Operando Scanning Electrochemical Probe Microscopy during Electrocatalysis

Scanning electrochemical probe microscopy (SEPM) techniques can disclose the local electrochemical reactivity of interfaces in single-entity and sub-entity studies. Operando SEPM measurements consist of using a SEPM tip to investigate the performance of electrocatalysts, while the reactivity of the interface is simultaneously modulated. This powerful combination can correlate electrochemical activity with changes in surface properties, e.g., topography and structure, as well as provide insight into reaction mechanisms. The focus of this review is to reveal the recent progress in local SEPM measurements of the catalytic activity of a surface toward the reduction and evolution of O₂ and H₂ and electrochemical conversion of CO₂. The capabilities of SEPMs are showcased, and the possibility of coupling other techniques to SEPMs is presented. Emphasis is given to scanning electrochemical microscopy (SECM), scanning ion conductance microscopy (SICM), electrochemical scanning tunneling microscopy (EC-STM), and scanning electrochemical cell microscopy (SECCM).

Multifrequency dielectric mapping of fixed mice colon tissues in cell culture media via scanning electrochemical microscopy

Alternating current scanning electrochemical microscopy (AC-SECM) is a powerful tool for characterizing the electrochemical reactivity of surfaces. Here, perturbation in the sample is induced by the alternating current and altered local potential is measured by the SECM probe. This technique has been used to investigate many exotic a range of biological interfaces including live cells and tissues, as well as the corrosive degradation of various metallic surfaces, etc. In principle, AC-SECM imaging is derived from electrochemical impedance spectroscopy (EIS) which has been used for a century to describe interfacial and diffusive behaviour of molecules in solution or on a surface. Increasingly bioimpedance centric medical devices have become an important tool to detect evolution of tissue biochemistry. Predictive implications of measuring electrochemical changes within a tissue is one of the core concepts in developing minimally invasive and smart medical devices. In this study, cross sections of mice colon tissue were used for AC-SECM imaging. A 10 micron sized platinum probe was used for two-dimensional (2D) tan δ mapping of histological sections at a frequency of 10 kHz, Thereafter, multifrequency scans were performed at 100 Hz, 10 kHz, 300 kHz, and 900 kHz. Loss tangent (tan δ) mapping of mice colon revealed microscale regions within a tissue possessing a discrete tan δ signature. This tan δ map may be an immediate measure of physiological conditions in biological tissues. Multifrequency scans highlight subtle changes in protein or lipid composition as a function of frequency which was recorded as loss tangent maps. Impedance profile at different frequencies could also be used to identify optimal contrast for imaging and extracting the electrochemical signature specific for a tissue and its electrolyte.

AI-Assisted Fusion of Scanning Electrochemical Microscopy Images Using Novel Soft Probe

Scanning electrochemical microscopy (SECM) is one of the scanning probe techniques that has attracted considerable attention because of its ability to interrogate surface morphology or electrochemical reactivity. However, the quality of SECM images generally depends on the sizes of the electrodes and many uncontrollable factors. Furthermore, manipulating fragile glass ultramicroelectrodes and blurred images sometimes frustrate researchers. To overcome the challenges of modern SECM, we developed novel soft gold probes and then established the AI-assisted methodology for image fusion. A novel gold microelectrode probe with high softness was developed to scan fragile samples. The distribution of EGFR (protein biomarker) in oral cancer was investigated. Then, we fused the optical microscopic and SECM images to enhance the image quality using Matlab software. However, thousands of fused images were generated by changing the parameters for image fusion, which is annoying for researchers. Thus, a deep learning model was built to select the best-fused images according to the contrast and clarity of the fused images. Therefore, the quality of the SECM images was improved using a novel soft probe and combining the image fusion technique. In the future, a new scanning probe with AI-assisted fused SECM image processing may be interpreted more preciously and contribute to the early detection of cancers.

Correlative Electrochemical Microscopy for the Elucidation of the Local Ionic and Electronic Properties of the Solid Electrolyte Interphase in Li-Ion Batteries

The solid-electrolyte interphase (SEI) plays a key role in the stability of lithium-ion batteries as the SEI prevents the continuous degradation of the electrolyte at the anode. The SEI acts as an insulating layer for electron transfer, still allowing the ionic flux through the layer. We combine the feedback and multi-frequency alternating-current modes of scanning electrochemical microscopy (SECM) for the first time to assess quantitatively the local electronic and ionic properties of the SEI varying the SEI formation conditions and the used electrolytes in the field of Li-ion batteries (LIB). Correlations between the electronic and ionic properties of the resulting SEI on a model Cu electrode demonstrates the unique feasibility of the proposed strategy to provide the two essential properties of an SEI: ionic and electronic conductivity in dependence on the formation conditions, which is anticipated to exhibit a significant impact on the field of LIBs.

Versatile Tools for Understanding Electrosynthetic Mechanisms

Electrosynthesis is a popular, green alternative to traditional organic methods. Understanding the mechanisms is not trivial yet is necessary to optimize reaction processes. To this end, a multitude of analytical tools is available to identify and quantitate reaction products and intermediates. The first portion of this review serves as a guide that underscores electrosynthesis fundamentals, including instrumentation, electrode selection, impacts of electrolyte and solvent, cell configuration, and methods of electrosynthesis. Next, the broad base of analytical techniques that aid in mechanism elucidation are covered in detail. These methods are divided into electrochemical, spectroscopic, chromatographic, microscopic, and computational. Technique selection is dependent on predicted reaction pathways and electrogenerated intermediates. Often, a combination of techniques must be utilized to ensure accuracy of the proposed model. To conclude, future prospects that aim to enhance the field are discussed.

A gateway for ion transport on gas bubbles pinned onto solids

Gas bubbles grown on solids are more than simple vehicles for gas transport. They are charged particles with surfaces populated with exchangeable ions. We here unveil a gateway for alkali metal ion transport between oxygen bubbles and semi-conducting (iron oxide) and conducting (gold) surfaces. This gateway was identified by electrochemical impedance spectroscopy using an ultramicroelectrode in direct contact with bubbles pinned onto these solid surfaces. We show that this gateway is naturally present at open circuit potentials, and that negative electric potentials applied through the solid enhance ion transport. In contrast, positive potentials or contact with an insulator (polytetrafluoroethylene) attenuates transport. We propose that this gateway is generated by overlapping electric double layers of bubbles and surfaces of contrasting (electro)chemical potentials. Knowledge of this ion transfer phenomenon is essential for understanding electric shielding and reaction overpotential caused by bubbles on catalysts. This has especially important ramifications for predicting processes including mineral flotation, microfluidics, pore water geochemistry, and fuel cell technology.

Direct Nanomachining on Semiconductor Wafer By Scanning Electrochemical Microscopy

Scanning electrochemical microscopy (SECM) is one of the most important instrumental methods of modern electrochemistry due to its high spatial and temporal resolution. We introduced SECM into nanomachining by feeding the electrochemical modulations of the tip electrode back to the positioning system, and we demonstrated that SECM is a versatile nanomachining technique on semiconductor wafers using electrochemically induced chemical etching. The removal profile was correlated to the applied tip current when the tip was held stationary and when it was moving slowly (<20 μm s^-1 ), and it followed Faraday's law. Both regular and irregular nanopatterns were translated into a spatially distributed current by the homemade digitally controlled SECM instrument. The desired nanopatterns were "sculpted" directly on a semiconductor wafer by SECM direct-writing mode. The machining accuracy was controlled to the sub-micrometer and even nanometer scales. This advance is expected to play an important role in electrochemical nanomachining for 3D micro/nanostructures in the semiconductor industry.

Mapping oscillating magnetic fields around rechargeable batteries

Power storage devices such as batteries are a crucial part of modern technology. The development and use of batteries has accelerated in the past decades, yet there are only a few techniques that allow gathering vital information from battery cells in a nonivasive fashion. A widely used technique to investigate batteries is electrical impedance spectroscopy (EIS), which provides information on how the impedance of a cell changes as a function of the frequency of applied alternating currents. Building on recent developments of inside-out MRI (ioMRI), a technique is presented here which produces spatially-resolved maps of the oscillating magnetic fields originating from the alternating electrical currents distributed within a cell. The technique works by using an MRI pulse sequence synchronized with a gated alternating current applied to the cell terminals. The approach is benchmarked with a current-carrying wire coil, and demonstrated with commercial and prototype lithium-ion cells. Marked changes in the fields are observed for different cell types.

Electrochemical Impedance Measurements in Scanning Ion Conductance Microscopy

Electrochemical impedance spectroscopy (EIS) is a versatile tool for electrochemistry, particularly when applied locally to reveal the properties and dynamics of heterogeneous interfaces. A new method to generate local electrochemical impedance spectra is outlined, by applying a harmonic bias between a quasi-reference counter electrode (QRCE) placed in a nanopipet tip of a scanning ion conductance microscope (SICM) and a conductive (working electrode) substrate (two-electrode setup). The AC frequency can be tuned so that the magnitude of the impedance is sensitive to the tip-to-substrate distance, whereas the phase angle is broadly defined by the local capacitive response of the electrical double layer (EDL) of the working electrode. This development enables the surface topography and the local capacitance to be sensed reliably, and separately, in a single measurement. Further, self-referencing the probe impedance near the surface to that in the bulk solution allows the local capacitive response of the working electrode substrate in the overall AC signal to be determined, establishing a quantitative footing for the methodology. The spatial resolution of AC-SICM is an order of magnitude larger than the tip size (100 nm radius), for the studies herein, due to frequency dispersion. Comprehensive finite element method (FEM) modeling is undertaken to optimize the experimental conditions and minimize the experimental artifacts originating from the frequency dispersion phenomenon, and provides an avenue to explore the means by which the spatial resolution could be further improved.

Mediator-Free SECM for Probing the Diffusion Layer pH with Functionalized Gold Ultramicroelectrodes

Probing pH gradients during electrochemical reactions is important to better understand reaction mechanisms and to separate the influence of pH and pH gradients from intrinsic electrolyte effects. Here, we develop a pH sensor to measure pH changes in the diffusion layer during hydrogen evolution. The probe was synthesized by functionalizing a gold ultramicroelectrode with a self-assembled monolayer of 4-nitrothiophenol (4-NTP) and further converting it to form a hydroxylaminothiophenol (4-HATP)/4-nitrosothiophenol (4-NSTP) redox couple. The pH sensing is realized by recording the tip cyclic voltammetry and monitoring the Nernstian shift of the midpeak potential. We employ a capacitive approach technique in our home-built Scanning Electrochemical Microscope (SECM) setup in which an AC potential is applied to the sample and the capacitive current generated at the tip is recorded as a function of distance. This method allows for an approach of the tip to the electrode that is electrolyte-free and consequently also mediator-free. Hydrogen evolution on gold in a neutral electrolyte was studied as a model system. The pH was measured with the probe at a constant distance from the electrode (ca. 75 μm), while the electrode potential was varied in time. In the nonbuffered electrolyte used (0.1 M Li₂SO₄), even at relatively low current densities, a pH difference of three units is measured between the location of the probe and the bulk electrolyte. The time scale of the diffusion layer transient is captured, due to the high time resolution that can be achieved with this probe. The sensor has high sensitivity, measuring differences of more than 8 pH units with a resolution better than 0.1 pH unit.

Spatially resolved electrical impedance methods for cell and particle characterization

Electrical impedance is an established technique used for cell and particle characterization. The temporal and spectral resolution of electrical impedance have been used to resolve basic cell characteristics like size and type, as well as to determine cell viability and activity. Such electrical impedance measurements are typically performed across the entire sample volume and can only provide an overall indication concerning the properties and state of that sample. For the study of heterogeneous structures such as cell layers, biological tissue, or polydisperse particle mixtures, an overall measured impedance value can only provide limited information and can lead to data misinterpretation. For the investigation of localized sample properties in complex heterogeneous structures/mixtures, the addition of spatial resolution to impedance measurements is necessary. Several spatially resolved impedance measurement techniques have been developed and applied to cell and particle research, including electrical impedance tomography, scanning electrochemical microscopy, and microelectrode arrays. This review provides an overview of spatially resolved impedance measurement methods and assesses their applicability for cell and particle characterization.

Recent Advances in Scanning Electrochemical Microscopy for Biological Applications

Scanning electrochemical microscopy (SECM) is a chemical microscopy technique with high spatial resolution for imaging sample topography and mapping specific chemical species in liquid environments. With the development of smaller, more sensitive ultramicroelectrodes (UMEs) and more precise computer-controlled measurements, SECM has been widely used to study biological systems over the past three decades. Recent methodological breakthroughs have popularized SECM as a tool for investigating molecular-level chemical reactions. The most common applications include monitoring and analyzing the biological processes associated with enzymatic activity and DNA, and the physiological activity of living cells and other microorganisms. The present article first introduces the basic principles of SECM, followed by an updated review of the applications of SECM in biological studies on enzymes, DNA, proteins, and living cells. Particularly, the potential of SECM for investigating bacterial and biofilm activities is discussed.

Single Ag nanoparticle collisions within a dual-electrode micro-gap cell

An adjustable width (between 600 nm and 20 μm) gap between two Au microelectrodes is used to probe the electrodissolution dynamics of single Ag nanoparticles. One Au microelectrode is used to drive the oxidation and subsequent dissolution of a single Ag nanoparticle, which displays a multi-peak oxidation behavior, while a second Au microelectrode is used to collect the Ag+ that is produced. Careful analysis of the high temporal resolution current-time traces reveals capacitive coupling between electrodes due to the sudden injection of Ag+ ions into the gap between the electrodes. The current-time traces allow measurement of the effect of citrate concentration on the electrodissolution dynamics of a single Ag nanoparticle, which reveals that the presence of 2 mM citrate significantly slows down the release of Ag+. Intriguingly, these experiments also reveal that only a portion (ca. 50%) of the oxidized Ag nanoparticle is released as free Ag+ regardless of citrate concentration.

Scanning electrochemical microscopy: an analytical perspective

Scanning electrochemical microscopy (SECM) has evolved from an electrochemical specialist tool to a broadly used electroanalytical surface technique, which has experienced exciting developments for nanoscale electrochemical studies in recent years. Several companies now offer commercial instruments, and SECM has been used in a broad range of applications. SECM research is frequently interdisciplinary, bridging areas ranging from electrochemistry, nanotechnology, and materials science to biomedical research. Although SECM is considered a modern electroanalytical technique, it appears that less attention is paid to so-called analytical figures of merit, which are essential also in electroanalytical chemistry. Besides instrumental developments, this review focuses on aspects such as reliability, repeatability, and reproducibility of SECM data. The review is intended to spark discussion within the community on this topic, but also to raise awareness of the challenges faced during the evaluation of quantitative SECM data.

Atomic force microscopy with nanoelectrode tips for high resolution electrochemical, nanoadhesion and nanoelectrical imaging

Multimodal nano-imaging in electrochemical environments is important across many areas of science and technology. Here, scanning electrochemical microscopy (SECM) using an atomic force microscope (AFM) platform with a nanoelectrode probe is reported. In combination with PeakForce tapping AFM mode, the simultaneous characterization of surface topography, quantitative nanomechanics, nanoelectronic properties, and electrochemical activity is demonstrated. The nanoelectrode probe is coated with dielectric materials and has an exposed conical Pt tip apex of ∼200 nm in height and of ∼25 nm in end-tip radius. These characteristic dimensions permit sub-100 nm spatial resolution for electrochemical imaging. With this nanoelectrode probe we have extended AFM-based nanoelectrical measurements to liquid environments. Experimental data and numerical simulations are used to understand the response of the nanoelectrode probe. With PeakForce SECM, we successfully characterized a surface defect on a highly-oriented pyrolytic graphite electrode showing correlated topographical, electrochemical and nanomechanical information at the highest AFM-SECM resolution. The SECM nanoelectrode also enabled the measurement of heterogeneous electrical conductivity of electrode surfaces in liquid. These studies extend the basic understanding of heterogeneity on graphite/graphene surfaces for electrochemical applications.

Kinetic Passivation Effect of Localized Differential Aeration on Brass

The formation of a localized differential aeration cell on metals, susceptible to both anodic and cathodic corrosion, is a serious threat because of multiple degradation processes commencing with the passivation layer destruction. By using local electrochemical and X-ray dispersive techniques, it has been demonstrated that the differential aeration cell formed on high brass (α-brass, Cu65-Zn35) in the presence of 1H-benzotriazole or 5-methyl-1H-benzotriazole plays both corrosion-inhibiting and accelerating roles, depending on the inhibitor exposure time. Alternating-current scanning electrochemical microscopy was used to image local electrochemical activity, whereas energy-dispersive X-ray spectroscopy provided evidence for the mechanism of the observed phenomena. Short-term exposure to the inhibitor (5 min) promotes the formation of a passivation layer in the waterline region. In contrast, after prolonged exposure (45 min), a deficient passivation layer develops for both inhibitors. An excess of zinc(II)-inhibitor complexes in the passivation layer is accountable for the corrosion resistance of the region with high differential aeration. Rapid dezincification and local alkalinization facilitate the initial rapid formation of a passivation layer in the area under differential aeration to preserve its composition upon further modification.

Reactive oxygen species formed in organic lithium–oxygen batteries

The generation of reactive oxygen species has been assumed to occur during the charging reaction of lithium-oxygen batteries with organic electrolytes. Here we show independently by fluorescence microscopy and scanning electrochemical microscopy that superoxide is also formed and released into the solution during the discharge reaction.

Mapping Electrochemical Heterogeneity at Iron Oxide Surfaces: A Local Electrochemical Impedance Study

Alternating current scanning electrochemical microscopy (AC-SECM) was used for the first time to map key electrochemical attributes of oriented hematite (α-Fe2O3) single crystal surfaces at the micron-scale. Localized electrochemical impedance spectra (LEIS) of the (001) and (012) faces provided insight into the spatial variations of local double layer capacitance (C(dl)) and charge transfer resistance (R(ad)). These parameters were extracted by LEIS measurements in the 0.4-8000 Hz range to probe the impedance response generated by the redistribution of water molecules and charge carriers (ions) under an applied AC. These were attributed to local variations in the local conductivity of the sample surfaces. Comparison with global EIS measurements on the same samples uncovered highly comparable frequency-resolved processes, that were broken down into contributions from the bulk hematite, the interface as well as the microelectrode/tip assembly. This work paves the way for new studies aimed at mapping electrochemical processes at the mesoscale on this environmentally and technologically important material.

Localized electrochemical impedance spectroscopy: visualization of spatial distributions of the key parameters describing solid/liquid interfaces

Acquisition of localized electrochemical impedance spectra as a function of spatial coordinates combined with novel approaches of data analysis brings a key for visualization of two-dimensional distributions of important parameters describing solid/liquid interfaces. They include the capacitance of the electric double layer, the resistance of the interfacial charge transfer, capacitances of adsorption, or other parameters depending on the properties of the system. Additionally, the proposed approach eliminates many common methodological problems of localized electrochemical impedance microscopies related to the frequency dependence of the actual pictures and difficulties with raw data interpretation. Thus, it offers a unique insight into the localized processes at the interface which is not possible to achieve using classical techniques.

Alternating current scanning electrochemical microscopy with simultaneous fast-scan cyclic voltammetry

Fast-scan cyclic voltammetry (FSCV) is combined with alternating current scanning electrochemical microscopy (AC-SECM) for simultaneous measurements of impedance and faradaic current. Scan rates of 10-1000 V s(-1) were used for voltammetry, while a high-frequency (100 kHz), low-amplitude (10 mV rms) sine wave was added to the voltammetric waveform for the ac measurement. Both a lock-in amplifier and an analog circuit were used to measure the amplitude of the resultant ac signal. The effect of the added sine wave on the voltammetry at a carbon fiber electrode was investigated and found to have negligible effect. The combined FSCV and ac measurements were used to provide simultaneous chemical and topographical information about a substrate using a single carbon fiber probe. The technique is demonstrated in living cell culture, where cellular respiration and topography were simultaneously imaged without the addition of a redox mediator. This approach promises to be useful for the topographical and multidimensional chemical imaging of substrates.