Semiconductor-driven "turn-off" surface-enhanced Raman scattering spectroscopy: application in selective determination of chromium(VI) in water

Semiconductor materials have been successfully used as surface-enhanced Raman scattering (SERS)-active substrates, providing SERS technology with a high flexibility for application in a diverse range of fields. Here, we employ a dye-sensitized semiconductor system combined with semiconductor-enhanced Raman spectroscopy to detect metal ions, using an approach based on the "turn-off" SERS strategy that takes advantage of the intrinsic capacity of the semiconductor to catalyze the degradation of a Raman probe. Alizarin red S (ARS)-sensitized colloidal TiO2 nanoparticles (NPs) were selected as an example to show how semiconductor-enhanced Raman spectroscopy enables the determination of Cr(VI) in water. Firstly, we explored the SERS mechanism of ARS-TiO2 complexes and found that the strong electronic coupling between ARS and colloidal TiO2 NPs gives rise to the formation of a ligand-to-metal charge transfer (LMCT) transition, providing a new electronic transition pathway for the Raman process. Secondly, colloidal TiO2 nanoparticles were used as active sites to induce the self-degradation of the Raman probe adsorbed on their surfaces in the presence of Cr(VI). Our data demonstrate the potential of ARS-TiO2 complexes as a SERS-active sensing platform for Cr(VI) in an aqueous solution. Remarkably, the method proposed in this contribution is relatively simple, without requiring complex pretreatment and complicated instruments, but provides high sensitivity and excellent selectivity in a high-throughput fashion. Finally, the ARS-TiO2 complexes are successfully applied to the detection of Cr(VI) in environmental samples. Thus, the present work provides a facile method for the detection of Cr(VI) in aqueous solutions and a viable application for semiconductor-enhanced Raman spectroscopy based on the chemical enhancement they contribute.