Surface-Enhanced Raman Scattering and Fluorescence Spectroscopy on Silver Plasmonic Nanoparticles as Innovative Techniques to Study Humic Substances. In Humic Substances and Natural Organic Matter

The application of normal Raman spectroscopy is not able to characterize the soil humic substances (HS) structure due to the intense fluorescence emission, which overlaps the Raman bands. Nevertheless, surface-enhanced Raman scattering (SERS) spectroscopy, based on localized surface plasmon (LSP) in nanostructured metals, leads to a remarkable local electromagnetic field enhancement and to the fluorescence quenching via a charge-transfer mechanism. The SERS technique was thus applied to investigate the structure and conformational changing of HS. However, the fluorescence is not quenched if the distance between fluorophores and LPS is relatively far. An intensification of the fluorescence emission through the surface-enhanced fluorescence (SEF) at optimal distance above 10–100 Å can be detected. At distances further than this value, SERS and SEF signals are simultaneously emitted and recorded for molecular species placed in the vicinity of metal nanoparticles. Since the average size of HS is larger than this value, the fluorophores included in the HS structure are valid candidates causing intense SEF plus SERS joint emission spectra, without the intrinsic quenching observed on a surface.