Adsorption of glycine on hydrophilic gold

Abstract

The adsorption of glycine (H 3N +CH 2CO − 2) onto evaporated gold substrates, treated to be hydrophilic, has been investigated using infrared reflection adsorption spectroscopy (IRAS) and angle-dependent X-ray photoelectron spectroscopy (XPS). Films only a few monolayers in thickness were formed by adsorption from a 0.1 mg/ml aqueous solution at pH 5.7. These films were characterized by strong infrared absorption bands at 3185 and 1528 cm −1 that were attributed to asymmetric stretching and symmetric deformation modes of the NH + 3 group. The asymmetric stretching mode of the CO − 2 group also appeared as a strong band near 1620 cm −1, while very weak bands were observed in regions where the corresponding symmetric stretching and scissoring modes are expected to absorb. These results indicated that a preferentially oriented film of zwitterionic glycine was formed on gold with the CC and CN bonds oriented essentially parallel and perpendicular to the metal surface, respectively. From the binding energy of the N(1 s) peak in the XPS spectrum, the species on the gold surface can be identified as zwitterionic glycine. The angle-dependent intensity of the N(1 s) relative to O(1 s) and C(1 s) spectra indicated that the zwitterionic species tended to be oriented with the N +-end closest to the gold surface.