Resonant-enhanced localized surface plasmon resonance spectroscopy

The extinction maximum of the localized surface plasmon resonance (LSPR) of noble metal nanoparticles is highly dependent upon the refractive index of the nanoparticles' surrounding environment. In this study, the effect that molecular resonances have on the intensity, LSPR peak width, and LSPR shift of the LSPR of Ag nanoparticles is monitored. By systematically tuning the LSPR extinction maxima of Ag nanoparticles versus molecular resonances, new phenomena are revealed. First, the LSPR peak shift induced by a resonant molecule varies with wavelength. In most instances, the trends in this data qualitatively track with the Kramer's-Kronig transformation of the molecular resonance spectrum; however, the magnitude of the response is severely underestimated. This was verified from both experimental data and theoretical calculations. Because this phenomenon is revealed to be electronic transition dependent, it is hypothesized that the coupling between the molecular and plasmon resonances is responsible for this wavelength dependent observation. These results will have implications in molecular enhanced LSPR sensing and in the understanding of surface-enhanced spectroscopy.