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 reduction (negative scan) of surface oxide (AuO). However, EQCM data indicate 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 explained on the basis of simultaneous contributions to the apparent change in surface mass from (i) cysteine adsorption, (ii) loss of surface oxygen, and (iii) decrease in surface hydration. These results indicate the difficulty of using EQCM data for quantitative determinations of adsorbate coverage when changes in surface hydration occur concurrently.