Promising Green NanomaterialsN A O M I L U B I C K

Researchers are creating green nanomaterials, with an eyetoward their hazards as well as cleanup potentials and pitfalls.

In the quest to clean water of unwanted pollutants, one ofthe latest tools is shaped like the roots of a tree and can reach100 nanometers from tip to tip. This multibranching mole-cule is based on a dendrimersa snowflake-shaped moleculewith 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 theCalifornia Institute of Technology. His team’s modifieddendrimer tailored to capture perchlorate was patented atthe end of last year. “We can make these very fast.”

The use of dendrimers as cleanup tools is a relatively newapplication that represents some of the many promisessandpossible perilssof nanotechnology in the environment.Newly discovered properties on small scales could lead toless chemical use and permit more chemical cleanup inindustrial settings. Nanomaterials might make it possible totackle 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 whatmight make them dangerous, from their antimicrobialbehavior to their strength and persistence. Yet even as mostexisting nanomaterials remain largely uncharacterized, in-

novators such as Diallo and his colleagues continue to forgeahead with entirely new nanofabrications.

Whether these materials’ advantages will ever be realizedremains to be seen, researchers in the field caution. But ifthey are, potentially immense savings in energy and materialsawait a society that embraces the new paradigm.

Transformations“We’re in a scientific revolution” like the one defined byThomas Kuhn, says Barbara Karn of the U.S. EPA. “Theparadigm that we have shifted to is that properties changewith size alone, not just composition,” Karn says. New toolsand approaches were necessary “to show that the phenomenawere real. [We’ve] never actually believed it before.”

Karn has actively encouraged the merger of greenchemistry and nanotechnology for almost a decade, and shehas followed nanotechnology’s meteoric rise with cautiousenthusiasm, keeping the unknowns of nanomaterials inmind. “Green nano” consists of two parts, she says: processesand products. Among the benefits, nanoscale manufacturingcan reduce the amount of source material that is necessary,get rid of nasty solvents, and use less water. Manufacturerscan design nanomaterialsswhether for inputs or endproductssto be safer. “The whole issue is to aim forsustainability,” Karn emphasizes.

Although difficult to monitor in proprietary industrialsettings, green manufacturing methods and product devel-opment are under way at such centers as the OregonNanoscience and Microtechnologies Institute and the Na-nomanufacturing Center at the University of MassachusettsLowell. Meanwhile, industry proceeds apace with the de-velopment of new nanoproducts, according to reports fromLux Research, a research and advisory firm that specializesin emerging technologies.

Almost nonexistent 5 years ago, the market for a broadrange of products containing emerging nanotechnologiesblossomed to $147 billion in 2007, according to Lux Research.The firm tracks about 300 companies working on nanoma-terials for wind power, photovoltaics, packaging materials,batteries for efficient and compact energy storage, and amultitude of other products or components in the works.Overall, established “nano-enabled” products, includingcarbon black or flash memory, for example, held a marketshare of $1.7 trillion in 2008.

Green nano is just being introduced, however, and “thewhole market overall was certainly not huge” in 2008, saysKristin Abkemeier, a Lux Research analyst. For example, in2008 the market share for nanocoatingsswhether antire-flective, strengthening, or protectivesamounted to onlyabout $2 billion, which was only 2% of the entire coatingsmarket. 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. [Mostare] still very much in the development stage,” Abkemeiersays, particularly the “emerging” nanotechnologies, such asthose for cleanup and other environmentally beneficialapplications. For example, some high-profile companies

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Multibranching molecules like this dendrimer may one daybe used to capture uranium in contaminated water.

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making thin-film photovoltaics claim to use nanotechnologyto lower the cost of solar power, but they have “had difficultytrying to scale up their technologies and make [them] stillwork effectively,” she says. “The same is true with othernanotechnologies; it’s not happening as soon as peoplethought it would.”

Coats of many nanomaterialsOne team of researchers is trying to bring its nanocoatingsto market, along with the benefits for energy efficiency andthe environment. The team hails from government, academia,and industrysa triumvirate that sometimes draws on venturecapitalists, too.

The team’s nanoboride coatings decrease friction inhydraulic pumps. With the coatings’ “unusually low coef-ficients of friction, ... pumps need less power to deliver thesame amount of work, because less energy is dissipated toovercome friction,” explains Alan Russell of Iowa StateUniversity, who works with researchers at the U.S. Depart-ment of Energy’s (DOE’s) Ames Laboratory and Eaton Corp.Cutting tools, when “coated with our borides, can cut fasterand wear out slower than uncoated tools,” he adds.

Further decreases in electricity use could come withtransitions to smaller engines that can run pumps made moreefficient by these coatings. For the Iowa researchers’nanoboride coatings, DOE calculations project that evenslight increases in pump efficiency could reduce energy useacross all U.S. industries: DOE estimates savings of 31 trillionBritish thermal units annually by 2030, which translates tosavings of $179 million per year.

Nanocoatings also present an opportunity to transitionto safer materials, Russell points out. In industrial settingsthat require lubricants, nanocoatings can be paired withwater-based hydraulic fluids, rather than petroleum-basedones. The switch reduces costs and avoids the use of materialknown to be environmentally hazardous.

Ready for cleanup?As researchers like Russell and his colleagues work tocommercialize their products following years of R&D, market-ready nanoproducts remain elusive. The same scalingproblems faced by photovoltaics are challenges to the useof nanoproducts for advanced water treatment (led by nano-zerovalent iron) or for cleaning up contaminants such asPCBs that may be spread over wide areas.

In the meantime, Diallo expects people to begin payingfor his team’s newly patented dendrimers as of January 2009.

Discovered nearly 30 years ago, dendrimers have garneredinterest as tools for environmental cleanup only during thepast several years, in part because visualization and ma-

nipulation techniques have improved. Lux Research reportson six companies working on dendrimers; most uses aremedical or drug-related, not for environmental remediation.

Diallo says the molecules’ main attraction is their multiple“flavors”. “We are developing special classes on one materialplatform,” he says. But he cautions that his team’s modifieddendrimers are a step below the pure star structuresdeveloped by materials scientists over the past two decades.

His team’s uneven “dendigraphs” still have a hyper-branched, macromolecular structure “that you can play with,”and they have “huge binding capacity,” he says. And whereaspure dendrimers are too expensive, costing $1000 or moreper pound, Diallo and his co-workers can make their lessperfect structures for $5-15 per pound. “A dendrimer wouldbe like a Mercedes; for a commercial application, we aredeveloping a Yugo with Mercedes performance,” he jokes.

Diallo’s resulting water-soluble dendrimers can be usedfor water recycling, for example, in a “polishing” step designedto be incorporated into existing treatment systems to removeperchlorate. The nanomaterials also can be recycled: onetailored dendrimer holds on to contaminants at pH 5 andreleases them at pH 9 for collection and disposal. Wheresuch pH-shifted recycling isn’t feasible, the particles andtheir tightly captured contaminants could be considereddisposable and encapsulated in wastewater treatment sludge,for example, Diallo says.

The possibilities seem endless to Diallo: dendrimers insidemicroparticles, for example, could be “loaded into a bedreactor like typical carbon” for water filtration, he says. Hewould like to look into membranes for groundwater treat-ment, particularly for concentrating perchlorate in place, oreven for removing uranium. More immediately, the den-drimers could find their way into household water filters atsmaller scales, Diallo hypothesizes.

But Diallo says his team has yet to put its products througha full battery of ecotoxicology tests. The fact remains thatthese nanoparticles could easily make it into the environmentby way of the waste stream or manufacturing processes, withas yet unknown impacts. Diallo suggests that industry mustdevelop nano-drug-delivery methods that are safe forpatients, and insights gleaned from medical uses may alsolead to ways to keep nanomaterials environmentally safe.He cites a cancer drug that one company reports is less toxicbecause its vehicle is a dendrimer targeted to a specifictreatment site, which is the only place where it will be active.

But although testing of dendrimers for medical uses maybe well under way, environmental testing is not as far along,and their environmental cleanup capabilities do not guar-antee dendrimers’ environmental safety. The end-groupchemistry controls the toxicity of dendrimers, as withpolymers in general, Diallo says, but multiple flavors meanmultiple possibilities for toxic effects, as well as for fate andtransport of these nanoparticles.

Environmental work, environmental contaminationProducts modified with nanomaterials have been on themarket for almost a decade now. But only last year didresearchers from the Swiss Federal Institute of AquaticScience and Technology (Eawag) and the Swiss FederalLaboratories for Materials Testing and Research (Empa)report what may be the first detection of an engineerednanoparticle in the environment, an endeavor akin to lookingfor 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 micro-organisms, replacing organic biocides that keep buildingsurfaces clean. And TiO2 photochemistry also breaks downparticulates, which means that nano-TiO2-coated windowscould eventually clean a city’s air, conjectures Bernd Nowack

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Nanocomposites containing boride nanoparticles couldmake super-hard cutting tools and super-smooth hydraulicpumpsssaving energy, water, and materials.

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of Empa. But Nowack emphasizes the trade-offs: even as fewerbiocides are used, the nanoparticles washing off these windowsand other coated surfaces into storm drains, streams, and riversmight pose problems for fish and other organisms.

The impacts of traditional chemicals, often made to targetspecific biological receptors in plants and humans, insects, orother creatures, stand as examples of what unexpected damagenanomaterials could do, notes Nora Savage of EPA. They alsoillustrate another problem, she cautions: “Mixtures of com-pounds are what’s going to be critical here.” Once in theenvironment, these substances do not act alone. No one knowsyet what nanomaterials will do in the presence of otherchemicals, or if they might heighten other chemicals’ risks.

Savage has coauthored previous dendrimer research withDiallo; she also administers EPA STAR grants for research onboth environmental benefits and impacts of nanomaterials.“I know people are trying to design environmentally benignnanomaterials,” she says, “but all toxicity tests to date showthat behaviors change with agglomeration, as coatingsdegrade, [and so on]. As they end up in the water, it’s goingto be much more complex.”

Paradigm continues to shiftSo far, for either good or bad nano-impacts, “the scientificdata don’t exist,” says Nowack. Only a handful of life-cycle

analyses have been conducted for any nanomaterials, in partbecause of the lack of data.

“We can’t just look at one part” of the existence ofnanomaterials, whether it’s the manufacturing, use, ordisposal stage, says Karn. Common materials can be “slicedand diced” until they “behave differently when they comein contact with the biosphere. We have reason to believethat there are some causes for concern, but we don’t knowenough.”

Karn hopes to see progress in the use of nanomaterialson multiple frontssas both catalysts and solvent removersin manufacturing processes, as biosensors to detect con-taminants in the environment, and more. But she sees noguarantees that inventors and producers will embrace thenew paradigm. For those who accept the challenge, Karncalls the green nanotechnology revolution an opportunityfor “a fresh way of designing new products, with theenvironment and sustainability in mind.”

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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 springACS meeting, March 22-26, 2009, in Salt Lake City, Utah.

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