Shearing triggers odd behavior in microscopic particles

 Microscopic spheres form strings in surprising alignments when suspended in a viscous fluid and sheared between two plates -- a finding that will affect the way scientists think about the properties of such wide-ranging substances as shampoo and futuristic computer chips.

A team of scientists at Cornell University and the University of Chicago have imaged this behavior and have explained the forces causing it for the first time. Its findings appear in the Dec. 19-23 early edition of the Proceedings of the National Academy of Sciences.

"The experimental breakthrough revealed that these string structures were perpendicular to the shear instead of parallel to it, contrary to what many in the field were expecting," said Aaron Dinner, associate professor in chemistry at UChicago and a study co-author.

The experiment was led by Itai Cohen, associate professor of physics at Cornell, who custom-built a device that would enable him simultaneously to exert shearing forces on suspended colloids (the spheres) and image the resulting motion at 100 frames per second with a confocal microscope. Imaging speed was critical to the experiment because the string-like structures appear only at certain shear rates.

"This issue of strings has been pretty controversial. I'm not sure that we've solved all the controversies associated with them, but at least we've made a step forward," Cohen said.

Shearing forces affect the dynamic behavior of paint, shampoo and other viscous household products, but an understanding of these and related phenomena at the microscopic level has largely eluded a detailed scientific understanding until the last decade, Dinner noted.

Futuristically speaking, these forces potentially could be harnessed to produce microscopic patterns on computer chips or biosensors via special paints that flow easily when layered in one direction, but becomes hard when layered in another direction.

Cohen's objective was more scientifically immediate: to devise an experiment that would overcome the technical difficulties associated with measuring the mechanical properties of the colloidal strings while also imaging their formation. "The holy grail is to be able to understand how the structure leads to the mechanical properties and then to be able to control the mechanical properties by influencing the structure," Cohen explained.

Cohen, PhD'01, received his doctorate in physics at UChicago, as did lead author Xiang Cheng, PhD'09, a postdoctoral associate at Cornell who assembled the team; and co-author Xinliang Xu, PhD'07, a postdoctoral scholar at UChicago. The study co-authors also included Stuart Rice, the Frank P. Hixon Distinguished Service Professor Emeritus in Chemistry at UChicago and a 1999 recipient of the National Medal of Science.

As members of UChicago's Materials Research Science and Engineering Center, Rice and Dinner are part of a larger effort to determine how materials behave under the influence of various dynamic forces. Some of their physics colleagues analyze forces operating on macroscopic scales, while chemists such as Rice and Dinner attempt to assess how those findings might apply to microscopic phenomena.

Rice and his UChicago co-authors used computer simulations to develop a precise explanation for the string-like colloidal structures that formed in the Cornell experiment. "The previous simulations all left out the consequences of the flow created in the supporting fluid as the particles move, the so-called hydrodynamic forces," Rice said.

"A very large fraction of the work in the field neglects hydrodynamic forces because it's hard. You try and get away with what you can," Rice noted with amusement. "But in this case it turns out that the inclusion of those forces is the crucial element."

The simulations allowed the UChicago team to control various experimental parameters to assess their relative importance. "You can play God," Rice said. "The important finding is the overwhelming role of the lubrication forces and the anti-intuitive result that they create."

The lubrication force comes into play when two colloids come together to behave much like macroscopic ball bearings soaking in a reservoir of goopy fluid.

"Pulling them apart would be working against the fluid and so it would be very hard," Dinner said. "So actually, when you get a collision in these colloidal systems, those lubrication forces hold them together much longer, and that actually allows for some of the unique dynamics that give rise to the structure. That was specifically what the simulations showed."

Xu, the UChicago postdoctoral scholar, adapted a mathematical formula developed by John Brady at the California Institute of Technology to simplify the simulations, which ran for days and weeks at a time. "Every time you rearrange the particles, the interactions are different," Rice said. "If you were to calculate that directly, it would be extremely tedious."

But Xu's adapation of Brady's formula enabled him to generate a table of hydrodynamic interactions that listed each particle configuration. Xu found that he could accurately simplify the simulation by focusing on just two of the experiment's seven layers of colloids.

The simulations and the experiment showed that even after three centuries of study, the field of hydrodynamics continues to yield surprising discoveries. "We are still discovering novel behavior that is fundamentally determined by the hydrodynamics," Rice noted.

Story Source:

The above story is reprinted from materials provided by University of Chicago. The original article was written by Steve Koppes.

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

1 room -- 63 different dust particles: Researchers aim to build dust library

Researchers recently isolated 63 unique dust particles from their laboratory – and that's just the beginning.

The chemists were testing a new kind of sensor when dust got stuck inside it, and they discovered that they could measure the composition of single dust particles.

In a recent issue of The Journal of Physical Chemistry C, they describe how the discovery could aid the study respiratory diseases caused by airborne particles.

Most dust is natural in origin, explained James Coe, professor of chemistry at Ohio State University. The 63 particles they identified were mainly irregular blobs containing bits of many different ingredients.

The most common ingredient of the dust particles was organic matter, Coe said. "Organic" indicates some kind of plant or animal material, though the researchers can't yet say precisely what kinds of organic matter they found. They are about to do an in-depth analysis to find out.

Quartz was the second-most common ingredient. Both quartz and organic matter were found in more than half of the dust particles the researchers classified. Man-made chemicals from air pollution, fertilizers, and construction materials were also present in small amounts.

"In that way, a single dust particle is like a snapshot of mankind's impact on the environment," Coe said.

 

This is a close up of a single dust particle on the sensor. Credit: Images courtesy of Ohio State University.

Scientists have had some difficulty getting precise measurements of dust composition, in part because standard techniques involve studying dust in bulk quantities rather than individual particles.

Nowhere is dust composition more important than in public health, where some airborne particulates have been linked to diseases. Coe cited silica dust from mining operations, which causes a lung disease called silicosis.

The patented sensor that Coe's team was testing – a type of metal mesh that transmits infrared light through materials caught in the holes – is ideal for picking up minute details in the composition of single dust grains.

"We can separate particles by size to isolate the ones that are small enough to get into people's lungs, and look at them in detail," he added.

Coe didn't set out to study dust. He and his team invented the metal mesh sensor in 2003, and discovered that they could use it to create surface plasmons – mixtures of conducting electrons and photons. The effect boosts the intensity of light passing through microscopic holes in the mesh, and lets scientists record a detailed infrared light spectrum. Any material stuck in the holes will leave a unique signature on the spectrum, so the sensor can be used to identify the chemicals in microscopic samples.

Early this year, the researchers were testing how light flows through the sensor, and they coated the mesh with a ring of tiny latex spheres to take a baseline measurement. The result should have been a spectrum unique to latex, but instead the spectrum carried the signature of several common minerals due to a single dust particle that had gotten inside the sensor – most likely from the laboratory air.

Coe launched a contest among his students to see who would be the first to take an infrared spectrum of a single dust particle – and an electron microscope image of the same particle. The winner got a free lunch and the chance to name the particle for publication.

Matthew McCormack, then an honors undergraduate student in the lab, won the contest and named the dust particle after his dog, Abby. His study of the particle formed the basis for his honors thesis, and the data has since been used by Coe and other members of the team in publications and presentations.

In subsequent tests, the students were able to isolate and study 63 individual dust particles from the air of their laboratory. The spectra they obtained with the sensor were free of scattering effects and stronger than expected.

The result is a library of common dust components from the lab. Forty of the particles (63 percent) contained organic material. The most common mineral was quartz, which was present in 34 (54 percent) of the particles, followed by carbonates (17 particles, or 27 percent), and gypsum (14 particles, or 22 percent).

Currently, Coe and his team are constructing computer algorithms to better analyze the mineral components and reveal details about the organic components.

A library of common dust components would be useful for many areas of science, he said.

Eventually, researchers in public health could use the sensor as a laboratory tool to analyze dust particles. It could also enable studies in astronomy, geology, environmental science, and atmospheric science.

Provided by The Ohio State University (news : web)

One room -- 63 different dust particles? Researchers aim to build dust library

 Researchers recently isolated 63 unique dust particles from their laboratory -- and that's just the beginning. The chemists were testing a new kind of sensor when dust got stuck inside it, and they discovered that they could measure the composition of single dust particles.

In a recent issue of The Journal of Physical Chemistry C, they describe how the discovery could aid the study respiratory diseases caused by airborne particles.

Most dust is natural in origin, explained James Coe, professor of chemistry at Ohio State University. The 63 particles they identified were mainly irregular blobs containing bits of many different ingredients.

The most common ingredient of the dust particles was organic matter, Coe said. "Organic" indicates some kind of plant or animal material, though the researchers can't yet say precisely what kinds of organic matter they found. They are about to do an in-depth analysis to find out.

Quartz was the second-most common ingredient. Both quartz and organic matter were found in more than half of the dust particles the researchers classified. Human-made chemicals from air pollution, fertilizers, and construction materials were also present in small amounts.

"In that way, a single dust particle is like a snapshot of mankind's impact on the environment," Coe said.

Scientists have had some difficulty getting precise measurements of dust composition, in part because standard techniques involve studying dust in bulk quantities rather than individual particles.

Nowhere is dust composition more important than in public health, where some airborne particulates have been linked to diseases. Coe cited silica dust from mining operations, which causes a lung disease called silicosis.

The patented sensor that Coe's team was testing -- a type of metal mesh that transmits infrared light through materials caught in the holes -- is ideal for picking up minute details in the composition of single dust grains.

"We can separate particles by size to isolate the ones that are small enough to get into people's lungs, and look at them in detail," he added.

Coe didn't set out to study dust. He and his team invented the metal mesh sensor in 2003, and discovered that they could use it to create surface plasmons -- mixtures of conducting electrons and photons. The effect boosts the intensity of light passing through microscopic holes in the mesh, and lets scientists record a detailed infrared light spectrum. Any material stuck in the holes will leave a unique signature on the spectrum, so the sensor can be used to identify the chemicals in microscopic samples.

Early this year, the researchers were testing how light flows through the sensor, and they coated the mesh with a ring of tiny latex spheres to take a baseline measurement. The result should have been a spectrum unique to latex, but instead the spectrum carried the signature of several common minerals due to a single dust particle that had gotten inside the sensor -- most likely from the laboratory air.

Coe launched a contest among his students to see who would be the first to take an infrared spectrum of a single dust particle -- and an electron microscope image of the same particle. The winner got a free lunch and the chance to name the particle for publication.

Matthew McCormack, then an honors undergraduate student in the lab, won the contest and named the dust particle after his dog, Abby. His study of the particle formed the basis for his honors thesis, and the data has since been used by Coe and other members of the team in publications and presentations.

In subsequent tests, the students were able to isolate and study 63 individual dust particles from the air of their laboratory. The spectra they obtained with the sensor were free of scattering effects and stronger than expected.

The result is a library of common dust components from the lab. Forty of the particles (63 percent) contained organic material. The most common mineral was quartz, which was present in 34 (54 percent) of the particles, followed by carbonates (17 particles, or 27 percent), and gypsum (14 particles, or 22 percent).

Currently, Coe and his team are constructing computer algorithms to better analyze the mineral components and reveal details about the organic components.

A library of common dust components would be useful for many areas of science, he said.

Eventually, researchers in public health could use the sensor as a laboratory tool to analyze dust particles. It could also enable studies in astronomy, geology, environmental science, and atmospheric science.

McCormack is a co-author on the paper, along with Katherine Cilwa, now a postdoctoral researcher in chemistry at the University of Michigan; Michelle Lew, now a doctoral student in chemistry at Indiana University; Christophe Robitaille, now in medical school at the University of Chicago; Lloyd Corwin, a former Ohio State undergraduate student in nuclear engineering; and Marvin Malone, a current doctoral student in Coe's laboratory.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by Ohio State University.

Journal Reference:

Katherine E. Cilwa, Matthew McCormack, Michelle Lew, Christophe Robitaille, Lloyd Corwin, Marvin A. Malone, James V. Coe. Scatter-Free IR Absorption Spectra of Individual, 3–5 µm, Airborne Dust Particles Using Plasmonic Metal Microarrays: A Library of 63 Spectra. The Journal of Physical Chemistry C, 2011; 115 (34): 16910 DOI: 10.1021/jp205383h

Ultrasensitive particles offer new way to find cancer

About 10 years ago, scientists discovered a new type of genetic material called microRNA, which appears to turn genes on or off inside a cell. More recently, they found that these genetic snippets often go haywire in cancer cells, contributing to tumors’ uncontrollable growth.

A team of researchers at MIT has now engineered a way to detect abnormal microRNA levels in the blood of cancer patients, raising the possibility of developing a simple blood test to diagnose or monitor the disease.

The technology, described in two recent papers in the journals Analytical Chemistry and Angewandte Chemie, consists of an array of tiny particles, each designed to latch onto a specific type of microRNA. By exposing blood samples or purified RNA to these particles, the researchers can generate a microRNA profile that reveals whether cancer is present. Each type of cancer — lung, pancreas, and so forth — has its own microRNA signature.

MicroRNAs, which are usually only about 20 nucleotides long, have been implicated in many other diseases, including HIV, Alzheimer’s disease, diabetes and cardiovascular disease. The human genome contains about 1,000 microRNAs, believed to fine-tune gene expression by blocking the messenger-RNA molecules that carry DNA’s protein-building instructions.

While measuring microRNA levels has clear potential benefits, there are many challenges to detecting microRNA, says Patrick Doyle, a professor of chemical engineering at MIT and leader of the research team. “There’s not an accepted gold standard,” Doyle says. “Everybody has their own favorite one.”

Fishing for microRNA

Most current microRNA-detection techniques require RNA to be isolated from a blood or tissue sample and purified — a time-consuming process. Detecting microRNA directly from a blood sample would be much more efficient, Doyle says.

In their Angewandte Chemie paper, published in January, Doyle, graduate student Stephen Chapin and their colleagues showed that they could use tiny hydrogel particles, about 200 micrometers in length, to rapidly detect microRNA dysregulation patterns in RNA taken from four individuals with four different types of cancer. In their Analytical Chemistry paper, which went online this month, their particles successfully detected microRNA in the blood serum of a prostate cancer patient.

Hydrogels are made of networks of water-loving polymer chains, which are conducive to the attachment of nucleic acids. Each of the researchers’ particles is decorated with millions of identical strands of DNA that are complementary to a specific microRNA target sequence.

When the particles are mixed with a blood sample, any microRNA present binds to its complementary DNA. Each DNA strand also contains a short sequence that binds to a fluorescent probe, added later. Using a custom-built microfluidic scanner, the researchers then rapidly measure each particle’s fluorescence, revealing how much microRNA is present. The scanner also reads a chemical “barcode” imprinted on each particle, which reveals the type of microRNA being detected. The entire process takes less than three hours.

In their second paper, the researchers bumped up their particles’ sensitivity by amplifying the fluorescence generated by each particle. They achieved this by attaching multiple DNA label sequences to each microRNA target captured on the gel microparticles. These label sequences could then be attached to fluorescent probes.

This approach is 100 times more sensitive than other particle technologies for detecting microRNA, according to Doyle. The technology can detect as few as 10,000 copies of a particular microRNA, and each serum assay requires only 25 microliters of sample.

Jun Lu, an assistant professor of genetics at the Yale School of Medicine, says that level of sensitivity makes the particle system “a very promising technology.”

“The reported sensitivity can detect low levels of microRNAs present in serum, and likely other body fluids. This can make the technology very useful, considering that serum and several other body fluids require minimal invasive operations on patients,” says Lu, who was not involved in this research.

The new MIT approach also gives more accurate results than existing techniques that directly label microRNA strands with a fluorescent probe. Different microRNA sequences can take on different shapes, which affects how easily they bind to the fluorescent probe.

Doyle is now starting to work with medical researchers to investigate using microRNA detection to study other diseases such as cardiovascular disease and HIV. He and one of his former graduate students, Daniel Pregibon, have started a company, Firefly Bioworks, which has licensed the technology to build and scan the particles, with plans to develop the system for commercial use.
This story is republished courtesy of MIT News (http://web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and teaching.

Provided by Massachusetts Institute of Technology (news : web)

Tiny gold particles boost organic solar cell efficiency: Plasmonic technique helps enhance power conversion by up to 20 percent

 In the world of solar energy, organic photovoltaic solar cells have a wide range of potential applications, but they are still considered an upstart. While these carbon-based cells, which use organic polymers or small molecules as semiconductors, are much thinner and less expensive to produce than conventional solar cells made with inorganic silicon wafers, they still lag behind in their ability to efficiently convert sunlight into electricity.

Now, UCLA researchers and their colleagues from China and Japan have shown that by incorporating gold nanoparticles into these organic photovoltaics -- taking advantage of the plasmonic effect, by which metal helps to enhance the absorption of sunlight -- they can significantly improve the cells' power conversion.

In a paper recently published in ACS Nano, the team of researchers, led by Yang Yang, a professor of materials science and engineering at the UCLA Henry Samueli School of Engineering and Applied Science and director of the Nano Renewable Energy Center at UCLA's California NanoSystems Institute, demonstrate how they sandwiched a layer of gold nanoparticles between two light-absorbing subcells in a tandem polymer solar cell in order to harvest a greater fraction of the solar spectrum.

They found that by employing the interconnecting gold-nanoparticle layer, they were able to enhance power conversion by as much as 20 percent. The gold nanoparticles create a strong electromagnetic field inside the thin organic photovoltaic layers by a plasmonic effect, which concentrates light so that much more of it can be absorbed by the subcells.

The team is the first to report a plasmonic-enhanced polymer tandem solar cell, having overcome the difficulties involved in incorporating metal nanostructures into the overall device structure.

"We have successfully demonstrated a highly efficient plasmonic polymer tandem solar cell by simply incorporating gold nanoparticles layer between two subcells," Yang said. "The plasmonic effect happening in the middle of the interconnecting layer can enhance both the top and bottom subcells simultaneously -- a 'sweet spot' -- leading to an improvement in the power conversion efficiency of the tandem solar cell from 5.22 percent to 6.24 percent. The enhancement ratio is as high as 20 percent."

The research team included Xing Wang Zhang from the Key Lab of Semiconductor Materials Science at the Institute of Semiconductors at Beijing's Chinese Academy of Science and Ziruo Hong from the Graduate School of Science and Engineering at Japan's Yamagata University.

Experimental and theoretical results demonstrate that the enhancement effect was attained from local near-field enhancement of the gold nanoparticles. The results show that the plasmonic effect has great potential for the future development of polymer solar cells. The team's proposed interlayer structures as an open platform can be applied to various polymer materials, opening up opportunities for highly efficient, multi-stacked tandem solar cells.

The research was financially supported by grants from the U.S. Office of Naval Research and the National Science Foundation.

The team also included Jun Yang, Jingbi You, Chun-Chao Chen, and Wan-Ching Hsu of the UCLA Department of Materials Science and Engineering and the California NanoSystems Institute.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by University of California - Los Angeles. The original article was written by Jennifer Marcus.

Journal Reference:

Jun Yang, Jingbi You, Chun-Chao Chen, Wan-Ching Hsu, Hai-ren Tan, Xing Wang Zhang, Ziruo Hong, Yang Yang. Plasmonic Polymer Tandem Solar Cell. ACS Nano, 2011; 110718133857056 DOI: 10.1021/nn202144b

Steering a beam of 'virtual particles' to manipulate ultra-small-scale particles in real time

The steady improvement in speed and power of modern electronics may soon hit the brakes unless new ways are found to pack more structures into microscopic spaces. Unfortunately, engineers are already approaching the limit of what light -- the choice tool for "tweezing" tiny features -- can achieve. But there may be a way of reaching beyond this so-called "diffraction limit" by precisely steering, in real time, a curve-shaped beam of weird "virtual particles" known as surface plasmons.

This technique, described in the Optical Society's (OSA) journal Optics Letters, opens the possibility of even smaller, faster communications systems and optoelectronic devices. Examples of optoelectronic devices used today include photodiodes such as solar cells, integrated optical circuits used in communications, and charged coupled imaging devices at the heart of cell phone cameras and receivers on the world's most advanced telescopes. This method also may yield new, important tools for research in chemistry, biology, and medicine.

The key to this innovation is the ability -- for the first time -- to actively manipulate a blended stream of light and plasma, known as a plasmonic Airy beam. The beam, owing to the laws of electromagnetism, travels, not in a straight line like the beams of light to which we are accustomed, but rather in an arc. "It's an odd thing for sure, as light is supposed to travel in a straight line," says Peng Zhang a member of the research team with the National Science Foundation (NSF) Nanoscale Science and Engineering Center of the University of California, Berkeley and Department of Physics and Astronomy at San Francisco State University (SFSU). "That's why people are so crazy about these kinds of interesting beams."

As the beam first strikes a metal surface (typically at an irregular feature called a grating structure), it stirs up small waves of electrons at the metal-insulator interface. These waves, which can be thought of as "virtual particles" known as surface plasmon polaritons (SPPs), then follow the curved trajectory of the Airy beams. And, just as ocean waves move objects on the surface of the water, the SPPs can be directed to manipulate ultrafine-scale features on the surface of a metal.

SPPs are already essential elements in the design and manufacture of optoelectronic devices. The reason they're so critical is that they can affect extremely small-scale objects, smaller than the diffraction limit, or half of the wavelength of light used to create SPPs.

The current systems, however, have a significant drawback: they required fixed, permanent nanostructures to direct the SPPs. This lack of flexibility severely limits their uses in nano-system design and manufacture. But by being able to manipulate the Airy beam, and therefore the SPPs, in real time, the new design gives scientists on-the-fly control.

"We have demonstrated a new way of routing the flow of surface plasmons without any guiding structures," says Xiang Zhang, who led this research and is the director of the NSF Nanoscale Science and Engineering Center at Berkeley and a faculty scientist with the Materials Sciences Division of the Lawrence Berkeley National Laboratory.

The lack of guiding structures, according to Xiang Zhang, is the critical innovation in their design. Currently, to manipulate surface plasmons over two-dimensional metal surfaces, different elements such as waveguides, lenses, beam splitters, and reflectors need to be created. This is done by either structuring metal surfaces (fabricating some permanent nanostructures) or placing insulators on metals. These permanent guiding structures cannot be reconfigured; once the structure is fabricated it cannot be changed in real time.

By using computer-controlled optics, however, the research team has developed a way to steer and manipulate the beams, precisely directing their trajectories to specific spots on an optical surface and adjusting them as needed. Due to their unique arc-shaped paths, the beams have the added ability to bypass surface roughness and defects, or even vault over obstacles.

"These on-the-fly adjustments are extremely desirable," says Zhigang Chen, a principal investigator with the Department of Physics and Astronomy at SFSU. "They enable reconfigurable optical interconnections in ultra-compact integrated photonic circuits, which are at the core of many high-speed computing technologies. They also would enable on-chip nanoparticle manipulations for chemical, medical, or biological research purposes."

The Airy beams used to direct the flow of plasmons also remain coherent, not fanning out or distorting as they travel along their curved trajectories, much in the same way that laser light remains coherent even after traveling great distances.

To create the Airy beams, the researchers used a laser beam and modulated its phase, or wave front, with a spatial light modulator (a device similar to a miniature liquid crystal display) controlled by a personal computer. By continuously changing the specially designed patterns in the computer, they were able to dynamically control the trajectories of the beam in real time.

"These results point out a new direction for dynamically routing surface energies without any permanent guiding structures," says Peng Zhang, "which could inspire researchers from different areas to develop new technologies or tools for a variety of applications." For example, in nano-photonics, researchers may design practical reconfigurable plasmonic devices for ultra-compact integrated photonic circuits. In biology and chemistry, researchers may establish new tools for dynamically manipulating nanoparticles or molecules, and improving the performance of sensors.

"The ultrafine wavelength nature of surface plasmons makes them a promising tool for future nanolithography or nanoimaging applications," says research team member Sheng Wang, also of the NSF Nanoscale Science and Engineering Center. "Now, with the dynamic tunable plasmonic Airy beams, researchers may also shed new light on ultrahigh resolution bioimaging. For example, by bypassing obstacles and directly shining a beam on a target sample, background noise can be greatly reduced, which would enable more accurate imaging."

"This method may also encourage researchers in other fields to manipulate the surface waves in other low-dimensional systems, including graphenes, topological insulators, and magnetic thin films," says fellow team member Yongmin Liu of the NSF Nanoscale Science and Engineering Center.

This research was supported by the U.S. Army Research Office, the Air Force Office of Scientific Research, and the National Science Foundation.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by Optical Society of America, via EurekAlert!, a service of AAAS.

Journal Reference:

Peng Zhang, Sheng Wang, Yongmin Liu, Xiaobo Yin, Changgui Lu, Zhigang Chen, Xiang Zhang. Plasmonic Airy beams with dynamically controlled trajectories. Optics Letters, 2011; 36 (16): 3191 DOI: 10.1364/OL.36.003191

Chemical imaging of individual salt particles advances aerosol research

Scientists recently combined experimental approaches and molecular dynamics modeling to gain new insights into the internal structure of sea salt particles and relate it to their fundamental chemical reactivity in the atmosphere. They used laboratory-proxy sea salt composed of mixed sodium methanesulfonate and sodium chloride salts (CH3SO3Na/NaCl). Sea salt particles are emitted into the atmosphere by the action of ocean waves and bubble bursting at the ocean surface.

They are ubiquitous in the atmospheric environment. Airborne particles impact and drive atmospheric chemical reactions that are known to influence Earth’s radiative balance and thereby physico-chemical processes that impact air quality and climate change. Using molecular dynamics simulations and surface tension measurements, the research team assessed the surfactant properties of CH3SO3- ions and their surface accumulation in wet, deliquesced particles. They investigated the internal structure of dry CH3SO3Na/NaCl particles using a combination of experimental chemical imaging techniques: scanning electron microscopy X-ray microanalysis and time-of-flight secondary ion mass spectrometry at EMSL and synchrotron-based X-ray microspectroscopy at Lawrence Berkeley National Laboratory.

The results indicate that the surfaces of aqueous (deliquesced) sea salt particles contain a substantial number of CH3SO3 ions, while in the dry (effloresced) particles, methanesulfonate salts form a coating layer that modifies the particles’ ability to absorb atmospheric moisture and contribute to chemical reactions. This research shows that surface enhancement or depletion of chemical components in marine particles can occur because of the difference in the chemical nature of the species. Because the atmospheric chemistry of the salt particles takes place at the gas-particle interface, understanding their complex surfaces provides new insights about their effect on the environment and climate change.

More information: Liu Y, B Minofar, Y Desyaterik, E Dames, Z Zhu, JP Cain, RJ Hopkins, MK Gilles, H Wang, P Jungwirth, and A Laskin. 2011. “Internal Structure, Hygroscopic and Reactive Properties of Mixed Sodium Methanesulfonate-Sodium Chloride Particles.” Phys. Chem. Chem. Phys. DOI: 10.1039/c1cp20444k

Provided by Environmental Molecular Sciences Laboratory (news : web)

Watching particles' jekyll-to-hyde transformation

Whether a common atmospheric particle stays in a stable form or twists into something else depends on the amount of water it encounters in the atmosphere, according to scientists at the University of Iowa and Pacific Northwest National Laboratory. This result for malonic acid challenges conventional wisdom; the results are based on the complementary experiments conducted at two Department of Energy user facilities in a collaborative project facilitated through PNNL's Chemical Imaging Initiative.

Managing today's fossil-fuel-based energy infrastructure for both economic and environmental health requires controlling the fate and transport of ubiquitous particles produced by combustion engines and other sources. Traditional approaches, which assume that the particles behave the same way in the air as they do in bulk aqueous solution, have created some faulty assumptions. The chemical imaging technique and subsequent study correct several of those assumptions, allowing scientists to work with more accurate, direct observations of these troublesome particles. This information could not have been obtained with conservative approaches.

"There were no means to probe in situ chemistry of individual particles as they take up water in the atmosphere," said Dr. Alexander Laskin, a chemist within PNNL's Chemical Imaging Initiative. "The X-ray spectro-microscopy technique allowed us to watch as the particles changed between the nonreactive and highly reactive forms."

It begins with malonic acid, one of the most abundant carboxylic acids in the atmosphere. This molecule exists in two forms called keto and enol that differ only with respect to the position of one of the hydrogen atoms. The two forms co-exist in equilibrium, and there is no physical or chemical way to separate these two forms.  With respect to their chemical reactivity, the keto form is stable and not very reactive, while the enol form is highly reactive. The common wisdom was that, in the atmosphere, the dominant form would be the keto form, the same as in aqueous solution. However, the team found that the aqueous chemistry results are not applicable in this case.

An abundant atmospheric species, malonic acid transforms from the stable, nonreactive keto form to the highly reactive enol form at elevated relative humidity. The reactive enol form is 4 to 5 orders of magnitude more abundant in the atmosphere than was expected from aqueous chemistry.

The team's approach began at EMSL, a DOE national scientific user facility at PNNL. Suman Ghorai and Prof. Alexei Tivanski of UI examined ensembles of malonic acid particles, characterizing hundreds of particles at a time using EMSL's optical microscopy and infrared spectroscopy techniques. "The experiments pointed out to unexpectedly high levels of the enol form, and allowed us to go to the light source with well-focused questions," said Ghorai, a graduate student who began the study at EMSL in 2009.

At the Advanced Light Source at Lawrence Berkeley National Laboratory, they probed individual particles using scanning transmission X-ray microscopy combined with near-edge X-ray absorption fine structure spectroscopy.

"Dr. Tivanski and his colleagues took a novel approach to the synchrotron based X-ray microscope," said Dr. Mary Gilles, an expert in the X-ray spectro-microscopy and a beam line scientist at the Advanced Light Source at LBNL. "Most people use it in static mode, but they did dynamic studies -- watching as the particles chemically changed in relation to the relative humidity."

Combining the light source and EMSL resources allowed the researchers to see the malonic acid molecules at the molecular level and determine which form was dominant. "The complementary capabilities were perfectly leveraged," said Tivanski, a UI professor who led the project. "It would have taken endless time at the synchrotron to come to the same results and conclusion if the EMSL part had not been available."

With the results from EMSL and ALS, the team determined that the highly reactive enol form is 4 to 5 orders of magnitude higher in particles than would be predicted from aqueous chemistry. "The implications could be very broad," said Laskin, EMSL scientist. "If you have more reactive enol intermediates, then many chemical reactions are triggered that you didn't expect."

What's next: "We saw the behavior in one carboxylic acid. Will we see it in others? That's the question we are planning to explore in future studies," said Laskin. The team and others will determine the answers using and refining the new chemical imaging approach.

More information: Ghorai S, A Laskin, and AV Tivanski. 2011. "Spectroscopic Evidence of Keto-Enol Tautomerism in Deliquesced Malonic Acid Particles." Journal of Physical Chemistry A 115, 4373-4380. DOI: 10.1021/jp112360x

Provided by Pacific Northwest National Laboratory (news : web)

Neutral atoms made to act like electrically charged particles

Completing the story they started by creating synthetic magnetic fields, scientists from the Joint Quantum Institute (JQI), a collaboration of the National Institute of Standards and Technology (NIST) and the University of Maryland, have now made atoms act as if they were charged particles accelerated by electric fields.

Reported in the journal Nature Physics, these synthetic electric fields make each atom in a gas act, individually, as if it were a charged particle, but collectively they remain neutral, uncharged particles. This dual personality will help researchers simulate and study fundamental electrical phenomena and may lead to a deeper understanding of exotic phenomena involving charged particles such as superconductivity, the flow of electricity without resistance, or the quantum Hall effect, used by NIST to create a standard of electrical resistance.

Some aspects of electricity are difficult to study because, although oppositely charged particles are attracted to one another, similarly charged particles are repelled by one another. To get around this, NIST physicist Ian Spielman and his colleagues realized that they could make atoms, which are typically electrically neutral, act as if they are charged particles in an electric field -- extending their earlier method for making neutral atoms act like charged particles in a magnetic field.

The researchers create their synthetic electric field in an ultracold gas of several hundred thousand rubidium atoms. Using lasers, the team alters the atoms' energy-momentum relationship. This had the effect of transferring a bit of the lasers' momentum to the atoms, causing them to move. The force on each atom is physically identical -- and mathematically equivalent -- to what a charged particle would feel in an electric field.

So while the neutral atoms each experience the force of this synthetic electric field individually, they do not repel each other as would true charged particles in an ordinary electric field. This is analogous to an experienced group of dancers all following the moves of their instructor without getting in each other's way.

According to Spielman, this work may enable scientists to study the Hall effect, a phenomenon where an electromagnetic field can cause charged particles traveling through a conductor to experience a sideways force, which has of yet been unobserved in cold-atom systems. The work may also facilitate measurements of the atomic equivalents of electrical quantities such as resistance and inductance. For neutral atoms in synthetic electric fields, inductance is a measure of the energy that is stored as a result of the atoms' motion, and resistance is a measure of the dissipation, or energy loss, in the system. Measuring these quantities could provide insights into the properties of charged particles in analogous systems, including superconductors.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by National Institute of Standards and Technology (NIST).

Journal Reference:

Y-J. Lin, R. L. Compton, K. Jiménez-García, W. D. Phillips, J. V. Porto, I. B. Spielman. A synthetic electric force acting on neutral atoms. Nature Physics, 2011; DOI: 10.1038/nphys1954

Twinkle, twinkle, quantum dot: New particles can change colors and tag molecules

 Engineers at Ohio State University have invented a new kind of nano-particle that shines in different colors to tag molecules in biomedical tests.

These tiny plastic nano-particles are stuffed with even tinier bits of electronics called quantum dots. Like little traffic lights, the particles glow brightly in red, yellow, or green, so researchers can easily track molecules under a microscope.

This is the first time anyone has created fluorescent nano-particles that can change colors continuously.

Jessica Winter, assistant professor of chemical and biomolecular engineering and biomedical engineering, and research scientist Gang Ruan describe their patent-pending technology in the online edition of the journal Nano Letters.

Researchers routinely tag molecules with fluorescent materials in order to see them under the microscope. Unlike the more common fluorescent molecules, quantum dots shine very brightly, and could illuminate chemical reactions especially well, allowing researchers to see the inner workings of living cells.

A bottleneck to combating major diseases like cancer is the lack of molecular or cellular-level understanding of biological processes, the engineers explained.

"These new nanoparticles could be a great addition to the arsenal of biomedical engineers who are trying to find the roots of diseases," Ruan said.

"We can tailor these particles to tag particular molecules, and use the colors to track processes that we wouldn't otherwise be able to," he continued. "Also, this work could be groundbreaking for the field of nanotechnology as a whole, because it solves two seemingly irreconcilable problems with using quantum dots."

Quantum dots are pieces of semiconductor that measure only a few nanometers, or billionths of a meter, across. They are not visible to the naked eye, but when light shines on them, they absorb energy and begin to glow. That's what makes them good tags for molecules.

Due to quantum mechanical effects, quantum dots "twinkle" -- they blink on and off at random moments. When many dots come together, however, their random blinking is less noticeable. So, large clusters of quantum dots appear to glow with a steady light.

Blinking has been a problem for researchers, because it breaks up the trajectory of a moving particle or tagged molecule that they are trying to follow. Yet, blinking is also beneficial, because when dots come together and the blinking disappears, researchers know for certain that tagged molecules have aggregated.

"Blinking is good and bad," Ruan explained. "But one day we realized that we could use the 'good' and avoid the 'bad' at the same time, by grouping a few quantum dots of different colors together inside a micelle."

A micelle is a nano-sized spherical container, and while micelles are useful for laboratory experiments, they are easily found in household detergents -- soap forms micelles that capture oils in water. Ruan created micelles using polymers, with different combinations of red and green quantum dots inside them.

In tests, he confirmed that the micelles appeared to glow steadily. Those stuffed with only red quantum dots glowed red, and those stuffed with green glowed green. But those he stuffed with red and green dots alternated from red to green to yellow.

The color change happens when one or another dot blinks inside the micelle. When a red dot blinks off and the green blinks on, the micelle glows green. When the green blinks off and the red blinks on, the micelle glows red. If both are lit up, the micelle glows yellow.

The yellow color is due to our eyes' perception of light. The process is the same as when a red pixel and green pixel appear close together on a television or computer screen: our eyes see yellow.

Nobody can control when color changes happen inside individual micelles. But because the particles glow continuously, researchers can use them to track tagged molecules continuously. They can also monitor color changes to detect when molecules come together.

Winter and Ruan said that the particles could also be used in fluid mechanics research -- specifically, micro-fluidics. Researchers who are developing tiny medical devices with fluid separation channels could use quantum dots to follow the fluid's path.

The same Ohio State research team is also developing magnetic particles to enhance medical imaging of cancer, and it may be possible to combine magnetism with the quantum dot technology for different kinds of imaging. But before the particles would be safe to use in the body, they would have to be made of biocompatible materials. Carbon-based nanomaterials are one possible option.

In the meantime, Winter and Ruan are going to continue developing the color-changing quantum dot particles for studies of cells and molecules under the microscope. They are also going to explore what happens when quantum dots of another color -- for instance, blue -- are added to the mix.

The university will look to license the technology for industry, and Winter and Ruan have created a Web site for the technologies they are developing: http://nanoforneuro.com.

This research was supported by the National Science Foundation, an endowment from the William G. Lowrie family to the Department of Chemical and Biomolecular Engineering, and the Center for Emergent Materials at Ohio State.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by Ohio State University. The original article was written by Pam Frost Gorder.

Journal Reference:

Gang Ruan, Jessica O. Winter. Alternating-Color Quantum Dot Nanocomposites for Particle Tracking. Nano Letters, 2011; 11 (3): 941 DOI: 10.1021/nl103233b

Molecular-level analysis of organic particles put in perspective

 

When it comes to air pollution, further development and integration of complementary analytical methods are needed to understand the effect of atmospheric particles, according to scientists at Pacific Northwest National Laboratory and University of California, Irvine. Dr. Julia Laskin and Dr. Alexander Laskin at PNNL and Prof. Sergey Nizkorodov at UCI share this other insights into the state of organic aerosol chemistry in Physical Chemistry Chemical Physics, March 2011. Artwork from the article graces the cover.

Air pollution by emissions related to energy production and the transportation fleet presents economic and environmental consequences. Understanding and mitigating these consequences requires answering challenging questions about the chemically complex particles emitted by these sources. By reporting on the state of the science and discussing future needs, the Laskins and Nizkorodov are providing other scientists with a solid, foundational reference.

High-resolution mass spectrometry provides researchers with the ability to characterize organic matter in aerosols and water samples. In the review article, the scientists discuss all studies published to date using high-resolution mass spectrometry to characterize aerosols and cloud water samples.

The authors also discuss new ionization techniques necessary to advance analysis of aerosol samples using high-resolution mass spectrometers, overcoming previous sample preparation limitations. One example is nanoDESI or Nanospray Desorption Electrospray Ionization. This approach provides a highly sensitive analysis of complex analytes, enabling a molecular-level understanding of the particles.

The authors also cover data analysis and visualization tools to aid in sorting through the hundreds of features on each mass spectrum. Using various tools and careful analysis, scientists are attaining solid information about the molecular composition and fundamental chemistry of particles. For example, researchers in 2010 found that N-heteroatom organic compounds produced through atmospheric aging of aerosols can contribute to the absorption of the visible light by pollutants.

More information: Nizkorodov SA, et al. 2011. "Molecular Chemistry of Organic Aerosols Through the Application of High Resolution Mass Spectrometry." Physical Chemistry Chemical Physics 13(9):3612-3629. DOI: 10.1039/c0cp02032j

Provided by Pacific Northwest National Laboratory (news : web)

A mix of tiny gold and viral particles, and the DNA ties that bind them

Scientists have created a diamond-like lattice composed of gold nanoparticles and viral particles, woven together and held in place by strands of DNA. The structure -- a distinctive mix of hard, metallic nanoparticles and organic viral pieces known as capsids, linked by the very stuff of life, DNA -- marks a remarkable step in scientists' ability to combine an assortment of materials to create infinitesimal devices.

The research, done by scientists at the University of Rochester Medical Center, Scripps Research Institute, and Massachusetts Institute of Technology, was published recently in Nature Materials.

While people commonly think of DNA as a blueprint for life, the team used DNA instead as a tool to guide the precise positioning of tiny particles just one-millionth of a centimeter across, using DNA to chaperone the particles.

Central to the work is the unique attraction of each of DNA's four chemical bases to just one other base. The scientists created specific pieces of DNA and then attached them to gold nanoparticles and viral particles, choosing the sequences and positioning them exactly to force the particles to arrange themselves into a crystal lattice.

When scientists mixed the particles, out of the brew emerged a sodium thallium crystal lattice. The device "self assembled" or literally built itself.

The research adds some welcome flexibility to the toolkit that scientists have available to create nano-sized devices.

"Organic materials interact in ways very different from metal nanoparticles. The fact that we were able to make such different materials work together and be compatible in a single structure demonstrates some new opportunities for building nano-sized devices," said Sung Yong Park, Ph.D., a research assistant professor of Biostatistics and Computational Biology at Rochester.

Park and M.G Finn, Ph.D., of Scripps Research Institute are corresponding authors of the paper.

Such a crystal lattice is potentially a central ingredient to a device known as a photonic crystal, which can manipulate light very precisely, blocking certain colors or wavelengths of light while letting other colors pass. While 3-D photonic crystals exist that can bend light at longer wavelengths, such as the infrared, this lattice is capable of manipulating visible light. Scientists foresee many applications for such crystals, such as optical computing and telecommunications, but manufacturing and durability remain serious challenges.

It was three years ago that Park, as part of a larger team of colleagues at Northwestern University, first produced a crystal lattice with a similar method, using DNA to link gold nanospheres. The new work is the first to combine particles with such different properties -- hard gold nanoparticles and more flexible organic particles.

Within the new structure, there are actually two distinct forces at work, Park said. The gold particles and the viral particles repel each other, but their deterrence is countered by the attraction between the strategically placed complementary strands of DNA. Both phenomena play a role in creating the rigid crystal lattice. It's a little bit like how countering forces keep our curtains up: A spring in a curtain rod pushes the rod to lengthen, while brackets on the window frame counter that force, creating a taut, rigid device.

Other authors of the paper include Abigail Lytton-Jean, Ph.D., of MIT, Daniel Anderson, Ph.D., of Harvard and MIT, and Petr Cigler, Ph.D., formerly of Scripps Research Institute and now at the Academy of Sciences of the Czech Republic. Park's work was supported by the National Institute of Allergy and Infectious Diseases.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by University of Rochester Medical Center.

Journal Reference:

Petr Cigler, Abigail K. R. Lytton-Jean, Daniel G. Anderson, M. G. Finn, Sung Yong Park. DNA-controlled assembly of a NaTl lattice structure from gold nanoparticles and protein nanoparticles. Nature Materials, 2010; 9 (11): 918 DOI: 10.1038/nmat2877