Nano parfait a treat for scientists

Rice researchers spin pure batches of nanotubes species

In two new papers, Rice University researchers report using ultracentrifugation (UCF) to create highly purified samples of carbon nanotube species.

One team, led by Rice Professor Junichiro Kono and graduate students Erik Haroz and William Rice, has made a small but significant step toward the dream of an efficient nationwide electrical grid that depends on highly conductive quantum nanowire.

The other, led by Rice Professor Bruce Weisman and graduate student Saunab Ghosh, employed UCF to prepare structurally sorted batches of semiconducting nanotubes that could find critical uses in medicine and electronics.

UCF is what it sounds like: a super-fast version of the centrifuge process medical lab technicians use to separate blood cells from plasma. The process involves suspending mixtures of single-walled carbon nanotubes in combinations of liquids of different densities.

When spun by a centrifuge at up to 250,000 g – that's 250,000 times the force of gravity – the nanotubes migrate to the liquids that match their own particular densities. After several hours in the centrifuge, the test tube becomes a colorful parfait with layers of purified nanotubes. Each species has its own electronic and optical characteristics, all of which are useful in various ways.

Weisman's lab reported its results in today's online edition of Nature Nanotechnology. Weisman is a professor of chemistry at Rice.

Kono's lab reported its results recently in the online edition of ACS Nano. Kono is a professor in electrical and computer engineering and professor of physics and astronomy.

The lack of pure batches of nanotubes species "has been a real hindrance in the field for nearly 20 years," Weisman said. While the UCF technique is not new, Ghosh found careful fine-tuning of the gradient structure let him sort at least 10 of the numerous species of nanotubes contained in a single sample produced by the Rice-created HiPco process.

Basic research is a big early winner, "because when you can get pure samples of nanotubes, you can learn so much more about them," Weisman said. "Secondly, some electronic applications become much simpler because the tube type determines the nanotube's band gap, a crucial electronic property." Biomedical applications may benefit by exploiting the optical properties of specific types of nanotubes.

In the Kono lab, metallic nanotubes rose to the top of the spinning vial while nearly all of the semiconducting nanotubes sank to the bottom. What surprised lead researchers Haroz and Rice was that nearly all of the metallic tubes were armchair SWNTs, the most desirable species for the manufacture of quantum nanowire. Zigzag and near-zigzag species, also considered metallic, would also sink out.

Armchair nanotubes are so-called because of their "U"-shaped end segments. Theoretically, armchairs are the most conductive nanotubes, letting electrons charge down the middle with nothing to slow them.

The composition of the gradient solution made a difference in the quality of the samples, Haroz said. "One of the surfactants we're using, sodium cholate, has a molecular structure that's similar to a nanotube -- basically hexagons put together," he said. "We think there's a match between the sodium cholate and the structure of nanotubes, and it binds just a little bit better to an armchair than it does to zigzags."

Hurdles remain in the path to quantum armchair nanowires that nanotechnology pioneer and Nobel laureate Richard Smalley, Haroz' first mentor at Rice who died in 2005, felt would be a panacea for many of the world's problems. Fix the distribution of energy and solutions to other challenges – clean water, food, environmental woes – will fall into place, he believed.

"Step 1 of the armchair quantum nanowire project is, 'Can we get armchairs?' We've done that," said Haroz. "Now let's make macroscopic structures -- not necessarily long cables, but small structures -- to test their conductivity." ###

Rice research scientist Sergei Bachilo is co-author of the Nature Nanotechnology paper with Weisman and Ghosh. Grants from the National Science Foundation and the Welch Foundation supported the research.

Co-authors of the ACS Nano paper with Kono, Haroz, Rice, Weisman and Ghosh are Robert Hauge, Distinguished Faculty Fellow in Chemistry at Rice, Rice junior Ben Lu and Los Alamos National Laboratory researcher Stephen Doorn. The Department of Energy office of Basic Energy Sciences, the Welch Foundation, the Air Force Research Laboratories, the National Science Foundation and the Laboratory Directed Research and Development program at Los Alamos National Laboratory supported the research.

Contact: David Ruth druth@rice.edu 713-348-6327 Rice University

Peptides may hold 'missing link' to life

Emory University scientists have discovered that simple peptides can organize into bi-layer membranes. The finding suggests a "missing link" between the pre-biotic Earth's chemical inventory and the organizational scaffolding essential to life.

"We've shown that peptides can form the kind of membranes needed to create long-range order," says chemistry graduate student Seth Childers, lead author of the paper recently published by the German Chemical Society's Angwandte Chemie. "What's also interesting is that these peptide membranes may have the potential to function in a complex way, like a protein."

Caption: Emory University scientists have discovered that simple peptides can organize into bi-layer membranes. The finding suggests a "missing link" between the pre-biotic Earth's chemical inventory and the organizational scaffolding essential to life. Credit: Emory University. Usage Restrictions: None.

Chemistry graduate student Yan Liang captured images of the peptides as they aggregated into molten globular structures, and self-assembled into bi-layer membranes. The results of that experiment were recently published by the Journal of the American Chemical Society. "In order to form nuclei, which become the templates for growth, the peptides first repel water," says Liang, who is now an Emory post-doctoral fellow in neuroscience. "Once the peptides form the template, we can now see how they assemble from the outer edges."

In addition to providing clues to the origins of life, the findings may shed light on protein assemblies related to Alzheimer's disease, Type 2 diabetes, and dozens of other serious ailments.

"This is a boon to our understanding of large, structural assemblies of molecules," says Chemistry Chair David Lynn, who helped lead the effort behind both papers, which were collaborations of the departments of chemistry, biology and physics. "We've proved that peptides can organize as bi-layers, and we've generated the first, real-time imaging of the self-assembly process. We can actually watch in real-time as these nano-machines make themselves."

The ability to organize things within compartments and along surfaces underpins all of biology. From the bi-layer phospholipids of cell membranes to information-rich DNA helices, self-assembling arrays define the architecture of life.

But while phospholipids and DNA are complicated molecules, peptides are composed of the simple amino acids that make up proteins. The Miller-Urey experiment demonstrated in 1953 that amino acids were likely to be present on the pre-biotic Earth, opening the question of whether simple peptides could achieve supra-molecular order.

To test how the hollow, tubular structure of peptides is organized, the researchers used specialized solid-state nuclear magnetic resonance (NMR) methods that have been developed at Emory during the past decade.

Working with Anil Mehta, a chemistry post-doctoral fellow, Childers tagged one end of peptide chains with an NMR label, and then allowed them to assemble to see if the ends would interact. The result was a bi-layer membrane with inner and outer faces and an additional, buried layer that localized functionality within the interior.

"The peptide membranes combine the long-range structure of cell membranes with the local order of enzymes," Childers said. "Now that we understand that peptide membranes are organized locally like a protein, we want to investigate whether they can function like a protein."

The goal is to direct molecules to perform as catalysts and create long-range order. "We'd really like to understand how to build something from the bottom up," Childers says. "How can we take atoms and make molecules? How can we get molecules that stick together to make nano-machines that will perform specific tasks?"

The research is part of "The Center for Chemical Evolution," a center based at Emory and Georgia Tech, for integrated research, education and public outreach focused on the chemistry that may have led to the origin of life. The National Science Foundation and the U.S. Department of Energy have funded the research.

Many groups studying the origins of life have focused on RNA, which is believed to have pre-dated living cells. But RNA is a much more complicated molecule than a peptide. "Our studies have now shown that, if you just add water, simple peptides access both the physical properties and the long-range molecular order that is critical to the origins of chemical evolution," Childers says. ###

Contact: Beverly Clark beverly.clark@emory.edu 404-712-8780 Emory University