THE trouble with carbon nanotubes is that they are sticky and always clump together. Trying to pick out a single one to make, say, a single-molecule transistor, is a nightmare. But a simple way to pluck a nanotube from such a sticky thicket might be to snare it with another giant molecule: DNA.
Nanotubes are minuscule cylinders made of rolled-up, honeycomb-like sheets of carbon atoms. They are made by causing an electrical spark to jump between two carbon electrodes in a high-pressure carbon monoxide atmosphere. The tiny super-strong filaments left on the electrodes are the nanotubes. Some are single-walled and some multi-walled, with perhaps 12 single-walled tubes inside them.
But scientists face a big problem when it actually comes to using them: the clumps comprise nanotubes with very different properties and sizes that are nigh on impossible to separate. They have different properties because a sheet of hexagonally arranged carbon atoms can be rolled up in many different directions, just as a poster can be rolled up lengthwise, widthwise or diagonally. Different directions produce tubes with different electronic properties. Some single-walled nanotubes are rolled such that they have a pointy, zig-zag edge of carbon atoms around the end of the tube, for example, and these conduct like metals. Others have a hexagon on their ends and behave more like semiconductors (91av, 15 March, p 30).
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So engineers who want to use single-walled nanotubes in electronic circuits have to take pot luck and see what kind of tubes they end up with. “It has been hit and miss until now. That’s fine for demonstrations and proof of principle, but it’s not good enough for manufacturing,” says Ming Zheng, a research engineer at the chemical firm DuPont’s lab in Wilmington, Delaware.
To solve both problems, Zheng’s team tried dousing nanotubes with samples of single- stranded DNA heated to separate the two strands into one. Other researchers have shown that long polymer molecules will wrap themselves round nanotubes, so they were not too surprised that DNA would do the same. But they were surprised by how strong the interaction was: the strands preferred to stick to the tubes rather than to each other.
Zheng thinks that the DNA is attracted to the nanotubes by the one of same forces that keep a DNA double helix stable. Interactions between the electrons in the carbon rings of DNA bases mean that the rings tend to “stack” on top of each other in DNA molecules. The same kind of interaction appears to bind DNA to the carbon nanotubes. This strong affinity makes DNA the most efficient way to stop nanotubes from clumping, they will propose in a future issue of Nature Materials.
And it also provides a way to separate out nanotubes with different properties. Single strands of DNA carry a negative charge, so when a strand wraps itself around a metallic or semiconducting nanotube, some charge will leak away depending upon the conductivity of the tube. Once all the tubes are coated in DNA, it’ll be simple to separate them based on the different charges they carry, says Zheng.
Besides transistors, another putative use for nanotubes is drug delivery. So could their ability to bind to DNA pose a danger? Kevin Ausman, an environmental nanotechnologist at Rice University in Texas, doesn’t think so. Nanotubes have to be extremely pure to bind to DNA he says – and would be certain to react with other body fluids first, preventing them breaking up DNA strands.
