Amino acid adsorption on hydrophilic TiO2: A sum frequency generation vibrational spectroscopy study

Sum frequency generation vibrational spectroscopy (SFG) is a powerful tool for in situ investigation of adsorption processes at biologically important solid–liquid interfaces. In this work adsorption of different amino acids was investigated by SFG at the titanium-dioxide–amino acid solution interface. While both naturally occurring acidic amino acids, aspartic acid and glutamic acid, were found to form an ordered adsorbate layer, amino acids with non-acidic side chains exhibited little affinity towards TiO2. A detailed study of aspartic acid on TiO2 pointed out that both carboxylic groups of the molecule are coordinated to the surface; data taken in the carboxyl stretching frequency range revealed that one of them is bonded monodentally to a surface Ti ion, while the other is most probably stabilized by a hydrogen bond. The results indicate that the acidic side chain is the prerequisite of the formation of a stable, ordered amino acid adsorbate layer on TiO2.