Jul 17 2009
Scientists at DuPont and Lehigh University have refined a technique, first published in 2003, to sort carbon nanotubes using specific sequences of DNA. This technique offers the first demonstration that nanotubes can be sorted by size, property and symmetry (chirality).
This new finding, reported in the current issue (Vol. 460 No. 7252) of the journal Nature, is titled "DNA Sequence Motifs for Structure-Specific Recognition and Separation of Carbon Nanotubes." The study was co-authored by DuPont researchers Ming Zheng and Xiamin Tu, with Lehigh University professor of chemical engineering Anand Jagota and student Suresh Manohar. The research was funded by a National Science Foundation grant to a collaborative team from Lehigh University, MIT and DuPont.
There has been great interest in the revolutionary electrical, mechanical and thermal properties of single walled carbon nanotubes (SWNTs) since their discovery in the early 1990s. However, single walled carbon nanotubes are produced as complex mixtures of different nanotube species with different properties, greatly limiting their applications. In 2003, a publication in Science by DuPont scientists, including Zheng, disclosed a method to separate carbon nanotubes using DNA. This was the first demonstration that the problem of sorting SWNTs could be solved. DuPont has continued to investigate these materials, most recently publishing a chemical approach to separating metallic and semi-conducting nanotubes in the Jan. 9 edition of Science. The current development is a significant advancement in this pioneering field, perfecting the only approach that uses biological molecules to carry out a refined sorting of carbon nanotubes, separating nanotubes with different optical, electronic and chemical properties.
"Our technique is similar to sorting snowflakes by wrapping DNA around each flake," Zheng said. "Nanotubes come in many sizes and designs, and each type offers unique properties for uses that can range from transistors for electronics, light sources for displays or conducting films for photovoltaic materials. The difficult part of our approach is identifying which DNA sequence is most efficient at separation. Our approach was a bit like probing into the DNA library to determine sequences. Through this approach we tried over 350 sequences and identified more than 20 that showed useful separation properties."
During the 18-month research program, Zheng and Tu set the course for the experimental work to identify the DNA sequences, and Jagota and Manohar developed the molecular models. The approach builds on the 2003 findings that a DNA sequence will wrap around a SWNT and then interact with micro-size beads in an anion exchange chromatography set-up in a way that depends on the type of nanotube to which the DNA is attached. This occurs because the carbon nanotube-DNA hybrids have different electrostatic properties that depend on the nanotubes' diameter and electronic behavior. The latest study has determined that the interaction is dependent on both the type of nanotube and the type of DNA. As a result, the research team focused on identifying the DNA sequences that performed the best with their corresponding SWNT species. The DNA library is vast, making the chance of finding these sequences through trial-and-error exceedingly low. The research team identified an approach called "sequence expansion" to systematically explore the DNA library in a confined and progressive manner. The result was the identification of more than 20 DNA sequences that reacted favorably with 12 species of nanotubes, sorting them with purity level of 80 to 90 percent.
"We are at a historic moment when biology and materials science meet at the nano meter scale, and this opens up lots of opportunities for new science and technology development," Zheng said. "We think this is the ultimate solution to isolate and identify every species of nanotube, allowing us to take advantage of the highest performance nanotube to create high performance nano-electronic and nano-photovoltaic materials and devices."
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