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

Abstract

The silver-nanoparticle-catalyzed decomposition of hydrogen peroxide (H 2 O 2 ) 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 H 2 O 2 was examined by measuring the decay of H 2 O 2 and the approach to steady state in AgNP and Ag + concentrations. Additionally, the generation of superoxide on reaction of AgNPs with H 2 O 2 was monitored using a chemiluminescence-based method. The second-order rate constants for reaction between AgNPs and H 2 O 2 correlated linearly with their average particle size ranging from 35.0 to 3.0 × 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 0 and superoxide is proposed to account for the results obtained.