Selenium is an essential trace element for humans, found primarily as the amino acid selenocysteine. Proteins containing selenocysteine in their active site, such as thioredoxin reductases, play an integral role in physiological processes. Mutation studies in which selenocysteine is replaced by its abundant sulfur analogue cysteine demonstrate the requirement of selenium for proper functioning. Several questions exist regarding the unique chemistry achieved by selenocysteine in the cellular context. Using a combination of electrochemical and spectroscopic techniques, we have examined the selenocysteine / selenocystine redox couple under physiological conditions. Reactivity of the diselenide affords stable modification of gold substrates, which in turn allows for voltammetric study of solution-based chalcogen species that would otherwise be impeded by surface chemistry. The potential use of selenium-modified substrates in biosensing applications will be discussed.