Low-temperature iodinated plasmonically tuned Ag@AgI core-shell nanostructured thin films for highly specific ascorbic acid SERS detection at ultra-low concentrations

A simple low temperature iodination route was developed to controllably iodinate silver nanoislands film (Ag) into silver@silver iodide core-shell nanoparticles film (Ag@AgI) as a chemically stable, near infrared active surface enhanced Raman scattering (SERS) substrate. The localised surface plasmon resonance (LSPR) of the core-shell film exhibits a huge red-shift of 100 nm. The formation of Ag core and a complete ultra-thin AgI shell layer on top of the core is confirmed by transmission electron microscope. Selected area electron diffraction reveals the amorphous nature of AgI shell layer. These results are further substantiated by Raman and x-ray diffraction measurements. The enhancement factor of Ag@AgI film estimated with methylene blue is 5 x 106, an order of magnitude higher compared to that of Ag film. It demonstrated excellent chemical stability and unwavering SERS performance for two months since fabrication. The Ag@AgI film fielded as a label -free SERS substrate exhibited a lowest limit-of-detection of 1 x 10-12 M for ascorbic acid and a linear detection range of 1 x 10-6 to 1 x 10-12 M. The ascorbic acid selective signal enhancement of Ag@AgI film at low concentration region offers a practical sample dilution strategy to suppress interference from non-target molecules in biofluids.