Elucidation of hydrated metal ions using flocculation-surface enhanced Raman scattering

Abstract

Hydrated metal ions such as Na+, Mg2+, and Cu2+ in addition to surface species like citrate, p-mercaptobenzoic acid (PMBA), and halide anions were identified by exploiting coupled localized surface plasmons of flocculated silver nanoparticles (AgNPs). Indeed, SERS spectra provided clear evidence for the formation of citrate complexes of Co2+ and Cu2+ as well as ion pairs of metal ions and PMBA− anions on AgNPs. For the first time, we have detected pronounced Raman bands from hydrated metal ions in flocculates of halide-covered AgNPs assigned to restricted rotation and translation at 600–200 cm−1 and OH stretching bands at 3500–3600 cm−1. Interestingly, the peak frequencies of OH stretching mode from hydrated metal ions are sensitive to metal and halide species, proving the observed water molecules are sandwiched between trapped metal ions and halide ions on adjacent AgNPs while forming a solvent shared ion pair.