Friday, July 29, 2011

Hydrogen may be key to growth of high-quality graphene

 A new approach to growing graphene greatly reduces problems that have plagued researchers in the past and clears a path to the crystalline form of graphite's use in sophisticated electronic devices of tomorrow.

Findings of researchers at the Department of Energy's Oak Ridge National Laboratory demonstrate that hydrogen rather than carbon dictates the graphene grain shape and size, according to a team led by ORNL's Ivan Vlassiouk, a Eugene Wigner Fellow, and Sergei Smirnov, a professor of chemistry at New Mexico State University. This research is published in ACS Nano.

"Hydrogen not only initiates the graphene growth, but controls the graphene shape and size," Vlassiouk said. "In our paper, we have described a method to grow well-defined graphene grains that have perfect hexagonal shapes pointing to the faultless single crystal structure."

In the past two years, graphene growth has involved the decomposition of carbon-containing gases such as methane on a copper foil under high temperatures, the so-called chemical vapor deposition method. Little was known about the exact process, but researchers knew they would have to gain a better understanding of the growth mechanism before they could produce high-quality graphene films.

Until now, grown graphene films have consisted of irregular- shaped graphene grains of different sizes, which were usually not single crystals.

"We have shown that, surprisingly, it is not only the carbon source and the substrate that dictate the growth rate, the shape and size of the graphene grain," Vlassiouk said. "We found that hydrogen, which was thought to play a rather passive role, is crucial for graphene growth as well. It contributes to both the activation of adsorbed molecules that initiate the growth of graphene and to the elimination of weak bonds at the grain edges that control the quality of the graphene."

Using their new recipe, Vlassiouk and colleagues have created a way to reliably synthesize graphene on a large scale. The fact that their technique allows them to control grain size and boundaries may result in improved functionality of the material in transistors, semiconductors and potentially hundreds of electronic devices.

Implications of this research are significant, according to Vlassiouk, who said, "Our findings are crucial for developing a method for growing ultra-large-scale single domain graphene that will constitute a major breakthrough toward graphene implementation in real-world devices."

Other authors of the paper, "Role of Hydrogen in Chemical Vapor Deposition Growth of Large Single-Crystal Graphene," are Murari Regmi, Pasquale Fulvio, Sheng Dai, Panos Datskos and Gyula Eres of ORNL.

The research was supported by the Department of Energy's Office of Science, in part through the Fluid Interface Reactions, Structures and Transport Center, a DOE Energy Frontier Research Center led by ORNL.

A portion of the work was performed at the Center for Nanophase Materials Sciences, one of the five DOE Nanoscale Science Research Centers supported by the DOE Office of Science, premier national user facilities for interdisciplinary research at the nanoscale. Together the NSRCs comprise a suite of complementary facilities that provide researchers with state-of-the-art capabilities to fabricate, process, characterize and model nanoscale materials, and constitute the largest infrastructure investment of the National Nanotechnology Initiative.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by DOE/Oak Ridge National Laboratory.

Journal Reference:

Ivan Vlassiouk, Murari Regmi, Pasquale Fulvio, Sheng Dai, Panos Datskos, Gyula Eres, Sergei Smirnov. Role of Hydrogen in Chemical Vapor Deposition Growth of Large Single-Crystal Graphene. ACS Nano, 2011; 110701132829005 DOI: 10.1021/nn201978y

Bacteria use Batman-like grappling hooks to 'slingshot' on surfaces, study shows

Bacteria use various appendages to move across surfaces prior to forming multicellular bacterial biofilms. Some species display a particularly jerky form of movement known as "twitching" motility, which is made possible by hairlike structures on their surface called type IV pili, or TFP.

"TFP act like Batman's grappling hooks," said Gerard Wong, a professor of bioengineering and of chemistry and biochemistry at the UCLA Henry Samueli School of Engineering and Applied Science and the California NanoSystems Institute (CNSI) at UCLA. "These grappling hooks can extend and bind to a surface and retract and pull the cell along."

In a study to be published online this week in Proceedings of the National Academy of Sciences, Wong and his colleagues at UCLA Engineering identify the complex sequence of movements that make up this twitching motility in Pseudomonas aeruginosa, a biofilm-forming pathogen partly responsible for the deadly infections seen in cystic fibrosis.

During their observations, Wong and his team made a surprising discovery. Using a high-speed camera and a novel two-point tracking algorithm, they noticed that the bacteria had the unique ability to "slingshot" on surfaces.

The team found that linear translational pulls of constant velocity alternated with velocity spikes that were 20 times faster but lasted only milliseconds. This action would repeat over and over again.

"The constant velocity is due to the pulling by multiple TFP; the velocity spike is due to the release of a single TFP," Wong said. "The release action leads to a fast slingshot motion that actually turns the bacteria efficiently by allowing it to over-steer."

The ability to turn and change direction is essential for bacteria to adapt to continually changing surface conditions as they form biofilms. The researchers found that the slingshot motion helped P. aeruginosa move much more efficiently through the polysaccharides they secrete on surfaces during biofilm formation, a phenomenon known as shear-thinning.

"If you look at the surfaces the bacteria have to move on, they are usually covered in goop. Bacterial cells secrete polysaccharides on surfaces, which are kind of like molasses," Wong said. "Because these polysaccharides are long polymer molecules that can get entangled, these are very viscous and can potentially impede movement. However, if you move very fast in these polymer fluids, the viscosity becomes much lower compared to when you're moving slowly. The fluid will then seem more like water than molasses. This kind of phenomenon is well known to chemical engineers and physicists."

Since the twitching motion of bacteria with TFP depends of the physical distributions of TFP on the surface of individual cells, Wong hopes that the analysis of motility patterns may in the future enable new methods for biometric "fingerprinting" of individual cells for single-cell diagnostics.

"It gives us the possibility of not just identifying species of bacteria but the possibility of also identifying individual cells. Perhaps in the future, we can look at a cell and try to find the same cell later on the basis of how it moves," he said.

The study was funded by the National Institutes of Health and the National Science Foundation. The lead authors are Fan Jin from the UCLA Department of Bioengineering, the UCLA Department Chemistry and Biochemistry, and the CNSI, and Jacinta C. Conrad of the department of chemical and biomolecular engineering at the University of Houston.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by University of California - Los Angeles. The original article was written by Wileen Wong Kromhout.

Journal Reference:

Fan Jin, Jacinta C. Conrad, Maxsim L. Gibiansky, Gerard C. L. Wong. Bacteria use type-IV pili to slingshot on surfaces. Proceedings of the National Academy of Sciences, 2011; DOI: 10.1073/pnas.1105073108

Deep below the Deepwater Horizon oil spill: New molecular model better explains diffusion of spill under water

 For the first time, scientists gathered oil and gas directly as it escaped from a deep ocean wellhead -- that of the damaged Deepwater Horizon oil rig. What they found allows a better understanding of how pollution is partitioned and transported in the depths of the Gulf of Mexico and permits superior estimation of the environmental impact of escaping oil, allowing for a more precise evaluation of previously estimated repercussions on seafloor life in the future.

The explosion of the Deepwater Horizon rig in April 2010 was both a human and an environmental catastrophe. Getting the spill under control was an enormous challenge. The main problem was the depth of the well, nearly 1,500 meters below the sea surface. It was a configuration that had never been tried before, and the pollution it unleashed after methane gas shot to the surface and ignited in a fiery explosion is also unequalled. Much research has been done since the spill on the effects on marine life at the ocean's surface and in coastal regions. Now, École Polytechnique Fédérale de Lausanne (EPFL) professor Samuel Arey and the Woods Hole Oceanographic Institute reveal in the advance online edition of Proceedings of the National Academy of Sciences how escaped crude oil and gas behave in the deep water environment.

Into the deep

In June 2010, with the help of a remotely operated vehicle (ROV), Woods Hole scientists reached the base of the rig and gathered samples directly from the wellhead using a robotic arm. The oceanographers also made more than 200 other measurements at various water depths over a 30-kilometer area. These samples were then analyzed with the help of the US National Oceanic and Atmospheric Administration and the dissolution of hydrocarbons was modeled at EPFL. This model showed how the properties of hydrocarbons are important in understanding the wellhead structure and pollution diffusion -- how pollution spreads out -- in the depths.

From the ROV to the lab

Lab analysis led the scientists to describe for the first time the physical basis for the deep sea trajectories of light-weight, water-soluble hydrocarbons such as methane, benzene, and naphthalene released from the base of the rig. The researchers observed, for example, that at a little more than 1,000 meters below the surface, a large plume spread out from the original gusher, moving horizontally in a southwest direction with prevailing currents. Unlike a surface spill, from which these volatile compounds evaporate into the atmosphere, in the deep water under pressure, light hydrocarbon components predominantly dissolve or form hydrates, compounds containing water molecules. And depending on its properties, the resulting complex mixture can rise, sink, or even remain suspended in the water, and possibly go on to cause damage to seafloor life far from the original spill.

By comparing the oil and gas escaping from the well with the mixture at the surface, EPFL's Samuel Arey, head of Environmental Chemistry Modeling Laboratory, and colleagues were able to show that the composition of the deep sea plumes could be explained by significant dissolution of light hydrocarbons at 1 kilometer depth. In other words, an important part of the oil spreads out in underwater plumes, so we need a more precise evaluation of previously estimated repercussions on seafloor life in the future. Arey's methodology offers a better estimation of how pollution travels and the potential deep sea consequences of spills.

"Modeling the environmental fate of hydrocarbons in deep water ecosystems required a new approach, with a global view, in order to correctly understand the impact of the pollution," explains Arey. This research will have a significant impact on assessments of the environmental impact of deep water oil spills.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by Ecole Polytechnique Fédérale de Lausanne, via EurekAlert!, a service of AAAS.

Journal Reference:

Christopher M. Reddy, J. Samuel Arey, Jeffrey S. Seewald, Sean P. Sylva, Karin L. Lemkau, Robert K. Nelson, Catherine A. Carmichael, Cameron P. McIntyre, Judith Fenwick, G. Todd Ventura, Benjamin A. S. Van Mooy, Richard Camilli. Science Applications in the Deepwater Horizon Oil Spill Special Feature: Composition and fate of gas and oil released to the water column during the Deepwater Horizon oil spill. Proceedings of the National Academy of Sciences, 2011; DOI: 10.1073/pnas.1101242108

Click chemistry with copper: A biocompatible version

Biomolecular imaging can reveal a great deal of information about the inner workings of cells and one of the most attractive targets for imaging are glycans -- sugars that are ubiquitous to living organisms and abundant on cell surfaces. Imaging a glycan requires that it be tagged or labeled. One of the best techniques for doing this is a technique called click chemistry. The original version of click chemistry could only be used on cells in vitro, not in living organisms, because the technique involved catalysis with copper, which is toxic at high micromolar concentrations.

A copper-free version of click chemistry that can safely be used in living organisms is available, but it is not always optimal in terms of reaction kinetics and target specificity. Now, a variation of click chemistry has been introduced that retains the copper catalyst of the original reaction -- along with its speed and specificity -- but is safe for cells in vivo.

Researchers with the Lawrence Berkeley National Laboratory (Berkeley Lab), in collaboration with researchers at the Albert Einstein College of Medicine at Yeshiva University in New York, have found a way to make copper-catalyzed click chemistry biocompatible. By adding a ligand that minimizes the toxicity of copper but still allows it to catalyze the click chemistry reaction, the researchers can safely use their reaction in living organisms. Compared to the copper-free click chemistry reaction, which can take up to an hour, the ligand-accelerated copper-catalyzed click chemistry reaction can achieve effective labeling within 3-5 minutes. The presence of the copper catalyst also enables this new formulation of click chemistry to be more target-specific with fewer background side reactions.

"The discovery of this new accelerating ligand for copper-catalyzed click chemistry should provide an effective complimentary tool to copper-free click chemistry," says Yi Liu, a chemist with Berkeley Lab's Molecular Foundry and the co-leader of this research with Peng Wu, of the Albert Einstein College of Medicine.

"While copper-free click chemistry may have advantages for whole animal imaging experiments such as imaging in mice," Liu says, "our ligand-accelerated copper reaction is better suited for enriching glycoproteins for their identification."

The ligand-accelerated copper-catalyzed reaction was used to label glycans in recombinant glycoproteins, glycoproteins in cell lysates, glycoproteins on live cell surfaces, and glycoconjugates in live zebrafish embryos. Because a zebrafish embryo is transparent in the first 24 hours of its development, it allows labeled glycans to be detected via molecular imaging techniques, making it a highly useful model for developmental biology studies.

"Based on our results," says Peng Wu, "we believe that ligand-accelerated copper-catalyzed click chemistry represents a powerful and highly adaptive bioconjugation tool that holds great promise for further improvement with the discovery of more versatile catalyst systems."

Click chemistry, which was introduced in 2002 by the Nobel laureate chemist Barry Sharpless of the Scripps Research Institute, utilizes a copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction that makes it possible for certain chemical building blocks to "click" together in an irreversible linkage, analogous to the snapping together of Lego blocks. While the technique immediately proved valuable for attaching small molecular probes to various biomolecules in a test tube or on fixed cells, it could not be used for biomolecule labeling in live cells or organisms because of the copper catalyst.

In 2007, Carolyn Bertozzi, a chemist who holds joint appointments with Berkeley Lab, the University of California (UC) Berkeley, and the Howard Hughes Medical Institute, led a research effort that produced a copper-free version of click chemistry. In this version, glycans were metabolically labeled with azides -- a functional group featuring three nitrogen atoms -- via reactions that were carried out through the use of cyclooctyne reagents that required no copper catalyst. With their latest reagent, biarylazacyclooctynone (BARAC), Bertozzi and her group have provided a copper-free click chemistry technique that delivers relatively fast reaction kinetics and the bioorthogonality needed for biomolecule labeling. However, the technique can only be used on biomolecules that can be tagged with azides.

"Our bio-benign ligand-accelerated copper-catalyzed click chemistry reaction liberates bioconjugation from the limitation where ligations could only be accomplished with azide-tagged biomolecules," Liu says. "Now terminal alkyne residues can also be incorporated into biomolecules and detected in vivo."

This work was supported by a grant from the National Institutes of Health, and in part as a User Project at the Molecular Foundry, which is funded through DOE's Office of Science.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by DOE/Lawrence Berkeley National Laboratory.

Journal Reference:

Christen Besanceney-Webler, Hao Jiang, Tianqing Zheng, Lei Feng, David Soriano del Amo, Wei Wang, Liana M. Klivansky, Florence L. Marlow, Yi Liu, Peng Wu. Increasing the Efficacy of Bioorthogonal Click Reactions for Bioconjugation: A Comparative Study. Angewandte Chemie International Edition, 2011; DOI: 10.1002/anie.201101817