Researchers have coopted DNA for a non-biological use -- sorting carbon nanotubes. A new study reports that synthetic DNA molecules can form paper-like sheets that can be used to separate nanotubes of different diameters, lengths, chiralities, and electronic properties.

The study reveals some of "the richness of the structural motifs that nucleic acids may have," said Ming Zheng, a biochemist and materials scientist at the DuPont Central Research and Development Experimental Station in Wilmington, Delaware, who led the research. "The structural variety of DNA is yet to be fully explored. There's a lot to be learned."

A nanotube wrapped by two hydrogen bonded strands of DNA Image: Xiaomin Tu, DuPont

Carbon nanotubes have been heralded as a wonder-material with superb mechanical and electrical building properties. But current synthesis techniques create non-uniform amalgams of nanotubes of different shapes and sizes, which hampers their large-scale utility. The new DNA molecules provide an efficient way to segregate the tiny carbon rods. "This is a seminal contribution," Michael Strano, a Massachusetts Institute of Technology chemical engineer who was not involved in the findings, told The Scientist. "This approach is the most comprehensive sorting that's been demonstrated."

Seven years ago, Zheng, Strano, and others first showed that single-stranded DNA could be wrapped around carbon nanotubes in a sequence-specific fashion and used for some crude nanotube sorting. Since then, many researchers have taken advantage of DNA to help separate the one-dimensional carbon crystals in water-based solutions, but why some DNA sequences work better or worse than others wasn't explored. In the current study, Zheng devised a new, targeted strategy to find DNA molecules capable of accurately sorting carbon nanotubes with different physical and electronic properties.

Zheng and his coauthors focused on 28 to 30 nucleotide sequences. Instead of exploring all quintillion possible combinations of bases, they restricted their search to simple one to four nucleotide repeats, which had shown some success in previous sorting efforts. From a library of around 350 sequences, they identified a couple dozen DNA molecules with alternating patterns of purines (As or Gs) and pyrimidines (Ts or Cs) that differentially recognized particular nanotubes. They used these molecules to pull out various types of nanotubes from a mixture of the carbon rods, and achieved purity levels as good as or better than other separation schemes.

The study "illustrates to a much greater extent [than previous work] how much specificity particular DNA oligonucleotides can have for these nanomolecules," said Strano.

Why that specificity exists is not fully understood. But Zheng, together with colleagues at Lehigh University in Pennsylvania, modeled the DNA sequences' shape and showed that it resembled a well-known protein motif called a beta-sheet. Zheng proposed that the DNA might form a two-dimensional sheet that selectively twists around and encapsulates certain carbon nanotubes to create a cylindrical barrel. "This is the best model we can put forward at this moment and we think it makes sense," Zheng said.

"It all seems very plausible to me and it seems very consistent with their data," said Mark Hersam, a materials engineer at Northwestern University in Evanston, Illinois, who wrote an accompanying commentary on the research. Overall, Zheng's technique "is not that complicated once you hear it. Most good ideas are pretty simple, but everyone else missed it."


Related stories:

Fake credentials in nanomed leader
[25th June 2009]

Nanoscience is out of the bottle
[28th July 2003]

From buckyballs to nanotubes
[19th February 2001]