Technology▼Solutions A greener route to gold nanoparticles Hutchison is but one of a number of researchers investigating ways to synthesize gold nanoparticles. However, he argues that his approach stands out both for being green and because the scientists are able to tightly control the size and chemical functionality of the synthesized particles so that they have the optimal properties for applications. Bio-based processes such as the use of alfalfa plants to generate gold nanoparticles, which has been demonstrated by a team led by Jorge GardeaTorresdey from the University of Texas at El Paso, are unquestionably greener methods, Hutchison acknowledges. But such approaches do not allow significant control over the size and functionality of the resulting particles, he says. Hutchison also admits that toluene is not the greenest of solvents, although he adds that substituting it for benzene is a maxim in green chemistry. But he also stresses that “green is a direction, not a destination,” an aphorism he attributes to Anastas. Hutchison says that he is actively researching how toluene might be eliminated by using even greener solvents in the process. Even in its present form, the new approach is safer and greener, “but the
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James E. Hutchison has been part of what’s becoming known as the “green nano” movement since before either term was a buzzword. Hutchison has been working on a greener method for synthesizing gold nanoparticles for nearly a decade, and he received a patent on the process last year. He and his colleagues at the University of Oregon are currently making the technology available to other noncommercial researchers in what could be termed a “field of dreams” approach to product development. A traditional method of synthesizing gold particles in the 1.4–1.5-nanometer (nm) range relies on diborane, which is both toxic and highly flammable at room temperature, and benzene, a toxic solvent. The new approach uses sodium borohydride and toluene and “is much less dangerous than the conventional one,” says Hutchison, who is a professor of chemistry and directs the university’s Materials Science Institute. In fact, generating gold nanoparticles using the new method “is one of the first things I have undergraduates do when they come into my lab,” he says. The applications of nanotechnology involving green chemistry are important, says Barbara Karn, director of the U.S. EPA’s Technology for a Sustainable Environment program, which is run jointly with the National Science Foundation. “If nanotechnology is to realize its full promise and potential, it is essential that these next-generation materials be designed from the outset to be benign to human health and the environment. The excellent work by Jim Hutchison not only lays out the conceptual template for how to design green chemistry nanosubstances but also puts these concepts into practice,” adds Paul Anastas, director of the Green Chemistry Institute, a nonprofit organization that is part of the American Chemical Society.
really cool part about it is that it gives much higher yields, is more rapid, and is significantly cheaper,” Hutchison says. Specifically, the traditional way of making these particles would result in 100–200 milligrams of material after nearly a week’s worth of work. Hutchison’s new method can be used to synthesize a gram or more in a day. A gram of material may not sound like much, but Hutchison argues persuasively that the process is already at the prototype stage and may never be scaled up to a larger reactor. “One of the things that is catching on throughout the country is [using] microreactors to make nanoparticles on a large scale. . . . What microreactors offer is exceptional control over mixing, and [good control] over the kinetics of the nanoparticle formation.” According to this new thinking, once scientists have demonstrated that kind of tight control with a microreactor, they’ve made a prototype. Scaling up involves construct ing a reactor with millions or billions of parallel channels, what engineers call numbering up, he says. Hutchison is working with engineers at Oregon State University to develop such multichanneled microreactors through the Oregon Nanoscience and Microtechnologies Institute (ONAMI), a research center with the
The gold nanoparticles that Jim Hutchison and his colleagues of the University of Oregon have synthesized are setting the stage for a new generation of electronics devices. The scientists have already overcome an important hurdle by demonstrating that the particles can self-assemble into chains of up to 1 micrometer in length.
104A ■ ENVIRONMENTAL SCIENCE & TECHNOLOGY / MARCH 1, 2005
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goal of accelerating “innovationbased economic development” in the Pacific Northwest that also counts the Pacific Northwest National Laboratory and Oregon Health and Sciences University as members. Gold nanoparticles have historically been used in colorimetric sensors such as those used for pregnancy tests. More recently, a team of researchers led by Chad Mirkin of Northwestern University has shown how such particles can be used to identify the DNA sequences associated with disease, as well as bodily invaders like viruses, or even bioterror agents. Mirkin’s particles are a bit larger—approximately 10 nm—Hutchison says. However, Hutchison’s particles can also be used as “seeds” to generate larger particles that could be used for such applications, he says. Because Hutchison’s gold nanoparticles can serve as a core that can be combined with a wide variety of what he calls functionalized nanomaterials, they could be used to create a tool that performs both diagnostic and therapeutic functions. “A dream would be to introduce these nanoparticles into people and . . . to use the metal core as an imaging tool and use the ligand shell as a drug delivery vehicle,” Hutchison says. Hutchison’s gold nanoparticles could also serve as the basis for new technologies that will render obsolete the energy- and materials-intensive methods currently used to fabricate computer semiconductors from silicon (Environ. Sci. Technol. 2003, 37, 8A). “What got us interested . . . in the first place . . . [were] proposals that these materials would be excellent components of nanoelectronics,” he says. Motorola Corp. is already investigating how Hutchison’s nanoparticles could be used to further the company’s goal of developing a molecular electronics “toolbox” containing organic molecules, nanoparticles, and carbon nanotubes that “can be self-arranged into a very advanced information processing system,” according to a recent research paper published by the company’s scientists. But the realization of such applications is many years—if not decades—away. In the shorter term, Hutchison says that the gold nanoparticles can also work with existing silicon-based semiconductors. “A lot of people in the
microelectronics industry today are looking . . . to use their silicon-based technology as platforms for something a bit more sophisticated, such as gene chips.” To that end, he says the nanoparticles can be thought of as building blocks that can expand the sensitivity and selectivity of silicon semiconductors. There are many ways to try to popularize a new technology, but by actively encouraging other researchers to use his gold particles, Hutchison is taking the kind of “field of dreams” approach that has already proven effective in other areas of nanotechnology. “We have this patent and we have a really efficient process. The question then becomes, if [we make these materials] commercially available, would people try a whole lot of things and develop new technologies in all of those areas. . . . I don’t think that anyone knows the answer to that, but people are certainly doing this on the research level,” Hutchison says. He adds that having readily available sources of buckeyballs and buckeytubes greatly helped research based on those materials take off. Dozens of scientists in the United States and Japan are already studying the electronic properties of Hutchison’s particles in hopes of developing nanoelectronics, such as tiny transistors, or using them in catalysis. For example, a team led by James Grebinski of the University of Notre Dame has reported some success in using the particles to make nanowires. In response to the growing concerns over nanotechnology’s potential health and environmental ramifications, Hutchison is currently studying whether his gold nanoparticles could have unintended consequences. “There aren’t a lot of data out there. . . . We are actually embarking on a study to test some of these in a variety of assays, including zebra-fish larvae and fruit flies, and look at about 100 different compositions to see what structural factors might have impacts on those organisms.” He adds that he doesn’t expect any particularly high level of toxicity to be associated with the gold nanoparticles because gold and silver colloids have a history of use as medicines and natural therapeutics. But he also stresses that the smaller size merits investigation, as do the ligand shells that can be combined with the metal core. —KELLYN BETTS
MARCH 1, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 105A
