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HOW TO MAKE THE INSOLUBLE SOLUBLE
Attaching large organic groups is key to unclumping nanotubes
If only they would swim freely in solution, nanotubes might easily realize their enormous potential for electronics and materials. But the chicken-wire-like carbon cylinders are notoriously insoluble: They tend to clump together in unmanageable globby ropes.
Chemists have tried to address the problem with everything from brute force--where tubes are throttled loose with high-intensity ultrasound in a process known as sonication--to functionalization with chemical groups that keep the tubes from associating.
The functionalization approach gets a dramatic boost with new work by organic chemistry professor Maurizio Prato and colleagues at the University of Trieste, in Italy; the University of Notre Dame; and the University of Erlangen-Nürnberg, in Germany. The group reports that a method they've developed for attaching organic groups to nanotubes makes the tubes soluble to the tune of 50 mg per mL--much higher than previously reported. In addition, the researchers say, the tubes remain in solution in water and a number of organic solvents indefinitely [J. Am. Chem. Soc., 124, 760 (2002)].
"I've never seen [nanotubes] with a solubility as high as this," says chemistry professor James M. Tour at Rice University. "That's really amazing."
Several years ago, Prato and his group developed a method that's been widely used to functionalize fullerenes, which they have now extended to nanotubes. This so-called Prato reaction generates reactive azomethine ylides that can attack the nanotubes through the condensation of an a-amino acid and an aldehyde.
The nature of the attached groups, which are quite large, is likely key to solubility, as the same reaction done with small organic groups yielded nanotubes that were not very soluble, Prato says.
Tour and his group are now trying to replicate the new work, in part for "the joy of saying, 'Voilà! We've got it, too.' "
FUNCTIONALIZED Carbon nanotubes with organic appendages are highly soluble.
COURTESY OF MAURIZIO PRATO & PAOLO BRAIUCA
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