Plasmonic detection of Cd2+ ions using surface-enhanced Raman scattering active core-shell nanocomposite.

The present study was structured to address development of an efficient devise for sensing of toxic Cd(2+) ions at trace level in aqueous media. In order to achieve this objective, the speckled core-shell nanocomposites (NCs) of silica-gold (SiO2@Au) using ~30 nm diameter of spherical gold nanoparticles (Au NPs) with 420 nm diameter of silica cores was synthesized. Au NPs showed the surface plasmon resonance (SPR) peak at 522 nm and spherical core-shell particles at 541 nm. Both Au NPs and SiO2@Au solutions were found to be sensitive to Cd(2+) ions in aqueous sample. The colour change occurred in presence of SiO2@Au at 0.1 ppm (100 ppb) of Cd(2+) ions whereas 2 ppm (2000 ppb) concentration of Cd(2+) ions was necessary for the colour change in Au NPs solution confirmed that SERS active SiO2@Au core-shell NCs 20 times more sensitive compared to Au NPs. The technique using SiO2@Au NCs is quantitative between 100 and 2000 ppb (0.1 to 2 ppm) while effective but non-quantitative above upto 10 ppm, the maximum concentration studied in present investigation. The detection limit using SiO2@Au NCs is 100 ppb (0.1 ppm) while Au NPs is able to detect Cd(2+) as low as 2000 ppb (2 ppm). The scanning electron microscopy (SEM) of Au NPs and SiO2@Au particles showed aggregation of Au NPs and SiO2@Au NCs in the presence of Cd(2+) ions. The surface enhanced Raman spectroscopy (SERS) was used to compare sensitivities of Au NPs and SiO2@Au towards Cd(2+) ions and confirmed that SiO2@Au core-shell NCs is 20 times more sensitive than Au NPs.