(S)-Cysteine chemisorption on Cu(110), from the gas or liquid phase:: An FT-RAIRS and XPS study

(S)-Cysteine has been deposited on a Cu(110) surface from sublimation of a crystalline phase. The surface was characterized by Fourier transform reflection absorption infrared spectroscopy (FT-RAIRS) during exposure and compared to the same copper surface after immersion into cysteine solutions at various pH values. X-ray photoelectron spectroscopy (XPS) measurements provided a chemical characterization of the surface at certain stages. The combination of these two techniques highlighted the importance of the cysteine "source" for the adsorbed form of the molecules and the mode of interaction. The zwitterionic amino acid was found to be predominant after adsorption at pH values close to the isoelectrice point (IEP) of the molecule but also when the layer was formed in the vapor phase. This state was very sensitive to the atmosphere, contained an excess of hydroxyls, and/or underwent reduction into the anionic form when in contact with water or air. Weakly bound cysteine or cystine molecules, formed in the adsorbed phase, were considered to explain the average thickness of the adsorbed layer that was close to 20 Angstrom. As expected, immersion in very acidic or very basic solutions led to cationic and anionic forms, respectively.