Investigation of the adsorption of L-cysteine on a polycrystalline silver electrode by surface-enhanced Raman scattering (SERS) and surface-enhanced second harmonic generation (SESHG)

The adsorption of L-Cysteine (L-CYS) onto a polycrystalline silver electrode surface was investigated by in situ spectroelectrochemical methods. Surface-enhanced Raman spectroscopy (SERS) and surface-enhanced second-harmonic generation (SESHG) measurements were performed in a 0.2 M KCl solution in the presence and absence of L-CYS. The experimental results indicated that L-CYS strongly adsorbs onto silver and remains on the surface at potentials as negative as -900 mV (vs Ag/AgCl). A peak around -650 mV was observed in the SESHG intensity versus applied potential plots obtained in the presence of L-Cys. The peak was indicative of an abrupt change in the electronic properties of the interface at that potential. The SERS spectra at potentials more negative than ca. -650 mV showed an increase in the intensity of vibrational modes assigned to the carboxylate group of L-CYS. The combination of the SERS and the SESHG results suggests a potential-induced reorientation of the adsorbed L-CYS Molecules for potentials more negative than -650 mV. The data interpretation considered the different possible conformational forms of L-Cys adsorbed on the Ag surface. At potentials more positive than ca. -650 MV, L-CYS molecules adsorb with the protonated amino group pointing toward the surface. In this case, the positively charged amino group is stabilized by the coadsorbed chloride anions. The molecule changes its conformation at potentials more negative than -650 mV as the chloride ions leave the surface. The C-alpha-C-beta bond rotation brings the carboxylate group closer to the surface at these potentials.