An SFG Study of Interfacial Amino Acids at the Hydrophilic SiO2 and Hydrophobic Deuterated Polystyrene Surfaces

Sum frequency generation (SFG) vibrational spectroscopy was employed to characterize the interfacial structure of eight individual amino acids—l-phenylalanine, l-leucine, glycine, l-lysine, l-arginine, l-cysteine, l-alanine, and l-proline—in aqueous solution adsorbed at model hydrophilic and hydrophobic surfaces. Specifically, SFG vibrational spectra were obtained for the amino acids at the solid−liquid interface between both hydrophobic d8-polystyrene (d8-PS) and SiO2 model surfaces and phosphate buffered saline (PBS) at pH 7.4. At the hydrophobic d8-PS surface, seven of the amino acids solutions investigated showed clear and identifiable C−H vibrational modes, with the exception being l-alanine. In the SFG spectra obtained at the hydrophilic SiO2 surface, no C−H vibrational modes were observed from any of the amino acids studied. However, it was confirmed by quartz crystal microbalance that amino acids do adsorb to the SiO2 interface, and the amino acid solutions were found to have a detectable and widely varying influence on the magnitude of SFG signal from water at the SiO2/PBS interface. This study provides the first known SFG spectra of several individual amino acids in aqueous solution at the solid−liquid interface and under physiological conditions.