Surface-enhanced raman scattering of uranyl-humic complexes using a silver-doped sol-gel substrate

A silver-doped, surface-enhanced Raman scattering (SERS) substrate was prepared by an acid-catalyzed sol-gel method. Silver nitrate was first doped into the sol-gel film followed by chemical reduction of silver ions with sodium borohydride to produce silver particles. This silver-doped sol-gel substrate exhibits strong enhancement of Raman scattering from adsorbed uranyl ions and soil humic materials. A uranyl ion peak at 713 cm^-1 and two humic acid peaks at about 1597-1610 and 1313-1338 cm^-1 were observed. In comparison with Raman spectra of uranyl ions (with no humic acid), the intensity of the symmetric stretching band of uranyl decreased and its frequency increased slightly after complexation with humic acid. The complexation weakened the interaction of uranyl ions with the silver surface. For humic acid, only two peaks of the backbone structure were observed before and after the complexation, suggesting that humic molecules were only weakly sorbed on the silver surface.