Friday, April 13, 2012

Quantum plasmons demonstrated in atomic-scale nanoparticles

 Addressing a half-decade-old debate, engineers at Stanford have positively identified the presence of plasmons, the collective oscillations of electrons, in individual metal particles as small as one nanometer in diameter. The discovery could impact nanotechnology.

The physical phenomenon of plasmon resonances in small metal particles has been apparent for centuries. They are visible in the vibrant hues of the great stained-glass windows of the world. More recently, plasmon resonances have been used by engineers to develop new, light-activated cancer treatments and to enhance light absorption in photovoltaics and photocatalysis.

"The stained-glass windows of Notre Dame Cathedral and Stanford Chapel derive their color from metal nanoparticles embedded in the glass. When the windows are illuminated, the nanoparticles scatter specific colors depending on the particle's size and geometry " said Jennifer Dionne, an assistant professor of materials science and engineering at Stanford and the senior author of a new paper on plasmon resonances to be published in the journal Nature.

In the study, the team of engineers report the direct observation of plasmon resonances in individual metal particles measuring down to one nanometer in diameter, just a few atoms across.

"Plasmon resonances at these scales are poorly understood," said Jonathan Scholl, a doctoral candidate in Dionne's lab and first author of the paper. "So, this class of quantum-sized metal nanoparticles has gone largely under-utilized. Exploring their size-dependent nature could open up some interesting applications at the nanoscale."

The research could lead to novel electronic or photonic devices based on excitation and detection of plasmons in these extremely small particles, the engineers said.

"Alternatively, there could be opportunities in catalysis, quantum optics, and bio-imaging and therapeutics," added Dionne.

Longstanding debate

The science of tiny metal particles has perplexed physicists and engineers for decades. As metallic particles near about 10 nanometers in diameter, classical physics breaks down. The particles begin to demonstrate unique physical and chemical properties that bulk counterparts of the very same materials do not. A nanoparticle of silver measuring a few atoms across, for instance, will respond to photons and electrons in ways profoundly different from a larger particle or slab of silver.

By clearly illustrating the details of this classical-to-quantum transition, Scholl and Dionne have pushed the field of plasmonics into a new realm that could have lasting consequences for catalytic processes such as artificial photosynthesis, for cancer research and treatment, and even quantum computing.

"Particles at this scale are more sensitive and more reactive than bulk materials," said Dionne. "But we haven't been able to take full advantage of their optical and electronic properties without a complete picture of the science. This paper provides the foundation for new avenues of nanotechnology entering the 100-to-10,000 atom regime."

Noble metals

In recent years, engineers have paid particular attention to nanoparticles of the noble metals: silver, gold, palladium, platinum and so forth. These metals are well known to support localized surface plasmon resonances in larger particles. Plasmons are the collective oscillation of electrons at the metal surface in response to light or an electric field.

Additionally, other important physical properties can be driven when plasmons are constrained in extremely small spaces, like the nanoparticles Dionne and Scholl studied, a phenomenon known as quantum confinement.

Depending on the shape and size of the particle, therefore, quantum confinement can dominate a particle's electronic and optical response. This research allows scientists, for the first time, to directly correlate a quantum-sized plasmonic particle's geometry -- its shape and size -- with its plasmon resonances.

Standing to benefit

Nanotechnology stands to benefit from this new understanding. Medical science, for instance, has devised a way to use nanoparticles excited by light to burn away cancer cells, a process known as photothermal ablation. Metal nanoparticles are affixed with molecular appendages called ligands that attach exclusively to chemical receptors on cancerous cells. When irradiated with infrared light, the plasmons begin to resonate and the metal nanoparticles heat up, burning away the cancerous cells while leaving the surrounding healthy tissue unaffected. The use of smaller nanoparticles in these therapies might improve their accuracy and the effectiveness, particularly since they can be more easily integrated into cells.

There is great promise for such small nanoparticles in catalysis, as well. The greater surface-area-to-volume ratios offered by atomic-scale nanoparticles could could significantly improve catalyic rates and efficiencies and provide advances in water-splitting and artificial photosynthesis, yielding clean and renewable energy sources from artificial fuels.

Aiding and abetting

The researchers' ability to observe plasmons in particles of such small size was abetted by the powerful, multi-million dollar environmental scanning transmission electron microscope (E-STEM) installed recently at Stanford's Center for Nanoscale Science and Engineering, one of just a handful of such microscopes in the world.

E-STEM imaging was used in conjunction with electron energy-loss spectroscopy (EELS) -- a research technique that measures the change in an electron's energy as it passes through a material -- to determine the shape and behavior of individual nanoparticles. Combined, STEM and EELS allowed the team to address many of the ambiguities of previous investigations.

"With this new microscope, we can resolve individual atoms within the nanoparticle," said Dionne, "and we can directly observe these particles' quantum plasmon resonances."

Ai Leen Koh, a research scientist at the Stanford Nanocharacterization Laboratory, and co-author of the paper, noted: "Even though plasmons can be probed using both light and electrons, electron excitation is advantageous in that it allows us to image the nanoparticle down to the atomic level and study its plasmon resonances at the same time."

Scholl added, "Someday, we might use this microscope to watch reactions in progress to better understand and optimize them."

Elegant and versatile

The researchers concluded by explaining the physics of their discovery through an elegant and versatile analytical model based on well-known quantum mechanical principles.

"Technically speaking, we've created a relatively simple, computationally light model that describes plasmonic systems where classical theories have failed," said Scholl.

"This paper represents fundamental research. We have clarified what was an ambiguous scientific understanding and, for the first time, directly correlated a particle's geometry with its plasmonic resonance for quantum-sized particles," summarized Dionne. "And this could have some very interesting, and very promising, implications and applications."

Story Source:

The above story is reprinted from materials provided by Stanford School of Engineering. The original article was written by Andrew Myers.

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

Journal Reference:

Jonathan A. Scholl, Ai Leen Koh, Jennifer A. Dionne. Quantum plasmon resonances of individual metallic nanoparticles. Nature, 2012; 483 (7390): 421 DOI: 10.1038/nature10904

New plastics 'bleed' when cut or scratched — and then heal like human skin

"Mother Nature has endowed all kinds of biological systems with the ability to repair themselves," explained Professor Marek W. Urban, Ph.D., who reported on the research. "Some we can see, like the skin healing and new bark forming in cuts on a tree trunk. Some are invisible, but help keep us alive and healthy, like the self-repair system that DNA uses to fix genetic damage to genes. Our new plastic tries to mimic nature, issuing a red signal when damaged and then renewing itself when exposed to visible light, temperature or pH changes."

Urban, who is with the University of Southern Mississippi in Hattiesburg foresees a wide range of potential applications for plastic with warn-and-self-repair capabilities. Scratches in automobile fenders, for instance, might be repaired by simply exposing the fender to intense light. Critical structural parts in aircraft might warn of damage by turning red along cracks so that engineers could decide whether to shine the light and heal the damage or undertake a complete replacement of the component. And there could be a range of applications in battlefield weapons systems.

have become so common, replacing steel, aluminum, glass, paper and other traditional materials because they combine desirable properties such as strength, light weight and corrosion resistance. Hundreds of around the world have been working, however, to remedy one of the downsides of these ubiquitous materials: Once many plastics get scratched or cracked, repairs can be difficult or impossible.

Self-healing plastics have become a Holy Grail of materials science. One approach to that goal involves seeding plastics with capsules that break open when cracked or scratched and release repairing compounds that heal scratches or cuts. Another is to make plastics that respond to an outside stimulus — like light, heat or a chemical agent — by repairing themselves.

Urban's group developed plastics with small molecular links or "bridges" that span the long chains of chemicals that compose plastic. When plastic is scratched or cracked, these links break and change shape. Urban tweaked them so that changes in shape produce a visible color change — a red splotch that forms around the defect. In the presence of ordinary sunlight or visible light from a light bulb, pH changes or temperature, the bridges reform, healing the damage and erasing the red mark.

Urban cited other advantages of the new plastic. Unlike self-healing plastics that rely on embedded healing compounds that can self-repair only once, this plastic can heal itself over and over again. The material also is more environmentally friendly than many other plastics, with the process for producing the plastic water-based, rather than relying on potentially toxic ingredients. And his team now is working on incorporating the technology into plastics that can withstand high temperatures.

More information:
Although the last decade has brought self-healing materials on the forefront of scientific interests, combining repair and sensing attributes into one material entity have not been addressed. These studies report the development of poly(methyl methacrylate/n-butylacrylate/2-[(1,3,3-trimethyl-1,3-dihydrospiro[indole-2,3'-naphtho[2,1-b][1,4]oxazin]-5-yl)amino]ethyl-2-methylacrylate) [p(MMA/nBA/SNO)] copolymer films that upon mechanical scratch undergo color changes from clear to red in the damaged area, but upon exposure to sunlight, temperature and/or acidic vapors, the damaged area is self-repaired and the initial colorless appearance is recovered. The process is reversible and driven by the ring-opening-closure of spironapthoxazine (SNO) segments to form merocyanine (MC), which are recovered back to the SNO form. Upon mechanical damage, SNO segments of the neighboring copolymer segments form inter-molecular H-bonding that stabilizes copolymer backbone, that remains in an extended conformation. External stimuli, such as light, temperature, or acidic environments cause a dissociation of the H-bonded MC pairs, which are converted back to SNO. This process is associated with the p(MMA/nBA/SNO) backbone collapse, thus pulling entangled neighboring copolymers to fill removed mass and repair a scratch. Mechanical nano-indentation analysis combined with molecular modeling and spectroscopic measurements confirm this behavior. The enclosed video clip illustrates molecular repair processes induced by visible light monitored by in-situ Raman imaging spectroscopy. These materials may find numerous future applications, where coupling of simultaneous color changes and reversible self-repair responses may lead to new technological paradigms.

Provided by American Chemical Society (news : web)

Brown liquor and solar cells to provide sustainable electricity

 A breakthrough for inexpensive electricity from solar cells, and a massive investment in wind power, will mean a need to store energy in an intelligent way. According to research at Linköping University, published in Science, batteries of biological waste products from pulp mills could provide the solution.

Organic solar cells based on conductive plastic is a low cost alternative that has achieved high enough performance to be upscaled and, in turn, become competitive. However, solar electricity must be able to be stored from day to night, as well as electricity from wind turbines from windy to calm days.

In conventional batteries metal oxides conduct the charge. Materials, such as cobalt, are expensive and a limited resource, therefore, low cost solutions are sought preferably with renewable materials.

"Nature solved the problem long ago," says Olle Inganäs, professor of biomolecular and organic electronics at Linköping University (LiU) and lead author of the article in a recent edition of Science.

He drew inspiration from the process of photosynthesis, where electrons charged by solar energy are transported by quinones; electrochemically active molecules based on benzene rings composed of six carbon atoms. Inganäs chose the raw material brown liquor that is a by-product from the manufacture of paper pulp. The brown liquor is largely composed of lignin, a biological polymer in the plant cell walls.

To utilise the quinones as charge carriers in batteries, Inganäs and his Polish colleague Grzegorz Milczarek devised a thin film from a mixture of pyrrole and lignin derivatives from the brown liquor. The film, 0.5 microns in thickness, is used as a cathode in the battery.

The goal is to offer ways to store renewable electricity where it is produced, without constructing up large grids. In several countries, major wind power investments are planned. Meanwhile, the performance of cheap organic solar cells has now reached a critical level. A research team at the University of California, Los Angeles, has recently reported efficiency of more than 10 percent of the energy of the captured sunlight.

According to Inganäs who for many years conducted research on organic solar cells, the efficiency is sufficient to initiate an industrial scale up of the technology.

"Now we need more research into new energy storage based on cheap and renewable raw materials. Lignin constitutes 20-30 percent of the biomass of a tree, so it's a source that never ends."

Story Source:

The above story is reprinted from materials provided by Linköping University.

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

Journal Reference:

Grzegorz Milczarek and Olle Inganäs. Renewable Cathode Materials from Biopolymer/Conjugated Polymer Interpenetrating Networks. Science, 2012 DOI: 10.1126/science.1215159]

Researchers create living human gut-on-a-chip

Building on the Wyss Institute's breakthrough "Organ-on-Chip" technology that uses microfabrication techniques to build living organ mimics, the gut-on-a-chip is a silicon polymer device about the size of a computer memory stick. Wyss Founding Director, Donald Ingber, M.D., Ph.D., led the research team, which included Postdoctoral Fellow, Hyun Jung Kim, Ph.D; Technology Development Fellow, Dan Huh, Ph.D.; and Senior Staff Scientist, Geraldine Hamilton, Ph.D. Ingber is also the Judah Folkman Professor of Vascular Biology at Harvard Medical School and the Vascular Biology Program at Children's Hospital Boston, and Professor of Bioengineering at Harvard's School of Engineering and Applied Sciences.

The new device mimics complex 3D features of the intestine in a miniaturized form. Inside a central chamber, a single layer of human intestinal epithelial cells grows on a flexible, porous membrane, recreating the intestinal barrier. The membrane attaches to side walls that stretch and recoil with the aid of an attached vacuum controller. This cyclic mechanical deformation mimics the wave-like peristaltic motions that move food along the digestive tract. The design also recapitulates the intestinal tissue-tissue interface, which allows fluids to flow above and below the intestinal cell layer, mimicking the luminal microenvironment on one side of the device and the flow of blood through capillary vessels on the other.

In addition, the researchers were able to grow and sustain common intestinal microbes on the surface of the cultured intestinal cells, thereby simulating some of the physiological features important to understanding many diseases. These combined capabilities suggest that gut-on-a-chip has the potential to become a valuable in vitro diagnostic tool to better understand the cause and progression of a variety of intestinal disorders and to help develop safe and effective new therapeutics, as well as probiotics. The gut-on-a-chip could also be used to test the metabolism and oral absorption of drugs and nutrients.

"Because the models most often available to us today do not recapitulate human disease, we can't fully understand the mechanisms behind many intestinal disorders, which means that the drugs and therapies we validate in animal models often fail to be effective when tested in humans," said Ingber. "Having better, more accurate in vitro disease models, such as the gut-on-a-chip, can therefore significantly accelerate our ability to develop effective new drugs that will help people who suffer from these disorders."

Gut-on-a-chip represents the most recent advance in the Wyss Institute's portfolio of engineered organ models. The platform technology was first reported on in Science in June 2010, where a living, breathing, human lung-on-a-chip was described. That same year, the Wyss received funding from the National Institutes of Health and the U.S. Food and Drug Administration to develop a heart-lung micromachine to test the safety and efficacy of inhaled drugs on the integrated heart and lung function. In September 2011, the Wyss was awarded a four-year grant from the Defense Advanced Research Projects Agency to develop a spleen-on-a-chip to treat sepsis, a commonly fatal bloodstream infection.

Provided by Harvard University (news : web)

New inhibitors of a cancer-causing protein may lead to targeted therapeutics

The activity of protein kinases, a large class of signaling molecules, must be closely regulated or signaling chaos arises within cells. Signaling chaos sets off a process that is implicated in the development of cancers, including solid tumors. Because protein kinases have a central role in cell signaling, researchers have devoted decades of investigation to developing kinase .

One kinase inhibitor, a drug called Imatinib (or Gleevec), illustrates the profound potential of kinase inhibitors in cancer treatment. The hyperactivity of the Abl kinase is the most common cause of chronic myelogenic leukemia (CML). Imatinab is routinely used to treat CML. Since Imatinib’s Food and Drug Administration-approval in 2001, deaths related to CML have dropped significantly.

Src kinase inhibitors have been used in clinical trials as experimental treatments for many types of solid tumors. However, in contrast to the success of Imatinib for treating CML, a drug based on inhibiting the Src kinase has not been effective in treating solid tumors.

“Think of kinases as a traffic light system that is regulated to keep traffic flowing properly,” says Dr. Seeliger. “But if all lights on a road system are green, chaos occurs. The Src kinase is a dangerous ‘traffic light’ when it stays green. The challenge is to inhibit Src kinase, in other words make it a ‘red light,’ without stopping other Src family of kinases from staying ‘green’ to keep the traffic moving safely.”

In “Highly specific, bisubstrate-competitive Src inhibitors from DNA-templated macrocycles,” the researchers successfully completed two steps that are necessary for the development of new Src kinase-targeted drugs.

First, under the direction of David Liu, Ph.D., Professor, Department of Chemistry, Harvard University, the team developed chemical inhibitors of Src kinase. Dr. Seeliger’s laboratory at Stony Brook then determined the three-dimensional structure of these inhibitors bound to the Src kinase. This second step enabled the team to explain why the inhibitors work to stop the Src kinase but not the other kinases in cultured mammalian cells.

“Using this method, the precise molecular basis of the inhibitory mechanism and Src kinase are revealed,” says Dr. Seeliger. “These results provide new insights into the development of Src-specific inhibitors with potential therapeutic relevance.”

Provided by Stony Brook University (news : web)