Chiral-discriminative amino acid films prepared by vacuum vaporization and/or plasma processing

D-Phenylalanine (D-Phe) films capable of discriminating optical isomers were prepared by dry processing: radiofrequency sputtering or vacuum vaporization with and without the assistance of an inductively coupled plasma (ICP). Chemical and spectroscopic analyses revealed that the majority of the constituents of the vaporized film are D-Phe molecules. However, the film prepared by ICP-assisted vaporization contains scarcely any D-Phe. The chiral-sensing properties of these D-Phe films were confirmed by sorption measurements on cyclic monoterpenes. Quartz crystal resonators coated with these films respond to the (-)-forms of limonene and alpha-pinene preferentially to their (+)-forms at the ppm level. This chiral preference was not observed for carvone, which induced comparable frequency shifts to limonene and alpha-pinene even at sub-ppm levels. The strong electrostatic interactions of the carbonyl group in carvone probably overcome the weak interactions of the discriminative optical isomers.