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<sup>+</sup> and melamine-Ag<sup>0</sup>, 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<sup>+</sup> 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.
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