SU R ES H M A NOHA R
NEWS OF THE WEEK
DNA SORTS CARBON NANOTUBES NANOTECHNOLOGY: Specific sequences separate nanotubes according to chirality
S A DNA sequence consisting of ATTT repeats forms a barrel-shaped structure around a single type of chiral carbon nanotube.
INGLE-WALLED carbon nanotubes (SWNTs)
show great promise as components of nanoscale electronic devices, but most commercial applications have been stymied by the difficulty in isolating nanotubes of identical chirality from a synthetic mixture. Now, Xiaomin Tu and Ming Zheng of DuPont Central Research & Development, together with Suresh Manohar and Anand Jagota of Lehigh University, have shown that the unique molecular properties of DNA can be exploited to sort SWNTs (Nature 2009, 460, 250). SWNT synthesis produces a mixture of nanotubes with nonuniform diameters and chiralities and, therefore, heterogeneous physicochemical properties. Having previously shown that a particular DNA sequence could form an ordered structure on SWNTs, Zheng and colleagues reasoned that they might be able to find a DNA sequence to purify each type of SWNT in a synthetic mixture. The problem was identifying the cor-
JUDGING DOCTORAL PROGRAMS EDUCATION: With methodology in place, long-awaited graduate program rankings may soon be released
A
FTER REPEATED delays, the National Re-
search Council on July 9 released the methodology behind its rankings of doctoral programs, including those in chemistry and chemical engineering. The methods represent a departure from previous assessments, abandoning reliance on reputation surveys in favor of a more data-intensive approach. A release date for the rankings themselves has not been scheduled. The assessments by the research council, which functions under the National Academy of Sciences, are a valued resource for students, educators, and administrators, but they were last updated in 1995. The release of the new rankings, to be based largely on data from the 2005–06 academic SHUTTERSTOCK
NRC’s methodology release has campuses nationwide eagerly awaiting the research council’s rankings.
WWW.CEN-ONLINE.ORG
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rect DNA molecules among an unfeasibly large number (1018) of possible 30-nucleotide sequences. To reduce the DNA library to a more manageable size of 350 oligonucleotides, the researchers devised a sequence-pattern-expansion scheme that considered all possible DNA sequences composed of mono-, di-, tri-, and tetranucleotide repeats. They added each DNA oligonucleotide to a random mixture of SWNTs. Then, they used ion-exchange chromatography to separate the 350 solutions into fractions, which they analyzed spectroscopically for the presence of specific DNA-SWNT hybrids. The study yielded more than 20 sequences that could together purify all 12 major chiral semiconducting SWNTs. To explain the SWNT-sorting ability of DNA, the authors propose a model in which the recognition sequences form stable hydrogen-bonded DNA barrels around specific SWNTs. The ordered structure minimizes interactions with the ion-exchange chromatography resin and causes early elution. “This is a very impressive study that reports the most selective method yet found for isolating specific structural forms of SWNTs from mixed samples,” R. Bruce Weisman of Rice University says. “The main limitation is the small scale and high expense.” Zheng tells C&EN that the major obstacle to scaling up the method is the high cost of DNA, which could decrease if oligonucleotide suppliers shift their business model to meet the demands of the nanoelectronics industry.—LAURA CASSIDAY
year, have been postponed on multiple occasions. “It’s frustrating for everybody—the longer it takes to get the rankings out, the more dated the information becomes,” says Charles A. Wight, a physical chemist and dean of the graduate school at the University of Utah. “But there’s also a sense that people would rather have things done right” than done hastily, he says. That’s also the council’s view, says Jeremiah P. Ostriker, chair of the committee overseeing the project. Each program will have a range of rankings, in order to stress inherent uncertainties in the analysis, he says. Rankings are based on 20 criteria in three areas: scholarly activity, student support, and diversity. In addition to an overall ranking range, programs will be ranked in those three areas. Data for several criteria, such as degree completion rates, have never before been collected or disseminated on a nationwide scale, Wight notes. Furthermore, weights given to criteria are driven by faculty input, says Ostriker, an astrophysicist and former provost at Princeton University. What’s deemed an important measure of program quality “turns out to be quite different in different fields,” he says. Where possible, measures will be reported per capita of fulltime faculty, in efforts to avoid giving artificial advantages to larger programs. Rankings may be released by the end of 2009, Ostriker speculates. “Now that the methodology is in place, we hope to update rankings more quickly in the future,” he says.—CARMEN DRAHL
JULY 20, 2009
