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novators such as Diallo and his colleagues continue to forge ahead with entirely new nanofabrications. Whether these materials’ advantages will ever be realized remains to be seen, researchers in the field caution. But if they are, potentially immense savings in energy and materials await a society that embraces the new paradigm.
Promising Green Nanomaterials NAOMI LUBICK
Researchers are creating green nanomaterials, with an eye toward their hazards as well as cleanup potentials and pitfalls. DIALLO ET AL., DOI 10.1021/ES0715905
Transformations
Multibranching molecules like this dendrimer may one day be used to capture uranium in contaminated water. In the quest to clean water of unwanted pollutants, one of the latest tools is shaped like the roots of a tree and can reach 100 nanometers from tip to tip. This multibranching molecule is based on a dendrimersa snowflake-shaped molecule with functionalized junctions that bind targeted contaminants. “We have produced these things that can bind fluoride, chloride, nitrate, bromide, phosphate,” and, in particular, carcinogenic perchlorate, says Mamadou Diallo of the California Institute of Technology. His team’s modified dendrimer tailored to capture perchlorate was patented at the end of last year. “We can make these very fast.” The use of dendrimers as cleanup tools is a relatively new application that represents some of the many promisessand possible perilssof nanotechnology in the environment. Newly discovered properties on small scales could lead to less chemical use and permit more chemical cleanup in industrial settings. Nanomaterials might make it possible to tackle seemingly intractable contaminants, such as PCBs. The materials can be recyclable, tailored to specific purposes, and are relatively cheap and are easy to make. But the attractive qualities of nanomaterials are also what might make them dangerous, from their antimicrobial behavior to their strength and persistence. Yet even as most existing nanomaterials remain largely uncharacterized, in10.1021/es900021v
2009 American Chemical Society
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“We’re in a scientific revolution” like the one defined by Thomas Kuhn, says Barbara Karn of the U.S. EPA. “The paradigm that we have shifted to is that properties change with size alone, not just composition,” Karn says. New tools and approaches were necessary “to show that the phenomena were real. [We’ve] never actually believed it before.” Karn has actively encouraged the merger of green chemistry and nanotechnology for almost a decade, and she has followed nanotechnology’s meteoric rise with cautious enthusiasm, keeping the unknowns of nanomaterials in mind. “Green nano” consists of two parts, she says: processes and products. Among the benefits, nanoscale manufacturing can reduce the amount of source material that is necessary, get rid of nasty solvents, and use less water. Manufacturers can design nanomaterialsswhether for inputs or end productssto be safer. “The whole issue is to aim for sustainability,” Karn emphasizes. Although difficult to monitor in proprietary industrial settings, green manufacturing methods and product development are under way at such centers as the Oregon Nanoscience and Microtechnologies Institute and the Nanomanufacturing Center at the University of Massachusetts Lowell. Meanwhile, industry proceeds apace with the development of new nanoproducts, according to reports from Lux Research, a research and advisory firm that specializes in emerging technologies. Almost nonexistent 5 years ago, the market for a broad range of products containing emerging nanotechnologies blossomed to $147 billion in 2007, according to Lux Research. The firm tracks about 300 companies working on nanomaterials for wind power, photovoltaics, packaging materials, batteries for efficient and compact energy storage, and a multitude of other products or components in the works. Overall, established “nano-enabled” products, including carbon black or flash memory, for example, held a market share of $1.7 trillion in 2008. Green nano is just being introduced, however, and “the whole market overall was certainly not huge” in 2008, says Kristin Abkemeier, a Lux Research analyst. For example, in 2008 the market share for nanocoatingsswhether antireflective, strengthening, or protectivesamounted to only about $2 billion, which was only 2% of the entire coatings market. But by 2015, Lux projects that share will be $20 billion, or 20% of the entire coatings market. “At best, products are only starting to be introduced. [Most are] still very much in the development stage,” Abkemeier says, particularly the “emerging” nanotechnologies, such as those for cleanup and other environmentally beneficial applications. For example, some high-profile companies VOL. 43, NO. 5, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Nanocomposites containing boride nanoparticles could make super-hard cutting tools and super-smooth hydraulic pumpsssaving energy, water, and materials. making thin-film photovoltaics claim to use nanotechnology to lower the cost of solar power, but they have “had difficulty trying to scale up their technologies and make [them] still work effectively,” she says. “The same is true with other nanotechnologies; it’s not happening as soon as people thought it would.”
Coats of many nanomaterials One team of researchers is trying to bring its nanocoatings to market, along with the benefits for energy efficiency and the environment. The team hails from government, academia, and industrysa triumvirate that sometimes draws on venture capitalists, too. The team’s nanoboride coatings decrease friction in hydraulic pumps. With the coatings’ “unusually low coefficients of friction, ... pumps need less power to deliver the same amount of work, because less energy is dissipated to overcome friction,” explains Alan Russell of Iowa State University, who works with researchers at the U.S. Department of Energy’s (DOE’s) Ames Laboratory and Eaton Corp. Cutting tools, when “coated with our borides, can cut faster and wear out slower than uncoated tools,” he adds. Further decreases in electricity use could come with transitions to smaller engines that can run pumps made more efficient by these coatings. For the Iowa researchers’ nanoboride coatings, DOE calculations project that even slight increases in pump efficiency could reduce energy use across all U.S. industries: DOE estimates savings of 31 trillion British thermal units annually by 2030, which translates to savings of $179 million per year. Nanocoatings also present an opportunity to transition to safer materials, Russell points out. In industrial settings that require lubricants, nanocoatings can be paired with water-based hydraulic fluids, rather than petroleum-based ones. The switch reduces costs and avoids the use of material known to be environmentally hazardous.
Ready for cleanup? As researchers like Russell and his colleagues work to commercialize their products following years of R&D, marketready nanoproducts remain elusive. The same scaling problems faced by photovoltaics are challenges to the use of nanoproducts for advanced water treatment (led by nanozerovalent iron) or for cleaning up contaminants such as PCBs that may be spread over wide areas. In the meantime, Diallo expects people to begin paying for his team’s newly patented dendrimers as of January 2009. Discovered nearly 30 years ago, dendrimers have garnered interest as tools for environmental cleanup only during the past several years, in part because visualization and ma1248
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nipulation techniques have improved. Lux Research reports on six companies working on dendrimers; most uses are medical or drug-related, not for environmental remediation. Diallo says the molecules’ main attraction is their multiple “flavors”. “We are developing special classes on one material platform,” he says. But he cautions that his team’s modified dendrimers are a step below the pure star structures developed by materials scientists over the past two decades. His team’s uneven “dendigraphs” still have a hyperbranched, macromolecular structure “that you can play with,” and they have “huge binding capacity,” he says. And whereas pure dendrimers are too expensive, costing $1000 or more per pound, Diallo and his co-workers can make their less perfect structures for $5-15 per pound. “A dendrimer would be like a Mercedes; for a commercial application, we are developing a Yugo with Mercedes performance,” he jokes. Diallo’s resulting water-soluble dendrimers can be used for water recycling, for example, in a “polishing” step designed to be incorporated into existing treatment systems to remove perchlorate. The nanomaterials also can be recycled: one tailored dendrimer holds on to contaminants at pH 5 and releases them at pH 9 for collection and disposal. Where such pH-shifted recycling isn’t feasible, the particles and their tightly captured contaminants could be considered disposable and encapsulated in wastewater treatment sludge, for example, Diallo says. The possibilities seem endless to Diallo: dendrimers inside microparticles, for example, could be “loaded into a bed reactor like typical carbon” for water filtration, he says. He would like to look into membranes for groundwater treatment, particularly for concentrating perchlorate in place, or even for removing uranium. More immediately, the dendrimers could find their way into household water filters at smaller scales, Diallo hypothesizes. But Diallo says his team has yet to put its products through a full battery of ecotoxicology tests. The fact remains that these nanoparticles could easily make it into the environment by way of the waste stream or manufacturing processes, with as yet unknown impacts. Diallo suggests that industry must develop nano-drug-delivery methods that are safe for patients, and insights gleaned from medical uses may also lead to ways to keep nanomaterials environmentally safe. He cites a cancer drug that one company reports is less toxic because its vehicle is a dendrimer targeted to a specific treatment site, which is the only place where it will be active. But although testing of dendrimers for medical uses may be well under way, environmental testing is not as far along, and their environmental cleanup capabilities do not guarantee dendrimers’ environmental safety. The end-group chemistry controls the toxicity of dendrimers, as with polymers in general, Diallo says, but multiple flavors mean multiple possibilities for toxic effects, as well as for fate and transport of these nanoparticles.
Environmental work, environmental contamination Products modified with nanomaterials have been on the market for almost a decade now. But only last year did researchers from the Swiss Federal Institute of Aquatic Science and Technology (Eawag) and the Swiss Federal Laboratories for Materials Testing and Research (Empa) report what may be the first detection of an engineered nanoparticle in the environment, an endeavor akin to looking for the proverbial needle in a haystack. The purported source: building facades coated with paints containing nano-TiO2. Nano-TiO2 inhibits the growth of algae and other microorganisms, replacing organic biocides that keep building surfaces clean. And TiO2 photochemistry also breaks down particulates, which means that nano-TiO2-coated windows could eventually clean a city’s air, conjectures Bernd Nowack
of Empa. But Nowack emphasizes the trade-offs: even as fewer biocides are used, the nanoparticles washing off these windows and other coated surfaces into storm drains, streams, and rivers might pose problems for fish and other organisms. The impacts of traditional chemicals, often made to target specific biological receptors in plants and humans, insects, or other creatures, stand as examples of what unexpected damage nanomaterials could do, notes Nora Savage of EPA. They also illustrate another problem, she cautions: “Mixtures of compounds are what’s going to be critical here.” Once in the environment, these substances do not act alone. No one knows yet what nanomaterials will do in the presence of other chemicals, or if they might heighten other chemicals’ risks. Savage has coauthored previous dendrimer research with Diallo; she also administers EPA STAR grants for research on both environmental benefits and impacts of nanomaterials. “I know people are trying to design environmentally benign nanomaterials,” she says, “but all toxicity tests to date show that behaviors change with agglomeration, as coatings degrade, [and so on]. As they end up in the water, it’s going to be much more complex.”
analyses have been conducted for any nanomaterials, in part because of the lack of data. “We can’t just look at one part” of the existence of nanomaterials, whether it’s the manufacturing, use, or disposal stage, says Karn. Common materials can be “sliced and diced” until they “behave differently when they come in contact with the biosphere. We have reason to believe that there are some causes for concern, but we don’t know enough.” Karn hopes to see progress in the use of nanomaterials on multiple frontssas both catalysts and solvent removers in manufacturing processes, as biosensors to detect contaminants in the environment, and more. But she sees no guarantees that inventors and producers will embrace the new paradigm. For those who accept the challenge, Karn calls the green nanotechnology revolution an opportunity for “a fresh way of designing new products, with the environment and sustainability in mind.”
Paradigm continues to shift
Naomi Lubick is a freelance writer based in Zurich and California. If you are interested in the most up-to-date nanomaterials research, she suggests attending the sessions chaired by Karn during the spring ACS meeting, March 22-26, 2009, in Salt Lake City, Utah.
So far, for either good or bad nano-impacts, “the scientific data don’t exist,” says Nowack. Only a handful of life-cycle
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