Molecular Coverage Modulates Chiral Surface-Enhanced Raman Scattering on Chiral Plasmonic Nanoparticles

Sensitive recognition of enantiomer has aroused extensive interest due to its importance in diverse fields ranging from pharmaceuticals to catalysis and biomedicine. Chiral surface-enhanced Raman scattering (SERS) by chiral plasmonic substrates has emerged as a promising tool for the recognition of enantiomers with high sensitivity as well as molecular fingerprinting capability. However, the impact of the molecular states including surface packing density and configurations of surface-adsorbed chiral molecules on chiral SERS has been largely unexplored. Herein, we demonstrate that chiral SERS by chiral plasmonic nanoparticles (NPs) is sensitively dependent on the molecular coverage of enantiomers. Au helical nanocubes with tunable optical properties and intense near-field enhancements were chosen as the chiral plasmonic substrates for chiral SERS. By changing the concentration and structure of enantiomers, the impact of the molecular states including surface packing density and configurations of enantiomers on chiral SERS is revealed. Finally, we demonstrate the use of achiral molecules as internal molecular spacers for achieving the ultrasensitive detection of enantiomer. The insights gained from this work not only shed light on the underlying mechanisms dictating the chiral SERS by chiral plasmonic NPs but also strongly suggest a promising approach to the sensitive detection of molecular chirality using Raman spectroscopy.