20 nm) confirming size distribution below 5 nm (reprinted with permission from Wiley, copyright © 2022).²³⁶

While flexibility is important, in the pursuit of transparent high-performance energy storage devices, researchers have faced a persistent challenge in balancing high transparency with efficient energy storage. The key to this issue lies in utilization of temporally and spatially shaped femtosecond laser to uniformly attach MQDs to laser-reduced graphene oxide (LRGO).²³⁶ The presence of nanosized Ti₃C₂T_x-MQDs and higher density of edges compared to basal sites provides a high-power density and areal capacitance of 2.04 × 10⁻³ mWh cm⁻² and 10.42 mF cm⁻², respectively. Well-distributed MQDs prevented restacking of graphene nanosheets (Fig. 16d), while providing more active sites for charge storage and transfer, thereby offering a flexible, ultrahigh transparency supercapacitor (areal capacitances = 10.42 mF cm⁻² and transmittance = 91%) with high durability, i.e., maintaining 97.6% of its capacitance after 12 000 cycles.²³⁶ In another study,¹³⁷ Ti₃C₂T_x-MQDs were introduced into the Ti₃C₂T_x nanosheet matrix to create a heterodimensional hybrid structure (0D/2D) through a microwave hydrothermal method that altered charge storage behavior. Electrochemical impedance spectroscopy (EIS) indicated that the ohmic resistance (1.439 Ω) and charge transfer resistance (0.04078 Ω) of MQD/MXene nanosheet electrodes are smaller than those of Ti₃C₂T_x nanosheets electrodes (R_s = 1.637 Ω, R_ct = 0.10032 Ω). An areal capacitance of 2202 mF cm⁻² at 3 mA cm⁻² was attained, which significantly outperformed conventional MXene-based electrodes. The material retained 84% of its capacity after 10 000 cycles and delivered an aerial energy density of 90.33 μWh cm⁻² at a power density of 450 mW cm⁻². No efficiency loss due to mechanical deformations was also noticed, highlighting their high flexibility and robust mechanical stability (Fig. 16c).^137,236

Beyond flexibility and mechanical properties, supercapacitors have long struggled with the trade-off between capacitance and stability.²⁴⁰ Despite the high theoretical capacitance of various classes of active materials, most of the structures suffer from low utilization of active sites due to limited electron mobility.²⁴¹ Recently, J. Wang et al.²⁰⁵ have demonstrated that by incorporating Ti₃C₂T_x-MQDs through a microwave-assisted DMF exfoliation process, the Ni(OH)₂ structure undergoes a remarkable transformation. As a result, a higher electroactive surface area, higher ion diffusion rate, and significantly improved charge transport efficiency are attained. The MQD-NH electrode on nickel foam offers a record-high specific capacitance of 1660 F g⁻¹ at 1 A g⁻¹ in 2 M KOH, far surpassing conventional Ni(OH)₂ composites such as Ni(OH)₂/rGO (802 F g⁻¹) and Ni(OH)₂ (1087.1 F g⁻¹).²⁰⁵

5.3.2. Energy conversion devices. Owing to exceptional electronics and surface properties of MQDs, their utility has been demonstrated across a diverse range of applications, such as energy conversion systems. In this section, we will go over research highlights on improving efficiency of MQDs as electrocatalysts.

• CO₂ reduction reaction

Electrochemical conversion can serve future energy demands by storing renewable energy and reducing anthropogenic emissions such as CO₂. For this aim, photocatalysis has long been regarded as a promising technology for tackling environmental pollution and converting CO₂ into valuable fuels.²⁴² Among many photocatalytic materials, g-C₃N₄ stands out for its stability, non-toxicity, and visible-light activity. Yet, despite its potential, a persistent challenge is its tendency to lose charge carriers too quickly before they can drive meaningful chemical reactions. Among different methods to improve the efficiency of CO₂ reduction electrodes, strategies such as reducing the activation energy of CO₂ redox reactions,²⁴³ introducing novel electrocatalysts to enhance selectivity for CO₂ reduction,²⁴⁴ and minimizing parasitic reactions like HER²⁴⁵ have been reported in literature.²⁴² One recent approach is the formation of heterojunctions with bismuth oxyiodide (BiOI), a p-type semiconductor known for its strong light absorption and complementary band structure.²⁴⁶ While this combination does improve charge separation to some extent, it still lacks the efficiency needed for high-performance photocatalysis. The real challenge is not just preventing recombination but actively driving charge migration to ensure efficient and prolonged reaction activity. To improve charge separation, a heterojunction of g-C₃N₄ with BiOI has recently been investigated.²⁴⁷ Although this combination enhances charge transfer to some extent, it remains insufficient for achieving optimal performance. The missing piece in this system is a mechanism to accelerate charge migration and suppress recombination more efficiently.²⁴⁷

Another strategy to accelerate CO₂ conversion is decreasing the activation energy of reaction. As an example, Ti₃C₂T_x-MQD heterostructure with g-C₃N₄/BiOI enhances the CO₂ photoreduction properties of the device.²⁴⁷ While the absorption band edge of g-C₃N₄ shows a strong UV absorption and BiOI displays a broad UV to visible light absorption, modifying with the MQDs will tune the bandgap, and extend the photo-response to NIR range (Fig. 17a). The CN/MQDs/BOI offers an observable photocurrent response of 2.3 μA and a CO production rate of 57.8 μmol g⁻¹ h⁻¹, which is more than 30 times higher than C₃N₄/TiO₂. The g-C₃N₄/MQDs/BOI exhibits a Nyquist semicircle, confirming enhanced charge transfer efficiency. However, it is noteworthy that NO_x reduction is susceptible to occur during the reduction reaction process, which affects CO₂ reduction yield and the corresponding production of CO and CH₄ (Fig. 17b).²⁴⁷

Fig. 17 (a) UV–vis diffuse reflectance absorption spectra of CN/MQDs/BOI confirming the effectiveness of hybrid structure to tune adsorption edge at 440 nm for CN/MQDs and ∼520 nm for CN/MQDs/BOI (reprinted with permission from Elsevier, copyright © 2025).²⁴⁷ (b) Schematic of the reactions based on potential vs. NHE (reprinted with permission from Elsevier, copyright © 2025).²⁴⁷

While metal chalcogenides, such as Bi₂S₃, have shown potential for CO₂ reduction due to their narrow bandgap and strong visible-light absorption, their efficiency is compromised by rapid recombination of carriers and insufficient adsorption sites for CO₂.²⁴⁸ To address these issues, 3D porous Ti₃C₂-MQDs-OH/Bi₂S₃ structure that leverages the synergy between protonated Bi₂S₃, and alkalized MQDs (MQDs-OH) were investigated for CO₂ reduction.²⁴⁹ The strong electron-withdrawing properties and abundant hydroxyl functional groups of MQDs-OH led to efficient charge carriers’ separation and retarded recombination, as evidenced by EIS. PL and photocurrent response measurements determined a high photocurrent density of 22 µA with 15-fold enhancement in the CO₂ adsorption capacity, i.e., from 1.6 mg CO₂ per g for pristine Bi₂S₃ to 23.0 mg CO₂ per g for MQDs-OH/Bi₂S₃. This massive improvement resulted in a record-high methanol yield of 694.7 µmol g⁻¹, more than double the output of unmodified Bi₂S₃-based catalysts (321.8 µmol g⁻¹). The electron spin resonance (ESR) measurements showed a significant increase in ROS (O₂˙⁻) generation, affirming that the presence of MQDs enhanced the activation of CO₂ molecules for more efficient reduction reactions.²⁴⁹

• Hydrogen evolution reaction

The generation of hydrogen from H₂O through semiconductor photocatalysis has garnered significant attention in recent years.²⁵⁰ Among various photocatalysts, conventional TiO₂/g-C₃N₄ composites without MQDs have found of great interest for photocatalytic hydrogen evolution.²⁵¹ It is noteworthy that the catalytic activity of TiO₂/g-C₃N₄ heterostructures is constrained by rapid charge carrier recombination and limited active sites. While the S-scheme heterostructure of TiO₂/g-C₃N₄ provides a foundation for improved charge separation, it is still not enough to sustain high reaction rates.¹⁴⁵ The modification of S-scheme TiO₂/g-C₃N₄ heterojunction with Ti₃C₂ MQDs is considered as an effective strategy for extending optical absorption into the visible range. Diffuse reflectance spectroscopy (DRS) measurements determine the bandgap energy of TiO₂ (T), g-C₃N₄ (CN), TiO₂/g-C₃N₄ (T-CN), and TiO₂/g-C₃N₄/Ti₃C₂ QDs (T-CN-TCQD) as 3.22 eV, 2.86 eV, 2.57 eV, and 2.45 eV, respectively (Fig. 18a). By serving as electron acceptors and facilitating charge separation, the MQDs address the critical challenge of rapid recombination of photoinduced carriers in g-C₃N₄, resulting in a hydrogen production rate of 5111.8 μmol g⁻¹ h⁻¹, which is nearly 26 times higher than pristine g-C₃N₄ nanosheets (196.8 μmol g⁻¹ h⁻¹), 3 times higher than Pt/g-C₃N₄ (1896.4 μmol g⁻¹ h⁻¹), and 10 times higher than Ti₃C₂ MXene sheet/g-C₃N₄ (524.3 μmol g⁻¹ h⁻¹). This drastic improvement is attributed to the superior electronic conductivity of Ti₃C₂ MQDs, which not only facilitated efficient carrier transfer but also increased the specific surface area of g-C₃N₄ from 27.575 m² g⁻¹ to 40.149 m² g⁻¹. Time-resolved photoluminescence measurements indicate a longer lifetime, i.e., 10.1242 μs for g-C₃N₄@Ti₃C₂ QDs compared to 9.6841 μs for g-C₃N₄. The enhanced photocurrent response of g-C₃N₄@Ti₃C₂ QDs, along with a reduced EIS arc radius compared to pristine g-C₃N₄, further demonstrate the superior charge transport and separation efficiency introduced by MQDs.¹⁴⁵

Fig. 18 (a) Flat-band potential (E_fb) estimated by Mott–Schottky curve obtained from DRS test showing the extended bandgap energy of g-C₃N₄@Ti₃C₂ MQDs have been expanded to visible region compared to other candidates without MQDs (reprinted with permission from American Chemical Society, copyright © 2019).¹⁴² (b) The Tafel slope and overpotential of Co(OH)₂-based catalysts at 10 mA cm⁻² for HER in an alkaline 1 M KOH solution (reprinted with permission from Elsevier, copyright © 2025).¹⁴¹ (c) H₂ evolution reaction as a function of time for (■) Ti₃C₂ QDs; (▼) Nb₂C QDs; (●) Ti₂C QDs; and (◊) V₂C MQDs (reprinted with permission from Wiley, copyright © 2023).²⁵² (d) Energy band positions of Ti₃C₂-MQD/Ni-MOF (reprinted with permission from American Chemical Society, copyright © 2020).⁴ (e) Energy level diagram of Ti₃C₂ MQDs/Cu₂O NWs/Cu and Ti₃C₂ sheets/Cu₂O NWs/Cu heterostructure (reprinted with permission from Wiley, copyright © 2018).¹⁴⁵

Hydrogen evolution reaction (HER) efficiency also depends on the H⁺ reduction barriers. M. Y. Solangi et al.²⁵³ demonstrated that an assembly of Co(OH)₂ and Ti₂C MQDs, accelerated the adsorption of H₂O and decreased the reduction barrier for H⁺. The presence of Co(OH)₂ provided active sites for hydroxyl (OH) adsorption, while Co sites with electron-deficiency protected the Co(OH)₂ from deteriorating under alkaline environment. The MQD introduction reduced the overall energy from −251.29 to −672.51 eV, while enhancing the structural stability of Co(OH)₂. The Gibbs free energy of water dissociation was also substantially decreased from 1.52 to 0.14 eV by the incorporation of MQDs. Additionally, a low overpotential of 91 mV at the current density of 10 mA cm⁻², which surpasses the performance of the CF (344 mV), Pt/C (111 mV), and the Co₂(OH)₃Cl/CF-R (268 mV) catalysts, was noticed (Fig. 18b).²⁵³

By utilizing HF-free laser ablation in an aqueous medium, not only can the need for HF be eliminated but also control over quantum confinement effects, surface functionalization, and bandgap modulation can be adopted. For instance, significantly enhanced photocatalytic performance of Ti₃C₂-MQDs for HER has been reported by this technique.²⁵² Here, a hydrogen production rate of 2.02 mmol g⁻¹ h⁻¹, which is comparable to noble-metal-based catalysts like Pt/g-C₃N₄ (2.5 mmol g⁻¹ h⁻¹), has been reported (Fig. 18c). It has been shown that not only fine-tuning of the valence band (VB) and conduction band (CB) positions can achieved by laser ablation,²⁵⁴ but also the oxygenated surface terminations (OH, and O) can improve stability and catalytic activity of MQDs.²⁵²

• NO_x reduction

NO_x reduction is critical for environmental protection because it aims to convert hazardous materials to value added products, while reducing nitrogen oxide emissions from industrial processes. However, the large bond energy (940.95 kJ mol⁻¹) of the nitrogen–nitrogen bond makes the nitrogen reduction reaction (NRR) difficult.²⁵⁵ Thus far different approaches, such as electrolyte engineering,²⁵⁶ crystallographic phase transformation engineering,²⁵⁷ transition metal dual-atom catalysts,²⁵⁸ transition metal oxides,²⁵⁹ covalent/metal organic frameworks, perovskite oxides, doping with transition metals to hybridizing with carbon-based materials like graphene,²⁶⁰ and incorporation of MQDs as a co-catalyst to tune NRR activity have been investigated.²⁶¹ Among various methods to develop efficient and selective electrocatalysts, utilization of MQDs with different surface terminations have found of interest in nitrogen reduction to produce ammonia at room temperature.^262,263 MQDs provide high density of edge sites, which are more catalytically active than basal planes, thus enhancing the adsorption and activation of nitrogenous molecules. Additionally, the surface chemistry of MQDs can be tailored with functional groups such as hydroxyl, or fluorine, optimizing their electronic structure and adsorption characteristics to lower energy barriers for nitrogen reduction.¹⁴³ For instance, J. Qin et al.⁴ demonstrated that incorporating surface-functionalized Ti₃C₂ MQDs to Ni-MOF led to photocatalytic NH₃ production rate of 88.79 μmol g⁻¹ h⁻¹, which was more than four times higher than pristine Ni-MOF (21.39 μmol g⁻¹ h⁻¹).^139,264 EIS indicated a low charge transfer resistance for the coupled structure, while transient PL decay determined an extended carrier lifetime of 5.636 ns, compared to 2.685 ns for the pristine Ni-MOF. DFT calculations also affirmed that the MQD incorporation decreased the free-energy barrier for N₂ activation, making the reaction more energetically favorable. Specifically, the initial N₂ adsorption step becomes exothermic, releasing 0.74 eV, while the N≡N bond elongated from 1.098 Å to 1.118 Å, facilitating subsequent reduction steps. Details of the heterostructure and reaction mechanism, shown in Fig. 18d, proposed that the synergistic MQD–MOF system altered the reaction pathway, making nitrogen fixation significantly more efficient.⁴

Another product of nitrogen containing compounds reduction is ammonia which is mostly obtained from NORR using copper-based electrocatalysts.¹⁴³ Ti₃C₂ MQDs have been proven to effectively boost the co-catalytic performance of NORR ammonia production performance of copper nanowires (Cu NWs).¹⁴³ An NH₃ yield of 5346.3 μg h⁻¹ mg⁻¹ and FE of 95.5%, which are much higher than those of unmodified Cu NWs (NH₃ yield of 3313.1 μg h⁻¹ mg⁻¹ and FE of 89.5%) have been reported (Fig. 19a). The onset potential of reaction (−0.4 V vs. reversible hydrogen electrode (RHE)) is also significantly less than that of Cu foam (−0.9 V vs. RHE). A record-high NH₃ yield, outperforming many well-performing catalysts previously reported in the literature like MoS₂,¹³⁹ MoC,²⁶⁵ Bi/C,²⁶⁶ and Ni₂P nanosheet array on carbon paper (Ni₂P/CP),^267,268 have been demonstrated. An assembled Zn–NO battery with Ti₃C₂ MQDs/Cu NWs offers a power density of 3.03 mW cm⁻² and a high NH₃ yield of 925.2 μg h⁻¹ mg⁻¹.¹⁴³

Fig. 19 (a) Linear Sweep Voltammetry (LSV) of Cu NWs and Ti₃C₂ MQDs/Cu NWs in Ar-saturated and NO-saturated 0.1 M K₂SO₄ exhibiting a high activity of MQDs modified Cu nanowires toward NO reduction (reprinted with the permission of Royal Society of Chemistry, copyright © 2023).¹⁴³ (b) The proposed reaction mechanism for photocatalytic phthalide synthesis and H₂ evolution by MQDs incorporation and electronic structure modification as a successful strategy to boost reactivity (reprinted with the permission of American Chemical Society, copyright © 2024).²⁶⁹ (c) Photocurrent vs. irradiation time for CdS, CdS/Ti₃C₂T_x, and Ni/CdS/Ti₃C₂T_x photocatalysts (reprinted with the permission of American Chemical Society, copyright © 2024).²⁶⁹ (d) H₂O₂ yield for Nb₂C MQDs/COF as compared to other photocatalysts, demonstrating the promising potential of MQDs to modulate the H₂O₂ production (reprinted with the permission of Elsevier, copyright © 2025).¹⁴⁷

• Photocatalyst

Copper oxide nanowires with p-type semiconductor properties and favorable band structure have been considered promising photocatalyst. Traditional approaches, such as integrating carbon nanomaterials,²⁷⁰ noble metals,²⁷¹ and other semiconductors,^272,273 have yielded only limited improvements on charge recombination without making the fabrication processes complex with reduced costs.¹⁴⁵ Recent studies have demonstrated that MQDs can serve as electron shuttles to accelerate charge transport, while suppress recombination losses.¹⁴⁵ For instance, by integrating Ti₃C₂ MQDs onto Cu₂O nanowires (NWs), photocatalytic CO₂ conversion results in a high rate of methanol production (153.38 ppm cm⁻²), which is 8.25 times higher than pristine Cu₂O NWs (18.58 ppm cm⁻²) and 2.15 times higher than Ti₃C₂ nanosheets/Cu₂O NWs (71.39 ppm cm⁻²).¹⁴⁵ Mott–Schottky analysis reveals that carrier density (N_D) increases dramatically from 1.09 × 10¹⁹ cm⁻³ in Cu₂O NWs to 2.75 × 10¹⁹ cm⁻³ in MQD-modified Cu₂O NWs, with prolonging photocarrier lifetimes. Additionally, DRS demonstrates significantly enhanced light absorption efficiency and shifts the bandgap from 2.2 eV (Cu₂O) to 2.02 eV (Ti₃C₂ MQDs/Cu₂O NWs) (Fig. 18e). This bandgap engineering facilitates a broader spectrum of light absorption and reduces the charge transfer resistance, as evidenced by EIS. Photostability studies of Ti₃C₂ MQDs/Cu₂O NWs/Cu over a wide range of potential also affirm the potential of the MQDs to prevent Cu₂O NWs from oxidation.¹⁴⁵

• Radical scavengers

Radical scavenging plays a pivotal role in mitigating oxidative stress and preserving functional electrode stability and preventing electrode degradation,²⁷⁴ stabilizing intermediates in reactions like ORR/CO₂RR.^275,276 Benefiting from a large surface-to-volume ratio, Cl, N-doped Ti₃C₂ MQDs (D = ∼3.4 nm) synthesized via electrochemical etching demonstrated outstanding hydroxyl (˙OH) radical scavenging activity (93.3%) at a concentration of 12.5 μg mL⁻¹, surpassing the performance of previously reported graphene-based nanoparticles.²⁷⁷ The scavenging mechanism is attributed to the electron transfer ability of the Ti₃C₂ structure and the electron donation of the double dopants.⁷⁵ In another study, nitrogen-doped Ti₃C₂ MQDs synthesized by a hydrothermal method exhibited exceptional scavenging capability against multiple ROS, including ˙OH, ¹O₂ (singlet oxygen), and O₂˙⁻ (superoxide anion) multi-ROS scavenging capabilities, achieving a hydroxyl radical scavenging efficiency of 93.2% at a concentration of 12.5 μg mL⁻¹, significantly outperforming graphene oxide (37.3%), and graphene QDs (53.3%) under comparable conditions. Detailed DFT simulations and voltammetry measurements revealed that nitrogen doping not only enhanced the electron density and created additional active sites but also dramatically strengthened the adsorption and charge-transfer processes with OH radical species, thereby underpinning the superior antioxidative activity of the N-Ti₃C₂ MQDs.¹¹⁸

• Oxidation of alcohols

Semiconductor photocatalysts often face the challenge of photoinduced decomposition, which hampers their efficiency and stability in selective oxidation of alcohols.²⁷⁸ To address this issue, recent studies have developed a robust photocatalyst by integrating nickel-decorated cadmium sulfide quantum dots (Ni/CdS QDs) with Ti₃C₂T_x MXene.²⁶⁹ This composite leverages the high conductivity and unique surface chemistry of Ti₃C₂T_x MXene to enhance charge separation and transfer, while the Ni decoration provides active sites that facilitate the oxidation process. This strategic combination not only mitigates the phot-corrosion of CdS but also significantly improves the efficiency and selectivity of alcohol oxidation reactions under visible light irradiation.²⁶⁹ Lactonization of 1,2-benzenedimethanol (1,2-BM) to phthalide is also integrated with concomitant H₂ production, as Ni clusters in CdS accelerates H₂ evolution (Fig. 19b). The anchored Ni as a single atom on Ti₃C₂T_x can also influence efficient adsorption and cyclization of diols. This composite material exhibits remarkably enhanced activity for lactone synthesis, which is 80.4 times higher than that of blank CdS, along with excellent selectivity of 91.6% for composite structure (Ni/CdS QDs) although blank CdS QDs had a much lower selectivity of only 48.4% under the same reaction conditions. While the blank CdS shows a significant decrease (43.6%) in the phthalide production after four repeated trials, no obvious deactivation phenomenon of phthalide and H₂ production could be observed over Ni/CdS/Ti₃C₂T MQDs. The transient photocurrent response of the samples increases in the following order: Ni/CdS/Ti₃C₂T_x > CdS/Ti₃C₂T_x > CdS, suggesting the higher charge separation efficiency in Ni/CdS/Ti₃C₂T_x composite (Fig. 19c).²⁶⁹

• H₂O₂ production reaction

Pyrene-based covalent organic frameworks (COFs) hold significant promise in photocatalytic applications; however, their practical use in hydrogen peroxide (H₂O₂) production is hindered by low charge-generation and charge-transfer efficiencies, as well as rapid charge recombination rate.^208,279 To address these challenges, recent research has focused on integrating MQDs as active sites within pyrene-based COFs through a self-assembly method.¹⁴⁷ This strategic combination enhances photocatalytic performance by improving charge separation and transfer, thereby facilitating efficient H₂O₂ photosynthesis from abundant seawater resources.¹⁴⁷ For instance, a H₂O₂ production rate of 3560 μmol g⁻¹ h⁻¹ was reported by integration of Nb₂C MQDs into high-crystalline COF structure.¹⁴⁷ An AQY of 12.8% at 400 nm together with SCC (Solar-to-Chemical Conversion) efficiency of 0.27% was also reported (Fig. 19d). From the mechanistic point of view, femtosecond transient absorption spectroscopy (fs-TAS) determined electronic coupling between Nb₂C MQDs and COF. Further supported by DFT simulations of the planar-averaged charge density difference along with Z direction it was proposed that the electrons mainly transferred from COF to Nb₂C MQDs through intermediates governed by a strong electron–phonon coupling at the interface, which promotes directional charge migration over thermal relaxation. This process is further amplified by the significant spin–orbit coupling of the heavy Nb atoms, which modifies the electronic band structure to favor rapid carrier extraction at the MQD active sites and corresponding free energy changes.^147,280

5.4. Biomedical applications

5.4.1. Immunomodulation. • In vitro assays

Transplantation immunology is critically important as it addresses the complex interplay between the immune system and transplanted tissues, biomaterials, and cells determining the success and safety of organ transplantation.²⁸¹ Graft rejection occurs when the recipient's immune system recognizes the transplanted organ as foreign and begins the expansion of effector immune cells and inflammation.²⁸¹ Inflammation, vital for the body's response to injury and infection to maintain the physiological balance, hinders the clinical use of tissue therapies in regenerative medicine by causing the rejection of transplanted biomaterials and stem cells.^282,283 Since a number of studies have indicated that transplanted tissue constructs frequently trigger pronounced proinflammatory responses leading to rejection, it is needed to control this response to realize the advantages of allografts.²⁸⁴

MQDs with anti-inflammatory properties can be applied for immunomodulation. As an example, Ti₃C₂ MQDs with intrinsic immunomodulatory properties were shown to decrease human CD4⁺ IFN-γ⁺ T-lymphocytes activation by about 20%.²⁸⁴ The QDs also encouraged a 3% increase in the proliferation of immunosuppressive CD4⁺ CD25⁺ FoxP₃⁺ regulatory T-cells within a stimulated lymphocyte population. Human naïve CD4⁺ T-lymphocytes were activated and turned into proinflammatory Th1 cells over 7 days with and without Ti₃C₂ MQDs. Flow cytometry indicated decreased expression of lymphocyte proliferation and upregulation of the regulatory T-cells. The mechanism behind the immunomodulatory effects of these MQDs is still not well-understood.⁵¹ On the other hand, Szuplewska et al.⁵³ have proposed that the ROS produced by Ti₃C₂T_x MXenes induces the release of proinflammatory cytokines from resident tissue macrophages and interferes with anti-inflammatory and immunomodulatory properties of implanted biomaterials in the body.²¹

Rafieerad et al.²⁴ have studied the immunomodulatory mechanism of Ta₄C₃T_x MQDs by activated HUVECs, human peripheral blood mononuclear cells, and Th1 cells. Activated HUVECs, PBMCs, and Th1 cells were selected as they represent key mediators of immune responses. Endothelial cells function as antigen-presenting cells during transplant rejection, PBMCs comprise essential immune effectors, and Th1 cells play a central role in proinflammatory responses and allograft rejection.^285,286 A 3.3-fold increase in the PD-L1 expression and a simultaneous 1.3-fold decrease in the CD86 expression in activated HUVECs treated with Ta₄C₃T_x.²⁸⁷ MQDs can elucidate the immunomodulatory mechanism of these MQDs. Both PD-L1 and CD86 play important roles in T-cell activation via antigen-presenting cells. PD-L1 inhibits T-cell activation, while CD86 activates it.²⁸⁸ Ta₄C₃T_x MQDs hold the potential to diminish host inflammation against allogeneic organs and tissues by modifying the PD-L1 and CD86 equilibrium within antigen-presenting endothelial cells (Fig. 20a).²⁴

Fig. 20 (a) Schematic representation of the immunomodulatory mechanisms of Ta₄C₃T_x MQDs. (b) H&E-staining of explanted abdominal aortic segments (reprinted with permission from Wiley, copyright © 2021).²⁴ (c) Typical photographs of 4T1 tumor-bearing mice and tumors after different treatments for 16 days (reprinted with permission from Elsevier, copyright © 2020).²³

• In vivo immunomodulation

To investigate the immunomodulatory potential of Ta₄C₃T_x MQDs within a living organism, Rafieerad et al.²⁸⁷ utilized a rat model of allograft vasculopathy, which is a significant factor in the rejection of transplanted organs.²⁸⁹ Following transplantation, Ta₄C₃T_x MQDs were promptly administered at a specific dose. No adverse effects were observed either over the course of a week or after that in blood and tissue samples. However, histologic examination of the abdominal aorta in transplanted animals revealed apparent inflammatory changes compared to sham animals (Fig. 20b). Nevertheless, animals treated with Ta₄C₃T_x MQDs showed reduced endothelial injury and immune cell infiltration when compared to those injected with saline, while both were different from control animals. Immunohistochemistry, flow cytometric analysis also revealed a decrease in alpha-smooth muscle actin to assess vascular injury. This protein is an early contributor to rejection.²⁹⁰ This reduction was ameliorated in animals treated with Ta₄C₃T_x MQDs. Tregs, which play a crucial role in immunologic tolerance after transplantation,²⁹¹ were reduced in transplanted animals but restored by Ta₄C₃T_x MQD treatment. The results of the in vivo examinations revealed the potential of Ta₄C₃T_x MQDs in reducing allograft vasculopathy and improving transplantation outcomes.²⁸⁷

5.4.2. Bioimaging. Overcoming the challenge of nanoparticle uptake from the bloodstream to the target site is a significant hurdle in the systemic delivery of subcellular nanomedicine. Vascular ECs, which act as a barrier between the blood and organs, facilitate the uptake of engineered nanoparticles.²⁸⁸ Recently, fluorescent Ti₃C₂ MQDs with tailored surface modifications were used for clinical subcellular nanomedicine.¹⁰ It was shown that a low concentration of the Ti-based MQDs (2–20 µg ml⁻¹) were efficiently taken up by live HUVECs in a dose-dependent manner. No significant alterations in cellular morphology or behavior over the course of the study was noticed.¹⁰

Recent studies have expanded the range of MQDs applied in bioimaging beyond conventional Ti₃C₂ composition. For example, niobium carbide (Nb₂C) MQDs have attracted particular attention owing to their bright fluorescence and excellent biocompatibility. Notably, at concentrations as high as 100 µg mL⁻¹, levels that are cytotoxic to Ti₃C₂ QDs, Nb₂C MQDs exhibited no detectable toxicity toward HUVEC cells and even induced protective autophagy, highlighting their superior biological tolerance.²⁹² Likewise, albumin-stabilized Ti₃C₂ MQDs have been engineered for targeted imaging of MDA-MB-231 breast cancer cells, producing strong and stable intracellular fluorescence signals in vitro.²⁹³

The structural versatility of MXene-based hybrid nanostructures further highlights their potential for advanced imaging applications. For example, Co–Mn Prussian-blue nanoparticles functionalized with Ti₃C₂ MQDs enabled in situ fluorescence sensing and spatial mapping of specific microRNAs within living cancer cells.²⁹⁴ Collectively, these studies underscore that rationally engineered MQDs, particularly those derived from non-Ti compositions, can serve as bright, stable, and biocompatible nanofluorophores suitable for live-cell imaging with minimal cytotoxicity. Overall, MQDs represent a newly emerging and promising material class of bioimaging materials. However, further research is required to achieve a deeper understanding of their behavior and potential in bioimaging.

5.4.3. Cancer therapy. Although research on 2D nanomaterial has led to the development of new PTT agents, potential long-term toxicity risks have emerged biodegradable QDs with satisfactory photothermal properties.¹⁰ Titanium-based MQDs, such as Ti₂N, have demonstrated great potential in this field by offering an effective platform for photoacoustic (PA) imaging – PTT within the NIR-I/II biowindows for cancer treatment.²¹ Ti₂N MQDs exhibit strong and broad NIR-I, and NIR-II absorption, achieving temperatures of around 60 °C with low concentrations (20 ppm) under 808 or 1064 nm laser irradiation. Their photothermal conversion efficiency (η) reaches as high as 48.62% and 45.51% for 808 and 1064 nm, surpassing conventional inorganic PTT agents, such as Au nanorods (21%), graphene oxide (25%), MoS₂ nanoflakes (27.6%), black phosphorus QDs (28.4%), and Bi₂Se₃ nanosheets (26.4%).^291,295 Ti₂N QDs maintain their photothermal properties over five lasers on/off cycles, and their morphology and structure remain unchanged indicating excellent photothermal stability. However, Ti₂N MQDs exhibit notable degradability in aqueous solution through oxidation and hydration processes, forming titanium oxide and hydrogen titanium oxide byproducts. Shao et al.²³ used Ti₂N MQDs in an in vivo setting, as PTT agents within the NIR-I/II windows. In an animal model, NIR laser irradiation resulted in a significant increase in temperature, effectively ablating the tumor tissues (Fig. 20c). Continuous monitoring of the animals revealed no significant weight loss or adverse effects, highlighting the negligible side effects of the treatments without signs of recurrence. Besides Ti₂N MQDs, titanium carbide MQDs have been used for cancer therapy via incorporating Ti₃C₂ into a doxorubicin (DOX)-based prodrug system.⁵⁶ This system demonstrated targeted drug release, photodynamic effects, and bioimaging capabilities in vitro.⁵⁶ Overall, this remains an essentially untouched research area, and MXene quantum dots hold remarkable potential for future applications in cancer therapy.

5.4.4. Other applications. Apart from the above biomedical applications, magnetic luminescent QDs are being explored as potential nanotherapeutics against viral infections, as they offer heightened immune response in infectious diseases like COVID-19.²⁹⁶ For instance, Ti₃C₂ MQDs enriched with bioactive functional groups have recently been used for testing SARS-CoV-2.²⁹⁶ It has been found that the MQDs can effectively mitigate virus multiplication, even at very low doses (for example, 0.15 μg mL⁻¹) in VeroE6 cells.²⁹⁶ In another study, Ta₄C₃ MQDs are suggested as nanocarrier to deliver sanguinarine (SAN) for enhanced antibacterial and wound healing. The SAN@AHEP@Ta₄C₃ formulation shows >85% antibacterial activity against S. aureus and E. coli in vitro and in vivo and, combined with NIR irradiation, demonstrated potential as a safe and effective antibacterial material.²⁹⁷

Some cations demonstrate effective reactivity with H₂O₂ within the specific acidic microenvironment of tumors.²⁹⁸ An efficient generation of highly toxic ˙OH leads to the destruction of tumor blood vessel integrity and causing cancer cell death. This process can be facilitated in the presence of MQDs. For example, it has been reported that Ti³⁺ in Ti₃C₂T_x MQDs, or Mn²⁺ in a complex Ti₃C₂T_x-based system, effectively suppress HeLa tumor xenografts.²⁹⁸ Due to the biocompatibility of Ti₃C₂T_x MQDs, no side effects on normal tissues and organs have been observed, offering a safe and efficient strategy for tumor therapy.²⁹⁸

6. Conclusions and outlook

In this review, the emergence of MQDs as a promising class of nanomaterials for various applications is comprehensively discussed. To unlock their potential in advancing nanoscience and technology, recent progress in their synthesis, including strategies for quantum state engineering and surface passivation, has been reviewed. Through surface functionalization, doping, co-doping, and molecular conjugation, MQDs with high PLQY, exceeding ∼28%, can successfully be synthesized. Taking advantage of intrinsic biocompatibility and exceptional optical properties, MQDs have been integrated into a wide range of applications, from sensing (various ions and molecules) and biomedicine (cancer therapy and cell imaging) to optoelectronics (LEDs, lasers, solar cells, and photonics). Their high surface area, layered structure, and abundant active sites male MQDs highly promising for applications in energy storage (batteries and supercapacitors) and energy conversion (hydrogen evolution and CO₂ reduction) systems.

Despite significant advancements in the properties of MQDs and their widespread applications, many promising opportunities remain unexplored, necessitating further comprehensive research. Some key areas for future perspectives are outlined below.

• Current fabrication methods suffer from low yields, temporal inefficiency, inconsistent quality, and high costs.²⁰ Scaling up production while maintaining uniformity in size, structure, and functionality remains a major challenge. One of the most recent techniques capable of synthesizing narrow-sized QDs from organic and inorganic semiconductors in a very short time scale (in less than hour) is pulsed laser ablation. This can be considered either top-down or bottom-up based on the pulse time, forming ultrasmall QDs. Using various additives, doped or surface functionalized MQDs can be synthesized. Further, the possibility of the hot injection method, according to which organometallic reagents rapidly inject into a hot solvent and produce homogeneous nuclei,²⁹⁹ that had been applied for other QDs, can be investigated for synthesizing MQDs.

• Regarding their optical properties, the precise contributions of quantum confinement (size-dependent behavior) and surface functional groups to enhanced radiative recombination by passivation of non-radiative trap states in MQDs are not yet clearly distinguished through in-depth investigation. To elucidate this, it is strongly recommended to combine DFT with PL, PLE, UV-Vis, and ultraviolet photoelectron spectroscopy to not only deepen interpretation about the electronic band structure but also find the excitation dependent or independent PL behavior of modified MQDs.³⁰⁰

• Heterojunctions have been capable of enhancing the sensing process. However, a few heterostructures based on MQDs and 2D structures have been fabricated. Transition metal dichalcogenides (TMDs) are chemically stable semiconductors showing ambipolar behavior.³⁰¹ Therefore, they can be a great career for transportation of the transferred electrons or holes between MQDs and analytes towards contacts, increasing the sensitivity. Moreover, decorated MQDs combined with tungsten disulfide (WS₂) or molybdenum disulfide (MoS₂) on a flexible polymer substrate (PTE) offer a potentially advantageous architecture for the development of future flexible wearable sensors.

• While the fabrication of single-component WLEDs and lasers based on MQDs has been demonstrated, their performances currently lag behind those achieved with lead-free perovskite QDs exhibiting PLQYs of 70%. Although surface-modified MQDs have reached PLQYs of about 30%, further enhancements via systematic investigations into the effects of size, dopant incorporation, and surface functionalization are necessary to compete with other materials.

• Given the recent significant advancements in PLQY enhancement through surface modification of MQDs, the absence, to our knowledge, of fabricated electroluminescent LED devices based on MQDs represents a notable research opportunity.

• MQDs have modified perovskite and ETL in solar cells, but their effects on hole transport layers like Spiro remain unexplored. Engineering the band structure of MQDs can be helpful to use it as a dopant or sensitizer for the conductivity improvement of Spiro-OMeTAD.

• MQDs show great promise in batteries, supercapacitors, and CO₂/N₂ reduction applications due to their high surface area, quantum confinement, and abundant edge sites.³⁰² A major challenge lies in controlling surface terminations, which strongly influence conductivity, ion intercalation, and long-term cycling stability. Current synthesis methods often yield a mixture of –O, –F, and –OH groups, limiting reproducibility and the ability to engineer their work function and surface catalytic energetics. Future strategies may include post-synthetic plasma treatments, thermal annealing in controlled environments, or selective chemical etching to achieve atomically precise surface control.

• While MQDs have shown promise in electrocatalytic systems such as Li–O₂ batteries, their application in intercalation-based batteries (e.g., Li-ion, Na-ion) remains limited. The quantum-confined size of MQDs facilitates ion diffusion and access to active sites but falls short in practical performance. Current MQD-based electrodes reach only ∼200 mA g⁻¹ and fewer than 300 cycles, compared to >5000 mA g⁻¹ and over 700 cycles in leading Li–air systems.²² To bridge this gap, efforts should focus on embedding MQDs into conductive, porous frameworks to improve electron transport and accommodate discharge products. Stabilizing surface terminations, controlling aggregation, and designing efficient catalytic interfaces will be critical. Incorporating redox mediators or solid electrolytes may also lower the overpotentials and enhance the kinetics of reversibility and cycling life.^303,304

• For photocatalytic and optical applications, most recent studies have focused on nanocomposites of MQDs with well-known catalysts such as TiO₂, Cu nanowires, and g-C₃N₄ to leverage interfacial charge-transfer mechanisms.³⁰⁵ Future research should explore less conventional co-catalysts, such as conductive or semiconducting MOFs and COFs, to expand the functional landscape of MQD-based systems. In parallel, the intrinsic quantum confinement effects of MQDs, which can independently tune bandgap and electronic transitions, remain underexplored and should be systematically studied. A deeper understanding of how surface terminations modulate exciton dynamics and charge separation efficiency is also needed. Moreover, integrating plasmonic materials or defect-engineered semiconductors could induce plasmonic resonance energy transfer (PRET) or broaden the light-absorption cross-section.

• In energy conversion systems, stabilizing MQDs against aggregation and surface oxidation is critical for long-term electrode stability. This can be achieved through encapsulation in conductive matrices or by anchoring MQDs onto chemically compatible, high-surface-area supports such as carbonaceous materials or transition metal oxides. These strategies not only prevent degradation but also form ohmic contacts and reduce interfacial Schottky in charge transfer mechanistic. Additionally, operando spectroscopy and electrochemical impedance analysis should be used to monitor catalyst evolution and reveal degradation mechanisms. Long-term studies under realistic conditions, such as continuous electrolysis, variable gas concentrations, and pH, are still lacking.^69,306 Future research must address these gaps to better evaluate and increase the activation energy of degradation of MQD stability in real-world environments.

• Selectivity in electrocatalytic reactions involving MQDs has received less attention than in other nanomaterials. Altering the adsorption energetics of competing reaction intermediates and selectivity is essential to reduce energy consumption and lower the free energy barrier for the desired efficiency in processes like CO₂ and N₂ reduction. Approaches such as heteroatom doping (e.g., N, S, B), anchoring single-metal atoms, or constructing well-defined active sites could offer greater control over catalytic pathways. Tailoring surface terminations can also steer intermediate adsorption energetics. Modulating the structural and electronic properties of MQDs, through strain engineering or controlled synthesis, could further enhance charge carrier mobility and reaction specificity.²¹⁵ These directions require combined experimental and computational studies to uncover structure–property relationships.

• Drug delivery is a key area of interest, particularly targeted smart drug delivery systems where drug-loaded QDs enter cell nuclei and release drugs for treatment.

• The intrinsic properties of MQDs, demonstrated in vitro and in vivo, require deeper investigation to understand underlying mechanisms.

• Research is needed to explore how functional groups influence reactive oxygen species generation to optimize medical applications. This could lead to innovative solutions in regenerative medicine and immune-related challenges, including transplantation.

• Fig. 21 gives a clear and comprehensive overview of the current progress and future directions of MQDs. It is organized into three main categories of synthesis, properties, and applications. Each part contains three sections of challenges, solutions, and outlook. This structure helps to show how different experimental methods and material designs influence MQD performance and practical applications. For example, it illustrates how controlling surface terminations can tune the photoluminescence behavior, or how adjusting synthesis routes can improve device stability and energy efficiency.

Fig. 21 Schematics overview of challenges, solutions, and future directions for MQDs.

• In the synthesis section, the Fig. 21 compares several key approaches, including top-down etching, laser ablation, bottom-up solvothermal synthesis, and surface engineering. It summarizes their main issues including oxidation, size distribution, and low yield and lists the corresponding solutions and improvements like HF-free synthesis, heteroatom doping, and ligand passivation. The properties section highlights how these strategies affect MQD optical response, charge transport, surface stability, and energy-related performance. Finally, the applications section outlines how MQDs are being developed for optoelectronics, energy storage, biomedical imaging, and environmental sensing.

• Overall, the scheme serves not only as a summary but also as a roadmap that guides readers to the next stage of MQD research. The outlook column points out key opportunities for progress, including precise surface functionalization, hybrid material design, environmentally friendly scalable production, and cross-disciplinary integration into energy, environmental, and biomedical technologies. Through this structured format, the figure helps readers understand the current state of the field while also identifying the challenges and emerging directions that will shape future MQD development.

Conflicts of interest

The authors declare no competing financial interest.

Abbreviations

AA	Ascorbic acid
AFM	Atomic force microscopy
APTES	3-Aminopropyltriethoxysilane
AQY	Apparent quantum yield
ARS	Alizarin red S
ATP	Adenosine triphosphate
BAW	Bulk acoustic wave
BiOI	Bismuth oxyiodide
BSA	Bovine serum albumin
CE	Coulombic efficiency
CIE	Commission Internationale de l’Éclairage
COF	Covalent organic framework
CRI	Color rendering index
Cu NWs	Copper nanowires
CUR	Curcumin
CV	Cyclic voltammetry
DA	Dopamine
DFT	Density functional theory
DMSO	Dimethyl sulfoxide
DMF	Dimethylformamide
DOX	Doxorubicin
DPA	2,6-Dipicolinic acid
DRS	Diffuse reflectance spectroscopy
DSSCs	Dye-sensitized solar cells
ECL	Electrochemiluminescence
EDD	Electron density difference
EDTA	Ethylenediaminetetraacetic acid
EIS	Electrochemical impedance spectroscopy
ESR	Electron spin resonance
ETL	Electron transport layer
FE	Faradaic efficiency
FRET	Förster resonance energy transfer
FWHM	Full width at half maximum
GCE	Glassy carbon electrode
GIXRD	Grazing-incidence X-ray diffraction
GSH	Glutathione
HAADF-STEM	High-angle annular dark-field scanning transmission electron microscopy
HER	Hydrogen evolution reaction
HOMO	Highest occupied molecular orbital
HRP	Horseradish peroxidase
IFE	Inner filter effect
iPSCs	Induced pluripotent stem cells
ITO	Indium tin oxide
KOH	Potassium hydroxide
LiPSs	Lithium polysulfides
LOD	Limit of detection
LRGO	Laser-reduced graphene oxide
LUMO	Lowest unoccupied molecular orbital
LSV	Linear sweep voltammetry
MQDs	MXene quantum dots
MSCs	Mesenchymal stem cells
MTT	3-(4,5-Dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide
MUC1	Mucin 1
NaF	Sodium fluoride
NDC	Nitrogen-doped carbon
NG	N-doped graphene
NMP	N-Methylpyrrolidone
NRR	Nitrogen reduction reaction
OER	Oxygen evolution reaction
OPD	O-Phenylenediamine
ORR	Oxygen reduction reaction
oxOPD	2,3-Diaminophenazine
PEC	Photoelectrochemical
PEDOT:PSS	Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate
PEI	Polyethyleneimine
PET	Photoinduced electron transfer
PL	Photoluminescence
PLQY	Photoluminescence quantum yield
PLL	ε-Poly-L-lysine
PNK	Polynucleotide kinase
POSS-PQDs	Silsesquioxane-perovskite quantum dots
PQDs	Perovskite quantum dots
PSCs	Perovskite solar cells
PTT	Photothermal therapy
PVP	Polyvinylpyrrolidone
RHE	Reversible hydrogen electrode
RES	Reticuloendothelial system
RGB	Red, green, blue
RRS	Resonance Rayleigh scattering
ROS	Reactive oxygen species
SCC	Solar-to-chemical conversion
SCN^−	Thiocyanate
SEM	Scanning electron microscopy
SWV	Square wave voltammetry
TC	Tetracycline
TMAOH	Tetramethylammonium hydroxide
TMCs	Transition metal carbides
TNP	Trinitrophenol
TPAOH	Tetrapropylammonium hydroxide
UA	Uric acid
UV-vis	Ultraviolet-visible spectroscopy
VP	Vibrio parahaemolyticus
WLED	White light-emitting diode
XPS	X-ray photoelectron spectroscopy
ZAB	Zinc–air battery

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d5nr03616j.

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