EVALUATION OF EQCM DATA FROM A STUDY OF CYSTEINE ADSORPTION ON COLD ELECTRODES IN ACIDIC MEDIA

Voltammetric adsorption and desorption of cysteine at gold electrodes were examined in 0.10 M HClO4 using a rotated ring-disk electrode and an electrochemical quartz crystal microbalance (EQCM). Ring-disk data support the conclusion that cysteine is adsorbed following cathodic re duction (negative scan) of surface oxide (AuO). However, EQCM data ind icate that the corresponding change in apparent surface mass measured in the presence of cysteine is smaller than that measured for the pure supporting electrolyte. The seemingly anomalous EQCM data are explain ed on the basis of simultaneous contributions to the apparent change i n surface mass from (i) cysteine adsorption, (ii) loss of surface oxyg en, and (iii) decrease in surface hydration. These results indicate th e difficulty of using EQCM data for quantitative determinations of ads orbate coverage when changes in surface hydration occur concurrently.