Plasmon-Induced Vibrational Circular Dichroism Bands of Achiral Molecules on Gold Nanostructures with Tunable Extrinsic Chiroptical Responses

Vibrational circular dichroism (VCD) is an important method used to assign absolute configuration to chiral molecules, but inherently weak signals necessitate high concentrations and long acquisition times for analysis. Plasmonic nanostructures are of interest for enhancement of VCD signals. However, understanding the influence of plasmonic structures on molecular VCD spectral features is important for assessing the information content of the spectra. Here, we present a study of plasmonic gold nanobowl hole (NBH) structures and the influence on the VCD spectra of achiral molecules. VCD bands for the symmetric and antisymmetric methylene stretching modes of 1-octadecanethiol (ODT) on NBHs were observed. Measurements of ODT on CaF2 substrates without plasmonic structures confirmed that the bands were not an artifact of tilting the substrate. The handedness of the VCD bands showed an inverse dependence on the handedness of the extrinsic CD response of the NBHs, producing a relative handedness of the opposite sign. By controlling the size of the NBHs, the localized surface plasmon resonance (LSPR) and CD bands were tuned relative to the spectral position of the ODT VCD bands. The amplitude and line shape of the methylene stretch VCD bands showed a strong dependence on overlap with the plasmonic CD band in the same spectral region. To further investigate the origin of the molecular VCD bands, nanobowls without holes (NBs) and nanodisks (NDs) were fabricated. The LSPR behavior of NBs and NDs was similar to NBHs, but the structures did not exhibit the extrinsic CD responses of NBHs, evidenced by the lack of defined CD bands and handedness conversion. Although the NBs and NDs exhibited small CD signals, VCD bands were not observed for ODT on the NBs and NDs. Stearic acid (SA) on NBHs also produced VCD bands, demonstrating the influence of the plasmonic nanostructures on molecules with different surface interactions and organization. Asymmetric line shapes of the VCD bands of ODT appeared similar to Fano resonance line shapes observed in surface-enhanced infrared absorption (SEIRA) spectroscopy; however, the NBH CD response was shown to be a key factor in producing the VCD bands, rather than the primary LSPR that influences the line shapes and enhancements in SEIRA. The studies of VCD spectra of achiral molecules on plasmonic structures highlight the importance of understanding how the plasmonic and chiroptical behaviors of the substrates impact the spectral features used to identify enantiomers in VCD spectroscopy, especially for configuration analysis and comparison with quantum calculations.