Ultrasensitive Plasmonic Chiral SERS Substrate for Ligand-Free Enantioselective Discrimination and Molecular Fingerprinting

In this work, we have fabricated plasmonic chiral silver nanohelices with a strong chiroptic response using the glancing angle deposition (GLAD) technique. These nanostructures have been explored for label-free enantioselective surface-enhanced Raman spectroscopy (SERS) discrimination. Three different biologically significant enantiomers (l-3-(3,4-dihydroxyphenyl) alanine (DOPA), cystine, and tartaric acid) with varying chemical features have been successfully differentiated. The homochiral analyte showed distinct and enhanced affinity toward the substrate of homogeneous handedness. Relative intensities of certain specific peaks changed, depending on the variations in the enantiomeric environment. The detection limit was calculated as low as 10-12 M for each homochiral analyte matching the handedness with the Ag helical substrate. The chiral distinction is further verified and explained by performing finite difference time domain (FDTD) simulations by studying the local field "hotspot" distribution in the case of each of the plasmonic chiral Ag substrates. Additionally, theoretical density functional theory (DFT) calculations were performed to get insights into the charge transfer (CT) process to understand the difference in the behavior of each analyte observed toward a specific chiral substrate. This ligand-free SERS discrimination may be of enormous significance in chiral separation.