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

Silver nanoparticles (Ag_NP) are widely utilized in increasing number of medical and consumer products due to their antibacterial properties. Once released to aquatic system, Ag_NP 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 Ag_NP and determine physico-chemicals parameters that control Ag_NP behavior in the natural environment. Nanoparticle size might have a dominant effect on Ag_NP dissolution in natural waters. In this work, we investigated size-dependent dissolution of Ag_NP exposed to ultrapure deionized water (pH ≈ 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 (Ag_NP_6), 9 nm (Ag_NP_9), 13 nm (Ag_NP_13), and 70 nm (Ag_NP_70) were prepared. The results of dissolution experiments showed that the extent of Ag_NP dissolution in acetic acid was larger than in water. Solubility of Ag_NP increased with the size decrease and followed the order Ag_NP_6 > Ag_NP_9 > Ag_NP_13 > Ag_NP_70 in both water and acetic acid. Transmission electron microscopy (TEM) was applied to characterize changes in size and morphology of the Ag_NP after dissolution in water. Analysis of Ag_NP 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 Ag_NP_6, Ag_NP_9, and Ag_NP_13 increased in size after dissolution likely due to Ostwald ripening