Environmental Impacts of Manufactured Nanoparticles

Katie Maginnis for TEI

Rape seeds inhibited by zinc nanoparticles.

As the 21st century unfolds, the emerging development of nanotechnology presents considerable benefits to society, as well as risks. Nanoparticles that measure 1-100 nanometers (nm) in size are already being used for a variety of purposes. For instance, carbon nanotubes have been considered for use in remediation of toxic chemicals in the environment, and metal oxide nanoparticles are present in many cosmetics, such as sunscreen and shampoo. As this technology develops, however, the need for information concerning the potential damages caused by nanoparticles becomes increasingly important.

Baoshan Xing, Professor of Environmental and Soil Chemistry, in Plant, Soil and Insect Soil Sciences , understands the necessity for research in this field. As Xing explains, the current regulations and protocols for the safety and testing of nanoparticles are either nonexistent, or in need of revision. In many cases, the same rules used for bulk chemicals are being applied to nanoparticles. This poses a serious problem, because the physiochemical properties and behavior of nanoparticles differ significantly from those of bulk chemicals. Due to their size – too large to behave as single atoms, yet too small to behave as bulk chemicals, nanoparticles represent a unique category of material.

Last year, Xing’s research group conducted research on the environmental behavior of carbon nanotubes, focusing on their ability to absorb toxic chemicals such as polycyclic aromatic hydrocarbons (PAHs), which are known to be carcinogens. Although the nanotubes displayed a high level of PAH adsorption, which could promote their use in toxin remediation, they also exhibited a higher tendency than bulk materials for re-release of the PAHs that were absorbed. If carbon nanotubes were to absorb toxins, enter the body of a live organism, and then release the toxins, the result could be quite harmful towards the health of the organism and to subsequent organisms in the food web.

In addition to studying carbon nanotubes, Xing’s group has been researching the nanoparticle forms of metal oxides as well as their effects on plants and microbes. Since mature plants and soil ecosystems are fairly complex entities, Xing began his research using simpler, easier-to-test systems involving seeds and hydroponic (water-based) environments.

In these preliminary experiments, they placed seeds from various plant species, including radish, rapegrass, ryegrass, lettuce, corn, and cucumber, into Petri dishes. The seeds were then submerged in a nutrient-enriched water solution, and exposed to an array of nanoparticles in varying concentrations. Out of the nanoparticles tested, including zinc, zinc oxide, aluminum, and aluminum oxide, the most drastic effects were caused by zinc oxide. The zinc oxide nanoparticles, commonly found in cosmetics such as shampoo, significantly impaired seed germination and stunted root growth in most of the tested species.

Aside from their toxic effects on plants, nanoparticles such as zinc oxide have also displayed negative impacts on microbes. At the 2007 International Conference on Soils, Sediments, and Water, Xing’s group presented their research involving the toxicity of metal oxide nanoparticles and carbon nanotubes on bacteria. Of the bacteria tested, including Bacillus subtilis, Pseudomonas fluorescence, and Escherichia coli, all suffered significant decreases in population, although the sensitivity of different bacteria species fluctuated depending on the particular nanoparticles used. Similar to the results of the tests involving plants, they found that zinc oxide nanoparticles produced the greatest damage among each of the bacteria species.

The reason that nanoparticles often exhibit greater microbial toxicity than bulk materials, Xing explains, involves the unique physiochemical properties attributed to materials of the nano-scale size. Because they are smaller, nanoparticles can attach to the bacterial cell more readily and may even enter the cell in many cases.

Although the mechanism for entry into the bacterial cell is still unclear, Xing speculates that the nanoparticles either open the membrane, or perhaps are small enough to infiltrate the cell without disturbing the membrane at all. In either case, the result of the nanoparticles’ invasion of the cell is a significant alteration of the cell’s normal functions, eventually leading to cell death.

As he continues to research the properties and effects of nanoparticles, Xing aspires to study more complex organisms. “For now we’re just studying germination, seedlings, and hydroponic systems,” he says. “Next we’ll do bigger plants, and then we’ll do soil.” With more data generated every day, the need for an interdisciplinary approach to studying nanotechnology becomes more apparent. Xing is organizing a symposium regarding the environmental fate of manufactured nanomaterials for the 2008 meeting of the American Chemical Society this coming April. He hopes to collaborate with scientists from various fields, such as microbiology, toxicology, analytical chemistry and engineering.

With the increasing diversity of uses for nanotechnology – cosmetics, industrial lubricants, tennis rackets, semiconductors, protective clothing, precision drug delivery – comes the necessity for a better understanding of nanoparticles’ behavior and negative effects. As Xing explains, “People are making a comparison: the last century was the century of plastics and antibiotics. This century will be the century of nanotechnology.” Although much of his research illuminates the negative aspects of nanoparticles, Xing views this as crucial information that, when passed on to manufacturers, will result in significant improvement of this new technology. There is no reason to stop or slow down the development of nanotechnology, according to Xing; the bottom line is that we “just make them safe, then we will have sustainable development of nanotechnology.”

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