Electrochemiluminescence (ECL)-Based Electrochemical Imaging Using a Massive Array of Bipolar Ultramicroelectrodes

A spatial-potential resolved bipolar electrode electrochemiluminescence biosensor based on polarity conversion for dual-mode detection of miRNA-122 and CEA

In this work, a spatial-potential resolved bipolar electrode electrochemiluminescence (BPE-ECL) biosensor based on polarity conversion strategy and CuHCF electrocatalyst was constructed for dual-mode detection of miRNA-122 and carcinoembryonic antigen (CEA). ECL technology was firstly used to systematically study the polarity conversion of BPE. It was found that changing the polarity of the driving voltage would cause the polarity change of BPE, and led to the change of the luminescent position of Ru(bpy)32+. As a "proof-of-concept application", we developed a shielded dual-channel BPE-ECL biosensor for dual-mode detection of miRNA-122 and CEA. In order to further improve the detection sensitivity, a non-precious metal electrocatalyst CuHCF with outstanding electrocatalytic reduction activity of H2O2 was firstly introduced to the BPE-ECL biosensor for signal amplification, which could generate high faradaic current under the excitation of negative potential. Based on the charge neutrality principle of BPE, the enhancement of the faradaic current resulted in the ECL signal amplification of Ru(bpy)32+. The targets in the sensing grooves caused the introduction or fall off of CuHCF, which led to the ECL signal change of Ru(bpy)32+ in the signal grooves, and realized the dual-mode detection of miRNA-122 and CEA. This work provided a deeper understanding of the polarity change of BPE. Furthermore, the introduction of non-precious metal electrocatalyst had broadened the application range of BPE-ECL sensors.

Flexible and Dynamic Light-Guided Electrochemiluminescence for Spatiotemporal Imaging of Photoelectrochemical Processes on Hematite

Here, we report an electrochemiluminescence (ECL)-based approach for imaging of local photoelectrochemical processes on hematite in a spatially and temporally controlled manner. The local processes were guided by flexible and dynamic light illumination, not requiring any prepatterned conductive features or photomasks, with a digital micromirror device (DMD). The imaging approach was based on light-addressable electrochemical reactions on hematite, resulting in photoinduced ECL emission for spatiotemporally resolved imaging of photoelectrochemical processes selectively guided by light illumination. After clarifying the capability of hematite as a photosensitive electrode, we validated that the illuminated hematite exhibited stable light-guided ECL emission in correspondence with the illuminated area, with a spatial resolution of 0.8 μm and a temporal resolution of 1 μs, even over a long period of 6 h. More importantly, this study exemplified the simple yet effective ECL-based approach for electrochemical visualization of local photoelectrochemical processes guided by flexible and dynamic adjustment of light illumination in a spatiotemporally controlled way.

Wireless Multimodal Light-Emitting Arrays Operating on the Principles of LEDs and ECL

Electrochemistry-based light-emitting devices have gained considerable attention in different applications such as sensing and optical imaging. In particular, such systems are an interesting alternative for the development of multimodal light-emitting platforms. Herein we designed a multicolor light-emitting array, based on the electrochemical switch-on of light-emitting diodes (LEDs) with a different intrinsic threshold voltage. Thermodynamically and kinetically favored coupled redox reactions, i. e. the oxidation of Mg and the reduction of protons on Pt, act as driving force to power the diodes. Moreover, this system enables to trigger an additional light emission based on the interfacial reductive-oxidation electrochemiluminescence (ECL) mechanism of the Ru(bpy)3 2+/S2O8 2- system. The synergy between these light-emission pathways offers a multimodal platform for the straightforward optical readout of physico-chemical information based on composition changes of the solution.

Complex electrochemiluminescence patterns shaped by hydrodynamics at a rotating bipolar electrode

Electrochemiluminescence (ECL) is a powerful analytical approach that enables the optical readout of electrochemical processes. Over the last few years, ECL has gained considerable attention due to its large number of applications, including chemical sensing, bioanalysis and microscopy. In these fields, the promotion of ECL at bipolar electrodes has offered unprecedented opportunities thanks to wireless electrochemical addressing. Herein, we take advantage of the synergy between ECL and bipolar electrochemistry (BE) for imaging light-emitting layers shaped by hydrodynamics, polarization effects and the nature of the electrochemical reactions taking place wirelessly on a rotating bipolar electrode. The proof-of-principle is established with the model ECL system [Ru(bpy)3]2+/tri-n-propylamine. Interestingly, the ECL-emitting region moves and expands progressively from the anodic bipolar pole to the cathodic one where ECL reactants should neither be generated nor ECL be observed. Therefore, it shows a completely unusual behavior in the ECL field since the region where ECL reagents are oxidized does not coincide with the zone where ECL light is emitted. In addition, the ECL patterns change progressively to an "ECL croissant" and then to a complete ring shape due to the hydrodynamic convection. Such an approach allows the visualization of complex light-emitting patterns, whose shape is directly controlled by the rotation speed, chemical reactivity and BE-induced polarization. Indeed, the bipolar electrochemical addressing of the electrode breaks the circular symmetry of the reported rotating system. This unexplored and a priori simple configuration yields unique ECL behavior and raises new curious questions from the theoretical and experimental points of view in analytical chemistry. Finally, this novel wireless approach will be useful for the development of original ECL systems for analytical chemistry, studies of electrochemical reactivity, coupling microfluidics with ECL and imaging.

Microarray-Based Electrochemical Biosensing

Microarrays are widely utilized in bioanalysis. Electrochemical biosensing techniques are often applied in microarray-based assays because of their simplicity, low cost, and high sensitivity. In such systems, the electrodes and sensing elements are arranged in arrays, and the target analytes are detected electrochemically. These sensors can be utilized for high-throughput bioanalysis and the electrochemical imaging of biosamples, including proteins, oligonucleotides, and cells. In this chapter, we summarize recent progress on these topics. We categorize electrochemical biosensing techniques for array detection into four groups: scanning electrochemical microscopy, electrode arrays, electrochemiluminescence, and bipolar electrodes. For each technique, we summarize the key principles and discuss the advantages, disadvantages, and bioanalysis applications. Finally, we present conclusions and perspectives about future directions in this field.

Galvanic Bipolar Electrode Arrays with Self-Driven Optical Readouts

In this work, we report a bipolar electrode (BPE) array system with self-driven optical readouts of the faradic current flowing through the BPEs. The BPE array system is based on the spontaneous redox reactions that are respectively occurring at opposite poles of the BPEs with appropriate electrocatalysts on the poles; this system is analogous to one consisting of galvanic electrochemical cells. The galvanic BPE array system operates in a self-powered mode that requires there to be neither a direct electrical connection nor external electrical polarization to each BPE. Importantly, the appropriate electrocatalysts on the poles play a critical role in the galvanic BPE array system to induce the spontaneous redox reactions occurring at the poles of BPEs. Moreover, the galvanic BPE array system provides self-driven optical readouts, including fluorometric and colorimetric ones, to report the faradaic current resulting from the spontaneous redox reactions on the BPE poles. Based on the unique benefits that the galvanic BPE array system has over conventional BPEs, we demonstrated the promising potential of galvanic BPE arrays for the simple yet rapid and quantitative screening of electrocatalysts for the oxygen reduction reaction as well as sensitive sensing of H2O2 in parallel.

Endogenous and exogenous wireless multimodal light-emitting chemical devices

Multimodal imaging is a powerful and versatile approach that integrates and correlates multiple optical modalities within a single device. This concept has gained considerable attention due to its potential applications ranging from sensing to medicine. Herein, we develop several wireless multimodal light-emitting chemical systems by coupling two light sources based on different physical principles: electrochemiluminescence (ECL) occurring at the electrode interface and a light-emitting diode (LED) switched on by an electrochemically triggered electron flow. Endogenous (thermodynamically spontaneous redox process) and exogenous (requiring an external power source) bipolar electrochemistry acts as a driving force to trigger both light emissions at different wavelengths. The results presented here interconnect optical imaging and electrochemical reactions, providing a novel and so far unexplored alternative to design autonomous hybrid systems with multimodal and multicolor optical readouts for complex bio-chemical systems.

Advances in electrochemiluminescence luminophores based on small organic molecules for biosensing

Electrochemiluminescence (ECL) has several advantages, such as a near-zero background signal, high sensitivity, wide dynamic range, simplicity, and is widely used for sensing, imaging, and single cell analysis. ECL luminophores are the key factors in the performance of various applications. Among various luminophores, small organic luminophores exhibit many intriguing features including good biocompatibility, facile modification, well-defined molecular structure, and sustainable raw materials, making small organic luminophores attractive for the use in the ECL field. Although many great achievements have been made in the synthesis of new small organic luminophores, solving various challenges, and expanding new applications, there are almost no comprehensive reviews on small organic ECL luminophores. In this review, we briefly introduce the advantages and emission mechanisms of small organic ECL luminophores, summarize the main types, molecular characteristics, and ECL properties of most existing small organic ECL luminophores, and present the important applications and design principles in sensors, imaging, single cell analysis, sterilization, and other fields. Finally, the challenges and outlook of organic ECL luminophores to be popularized in biosensing applications are also discussed.

Elucidation of an Aggregate Excited State in the Electrochemiluminescence and Chemiluminescence of a Thermally Activated Delayed Fluorescence (TADF) Emitter

The electrochemistry, electrochemiluminescence (ECL), and chemiluminescence (CL) properties of a thermally activated delayed fluorescence (TADF) emitter 4,4'-(1,2-dihydroacenaphthylene-5,6-diyl)bis(N,N-diphenylaniline) (TPA-ace-TRZ) and three of its analogues were investigated. TPA-ace-TRZ exhibits both (a) delayed onset of ECL and (b) long-persistent luminescence, which we have attributed to the formation of an aggregate excited state in excimer or exciplex form. The evidence of this aggregate excited state was consistent across ECL annihilation and coreactant pathways as well as in CL. The absolute ECL efficiency of TPA-ace-TRZ using benzoyl peroxide (BPO) as a coreactant was found to be 0.028%, which was 9-fold stronger than the [Ru(bpy)₃]²⁺/BPO reference coereactant system. Furthermore, the absolute CL quantum efficiency of TPA-ace-TRZ was determined to be 0.92%. The performance and flexibility of the TADF emitter TPA-ace-TRZ under these various emissive pathways are highly desirable toward applications in sensing, imaging, and light-emitting devices.

Nanoparticle Printing for Microfluidic Applications: Bipolar Electrochemistry and Localized Raman Sensing Spots

The local integration of metal nanoparticle films on 3D-structured polydimethylsiloxane (PDMS)-based microfluidic devices is of high importance for applications including electronics, electrochemistry, electrocatalysis, and localized Raman sensing. Conventional processes to locally deposit and pattern metal nanoparticles require multiple steps and shadow masks, or access to cleanroom facilities, and therefore, are relatively imprecise, or time and cost-ineffective. As an alternative, we present an aerosol-based direct-write method, in which patterns of nanoparticles generated via spark ablation are locally printed with sub-mm size and precision inside of microfluidic structures without the use of lithography or other masking methods. As proof of principle, films of Pt or Ag nanoparticles were printed in the chambers of a multiplexed microfluidic device and successfully used for two different applications: Screening electrochemical activity in a high-throughput fashion, and localized sensing of chemicals via surface-enhanced Raman spectroscopy (SERS). The versatility of the approach will enable the generation of functional microfluidic devices for applications that include sensing, high-throughput screening platforms, and microreactors using catalytically driven chemical conversions.

Closed Bipolar Electrode Array for Optical Reporting Reaction-Coupled Electrochemical Sensing and Imaging

This review centers on a closed bipolar electrode (BPE) array using an electro-fluorochromism (EFC) or electro-chemiluminescence (ECL) reaction as the reporting reaction. Electrochemical signals at one pole of the closed BPE array can be transduced into the EFC or ECL signals at the opposite pole. Therefore, the current signal of a redox reaction can be easily detected and imaged by monitoring the luminescence signal. Recent developments in closed BPE array-based EFC and ECL sensing and imaging are summarized and discussed in detail. Finally, we consider the challenges and opportunities for improving the spatial resolution of closed BPE array-based electrochemical imaging, and emphasize the important application of this technique to the imaging of cellular activities at the single-cell level.

Advances in electrochemiluminescence for single-cell analysis

Recent years have witnessed the emergence of innovative analytical methods with high sensitivity and spatiotemporal resolution that allowed qualitative and quantitative analysis to be carried out at single-cell and subcellular levels. Electrochemiluminescence (ECL) is a unique chemiluminescence of high-energy electron transfer triggered by electrical excitation. The ingenious combination of electrochemistry and chemiluminescence results in the distinct advantages of high sensitivity, a wide dynamic range and good reproducibility. Specifically, single-cell ECL (SCECL) analysis with excellent spatiotemporal resolution has emerged as a promising toolbox in bioanalysis for revealing individual cells' heterogeneity and stochastic processes. This review focuses on advances in SCECL analysis and bioimaging. The history and recent advances in ECL probes and strategies for system design are briefly reviewed. Subsequently, the latest advances in representative SCECL analysis techniques for bioassays, bioimaging and therapeutics are also highlighted. Then, the current challenges and future perspectives are discussed.

Fabrication of High-Density Vertical Closed Bipolar Electrode Arrays by Carbon Paste Filling Method for Two-Dimensional Chemical Imaging

In this study, a carbon paste filling method was proposed as a simple strategy for fabricating high-density bipolar electrode (BPE) arrays for bipolar electrochemical microscopy (BEM). High spatiotemporal resolution imaging was achieved using the fabricated BPE array. BEM, which is an emerging microscopic system in recent years, achieves label-free and high spatiotemporal resolution imaging of molecular distributions using high-density BPE arrays and electrochemiluminescence (ECL) signals. We devised a simple method to fabricate a BPE array by filling a porous plate with carbon paste and succeeded in fabricating a high-density BPE array (15 μm pitch). After a detailed observation of the surface of the BPE array using a scanning electron microscope, the basic electrochemical and ECL emission characteristics were evaluated using potassium ferricyanide solution as a sample solution. Moreover, inflow imaging of the sample molecules was conducted to evaluate the imaging ability of the prepared BPE array. In addition, Prussian Blue containing carbon ink was applied to the sample solution side of the BPE array to provide catalytic activity to hydrogen peroxide, and the quantification and inflow imaging of hydrogen peroxide by ECL signals was achieved. This simple fabrication method of the BPE array can accelerate the research and development of BEM. Furthermore, hydrogen peroxide imaging by BEM is an important milestone for achieving bioimaging with high spatiotemporal resolution such as biomolecule imaging using enzymes.

Wireless Anti-Stokes Photoinduced Electrochemiluminescence at Closed Semiconducting Bipolar Electrodes

Wireless electrochemical systems constitute a rapidly developing field. Herein, photoinduced electrochemiluminescence (PECL) is studied at Si-based closed bipolar electrodes (BPEs) for designing anti-Stokes systems that can convert IR into visible photons, without direct electrical contact. We show that protection of the anodic emitting pole of the BPE allows the triggering of bright and longstanding emission under the synergetic actions of an external bias and IR illumination. Photoactive n- and p-type Si BPEs are studied with front-side and back-side illumination, respectively, and nonphotoactive n⁺-Si BPEs are studied in the dark. Two electrochemiluminescent (ECL) systems ([Ru(bpy)₃]²⁺/TPrA and L-012) are tested, and we show that the onset bias and the anti-Stokes shift can be controlled by the ECL system that is employed. These advances, rationalized by simulations, will be useful for the design of original PECL systems for chemical sensing or photodetection.

Bipolar Electrode Arrays for Chemical Imaging and Multiplexed Sensing

Bipolar electrodes (BPEs) with arrays of cathodic and anodic poles were developed for use in closed bipolar systems. To increase the number of BPEs in the array, the anodic and cathodic poles were connected with each other using thin leads. A further increase in the number of BPEs was achieved by forming the cathodic and anodic poles of the BPEs and the leads in different layers. A device with 9 × 10 arrays of cathodes and anodes was thus realized. When using this device to sense hydrogen peroxide (H₂O₂), the sensitivity and linear range of calibration plots could be adjusted by changing the driving voltage and the area ratio between the cathodic and anodic poles. The devices were used to image H₂O₂ and obtain time-lapse images for the diffusion and dilution of H₂O₂. Furthermore, DNA detection was demonstrated using an electroactive intercalator. The sensitivity could be improved by making the anodic poles smaller with respect to the cathodic pole and concentrating the electrochemiluminescence (ECL) in a small area. The ECL intensity changed according to the target DNA concentration in the solution.

Review on combining surface-enhanced Raman spectroscopy and electrochemistry for analytical applications

The discovery of surface enhanced Raman scattering (SERS) from an electrochemical (EC)-SERS experiment is known as a historic breakthrough. Five decades have passed and Raman spectroelectrochemistry (SEC) has developed into a common characterization tool that provides information about the electrode-electrolyte interface. Recently, this technique has been successfully explored for analytical purposes. EC was found to highly improve the performances of SERS sensors, providing, among others, controlled adsorption of analytes and increased reproducibility. In this review, we highlight the potential of EC-SERS sensors to be implemented for point-of-need (PON) analyses as miniaturized devices, and their ability to revolutionize fields like quality control, diagnosis or environmental and food safety. Important developments have been achieved in Raman spectroelectrochemistry, which now represents a promising alternative to conventional analytical methods and interests more and more researchers. The studies included in this review open endless possibilities for real-life EC-SERS analytical applications.

Bipolar Electrode Array for Multiplexed Detection of Prostate Cancer Biomarkers

Owing to the characteristics of high throughput, high flexibility, and convenient separation of the sensing and reporting reactions, the bipolar electrode (BPE) shows great potential in clinical analysis. However, there are some difficulties in the combination of BPEs and multiplex electrochemiluminescence (ECL) biosensing, such as the need for small sample consumption, multistep operations, and separated sample loading. In this paper, a microfluidic BPE array chip was fabricated toward multiplex detection of cancer biomarkers. With a special channel structure and the difference in flow resistance of channels of different sizes, the direction of liquid flow was successfully controlled. In this way, rapid and automatic multiplex sampling was achieved on the array, which would help improve the sensing efficiency and reduce the reagent consumption. The ECL BPE array chip served as an immunosensor for multiple prostate cancer biomarkers including prostate-specific antigen (PSA), interleukin-6 (IL-6), and prostate-specific membrane antigen (PSMA). The microfluidic BPE chip shows good reproducibility and high sensitivity. The limits of detection for PSA, IL-6, and PSMA are 0.093 ng/mL, 0.061 pg/mL, and 0.059 ng/mL, respectively. It also exhibits excellent performance in real sample analysis. The integrated ECL BPE array shows a good application prospect in clinical sensing of cancer biomarkers, as well as point-of-care testing.

Monitoring the electrochemical reactions on a gold nanoelectrode array via fluorescence-enabled electrochemical imaging

We first observed the electrochemical reaction on an independent gold nanoelectrode via super-resolution fluorescence imaging achieved by anchoring fluorescent molecules on the surface of the nanoelectrode and using an ionic liquid as the electrolyte.

Label-Free Electrochemiluminescence Imaging of Single-Cell Adhesions by Using Bipolar Nanoelectrode Array

A label-free and fast approach for positive electrochemiluminescence (ECL) imaging of single cells by bipolar nanoelectrode array is proposed. The reduction of oxygen at a platinized gold nanoelectrode array in a closed bipolar electrochemical system is coupled with an oxidative ECL process at the anodic side. For elevating the ECL imaging contrast of single cells, a driving voltage of -2.0 V is applied to in situ generate oxygen confined beneath cells that is subsequently used for ECL imaging at 1.1 V. High oxygen concentration in the confined space resulting from steric hindrance generates prominent oxygen reduction current at the cathodic side and higher ECL intensity at the anodic side, allowing positive ECL imaging of the cells adhesion region with excellent contrast. Cell morphology and adhesion strength can be successfully imaged with high image acquisition rate. This approach opens a new avenue for label-free imaging of single cells.

Wired Microfabricated Electrochemical Systems

Metal wires have been used as an alternative to liquid junctions for the connection of solutions in microfabricated electrochemical devices. They exhibit similar performance to liquid junctions, provided that the interfacial potentials at both ends of the wires were appropriately canceled. Cyclic voltammograms of devices with liquid junctions and metal wires were very similar when no current or a low current flowed through the metal wire between the working and reference electrodes. Iridium wires with iridium oxide at both ends facilitated canceling of the interfacial potentials at either end of the junction particularly well, and were used effectively for voltammetry, amperometry, and potentiometry by adjusting the pH of the solutions in the working and reference electrode compartments to be equal. This approach was used to effectively integrate a reliable common reference electrode between multiple working electrodes and to conduct automated electrochemical control of solution transport in microfluidic systems.

Visualization analysis of lecithin in drugs based on electrochemiluminescent single gold microbeads

Fast and high-throughput determination of drugs is a key trend in clinical medicine. Single particles have increasingly been adopted in a variety of photoanalytical and electroanalytical applications, and microscopic analysis has been a hot topic in recent years, especially for electrochemiluminescence (ECL). This paper describes a simple ECL method based on single gold microbeads to image lecithin. Lecithin reacts to produce hydrogen peroxide under the successive enzymatic reaction of phospholipase D and choline oxidase. ECL was generated by the electrochemical reaction between a luminol analog and hydrogen peroxide, and ECL signals were imaged by a camera. Despite the heterogeneity of single gold microbeads, their luminescence obeyed statistical regularity. The average luminescence of 30 gold microbeads is correlated with the lecithin concentration, and thus, a visualization method for analyzing lecithin was established. Calibration curves were constructed for ECL intensity and lecithin concentration, achieving detection limits of 0.05 mM lecithin. This ECL imaging platform based on single gold microbeads exhibits outstanding advantages, such as high throughput, versatility and low cost, and holds great potential in disease diagnostics, environmental monitoring and food safety.

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The ECL imaging strategy reflected the advantage of spatial resolution and high-throughput analysis of lecithin in drugs.

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Micrometer-scale gold microbeads were easily synthesized by adding H₂SO₄ in Na₃Au(SO₃)₂ solution.

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The ECL spots exhibited the heterogeneity of single gold microbeads in ECL reaction.

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ECL microscopic quantitative analysis for lecithin was established via single gold microbeads with PLD and COD.

Light in Electrochemistry

Electrochemistry represents an important analytical technique used to acquire and assess chemical information in detail, which can aid fundamental investigations in various fields, such as biological studies. For example, electrochemistry can be used as simple and cost-effective means for bio-marker tracing in applications, such as health monitoring and food security screening. In combination with light, powerful spatially-resolved applications in both the investigation and manipulation of biochemical reactions begin to unfold. In this article, we focus primarily on light-addressable electrochemistry based on semiconductor materials and light-readable electrochemistry enabled by electrochemiluminescence (ECL). In addition, the emergence of multiplexed and imaging applications will also be introduced.

Bipolar (Bio)electroanalysis

This contribution reviews a selection of the most recent studies on the use of bipolar electrochemistry in the framework of analytical chemistry. Despite the fact that the concept is not new, with several important studies dating back to the middle of the last century, completely novel and very original approaches have emerged over the last decade. This current revival illustrates that scientists still (re)discover some exciting virtues of this approach, which are useful in many different areas, especially for tackling analytical challenges in an unconventional way. In several cases, this "wireless" electrochemistry strategy enables carrying out measurements that are simply not possible with classic electrochemical approaches. This review will hopefully stimulate new ideas and trigger scientists to integrate some aspects of bipolar electrochemistry in their work in order to drive the topic into yet unexplored and eventually completely unexpected directions.

Rational Design of Electrochemiluminescent Devices

Electrochemiluminescence (ECL) is a light-emitting process which combines the intriguing merits of both electrochemical and chemiluminescent methods. It is an extensively used method especially in clinical analysis and biological research due to its high sensitivity, wide dynamic range, and good reliability. ECL devices are critical for the development and applications of ECL. Much effort has been expended to improve the sensitivity, portability, affordability, and throughput of new ECL devices, which allow ECL to adapt broad usage scenarios.In this Account, we summarize our efforts on the recent development of ECL devices including new electrodes, ECL devices based on a wireless power transfer (WPT) technique, and novel bipolar electrochemistry. As the essential components in the ECL devices, electrodes play an important role in ECL detection. We have significantly improved the sensitivity of luminol ECL detection of H₂O₂ by using a stainless steel electrode. By using semiconductor materials (e.g., silicon and BiVO₄), we have exploited photoinduced ECL to generate intense emission at much lower potentials upon illumination. For convenience, portability, and disposability, ECL devices based on cheap WPT devices have been designed. A small diode has been employed to rectify alternating current into direct current to dramatically enhance ECL intensity, enabling sensitive ECL detection using a smart phone as a detector. Finally, we have developed several ECL devices based on bipolar electrochemistry in view of the convenience of multiplex ECL sensing using a bipolar electrode (BPE). On the basis of the wireless feature of BPE, we have employed movable BPEs (e.g., BPE swimmers and magnetic rotating BPE) for deep exploration of the motional and ECL properties of dynamic BPE systems. To make full use of the ECL solution, we have dispersed numerous micro-/nano-BPEs in solution to produce intense 3D ECL in the entire solution, instead of 2D ECL in conventional ECL devices. In addition, the interference of ECL noise from driving electrodes was minimized by introducing the stainless steel with a passivation layer as the driving electrode. To eliminate the need for the fabrication of electrode arrays and the interference from the driving electrode and to decrease the applied voltage, we develop a new-type BPE device consisting of a single-electrode electrochemical system (SEES) based on a resistance-induced potential difference. The SEES is fabricated easily by attaching a multiperforated plate to a single film electrode. It enables the simultaneous detection of many samples and analytes using only a single film electrode (e.g., screen-printed electrode) instead of electrode arrays. It is of great potential in clinical analysis especially for multiple-biomarker detection, drug screening, and biological studies. Looking forward, we believe that more ECL devices and related ECL materials and detection methods will be developed for a wide range of applications, such as in vitro diagnosis, point-of-care testing, high-throughput analysis, drug screening, biological study, and mechanism investigation.

Electrochemiluminescence imaging of respiratory activity of cellular spheroids using sequential potential steps

The respiratory activity of cultured cells can be electrochemically monitored using scanning electrochemical microscopy (SECM) with high spatial resolution. However, in SECM, the electrode takes a long time to scan, limiting simultaneous measurements with large biological samples such as cell spheroids. Therefore, for rapid electrochemical imaging, a novel strategy is needed. Herein, we report electrochemiluminescence (ECL) imaging of spheroid respiratory activity for the first time using sequential potential steps. L-012, a luminol analog, was used as an ECL luminophore, and H₂O₂, a sensitizer for ECL of L-012, was generated by the electrochemical reduction of dissolved O₂. The ECL imaging visualized spheroid respiratory activity-evidenced by ECL suppression-corresponding to O₂ distribution around the spheroids. This method enabled the time-lapse imaging of respiratory activity in multiple spheroids with good spatial resolution comparable to that of SECM. Our work provides a promising high-throughput imaging strategy for elucidating spheroid cellular dynamics.

Glazunov’s electrography—the first electrochemical imaging and the first solid-state electroanalysis

Aleksandr Il’ich Glazunov developed the first electrochemical technique to image the surface of conducting solids giving the technique the name electrography. The electrographic images can mirror the distribution of elements on the surface of solid materials and also the electrochemical activity, caused by variations of “dissolution tension”. Thus, he has established for the first time a kind of spatially resolved electrochemistry. Electrography is also the first direct electroanalytical technique for solid materials. The present paper gives an account of his turbulent life in Russia, Czechoslovakia and Chile, and a discussion of his main scientific achievement, the development of electrography.

Electrofluorochromic Imaging Analysis of Glycan Expression on Living Single Cell with Bipolar Electrode Arrays

The in situ glycan profiling of a single tumor cell plays an important role in personalized cancer treatment. Herein, an integrated microfluidic system was designed for living single-cell trapping and real-time monitoring of galactosyl expression on the surface, combining closed bipolar electrode (BPE) arrays and electrofluorochromic (EFC) imaging. Galactosyl groups on human liver cancer HepG2 cells were used as the model analysts, galactose oxidase (GAO) could selectively oxidize hydroxyl sites of galactosyl groups on the cell surface to aldehydes, and then biotin hydrazide (BH) was used to label the aldehydes by aniline-catalyzed hydrazone ligation. With the biotin-avidin system, nanoprobes were finally introduced to the galactosyl groups on the cell surface with avidin as a bridge, which was prepared by simultaneously assembling ferrocene-DNA (Fc-DNA) and biotin-DNA (Bio-DNA) on gold nanoparticles (AuNPs) due to their large surface area and excellent electrical conductivity. After a labeled single cell was captured in the anodic microchannel, the Fc groups attached on the cell surface were oxidized under suitable potential, and the nonfluorescent resazurin on the cathode was correspondingly reduced to produce highly fluorescent resorufin, collected by fluorescence confocal microscope. The combination of EFC imaging and BPE realized monitoring galactosyl group expression of 5.0 × 10⁸ molecules per cell. Furthermore, the proposed platform had the ability to distinguish a single cancer cell from a normal cell according to the expression level of galactosyl groups and to dynamically monitor the galactosyl group variation on the cell surface, providing a simple and accessible method for the single-cell analysis.

Electrogenerated Chemiluminescence Immunoassays on Nanoelectrode Ensembles Platform with Magnetic Microbeads for the Determination of Carbohydrate Antigen

This work reports a gold nanoelectrode ensembles (Au-NEE) platform taken as a disposable electrogenerated chemiluminescence (ECL) platform with immunomagnetic microbeads for ECL immunoassays for the first time. The peak-shaped voltammograms were obtained at the Au-NEE, attributed to the total diffusional overlap. The ECL intensity at Au-NEE was 12.9 folds in the Ru(bpy)₃²⁺-tri-n-propylamine (TPA) ECL system and 19.6 folds in the luminol-H₂O₂ system, compared with that at the Au macroelectrode using the normalized active area of the electrodes, mainly attributed to the diffusion overlap of the Au-NEE and the edge effect of the individual gold nanodisks of the Au-NEE. The ECL immunoassay on the Au-NEE platform with magnetic microbeads for the determination of cancer biomarkers was developed. Carbohydrate antigen 19-9 (CA 19-9) was chosen as a model analyte while CA 19-9 antibody on the magnetic microbeads was taken as the capture probe, and ruthenium complex-labeled CA 19-9 antibody was used as the signal probe. A "sandwich" bioconjugates on the magnetic beads were transferred onto the ECL platform, and then the ECL measurements were performed in TPA solution. The developed method showed that the ECL peak intensity was directly in proportion to the concentration of CA 19-9 in the range from 0.5 to 20 U/mL with a limit of detection of 0.4 U/mL. This work demonstrates that the Au-NEE can be employed as a useful disposable ECL platform with the merits of cheapness, low nonspecific adsorption and practical application. The proposed approach will open a new avenue in the point-of-care test for the determination of protein biomarkers.

Switching the Photoluminescence and Electrochemiluminescence of Liposoluble Porphyrin in Aqueous Phase by Molecular Regulation

By a facile peripheral decoration of 5-(4-aminophenyl)-10,15,20-triphenylporphyrin (ATPP) with inherent aggregation-induced emission (AIE) active tetraphenylethene (TPE), a versatile AIEgenic porphyrin derivative (ATPP-TPE) was obtained, which greatly abolishes the detrimental π-π stacking and thus surmounts the notorious aggregation-caused quenching (ACQ) effect of ATPP in aqueous phase. The photoluminescence of ATPP-TPE is 4.5-fold stronger than ATPP at aggregation state. Moreover, an unequivocal aggregation induced electrochemiluminescence (AIECL) of ATPP-TPE was found to be seriously dependent on its aggregation property in aqueous solution with efficiency of 34 %, which is 6 times higher than pure ATPP. The versatility of this molecular structure modulation strategy along with the ACQ-to-AIE transformation in this work provides direction to guide for applying liposoluble porphyrins in aqueous phase by designs of synthetic porphyrin AIEgens.

Recent Advances in Electrochemiluminescence-Based Systems for Mammalian Cell Analysis

Mammalian cell analysis is essential in the context of both fundamental studies and clinical applications. Among the various techniques available for cell analysis, electrochemiluminescence (ECL) has attracted significant attention due to its integration of both electrochemical and spectroscopic methods. In this review, we summarize recent advances in the ECL-based systems developed for mammalian cell analysis. The review begins with a summary of the developments in luminophores that opened the door to ECL applications for biological samples. Secondly, ECL-based imaging systems are introduced as an emerging technique to visualize single-cell morphologies and intracellular molecules. In the subsequent section, the ECL sensors developed in the past decade are summarized, the use of which made the highly sensitive detection of cell-derived molecules possible. Although ECL immunoassays are well developed in terms of commercial use, the sensing of biomolecules at a single-cell level remains a challenge. Emphasis is therefore placed on ECL sensors that directly detect cellular molecules from small portions of cells or even single cells. Finally, the development of bipolar electrode devices for ECL cell assays is introduced. To conclude, the direction of research in this field and its application prospects are described.

Bioimaging using bipolar electrochemical microscopy with improved spatial resolution

In this study, we developed bipolar electrochemical microscopy (BEM) using a closed bipolar electrode (cBPE) array with an electrochemiluminescence (ECL) detecting system.