Imaging Electrogenerated Chemiluminescence at Single Gold Nanowire Electrodes

Shadow electrochemiluminescence imaging of giant liposomes opening at polarized electrodes

In this work, the release of giant liposome (∼100 μm in diameter) content was imaged by shadow electrochemiluminescence (ECL) microscopy. Giant unilamellar liposomes were pre-loaded with a sucrose solution and allowed to sediment at an ITO electrode surface immersed in a solution containing a luminophore ([Ru(bpy)3]2+) and a sacrificial co-reactant (tri-n-propylamine). Upon polarization, the electrode exhibited illumination over its entire surface thanks to the oxidation of ECL reagents. However, as soon as liposomes reached the electrode surface, dark spots appeared and then spread over time on the surface. This observation reflected a blockage of the electrode surface at the contact point between the liposome and the electrode surface, followed by the dilution of ECL reagents after the rupture of the liposome membrane and release of its internal ECL-inactive solution. Interestingly, ECL reappeared in areas where it initially faded, indicating back-diffusion of ECL reagents towards the previously diluted area and thus confirming liposome permeabilization. The whole process was analyzed qualitatively and quantitatively within the defined region of interest. Two mass transport regimes were identified: a gravity-driven spreading process when the liposome releases its content leading to ECL vanishing and a diffusive regime when ECL recovers. The reported shadow ECL microscopy should find promising applications for the imaging of transient events such as molecular species released by artificial or biological vesicles.

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.

Electrochemiluminescent imaging of a NADH-based enzymatic reaction confined within giant liposomes

Herein, transient releases either from NADH-loaded liposomes or enzymatic reactions confined in giant liposomes were imaged by electrochemiluminescence (ECL). NADH was first encapsulated with the [Ru(bpy)3]2+ luminophore inside giant liposomes (around 100 µm in diameter) made of DOPC/DOPG phospholipids (i.e., 1,2-dioleolyl-sn-glycero-3-phosphocholine/1,2-dioleoyl-sn-glycerol-3-phospho-(1'-rac-glycerol) sodium salt) on their inner- and outer-leaflet, respectively. Then, membrane permeabilization triggered upon contact between the liposome and a polarized ITO electrode surface and ECL was locally generated. Combination of amperometry, photoluminescence, and ECL provided a comprehensive monitoring of a single liposome opening and content release. In a second part, the work is focused on the ECL characterization of NADH produced by glucose dehydrogenase (GDH)-catalyzed oxidation of glucose in the confined environment delimited by the liposome membrane. This was achieved by encapsulating both the ECL and catalytic reagents (i.e., the GDH, glucose, NAD+, and [Ru(bpy)3]2+) in the liposome. In accordance with the results obtained, NADH can be used as a biologically compatible ECL co-reactant to image membrane permeabilization events of giant liposomes. Under these conditions, the ECL signal duration was rather long (around 10 s). Since many enzymatic reactions involve the NADH/NAD+ redox couple, this work opens up interesting prospects for the characterization of enzymatic reactions taking place notably in artificial cells and in confined environments.

Site-selective heat boosting electrochemiluminescence for single cell imaging

In operando visualization of local electrochemical reactions provides mechanical insights into the dynamic transport of interfacial charge and reactant/product. Electrochemiluminescence is a crossover technique that quantitatively determines Faraday current and mass transport in a straightforward manner. However, the sensitivity is hindered by the low collision efficiency of radicals and side reactions at high voltage. Here, we report a site-selective heat boosting electrochemiluminescence microscopy. By generating a micron-scale heat point in situ at the electrode-solution interface, we achieved an enhancement of luminescence intensity up to 63 times, along with an advance of 0.2 V in applied voltage. Experimental results and finite element simulation demonstrate that the fundamental reasons are accelerated reaction rate and thermal convection via a photothermal effect. The concentrated electrochemiluminescence not only boosts the contrast of single cells by 20.54 times but also enables the site-selective cell-by-cell analysis of the heterogeneous membrane protein abundance. This electrochemical visualization method has great potential in the highly sensitive and selective analysis of local electron transfer events.

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.

Bimodal Electrochemiluminescence Microscopy of Single Cells

Electrochemiluminescence (ECL) microscopy is an emerging technique with new applications such as imaging of single entities and cells. Herein, we have developed a bimodal and bicolor approach to record both positive ECL (PECL: light-emitting object on dark background) and shadow label-free ECL (SECL: nonemissive object shadowing the background luminescence) images of single cells. This bimodal approach is the result of the simultaneous emissions of [Ru(bpy)₃]²⁺ used to label the cellular membrane (PECL) and [Ir(sppy)₃]^3- dissolved in solution (SECL). By spectrally resolving the ECL emission wavelengths, we recorded the images of the same cells in both PECL and SECL modes using the [Ru(bpy)₃]²⁺ (λ_max = 620 nm) and [Ir(sppy)₃]^3- (λ_max = 515 nm) luminescence, respectively. PECL shows the distribution of the [Ru(bpy)₃]²⁺ labels attached to the cellular membrane, whereas SECL reflects the local diffusional hindrance of the ECL reagents by each cell. The high sensitivity and surface-confined features of the reported approach are demonstrated by imaging cell-cell contacts during the mitosis process. Furthermore, the comparison of PECL and SECL images demonstrates the differential diffusion of tri-n-propylamine and [Ir(sppy)₃]^3- through the permeabilized cell membranes. Consequently, this dual approach enables the imaging of the morphology of the cell adhering on the surface and can significantly contribute to multimodal ECL imaging and bioassays with different luminescent systems.

Direct Visualization of Nanoconfinement Effect on Nanoreactor via Electrochemiluminescence Microscopy

Nanoconfinement in mesoporous nanoarchitectures could dramatically change molecular transport and reaction kinetics during electrochemical process. A molecular-level understanding of nanoconfinement and mass transport is critical for the applications, but a proper route to study it is lacking. Herein, we develop a single nanoreactor electrochemiluminescence (SNECL) microscopy based on Ru(bpy)₃ ²⁺ -loaded mesoporous silica nanoparticle to directly visualize in situ nanoconfinement-enhanced electrochemical reactions at the single molecule level. Meanwhile, mass transport capability of single nanoreactor, reflected as long decay time and recovery ability, is monitored and simulated with a high spatial resolution. The nanoconfinement effects in our system also enable imaging single proteins on cellular membrane. Our SNECL approach may pave the way to decipher the nanoconfinement effects during electrochemical process, and build bridges between mesoporous nanoarchitectures and potential electrochemical applications.

Recent Progress in Plasmonic based Electrochemiluminescence Biosensors: A Review

Electrochemiluminescence (ECL) analysis has become a powerful tool in recent biomarker detection and clinic diagnosis due to its high sensitivity and broad linear range. To improve the analytical performance of ECL biosensors, various advanced nanomaterials have been introduced to regulate the ECL signal such as graphene, gold nanomaterials, and quantum dots. Among these nanomaterials, some plasmonic nanostructures play important roles in the fabrication of ECL biosensors. The plasmon effect for the ECL signal includes ECL quenching by resonant energy transfer, ECL enhancement by surface plasmon resonance enhancement, and a change in the polarized angle of ECL emission. The influence can be regulated by the distance between ECL emitters and plasmonic materials, and the characteristics of polarization angle-dependent surface plasmon coupling. This paper outlines the recent advances of plasmonic based ECL biosensors involving various plasmonic materials including noble metals and semiconductor nanomaterials. The detection targets in these biosensors range from small molecules, proteins, nucleic acids, and cells thanks to the plasmonic effect. In addition to ECL biosensors, ECL microscopy analysis with plasmonic materials is also highlighted because of the enhanced ECL image quality by the plasmonic effect. Finally, the future opportunities and challenges are discussed if more plasmonic effects are introduced into the ECL realm.

Plasmon-Enhanced Electrochemiluminescence at the Single-Nanoparticle Level

Plasmon-enhanced electrochemiluminescence (ECL) at the single-nanoparticle (NP) level was investigated by ECL microscopy. The Au NPs were assembled into an ordered array, providing a high-throughput platform that can easily locate each NP in sequential characterizations. A strong dependence of ECL intensity on Au NP configurations was observed. We demonstrate for the first time that at the single-particle level, the ECL of Ru(bpy)₃ ²⁺ -TPrA was majorly quenched by small Au NPs (<40 nm), while enhanced by large Au ones (>80 nm) due to the localized surface plasmon resonance (LSPR). Notably, the ECL intensity was further increased by the coupling effect of neighboring Au NPs. Finite Difference Time Domain (FDTD) simulations conformed well with the experimental results. This plasmon enhanced ECL microscopy for arrayed single NPs provides a reliable tool for screening electrocatalytic activity at a single particle.

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.

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

Thus far, no correlation between nanocluster structures and their electrochemiluminescence (ECL) has been identified. Herein, we report how face-centered-cubic and hexagonal close-packed structures of two Au₂₁(SR)₁₅ nanocluster isomers determine their chemical reactivity. The relationships were explored by means of ECL and photoluminescence spectroscopy. Both isomers reveal unprecedented ECL efficiencies in the near-infrared region, which are >10- and 270-fold higher than that of standard Ru(bpy)₃²⁺, respectively. Photoelectrochemical reactivity as well as ECL mechanisms were elucidated based on electrochemistry, spooling photoluminescence, and ECL spectroscopy, unfolding the three emission enhancement origins: (i) effectively exposed reactive facets available to undergo electron transfer reactions; (ii) individual excited-state regeneration loops; (iii) cascade generations of various exited states. Indeed, these discoveries will have immediate impacts on various applications including but not limited to single molecular detection as well as photochemistry and electrocatalysis toward clean photon-electron conversion processes such as light-harvesting and light-emitting technologies.

Single Biomolecule Imaging by Electrochemiluminescence

Herein, a single biomolecule is imaged by electrochemiluminescence (ECL) using Ru(bpy)₃²⁺-doped silica/Au nanoparticles (RuDSNs/AuNPs) as the ECL nanoemitters. The ECL emission is confined to the local surface of RuDSNs leading to a significant enhancement in the intensity. To prove the concept, a single protein molecule at the electrode is initially visualized using the as-prepared RuDSN/AuNPs nanoemitters. Furthermore, the nanoemitter-labeled antibody is linked at the cellular membrane to image a single membrane protein at one cell, without the interference of current and optical background. The success in single-biomolecule ECL imaging solves the long-lasting task in the ultrasensitive ECL analysis, which should be able to provide more elegant information about the protein in cellular biology.

Plasmonic Imaging of Electrochemical Reactions at Individual Prussian Blue Nanoparticles

Prussian blue is an iron-cyanide-based pigment steadily becoming a widely used electrochemical sensor in detecting hydrogen peroxide at low concentration levels. Prussian blue nanoparticles (PBNPs) have been extensively studied using traditional ensemble methods, which only provide averaged information. Investigating PBNPs at a single entity level is paramount for correlating the electrochemical activities to particle structures and will shed light on the major factors governing the catalyst activity of these nanoparticles. Here we report on using plasmonic electrochemical microscopy (PEM) to study the electrochemistry of PBNPs at the individual nanoparticle level. First, two types of PBNPs were synthesized; type I synthesized with double precursors method and type II synthesized with polyvinylpyrrolidone (PVP) assisted single precursor method. Second, both PBNPs types were compared on their electrochemical reduction to form Prussian white, and the effect from the different particle structures was investigated. Type I PBNPs provided better PEM sensitivity and were used to study the catalytic reduction of hydrogen peroxide. Progressively decreasing plasmonic signals with respect to increasing hydrogen peroxide concentration were observed, demonstrating the capability of sensing hydrogen peroxide at a single nanoparticle level utilizing this optical imaging technique.

Single-nanoparticle spectroelectrochemistry studies enabled by localized surface plasmon resonance

This review describes recent progress of spectroelectrochemistry (SEC) analysis of single metallic nanoparticles (NPs) which have strong surface plasmon resonance properties. Dark-field scattering (DFS), photoluminescence (PL), and electrogenerated chemiluminescence (ECL) are three commonly used optical methods to detect individual NPs and investigate their local redox activities in an electrochemical cell. These SEC methods are highly dependent on a strong light-scattering cross-section of plasmonic metals and their electrocatalytic characteristics. The surface chemistry and the catalyzed reaction mechanism of single NPs and their chemical transformations can be studied using these SEC methods. Recent progress in the experimental design and fundamental understanding of single-NP electrochemistry and catalyzed reactions using DFS, PL, and ECL is described along with selected examples from recent publications in this field. Perspectives on the challenges and possible solutions for these SEC methods and potential new directions are discussed.

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.

Crystallization-Induced Enhanced Electrochemiluminescence from Tetraphenyl Alkene Nanocrystals for Ultrasensitive Sensing

Organic materials with diverse structures and brilliant glowing colors have been attracting extensive attention in optical electronic devices and electrochemiluminescence (ECL) fields and are currently faced with the issue of low ECL efficiency. Herein, a series of tetraphenyl alkene nanocrystals (TPA NCs) with an ordered molecular structure were synthesized to explore regularities in the crystallization-induced enhanced (CIE) ECL emission effects by altering the number and position of vinyl on the backbone of TPA molecules. Among those TPA NCs, tetraphenyl-1,3-butadiene (TPB) NCs exhibit the brightest ECL emission via a coreactant pathway, with the relative ECL efficiency of up to 31.53% versus the standard [Ru(bpy)₃]²⁺/TEA system, which is thousands of times higher than that of free TPB molecules. The high ECL efficiency of TPB NCs originates from the effective electron transfer of unique J-aggregates on the a axis of the nanocrystals to notably promote radiative transition and the restriction on the free rotation of TPB molecules to further suppress the nonradiative transition, which has exhibited great potential in ultrasensitive biosensing, efficient light-emitting devices, and clear ECL imaging fields. As a proof of concept, since dopamine (DA) can form benzoquinone species by electrochemical oxidation to realize intermediate radical quenching and excited-state quenching on the TPB NCs/TEA system, the TPB NCs with the CIE ECL effect are used to construct an ultrasensitive ECL-sensing platform for the determination of DA with a lower detection limit of 3.1 nM.

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.

Tuning Electrogenerated Chemiluminescence Intensity Enhancement Using Hexagonal Lattice Arrays of Gold Nanodisks

Electrogenerated chemiluminescence (ECL) microscopy shows promise as a technique for mapping chemical reactions on single nanoparticles. The technique's spatial resolution is limited by the quantum yield of the emission and the diffusive nature of the ECL process. To improve signal intensity, ECL dyes have been coupled with plasmonic nanoparticles, which act as nanoantennas. Here, we characterize the optical properties of hexagonal arrays of gold nanodisks and how they impact the enhancement of ECL from the coreaction of tris(2,2'-bipyridyl)dichlororuthenium(II) hexahydrate and tripropylamine. We find that varying the lattice spacing results in a 23-fold enhancement of ECL intensity because of increased dye-array near-field coupling as modeled using finite element method simulations.

Electrochemiluminescence Based on a Dual Carbon Ultramicroelectrode with Confined Steady-State Annihilation

Developing novel microelectronic devices for electrochemical measurements and electrochemiluminescence (ECL) study is of great importance. Herein, we fabricated a submicrometer-sized dual carbon electrode (DCE) and investigated its annihilation ECL behavior under steady-state conditions for the first time. The oxidation and reduction of the model luminophore, [Ru(bpy)₃]²⁺, occurred separately at the two sides of the DCE, and the electrogenerated ions then diffused to the gap between the two electrodes to generate the excited-state intermediate [Ru(bpy)₃]^2+* and ECL emission. Compared with other types of two-electrode systems, the prepared DCE possesses a smaller total size and an ultrasmall interelectrode distance of 60 nm or less, which could result in a shorter diffusion time and an amplified ECL signal without the purification of the solvent and supporting electrolytes. On the basis of the constructed ECL microscopic platform, we successfully obtained a stable and confined ECL signal in the vicinity of the electrode tip. Furthermore, a two-dimensional finite element method simulation of this model system was performed to quantitively analyze the concentration profiles of the electrogenerated species around the tip of the DCE and predict the concentrations of [Ru(bpy)₃]^2+* with various gap distances. The simulation results also proved that the higher concentrations of [Ru(bpy)₃]^2+* could be achieved with a smaller distance with a possible amplification factor of 6 (compared with the concentration when the gap distance is greater than 300 nm). This work provides an experimental model for further improvement of ECL efficiency and broadens the availability for annihilation ECL applications in small confined spaces.

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.

Correlated Optical-Electrochemical Measurements Reveal Bidirectional Current Steps for Graphene Nanoplatelet Collisions at Ultramicroelectrodes

Single-entity electrochemistry has emerged as a powerful tool to study the adsorption behavior of single nanoscale entities one-at-a-time on an ultramicroelectrode surface. Classical single-entity collision studies have focused on the behavior of spherical nanoparticles or entities where the orientation of the colliding entity does not impact the electrochemical response. Here, we report a detailed study of the collision of asymmetric single graphene nanoplatelets onto ultramicroelectrodes. The collision of conductive graphene nanoplatelets on biased ultramicroelectrode surfaces can be observed in an amperometric i-t trace, revealing a variety of current transients (both positive and negative steps). To elucidate the dynamics of nanoplatelet adsorption processes and probe response heterogeneity, we correlated the collision events with optical microscopy. We show that positive steps are due to nanoplatelets coming into contact with the ultramicroelectrode, making an electrical connection, and adsorbing partly on the glass surrounding the ultramicroelectrode. Negative steps occur when nanoplatelets adsorb onto the glass without an electrical connection, effectively blocking flux of ferrocenemethanol to the ultramicroelectrode surface. These measurements allow rigorous quantification of current transients and detailed insights into the adsorption dynamics of asymmetric objects at the nanoscale.

Spatially resolved electrochemiluminescence through a chemical lens

Electrochemiluminescence (ECL) microscopy is an emerging technique with a wide range of imaging applications and unique properties in terms of high spatial resolution, surface confinement and favourable signal-to-noise ratio. Despite its successful analytical applications, tuning the depth of field (i.e., thickness of the ECL-emitting layer) is a crucial issue. Indeed, the control of the thickness of this ECL region, which can be considered as an "evanescent" reaction layer, limits the development of cell microscopy as well as bioassays. Here we report an original strategy based on chemical lens effects to tune the ECL-emitting layer in the model [Ru(bpy)3]2+/tri-n-propylamine (TPrA) system. It consists of microbeads decorated with [Ru(bpy)3]2+ labels, classically used in bioassays, and TPrA as the sacrificial coreactant. In particular we exploit the buffer capacity of the solution to modify the rate of the reactions involved in the ECL generation. For the first time, a precise control of the ECL light distribution is demonstrated by mapping the luminescence reactivity at the level of single micrometric bead. The resulting ECL image is the luminescent signature of the concentration profiles of diffusing TPrA radicals, which define the ECL layer. Therefore, our findings provide insights into the ECL mechanism and open new avenues for ECL microscopy and bioassays. Indeed, the reported approach based on a chemical lens controls the spatial extension of the "evanescent" ECL-emitting layer and is conceptually similar to evanescent wave microscopy. Thus, it should allow the exploration and imaging of different heights in substrates or in cells.

Hydrogen Evolution Reaction Monitored by Electrochemiluminescence Blinking at Single-Nanoparticle Level

Monitoring and characterization methods that provide performance tracking of hydrogen evolution reaction (HER) at the single-nanoparticle level can greatly advance our understanding of catalysts' structure and activity relationships. Electrochemiluminescence (ECL) microscopy is implemented for the first time to identify HER activities of single nanocatalysts and to provide a direction for further optimization. Here, we develop a novel ECL blinking technique at the single-nanoparticle level to directly monitor H₂ nanobubbles generated from hollow carbon nitride nanospheres (HCNSs). The ECL ON and OFF mechanisms are identified being closely related to the generation, growth, and collapse of H₂ nanobubbles. The power-law distributed durations of ON and OFF states demonstrate multiple catalytic sites with stochastic activities on a single HCNS. The power-law coefficients of ECL blinking increase with improved HER activities from modified HCNSs with other active HER catalysts. Besides, ECL blinking phenomenon provides an explanation for the low cathodic ECL efficiency of semiconductor nanomaterials.

A simple and convenient electrochemiluminescence immunoassay by using gold nanoparticles as both label and co-reactant

A simple and convenient electrochemiluminescence immunoassay (ECLIA) has been developed by using gold nanoparticles (AuNPs) as both label and co-reactant. In this novel ECLIA, the detection antibodies labeled with AuNPs as co-reactant reacts directly with ruthenium complex to produce electrochemiluminescence (ECL), which avoids the weakness of luminophore as a label. The feasibility of the new method was investigated through the sandwich ECL immunosensor for the determination of human immunoglobulin (HIgG), and excitation of luminescence was respectively driven by linear sweep voltammetry and step potential. Under the optimal conditions, ECL intensity of the immunosensor increased with HIgG concentration in a wide range from 0.01 to 10.0 ng/mL and displayed linear response to logarithm of HIgG concentration. The results showed that the detection limit was 5 pg/mL, and the relative error was no more than 4.6% for the repeated measurements. The relative standard deviation was less than 2.9% for the determination of the standard sample, which can meet the requirement of ECLIA.

Electrochemiluminescence Imaging for the Morphological and Quantitative Analysis of Living Cells under External Stimulation

In this work, a simple electrochemiluminescence (ECL) imaging method based on the cell shield of the ECL emission was developed for the morphological and quantitative analysis of living cells under external stimulation. ECL images of MCF-7 cells cultured on or captured at the glassy carbon electrode (GCE) surface in a solution of tris(2,2'-bipyridyl)ruthenium(II)-tri-n-propylamine were recorded. Important morphological characteristics of living cells, including cell shape, cell area, average cell boundary, and junction distance between two adjacent cells, were directly obtained using the developed negative ECL imaging method. The ECL images revealed gradual morphological changes in cells on the GCE surface. During the course of H₂O₂ stimulation of cells on the GCE surface, cells shrunk, rounded up, disengaged from surrounding cells, and finally detached from the electrode surface. During the course of electrical stimulation (0.8 V), the cells on the GCE surface exhibited aggregation as demonstrated by increases in the average cell boundary and decreases in the junction distance between two adjacent cells. Additionally, a quantitative method for the sensitive determination of MCF-7 cells with a limit of detection of 29 cells/mL was developed using the negative ECL imaging strategy. This work demonstrates that the proposed negative ECL imaging strategy is a promising approach to assess important morphological characteristics of living cells during the course of external stimulation and to obtain quantitative information on cell concentrations in solution.

Electrochemiluminescence Waveguide in Single Crystalline Molecular Wires

Here we report the first observation of active waveguide of electrochemiluminescence (ECL) in single crystalline molecular wires self-assembled from cyclometalated iridium(III) complexes, namely tris(1-phenylisoquinoline-C² , N) (Ir(piq)₃ ). Under dark conditions, the molecular wires deposited on the electrode surface can act as both ECL emitters and active waveguides. As revealed by ECL microscopy, they exhibit the typical characteristics of optical waveguides, transmitting ECL and generating much brighter ECL emission at their terminals. Moreover, self-generated ECL can be confined inside the molecular wire and propagates along the longitudinal direction as far as ≈100 μm to the terminal out of touch with the electrode. Therefore, this one-dimensional crystalline molecular wire-based waveguide offers the opportunity to switch the electrochemically generated ECL to remote light emission in non-conductive regions and is promising for contactless electrochemical analysis and study of (bio)chemical systems.

Operando Studies of the Electrochemical Dissolution of Silver Nanoparticles in Nitrate Solutions Observed With Hyperspectral Dark-Field Microscopy

Since nanoparticles are frequently used in commercial applications, there is a huge demand to obtain deeper insights into processes at the nanoscale. Especially, catalysis, chemical and electrochemical reaction dynamics are still poorly understood. Thus, simultaneous and coupled opto-and spectro-electrochemical dark-field microscopy is used to study in situ and operando the electrochemically driven dissolution mechanism of single silver nanoparticles in the presence of nitrate ions as non-complexing counter-ions, herein. Hyperspectral imaging is used to probe the intrinsic localized surface plasmon resonance of individual silver nanospheres before, during and after their electrochemical oxidation on a transparent indium tin oxide (ITO) electrode. Furthermore, optical video imaging was performed for additional information. Based on the complete loss of spectral information and intensity, a dissolution of the particles during the reaction was concluded. This way it is revealed that the dissolution of individual particles proceeds over several seconds, indicating a hindrance by the nitrate ions. Only electrochemical analysis does not provide this insight as the measured current does not allow distinguishing between successive fast dissolution of one particle after another or slow dissolution of several particles in a concerted manner. For comparison, experiments were performed in the presence of chloride ions. It was observed that the silver chloride formation is an instantaneous process. Thus, it is possible to study and define the reaction dynamics on the single nanoparticle level in various electrochemical systems and electrolyte solutions. Accordingly, operando opto- and spectro-electrochemical studies allow us to conclude, that the oxidation of silver to solvated silver cations is a kinetically slow process, while the oxidation to silver chloride is fast. We propose this approach as a new method to study electrocatalyst materials, their transformation and degradation under operando conditions.

Observing the structure-dependent electrocatalytic activity of bimetallic Pd-Au nanorods at the single-particle level

We developed an electrochemiluminescence (ECL) microscopy technique to image the structure-dependent electrocatalytic reactivity of bimetallic Pd-Au nanorods (NRs) at the single-particle level.

Nanoelectrode-emitter spectral overlap amplifies surface enhanced electrogenerated chemiluminescence

Electrogenerated chemiluminescence (ECL) is a promising technique for low concentration molecular detection. To improve the detection limit, plasmonic nanoparticles have been proposed as signal boosting antennas to amplify ECL. Previous ensemble studies have hinted that spectral overlap between the nanoparticle antenna and the ECL emitter may play a role in signal enhancement. Ensemble spectroscopy, however, cannot resolve heterogeneities arising from colloidal nanoparticle size and shape distributions, leading to an incomplete picture of the impact of spectral overlap. Here, we isolate the effect of nanoparticle-emitter spectral overlap for a model ECL system, coreaction of tris(2,2'-bipyridyl)dichlororuthenium(ii) hexahydrate and tripropylamine, at the single-particle level while minimizing other factors influencing ECL intensities. We found a 10-fold enhancement of ECL among 952 gold nanoparticles. This signal enhancement is attributed exclusively to spectral overlap between the nanoparticle and the emitter. Our study provides new mechanistic insight into plasmonic enhancement of ECL, creating opportunities for low concentration ECL sensing.

Electrochemiluminescence-Microscopy for microRNA Imaging in Single Cancer Cell Combined with Chemotherapy-Photothermal Therapy

In this work, a new technology using ECL as a microscopy to parallel image miRNA-21 in single cancer cell was built. Phorbol-12-myristate-13-acetate (PMA) loaded gold nanocages (Au NCs) was closed with DNA gate which could be recognized and opened by miRNA-21 in HeLa cell. PMA was then released and further induced HeLa cells to produce reactive oxygen species (ROS; including O^2-•, •OH and H₂O₂ etc.). With H₂O₂ as coreactant and luminol as ECL active material, ECL imaging of intracellular miRNA-21 in single HeLa cell was obtained by EMCCD. Moreover, ROS therapy and photothermal therapy (PPT) of Au NCs@PMA probe were also motivated by in situ miRNA-21 marker instead of the external source. The combined therapy leads to dramatically enhanced ability for cancer cell killing. Au NCs@PMA probe alone could not only achieve a high sensitivity and high resolution ECL-microscopy for imaging of intracellular miRNA-21 for the first time, but also realize the integrated diagnosis like ROS induced tumor damage and photothermal-induced intelligent therapy. This multifunctional platform is believed to be capable of playing an important role in future oncotherapy by the synergistic effects between chemotherapy and photothermal therapy.