A Biological Solution
Researchers harness bacteria to produce uniformly sized magnetic nanoparticles.
Magnetic nanoparticles for refrigerator magnets and computer memory are still being made the old-fashioned way. The mining, milling and grinding technologies of the 1920s continue to be applied to iron to produce magnetic particles ranging in size between 10 and 100 nanometers, or 1/500th the width of a human hair.
A strain of bacteria identified nearly 15 years ago by ORNL's Tommy Phelps can churn out a high yield of zinc-doped, iron oxide nanoparticles in the size range of 20 to 30 nm. By altering the chemical environment in which the bacteria are grown in a bioreactor and by feeding the microorganisms the correct energy source, researchers can induce the biological production of nanoparticles with a uniform size and desirable magnetic properties.
"We can produce zinc-doped, magnetite nanoparticles in large, scalable batches," Phelps says. "These magnetic powders can be used for coatings, stronger magnets, hand-held battery drills with direct-current motors, and magnetic media for data storage for computers."
Customers who want zinc-doped, magnetite nanoparticles in the size range of 20 to 30 nm may be surprised to learn that the powder is up to 100 times less expensive if produced by ORNL's NanoFermentation™ process instead of the conventional method. Conversely, those content with powders of various sizes in the range of 50 to 100 nm will find the milled and ground particles more economical. Internally funded ORNL research is directed at improving the economics of NanoFermentation™.
Other potential applications of NanoFermentation™ powders are ferrofluids, which can be used for brakes to securely hold suspended airplanes on aircraft carriers; catalytic iron nanoparticles, which offer an extremely high surface area and, therefore, more chemical reactivity; and water treatment in which specially coated magnetic particles theoretically attract contaminants and the loaded particles are recovered by a magnetic field.
In 1992, while examining core samples obtained from a depth of 10,000 feet by a natural-gas drilling project in Virginia, Phelps discovered a new strain of the thermophile Thermoanaerobacter ethanolicus. This species of bacteria, which thrives at temperatures of 60 to 70°C, was first discovered in the 1970s at Yellowstone National Park in Wyoming and later in 1994 at the Piceance Basin in Colorado. The three strains of the same species exhibit slight metabolic differences. For example, the Oak Ridge strain cannot use hydrogen as an energy source, but hydrogen can be fed to the Piceance Basin strain.
Thermoanaerobacter ethanolicus produces ethanol and acetic acid as waste products. Phelps believes an excess of these products is toxic to the species, so the bacteria limit their waste production by donating electrons to iron or other metals in their environment, forming nanoparticles of iron compounds.
Bob Lauf, a materials scientist and inventor, has collaborated with Phelps on the biological production of iron oxide (Fe3O4) doped with eight different metals, including chromium, cobalt, manganese, nickel, lanthanides (rare earths) and zinc. By growing the bacteria in a solution containing zinc, the microorganisms incorporate the metal into the magnetite structure, changing its chemical formula. Some of the iron atoms bonded to oxygen atoms are replaced by zinc atoms. ORNL researchers Bryan Chakoumakas and Claudia Rawn used neutron scattering to verify that zinc atoms are part of the magnetite structure rather than on the particle surface, as some people believe.
ORNL's Lonnie Love and Adam Rondinone measured the changes in magnetization manifested by nanoparticles doped with different metals. The researchers found different techniques for changing the size and shape of magnetic nanoparticles. Several patent applications have been filed, and at least 15 scientific papers on the bacteria and their products have been published.
Compared with mining, milling and grinding, Phelps sees NanoFermentation™, which received R&D magazine's inaugural MICRO/NANO 25 award and an R&D 100 Award in 2006, as a "green" process that does not harm the environment and uses much less labor and energy and fewer hazardous solvents. He envisions this new way of fashioning nanoparticles as a 21st century nanomanufacturing technology.
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