Identifying Highly Photoelectrochemical Active Sites of Two Au21 Nanocluster Isomers toward Bright Near-Infrared Electrochemiluminescence

Eight-Electron Copper Nanoclusters for Photothermal Conversion

Owing to distinct physicochemical properties in comparison to gold and silver counterparts, atomically precise copper nanoclusters are attracting embryonic interest in material science. The introduction of copper cluster nanomaterials in more interesting fields is currently urgent and desired. Reported in this work are novel copper nanoclusters of [XCu54Cl12(tBuS)20(NO3)12] (X=S or none, tBuSH=2-methyl-2-propanethiol), which exhibit high performance in photothermal conversion. The clusters have been prepared in one pot and characterized by combinatorial techniques including ultraviolet-visible spectroscopy (UV-vis), electrospray ionization mass spectrometry (ESI-MS), and X-ray photoelectron spectroscopy (XPS). The molecular structure of the clusters, as revealed by single crystal X-ray diffraction analysis (SCXRD), shows the concentric three-shell Russian doll arrangement of X@Cu14@Cl12@Cu40. Interestingly, the [SCu54Cl12(tBuS)20(NO3)12] cluster contains 8 free valence electrons in its structure, making it the first eight-electron copper nanocluster stabilized by thiolates. More impressively, the clusters possess an effective photothermal conversion (temperature increases by 71 °C within ~50 s, λex=445 nm, 0.5 W cm-2) in a wide wavelength range (either blue or near-infrared). The photothermal conversion can be even driven under irradiation of simulated sunlight (3 sun), endowing the clusters with great potency in solar energy utilization.

Enhanced Aggregation-Induced Delayed Electrochemiluminescence Triggered by Spatial Perturbation of Organic Dots

Development of highly efficient, heavy-metal-free electrochemiluminescence (ECL) materials is attractive but still challenging. Herein, we report an aggregation-induced delayed ECL (AIDECL) active organic dot (OD) composed of a tert-butoxy-group-substituted benzophenone-dimethylacridine compound, which shows high ECL efficiency. The resultant ODs exhibit 2.1-fold higher ECL efficiency compared to control AIDECL-active ODs. Molecular stacking combined with theoretical calculations suggests that tert-butoxy groups effectively participate in the intermolecular interactions, further inhibiting the molecular motions in the aggregated states and thus accelerating radiative decay. On the basis of these ODs exhibiting excellent ECL performance, a proof-of-concept biosensor is constructed for the detection of miR-16 associated with Alzheimer's disease, which demonstrates excellent detection ability with the limit of detection of 1.7 fM. This work provides a new approach to improve the ECL efficiency and enriches the fundamental understanding of the structure-property relationship.

Near-infrared electrochemiluminescence biosensors facilitated by thermally activated delayed fluorescence (TADF) emitters for ctDNA analysis

The near-infrared electrochemiluminescence technique (NIR ECL) has gained significant attention as a powerful analytical tool in biomedicine and clinical diagnosis due to its inherent advantages. In this work, we successfully synthesized a novel NIR ECL emitter of TPA-DCPP nanoparticles (NPs) with a D-π-A-π-D configuration. By utilizing the thermally activated delayed fluorescence (TADF) property, we achieved enhanced electrochemiluminescence (ECL) emission through complete exciton harvesting for radiative decay. Specifically, when BDEA was used as a co-reactant, the TPA-DCPP NPs exhibited strong bandgap ECL emission. Additionally, they demonstrated an exceptionally higher ECL efficiency compared to conventional near-infrared fluorescence organic nanomaterials (BSeT-BT NPs). By integrating the efficient anodic ECL performance of TPA-DCPP NPs with Exo III-assisted polymerase enzyme reaction cascade amplification, a highly efficient ECL resonance energy transfer (ECL-RET) platform was developed for ultrasensitive detection of circulating tumor DNA (ctDNA). The established biosensor demonstrated an exceptional linear dynamic range and achieved attomolar-level detection limit. This study highlights the immense potential of TADF emitters in enhancing ECL efficiency and extends the emission wavelength of organic nanomaterials to the NIR region, thereby expanding their applications in biological analysis.

Spectroscopy and absolute quantum efficiency of near-infrared electrochemiluminescence for a macrocyclic palladium complex

Electrochemiluminescence (ECL) is widely applied as a reliable tool in clinical diagnosis, including immunoassays, cancer biomarker detection, etc. Metal complexes with emission in the near-infrared (NIR) range possess distinct features such as high transmission and minimal tissue auto-absorption, making them versatile for applications in biosensing and other fields. Through ECL spectral studies of an O-linked nonaromatic benzitripyrrin (C^N^N^N) macrocyclic palladium complex (Pd1) with multiple pyrrole structures, we observed emission peaks from the Qx(0,0) and its vibronic Qx(0,1) bands during both photoluminescence (PL) and ECL. Notably, the emission from the Qx(0,1) band was significantly enhanced in the ECL spectrum, demonstrating higher selectivity for near-infrared light at 743 nm. In the ECL annihilation pathway, the appearance of ECL signals showed a strong correlation with the redox processes of the tri-pyrrin structure, revealing a cyclic tri-pyrrin ligand-centered nature with contributions from the metal center. Upon the introduction of tripropylamine (TPrA) and benzoyl peroxide (BPO) coreactants, the ECL signals exhibited enhancements ranging from several hundred to tens of times. Various reaction routes within different coreactant systems are extensively discussed. Additionally, the absolute quantum efficiencies of the Pd1/TPrA coreactant system were determined, showing efficiencies of 0.0032% ± 0.0005% and 0.000074% ± 0.000016% during pulsing and CV scan processes, respectively. This work addresses gaps in the study of palladacycle complexes in ECL and provides insights into the design of NIR luminescent structures that contribute to the fast screening and deep tissue penetration bioimaging techniques.

Solvent-Mediated Hetero/Homo-Phase Crystallization of Copper Nanoclusters and Superatomic Kernel-Related NIR Phosphorescence

Atomically precise superatomic copper nanoclusters (Cu NCs) have been the subject of immense interest for their intriguing structures and diverse properties; nonetheless, the variable oxidation state of copper ions and complex solvation effects in wet synthesis systems pose significant challenges for comprehending their synthesis and crystallization mechanism. Herein, we present a solvent-mediated approach for the synthesis of two Cu NCs, namely, superatomic Cu26 and pure-Cu(I) Cu16. They initially formed as a hetero-phase and then separated as a homo-phase via modulating binary solvent composition. In situ UV/vis absorption and electrospray ionization mass spectra revealed that the solvent-mediated assembly was determined to be the underlying mechanism of hetero/homo-phase crystallization. Cu26 is a 2-electron superatom with a kernel-shell structure that includes a [Cu20Se12]4- shell and [Cu6]4+ kernel, containing two 1S jellium electrons. Conversely, Cu16 is a pure-Cu(I) Cu/Se nanocluster that features a [Cu16Se6]4+ core protected by extra dimercaptomaleonitrile ligands. Remarkably, Cu26 exhibits unique near-infrared phosphorescence (NIR PH) at 933 nm due to the presence of a superatomic kernel-related charge transfer state (3MM(Cu)CT). Overall, this work not only showcases the hetero/homo-phase crystallization of Cu NCs driven by a solvent-mediated assembly mechanism but also enables the rare occurrence of NIR PH within the 2-electron copper superatom family.

Dynamic surface reconstruction of individual gold nanoclusters by using a co-reactant enables color-tunable electrochemiluminescence

Here we report for the first time the phenomenon of continuously color-tunable electrochemiluminescence (ECL) from individual gold nanoclusters (Au NCs) confined in a porous hydrogel matrix by adjusting the concentration of the co-reactant. Specifically, the hydrogel-confined Au NCs exhibit strong dual-color ECL in an aqueous solution with triethylamine (TEA) as a co-reactant, with a record-breaking quantum yield of 95%. Unlike previously reported Au NCs, the ECL origin of the hydrogel-confined Au NCs is related to both the Au(0) kernel and the Au(i)-S surface. Surprisingly, the surface-related ECL of Au NCs exhibits a wide color-tunable range of 625-829 nm, but the core-related ECL remains constant at 489 nm. Theoretical and experimental studies demonstrate that the color-tunable ECL is caused by the dynamic surface reconstruction of Au NCs and TEA radicals. This work opens up new avenues for dynamically manipulating the ECL spectra of core-shell emitters in biosensing and imaging research.

Co-Reactive Ligand In Situ Engineered Gold Nanoclusters with Ultra-Bright Near-Infrared Electrochemiluminescence for Ultrasensitive and Label-Free Detection of Carboxylesterase Activity

Ultrasensitive and accurate monitoring of carboxylesterase (CE) activity is extremely crucial for the early diagnosis of hepatocellular carcinoma (HCC), which is still a considerable challenge. Herein, using a co-reactive ligand engineering strategy, ultra-bright near-infrared (λmax = 830 nm) and self-enhanced electrochemiluminescence (ECL) Au nanoclusters (NCs) were in situ prepared with 2-(diethylamino) ethanethiol (DEAET) as a co-reactive ligand. Remarkably, the co-reactive ligand not only acts as a stabilizer like traditional ligands but also plays a crucial role as a co-reactant to ensure a confinement effect to shorten the charge transfer distance and increase the local concentration, significantly improving the collision efficiency between the electrogenerated free radicals. Consequently, the DEAET Au NCs exhibited a record and stable anodal ECL without the addition of an exogenous co-reactant, dramatically superior to classical Au NCs and Ru(bpy)32+ with a certain amount of the co-reactant. As a proof of concept, a convenient and label-free CE biosensor was innovatively constructed using 1-naphthyl acetate as a selective substrate, achieving ultrasensitive detection for CE activity with a low limit of detection of 9.1 × 10-7 U/L. Therefore, this work not only paves a co-reactive ligand engineering strategy for in situ preparation of high-efficiency metal NCs but also provides an ultrasensitive and convenient platform for the early diagnosis of HCC.

Silver Nanocluster-Tagged Electrochemiluminescence Immunoassay with a Sole and Narrow Triggering Potential Window

The commercialized electrochemiluminescence (ECL) immunoassay is carried out by holding luminophore Ru(bpy)32+ at a given potential. Designing an electrochemiluminophore with a narrow triggering potential window is strongly anticipated to decrease the electrochemical cross-talk and improve the flux of the commercialized ECL immunoassay in a potential-resolved way. Herein, L-penicillamine-capped silver nanoclusters (LPA-AgNCs) are facilely synthesized and utilized as tags to perform the ECL immunoassay with a sole and narrow triggering potential window of 0.24 V by employing hydrazine (N2H4) as a coreactant. The maximum ECL emission of the LPA-AgNCs/N2H4 system is located ca. +1.27 V. Upon immobilizing LPA-AgNCs onto the electrode surface via forming a sandwich immunocomplex, the ECL of LPA-AgNCs/N2H4 can be utilized to sensitively and selectively determine human carcinoembryonic antigen from 0.5 to 1000 pg/mL with a low limit of detection of 0.1 pg/mL (S/N = 3). This work might open a way to screen electrochemiluminophores for the multiple ECL immunoassay in a potential-resolved way.

Self-assembled copper nanoclusters used to mimic peroxidase for glucose detection

A sensing system for glucose was established based on a self-assembled copper nanoclusters (Cu NCs)-based nano-enzyme and glucose oxidase (GOD). The assembled copper nanosheets (Cu NSs) were prepared in a one-step method using 2,3,4,5,6-pentafluorothiophenol (PFTP) as a reducing agent and protecting ligand. Cu NSs could be used to mimic the enzyme horseradish peroxidase. Cu NSs were endowed with excellent enzymatic catalytic activity in the oxidation of o-phenyldiamine (OPD) in the presence of H2O2. The latter could be generated in the aerobic oxidation of glucose catalyzed by GOD. Therefore, a detection method for glucose was constructed based on a Cu NSs-OPD-GOD catalytic system. This proposed sensing platform showed a standard linear range from 10 μM to 5 mM towards glucose, and the limit of detection was 5.5 μM. Finally, practical application of a sensor based on the Cu NSs nano-enzyme was verified in three sugared beverages as real samples. Our data reveal that the prepared Cu NSs could mimic peroxidase and be applied to a mixed catalytic system with GOD for glucose detection.

Near-infrared II aggregation-induced electrochemiluminescence of organic dots

For the first time, we report novel aggregation-induced electrochemiluminescence (AIECL) of organic dots in aqueous media, with near-infrared II (NIR-II) luminescence peaked at 906 nm. Furthermore, a hybrid mechanism of ECL generation is revealed by various experiments in conjunction with theoretical calculations. This work opens a window for exploring efficient organic dye-based NIR-II AIECL emitters.

Supramolecular Anchored Copper Nanoclusters for a Multipath Electrochemiluminescence Probe

Copper nanoclusters (Cu NCs) are highly promising nanomaterials in the field of electrochemiluminescence (ECL). Nevertheless, their limited stability and efficiency have impeded their practical applications. Here, we introduced a novel supramolecular anchoring strategy resulting in the creation of exceptionally stable Cu NCs (CET-Cu NCs) with remarkable ECL properties. Specifically, CET-Cu NCs exhibited a relative ECL efficiency (ΦECL) of 62% based on the annihilation ECL efficiency of [Ru(bpy)3]2+ (100%), with tripropylamine employed as a coreactant. Moreover, CET-Cu NCs can generate ECL emission through multiple different paths, which enables them to serve as signal probes in a wider range of testing scenarios, thereby enhancing the reliability and robustness of sensing and analytical systems. To demonstrate the practical utility, CET-Cu NCs were selected as an ECL signal probe for a sensing platform that facilitated ultrasensitive detection of progesterone via oriented immobilization technology and antibody/aptamer sandwich assays. This study surmounted the barriers to the practical application of Cu NCs through the implementation of a supramolecular anchoring strategy, thereby providing enhanced utility of Cu NCs in ECL sensing and analysis.

Precise Modulation of Intramolecular Aggregation-induced Electrochemiluminescence by Tetraphenylethylene-based Supramolecular Architectures

The precisely modulated synthesis of programmable light-emitting materials remains a challenge. To address this challenge, we construct four tetraphenylethylene-based supramolecular architectures (SA, SB, SC, and SD), revealing that they exhibit higher electrochemiluminescence (ECL) intensities and efficiencies than the tetraphenylethylene monomer and can be classified as highly efficient and precisely modulated intramolecular aggregation-induced electrochemiluminescence (PI-AIECL) systems. The best-performing system (SD) shows a high ECL cathodic efficiency exceeding that of the benchmark tris(2,2'-bipyridyl)ruthenium(II) chloride in aqueous solution by nearly six-fold. The electrochemical characterization of these architectures in an organic solvent provides deeper mechanistic insights, revealing that SD features the lowest electrochemical band gap. Density functional theory calculations indicate that the band gap of the guest ligand in the SD structure is the smallest and most closely matched to that of the host scaffold. Finally, the SD system is used to realize ECL-based cysteine detection (detection limit=14.4 nM) in real samples. Thus, this study not only provides a precisely modulated supramolecular strategy allowing chromophores to be controllably regulated on a molecular scale, but also inspires the programmable synthesis of high-performance aggregation-induced electrochemiluminescence emitters.

Copper nanoclusters electrochemiluminescence with tunable near-infrared emission wavelength for ultrasensitive detection of matrix metalloproteinase-2

Herein, the methionine (Met)/N-acetyl-L-cysteine (NAC) templated copper nanoclusters (Met/NAC-Cu NCs) with tunable near-infrared region (NIR) electrochemiluminescence (ECL) emission wavelength was firstly synthesized as emitter for the ultrasensitive detection of matrix metalloproteinase-2 (MMP-2). Significantly, the NAC played the role of template and reductant of cupric to acquire Cu NCs, and the surface defect regulator Met was used to connect NAC through -S-S- bond, which could heighten the surface defect of Cu NCs to continuously regulate the maximum ECL emission by successively controlling the molar ratio of Met and NAC, leading to the ECL emission wavelength of Cu NCs ranged from 680 nm to 750 nm. In addition, a rapid target triggered catalyst hairpin assembly (CHA) recycling amplification strategy was constructed through orderly and equidistantly arranging hairpin to increase its local concentration, resulting in greatly accelerated signal amplification efficiency and reaction rate. As a proof of concept, based on Met/NAC-Cu NCs as NIR ECL emitter and effective signal amplification tactic, a super-sensitive ECL biosensor was fabricated to detect target MMP-2 with the detection limit (LOD) as low as 1.65 fg/mL and successfully utilized for detecting of MMP-2 that from Hela and MCF-7 cancer cells. This research provided a wonderful avenue for regulating the optical performance of metal nanoclusters-based ECL emitters, and the developed neoteric NIR ECL emitter with the merits of less photochemical damage and deeper tissue penetration exhibited great potential in ultrasensitive biosensing and high-definition ECL imaging.

Engineering Molecular Emission Centers of Carbon Dots to Boost the Electrochemiluminescence for the Detection of Cancer Cells

Despite the fact that electrochemiluminescent (ECL) performance of carbon dots (CDs) could be improved by modulating their surface defects, they are still restricted by inferior controllability and poor reproducibility. In this work, we disclosed a new approach for synthesizing luminescent groups rich in CDs (Lu-CDs) by engineering the luminol as molecular emission centers into the CDs, which exhibited an 80-fold stronger ECL intensity at an ECL onset potential of 0.6 V than the CDs without pre-implanted luminol. Different from the significant deviation between the ECL and fluorescence emission of other surface state-dominated CDs, the ECL emission of Lu-CDs was nearly consistent with its fluorescence emission at 465 nm, which was defined as the molecular emission dominated-ECL CDs herein. To prove this principle, the Lu-CDs were employed to construct an ECL biosensor for MCF-7 cell analysis based on the cell direct recognition and amplification strategy, which made the MCF-7 cells as nanomachines via specific binding with aptamer signal probes on the DNA triangular scaffold. The proposed biosensor displayed a wide detection range from 101 to 104 cell mL-1 and a low detection limit of 8.91 cells mL-1. Overall, this work not only presents a new strategy for preparing CDs with high controllability and excellent reproducibility but also provides a platform for tumor cell sensing.

Gold Microbeads Enabled Proximity Electrochemiluminescence for Highly Sensitive and Size-Encoded Multiplex Immunoassays

Developing highly sensitive multiplex immunoassays is urgently needed to guide medical research and improve clinical diagnosis. Here, we report the proximity electrochemiluminescence (ECL) generation enabled by gold microbeads (GMBs) for improving the detection sensitivity and multiplexing capacity of ECL immunoassays (ECLIAs). As demonstrated by microscopy and finite element simulation, GMBs can function as spherical ultramicroelectrodes for triggering ECL reactions in solutions. Employing GMBs as solid carriers in the bead-based ECLIA, the electrochemical oxidation of a coreactant can occur at both the GMB surface and the substrate electrode, allowing the coreactant radicals to diffuse only a short distance of ∼100 nm to react with ECL luminophores that are labeled on the GMB surface. The ECL generation via this proximity low oxidation potential (LOP) route results in a 21.7-fold increase in the turnover frequency of ECL generation compared with the non-conductive microbeads that rely exclusively on the conventional LOP route. Moreover, the proximity ECL generation is not restricted by the diffusion distance of short-lived coreactant radicals, which enables the simultaneous determination of multiple acute myocardial infarction biomarkers using size-encoded GMB-based multiplex ECLIAs. This work brings new insight into the understanding of ECL mechanisms and may advance the practical use of multiplex ECLIAs.

Array electrochemiluminescence device with ultra-high sensitivity and selectivity for rapid visualized monitoring of trace radon in environment

The World Health Organization has reported radioactive Rn gas as the second leading cause of lung cancer and gives an extreme limit to indoor Radon (Rn) concentration as 100 Bq/m³. To realize rapid and accurate Rn monitoring, we report an efficient visualized electrochemiluminescence (ECL) device for Rn detection with the lowest limit of detection (0.9 Bq/m³/3.6 Bq h m^-3) compared to known Rn detection methods and the shortest measurement time (less than 5 h) among non-pump methods. In detail, an efficient Rn probe is prepared by Au nanoparticles, Pb²⁺ aptamer, as well as NH₂-ssDNA co-reactant and then modified on ITO electrodes to obtain Rn detection devices. With tris(2,2'-bipyridyl)ruthenium(II)chloride (Ru(bpy)₃Cl₂) as an ECL emitter, the devices can exhibit ultra-high sensitivity and selectivity to trace Rn in environment via the ECL quenching caused by ²¹⁰Pb, the relatively stable decay product of Rn. Furthermore, ECL imaging technology can be applied to realize the visualized Rn detection. An efficient up-response ECL detector was also invented to support this detection device to achieve accurate Rn detection in environment. This work reports noble gas ECL detection for the first time and provides an efficient strategy for rapid and accurate monitoring of trace Rn in environment.

Programmable kernel structures of atomically precise metal nanoclusters for tailoring catalytic properties

The unclear structures and polydispersity of metal nanoparticles (NPs) seriously hamper the identification of the active sites and the construction of structure-reactivity relationships. Fortunately, ligand-protected metal nanoclusters (NCs) with atomically precise structures and monodispersity have become an ideal candidate for understanding the well-defined correlations between structure and catalytic property at an atomic level. The programmable kernel structures of atomically precise metal NCs provide a fantastic chance to modulate their size, shape, atomic arrangement, and electron state by the precise modulating of the number, type, and location of metal atoms. Thus, the special focus of this review highlights the most recent process in tailoring the catalytic activity and selectivity over metal NCs by precisely controlling their kernel structures. This review is expected to shed light on the in-depth understanding of metal NCs' kernel structures and reactivity relationships.

Impact of the metal core on the electrochemiluminescence of a pair of atomically precise Au20 nanocluster isomers

Although the electrochemiluminescence (ECL) of metal nanoclusters has been reported, revealing the correlation between structure and ECL at an atomic level is highly challenging. Here, we reported the impact of the metal core of Au₂₀(SAdm)₁₂(CHT)₄ (Au₂₀-AC for short; SAdm = 1-adamantanethiolate; CHT= cyclohexanethiol) and its isomer Au₂₀(TBBT)₁₆ (TBBT = 4-tert-butylthiophenol) on their solution-state and solid-state electrochemiluminescence. In self-annihilation ECL experiments, Au₂₀-AC showed a strong cathodic ECL but a weak anodic ECL, while the ECL signal of Au₂₀(TBBT)₁₆ was weak and barely detectable. Density functional theory (DFT) calculations showed that the Au₇ kernel of [Au₂₀-AC]^- is metastable, weakening its anodic ECL. Au₂₀-AC in solution-state displayed an intense co-reactant ECL in the near-infrared region, which is 7 times higher than that of standard Ru(bpy)₃²⁺. The strongest solid-state ECL emissions of Au₂₀-AC and Au₂₀(TBBT)₁₆ were at 860 and 770 nm, respectively - 15 nm red-shifted for Au₂₀-AC and 20 nm blue-shifted for Au₂₀(TBBT)₁₆, compared to their corresponding solid-state photoluminescence (PL) emissions. This work shows that ECL is significantly affected by the subtle differences of the metal core, and offers a potential basis for sensing and immunoassay platforms based on atomically precise emissive metal nanoclusters.

Non-enzymatic electrochemiluminescence biosensor for ultrasensitive detection of ochratoxin A based on efficient DNA walker

Ochratoxin A (OTA) with high toxicity represents a serious threat to the agriculture and food chain, consequently to human health. Herein, a simple electrochemiluminescence (ECL) biosensor was constructed for ultrasensitive detection of OTA based on mercaptopropionic acid templated Au nanoclusters (Au NCs) as intensive signal probe and a non-enzymatic 2D DNA walking machine as the effective amplification strategy. Specifically, the target related bipedal DNA walker efficiently moved along 2D DNA tracks through toehold-mediated DNA strand displacement, which triggered abundant signal probes for combining to the DNA tracks. Moreover, the Au NCs could exhibit strong ECL emission due to fast electron transfer from massive Au-S electronic pathways under the electrochemical excitation. Thus, the biosensor possessed significant ECL response for achieving ultrasensitive detection toward OTA with low detection limit of 3.19 fg/mL. Impressively, the sensing platform was also applied to detect OTA from edible oils, exhibiting great application potential in food analysis.

Ultra-Sensitive MicroRNA Biosensor Based on Strong Aggregation-Induced Electrochemiluminescence from Bidentate Ligand-Stabilized Copper Nanoclusters in Polymer Hydrogel

Herein, dihydrolipoic acid (DHLA)-stabilized copper nanoclusters (Cu NCs) with high aggregation-induced electrochemiluminescence (AIECL) in polymer hydrogel were prepared to construct an ECL biosensor for detection of microRNA-21. DHLA, a small molecule ligand with two sulfhydryl groups, was used as a protective agent to synthesize Cu NCs, which improved the ECL stability and intensity of Cu NCs. Furthermore, the Cu NCs were loaded into the (PVP-PVA)hydrogel to form the DHLA-Cu NCs@(PVP-PVA)hydrogel composite, which showed effective AIECL performance. The confinement of Cu NCs into the hydrogel increased the local concentration of Cu NCs, which could not only prevent oxides from entering the copper core, but also limit the vibration to reduce non-radiative transitions of Cu NCs, leading to a distinct AIECL emission. Then, combined with the self-priming clip trigger isothermal amplification (SCTIA) technology, an ECL biosensor was constructed to realize the sensitive detection of miRNA-21. Interestingly, SCTIA technology was a simple and efficient strategy that realized multiple-cycle amplified processes to acquire a mass of output DNA, achieving remarkable signal amplification. Therefore, this strategy provided an efficient approach in the preparation of Cu NCs with high AIECL emission and target amplification technology, which might have promising potential in clinical application.

Aggregation-Induced Electrochemiluminescence of Copper Nanoclusters by Regulating Valence State Ratio of Cu(I)/Cu(0) for Ultrasensitive Detection of MicroRNA

In this work, Cu nanoclusters (Cu NCs) with strong aggregation-induced electrochemiluminescence (AIECL) as emitters were used to construct an ECL biosensor for ultrasensitive detection of microRNA-141 (miR-141). Impressively, the ECL signals enhanced with the increased content of Cu(I) in the aggregative Cu NCs. When the ratio of Cu(I)/Cu(0) in aggregative Cu NCs was 3.2, Cu NCs aggregates showed the highest ECL intensity, in which Cu(I) could enhance the cuprophilic Cu(I)···Cu(I) interaction to form rod-shaped aggregates for restricting nonradiative transitions to obviously improve the ECL response. As a result, the ECL intensity of the aggregative Cu NCs was 3.5 times higher than that of the monodispersed Cu NCs. With the aid of the cascade strand displacement amplification (SDA) strategy, an outstanding ECL biosensor was developed to achieve the ultrasensitive detection of miR-141, whose linear range varied from 10 aM to 1 nM with a detection limit of 1.2 aM. This approach opened an avenue to prepare non-noble metal nanomaterials as robust ECL emitters and provided a new idea for detection of biomolecules for diagnosis of disease.

Electrochemiluminescence Distance and Reactivity of Coreactants Determine the Sensitivity of Bead-Based Immunoassays

Herein we report the study of electrochemiluminescence (ECL) generation by tris(2,2'-bipyridyl)ruthenium (Ru(bpy)₃ ²⁺ ) and five tertiary amine coreactants. The ECL distance and lifetime of coreactant radical cations were measured by ECL self-interference spectroscopy. And the reactivity of coreactants was quantitatively evaluated in terms of integrated ECL intensity. By statistical analysis of ECL images of single Ru(bpy)₃ ²⁺ -labeled microbeads, we propose that ECL distance and reactivity of coreactant codetermine the emission intensity and thus the sensitivity of immunoassay. 2,2-bis(hydroxymethyl)-2,2',2''-nitrilotriethanol (BIS-TRIS) can well balance ECL distance-reactivity trade-off and enhance the sensitivity by 236 % compared with tri-n-propylamine (TPrA) in the bead-based immunoassay of carcinoembryonic antigen. The study brings an insightful understanding of ECL generation in bead-based immunoassay and a way of maximizing the analytical sensitivity from the aspect of coreactant.

Electrochemiluminescence nanoemitters for immunoassay of protein biomarkers

The family of electrochemiluminescent luminophores has witnessed quick development since the electrochemiluminescence (ECL) phenomenon of silicon nanoparticles was first reported in 2002. Moreover, these developed ECL nanoemitters have extensively been applied in sensitive detection of protein biomarker by combining with immunological recognition. This review firstly summarized the origin and development of various ECL nanoemitters including inorganic and organic nanomaterials, with an emphasis on metal-organic frameworks (MOFs)-based ECL nanoemitters. Several effective strategies to amplify the ECL response of nanoemitters and improve the sensitivity of immunosensing were discussed. The application of ECL nanoemitters in immunoassay of protein biomarkers for diagnosis of cancers and other diseases, especially lung cancer and heart diseases, was comprehensively presented. The recent development of ECL imaging with the nanoemitters as ECL tags for detection of multiplex protein biomarkers on single cell membrane also attracted attention. Finally, the future opportunities and challenges in the ECL biosensing field were highlighted.

Valence-State-Engineered Electrochemiluminescence from Au Nanoclusters

To determine the intrinsic effects of body elements on the electrochemiluminescence (ECL) of metal nanoclusters (NCs), herein, a valence-state engineering strategy is developed to adjust the NCs' ECL with bovine serum albumin (BSA)-stabilized AuNCs as a model, in which engineering the valence state of the Au body element, i.e., Au(0) and Au(I), is performed via successively reducing the precursor AuCl₄^- to Au(I) and Au(0) with BSA. The obtained BSA-AuNCs/N₂H₄ system leads to three anodic ECL processes at 0.37 (ECL-1), 0.85 (ECL-2), and 1.45 V (ECL-3). ECL-1 is generated from the BSA-Au(0) section of BSA-AuNCs in a surface-defect-involved route and is much stronger and red-shifted compared to ECL-2 and ECL-3, which are generated from the BSA-Au(I) section of BSA-AuNCs in the band-gap-engineered route. Each of the anodic ECL processes can be selectively generated and/or suppressed via adjusting the Au(I)/Au(0) ratio of BSA-AuNCs, tunable ECL generation route, and triggering potential, and the emission intensity and waveband of metal NCs are conveniently achieved in body-element-involved valence-state engineering.

Covalent organic framework linked with amination luminol derivative as enhanced ECL luminophore for ultrasensitive analysis of cytochrome c

Cytochrome c (cyt c) plays a critical role in mitochondrial respiratory chain, whose absence is detrimental to electron transport and reduce adenosine triphosphate. For ultrasensitive detection of cyt c, sheet-like covalent organic frameworks (COFs) were prepared by orderly accumulation of 1,3,5-benzenetricarboxaldehyde (BTA) and p-phenylenediamine (PDA), and further grafted with N-(4-aminobutyl)-N-ethylisoluminol (ABEI) - an electrochemiluminescence (ECL) emitter. Specifically, the morphology and structure of the COFs-ABEI were mainly characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis, and X-ray photoelectron spectroscopy (XPS). In parallel, the optical properties of the emitter were certified by UV-vis absorbance spectroscopy, Fourier infrared spectroscopy (FTIR), fluorescence (FL), and ECL measurements, showing 2.25-time enhanced ECL efficiency over pure ABEI, coupled by illustrating the interfacial electron transport mechanism. On the above foundation, a label-free "signal off" ECL biosensor was constructed by virtue of the specific immune recognition between the aptamer of the target cyt c with its capture DNA (cDNA) anchored on the biosensing platform, exhibiting a wider linear range of 1.00 fg mL^-1-0.10 ng mL^-1 (R² = 0.998) and a lower limit of detection (LOD) down to 0.73 fg mL^-1. This work offers some constructive guidelines for sensitive bioassays of disease-related biomarkers in the clinical field.

Nanocluster Transformation Induced by SbF6- Anions toward Boosting Photochemical Activities

The interactions between SbF₆^- and metal nanoclusters are of significance for customizing clusters from both structure and property aspects; however, the whole-segment monitoring of this customization remains challenging. In this work, by controlling the amount of introduced SbF₆^- anions, the step-by-step nanocluster evolutions from [Pt₁Ag₂₈(S-Adm)₁₈(PPh₃)₄]Cl₂ (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) have been mapped out with X-ray crystallography, with which atomic-level SbF₆^- counterion effects in reconstructing and rearranging nanoclusters are determined. The structure-dependent optical properties, including optical absorption, photoluminescence, and electrochemiluminescence (ECL), of these nanoclusters are then explored. Notably, the Pt₁Ag₃₀-SbF₆ nanocluster was ultrabright with a high phosphorescence quantum yield of 85% in N₂-purged solutions, while Pt₁Ag₂₈ nanoclusters were fluorescent with weaker emission intensities. Furthermore, Pt₁Ag₃₀-SbF₆ displayed superior ECL efficiency over Pt₁Ag₂₈-SbF₆, which was rationalized by its increased effectively exposed reactive facets. Both Pt₁Ag₃₀-SbF₆ and Pt₁Ag₂₈-SbF₆ demonstrated unprecedented high absolute ECL quantum efficiencies at sub-micromolar concentrations. This work is of great significance for revealing the SbF₆^- counterion effects on the control of both structures and luminescent properties.

Surface Defect-Involved and Single-Color Electrochemiluminescence of Gold Nanoclusters for Immunoassay

Single-color electrochemiluminescence (ECL) of nanoparticles is normally achieved in a bandgap engineered route via passivating the nanoparticle surface. Herein, when linear mercaptoalkanoic acids are employed as the thiol-capping agent of unary Au nanoclusters (NCs), a single-stabilizer-capped strategy is proposed to achieve surface defect-involved and single-color ECL from the AuNCs with hydrazine (N2H4) as the coreactant. The carbon skeleton of the linear mercaptoalkanoic acids exhibits important effects on the ECL of the AuNCs, and efficient oxidative-reductive ECL is achieved with 8-mercaptooctanoic acid (MOA), 11-mercaptoundecanoic acid (MUA), and 12-mercaptododecanoic acid (MDA) capped AuNCs, respectively. The ECL of these AuNCs not only exhibits similar ECL intensity-potential profiles with the same maximum emission potential of ∼1.20 V (vs Ag/AgCl), but also demonstrates almost identical spectral ECL profiles of the same maximum emission wavelength around 713 nm as well as the same fwhm of 64 nm. The ECL of AuNCs/N2H4 is obviously red-shifted to the photoluminescence of AuNCs, which not only provides unambiguous evidence that bandgap-engineered ECL of these AuNCs is quenched but also manifests that the capping agent of linear mercaptoalkanoic acid is promising for the achievement of surface defect-involved and single-color ECL from AuNCs. The MUA capped AuNCs can be utilized as an ECL tag for a sensitive and selective immunoassay, which exhibits a broad linear range from 0.5 mU/mL to 1 U/mL with a low limit of detection of 0.1 mU/mL (S/N = 3) with CA125 as the model analyte. This work provides a promising alternative to the traditional surface-passivating strategy for the achievement of single-color ECL from nanoparticle luminophores.

Enhanced Electrochemiluminescence of A Macrocyclic Tetradentate Chelate Pt(II) Molecule via Its Collisional Interactions with the Electrode

A macrocyclic tetradentate chelate Pt(II) molecule (Pt1) served as an excellent luminophore in electrochemiluminescence (ECL) processes. The blue ECL of Pt1/S2O82- coreactant system in N,N'-dimethylformamide was found to be 46 times higher than that of the Ru(bpy)2+/S2O82- system or 30 times higher than that of the 9,10-diphenylanthracene/S2O82- system. The unprecedented high ECL quantum efficiencies were caused by the cyclic generation of monomer excited states through collisional interactions of Pt1 molecules with the electrode at an elevated frequency. The ECL is tunable from bright blue to pure white by simply changing the solvent from N,N'-dimethylformamide to dichloromethane. The white ECL of Pt(II) molecule was reported for the first time and the mechanism was proposed to be the simultaneous emissions from the monomer excited state (blue) and excimer (red).

Recent Advances in Single Cell Analysis by Electrochemiluminescence

Understanding biological mechanisms operating in cells is one of the major goals of biology. Since heterogeneity is the fundamental property of cellular systems, single cell measurements can provide more accurate information about the composition, dynamics, and regulatory circuits of cells than population-averaged assays. Electrochemiluminescence (ECL), the light emission triggered by electrochemical reactions, is an emerging approach for single cell analysis. Numerous analytes, ranging from small biomolecules such as glucose and cholesterol, proteins and nucleic acids to subcellular structures, have been determined in single cells by ECL, which yields new insights into cellular functions. This review aims to provide an overview of research progress on ECL principles and systems for single cell analysis in recent years. The ECL reaction mechanisms are briefly introduced, and then the advances and representative works in ECL single cell analysis are summarized. Finally, outlooks and challenges in this field are addressed.

Insight into the Mechanism of Single-Metal-Atom Tailoring on the Surface of Au-Cu Alloy Nanoclusters

Tailoring the surface structure of nanomaterials is desirable for investigating their mechanisms and properties from a nanochemistry perspective. The modification of the surface of metal nanoparticles with a single metal atom has proven difficult, which has hindered the understanding of the contribution of different motifs in nanoclusters to their properties. Herein, we report single-metal-atom surface tailoring by thermally etching the nanocluster Au_xCu_15-x(DPPMH)₃(SPhCl₂)₉ (x = 8 or 9) to obtain Au_xCu_16-x(DPPMH)₂(DPPM)(SPhCl₂)₉ (x = 9 or 10) nanoclusters. An Au₇Cu₄ core was observed in both nanoclusters, which can be regarded as part of an icosahedron. Experiments and theoretical simulations revealed the tailoring processes of the icosahedron. Both nanoclusters displayed an NIR-II emission, and the introduction of the surface metal atom led to a red-shift in the emission band from 983 to 1025 nm. This work contributes to the development of precisely tailored nanocluster structures and provides an understanding of structure-property correlations.

Polymer Carbon Nanodots: A Novel Electrochemiluminophore for Dual Mode Detection of Ferric Ions

The development of simple and effective dual-mode analytical methods plays crucial regulatory roles in the discrimination of relevant target species, because of their built-in cross reference correction and high accuracy. In this work, a novel polymer carbon nanodots (PCNDs) prepared from a facile one-pot hydrothermal procedure using readily available l-tryptophan and l-phenylalanine as precursors, showed excellent aqueous solubility and blue fluorescence property with a high quantum yield of 29%. Moreover, the PCNDs was demonstrated to be a robust luminophore with electrochemiluminescence (ECL) efficiency of 43% was achieved by using K₂S₂O₈ as a coreactant. The spooling ECL spectroscopy was employed to take insight into excited states responsible for the potential-dependent ECL emissions. Most importantly, when introduced into construction of the fluorescence and ECL dual mode sensing platform, for the first time, the PCNDs displayed prominent performance for the detection of ferric ions (Fe³⁺). The ferric ions could be quantified ranging from micromolar to millimolar with a detection limit of 0.22 and 5.3 μM, respectively. Such a dual-functional sensing platform also exhibits excellent selectivity, reproducibility and stability. Results from this work indicate that PCNDs showing great promise as a bright luminophore for the fabrication of low-cost, high-performance dual-signal readout platforms for ferric ions determination.

Near-Infrared Electrogenerated Chemiluminescence from Simple Copper Nanoclusters for Sensitive Alpha-Fetoprotein Sensing

Exploiting low cost, water-soluble, and near-infrared (NIR) emissive electrochemiluminophores (ECLphores) is significantly important for biological applications. In this study, bright and NIR electrogenerated chemiluminescence (ECL) emissive copper nanoclusters (Cu NCs) were synthesized through a facile one-pot wet chemical reduction method. ECL properties of obtained Cu NCs were examined in the presence of potassium persulfate, resulting in maximum intensity at 735 nm, at least 135 nm red-shifted with respect to all other Cu NCs. Electrochemistry, photoluminescence (PL), and spooling ECL spectroscopies were used to track NIR ECL emission of Cu NCs ascribed to the monomeric excited states. Due to the abundant binding sites of bovine serum albumin (BSA) to anchor target biomolecules, a sandwich-type ECL immunosensor was thus fabricated using such BSA-templated Cu NCs as tags and alpha fetoprotein antigen (AFP) as a model protein for the first time. Without assisting any signal amplification strategies, the proposed NIR ECL biosensor exhibited a wide linear range (1-400 ng mL^-1) and low detection limit (0.02 ng mL^-1) as well as superior selectivity and reproducibility and was successfully applied in real human serum sample determination. This work sets the stage for the development of novel non-noble metal nanoclusters for large-scale and emerging nanotechnology applications.