Protonation and deprotonation of cysteine and cystine monolayers probed by impedance spectroscopy

The desorption mechanism for electrochemically-formed self-assembled monolayers from cysteine and cystine on a Au(111) electrode was investigated. The monolayer desorption in 0.1 M KClO4 is found to occur in two steps with differing potentials. At potentials more positive than the desorption potential (circa -0.65 V vs. SCE), a quasi-reversible process, assigned to protonation/deprotonation, is observed. This process is found to occur at very discreet potentials when cysteine monolayers are probed by impedance spectroscopy. Monolayers derived from the disulfide (cystine) are found to protonate/deprotonate over a. wider potential range. This suggests that electrochemically-formed monolayers; obtained from disulfides do not afford the same level of organization as those derived from thiol species. The value of the double layer capacitance increases from 15 to 30 mu F cm(-2) after deprotonation. This suggests that the adsorption of cysteine weakens following the deprotonation. As expected, the protonation/deprotonation step is pH dependent and it is not observed in strong basic medium (0.1 M NaOH). (C) 2008 Elsevier B.V. All rights reserved.