Enantiomeric discrimination through tip-enhanced Raman scattering using a chemically modified silver tip with an asymmetric atomic arrangement.

Enantiomeric discrimination is achieved by tip-enhanced Raman scattering (TERS) using a silver tip chemically modified by an achiral para -mercaptopyridine ( p MPY) probe molecule. Differences in the relative intensities of the p MPY spectra were monitored for three pairs of enantiomers containing hydroxyl (-OH) and/or amino (-NH 2 ) groups. The N: or N + -H functionality of the p MPY-modified tip participates in hydrogen bond interactions with a particular molecular orientation of each chiral isomer. The asymmetric arrangement of silver atoms at the apex of the tip induces an asymmetric electric field, which causes the tip to become a chiral center. Differences in the charge-transfer (CT) states of the metal-achiral probe system in conjunction with the asymmetric electric field produce different enhancements in the Raman signals of the two enantiomers. The near-field effect of the asymmetric electric field, which depends on the number of analyte functional groups capable of hydrogen bond formation, improves the degree of discrimination.