Perspective pubs.acs.org/est
The Mysterious Fates of Nanoparticles: ES&T’s Top Feature Article 2012 Erika Gebel
oxidizing, while groundwater and carbon-rich soils can be reducing. Besides such chemical alterations, nanoparticles also tend to aggregate over time. This physical transformation may not be a bad thing, says Benjamin Twining of the Bigelow Laboratory of Ocean Sciences, who was not an author of the feature. “As these small particles aggregate, the expectation is they will become less toxic.” In part, aggregation reduces the particles’ available surface area, decreasing their reactivity. Also, while individual nanoparticles tend to disperse in liquids, aggregates settle out as sediments, which slows the movement of the particles through the environment, he says. Predicting where a nanomaterial will accumulate is important for assessing the toxicity risk for organisms in an ecosystem, Twining says. Nanomaterials also encounter many different biological molecules in the environment. For example, Lowry describes how horseradish peroxidase can alter the properties of carbon nanotubes, oxidizing them and priming them for further transformations. This alters the surface charge of the nanotubes and may increase or decrease their toxicity depending on other conditions. Perhaps the most important transformation, the authors note, is the attachment of biological macromolecules to the surface of nanomaterials. In a snowball effect, biological molecules accumulate on the surfaces of nanoparticles, creating what some scientists call a protein corona. This halo of biological molecules may mask the identity of the nanomaterial, allowing it to slip easily into cells or to generate biological responses through interactions between the halo molecules and cellular receptors. To complicate matters, nanomaterials exposed to multiple environments may experience unique transformations. A nanomaterial that first enters an oxidizing environment and then interacts with a mix of biological molecules may end up different from a material that encounters those scenarios in the reverse order. Although the laundry list of possible transformations is long, and as a result, the task of predicting nanomaterial behavior in the environment seems daunting, Lowry proposes a solution. He thinks researchers should find a handful of important environments that nanomaterials are most likely to land in. Then they should test nanomaterials in each of those environments to catalogue what transformations occur, and to identify classes of nanomaterials based on their reactivity. Which environments are important? “That’s the million-dollar question,” says Lowry.
Environ. Sci. Technol.
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anomaterials pop up in many products, such as smartphones, cosmetics, food, and medicines. Despite the ubiquity of nanomaterials, their environmental impact remains only partially understood. One major problem, according to Gregory V. Lowry of Carnegie Mellon University, is that nanomaterials can change drastically in the environment, and scientists do not yet know which transformations can occur or how these changes affect the materials’ toxicity. In ES&T’s Best Feature of 2012, Lowry and his colleagues assert that such information is critical for an accurate assessment of the risks associated with nanomaterials (Environ. Sci. Technol., DOI: 10.1021/es300839e). “We are introducing nanomaterials into commercial products at a faster rate than we are acquiring information on their safe disposal,” says Pedro J. Alvarez of Rice University, who was not an author on the feature. “This paper essentially highlights that critical knowledge gap.” Scientists have long been worried about nanomaterials because the materials have novel chemical and electronic properties that could present unique risks in the environment. Research teams have studied common nanomaterials in the laboratory, checking for toxicity in model organisms. However, these studies tend to focus on the materials as they exist in products, Lowry says. One of the desirable properties of nanomaterials is that, because of their high surface-to-volume ratio, they tend to be highly reactive. As a result, nanomaterials likely change dramatically when released into the environment. And scientists have rarely studied these altered states of the substances, Lowry says. In the feature, Lowry and his colleagues explore the various scenarios a nanomaterial may encounter after escaping into the environment and the likely transformations that could occur. They discuss potential chemical reactions, physical transformations, and interactions with biological macromolecules. In the environment, oxidizing or reducing conditions can alter metal-based nanomaterials, changing their toxicity or physical states. For example, when quantum dots oxidize, they may release toxic metals such as cadmium. And there are many redoxactive environments: Aerated soils and surface waters can be © 2013 American Chemical Society
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest. Received: February 28, 2013 Accepted: February 28, 2013 Published: March 8, 2013 3020
dx.doi.org/10.1021/es400928h | Environ. Sci. Technol. 2013, 47, 3020−3020
