martedì 16 ottobre 2012

Magnetic nanoparticles used to control thousands of cells simultaneously.

Using clusters of tiny magnetic particles about 1,000 times smaller than the width of a human hair, researchers from the UCLA Henry Samueli School of Engineering and Applied Science have shown that they can manipulate how thousands of cells divide, morph and develop finger-like extensions.
This new tool could be used in developmental biology to understand how tissues develop, or in cancer research to uncover how cancer cells move and invade surrounding tissues, the researchers said. The UCLA team's findings were published online Oct. 14 in the journal Nature Methods. A cell can be considered a complex biological machine that receives an assortment of "inputs" and produces specific "outputs," such as growth, movement, division or the production of molecules. Beyond the type of input, cells are extremely sensitive to the location of an input, partly because cells perform "spatial multiplexing," reusing the same basic biochemical signals for different functions at different locations within the cell. Understanding this localization of signals is particularly challenging because scientists lack tools with sufficient resolution and control to function inside the miniature environment of a cell. And any usable tool would have to be able to perturb many cells with similar characteristics simultaneously to achieve an accurate distribution of responses, since the responses of individual cells can vary. To address this problem, an interdisciplinary UCLA team that included associate professor of bioengineering Dino Di Carlo, postdoctoral scholar Peter Tseng and professor of electrical engineering Jack Judy developed a platform to precisely manipulate magnetic nanoparticles inside uniformly shaped cells. These nanoparticles produced a local mechanical signal and yielded distinct responses from the cells.
By determining the responses of thousands of single cells with the same shape to local nanoparticle-induced stimuli, the researchers were able to perform an automated averaging of the cells' response. To achieve this platform, the team first had to overcome the challenge of moving such small particles (each measuring 100 nanometers) through the viscous interior of a cell once the cells engulfed them. Using ferromagnetic technologies, which enable magnetic materials to switch "on" and "off," the team developed an approach to embed a grid of small ferromagnetic blocks within a microfabricated glass slide and to precisely place individual cells in proximity to these blocks with a pattern of proteins that adhere to cells. When an external magnetic field is applied to this system, the ferromagnetic blocks are switched "on" and can therefore pull the nanoparticles within the cells in specific directions and uniformly align them. The researchers could then shape and control the forces in thousands of cells at the same time. Using this platform, the team showed that the cells responded to this local force in several ways, including in the way they divided. When cells go through the process of replication to create two cells, the axis of division depends on the shape of the cell and the anchoring points by which the cell holds on to the surface. The researchers found that the force induced by the nanoparticles could change the axis of cell division such that the cells instead divided along the direction of force. The researchers said this sensitivity to force may shed light on the intricate forming and stretching of tissues during embryonic development. Besides directing the axis of division, they found that nanoparticle-induced local force also led to the activation of a biological program in which cells generate filopodia, which are finger-like, actin-rich extensions that cells often use to find sites to adhere to and which aid in movement.
Di Carlo, the principal investigator on the research, envisions that the technique can apply beyond the control of mechanical stimuli in cells. "Nanoparticles can be coated with a variety of molecules that are important in cell signaling," he said. "We should now have a tool to quantitatively investigate how the precise location of molecules in a cell produces a specific behavior. This is a key missing piece in our tool-set for understanding cell programs and for engineering cells to perform useful functions." More information: Provided by University of California, Los Angeles.

lunedì 15 ottobre 2012

Graphene researchers make a layer cake with atomic precision.

Graphene and associated one-atom-thick crystals offer the possibility of a vast range of new materials and devices by stacking individual atomic layers on top of each other, new research from the University of Manchester shows.
In a report published in Nature Physics, a group led Dr Leonid Ponomarenko and Nobel prize-winner Professor Andre Geim has assembled individual atomic layers on top of each other in a desired sequence. The team used individual one-atom-thick crystals to construct a multilayer cake that works as a nanoscale electric transformer. Graphene, isolated for the first time at The University of Manchester in 2004, has the potential to revolutionise diverse applications from smartphones and ultrafast broadband to drug delivery and computer chips. It has the potential to replace existing materials, such as silicon, but the Manchester researchers believe it could truly find its place with new devices and materials yet to be invented. In the nanoscale transformer, electrons moving in one metallic layer pull electrons in the second metallic layer by using their local electric fields. To operate on this principle, the metallic layers need to be insulated electrically from each other but separated by no more than a few interatomic distances, a giant leap from the existing nanotechnologies. These new structures could pave the way for a new range of complex and detailed electronic and photonic devices which no other existing material could make, which include various novel architectures for transistors and detectors. The scientists used graphene as a one-atom-thick conductive plane while just four atomic layers of boron nitride served as an electrical insulator.
The researchers started with extracting individual atomic planes from bulk graphite and boron nitride by using the same technique that led to the Nobel Prize for graphene, a single atomic layer of carbon. Then, they used advanced nanotechnology to mechanically assemble the crystallites one by one, in a Lego style, into a crystal with the desired sequence of planes. The nano-transformer was assembled by Dr Roman Gorbachev, of The University of Manchester, who described the required skills. He said: "Every Russian and many in the West know The Tale of the Clockwork Steel Flea. "It could only be seen through the most powerful microscope but still danced and even had tiny horseshoes. Our atomic-scale Lego perhaps is the next step of craftsmanship". Professor Geim added: "The work proves that complex devices with various functionalities can be constructed plane by plane with atomic precision. "There is a whole library of atomically-thin materials. By combining them, it is possible to create principally new materials that don't exist in nature. This avenue promises to become even more exciting than graphene itself." More information: 'Strong Coulomb drag and broken symmetry in double-layer graphene', by L Ponomarenko, R Gorbachev and A Geim, Nature Physics, 2012.Journal reference: Nature Physics Provided by University of Manchester.

Accelerators can search for signs of Planck-scale gravity.

Although quantum theory can explain three of the four forces in nature, scientists currently rely on general relativity to explain the fourth force, gravity. However, no one is quite sure of how gravity works at very short distances, in particular the shortest distance of all: the Planck length, or 10^-35 m. So far, the smallest distance accessible in experiments is about 10^-19 m at the LHC.
Now in a new paper published in Physical Review Letters, physicist Vahagn Gharibyan of Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Germany, has proposed a test of quantum gravity that can reach a sensitivity of 10^-31 m down to the Planck length, depending on the energy of the particle accelerator. As Gharibyan explains, several models of quantum gravity predict that empty space near the Planck length may behave like a crystal in the sense that the space is refractive (light is bent due to "gravitons," the hypothetical particles that mediate gravity) and has birefringence/chirality (the light's bending degree also depends on the light's polarization). In quantum gravity, both refractivity and birefringence are energy-dependent: the higher the photon energy, the stronger the photon-graviton interaction and the more bending. This correlation is the opposite of what happens when photons interact with electromagnetic fields or matter, where these effects are suppressed by photon energy. The predicted correlation also differs from what happens according to Newtonian gravity and Einstein's general relativity, where any bending of light is independent of the light's energy. "If one describes gravity at the quantum level, the bending of light by gravitation becomes energy-dependent – unlike in Newtonian gravity or Einstein's general relativity," Gharibyan told "The higher the energy of the photons, the larger the bending, or the stronger the photon-graviton interaction should be."
Gharibyan suggests that this bending of light according to quantum gravity models may be studied using high-energy accelerator beams that probe the vacuum symmetry of empty space at small scales. Accelerators could use high-energy Compton scattering, in which a photon that scatters off another moving particle acquires energy, causing a change in its momentum. The proposed experiments could detect how the effects of quantum gravity change the photon's energy-momentum relation compared with what would be expected on a normal scale. For these experiments, the beam energy is vital in determining the sensitivity to small-scale effects. Gharibyan estimates that a 6 GeV energy lepton accelerator, such as PETRA-III at DESY, could test space birefringence down to 10^-31 m. Future accelerators that could achieve energies of up to 250 GeV, such as the proposed International Linear Collider (ILC), could test birefringence all the way down to the Planck length. For probing refractivity, Gharibyan estimates that a 6 GeV machine would have a sensitivity down to 10^-27 m, while a 250 GeV machine could reach about 10^-31 m. As Gharibyan explains, probing Planck-scale gravity in this way is somewhat similar to investigating nanoscale crystal structures.
"Conventional crystals have cell sizes around tens of nanometers and are transparent to, or do not interact with, photons with much larger (m or mm) wavelengths," Gharibyan said. "In order to investigate crystal cells/structures, one needs photons with compatible nm wavelength: X-rays. However, visible light with wavelengths 1000 times more than the crystal cell can still feel the averaged influence of the cells: the light could be reflected singly or doubly. Comparing this to the Planck-length crystal, we don't have photons with a Planck wavelength or that huge energy. Instead, we are able to feel the averaged effects of Planck crystal cells – or space grains – by using much [relatively] lower-energy photons." In fact, as Gharibyan has found, there are already experimental hints of gravitons. "This work presents evidence for quantum gravity interactions by applying the developed method to gamma rays faster than light, which I found earlier in data from the largest US and German electron accelerators," he said. "The absence of any starlight deflection in the cosmic vacuum hints that Earth's gravitons should be considered responsible for the observed bending of the accelerators' gamma rays." Gharibyan found that data from the now-closed 26.5 GeV Hadron-Electron Ring Accelerator (HERA) at DESY measured a Planck cell size of 2.6x10^-28 m, and data from the mothballed 45.6 GeV Stanford Linear Collider (SLC) at Stanford University in the US measured a space grain size of 3.5x10^-30 m. While these results provide some hints of Planck-scale gravity, neither of these experiments was designed as a tool to specifically test gravity, so Gharibyan warns that uncontrolled pieces of setups could mimic observed effects.
If Gharibyan's newly proposed experiments are performed, they would provide the first direct measurements of space near or even at the Planck scale, and by doing so, offer a closer glimpse of gravity in this enigmatic regime. More information: Vahagn Gharibyan. "Testing Planck-Scale Gravity with Accelerators." Physical Review Letters 109, 141103 (2012). DOI: 10.1103/PhysRevLett.109.141103 Vahagn Gharibyan. "Possible observation of photon speed energy dependence." Physics Letters B 611 231-238 (2005). DOI: 10.1016/j.physletb.2005.02.053

Quantum oscillator responds to pressure.

In the far future, superconducting quantum bits might serve as components of high-performance computers. Today already do they help better understand the structure of solids, as is reported by researchers of Karlsruhe Institute of Technology in the Science magazine. By means of Josephson junctions, they measured the oscillations of individual atoms "tunneling" be-tween two positions. This means that the atoms oscillated quantum mechanically. Deformation of the specimen even changed the frequency. 
"We are now able to directly control the frequencies of individual tunneling atoms in the solid," say Alexey Ustinov and Georg Weiß, Professors at the Physikalisches Institut of KIT and members of the Center for Functional Nanostructures CFN. Metaphorically speaking, the researchers so far have been confronted with a closed box. From inside, different clattering noises could be heard. Now, it is not only possible to measure the individual objects contained, but also to change their physical properties in a controlled manner. The specimen used for this purpose consists of a superconducting ring interrupted by a nanometer-thick non-conductor, a so-called Josephson junction. The qubit formed in this way can be switched very precisely between two quantum states. "Interestingly, such a Josephson qubit couples to the other atomic quantum systems in the non-conductor," explains Ustinov. "And we measure their tunneling frequencies via this coupling." At temperatures slightly above absolute zero, most sources of noise in the material are switched off. The only remaining noise is produced by atoms of the material when they jump between two equivalent positions. "These frequency spectra of atom jumps can be measured very precisely with the Josephson junction," says Ustinov. "Metaphorically speaking, we have a microscope for the quantum mechanics of individual atoms." In the experiment performed, 41 jumping atoms were counted and their frequency spectra were measured while the specimen was bent slightly with a piezo element. Georg Weiß explains: "The atomic dis-tances are changed slightly only, while the frequencies of the tunneling atoms change strongly." So far, only the sum of all tunneling atoms could be measured. The technology to separately switch atomic tunneling systems only emerged a few years ago. The new method developed at KIT to control atomic quantum systems might provide valuable insights into how qubits can be made fit for applica-tion. However, the method is also suited for studying materials of conventional electronic components, such as transistors, and estab-lishing the basis of further miniaturization. More information: DOI: 10.1126/science.1226487 Provided by Karlsruhe Institute of Technology.

sabato 13 ottobre 2012

Physicists propose method to determine if the universe is a simulation.

(—A common theme of science fiction movies and books is the idea that we're all living in a simulated universe—that nothing is actually real. This is no trivial pursuit: some of the greatest minds in history, from Plato, to Descartes, have pondered the possibility. Though, none were able to offer proof that such an idea is even possible. Now, a team of physicists working at the University of Bonn have come up with a possible means for providing us with the evidence we are looking for; namely, a measurable way to show that our universe is indeed simulated. They have written a paper describing their idea and have uploaded it to the preprint server arXiv.
The team's idea is based on work being done by other scientists who are actively engaged in trying to create simulations of our universe, at least as we understand it. Thus far, such work has shown that to create a simulation of reality, there has to be a three dimensional framework to represent real world objects and processes. With computerized simulations, it's necessary to create a lattice to account for the distances between virtual objects and to simulate the progression of time. The German team suggests such a lattice could be created based on quantum chromodynamics—theories that describe the nuclear forces that bind subatomic particles. To find evidence that we exist in a simulated world would mean discovering the existence of an underlying lattice construct by finding its end points or edges. In a simulated universe a lattice would, by its nature, impose a limit on the amount of energy that could be represented by energy particles. This means that if our universe is indeed simulated, there ought to be a means of finding that limit. In the observable universe there is a way to measure the energy of quantum particles and to calculate their cutoff point as energy is dispersed due to interactions with microwaves and it could be calculated using current technology. Calculating the cutoff, the researchers suggest, could give credence to the idea that the universe is actually a simulation. Of course, any conclusions resulting from such work would be limited by the possibility that everything we think we understand about quantum chromodynamics, or simulations for that matter, could be flawed.
More information: Constraints on the Universe as a Numerical Simulation, arXiv:1210.1847 [hep-ph]  
Observable consequences of the hypothesis that the observed universe is a numerical simulation performed on a cubic space-time lattice or grid are explored. The simulation scenario is first motivated by extrapolating current trends in computational resource requirements for lattice QCD into the future. Using the historical development of lattice gauge theory technology as a guide, we assume that our universe is an early numerical simulation with unimproved Wilson fermion discretization and investigate potentially-observable consequences. Among the observables that are considered are the muon g-2 and the current differences between determinations of alpha, but the most stringent bound on the inverse lattice spacing of the universe, b^(-1) >~ 10^(11) GeV, is derived from the high-energy cut off of the cosmic ray spectrum. The numerical simulation scenario could reveal itself in the distributions of the highest energy cosmic rays exhibiting a degree of rotational symmetry breaking that reflects the structure of the underlying lattice.
Journal reference: arXiv

giovedì 11 ottobre 2012

Nanoparticles: Making Gold Economical for Sensing.

Source: ScienceDaily
ScienceDaily (Oct. 10, 2012) — Gold nanocluster arrays developed at A*STAR are well suited for commercial applications of a high-performance sensing technique.
Cancer, food pathogens and biosecurity threats can all be detected using a sensing technique called surface enhanced Raman spectroscopy (SERS). To meet ever-increasing demands in sensitivity, however, signals from molecules of these agents require massive enhancement, and current SERS sensors require optimization. An A*STAR-led research team recently fabricated a remarkably regular array of closely packed gold nanoparticle clusters that will improve SERS sensors.
So-called 'Raman scattering' occurs when molecules scatter at wavelengths not present in the incident light. These molecules can be detected with SERS sensors by bringing them into contact with a nanostructured metal surface, illuminated by a laser at a particular wavelength. An ideal sensor surface should have: dense packing of metal nanostructures, commonly gold or silver, to intensify Raman scattering; a regular arrangement to produce repeatable signal levels; economical construction; and robustness to sustain sensing performance over time.
Few of the many existing approaches succeed in all categories. However, Fung Ling Yap and Sivashankar Krishnamoorthy at the A*STAR Institute of Materials Research and Engineering, Singapore, and co-workers produced closely packed nanocluster arrays of gold that incorporate the most desirable aspects for fabrication and sensing. In addition to flat surfaces, they also succeeded in coating fiber-optic tips with similarly dense nanocluster arrays (see image), which is a particularly promising development for remote-sensing applications, such as hazardous waste monitoring.
The researchers self-assembled their arrays by using surfaces coated with self-formed polymer nanoparticles, to which smaller gold nanoparticles spontaneously attached to form clusters. "It was surprising to reliably attain feature separations of less than 10 nanometers, at high yield, across macroscopic areas using simple processes such as coating and adsorption," notes Krishnamoorthy.
By varying the size and density of the polymer features, Krishnamoorthy, Yap and co-workers tuned the cluster size and density to maximize SERS enhancements. Their technique is also efficient: less than 10 milligrams of the polymer and 100 milligrams of gold nanoparticles are needed to coat an entire 100 millimeter diameter wafer, or approximately 200 fiber tips. Both the polymer and the nanoparticles can be mass-produced at low cost. By virtue of being entirely 'self-assembled', the technique does not require specialized equipment or a custom-built clean room, so it is well suited to low-cost commercial implementation.
"We have filed patent applications for the work in Singapore, the USA and China," says Krishnamoorthy. "The arrays are close to commercial exploitation as disposable sensor chips for use in portable SERS sensors, in collaboration with industry."