Silver Nanoparticle-Reactive Oxygen Species Interactions: Application of a Charging-Discharging Model

The silver-nanoparticle-catalyzed decomposition of hydrogen peroxide (H2O2) in pH 9.5 bicarbonate buffer is investigated here with attention given to (i) the mechanism of decomposition, (ii) the role of superoxide in mediating silver nanoparticle re-formation, and (iii) the effect of nanoparticle size on decomposition rate. Silver nanoparticles (AgNPs) of average size between 25.0 and 69.4 nm were synthesized via the reduction of Ag+ [the dominant Ag(I) species present] by photochemically produced superoxide at pH 9.5 and characterized by UV-visible spectroscopy and dynamic light scattering. The ability of these particles to catalytically decompose H2O2 was examined by measuring the decay of H2O2 and the approach to steady state in AgNP and Ag+ concentrations. Additionally, the generation of superoxide on reaction of AgNPs with H2O2 was monitored using a chemiluminescence-based method. The second-order rate constants for reaction between AgNPs and H2O2 correlated linearly with their average particle size ranging from 35.0 to 3.0 x 10(2) M-1 s(-1) for average sizes between 69.4 and 25.0 nm. A sensitive trap-and-trigger chemiluminescence-based method for hydroxyl radical detection showed no evidence for the presence of hydroxyl radicals, though an inhibitory effect of tert-butyl alcohol suggested the presence of a strongly oxidizing species. A process involving the superoxide-mediated charging of silver nanoparticles with subsequent discharge by reaction with oxygen and Ag+ leading to regeneration of Ag and superoxide is proposed to account for the results obtained.