Electrochemiluminescence Waveguide in Single Crystalline Molecular Wires

Electrochemiluminescence Microscopy

Electrochemiluminescence (ECL) is rapidly evolving from an analytical method into an optical microscopy. The orthogonality of the electrochemical trigger and the optical readout distinguishes it from classic microscopy and electrochemical techniques, owing to its near-zero background, remarkable sensitivity, and absence of photobleaching and phototoxicity. In this minireview, we summarize the recent advances in ECL imaging technology, emphasizing original configurations which enable the imaging of biological entities and the improvement of the analytical properties by increasing the complexity and multiplexing of bioassays. Additionally, mapping the (electro)chemical reactivity in space provides valuable information on nanomaterials and facilitates deciphering ECL mechanisms for improving their performances in diagnostics and (electro)catalysis. Finally, we highlight the recent achievements in imaging at the ultimate limits of single molecules, single photons or single chemical reactions, and the current challenges to translate the ECL imaging advances to other fields such as material science, catalysis and biology.

Controllable and Low-Loss Electrochemiluminescence Waveguide Supported by a Micropipette Electrode

One-dimensional molecular crystal waveguide (MCW) can transmit self-generated electrochemiluminescence (ECL), but heavy optical loss occurs because of the small difference in the refractive index between the crystal and its surroundings. Herein, we report a micropipette electrode-supported MCW (MPE/MCW) for precisely controlling the far-field transmission of ECL in air with a low optical loss. ECL is generated from one terminal of the MCW positioned inside the MPE, which is transmitted along the MCW to the other terminal in air. In comparison with conventional waveguides on solid substrates or in solutions, the MPE/MCW is propitious to the total internal reflection of light at the MCW/air interface, thus confining the ECL efficiently in MCW and improving the waveguide performance with an extremely low-loss coefficient of 4.49 × 10-3 dB μm-1. Moreover, by regulation of the gas atmosphere, active and passive waveguides can be resolved simultaneously inside MPE and in air. This MPE/MCW offers a unique advantage of spatially controlling and separating ECL signal readout from its generation, thus holding great promise in biosensing without or with less electrical/chemical disturbance.

Nanoscale Luminescence Imaging/Detection of Single Particles: State-of-the-Art and Future Prospects

Single-particle-level measurements, during the reaction, avoid averaging effects that are inherent limitations of conventional ensemble strategies. It allows revealing structure-activity relationships beyond averaged properties by considering crucial particle-selective descriptors including structure/morphology dynamics, intrinsic heterogeneity, and dynamic fluctuations in reactivity (kinetics, mechanisms). In recent years, numerous luminescence (optical) techniques such as chemiluminescence (CL), electrochemiluminescence (ECL), and fluorescence (FL) microscopies have been emerging as dominant tools to achieve such measurements, owing to their diversified spectroscopy principles, noninvasive nature, higher sensitivity, and sufficient spatiotemporal resolution. Correspondingly, state-of-the-art methodologies and tools are being used for probing (real-time, operando, in situ) diverse applications of single particles in sensing, medicine, and catalysis. Herein, we provide a concise and comprehensive perspective on luminescence-based detection and imaging of single particles by putting special emphasis on their basic principles, mechanistic pathways, advances, challenges, and key applications. This Perspective focuses on the development of emission intensities and imaging based individual particle detection. Moreover, several key examples in the areas of sensing, motion, catalysis, energy, materials, and emerging trends in related areas are documented. We finally conclude with the opportunities and remaining challenges to stimulate further developments in this field.

Nanoconfined Cathodic Electrochemiluminescence for Self-Sensitized Bioimaging of Membrane Protein

Self-enhanced electrochemiluminescence (ECL) can be achieved via the confinement of coreactants and ECL emitters in a single nanostructure. This strategy has been used for the design of anodic ECL systems with amine compounds as coreactants. In this work, a novel confinement system was proposed by codoping positively charged ECL emitter tris(2,2'-bipyridine)ruthenium(II) (Ru(bpy)32+) and negatively charged coreactant peroxydisulfate (S2O82-) in silica nanoparticles. The codoping process could be performed by introducing S2O82- in cationic poly(diallyldimethylammonium chloride) (PDDA) to form PDDA@S2O82- and then encapsulating it and Ru(bpy)32+ in the Triton X-100 vesicle followed by the hydrolysis of tetraethyl ortosilicate, surface modification, and demulsification. The obtained RuSSNs exhibited good homogeneity, excellent monodispersity, acceptable biocompatibility, and 2.9-fold stronger ECL emission than Ru(bpy)32+-doped silica nanoparticles at an equal amount of nanoparticles in the presence of 0.1 M K2S2O8. Thus, an in situ self-sensitized cathodic ECL imaging method was designed for the monitoring of glycoprotein on living cell membranes. This work provides a new way for the modification, enhancement, and application of nano-ECL emitters in biological analysis.

Dynamic Mapping of Electrochemiluminescence Reactivity in Space: Application to Bead-Based Assays

As an electrochemical technique offering an optical readout, electrochemiluminescence (ECL) evolved recently into a powerful microscopy technique with the visualization of a wide range of microscopic entities. However, the dynamic imaging of transient ECL events did not receive intensive attention due to the limited number of electrogenerated photons. Here, the reaction kinetics of the model ECL bioassay system was revealed by dynamic imaging of single [Ru(bpy)3]2+-functionalized beads in the presence of the efficient tripropylamine coreactant. The time profile behavior of ECL emission, the variations of the ECL layer thickness, and the position of maximum ECL intensity over time were investigated, which were not achieved by static imaging in previous studies. Moreover, the dynamics of the ECL emission were confronted with the simulation. The reported dynamic ECL imaging allows the investigation of the ECL kinetics and mechanisms operating in bioassays and cell microscopy.

Facile preparation of poly-(styrene-co-maleic anhydride) encapsulated Iridium(III) complexes as highly efficient electrochemiluminescence indicators for sensitive immunoassay of CYFRA 21-1

Exploring facile strategy for developing highly efficient emitters using water-insoluble luminophores has become a vital topic in electrochemiluminescence (ECL) immunoassay. In this work, an ECL-active and water-dispersive iridium(III) complex-based polymer dots (IrPdots) was fabricated by encapsulating water-insoluble tris[1-phenylisoquinolinato-C2, N] iridium(III) complexes [Ir(piq)₃] into poly-(styrene-co-maleic anhydride) (PSMA) matrix by a controllable nanoprecipitation process. The obtained IrPdots generated strong ECL signals in the presence of tri-n-propylamine (TPrA) and were used to label detection antibody (Ab₂) to act as ECL probes to indicate the signal changes when analyzing target antigen. To construct a sandwich immunosensor, Pd nanoparticles (NPs) decorated MoS₂/Ti₃C₂T_x MXene nanocomposites (MoS₂/Ti₃C₂T_x MXene/Pd) were fabricated as substrates to bind capture antibody (Ab₁), which could further amplify ECL signals via a coreaction-accelerating pathway to improve the detection sensitivity. When the cytokeratin 19 fragment 21-1 (CYFRA 21-1) was chosen as model analyte, the developed immunosensor displayed a good linear relationship ranging from 0.1 pg/mL to 50 ng/mL with a low detection limit of 95 fg/mL (S/N = 3) was achieved as well. This research proposed a facile and effective method of fabricating IrPdots as ECL probes for immunoassay using water-insoluble iridium complexes, which expanded the application scope of those water-insoluble luminophores for aqueous bioanalysis.

Bright near-infrared circularly polarized electrochemiluminescence from Au9Ag4 nanoclusters

Metal nanoclusters are excellent electrochemiluminescent luminophores owing to their rich electrochemical and optical properties. However, the optical activity of their electrochemiluminescence (ECL) is unknown. Herein, we achieved, for the first time, the integration of optical activity and ECL, i.e., circularly polarized electrochemiluminescence (CPECL), in a pair of chiral Au₉Ag₄ metal nanocluster enantiomers. Chiral ligand induction and alloying were employed to endow the racemic nanoclusters with chirality and photoelectrochemical reactivity. S-Au₉Ag₄ and R-Au₉Ag₄ exhibited chirality and bright-red emission (quantum yield = 4.2%) in the ground and excited states. The enantiomers showed mirror-imaged CPECL signals at 805 nm owing to their highly intense and stable ECL emission in the presence of tripropylamine as a co-reactant. The ECL dissymmetry factor of the enantiomers at 805 nm was calculated to be ±3 × 10^-3, which is comparable with that obtained from their photoluminescence. The obtained nanocluster CPECL platform shows the discrimination of chiral 2-chloropropionic acid. The integration of optical activity and ECL in metal nanoclusters provides the opportunity to achieve enantiomer discrimination and local chirality detection with high sensitivity and contrast.

All-Silicon Photoelectric Biosensor on Chip Based on Silicon Nitride Waveguide with Low Loss

Compared to the widely used compound semiconductor photoelectric sensors, all-silicon photoelectric sensors have the advantage of easy mass production because they are compatible with the complementary metal-oxide-semiconductor (CMOS) fabrication technique. In this paper, we propose an all-silicon photoelectric biosensor with a simple process and that is integrated, miniature, and with low loss. This biosensor is based on monolithic integration technology, and its light source is a PN junction cascaded polysilicon nanostructure. The detection device utilizes a simple refractive index sensing method. According to our simulation, when the refractive index of the detected material is more than 1.52, evanescent wave intensity decreases with the growth of the refractive index. Thus, refractive index sensing can be achieved. Moreover, it was also shown that, compared to a slab waveguide, the embedded waveguide designed in this paper has a lower loss. With these features, our all-silicon photoelectric biosensor (ASPB) demonstrates its potential in the application of handheld biosensors.

Artificial Light-Harvesting System Based on Zinc Porphyrin and Benzimidazole: Construction, Resonance Energy Transfer, and Amplification Strategy for Electrochemiluminescence

Constructing robust and efficient luminophores is of significant importance in the development of electrochemiluminescence (ECL) amplification strategies. Inspired by the resonance energy transfer in natural light-harvesting systems, we propose a novel ECL amplification system based on ECL resonance energy transfer (ECL-RET), which integrates two luminophores, benzimidazole (BIM) and zinc(II) tetrakis(4-carboxyphenyl)porphine (ZnTCPP), into one framework. Through disassembling and reconstruction processes, numerous BIM surround ZnTCPP in the constructed ZIF-9-ZnTCPP. Combined with the overlapped spectra between the emission of BIM and the absorption of ZnTCPP, the energy of multiple BIM (donor) can be concentrated to a single ZnTCPP (acceptor) to amplify the ECL emission of the acceptor. This work provides a convenient way to design an efficient ECL-RET system, which initiates a brand-new chapter in the development of ECL amplification strategies.

A temperature-tuned electrochemiluminescence layer for reversibly imaging cell topography

Investigating electrochemiluminescence (ECL) scenarios under different temperatures is important to expand its imaging scope near an electrode surface. Here, we develop a temperature-tuned ECL layer by recording the evolution of shadow regions of adherent cells. Finite element simulation and experimental results demonstrate that the thickness of the ECL layer (TEL) is reversibly regulated by electrode temperature (T _e), so that single cell topography at different heights is imaged. The TEL in two ECL routes shows different regulation ranges with elevated T _e, thus providing a flexible approach to adjust the imaging scope within specific heights. In addition, a heated electrode significantly improves the image quality of cell adhesion in heterogeneous electrochemical rate-determined situations. Thus, the contrast in cell regions shows a reversible response to T _e. This work provides a new approach to regulate the TEL and is promising for monitoring transient heat generation from biological entities.

Arginine-modified black phosphorus quantum dots with dual excited states for enhanced electrochemiluminescence in bioanalysis

The electrochemiluminescence (ECL) is generally emitted via radiative transition of singlet or triplet excited state (S₁ or T₁). Herein, an ECL mechanism with the transitions of both S₁ and T₁ of black phosphorus quantum dots (BPQDs) is found, and an arginine (Arg) modification strategy is proposed to passivate the surface oxidation defects of BPQDs, which could modulate the excited states for enhancing the ECL efficiency of BPQDs. The Arg modification leads to greater spatial overlap of highest and lowest occupied molecular orbitals, and spectral shift of radiative transitions, and improves the stability of anion radical of BPQDs. To verify the application of the proposed mechanism, it is used to construct a sensitive method for conveniently evaluating the inhibiting efficiency of cyclo-arginine-glycine-aspartic acid-d-tyrosine-lysine to cell surface integrin by using Arg containing peptide modified BPQDs as signal tag. The dual excited states mediated ECL emitters provide a paradigm for adjustable ECL generation and extend the application of ECL analysis.

2D Zn-Porphyrin-Based Co(II)-MOF with 2-Methylimidazole Sitting Axially on the Paddle-Wheel Units: An Efficient Electrochemiluminescence Bioassay for SARS-CoV-2

High electrocatalytic activity with tunable luminescence is crucial for the development of electrochemiluminescence (ECL) luminophores. In this study, a porphyrin-based heterobimetallic 2D metal organic framework (MOF), [(ZnTCPP)Co2(MeIm)] (1), is successfully self-assembled from the zinc(II) tetrakis(4-carboxyphenyl)porphine (ZnTCPP) linker and cobalt(II) ions in the presence of 2-methylimidazole (MeIm) by a facile one-pot reaction in methanol at room temperature. On the basis of the experimental results and the theoretical calculations, the MOF 1 contains paddle-wheel [Co2(-CO2)4] secondary building units (SBUs) axially coordinated by a MeIm ligand, which is very beneficial to the electron transfer between the Co(II) ions and oxygen. Combining the photosensitizers ZnTCPP and the electroactive [Co2(-CO2)4] SBUs, the 2D MOF 1 possesses an excellent ECL performance, and can be used as a novel ECL probe for rapid nonamplified detection of the RdRp gene of SARS-CoV-2 with an extremely low limit of detection (≈30 aM).

Dual-wavelength electrochemiluminescence biosensor based on a multifunctional Zr MOFs@PEI@AuAg nanocomposite with intramolecular self-enhancing effect for simultaneous detection of dual microRNAs

Rapid parallel detection of multi-targets has always been an exploration aim in electrochemiluminescence (ECL) assays. Herein, a multifunctional nanocomposite of Zr metal-organic frameworks (MOFs) @PEI@AuAg nanoclusters (NCs) with intense and stable dual-wavelength ECL was synthesized for the first time, and used to construct a new ECL biosensor for rapid simultaneous detection of dual targets. Notably, the novel ECL emitter Zr MOFs with high-performance was not only integrated with a co-reactant polyethyleneimine (PEI) to form a unique intramolecular self-enhancing structure, but also loaded a large number of another ECL emitter AuAg NCs, furthermore, AuAg NCs with superior electron transfer property can much enhance the electrical conductivity of the composites, thus achieving the goal of "killing three birds with one stone". Moreover, a unique stable and rigid three-dimensional DNA tetrahedron (TDN) structure was connected with two quenching probes BHQ1 and BHQ3 and immobilized on the composites-modified electrode, so ECL emission of the nanocomposites at two wavelengths of 535 nm and 644 nm were both quenched by resonance energy transfer (RET). In the presence of target miRNAs, the efficient DNA cycling double-amplification processes were performed by using exonuclease (T7 Exo) combined with DNA Walker, thus both quenching groups were separated to restore the ECL at two wavelengths, achieving simultaneous and rapid ECL detection of two miRNAs. Therefore, this present work not only opens a unique nanocomplex with dual wavelength ECL and self-enhancing performance, but also develops a highly sensitive ECL biosensor with promising value for rapid multi-target analysis in clinical fields.

Deciphering electrochemiluminescence generation from luminol and hydrogen peroxide by imaging light emitting layer

Electrochemiluminescence (ECL) of luminol is a luminescence process that proceeds in the presence of reactive oxygen species (e.g. hydrogen peroxide (H2O2)) at a suitable electrode potential, the reaction mechanism of which is complicated and remains ambiguous. In this work, we report a visualization approach for measuring the thickness of the ECL layer (TEL) of the luminol/H2O2 system to decipher the reaction process by combined use of the microtube electrode, ECL microscopy, and finite element simulations. With the increase of solution pH, the ECL image captured with the microtube electrode tends to vary from spot to ring, corresponding to the decrease of TEL from >9.1 μm to ca. 4.3 μm. We propose that different intermediates are involved in the course of ECL reaction. At a low pH (e.g. pH < 9), a relatively large TEL is most likely determined by the diffusion of oxidized and deprotonated luminol intermediate that is neutral and has a long lifetime. While at a high pH (e.g. pH in the range of 10 to 12), the ECL reaction is controlled by short-lived radical intermediates of both luminol and superoxide anion. The proposed mechanism is proved theoretically by finite element simulations and experimentally by the apparent effect of concentration ratio of luminol/H2O2.

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.

Determining the depth of surface charging layer of single Prussian blue nanoparticles with pseudocapacitive behaviors

Understanding the hybrid charge-storage mechanisms of pseudocapacitive nanomaterials holds promising keys to further improve the performance of energy storage devices. Based on the dependence of the light scattering intensity of single Prussian blue nanoparticles (PBNPs) on their oxidation state during sinusoidal potential modulation at varying frequencies, we present an electro-optical microscopic imaging approach to optically acquire the Faradaic electrochemical impedance spectroscopy (oEIS) of single PBNPs. Here we reveal typical pseudocapacitive behavior with hybrid charge-storage mechanisms depending on the modulation frequency. In the low-frequency range, the optical amplitude is inversely proportional to the square root of the frequency (∆I ∝ f^−0.5; diffusion-limited process), while in the high-frequency range, it is inversely proportional to the frequency (∆I ∝ f⁻¹; surface charging process). Because the geometry of single cuboid-shaped PBNPs can be precisely determined by scanning electron microscopy and atomic force microscopy, oEIS of single PBNPs allows the determination of the depth of the surface charging layer, revealing it to be ~2 unit cells regardless of the nanoparticle size.

The surface charging layer in nanomaterials, which determines their pseudocapacitive behavior, is challenging to characterize. Here the authors perform Faradic electrochemical impedance spectroscopy measurements of single cuboid Prussian blue nanoparticles, displaying a hybrid charge storage mechanism, and determine the depth of the surface charging layer.

Lighting Up Electrochemiluminescence-Inactive Dyes via Grafting Enabled by Intramolecular Resonance Energy Transfer

Due to near-zero optical background and photobleaching, electrochemiluminescence (ECL), an optical phenomenon excited by electrochemical reactions, has drawn extensive attention, especially for ultrasensitive bioassays. Developing diverse ECL emitters is crucial to unlocking their multiformity and performances but remains a formidable challenge due to the rigorous requirements for ECL. Herein, we report a general strategy to light up ECL-inactive dyes in an aqueous solution via grafting, a well-developed concept for plant propagation since 500 BCE. As a proof of concept, a series of luminol donor-dye acceptor-based ECL emitters were grafted with near-unity resonance energy transfer (RET) efficiency and coarse/fine-tunable emission wavelengths. Rather than the sophisticated design of new skeleton-based molecules to meet all of the prerequisites for ECL in a constrained manner, each unit in the proposed ECL ensemble performed its functions maximally. As a result, beyond traditional two-dimensional (2D) ones, a three-dimensional (3D) coordinate biosensing system, simultaneously showing a calibration curve and selectivity, was established using the new ECL emitter. This lighting up strategy would generally address the scarcity of ECL emitters and enable unprecedented functions.

Electrogenerated chemiluminescence of a Ru(bpy)32+/arginine system: a specific and sensitive detection of acetaminophen

Ru(bpy)₃Cl₂/TPrA is a prominent and widely used ECL system in analytical science. However, the co-reactant TPrA restricts the variety of applications because of its toxicity, volatility, and high cost. Here, we use arginine (Arg) as an alternative co-reactant for Ru(bpy)₃²⁺ by taking advantage of its low cost, non-toxicity, and biocompatibility. The mechanism of the Ru(bpy)₃²⁺/Arg system is that the deprotonated Arg can react with Ru(bpy)₃²⁺ to release emission. The similarity between the Ru(bpy)₃²⁺/Arg, Ru(bpy)₃²⁺/TPrA, and Ru(bpy)₃²⁺/DBAE systems demonstrates that Arg can be used as an alternative co-reactant for Ru(bpy)₃²⁺ ECL. As a proof of concept, we achieve an excellent performance for acetaminophen (Ace) detection based on the specificity of Arg and Ace, with excellent linearity, low detection limits, and good recoveries. This work is promising to expand the scope of the Ru(bpy)₃²⁺/Arg system and move forward their applications in bioassays.

We use arginine as an alternative co-reactant for Ru(bpy)₃²⁺ by taking advantage of its low cost, non-toxicity, and biocompatibility. It is promising to expand the scope of the Ru(bpy)₃²⁺/Arg system and move forward their applications in bioassays.

Covalent Organic Frameworks as Advanced Uranyl Electrochemiluminescence Monitoring Platforms

Electrochemiluminescence (ECL), as an advanced sensing process, can selectively control the generation of excited states by changing the potential. However, most of the existing ECL systems rely on poisonous coreactants to provide radicals for luminescence; although the ECL efficiency was improved, the athematic coreactants will cause unpredictable interference to the accurate analysis of trace targets. Herein, we realized the ECL of nonemitting molecules by performing intramolecular electron transfer in the olefin-linked covalent organic frameworks (COFs), with a high efficiency of 63.7%. Employing internal dissolved oxygen as the coreactant, it is well suitable for the analysis of various complex samples in the environment. Taking nuclear contamination analysis as the goal orientation, we further illustrated a design of a "turn-on" uranyl ion monitoring system integrating fast response, low detection limit, and high selectivity, showing that new ECL-COFs are promising to facilitate environment-related sensing analysis and structure-feature correlation mechanism exploration.

Shadow Electrochemiluminescence Microscopy of Single Mitochondria

Mitochondria are the subcellular bioenergetic organelles. The analysis of their morphology and topology is essential to provide useful information on their activity and metabolism. Herein, we report a label-free shadow electrochemiluminescence (ECL) microscopy based on the spatial confinement of the ECL-emitting reactive layer to image single living mitochondria deposited on the electrode surface. The ECL mechanism of the freely-diffusing [Ru(bpy)₃ ]²⁺ dye with the sacrificial tri-n-propylamine coreactant restrains the light-emitting region to a micrometric thickness allowing to visualize individual mitochondria with a remarkable sharp negative optical contrast. The imaging approach named "shadow ECL" (SECL) reflects the negative imprint of the local diffusional hindrance of the ECL reagents by each mitochondrion. The statistical analysis of the colocalization of the shadow ECL spots with the functional mitochondria revealed by classical fluorescent biomarkers, MitoTracker Deep Red and the endogenous intramitochondrial NADH, validates the reported methodology. The versatility and extreme sensitivity of the approach are further demonstrated by visualizing single mitochondria, which remain hardly detectable with the usual biomarkers. Finally, by alleviating problems of photobleaching and phototoxicity associated with conventional microscopy methods, SECL microscopy should find promising applications in the imaging of subcellular structures.

Spatially Selective Imaging of Cell-Matrix and Cell-Cell Junctions by Electrochemiluminescence

Cell junctions are protein structures located at specific cell membrane domains that determine key processes in multicellular development. Here we report spatially selective imaging of cell junctions by electrochemiluminescence (ECL) microscopy. By regulating the concentrations of luminophore and/or co-reactant, the thickness of ECL layer can be controlled to match with the spatial location of different cell junctions. At a low concentration of luminophore, ECL generation is confined to the electrode surface, thus revealing only cell-matrix adhesions at the bottom of cells. While at a high concentration of luminophore, the ECL layer can be remarkably extended by decreasing the co-reactant concentration, thus allowing the sequential imaging of cell-matrix and cell-cell junctions at the bottom and near the apical surface of cells, respectively. This strategy not only provides new insights into the ECL mechanisms but also promises wide applications of ECL microscopy in bioimaging.

Electrogenerated chemiluminescence detection of single entities

Electrogenerated chemiluminescence, also known as electrochemiluminescence (ECL), is an electrochemically induced production of light by excited luminophores generated during redox reactions. It can be used to sense the charge transfer and related processes at electrodes via a simple visual readout; hence, ECL is an outstanding tool in analytical sensing. The traditional ECL approach measures averaged electrochemical quantities of a large ensemble of individual entities, including molecules, microstructures and ions. However, as a real system is usually heterogeneous, the study of single entities holds great potential in elucidating new truths of nature which are averaged out in ensemble assays or hidden in complex systems. We would like to review the development of ECL intensity and imaging based single entity detection and place emphasis on the assays of small entities including single molecules, micro/nanoparticles and cells. The current challenges for and perspectives on ECL detection of single entities are also discussed.

Dual Intrareticular Oxidation of Mixed-Ligand Metal-Organic Frameworks for Stepwise Electrochemiluminescence

This work presents a mixed-ligand metal-organic framework (m-MOF) integrated with two ligands, one as a luminophore and the other as a coreactant, on one metal node for self-enhanced electrochemiluminescence (ECL). Both 9,10-di(p-carboxyphenyl)anthracene (DPA) and 1,4-diazabicyclo[2.2.2]octane (D-H₂) ligands can be oxidized, generating the cation radicals DPA^+• and D-H₂^+•, respectively. The latter can be deprotonated to form the neutral radical (D-H^•) and then react with DPA^+• to produce excited DPA* for ECL emission without exogenous coreactants. As a result of the incorporation into the MOF framework and the intrareticular charge transfer between the two ligands, the ECL intensity of the m-MOF was increased 26.5-fold compared with that of the mixture of DPA and D-H₂ in aqueous solution. Moreover, with the process of second oxidation of D-H₂, stepwise ECL emission was observed as a result of local excitation in the DPA unit, which was identified through density functional theory calculations. Overall, the implementation of the mixed-ligand approach, which combines the luminophore and coreactant as linkers in reticular materials, enriches the fundamentals and applications of ECL systems.

Electrochemiluminescence Loss in Photobleaching

The effects of photobleaching on electrochemiluminescence (ECL) was investigated for the first time. The plasma membrane of Chinese Hamster Ovary (CHO) cells was labeled with a [Ru(bpy)₃ ]²⁺ derivative. Selected regions of the fixed cells were photobleached using the confocal mode with sequential stepwise illumination or cumulatively and they were imaged by both ECL and photoluminescence (PL). ECL was generated with a model sacrificial coreactant, tri-n-propylamine. ECL microscopy of the photobleached regions shows lower ECL emission. We demonstrate a linear correlation between the ECL decrease and the PL loss due to the photobleaching of the labels immobilized on the CHO membranes. The presented strategy provides valuable information on the fundamentals of the ECL excited state and opens new opportunities for exploring cellular membranes by combining ECL microscopy with photobleaching techniques such as fluorescence recovery after photobleaching (FRAP) or fluorescence loss in photobleaching (FLIP) methods.

High-resolution imaging of catalytic activity of a single graphene sheet using electrochemiluminescence microscopy

Here, the electrocatalytic activity of a single graphene sheet is mapped using electrochemiluminescence (ECL) microscopy with a nanometer resolution. The achievement of this high-spatial imaging relies on the varied adsorption of hydrogen peroxide at different sites on the graphene surface, leading to unsynchronized ECL emission. By shortening the exposure time to 0.2 ms, scattered ECL spots are observed in the ECL image that are not overlaid with the spots in the consecutive images. Accordingly, after stacking all the images into a graph, the ECL intensity of each pixel could be used to reflect the electrocatalytic features of the graphene surface with a resolution of 400 nm. This novel ECL method efficiently avoids the long-standing problem of classic ECL microscopy regarding the overlap of ECL emissions from adjacent regions and enables the nanometer spatial resolution of ECL microscopy for the first time.

Mechanical single-molecule potentiometers with large switching factors from ortho-pentaphenylene foldamers

Molecular potentiometers that can indicate displacement-conductance relationship, and predict and control molecular conductance are of significant importance but rarely developed. Herein, single-molecule potentiometers are designed based on ortho-pentaphenylene. The ortho-pentaphenylene derivatives with anchoring groups adopt multiple folded conformers and undergo conformational interconversion in solutions. Solvent-sensitive multiple conductance originating from different conformers is recorded by scanning tunneling microscopy break junction technique. These pseudo-elastic folded molecules can be stretched and compressed by mechanical force along with a variable conductance by up to two orders of magnitude, providing an impressively higher switching factor (114) than the reported values (ca. 1~25). The multichannel conductance governed by through-space and through-bond conducting pathways is rationalized as the charge transport mechanism for the folded ortho-pentaphenylene derivatives. These findings shed light on exploring robust single-molecule potentiometers based on helical structures, and are conducive to fundamental understanding of charge transport in higher-order helical molecules.

Visualization of an Accelerated Electrochemical Reaction under an Enhanced Electric Field

Locally enhanced electric fields produced by high-curvature structures have been reported to boost the charge transport process and improve the relevant catalytic activity. However, no visual evidence has been achieved to support this new electrochemical mechanism. Here, accelerated electrochemiluminescence (ECL) reactions emitting light are visualized for the first time at the heterogeneous interfaces between microbowls and the supporting electrode surface. The simulation result shows that the electric intensity at the interface with a high curvature is 40-fold higher than that at the planar surface. Consequently, local high electric fields concentrate reactive species to the heterogeneous interfaces and efficiently promote the charge transport reactions, which directly leads to the enhancement of ECL emission surrounding the microbowls. Additionally, the potential to induce visual ECL from a ruthenium complex drops to 0.9 V, which further illustrates the promotion of an electrochemical reaction with the aid of an enhanced electric field. This important visualization of electric field boosted electrochemical reactions helps to establish the proposed electron transfer mechanism and provide an alternative strategy to improve electrocatalytic efficiency.

Mechanophotonics: Flexible Single-Crystal Organic Waveguides and Circuits

We present the one-dimensional optical-waveguiding crystal dithieno[3,2-a:2',3'-c]phenazine with a high aspect ratio, high mechanical flexibility, and selective self-absorbance of the blue part of its fluorescence (FL). While macrocrystals exhibit elasticity, microcrystals deposited at a glass surface behave more like plastic crystals due to significant surface adherence, making them suitable for constructing photonic circuits via micromechanical operation with an atomic-force-microscopy cantilever tip. The flexible crystalline waveguides display optical-path-dependent FL signals at the output termini in both straight and bent configurations, making them appropriate for wavelength-division multiplexing technologies. A reconfigurable 2×2-directional coupler fabricated via micromanipulation by combining two arc-shaped crystals splits the optical signal via evanescent coupling and delivers the signals at two output terminals with different splitting ratios. The presented mechanical micromanipulation technique could also be effectively extended to other flexible crystals.