Thermochemical redox catalysis is critical to a wide array of key chemical transformations and can proceed via the coupling of two electrochemical half-reactions. This electrochemical mechanism is exemplified by the platinum-catalysed aerobic oxidation of formic acid, wherein the oxygen reduction reaction is coupled to the formic acid oxidation reaction. Here using scanning electrochemical cell microscopy, we show there are grain-dependent variations in catalytic rates for the oxygen reduction and formic acid oxidation reactions at a platinum catalyst. Quantitative spatially resolved images of catalytic rates imply inter-grain cooperativity during ensemble thermochemical catalysis via lateral current flows that galvanically couple disparate active sites. Moreover, by comparing current-potential profiles of the half-reactions in isolation and in the presence of both reactants, we reveal additional site-specific chemical interactions that modify the two constituent half-reactions. These studies establish a methodology that exposes how electrochemical half-reactions couple and interact across surface structures to enable redox transformations.

Nanopipettes are important tools across diverse disciplines, including biology, physics, and materials science. Precisely controlling their characteristics is crucial for many applications. Recent progress in this endeavor has involved using the asymmetric-conductivity configuration with different electrolyte solutions inside and outside the nanopipette, which can greatly improve nanopipette sensing. However, understanding such measurements remains challenging due to the complex interplay of diffusion, electromigration, and electroosmosis. Here, we systematically explore a fundamental regime of the asymmetric-conductivity configuration where classical ion current rectification due to ion-selective migration is minimized and the effect of electroosmotic flow is maximized. We characterized the current-potential and current-distance relationship and revealed that this experimental configuration exhibits many of the characteristics of traditionally rectifying nanopipettes, such as surface charge sensitivity, while the current response can be understood simply from the rate and direction of solution mixing due to electroosmotic flow. To optimize the sensitivity in the asymmetric-conductivity configuration, we introduced a method that uses external pressure to control the fluid flow rates at the aperture, tuning the local ionic environment in situ.

The surface structure of an electrocatalyst plays a crucial role in determining the activity. As a model system, gold has been widely investigated as an electro-oxidation catalyst, although there has been much less research on the oxygen evolution reaction (OER) in the potential region of gold oxidation. Here, we combine voltammetric scanning electrochemical cell microscopy (SECCM) and electron backscatter diffraction (EBSD), at different spatial and angular resolutions, respectively, to correlate the local crystallographic structure of polycrystalline goldfocusing on grains close to (113), (011), (114), and (111) orientationswith the electrocatalytic behavior for the OER. By recording potential-dependent electrochemical movies, grain-dependent gold oxidation and gold oxide reduction are clearly visualized. In contrast, the OER demonstrates less grain dependence. High-resolution SECCM also provides a powerful way to map the electrochemical activity for OER at grain boundaries, where distinct boundaries exhibit either enhanced or suppressed activity, revealing a strong correlation with the electrochemical active surface area, together with effects from surface roughness and dislocation densities. This latter aspect of the work further emphasizes the important role of grain boundaries in electrocatalysis and the ability of SECCM to directly target and map the activity of these underexplored regions.

This work presents an experimental platform combining scanning ion conductance microscopy (SICM) with confocal laser scanning microscopy (CLSM), using intra- and extracellular pH indicator dyes to study the impact of acid delivery on individual HeLa cells within a population. The proton gradient generated by the SICM delivery is highly confined by the action of the media buffer, making the challenge local. Temporal and spatial aspects of the delivery are modeled by simulations, allowing for pH gradients across individual cells, even within a group, to be calculated. We find a strong dependency between the intracellular pH and the extracellular pH gradient imposed by local acid delivery. Postdelivery intracellular pH recovery depends on the extent of the acid challenge, with cells exposed to lower pH not returning to basal intracellular pH values after the extracellular pH recovers. This is a unique method for concentration-gradient challenge studies of cell populations that will have broad applications in cell biology. SICM can be used to deliver different chemicals and enables a wide range of local conditions to be applied across a cell population, for which the effects can be investigated at the single-cell level.

Studying cells exposed to low and controllable oxygen levels is key to investigating various fundamental aspects of pathological states, such as stroke and cancer. At present, available methodologies applied in vitro focus on large groups of cells exposed to low oxygen conditions through slow-time approaches, such as environmental incubators or microfluidic devices. Here, we demonstrate a novel approach for titrating the local oxygen concentration around individual adhered PC12 cells, enabling single cells within a population to be exposed to hypoxic-like conditions. A 25 μm diameter platinum disk microelectrode performing the oxygen reduction reaction (ORR) at constant current (galvanostatic control) is used as a microscale oxygen scavenger that can be positioned precisely over individual cells. By coupling the galvanostatic oxygen challenge with confocal laser scanning microscopy (CLSM) and a commercially available hypoxia dye (Image-iT Green hypoxia reagent), we monitor the response of single cells when exposed to depleted oxygen concentrations over time. Numerical simulations are used to characterize the oxygen and pH gradient imposed by the microelectrode at different cathodic currents, revealing that within seconds, the oxygen depletion zone reaches a steady-state condition, extending a few microelectrode radii into solution, while the corresponding pH gradient is strongly compressed by the buffer solution. Cells under the microelectrode show a marked increase in average fluorescence rate relative to control, reporting their hypoxic conditions and demonstrating the effectiveness of the proposed method. Heterogenous cell response in a challenged group is also observed, highlighting the ability of this approach to investigate the natural heterogeneity in cell populations. This work provides a platform and roadmap for future studies of cellular systems where the ability to control and vary oxygen concentration on a rapid time scale would be beneficial.

We present a thorough description of the scanning electrochemical cell microscopy (SECCM) meniscus probe, in operation, by combining dual-channel SECCM measurements with in situ interference reflection microscopy (IRM). SECCM is a pipette-based nanoscale characterization tool with an unparalleled capacity for mapping the electrochemical activity of material surfaces, with high precision and at high throughput. In hopping mode, it operates by bringing the electrolyte meniscus, at the scanned pipette tip, in contact with the sample, restricting the probed area each time to a separate, newly wetted site, and forming a small-scale reactor. Each contact area can normally be imaged post-experiment, to inform on the wetted area stability and enable quantitative data interpretation (e.g., to calculate current density). However, the description of meniscus behavior during measurements would be beneficial. Herein, we utilize semi-transparent electrode substrates, to enable the direct optical observation, by IRM, of the meniscus status, with high spatial and temporal resolution, and synchronously to SECCM operation. The surface-sensitive optical method allows us to accurately capture the nature of the miniature electrochemical cell during all phases of the experiment-during approach, meniscus contact, wetting, and pipette withdrawal-and to follow subtle changes while in contact with the electrode substrate. Through the use of a dual-channel probe, we are able to monitor both the ionic current across the meniscus, between quasi-reference counter electrodes (QRCEs) under bias, and between the working electrode surface and the QRCEs. Correlating these electrochemical data and operando optical information via the hybrid SECCM-IRM approach aids the design of experimental protocols, streamlines the interpretation of results, and paints a comprehensive picture of meniscus wetting behavior.

Silver nanoparticles (AgNPs) conjugated with polymers are well-known for their powerful and effective antimicrobial properties. In particular, the incorporation of AgNPs in biocompatible catecholamine-based polymers, such as polydopamine (PDA), has recently shown promising antimicrobial activity, due to the synergistic effects of the AgNPs, silver(I) ions released and PDA. In this study, we generated AgNPs-PDA-patterned surfaces by localised electrochemical depositions, using a double potentiostatic method via scanning electrochemical cell microscopy (SECCM). This technique enabled the assessment of a wide parameter space in a high-throughput manner. The optimised electrodeposition process resulted in stable and homogeneously distributed AgNP-microspots, and their antimicrobial activity against Escherichia coli was assessed using atomic force microscopy (AFM)-based force spectroscopy, in terms of bacterial adhesion and cell elasticity. We observed that the bacterial outer membrane underwent significant structural changes, when in close proximity to the AgNPs, namely increased hydrophilicity and stiffness loss. The spatially varied antimicrobial effect found experimentally was rationalised by numerical simulations of silver(I) concentration profiles.

The development of Li metal batteries requires a detailed understanding of complex nucleation and growth processes during electrodeposition. In situ techniques offer a framework to study these phenomena by visualizing structural dynamics that can inform the design of uniform plating morphologies. Herein, we combine scanning electrochemical cell microscopy (SECCM) with in situ interference reflection microscopy (IRM) for a comprehensive investigation of Li nucleation and growth on lithiophilic thin-film gold electrodes. This multimicroscopy approach enables nanoscale spatiotemporal monitoring of Li plating and stripping, along with high-throughput capabilities for screening experimental conditions. We reveal the accumulation of inactive Li nanoparticles in specific electrode regions, yet these regions remain functional in subsequent plating cycles, suggesting that growth does not preferentially occur from particle tips. Optical-electrochemical correlations enabled nanoscale mapping of Coulombic Efficiency (CE), showing that regions prone to inactive Li accumulation require more cycles to achieve higher CE. We demonstrate that electrochemical nucleation time (tnuc) is a lagging indicator of nucleation and introduce an optical method to determine tnuc at earlier stages with nanoscale resolution. Plating at higher current densities yielded smaller Li nanoparticles and increased areal density, and was not affected by heterogeneous topographical features, being potentially beneficial to achieve a more uniform plating at longer time scales. These results enhance the understanding of Li plating on lithiophilic surfaces and offer promising strategies for uniform nucleation and growth. Our multimicroscopy approach has broad applicability to study nanoscale metal plating and stripping phenomena, with relevance in the battery and electroplating fields.

Defect-free graphene is impermeable to all atoms1-5 and ions6,7 under ambient conditions. Experiments that can resolve gas flows of a few atoms per hour through micrometre-sized membranes found that monocrystalline graphene is completely impermeable to helium, the smallest atom2,5. Such membranes were also shown to be impermeable to all ions, including the smallest one, lithium6,7. By contrast, graphene was reported to be highly permeable to protons, nuclei of hydrogen atoms8,9. There is no consensus, however, either on the mechanism behind the unexpectedly high proton permeability10-14 or even on whether it requires defects in graphene's crystal lattice6,8,15-17. Here, using high-resolution scanning electrochemical cell microscopy, we show that, although proton permeation through mechanically exfoliated monolayers of graphene and hexagonal boron nitride cannot be attributed to any structural defects, nanoscale non-flatness of two-dimensional membranes greatly facilitates proton transport. The spatial distribution of proton currents visualized by scanning electrochemical cell microscopy reveals marked inhomogeneities that are strongly correlated with nanoscale wrinkles and other features where strain is accumulated. Our results highlight nanoscale morphology as an important parameter enabling proton transport through two-dimensional crystals, mostly considered and modelled as flat, and indicate that strain and curvature can be used as additional degrees of freedom to control the proton permeability of two-dimensional materials.

Scanning electrochemical probe microscopies (SEPMs) have played a key role in advancing small-scale electrochemistry. SEPMs use an electrochemical probe (micro/nanoelectrode or pipet) to quantify and map local interfacial fluxes of electroactive species and have found increasingly wide applications. Our contribution to the Fundamental and Applied Reviews in Analytical Chemistry 2019 discussed how advances in SEPMs converged towards nanoscale electrochemical mapping. This inflection in experimental capability has opened up myriad opportunities for SEPMs in many types of systems, from material and energy sciences to the life sciences. The enhancement in the spatial resolution of imaging techniques, and instrumental developments, have resulted in significant increases in the size of electrochemical datasets from a typical experiment and served to speed up measurement throughput. Next generation nanoelectrochemistry will thus see an emphasis on “big data”, its analysis, storage and curation, high throughput analysis and parallelization, “intelligent” instruments and experiments, active control of nanoscale systems, and the integration of nanoelectrochemistry and nanoscale micro(spectro)scopy. For this review article, we focus on recent advances in frontier nanoscale electrochemistry, analysis and imaging techniques that are already addressing some of these key targets, and are well-placed to embrace other aspects in the near future. Our goal is to provide an overview of the present state-of-the-art in high throughput nanoelectrochemistry and imaging, and signpost promising new avenues for nanoscale electrochemical methods.

AbstractAnatas‐TiO2 ist ein vielversprechendes Material für Li‐Ionen‐Batterien (Li+) mit Schnellladefähigkeit. Die (De‐)Interkalationsdynamik von Li+ in TiO2 ist jedoch nach wie vor schwer fassbar, und die beschriebenen Diffusivitäten überspannen viele Größenordnungen. Wir entwickeln ein intelligentes Protokoll für die rasterelektrochemische Zellmikroskopie (SECCM) in Kombination mit in situ optischer Mikroskopie (OM), um die Lade‐/Entladeanalyse einzelner TiO2‐Nanopartikel‐Cluster mit hohem Durchsatz zu beobachten. Die direkte Untersuchung aktiver Nanopartikel ergab, dass TiO2 mit einer Größe von ca. 50 nm mehr als 30 % der theoretischen Kapazität bei einer äußerst schnellen Lade‐/Entladerate von ca. 100 C speichern kann. Dieser Nachweis einer schnellen Li+‐Speicherung in TiO2‐Partikeln stärkt ihr Potenzial für schnell ladende Batterien. Die hier vorgeschlagene SECCM‐OM ist generell für das elektrochemische Hochdurchsatz‐Screening von nanostrukturierten Materialien breit anwendbar.

Anatase TiO2 is a promising material for Li-ion (Li+ ) batteries with fast charging capability. However, Li+ (de)intercalation dynamics in TiO2 remain elusive and reported diffusivities span many orders of magnitude. Here, we develop a smart protocol for scanning electrochemical cell microscopy (SECCM) with in situ optical microscopy (OM) to enable the high-throughput charge/discharge analysis of single TiO2 nanoparticle clusters. Directly probing active nanoparticles revealed that TiO2 with a size of ≈50 nm can store over 30 % of the theoretical capacity at an extremely fast charge/discharge rate of ≈100 C. This finding of fast Li+ storage in TiO2 particles strengthens its potential for fast-charging batteries. More generally, smart SECCM-OM should find wide applications for high-throughput electrochemical screening of nanostructured materials.

The ability to control crystal nucleation through the simple addition of a nucleating agent (nucleant) is desirable for a huge range of applications. However, effective nucleating agents are known for only a small number of systems, and many questions remain about the mechanisms by which they operate. Here, we explore the features that make an effective nucleant and demonstrate that the biological material hair-which naturally possesses a chemically and topographically complex surface structure-has excellent potential as an effective nucleating agent. Crystallization of poorly soluble compounds in the presence of hairs from a range of mammals shows that nucleation preferentially occurs at the cuticle step edges, while a novel microdroplet-based methodology was used to quantify the nucleating activities of different hairs. This showed that the activities of the hairs can be tuned over a wide range using chemical treatments. Analysis of the hair structure and composition using atomic force microscopy, scanning ion conductance microscopy, and X-ray photoelectron spectroscopy demonstrates that surface chemistry, surface topography, and surface charge all act in combination to create effective nucleation sites. This work therefore contributes to our understanding of heterogeneous nucleating agents and shows that surface topography as well as surface chemistry can be used in the design or selection of universal nucleating agents.

The Front Cover draws a parallel between the hybrid scanning electrochemical cell microscopy-interference reflection microscopy (SECCM-IRM) technique, and its use to monitor the nucleation and growth of silver nanoparticles, and the recording of a performance by a rock band. The stage corresponds to the meniscus-electrode wetted area formed with the SECCM probe. It is illuminated by optical lighting. Silver particles rock and roll; they are dynamic and interact with each other just like the band. The behaviour of each individual particle is revealed through optical-guided localisation, just as post-processing in a multitrack sound recording can be used to isolate individual tracks. Cover design by Somersault18:24. More information can be found in the Research Article by P. Ciocci, D. Valavanis et al.

Correlative co-located electrochemical multi-microscopy is transforming our understanding of property-function relationships in electrode materials. By coupling scanning electrochemical cell microscopy (SECCM) with complementary characterization techniques applied to identical locations of a surface, we can now unravel the intricate interplay between various physicochemical properties of electrode materials and interfaces and their impact on electrochemical phenomena, with high spatial resolution. This review explores recent advances in this correlative approach to showcase how it can reveal major new insights into the activity of materials and interfaces. Applications span diverse electrochemical fields, including energy conversion and storage, sensing, and corrosion science. We also envision future developments that will open up new possibilities for rational materials design, accelerated mechanistic understanding, and automated materials discovery.

Two-dimensional (2D) materials offer a prospect of membranes that combine negligible gas permeability with high proton conductivity and could outperform the existing proton exchange membranes used in various applications including fuel cells. Graphene oxide (GO), a well-known 2D material, facilitates rapid proton transport along its basal plane but proton conductivity across it remains unknown. It is also often presumed that individual GO monolayers contain a large density of nanoscale pinholes that lead to considerable gas leakage across the GO basal plane. Here we show that relatively large, micrometer-scale areas of monolayer GO are impermeable to gases, including helium, while exhibiting proton conductivity through the basal plane which is nearly two orders of magnitude higher than that of graphene. These findings provide insights into the key properties of GO and demonstrate that chemical functionalization of 2D crystals can be utilized to enhance their proton transparency without compromising gas impermeability.

Abstract The formation of metal nanoparticles (NPs) on surfaces by electrodeposition is of significant interest, particularly with a view to understand the early stages of nucleation and growth. Here, the combination of scanning electrochemical cell microscopy (SECCM) and interference reflection microscopy (IRM) is demonstrated to be a compelling approach for real‐time monitoring of NP dynamics within the SECCM meniscus‐electrode wetted area, through synchronous monitoring in the millisecond range of the electrochemical and optical signatures. Diffraction‐limited entities, undergoing phase changes at the electrode substrate, are readily highlighted and tracked in time, including the onset time for the appearance of NPs and their movement over time. The results strongly implicate the rapid formation, surface diffusion and aggregation of smaller entities (not detectable optically) to produce the larger electrodeposited NPs. By applying SECCM tips of different size, it is also possible to understand how the wetted area (meniscus size) plays a key role in the number of NPs formed, with small tip sizes allowing the formation of single NPs. The SECCM‐IRM approach is expected to be a powerful platform for the study of myriad phase‐formation processes at the nanoscale, particularly by drawing on the possibility of making hundreds or thousands of measurements in fresh surface locations through SECCM technology.

Nanostructured electrocatalysts exhibit variations in electrochemical properties across different length scales, and the intrinsic catalytic characteristics measured at the nanoscale often differ from those at the macro-level due to complexity in electrode structure and/or composition. This aspect of electrocatalysis is addressed herein, where the oxygen evolution reaction (OER) activity of β-Co(OH)2 platelet particles of well-defined structure is investigated in alkaline media using multiscale scanning electrochemical cell microscopy (SECCM). Microscale SECCM probes of ∼50 μm diameter provide voltammograms from small particle ensembles (ca. 40-250 particles) and reveal increasing dispersion in the OER rates for samples of the same size as the particle population within the sample decreases. This suggests the underlying significance of heterogeneous activity at the single-particle level that is confirmed through single-particle measurements with SECCM probes of ∼5 μm diameter. These measurements of multiple individual particles directly reveal significant variability in the OER activity at the single-particle level that do not simply correlate with the particle size, basal plane roughness, or exposed edge plane area. In combination, these measurements demarcate a transition from an "individual particle" to an "ensemble average" response at a population size of ca. 130 particles, above which the OER current density closely reflects that measured in bulk at conventional macroscopic particle-modified electrodes. Nanoscale SECCM probes (ca. 120 and 440 nm in diameter) enable measurements at the subparticle level, revealing that there is selective OER activity at the edges of particles and highlighting the importance of the three-phase boundary where the catalyst, electrolyte, and supporting carbon electrode meet, for efficient electrocatalysis. Furthermore, subparticle measurements unveil heterogeneity in the OER activity among particles that appear superficially similar, attributable to differences in defect density within the individual particles, as well as to variations in electrical and physical contact with the support material. Overall this study provides a roadmap for the multiscale analysis of nanostructured electrocatalysts, directly demonstrating the importance of multilength scale factors, including particle structure, particle-support interaction, presence of defects, etc., in governing the electrochemical activities of β-Co(OH)2 platelet particles and ultimately guiding the rational design and optimization of these materials for alkaline water electrolysis.

Ultramicroelectrode (UME), or, equivalently, microelectrode, probes are increasingly used for single-cell measurements of cellular properties and processes, including physiological activity, such as metabolic fluxes and respiration rates. Major challenges for the sensitivity of such measurements include: (i) the relative magnitude of cellular and UME fluxes (manifested in the current); and (ii) issues around the stability of the UME response over time. To explore the extent to which these factors impact the precision of electrochemical cellular measurements, we undertake a systematic analysis of measurement conditions and experimental parameters for determining single cell respiration rates via the oxygen consumption rate (OCR) in single HeLa cells. Using scanning electrochemical microscopy (SECM), with a platinum UME as the probe, we employ a self-referencing measurement protocol, rarely employed in SECM, whereby the UME is repeatedly approached from bulk solution to a cell, and a short pulse to oxygen reduction reaction (ORR) potential is performed near the cell and in bulk solution. This approach enables the periodic tracking of the bulk UME response to which the near-cell response is repeatedly compared (referenced) and also ensures that the ORR near the cell is performed only briefly, minimizing the effect of the electrochemical process on the cell. SECM experiments are combined with a finite element method (FEM) modeling framework to simulate oxygen diffusion and the UME response. Taking a realistic range of single cell OCR to be 1 × 10-18 to 1 × 10-16 mol s-1, results from the combination of FEM simulations and self-referencing SECM measurements show that these OCR values are at, or below, the present detection sensitivity of the technique. We provide a set of model-based suggestions for improving these measurements in the future but highlight that extraordinary improvements in the stability and precision of SECM measurements will be required if single cell OCR measurements are to be realized.