Investigation of the electrochemical behavior of l-cysteine in acidic media

• Adsorption of l-cysteine follows a Temkin model.
• Borrowing SERS is suitable to characterize the adsorbed species on Pt.
• Adsorbed species are similar to those on Ag in basic media.
• Claimed multielectronic oxidation is not a single step.

Cysteine is an important organic molecule in electrochemistry. It can be used as an electrode modifier and is responsible for the adsorption of many proteins. Although it is a well-studied molecule, cysteine's behavior and mechanism of oxidation remain unclear in the literature. The current study is focused on investigating the electrochemical behavior of this molecule in acidic media on a Pt electrode using electrochemical techniques and Raman spectroscopy. The adsorption process appears to be similar to the one proposed in the literature (i.e. a monoelectronic oxidation). The adsorbed species appear to strongly interact with one another, and therefore, this adsorption is more suitably described by a Temkin model. The Raman spectra indicate that the adsorption is most likely due to the thiol group on the side chain of the amino acid. In addition, the carboxyl group interacts more with the electrode than the amino group. The oxidation of these adsorbed species is achieved by 5 electrons, and supports the formation of the previously proposed species. Other experiments suggest that the adsorption does not occur in a single step. Using voltammetry and pseudocapacitance adsorption model interpretations, a monoelectronic rate-limiting step was detected, which indicates the existence of a more complex mechanism for the oxidation of cysteine on platinum.