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Abstract:
DNA, a molecule famous for storing the genetic blueprints for all living things, can do other things as well. In a new paper,* researchers at the National Institute of Standards and Technology (NIST) describe how tailored single strands of DNA 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.

Armchair Science: DNA Strands That Select Nanotubes Are First Step to a Practical ‘Quantum Wire’

Gaithersburg, MD | Posted on August 3rd, 2011

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."

* X. Tu, A.R. Hight Walker, C.Y. Khripin and M. Zheng. Evolution of DNA sequences towards recognition of metallic armchair carbon nanotubes. J. Am. Chem. Soc., Just Accepted Manuscript, Web publication: July 21, 2011.

** From the distinctive shape of the edge of the cylinder.

*** X. Tu, S. Manohar, A. Jagota and M. Zheng. DNA sequence motifs for structure-specific recognition and separation of carbon nanotubes. Nature, 460, 250-253, July 9, 2009.

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About NIST
The National Institute of Standards and Technology (NIST) is an agency of the U.S. Department of Commerce.

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Doctor Robert Morris may have illustrated more about the scientific world than he intended, in last Wednesday’s lecture, “Chemists Have Solutions.”

There’s a divide amongst scientists who want to get the public to recognize the value of what they do. (Read this book review of Don’t be SUCH a Scientist for a more detailed discussion.) Some like to use a lot of pizazz when they talk to a general audience. Others prefer to lay out all the detailed scientific information and “let the facts speak for themselves.” The criticism of the “pizazz” approach is that it often means using flashy graphics and doing a lot of pep talking at the expense of some scientific detail. On the other hand, a dry recitation of every pertinent fact tends to put an audience to sleep, and their appreciation for the value of the work is not enhanced.

As he presented his lecture at the Toronto Reference Library, Morris was clearly aware that you have to do something to grab your audience’s attention, so they’re keen to hear what you’ll say next. He started by mixing three liquids in a lab flask, promising an interesting result after it “cooked” for a while. And he made liberal use of slides, and interspersed the occasional trivia question, making a point of letting a child in the front row give one of the answers.

Yet for all that, Morris is still clearly of the “lay out all the facts” school. Most of the first half hour was about the Chemistry Department at the University of Toronto. We heard how many faculty there were, how many grad students they teach, how many awards they had won, where they were ranked compared to other university Chemistry departments…we heard it all. And then, since it’s the International Year of Chemistry, we also heard about and saw slides from events held several months ago. Yes, it’s really too bad the Toronto press didn’t properly publicize or cover those events, but they really weren’t that interesting — in the present — to an audience  for whom attendance was now impossible.

I suspect what the public really wanted to hear about was new chemical breakthroughs that allowed the production of cool new gadgets or processes. Forget how many grad students are in the university department! Show us that piece of cloth made with nanotechnology, which repels dirt and water and can never get soiled!

Morris did actually show slides of a couple of those breakthroughs, later on. The most intriguing was the Lab on a Chip — a tiny computer chip that can do medical analysis of an even tinier drop of blood placed on it. When that chip becomes common, no lab tech will ever take four vials of blood from us again, for tests. And instead of waiting five hours for results, we’ll have them in twenty minutes. We also learned about a newly developed process that can send antibodies into a person with leukemia, to target the specific family of blood cells causing the disease.

And that liquid compound Morris had made at the beginning? It provided a “traffic light” type of display. It had turned to amber while it sat, but with a little shaking, it turned red. And with much more shaking, it turned green. So there were a few of the “Wow, that’s cool”!” moments. Just not enough of them.

Doctor Morris was a great guy who was clearly excited about the work he and the other faculty and grad students were doing. His department deserves its high world rank, and is making a significant contribution in many areas. But in trying to get a general audience excited about chemistry, he may not be the person to grab their attention and keep it.

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