In Situ Detection of the Adsorbed Fe(II) Intermediate and the Mechanism of Magnetite Electrodeposition by Scanning Electrochemical Microscopy

Transient theory for scanning electrochemical microscopy of biological membrane transport: uncovering membrane-permeant interactions

Scanning electrochemical microscopy (SECM) has emerged as a powerful method to quantitatively investigate the transport of molecules and ions across various biological membranes as represented by living cells. Advantageously, SECM allows for the in situ and non-destructive imaging and measurement of high membrane permeability under simple steady-state conditions, thereby facilitating quantitative data analysis. The SECM method, however, has not provided any information about the interactions of a transported species, i.e., a permeant, with a membrane through its components, e.g., lipids, channels, and carriers. Herein, we propose theoretically that SECM enables the quantitative investigation of membrane-permeant interactions by employing transient conditions. Specifically, we model the membrane-permeant interactions based on a Langmuir-type isotherm to define the strength and kinetics of the interactions as well as the concentration of interaction sites. Finite element simulation predicts that each of the three parameters uniquely affects the chronoamperometric current response of an SECM tip to a permeant. Significantly, this prediction implies that all three parameters are determinable from an experimental chronoamperometric response of the SECM tip. Complimentarily, the steady-state current response of the SECM tip yields the overall membrane permeability based on the combination of the three parameters. Interestingly, our simulation also reveals the optimum strength of membrane-permeant interactions to maximize the transient flux of the permeant from the membrane to the tip.

Investigation of the Electrocatalytic Reduction of Peroxydisulfate Using Scanning Electrochemical Microscopy

The elementary steps of the electrocatalytic reduction of S2O82- using the Ru(NH3)63+/2+ redox couple were investigated using scanning electrochemical microscopy (SECM) and steady-state voltammetry (SSV). SECM investigations were carried out in a 0.1 M KCl solution using a 3.5 μm radius carbon ultramicroelectrode (UME) as the SECM tip and a 25 μm radius platinum UME as the substrate electrode. Approach curves were recorded in the positive feedback mode of SECM by reducing Ru(NH3)63+ at the tip electrode and oxidizing Ru(NH3)62+ at the substrate electrode, as a function of the tip-substrate separation and S2O82- concentration. The one-electron reaction between electrogenerated Ru(NH3)62+ and S2O82- yields the unstable S2O83•-, which rapidly dissociates to produce highly oxidizing SO4•-. Because SO4•- is such a strongly oxidizing species, it can be further reduced at both the tip and the substrate, or it can react with Ru(NH3)62+ to regenerate Ru(NH3)63+. SECM approach curves display a complex dependence on the tip-substrate distance, d, due to redox mediation reactions at both the tip and the substrate. Finite element method (FEM) simulations of both SECM approach curves and SSV confirm a previously proposed mechanism for the mediated reduction of S2O82- using the Ru(NH3)63+/2+ redox couple. Our results provide a lower limit for dissociation rate constant of S2O83•- (∼1 × 106 s-1), as well as the rate constants for electron transfer between SO4•- and Ru(NH3)62+ (∼1 × 109 M-1 s-1) and between S2O82- and Ru(NH3)62+ (∼7 × 105 M-1 s-1).

A nanoelectrode-based study of water splitting electrocatalysts

The development of low-cost and efficient catalytic materials for key reactions like water splitting, CO₂ reduction and N₂ reduction is crucial for fulfilling the growing energy consumption demands and the pursuit of renewable and sustainable energy. Conventional electrochemical measurements at the macroscale lack the potential to characterize single catalytic entities and nanoscale surface features on the surface of a catalytic material. Recently, promising results have been obtained using nanoelectrodes as ultra-small platforms for the study of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) on innovative catalytic materials at the nanoscale. In this minireview, we summarize the recent progress in the nanoelectrode-based studies on the HER and OER on various nanostructured catalytic materials. These electrocatalysts can be generally categorized into two groups: 0-dimensional (0D) single atom/molecule/cluster/nanoparticles and 2-dimensional (2D) nanomaterials. Controlled growth as well as the electrochemical characterization of single isolated atoms, molecules, clusters and nanoparticles has been achieved on nanoelectrodes. Moreover, nanoelectrodes greatly enhanced the spatial resolution of scanning probe techniques, which enable studies at the surface features of 2D nanomaterials, including surface defects, edges and nanofacets at the boundary of a phase. Nanoelectrode-based studies on the catalytic materials can provide new insights into the reaction mechanisms and catalytic properties, which will facilitate the pursuit of sustainable energy and help to solve CO₂ release issues.

Voltammetric Measurement of Adsorption Isotherm for Ferrocene Derivatives on Highly Oriented Pyrolytic Graphite

Reversible and specific adsorption of redox-active molecules from the electrolyte solution to the electrode surface is an important process and is often diagnosed by cyclic voltammetry (CV). The entire voltammogram, however, is rarely analyzed quantitatively, thereby completely missing or incorrectly extracting inherent information about the adsorption isotherm. Herein, we report CV measurements of the adsorption isotherm for ferrocene derivatives on the basal plane of highly oriented pyrolytic graphite (HOPG) to quantitatively understand the thermodynamics of ferrocene-HOPG and ferrocene-ferrocene interactions at HOPG/water interfaces. Specifically, reversible CV of (ferrocenylmethyl)trimethylammonium, ferrocenemethanol, and 1,1'-ferrocenedimethanol is obtained at 0.05-10 V/s to confirm that only reduced forms of ferrocene derivatives are adsorbed on HOPG. Finite element analysis of the entire voltammogram yields the Frumkin isotherm to separately parametrize ferrocene-HOPG and ferrocene-ferrocene interactions. Adsorption of all ferrocene derivatives is driven by similarly weak ferrocene-HOPG interactions with free energy changes of approximately -20 kJ/mol. Adsorption of ferrocenemethanol is strengthened by intermolecular hydrogen bonding, which is quantitatively represented by a free energy change of -8 kJ/mol for surface saturation and is qualitatively characterized by a pair of sharp adsorption and desorption peaks following a pair of diffusional peaks. By contrast, adsorption of (ferrocenylmethyl)trimethylammonium and 1,1'-ferrocenedimethanol remains weak because of electrostatic repulsion and weak hydrogen bonding, respectively, which correspond to the respective free energy changes of +0.7 and -3 kJ/mol for surface saturation. The unfavorable or weakly favorable intermolecular interactions broaden or narrow a diffusional peak during the forward scan, respectively, without yielding a post peak.

Self-Inhibitory Electron Transfer of the Co(III)/Co(II)-Complex Redox Couple at Pristine Carbon Electrode

Applications of conducting carbon materials for highly efficient electrochemical energy devices require a greater fundamental understanding of heterogeneous electron-transfer (ET) mechanisms. This task, however, is highly challenging experimentally, because an adsorbing carbon surface may easily conceal its intrinsic reactivity through adventitious contamination. Herein, we employ nanoscale scanning electrochemical microscopy (SECM) and cyclic voltammetry to gain new insights into the interplay between heterogeneous ET and adsorption of a Co(III)/Co(II)-complex redox couple at the contamination-free surface of electron-beam-deposited carbon (eC). Specifically, we investigate the redox couple of tris(1,10-phenanthroline)cobalt(II), Co(phen)₃²⁺, as a promising mediator for dye-sensitized solar cells and redox flow batteries. A pristine eC surface overlaid with KCl is prepared in vacuum, protected from contamination in air, and exposed to an ultrapure aqueous solution of Co(phen)₃²⁺ by the dissolution of the protective KCl layer. We employ SECM-based nanogap voltammetry to quantitatively demonstrate that Co(phen)₃²⁺ is adsorbed on the pristine eC surface to electrostatically self-inhibit outer-sphere ET of nonadsorbed Co(phen)₃³⁺ and Co(phen)₃²⁺. Strong electrostatic repulsion among Co(phen)₃²⁺ adsorbates is also demonstrated by SECM-based nanogap voltammetry and cyclic voltammetry. Quantitatively, self-inhibitory ET is characterized by a linear decrease in the standard rate constant of Co(phen)₃²⁺ oxidation with a higher surface concentration of Co(phen)₃²⁺ at the formal potential. This unique relationship is consistent not with the Frumkin model of double layer effects, but with the Amatore model of partially blocked electrodes as extended for self-inhibitory ET. Significantly, the complicated coupling of electron transfer and surface adsorption is resolved by combining nanoscale and macroscale voltammetric methods.

Simulation of Fast-Scan Nanogap Voltammetry at Double-Cylinder Ultramicroelectrodes

High temporal resolution of fast-scan cyclic voltammetry (FSCV) is widely appreciated in fundamental and applied electrochemistry to quantitatively investigate rapid dynamics of electron transfer and neurotransmission using ultramicroelectrodes (UMEs). Faster potential scan, however, linearly increases the background current, which must be subtracted for quantitative FSCV. Herein, we numerically simulate fast-scan nanogap voltammetry (FSNV) for quantitative detection of diffusing redox species under quasi-steady states without the need of background subtraction while maintaining high temporal resolution of transient FSCV. These advantages of FSNV originate from the use of a parallel pair of cylindrical UMEs with nanometer-wide separation in contrast to FSCV with single UMEs. In FSNV, diffusional redox cycling across the nanogap is driven voltammetrically at the generator electrode and monitored amperometrically at the collector electrode without the transient background. We reveal that the cylindrical collector electrode can reach quasi-steady states ~10⁴ times faster than the generator electrode with identical sizes to allow for fast scan. Double-microcylinder and nanocylinder UMEs enable quasi-steady-state FSNV at hundreds volts per second as practiced for in-vivo FSCV and megavolts per second as achieved for ultra-FSCV, respectively. Rational design and simple fabrication of double-cylinder UMEs are proposed to broaden the application of nanogap voltammetry.