Oxidative Dissolution of Silver Nanoparticles by Biologically Relevant Oxidants: A Kinetic and Mechanistic Study

Abstract

The oxidative dissolution of silver nanoparticles (AgNPs) plays an important role in the synthesis of well-defined nanostructured materials, and may be responsible for their activities in biological systems. In this study, we use stopped-flow spectrophotometry to investigate the kinetics and mechanism of the oxidative dissolution of AgNPs by H(2)O(2) in quasi-physiological conditions. Our results show that the reaction is first order with respect to both [Ag(0)] and [H(2)O(2)], and parallel pathways that involve the oxidation of H(2)O(2) and HO(2)(-) are proposed. The order of the reaction is independent of the size of the AgNPs (approximately 5-20 nm). The rate of dissolution increases with increasing pH from 6.0 to 8.5. At 298 K and I=0.1 M, the value of k(b) is five orders of magnitude higher than that of k(a) (where k(a) and k(b) are the rate constants for the oxidative dissolution of AgNPs by H(2)O(2) and HO(2)(-), respectively). In addition, the effects of surface coating and the presence of halide ions on the dissolution rates are investigated. A possible mechanism for the oxidative dissolution of AgNPs by H(2)O(2) is proposed. We further demonstrate that the toxicities of AgNPs in both bacteria and mammalian cells are enhanced in the presence of H(2)O(2), thereby highlighting the biological relevance of investigating the oxidative dissolution of AgNPs.

Controlled Evaluation of Silver Nanoparticle Dissolution Using Atomic Force Microscopy

“…Dissolution kinetics of AgNPs are much faster in the presence of H 2 O 2 than in the presence of oxygen alone. 13,32,55 Some living organisms, such as algae and macrophages, can release H 2 O 2 and other reactive oxygen species into solution; thus, interactions between AgNPs and organisms may cause more rapid Ag + release than would be predicted based on kinetic models for the oxidation AgNPs by oxygen. 15,55 In fact, one study claimed that algae caused AgNPs to release more Ag + , which increased the observed toxic effects of AgNPs on the algae.…”

“…10,13,35,55 All of these attempts have arrived at rate laws that are pseudo-first-order in silver concentration. Results from Liu and Hurt suggested that AgNPs may persist in the environment in their colloidal form for only weeks to months.…”

“…8,9,35,54,55 This result is partially explained by the larger surface area of smaller particles on an equivalent mass basis; if more surface atoms are exposed, then more reaction sites are available for oxidation and subsequent dissolution. 9 However, normalizing dissolution rates by surface area cannot fully account for the size-dependent dissolution of nanoparticles.…”

“…13 On the other hand, dissolution rates increased with increasing pH in a study where hydrogen peroxide was used as the primary oxidant instead of oxygen. 55 This observation is probably due to the dissociation of H 2 O 2 at high pH to form the hydroperoxide anion, HO 2 -, which is expected to have a stronger interaction with the silver surface since it is a stronger nucleophile than the undissociated form. 62 This result may be important for conditions that are relevant to textile washing where pH may be high and H 2 O 2 may be present in bleach.…”

“…In the case of hydrogen peroxide, ionic strength did not appear to have any effect on AgNP dissolution. 55 It is evident from data presented by Liu and Hurt that ionic strength had some influence on the reaction kinetics, probably mostly due to aggregation, but the authors dismissed this effect as minor compared to the influence of pH. 13 …”