Comparison of metal-amino acid interaction in Phe-Ag and Tyr-Ag complexes by spectroscopic measurements

In this article, we have compared the metal–amino acid interactions in Tyr–Ag and Phe–Ag complexes through pH dependent SERS measurements. By analyzing the variation in relative intensities of SERS bands with the pH of the amino acid solution, we have obtained the orientation and conformation of the amino acid molecules on the Ag surface. The results obtained from our experimental studies are supported by the energy minimized structures and the observed charge distributions in different terminals of the molecules. This, in a way, shows that SERS measurements not only exhibit the interaction of the amino acid molecules with Ag clusters but also demonstrate their orientation around it. We have addressed a long standing query on whether the amine group is directly attached to the Ag surface along with the carboxylate group and π-electrons in these systems. In addition, pH dependent optical absorption and transmission electron microscopy measurements have been performed to understand the required conditions for the appearance of the SERS spectra in the light of the aggregation of metal particles and the number of hot sites in the sol. Our results confirm that the formation of hot sites in the sol plays a direct role in forming a stable Ag–ligand complex. Furthermore, the interaction kinetics of metal–amino acid complexes have been analyzed via both Raman and absorption measurements.