Distinguishing Enantiomers by Tip‐Enhanced Raman Scattering: Chemically Modified Silver Tip with an Asymmetric Atomic Arrangement

Abstract

Discrimination between enantiomers is achieved by tip‐enhanced Raman scattering (TERS) using a silver tip that is chemically modified by an achiral para‐mercaptopyridine (pMPY) probe molecule. Differences in the relative intensities of the pMPY spectra were monitored for three pairs of enantiomers containing hydroxy (−OH) and/or amino (−NH2) groups. The N: or N+−H functionality of the pMPY‐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.

Chiral detection by induced surface-enhanced Raman optical activity

“…23,34 Lately, a similar effect, chirality sensing through a mercaptopyridine ''mediator'', has been observed for tipenhanced Raman scattering. 39 We could detect strong ROA, with a CID up to B10 À3 , universally for all the four linkers and several analytes (''chiral modifiers'') investigated. The chiral signal could be verified by ''mirror image'' enantiomer spectra, measurements on many combinations of the linkers and modifiers, and by reproducing the experiment in two different laboratories (Academy of Sciences, Prague, and Palacký University, Olomouc).…”

Electric field analysis, polarization, excitation wavelength dependence, and novel applications of tip‐enhanced Raman scattering

“…Normalized tip‐enhanced Raman scattering (TERS) spectra of a p MPY‐modified tip before (A1–C1) and after immersion in (A2–C2) (R) ‐ and (A3–C3) (S) ‐enantiomers. Bar graphs of I 1577 /I 1615 before and after tip immersion in pure enantiomers of (A4) 2‐octanol, (B4) α‐methylbenzylamine, and (C4) 2AP [ 104 ] (reproduced from Sukmanee et al [ 104 ] with permission)…”

“…Figure 18 also shows that the degree of enantiomeric discrimination of 2AP, which has two functional groups (–OH and –NH 2 ) to form hydrogen bonds, is greater than that of 2‐octanol and α‐methylbenzylamine, which have only one such functional group. [ 104 ] The degree of enantiomeric discrimination was roughly equal for 2‐octanol and α‐methylbenzylamine. This comparison suggests that a larger number of hydrogen‐bonding functional groups will bring multimer complexes closer to the tip with lower degrees of freedom in their molecular orientation.…”

“…Figure 19 shows the I 1577 /I 1615 versus the enantiomeric excess (%ee) of (R) ‐ and (S) ‐2AP. [ 104 ] The results shown in Figure 19 indicate that the quantity of (S) ‐enantiomer in mixtures can be clearly determined when the amount of (S) ‐enantiomer exceeds 50%ee. The difference in response suggests that the (S) ‐enantiomer is less sensitive to the modified tip than the (R) ‐enantiomer, which is consistent with different modes of hydrogen bond formation.…”

“…[101][102][103] Recently, the concept of enantiomeric discrimination using an achiral probe molecule was adapted to TERS. [104] The silver TERS tip was chemically modified via Ag-S bonding with para-mercaptopyridine (pMPY), which is very sensitive to pH, potential, temperature, and laser energy, [105,106] as a probe molecule. Enantiomeric discrimination is based on the CT mechanism, as in the case of SERS studies.…”