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  • richardmitnick 8:26 am on May 15, 2015 Permalink | Reply
    Tags: , , , , Nanotechnology   

    From BNL: “Intense Lasers Cook Up Complex, Self-Assembled Nanomaterials” 

    Brookhaven Lab

    May 13, 2015
    Justin Eure

    New technique developed at Brookhaven Lab makes self-assembly 1,000 times faster and could be used for industrial-scale solar panels and electronics

    1
    Brookhaven Lab scientist Kevin Yager (left) and postdoctoral researcher Pawel Majewski with the new Laser Zone Annealing instrument at the Center for Functional Nanomaterials.

    Nanoscale materials feature extraordinary, billionth-of-a-meter qualities that transform everything from energy generation to data storage. But while a nanostructured solar cell may be fantastically efficient, that precision is notoriously difficult to achieve on industrial scales. The solution may be self-assembly, or training molecules to stitch themselves together into high-performing configurations.

    Now, scientists at the U.S. Department of Energy’s Brookhaven National Laboratory have developed a laser-based technique to execute nanoscale self-assembly with unprecedented ease and efficiency.

    “We design materials that build themselves,” said Kevin Yager, a scientist at Brookhaven’s Center for Functional Nanomaterials (CFN). “Under the right conditions, molecules will naturally snap into a perfect configuration. The challenge is giving these nanomaterials the kick they need: the hotter they are, the faster they move around and settle into the desired formation. We used lasers to crank up the heat.”

    Yager and Brookhaven Lab postdoctoral researcher Pawel Majewski built a one-of-a-kind machine that sweeps a focused laser-line across a sample to generate intense and instantaneous spikes in temperature. This new technique, called Laser Zone Annealing (LZA), drives self-assembly at rates more than 1,000 times faster than traditional industrial ovens. The results are described in the journal ACS Nano.

    “We created extremely uniform self-assembled structures in less than a second,” Majewski said. “Beyond the extraordinary speed, our laser also reduced the defects and degradations present in oven-heated materials. That combination makes LZA perfect for carrying small-scale laboratory breakthroughs into industry.”

    The scientists prepared the materials and built the LZA instrument at the CFN. They then analyzed samples using advanced electron microscopy at CFN and x-ray scattering at Brookhaven’s now-retired National Synchrotron Light Source (NSLS)—both DOE Office of Science User Facilities.

    “It was enormously gratifying to see that our predictions were accurate—the enormous thermal gradients led to a correspondingly enormous acceleration!” Yager said.

    2
    Illustration of the Lazer Zone Annealing instrument showing the precise laser (green) striking the un-assembled polymer (purple). The extreme thermal gradients produced by the laser sweeping across the sample cause rapid and pristine self-assembly.

    Ovens versus lasers

    Imagine preparing a complex cake, but instead of baking it in the oven, a barrage of lasers heats it to perfection in an instant. Beyond that, the right cooking conditions will make the ingredients mix themselves into a picture-perfect dish. This nanoscale recipe achieves something equally extraordinary and much more impactful.

    The researchers focused on so-called block copolymers, molecules containing two linked blocks with different chemical structures and properties. These blocks tend to repel each other, which can drive the spontaneous formation of complex and rigid nanoscale structures.

    “The price of their excellent mechanical properties is the slow kinetics of their self-assembly,” Majewski said. “They need energy and time to explore possibilities until they find the right configuration.”

    In traditional block copolymer self-assembly, materials are heated in a vacuum-sealed oven. The sample is typically “baked” for a period of 24 hours or longer to provide enough kinetic energy for the molecules to snap into place—much too long for commercial viability. The long exposure to high heat also causes inevitable thermal degradation, leaving cracks and imperfections throughout the sample.

    The LZA process, however, offers sharp spikes of heat to rapidly excite the polymers without the sustained energy that damages the material.

    “Within milliseconds, the entire sample is beautifully aligned,” Yager said. “As the laser sweeps across the material, the localized thermal spikes actually remove defects in the nanostructured film. LZA isn’t just faster, it produces superior results.”

    LZA generates temperatures greater than 500 degrees Celsius, but the thermal gradients—temperature variations tied to direction and location in a material—can reach more than 4,000 degrees per millimeter. While scientists know that higher temperatures can accelerate self-assembly, this is the first proof of dramatic enhancement by extreme gradients.

    Built from scratch

    “Years ago, we observed a subtle hint that thermal gradients could improve self-assembly,” Yager said. “I became obsessed with the idea of creating more and more extreme gradients, which ultimately led to building this laser setup, and pioneering a new technique.”

    The researchers needed a high concentration of technical expertise and world-class facilities to move the LZA from proposal to execution.

    “Only at the CFN could we develop this technique so quickly,” Majewski said. “We could do rapid instrument prototyping and sample preparation with the on-site clean room, machine shop, and polymer processing lab. We then combined CFN electron microscopy with x-ray studies at NSLS for an unbeatable evaluation of the LZA in action.”

    Added Yager, “The ability to make new samples at the CFN and then walk across the street to characterize them in seconds at NSLS was key to this discovery. The synergy between these two facilities is what allowed us to rapidly iterate to an optimized design.”

    The scientists also developed a new microscale surface thermometry technique called melt-mark analysis to track the exact heat generated by the laser pulses and tune the instrument accordingly.

    “We burned a few films initially before we learned the right operating conditions,” Majewski said. “It was really exciting to see the first samples being rastered by the laser and then using NSLS to discover exactly what happened.”

    Future of the technique

    The LZA is the first machine of its kind in the world, but it signals a dramatic step forward in scaling up meticulously designed nanotechnology. The laser can even be used to “draw” structures across the surface, meaning the nanostructures can assemble in well-defined patterns. This unparalleled synthesis control opens the door to complex applications, including electronics.

    “There’s really no limit to the size of a sample this technique could handle,” Yager said. “In fact, you could run it in a roll-to-roll mode—one of the leading manufacturing technologies.”

    The scientists plan to further develop the new technique to create multi-layer structures that could have immediate impacts on anti-reflective coatings, improved solar cells, and advanced electronics.

    This research and operations at CFN and NSLS were funded by the DOE Office of Science.

    See the full article here.

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    One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. The Laboratory’s almost 3,000 scientists, engineers, and support staff are joined each year by more than 5,000 visiting researchers from around the world.Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit, applied science and technology organization.
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  • richardmitnick 7:56 am on May 14, 2015 Permalink | Reply
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    From MIT: “Researchers build new fermion microscope” 


    MIT News

    May 13, 2015
    Jennifer Chu

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    Graduate student Lawrence Cheuk adjusts the optics setup for laser cooling of sodium atoms. Photo: Jose-Luis Olivares/MIT

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    Laser beams are precisely aligned before being sent into the vacuum chamber. Photo: Jose-Luis Olivares/MIT

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    Sodium atoms diffuse out of an oven to form an atomic beam, which is then slowed and trapped using laser light. Photo: Jose-Luis Olivares/MIT

    4
    A Quantum gas microscope for fermionic atoms. The atoms, potassium-40, are cooled during imaging by laser light, allowing thousands of photons to be collected by the microscope. Credit: Lawrence Cheuk/MIT

    5
    The Fermi gas microscope group: (from left) graduate students Katherine Lawrence and Melih Okan, postdoc Thomas Lompe, graduate student Matt Nichols, Professor Martin Zwierlein, and graduate student Lawrence Cheuk. Photo: Jose-Luis Olivares/MIT

    Instrument freezes and images 1,000 individual fermionic atoms at once.

    Fermions are the building blocks of matter, interacting in a multitude of permutations to give rise to the elements of the periodic table. Without fermions, the physical world would not exist.

    Examples of fermions are electrons, protons, neutrons, quarks, and atoms consisting of an odd number of these elementary particles. Because of their fermionic nature, electrons and nuclear matter are difficult to understand theoretically, so researchers are trying to use ultracold gases of fermionic atoms as stand-ins for other fermions.

    But atoms are extremely sensitive to light: When a single photon hits an atom, it can knock the particle out of place — an effect that has made imaging individual fermionic atoms devilishly hard.

    Now a team of MIT physicists has built a microscope that is able to see up to 1,000 individual fermionic atoms. The researchers devised a laser-based technique to trap and freeze fermions in place, and image the particles simultaneously.

    The new imaging technique uses two laser beams trained on a cloud of fermionic atoms in an optical lattice. The two beams, each of a different wavelength, cool the cloud, causing individual fermions to drop down an energy level, eventually bringing them to their lowest energy states — cool and stable enough to stay in place. At the same time, each fermion releases light, which is captured by the microscope and used to image the fermion’s exact position in the lattice — to an accuracy better than the wavelength of light.

    With the new technique, the researchers are able to cool and image over 95 percent of the fermionic atoms making up a cloud of potassium gas. Martin Zwierlein, a professor of physics at MIT, says an intriguing result from the technique appears to be that it can keep fermions cold even after imaging.

    “That means I know where they are, and I can maybe move them around with a little tweezer to any location, and arrange them in any pattern I’d like,” Zwierlein says.

    Zwierlein and his colleagues, including first author and graduate student Lawrence Cheuk, have published their results today in the journal Physical Review Letters.

    Seeing fermions from bosons

    For the past two decades, experimental physicists have studied ultracold atomic gases of the two classes of particles: fermions and bosons — particles such as photons that, unlike fermions, can occupy the same quantum state in limitless numbers. In 2009, physicist Marcus Greiner at Harvard University devised a microscope that successfully imaged individual bosons in a tightly spaced optical lattice. This milestone was followed, in 2010, by a second boson microscope, developed by Immanuel Bloch’s group at the Max Planck Institute of Quantum Optics.

    These microscopes revealed, in unprecedented detail, the behavior of bosons under strong interactions. However, no one had yet developed a comparable microscope for fermionic atoms.

    “We wanted to do what these groups had done for bosons, but for fermions,” Zwierlein says. “And it turned out it was much harder for fermions, because the atoms we use are not so easily cooled. So we had to find a new way to cool them while looking at them.”

    Techniques to cool atoms ever closer to absolute zero have been devised in recent decades. Carl Wieman, Eric Cornell, and MIT’s Wolfgang Ketterle were able to achieve Bose-Einstein condensation in 1995, a milestone for which they were awarded the 2001 Nobel Prize in physics. Other techniques include a process using lasers to cool atoms from 300 degrees Celsius to a few ten-thousandths of a degree above absolute zero.

    A clever cooling technique

    And yet, to see individual fermionic atoms, the particles need to be cooled further still. To do this, Zwierlein’s group created an optical lattice using laser beams, forming a structure resembling an egg carton, each well of which could potentially trap a single fermion. Through various stages of laser cooling, magnetic trapping, and further evaporative cooling of the gas, the atoms were prepared at temperatures just above absolute zero — cold enough for individual fermions to settle onto the underlying optical lattice. The team placed the lattice a mere 7 microns from an imaging lens, through which they hoped to see individual fermions.

    However, seeing fermions requires shining light on them, causing a photon to essentially knock a fermionic atom out of its well, and potentially out of the system entirely.

    “We needed a clever technique to keep the atoms cool while looking at them,” Zwierlein says.

    His team decided to use a two-laser approach to further cool the atoms; the technique manipulates an atom’s particular energy level, or vibrational energy. Each atom occupies a certain energy state — the higher that state, the more active the particle is. The team shone two laser beams of differing frequencies at the lattice. The difference in frequencies corresponded to the energy between a fermion’s energy levels. As a result, when both beams were directed at a fermion, the particle would absorb the smaller frequency, and emit a photon from the larger-frequency beam, in turn dropping one energy level to a cooler, more inert state. The lens above the lattice collects the emitted photon, recording its precise position, and that of the fermion.

    Zwierlein says such high-resolution imaging of more than 1,000 fermionic atoms simultaneously would enhance our understanding of the behavior of other fermions in nature — particularly the behavior of electrons. This knowledge may one day advance our understanding of high-temperature superconductors, which enable lossless energy transport, as well as quantum systems such as solid-state systems or nuclear matter.

    “The Fermi gas microscope, together with the ability to position atoms at will, might be an important step toward the realization of a quantum computer based on fermions,” Zwierlein says. “One would thus harness the power of the very same intricate quantum rules that so far hamper our understanding of electronic systems.”

    Zwierlein says it is a good time for Fermi gas microscopists: Around the same time his group first reported its results, teams from Harvard and the University of Strathclyde in Glasgow also reported imaging individual fermionic atoms in optical lattices, indicating a promising future for such microscopes.

    Zoran Hadzibabic, a professor of physics at Trinity College, says the group’s microscope is able to detect individual atoms “with almost perfect fidelity.”

    “They detect them reliably, and do so without affecting their positions — that’s all you want,” says Hadzibabic, who did not contribute to the research. “So far they demonstrated the technique, but we know from the experience with bosons that that’s the hardest step, and I expect the scientific results to start pouring out.”

    This research was funded in part by the National Science Foundation, the Air Force Office of Scientific Research, the Office of Naval Research, the Army Research Office, and the David and Lucile Packard Foundation.

    See the full article here.

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  • richardmitnick 7:16 am on May 8, 2015 Permalink | Reply
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    From MIT: “Plugging up leaky graphene” 


    MIT News

    May 8, 2015
    Jennifer Chu

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    In a two-step process, engineers have successfully sealed leaks in graphene. First, the team fabricated graphene on a copper surface (top left) — a process that can create intrinsic defects in graphene, shown as cracks on the surface. After lifting the graphene and depositing it on a porous surface (top right), the transfer creates further holes and tears. In a first step (bottom left), the team used atomic layer deposition to deposit hafnium (in gray) to seal intrinsic cracks, then plugged the remaining holes (bottom left) with nylon (in red), via interfacial polymerization.
    Courtesy of the researchers.

    For faster, longer-lasting water filters, some scientists are looking to graphene —thin, strong sheets of carbon — to serve as ultrathin membranes, filtering out contaminants to quickly purify high volumes of water.

    Graphene’s unique properties make it a potentially ideal membrane for water filtration or desalination. But there’s been one main drawback to its wider use: Making membranes in one-atom-thick layers of graphene is a meticulous process that can tear the thin material — creating defects through which contaminants can leak.

    Now engineers at MIT, Oak Ridge National Laboratory, and King Fahd University of Petroleum and Minerals (KFUPM) have devised a process to repair these leaks, filling cracks and plugging holes using a combination of chemical deposition and polymerization techniques. The team then used a process it developed previously to create tiny, uniform pores in the material, small enough to allow only water to pass through.

    Combining these two techniques, the researchers were able to engineer a relatively large defect-free graphene membrane — about the size of a penny. The membrane’s size is significant: To be exploited as a filtration membrane, graphene would have to be manufactured at a scale of centimeters, or larger.

    In experiments, the researchers pumped water through a graphene membrane treated with both defect-sealing and pore-producing processes, and found that water flowed through at rates comparable to current desalination membranes. The graphene was able to filter out most large-molecule contaminants, such as magnesium sulfate and dextran.

    Rohit Karnik, an associate professor of mechanical engineering at MIT, says the group’s results, published in the journal Nano Letters, represent the first success in plugging graphene’s leaks.

    “We’ve been able to seal defects, at least on the lab scale, to realize molecular filtration across a macroscopic area of graphene, which has not been possible before,” Karnik says. “If we have better process control, maybe in the future we don’t even need defect sealing. But I think it’s very unlikely that we’ll ever have perfect graphene — there will always be some need to control leakages. These two [techniques] are examples which enable filtration.”

    Sean O’Hern, a former graduate research assistant at MIT, is the paper’s first author. Other contributors include MIT graduate student Doojoon Jang, former graduate student Suman Bose, and Professor Jing Kong.

    A delicate transfer

    “The current types of membranes that can produce freshwater from saltwater are fairly thick, on the order of 200 nanometers,” O’Hern says. “The benefit of a graphene membrane is, instead of being hundreds of nanometers thick, we’re on the order of three angstroms — 600 times thinner than existing membranes. This enables you to have a higher flow rate over the same area.”

    O’Hern and Karnik have been investigating graphene’s potential as a filtration membrane for the past several years. In 2009, the group began fabricating membranes from graphene grown on copper — a metal that supports the growth of graphene across relatively large areas. However, copper is impermeable, requiring the group to transfer the graphene to a porous substrate following fabrication.

    However, O’Hern noticed that this transfer process would create tears in graphene. What’s more, he observed intrinsic defects created during the growth process, resulting perhaps from impurities in the original material.

    Plugging graphene’s leaks

    To plug graphene’s leaks, the team came up with a technique to first tackle the smaller intrinsic defects, then the larger transfer-induced defects. For the intrinsic defects, the researchers used a process called “atomic layer deposition,” placing the graphene membrane in a vacuum chamber, then pulsing in a hafnium-containing chemical that does not normally interact with graphene. However, if the chemical comes in contact with a small opening in graphene, it will tend to stick to that opening, attracted by the area’s higher surface energy.

    The team applied several rounds of atomic layer deposition, finding that the deposited hafnium oxide successfully filled in graphene’s nanometer-scale intrinsic defects. However, O’Hern realized that using the same process to fill in much larger holes and tears — on the order of hundreds of nanometers — would require too much time.

    Instead, he and his colleagues came up with a second technique to fill in larger defects, using a process called “interfacial polymerization” that is often employed in membrane synthesis. After they filled in graphene’s intrinsic defects, the researchers submerged the membrane at the interface of two solutions: a water bath and an organic solvent that, like oil, does not mix with water.

    In the two solutions, the researchers dissolved two different molecules that can react to form nylon. Once O’Hern placed the graphene membrane at the interface of the two solutions, he observed that nylon plugs formed only in tears and holes — regions where the two molecules could come in contact because of tears in the otherwise impermeable graphene — effectively sealing the remaining defects.

    Using a technique they developed last year, the researchers then etched tiny, uniform holes in graphene — small enough to let water molecules through, but not larger contaminants. In experiments, the group tested the membrane with water containing several different molecules, including salt, and found that the membrane rejected up to 90 percent of larger molecules. However, it let salt through at a faster rate than water.

    The preliminary tests suggest that graphene may be a viable alternative to existing filtration membranes, although Karnik says techniques to seal its defects and control its permeability will need further improvements.

    “Water desalination and nanofiltration are big applications where, if things work out and this technology withstands the different demands of real-world tests, it would have a large impact,” Karnik says. “But one could also imagine applications for fine chemical- or biological-sample processing, where these membranes could be useful. And this is the first report of a centimeter-scale graphene membrane that does any kind of molecular filtration. That’s exciting.”

    De-en Jiang, an assistant professor of chemistry at the University of California at Riverside, sees the defect-sealing technique as “a great advance toward making graphene filtration a reality.”

    “The two-step technique is very smart: sealing the defects while preserving the desired pores for filtration,” says Jiang, who did not contribute to the research. “This would make the scale-up much easier. One can produce a large graphene membrane first, not worrying about the defects, which can be sealed later.”

    This research was supported in part by the Center for Clean Water and Clean Energy at MIT and KFUPM, the U.S. Department of Energy, and the National Science Foundation.

    See the full article here.

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  • richardmitnick 3:48 pm on April 16, 2015 Permalink | Reply
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    From LBL: “News Center Major Advance in Artificial Photosynthesis Poses Win/Win for the Environment” 

    Berkeley Logo

    Berkeley Lab

    April 16, 2015
    Lynn Yarris (510) 486-5375

    1
    A major advance in artificial photosynthesis poses win/win for the environment – using sequestered CO2 for green chemistry, including renewable fuel production. (Photo by Caitlin Givens)

    A potentially game-changing breakthrough in artificial photosynthesis has been achieved with the development of a system that can capture carbon dioxide emissions before they are vented into the atmosphere and then, powered by solar energy, convert that carbon dioxide into valuable chemical products, including biodegradable plastics, pharmaceutical drugs and even liquid fuels.

    Scientists with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley have created a hybrid system of semiconducting nanowires and bacteria that mimics the natural photosynthetic process by which plants use the energy in sunlight to synthesize carbohydrates from carbon dioxide and water. However, this new artificial photosynthetic system synthesizes the combination of carbon dioxide and water into acetate, the most common building block today for biosynthesis.

    “We believe our system is a revolutionary leap forward in the field of artificial photosynthesis,” says Peidong Yang, a chemist with Berkeley Lab’s Materials Sciences Division and one of the leaders of this study. “Our system has the potential to fundamentally change the chemical and oil industry in that we can produce chemicals and fuels in a totally renewable way, rather than extracting them from deep below the ground.”

    2
    This break-through artificial photosynthesis system has four general components: (1) harvesting solar energy, (2) generating reducing equivalents, (3) reducing CO2 to biosynthetic intermediates, and (4) producing value-added chemicals.

    Yang, who also holds appointments with UC Berkeley and the Kavli Energy NanoSciences Institute (Kavli-ENSI) at Berkeley, is one of three corresponding authors of a paper describing this research in the journal Nano Letters. The paper is titled Nanowire-bacteria hybrids for unassisted solar carbon dioxide fixation to value-added chemicals. The other corresponding authors and leaders of this research are chemists Christopher Chang and Michelle Chang. Both also hold joint appointments with Berkeley Lab and UC Berkeley. In addition, Chris Chang is a Howard Hughes Medical Institute (HHMI) investigator. (See below for a full list of the paper’s authors.)

    The more carbon dioxide that is released into the atmosphere the warmer the atmosphere becomes. Atmospheric carbon dioxide is now at its highest level in at least three million years, primarily as a result of the burning of fossil fuels. Yet fossil fuels, especially coal, will remain a significant source of energy to meet human needs for the foreseeable future. Technologies for sequestering carbon before it escapes into the atmosphere are being pursued but all require the captured carbon to be stored, a requirement that comes with its own environmental challenges.

    3
    (From left) Peidong Yang, Christopher Chang and Michelle Chang led the development of an artificial photosynthesis system that can convert CO2 into valuable chemical products using only water and sunlight. (Photo by Roy Kaltschmidt)

    The artificial photosynthetic technique developed by the Berkeley researchers solves the storage problem by putting the captured carbon dioxide to good use.

    “In natural photosynthesis, leaves harvest solar energy and carbon dioxide is reduced and combined with water for the synthesis of molecular products that form biomass,” says Chris Chang, an expert in catalysts for carbon-neutral energy conversions. “In our system, nanowires harvest solar energy and deliver electrons to bacteria, where carbon dioxide is reduced and combined with water for the synthesis of a variety of targeted, value-added chemical products.”

    By combining biocompatible light-capturing nanowire arrays with select bacterial populations, the new artificial photosynthesis system offers a win/win situation for the environment: solar-powered green chemistry using sequestered carbon dioxide.

    “Our system represents an emerging alliance between the fields of materials sciences and biology, where opportunities to make new functional devices can mix and match components of each discipline,” says Michelle Chang, an expert in biosynthesis. “For example, the morphology of the nanowire array protects the bacteria like Easter eggs buried in tall grass so that these usually-oxygen sensitive organisms can survive in environmental carbon-dioxide sources such as flue gases.”

    The system starts with an “artificial forest” of nanowire heterostructures, consisting of silicon and titanium oxide nanowires, developed earlier by Yang and his research group.

    “Our artificial forest is similar to the chloroplasts in green plants,” Yang says. “When sunlight is absorbed, photo-excited electron−hole pairs are generated in the silicon and titanium oxide nanowires, which absorb different regions of the solar spectrum. The photo-generated electrons in the silicon will be passed onto bacteria for the CO2 reduction while the photo-generated holes in the titanium oxide split water molecules to make oxygen.”

    3
    Cross-sectional SEM image of the nanowire/bacteria hybrid array used in a revolutionary new artificial photosynthesis system.

    Once the forest of nanowire arrays is established, it is populated with microbial populations that produce enzymes known to selectively catalyze the reduction of carbon dioxide. For this study, the Berkeley team used Sporomusa ovata, an anaerobic bacterium that readily accepts electrons directly from the surrounding environment and uses them to reduce carbon dioxide.

    “S. ovata is a great carbon dioxide catalyst as it makes acetate, a versatile chemical intermediate that can be used to manufacture a diverse array of useful chemicals,” says Michelle Chang. “We were able to uniformly populate our nanowire array with S. ovata using buffered brackish water with trace vitamins as the only organic component.”

    Once the carbon dioxide has been reduced by S. ovata to acetate (or some other biosynthetic intermediate), genetically engineered E.coli are used to synthesize targeted chemical products. To improve the yields of targeted chemical products, the S. ovata and E.coli were kept separate for this study. In the future, these two activities – catalyzing and synthesizing – could be combined into a single step process.

    A key to the success of their artificial photosynthesis system is the separation of the demanding requirements for light-capture efficiency and catalytic activity that is made possible by the nanowire/bacteria hybrid technology. With this approach, the Berkeley team achieved a solar energy conversion efficiency of up to 0.38-percent for about 200 hours under simulated sunlight, which is about the same as that of a leaf.

    The yields of target chemical molecules produced from the acetate were also encouraging – as high as 26-percent for butanol, a fuel comparable to gasoline, 25-percent for amorphadiene, a precursor to the antimaleria drug artemisinin, and 52-percent for the renewable and biodegradable plastic PHB. Improved performances are anticipated with further refinements of the technology.

    “We are currently working on our second generation system which has a solar-to-chemical conversion efficiency of three-percent,” Yang says. “Once we can reach a conversion efficiency of 10-percent in a cost effective manner, the technology should be commercially viable.”

    In addition to the corresponding authors, other co-authors of the Nano Letters paper describing this research were Chong Liu, Joseph Gallagher, Kelsey Sakimoto and Eva Nichols.

    This research was primarily funded by the DOE Office of Science.

    See the full article here.

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  • richardmitnick 7:51 am on April 6, 2015 Permalink | Reply
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    From NOVA: “Silver Nanoparticles Could Give Millions Microbe-free Drinking Water” 

    PBS NOVA

    NOVA

    24 Mar 2015
    Cara Giaimo

    1
    Microbe-free drinking water is hard to come by in many areas of India.

    Chemists at the Indian Institute of Technology Madras have developed a portable, inexpensive water filtration system that is twice as efficient as existing filters. The filter doubles the well-known and oft-exploited antimicrobial effects of silver by employing nanotechnology. The team, led by Professor Thalappil Pradeep, plans to use it to bring clean water to underserved populations in India and beyond.

    Left alone, most water is teeming with scary things. A recent study showed that your average glass of West Bengali drinking water might contain E. coli, rotavirus, cryptosporidium, and arsenic. According to the World Health Organization, nearly a billion people worldwide lack access to clean water, and about 80% of illnesses in the developing world are water-related. India in particular has 16% of the world’s population and less than 3% of its fresh water supply. Ten percent of India’s population lacks water access, and every day about 1,600 people die of diarrhea, which is caused by waterborne microbes.

    Pradeep has spent over a decade using nanomaterials to chemically sift these pollutants out. He started by tackling endosulfan, a pesticide that was hugely popular until scientists determined that it destroyed ozone and brain cells in addition to its intended insect targets. Endosulfan is now banned in most places, but leftovers persist in dangerous amounts. After a bout of endosulfan poisoning in the southwest region of Kerala, Pradeep and his colleagues developed a drinking water filter that breaks the toxin down into harmless components. They licensed the design to a filtration company, who took it to market in 2007. It was “the first nano-chemistry based water product in the world,” he says.

    But Pradeep wanted to go bigger. “If pesticides can be removed by nanomaterials,” he remembers thinking, “can you also remove microbes without causing additional toxicity?” For this, Pradeep’s team put a new twist on a tried-and-true element: silver.

    Silver’s microbe-killing properties aren’t news—in fact, people have known about them for centuries, says Dr. David Barillo, a trauma surgeon and the editor of a recent silver-themed supplement of the journal Burns.

    “Alexander the Great stored and drank water in silver vessels when going on campaigns” in 335 BC, he says, and 19th century frontier-storming Americans dropped silver coins into their water barrels to suppress algae growth. During the space race, America and the Soviet Union both developed silver-based water purification techniques (NASA’s was “basically a silver wire sticking in the middle of a pipe that they were passing electricity through,” Barillo says). And new applications keep popping up: Barillo himself pioneered the use of silver-infused dressings to treat wounded soldiers in Afghanistan. “We’ve really run the gamut—we’ve gone from 300 BC to present day, and we’re still using it for the same stuff,” he says.

    No one knows exactly how small amounts of silver are able to kill huge swaths of microbes. According to Barillo, it’s probably a combination of attacks on the microbe’s enzymes, cell wall, and DNA, along with the buildup of silver free radicals, which are studded with unpaired electrons that gum up cellular systems. These microbe-mutilating strategies are so effective that they obscure our ability to study them, because we have nothing to compare them to. “It’s difficult to make something silver-resistant, even in the lab where you’re doing it intentionally,” Barillo says.

    But unlike equal-opportunity killers like endosulfan, silver knocks out the monsters and leaves the good guys alone. In low concentrations, it’s virtually harmless to humans. “It’s not a carcinogen, it’s not a mutagen, it’s not an allergen,” Barillo says. “It seems to have no purpose in human physiology—it’s not a metal that we need to have in our bodies like copper or magnesium. But it doesn’t seem to do anything bad either.”

    Though silver’s mysterious germ-killing properties are old news, Pradeep is taking advantage of them in new ways. The particles his team works with are less than 50 nanometers long on any one side—about four times smaller than the smallest bacteria. Working at this level allows him greater control over desired chemical reactions, and the ability to fine-tune his filters to improve efficiency or add specific effects. Two years ago, his team developed their biggest hit yet—a combination filter that kills microbes with silver and breaks down chemical toxins with other nanoparticles. It’s portable, works at room temperature, and doesn’t require electricity. Pradeep is working with the government to make these filters available to underserved communities. Currently 100,000 households have them; “by next year’s end,” he hopes, “it will reach 600,000 people.”

    The latest filter goes one better: it “tunes” the silver with carbonate, a negatively-charged ion that strips protective proteins from microbe cell membranes. This leaves the microbes even more vulnerable to silver’s attack. “In the presence of carbonate, silver is even more effective,” he explains, so he can use less of it: “Fifty parts per billion can be brought down to [25].” Unlike the earlier filter, this one kills viruses, too—good news, since according to the National Institute of Virology, most do not.

    Going from 50 parts per billion of silver to 25 may not seem like a huge leap. But for Pradeep—who aims to help a lot of people for a long time—every little bit counts. Filters that contain less silver are less expensive to produce. This is vital if you want to keep costs low enough for those who need them most to buy them, or to entice the government into giving them away. He estimates that one of his new filter units will cost about $2 per year, proportionately less than what the average American pays for water.

    Using less silver also improves sustainability. “Globally, silver is the most heavily used nanomaterial,” Pradeep says, and it’s not renewable: anything we use “is lost for the world.” If all filters used his carbonate trick, he points out, we could make twice as many of them before we run out of raw materials—and even more if, as he hopes, his future tunings bring the necessary amount down further. This will become especially important if his filters catch on in other places with no infrastructure and needy populations. “Ultimately, I want to use the very minimum quantity of silver,” he says.

    “Pradeep’s work shows enormous potential,” says Dr. Theresa Dankovich, a water filtration expert at the University of Virginia’s Center for Global Health. But, she points out, “carbonate anions are naturally occurring in groundwater and surface waters,” so “it warrants further study to determine how they are already enhancing the effect of silver ions and silver nanoparticles,” even without purposeful manipulation by chemists. Others see potential shortcomings. James Smith, a professor of environmental engineering at the University of Virginia and the inventor of a nanoparticle-coated clay filtering pot, worries that the nanotech-heavy production process “would not allow for manufacturing in a developing world setting,” especially if Pradeep’s continuous tweaking of the model deters large-scale companies from actually producing it.

    Nevertheless, Pradeep plans to continue scaling up. “If you can provide clean water, you have provided a solution for almost everything,” he says. When you have the lessons of history and the technology of the future, why settle for anything less?

    See the full article here.

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  • richardmitnick 7:29 am on April 6, 2015 Permalink | Reply
    Tags: , , , Nanotechnology   

    From AAAS: “U.S. takes possible first step toward regulating nanochemicals” 

    AAAS

    AAAS

    2 April 2015
    Puneet Kollipara

    1
    Nanocubes, which researchers have explored as a possible way to store hydrogen for energy. BASF/Flickr

    The U.S. Environmental Protection Agency (EPA) is ratcheting up its scrutiny of nanoscale chemicals amid concerns that they could pose unique environmental and health risks. Late last month, the agency proposed requiring companies to submit data on industrial nanomaterials that they already make and sell. Observers say EPA’s move could be a prelude to tighter federal regulation of nanomaterials, which have begun to show up in consumer products.

    For years, EPA has grappled with whether and how to use the Toxic Substances Control Act (TSCA), the nation’s leading chemical regulation law, to handle nanomaterials. TSCA is silent on nanoproducts, generally defined as materials composed of structures between 1 and 100 billionths of a meter. But many environmental groups worry that they potentially carry unknown risks by virtue of their size. Other observers, however, have argued that size alone shouldn’t trigger new regulation and that existing rules are adequate to deal with the new products.

    EPA’s 25 March proposal actually walks back an earlier version—now scrapped—that would have let the agency more easily clamp down on any new uses of nanomaterials. Still, the weaker version being proposed now represents the first time EPA would use its powers under TSCA to request information specifically on nanomaterials. (The proposal comes as Congress is debating revamping TSCA, which has drawn extensive criticism.)

    Under the rule, manufacturers would have to submit a range of data regarding the nanoscale substances they now make and that fall under TSCA’s scope—such as substances used in industrial applications. EPA wants to know how much the company is producing, for example, as well as potential public exposures, and manufacturing and processing methods. It also wants see any existing health and safety data. In addition, the agency would require manufacturers of proposed new nanomaterials to submit existing data before they want to start making and selling those substances.

    The rule wouldn’t force companies to generate any new health and safety data. And by itself, the rule wouldn’t restrict any nanomaterials’ use, EPA notes in its draft proposal. The agency’s actions “do not conclude and are not intended to conclude that nanoscale materials as a class, or specific uses of nanoscale materials, necessarily give rise to or are likely to cause harm,” the notice states. Rather, EPA says the information would let it better assess nanomaterials’ risks.

    And the agency states that its approach would help protect human health and the environment “without prejudging new technologies or creating unnecessary barriers to trade or hampering innovation.” EPA argues that case-by-case approach would jibe with a set of nanotech regulation principles released in 2011 by the White House Office of Science and Technology Policy. Those principles advise agencies against making one-size-fits-all judgments.

    The American Chemistry Council (ACC), the largest chemical industry trade group, is still evaluating the proposal, it said in a statement. But it “is particularly interested in how EPA defines the materials to be covered by the proposed rule,” says Jay West, manager of ACC’s Nanotechnology Panel, says in the statement.

    The proposal is “logical” and “creatively written,” says Lynn Bergeson, a managing partner with the law firm Bergeson & Campbell, P.C. in Washington, D.C., which advises companies on EPA regulatory compliance. Some companies may argue the rule is too broad or burdensome, she says, or worry that EPA’s move could stigmatize their products. But the government effort to collect information could potentially help the industry by reassuring a skeptical public, she adds. “If there are no data on which EPA is able to rely to conclude that there is no risk, then the agency really is not doing its job,” she says.

    The proposal is a good first step for EPA, says Jaydee Hanson, policy director at the International Center for Technology Assessment, a group in Washington, D.C., that has raised concerns about nanotechnology’s potential risks. But he worries that many companies might simply not respond and that the cash-strapped EPA would struggle to crack down on violators. And he worries that the proposal would let companies keep too much information secret, by claiming it as confidential business information. (TSCA reforms that Congress is debating would limit the types of information that companies could claim as confidential, he notes.) But Hanson is looking on the bright side. “We wish [EPA was] doing more, but we’re excited that they are doing it,” he says.

    Still, even with all the new information in hand, it’s unclear how much action EPA could take to restrict nanomaterials under current law. In general, EPA has moved slowly to regulate new chemicals, and struggled to meet the burden that TSCA sets on it for removing, restricting, or preventing the sale of chemicals found to be unsafe. Congress says it wants to make that process easier, but it is unclear how any new rules would apply to nanotechnologies.

    See the full article here.

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  • richardmitnick 11:47 am on March 25, 2015 Permalink | Reply
    Tags: , Nanotechnology,   

    From UW: “UW scientists build a nanolaser using a single atomic sheet” 

    U Washington

    University of Washington

    March 23, 2015
    Jennifer Langston

    1
    The ultra-thin semiconductor, which is about 100,000 times thinner than a human hair, stretches across the top of the photonic cavity.U of Washington

    University of Washington scientists have built a new nanometer-sized laser — using the thinnest semiconductor available today — that is energy efficient, easy to build and compatible with existing electronics.

    Lasers play essential roles in countless technologies, from medical therapies to metal cutters to electronic gadgets. But to meet modern needs in computation, communications, imaging and sensing, scientists are striving to create ever-smaller laser systems that also consume less energy.

    The UW nanolaser, developed in collaboration with Stanford University, uses a tungsten-based semiconductor only three atoms thick as the “gain material” that emits light. The technology is described in a paper published in the March 16 online edition of Nature.

    “This is a recently discovered, new type of semiconductor which is very thin and emits light efficiently,” said Sanfeng Wu, lead author and a UW doctoral candidate in physics. “Researchers are making transistors, light-emitting diodes, and solar cells based on this material because of its properties. And now, nanolasers.”

    Nanolasers — which are so small they can’t be seen with the eye — have the potential to be used in a wide range of applications from next-generation computing to implantable microchips that monitor health problems. But nanolasers so far haven’t strayed far from the research lab.

    Other nanolaser designs use gain materials that are either much thicker or that are embedded in the structure of the cavity that captures light. That makes them difficult to build and to integrate with modern electrical circuits and computing technologies.

    The UW version, instead, uses a flat sheet that can be placed directly on top of a commonly used optical cavity, a tiny cave that confines and intensifies light. The ultrathin nature of the semiconductor — made from a single layer of a tungsten-based molecule — yields efficient coordination between the two key components of the laser.

    The UW nanolaser requires only 27 nanowatts to kickstart its beam, which means it is very energy efficient.

    Other advantages of the UW team’s nanolaser are that it can be easily fabricated, and it can potentially work with silicon components common in modern electronics. Using a separate atomic sheet as the gain material offers versatility and the opportunity to more easily manipulate its properties.

    “You can think of it as the difference between a cell phone where the SIM card is embedded into the phone versus one that’s removable,” said co-author Arka Majumdar, UW assistant professor of electrical engineering and of physics.

    “When you’re working with other materials, your gain medium is embedded and you can’t change it. In our nanolasers, you can take the monolayer out or put it back, and it’s much easier to change around,” he said.

    2
    This emission map of the nano-device shows the light is confined by and emitted from the photonic cavity.U of Washington

    The researchers hope this and other recent innovations will enable them to produce an electrically-driven nanolaser that could open the door to using light, rather than electrons, to transfer information between computer chips and boards.

    The current process can cause systems to overheat and wastes power, so companies such as Facebook, Oracle, HP, Google and Intel with massive data centers are keenly interested in more energy-efficient solutions.

    Using photons rather than electrons to transfer that information would consume less energy and could enable next-generation computing that breaks current bandwidth and power limitations. The recently proven UW nanolaser technology is one step toward making optical computing and short distance optical communication a reality.

    “We all want to make devices run faster with less energy consumption, so we need new technologies,” said co-author Xiaodong Xu, UW associate professor of materials science and engineering and of physics. “The real innovation in this new approach of ours, compared to the old nanolasers, is that we’re able to have scalability and more controls.”

    Still, there’s more work to be done in the near future, Xu said. Next steps include investigating photon statistics to establish the coherent properties of the laser’s light.

    Co-authors are John Schaibley of the UW, Liefeng Feng of the UW and Tianjin University in China, Sonia Buckley and Jelena Vuckovic of Stanford University, Jiaqiang Yan and David G. Mandrus of Oak Ridge National Laboratory and the University of Tennessee, Fariba Hatami of Humboldt University in Berlin and Wang Yao of the University of Hong Kong.

    Primary funding came from the Air Force Office of Scientific Research. Other funders include the National Science Foundation, the state of Washington through the Clean Energy Institute, the Presidential Early Award for Scientists and Engineers administered through the Office of Naval Research, the U.S. Department of Energy, and the European Commission.

    For more information, contact Xu at xuxd@uw.edu and Majumdar at arka@uw.edu.

    See the full article here.

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    The University of Washington is one of the world’s preeminent public universities. Our impact on individuals, on our region, and on the world is profound — whether we are launching young people into a boundless future or confronting the grand challenges of our time through undaunted research and scholarship. Ranked number 10 in the world in Shanghai Jiao Tong University rankings and educating more than 54,000 students annually, our students and faculty work together to turn ideas into impact and in the process transform lives and our world. For more about our impact on the world, every day.

    So what defines us — the students, faculty and community members at the University of Washington? Above all, it’s our belief in possibility and our unshakable optimism. It’s a connection to others, both near and far. It’s a hunger that pushes us to tackle challenges and pursue progress. It’s the conviction that together we can create a world of good. Join us on the journey.

     
  • richardmitnick 8:41 am on March 19, 2015 Permalink | Reply
    Tags: , , Nanotechnology, , ,   

    From SLAC: “Scientists Watch Quantum Dots ‘Breathe’ in Response to Stress” 


    SLAC Lab

    March 18, 2015

    Nanocrystal Study at SLAC’s X-ray Laser Could Aid in the Design of New Materials

    1
    In this illustration, intense X-rays produced at SLAC’s Linac Coherent Light Source strike nanocrystals of a semiconductor material. Scientists used the X-rays to study an ultrafast “breathing” response in the crystals induced quadrillionths of a second earlier by laser light. (SLAC National Accelerator Laboratory)

    Researchers at the Department of Energy’s SLAC National Accelerator Laboratory watched nanoscale semiconductor crystals expand and shrink in response to powerful pulses of laser light. This ultrafast “breathing” provides new insight about how such tiny structures change shape as they start to melt – information that can help guide researchers in tailoring their use for a range of applications.

    In the experiment using SLAC’s Linac Coherent Light Source (LCLS) X-ray laser, a DOE Office of Science User Facility, researchers first exposed the nanocrystals to a burst of laser light, followed closely by an ultrabright X-ray pulse that recorded the resulting structural changes in atomic-scale detail at the onset of melting.

    SLAC LCLS Inside
    LCLS

    “This is the first time we could measure the details of how these ultrasmall materials react when strained to their limits,” said Aaron Lindenberg, an assistant professor at SLAC and Stanford who led the experiment. The results were published March 12 in Nature Communications.

    Getting to Know Quantum Dots

    The crystals studied at SLAC are known as “quantum dots” because they display unique traits at the nanoscale that defy the classical physics governing their properties at larger scales. The crystals can be tuned by changing their size and shape to emit specific colors of light, for example.

    So scientists have worked to incorporate them in solar panels to make them more efficient and in computer displays to improve resolution while consuming less battery power. These materials have also been studied for potential use in batteries and fuel cells and for targeted drug delivery.

    Scientists have also discovered that these and other nanomaterials, which may contain just tens or hundreds of atoms, can be far more damage-resistant than larger bits of the same materials because they exhibit a more perfect crystal structure at the tiniest scales. This property could prove useful in battery components, for example, as smaller particles may be able to withstand more charging cycles than larger ones before degrading.

    A Surprise in the ‘Breathing’ of Tiny Spheres and Nanowires

    In the LCLS experiment, researchers studied spheres and nanowires made of cadmium sulfide and cadmium selenide that were just 3 to 5 nanometers, or billionths of a meter, across. The nanowires were up to 25 nanometers long. By comparison, amino acids – the building blocks of proteins – are about 1 nanometer in length, and individual atoms are measured in tenths of nanometers.

    By examining the nanocrystals from many different angles with X-ray pulses, researchers reconstructed how they change shape when hit with an optical laser pulse. They were surprised to see the spheres and nanowires expand in width by about 1 percent and then quickly contract within femtoseconds, or quadrillionths of a second. They also found that the nanowires don’t expand in length, and showed that the way the crystals respond to strain was coupled to how their structure melts.

    In an earlier, separate study, another team of researchers had used LCLS to explore the response of larger gold particles on longer timescales.

    “In the future, we want to extend these experiments to more complex and technologically relevant nanostructures, and also to enable X-ray exploration of nanoscale devices while they are operating,” Lindenberg said. “Knowing how materials change under strain can be used together with simulations to design new materials with novel properties.”

    Participating researchers were from SLAC, Stanford and two of their joint institutes, the Stanford Institute for Materials and Energy Sciences (SIMES) and Stanford PULSE Institute; University of California, Berkeley; University of Duisburg-Essen in Germany; and Argonne National Laboratory. The work was supported by the DOE Office of Science and the German Research Council.

    See the full article here.

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    SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the DOE’s Office of Science.
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  • richardmitnick 9:04 am on February 19, 2015 Permalink | Reply
    Tags: , , , Nanotechnology   

    From MIT: “New nanogel for drug delivery” 


    MIT News

    February 19, 2015
    Anne Trafton | MIT News Office

    1

    Self-healing gel can be injected into the body and act as a long-term drug depot.

    Scientists are interested in using gels to deliver drugs because they can be molded into specific shapes and designed to release their payload over a specified time period. However, current versions aren’t always practical because must be implanted surgically.

    To help overcome that obstacle, MIT chemical engineers have designed a new type of self-healing hydrogel that could be injected through a syringe. Such gels, which can carry one or two drugs at a time, could be useful for treating cancer, macular degeneration, or heart disease, among other diseases, the researchers say.

    The new gel consists of a mesh network made of two components: nanoparticles made of polymers entwined within strands of another polymer, such as cellulose.

    “Now you have a gel that can change shape when you apply stress to it, and then, importantly, it can re-heal when you relax those forces. That allows you to squeeze it through a syringe or a needle and get it into the body without surgery,” says Mark Tibbitt, a postdoc at MIT’s Koch Institute for Integrative Cancer Research and one of the lead authors of a paper describing the gel in Nature Communications on Feb. 19.

    Koch Institute postdoc Eric Appel is also a lead author of the paper, and the paper’s senior author is Robert Langer, the David H. Koch Institute Professor at MIT. Other authors are postdoc Matthew Webber, undergraduate Bradley Mattix, and postdoc Omid Veiseh.

    Heal thyself

    Scientists have previously constructed hydrogels for biomedical uses by forming irreversible chemical linkages between polymers. These gels, used to make soft contact lenses, among other applications, are tough and sturdy, but once they are formed their shape cannot easily be altered.

    The MIT team set out to create a gel that could survive strong mechanical forces, known as shear forces, and then reform itself. Other researchers have created such gels by engineering proteins that self-assemble into hydrogels, but this approach requires complex biochemical processes. The MIT team wanted to design something simpler.

    “We’re working with really simple materials,” Tibbitt says. “They don’t require any advanced chemical functionalization.”

    The MIT approach relies on a combination of two readily available components. One is a type of nanoparticle formed of PEG-PLA copolymers, first developed in Langer’s lab decades ago and now commonly used to package and deliver drugs. To form a hydrogel, the researchers mixed these particles with a polymer — in this case, cellulose.

    Each polymer chain forms weak bonds with many nanoparticles, producing a loosely woven lattice of polymers and nanoparticles. Because each attachment point is fairly weak, the bonds break apart under mechanical stress, such as when injected through a syringe. When the shear forces are over, the polymers and nanoparticles form new attachments with different partners, healing the gel.

    Using two components to form the gel also gives the researchers the opportunity to deliver two different drugs at the same time. PEG-PLA nanoparticles have an inner core that is ideally suited to carry hydrophobic small-molecule drugs, which include many chemotherapy drugs. Meanwhile, the polymers, which exist in a watery solution, can carry hydrophilic molecules such as proteins, including antibodies and growth factors.

    Long-term drug delivery

    In this study, the researchers showed that the gels survived injection under the skin of mice and successfully released two drugs, one hydrophobic and one hydrophilic, over several days.

    This type of gel offers an important advantage over injecting a liquid solution of drug-delivery nanoparticles: While a solution will immediately disperse throughout the body, the gel stays in place after injection, allowing the drug to be targeted to a specific tissue. Furthermore, the properties of each gel component can be tuned so the drugs they carry are released at different rates, allowing them to be tailored for different uses.

    The researchers are now looking into using the gel to deliver anti-angiogenesis drugs to treat macular degeneration. Currently, patients receive these drugs, which cut off the growth of blood vessels that interfere with sight, as an injection into the eye once a month. The MIT team envisions that the new gel could be programmed to deliver these drugs over several months, reducing the frequency of injections.

    Another potential application for the gels is delivering drugs, such as growth factors, that could help repair damaged heart tissue after a heart attack. The researchers are also pursuing the possibility of using this gel to deliver cancer drugs to kill tumor cells that get left behind after surgery. In that case, the gel would be loaded with a chemical that lures cancer cells toward the gel, as well as a chemotherapy drug that would kill them. This could help eliminate the residual cancer cells that often form new tumors following surgery.

    “Removing the tumor leaves behind a cavity that you could fill with our material, which would provide some therapeutic benefit over the long term in recruiting and killing those cells,” Appel says. “We can tailor the materials to provide us with the drug-release profile that makes it the most effective at actually recruiting the cells.”

    The research was funded by the Wellcome Trust, the Misrock Foundation, the Department of Defense, and the National Institutes of Health.

    See the full article here.

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  • richardmitnick 2:38 am on February 14, 2015 Permalink | Reply
    Tags: , Nanotechnology,   

    From phys.org: “Getting two for one: ‘Bonus’ electrons in germanium nanocrystals can lead to better solar cells” 

    physdotorg
    phys.org

    February 14, 2015
    Ans Hekkenberg

    1
    The material is illuminated with photons. In some of the germanium nanocrystals, the photons cause electrons to be excited, and thus form an electron-hole (e-h) pair. There are two possibilities. (1) The incoming photon has an energy in the range between once and twice the bandgap energy. One e-h pair is formed. (2) The incoming photon has an energy of more than two times the bandgap energy. The excess energy of the electron – the ‘kinetic’ energy of the electron which is excited high up in the conduction band – is sufficient to create a second e-h pair in the same nanocrystal. In that way, carrier multiplication is achieved. Credit: Fundamental Research on Matter (FOM)

    Researchers from FOM, the University of Amsterdam, the Delft University of Technology and the University of the Algarve have discovered that when light hits germanium nanocrystals, the crystals produce ‘bonus electrons’. These additional electrons could increase the yield of solar cells and improve the sensitivity of photodetectors. The researchers will publish their work in Light: Science & Applications today.

    In nanocrystals, the absorption of a single photon can lead to the excitation of multiple electrons: two for one! This phenomenon, known as carrier multiplication, was already well known in silicon nanocrystals. Silicon is the most commonly used material in solar cells. However, the researchers found that carrier multiplication also occurs in germanium nanocrystals, which are more suitable for optimizing the efficiency than silicon nanocrystals. Their discovery could lead to better solar cells.

    Semiconductor physics

    Germanium and silicon are examples of semiconductors: materials that have an energy bandgap. When these materials absorb light, electrons from the band below this energy gap (valence band) leap to the band above the gap (conduction band). These excited ‘hot’ electrons and the holes they leave behind can be harvested to form an electrical current. They form the basic fuel for a solar cell.

    Nanocrystals and carrier multiplication

    If an absorbed photon contains more energy than an electron requires to leap over the bandgap, the excess energy can be used to excite a second electron. Earlier research has shown that a bandgap energy from 0.6 to 1.0 electronvolts is ideal to achieve this carrier multiplication.

    Nanocrystals are extremely small, about a thousand times smaller than the width of a human hair. Due to their size, the energy structure of the crystals is dramatically different from that of bulk material. In fact, the bandgap energy depends on the nanocrystal size. Bulk germanium has an energy bandgap of 0.67 electronvolts. By tuning the germanium nanocrystals’ size, the researchers can change the bandgap energy to values between 0.6 and 1.4 electronvolts. This is within the ideal range for optimizing carrier multiplication, or the amount of ‘bonus electrons’.

    Performing the experiment

    To investigate carrier multiplication in nanocrystals, the researchers used an optical technique called pump-probe spectroscopy. An initial laser pulse, called the pump, emits photons that excite the nanocrystal by creating one free electron in the conduction band. A second pulse of photons, called the probe, can then be absorbed by this electron.

    The researchers found that if the energy of the pump photon is twice the bandgap energy of the germanium nanocrystals, the probe light is absorbed by two electrons instead of one. This effect is the well-known fingerprint of carrier multiplication. In other words, if the pump photon carries sufficient energy, the hot electron contains enough excess energy to excite a second electron in the same nanocrystal. Using this carrier multiplication, germanium nanocrystals can help achieve the maximum efficiency of solar cells.

    See the full article here.

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    About Phys.org in 100 Words

    Phys.org™ (formerly Physorg.com) is a leading web-based science, research and technology news service which covers a full range of topics. These include physics, earth science, medicine, nanotechnology, electronics, space, biology, chemistry, computer sciences, engineering, mathematics and other sciences and technologies. Launched in 2004, Phys.org’s readership has grown steadily to include 1.75 million scientists, researchers, and engineers every month. Phys.org publishes approximately 100 quality articles every day, offering some of the most comprehensive coverage of sci-tech developments world-wide. Quancast 2009 includes Phys.org in its list of the Global Top 2,000 Websites. Phys.org community members enjoy access to many personalized features such as social networking, a personal home page set-up, RSS/XML feeds, article comments and ranking, the ability to save favorite articles, a daily newsletter, and other options.

     
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