Silver nanoparticles stabilized by polyoxotungstates. Influence of the silver – Polyoxotungstate molar ratio on UV/Vis spectra and SERS characteristics

Metal nanoparticles capped with polyoxometalates show unique optical properties due to the redox activity of the stabilizing ligands. Polyoxosilicotungstate stabilized silver nanoparticles (AgNPs) were synthesized using partially reduced Keggin-type anions (heteropolyblue) as reducing agent for the silver precursor (AgNO3). The solution of heteropolyblue was added to the AgNO3 solution. The heteropolyblue was reoxidized by silver ions giving stable colloidal solutions of AgNPs stabilized with SiW12O40 4 (SiW12) ions. We studied the influence of the molar ratio between the heteropolyblue and silver on the plasmonic properties of resulting nanoparticles. The SiW12 to AgNO3 molar ratio was equal to 1:2, 3:2 or 6:2. The nanoparticles with the lowest SiW12 to AgNO3 ratio (AgSiW12L) had largest average dimension and their UV/VIS spectra were characterized by strong plasmonic absorption band and weak band due to the oxygen – tungsten charge transfer. The nanoparticles with the higher SiW12 to AgNO3 ratio (AgSiW12M and AgSiW12H) had smaller dimension, relatively weaker plasmonic bands and stronger O→W charge transfer bands. All nanoparticles were studied as platforms for Surface Enhanced Raman Spectroscopy (SERS). AgSiW12L yield highest SERS enhancement. High quality Raman spectra of rhodamine 6 G (R6G) were obtained at femtomolar concentration of the analyte and the spectra of the p-aminothiophenol (PATP) at micromolar level. The differences between SERS enhancement observed for various AgSiW12 are discussed and co-related with their different UV/VIS, infrared spectra and size differences. We studied further the effect of electrochemical decomposition of SiW12 ligands on the SERS enhancement ability of the nanoparticles.