Concluding remarks: next generation nanoelectrochemistry - next generation nanoelectrochemists

Spiers Memorial Lecture: New horizons in nanoelectrochemistry

This introductory lecture prefaces the 2024 New Horizons in Nanoelectrochemistry Faraday Discussion. A broad view of the previous Discussions related to nanoelectrochemistry is taken. Big ideas or concepts discussed at these previous meetings are identified, along with specific examples in each area. Closing comments aimed at a high level and related to where we are today and what is needed to continue to drive nanoelectrochemistry towards the horizon are considered.

Electrochemistry Beyond Solutions: Modeling Particle Self-Crowding of Nanoparticle Suspensions

Nanoparticle suspensions hold promise to transform functionality of next-generation electrochemical systems including batteries, capacitors, wastewater treatment, and sensors, challenging the limits of existing electrochemical models. Classical solution-based electrochemistry assumes that charge is transported and transferred by point-like carriers. Herein, we examine the electrochemistry of a model aqueous suspension of nondissolvable electroactive nanoparticles over a wide concentration range using a rotating disk electrode. Past a concentration and rotation rate threshold, the electrochemistry deviates from solution theory with a maximum attainable current due to particle "self-crowding" where reacted particles on the electrode surface reduce the area accessible for charge transfer by unreacted particles. The observed response is rationalized with an analytical model considering the physical adsorption/desorption kinetics and interfacial transport of nondissolvable finite-size charge carriers. Experimental validation shows the model to be applicable across a range of electrode sizes and thus suitable for engineering electrochemical systems employing nondissolvable nanoparticle suspensions.

Nanobiopsy investigation of the subcellular mtDNA heteroplasmy in human tissues

Mitochondrial function is critical to continued cellular vitality and is an important contributor to a growing number of human diseases. Mitochondrial dysfunction is typically heterogeneous, mediated through the clonal expansion of mitochondrial DNA (mtDNA) variants in a subset of cells in a given tissue. To date, our understanding of the dynamics of clonal expansion of mtDNA variants has been technically limited to the single cell-level. Here, we report the use of nanobiopsy for subcellular sampling from human tissues, combined with next-generation sequencing to assess subcellular mtDNA mutation load in human tissue from mitochondrial disease patients. The ability to map mitochondrial mutation loads within individual cells of diseased tissue samples will further our understanding of mitochondrial genetic diseases.

Practical guidelines for the use of scanning electrochemical cell microscopy (SECCM)

Scanning electrochemical cell microscopy (SECCM) has emerged as a transformative technology for electrochemical materials characterisation and the study of single entities, garnering global adoption by numerous research groups. While details on the instrumentation and operational principles of SECCM are readily available, the growing need for practical guidelines, troubleshooting strategies, and a systematic overview of applications and trends has become increasingly evident. This tutorial review addresses this gap by offering a comprehensive guide to the practical application of SECCM. The review begins with a discussion of recent developments and trends in the application of SECCM, before providing systematic approaches to (and the associated troubleshooting associated with) instrumental set up, probe fabrication, substrate preparation and the deployment of environmental (e.g., atmosphere and humidity) control. Serving as an invaluable resource, this tutorial review aims to equip researchers and practitioners entering the field with a comprehensive guide to essential considerations for conducting successful SECCM experiments.

Emerging Data Processing Methods for Single-Entity Electrochemistry

Single-entity electrochemistry is a powerful tool that enables the study of electrochemical processes at interfaces and provides insights into the intrinsic chemical and structural heterogeneities of individual entities. Signal processing is a critical aspect of single-entity electrochemical measurements and can be used for data recognition, classification, and interpretation. In this review, we summarize the recent five-year advances in signal processing techniques for single-entity electrochemistry and highlight their importance in obtaining high-quality data and extracting effective features from electrochemical signals, which are generally applicable in single-entity electrochemistry. Moreover, we shed light on electrochemical noise analysis to obtain single-molecule frequency fingerprint spectra that can provide rich information about the ion networks at the interface. By incorporating advanced data analysis tools and artificial intelligence algorithms, single-entity electrochemical measurements would revolutionize the field of single-entity analysis, leading to new fundamental discoveries.

Variable Nanoelectrode at the Air/Water Interface by Hydrogel-Integrated Atomic Force Microscopy Electrochemical Platform

A nanoelectrode with a controllable area was developed using commercial atomic force microscopy and a hydrogel. Although tremendous advantages of small electrodes from micrometer scale down to nanometer scale have been previously reported for a wide range of applications, precise and high-throughput fabrication remains an obstacle. In this work, the set-point feedback current in a modified scanning ionic conductance microscopy system controlled the formation of electrodes with a nanometer-sized area by contact between the boron-doped diamond (BDD) tip and the agarose hydrogel. The modulation of the electroactive area of the BDD-coated nanoelectrode in the hydrogel was successively investigated by the finite element method and cyclic voltammetry with the use of a redox-contained hydrogel. Moreover, this nanoelectrode enables the simultaneous imaging of both the topography and electrochemical activity of a polymeric microparticle embedded in a hydrogel.

Electrochemical Imaging of Interfaces in Energy Storage via Scanning Probe Methods: Techniques, Applications, and Prospects

Developing a deeper understanding of dynamic chemical, electronic, and morphological changes at interfaces is key to solving practical issues in electrochemical energy storage systems (EESSs). To unravel this complexity, an assortment of tools with distinct capabilities and spatiotemporal resolutions have been used to creatively visualize interfacial processes as they occur. This review highlights how electrochemical scanning probe techniques (ESPTs) such as electrochemical atomic force microscopy, scanning electrochemical microscopy, scanning ion conductance microscopy, and scanning electrochemical cell microscopy are uniquely positioned to address these challenges in EESSs. We describe the operating principles of ESPTs, focusing on the inspection of interfacial structure and chemical processes involved in Li-ion batteries and beyond. We discuss current examples, performance limitations, and complementary ESPTs. Finally, we discuss prospects for imaging improvements and deep learning for automation. We foresee that ESPTs will play an enabling role in advancing EESSs as we transition to renewable energies.

Monitoring Photoinduced Interparticle Chemical Communication In Situ

Monitoring interparticle chemical communication plays a critical role in the nanomaterial synthesis as this communication controls the final structure and stability of global nanoparticles (NPs). Yet most ensemble analytical techniques, which could only reveal average macroscopic information, are unable to elucidate NP-to-NP interactions. Herein, we employ stochastic collision electrochemistry to track the morphology transformation of Ag NPs in photochemical process at the single NP level. By further statistical analysis of time-resolved current transients, we quantitatively determine the dynamic chemical potential difference and interparticle communication between populations of large and small Ag NPs. The high sensitivity of stochastic collision electrochemistry enables the in situ investigation of chemical communication-dependent transformation kinetics of NPs in photochemical process, shedding light on designing nanomaterials.

Mass Transport and Electron Transfer at the Electrochemical-Confined Interface

Single entity measurements based on the stochastic collision electrochemistry provide a promising and versatile means to study single molecules, single particles, single droplets, etc. Conceptually, mass transport and electron transfer are the two main processes at the electrochemically confined interface that underpin the most transient electrochemical responses resulting from the stochastic and discrete behaviors of single entities at the microscopic scale. This perspective demonstrates how to achieve controllable stochastic collision electrochemistry by effectively altering the two processes. Future challenges and opportunities for stochastic collision electrochemistry are also highlighted.

A Generalized Transformer-Based Pulse Detection Algorithm

Pulse-like signals are ubiquitous in the field of single molecule analysis, e.g., electrical or optical pulses caused by analyte translocations in nanopores. The primary challenge in processing pulse-like signals is to capture the pulses in noisy backgrounds, but current methods are subjectively based on a user-defined threshold for pulse recognition. Here, we propose a generalized machine-learning based method, named pulse detection transformer (PETR), for pulse detection. PETR determines the start and end time points of individual pulses, thereby singling out pulse segments in a time-sequential trace. It is objective without needing to specify any threshold. It provides a generalized interface for downstream algorithms for specific application scenarios. PETR is validated using both simulated and experimental nanopore translocation data. It returns a competitive performance in detecting pulses through assessing them with several standard metrics. Finally, the generalization nature of the PETR output is demonstrated using two representative algorithms for feature extraction.

Bridging the Gap between Single Nanoparticle Imaging and Global Electrochemical Response by Correlative Microscopy Assisted By Machine Vision

The nanostructuration of an electrochemical interface dictates its micro- and macroscopic behavior. It is generally highly complex and often evolves under operating conditions. Electrochemistry at these nanostructurations can be imaged both operando and/or ex situ at the single nanoobject or nanoparticle (NP) level by diverse optical, electron, and local probe microscopy techniques. However, they only probe a tiny random fraction of interfaces that are by essence highly heterogeneous. Given the above background, correlative multimicroscopy strategy coupled to electrochemistry in a droplet cell provides a unique solution to gain mechanistic insights in electrocatalysis. To do so, a general machine-vision methodology is depicted enabling the automated local identification of various physical and chemical descriptors of NPs (size, composition, activity) obtained from multiple complementary operando and ex situ microscopy imaging of the electrode. These multifarious microscopically probed descriptors for each and all individual NPs are used to reconstruct the global electrochemical response. Herein the methodology unveils the competing processes involved in the electrocatalysis of hydrogen evolution reaction at nickel based NPs, showing that Ni metal activity is comparable to that of platinum.

Learning from the Heterogeneity at Electrochemical Interfaces

Understanding the structure-activity relationship at electrochemical interfaces is crucial in improving the performance of practical electrochemical devices, ranging from fuel cells, electrolyzers, and batteries to electrochemical sensors. However, functional electrochemical interfaces are often complex and contain various surface structures, creating heterogeneity in electrochemical activity. In this Perspective, we highlight the role of heterogeneity in electrochemistry, especially in the context of electrocatalysis. Current methods for revealing the heterogeneity at electrochemical interfaces, including nanoelectrochemistry tools and single-entity approaches, are discussed. Lastly, we provide perspectives on what one can learn by studying heterogeneity and how one can use heterogeneity to design more efficient electrochemical devices.

High-Resolution Low-Power Hyperspectral Line-Scan Imaging of Fast Cellular Dynamics Using Azo-Enhanced Raman Scattering Probes

Small-molecule Raman probes for cellular imaging have attracted great attention owing to their sharp peaks that are sensitive to environmental changes. The small cross section of molecular Raman scattering limits dynamic cellular Raman imaging to expensive and complex coherent approaches that acquire single-channel images and lose hyperspectral Raman information. We introduce a new method, dynamic azo-enhanced Raman imaging (DAERI), to couple the new class of azo-enhanced Raman probes with a high-speed line-scan Raman imaging system. DAERI achieved high-resolution low-power imaging of fast cellular dynamics resolved at ∼270 nm along the confocal direction, 75 μW/μm² and 3.5 s/frame. Based on the azo-enhanced Raman probes with characteristic signals 10²-10⁴ stronger than classic Raman labels, DAERI was not restricted to the cellular Raman-silent region as in prior work and enabled multiplex visualization of organelle motions and interactions. We anticipate DAERI to be a powerful tool for future studies in biophysics and cell biology.

Dynamic Chemistry Interactions: Controlled Single-Entity Electrochemistry

Single-entity electrochemistry (SEE) provides powerful means to measure single cells, single particles, and even single molecules at the nanoscale by diverse well-defined interfaces. The nanoconfined electrode interface has significantly enhanced structural, electrical, and compositional characteristics that have great effects on the assay limitation and selectivity of single-entity measurement. In this Perspective, after introducing the dynamic chemistry interactions of the target and electrode interface, we present a fundamental understanding of how these dynamic interactions control the features of the electrode interface and thus the stochastic and discrete electrochemical responses of single entities under nanoconfinement. Both stochastic single-entity collision electrochemistry and nanopore electrochemistry as examples in this Perspective explore how these interactions alter the transient charge transfer and mass transport. Finally, we discuss the further challenges and opportunities in SEE, from the design of sensing interfaces to hybrid spectro-electrochemical methods, theoretical models, and advanced data processing.