Carbon nanotubes come in all shapes and sizes. As with many other materials, properties of carbon nanotubes depend upon the geometric arrangement of the constituent atoms. You may recall the paradigm example of graphite versus diamond: even though both are composed of pure carbon, the former is soft, opaque, and a good conductor while the latter, in addition to being a girl’s best friend, is hard, transparent, and a good insulator .
Most single-walled carbon nanotubes (SWNT), which are about 1 nm in diameter but can extend for thousands of nanometers, behave like semiconductors. However, a useful form called the “armchair carbon nanotube” behaves like a pure metal and, according to a National Institute of Standards and Technology (NIST) announcement, has the potential to:
…revolutionize electric power systems, large and small…Wires made from them are predicted to conduct electricity 10 times better than copper, with far less loss, at a sixth the weight. But researchers face two obstacles: producing totally pure starting samples of armchair nanotubes, and “cloning” them for mass production. The first challenge…has been “an elusive goal.”
That may change due to developments in medical technology – specifically, in our ability to manipulate DNA. NIST scientists recently reported in the Journal of the American Chemical Society their ability to purify certain forms of carbon nanotubes using specific DNA sequences. For example, they found that the sequence TTATTATTATTATT could isolate (8,3) nanotubes with some metallic contamination, whereas the sequence ATTAATTAATTAAT allowed for the purification of (6,6) armchair tubes. Using trial-and-error, they:
…methodically stepped through simple mutations of the semiconductor-friendly DNA to “evolve” a pattern that preferred the metallic armchair nanotubes instead.
“We believe that what happens is that, with the right nanotube, the DNA wraps helically around the tube,” explains Constantine Khripin, “and the DNA nucleotide bases can connect with each other in a way similar to how they bond in double-stranded DNA.” According to [Ming] Zheng [, the team leader], “The DNA forms this tight barrel around the nanotube. I love this idea because it’s kind of a lock and key. The armchair nanotube is a key that fits inside this DNA structure—you have this kind of molecular recognition.”
Once the target nanotubes are enveloped with the DNA, standard chemistry techniques such as chromatography can be used to separate them from the mix with high efficiency.
“Now that we have these pure nanotube samples,” says team member Angela Hight Walker, “we can probe the underlying physics of these materials to further understand their unique properties.”
Flashback:DNA-decorated Carbon Nanotubes …