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

Abstract

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.