Design strategies and applications of electrochemiluminescence from metal nanoclusters

Abstract

Metal nanoclusters have recently gained significant attention in the field of electrochemiluminescence (ECL) due to their unique redox properties as well as optical properties, holding great potential for applications in biosensors, devices, and diagnostics. Constrained by complex pumping kinetics, elusive oxidant–reductant interactions, and the unclear non-radiative transition mechanism, the ECL efficiency is often unsatisfactory thus significantly affecting their sensitivity, stability, and functionality. This review aims to summarize the efforts made to raise the ECL efficiency, providing new ideas and insights for a deeper exploration of metal nanocluster ECL.

---

Tingting Li

Tingting Li joined Jilin Jianzhu University in 2021 and is currently a lecturer in the School of Materials Science and Engineering. She received her PhD degree from Jilin University in 2016. From 2016 to 2018, she worked as a postdoctoral researcher at the National University of Singapore and King Abdullah University of Science and Technology. Her research interests are mainly focused on the property modulation and device fabrication of metal nanocluster luminescent materials.

Jiyu Sui

Jiyu Sui is a master's student, College of Materials Science and Engineering, Jilin Jianzhu University, China. His research interests are mainly focused on the property modulation and device fabrication of metal nanocluster luminescent materials.

Weinan Dong

Weinan Dong is now a Ph.D. student at the State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, China. His research interests include nanophotonics, nano-optoelectronics, and optical physics of functional nanocrystals.

Ying Zhang

Ying Zhang is currently a consultant physician in the Department of Pediatric Respiratory Medicine at the First Hospital of Jilin University. Her research focuses on extracellular vesicles and their roles in respiratory diseases, including COPD, asthma, and allergic rhinitis. She explores engineering coinage metal nanoclusters and pH-responsive intracellular probes and employs single-cell Raman spectroscopy in the fields of optical applications and disease diagnostics. Zhang has secured funding from the Jilin Provincial Finance Department and the Jilin Provincial Health Department. Her research has resulted in several publications in SCI journals.

Yanan Li

Yanan Li is Chief physician, professor, doctoral supervisor and deputy director of pediatric Respiratory Medicine, First Hospital of Jilin University. She received her B.S degree from Peking University (2003) and her Ph.D degree from Huazhong University of Science and Technology (2008), and served as a nationally sponsored visiting scholar at the Neurophysiology Laboratory of the University of Connecticut in 2017–2018. She is mainly engaged in clinical and basic research in respiratory diseases. She has published several papers as the corresponding author and first author in JCR Q1 Liver Journal and chaired a National Natural Science Foundation Project, with a wealth of professional experience and academic achievements.

Zhennan Wu

Zhennan Wu joined Jilin University in 2021 and is currently a full professor at the College of Electronic Science and Engineering. He obtained his Ph.D. degree (2016) from the State Key Laboratory of Supramolecular Structure and Materials at Jilin University under the supervision of Prof. Hao Zhang. Thereafter, he joined the team of Prof. Osman. M. Bakr in KAUST (2016–2018) and Prof. Xie Jianping in NUS (2018–2020) for post-doctoral research. His research interests are centered on the colloidal synthesis dynamics, self-assembly chemistry, and optical physics of functional nanocrystals.

1 Introduction

Metal nanoclusters (NCs), surface motifs formed by the wrapping of metal–organic complexes around the stacked metal core, possess unique optical and electronic properties compared to larger-sized metal nanoparticles and bulk materials.^1–6 The unique structure enables metal NCs with wide applications based on electrochemiluminescence (ECL).^7–10 ECL occurs when free radicals are generated near an electrode. These radicals undergo an electron transfer reaction to form an excited state and then undergo relaxation back to the ground state and emit light.⁶ Compared with other analytical techniques such as colorimetry and photoluminescence, ECL technology has attracted much attention in the analytical field because of its high sensitivity, low background, simplicity of equipment, and good controllability.¹¹

Among various ECL nanomaterials, such as classical Ru(bpy)₃²⁺, chalcogenide nanocrystals, carbon nitride nanosheets, and metal nanoclusters,^12–18 satisfactory ECL efficiencies and easy synthesis processes remain challenging.^19–22 The good biocompatibility and low toxicity of metal NCs confer remarkable prospects in the biological application field.^23–27 The conventional ECL luminophores, such as Ru(bpy)₃²⁺/triethylamine (TEA) systems and quantum dots, have limited usefulness due to their poor biocompatibility and high toxicity (Table 1).^28–30 More importantly, diverse luminescent centers and target effects of the metal–organic ligand shell layer greatly enhance sensitivity and responsiveness. However, the high trigger potential and limited ECL efficiency of metal NCs severely limit their application in practice, mainly because of the following: 1 complex charge transfer during electrochemical processes; 2 elusive oxidant–reductant interactions; 3 the unclear non-radiative relaxation mechanism during the excited state electron transition, where the nuclear size, nuclear structure, nuclear composition, ligands, the charge state of metal nanoclusters and solvent as well as the electrolyte could influence their electrochemiluminescence properties.^31–33 In this regard, the ECL efficiency of metal nanoclusters has been improved by various methods, but it is still unsatisfactory.

Table 1 Comparison of properties of different substances within each system

	Nanomaterial systems						Inorganic system	Organic system
	Metal nanoclusters	QDs	Carbon dots	Polymer dots	MOF	Luminescent cluster doped SiO2 nanoparticles	Ru(bpy)3^2+	Luminol	Organic nanoparticles
Solubility	Better	Moderate	Moderate	Moderate	Moderate	Moderate	Better	Better	Moderate
Poisonous	Low	High	Low	Low	Low	Moderate	High	Moderate	Moderate
Redox reversibility	Reversible/irreversible	Irreversible	Irreversible	Reversible/irreversible	Reversible/irreversible	Reversible	Reversible	Reversible/irreversible	Reversible/irreversible
ECL type	Anodic/cathodic	Anodic/cathodic	Anodic/cathodic	Anodic	Anodic/cathodic	Anodic	Anodic/cathodic	Anodic/cathodic	Anodic/cathodic
Wavelength band	Visible/infrared	Visible	Visible/infrared	Visible	Visible	Visible	610 nm	425 nm	Visible
ECL stability	Moderate	Moderate	Mediocre	Mediocre	Moderate	Moderate	Better	Better	Moderate
ECL efficiency	Dozens of times	Tens of thousands of times	<1	Several times	Several times	Several times	1	1	—
Biocompatibility	Better	Mediocre	Moderate	Moderate	Moderate	Moderate	Better	Moderate	Better

---

In this review, we begin with an illustration of the ECL mechanism for metal NCs and then parse methods of improving ECL, including 1 aggregation-induced emission; 2 anodic pre-oxidation; 3 aggregation-induced self-loading; 4 anionic substitution; 5 ion doping; 6 valence engineering. Finally, for the application of metal nanocluster ECL in the field of biology, the framework of ECL application of metal nanoclusters is established to expand the scope of its application by looking at both biosensors and diagnostics. Novel sensing methods, signal amplification strategies, and simplified procedures were introduced to build highly stable, sensitive, and selective platforms of metal nanoclusters. We provide a comprehensive mechanism and exploration for the development of high-efficiency ECL metal nanoclusters, which will be helpful for the development of metal nanoclusters in the field of electrochemiluminescence.

2 ECL properties of metal nanoclusters

ECL involves the generation of free radical substances in the vicinity of the electrode. These radicals then undergo an electron transfer reaction to form an excited state and then emit light when the electron returns to the ground state. The annihilation and co-reaction pathways constitute the two main pathways of the ECL.³⁴ In the annihilation pathway, the excited state material originates from an electron transfer reaction from an electrically generated radical anion to a cation.³⁵ The general mechanism of NCs is listed below:

NCs − e → NCs^*+ (1)

NCs − e → NCs^*− (2)

NCs^*+ + NCs^*− → NCs^* + NCs (3)

NCs^* → NCs + hv. (4)

Rapid changes in the potential of the working electrode between two different values have generated the oxidizing substance NCs^*+ and the reducing substance NCs^*− (eqn (1) and (2), respectively), which will react near the surface of the electrode to form the excited state NCs^* (eqn (3)). Then the excited state NCs^* reverts to the ground state and emits light (eqn (4)).

In the co-reactant pathway, excited state substances are produced by electron transfer reactions between redox or oxidation intermediates (derived from co-reactants) and radical cations or anions of the luminophore.^36,37 The general mechanism of NCs^* is listed below (co-reactants are exemplified by oxalate ions (C₂O₄²⁻)):

C₂O₄²⁻ − e → [C₂O₄^*−] → CO₂^*− + CO₂ (5)

NCs − e → NCs^*+ (6)

NCs^*+ + CO₂^*− → NCs^* + CO₂ (7)

NCs^* → NCs + hv. (8)

Oxalate ions produce a strong reductant CO₂^*− when oxidized (eqn (5)), and NCs produce oxidized substances NCs^*+ at the anode (eqn (6)), after which NCs^*+ reacts with CO₂^*− on the electrode surface to form an excited state NCs^* (eqn (7)). Then excited state electrons return from the excited state to the ground state and emit light (eqn (8)). If the free radical substance is chemically unstable or too short-lived, ECL cannot occur.

The first case of ECL metal nanoclusters was reported in 2009, i.e., polymethacrylic acid-coated AgNCs containing a small number of atoms.³⁸ Metal nanoclusters with variable valence states can be used as electron donors or acceptors as well as multifunctional light emitters to generate cathodic or anodic ECL.²⁷ If the redox potential of the metal nanoclusters is close to the redox potential of the co-reactants, their ECL strength will be greatly increased. Highly triggered potentials and limited ECL efficiency (Φ_ECL), due to the inhibition of charge transfer during electrochemical processes and the increase of non-radiative leaps during energy relaxation, are still the main impediments to the widespread use of metal nanoclusters for ECL. To expand the application of metal nanoclusters, low emission potential and high emission efficiency should be realized.

The main detection techniques for electrochemiluminescence of metal nanoclusters are cyclic voltammetry (CV), which determines the potential at which the desired reactants can be produced, and differential pulse voltammetry (DPV), which measures the current before the potential changes to minimize the effect of the charging current. The simplicity of their instrumentation and the low cost of testing make them more widely applicable to a wider range of environments.^35–38

Thus, ECL metal nanoclusters also benefit from their simple testing technique to become a powerful tool for bioanalysis and clinical diagnostics.^39,40 The ECL quantum efficiency (Φ_ECL) is described as the ratio of the number of photons emitted to the number of photons in the chemiluminescence reaction between redox substances.⁴¹Φ_ECL was obtained by relevant literature methods with Ru(bpy)₃²⁺ as the reference system.⁴²Φ_ECL can be calculated by the following equation:⁴³

where is the ECL efficiency of Ru(bpy)₃²⁺, which is 5.0% for Ru(bpy)₃²⁺ (1 mM) in TBAP/CAN (0.1 M), I and I° are the ECL intensities of the integral metal nanoclusters and Ru(bpy)₃²⁺, and Q_f and are the pass-through charge of metal nanoclusters and Ru(bpy)₃²⁺, respectively.^44,45Table 2 compares the ECL performance of previously reported ECL emitters and shows that the ECL performance of metal nanoclusters is universally low.

Table 2 Comparison of ECL efficiency of previously reported ECL transmitters

Emitter	Co-reactants	ECL	Standard (100%)	Φ ECL (100%)	Ref.
Cd-ln-S NCs	TPA	Anodic	Ru(bpy)3^2+/TPA	2.1	46
Cd-ln-S NCs	K2S2O8	Cathodic	Ru(bpy)3^2+/S2O8^2−	0.13	46
Met-AuNCs	K2S2O8	Cathodic	Ru(bpy)3^2+/S2O8^2−	2.23	47
BSA-AuNCs	K2S2O8	Cathodic	Ru(bpy)3^2+/S2O8^2−	0.33	47
NAC-AuNCs	K2S2O8	Cathodic	Ru(bpy)3^2+/S2O8^2−	4.11	31
DNA-CuNCs	K2S2O8	Cathodic	Ru(bpy)3^2+/S2O8^2−	46.8	48
GSH-AuNCs	TEA	Anodic	Ru(bpy)3^2+/TEA	0.42	49
BSA-AuNCs	TEA	Anodic	Ru(bpy)3^2+/TEA	9.8	49
Ox-Met-AuNCs	TEA	Anodic	Ru(bpy)3^2+/TEA	66.1	49
ATT-AuNCs	TEA	Anodic	Ru(bpy)3^2+/TEA	78	47
MPP-CuNCs	TPA	Anodic	Ru(bpy)3^2+/TPA	0.74	50
BSA-CuNCs	K2S2O8	Cathodic	Ru(bpy)3^2+/S2O8^2−	50.2	51
CuNCsAssy	TEA	Anodic	Ru(bpy)3^2+/TEA	10	52
Au12Ag13NCs	TEA	Anodic	Ru(bpy)3^2+/TPA	40	53
BSA-AuNCs	DIPEA-OH	Anodic	Ru(bpy)3^2+/TPA	68.17	54
Au-LA-NCs	DEDA	Anodic	Ru(bpy)3^2+/TPA	17	21
Arg-ATT-AuNCs	TPA	Anodic	Ru(bpy)3^2+/TPA	67.02	23
Co^2+-AuNCs	TEA	Anodic	Ru(bpy)3^2+/TPA	33.8	55
Zn^2+-MHA-AuNCs	K2S2O8	Anodic	Ru(bpy)3^2+/S2O8^2−	10.54	56
Au20-AC	TBBT	Anodic	Ru(bpy)3^2+/TPA	7	57
(Tmead)6Zn8Mn6Se13C12	K2S2O8	Cathodic	[Ru(bpy)3](PF)6/S2O8^2−	0.3	58
(Tmead)6Zn14Se13C12	K2S2O8	Cathodic	[Ru(bpy)3](PF)6/S2O8^2−	0.3	58
Polymer dots	TPA	Anodic	Ru(bpy)3^2+/TPA	23.1	59
CdSe nanocrystals	CHCl2	Anodic	Ru(bpy)3^2+/TPA	2.5	60
[(bpy)2Ru]2(bphb)^2+	TPA	Anodic	Ru(bpy)3^2+/S2O8^2−	5.8	61
Copper metal cluster	TPA	Cathodic	Ru(bpy)3^2+/TPA	0.74	50
[((phen)2Ru(dpp))2RhCl2]^5+	TPA	Cathodic	Ru(bpy)3^2+/TPA	1.4	62
Iridium(III) complexes	BPO	Cathodic	Ru(bpy)3^2+/TPA	25	63

---

3 ECL-enhanced emission strategy for metal nanoclusters

Because of low quantum yield (Φ_ECL) and poor mechanistic understanding, enhancing the ECL response has been long pursued. So, this section summarizes the design strategy to achieve efficient emission from metal nanocluster ECL through aggregation-induced emission (AIE), anodic pre-oxidation, aggregation-induced self-assembly, anion substitution, ion doping, and metal valence engineering, aiming to provide a comprehensive analysis of ECL promotion.

3.1 Aggregation-induced emission (AIE)

Aggregation induced by ligand/ligand and ligand/metal interactions is an effective way to enhance ECL of metal nanoclusters through suppressing intermolecular vibrations and rotations.^64,65 Since Cola reported the first aggregation-induced ECL material, i.e., Pt complexes, various material systems based on aggregation-induced emission (AIE) were developed, such as metal complexes (Ir), polymer dots, organic dots, and metal nanoclusters.^66–72

Peng et al. proposed that drying AuNCs on electrodes can enhance electrochemical excitation through electrocatalytic effects and induce ECL-enhanced emission through the aggregation of 6-aza-thiothymidine (ATT) AuNCs and co-reactant triethylamine (TEA).⁴⁷ As shown in Fig. 1, the dry ATT-AuNCs/TEA system produces a highly stable ECL with an emission efficiency of 78% based on a simultaneous enhancement of both electrochemical excitation and emission efficiency. Conventional electrochemical luminophores are mainly polycyclic aromatic hydrocarbons and metal complexes.⁷³ Inspired by the enhanced photoluminescence through rigid shell layers, Yang and co-workers achieved host–guest assemblies on the surface of AuNCs for the first time, where L-arginine (ARG) was branched into ATT-protected AuNCs (ARG-ATT-AuNCs) conferring water-soluble ARG-ATT-AuNCs with a significantly enhanced ECL compared to ATT-AuNCs. More importantly, one cathodic ECL process (1.30 V) was observed in the so-called “half-scan” experiments without co-reactants and three anodic ECL processes, and a 70-fold enhanced redox ECL (0.78 V) with tri-n-propylamine as the co-reactant. The host–guest assembly formed by hydrogen bonding between the ATT of ARG could impede intramolecular vibrations and rotations to reduce the nonradiative relaxation of the excited state.^74–76 Annihilation processes between electrogenerated anionic and cationic radicals of NCs can generate excited states of ECL.³⁵ Thus ECL of NCs can be achieved by pulsing or cycling between oxidation and reduction potentials.⁷⁷ No annihilated ECL was detected for ATT-AuNCs compared to the apparent annihilated ECL for ARG-ATT-AuNCs, suggesting that the rigid host–guest assembly on the surface of ARG and ATT reduces the nonradiative transition of the ECL process, thus improving the luminescence performance. The redox ECL of ATT-AuNCs and ARG/ATT-AuNCs has a single emission peak at around 532 nm and a full width at half maximum (FWHM) of about 36 nm, which is very close to the photoluminescence (PL) spectra of ATT-AuNCs and ARG-ATT-AuNCs. Peng et al. recently realized a host–guest structure through hydrogen-bonding interactions, which is also similar to the former. This structure restricts the rotation of the ligand molecule and inhibits non-radiative leaps, thus improving the ECL efficiency.⁷⁸

Fig. 1 Schematic illustration of aggregation-enhanced ECL through the dry method.

Shen et al. reported the high-efficiency ECL through Zn²⁺-induced aggregation of Zn²⁺-MHA-AuNCs (MHA, mercaptohexanoic acid), whose Φ_ECL was as high as 10.54% compared to the Ru(bpy)₃²⁺/S₂O₈²⁻ system.⁵⁶ The ECL mechanism of the Zn²⁺-MHA-AuNCs and the co-reactor S₂O₈²⁻ system was investigated by differential pulse voltammetry (DPV) and cyclic voltammetry (CV). In the absence of co-reactants, the DPV test showed two reduction peaks (−1.56 V and −1.8 V). While three peaks (−1.15 V, −1.56 V and −1.78 V) appeared after the introduction of S₂O₈²⁻ and an intense ECL signal was obtained at −1.78 V. Zn²⁺ ions induce aggregation of monodisperse AuNCs, restrain the ligand vibrations of AuNCs, and serve as a co-reaction accelerator that catalyze the dissociation of the co-reactant S₂O₈²⁻ into sulfate (SO₄²⁻) to increase the efficiency of the interactions between Zn²⁺-MHA-AuNCs and S₂O₈²⁻, thus improving ECL efficiency finally.

3.2 Anodic pre-oxidation

Besides decreasing the non-radiative relaxation, another way to realize efficient ECL is by reducing the oxidation potential. Anodic pre-oxidation can effectively reduce the oxidation potential by oxidizing the host material at the electrode first and adding co-reactants subsequently.^79,80 In addition, pre-oxidized ECL excitation requires a lower potential than conventional methods and reduces additional operation.

Peng and co-workers reported that the ECL of L-methionine (MET) stabilized AuNCs, using triethylamine (TEA) as a co-reactant, was significantly enhanced by single-electrode pre-oxidation (Fig. 2).⁴⁹ Electrochemical oxidation was employed to oxidize Met-AuNCs to obtain oxidized Ox-Met-AuNCs. The Φ_ECL of Ox-MET-AuNCs was up to 66%. Exploring the anodic ECL behavior and mechanism of Met-AuNCs found that electrochemical pre-oxidation enhanced the ECL signal. Starting near 0.6 V, the ECL intensity reaches a maximum near 0.97 V, which is consistent with the oxidation potential of the CV response. The low potential required is superior to that of other ECL materials including dithiothreitol (DTT)-AuNCs, bovine serum albumin (BSA)-AuNCs, calcite quantum dots, and graphene quantum dots.^81,82 Of note, the ECL signal is also influenced by the rate of electron transfer from the ligand shell to the metal nucleus. Strong and stable ECL signals were also generated by peroxidation of dithiothreitol (DTT)-AuNCs and bovine serum albumin (BSA)-AuNCs.⁸³ The ECL efficiency was enhanced 20.3-fold and 5-fold, respectively. The electron-rich ligand shell allows for more efficient electron transfer and thus enhanced ECL.

Fig. 2 Schematic illustration of the enhanced ECL performance after pre-oxidation treatment.

3.3 Aggregation-induced self-assembly

Aggregation-induced self-assembly can improve the overall electron transfer efficiency by enhancing ordering.^84–87 The metal NCs tend to realize the assembled structure through the interactions between ligands, such as hydrogen bonding, van der Waals forces, and π–π stacking, which not only enhances the ordering of the clusters, but also improves their luminescence properties.^88,89

Copper nanoclusters (CuNCs) have attracted much interest in ECL applications due to their relatively low cost.⁹⁰ Pyrimidine derivatives, which are aromatic heterocyclic scaffolds with two nitrogen atoms in the ring, have been widely used in the construction of organic semiconductors and organic thin-film transistors due to their excellent electron transport properties. Aryl rings on pyrimidine derivatives have a strong tendency to form assembly via π–π interactions.⁹¹ Sun et al. reported a simple gram-scale method to prepare self-assembled CuNCs (CuNCs_Assy) using 4,6-dimethyl-2-hydrophobic pyrimidine (DMMP) as a reducing agent.⁵² CuNCs_Assy shows an anodic ECL efficiency of 10% relative to Ru(bpy)₃Cl₂/TEA, which is higher compared to disordered aggregates. The disordered structure of the aggregated metal nanoclusters is unconducive to efficient long electron transfer path thus greatly limiting the further enhancement of ECL. Due to the π–π interaction of the ligand-induced subsequent self-assembly arrangement of the CuNCs, the higher oxidation current suggests a more efficient electron transport. CuNCs_Assy further hinders the oxidation of Cu(0)/Cu(I) enabling the conversion of Cu (Cu–S/Cu–O) in CuNCs, and thus the tighter stacking of ligands after assembly, enhancing the stability of CuNCs_Assy.

Wei et al. also successfully employed a self-assembly-induced enhancement strategy to enhance the cathodic ECL performance of flexible ligand-stabilized CuNCs.⁹² Specifically, CuNCs form an ordered lamellar structure through intermolecular forces. Here, the limitation of ligand torsion in this self-assembled structure leads to a significant improvement in the ECL performance of CuNCs. The cathodic ECL emission of the assembled nanoscale CuNCs sheets increased approximately threefold compared to that of CuNCs in the dispersed state.

DNA is an attractive functional polymer for assembling nanoparticles into rigid framework structures due to its programmable interactions and highly tunable structure.^93,94 In particular, the advent of DNA nanotechnology has paved the way for the precise organization of nanoparticles, proteins, fluorescent dyes, and polymers in the limited space of the nanoscale.^95–101 Thus, programmable DNA nanostructures can be used to fine-tune nanocluster aggregation. Ouyang and co-workers realized the ordered self-assembly of copper nanoclusters (CuNCs) using “framework nucleic acids” – DNA nanosheets and DNA nanoribbons as templates.⁴⁸ Compared to discrete CuNCs (dsDNA/CuNCs), the assembly exhibits (DNR/CuNCs) enhanced fluorescence quantum yield (∼62-fold) and higher ECL efficiency (∼68-fold) with the addition of co-reactants (Fig. 3). The ECL intensity of DNR/CuNCs at 519 nm is about 3.5-fold stronger than that of dsDNA/CuNCs in aqueous medium and the corresponding potential was positively shifted from −2.0 V to −1.7 V. This strategy was realized by DNA as templates for constructing ordered assembly of CuNCs, and due to the accelerated electron transfer reaction and reduced bandgap, CuNCs exhibited stronger ECL compared to dsDNA/CuNCs in both annihilation and co-reaction ECL.

Fig. 3 Schematic illustration of the ECL process for DNR/CuNCs and dsDNA/CuNCs.

3.4 Anionic substitution

Appropriate modulation of co-reducers is also an effective way to enhance the ECL luminescence. Hong et al. developed high-performance and low-potential ECL AuNCs based on a co-reactant-mediated approach.⁵⁴ The BSA-AuNCs/co-reactant 2-(diisopropylamine) ethanol (DIPEA-OH) system achieves high-efficiency ECL up to 68.17% and a lower potential at 0.75 V. Benefiting from the isopropyl substitution and carboxyl addition of triethylamine (TEA), DIPEA-OH as the co-reactant were 13-fold compared to the original BSA-AuNC/TEA system. For the BSA-AuNCs/DIPEA-OH system, the anodic current increases linearly with the scan rate, indicating that the electrochemical oxidation of DIPEA-OH on BSA-AuNCs/AuE is a surface-controlled process.⁴⁷ Through using DIPEA-OH^*+ and BSA-AuNCs at high concentrations on the electrode surface and under the potential cycling range 0 to 0.8 V, high reaction efficiency was realized between DIPEA-OH^*+ and BSA-AuNCs. It is noteworthy that the oxidation potential of DIPEA-OH is +0.71 V, whereas the voltage of BSA-AuNC is +1.39 V. In the low potential window of 0–0.8 V, only DIPEA-OH can be electrochemically oxidized on the electrode surface, which is different from the classical “redox” pathway. However, AuNCs can be chemically oxidized to AuNCs⁺ by the strong oxidant DIPEA-OH˙⁺ (Fig. 4).^101,102

Fig. 4 Schematic illustration of the ECL enhancement mechanism by anionic substitution.

Yuan et al. obtained different ECL signals by selecting different co-reactants (e.g., H₂O₂, TPA, and hydrazine (Hz)).¹⁰³ The ECL of BSA-CuNCs with TPA and H₂O₂ as co-reactants was improved compared to the ECL of pure BSA-CuNCs. When HZ is used as a co-reactant, the ECL response significantly increases from 60 a.u. to 2090 a.u. (∼34 fold). The strong ECL of BSA-CuNCs was also detected using HZ-modified glassy carbon electrodes (HZ/GCE) in the potential scanning range of 0–1.45 V. Therefore, the selection of effective co-reactants can significantly improve the luminescence efficiency of NCs.

Wei et al. suggested that SbF₆⁻ is important for customizing both the structure and performance of [Pt₁Ag₂₈(S-Adm)₁₈(PPh₃)₄] Cl₂ (Pt₁Ag₂₈-Cl).¹⁰⁴ X-ray crystallography was used to monitor the transition from Pt₁Ag₂₈-Cl to [Pt₁Ag₂₈(S-Adm)₁₈(PPh₃)₄] (SbF₆)₂ (Pt₁Ag₂₈-SbF₆) and then to [Pt₁Ag₃₀Cl₁(S-Adm)₁₈(PPh₃)₃] (SbF₆)₃ (Pt₁Ag₃₀-SbF₆). By controlling the amount of introduced SbF₆⁻, they found that the presence of SbF₆⁻ not only reconfigures the structure but also rearranges the superlattice. The ultra-bright Pt₁Ag₃₀-SbF₆ exhibits an unprecedented absolute quantum yield 120 times higher than that of Ru(bpy)₃²⁺, and Pt₁Ag₂₈-SbF₆ exhibits a 55-fold higher ECL quantum yield than that of Ru(bpy)₃²⁺, highlighting its superior electrowinning emission capability at submicromolar concentrations. The high efficiency could be further supported by the increasing reactive surface conducive to the electron transfer reaction.

3.5 Ion doping

Doping heterogeneous metals in AuNCs can change the electronic structure and enhance optical properties and synthetic controllability. The synergistic effect of the metal elements leads to the facilitation of the ECL response as well as the alteration of the charge recombination pathway.^105–107

Li et al. doped AuNCs with Ag to form Au–Ag bimetallic nanoclusters (BNCs) using methionine (Met) as a ligand (Met-AuNCs) and triethanolamine (TEOA) as a co-reactant. The Au–Ag BNCs showed a 54-fold enhancement of ECL and a 3-fold enhancement of PL in aqueous media. This suggests that the synergistic effect of metal NCs is favorable for enhancing ECL induced by electrochemical redox. The synergistic effect not only enabled Au–Ag NCs to enhance ECL through gap-engineering at around 520 nm, but also produced strong ECL induced by surface defects at around 710 nm.¹⁰⁸

Wei et al. developed a ligand modulation strategy to enhance the anodic ECL of CuNCs. Specifically, we utilized ovalbumin (OVA), a naturally occurring protein, as a stabilizing ligand for CuNCs and introduced terbium ions into the ligand. This approach optimizes the ECL performance by improving the electron transfer efficiency and suppressing non-radiative relaxation.¹⁰⁹

Jia et al. proposed a Co²⁺ doping strategy in cysteamine and N-acetyl-L-cysteine co-stabilized AuNCs, which not only generates tunable hole injection channels for ECL emission but also reduces surface defects to facilitate electron transfer.⁵⁵ The obtained Co²⁺ doped NCs have a lower anodic emission potential and higher ECL efficiency compared to original AuNCs (Fig. 5). Particularly, Co²⁺ doped NCs possess near-infrared emission, which showed significant value in biological applications. Differential pulse voltammetry (DPV) was used to examine its anodic and cathodic processes to better understand the electrochemical behavior. The enhanced ECL strength of Co²⁺-AuNCs compared to original AuNCs is related to the synergistic effect of Au and Co.^110,111 The ECL quantum yield of Co²⁺-AuNCs was 33.8% as standardized by the Ru(bpy)₃²⁺/TPrA system.

Fig. 5 Schematic illustration of the ECL enhancement mechanism by ion doping.

3.6 Valence engineering

The valence state of metal elements is always an important factor affecting the emission efficiency of ECL. Peng et al. elucidated the effect of the valence state on the ECL properties of N-acetyl-L-cysteine-protected Au clusters.³¹ When K₂S₂O₈ was used as a co-reactant, the Φ_ECL value of N-acetyl-L-cysteine-protected AuNCs could reach 4.11%. The results show that the oxidation state of Au clusters is dependent on the reduction potential, and the ECL signal is dependent on the oxidation state of Au clusters.

Wang proposed a valence engineering strategy to explore its effects on the ECL of metal NCs. Based on bovine serum albumin (BSA) stabilized AuNCs, the valence of the Au element was regulated by reduction of the precursor AuCl₄⁻ with BSA. The BSA-AuNCs exhibit four different internal structures, i.e., different ratios of Au(I) and Au(0), which have different effects on anodic ECL processes (Fig. 6).¹¹² Interestingly, this valence engineering can modulate the source of the ECL from the surface state emission to the defect state emission with smaller trigger potentials. Valence engineering demonstrates that metal valence is crucial for achieving efficient and low trigger potential ECL.

Fig. 6 Schematic illustration of valence engineering ECL.

4 Applications of metal nanocluster ECL

4.1 Metal nanoclusters ECL in biosensor applications

Nowadays, huge efforts have been devoted to the practical application based on the ECL metal nanoclusters and remarkable progress has been achieved. Benefiting from processable selectivity and biocompatibility, ECL metal nanoclusters have been widely used as biosensors. Jie et al. reported a multifunctional ECL Ag cluster probe (persulfate as co-reactor and Fe₃O₄-CeO₂ as co-reaction accelerator) for the detection of thrombin in 0.1 M PBS (pH 7.4) containing 0.05 M K₂S₂O₈ and 0.1 M KCl.¹¹³ The ECL biosensor was designed based on a DNAzyme-assisted recovery target strategy and hybridization chain reaction amplification strategy, which exhibits excellent sensitivity for the detection of both thrombin and Cyclin-D1.¹¹⁴ Li et al. synthesized Au–Ag alloy nanoclusters using bovine serum albumin (BSA) as a ligand, which showed a highly efficient ECL and thus served as a biosensor for noninvasive glucose detection through saliva specimen.¹¹⁵ Sun et al. reported a simple gram-scale method to prepare self-assembled CuNCs (CuNCs_Assy) using 4,6-dimethyl-2-hydrophobic pyrimidine (DMMP) as a reducing agent.⁵² ALP, one of the most important hydrolytic enzymes in the mammalian liver, body fluids, and tissues, represents an important biomarker for the clinical diagnosis of bone and hepatobiliary diseases. The limit of detection of the established ECL biosensor was 8.1 × 10⁻⁶ U L⁻¹ (S/N = 3), which has been superior to date. Interestingly, a ferrocene-driven “on–off–on” biosensor was also achieved for the detection of amyloid-β through a composite BSA-protected Ag cluster and titanium oxide nanoflower as signal labels.¹¹⁶ The selectivity of the composite was confirmed by the comparison of several potential interferences, including galactosidase, tyrosinase, glucose oxidase, peroxidase, catalase, and protease. Weng et al. proposed a high-performance enzyme-linked immunosorbent assay (ELISA) ECL, which combines the advantages of a high-quantum-yield ECL gold nanocluster and an efficient ECL resonance energy transfer (ECL-RET) strategy. The ECISA technique through recyclable AuNC probes and MnO₂ nanomaterials, boosts the ECL at the interface of MnO₂/AuNCs-modified glassy carbon electrodes. Then the ECL signal was recovered by etching the MnO₂ nanomaterials with the products of ALP-based ELISA.^117–119

Yuan et al. greatly stimulated the promising applications of efficient ECL metal NCs by gaining insights into the critical role of ligands in enhancing the stability of reactive co-reactants with free radicals. Using β-CD-Au NCs as emitters, a “signal-off” ECL sensing platform was constructed to detect norepinephrine as a model target with a lower detection limit of 0.91 nM.¹²⁰

A dopamine biosensor was fabricated based on the ECL properties of methionine-protected gold clusters.¹²¹ The GCE (Glassy Carbon Electrode) modified by the NC exhibited a linear relationship between ECL intensity and dopamine concentration in the range of 1.0 × 10⁻⁷ to 4.0 × 10⁻⁶ M, with a high detection limit of 3.2 × 10⁻⁸ M. Another dopamine detection platform was later developed on a copper cluster/hydrazine system, which showed a linear relationship between the ECL intensity and the dopamine concentration in the range of 1.0 × 10⁻¹² to 1.0 × 10⁻⁸ M.¹⁰³ The detection limit was as low as 3.5 × 10⁻¹³ M with high selectivity and good stability. In addition, DNA/CuNCs are also sensitive to dopamine after copper nanocluster assembly was induced by the DNA nanoband template.

With the development of versatile sensing methods and signal amplification strategies, ECL has evolved into an indispensable analysis technique beyond biosensors. Metal nanoclusters (NCs) are expected as versatile probes based on overcoming their inefficiencies and revealing their mechanisms.

4.2 Metal nanocluster ECL in biodiagnostic applications

The ECL-responsive behavior of metal nanoclusters has a targeting effect on the etiology of specific diseases and thus has also been widely used in bio-diagnostics. Highly efficient ECL emitters were developed for the ultrasensitive detection of biomolecules for the early diagnosis of cancer. MicroRNA-21 is a carcinogen that is expressed in a variety of cancers. It is closely related to cancer development, apoptosis, inhibition, proliferation, and drug resistance. Therefore, accurate detection of microRNA-21 is becoming increasingly important. Shen et al. used Zn²⁺-induced gold cluster aggregation (Zn²⁺-MHA-AuNCs) as a high-efficiency ECL emitter for the ultra-sensitive detection of microRNA-21 (miRNA-21).⁵⁶ The strong adsorption affinity between polyA and AuNPs enabled a detection limit of 44.7 aM miRNA-21.

Yuan et al. constructed a fast target-triggered catalyst hairpin assembly (CHA) cycle amplification strategy by ordering equally spaced hairpins to increase their local concentration, greatly accelerating the signal amplification efficiency and reaction rate.¹²² Based on Met/NAC-CuNCs as ECL emitters and an effective signal amplification strategy, an ultrasensitive ECL biosensor was fabricated to detect the target MMP-2 with a limit of detection (LOD) as low as 1.65 fg mL⁻¹ and was successfully used for the detection of MMP-2 from HeLa and MCF-7 cancer cells. This study provides a good opportunity to modulate the metal nanocluster-based ECL emitters’ optical performance of metal nanocluster-based ECL emitters.

Enolase (NSE), around 12 ng mL⁻¹ in serum samples, is a perfect biomarker for the diagnosis of small cell lung cancer (SCLC).¹²³ The majority of SCLC patients have serum NSE concentrations between 12.5 and 25 ng mL⁻¹.^124,125 Benefiting from the enhanced NIR ECL emission and the excellent biocompatibility of Co²⁺-AuNCs, a sensitive “signal-on” immunosensor based on neuron-specific NSE was proposed with a limit of detection (LOD) of 0.16 fg mL⁻¹ (S/N = 3) and a linear range of 0.5 fg mL⁻¹ to 1 ng mL⁻¹.⁵⁵ More importantly, the proposed sensitive immunosensing method for NSE detection showed advantages of a wide concentration range, low detection limit, good specificity and good utility, demonstrating excellent application potential in bioanalysis through ECL properties. Applying metal clusters in the biological field based on their good biocompatibility, low toxicity, and ligand-targeting provides a brand-new idea for constructing ultrasensitive and selective platforms.^126–129

5. Conclusions

Metal nanoclusters are particularly prominent as novel ECL materials for their redox properties and unique optical properties. Electro-pumping kinetics, oxidant-reducer interactions, and non-vibrational relaxation of clusters play a crucial role in achieving efficient ECL. This article summarizes the design strategy to achieve efficient emission from metal nanocluster ECL through aggregation-induced emission (AIE), anodic pre-oxidation, aggregation-induced self-assembly, anion substitution, ion doping, and metal valence engineering, aiming to provide a comprehensive analysis of ECL promotion. In biosensors and biodiagnostics, novel sensing methods, signal amplification strategies, and simplified procedures are expected to build highly stable, sensitive, and selective platforms based on metal nanoclusters. We provide a comprehensive mechanism and exploration for the development of high-efficiency ECL metal nanoclusters, which will be helpful for the development of metal nanoclusters in the field of electrochemiluminescence.

Author contributions

The manuscript was written through the contributions of all authors. All authors have approved the final version of the manuscript.

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.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (T2325015, U21A2068, 61935009, 12174151, and 12304448).

References

1. R. W. Murray, Nanoelectrochemistry: metal nanoparticles, nanoelectrodes, and nanopores, Chem. Rev., 2008, 108, 2688–2720.
2. L. Qian, Z. Wang, E. V. Beletskiy, J. Liu, H. J. Dos Santos, T. Li, M. d, C. Rangel, M. C. Kung and H. H. Kung, Stable and solubilized active Au atom clusters for selective epoxidation of cis-cyclooctene with molecular oxygen, Nat. Commun., 2017, 8, 14881.
3. X. Chen, X. Ren and X. Gao, Peptide or Protein-Protected Metal Nanoclusters for Therapeutic Application, Chin. J. Chem., 2022, 40, 267–274.
4. Q. He and T.-T. Li, Tandem Electroreduction of CO₂ to C²⁺ Products based on M-SACs/Cu Catalyst, Chem. – Eur. J., 2025, 31, e202403297.
5. M. Qu, F. Xue, J. Y. Wei, M. M. Qiao, W. Q. Ren, S. L. Li and X. M. Zhang, Kernels-Different Au25Nanoclusters Enhanced Catalytic Performance via Modification of Ligand and Electronic Effects, Chin. J. Chem., 2022, 40, 2575–2581.
6. M. M. Richter, Electrochemiluminescence (ecl), Chem. Rev., 2004, 104, 3003–3036.
7. R. Jin, C. Zeng, M. Zhou and Y. Chen, Atomically Precise Colloidal Metal Nanoclusters and Nanoparticles: Fundamentals and Opportunities, Chem. Rev., 2016, 10346.
8. J. Xie, Y. Zheng and J. Y. Ying, Protein-directed synthesis of highly fluorescent gold nanoclusters, J. Am. Chem. Soc., 2009, 131, 888–889.
9. Q. Yao, X. Yuan, V. Fung, Y. Yu, D. T. Leong, D.-E. Jiang and J. Xie, Understanding seed-mediated growth of gold nanoclusters at molecular level, Nat. Commun., 2017, 8, 927.
10. Q. Yao, Y. Yu, X. Yuan, Y. Yu, D. Zhao, J. Xie and J. Y. Lee, Counterion–Assisted Shaping of Nanocluster Supracrystals, Angew. Chem., 2015, 127, 186–191.
11. A. Zanut, A. Fiorani, S. Canola, T. Saito, N. Ziebart, S. Rapino, S. Rebeccani, A. Barbon, T. Irie and H.-P. Josel, Insights into the mechanism of coreactant electrochemiluminescence facilitating enhanced bioanalytical performance, Nat. Commun., 2020, 11, 2668.
12. P. Wu, X. Hou, J.-J. Xu and H.-Y. Chen, Electrochemically generated versus photoexcited luminescence from semiconductor nanomaterials: bridging the valley between two worlds, Chem. Rev., 2014, 114, 11027–11059.
13. Z. Liu, W. Qi and G. Xu, Recent advances in electrochemiluminescence, Chem. Soc. Rev., 2015, 44, 3117–3142.
14. Z. Cao, Y. Shu, H. Qin, B. Su and X. Peng, Quantum dots with highly efficient, stable, and multicolor electrochemiluminescence, ACS Cent. Sci., 2020, 6, 1129–1137.
15. L. Li, Y. Chen and J.-J. Zhu, Recent advances in electrochemiluminescence analysis, Anal. Chem., 2017, 89, 358–371.
16. L. Zheng, Y. Chi, Q. Shu, Y. Dong, L. Zhang and G. Chen, Electrochemiluminescent reaction between Ru(bpy)₃²⁺ and oxygen in nafion film, J. Phys. Chem., 2009, 113, 20316–20321.
17. G. Xu, X. Zeng, S. Lu, H. Dai, L. Gong, Y. Lin, Q. Wang, Y. Tong and G. Chen, Electrochemiluminescence of luminol at the titanate nanotubes modified glassy carbon electrode, Luminescence, 2013, 28, 456–460.
18. M. Mayer, S. Takegami, M. Neumeier, S. Rink, A. Jacobi von Wangelin, S. Schulte, M. Vollmer, A. G. Griesbeck, A. Duerkop and A. J. Baeumner, Electrochemiluminescence bioassays with a water-soluble luminol derivative can outperform fluorescence assays, Angew. Chem., Int. Ed., 2018, 57, 408–411.
19. Z. Cai, F. Li, W. Xu, S. Xia, J. Zeng, S. He and X. Chen, Colloidal CsPbBr₃ perovskite nanocrystal films as electrochemiluminescence emitters in aqueous solutions, Nano Res., 2018, 11, 1447–1455.
20. X. Tan, B. Zhang and G. Zou, Electrochemistry and electrochemiluminescence of organometal halide perovskite nanocrystals in aqueous medium, J. Am. Chem. Soc., 2017, 139, 8772–8776.
21. T. Wang, D. Wang, J. W. Padelford, J. Jiang and G. Wang, Near-infrared electrogenerated chemiluminescence from aqueous soluble lipoic acid Au nanoclusters, J. Am. Chem. Soc., 2016, 138, 6380–6383.
22. S. Chen, Y. Lv, Y. Shen, J. Ji, Q. Zhou, S. Liu and Y. Zhang, Highly sensitive and quality self-testable electrochemiluminescence assay of DNA methyltransferase activity using multifunctional sandwich-assembled carbon nitride nanosheets, ACS Appl. Mater. Interfaces, 2018, 10, 6887–6894.
23. L. Yang, B. Zhang, L. Fu, K. Fu and G. Zou, Efficient and Monochromatic Electrochemiluminescence of Aqueous-Soluble Au Nanoclusters via Host–Guest Recognition, Angew. Chem., 2019, 131, 6975–6979.
24. A. S. Danis, K. P. Potts, S. C. Perry and J. Mauzeroll, Combined spectroelectrochemical and simulated insights into the electrogenerated chemiluminescence coreactant mechanism, Anal. Chem., 2018, 90, 7377–7382.
25. H.-R. Zhang, J.-J. Xu and H.-Y. Chen, Electrochemiluminescence ratiometry: a new approach to DNA biosensing, Anal. Chem., 2013, 85, 5321–5325.
26. S. Carrara, F. Arcudi, M. Prato and L. De Cola, Amine-rich nitrogen-doped carbon nanodots as a platform for self-enhancing electrochemiluminescence, Angew. Chem., 2017, 129, 4835–4839.
27. M. Hesari and Z. Ding, A grand avenue to Au nanocluster electrochemiluminescence, Acc. Chem. Res., 2017, 50, 218–230.
28. X. Zhou, D. Zhu, Y. Liao, W. Liu, H. Liu, Z. Ma and D. Xing, Synthesis, labeling and bioanalytical applications of a tris (2, 2′-bipyridyl) ruthenium(II)-based electrochemiluminescence probe, Nat. Protoc., 2014, 9, 1146–1159.
29. X. Gao, G. Jiang, C. Gao, A. Prudnikau, R. Hübner, J. Zhan, G. Zou, A. Eychmüller and B. Cai, Interparticle Charge-Transport-Enhanced Electrochemiluminescence of Quantum-Dot Aerogels, Angew. Chem., Int. Ed., 2023, 62, e202214487.
30. S. Yu, Y. Du, X. Niu, G. Li, D. Zhu, Q. Yu, G. Zou and H. Ju, Arginine-modified black phosphorus quantum dots with dual excited states for enhanced electrochemiluminescence in bioanalysis, Nat. Commun., 2022, 13, 7302.
31. H. Peng, M. Jian, H. Deng, W. Wang, Z. Huang, K. Huang, A. Liu and W. Chen, Valence states effect on electrogenerated chemiluminescence of gold nanocluster, ACS Appl. Mater. Interfaces, 2017, 9, 14929–14934.
32. W. Miao, J.-P. Choi and A. J. Bard, Electrogenerated chemiluminescence 69: The Tris (2, 2 ‘-bipyridine) ruthenium(II), (Ru(bpy)₃²⁺)/Tri-n-propylamine (TPrA) system revisited A new route involving TPrA˙⁺ Cation Radicals, J. Am. Chem. Soc., 2002, 124, 14478–14485.
33. S. Antonello and F. Maran, Molecular electrochemistry of monolayer-protected clusters, Curr. Opin. Electrochem., 2017, 2, 18–25.
34. J.-P. Choi and A. J. Bard, Electrogenerated chemiluminescence 73: acid–base properties, electrochemistry, and electrogenerated chemiluminescence of neutral red in acetonitrile, J. Electroanal. Chem., 2004, 573, 215–225.
35. W. Miao, Electrogenerated chemiluminescence and its biorelated applications, Chem. Rev., 2008, 108, 2506–2553.
36. J. Ludvík, DC-electrochemiluminescence (ECL with a coreactant)—principle and applications in organic chemistry, J. Solid State Electrochem., 2011, 15, 2065–2081.
37. M. Hesari and Z. Ding, Electrogenerated chemiluminescence: light years ahead, J. Electrochem. Soc., 2015, 163, H3116.
38. I. Díez, M. Pusa, S. Kulmala, H. Jiang, A. Walther, A. S. Goldmann, A. H. Müller, O. Ikkala and R. H. Ras, Color tunability and electrochemiluminescence of silver nanoclusters, Angew. Chem., Int. Ed., 2009, 48, 2122–2125.
39. Y.-M. Fang, J. Song, J. Li, Y.-W. Wang, H.-H. Yang, J.-J. Sun and G.-N. Chen, Electrogenerated chemiluminescence from Au nanoclusters, Chem. Commun., 2011, 47, 2369–2371.
40. G. Hong, C. Su, Z. Huang, Q. Zhuang, C. Wei, H. Deng, W. Chen and H. Peng, Electrochemiluminescence immunoassay platform with immunoglobulin G-encapsulated gold nanoclusters as a “two-in-one” probe, Anal. Chem., 2021, 93, 13022–13028.
41. L. Dennany, C. F. Hogan, T. E. Keyes and R. Forster, Effect of surface immobilization on the electrochemiluminescence of ruthenium-containing metallopolymers, Anal. Chem., 2006, 78, 1412–1417.
42. I. Rubinstein and A. J. Bard, Electrogenerated chemiluminescence. 37. Aqueous ecl systems based on tris (2, 2′-bipyridine) ruthenium (2+) and oxalate or organic acids, J. Am. Chem. Soc., 1981, 103, 512–516.
43. M. M. Richter, J. D. Debad, D. R. Striplin, G. Crosby and A. J. Bard, Electrogenerated chemiluminescence. 59. Rhenium complexes, Anal. Chem., 1996, 68, 4370–4376.
44. W. L. Wallace and A. J. Bard, Electrogenerated chemiluminescence. 35. temperature dependence of the ECL efficiency of Ru(bpy)²⁺ in acetonitrile and evidence for very high excited state yields from electron transfer reactions, J. Phys. Chem., 1979, 83, 1350–1357.
45. H. S. White and A. J. Bard, Electrogenerated chemiluminescence. 41. Electrogenerated chemiluminescence and chemiluminescence of the Ru(2, 21-bpy)₃²⁺-S₂O₈²⁻system in acetonitrile-water solutions, J. Am. Chem. Soc., 1982, 104, 6891–6895.
46. F. Wang, J. Lin, T. Zhao, D. Hu, T. Wu and Y. Liu, Intrinsic “vacancy point defect” induced electrochemiluminescence from coreless supertetrahedral chalcogenide nanocluster, J. Am. Chem. Soc., 2016, 138, 7718–7724.
47. H. Peng, Z. Huang, H. Deng, W. Wu, K. Huang, Z. Li, W. Chen and J. Liu, Dual enhancement of gold nanocluster electrochemiluminescence: electrocatalytic excitation and aggregation-induced emission, Angew. Chem., Int. Ed., 2020, 59, 9982–9985.
48. X. Ouyang, Y. Wu, L. Guo, L. Li, M. Zhou, X. Li, T. Liu, Y. Ding, H. Bu and G. Xie, Self-assembly Induced Enhanced Electrochemiluminescence of Copper Nanoclusters Using DNA Nanoribbon Templates, Angew. Chem., 2023, 135, e202300893.
49. H. Peng, Z. Huang, Y. Sheng, X. Zhang, H. Deng, W. Chen and J. Liu, Pre-oxidation of Gold nanoclusters results in a 66% anodic electrochemiluminescence yield and drives mechanistic insights, Angew. Chem., 2019, 131, 11817–11820.
50. A. Han, Y. Yang, Q. Zhang, Q. Tu, G. Fang, J. Liu, S. Wang and R. Li, Electrochemistry and electrochemiluminescence of copper metal cluster, J. Electroanal. Chem., 2017, 795, 116–122.
51. H. Lv, R. Zhang, S. Cong, J. Guo, M. Shao, W. Liu, L. Zhang and X. Lu, Near-infrared electrogenerated chemiluminescence from simple copper nanoclusters for sensitive alpha-fetoprotein sensing, Anal. Chem., 2022, 94, 4538–4546.
52. Q. Sun, Z. Ning, E. Yang, F. Yin, G. Wu, Y. Zhang and Y. Shen, Ligand-induced Assembly of Copper Nanoclusters with Enhanced Electrochemical Excitation and Radiative Transition for Electrochemiluminescence, Angew. Chem., 2023, 135, e202312053.
53. S. Chen, H. Ma, J. W. Padelford, W. Qinchen, W. Yu, S. Wang, M. Zhu and G. Wang, Near infrared electrochemiluminescence of rod-shape 25-atom AuAg nanoclusters that is hundreds-fold stronger than that of Ru (bpy)₃ standard, J. Am. Chem. Soc., 2019, 141, 9603–9609.
54. G. Hong, C. Su, M. Lai, Z. Huang, Z. Weng, Y. Chen, H. Deng, W. Chen and H. Peng, Co-reactant-mediated low-potential anodic electrochemiluminescence platform and its immunosensing application, Anal. Chem., 2022, 94, 12500–12506.
55. H. Jia, L. Yang, D. Fan, X. Kuang, X. Sun, Q. Wei and H. Ju, Cobalt ion doping to improve electrochemiluminescence emisssion of gold nanoclusters for sensitive NIR biosensing, Sens. Actuators, B, 2022, 367, 132034.
56. Z.-C. Shen, Y.-T. Yang, Y.-Z. Guo, Y.-Q. Chai, J.-L. Liu and R. Yuan, Zn²⁺-Induced gold cluster aggregation enhanced electrochemiluminescence for ultrasensitive detection of MicroRNA-21, Anal. Chem., 2023, 95, 5568–5574.
57. S. Chen, Y. Liu, K. Kuang, B. Yin, X. Wang, L. Jiang, P. Wang, Y. Pei and M. Zhu, Impact of the metal core on the electrochemiluminescence of a pair of atomically precise Au₂₀ nanocluster isomers, Commun. Chem., 2023, 6, 105.
58. G. R. Berdiyorov and H. Hamoudi, Electronic transport through molecules containing pyrimidine units: First-principles calculations, Int. J. Comput. Mater. Sci. Eng., 2021, 48, 101261.
59. Y. Liu, D. Yao and H. Zhang, Self-assembly driven aggregation-induced emission of copper nanoclusters: a novel technology for lighting, ACS Appl. Mater. Interfaces, 2017, 10, 12071–12080.
60. J. Zhou, J. Zhu, J. Brzezinski and Z. Ding, Tunable electrogenerated chemiluminescence from CdSe nanocrystals, Can. J. Chem., 2009, 87, 386–391.
61. M. M. Richter, A. J. Bard, W. Kim and R. H. Schmehl, Electrogenerated chemiluminescence. 62. Enhanced ECL in bimetallic assemblies with ligands that bridge isolated chromophores, Anal. Chem., 1998, 70, 310–318.
62. S. Wang, J. Milam, A. C. Ohlin, V. H. Rambaran, E. Clark, W. Ward, L. Seymour, W. H. Casey, A. A. Holder and W. Miao, Electrochemical and electrogenerated chemiluminescent studies of a trinuclear complex,[((phen)₂Ru(dpp))₂RhCl₂]⁵⁺, and its interactions with calf thymus DNA, Anal. Chem., 2009, 81, 4068–4075.
63. K. N. Swanick, S. Ladouceur, E. Zysman-Colman and Z. Ding, Bright electrochemiluminescence of iridium(III) complexes, Chem. Commun., 2012, 48, 3179–3181.
64. X. Wei, M. J. Zhu, H. Yan, C. Lu and J. J. Xu, Recent advances in aggregation-induced electrochemiluminescence, Chem. – Eur. J., 2019, 25, 12671–12683.
65. Y. Zhou, H. Wang, H. Zhang, Y. Chai and R. Yuan, Programmable modulation of copper nanoclusters electrochemiluminescence via DNA nanocranes for ultrasensitive detection of microRNA, Anal. Chem., 2018, 90, 3543–3549.
66. S. Carrara, A. Aliprandi, C. F. Hogan and L. De Cola, Aggregation-induced electrochemiluminescence of platinum(II) complexes, J. Am. Chem. Soc., 2017, 139, 14605–14610.
67. T.-B. Gao, J.-J. Zhang, R.-Q. Yan, D.-K. Cao, D. Jiang and D. Ye, Aggregation-induced electrochemiluminescence from a cyclometalated iridium(III) complex, Inorg. Chem., 2018, 57, 4310–4316.
68. S. Carrara, B. Stringer, A. Shokouhi, P. Ramkissoon, J. Agugiaro, D. J. Wilson, P. J. Barnard and C. F. Hogan, Unusually strong electrochemiluminescence from iridium-based redox polymers immobilized as thin layers or polymer nanoparticles, ACS Appl. Mater. Interfaces, 2018, 10, 37251–37257.
69. H. Gao, N. Zhang, Y. Li, W. Zhao, Y. Quan, Y. Cheng, H.-Y. Chen and J.-J. Xu, Trace Ir(III) complex enhanced electrochemiluminescence of AIE-active Pdots in aqueous media, Sci. China: Chem., 2020, 63, 715–721.
70. Z. Wang, Y. Feng, N. Wang, Y. Cheng, Y. Quan and H. Ju, Donor–acceptor conjugated polymer dots for tunable electrochemiluminescence activated by aggregation-induced emission-active moieties, J. Phys. Chem. Lett., 2018, 9, 5296–5302.
71. X. Wei, M. J. Zhu, Z. Cheng, M. Lee, H. Yan, C. Lu and J. J. Xu, Aggregation-induced electrochemiluminescence of carboranyl carbazoles in aqueous media, Angew. Chem., 2019, 131, 3194–3198.
72. Z. Han, Z. Yang, H. Sun, Y. Xu, X. Ma, D. Shan, J. Chen, S. Huo, Z. Zhang and P. Du, Electrochemiluminescence platforms based on small water-insoluble organic molecules for ultrasensitive aqueous-phase detection, Angew. Chem., Int. Ed., 2019, 58, 5915–5919.
73. A. B. Nepomnyashchii and A. J. Bard, Electrochemistry and electrogenerated chemiluminescence of BODIPY dyes, Acc. Chem. Res., 2012, 45, 1844–1853.
74. M. Walter, J. Akola, O. Lopez-Acevedo, P. D. Jadzinsky, G. Calero, C. J. Ackerson, R. L. Whetten, H. Grönbeck and H. Häkkinen, A unified view of ligand-protected gold clusters as superatom complexes, Proc. Natl. Acad. Sci. U. S. A., 2008, 105, 9157–9162.
75. K. Pyo, V. D. Thanthirige, K. Kwak, P. Pandurangan, G. Ramakrishna and D. Lee, Ultrabright luminescence from gold nanoclusters: rigidifying the Au(I)–thiolate shell, J. Am. Chem. Soc., 2015, 137, 8244–8250.
76. H.-H. Deng, X.-Q. Shi, F.-F. Wang, H.-P. Peng, A.-L. Liu, X.-H. Xia and W. Chen, Fabrication of water-soluble, green-emitting gold nanoclusters with a 65% photoluminescence quantum yield via host–guest recognition, Chem. Mater., 2017, 29, 1362–1369.
77. Z. Ding, B. M. Quinn, S. K. Haram, L. E. Pell, B. A. Korgel and A. J. Bard, Electrochemistry and electrogenerated chemiluminescence from silicon nanocrystal quantum dots, Science, 2002, 296, 1293–1297.
78. S. Hu, H. Zhao, P. Xie, X. Zhu, L. Liu, N. Yin, Z. Tang, K. Peng and R. Yuan, High-efficiency electrochemiluminescence of host-guest ligand-assembled gold nanoclusters preoxidated for ultrasensitive biosensing of protein, Sens. Actuators, B, 2024, 419, 136347.
79. H. Jia, L. Yang, X. Dong, L. Zhou, Q. Wei and H. Ju, Cysteine modification of glutathione-stabilized Au nanoclusters to red-shift and enhance the electrochemiluminescence for sensitive bioanalysis, Anal. Chem., 2022, 94, 2313–2320.
80. Y. Zhao, J. Yu, G. Xu, N. Sojic and G. Loget, Photoinduced electrochemiluminescence at silicon electrodes in water, J. Am. Chem. Soc., 2019, 141, 13013–13016.
81. G.-H. Yang, J.-J. Shi, S. Wang, W.-W. Xiong, L.-P. Jiang, C. Burda and J.-J. Zhu, Fabrication of a boron nitride–gold nanocluster composite and its versatile application for immunoassays, Chem. Commun., 2013, 49, 10757–10759.
82. G. Nie, Y. Wang, Y. Tang, D. Zhao and Q. Guo, A graphene quantum dots based electrochemiluminescence immunosensor for carcinoembryonic antigen detection using poly (5-formylindole)/reduced graphene oxide nanocomposite, Biosens. Bioelectron., 2018, 101, 123–128.
83. H. Ding, C. Liang, K. Sun, H. Wang, J. K. Hiltunen, Z. Chen and J. Shen, Dithiothreitol-capped fluorescent gold nanoclusters: an efficient probe for detection of copper(II) ions in aqueous solution, Biosens. Bioelectron., 2014, 59, 216–220.
84. D. Le, D. Keller and G. Delaittre, Reactive and Functional Nanoobjects by Polymerization-Induced Self-Assembly, Macromol. Rapid Commun., 2019, 40, 1800551.
85. A. Abdilla, N. D. Dolinski, P. De Roos, J. M. Ren, E. Van Der Woude, S. E. Seo, M. S. Zayas, J. Lawrence, J. Read de Alaniz and C. J. Hawker, Polymer stereocomplexation as a scalable platform for nanoparticle assembly, J. Am. Chem. Soc., 2020, 142, 1667–1672.
86. J. Zhang, P. J. Santos, P. A. Gabrys, S. Lee, C. Liu and R. J. Macfarlane, Self-assembling nanocomposite tectons, J. Am. Chem. Soc., 2016, 138, 16228–16231.
87. X. Ye, C. Zhu, P. Ercius, S. N. Raja, B. He, M. R. Jones, M. R. Hauwiller, Y. Liu, T. Xu and A. P. Alivisatos, Structural diversity in binary superlattices self-assembled from polymer-grafted nanocrystals, Nat. Commun., 2015, 6, 10052.
88. H. Ye, D. Chen, M. Liu, S. J. Su, Y. F. Wang, C. C. Lo, A. Lien and J. Kido, Pyridine-containing electron-transport materials for highly efficient blue phosphorescent OLEDs with ultralow operating voltage and reduced efficiency roll-off, Adv. Funct. Mater., 2014, 24, 3268–3275.
89. S. J. Su, Y. Takahashi, T. Chiba, T. Takeda and J. Kido, Structure–Property Relationship of Pyridine-Containing Triphenyl Benzene Electron-Transport Materials for Highly Efficient Blue Phosphorescent OLEDs, Adv. Funct. Mater., 2009, 19, 1260–1267.
90. W. F. Lai, W. T. Wong and A. L. Rogach, Development of copper nanoclusters for in vitro and in vivo theranostic applications, Adv. Mater., 2020, 32, 1906872.
91. J. Davis, Scanning tunnelling microscopy study of the self assembly of 2-mercaptopyrimidine and 4, 6-dimethyl-2-mercaptopyrimidine on Au (111), J. Chem. Soc., 1998, 94, 1315–1319.
92. X. Zhang, Y. Jia, N. Zhang, D. Wu, H. Ma, X. Ren, H. Ju and Q. Wei, Self-Assembly-Induced Enhancement of Cathodic Electrochemiluminescence of Copper Nanoclusters for a Split-Type Matrix Metalloproteinase 14 Sensing Platform, Anal. Chem., 2024, 96, 7265–7273.
93. C. M. Platnich, F. J. Rizzuto, G. Cosa and H. F. Sleiman, Single-molecule methods in structural DNA nanotechnology, Chem. Soc. Rev., 2020, 49, 4220–4233.
94. M. Madsen and K. V. Gothelf, Chemistries for DNA nanotechnology, Chem. Rev., 2019, 119, 6384–6458.
95. A. Kuzyk, R. Schreiber, Z. Fan, G. Pardatscher, E.-M. Roller, A. Högele, F. C. Simmel, A. O. Govorov and T. Liedl, DNA-based self-assembly of chiral plasmonic nanostructures with tailored optical response, Nature, 2012, 483, 311–314.
96. G. Acuna, F. Möller, P. Holzmeister, S. Beater, B. Lalkens and P. Tinnefeld, Fluorescence enhancement at docking sites of DNA-directed self-assembled nanoantennas, Science, 2012, 338, 506–510.
97. S. Rinker, Y. Ke, Y. Liu, R. Chhabra and H. Yan, Self-assembled DNA nanostructures for distance-dependent multivalent ligand–protein binding, Nat. Nanotechnol., 2008, 3, 418–422.
98. Y. Ke, T. Meyer, W. M. Shih and G. Bellot, Regulation at a distance of biomolecular interactions using a DNA origami nanoactuator, Nat. Commun., 2016, 7, 10935.
99. J. B. Woehrstein, M. T. Strauss, L. L. Ong, B. Wei, D. Y. Zhang, R. Jungmann and P. Yin, Sub–100 nm metafluorophores with digitally tunable optical properties self-assembled from DNA, Sci. Adv., 2017, 3, e1602128.
100. F. Nicoli, A. Barth, W. Bae, F. Neukirchinger, A. H. Crevenna, D. C. Lamb and T. Liedl, Directional photonic wire mediated by homo-forster resonance energy transfer on a DNA origami platform, ACS Nano, 2017, 11, 11264–11272.
101. S. Liu, Y. Jia, J. Xue, Y. Li, Z. Wu, X. Ren, H. Ma, Y. Li and Q. Wei, Bifunctional peptide-biomineralized gold nanoclusters as electrochemiluminescence probe for optimizing sensing interface, Sens. Actuators, B, 2020, 318, 128278.
102. L. Chen, D. J. Hayne, E. H. Doeven, J. Agugiaro, D. J. Wilson, L. C. Henderson, T. U. Connell, Y. H. Nai, R. Alexander and S. Carrara, A conceptual framework for the development of iridium(III) complex-based electrogenerated chemiluminescence labels, Chem. Sci., 2019, 10, 8654–8667.
103. M. Zhao, A.-Y. Chen, D. Huang, Y. Zhuo, Y.-Q. Chai and R. Yuan, Cu Nanoclusters: Novel Electrochemiluminescence Emitters for Bioanalysis, Anal. Chem., 2016, 88, 11527–11532.
104. X. Wei, K. Chu, J. R. Adsetts, H. Li, X. Kang, Z. Ding and M. Zhu, Nanocluster transformation induced by SbF6−anions toward boosting photochemical activities, J. Am. Chem. Soc., 2022, 144, 20421–20433.
105. L. E. Marbella, C. M. Andolina, A. M. Smith, M. J. Hartmann, A. C. Dewar, K. A. Johnston, O. H. Daly and J. E. Millstone, Gold-Cobalt Nanoparticle Alloys Exhibiting Tunable Compositions, Near-Infrared Emission, and High T2 Relaxivity, Adv. Funct. Mater., 2014, 24, 6532–6539.
106. S. E. Crawford, M. J. Hartmann and J. E. R. Millstone, Surface chemistry-mediated near-infrared emission of small coinage metal nanoparticles, Acc. Chem. Res., 2019, 52, 695–703.
107. B. Gao, M. A. Haghighatbin and H. Cui, Polymer-encapsulated cobalt/gold bimetallic nanoclusters as stimuli-responsive chemiluminescent nanoprobes for reactive oxygen species, Anal. Chem., 2020, 92, 10677–10685.
108. L. Fu, X. Gao, S. Dong, H.-Y. Hsu and G. Zou, Surface-defect-induced and synergetic-effect-enhanced NIR-II electrochemiluminescence of Au–Ag bimetallic nanoclusters and its spectral sensing, Anal. Chem., 2021, 93, 4909–4915.
109. X. Zhang, X. Dong, Y. Jia, X. Ren, L. Xu, X. Liu, F. Li, H. Ju and Q. Wei, Ligand regulation strategy enhanced anodic electrochemiluminescence of copper nanoclusters for enrofloxacin trace determination, Chin. J. Chem., 2024, 417, 136193.
110. Z. Luo, X. Yuan, Y. Yu, Q. Zhang, D. T. Leong, J. Y. Lee and J. Xie, From aggregation-induced emission of Au(I)–thiolate complexes to ultrabright Au (0)@ Au(I)–thiolate core–shell nanoclusters, J. Am. Chem. Soc., 2012, 134, 16662–16670.
111. A. Lu, D.-L. Peng, F. Chang, Z. Skeete, S. Shan, A. Sharma, J. Luo and C.-J. Zhong, Composition-and structure-tunable gold–cobalt nanoparticles and electrocatalytic synergy for oxygen evolution reaction, ACS Appl. Mater. Interfaces, 2016, 8, 20082–20091.
112. D. Wang, X. Gao, J. Jia, B. Zhang and G. Zou, Valence-state-engineered electrochemiluminescence from Au nanoclusters, ACS Nano, 2022, 17, 355–362.
113. G. Jie, L. Tan, Y. Zhao and X. Wang, A novel silver nanocluster in situ synthesized as versatile probe for electrochemiluminescence and electrochemical detection of thrombin by multiple signal amplification strategy, Biosens. Bioelectron, 2017, 94, 243–249.
114. Y. Zhou, M. Chen, Y. Zhuo, Y. Chai, W. Xu and R. Yuan, In situ electrodeposited synthesis of electrochemiluminescent Ag nanoclusters as signal probe for ultrasensitive detection of Cyclin-D1 from cancer cells, Anal. Chem., 2017, 89, 6787–6793.
115. D. Li, R. Tan, X. Mi, C. Fang and Y. Tu, An electrochemiluminescent biosensor for noninvasive glucose detection based on cluster-like AuAg hollowed-nanoparticles, Microchem. J., 2021, 167, 106271.
116. Y. Zhou, H. Wang, Y. Zhuo, Y. Chai and R. Yuan, Highly efficient electrochemiluminescent silver nanoclusters/titanium oxide nanomaterials as a signal probe for ferrocene-driven light switch bioanalysis, Anal. Chem., 2017, 89, 3732–3738.
117. S. S. Dasary, D. Senapati, A. K. Singh, Y. Anjaneyulu, H. Yu and P. C. Ray, Highly sensitive and selective dynamic light-scattering assay for TNT detection using p-ATP attached gold nanoparticle, ACS Appl. Mater. Interfaces, 2010, 2, 3455–3460.
118. Y. Yu, Q. Cao, M. Zhou and H. Cui, A novel homogeneous label-free aptasensor for 2, 4, 6-trinitrotoluene detection based on an assembly strategy of electrochemiluminescent graphene oxide with gold nanoparticles and aptamer, Biosens. Bioelectron., 2013, 43, 137–142.
119. H. Peng, Z. Huang, W. Wu, M. Liu, K. Huang, Y. Yang, H. Deng, X. Xia and W. Chen, Versatile high-performance electrochemiluminescence ELISA platform based on a gold nanocluster probe, ACS Appl. Mater. Interfaces, 2019, 11, 24812–24819.
120. Y. Nie, Y. Zhang, W. Cao, Y.-Q. Chai and R. Yuan, Ligand-based shielding effect induced efficient near-infrared electrochemiluminescence of gold nanoclusters and its sensing application, Anal. Chem., 2023, 95, 6785–6790.
121. H. Peng, H. Deng, M. Jian, A. Liu, F. Bai, X. Lin and W. Chen, Electrochemiluminescence sensor based on methionine-modified gold nanoclusters for highly sensitive determination of dopamine released by cells, Microchim. Acta, 2017, 184, 735–743.
122. X. Zhu, Y. Song, X. Wang, Y. Zhou, Y. Chai and R. Yuan, Copper nanoclusters electrochemiluminescence with tunable near-infrared emission wavelength for ultrasensitive detection of matrix metalloproteinase-2, Biosens. Bioelectron., 2023, 238, 115580.
123. C. Zhang and Z. Ma, PtCu nanoprobe-initiated cascade reaction modulated iodide-responsive sensing interface for improved electrochemical immunosensor of neuron-specific enolase, Biosens. Bioelectron., 2019, 143, 111612.
124. X. Huang, J. Miao, J. Fang, X. Xu, Q. Wei and W. Cao, Ratiometric electrochemical immunosensor based on L-cysteine grafted ferrocene for detection of neuron specific enolase, Talanta, 2022, 239, 123075.
125. E. B. Aydın, M. Aydın and M. K. Sezgintürk, Selective and ultrasensitive electrochemical immunosensing of NSE cancer biomarker in human serum using epoxy-substituted poly (pyrrole) polymer modified disposable ITO electrode, Sens. Actuators, B, 2020, 306, 127613.
126. W. Dong, F. Zhang, T. Li, Y. Zhong, L. Hong, Y. Shi, F. Jiang, H. Zhu, M. Lu and Q. Yao, Triple-Phosphorescent Gold Nanoclusters Enabled by Isomerization of Terminal Thiouracils in the Surface Motifs, J. Am. Chem. Soc., 2024, 146, 22180–22192.
127. Y. Shi, Z. Wu, M. Qi, C. Liu, W. Dong, W. Sun, X. Wang, F. Jiang, Y. Zhong and D. Nan, Multiscale Bioresponses of Metal Nanoclusters, Adv. Mater., 2024, 36, 2310529.
128. Y. Zhong, X. Wang, T. Li, Q. Yao, W. Dong, M. Lu, X. Bai, Z. Wu, J. Xie and Y. Zhang, White-Emitting Gold Nanocluster Assembly with Dynamic Color Tuning, Nano Lett., 2024, 24, 6997–7003.
129. Y. Zhong, J. Zhang, T. Li, W. Xu, Q. Yao, M. Lu, X. Bai, Z. Wu, J. Xie and Y. Zhang, Suppression of kernel vibrations by layer-by-layer ligand engineering boosts photoluminescence efficiency of gold nanoclusters, Nat. Commun., 2023, 14, 658.

---