Anodic response of cystine at preanodized Au and Au-Ag electrodes in perchloric acid media

This study involves a comparison of the anodic response of the disulfide mo iety of cystine at preanodized Au and Au-Ag rotated disk electrodes (RDEs). The goal of the study was the determination of beneficial effects that mig ht result from the introduction of Ag-sites into Au surfaces. An anodic wav e is observed for cystine at oxide covered Au and Au-Ag electrodes appearin g as a shoulder to the wave for anodic discharge of H2O with evolution of O -2. Both the half-wave potential for the cystine wave (E-1/2,E-RSSR) and th e overpotential for anodic evolution of O-2 (eta(O2) up arrow) are shifted by -3.6 mV per atomic percent Ag. These negative shifts are attributed to t he higher rate of the anodic discharge of H2O at Ag sites in comparison to Au sites. Values for the apparent number of electrons (n(app)) transferred during oxidation of 1.0 mM cystine at 1.50 and 1.55 V (vs. SCE) are approxi mately 5 eq mol(-1) for Au and 6 eq mol(-1) for Au-80-Ag-20, as calculated from the slopes of Koutecky-Levich plots. The larger value of n(app) at the Au-Ag electrode is attributed to the higher rate for H2O discharge at the Ag sites. Cysteine sulfonic acid is identified as the product of extensive electrolysis (n = 10 eq mol(-1)) at a Au electrode. A preanodized Au electr ode in a flow-through cell is demonstrated for cystine detection using a fl ow-injection system. A 6% loss in peak response observed over a 30-min peri od is attributed to the slow conversion of an active surface oxide, perhaps corresponding to the equivalent of a monolayer, to an inert oxide, probabl y corresponding to oxide coverage exceeding a monolayer.