Single‐Entity Electrochemistry: Fundamentals and Applications

Nanomaterials have been widely used for catalysis, medicine, and sensing due to developments in nanoscience and nanotechnology. However, owing to intrinsic heterogeneities in structures and properties, nanomaterials are challenging to characterize in ensemble measurements. Indeed, the past several decades have seen an explosion in methods capable of addressing individual nanoparticles, with electrochemical experimental methods proving increasingly popular. Nanoscale electrochemistry is adaptable and versatile and can be used to investigate single nanoparticles, wires, vesicles, nanobubbles, nanotubes, cells, and viruses, and even single ions and small molecules, as well as target and investigate particular sites on complex electrode surfaces. As a result, so-called “single-entity electrochemistry” has captured the imagination of electrochemists since 20161 and is a rapidly growing field. Considering the transient nature and ultralow amplitude of the electrochemical responses of single entities, ongoing efforts focus on obtaining high-fidelity current signals. Effective tools for identifying well-defined signals include small-sized electrodes (e.g. nanoelectrodes, ultramicroelectrodes, and nanopore-based electrodes) to reduce the background current noise, whereas a low-noise electrochemical measurement system with high temporal resolution and high current resolution is needed to acquire these signals. Benefiting from new instrumental and methodology developments, single-entity electrochemistry measurements offer new insights into electrochemical reactions at interfaces and provide intrinsic electrochemical activities at the individual entity level. Moreover, this class of measurement also brings further advantages including rapid and sensitive measurements that can be low cost, label free, high throughput, and requiring very little sample. Recently, advances in single-entity electrochemistry research, both experimental and theoretical, have been coupled with enhanced information from in situ optical microscopy and spectroscopy (e.g. fluorescence, plasmonic, and Raman scattering techniques) as well as scanning-probe-based imaging techniques with greatly improved spatial resolution. This growing, exciting, and captivating field is represented by 25 original contributions and focused topical reviews on single-entity electrochemistry research in this Special Issue of ChemElectroChem. Topics include theoretical and fundamental studies, development of new methods, and analytical assays for various applications. For example, stochastic nanoparticle collision electrochemistry, a powerful and convenient method for detecting single entities at electrode surfaces, has been used to investigate dynamic charge transfer during single collision processes at an ultramicroelectrode surface. Nanoparticle collision electrochemistry has been applied in areas ranging from electrocatalytic amplification and direct electrochemical stripping of individual metal nanoparticles to soft particles and biologically relevant samples. Moreover, attention is being paid to the expansion of electrochemical interfaces to achieve the detection of single-entity electrochemical signals, including the use of nanopores, nanopipettes, microphase-separated block copolymer thin films, and others. Select reports in this Special Issue address the need for high spatial resolution achieved by techniques such as scanning electrochemical microscopy, scanning ion conductance microscopy, and dark-field optical microscopy. The Reviews and Minireviews in this Special Issue outline state-of-art research in single-atom catalysis areas, highlighting present gaps and shortcomings of single-entity collision methods and offering new perspectives. As demonstrated, single-entity electrochemistry offers high sensitivity in terms of both current magnitude and temporal duration for the quantitative measurement of single events. Nowadays, we are in a position to enhance the possibilities by applying single-entity electrochemistry to a broad range of electrochemical measurements. This field will help to achieve an improved understanding of interfacial electron and ion transfer, particularly with the convergence in scale of experimental and simulation methods, and the enhanced information from complementary spectroscopic methods. Beyond fundamental science, owing to the remarkable features of individual event measurements, we also anticipate that single-entity electrochemistry will find success in practical applications in the future. Yi-Tao Long received his PhD in Bioelectroanalytical Chemistry from Nanjing University (P.R. China) in 1998. After completing his two-year postdoctoral studies at Heidelberg University (Germany), he moved to Canada and worked for five years as a Research Scientist at the University of Saskatchewan and University of Alberta. Yi-Tao was appointed as Full Professor at East China University of Science and Technology (Shanghai, P.R. China) in 2007, after working in the Department of Bioengineering at UC Berkeley (USA). His main research expertise involves nanopore single-molecule analysis, nanospectroscopy, spectroelectrochemistry, integrated biosensors, and biointerphases. Patrick Unwin earned his BSc at the University of Liverpool (UK) in 1985, DPhil at the University of Oxford (UK) in 1989, and DSc at the University of Warwick (UK) in 2008. He is Professor of Chemistry (since 1998) and Director of the Centre for Doctoral Training in Molecular Analytical Science (since 2014) at the University of Warwick. Pat has won numerous awards for his research, most recently the 2017 ISE–Elsevier Award in Experimental Electrochemistry and the 2018 Charles N. Reilley Award of the Society of Electroanalytical Chemistry. Pat and his group are especially known for the development of high-resolution electrochemical imaging techniques that have broad applications for fundamental investigations, materials characterization, electrocatalysis, and in the life sciences. Lane Baker attended Missouri State University (USA) and completed his BS degree in 1996, followed by graduate studies at Texas A&M University (USA), obtaining his PhD in 2001, and postdoctoral appointments at the Naval Research Laboratory and the University of Florida (USA). He is currently a James F. Jackson Professor of Chemistry at Indiana University (USA). Lane and his research group pursue research in electrochemical imaging, transport in biological systems and small-scale analysis at the interface of electrochemistry and mass spectrometry.

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