Although carbon nanotubes have been proposed for myriad applications - from miniature motors and chemical sensors to molecule-size electronic circuits - their actual uses have been severely limited by an inability to isolate and manipulate nanotubes having different characteristics.

Now, researchers at the University of Illinois at Urbana-Champaign and at Rice University have discovered a way to chemically select and separate carbon nanotubes based on their electronic structure. The new process also represents a fundamental shift in the way scientists think about the chemistry of nanotubes.

"Separating nanotubes based on their electronic properties has been a long sought goal of the carbon nanotube community," said Michael Strano, a professor of chemical and biomolecular engineering at Illinois and lead author of a paper to appear in the Sept. 12 issue of the journal Science.

"In addition to presenting a broad class of materials, nanotubes tend to stick together and form bundles, making it even more difficult to separate and use them."

Several years ago, Strano and co-workers at Rice developed a technique for breaking up the bundles and dispersing the nanotubes in water laced with a surfactant.

In the present work, Strano and his graduate students, Monica Usrey and Paul Barone, teamed up with organic chemist James Tour at Rice and his postdoctoral researcher Christopher Dyke to apply reaction chemistry to the surfaces of nanotubes in order to select metallic tubes over the semiconductors.

To control nanotube chemistry, Strano and his colleagues add water-soluble diazonium salts to nanotubes suspended in an aqueous solution. The diazonium reagent extracts an electric charge and chemically bonds to the nanotubes under certain controlled conditions.

By adding a functional group to the end of the reagent, the researchers can create a "handle" that they can then use to selectively manipulate the nanotubes. There are different techniques for pulling on the handles, including chemical deposition and capillary electrophoresis.

"The electronic properties of nanotubes are determined by their structure, so we have a way of grabbing hold of different nanotubes by utilizing the differences in this electronic structure," Strano said. "Because metals give up an electron faster than semiconductors, the diazonium reagent can be used to separate metallic nanotubes from semiconducting nanotubes."

The chemistry is reversible, Strano said. After manipulating the nanotubes, the scientists can remove the chemical handles by applying heat. The thermal treatment also restores the pristine electronic structure of the nanotubes.

"Until now, the consensus has been that the chemistry of a nanotube is dependent only on its diameter, with smaller tubes being less stable and more reactive," Strano said. "But that's clearly not the case here. Our reaction pathways are based on the electronic properties of the nanotube, not strictly on its geometric structure. This represents a new paradigm in the solution phase chemistry of carbon nanotubes."

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