L-cysteine oxidation on Pt and Au rotating disk electrodes: Insights on mixed controlled kinetics

L-Cysteine (L-Cys) oxidation on gold and platinum exhibits different behaviors due to adsorption phenomena related to the affinity between the S atom present on the L-Cys side chain and the metal sites. The adsorption of S on Au is very strong, which renders the determination of the L-Cys adsorption reaction order and its symmetry coefficient impossible. By contrast, on Pt electrodes, the L-Cys protonation state can be controlled by the electrolyte pH, and it is shown that the Tafel slope for L-Cys oxidation is pH dependent, with the most favorable kinetics being observed at pH = 9. Under this condition, the thiol is deprotonated, and thiolate is present, with L-Cys presenting a nucleophilic character. Differences between the two metals during L-Cys oxidation were also investigated. In rotating disk electrode measurements, the oxidation peak was observed to shift with the rotation rate, highlighting the kinetic activation control over this process and showing that diffusion influences this activation control as well. The combination of triangular potential perturbation and convection techniques allowed a chemometric soft model to be used to investigate reactions that are simultaneously controlled by activation and mass transport.