Magnetic Microbes Show a Knack for Nano-Engineering

Going my way? Aligned carbon nanotubes (streaks) grow from the magnetic crystals produced within the bacterium Magnetospirillum magnetotacticum (inset).

In an odd confluence of bio- and nanotechnologies, researchers have employed bacteria to accomplish a task much harder than it sounds: Lining up nanometer-sized tubes of carbon on the surface of a silicon chip. That's one small step for microbe-kind, but such biological builders might someday help assemble nanotubes into electrical circuits or miniscule motors and other machines.

Resembling miniature rolls of chicken wire, nanotubes are super-strong and can conduct electricity much like the materials in microchips. To be useful as circuits, however, scientists must control the placement and alignment of individual tubes. But even producing tubes that lie parallel on a surface is a big challenge. The techniques are difficult and expensive, and none works in all situations.

However, bacteria can line up the tubes relatively easily and inexpensively, report physical chemist Jong-in Hahm and colleagues at Pennsylvania State University in University Park. Hahm and her team knew that the bacterium Magnetospirillum magnetotacticum naturally contains nanometer-sized crystals of iron oxide that enable the elongated microbe to sense the Earth's magnetic field and line up with it. The magnetic nanoparticles might also serve as the seeds from which to grow aligned nanotubes, the researchers reasoned.

To test the idea, the team covered silicon surfaces with the bacteria and let them sit overnight. Some samples were placed in a weak magnetic field to align the bacteria's magnetic particles, while others were not. The researchers then washed the silicon surfaces with distilled water, which ruptured the bacteria and exposed the iron oxide crystals. The team grew nanotubes on the nanoparticles and gauged their direction with a device called an atomic force microscope. The samples that had been exposed to a magnetic field produced parallel nanotubes, while the other samples produced nanotubes oriented at random, the researchers report online this week in Applied Physics Letters.

Opinions vary about the utility of the technique. There are other ways to synthesize magnetic nanoparticles, says Charles Lieber, a chemist at Harvard University in Cambridge, Massachusetts, so the new approach may be needlessly complicated. But Wolfgang Sigmund, a materials scientist at the University of Florida in Gainesville, says that the bacteria are easy and cheap to grow and produce nanoparticles of a very uniform size and structure. Still, he notes, a micron-sized bacterium cannot deposit the nanoparticles with nanometer precision. "A bacterium is way too big," he says. "You need a virus."

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