Voltammetry in the presence of ultrasound: Sonovoltammetric detection of cytochrome c under very fast mass transport conditions

In this study sonovoltammetry, voltammetry in the presence of power ultrasound, is used to investigate aspects of cytochrome c redox processes at conventional-sized electrodes under extreme mass transport conditions comparable to those found in microelectrode steady state experiments. A new small volume sonovoltammetry cell (10-20 mL) equipped with a high-intensity 3 mm diameter titanium horn was built and characterized. By variation of the electrode-to-horn separation, a range of accessible average diffusion layer thicknesses from typically 1 to 7 μm was determined and an approximate mass transport model based on a "uniformly accessible electrode" is suggested. On gold electrodes modified with 4,4′-bipyridyl disulfide, well-defined "steady state" sonovoltammograms for the reduction of ferricytochrome c corresponding to very fast electron transfer (ks > 0.1 cm s-1) were obtained, although the activity of the electrode surface was found to be sensitive to the applied potential and to some degree to the period and intensity of insonation. A model based on the adsorption of cytochrome c on bare gold and surface modified gold [Szucs et al. Electrochim. Acta 1992, 37, 403] is used to explain irreversible and reversible electrode deactivation processes. Voltammograms obtained on glassy carbon, basal, and edge plane pyrolytic graphite in the presence of ultrasound under high mass transport conditions were ill-defined possibly due to competing adsorption of impurities or the effect of high shear forces induced by ultrasound. © 1996 American Chemical Society