Experimental (SERS) and theoretical (DFT) studies on the adsorption of p-, m-, and o-nitroaniline on gold nanoparticles.

Surface-enhanced Raman scattering (SERS) spectra of p-, m-, and o-nitroaniline (PNA, MNA, and ONA) adsorbed on gold nanoparticles were studied, respectively, in a gold colloidal solution and on dried gold-coated filter paper. Significant changes were found in the SERS spectra of p-, m-, and o-nitroaniline molecules adsorbed on the two substrates, indicating that the behavior of p-, m-, and o-nitroaniline adsorbed on gold nanoparticles coated on filter paper was different from that in gold aqueous colloids. Moreover, on the filter paper it was found that the SERS spectra of the PNA molecules changed with the coverage density of the gold nanoparticles, while the MNA and ONA did not evidently change. The analyses showed that the origins of these changes were the different adsorption behavior of molecules adsorbed on the gold nanoparticles and that the adsorption behavior of molecules was greatly affected by the surface characteristic of the substrate and the surface configuration of the adsorbate. In addition, the vibrational frequencies and Raman scattering intensities of the three nitroanilines were calculated by use of the density functional theory (DFT-B3LYP/6-31G(d)) method. The combination of SERS spectroscopy and DFT calculation is quite useful for studies of adsorption orientation of a molecule on a metal nanoparticle.