Environmental Transformations of Silver Nanoparticles: Impact on Stability and Toxicity Cle ́ ment Levard,* ,†,‡ E. Matt Hotze, ‡,§ Gregory V. Lowry, ‡,§ and Gordon E. Brown, Jr. †,‡,∥ † Surface and Aqueous Geochemistry Group, Department of Geological & Environmental Sciences, Stanford University, Stanford, California 94305-2115, United States ‡ Center for Environmental Implications of NanoTechnology (CEINT), P.O. Box 90287, Duke University, Durham, North Carolina 27708-0287, United States § Department of Civil and Environmental Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States ∥ Department of Photon Science and Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States ABSTRACT: Silver nanoparticles (Ag-NPs) readily transform in the environ- ment, which modifies their properties and alters their transport, fate, and toxicity. It is essential to consider such transformations when assessing the potential environmental impact of Ag-NPs. This review discusses the major transformation processes of Ag-NPs in various aqueous environments, particularly trans- formations of the metallic Ag cores caused by reactions with (in)organic ligands, and the effects of such transformations on physical and chemical stability and toxicity. Thermodynamic arguments are used to predict what forms of oxidized silver will predominate in various environmental scenarios. Silver binds strongly to sulfur (both organic and inorganic) in natural systems (fresh and sea waters) as well as in wastewater treatment plants, where most Ag-NPs are expected to be concentrated and then released. Sulfidation of Ag-NPs results in a significant decrease in their toxicity due to the lower solubility of silver sulfide, potentially limiting their short-term environmental impact. This review also discusses some of the major unanswered questions about Ag- NPs, which, when answered, will improve predictions about their potential environmental impacts. Research needed to address these questions includes fundamental molecular-level studies of Ag-NPs and their transformation products, particularly Ag 2 S- NPs, in simplified model systems containing common (in)organic ligands, as well as under more realistic environmental conditions using microcosm/mesocosm-type experiments. Toxicology studies of Ag-NP transformation products, including different states of aggregation and sulfidation, are also required. In addition, there is the need to characterize the surface structures, compositions, and morphologies of Ag-NPs and Ag 2 S-NPs to the extent possible because they control properties such as solubility and reactivity. 1. INTRODUCTION Silver nanoparticles (Ag-NPs) have recently been the focus of intense research because of the potential risk they pose to humans and other biological organisms. 1−9 Indeed, the toxicity of silver nanoparticles to a variety of organisms has been demonstrated in a number of recent studies. For example, toxicity has been observed for aquatic (Lemna minor) 10,11 and terrestrial (Lolium multif lorum) 12 plants, algae, and fungi, 4 vertebrates (zebra fish), 13 invertebrates (Caenorhabditis ele- gans), 14,15 microorganisms (Escherichia coli, 16,17 Pseudomonas putida 18 ), and human cells (skin keratinocytes, lung fibroblast cells, and glioblastoma cells). 19,20 The list of studies showing the negative impact of Ag-NPs on the environment and potentially on humans is long and has been reviewed many times over the past decade. 1−9 Although the toxicity of Ag-NPs is partly explained by the release of Ag ions, it remains unclear if Ag-NPs are a direct cause of enhanced toxicity. For example, Navarro et al. 10 presented evidence that toxicity is mainly the result of Ag ions and that Ag-NPs contribute to toxicity as a source of dissolved Ag ions. In contrast, Fabrega et al. 21 showed a specific nanoparticle effect that could not be explained by dissolved Ag + . Similarly, Yin et al. 12 demonstrated that gum arabic-stabilized Ag-NPs more strongly affected the growth of Lolium multif lorum, a common grass, more than the equivalent dose of Ag ions added as AgNO 3 . They concluded that growth inhibition and cell damage can be directly attributed either to the nanoparticles themselves or to the ability of Ag-NPs to deliver dissolved Ag + to critical biotic receptors. Recently Sotiriou et al. 22 proposed that the antibacterial activity of Ag- NPs depends on their size. They provide some evidence that when Ag-NPs are small and release many Ag ions, the Special Issue: Transformations of Nanoparticles in the Environment Received: October 21, 2011 Revised: February 1, 2012 Accepted: February 16, 2012 Published: February 16, 2012 Critical Review pubs.acs.org/est © 2012 American Chemical Society 6900 dx.doi.org/10.1021/es2037405 | Environ. Sci. Technol. 2012, 46, 6900−6914