DFT and SERS Study of l-Cysteine Adsorption on the Surface of Gold Nanoparticles

Both surface enhanced Raman spectroscopy (SERS) experiments and density functional theory (DFT) calculations have been carried out to investigate the adsorption of cysteine on gold nanoparticle surface. Cysteine is one of the amino acids that have a rotating dihedral angle and several possible adsorption sites, so there could be many possible adsorption conformations. At present, the adsorption conformation of cysteine on the gold surface has not been solved, although previous work had explored the conformations based on DFT method, but no explicit experimental evidence was provided. Nevertheless, the previous work had indicated that the adsorption conformations of some simple rigid molecules on the surface of metal nanoparticles could be identified through SERS experimental data. Therefore, in this work, we attempted to figure out the conformation and adsorption sites of the flexible typical amino acid cysteine on gold nanoparticle surface by combining both DFT and SERS methods. We found that the adsorption of cysteine on the surface gold nanoparticle adopted the most predominant configuration in which cysteine chemically interacted with Au cluster through both S and O sites, with cysteine taking the configuration of a gauche position for the protonated amino group and an antiposition for the carboxylate group in the molecule.