Size-Controlled Dissolution of Silver Nanoparticles at Neutral and Acidic pH Conditions: Kinetics and Size Changes

Silver nanoparticles (AgNP) are widely utilized in increasing number of medical and consumer products due to their antibacterial properties. Once released to aquatic system, AgNP undergoes oxidative dissolution leading to production of toxic Ag+. Dissolved Ag+ can have a severe impact on various organisms, including indigenous microbial communities, fungi, alga, plants, vertebrates, invertebrates, and human cells. Therefore, it is important to investigate fate of AgNP and determine physico-chemicals parameters that control AgNP behavior in the natural environment. Nanoparticle size might have a dominant effect on AgNP dissolution in natural waters. In this work, we investigated size-dependent dissolution of AgNP exposed to ultrapure deionized water (pH approximate to 7) and acetic acid (pH 3) and determined changes in nanoparticle size after dissolution. Silver nanoparticles stabilized by thiol functionalized methoxyl polyethylene glycol (PEGSH) of 6 nm (AgNP-6), 9 nm (AgNP-6), 13 nm (AgNP-13), and 70 nm (AgNP-70) were prepared. The results of dissolution experiments showed that the extent of AgNP dissolution in acetic acid was larger than in water. Solubility of AgNP increased with the size decrease and followed the order AgNP-6> AgNP-9 > AgNP-13 > AgNP-70 in both water and acetic acid. Transmission electron microscopy (TEM) was applied to characterize changes in size and morphology of the AgNP after dissolution in water. Analysis of AgNP by TEM revealed that the particle morphology did not change during dissolution. The particles remained approximately spherical in shape, and no visible aggregation was observed in the samples. TEM analysis also demonstrated that AgNP-6, AgNP-9, and AgNP-13 increased in size after dissolution likely due to Ostwald ripening.