Highly stable In@SiO2 core-shell nanostructures for ultraviolet surface-enhanced Raman spectroscopy

We report trace level detection of tryptophan and adenine molecules over indium (In) and In@SiO2 core-shell nanostructures using ultraviolet surface-enhanced Raman spectroscopy (UV-SERS). The In nanostructure-based SERS substrates display highly stable and reproducible SERS spectra of tryptophan and adenine under 325 nm UV excitation. With 325 nm wavelength, intrinsic to the plasmon resonance wavelength of the In@SiO2 nanoparticles (NPs) and near the molecular absorption band, intensity enhancement factor of similar to 10(5) and an ultrasensitive limit of detection (LOD) up to similar to 0.18 fM for tryptophan and similar to 0.05 fM for adenine have been achieved. The SERS enhancement exhibits sensitive to SiO2 shell thickness, i.e. to the distance of the probe molecule from the In core. Our results show that a similar to 2 nm thick SiO2 layer serves as an efficient protective layer for preventing Raman signal deterioration by oxidative degradation of In nanoparticles (NPs). Also, finite-difference-time-domain (FDTD) based simulations have been carried out to study the plasmonic response of indium and core-shell In@SiO2 NP, enhanced electric field distribution under 325 nm excitation wavelength to analyse the experimentally observed SERS spectra. The simulation results are in good agreement with the experimental observations supporting the strong plasmonic response of In NPs in the ultraviolet region.