The reduction of seleno-L-cystine is examined on gold and platinum electrodes in aqueous solution. The voltammetric behavior is indicative of a two-step process, in which the metallic substrate is modified by the diselenide analyte prior to observation of diffusionally controlled, proton-coupled electron transfer. X-ray photoelectron spectroscopy confirms the presence of selenium on gold foil samples that have been reductively cycled in selenocystine solution. The selenium-based surface modification, which is accessible through multiple preparation routes, can be removed by anodic stripping and fully restored through subsequent cycling. The described electrochemical approach allows for the analysis of redox-active chalcogen-containing species under physiologically relevant conditions. Results and Discussion Voltammetric Behavior of Seleno-L-cystine on GoldThe voltammetric response of selenocystine on gold features a quasi-reversible redox couple centered at À486 mV vs. Ag/ AgCl. [10] Figure 1 shows a comparison of cyclic voltammograms recorded for increasing selenocystine concentrations (0.2-2 mM) in phosphate buffered saline (PBS, pH 7). For each concentration, the electrode has been reductively cycled (À0.2 to À0.7 V vs. Ag/AgCl) for 100 consecutive scans in the selenium-containing solution to reach a steady signal. [10] The dependence of peak current on concentration is linear (Figure S1), consistent with the measured redox process corre- [a]
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