Lanthanide ion-doped carbon dots (Ln-CDs) as fluorescent nanoprobes: a review

Abstract

Breakthroughs in nanoscale science and engineering have facilitated the design of nanomaterials with exceptional optical behaviors. Over the past decade, numerous fluorescent nanoprobes have been reported for the highly sensitive and selective detection of a wide range of analytes. Among these, doped carbon dots (CDs) have become a major focus of researchers across multiple disciplines due to their exceptional physicochemical and optical properties. Lanthanide ion-doped carbon dots (Ln-CDs) have attracted increasing attention in recent years as specialized nanoprobes for luminescent detection because of their unique optical behavior, chemical stability, and low toxicity. However, most existing studies are limited to luminescence color modulation under different excitation wavelengths, and only a few reports have exploited dual-emission sources for nanoprobe construction. This review summarizes lanthanide-ion-doped carbon dots, highlighting the unique roles of lanthanide ions, doping mechanisms, and their advantages for analytical applications. In addition, the applications of these nanoprobes in various fields, including metal-ion detection, pharmaceutical analysis, contaminant monitoring, anthrax biomarker detection, and biomolecule sensing, are systematically discussed. In summary, Ln-CD-based nanoprobes show potential as promising candidates for future analytical platforms for the detection of multiple analytes.

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1. Introduction

Chemical sensors play important roles in various fields including chemistry, biology, medical diagnostics, and environmental science.^1–4 Typically, these sensors contain receptors that convert chemical interactions into measurable analytical signals when they bind with specific target species.⁵ Numerous techniques have been developed for the sensitive detection of analytes, including atomic absorption spectroscopy (AAS),⁶ inductively coupled plasma mass spectrometry (ICP-MS),⁷ electrochemical sensing,⁸ chemiluminescence sensing,⁹ and fluorescence (FL)-based sensing. The fluorescence sensing strategy is prevalent, as it is an easy-to-use, low-cost, and sensitive method that enables real-time online detection.¹⁰ Researchers have exploited many kinds of fluorescent nanomaterials to detect multiple analytes, including nanoclusters (NCs),^11,12 semiconductor quantum dots,^13,14 metal–organic frameworks (MOFs),^15,16 covalent organic frameworks (COFs),¹⁷ sulfur dots,¹⁸ nanocomposite fluorescent sensors,^19,20 and CDs.²¹

Despite their advantages, the previously reported sensors still exhibit several limitations. These drawbacks include the need for complex modifications in hybrid fluorescent dyads and the relatively high cost associated with gold and silver nanoclusters.²² For example, MOFs often demonstrate limited chemical selectivity,²³ while COFs and near-infrared nanosensors typically involve complicated and time-consuming synthesis procedures.²⁴ In addition, quantum dots (QDs) may present potential toxicity due to the presence of metal ions, and NCs generally suffer from poor stability.²⁵ Consequently, there is a significant need to develop fluorescent nanoprobes that are simple and cost-effective and provide rapid responses.

Selecting appropriate fluorescent materials is crucial for their application as sensors for various analytes, including metal ions,^26,27 anions,²⁸ drugs,²⁹ biomarkers,³⁰ and pollutant detection.³¹ CDs are promising fluorescent sensors because of their readily available and widely distributed precursor sources, good water solubility, strong and stable FL, good biocompatibility, low toxicity, and ease of preparation.³² In addition, CDs are a novel family of fluorescent nanomaterials that are usually less than 10 nm in size.³³ Since their discovery in 2004, CDs have attracted significant attention owing to their superior photostability, water solubility, and resistance to photobleaching.^34–36

Because of these characteristics of CDs, they are widely used in a variety of fields. There are two primary methods for creating CDs: “top-down” and “bottom-up”. Larger carbon structures are broken down by top-down techniques, including ball milling,³⁷ chemical exfoliation,³⁸ and ultrasonic treatment,³⁹ which are helpful for large-scale production. However, they frequently entail challenging conditions, protracted processing periods, and costly equipment. By contrast, more popular bottom-up approaches involve chemical vapour deposition,⁴⁰ hydrothermal treatment,⁴¹ and microwave pyrolysis⁴² to create CDs from small molecular precursors. These methods generally yield CDs with fewer defects and allow better control over the final outcome.^43,44

In semiconductor research, doping is a fundamental technique for precisely tuning a material's optical, magnetic and electrical properties. The atomic orbitals of heteroatoms can overlap with those of carbon atoms when they are introduced in CDs' nanostructure. This interaction can have a significant impact on the electronic structure, morphology, and chemical composition of CDs when combined with the electron-withdrawing or -donating characteristics of the heteroatoms.⁴⁵ The incorporation of nonmetallic heteroatoms such as N, S, B and P is common in most doped CDs reported to date.⁴⁶ Metal ions are able to cause even more pronounced changes because of their greater number of electrons, larger atomic radii, and available orbitals. The electrostatic distribution and energy bandgap of CDs can be fine-tuned by doping them with metal ions, which greatly alters their physical and chemical properties.^47,48

Although various metal ions, such as Fe, Ni, Mn, and Cu, are used for doping carbon dots, nonfluorescent metal-ion-doped CDs generally do not introduce new emission peaks. Instead, they influence FL indirectly by modifying surface states or electron density, resulting in intensity-based turn-on/off sensing. The main advantages of fluorescent metal-doped CDs are their simplicity, cost-effectiveness, and suitability for general sensing or photostability enhancement,^48–52 whereas their main disadvantage is the lack of intrinsic FL. In contrast, lanthanide ions exhibit remarkable spectral features, including long-lived excited states, a large Stokes shift, and narrow emission bands.^53,54 Lanthanide-based luminescent materials are widely employed in chemical and biological sensing systems because of their strong and selective responses to specific analytes.⁵⁵ These materials have been extensively studied due to their unique optical properties. However, generating FL through direct excitation is challenging because the f–f transitions of lanthanide ions (Ln³⁺) are forbidden. The antenna effect addresses this issue: when chromophore-containing ligands bind to Ln³⁺, they form lanthanide complexes that enhance luminescence through efficient energy transfer.⁵⁶ Therefore, Ln³⁺ coupled with CDs is highly promising for fluorescent sensing applications owing to its simple preparation, long-wavelength emissions, multiple emission bands and the large Stokes shift characteristic of fluorescent Ln³⁺.

Recently, several reviews on the properties, characterization, and various technological applications of CDs have been published. However, a focused review on lanthanide ion-doped CDs in promising analytical applications is still lacking in the literature. Therefore, in this review, we first introduce CDs and their properties and then discuss lanthanide ion-doped CDs and their functionalization mechanisms. As shown in Fig. 1, these observations provide a clearer understanding of the differences between undoped CDs and lanthanide ion-doped CDs, and finally, we describe their applications in diverse fields as fluorescent nanoprobes.

Fig. 1 Comparison between undoped CDs and Ln-CDs.

1.1 Novelty statement

Based on a search on lanthanide ion-doped CDs, the first publication on this topic appeared in 2011 by Zhao et al.,⁵⁷ who explored europium-doped CDs for the detection of phosphate in complex matrices. Since then, publications on this topic have increased, with most lanthanide ions being chosen for doping or coordination with CDs for various purposes. In this review, for the first time, all relevant articles on lanthanide ion-doped CDs used as fluorescent nanoprobes are compiled and evaluated with respect to their properties and the types of bonding between lanthanide ions and CDs. In addition, the applications of these fluorescent nanoprobes in various fields, including metal-ion detection, pharmaceutical analysis, contaminant monitoring, anthrax biomarker detection, and biomolecule sensing, are systematically discussed. In summary, lanthanide ion-doped carbon dot-based nanoprobes demonstrate strong potential as promising candidates for future analytical platforms for the detection of multiple analytes. Table 1 discusses the main differences between this work and previously published reviews on doped CDs, particularly in terms of their applications.

Table 1 Comparison of this work with previous studies on doped CDs

Types of dopants	Summary of published works on doped carbon dots	Year of publication	Ref.
Heteroatom	Li et al. published a study on non-metal heteroatom-doped carbon dots, focusing on their synthesis and properties, without discussing their applications	2018	58
Heteroatom	Miao et al. published a review article on heteroatom-doped carbon dots, discussing doping strategies and applications, including optoelectronic devices, nano-probes, catalysis, and biomedicine	2020	59
Metal	Li et al. published a review article on metal-doped carbon dots, focusing on their synthesis methods and applications in sensing, optoelectronics, imaging, phototherapy, and catalysis	2022	60
Nitrogen	Munusamy et al. published a review on nitrogen-doped carbon dots and their applications as fluorescent sensors	2023	61
Boron	Fu et al. published a review on boron-doped carbon dots, discussing their doping strategies, performance effects, and applications in information encryption, optical sensors, and anti-counterfeiting	2024	62
Nitrogen	Mohammed et al. published a review on nitrogen-doped carbon dots and their applications for anti-bacterial purposes	2025	46
Phosphorus	Kayani et al. published work focusing on phosphorus-doped carbon dots and their applications in analytical and bioanalytical fields	2025	63
Lanthanides	As mentioned above, no review has specifically focused on lanthanide ion-doped carbon dots as fluorescent nanoprobes. Based on the literature survey, this topic is considered novel and unique	2026	This work

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2. Advantages of lanthanide ion-doped carbon dots

Lanthanides, also known as lanthanoids, comprise a group of elements located in the f-block at the bottom of the periodic table.⁶⁴ This block is defined by the presence of f electrons, which play a crucial role in determining the chemical behavior of these elements. Although commonly referred to as “rare-earth elements”, lanthanides are actually more abundant in the Earth's crust than gold. For many years, they attracted relatively limited attention; however, beginning in the 1980s, interest in lanthanide chemistry has increased significantly, leading to rapid advances in the field.^65,66 This renewed focus is largely attributed to the unique characteristics of their 4f electrons, which endow lanthanides with exceptional optical and magnetic properties that are now widely exploited in various commercial technologies.⁶⁷ In solution, lanthanide chemistry is predominantly governed by the +3 oxidation state under oxygen-rich conditions, a feature that initially contributed to the limited exploration of these elements.⁶⁸ Notable exceptions occur for ions with empty, half-filled, or completely filled 4f subshells. A representative example is Eu(II), whose half-filled 4f⁷ electronic configuration provides enhanced stability to the +2 oxidation state under reducing environments. Although most lanthanides have since been stabilized in the divalent state through advances in organometallic chemistry, these species typically require carefully designed ligands and can only be maintained under controlled atmospheric conditions.⁶⁷ Despite their sharp and characteristic emission features, Ln³⁺ ions exhibit inherently weak light absorption because of the Laporte-forbidden nature of the f–f electronic transitions. As a result, the molar absorption coefficients of most Ln³⁺ ions are very low, and only a limited fraction of incident radiation can be absorbed through the direct excitation of the 4f energy levels. This drawback can be mitigated by coordinating Ln³⁺ ions with organic ligands capable of efficient light absorption and energy transfer (ET) processes, commonly referred to as the “antenna effect” or “luminescence sensitization”. The resulting luminescence behavior is governed by the efficiency of energy transfer from the ligand (antenna) to the Ln³⁺ ion (emissive center) as well as by the presence and concentration of quenching species within the primary coordination sphere of the emitting ion.⁶⁹ Because the direct excitation of lanthanide ions is inefficient due to the weak absorption of f–f transitions arising from low absorption cross-sections, indirect excitation via an appropriate chromophore represents an effective strategy.⁷⁰ In this process, the antenna absorbs UV-visible light and undergoes intersystem crossing from the singlet to the triplet excited state. Energy is then transferred intramolecularly from the antenna to the lanthanide ion, promoting its excitation through an energy transfer (ET) mechanism. The stored energy is subsequently released through radiative and/or nonradiative pathways, producing characteristic lanthanide luminescence before the ion relaxes back to its ground state. Notably, Tb³⁺ and Eu³⁺ display strong visible emissions, whereas Er³⁺, Nd³⁺, and Yb³⁺ primarily emit in the near-infrared region.⁷¹ The properties of all lanthanide metals are summarized in Table 2.

Table 2 Summary of all lanthanide elements

Element	Symbol	Atomic number	4f Configuration (Ln^3+)	Characteristic optical properties
Lanthanum	La^3+	57	4f^0	No f–f emission
Cerium	Ce^3+	58	4f^1	Broad UV-blue (5d–4f)
Praseodymium	Pr^3+	59	4f^2	Green-red emissions
Neodymium	Nd^3+	60	4f^3	Near-IR emission
Promethium	Pm^3+	61	4f^4	Weak emission
Samarium	Sm^3+	62	4f^5	Orange-red emission
Europium	Eu^3+	63	4f^6	Intense-red emission
Gadolinium	Gd^3+	64	4f^7	No visible emission
Terbium	Tb^3+	65	4f^8	Strong-green emission
Dysprosium	Dy^3+	66	4f^9	Blue-yellow emission
Holmium	Ho^3+	67	4f^10	Green emission
Erbium	Er^3+	68	4f^11	Green and IR emissions
Thulium	Tm^3+	69	4f^12	Blue emission
Ytterbium	Yb^3+	70	4f^13	Near-IR emission
Lutetium	Lu^3+	71	4f^14	No f–f emission

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The “antenna effect” refers to a mechanism in which an organic ligand functions as a light-harvesting antenna, efficiently absorbing optical energy and subsequently transferring it to a coordinated rare-earth ion through a molecular energy transfer pathway.^72,73 Typically, upon the absorption of excitation light at an appropriate wavelength, the ligand chromophore is promoted from the ground state (S₀) to the first excited singlet state (S₁). The excited molecule may then relax through either radiative or nonradiative processes. The transition from the singlet excited state (S₁) to the triplet excited state (T₁), known as intersystem crossing (ISC), plays a key role in this mechanism. Radiative relaxation results in molecular luminescence, which can occur as FL (S₁ → S₀) or phosphorescence (T₁ → S₀).⁷⁴

In lanthanide-CD systems, carbon dots serve as sensitizers due to their abundant surface functional groups, enabling effective coordination with lanthanide ions. By strongly absorbing UV-visible light and transferring the harvested energy to the lanthanide centers, CDs act as sensitizers that enhance the lanthanide's luminescence. As a result, lanthanide ion-doped CDs often exhibit dual or multiple emission bands originating from both the CDs and lanthanide ions. Owing to this unique sensitization behavior, extensive research has been devoted to lanthanide ion-doped CDs and lanthanide-CD complexes for diverse applications. In this review, we comprehensively compile and organize, for the first time, reported studies on these systems and summarize their applications as fluorescent nanoprobes for the detection of a wide range of analytes.

3. Functionalization of CDs by lanthanide ions

Recently, most synthesized CDs exhibit relatively low emission efficiencies compared with conventional semiconductor quantum dots, even after surface passivation and/or functionalization. In many cases, additional passivation steps are necessary to achieve satisfactory luminescence, which complicates the synthesis process and restricts its scalability for large-scale applications. Consequently, developing effective strategies to enhance the FL performance of CDs remains a significant challenge and is highly desirable to broaden their practical applications. In recent years, element doping has recently emerged as an effective strategy to tune the optical properties of CDs, producing highly luminescent materials and enhancing their electronic structure. Dopants are generally classified as metal atoms or heteroatoms (non-metal elements).^75,76

Non-covalent surface modification serves as an effective approach for the surface functionalization of CDs with metal ions, enabling the introduction of new functionalities without substantially perturbing the core structure of the CDs. This approach is governed by non-covalent interactions, including electrostatic attraction, π–π stacking, and coordination (complexation), rather than covalent bonding. A key advantage of this method is the preservation of the CDs' core–shell architecture composed of sp²-conjugated domains, with minimal impact on the surface electron cloud distribution.⁷⁷ Among these interactions, complexation is particularly important for metal-doped CDs, involving coordination between metal ions and surface functional groups such as amino, hydroxyl, and carboxyl moieties. Additionally, the complexation of amphiphilic molecules onto the CD surface enhances their solubility and stability in organic solvents through hydrophobic interactions. Overall, complexation enables effective tuning of CD surface properties, thereby extending their functional versatility and broadening their potential applications (Fig. 2).^45,78 For example, Liang et al. prepared Eu-CDs for the detection of ciprofloxacin and Ga³⁺. Their work demonstrated that the interaction between Eu ions and CDs occurs through complexation, as confirmed by FTIR and XPS analyses, and their work further verified the successful formation of europium-doped CDs.⁷⁹ In addition, Yao et al. prepared novel lanthanide-doped CDs. Their results showed that Eu³⁺ can efficiently chelate with Cu-CDs, and the resulting Cu-CDs-Eu exhibits a larger average diameter than Cu-CDs, confirming the successful coordination between Cu-CDs and Eu³⁺.⁸⁰

Fig. 2 Schematic of the complexation of CDs with lanthanide ions.

The photoluminescence (PL) behavior of CDs is governed by multiple electronic transition pathways that are strongly influenced by their core structure, surface chemistry, and defect states. In undoped CDs, the intrinsic emission originates from π → π* transitions within sp²-hybridized carbon domains embedded in a sp³ matrix, which typically results in size-dependent emission due to quantum confinement effects. In contrast, lanthanide ion-doped CDs (Ln-CDs) exhibit fundamentally different emission mechanisms dominated by the unique 4f electronic transitions of lanthanide ions. Upon doping, Ln³⁺ ions coordinate with oxygen- or nitrogen-containing surface groups of CDs, enabling ligand-to-metal charge transfer (LMCT) and efficient non-radiative energy transfer from the photoexcited CDs to the lanthanide centers via the antenna effect. The subsequent radiative relaxation of Ln³⁺ occurs through characteristic 4f–4f transitions, yielding narrow emission bands with long lifetimes and high color purity, as exemplified by the red emission of Eu³⁺ (⁵D₀ → ⁷F_j) and the green emission of Tb³⁺ (⁵D₄ → ⁷F_j). In some systems, the co-existence of CD-centered and lanthanide-centered emissions enables synergistic dual-emission behavior. The effect of CDs as sensitizers with lanthanide ions is shown in Fig. 3.

Fig. 3 Schematic of CDs as sensitizers with lanthanide ions.

3.1 Types of doping

Doping is a widely used strategy to improve the physicochemical properties of CDs, with heteroatom doping and surface functionalization being among the most effective approaches for tuning their optical, electrical, and chemical characteristics. In heteroatom doping, non-metal atoms such as nitrogen, boron, sulfur, and phosphorus are introduced into the carbon framework, which modifies the electronic structure of CDs and generates n-type or p-type charge carriers. As a result, the optical and electronic properties of CDs can be precisely controlled by adjusting the type and concentration of dopant atoms.⁵⁹ Metal-ion doping represents another important approach to enhance the functionality of CDs. Transition and rare-earth metal ions, including Fe³⁺, Ni²⁺, Cu²⁺, and Mn²⁺, possess partially filled d- or f-orbitals and numerous unoccupied orbitals that can interact with the carbon matrix. Their incorporation introduces localized electronic states within the band structure and disrupts the intrinsic sp²-conjugated π-system, thereby modulating the energy bandgap and facilitating new electronic transitions and charge transfer processes. Consequently, properties such as optical absorption, electrical conductivity, PL, and even magnetic behavior can be significantly improved.⁴⁵ In addition, halogen doping has been shown to enhance the quantum yield and photophysical properties of CDs. For instance, fluorine, which has the highest electronegativity among all elements, can strongly attract neighboring electrons, promoting charge separation and improving luminescence efficiency. Fluorine edge doping can also replace hydrogen atoms in conjugated organic structures while preserving the sp²-hybridized π-conjugated system of the carbon core.^81,82 Furthermore, lanthanide doping provides unique optical advantages due to the characteristic 4f–4f electronic transitions of rare-earth ions such as Eu³⁺ and Tb³⁺. When incorporated into CDs, these ions act as luminescent centers and receive excitation energy from the carbon matrix through energy transfer processes. This dual effect, involving both electronic structure modification and energy transfer, enables the precise tuning of emission wavelengths, overall sensing performance of CDs, and quantum yields.⁸³ In Table 3, the different types of doping and their effects on the characteristics of CDs are summarized.

Table 3 Summary of typical dopants in CDs and their effects on optical and structural properties

Doping type	Examples	Key effect on carbon dots
Heteroatom doping	N, S, P, B	Modifies the electronic structure, generates n- or p-type carriers, and enhances the photoluminescence and quantum yield
Metal-ion doping	Fe^3+, Ni^2+, Cu^2+, Mn^2+	Introduces localized electronic states, alters the HOMO–LUMO gap, and improves charge transfer and sensing performance
Halogen doping	F, Cl, Br	Alters the electron distribution due to high electronegativity and increases the quantum yield and photophysical stability
Lanthanide doping	Eu^3+, Tb^3+, Sm^3+, Gd^3+	Provides characteristic 4f–4f luminescence, enabling tunable emission and improved fluorescence sensing

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4. Optical properties of carbon dots

4.1 UV-vis absorbance of CDs

CDs typically exhibit strong absorption in the ultraviolet region, along with a broad tail extending into the visible range. Most absorption bands are commonly observed between 260 and 350 nm. The prominent absorption peak around 230 nm is mainly associated with the π–π* electronic transitions of aromatic C=C bonds.⁸⁴ The shoulder peak near 300 nm is assigned to n–π transitions arising from surface functional groups, such as C=O bonds⁸⁵ as well as other heteroatom-containing groups, including nitrogen and oxygen.⁸⁶

4.2 Fluorescence of CDs

FL is considered one of the most remarkable properties of CDs. CDs synthesized through various methods and precursor materials often exhibit different chemical compositions and structural features. Generally, the FL emission of CDs can originate from several factors, including the quantum size effect, surface states, and molecular states.⁸⁷ However, the precise mechanism responsible for the FL behavior of CDs is still debated and requires further investigation.⁸⁸ Among the proposed mechanisms, the quantum confinement effect, also known as the size effect, is one of the most widely accepted models used to explain the FL properties of CDs.^89,90 Typically, the bandgap energy of CDs is influenced by their particle size and morphology. In recent years, increasing attention has been directed toward studying the role of quantum size effects in CDs.⁹¹

4.3 Quantum yield (QY)

CDs exhibit notable optical properties, among which the QY is a key parameter that reflects their efficiency in converting absorbed photons into emitted light. This value represents the ratio of emitted photons to absorbed photons and is commonly used to evaluate the efficiency of the PL process.⁹² CDs with high QYs are particularly desirable for various applications, especially FL imaging and sensing, where a strong and stable emission is essential for precise detection and visualization.⁹³ The QY of carbon dots is influenced by several factors, including synthesis methods, surface functionalization, chemical composition, and particle size distribution.⁹⁴ Tian et al. used folic acid and terbium nitrate as precursors to prepare CDs, Tb-CDs, and Tb@CDs. The QYs of the prepared CDs, Tb@CDs, and Tb-CDs were calculated to be 57.12%, 48.2%, and 73.8%, respectively.⁹⁵ Wu et al. developed novel lanthanide-doped CDs. The probe was prepared from citric acid, ethylenediamine, and lanthanide ions using a hydrothermal method. The obtained CDs exhibited excitation-dependent FL behavior. The FL quantum yields of Yb-CDs and Nd-CDs at 360 nm were calculated to be 52.32% and 49.06%, respectively.⁹⁶

4.4 Lifetime measurement

The fluorophore lifetime refers to the average duration that a molecule remains in the excited state before returning to the ground state.⁹⁷ The FL lifetime is considered an intrinsic property of fluorescent materials. CDs that exhibit relatively long FL lifetimes may have promising applications in sensing and imaging. However, the FL lifetime of CDs is typically only a few nanoseconds.⁹⁸ In sensing studies, a decrease in FL lifetime after the addition of an analyte generally suggests a dynamic quenching mechanism. In contrast, if the lifetime remains unchanged while the FL intensity decreases, the quenching process is often attributed to the inner filter effect (IFE) or static quenching, as reported in several studies.^99,100 For example, Meng et al. prepared Eu-CDs for the detection of tetracycline (TC). The sensing mechanism was investigated using FL lifetime measurements. The fluorescence lifetimes of Eu-CDs were measured to be 5.13 ns in the absence of TC and 4.92 ns in the presence of TC, indicating that the addition of TC caused only a negligible change in the FL lifetime. These results suggest that energy transfer between the CDs and TC is minimal, and the decrease in the blue FL emission intensity of the CDs can be attributed to the IFE rather than dynamic quenching.¹⁰¹ Similarly, Yan et al. demonstrated that the sensing mechanism for caffeic acid detection was also based on the IFE mechanism. Their fluorescence lifetime measurements showed that the FL lifetime remained almost unchanged before and after the addition of caffeic acid, further confirming that the observed FL quenching was mainly caused by the IFE.¹⁰²

5. Sensing mechanisms

Several mechanisms can explain FL quenching and sensing behavior in CD systems, including the IFE, Förster resonance energy transfer (FRET), photo-induced electron transfer (PET), and static or dynamic quenching processes. The inner filter effect occurs when the absorption of excitation or emitted light reduces the observed FL intensity, leading to fewer emission signals; this attenuation may arise from the primary absorption of the excitation light or the secondary absorption of the emitted FL. Unlike energy transfer mechanisms, the FL lifetime in IFE-based quenching generally remains unchanged, which allows it to be distinguished from other processes.¹⁰³ In contrast, FRET involves non-radiative energy transfer from an excited donor to an acceptor through long-range dipole–dipole interactions, often requiring spectral overlap between the donor emission and acceptor absorption as well as an appropriate intermolecular distance. In CD-based sensors, CDs commonly act as energy donors, and FRET can either reduce the FL intensity or cause emission wavelength shifts, depending on the interaction with the target or quencher.¹⁰⁴ PET is another quenching mechanism in which electron transfer occurs between the excited fluorophore and an electron donor or acceptor analyte, forming a transient complex that relaxes to the ground state without photon emission.¹⁰⁵ Additionally, FL quenching can be categorized into static and dynamic processes: static quenching arises from the formation of a non-fluorescent ground-state complex between the fluorophore and quencher without altering the FL lifetime, whereas dynamic quenching occurs through collisional interactions in the excited state, typically resulting in a reduced FL lifetime while leaving the absorption spectrum largely unchanged.³²

6. Applications of lanthanide ion-doped carbon dots

6.1 Metal-ion detection

Metal ions are widely distributed in the natural environment, and their detection is essential for environmental and biomedical applications. Variations in the concentrations of specific metal ions can indicate pathological conditions in the body, offering potential tools for early diagnosis and disease monitoring.^106–108

Xu et al. developed an Eu-CDs/MOF-based sensor for Hg²⁺ detection, exhibiting a wide linear working range of 0.065–150 µM and a limit of detection (LOD) of 13 ppb.¹⁰⁹ Sang et al.¹¹⁰ reported Eu-CDs for the dual sensing of TC and Al³⁺. Upon increasing the TC concentration, the FL emission color of the Eu-CDs progressively shifted from blue to light purple and finally to red. The FL intensity ratio showed a good linear correlation in the concentration range of 0–100 µM, with a LOD of 6.9 nM. Conversely, the subsequent introduction of Al³⁺ at different concentrations induced a reverse color transition from red to pink and ultimately back to blue. A linear correlation was observed between the FL intensity and Al³⁺ concentration in the working range of 0–50 µM, with a LOD of 28.6 nM. In addition, a smartphone-based visual assay platform was developed, achieving visual detection limits as low as 13.2 nM for TC and 160 nM for Al³⁺, as illustrated in Fig. 4.

Fig. 4 Schematic of Eu-CD synthesis and colorimetric sensing for TC and Al³⁺, reproduced with permission from ref. 110. Copyright 2022, Elsevier.

Desai et al. fabricated a fluorescent nanosensor based on Eu³⁺-CDs. The addition of Hg²⁺ ions led to significant quenching of the Eu³⁺-CD FL emission, and a linear correlation between the FL quenching intensity and Hg²⁺ concentration was observed in the working range of 5.0–250 µM, with a LOD of 2.2 µM.¹¹¹ Similarly, Correia et al. synthesized Eu-CDs and demonstrated that their luminescence was selectively quenched by Hg²⁺ and Ag⁺ ions, while remaining largely unaffected by other metal cations. Notably, the quenching mechanism depended on the specific ion: both the blue emission of the carbon dots and the red emission of Eu³⁺ were suppressed in the presence of Hg²⁺, whereas only the carbon dot emission was quenched by Ag⁺. Based on these distinct responses, a ratiometric sensing strategy was established using the intensity ratio of carbon dot emission to Eu³⁺ emission. Eu-doped carbon dots show higher sensitivity to metal cations than undoped CDs due to structural changes caused by the incorporation of Eu ions. These ions coordinate with functional groups during synthesis, act as nucleation centers, and modify the carbon network structure. As a result, additional defects and active sites are formed, enhancing the interaction of Eu-CDs with metal cations and improving their sensing performance. The limits of detection were approximately 5 µM for Ag⁺ and 4 µM for Hg²⁺, with an effective working concentration range of 10–100 µM.¹¹²

Other lanthanide ion-doped carbon dots (CDs), such as Ce-doped CDs, have also been reported. For example, Zhang et al. used Ce-doped CDs for the determination of As(III), achieving a working range of 0.5–5.8 µg L⁻¹ and a LOD of 0.2 µg L⁻¹.¹¹³ Another example is the work by Mohandoss et al., who developed a sensor for the dual detection of Fe³⁺ and pyrophosphate (PPi) ions. The sensor exhibited high sensitivity, with LODs of 34.6 nM for Fe³⁺ and 25.4 nM for PPi.¹¹⁴ Chai et al. reported a Dy-CD system in which Dy³⁺ ions induced the aggregation of CDs. In the presence of phosphate (PO₄³⁻), strong Dy³⁺-phosphate coordination disrupted these aggregates, resulting in FL recovery of the CDs. This mechanism enabled the selective detection of PO₄³⁻ in artificial wetland samples, with a working range of 0.2–30 µM and a LOD of 0.1 µM.¹¹⁵ In another study, Kumar et al. developed Er-CDs with an enhanced quantum yield, which were employed as a FL probe for the turn-off/turn-on sensing of Fe²⁺ ions and H₂O₂. The FL of Er-CDs was quenched upon the addition of Fe²⁺ and subsequently restored after introducing H₂O₂, achieving detection limits of 51.7 nM for Fe²⁺ and 55 µM for H₂O₂.⁸³ More research on metal-ion detection using Ln-CDs is summarized in Table 4.

Table 4 An overview of Ln-CDs as fluorescent nanoprobes for multiple metal ions, highlighting their analytical features and merits

Types of Ln-CDs	Analyte	Sample	LOD (nM)	Working range	Ref.
Gd-CDs	Hg^2+	Water	104.4, 60.2	0–50 µM	116
Gd-CDs	Cu^2+	Water	380	0–38.4 µM	117
Ln-CDs	Hg^2+	Cancer cells	100	0 mM–50 µM	118
Ln-CDs	Fe^3+, Sn^2+, ClO^−	Zebrafish	990, 110, 110	0–100 µM	119
Ln-CDs	Fe^3+	HeLa cells	91	0.15–10.0 µM	120
Eu/Tb-CD	Cr(VI)	—	175	1–20 µM	121
Eu/Tb-CDs	Ag^+ and Hg^2+	Water	50.1, 33.3	0–10 µM	122
La/Ce-CDs	Fe^3+	Water	753	0–60 µM	123
Pr-CDs	Hg^2+	Water	90.3	0–10 µM	124
Tb-CDs	Hg^2+	Seafood	37	1–161.51 µM	125

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6.2 Pharmaceutical detection

In pharmaceutical applications, analytical chemistry plays a crucial role in the identification and quantification of active compounds, evaluation of their purity and stability, and assurance of pharmaceutical product quality throughout manufacturing and storage.¹²⁶

Li et al. developed a ratiometric fluorescent probe to detect TC, achieving a LOD of 31 nM and a working range of 1–100 µM.¹²⁷ Similarly, Liang et al.⁷⁹ developed a novel probe based on Eu-CDs for the detection of ciprofloxacin (CIP) and gallium ions (Ga³⁺). In addition to fluorescence sensing, a hydrogel-based sensing chip was fabricated, with a smartphone employed as the detector. Under irradiation with a 360-nm ultraviolet lamp, the emission of the chip containing increasing concentrations of CIP gradually changed from negligible fluorescence to bright red. Within the concentration range of 5.0–200.0 µM, the CIE-R value of the chip showed a linear correlation with the CIP concentration. After the addition of Ga³⁺ (0.5–70.0 µM) to the chip pretreated with 215 µM CIP, the FL color gradually changed from bright red to bright blue.

Wu et al. reported a novel probe for oxytetracycline (OTC) and tetracycline (TC) detection based on Eu-CDs. In this system, Eu³⁺ ions chelated with CDs and specifically recognized OTC and TC. Owing to the dual and reverse FL responses, the ratiometric detection of OTC and TC was achieved in the working ranges of 0.1–45 µM and 0.1–30 µM, with detection limits of 17 nM and 41 nM, respectively. In addition, a smartphone-assisted portable device was developed for OTC detection. As shown in Fig. 5, the probe was prepared in 24-well plates, and upon the addition of OTC, a gradual change in the FL color was observed. FL images were captured using a smartphone camera and converted into RGB values. With increasing OTC concentration, the R value increased, accompanied by a decrease in the B value.¹⁰⁰

Fig. 5 Smartphone-based platform for the quantitative detection of OTC, reproduced with permission from ref. 100. Copyright 2023, Elsevier.

Liu et al. also developed an Eu-CD-based sensor for TC detection. The sensor enabled rapid and sensitive TC detection using both FL and colorimetric methods, with LODs as low as 2.82 nM and 3.74 nM, respectively.¹²⁸ Ma et al. developed a ratiometric fluorescent probe based on Eu-CDs for the detection of TC. The proposed sensor exhibited a wide working range of 0.05–45 µM and a low LOD of 5.92 nM for TC.¹²⁹ In addition, Hu et al.¹³⁰ designed a multiple-response fluorescent probe composed of dual-emission Eu-CDs for the determination and discrimination of TCs. The carboxyl and amino groups on the dual-emission carbon dots coordinated with Eu³⁺ to form the Eu-CD complex. Upon the addition of TCs, the FL intensities of CDs were quenched due to the IFE and localized FL resonance energy transfer between Eu-CDs and TCs. Simultaneously, the chelation between Eu-CDs and TCs enhanced the antenna effect, resulting in increased characteristic Eu³⁺ emission peaks. Based on these responses, TCs were detected with low detection limits ranging from 46.7 to 72.0 nM. In addition, discrimination among different TCs was achieved through variations in the L-FRET efficiency. Furthermore, a novel centrifuged lateral flow assay strip (CLFAS) device was developed by integrating Eu-CDs, lateral flow assay strips, and a smartphone, enabling TC detection through RGB value variations, as illustrated in Fig. 6.

Fig. 6 Schematic representation of (a) the Eu-CD probes, (b) discrimination among four TCs, and (c) the CLFAS device, reproduced with permission from ref. 130. Copyright 2024, Elsevier.

More research on pharmaceutical product detection using Ln-CDs is summarized in Table 5.

Table 5 An overview of Ln-CDs as fluorescent nanoprobes for multiple pharmaceuticals, highlighting their analytical features and merits

Types of Ln-CDs	Analyte	Sample	LOD (nM)	Working range	Ref.
Eu-CDs	TC	Milk, honey, water	36.1	0.4–60 µM	80
Eu-CDs	TC	Water	5.8	0.01–3 µM	131
Eu-CDs	OTC, TC, DC, CTC	Chicken, pork, fish	9.50, 15.80, 10.40, 90.30	0.00–603.75 µM, 0.00–623.82 µM, 0.00–594.61 µM, 0.00–601.54 µM	132
Eu-CDs	TC	Water, milk, honey, serum	3	0–80 µmol L^−1	133
Eu-CDs	TC	Water, milk	3.83	0.01–240 µM	134
Eu-CDs	TC	Milk, water	7.9	25–2000 nM	135
Eu-CDs	TC	Water	12	0.08–5 µM	136
Eu-CDs	TC	Milk, honey, water	8.7	25 nM to 20 µM	137
Eu-CDs	OTC	Honey, milk	150	0–100 µM	138
Eu-CDs	TC	Milk, water	22.6	0.1–50 µM	139
Eu-CDs	TC	Water, milk	8.2	0–20 µM	140
Eu-CDs	TC	—	11.7	0–3.5 µg mL^−1	141
Eu-CDs	OTC	Water, milk	29	0.1–25 µM	142
Eu-CDs	TC, OTC	Water	2.96, 2.85	0.0067–25 µM, 0.0067–21 µM	143
Eu-CDs	TC, OTC	Bovine, goat, milk	29, 16	0.25–20 µM, 20–70 µM	144
Eu-CDs	TC	Water	30	0.1–100 µM	145
Eu-CDs	OTC	Water	9.6	0–10 µM	146
Eu-CDs	TC	Water	2.7	0.04–2.4 µM	147
Ln-CDs	OTC, TC	Pork, fish	42.90, 83.30	0.00–805.20 µM, 0.00–1039.50 µM	148
Tb-CDs	Fluoroquinolones	Milk	9.1	0.5–58 µM	149
Tb-CDs	Ciprofloxacin	Milk, urine	1.6	0.008–120 µM	150
Tb-CDs	Alogliptin	Water, plasma	88.09	0.01–1.5 µg mL^−1	151
Tb-CDs	Edoxaban	Human plasma	11.04	30.0–350.0 ng mL^−1	152
Tb-CDs	Deferasirox	Serum	80	0.1–2.5 µM	153

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6.3 Contaminant detection

Rapid population increase and industrialization have intensified environmental pollution, endangering ecosystems and human health through contaminants such as dyes, mycotoxins, organic compounds, and pathogens.

Abdel-Lateef et al. developed Eu-CDs for the detection of curcumin adulteration by metanil yellow. The probe was synthesized through the carbonization and conversion of a tannic acid–Eu complex into highly fluorescent Eu-CDs. The proposed method exhibited a LOD of 1.05 nM with a linear detection range of 1.0–15.0 µg mL⁻¹.¹⁵⁴ Similarly, Zhang et al. designed a novel probe for ATP detection by simply mixing CDs with calcein-Eu. In this system, the carbon dots served as the internal reference, while calcein-Eu acted as the recognition unit. The probe showed a dynamic linear range of 0.05–2.0 µM, with a LOD of 20 nM.¹⁵⁵ Furthermore, Zhang et al. developed a sensor based on Eu-CDs. The sensor exhibited a broad working range (0–2.5 µM), an extremely low LOD (1.25 nM), and a rapid response time (<40 s). Furthermore, a smartphone-based method enabled the portable, visual quantification of Fe³⁺ with a practical LOD of 6.588 nM.¹⁵⁶ In addition, Qin et al. prepared a novel ratiometric probe for the biomarker diaminotoluene using a Eu-CD-encapsulated MOF, achieving a LOD of 6.8 µg mL⁻¹.¹⁵⁷

Amin et al. developed Eu-CDs for the determination of selenite. The method had a simple procedure that enabled the selective detection of selenite, with a working range of 0.078–21.4 µg mL⁻¹ and a LOD of 53.0 ng mL⁻¹.¹⁵⁸ Similarly, Zhou et al. prepared CDs doped with Eu via direct coordination for the detection of UO₂²⁺ within the working range of 25–200 nM, achieving a LOD of 0.84 nM (0.42 µg L⁻¹).¹⁵⁹ Lastly, Albalawi et al. prepared Eu-doped CDs for the detection of indigo carmine dye over the working range of 1.5–10.0 µg mL⁻¹, with a LOD of 0.40 µg mL⁻¹.¹⁶⁰

More research on contaminant detection using Ln-CDs is summarized in Table 6.

Table 6 An overview of Ln-CDs as fluorescent nanoprobes for multiple contaminants, highlighting their analytical features and merits

Types of Ln-CDs	Analyte	Sample	LOD (nM)	Working range	Ref.
Tb-CDs	Phosphates (Pi)	Frozen seafoods	410	0.5–12 µM	161
Tb-CDs	2,4,6-Trinitrophenol	Water	200	500 nM–100 µM	162
Tb-CDs	Melamine	Milk, infant formula	2.6	0.01–300 µM	163

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6.4 Anthrax biomarker detection

Anthrax is a zoonotic infectious disease caused by Bacillus anthracis.¹⁶⁴ Individuals who inhale more than 10⁴ Bacillus anthracis spores at a single exposure and fail to receive effective treatment within 48 h face a high risk of mortality.¹⁶⁵ As an emerging class of biological contaminants, anthrax spores pose an extreme threat not only to animals and plants but also to humans. Among their components, 2,6-pyridinedicarboxylic acid (DPA) is a key constituent and serves as a major biomarker of anthrax spores.¹⁶⁶ Even at low levels (around 60 µM), Bacillus anthracis spores can severely harm human health and are associated with a high fatality rate.¹⁶⁷

Li et al. reported the preparation of a europium-containing CD probe for the detection of DPA and copper ions. In this system, the red emission originating from Eu³⁺ increased with increasing DPA concentration, whereas it decreased after the addition of Cu²⁺. Subsequently, the introduction of glutathione (GSH) restored the red emission. Therefore, the probe operated based on an “on–off–on” sensing mechanism. The corresponding LODs were determined to be 137 nM for DPA, 80 nM for Cu²⁺, and 50 nM for GSH, respectively.⁵⁵ Similarly, Song et al. developed a ratiometric probe based on Eu/CDs/guanosine monophosphate disodium (GMP) for the detection of DPA. In this system, the CDs served as the reference signal, while Eu³⁺ acted as the analytical signal. The FL intensity of Eu³⁺ increased with increasing DPA concentration. The probe exhibited a LOD of 5.1 nM and a linear working range from 15 nM to 5 µM.¹⁶⁸ Additionally, Zhou et al. developed another Eu-CD-based ratiometric probe for DPA detection using both colorimetric and visual assays. The LOD for the ratiometric colorimetric assay was 10.6 nM. More notably, naked-eye detection of DPA without instrumental assistance was achieved at concentrations as low as 1.0 µM.¹⁶⁹ Lastly, Du et al.¹⁷⁰ developed Eu-CDs via complexation for the detection of the DPA biomarker. The probe was successfully applied in two modes: ratiometric sensing and paper-based sensing. The ratiometric method exhibited a working range of 0–120 µM with a LOD of 7.566 nM, while the paper-based method showed a working range of 0–100 µM, as depicted in Fig. 7.

Fig. 7 Schematic of DPA detection based on Eu-CDs, reproduced with permission from ref. 170. Copyright 2025, Elsevier.

More research on anthrax biomarker detection using Ln-CDs is summarized in Table 7.

Table 7 An overview of Ln-CDs as fluorescent nanoprobes for multiple anthrax biomarkers, highlighting their analytical features and merits

Types of Ln-CDs	Analyte	Sample	LOD (nM)	Working range	Ref.
Eu-CDs	DPA	Water, FBS, soil	5	5–700 nM	171
Eu-CDs	DPA	Water, bacteria	23	0.1–100 µM	172
Tb-CDs	DPA	Water	44	0.05–8 µM	95
Tb-CDs	DPA	—	5	0.005–1.2 µM	173
Tb-CDs	DPA	Orange juice, water	0.1	0.0005–2.5 µM	174
Tb-CDs	DPA	Spinach, cucumber, pear	3.0	0–0.091 mM	175
Tb-CDs	DPA	Rice	680	0–20 µM	176
Tb-CDs	DPA	—	5	0.005–1.2 µM	173
Tb-CDs	DPA	Water	35.9	0.5–6 µM	177

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6.5 Biomolecule detection

Biomolecules are essential to physiological functions and are intimately linked to the initiation and progression of numerous diseases. Consequently, developing analytical methods that are efficient, sensitive, rapid, selective, and cost-effective for biomolecule detection is of great significance.¹⁷⁸

GSH plays a vital role in biological systems and performs diverse cellular functions. Al-Mashriqi et al. prepared Eu-CDs as nanoprobes for the detection of GSH, which exhibited a high quantum yield of 40.67%. The LOD was 30 nM, and the linear working range was 0–50 µM.²⁴ Similarly, Liu et al. developed a new ratiometric FL probe based on Eu-CDs for the detection of cerebrospinal Aβ monomers, with a linear range of 0.5–100 nM and a LOD of 0.17 nM. The probe was successfully applied to rat samples.¹⁷⁹ Furthermore, Li et al. prepared Eu-CDs for the detection of Fe³⁺ and ascorbic acid based on an ‘off–on’ FL strategy. The FL intensity ratio of the Eu-CDs showed an excellent linear relationship with the concentrations of Fe³⁺ and ascorbic acid, with limits of detection of 0.15 and 0.13 µM, respectively. The method was successfully applied to water and food samples.¹⁸⁰ Lastly, Li et al.¹⁸¹ reported an effective synthesis strategy for imidazole-based ionic liquid-functionalized carbon dot hybrids (APM-CDs). Using these materials, a novel photofunctional hybrid probe, Eu-CDs, was fabricated. The FL intensity of Eu-CDs exhibited a strong linear correlation with the ascorbic acid concentration in the range of 25–500 µM, achieving a LOD of 80 nM, as illustrated in Fig. 8.

Fig. 8 Schematic of ascorbic acid detection using Eu-CDs, reproduced with permission from ref. 181. Copyright 2024, Elsevier.

Tian et al. prepared a ratiometric FL probe for ascorbic acid based on Eu-CDs. A wide linear detection range from 0.5 to 820 µM was achieved, with a LOD of 60 nM.¹⁸² In addition, Wang et al. developed a ratiometric probe based on Gd-CDs for the determination of morin in biological samples. Owing to the specific chelation between Gd³⁺ and morin, a ratiometric FL nanosensor was constructed, which exhibited a linear detection range of 0.04–80.0 µM and a LOD of 25.0 nM. In addition, the color of the Gd-CDs changed from colorless to deep yellow upon interaction with morin, enabling the development of a colorimetric nanosensor. This method showed two working ranges of 0.04–15.0 µM and 15.0–93.0 µM, with corresponding LODs of 10.5 nM and 21.2 nM, respectively,¹⁸³ as illustrated in Fig. 9.

Fig. 9 Schematic of the Gd-CD preparation and applications, reproduced with permission from ref. 183. Copyright 2022, Elsevier.

More research on biomolecule detection using Ln-CDs is summarized in Table 8.

Table 8 An overview of Ln-CDs as fluorescent nanoprobes for multiple biomolecules, highlighting their analytical features and merits

Types of Ln-CDs	Analyte	Sample	LOD (nM)	Working range	Ref.
Gd-CDs	Dopamine hydrochloride (DH)	Human urine, mouse serum	1.26	1–10 µM	184
Ln-CDs	Serotonin	Urine	7.4	0–38 nM	185
Sm-CDs	Epinephrine	Urine	28.1	0–105 nM	186
Tb-CDs	Bovine serum albumin	Bovine serum	42	0.1–650 µM	187
Tb-CDs	5-Hydroxytryptamine, 5-hydroxyindole-3-acetic acid	Artificial blood Plasma, synthetic urine	0.44, 0.49	0.0–2.60 × 10^−7 M, 0.0–2.91 × 10^−7 M	188
Tb-CDs	H2O2	Rat serum	60	0–40 µM	189
Tb-CDs	Adenosine 5′-triphosphate	Human serum	8.5	40 nM-4.0 µM	190
Tb-CDs	Cytochrome c	Human serum	0.35	1–50 nM	191
Tb-CDs	Guanosine 3′-diphosphate-5′-diphosphate (ppGpp)	Serum, urine	50	0.5–15 µM	192
Tb-CDs	DNA	Synthetic samples	0.80	80 ng mL^−1–50 µg mL^−1	193

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7. Conclusion and future perspectives

Fluorescent nanomaterials have seen rapid development as sensing probes in recent years owing to their unique optical and physicochemical characteristics, along with their high sensitivity, selectivity, targeting capability, and strong suitability for detection applications. Researchers are increasingly focused on developing analyte nanosensors that combine high selectivity and sensitivity, as such systems that offer cost-effective solutions for environmental and analytical monitoring, and this review highlights the significant role of Ln-CDs in this context. Owing to their simple and economical synthesis and strong reactivity toward a broad range of targets, Ln-CDs represent promising platforms for the detection of diverse analytes, including metal ions, pharmaceuticals, contaminants, anthrax biomarkers, and biomolecules, often achieving nanomolar level detection limits that reflect their high analytical performance.

Although Ln-CDs are promising platforms and they are widely used for the detection of various analytes, several challenges remain, including the need for a deeper understanding of lanthanide doping mechanisms and their influence on the electronic structure and photophysical properties of carbon dots, the development of scalable and environmentally friendly synthesis strategies, the expansion of multianalyte sensing capabilities for complex sample matrices, and the integration of Ln-CDs into portable or wearable sensing devices. Sustained interdisciplinary advances will likely make Ln-CDs vital to upcoming smart and sustainable analytical technologies.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data sharing is not applicable to this article, as no datasets were generated or analysed during the current study.

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