The electro-oxidative activity of cysteine on the Au electrode as evidenced by surface enhanced Raman scattering.

It is demonstrated from the surface-enhanced Raman scattering that cysteine is adsorbed on the Au electrode via its carboxylate moiety. The mechanism is mainly via the charge transfer involving the pi electron-rich carboxylate moiety. The adsorption of cysteine is weak that its diffusion on the Au surface is possible. The collision of two cysteine molecules may lead to the electro-oxidative formation of cystine in an irreversible way due to that the disulfide bond is not in the close vicinity of the electrode and its rupture is hampered. The physical difference as compared to its adsorption on the Ag electrode is stressed.