Photoinduced Enhanced Raman Spectroscopy for the Ultrasensitive Detection of a Low-Cross-Section Chemical, Urea, Using Silver–Titanium Dioxide Nanostructures

Surface-enhanced Raman scattering (SERS) is a powerful analysis technique that allows both the identification and detection of analytes at trace levels. However, the low rate of charge transfer (CT) between noble-metal nanoparticles and several analytes prevents them from being effectively detected by SERS-based sensors. They are regarded as low Raman cross-section molecules. In this study, we enhanced the performance of the silver nanoparticles (AgNPs)-based SERS sensing platform for a low Raman cross-section molecule, urea, focusing on improving the rate of CT. First, a set of Ag/titanium dioxide (TiO2) nanocomposites were synthesized. The presence of TiO2 improved the intensity of the SERS signal of urea, in comparison to the use of bare AgNPs. Second, a photoinduced enhanced Raman spectroscopy (PIERS) technique was employed to further elevate the Raman signal of urea. Thanks to the step of preirradiation using UV light at λ = 365 nm, with the use of the substrates containing 25%, 33%, and 50% TiO2 content, enhancements of 1.93, 3.42, and 7.45 times were achieved, respectively, compared to the use of Ag/TiO2 composites without UV irradiation. Through modification of the substrate, combined with application of the PIERS technique, the SERS system for urea detection using Ag/3TiO2 (50% TiO2) achieved a competitive detection limit of 4.6 × 10–6 M. It also allowed the detection of urea in milk at concentrations down to 10–5 M. This substrate modification and PIERS technique are promising for improvement of the sensing performance of other low-cross-section molecules.