H2O2-Mediated Oxidation of Zero-Valent Silver and Resultant Interactions among Silver Nanoparticles, Silver Ions, and Reactive Oxygen Species

The H2O2-mediated oxidation of silver nanoparticles (AgNPs) over a range of pH (3.0–14.0) is investigated here, and an electron charging–discharging model capable of describing the experimental results obtained is developed. AgNPs initially react with H2O2 to form Ag+ and superoxide, with these products subsequently reacting to reform AgNPs (in-situ-formed AgNPs) via an electron charging–discharging mechanism. Our experimental results show that the AgNP reactivity toward H2O2 varies significantly with pH, with the variation at high pH (>10) due particularly to the differences in the reactivity of H2O2 and its conjugate base HO2 – with AgNPs whereas at lower pH (3–10) the pH dependence of H2O2 decay is accounted for, at least in part, by the pH dependence of the rate of superoxide disproportionation. Our results further demonstrate that the in-situ-formed AgNPs resulting from the superoxide-mediated reduction of Ag+ have a different size and reactivity compared to those of the citrate-stabilized particles initially present. The turnover frequency for AgNPs varies significantly with pH and is as high as 1776.0 min–1 at pH 11.0, reducing to 144.2 min–1 at pH 10.0 and 3.2 min–1 at pH 3.0.