Single-walled carbon nanotubes (SWNTs), for all their remarkable and promising properties, will never totally fulfill their potential until an efficient way is found to manipulate and organize them into ordered arrays. One strategy that scientists have begun to explore is to attach organic molecules-- "handles"--to these tubular nanostructures in a noncovalent way, which preserves the nanotubes' networks--and thus their electronic characteristics.

	ANCHORED The pyrene group of a succinimidyl ester adheres to the nanotube wall, allowing proteins or other molecules to be attached at the ester function.

Preliminary success in using this strategy has now been reported independently by two research teams. One team--led by Hongjie Dai, assistant professor of chemistry at Stanford University--has found "a simple and general approach" for noncovalently anchoring aromatic molecules to the sides of SWNTs [J. Am. Chem. Soc., 123, 3838 (2001)]. The anchored molecules have a "tail" to which proteins or a variety of other molecules can be covalently attached. The end result is that these molecules are immobilized on the sidewall of the nanotube with high specificity and efficiency.

For the anchor, Dai and coworkers use a molecule containing a planar pyrenyl group that irreversibly adsorbs to the surface of a SWNT using -stacking forces. The molecule's tail is tipped with a succinimidyl ester group, which is readily displaced when an amine group attacks the ester function and forms an amide bond. The Stanford researchers have used this reaction to immobilize proteins, DNA, and smaller biomolecules on the nanotube sidewalls.

	MAYPOLE Molecular model shows how conjugated polymer would helically wrap itself around a carbon nanotube.

Dai believes that coupling the electronic properties of nanotubes with the specific recognition properties of the immobilized biomolecules could provide the ideal miniaturized sensor.

In addition, this approach to functionalizing nanotubes could be used to make the tubes assemble into larger, more complex architectures with their electronic properties intact.

Noncovalent functionalization of nanotubes also may solve another problem: SWNTs are notoriously insoluble. Researchers have been struggling to find ways to make them soluble or at least suspendable in solution.

The second team, led by chemistry professors J. Fraser Stoddart and James R. Heath of the University of California, Los Angeles, has tackled this problem by producing bundles of SWNTs that appear to have a conjugated polymer helically wrapped around them [Angew. Chem. Int. Ed., 40, 1721 (2001)].

The polymer they used is a poly(m-phenylenevinylene) substituted with octyloxy chains. By adding SWNTs to a solution of this polymer and sonicating the mixture, the UCLA chemists were able to produce a stable suspension of nanotubes. Using a variety of techniques, they studied both the suspension and single nanotube/polymer complexes that they isolated from it. They believe their experimental evidence shows that the polymer wraps itself around the SWNT bundles, with the phenylene rings and vinyl units of the polymer backbone hugging the nanotube surfaces, presumably as a result of - interactions.

Furthermore, the UCLA team finds that the polymer chain is in "intimate electrical contact" with the SWNT bundle. In fact, Heath tells C&EN, the nanotube/polymer complex behaves like a photoamplifier, producing a current of a thousand or more electrons for each photon the polymer absorbs. The results indicate that the electrical properties of SWNTs are "largely unperturbed by the associated polymer," the researchers note.

That's good because Heath, Stoddart, and their coworkers plan to graft molecular switches onto the polymer and then assemble the wrapped nanotubes, which serve as nanowires, into crossbar lattices in which switches are located between individual crossed nanowires. It's all part of their effort to build a nanoscale computer (C&EN, Oct. 16, 2000, page 27).

Stoddart is enthusiastic about the polymer approach, but he also thinks that Dai's pyrene-based "sticky labels" could benefit the UCLA project as an alternative way to attach molecular switches to nanotube wires.

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