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 ul trasound, is used to investigate aspects of cytochrome c redox process es at conventional-sized electrodes under extreme mass transport condi tions comparable to those found in microelectrode steady state experim ents. A new small volume sonovoltammetry cell (10-20 mt) equipped with a high-intensity 3 mm diameter titanium horn was built and characteri zed. By variation of the electrode-to-horn separation, a range of acce ssible average diffusion layer thicknesses from typically 1 to 7 mu m was determined and an approximate mass transport model based on a ''un iformly accessible electrode'' is suggested. On gold electrodes modifi ed with 4,4'-bipyridyl disulfide, well-defined ''steady state'' sonovo ltammograms for the reduction of ferricytochrome c corresponding to ve ry fast electron transfer (k(s) > 0.1 cm s(-1)) were obtained, althoug h the activity of the electrode surface was found to be sensitive to t he applied potential and to some degree to the period and intensity of insonation. A model based on the adsorption of cytochrome c on bare g old and surface modified gold [Szucs et al. Electrochim. Acta 1992, 37 , 403] is used to explain irreversible and reversible electrode deacti vation 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.