Unlike Rubber Bands, Molecular Bonds May Not Break Faster When Pulled

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ScienceDaily (June 22, 2009) — From balloons to rubber bands, things always break faster when stretched. Or do they? University of Illinois scientists studying chemical bonds now have shown this isn't always the case, and their results may have profound implications for the stability of proteins to mechanical stress and the design of new high-tech polymers.

"Our findings contradict the intuitive notion that molecules are like rubber bands in that when we pull on a chemical bond, it should always break faster," said chemistry professor Roman Boulatov, who led the study. "When we stretch a sulfur-sulfur bond, for example, how fast it breaks depends on how the nearby atoms move."
The findings also contradict the conventional interpretation of experimental results obtained by other researchers studying the fragmentation rate of certain proteins containing sulfur-sulfur bonds when stretched with a microscopic force probe. In those experiments, as the force increased, the proteins fragmented faster, leading the researchers to conclude that as the sulfur-sulfur bond was stretched, it reacted faster and broke faster.
"Our experiments suggest a different conclusion," Boulatov said. "We believe the acceleration of the fragmentation was caused by a change in the protein's structure as it was stretched, and had little or nothing to do with increased reactivity of a stretched sulfur-sulfur bond."
In their experiments, the researchers use stiff stilbene as a molecular force probe to generate well-defined forces on molecules atom by atom.
The probe allows reaction rates to be measured as a function of the restoring force. Similar to the force that develops when a rubber band is stretched, the molecular restoring force contains information about how much the molecule was distorted, and in what direction.
In previous work, when Boulatov's team pulled on carbon-carbon bonds with the same force they would later apply to sulfur-sulfur bonds, they found the carbon-carbon bonds broke a million times faster than when no force was applied.
"Because the sulfur-sulfur bond is much weaker than the carbon-carbon bond, you might think it would be much more sensitive to being pulled on," Boulatov said. "We found, however, that the sulfur-sulfur bond does not break any faster when pulled."
Boulatov and his team report their findings in a paper accepted for publication in Angewandte Chemie, and posted on the journal's Web site.
"When we pulled on the sulfur-sulfur bond, the nearby methylene groups prevented the rest of the molecule from relaxing," Boulatov said, "thus eliminating the driving force for the sulfur-sulfur bond to break any faster."
Chemists must bear in mind that even in simple chemical reactions, such as a single bond dissociation, "we must take into account other structural changes in the molecule," Boulatov said. "The elongation alone, which occurs when a bond is stretched, does not represent the full picture of what happens when the reaction occurs."
The good news, Boulatov said, is that not every polymer that is stretched will break faster. "We might be able to design polymers, for example, that would resist fragmentation under modest mechanical stresses," he said, "or will not break along the stretched direction, but in some other desired direction."
With Boulatov, co-authors of the paper are graduate student and lead author Timothy Kucharski, research associate Qing-Zheng Yang, postdoctoral researcher Yancong Tian, and graduate students Zhen Huang, Nicholas Rubin and Carlos Concepcion.
Funding was provided by the National Science Foundation, the U.S. Air Force Office of Scientific Research, the American Chemical Society Petroleum Research Fund, and the U. of I.
Adapted from materials provided by University of Illinois at Urbana-Champaign.

Carbon Nanotubes: Innovative Technology Or Risk To Health Or Environment?

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ScienceDaily (May 10, 2009) — Carbon nanotubes have made a meteoric career in the past 15 years, even if their applications are still limited. Recent research results show that – apart from their favorable mechanical and electrical properties – they also have disadvantageous characteristics.

One aspect which has rarely been considered so far is now addressed by researchers of the research center Forschungszentrum Dresden-Rossendorf. “If the application of products and commodities containing carbon nanotubes will increase in the future, then there will be a higher probability for the tubes to get into the environment during their production, usage or disposal, to be distributed there, and to bind pollutants such as heavy metals on their way trough the environment”, says Harald Zaenker, scientist at the FZD.
Via water into the environment
An important way for carbon nanotubes of getting into the environment is the way via the water. In their original state, the flimsy carbon fibers with a diameter of less than 50 nanometers (1 nanometer = 1 millionth of a millimeter) are hardly water-soluble. At first glance, they should therefore not be mobile in groundwater, lakes etc., i.e. they should rapidly settle or deposit. However, carbon nanotubes are able to form colloidal solutions if their surface structure is changed. Changes in the surface structure can be brought about deliberately during the production of the tubes or can be induced by natural processes if the tubes are released into the environment.
A colloidal solution, unlike a true solution of water-soluble substances, is a solution in which the apparently dissolved substance is finely dispersed in the solvent forming tiny particles. These particles are still much bigger than the molecules of a dissolved substance in a true solution. As colloids, carbon nanotubes might be transported anywhere in environmental waters. It is known meanwhile that the tubes can even penetrate cell walls and, thus, might theoretically be able to enter also animal or human cells. In addition, changes in the surface structure of carbon nanotubes cause another effect: their capability to bind heavy metals is increased.
Tubes with changed surface
The scientists investigated carbon nanotubes both in their original state and in a state changed by oxidizing acids (such as a mixture of nitric and sulfuric acid). They found out that solutions of treated carbon nanotubes scatter light more strongly. “This is an indication that colloids have formed which do not settle”, Harald Zaenker says.
The researchers provided evidence for the first time that the heavy metal uranium, which is ubiquitous in the environment and, hence, also in the water, is particularly attached to the surface of treated carbon nanotubes. The scientists found out that the uranium uptake capacity is increased by an order of magnitude in comparison to untreated carbon nanotubes. “Therefore, it is plausible to assume that carbon nanotubes, if released to the environment, influence the transport of uranium in environmental waters and even in biological systems. The possible impact on the environment and on human health has in general been considered too little”, Harald Zaenker says.
On the other hand, the high bonding capacity of carbon nanotubes for uranium and other heavy metals also suggests using them for the removal of heavy metals from waters. However, they are not yet a cost-efficient alternative to classic water purifiers, Zaenker says. “Eventually, it is important to further study the behavior of carbon nanotubes in waters”, the scientist says. “Only then can the positive and negative aspects of carbon nanotubes be better assessed.”
Journal reference:
Schierz et al. Aqueous suspensions of carbon nanotubes: Surface oxidation, colloidal stability and uranium sorption. Environmental Pollution, 2009; 157 (4): 1088 DOI: 10.1016/j.envpol.2008.09.045
Adapted from materials provided by Forschungszentrum Dresden Rossendorf.

Nanowire Device Fabrication Moves Into High Gear

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http://www.sciencedaily.com/releases/2007/10/071026162149.htm

ScienceDaily (Oct. 30, 2007) — In the growing catalog of nanoscale technologies, nanowires--tiny rows of conductor or semiconductor atoms--have attracted a great deal of interest for their potential to build unique atomic-scale electronics. But before you can buy some at your local Nano Depot, manufacturers will need efficient, reliable methods to build them in quantity. Researchers at the National Institute of Standards and Technology (NIST) believe they have one solution--a technique that allows them to selectively grow nanowires on sapphire wafers in specific positions and orientations accurately enough to attach contacts and layer other circuit elements, all with conventional lithography techniques.

Despite their name, nanowires are more than just electrical connectors. Researchers have used nanowires to create transistors like those used in memory devices and prototype sensors for gases or biomolecules. However working with objects only tens of nanometers wide is challenging. A common approach in the lab is to grow nanowires like blades of grass on a suitable substrate, mow them off and mix them in a fluid to transfer them to a test surface, using some method to give them a preferred orientation.
When the carrier fluid dries, the nanowires are left behind like tumbled jackstraws. Using scanning probe microscopy or similar tools, researchers hunt around for a convenient, isolated nanowire to work on, or place electrical contacts without knowing the exact positions of the nanowires. It's not a technique suitable for mass production.
Building on earlier work to grow nanowires horizontally on the surface of wafers (see "Gold Nano Anchors Put Nanowires in Their Place), NIST researchers used conventional semiconductor manufacturing techniques to deposit small amounts of gold in precise locations on a sapphire wafer. In a high-temperature process, the gold deposits bead up into nanodroplets that act as nucleation points for crystals of zinc oxide, a semiconductor.
A slight mismatch in the crystal structures of zinc oxide and sapphire induces the semiconductor to grow as a narrow nanowire in one particular direction across the wafer. Because the starting points and the growth direction are both well known, it is relatively straightforward to add electrical contacts and other features with additional lithography steps.
As proof of concept, the NIST researchers have used this procedure to create more than 600 nanowire-based transistors, a circuit element commonly used in digital memory chips, in a single process. In the prototype process, they report, the nanowires typical grew in small bunches of up to eight wires at a time, but finer control over the size of the initial gold deposits should make it possible to select the number of wires in each position. The technique, they say, should allow industrial-scale production of nanowire-based devices.
Reference: B. Nikoobakht. Toward industrial-scale fabrication of nanowire-based devices. Chem. Mater., ASAP Article 10.1021/cm071798p S0897-4756(07)01798-X. Web Release Date: October 9, 2007.
Adapted from materials provided by National Institute of Standards and Technology.

Fausto Intilla

www.oloscience.com

Unveiling The Structure Of Microcrystals

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http://www.sciencedaily.com/releases/2007/10/071004092126.htm

Science Daily — Microcrystals take the form of tiny grains, so small that they resemble a powder. How can we determine their structure?
Until now, the technique of X-ray diffraction, normally used to study crystals, was not an appropriate solution. For the first time, researchers from the ESRF and the CNRS have used X-ray diffraction to determine the structure of microcrystal grains of only one cubic micrometre in size.
They gained a factor of a thousand on the size of the analysable samples thanks to new equipment created at the ESRF. This breakthrough opens up new possibilities of research to chemists, physicists and biologists.
The properties of a crystal are determined by the arrangement of its atom in space, its crystalline structure. Scientists use X-ray or neutron diffraction to study crystalline structure when the size of the crystal is more than 10 cubic micrometres. Below this limit, the solid material is considered a powder.
Scientists can apply powder diffraction to analyse such a material but this technique is not easy to exploit. Moreover, powder diffraction can only be used for materials with grain sizes of less than three millionths of a cubic micrometre. Due to these limitations, a determination of the structure of new synthetic solids in powder form is not always possible because the crystals are too small.
The teams from the ESRF and the Institute Lavoisier (CNRS/Université de Versailles Saint-Quentin) have used new set-up permitting X-ray diffraction on crystals of a size of one cubic micrometre, a volume a thousand times smaller than that ever attainable before. This new set-up consists of a focusing system for the ESRF beam, coupled with a goniometer, an instrument to position the sample with maximum precision.
The researchers studied the structure of an organic-inorganic hybrid compound (a microporous aluminium carboxylate), which could be used for gas absorption or to encapsulate various organic molecules. This study confirms that the new set-up allows pushing back the limits in crystal dimension accessible to X-ray diffraction.
“It is a revolution: what was considered a powder in the past has become a crystal today. Researchers can now bring forward samples left in their cupboards because the sizes had previously prevented their study. Now they will be able to elucidate the structures of these samples, with potentially great scientific advances on the horizon”, explains Thierry Loiseau, from the Institut Lavoisier.
Reference: A Microdiffraction Set-up for Nanoporous Metal-Organic-Framework-Type Solids. C. Volkringer, D. Popov, T. Loiseau, N. Guillou, G. Férey, M. Haouas, F. Taulelle, C. Mellot-Draznieks, M. Burghammer and C. Riekel, Nature Materials, 6 (2007) 760-4.
Note: This story has been adapted from material provided by European Synchrotron Radiation Facility.

Fausto Intilla
www.oloscience.com

New Giant Molecule Created

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http://www.sciencedaily.com/releases/2007/10/071005214924.htm

Science Daily — Ulrich Kortz, Professor of Chemistry at Jacobs University, and his team successfully synthesized a polyoxometalate with 100 Tungsten and 20 Cerium atoms that has a molar mass of about 30 kilo Dalton. With a maximum diameter of 4.2 nm the inorganic molecule is comparable in size to large complex bio-molecules or even small viruses.

Polyoxometalates are anionic metal-oxygen clusters of large structural diversity with chemical properties, which make them especially interesting for applications in catalysis, but also in materials science and nanotechnology.
Ulrich Kortz and his co-workers now achieved the synthesis of the tungstogermanate*, which belongs to the polyoxometalates, by condensation of the precursors [α-GeW9O34]10- and Cerium(III) ions in aqueous solution.
With about 600 atoms in total, amongst them 100 atoms of the heavy metal Tungsten, the new compound is the third largest molecular polytungstate ever synthesized. In addition it contains the largest number of atoms of the Rare Earth Cerium ever incorporated in such a compound.
„A single molecule of our new giant tungstate has many catalytically active centers and therefore a very high catalytic potential, which normally applies only to biological catalyst molecules. Being a lot less temperature and oxidatively sensitive than bio-catalysts though and in crystalline form applicable as a heterogenic solid catalyst in liquid phase reactions our new tungstogermanate is predestined for industrial purposes,“ says Ulrich Kortz about the possible applications of the newly created molecule.
“In addition our successful synthesis allows very good inferences about the mechanism of formation by stepwise self-assembly of the simple precursors in a classic one-pot synthesis, which is vitally important for the development of other so-called ‘molecular machines’, large molecules designed to have very specific functions,“ the Jacobs chemist concludes.
*Tungstogermanate [Ce20Ge10W100O376(OH)4(H2O)30]56-
The reaction conditions and the molecular structure were published in Angewandte Chemie (doi: 10.1002/anie.200701422).
Note: This story has been adapted from material provided by Jacobs University.

Fausto Intilla

www.oloscience.com