Sunday, November 13, 2011

Viruses coaxed to form synthetics with microstructures akin to those of corneas, teeth and skin

 Using a simple, single-step process, engineers and scientists at the University of California at Berkeley recently developed a technique to direct benign, filamentous viruses called M13 phages to serve as structural building blocks for materials with a wide range of properties.

By controlling the physical environment alone, the researchers caused the viruses to self-assemble into hierarchically organized thin-film structures, with complexity that ranged from simple ridges, to wavy, chiral strands, to truly sophisticated patterns of overlapping strings of material--results that may also shed light on the self-assembly of biological tissues in nature.

Each film presented specific properties for bending light, and several films were capable of guiding the growth of cells into structures with precise physical orientations.

Led by University of California at Berkeley bioengineer Seung-Wuk Lee and his student and lead author Woo-Jae Chung, the researchers published their findings in the Oct. 20, 2011, issue of Nature.

"We are very curious how nature can create many diverse structures and functions from single structural building blocks, such as collagens for animals and celluloses for plants," says Lee. "We have thought that periodic changes in cell activity--such as from day to night, or summer to winter--cause cells to secrete different amounts of macromolecules into confined and curved micro-environments, which might play critical roles in the formation of such sophisticated structures. We believe that biological helical nanofiber structures play a critical role in that process, yet for collagen and cellulose, it has proven quite difficult to engineer their chemical and physical properties to study their assembly process. Therefore, we have been looking for new, helical engineering materials."

The fundamental unit of the novel films is the bacteria-hunting virus, M13. In nature, the virus attacks Escherichia coli (E.coli), but in bioengineering laboratories, the virus is emerging as a nanoscale tool that can assemble in complex ways due to its long, slender shape and its chiral twist.

"Fortunately," adds Lee, "M13 also possesses an elegant helical surface that makes it a best fit for this study."

In the Berkeley laboratory, the viruses are suspended in a buffered salt solution, into which the engineers dip a thin substrate onto which the viruses can adhere.

By varying the speed at which they withdrew the substrates from the virus-rich solution, the concentration of viruses in the solution, and the ionic concentration, the researchers were able to craft three distinct categories of films.

The simplest film consisted of alternating bands of filaments, with the viral filaments in each band oriented perpendicular to the filaments in the adjacent band. Created using a relatively low concentration of viruses in the starter solution, the bands formed as the substrate rose out of the liquid with a repeated stick-slip motion.

To create films at the next hierarchical level of complexity, the researchers increased the concentration of viruses in the solution, which added more physical constraints to each filament's movement within its environment. As a result, the filaments bunched together into helical ribbons, with a handedness at a broader scale than the handedness of each individual virus.

With even higher concentrations-and in some experiments, greater substrate-pulling speed-the withdrawal yielded ever more complex, yet ordered, bundles of filaments that the researchers referred to as "ramen-noodle-like."

"Nature can dynamically change environmental variables when building new tissues to control an assembly process," adds Chung, the first author. "The beauty of our system is that we can do the same. By altering various parameters we drive assembly towards specific structures in a controlled manner. We can even make different structures on the same substrate."

By varying their techniques, the researchers altered the physical environment for the viral filaments, ultimately forcing the viruses to align into the highly specialized structural films. Each film is different, as expressed by differences in color, iridescence, polarity and other properties.

In one expression of those differences, structures built using faster-pulled substrates yielded patterns that reflected ever-shorter wavelengths of light--50 microns per minute yielded material that reflected light in the orange color range of the spectrum (600 nm), while 80 micrometers per minute yielded blue light (450 nm). The process was precise, allowing the researchers to tune the films to various wavelengths and colors, and induce polarization.

The researchers believe the hierarchical nature of the structures reflects the hierarchical growth patterns of similar biomolecules in nature, processes that result in chiral materials, like collagen, expressing themselves as the building blocks of a cornea in one level of self-assembly and the building blocks of skin tissue at a more complex level. Such self-assembly yields stunning macroscale structures--for example, skin tissue that appears blue on birds and blue-faced monkeys is actually not expressing the light absorption from blue pigment, but the blue light scattered by complex arrays of chiral, molecular building blocks.

"We strongly believe that our novel approach to constructing biomimetic 'self-templated', supramolecular structures closely mimics natural helical fiber assembly," says Lee. "One important reason is that we not only mimicked the biological structures, but we also discovered structures that have not been seen in nature or the laboratory, like the self-assembled 'ramen-noodle structures' with six distinct order-parameters."

In addition to crafting novel biomolecular films with unique traits, the researchers also demonstrated that the films can serve as biological substrates. The team was able to grow sheets of cells that were oriented based on the texture of such substrates, with one variation incorporating calcium and phosphate to create a biomaterial similar to tooth enamel.

"This novel, self-templating, biomaterials assembly process could be used in many other organic and inorganic materials to build hierarchical structures to tune optical, mechanical and even electrical properties from nano to macro scales," adds NSF Biomaterials program director Joseph Akkara, who helped fund the project. "The reported approaches could be used to investigate mechanisms for diseases such as Alzheimer's, which is caused by amyloid aggregation in our brain tissues. More broadly, the breakthroughs could potentially yield scientific impacts in the area of tissue regeneration and repair."

Story Source:

The above story is reprinted from materials provided by National Science Foundation.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Woo-Jae Chung, Jin-Woo Oh, Kyungwon Kwak, Byung Yang Lee, Joel Meyer, Eddie Wang, Alexander Hexemer, Seung-Wuk Lee. Biomimetic self-templating supramolecular structures. Nature, 2011; 478 (7369): 364 DOI: 10.1038/nature10513

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Impurity atoms introduce waves of disorder in exotic electronic material

 It's a basic technique learned early, maybe even before kindergarten: Pulling things apart -- from toy cars to complicated electronic materials -- can reveal a lot about how they work. "That's one way physicists study the things that they love; they do it by destroying them," said Séamus Davis, a physicist at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory and the J.G. White Distinguished Professor of Physical Sciences at Cornell University.

Davis and colleagues recently turned this destructive approach -- and a sophisticated tool for "seeing" the effects -- on a material they've been studying for its own intrinsic beauty, and for the clues it may offer about superconductivity, the ability of some materials to carry electric current with no resistance. The findings, published in the Proceedings of the National Academy of Sciences the week of October 17, 2011, reveal how substituting just a few atoms can cause widespread disruption of the delicate interactions that give the material its unique properties, including superconductivity.

The material, a compound of uranium, ruthenium, and silicon, is known as a "heavy-fermion" system. "It's a system where the electrons zooming through the material stop periodically to interact with electrons localized on the uranium atoms that make up the lattice, or framework of the crystal," Davis said. These stop-and-go magnetic interactions slow down the electrons, making them appear as if they've taken on extra mass, but also contribute to the material's superconductivity.

In 2010, Davis and a group of collaborators visualized these heavy fermions for the first time using a technique developed by Davis, known as spectroscopic imaging scanning tunneling microscopy (SI-STM), which measures the wavelength of electrons of the material in relation to their energy.

The idea of the present study was to "destroy" the heavy fermion system by substituting thorium for some of the uranium atoms. Thorium, unlike uranium, is non-magnetic, so in theory, the electrons should be able to move freely around the thorium atoms, instead of stopping for the brief magnetic encounters they have at each uranium atom. These areas where the electrons should flow freely are known as "Kondo holes," named for the physicist who first described the scattering of conductive electrons due to magnetic impurities.

Free-flowing electrons might sound like a good thing if you want a material that can carry current with no resistance. But Kondo holes turn out to be quite destructive to superconductivity. By visualizing the behavior of electrons around Kondo holes for the first time, Davis' current research helps to explain why.

"There have been beautiful theories that predict the effects of Kondo holes, but no one knew how to look at the behavior of the electrons, until now," Davis said.

Working with thorium-doped samples made by physicist Graeme Luke at McMaster University in Ontario, Davis' team used SI-STM to visualize the electron behavior.

"First we identified the sites of the thorium atoms in the lattice, then we looked at the quantum mechanical wave functions of the electrons surrounding those sites," Davis said.

The SI-STM measurements bore out many of the theoretical predictions, including the idea proposed just last year by physicist Dirk Morr of the University of Illinois that the electron waves would oscillate wildly around the Kondo holes, like ocean waves hitting a lighthouse.

"Our measurements revealed waves of disturbance in the 'quantum glue' holding the heavy fermions together," Davis said.

So, by destroying the heavy fermions -- which must pair up for the material to act as a superconductor -- the Kondo holes disrupt the material's superconductivity.

Davis' visualization technique also reveals how just a few Kondo holes can cause such widespread destruction: "The waves of disturbance surrounding each thorium atom are like the ripples that emanate from raindrops suddenly hitting a still pond on a calm day," he said. "And like those ripples, the electronic disturbances travel out quite a distance, interacting with one another. So it takes a tiny number of these impurities to make a lot of disorder."

What the scientists learn by studying the exotic heavy fermion system may also pertain to the mechanism of other superconductors that can operate at warmer temperatures.

"The interactions in high-temperature superconductors are horribly complicated," Davis said. "But understanding the magnetic mechanism that leads to pairing in heavy fermion superconductors -- and how it can so easily be disrupted -- may offer clues to how similar magnetic interactions might contribute to superconductivity in other materials."

This research was supported by the DOE's Office of Science, the Natural Sciences and Engineering Research Council of Canada, and the Canadian Institute for Advanced Research. Additional collaborators included Mohammad Hamidian and Ines Firmo of Brookhaven Lab and Cornell, and Andy Schmidt now at the University of California, Berkeley.

Story Source:

The above story is reprinted from materials provided by DOE/Brookhaven National Laboratory.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Mohammad H. Hamidian, Andrew R. Schmidt, Ines A. Firmo, Milan P. Allan, Phelim Bradley, Jim D. Garrett, Travis J. Williams, Graeme M. Luke, Yonatan Dubi, Alexander V. Balatsky, and J. C. Davis. How Kondo-holes create intense nanoscale heavy-fermion hybridization disorder. PNAS, October 17, 2011 DOI: 10.1073/pnas.1115027108

Taking the pulse of charge-separation processes: Self-organization gives rise to more efficient organic solar cells

 Organic solar cells have the potential to convert sunlight into electrical energy in an economical and environmentally friendly fashion. The challenge is that they still work less efficiently than inorganic semiconductors. Ultrafast measurements on hybrid cells now reveal a route to double their efficiency.

The use of organic photovoltaics for the production of electricity from sunlight offers an attractive and promising basis for an innovative and environmentally friendly means of energy supply. They can be manufactured quite economically and, because they are as bendable as plastic wrap, they can be processed flexibly. The problem is that they are yet markedly less efficient than conventional inorganic semiconductor cells.

The most crucial process in the conversion of light into electric current is the generation of free charge carriers. In the first step of photoconversion, upon absorption of light one component of the organic solar cell, usually a polymer, releases electrons that are taken up by the second component of the cell -- in this case silicon nanoparticles -- and can then be transported further.

"The mechanisms and the timescale of charge separation have been the subject of controversial scientific debate for many years," says LMU physics professor Eberhard Riedle. In cooperation with investigators at the Technical University in Munich and at Bayreuth University, Riedle and his group have now been able to dissect the process in detail. To do so, the researchers used a novel hybrid cell type containing both organic and inorganic constituents, in which silicon serves as the electron acceptor. Based on the insights obtained with this system, they developed a processing strategy to improve the structural order of the polymer -- and found that this enhances the efficiency of charge separation in organic semiconductors by up to twofold. Their findings provide a new way to optimize the performance of organic solar cells.

The key to this breakthrough lies in a unique, laser-based experimental setup, which combines extremely high temporal resolution of 40 femtoseconds (fs) with a very broadband detection. This allowed the team to follow the ultrafast processes induced by photon absorption in real time as they occur. Instead of the fullerenes used in typical organic cells, the researchers used silicon as the electron acceptor, a choice that has two major advantages.

"First, with these novel hybrid solar cells, we were able to probe the photophysical processes taking place in the polymer with greater precision than ever before, and secondly through the use of silicon, a much larger segment of the solar spectrum can be harnessed for electricity," says Riedle. It turns out that free charge carriers -- so called polarons -- are not generated immediately upon photoexcitation, but with a delay of about 140 fs. Primary photoexcitation of a polymer molecule first leads to the formation of an excited state, called an exciton. This then dissociates, releasing an electron, which is then transferred to the electron acceptor.

The loss of electrons leaves behind positively charged "holes" in the polymer and, as oppositely charged entities are attracted to one another by the Coulomb force, the two have a tendency to recombine. "In order to obtain free charge carriers, electron and hole must both be sufficiently mobile to overcome the Coulomb force," explains Daniel Herrmann, the first author of the new study. The team was able to show, for the first time, that this is much easier to achieve in polymers with an ordered, regular structure than with polymers that are chaotically arranged. In other words, a high degree of self-organization of the polymer significantly increases the efficiency of charge separation.

"The polymer that we used is one of the few known to have a tendency to self-organize. This tendency can be inhibited, but one can also increase the polymer's intrinsic propensity for self-organization by choosing appropriate processing parameters," Herrmann explains. By cleverly optimizing the processing of the polymer P3HT, the researchers succeeded in doubling the yield of free charge carriers -- and thereby significantly enhancing the efficiency of their experimental solar cells.

Story Source:

The above story is reprinted from materials provided by Ludwig-Maximilians-Universitaet Muenchen (LMU).

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Daniel Herrmann, Sabrina Niesar, Christina Scharsich, Anna Köhler, Martin Stutzmann, Eberhard Riedle. Role of Structural Order and Excess Energy on Ultrafast Free Charge Generation in Hybrid Polythiophene/Si Photovoltaics Probed in Real Time by Near-Infrared Broadband Transient Absorption. Journal of the American Chemical Society, 2011; 111021142308005 DOI: 10.1021/ja207887q


Nano funnel used to generate extreme ultraviolet light pulses

If you want to avoid spilling when you are pouring liquids in the kitchen you may appreciate a funnel. Funnels are not only useful tools in the kitchen. Light can also be efficiently concentrated with funnels. In this case, the funnels have to be about 10.000-times smaller.

An international team of scientists from the Korea Advanced Institute of Science and Technology (KAIST) in Daejeon (South Korea), the Max Planck Institute of Quantum Optics (MPQ) in Garching (Germany), and the Georgia State University (GSU) in Atlanta (USA) has now managed to concentrate the energy of infrared light pulses with a nano funnel and use the concentrated energy to generate extreme ultraviolet light flashes. These flashes, which repeated 75 million times per second, lasted only a few femtoseconds. The new technology can help in the future to measure the movement of electrons with the highest spatial and temporal resolution.

Light is convertible. The wavelengths composing the light can change through interactions with matter, where both the type of material and shape of the material are important for the frequency conversion. An international team of scientists from the Korea Advanced Institute of Science and Technology (KAIST), the Max Planck Institute of Quantum Optics (MPQ), and the Georgia State University (GSU) has now modified light waves with a nano funnel made out of silver. The scientists converted femtosecond laser pulses in the infrared spectral range to femtosecond light flashes in the extreme ultraviolet (EUV). Ultrashort, pulsed EUV light is used in laser physics to explore the inside of atoms and molecules. A femtosecond lasts only a millionth of a billionth of a second.

Light in the infrared (IR) can be converted to the EUV by a process known as high-harmonic generation, whereby the atoms are exposed to a strong electric field from the IR laser pulses. These fields have to be as strong as the fields holding the atom together. With these fields electrons can be extracted from the atoms and accelerated with full force back onto the atoms. Upon impact highly energetic radiation in the EUV is generated.

To reach the necessary strong electric fields for the production of EUV light, the team of scientists has now combined this scheme with a nano funnel in order to concentrate the electric field of the light. With their new technology, they were able to create a powerful EUV light source with wavelengths down to 20 nanometers. The light source exhibits a so far unreached high repetition rate: the few femtoseconds lasting EUV light flashes are repeated 75 million times per second.

The core of the experiment was a small, only a few micrometers long, slightly elliptical funnel made out of silver and filled with xenon gas. The tip of the funnel was only ca. 100 nanometers wide. The infrared light pulses were sent into the funnel entrance where they travel through towards the small exit. The electromagnetic forces of the light result in density fluctuations of the electrons on the inside of the funnel. Here, a small patch of the metal surface was positively charged, the next one negative and so on, resulting in new electromagnetic fields on the inside of the funnel, which are called surface plasmon polaritons. The surface plasmon polaritons travel towards the tip of the funnel, where the conical shape of the funnel results in a concentration of their fields. “The field on the inside of the funnel can become a few hundred times stronger than the field of the incident infrared light. This enhanced field results in the generation of EUV light in the Xe gas.”, explains Prof. Mark Stockman from GSU.

The nano funnel has yet another function. Its small opening at the exit acts as “doorman” for light wavelengths. Not every opening is passable for light. If the opening is smaller than half of a wavelength, the other side remains dark. The 100 nanometer large opening of the funnel did not allow the infrared light at 800 nm to pass. The generated EUV pulses with wavelengths down to 20 nanometers passed, however, without problems. “The funnel acts as an efficient wavelength filter: at the small opening only EUV light comes out.”, explains Prof. Seung-Woo Kim from KAIST, where the experiments were conducted.

“Due to their short wavelength and potentially short pulse duration reaching into the attosecond domain, extreme ultraviolet light pulses are an important tool for the exploration of electron dynamics in atoms, molecules and solids”, explains Seung-Woo Kim. Electrons are extremely fast, moving on attosecond timescales (an attosecond is a billionth of a billionth of a second). In order to capture a moving electron, light flashes are needed, which are shorter than the timescale of the motion. Attosecond light flashes have become a familiar tool in the exploration of electron motion. With the conventional techniques, they can only be repeated a few thousand times per second. This can change with the nano funnel. “We assume that the few femtosecond light flashes consist of trains of attosecond pulses”, argues Matthias Kling, group leader at MPQ. “With such pulse trains, we should be able to conduct experiments with attosecond time resolution at very high repetition rate.”

The repetition rate is important for e.g. the application of EUV pulses in electron spectroscopy on surfaces. Electrons repel each other by Coulomb forces. Therefore, it may be necessary to restrict the experimental conditions such that only a single electron is generated per laser shot. With low repetition rates, long data acquisition times would be required in order to achieve sufficient experimental resolution. “In order to conduct experiments with high spatial and temporal resolution within a sufficiently short time, a high repetition rate EUV source is needed”, explains Kling. The novel combination of laser technology and nanotechnology can help in the future to record movies of ultrafast electron motion on surfaces with so far unreached temporal and spatial resolution in the attosecond-nanometer domain.

Story Source:

The above story is reprinted from materials provided by Max Planck Institute of Quantum Optics.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

In-Yong Park, Seungchul Kim, Joonhee Choi, Dong-Hyub Lee, Young-Jin Kim, Matthias F. Kling, Mark I. Stockman, Seung-Woo Kim. Plasmonic generation of ultrashort extreme-ultraviolet light pulses. Nature Photonics, 2011; DOI: 10.1038/NPHOTON.2011.258

Research into energy flow features on the cover of Nature Chemistry

Molecules are made out of that are attached to one another by (which can be compared to little elastic bands that hold the atoms together). In chemical reactions, undergo processes that break old bonds and make new ones, resulting in different products. The skill of the chemist lies in knowing which molecules to mix, under what conditions, to form the desired product.

Most of the time, molecules are not reacting: the elastic bands holding the atoms together remain intact, and the atoms undergo small jiggles and weak vibrations that arise from the small amount of that atoms have at any given temperature.  This state of weak vibrations is called thermal equilibrium.

Chemical reactions happen far from equilibrium, however.  They require large amounts of energy to be located in the atoms whose bonds are going to break, giving way to strong vibrations that cause the elastic bands to stretch and ultimately snap.  In the moments immediately after a reaction has occurred and a new bond is formed, there is a complementary situation, and the atoms in that particular new bond vibrate very strongly.

Most chemical reactions take place with reactant molecules embedded in a sea of unreactive liquid (or solvent) molecules.  Common solvents, including water and a number of organic liquids, play an important role in both shuffling energy to reacting molecules, and subsequently shuffling it away after reaction has occurred.  However, when chemists think about reactions in liquids, they tend to overlook the underlying energy shuffle that transports energy to and from the chemical reaction.  Instead, they focus on the equilibrium states that occur well before, and well after, a reaction occurs.

The study by Dr David Glowacki and colleagues in Bristol’s School of Chemistry provides fundamental microscopic insight into ultrafast laser experiments that track the energy levels of products formed one millionth of a millionth of a second after a chemical reaction.

Using state-of-the-art computational models run on Bristol’s BlueCrystal supercomputer, Dr Glowacki and colleagues were able to resolve the energy shuffle associated with individual bond making and breaking in liquids.  The unprecedented level of detail afforded by their study allowed them to construct a ‘map’ of how energy flows in the immediate wake of a chemical reaction.

Their ‘ map’ reveals clear shortcomings in the physical models commonly used to describe the energy shuffle occurring alongside .  The new insight afforded by such ‘energy flow maps’ has the scope to help chemists working in areas as diverse as biochemistry, pharmaceutical chemistry, polymer chemistry, and nanoscience.

This paper is featured on the cover of the November issue of (image by Becca Rose and Dr David Glowacki). It follows on from recent work featured on the cover of Science, and involves a collaboration between theoretical chemists Dr David Glowacki and Professor Jeremy Harvey, and experimental physical chemists led by Professor Andrew Orr-Ewing.

More information: D. R. Glowacki, R. A. Rose, S. J. Greaves, A. J. Orr-Ewing, and J. N. Harvey, ‘Mapping ultrafast energy flow in the wake of solution phase bimolecular reactions’ Nature Chemistry, doi:10.1038/nchem.1154

Provided by University of Bristol (news : web)