Posted: June 20, 2008

Selection of carbon nanotubes with specific chiralities demonstrated

(Nanowerk News) A chemistry professor in the College of Liberal Arts and Sciences at the University of Connecticut and his graduate students have published new results in Nature Nanotechnology showing how they isolated a particular type of carbon nanotube from a sample and manipulated it in a way that could have broad applicability in drug and gene delivery, electronic devices, and nanotechnology research ("Selection of carbon nanotubes with specific chiralities using helical assemblies of flavin mononucleotide").
Fotios Papadimitrakopoulos and his graduate students found a way for a biological molecule, a form of vitamin B2, to wrap around a single-walled carbon nanotube – a tube so small that it has the highest curvature on earth.
Wrapping a carbon nanotube was a difficult achievement and instrumental to their research, since it was a step that eventually enabled them to isolate a particular type of nanotube from a sample that contained 50 different kinds.
Papadimitrakopoulos has spent seven years investigating how to efficiently separate the various nanotubes in a sample into like types.
Nanotubes that are alike can be interlocked to create a material that is extremely strong, even if each nanotube is as small as one micron.
Homogenous nanotubes also have the same electrical and optical properties, and they form a material that is extremely pure.
The research opens the possibility of wrapping nanotubes with proteins or other molecules, which would be useful in a variety of applications.
“We have learned how to manipulate this molecule,” says Papadimitrakopoulos.
The lead author of the Nature Nanotechnology paper is Sang-Young Ju, a polymer science Ph.D. candidate in his fifth year of study. Other authors are Jonathan Doll, a fourth-year polymer science Ph.D. student, and Ity Sharma, a second-year chemistry Ph.D. candidate .
Two undergraduates, William Kopcha, CLAS ’08, a chemistry major, and Christopher Badalucco, a junior majoring in physiology and neurobiology, also were involved in the research.
The researchers worked with single-walled carbon nanotubes formed from graphene. If you drag a pencil across paper, Papadimitrakopoulos says, you leave thousands of graphene “seeds” behind, a deposit from the friction of the graphite pencil tip against the paper.
At the molecular level, graphene seeds look like a honeycomb. If you form these graphene sheets into a tube, they can become the basis of single-walled carbon nanotubes.
Getting another material to wrap around them was the next challenge.
The researchers discovered that the vitamin B2 molecule stitches itself into a ribbon, using soft hydrogen bonds, and seamlessly wraps itself around the carbon nanotube. The ribbon, in a sense, acted as a detergent, dispersing the oil-loving nanotube in water.
“Nobody has shown this before,” says Papadimitrakopoulos.
By introducing a second detergent, they managed to destabilize the ribbon, breaking its hydrogen bonds and leaving the second detergent in its place.
Varying the concentration of the second detergent allowed them to separate nanotubes that had a given chirality, or pitch.
Identifying carbon nanotubes of like chirality, or pitch, has important implications.
If the chirality is the same, the nanotubes have the potential to interlock themselves in a hexagonal pattern and create an extremely strong material, even if the nanotubes are not very long.
Papadimitrakopoulos says that this is an important step toward minimizing the potential negative health impact of carbon nanotubes, which recently were associated with asbestos-like contamination in the lung linings of laboratory animals.
In that recent study, it was shown that carbon nanotubes larger than 20 microns behaved like asbestos, while those smaller than 20 microns could be cleared out of the lungs, much like pollen.
The carbon nanotubes that his research group works on are far smaller, at approximately one-micron in length.
Carbon nanotubes began to receive widespread attention in 1991, but it is only in the past 10 years or so that research on their applications has heated up.
Nanotubes are small, strong, and special because of their potential for use in drug delivery and electronics applications.
Some have described carbon nanotubes as the reigning celebrities of the advanced materials world. Papadimitrakopoulos describes them as the “Cinderella” molecules of nanotechnology.
Hydrocarbons can be burned and still be used to make strong materials, he notes. Carbon is inexpensive, and carbon nanotubes can transform products, making stronger tennis rackets or bullet-proof vests, for example.
The Air Force, which funds his research, is interested in advanced materials that are light, strong, and can withstand high temperatures, he says. In the future, he predicts, planes will be made from carbon nano-fibers.
Papadimitrakopoulos is a chemistry professor in CLAS, but his work is interdisciplinary, involving physics as well. He also serves as the associate director of the Institute of Materials Science and is a member of the Polymer Program.
Papadimitrakopoulos says his research could not have proceeded without the use of a high resolution transmission electron microscope, which allowed his research group to confirm and verify visually that the B2 molecule was wrapping around the carbon nanotube.
Source: University of Connecticut (Cindy Weiss)
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