Surface adsorption of L-phenylalanine on silver colloidal nanoparticles investigated by surface-enhanced Raman spectroscopy

Abstract

L-phenylalanine (L-Phe) is one of three aromatic amino acids frequently used in numerous vibrational applications, including surface-enhanced Raman spectroscopy (SERS). Despite the wide applications, the spectral analyses and vibrational assignments of the Raman spectra of L-Phe based on surface adsorption on metallic nanosurfaces are limited. In this work, we report pH-dependent SERS of L-Phe adsorbed on borohydride-reduced silver colloidal nanoparticles, where the spectral changes upon protonation of amine group are understood as the modification in the surface adsorption via the carboxylate and amino group. In-plane and out-of-plane ring stretching and aromatic ν(CH) modes appear sensitive to the surface orientation of the phenyl ring, and the peripheral vibrational modes, including the stretching and bending of carboxylate and amine groups, are crucial to understanding the detailed surface adsorption mechanisms of L-Phe in anionic and zwitterionic forms depending on the solution pH. The pH-dependent intensity changes of the major ring stretching modes show that the surface pKa of L-Phe decreases to ∼6 from the bulk value of 9.13.