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

Highlights: • For the first time, we successfully trapped and elucidated hydrated metal ions on negatively charged silver nanoparticles using flocculation-SERS method. • In particular, we found that solvent shared ion pairs are formed between various metal ions and halide-covered AgNPs, of which OH stretching band at 3500–3600 cm⁻¹ shifts dependent on metal ions and halide species. • These water bands have not been reported before, mainly because of much smaller enhancement on roughened metal surfaces, and appreciably small scattering cross section of water. • We have exploited sufficiently high enhancement in flocculates of concentrated AgNPs. • Since the observed water molecules bound to metal ions possess distinct features from those in concentrated solutions in addition to those of neat water, flocculation-SERS will provide a new insight into the structure and interaction of water associating to various cations and anions. 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.