Experimental (SERS) and theoretical (DFT) studies on the adsorption behaviors of l-cysteine on gold/silver nanoparticles

It is important to apply the surface-enhanced Raman scattering (SERS) spectra and density functional theory (DFT) to study the interfacial properties of nanoparticles and the adsorption behavior of molecules on the substrates. In the paper, the adsorption behaviors of l-cysteine molecules on gold/silver nanoparticles were studied. It was demonstrated that in gold colloidal solution, l-cysteine molecules were adsorbed in PH conformation and interacted with the gold surface simultaneously through sulfur, NH3+ and COO− groups. However, in silver colloidal solution activated by coadsorption of chloride anions, we could see that for most of the l-cysteine molecules, the collision of two l-cysteine molecules may lead to the formation of one l-cystine molecule by the S-S bond, and a few of l-cysteine molecules existed as the PH and PN conformations, and the molecules attached to the silver surface via the carboxylate and amino groups. The calculated Raman frequencies using DFT-B3LYP/6-31++G(d,p) of l-cysteine molecule were in good agreement with experimental values, which demonstrated that the DFT method can be taken as a very useful tool to assign the Raman band.