Surface-Charge Effects on Voltammetry in Carbon Nanocavities

Ion transport controlled by electrostatic interactions is an important phenomenon in biological and artificial membranes, channels, and nanopores. Here, we employ carbon-coated nanopipets (CNPs) for studying permselective electrochemistry in a conductive nanopore. A significant accumulation (up to 2000-fold) of cationic redox species and anion depletion inside a CNP by diffuse-layer and surface-charge effects in a solution of low ionic strength were observed as well as the shift of the voltammetric midpeak potential. Finite-element simulations of electrostatic effects on CNP voltammograms show permselective ion transport in a single conducting nanopore and semiquantitatively explain our experimental data. The reported results are potentially useful for improving sensitivity and selectivity of CNP sensors for ionic analytes.

Diffuse Layer Effect on Electron-Transfer Kinetics Measured by Scanning Electrochemical Microscopy (SECM)

Recent theoretical and experimental studies revealed strong effects of the electrical double layer (EDL) on mass transfer at nanometer-sized electrodes and in electrochemical nanogaps. Although the EDL effect is much stronger in weakly supported media, it can significantly influence the kinetics of electron-transfer processes involving multicharged ionic redox species, even at high concentrations of supporting electrolyte. We measured the kinetics of Fe(CN)₆^4- oxidation in 1 M KCl solution at the Pt nanoelectrode used as a tip in the scanning electrochemical microscope. The apparent standard rate constant values extracted from tip voltammograms without double-layer correction increased markedly with the decreasing separation distance between the tip and substrate electrodes. The same steady-state voltammograms were fitted to the theory including the EDL effect and yielded the rate constant essentially independent of the separation distance.