The contribution of photoinduced charge-transfer enhancement to the SERS of uranyl(VI) in a uranyl-Ag2O complex

Charge-transfer (CT) is an important enhancement mechanism in the field of surface-enhanced Raman scattering (SERS) that typically increases the Raman intensity of molecules by as much as 10-100 times. Herein, a low-cost Ag2O aggregates substrate was prepared via a facile chemical precipitation method, and the calculated CT-based enhancement factor of the uranyl ions adsorbed on it reached as high as 105, a metal-comparable value. The efficient photoinduced CT process from the valence band of Ag2O to the LUMO of uranyl ions under appropriate excitation sources resulted in the repulsion of the axial oxygen atoms of the O U O bond, which enhanced its polarizability, creating a more intense Raman mode. To the best of our knowledge, this study firstly reports such a strong photoinduced CT enhancement of uranyl ions, with concentrations of 10−8 mol L−1 or lower being detected using this Ag2O substrate. Most importantly, this research has shown that the photoinduced CT enhancement also contributes to the SERS of uranyl ions on pure Ag substrates which have often been ascribed to the electromagnetic enhancement in previous studies. In addition, Ag2O can be used to selectively detect uranyl ions without interference from many other molecules or ions because of the energy matching rule of the photoinduced CT process, which was readily available for uranyl detection in the environmental aqueous solution.