Enhanced localized surface plasmon resonance dependence of silver nanoparticles on the stoichiometric ratio of citrate stabilizers

Abstract

A stoichiometric approach to the synthesis of silver nanoparticles (AgNPs) with appreciable enhancements in the localized surface plasmon resonance is presented. Microwave irradiation afforded AgNPs, optimized to a thermodynamic equilibrium by varying the silver to trisodium citrate (Ag0/citrate3−) stoichiometric ratio from 1:1 to 1:10, and ranging in size from 32 to 65 nm (±1–9 nm, hydrodynamic diameter). The concentration-dependent plasmonic enhancements were monitored by UV–Vis absorption spectrophotometry, showing absorption maxima typical of AgNPs, at 440–450 nm. A linear accession in plasmon absorbance intensity, approaching 1:5 (Ag0/citrate3−), followed by a linear depletion, at larger stoichiometries (1:6–1:10), was observed. Size distribution measurements, using dynamic light scattering, showed the highest polydispersity index, 0.547, for 1:10 suspensions and the lowest, 0.305, for the thermodynamic maximum, determined to occur at 1:5. Surface charge measurements approaching 0 mV confirm the destabilizing effect of high concentrations of citrate, leading to greater instances of aggregation and large hydrodynamic diameters. Reaction kinetics data suggests an increased preference for Agn+-citrate, metal/ligand complexation, at 1:10, diminishing nanoparticle production.