| Observations of time dependent enhancements in the absorbance spectra of citrate3- stabilized silver nanoparticles (AgNPs), led to chemical optimization, for the control and reproducible synthesis of AgNP suspensions, with enhanced localized surface plasmon resonance (LSPR).;Optimization is executed by microwave thermochemical reduction to synthesize stoichiometrically controlled AgNPs. The mass action study of citrate 3-/AgNPs show a linear growth in LSPR, approaching a 5:1, citrate 3-/Ag0, stoichiometry. LSPR enhancements result from maximum citrate3- electrostatic stabilization, leading to stable and separated AgNPs. Stoichiometries greater than 5:1, show a decay in plasmon absorbance as a function of the decreased particle concentration, and weakened stabilizing layer. At the largest stoichiometry, 10:1, the decay in LSPR and 31% Ag+ reduction, suggests the preference for Ag n+-citrate3-, metal-ligand complexes. AgNP surface charge, determined by zeta potential measurements, approach 0 mV, leading to the destabilizing effect of high citrate 3- concentrations, and increased hydrodynamic diameters, 65 nm, at 10:1. Also, slowed reaction constants at 10:1, confirm the influence of citrate 3- concentration toward Agn+-citrate 3- complexes; also noted by a decrease in chemical reactivity toward pyridine.;Measured by surface enhanced Raman scattering (SERS), no reactivity toward pyridine is observed for 9:1 and 10:1. The largest chemical influence to pyridine is noted for 3:1 and 5:1. This reactivity is further evaluated using Cd2+, demonstrating a rapid plasmon decay of citrate 3-/AgNPs, resulting from an available carboxyl functional group at the surface of the nanoparticle, assigned by X-ray photoelectron spectroscopy (XPS), and not observed for ethylene glycol (EG) stabilized AgNPs. Similar reactivity with an organic molecule, 4, 5-diazafluoren-9-one (dafone), along with a 2 eV (46 kcal/mol) increase in O 1s XPS binding energy in EG/AgNPs, suggest the presence of a higher energy coordinate covalent Ag-O bond, relative to that of the sterically bulky citrate3-/AgNPs. The weakened Ag-O bond can be attributed to torsional strain of the charge dense citrate 3- at the nanoparticle surface, compared to the small and symmetric EG. Density functional theory (DFT) calculations of overall binding energy of Agn+-citrate3- complexes versus ethylene glycol-Agn+ confirm stronger Ag-O bonding with ethylene glycol stabilizers, explaining the ease of displacement of citrate 3- stabilized AgNPs during reactivity experiments. |
