Wrapped up in their work: Molecular model shows a single-strand DNA molecule (yellow ribbon) coiled around an “armchair” carbon nanotube. Source: Roxbury, Jagota/NIST
Researchers at the National Institute of Standards and Technology (NIST) have tailored single strands of DNA that can be used to purify the highly desired “armchair” form of carbon nanotubes. Armchair-form single wall carbon nanotubes are needed to make “quantum wires” for low-loss, long distance electricity transmission and wiring.
Rice University uses an alternative approach by amplifying the growth of desired nanotubes. Armchair quantum wire is probably in milligram non-pure quantities now and the breakthrough may bump it up to semi-pure gram or kilogram quantities in 5 years. Rice University has 90% purity. The combination of the two approaches could achieve higher purity.
The armchair carbon nanotube is an ideal system to study fundamental physics in one-dimensional metals and potentially a superb material for applications such as electrical power transmission. Synthesis and purification efforts to date have failed to produce a homogeneous population of such a material. Here we report evolutionary strategies to find DNA sequences for the recognition and subsequent purification of (6,6) and (7,7) armchair species from synthetic mixtures. The new sequences were derived by single-point scanning mutation and sequence motif variation of previously identified ones for semiconducting tubes. Optical absorption spectroscopy of the purified armchair tubes revealed well-resolved first- and second-order electronic transitions accompanied by prominent sideband features that have neither been predicted nor observed previously. Resonance Raman spectroscopy showed a single Lorentzian peak for the in-plane carbon–carbon stretching mode (G band) of the armchair tubes, repudiating the common practice of using such a line shape to infer the absence of metallic species. Our work demonstrates the exquisite sensitivity of DNA to nanotube metallicity and makes the long-anticipated pure armchair tubes available as seeds for their mass amplification.
Single-wall carbon nanotubes are usually about a nanometer in diameter, but they can be millions of nanometers in length. It’s as if you took a one-atom-thick sheet of carbon atoms, arranged in a hexagonal pattern, and curled it into a cylinder, like rolling up a piece of chicken wire. If you’ve tried the latter, you know that there are many possibilities, depending on how carefully you match up the edges, from neat, perfectly matched rows of hexagons ringing the cylinder, to rows that wrap in spirals at various angles—“chiralities” in chemist-speak.
Chirality plays an important role in nanotube properties. Most behave like semiconductors, but a few are metals. One special chiral form—the so-called “armchair carbon nanotube”**—behaves like a pure metal and is the ideal quantum wire, according to NIST researcher Xiaomin Tu.
Armchair carbon nanotubes could revolutionize electric power systems, large and small, Tu says. 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, as the authors note, has been “an elusive goal.”
Separating one particular chirality of nanotube from all others starts with coating them to get them to disperse in solution, as, left to themselves, they’ll clump together in a dark mass. A variety of materials have been used as dispersants, including polymers, proteins and DNA. The NIST trick is to select a DNA strand that has a particular affinity for the desired type of nanotube. In earlier work,*** team leader Ming Zheng and colleagues demonstrated DNA strands that could select for one of the semiconductor forms of carbon nanotubes, an easier target. In this new paper, the group describes how 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 Zheng, “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. As an example, some optical features once thought to be indicative of metallic carbon nanotubes are not present in these armchair samples.”