The Nano Sorter

You think it’s hard keeping your tube socks organized? Try sorting carbon nanotubes, those remarkable molecules whose electrical properties make them potential building blocks for everything from ultrasensitive diagnostic devices to transistors 100 times smaller than those in today’s fastest microchips. Trouble is, when nanotubes are fabricated, they’re a mixed bag; some are electricity conductors, while others are semiconductors. Since a number of practical electronics applications demand nanotubes of uniform conductivity, sorting technologies are needed.

Researchers at DuPont in Wilmington, DE, say they’re beginning to solve the problem using another remarkable molecule: DNA. The results are literally visible. A pink-colored vial of nanotubes in solution contains highly conducting nanotubes; other vials, with greenish hues, hold semiconducting ones. “One of the central goals of the field at the moment is to separate nanotubes, because there are applications where having mixtures of semiconducting and metal versions is a real hindrance,” says R. Bruce Weisman, a chemist and nanotube researcher at Rice University. “If they’ve got vials of separated nanotubes, that is a big result.”

The DuPont researchers found that single-stranded DNA tends to wrap around the nanotubes, forming a stable structure. To enlist this property to sort nanotubes, they engineered DNA to selectively attach to nanotubes with specific conductivities. Then they used standard lab techniques to separate the DNA-nanotube hybrids according to the natural charge of the DNA, which is different for different sequences. The attached nanotubes go along for the ride.

The company has several goals in mind, says Tim Gierke, a chemist at DuPont, which published the results in the journal Nature Materials. First, the researchers plan within a year or so to use DNA-nanotube hybrids in prototype sensors for medical diagnostics. For this application, they would marry nanotubes with DNA sequences that bind to complementary sequences of a pathogen’s DNA; the nanotube would register the binding electrically. Longer term, sorted nanotubes-after being heated to unravel the DNA-nanotube hybrid-could be used as switches or other elements in molecular electronic devices. DuPont might even find itself becoming a supplier of custom batches of nanotubes. “We haven’t sorted out just exactly how we want to consider that business opportunity,” Gierke says. But at least the nanotubes themselves are now beginning to get sorted.