Environmental Transformations of Silver and Zinc Oxide Nanoparticles

Overview: 

Characterization of the corrosion processes of Ag-NPs and impact on properties.

Ag-NPs readily transform in the environment. These transformations modify the nanoparticle properties, impacting their transport, fate, and toxicity. Therefore, it is essential to consider such transformations when assessing their potential environmental risks. A review presenting what is known about the major transformation processes of Ag-NPs and their effects on nanoparticle properties, including physical and chemical stability and toxicity, was submitted to ES&T in October 2011. We also discuss the important research needs that are required to better predict the impact of Ag-NPs on the environment

Among the important environmental transformations, we have shown that sulfidation strongly affects the properties of the Ag-NPs. Strong aggregation of the particles was observed by TEM following sulfidation. Ag2S nano-bridges are formed between Ag-NPs. Sulfidation leads to modification of the surface charge and affects the aggregation and transport of Ag-NPs. Finally, solubility of Ag-NPs decreases as the sulfidation rate (S/Ag) increases. As a consequence, sulfidation was shown to strongly limit Ag-NP toxicity because released Ag+ ions are known to contribute significantly to Ag-NPs toxicity. This effect was tested on E. coli in collaboration with Ph.D. student Brian C. Reinsch and Prof. Greg Lowry at Carnegie Mellon University and also on Zebra fish embryo, killifish and C. elegans in collaboration with Lisa Truong (Oregon State University), Audrey Bone and Joel Meyer (Duke University).

In addition to sulfidation, we have studied the reaction of PVP-coated Ag-NPs in the presence of chloride ions and a simple surrogate for natural organic matter (polyacrylic acid-PAA). Our grazing-incidence XRD and XSW-based XRD measure-ments show that although most of the released Ag+ ions are associated with Cl- ions in the form of crystalline AgCl nanoparticles, a significant fraction is present in the form of an amorphous AgCl phase based on a comparison of XAFS spectroscopy and XRD data. Thus, we have shown that a stable AgCl corrosion product is formed despite the presence of potential surface stabilizers (PVP and PAA). This is an important result in the context of Ag-NP toxicity. The strong affinity of Cl- for Ag+ and, more particularly, the Cl/Ag ratio may control the toxic effects of Ag-NPs on organisms. This hypothesis is currently being tested on E. coli by Stanford sophomore Sumit Mitra. First results indicate that toxicity cannot be explained solely by the presence of soluble AgCl species and suggest a nanoparticle effect as well. Further experiments are underway to help define what controls the toxicity of Ag-NPs in presence of various amounts of chloride.

Synthesis and corrosion of ZnO-NPs

We are interested in determining how ZnO-NPs transform and how these transformations affect their properties. As for Ag-NPs, sulfidation is one of the most likely corrosion processes that may occur for ZnO-NPs. However, sulfidation mechanisms are different, mostly because no initial oxidation of Zn is required before it can react with sulfur. One important aspect we are looking at is how nanoparticle size affects the kinetics of sulfidation of ZnO-NPs.

First, 4-5 nm ZnO-NPs were synthesized. Preliminary experiments show that sulfidation of the 4nm ZnO-NPs occurred in less than a minute at pH7 under anaerobic conditions. The ZnS particles are approximately 5nm in diameter, and based on lattice fringe analysis, part of the ZnS crystallizes with the wurtzite structure and not exclusively within the sphalerite structure which is the most stable ZnS phase in nature. Similar experiments will be done for 30 and 250 nm nanoparticles to compare transformation rates.

Other experiments in collaboration with Rui Ma (Carnegie Mellon) have shown that 30nm ZnO-NPs are completely sulfidized and transformed into 4 nm ZnS particles. High energy X-ray Total Scattering has shown that the resulting ZnS nanoparticles have the sphalerite structure.