Friday, May 30, 2014

Hitchhiking nanotubes show how cells stir themselves

Rice, Göttingen, VU researchers track single-molecule proteins in living cells 

Chemical engineers from Rice University and biophysicists from Georg-August Universität Göttingen in Germany and the VU University Amsterdam in the Netherlands have successfully tracked single molecules inside living cells with carbon nanotubes.
Through this new method, the researchers found that cells stir their interiors using the same motor proteins that serve in muscle contraction.
The study, which sheds new light on biological transport mechanisms in cells, appears this week in Science.
The team attached carbon nanotubes to transport molecules known as kinesin motors to visualize and track them as they moved through the cytoplasm of living cells.
“I am amazed how versatile carbon nanotubes are,” said co-author Matteo Pasquali, a Rice professor of chemical and biomolecular engineering and of chemistry. “We use them for a wide range of applications, from engineering conducting fibers to imaging in cells.”
Carbon nanotubes are hollow cylinders of pure carbon with one-atom-thick walls. They naturally fluoresce with near-infrared wavelengths when exposed to visible light, a property discovered at Rice by Professor Rick Smalley a decade ago and then leveraged by Rice Professor Bruce Weisman to image carbon nanotubes. When attached to a molecule, the hitchhiking nanotubes serve as tiny beacons that can be precisely tracked over long periods of time to investigate small, random motions inside cells.
“Any probe that can hitch the length and breadth of the cell, rough it, slum it, struggle against terrible odds, win through and still know where its protein is, is clearly a probe to be reckoned with,” said lead author Nikta Fakhri, paraphrasing “The Hitchhiker’s Guide to the Galaxy.” Fakhri, who earned her Rice doctorate in Pasquali’s lab in 2011, is currently a Human Frontier Science Program Fellow at Göttingen.
“In fact, the exceptional stability of these probes made it possible to observe intracellular motions from times as short as milliseconds to as long as hours,” she said.
For long-distance transport, such as along the long axons of nerve cells, cells usually employ motor proteins tied to lipid vesicles, the cell’s “cargo containers.” This process involves considerable logistics: Cargo needs to be packed, attached to the motors and sent off in the right direction.
“This research has helped uncover an additional, much simpler mechanism for transport within the cell interior,” said principal investigator Christoph Schmidt, a professor of physics at Göttingen. “Cells vigorously stir themselves, much in the way a chemist would accelerate a reaction by shaking a test tube. This will help them to move objects around in the highly crowded cellular environment.”
The researchers showed the same type of motor protein used for muscle contraction is responsible for stirring. They reached this conclusion after exposing the cells to drugs that suppressed these specific motor proteins. The tests showed that the stirring was suppressed as well.
The mechanical cytoskeleton of cells consists of networks of protein filaments, like actin. Within the cell, the motor protein myosin forms bundles that actively contract the actin network for short periods. The researchers found random pinching of the elastic actin network by many myosin bundles resulted in the global internal stirring of the cell. Both actin and myosin play a similar role in muscle contraction. 
The highly accurate measurements of internal fluctuations in the cells were explained in a theoretical model developed by VU co-author Fred MacKintosh, who used the elastic properties of the cytoskeleton and the force-generation characteristics of the motors.
“The new discovery not only promotes our understanding of cell dynamics, but also points to interesting possibilities in designing ‘active’ technical materials,” said Fakhri, who will soon join the Massachusetts Institute of Technology faculty as an assistant professor of physics. “Imagine a microscopic biomedical device that mixes tiny samples of blood with reagents to detect disease or smart filters that separate squishy from rigid materials.”
Co-authors of the study include graduate student Alok Wessel, technical assistant Charlotte Willms and research scientist Dieter Klopfenstein, all of the University of Göttingen.
The German Research Foundation, the Dutch Foundation for Fundamental Research on Matter, the Netherlands Organization for Scientific Research, the Welch Foundation, the National Science Foundation and the Human Frontier Science Program supported the research.
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View a short movie of nanotube-tagged proteins moving via stirring inside cells:


A thin carbon nanotube is attached to a molecular motor (yellow) that moves along microtubule filaments (green) that form the transport network of cells. This transport occurs in the highly crowded environment of the cytoplasm that includes a network of actin filaments (red). The fluorescent nanotube serves as a beacon for both the transport along the microtubule, as well as the buffeting of the microtubule by the highly agitated surrounding cytoplasm. (Credit: M. Leunissen, Dutch Data Design)










- See more at: http://news.rice.edu/2014/05/29/hitchhiking-nanotubes-show-how-cells-stir-themselves/#sthash.cQhqzdGm.dpuf

Thursday, May 29, 2014

Scientists Pinpoint the Creeping Nanocrystals Behind Lithium-Ion Battery Degradation


Two breakthrough studies track the nanoscale structural changes that degrade battery performance during cycles of charge and discharge
Huolin Xin
Materials scientist Huolin Xin in Brookhaven Lab's Center for Functional Nanomaterials.
Batteries do not age gracefully. The lithium ions that power portable electronics cause lingering structural damage with each cycle of charge and discharge, making devices from smartphones to tablets tick toward zero faster and faster over time. To stop or slow this steady degradation, scientists must track and tweak the imperfect chemistry of lithium-ion batteries with nanoscale precision.
“We discovered surprising and never-before-seen evolution and degradation patterns in two key battery materials,” said Huolin Xin, a materials scientist at Brookhaven Lab’s Center for Functional Nanomaterials (CFN) and coauthor on both studies. “Contrary to large-scale observation, the lithium-ion reactions actually erode the materials non-uniformly, seizing upon intrinsic vulnerabilities in atomic structure in the same way that rust creeps unevenly across stainless steel.”In two recent Nature Communicationspapers, scientists from several U.S. Department of Energy national laboratories—Lawrence Berkeley, Brookhaven, SLAC, and the National Renewable Energy Laboratory—collaborated to map these crucial billionths-of-a-meter dynamics and lay the foundation for better batteries.
Scientists used electron tomography techniques to create this 3D animation of the nickel-oxide nanosheet transformations during the lithium-ion battery charging process.
Xin used world-leading electron microscopy techniques in both studies to directly visualize the nanoscale chemical transformations of battery components during each step of the charge-discharge process. In an elegant and ingenious setup, the collaborations separately explored a nickel-oxide anode and a lithium-nickel-manganese-cobalt-oxide cathode—both notable for high capacity and cyclability—by placing samples inside common coin-cell batteries running under different voltages.
“Armed with a precise map of the materials’ erosion, we can plan new ways to break the patterns and improve performance,” Xin said.
In these experiments, lithium ions traveled through an electrolyte solution, moving into an anode when charging and a cathode when discharging. The processes were regulated by electrons in the electrical circuit, but the ions’ journeys—and the battery structures—subtly changed each time.

Chinks in Nano-Armor

For the nickel-oxide anode, researchers submerged the batteries in a liquid organic electrolyte and closely controlled the charging rates. They stopped at predetermined intervals to extract and analyze the anode. Xin and his collaborators rotated 20-nanometer-thick sheets of the post-reaction material inside a carefully calibrated transmission electron microscope (TEM) grid at CFN to catch the contours from every angle—a process called electron tomography.
experimental coin cell setup
In the experimental coin cell setup, a carbon supported transmission electron microscopy (TEM) grid loaded with a small amount of the nickel-oxide material was pressed against the bulk anode and submerged in the same electrolyte environment.
To see the way the lithium-ions reacted with the nickel oxide, the scientists used a suite of custom-written software to digitally reconstruct the three-dimensional nanostructures with single-nanometer resolution. Surprisingly, the reactions sprang up at isolated spatial points rather than sweeping evenly across the surface.
“Consider the way snowflakes only form around tiny particles or bits of dirt in the air,” Xin said. “Without an irregularity to glom onto, the crystals cannot take shape. Our nickel oxide anode only transforms into metallic nickel through nanoscale inhomogeneities or defects in the surface structure, a bit like chinks in the anode’s armor.”
The electron microscopy provided a crucial piece of the larger puzzle assembled in concert with Berkeley Lab materials scientists and soft x-ray spectroscopy experiments conducted at SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL). The combined data covered the reactions on the nano-, meso-, and microscales.

Rock-Salt Buildups

In the other study, scientists sought the voltage sweet-spot for the high-performing lithium-nickel-manganese-cobalt-oxide (NMC) cathode: How much power can be stored, at what intensity, and across how many cycles?
The answers hinged on intrinsic material qualities and the structural degradation caused by cycles at 4.7 volts and 4.3 volts, as measured against a lithium metal standard.
As revealed through another series of coin-cell battery tests, 4.7 volts caused rapid decomposition of the electrolytes and poor cycling—the higher power comes at a price. A 4.3-volt battery, however, offered a much longer cycling lifetime at the cost of lower storage and more frequent recharges.
In both cases, the chemical evolution exhibited sprawling surface asymmetries, though not without profound patterns.
atomic column in the NMC cathode
Each orange dot in these scanning transmission electron microscopy (STEM) images represents one atomic column in the NMC cathode. The scientists found that the lithium ions tended to travel along the vertical channels between atomic layers. After one full charge/discharge cycle, the surface layers (the edge beyond the blue line) exhibited the atomic disordering that ultimately diminishes battery performance.
“As the lithium ions race through the reaction layers, they cause clumping crystallization—a kind of rock-salt matrix builds up over time and begins limiting performance,” Xin said. “We found that these structures tended to form along the lithium-ion reaction channels, which we directly visualized under the TEM. The effect was even more pronounced at higher voltages, explaining the more rapid deterioration.”
Identifying this crystal-laden reaction pathways hints at a way forward in battery design.
“It may be possible to use atomic deposition to coat the NMC cathodes with elements that resist crystallization, creating nanoscale boundaries within the micron-sized powders needed at the cutting-edge of industry,” Xin said. “In fact, Berkeley Lab battery experts Marca Doeff and Feng Lin are working on that now.”
Shirley Meng, a professor at UC San Diego’s Department of NanoEngineering, added, “This beautiful study combines several complementary tools that probe both the bulk and surface of the NMC layered oxide—one of the most promising cathode materials for high-voltage operation that enables higher energy density in lithium-ion batteries. The meaningful insights provided by this study will significantly impact the optimization strategies for this type of cathode material.”
The TEM measurements revealed the atomic structures while electron energy loss spectroscopy helped pinpoint the chemical evolution—both carried out at the CFN. Further crucial research was conducted at SLAC’s SSRL and Berkeley Lab’s National Center for Materials Synthesis, Electrochemistry, and Electron Microscopy, with computational support from the National Energy Research Supercomputer Center and the Extreme Science and Engineering Discovery Environment.  

Toward Real-Time, Real-World Analyses

“The chemical reactions involved in these batteries are startlingly complex, and we need even more advanced methods of interrogation,” Xin said. “My CFN colleagues are developing ways to watch the reactions in real-time rather than the stop-and-go approach we used in these studies.” 
These in operando microscopy techniques, led in part by Brookhaven Lab materials scientists Dong Su, Feng Wang, and Eric Stach, will image reactions as they unfold in liquid environments. Custom-designed electrochemical contacts and liquid flow holders will usher in unprecedented insights.
Research at Brookhaven Lab’s CFN and SLAC’s SSRL—both DOE user facilities—was supported by DOE’s Office of Science. The NMC work was also supported through the Batteries for Advanced Transportation Technologies (BATT) program funded by DOE’s Office of Energy Efficiency and Renewable Energy and led by Berkeley Lab.
DOE's Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.
http://www.bnl.gov/newsroom/news.php?a=24805

Monday, May 26, 2014

Fractal Nanotruss Work



Fancy Erector Set? Nope. The elaborate fractal structure shown at right is many, many times smaller than that and is certainly not child's play. It is the latest example of what Julia Greer, professor of materials science and mechanics, calls a fractal nanotruss—nano because the structures are made up of members that are as thin as five nanometers (five billionths of a meter); truss because they are carefully architected structures that might one day be used in structural engineering materials.



Greer's group has developed a three-step process for building such complex structures very precisely. They first use a direct laser writing method called two-photon lithography to "write" a three-dimensional pattern in a polymer, allowing a laser beam to crosslink and harden the polymer wherever it is focused. At the end of the patterning step, the parts of the polymer that were exposed to the laser remain intact while the rest is dissolved away, revealing a three-dimensional scaffold. Next, the scientists coat the polymer scaffold with a continuous, very thin layer of a material—it can be a ceramic, metal, metallic glass, semiconductor, "just about anything," Greer says. In this case, they used alumina, or aluminum oxide, which is a brittle ceramic, to coat the scaffold. In the final step they etch out the polymer from within the structure, leaving a hollow architecture.

Taking advantage of some of the size effects that many materials display at the nanoscale, these nanotrusses can have unusual, desirable qualities. For example, intrinsically brittle materials, like ceramics, including the alumina shown, can be made deformable so that they can be crushed and still rebound to their original state without global failure.

"Having full control over the architecture gives us the ability to tune material properties to what was previously unattainable with conventional monolithic materials or with foams," says Greer. "For example, we can decouple strength from density and make materials that are both strong (and tough) as well as extremely lightweight. These structures can contain nearly 99 percent air yet can also be as strong as steel. Designing them into fractals allows us to incorporate hierarchical design into material architecture, which promises to have further beneficial properties."

The members of Greer's group who helped develop the new fabrication process and created these nanotrusses are graduate students Lucas Meza and Lauren Montemayor and Nigel Clarke, an undergraduate intern from the University of Waterloo.
Written by Kimm Fesenmaier

Source: http://www.caltech.edu/content/miniature-truss-work#sthash.xl98f5Tu.dpuf

Saturday, May 24, 2014

Repair protein’s DNA recognition motif

DNA replication – the process of copying the DNA each time a cell divides – must be completed accurately to avoid mutations that cause cancer and other diseases. The DNA damage response protein SMARCAL1 recognizes stalled replication “forks” and remodels the DNA to allow repair and restored replication. SMARCAL1 is essential to maintaining genome integrity during replication, but how it works is poorly understood.
Now, Brandt Eichman, Ph.D., and colleagues have determined the crystal structure of a region of SMARCAL1 (the HARP domain), which is fused to a motor domain. They used X-ray scattering to examine the conformation and assembly of the HARP domain in solution and found that the domain is conserved with DNA damage recognition domains from other DNA repair proteins. They showed that the HARP domain is a functional substitute for one of these regions and that mutations of predicted DNA-binding amino acids in the HARP domain reduced its ability to bind to replication forks and facilitate repair.
The studies, reported in the Proceedings of the National Academy of Sciences, uncovered a conserved recognition domain in DNA repair enzymes. This domain couples DNA recognition and remodeling and plays an important role in stabilizing replication forks and maintaining genome integrity.
The structure also illustrates the location of several SMARCAL1 mutations that cause Schimke immuno-osseous dysplasia (SIOD), a multi-system disorder characterized by growth defects, immune deficiencies and renal failure.
The findings are the latest in an ongoing collaboration between the teams of Eichman, associate professor of Biological Sciences and Biochemistry, David Cortez, Ph.D., professor of Biochemistry and Cancer Biology, and Walter Chazin, Ph.D., Chancellor’s Professor of Biochemistry and Chemistry. Together, the researchers aim to understand how DNA replication happens faithfully so that every cell ends up with exactly the same DNA – and without damaging mutations.
The research was supported by a pilot grant from the Vanderbilt Center in Molecular Toxicology and by National Institutes of Health grant CA136933.
http://news.vanderbilt.edu/2014/05/repair-proteins-dna-recognition-motif/

Friday, May 23, 2014

New Details on Microtubules and How the Anti-Cancer Drug Taxol Works

The most detailed look ever at the assembly and disassembly
of microtubules, tiny fibers of tubulin protein that
play a crucial role in cell division, provides new insight
into the success of the anti-cancer drug Taxol.

Berkeley Lab Researchers Take an Atomic-Scale Look at Key Cellular Protein


A pathway to the design of even more effective versions of the powerful anti-cancer drug Taxol has been opened with the most detailed look ever at the assembly and disassembly of microtubules, tiny fibers of tubulin protein that form the cytoskeletons of living cells and play a crucial role in mitosis. 

Through a combination of high-resolution cryo-electron microscopy (cryo-EM) and new methodology for image analysis and structure interpretation, researchers with the Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley have produced images of microtubule assembly and disassembly at the unprecedented resolution of 5 angstroms (Å). Among other insights, these observations provide the first explanation of Taxol’s success as a cancer chemotherapy agent.

“This is the first experimental demonstration of the link between nucleotide state and tubulin conformation within the microtubules and, by extension, the relationship between tubulin conformation and the transition from assembled to disassembled microtubule structure,” says Eva Nogales, a biophysicist with Berkeley Lab’s Life Sciences Division who led this research. “We now have a clear understanding of how hydrolysis of guanosine triphosphate (GTP) leads to microtubule destabilization and how Taxol works to inhibit this activity.”

Nogales, who is also a professor of biophysics and structural biology at UC Berkeley, as well as an investigator with the Howard Hughes Medical Institute, is the corresponding author of a paper describing this research in the journal Cell. The paper is entitled “High resolution αβ microtubule structures reveal the structural transitions in tubulin upon GTP hydrolysis.” Co-authors are Gregory Alushin, Gabriel Lander, Elizabeth Kellogg, Rui Zhang and David Baker.

Gregory Alushin and Eva Nogales led a team of researchers that produced images of microtubule assembly and disassembly at the unprecedented resolution of 5 angstroms. (Photo by Roy Kaltschmidt)
Gregory Alushin and Eva Nogales led a team of researchers that produced images of microtubule assembly and disassembly at the unprecedented resolution of 5 angstroms. (Photo by Roy Kaltschmidt)
During mitosis, the process by which a dividing cell duplicates its chromosomes and distributes them between two daughter cells, microtubules disassemble and reform into spindles across which the duplicate sets of chromosomes migrate. For chromosome migration to occur, the microtubules attached to them must disassemble, carrying the chromosomes in the process. 

The crucial ability of microtubules to transition from a rigid polymerized or “assembled” state to a flexible depolymerized or “disassembled” state – called “dynamic instability” – is driven by GTP hydrolysis in the microtubule lattice. Taxol prevents or dramatically slows down the unchecked cell division that is cancer by binding to a microtubule in such a manner as to block the effects of hydrolysis. However, until now the atomic details as to how microtubules transition from polymerized to depolymerized structures and the role that Taxol can play have been sketchy.

“Uncovering the atomic details of the conformational cycle accompanying polymerization, nucleotide hydrolysis, and depolymerization is essential for a complete description of microtubule dynamics,” Nogales says. “Such details should significantly aid in improving the potency and selectivity of existing anti-cancer drugs, as well as facilitate the development of novel agents.”

To find these details, Nogales, an expert in electron microscopy and image analysis and a leading authority on the structure and dynamics of microtubules, employed cryo-EM, in which protein samples are flash-frozen at liquid nitrogen temperatures to preserve their natural structure. Using an FEI 300 kV Titan cryo-EM from the laboratory of Robert Glaeser, she and her colleagues generated cryo-EM reconstructions of tubulin proteins whose structures were either stabilized by GMPCPP, a GTP analogue, or were unstable and bound to guanosine diphosphate (GDP), or were bound to GDP but stabilized by the presence of Taxol.
Alushin-Fig7_revised
The tubulin protein is a heterodimer consisting of alpha (α) and beta (β) monomer subunits. It features two guanine nucleotide binding sites, an “N-site” on the α-tubulin that is buried, and an “E-site” on the β-tubulin that is exposed when the tubulin is depolymerized. Previous microtubule reconstruction studies were unable to distinguish the highly similar α-tubulin and β-tubulin from each other.

“To be able to distinguish the α-tubulin from the β-tubulin, we had to resolve our images at better than 8 Å, which most prior cryo-EM studies were unable to do,” Nogales says. “For that, we marked the subunits with kinesin, a protein motor that distinguishes between α- and β-tubulin.”
Nogales and her colleagues found that GTP hydrolysis and the release of the phosphate (GTP becomes GDP) leads to a compaction of the E-site and a rearrangement of the α-tubulin monomer that generates a strain on the microtubule that destabilizes its structure. Taxol binding leads to a reversal of this E-site compaction and α-tubulin rearrangement that restores structural stabilization.

“Remarkably, Taxol binding globally reverses the majority of the conformational changes we observe when comparing the GMPCPP and GDP states,” Nogales says. “We propose that GTP hydrolysis leads to conformational strain in the microtubule that would be released by bending during depolymerization. This model is consistent with the changes we observe upon taxol binding, which dramatically stabilizes the microtubule lattice. Our analysis supports a model in which microtubule-stabilizing agents like Taxol modulate conformational strain and longitudinal contacts in the microtubule lattice.”
This research was supported by NIH’s National Institute of General Medical Sciences, the Damon Runyon Cancer Research Foundation, and the Howard Hughes Medical Institute.

http://newscenter.lbl.gov/news-releases/2014/05/22/new-details-on-microtubules/

Not all diamonds are forever

Images taken by Rice University scientists show that some diamonds are not forever.


Rice University researchers see nanodiamonds created in coal fade away in seconds

The Rice researchers behind a new study that explains the creation of nanodiamonds in treated coal also show that some microscopic diamonds only last seconds before fading back into less-structured forms of carbon under the impact of an electron beam.
The research by Rice chemist Ed Billups and his colleagues appears in the American Chemical Society’s Journal of Physical Chemistry Letters.
Nanodiamonds fading
A series of images shows a small nanodiamond (the dark spot in the lower right corner) reverting to anthracite. Rice University scientists saw nanodiamonds form in hydrogenated coal when hit by the electron beam used in high-resolution transmission electron microscopes. But smaller diamonds like this one degraded with subsequent images. The scale bar is 1 nanometer. Courtesy of the Billups Lab
Billups and Yanqiu Sun, a former postdoctoral researcher in his lab, witnessed the interesting effect while working on ways to chemically reduce carbon from anthracite coal and make it soluble. First they noticed nanodiamonds forming amid the amorphous, hydrogen-infused layers of graphite.
It happened, they discovered, when they took close-ups of the coal with an electron microscope, which fires an electron beam at the point of interest. Unexpectedly, the energy input congealed clusters of hydrogenated carbon atoms, some of which took on the lattice-like structure of nanodiamonds.
“The beam is very powerful,” Billups said. “To knock hydrogen atoms off of something takes a tremendous amount of energy.”
Even without the kind of pressure needed to make macroscale diamonds, the energy knocked loose hydrogen atoms to prompt a chain reaction between layers of graphite in the coal that resulted in diamonds between 2 and 10 nanometers wide.
But the most “nano” of the nanodiamonds were seen to fade away under the power of the electron beam in a succession of images taken over 30 seconds.
“The small diamonds are not stable and they revert to the starting material, the anthracite,” Billups said.
Billups turned to Rice theoretical physicist Boris Yakobson and his colleagues at the Technological Institute for Superhard and Novel Carbon Materials in Moscow to explain what the chemists saw. Yakobson, Pavel Sorokin and Alexander Kvashnin had already come up with a chart – called a phase diagram — that demonstrated how thin diamond films might be made without massive pressure.
They used similar calculations to show how nanodiamonds could form in treated anthracite and subbituminous coal. In this case, the electron microscope’s beam knocks hydrogen atoms loose from carbon layers. Then the dangling bonds compensate by connecting to an adjacent carbon layer, which is prompted to connect to the next layer. The reaction zips the atoms into a matrix characteristic of diamond until pressure forces the process to halt.
Natural, macroscale diamonds require extreme pressures and temperatures to form, but the phase diagram should be reconsidered for nanodiamonds, the researchers said.
“There is a window of stability for diamonds within the range of 19-52 angstroms (tenths of a nanometer), beyond which graphite is more stable,” Billups said. Stable nanodiamonds up to 20 nanometers in size can be formed in hydrogenated anthracite, they found, though the smallest nanodiamonds were unstable under continued electron-beam radiation.
Billups noted subsequent electron-beam experiments with pristine anthracite formed no diamonds, while tests with less-robust infusions of hydrogen led to regions with “onion-like fringes” of graphitic carbon, but no fully formed diamonds. Both experiments lent support to the need for sufficient hydrogen to form nanodiamonds.
Kvashnin is a former visiting student at Rice and a graduate student at the Moscow Institute of Physics and Technology (MIPT). Sorokin holds appointments at MIPT and the National University of Science and Technology, Moscow. Yakobson is Rice’s Karl F. Hasselmann Professor of Mechanical Engineering and Materials Science, a professor of chemistry and a member of the Richard E. Smalley Institute for Nanoscale Science and Technology. Billups is a professor of chemistry at Rice.
The Robert A. Welch Foundation, the Ministry of Education and Science of the Russian Federation and the Russian Foundation for Basic Research supported the research.
http://news.rice.edu/2014/05/22/not-all-diamonds-are-forever-2/#sthash.42KRKrBn.dpuf

Thursday, May 22, 2014

Resonant Energy Transfer from Quantum Dots to Graphene

Schematic of a quantum dot-graphene nano-photonic device,
as described in this research project.
Semiconductor quantum dots (QDs) are nanoscale semiconductors that exhibit size dependent physical properties. For example, the color (wavelength) of light that they absorb changes dramatically as the diameter decreases. 
Graphene is an atomically thick sheet of carbon atoms, arranged in a hexagonal lattice pattern. In this work, QDs have been combined with graphene to develop nanoscale photonic devices that can dramatically improve our ability to detect light. Quantum dots can absorb light and transfer it to graphene, but the efficiency of the transfer depends on how far the QDs and the graphene are separated from each other. 
This study demonstrated that the thickness of the organic molecule layer that typically surrounds the QDs is crucial in attaining sufficiently high efficiency of this light/energy transfer into the graphene. In other works, the thinner the organic layer, the better. This transfer can be further optimized by engineering the interface between the two nanomaterials, specifically optimizing the thickness of the organic capping molecules on the quantum dots. Based on this work, further improvement of the performance of these nano-photonic devices can be expected.

Why Does This Matter?

chloride-terminated CdSe quantum dot
a) Schematic of a chloride-terminated CdSe quantum dot. b) A high resolution transmission electron microscopy image of such quantum dots.
Commercial cadmium selenide (CdSe) quantum dots have long insulating organic ligands that prevent their utilization in energy and charge transfer applications for which short distances between the QDs and other materials are critical.  Short, chlorine ligands that passivated CdSe QDs are an intriguing alternative material to enhance the interaction with materials into which charge carriers, such as electrons, can easily conduct.  Graphene is such a material.  The combination of CdSe quantum dots and graphene could hold the key to the development and implementation of nanoscale materials systems in flexible electronics and photodetectors.
Photoluminescence lifetime decay of isolated quantum dots
Photoluminescence lifetime decay of isolated quantum dots on glass (blue) and graphene (red) demonstrate efficient energy transfer between the quantum dots and graphene.

What Are The Details?

  • CFN Capabilities: The Advanced Optical Microscopy Facility measured the time-resolved photoluminescence from isolated CdSe quantum dots deposited on graphene.
  • The team discovered that short, chloride-capped CdSe quantum dots, deposited on chemical-vapor-deposited, monolayer layer graphene, exhibited highly efficient energy transfer to the graphene with a 4x observed reduction in the excitonic lifetime.  This demonstrated significant near-field coupling between quantum dots and the graphene.  

http://www.bnl.gov/newsroom/news.php?a=24906

Wednesday, May 21, 2014

Engineers Build World’s Smallest, Fastest Nanomotor




Researchers at the Cockrell School of Engineering at The University of Texas at Austin have built the smallest, fastest and longest-running tiny synthetic motor to date. The team’s nanomotor is an important step toward developing miniature machines that could one day move through the body to administer insulin for diabetics when needed, or target and treat cancer cells without harming good cells.
With the goal of powering these yet-to-be invented devices, UT Austin engineers focused on building a reliable, ultra-high-speed nanomotor that can convert electrical energy into mechanical motion on a scale 500 times smaller than a grain of salt.
Mechanical engineering assistant professor Donglei “Emma” Fan led a team of researchers in the successful design, assembly and testing of a high-performing nanomotor in a nonbiological setting. The team’s three-part nanomotor can rapidly mix and pump biochemicals and move through liquids, which is important for future applications. The team’s study was published in the April issue of Nature Communications.
Fan and her team are the first to achieve the extremely difficult goal of designing a nanomotor with large driving power.
With all its dimensions under 1 micrometer in size, the nanomotor could fit inside a human cell and is capable of rotating for 15 continuous hours at a speed of 18,000 RPMs, the speed of a motor in a jet airplane engine. Comparable nanomotors run significantly more slowly, from 14 RPMs to 500 RPMs, and have only rotated for a few seconds up to a few minutes.
Looking forward, nanomotors could advance the field of nanoelectromechanical systems (NEMS), an area focused on developing miniature machines that are more energy efficient and less expensive to produce. In the near future, the Cockrell School researchers believe their nanomotors could provide a new approach to controlled biochemical drug delivery to live cells.
emma fan
Mechanical engineering assistant professor Donglei "Emma" Fan
To test its ability to release drugs, the researchers coated the nanomotor’s surface with biochemicals and initiated spinning. They found that the faster the nanomotor rotated, the faster it released the drugs.
“We were able to establish and control the molecule release rate by mechanical rotation, which means our nanomotor is the first of its kind for controlling the release of drugs from the surface of nanoparticles,” Fan said. “We believe it will help advance the study of drug delivery and cell-to-cell communications.”
The researchers address two major issues for nanomotors so far: assembly and controls. The team built and operated the nanomotor using a patent-pending technique that Fan invented while studying at Johns Hopkins University. The technique relies on AC and DC electric fields to assemble the nanomotor’s parts one by one.
In experiments, the researchers used the technique to turn the nanomotors on and off and propel the rotation either clockwise or counterclockwise. The researchers found that they could position the nanomotors in a pattern and move them in a synchronized fashion, which makes them more powerful and gives them more flexibility.
Fan and her team plan to develop new mechanical controls and chemical sensing that can be integrated into nanoelectromechanical devices. But first they plan to test their nanomotors near a live cell, which will allow Fan to measure how they deliver molecules in a controlled fashion.
Cockrell School graduate students Kwanoh Kim, Xiaobin Xu and Jianhe Guo co-authored the study. The National Science Foundation Career Award, the Welch Foundation and startup funds from the Cockrell School supported the study.
http://www.engr.utexas.edu/features/nanomotors

Tuesday, May 20, 2014

Scientists discover how to turn light into matter after 80-year quest


Imperial physicists have discovered how to create matter from light - a feat thought impossible when the idea was first theorised 80 years ago.

In just one day over several cups of coffee in a tiny office in Imperial’s Blackett Physics Laboratory, three physicists worked out a relatively simple way to physically prove a theory first devised by scientists Breit and Wheeler in 1934.

Breit and Wheeler suggested that it should be possible to turn light into matter by smashing together only two particles of light (photons), to create an electron and a positron – the simplest method of turning light into matter ever predicted. The calculation was found to be theoretically sound but Breit and Wheeler said that they never expected anybody to physically demonstrate their prediction. It has never been observed in the laboratory and past experiments to test it have required the addition of massive high-energy particles.

"What was so surprising to us was the discovery of how we can create matter directly from light using the technology that we have today in the UK." – Professor Steve Rose, Department of Physics


The new research, published in Nature Photonics, shows for the first time how Breit and Wheeler’s theory could be proven in practice. This ‘photon-photon collider’, which would convert light directly into matter using technology that is already available, would be a new type of high-energy physics experiment. This experiment would recreate a process that was important in the first 100 seconds of the universe and that is also seen in gamma ray bursts, which are the biggest explosions in the universe and one of physics’ greatest unsolved mysteries.

The scientists had been investigating unrelated problems in fusion energy when they realised what they were working on could be applied to the Breit-Wheeler theory. The breakthrough was achieved in collaboration with a fellow theoretical physicist from the Max Planck Institute for Nuclear Physics, who happened to be visiting Imperial.

Demonstrating the Breit-Wheeler theory would provide the final jigsaw piece of a physics puzzle which describes the simplest ways in which light and matter interact (see image). The six other pieces in that puzzle, including Dirac’s 1930 theory on the annihilation of electrons and positrons and Einstein’s 1905 theory on the photoelectric effect, are all associated with Nobel Prize-winning research (see image).

Professor Steve Rose from the Department of Physics at Imperial College London said: “Despite all physicists accepting the theory to be true, when Breit and Wheeler first proposed the theory, they said that they never expected it be shown in the laboratory. Today, nearly 80 years later, we prove them wrong. What was so surprising to us was the discovery of how we can create matter directly from light using the technology that we have today in the UK. As we are theorists we are now talking to others who can use our ideas to undertake this landmark experiment.”



Theories describing light and matter interactions.
Credit: Oliver Pike, Imperial College London



"Within a few hours of looking for applications of hohlraums outside their traditional role in fusion energy research, we were astonished to find they provided the perfect conditions for creating a photon collider. The race to carry out and complete the experiment is on!" – Oliver Pike, Department of Physics

The collider experiment that the scientists have proposed involves two key steps. First, the scientists would use an extremely powerful high-intensity laser to speed up electrons to just below the speed of light. They would then fire these electrons into a slab of gold to create a beam of photons a billion times more energetic than visible light.

The next stage of the experiment involves a tiny gold can called a hohlraum (German for ‘empty room’). Scientists would fire a high-energy laser at the inner surface of this gold can, to create a thermal radiation field, generating light similar to the light emitted by stars.

They would then direct the photon beam from the first stage of the experiment through the centre of the can, causing the photons from the two sources to collide and form electrons and positrons. It would then be possible to detect the formation of the electrons and positrons when they exited the can.

Lead researcher Oliver Pike who is currently completing his PhD in plasma physics, said: “Although the theory is conceptually simple, it has been very difficult to verify experimentally. We were able to develop the idea for the collider very quickly, but the experimental design we propose can be carried out with relative ease and with existing technology. Within a few hours of looking for applications of hohlraums outside their traditional role in fusion energy research, we were astonished to find they provided the perfect conditions for creating a photon collider. The race to carry out and complete the experiment is on!”

http://www3.imperial.ac.uk/newsandeventspggrp/imperialcollege/newssummary/news_16-5-2014-15-32-44

Friday, May 16, 2014

Fast and curious: Electrons hurtle into the interior of a new class of quantum materials

Scientists at Princeton University have shown that negatively charged particles known as electrons can flow extremely rapidly due to quantum behaviors in a type of material known as a topological Dirac semi-metal. Previous work by the same group indicated that these electrons can flow on the surface of certain materials, but the new research indicates that they can also flow through the bulk of the material, in this case cadmium arsenide. Using a technique called angle-resolved photoemission spectroscopy (left), the researchers measured the energy and momentum of electrons as they were ejected from the cadmium arsenide. The resulting data revealed each electron as two cones oriented opposite each other that converge at a point, a telltale sign of the quantum behavior that allows electrons to act like light, which has no mass. A 3-D reconstruction (right) shows that the cone-shaped electrons are able to move in all directions in the material. The top-right panel reveals that these electrons are linked, allowing them to move even when deformed by bending or stretching, an attribute that gives them their topological nature. (Image courtesy of M. Zahid Hasan and Suyang Xu)

As smartphones get smarter and computers compute faster, researchers actively search for ways to speed up the processing of information. Now, scientists at Princeton University have made a step forward in developing a new class of materials that could be used in future technologies.
They have discovered a new quantum effect that enables electrons — the negative-charge-carrying particles that make today's electronic devices possible — to dash through the interior of these materials with very little resistance.
The discovery is the latest chapter in the story of a curious material known as a "topological insulator," in which electrons whiz along the surface without penetrating the interior. The newest research indicates that these electrons also can flow through the interior of some of these materials.
"With this discovery, instead of facing the challenge of how to use only the electrons on the surface of a material, now you can just cut the material open and you have light-like electrons flowing in three dimensions inside the materials," said M. Zahid Hasan, a professor of physics at Princeton, who led the discovery.
The finding was conducted by a team of scientists from the United States, Taiwan, Singapore, Germany and Sweden and published in two papers in the journal Nature Communications. The first paper, published May 7, demonstrates that fast electrons can flow in the interior of crystals made from cadmium and arsenic, or cadmium arsenide. The second paper, published May 12, explores fast electrons in a material made from the elements bismuth and selenium.
In most materials, including copper and other metals that conduct electricity, electrons navigate an obstacle course of microscopic outcroppings, ledges and other imperfections that obstruct the tiny particles and send them scattering in the wrong directions. This causes resistance and the conversion of electrical current into heat, which is why electronic appliances become warm during use.
In topological insulators and the new class of materials the Princeton researchers studied, the unique properties of the atoms combine to create quantum effects that coax electrons into acting similar to a light wave instead of like individual particles. These waves can weave around and dodge — and even move through — barriers that would typically stop most electrons. These properties were theoretically proposed by Charles Kane and a team at the University of Pennsylvania from 2005 to 2007 and first observed experimentally in solid materials by the Hasan group in 2007 and 2008.
In 2011, the Hasan group detected this fast electron-flow in the interior of a material made from combining several elements — bismuth, thallium, sulfur and selenium. The results were published in the journal Science.
In the new study in cadmium arsenide, the electrons have an average velocity that is 10,000 times more than that of the previous bismuth-based materials identified by the group. "This is a big deal," Hasan said. "It means the electrons can flow quite easily in the material and many more exotic quantum effects can now be studied. That just wasn't possible in the past."
The most promising application for these materials may be for a proposed "topological quantum computer" based on novel electronics that would use a property of electrons known as "spin" to do calculations and transmit information.
The quantum behavior in this new class of materials has led them to be called "topological Dirac semi-metals" in reference to English quantum physicist and 1933 Nobel Prize winner Paul Dirac, who noted that electrons could behave like light. Semi-metals that are "topological" are ones that retain their spatial electronic properties — and their speedy electrons — even when deformed by certain types of stretching and twisting.
The speeds achieved by these electrons have led to comparisons to another novel electronic material, graphene. The new class of materials has the potential to be superior to graphene in some aspects, Hasan said, because graphene is a single layer of atoms in which electrons can flow only in two dimensions. Cadmium arsenide permits electrons to flow in three dimensions.
The new study redefines what it means to be a topological material, according to Su-Yang Xu, a graduate student in Hasan's lab and co-first author of the May 7 paper with postdoctoral research associate Madhab Neupane at Princeton and Raman Sankar of National Taiwan University.
"The term topological insulator is now quite famous, and the yet term 'insulator' means that there are no electrons flowing in the bulk of the material," Xu said. "Our study shows that electrons are flowing in the bulk of the material, so clearly cadmium arsenide is not an insulator, but it is still topological in nature, so this is a totally new type of quantum matter," he said.
The team made the discovery using a technique called angle-resolved photoemission spectroscopy. The researchers shined a very powerful X-ray beam — using a particle accelerator at the Advanced Light Source at Lawrence Berkeley National Laboratory — onto the surface of the material then monitored the electrons as they were knocked out of the interior.
"When the electron comes out, we measure its energy and velocity, and what we found is that electrons coming out of the cadmium arsenide had measurements that were similar to what is seen in particles that are massless," Neupane said.
In the second paper in Nature Communications, Neupane and co-authors presented a model for controlling the spin direction of the electron particles in a different material, bismuth selenide.
"The Princeton group showed in exquisite details that electrons in certain solids obey the three- dimensional massless Dirac equation," said Patrick Lee, a professor of physics at the Massachusetts Institute of Technology who was not involved in the work. "While predicted by theoretical calculations, this behavior has never been seen before in real materials until this past year. This work adds greatly to the ongoing excitement of how topology can impact electronic states in real materials."
The first study, "Observation of a three-dimensional topological Dirac semimetal phase in high-mobility Cd3As2" appeared in the journal Nature Communications on May 7, 2014. The co-first-authors were Madhab Neupane and Su-Yang Xu of Princeton University and Raman Sankar of National Taiwan University. Additional researchers at Princeton who contributed to the work were graduate students Nasser Alidoust and Ilya Belopolski, and postdoctoral research associates Guang Bian and Chang Liu. The team also included Tay-Rong Chang of National Tsing Hua University in Taiwan; Horng-Tay Jeng of National Tsing Hua University and Academia Sinica in Taiwan; Hsin Lin of National University of Singapore; Arun Bansil of Northeastern University; and Fangcheng Chou of National Taiwan University.
The second study, "Observation of a quantum-tunnelling-modulated spin texture in ultrathin topological insulator Bi2Se3 films," appeared in the journal Nature Communications on May 12, 2014. The first author was Madhab Neupane. Co-authors at Princeton were Su-Yang Xu, Nasser Alidoust, Ilya Belopolski, Chang Liu and Guang Bian. Also on the team were Anthony Richardella, Duming Zhang and Nitin Samarth of Pennsylvania State University; Jaime Sánchez-Barriga, Dmitry Marchenko, Oliver Rader and Andrei Varykhalov of Helmholtz Centre Berlin for Materials and Energy; Mats Leandersson and Thiagarajan Balasubramanian of MAX-lab, Sweden; Tay-Rong Chang of National Tsing Hua University in Taiwan; Horng-Tay Jeng of National Tsing Hua University and Academia Sinica in Taiwan; Hsin Lin of the National University of Singapore; and Susmita Basak and Arun Bansil of Northeastern University.
http://www.princeton.edu/main/news/archive/S40/01/00C88/index.xml?section=topstories,featured