Interaction of melamine molecules with silver nanoparticles explored by surface-enhanced Raman scattering and density functional theory calculations.

Recently, unethical manufacturers illegally adulterated foods to increase the nitrogen content of protein through the use of melamine. Although surface-enhanced Raman scattering (SERS) methods have been widely used to detect melamine, the details of the interaction mechanism between a melamine molecule and silver colloids are still unclear. In this paper, we present the adsorption behavior of melamine on the surface of silver nanoparticles, which we explored by using SERS and density functional theory calculation methods. The calculated results demonstrate that the melamine molecule interacts with the silver surface mainly via the heterocyclic nitrogen. When comparing the predicted spectra of the complexes of melamine-Ag(+) and melamine-Ag(0), we found that the spectrum of the former agrees better with the experimental spectra than that of the latter. In the structure of the melamine-Ag(+) complex, we found that the melamine molecule "stands" on the surface of nanoparticles, with a small tilt angle. The concentration-dependent SERS spectra reveal that the melamine molecule stands on the silver surface with a small tilt angle in high concentration, and the tilt angle becomes larger when the concentrations are diluted.