Planning Nanotech From The Ground Up

"An ounce of prevention is worth a pound of cure." That timeless admonition, attributed to Benjamin Franklin, could become a mantra for the fledgling nanotechnology industry, according to a developing community of chemists who advocate infusing green chemistry and engineering into the design of nanotech-based products. It's an opportunity to build the new industry to be environmentally clean from the beginning, they believe, by using pollution-prevention strategies to control risks.

"In the past, society did not always think about the consequences of introducing new technologies," observed Barbara P. Karn, an environmental scientist with the Environmental Protection Agency. "But we have come to a point in our civilization where we do think about it, where we have started to ask questions when there's a new technology. Is it harmful? Can the products be made without pollution? Do we have an adequate infrastructure to handle it? How will it affect society?"

Karn posed these rhetorical questions and brought up Franklin's sage advice during a "state of the science" overview of nanotechnology at the American Chemical Society's national meeting in Atlanta last month. Her presentation was part of a plenary session that kicked off a weeklong symposium on nanotechnology and the environment that was sponsored by the Division of Industrial & Engineering Chemistry (I&EC). The symposium included 50 talks over four days and anchored the meeting's nanotech-based programming in which 13 ACS divisions combined to host 175 sessions. This was the fourth ACS meeting to include sessions on nanotechnology and the environment, Karn noted, but it was the first time that the emphasis was on green nanoscience.

An ongoing concern in Congress is how a lack of risk studies regarding toxicity or persistence of nanomaterials in the environment is impeding its ability to make informed and timely policy decisions and establish a regulatory framework for nanotechnology, Karn said. Still to be determined is if new laws are required to cover some areas of nanotechnology that aren't addressed by current laws covering chemicals and workplace safety (C&EN, Jan. 30, page 34).

The focus of the I&EC symposium in Atlanta, though, was to approach nanotechnology from another viewpoint: By establishing a green foundation for the industry, potential health and environmental problems that could arise during the expected rapid growth of nanotechnology can be avoided. The goal, according to Karn and her green nanoscience colleagues, is to develop economically viable products and processes that require fewer reagents, less solvent, and less energy to produce, while being safer, generating less waste, and having a milder environmental impact than current technology.

"Nanotechnology promises to dramatically change the products we manufacture and the way we manufacture in virtually every area," Karn noted. "It offers the opportunity to make products greener from the beginning. We simply can't let this opportunity pass by."

Karn has led EPA's effort during the past five years to study the potential impact of nanotechnology on the environment. She currently also is a visiting scientist at the Woodrow Wilson International Center for Scholars in Washington, D.C., which launched a GreenNano initiative earlier this year. The initiative is part of the Wilson Center's Project on Emerging Nanotechnologies, which is developing a major report about how scientists and policymakers can apply green chemistry and engineering to nanotechnology development.

"Nano doesn't refer just to a single material or class of materials, it doesn't just include a single industry or industrial sector, and it converges with other technologies," Karn continued. And as new products are entering the marketplace, "we need to apply what we already know to try to do things right the first time."

About 230 consumer products currently have nanotech-based components, she noted, such as metal oxide nanoparticles, nanotubes, and fullerenes. These products include cosmetics and sunscreens (C&EN, March 27, page 47), paint, wrinkle-resistant clothing, and sports equipment. A nanotech glass-cleaning spray, for example, was recently recalled in Germany amid health concerns (see page 10).

Another 600 products, such as films, coatings, and fillers, are being used in electronics production, drug delivery technologies, and instrumentation for research, Karn said. And an effort is being made to gather data on early stages of nanotech-based food and agricultural science research (see page 31). The global marketplace for nanotech-based goods and services, already a multi-billion-dollar industry, is expected to grow significantly to more than $1 trillion annually and employ two million people by 2015, according to the National Science Foundation.

"Nanotechnology can provide new tools and applications to benefit society, including ones that both clean up and protect the environment," commented James E. Hutchison, one of the plenary speakers and a chemistry professor at the University of Oregon. "It's a two-prong design problem to make high-performance nanomaterials that are inherently safe," Hutchison said. That challenge makes it exciting for synthetic chemists, and students get excited about the opportunity to improve the quality of life through new products, he added. For industry, the excitement is in the ability to make products for a competitive advantage. "There's something for everyone to be excited about in green nanoscience," he said.

Hutchison elaborated by discussing research in his lab to develop new processes to make metal nanoparticles, which in turn can be used to fabricate electronic devices. In one example, he described an improved synthesis of 1.5-nm-diameter gold particles functionalized with triphenylphosphine groups. The phosphine groups can be readily replaced by various thiol groups, and the thiol-functionalized nanoparticles are capable of self-assembling with the help of DNA or other templates that control the shape and spacing of the nanoparticles (Inorg. Chem. 2005, 44, 6149).

The original synthesis involved reacting HAuCl₄ with triphenylphosphine in ethanol to form an intermediate complex, which was reduced by diborane gas (B₂H₆) in benzene solvent to form the gold nanoparticles, Hutchison explained. Both benzene and diborane are toxic and should be avoided if possible, he said. The process also is slow and labor-intensive: The product purification step requires a lot of organic solvent.

Hutchison's group devised a greener method that uses sodium borohydride (NaBH₄) instead of diborane and toluene instead of benzene. But because toluene doesn't dissolve sodium borohydride, the chemists used a two-phase toluene-water system in combination with tetraoctylammonium bromide as a phase-transfer catalyst. The new process reduces the cost of the gold nanoparticles from about $300,000 per gram in the case of a commercially available product to about $500 per gram. "That's economically competitive," he said emphatically.

Often the most wasteful steps in preparing nanomaterials, and in chemical synthesis, are the purification steps, Hutchison continued. For purification, the original method required about 15 L of solvent per gram of functionalized nanoparticles for several filtering and washing steps. Hutchison's group has developed an alternative system that utilizes nanoporous diafiltration membranes, which are tubes that contain a polymer film with nanoscale pores (J. Am. Chem. Soc. 2006, 128, 3190).

"For water-soluble nanoparticles, we no longer need to use organic solvents for purification, and it's much faster," he said. In addition, the method can be used as a type of size-exclusion chromatography to provide very pure nanoparticles with a uniform size. As an example, he described experiments to efficiently separate mixtures of 1.5-nm particles and 3-nm particles.

"There are three take-home messages about green nanotechnology," Hutchison concluded. "This is a great opportunity to design and manufacture new products right the first time. There's no compromise necessary: We can have the high performance and technological advantages that we want using less expensive, economically viable processes and materials that are greener and don't cause harm to human health and the environment. And there are real solutions that are available right now to make this happen."

Other scientists also offered innovations at the symposium.In two talks, Joseph M. DeSimone of the University of North Carolina, Chapel Hill, and North Carolina State University described the use of pourable perfluoropolyethers that can be photochemically cross-linked in a "solventless" process to form thin films. The fluorinated films offer better performance than polydimethylsiloxane polymers, which swell when exposed to organic solvents, he said. DeSimone described several applications for the perfluoropolyether films, including fabrication of improved proton-exchange membranes for methanol or hydrogen fuel cells and as molds to make shape-specific nanomaterials that can carry functional agents such as drugs (J. Am. Chem. Soc. 2005, 127, 10096).

Vicki L. Colvin of Rice University presented work on the role of surface chemistry in designing potentially nontoxic nanoparticles. Colvin's research on the in vitro properties of carbon and titania nanomaterials has revealed that the surface coating of the materials, not their size and shape, is the primary factor governing their biological activity.

Somenath Mitra of New Jersey Institute of Technology discussed a fast, energy-efficient microwave synthesis technique for making functionalized single-walled carbon nanotubes that have improved water solubility (J. Am. Chem. Soc. 2006, 128, 95). The microwave method, which heats nanotubes in acid solutions, is inexpensive and has a synthesis time of only a few minutes, compared with hours needed in traditional reflux methods. In a second talk, Mitra described a similar microwave method for preparing silicon carbide-carbon nanotube composites.

Wrapping up the plenary session, Paul T. Anastas, director of ACS's Green Chemistry Institute, spoke about the history, growth, and benefits of green chemistry and discussed the role of product life cycle analysis and risk assessment when it comes to nanotechnology.

"The old paradigm was that if you wanted to do something good for the environment, it was going to cost," he noted. "Or if you wanted to make money, you were going to have to do a little damage to the environment. Green chemistry is smashing that old paradigm."

Green chemistry is "not just an expression of a noble goal," Anastas added, but provides a framework for approaching the design of new materials. "When it comes to global sustainability, nanotechnology is going to play an important role," he said. "And for nano, green chemistry is going to be essential not only for doing the right thing, but for doing the right things right."