Electrochemistry at Microscopic Liquid-Liquid Interfaces

Abstract

A significant progress in studies of electrochemical processes occurring at the interface between two immiscible electrolyte solutions (ITIES) over the last decade is due to the development of a number of new spectroscopic and electrochemical techniques. This review is focused on microelectrochemical approaches to probing charge transfer processes and coupled homogeneous chemical reactions at the ITIES. Both theoretical and experimental aspects of electrochemical experiments with the two most important types of the micro‐ITIES, i.e., micropipettes and microholes, will be considered along with the more recently introduced θ‐pipettes and nanopipette electrodes. The surveyed applications include kinetic studies of ion and electron transfers, sensors, and electrochemical imaging.

Fast Ion‐Transfer Processes at Nanoscopic Liquid/Liquid Interfaces

“…Assuming that for a thin‐wall pipette (e.g., RG≤2, RG= r g / r , where r g is the outer radius of the glass insulator and r the inner radius, that is, the radius of the interface) the micro‐ and nano‐ITIES are essentially uniformly accessible, and the potential dependence of the rate constant follows the Butler–Volmer equation as corroborated by the excellent agreement between theory and experimental voltammograms, the standard rate constant ( k 0 ), and the charge‐transfer coefficient ( α ), which can be determined by fitting an experimental voltammogram to a theoretical equation 8. Although several groups have reported kinetic measurements of IT reactions at micro‐ITIES,1e many IT reactions are too fast to be probed with a micro‐ITIES under steady‐state conditions. The nano‐ITIES supported at nanopipettes should be advantageous because the mass‐transport rate can be enhanced according to Equation (1),5 where m 0 is the mass‐transport rate, D the ion diffusion coefficient, and r app the effective radius.…”

“…further reduced the liquid/liquid interface to the nanometer range by using nanopipettes. Voltammetry at nanopipettes and scanning electrochemical microscopy are currently the most reliable techniques used to extract the fast kinetic rate constants of ion‐transfer reactions at ITIES 1e. 4 The fastest rate constants reported so far are in the range of a few cm s −1 3.…”

“…Steady‐state voltammetry at solid ultramicroelectrodes has been employed extensively to measure the rapid kinetics of heterogeneous electron‐transfer reactions, and the methodologies developed are transposable to IT reactions at micrometer‐scale or nanometer‐scale ITIES (micro‐ and nano‐ITIES, respectively) 1e. Assuming that for a thin‐wall pipette (e.g., RG≤2, RG= r g / r , where r g is the outer radius of the glass insulator and r the inner radius, that is, the radius of the interface) the micro‐ and nano‐ITIES are essentially uniformly accessible, and the potential dependence of the rate constant follows the Butler–Volmer equation as corroborated by the excellent agreement between theory and experimental voltammograms, the standard rate constant ( k 0 ), and the charge‐transfer coefficient ( α ), which can be determined by fitting an experimental voltammogram to a theoretical equation 8.…”

Investigation of modified nanopore arrays using FIB/SEM tomography

“…12,13 Similar to micro-and nanoelectrodes, sensing applications benet from an enhanced mass-transport/higher charge transfer due to hemispherical diffusion compared to macroscopic electrodes or interfaces, reduced capacitive current and a reduced ohmic drop. 14,15 The diffusional behavior through micro-and nanopores in ITIES is thereby an important factor, determining the mass transport and charge transfer processes. The diffusion processes have been studied using electrochemical measurements and the obtained results have been compared with theoretical values [16][17][18][19] providing information on the location of the interface.…”

Electrochemical sensing and imaging based on ion transfer at liquid/liquid interfaces

“…In contrast, the sensing applications of IT voltammetry and amperometry at the macroscopic ITIES [13] had been limited by the traditional use of a four-electrode setup for the compensation of the large ohmic potential (or iR) drop in the resistive non-aqueous media [14]. To overcome this limitation, microscopic ITIES [15] was developed by supporting it at the tip of a glass micropipet [16, 17] or at the microhole drilled through a polymer film [18]. A micropipet-supported ITIES was also useful as an amperometric tip for scanning electrochemical microscopy (SECM) [19].…”