Enhancing and manipulating the light emission of organic molecules is at heart of many important technological and scientific advances, including in the fields of organic light emitting devices, bio-imaging, bio-molecular detection.
Researchers at MIT have now discovered a new platform that enables dramatic manipulation of the emission of organic molecules when simply suspended on top of a carefully designed planar slab with a periodic array of holes: so-called photonic crystal surface.
Influenced by the fast and directional emission channels (called ‘resonances’) provided by the photonic crystal surface, molecules in the solution that are suspended on top of the surface no longer behave in their usual fashion: instead of sending light isotropically into all directions, they rather send light into specific directions.
The researchers say that this platform could also be applied to enhance other type of interactions of light with matter, such as Raman scattering. Furthermore, this process applies to any other nano-emitters as well, such as quantum dots.
Physics Professors Marin Soljacic and John Joannopoulos, Associate Professor of Applied Mathematics Steven Johnson, Research scientist Dr. Ofer Shapira, Postdocs Dr. Alejandro Rodriguez, Dr. Xiangdong Liang, and graduate students Bo Zhen, Song-Liang Chua, Jeongwon Lee report this discovery as featured in Proceedings of the National Academy of Sciences.
“Most fluorescing molecules are like faint light bulbs uniformly emitting light into all directions,” says Soljacic. Researchers have often sought to enhance this emission by incorporating organic emitters into sub-wavelength structured cavities that are usually made out of inorganic materials. However, the challenge lies in an inherent incompatibility in the fabrication of cavities for such hybrid systems.
Zhen et al present a simple and direct methodology to incorporate the organic emitters into their structures. By introducing a microfluidic channel on top of the photonic crystal surface, organic molecules in solution are delivered to the active region where interaction with light is enhanced. Each molecule then absorbs and emits significantly more energy with an emission pattern that can be designed to be highly directional. “Now we can turn molecules from being simple light bulbs to powerful flashlights that are thousands of times stronger and can all be aligned towards the same direction,” says Shapira, the senior author of the paper.
This discovery lends itself to a number of practical applications. “During normal blood tests, for example,” adds Shapira, “cells and proteins are labeled with antibodies and fluorescing molecules that allow their recognition and detection. Their detection limit could be significantly improved using such a system due to the enhanced directional emission from the molecules.”
The researchers also demonstrated that the directional emission can be turned into organic lasers with low input powers. “This lasing demonstration truly highlights the novelty of this system,” says the first author Zhen. For almost any lasing system to work there is a barrier on the input power level, named the lasing threshold, below which lasing will not happen. Naturally, the lower the threshold, the less power it takes to turn on this laser. Exploring the enhancement mechanisms present in the current platform, lasing was observed with a substantially lower barrier than before: the measured threshold in this new system is at least an order of magnitude lower than any previously reported results using the same molecules.
|(Nanowerk News) Nanotechnologies are capable of introducing promising applications that could impact upon our daily lives; it is through the visualisation and control of matter at the scale of a billionth of a metre that allows nanotechnologies to modify and enhance the properties of products across all industry sectors. Even though nanotechnologies have immense potential, they are only in their infancy and have yet to reach full maturity. When considering the changes they could bring, it must be asked: are nanotechnologies going to reduce the rich-poor divide, or will they have the opposite effect?|
In light of debates that make nanotechnologies responsible for a further widening of the aforementioned divide, the Nanotechnology Industries Association (NIA) has published a report analysing this Nano-Gap, or Nano-Divide, by examining the pros and cons of nanotechnologies and their impact global development and the on-going fight against poverty.
|This subsequently leads the report into looking at the possible ways forward for the fair development of nanotechnologies. Finally, the report looks at the possibilities for scientists and entrepreneurs from low- and middle-income countries to scale-up the benefits for their countries with the help of international cooperation and global dialogue.|
|Source: Nanotechnology Industries Association|
17 Dec 2013
|(Nanowerk News) Graphene holds potential for diverse applications, including battery materials, electrodes, high-speed electronics, water filtration, and solar energy harvesting. We’ve discussed most of those applications in earlier blog posts, and not a day passes without some progress in one of those directions hitting the world headlines. Little media attention, however, has been paid to a young and exciting application of graphene – oil exploration.|
|Most of the world’s growing energy demand is fulfilled from some form of fossil fuel, like coal and oil. It is well known that oil exploration and the energy sector are big business, but also potentially damaging to the environment. Oil spills and uncontrolled oil well explosions form just a part of the risk involved in oil exploration. Another cause for concern is the efficiency of extraction, and potential losses, or leaks of oil into the environment. Graphene is being explored for its use in various stages of the exploration and extraction process.|
|Much of the research on graphene for oil has come out of the lab of Prof. James Tour at Rice University. In their early work (published in 2012: “Graphene Oxide as a High-Performance Fluid-Loss-Control Additive in Water-Based Drilling Fluids”), the group first showed that adding platelets of graphene oxide to a common water-based drilling fluid decreased the losses of the fluid to the surrounding rock, as compared to a standard mixture of clays and polymers used in the drilling industry today.|
|Graphene platelet plugging a nanopore (from ACS Applied Materials and Interfaces 4, 222 (2012))|
|These fluids are pumped downhole as part of the process to keep drill bits clean and remove cuttings. With traditional clay-enhanced fluids, differential pressure forms a layer on the wellbore called a filter cake, which both keeps the oil from flowing out and drilling fluids from invading the tiny, oil-producing pores.|
|When the drill bit is removed and drilling fluid displaced, the formation oil forces remnants of the filter cake out of the pores as the well begins to produce. But sometimes the clay won’t budge, and the well’s productivity is reduced.|
|The Tour Group discovered that microscopic, pliable flakes of graphene can form a thinner, lighter filter cake (“Functionalized graphene oxide plays part in next-generation oil-well drilling fluids”). When they encounter a pore, the flakes fold in upon themselves and look something like starfish sucked into a hole. But when well pressure is relieved, the flakes are pushed back out by the oil. The thinner graphene layer budged much more easily than the the layer which would remain after a traditional clay-enhanced liquid was used. A drilling fluid with 2 percent functionalized graphene oxide formed a filter cake an average of 22 micrometers wide — substantially smaller than the 278-micrometer cake formed by traditional drilling fluids. GO blocked pores many times smaller than the flakes’ original diameter by folding.|
|Graphene can also be put to use for well logging. Well logging techniques provide data on the geological properties of reservoirs of interest to the oil and gas exploration industry. A commonly used logging technique uses wirelines to provide information about an oil or gas well. Wirelines are long wires with sensors attached to them, which are lowered into an exploration hole to provide information about the hole and its contents. An extension of wireline logging is logging-while-drilling, which relies on sensors at the end of the drill itself. Both methods utilize oil-based fluids for drilling and lubrication. Oil-based fluids, however, are not very good conductors of electricity, which is where graphene enters the scene. The group of Tour developed a solution that contains magnetic graphene nanoribbons (MGNRs). The MGNRs form part of a conductive coating in oil-based drilling fluids, improving the reliability of the information that is sent back up the hole by the sensors. Furthermore, the magnetic properties of the ribbons could also be exploited for using the ribbons themselves as advanced sensors. The Tour group filed a patent for this application.|
|Finally, since graphene nanoribbons can be made small enough to pass into tiny crevices of the rock which holds precious oil, some envision little graphene-based robots creeping through rocks, sending wireless data which contains information on oil location and concentration.|
|Source: By Marko Spasenovic, Graphenea|
The prospect of turning coal into fluorescent particles may sound too good to be true, but the possibility exists, thanks to scientists at Rice University.
The Rice lab of chemist James Tour found simple methods to reduce three kinds of coal into graphene quantum dots (GQDs), microscopic discs of atom-thick graphene oxide that could be used in medical imaging as well as sensing, electronic and photovoltaic applications.
Watch the video Here:
Band gaps determine how a semiconducting material carries an electric current. In quantum dots, band gaps are responsible for their fluorescence and can be tuned by changing the dots’ size. The process by Tour and company allows a measure of control over their size, generally from 2 to 20 nanometers, depending on the source of the coal.
An illustration shows the nanostructure of bituminous coal before separation into graphene quantum dots. Courtesy of the Tour Group
There are many ways to make GQDs now, but most are expensive and produce very small quantities, Tour said. Though another Rice lab found a way last year to make GQDs from relatively cheap carbon fiber, coal promises greater quantities of GQDs made even cheaper in one chemical step, he said.
“We wanted to see what’s there in coal that might be interesting, so we put it through a very simple oxidation procedure,” Tour explained. That involved crushing the coal and bathing it in acid solutions to break the bonds that hold the tiny graphene domains together.
“You can’t just take a piece of graphene and easily chop it up this small,” he said.
Tour depended on the lab of Rice chemist and co-author Angel Martí to help characterize the product. It turned out different types of coal produced different types of dots. GQDs were derived from bituminous coal, anthracite and coke, a byproduct of oil refining.
An electron microscope image shows the stacking layer structure of graphene quantum dots extracted from anthracite. The scale bar equals 100 nanometers. Courtesy of the Tour Group.
The coals were each sonicated in nitric and sulfuric acids and heated for 24 hours. Bituminous coal produced GQDs between 2 and 4 nanometers wide. Coke produced GQDs between 4 and 8 nanometers, and anthracite made stacked structures from 18 to 40 nanometers, with small round layers atop larger, thinner layers. (Just to see what would happen, the researchers treated graphite flakes with the same process and got mostly smaller graphite flakes.)
Tour said the dots are water-soluble, and early tests have shown them to be nontoxic. That offers the promise that GQDs may serve as effective antioxidants, he said.
Medical imaging could also benefit greatly, as the dots show robust performance as fluorescent agents.
“One of the problems with standard probes in fluorescent spectroscopy is that when you load them into a cell and hit them with high-powered lasers, you see them for a fraction of a second to upwards of a few seconds, and that’s it,” Martí said. “They’re still there, but they have been photo-bleached. They don’t fluoresce anymore.”
Testing in the Martí lab showed GQDs resist bleaching. After hours of excitation, Martí said, the photoluminescent response of the coal-sourced GQDs was barely affected.
Rice University chemist James Tour, left, and graduate student Ruquan Ye show the source and destination of graphene quantum dots extracted from coal in a process developed at Rice. Tour said the fluorescent particles can be drawn in bulk from coal in a one-step process. Photo by Jeff Fitlow
That could make them suitable for use in living organisms. “Because they’re so stable, they could theoretically make imaging more efficient,” he said.
A small change in the size of a quantum dot – as little as a fraction of a nanometer – changes its fluorescent wavelengths by a measurable factor, and that proved true for the coal-sourced GQDs, Martí said.
“Coal is the cheapest material you can get for producing GQDs, and we found we can get a 20 percent yield. So this discovery can really change the quantum dot industry. It’s going to show the world that inside of coal are these very interesting structures that have real value.”
Co-authors of the work include graduate students Ruquan Ye, Changsheng Xiang, Zhiwei Peng, Kewei Huang, Zheng Yan, Nathan Cook, Errol Samuel, Chih-Chau Hwang, Gedeng Ruan, Gabriel Ceriotti and Abdul-Rahman Raji and postdoctoral research associate Jian Lin, all of Rice. Martí is an assistant professor of chemistry and bioengineering. Tour is the T.T. and W.F. Chao Chair in Chemistry as well as a professor of mechanical engineering and materials science and of computer science.
For more: http://news.rice.edu/2013/12/06/coal-…
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By Michael Berger. Copyright ©
Nanowerk (Nanowerk Spotlight) The construction of artificial micro- and nanomotors is a high priority in the nanotechnology field owing to their great potential for diverse potential applications, ranging from targeted drug delivery, on-chip diagnostics and biosensing, or pumping of fluids at the microscale to environmental remediation.
Particular attention has been given to self-propelled chemically-powered micro/nanoscale motors, such as catalytic nanowires (read more: “”Another nanotechnology step towards ‘Fantastic Voyage'”), microtube engines (read more: “Microbots transport, assemble and deliver micro- and nanoscale objects”) or spherical Janus microparticles (read more: “Novel motor system powered by polymerization”). In new work, researchers in Germany have now reported the first example of micromotors for the active degradation of organic pollutants in solution. The novelty of this work lies in the synergy between internal and external functionality of the micromotors.
“Previously, some groups tried to demonstrate the use of catalytic nanomotors for biomedical applications – including ours – on-chip biosensors and capture of bio species,” Dr. Samuel Sánchez, Group Leader Smart Nano-Bio-Systems, Max Planck Institute for Intelligent Systems, tells Nanowerk. “However, the toxicity of the fuel employed still limits their real applications. We imagined that environmental applications might be another field to explore, where the use of hydrogen peroxide is not controversial.” In that direction, Wang’s group reported the removal of oil droplets (“‘Microsubmarines’ designed to help clean up oil spills”) from solution, not degrading them.
Now, Sánchez and his collaborators went one step beyond that and demonstrated the total removal of contaminants using micromotors. Indeed, the chemical is used for the self-propulsion and for the remediation when interacts with the outer layer of the micromachine. “We have demonstrated the ability of self-propelled micromotors to oxidize organic pollutants in aqueous solutions through a Fenton process,” explains Sánchez. “The combination of mixing and releasing iron ions in liquids results in a rate of removal of model pollutant (rhodamine 6G) ca. 12 times higher than when the Fenton oxidation process is carried out with nonpropelling metallic iron tubes.”
Reporting their results in The November 1, 2013 online edition of ACS Nano (“Self-Propelled Micromotors for Cleaning Polluted Water”), the research team from Max Planck, the Leibniz Institute for Solid State and Materials Research Dresden, and the Chemnitz University of Technology demonstrates that micromotors boost the Fenton oxidation process (read more about Fenton reactions at the bottom of this article) without applying external energy, and complete degradation of organic pollutants is achieved.
Schematic process for the degradation of polluted water (rhodamine 6G as model contaminant) into inorganic products by multifunctional micromotors. The self propulsion is achieved by the catalytic inner layer (Pt), which provides the motion of the micromotors in H2O2 solutions. The remediation of polluted water is achieved by the combination of Fe2+ ions with peroxide, generating OH• radicals, which degrade organic pollutants. (Reprinted with permission from American Chemical Society)
Sánchez notes that, if desired, the micromotors can be easily recovered using a magnet once the water purification process has been completed and the excess of hydrogen peroxide can be easily decomposed to pure water and oxygen under visible light. The team fabricated their tubular bubble-propelled micromotors containing small amounts of metallic iron (from 20 to 200 nm layer thickness) as outer layer and platinum as inner layer. The mechanism of degradation is based on Fenton reactions relying on spontaneous acidic corrosion of the iron metal surface of the micromotors in the presence of hydrogen peroxide, which acts both as a reagent for the Fenton reaction and as main fuel to propel the micromotors.
Moreover, the ability of self-propelled, tubular micromotors to improve mixing results in a synergetic effect that enhances water remediation without applying external energy. This work can pave the way for the use of multifunctional micromotors for environmental applications where the use of hydrogen peroxide is not a major drawback but a co-reagent. Sánchez adds that the high efficiency of the oxidation of organic pollutants achieved by the Fe/Pt catalytic micromotors reported in this work is of importance for the design of new and faster water treatments, such as the decontamination of organic compounds in wastewaters and industrial effluents. The aim of this study was to fabricate an autonomous microscopic cleaning system that is working without external energy input in a much faster and convenient way.
The micromotors offer this ability to move the catalyst around without external actuation or addition of catalyst (iron salts) to achieve water remediation, removal of organic dyes, etc. However, as the researchers point out, this is an application especially for microscale environments. “It is, unfortunately, clear that we would not use the micromotors in a large reactor vessel to clean huge amounts of water,” says Sánchez. Nevertheless, the high efficiency of the oxidation of organic pollutants achieved by the Fe/Pt catalytic micromotors reported here is of importance for the design of new and faster water treatments, such as the decontamination of organic compounds in wastewaters and industrial effluents. “We have proven that the usefulness of the micromotors lies not solely in their capacity to move, but to exploit their motion using their external surface to enhance useful catalytic reactions,” says Sánchez. “This work could open a new research line towards coupling a variety of catalytic reactions in self-propelled devices where the presence of hydrogen peroxide is not a disadvantage. We expect that a rich variety of contaminants can be in the next years be cleaned.”
Watch Short Video Here:
Watch the micromotor in action. A synergetic effect is achieved taking advantage of the release of the iron ions from the outer layer of the micromotors and their active motion in the solution.
About Fenton reactions The Fenton method is one of the most popular advanced oxidation processes for the degradation of organic pollutants, utilizing the hydroxyl radical (OH•) as its main oxidizing agent. The generation of OH• in the Fenton method occurs by reaction of H2O2 in the presence of Fe(II). However, one disadvantage of these processes is that Fe ions in solution must be removed after the treatment to meet regulations for drinking water. In order to diminish and, in the best scenario, solve the problems caused by the presence of Fe ions in treated effluents and decrease the costs of recovery, the use of heterogeneous Fenton catalysts is a promising strategy that could allow for the degradation of pollutants by Fenton processes without the requirement of dissolved iron salts. The micromotors fabricated in this work can be included as a new type of heterogeneous Fenton catalyst. With this method the remaining iron in the solution is one to three orders of magnitude lower than in conventional Fenton processes.
Read more: http://www.nanowerk.com/spotlight/spotid=33493.php#ixzz2mN10wW9l
The 7″ display includes a Quantum Dot Enhancement Film (QDEF) produced by 3M in collaboration with Nanosys, Inc. Compared to the traditional LED-LCD display, the QDEF essentially replaces the YAG phosphor of the white LED backlight and functions as a high-efficiency photoluminescent emitter. The ODEF includes quantum dots of different sizes, which would emit different colors when excited due to quantum confinement effect. More detail of the QDEF can be found via the link below.
It is noted that the quantum-dot-enhanced display of Kindle Fire HDX 7 does not utilize the electroluminescent property of quantum dots, and thus is not actually a quantum dot light emitting diode (QLED). Nevertheless, it could signal the beginning of the mass commercialization of quantum dots technology in consumer markets.
And the “very best” of today’s display technologies according to Displaymate, quote,
“The very best of today’s display technologies? The Quantum Dots displays used in the Kindle Fire HDX 7 according to the report.
Quantum Dots are almost magical because they use Quantum Physics to produce highly saturated primary colors for LCDs that are similar to those produced by OLED displays. They not only significantly increase the size of the Color Gamut by 40-50 percent but also improve the power efficiency by an additional 15-20 percent. Instead of using White LEDs (which have yellow phosphors) that produce a broad light spectrum that makes it hard to efficiently produce saturated colors, Quantum Dots directly convert the light from Blue LEDs into highly saturated primary colors for LCDs. You can see the remarkable difference in their light spectra in Figure 4. Quantum Dots are going to revolutionize LCDs for the next 5+ years.”
The content of this article is intended to provide a general guide to the subject matter. Specialist advice should be sought about your specific circumstances.
How does QDEF work?
Nanosys QDEF™ enables deep color and high efficiency by providing displays with an ideal light source. How does it do that?
Each sheet of QDEF contains trillions of tiny (by tiny we mean: a bit bigger than a water molecule but smaller than a virus in size) nanoscrystal phosphors, called “Quantum Dots .” Not found naturally occurring anywhere on Earth, these “dots” can be tuned, by changing their size, to emit light at just the right wavelengths for our displays and do so very efficiently.
Unlike conventional phosphor technologies such as YAG that emit with a fixed spectrum, quantum dots can actually convert light to nearly any color in the visible spectrum. Pumped with a blue source, such as the GaN LED, they can be made to emit at any wavelength beyond the pump source wavelength with very high efficiency (over 90% quantum yield) and with very narrow spectral distribution (only 30 – 40nm FWHM.) The real magic of quantum dots is in the ability to tune the color output of the dots, by carefully controlling the size of the crystals as they are synthesized so that their spectral peak output can be controlled within 2 nanometers to nearly any visible wavelength.
For the first time, display designers will have the ability to tune and match the backlight spectrum to the color filters. This means displays that are brighter, more efficient, and produce truly vibrant colors.
How does it all come together?
Engineering the quantum dots to precise display industry specifications isn’t enough to revolutionize the way LCDs are experienced on its own. The dots need to be easily integrated into current manufacturing operations with minimal impact on display system design if they are to be widely adopted. To do this, Nanosys spent a lot of time working with major display manufacturers to get the packaging just right so that it would be a simple, drop-in product that did not require any line retooling or process changes. The end result is called Quantum Dot Enhancement Film or QDEF.
Designed as a replacement for the an existing film in LCD backlights called the diffuser, QDEF combines red and green emitting quantum dots in a thin, optically clear sheet that emits white light when stimulated by blue (some of that blue is allowed to pass through to make the B in RGB at the LCM of course). So manufacturers who’ve invested billions in plant and equipment for LCD production can simply slip this sheet into their process, change their ‘white’ LEDs to blue (the same LEDs but without the phosphor) and start producing LCD panels with the colors and efficiencies of the best OLEDs, at a fraction of the cost and current industrial scale.
Nanosys is currently shipping production samples to display manufacturers and is on track to begin producing at commercial volumes fall of 2013.
|(Nanowerk News) Jeong-Yeol Yoon, associate professor of agricultural and biosystems engineering, and Dr. Marvin Slepian, professor of cardiology and biomedical engineering, collaborated to test how nanotechnology-based techniques can be used to better facilitate adhesion between tissue and implanted devices.|
|“When we created the nanotexture surface, we thought it could be used as a sticky surface for the implants,” Yoon says.|
|Cell-substrate adhesion involves the interplay of mechanical properties, surface topographic features, electrostatic charge and biochemical mechanisms. By working at the nanoscale level, Yoon was able to maximize the physical properties of the underlying substrate in promoting adhesion.|
|But beyond simply creating a sticky surface, the researchers’ goal was to create a selectively sticky surface, favoring endothelial cell attachment, without favoring platelet attachment, Slepian says.|
|The connection between Yoon, a specialist in biosensors and nanotechnology from the College of Agriculture and Life Sciences, and Slepian, co-founder and chief scientific officer of artificial-heart manufacturer SynCardia, came about by chance. A graduate student in Yoon’s lab met Slepian through their shared interest in bicycling.|
|“It’s very rare for the agriculture people to work with the cardiovascular people in the medical school,” Yoon says.|
|But their research specialties clicked.|
|One particular challenge to overcome in cardiovascular implants is the potential for devices – such as stents placed inside coronary arteries – to become detached as a result of blood flow, Yoon says.|
|“We’re particularly focused on the cardiovascular applications because there’s a blood flow involved and our system is very good when there’s a flow situation,” Yoon says.|
|The results of the study, published in the journal Advanced Healthcare Materials (“Nanowell-Trapped Charged Ligand-Bearing Nanoparticle Surfaces: A Novel Method of Enhancing Flow-Resistant Cell Adhesion”), reveal that the researchers’ strategy leads to enhanced endothelial cell adhesion under both static and flow conditions.|
|The adhesive properties derive from optimized surface texturing, electrostatic charge and cell adhesive ligands (molecular binding substances) that are uniquely assembled on the substrata surface as an ensemble of nanoparticles trapped in nanowells.|
|“There are lot of other people out there who use nanotechnology for improving the implants, but this is stronger than other adhesive methods using nanotechnology,” Yoon says.|
|“Obviously it can be used for everything else – lungs, digestive track and other systems. There are lots of other opportunities we haven’t explored,” he says.|
|The research is a perfect fit for Advanced Healthcare Materials, a new journal that spun off from the longstanding Advanced Materials journal.|
|“The use of the materials for the health care applications is probably the hottest area in materials science and engineering,” Yoon says. “We believe the journal will become even stronger than the mother journal.”|
|Just as the new journal marks an exciting intersection of disciplines, Yoon says the environment at the UA encourages such interdisciplinary approaches.|
|“I joined the University of Arizona because there are so many interdisciplinary activities going on. I see a lot of collaboration between departments in the same college at other universities, but at the University of Arizona, the environment is more open and you see collaboration across colleges,” Yoon says.|
|Slepian agreed, saying the pair has already filed grant applications for future work together.|
|“It has been fun and exciting to have an interdisciplinary collaborator,” he says.|
|Source: University of Arizona|
|(Nanowerk News) Chemical engineers at Rice University have found a new catalyst that can rapidly break down nitrites, a common and harmful contaminant in drinking water that often results from overuse of agricultural fertilizers.|
|Nitrites and their more abundant cousins, nitrates, are inorganic compounds that are often found in both groundwater and surface water. The compounds are a health hazard, and the Environmental Protection Agency places strict limits on the amount of nitrates and nitrites in drinking water. While it’s possible to remove nitrates and nitrites from water with filters and resins, the process can be prohibitively expensive.|
|Researchers at Rice University’s Catalysis and Nanomaterials Laboratory have found that gold and palladium nanoparticles can rapidly break down nitrites.|
|“This is a big problem, particularly for agricultural communities, and there aren’t really any good options for dealing with it,” said Michael Wong, professor of chemical and biomolecular engineering at Rice and the lead researcher on the new study. “Our group has studied engineered gold and palladium nanocatalysts for several years. We’ve tested these against chlorinated solvents for almost a decade, and in looking for other potential uses for these we stumbled onto some studies about palladium catalysts being used to treat nitrates and nitrites; so we decided to do a comparison.”|
|Catalysts are the matchmakers of the molecular world: They cause other compounds to react with one another, often by bringing them into close proximity, but the catalysts are not consumed by the reaction.|
|In a new paper in the journal Nanoscale (“Supporting palladium metal on gold nanoparticles improves its catalysis for nitrite reduction “), Wong’s team showed that engineered nanoparticles of gold and palladium were several times more efficient at breaking down nitrites than any previously studied catalysts. The particles, which were invented at Wong’s Catalysis and Nanomaterials Laboratory, consist of a solid gold core that’s partially covered with palladium.|
|Over the past decade, Wong’s team has found these gold-palladium composites have faster reaction times for breaking down chlorinated pollutants than do any other known catalysts. He said the same proved true for nitrites, for reasons that are still unknown.|
|“There’s no chlorine in these compounds, so the chemistry is completely different,” Wong said. “It’s not yet clear how the gold and palladium work together to boost the reaction time in nitrites and why reaction efficiency spiked when the nanoparticles had about 80 percent palladium coverage. We have several hypotheses we are testing out now. “|
|He said that gold-palladium nanocatalysts with the optimal formulation were about 15 times more efficient at breaking down nitrites than were pure palladium nanocatalysts, and about 7 1/2 times more efficient than catalysts made of palladium and aluminum oxide.|
|Wong said he can envision using the gold-palladium catalysts in a small filtration unit that could be attached to a water tap, but only if the team finds a similarly efficient catalyst for breaking down nitrates, which are even more abundant pollutants than nitrites.|
|“Nitrites form wherever you have nitrates, which are really the root of the problem,” Wong said. “We’re actively studying a number of candidates for degrading nitrates now, and we have some positive leads.”|
|Source: Rice University|
Berkeley Lab Researchers Use Fluorescent Tetrapod Quantum Dots to Measure the Mechanical Strength of Polymer Fibers
Fluorescent tetrapod nanocrystals could light the way to the future design of stronger polymer nanocomposites. A team of researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) has developed an advanced opto-mechanical sensing technique based on tetrapod quantum dots that allows precise measurement of the tensile strength of polymer fibers with minimal impact on the fiber’s mechanical properties.
In a study led by Paul Alivisatos, Berkeley Lab director and the Larry and Diane Bock Professor of Nanotechnology at the University of California (UC) Berkeley, the research team incorporated into polymer fibers a population of tetrapod quantum dots (tQDs) consisting of a cadmium-selenide (CdSe) core and four cadmium sulfide (CdS) arms. The tQDs were incorporated into the polymer fibers via electrospinning, among today’s leading techniques for processing polymers, in which a large electric field is applied to droplets of polymer solution to create micro- and nano-sized fibers. This is the first known application of electrospinning to tQDs.
Fluorescent tetrapod quantum dots or tQDs (brown) serve as stress probes that allow precise measurement of polymer fiber tensile strength with minimal impact on mechanical properties. Inserts show relaxed tQDs (upper) and stressed tQDs (lower).
“The electrospinning process allowed us to put an enormous amount of tQDs, up to 20-percent by weight, into the fibers with minimal effects on the polymer’s bulk mechanical properties,” Alivisatos says. “The tQDs are capable of fluorescently monitoring not only simple uniaxial stress, but stress relaxation and behavior under cyclic varying loads. Furthermore, the tQDs are elastic and recoverable, and undergo no permanent change in sensing ability even upon many cycles of loading to failure.”
Alivisatos is the corresponding author of a paper describing this research in the journal NANO Letters titled “Tetrapod Nanocrystals as Fluorescent Stress Probes of Electrospun Nanocomposites.” Coauthors were Shilpa Raja, Andrew Olson, Kari Thorkelsson, Andrew Luong, Lillian Hsueh, Guoqing Chang, Bernd Gludovatz, Liwei Lin, Ting Xu and Robert Ritchie.
From left, Andrew Olson, Shilpa Raja and Andrew Luong are members of Paul Alivisatos’s research group who used electrospinning to incorporate tetrapod quantum dot stress probes into polymer fibers. (Photo by Roy Kaltschmidt)
Polymer nanocomposites are polymers that contain fillers of nanoparticles dispersed throughout the polymer matrix. Exhibiting a wide range of enhanced mechanical properties, these materials have great potential for a broad range of biomedical and material applications. However, rational design has been hampered by a lack of detailed understanding of how they respond to stress at the micro- and nanoscale.
“Understanding the interface between the polymer and the nanofiller and how stresses are transferred across that barrier are critical in reproducibly synthesizing composites,” Alivisatos says. “All of the established techniques for providing this information have drawbacks, including altering the molecular-level composition and structure of the polymer and potentially weakening mechanical properties such as toughness. It has therefore been of considerable interest to develop optical luminescent stress-sensing nanoparticles and find a way to embed them inside polymer fibers with minimal impact on the mechanical properties that are being sensed.”
The Berkeley Lab researchers met this challenge by combining semiconductor tQDs of CdSe/CdS, which were developed in an earlier study by Alivisatos and his research group, with electrospinning. The CdSe/CdS tQDs are exceptionally well-suited as nanoscale stress sensors because an applied stress will bend the arms of the tetrapods, causing a shift in the color of their fluorescence. The large electric field used in electrospinning results in a uniform dispersal of tQD aggregates throughout the polymer matrix, thereby minimizing the formation of stress concentrations that would act to degrade the mechanical properties of the polymer. Electrospinning also provided a much stronger bond between the polymer fibers and the tQDs than a previous diffusion-based technique for using tQDs as stress probes that was reported two years ago by Alivisatos and his group. Much higher concentrations of tQDs could also be a achieved with electrospinning rather than diffusion.
When stress was applied to the polymer nanocomposites, elastic and plastic regions of deformation were easily observed as a shift in the fluorescence of the tQDs even at low particle concentrations. As particle concentrations were increased, a greater fluorescence shift per unit strain was observed. The tQDs acted as non-perturbing probes that tests proved were not adversely affecting the mechanical properties of the polymer fibers in any significant way.
“We performed mechanical tests using a traditional tensile testing machine with all of our types of polymer fibers,” says Shilpa Raja, a lead author of the Nano Letters paper along with Andrew Olson, both members of Alivisatos’ research group. “While the tQDs undoubtedly change the composition of the fiber – it is no longer pure polylactic acid but instead a composite – we found that the mechanical properties of the composite and crystallinity of the polymer phase show minimal change.”
The research team believes their tQD probes should prove valuable for a variety of biological, imaging and materials engineering applications.
“A big advantage in the development of new polymer nanocomposites would be to use tQDs to monitor stress build-ups prior to material failure to see how the material was failing before it actually broke apart,” says co-lead author Olson. “The tQDs could also help in the development of new smart materials by providing insight into why a composite either never exhibited a desired nanoparticle property or stopped exhibiting it during deformation from normal usage.”
For biological applications, the tQD is responsive to forces on the nanoNewton scale, which is the amount of force exerted by living cells as they move around within the body. A prime example of this is metastasizing cancer cells that move through the surrounding extracellular matrix. Other cells that exert force include the fibroblasts that help repair wounds, and cardiomyocytes, the muscle cells in the heart that beat.
“All of these types of cells are known to exert nanoNewton forces, but it is very difficult to measure them,” Raja says.
“We’ve done preliminary studies in which we have shown that cardiomyocytes on top of a layer of tQDs can be induced to beat and the tQD layer will show fluorescent shifts in places where the cells are beating. This could be extended to a more biologically-relevant environment in order to study the effects of chemicals and drugs on the metastasis of cancer cells.”
Another exciting potential application is the use of tQDs to make smart polymer nanocomposites that can sense when they have cracks or are about to fracture and can strengthen themselves in response.
“With our technique we are combining two fields that are usually separate and have never been combined on the nanoscale, optical sensing and polymer nanocomposite mechanical tunability,” Raja says. “As the tetrapods are incredibly strong, orders of magnitude stronger than typical polymers, ultimately they can make for stronger interfaces that can self-report impending fracture.”
This research was primarily supported by the DOE Office of Science.
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Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit www.lbl.gov.
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For more about the research of Paul Alivisatos go here
|Source: American Associates, Ben-Gurion University of the Negev|