Nonresonant Surface‐Enhanced Hyper Raman Scattering of Aromatic Amino Acids on Gold Nanoparticles

ABSTRACT Surface‐enhanced hyper Raman scattering (SEHRS), the two‐photon excited spontaneous Raman process that takes place in high local fields of plasmonic nanostructures, can provide information on molecular structure and interaction that is complementary to the information obtained by surface‐enhanced Raman scattering (SERS). In this work, we report the nonresonant SEHRS spectra of the aromatic amino acids tryptophan, phenylalanine, and tyrosine using two types of citrate‐stabilized gold nanoparticles. The SEHRS spectra are excited with a wavelength of 1064 nm and are compared to nonresonant SERS spectra, excited at a wavelength of 633 nm. Important bands in the spectra are assigned and reveal differences in the interaction of the amino acids with the gold nanostructures compared to previous reports of their spectra obtained with silver nanoparticles as plasmonic substrate. A greater variability of the SEHRS spectra of tyrosine with the similar gold substrates is found and discussed in the context of possible intermolecular interactions of the tyrosine molecule at the probed millimolar concentration. The nonresonant SEHRS spectra of the pure amino acids are a basis for probing the structure and interaction of whole proteins and supramolecular assemblies by this method using biocompatible gold nanoparticles rather than silver structures.