Scientists solve mystery of colorful armchair nanotubes

 Rice University researchers have figured out what gives armchair nanotubes their unique bright colors: hydrogen-like objects called excitons.

Their findings appear in the online edition of the Journal of the American Chemical Society.

Armchair carbon nanotubes -- so named for the "U"-shaped configuration of the atoms at their uncapped tips -- are one-dimensional metals and have no band gap. This means electrons flow from one end to the other with little resistivity, the very property that may someday make armchair quantum wires possible.

The Rice researchers show armchair nanotubes absorb light like semiconductors. An electron is promoted from an immobile state to a conducting state by absorbing photons and leaving behind a positively charged "hole," said Rice physicist Junichiro Kono. The new electron-hole pair forms an exciton, which has a neutral charge.

"The excitons are created by the absorption of a particular wavelength of light," said graduate student and lead author Erik Hároz. "What your eye sees is the light that's left over; the nanotubes take a portion of the visible spectrum out." The diameter of the nanotube determines which parts of the visible spectrum are absorbed; this absorption accounts for the rainbow of colors seen among different batches of nanotubes.

Scientists have realized that gold and silver nanoparticles could be manipulated to reflect brilliant hues -- a property that let artisans who had no notions of "nano" create stained glass windows for medieval cathedrals. Depending on their size, the particles absorbed and emitted light of particular colors due to a phenomenon known as plasma resonance.

In more recent times, researchers noticed semiconducting nanoparticles, also known as quantum dots, show colors determined by their size-dependent band gaps.

But plasma resonance happens at wavelengths outside the visible spectrum in metallic carbon nanotubes. And armchair nanotubes don't have band gaps.

Kono's lab ultimately determined that excitons are the source of color in batches of pure armchair nanotubes suspended in solution.

The results seem counterintuitive, Kono said, because excitons are characteristic of semiconductors, not metals. Kono is a professor of electrical and computer engineering and of physics and astronomy.

While armchair nanotubes don't have band gaps, they do have a unique electronic structure that favors particular wavelengths for light absorption, he said.

"In armchair nanotubes, the conduction and valence bands touch each other," Kono said. "The one-dimensionality, combined with its unique energy dispersion, makes it a metal. But the bands develop what's called a van Hove singularity," which appears as a peak in the density of states in a one-dimensional solid. "So there are lots of electronic states concentrated around this singularity."

Exciton resonance tends to occur around these singularities when hit with light, and the stronger the resonance, the more distinguished the color. "It's an unusual quality of these particular one-dimensional materials that these excitons can actually exist," Hároz said. "In most metals, that's not possible; there's not enough Coulomb interaction between the electron and the hole for an exciton to be stable."

The new paper follows on the heels of work by Kono and his team to create batches of pure single-walled carbon nanotubes through ultracentrifugation. In that process, nanotubes were spun in a mix of solutions with different densities up to 250,000 times the force of gravity. The tubes naturally gravitated toward separated solutions that matched their own densities to create a colorful "nano parfait."

As a byproduct of their current work, the researchers proved their ability to produce purified armchair nanotubes from a variety of synthesis techniques. They now hope to extend their investigation of the optical properties of armchairs beyond visible light. "Ultimately, we'd like to make one collective spectrum that includes frequency ranges all the way from ultraviolet to terahertz," Hároz said. "From that, we can know, optically, almost everything about these nanotubes."

Co-authors of the paper include Robert Hauge, a distinguished faculty fellow in chemistry at Rice; Rice alumnus Benjamin Lu; and professors Pavel Nikolaev and Sivaram Arepalli of Sungkyunkwan University, Suwon, Korea.

The research was supported by the Department of Energy, the Robert A. Welch Foundation, the Air Force Research Laboratory and the World Class University Program at Sungkyunkwan University.

Story Source:

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

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

Journal Reference:

Erik H. Hároz, Benjamin Y. Lu, Pavel Nikolaev, Sivaram Arepalli, Robert H. Hauge, Junichiro Kono. Unique Origin of Colors of Armchair Carbon Nanotubes. Journal of the American Chemical Society, 2012; : 120103131856006 DOI: 10.1021/ja209333m

First certified reference material for single-wall carbon nanotubes

The National Institute of Standards and Technology (NIST) has issued the world's first reference material for single-wall carbon nanotube soot. Distantly related to the soot in your fireplace or in a candle flame, nanotube-laden soot is the primary industrial source of single-wall carbon nanotubes, perhaps the archetype of all nanoscale materials. The new NIST material offers companies and researchers a badly needed source of uniform and well-characterized carbon nanotube soot for material comparisons, as well as chemical and toxicity analysis.

With walls of carbon only one atom thick and looking like a sheet of chicken wire curled into a cylinder, single-wall carbon nanotubes are one of several families of pure carbon materials that, because of their nanoscale size, have special properties. "Single-wall carbon nanotubes," says NIST chemical engineer Jeffery Fagan, "have exquisite optical, mechanical, thermal and electronic properties, and because of their small width but long lengths -- think of something like a long piece of hair but 10,000 times thinner -- full development of these materials should enable lighter, stronger materials, as well as improve many technologies from sensors to electronics and batteries."

Unfortunately, nanotubes are difficult to produce without significant impurities or in large quantities. Single-wall nanotubes, in particular, have been notorious for their relatively low quality and batch-to-batch variability. They typically are produced in complex processes using small particles of metal catalysts that promote the growth of the nanotubes. The resulting material -- often a powder not unlike the soot you would find in your fireplace -- has frequently contained large amounts of impurities, such as other forms of carbon, and sometimes significant levels of catalysts.

"One of the issues that this reference material addresses is that there's no homogeneous lot that people can buy to do comparative measurements," says Fagan. "Even batch-to-batch, raw carbon nanotube powder samples have varied so much that there is no interlaboratory consistency. And that's particularly a problem for comparisons such as toxicity measurements. If you bought carbon nanotubes, you were pretty much guaranteed that your sample could be so different from anyone else's samples that either your measurements could be specific to some flaw of your material, or that others might not be able to reproduce what you were doing."

To address these issues, a multidisciplinary research team at NIST has worked to develop the metrology necessary for quantitative single-wall carbon nanotube measurements through a three-prong approach: basic measurement and separation science, documentary protocols and standards through international standards organizations, and now certified reference materials.

The new NIST product, Standard Reference Material (SRM) 2483, "Single-Wall Carbon Nanotubes (Raw Soot)," will directly address the issue of comparability. It is possibly the world's single largest supply of homogeneous, chemically analyzed, carbon nanotube soot where the uniformity of the samples from unit to unit is assured. Each unit of SRM 2483, a glass vial containing 250 milligrams of soot, is certified by NIST for the mass fraction values of several common contaminants: barium, cerium, chlorine, cobalt, dysprosium, europium, gadolinium, lanthanum, molybdenum and samarium. Reference values (values believed to be accurate, but not rising to the level of confidence that NIST certifies) are provided for an additional seven elements.

NIST also provides additional reference data useful for nanotube analysis, including thermal gravimetric and Raman data, as well as informational values for ultraviolet-visible-near-infrared absorbance spectra, near-infrared fluorescence spectra, Raman scattering spectra and scanning electron microscopy images. With these sets of information, purchasers of the material should be able to compare their results against the NIST values and against those from suppliers or after processing, ensuring a consistent point of comparison.

Single units of SRM 2483, "Single-Wall Carbon Nanotubes (Raw Soot)," are available from the NIST Standard Reference Materials Program at www.nist.gov/srm/.

Standard Reference Materials are among the most widely distributed and used products from NIST. The agency prepares, analyzes and distributes more than a thousand different materials that are used throughout the world to check the accuracy of instruments and test procedures used in manufacturing, clinical chemistry, environmental monitoring, electronics, criminal forensics and dozens of other fields.

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

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'Label-free' imaging tool tracks nanotubes in cells, blood for biomedical research

 Researchers have demonstrated a new imaging tool for tracking structures called carbon nanotubes in living cells and the bloodstream, which could aid efforts to perfect their use in biomedical research and clinical medicine.

The structures have potential applications in drug delivery to treat diseases and imaging for cancer research. Two types of nanotubes are created in the manufacturing process, metallic and semiconducting. Until now, however, there has been no technique to see both types in living cells and the bloodstream, said Ji-Xin Cheng, an associate professor of biomedical engineering and chemistry at Purdue University.

The imaging technique, called transient absorption, uses a pulsing near-infrared laser to deposit energy into the nanotubes, which then are probed by a second near-infrared laser.

The researchers have overcome key obstacles in using the imaging technology, detecting and monitoring the nanotubes in live cells and laboratory mice, Cheng said.

"Because we can do this at high speed, we can see what's happening in real time as the nanotubes are circulating in the bloodstream," he said.

Findings are detailed in a research paper posted online Dec. 4 in the journal Nature Nanotechnology.

The imaging technique is "label free," meaning it does not require that the nanotubes be marked with dyes, making it potentially practical for research and medicine, Cheng said.

"It's a fundamental tool for research that will provide information for the scientific community to learn how to perfect the use of nanotubes for biomedical and clinical applications," he said.

The conventional imaging method uses luminescence, which is limited because it detects the semiconducting nanotubes but not the metallic ones.

The nanotubes have a diameter of about 1 nanometer, or roughly the length of 10 hydrogen atoms strung together, making them far too small to be seen with a conventional light microscope. One challenge in using the transient absorption imaging system for living cells was to eliminate the interference caused by the background glow of red blood cells, which is brighter than the nanotubes.

The researchers solved this problem by separating the signals from red blood cells and nanotubes in two separate "channels." Light from the red blood cells is slightly delayed compared to light emitted by the nanotubes. The two types of signals are "phase separated" by restricting them to different channels based on this delay.

Researchers used the technique to see nanotubes circulating in the blood vessels of mice earlobes.

"This is important for drug delivery because you want to know how long nanotubes remain in blood vessels after they are injected," Cheng said. "So you need to visualize them in real time circulating in the bloodstream."

The structures, called single-wall carbon nanotubes, are formed by rolling up a one-atom-thick layer of graphite called graphene. The nanotubes are inherently hydrophobic, so some of the nanotubes used in the study were coated with DNA to make them water-soluble, which is required for them to be transported in the bloodstream and into cells.

The researchers also have taken images of nanotubes in the liver and other organs to study their distribution in mice, and they are using the imaging technique to study other nanomaterials such as graphene.

The paper was written by doctoral student Ling Tong; postdoctoral research associate Yuxiang Liu; doctoral students Bridget D. Dolash and Yookyung Jung; biomedical engineering research scientist Mikhail N. Slipchenko; Donald E. Bergstrom, the Walther Professor of Medicinal Chemistry; and Cheng.

The research is funded by the National Science Foundation.

Story Source:

The above story is reprinted from materials provided by Purdue University. The original article was written by Emil Venere.

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

Journal Reference:

Ling Tong, Yuxiang Liu, Bridget D. Dolash, Yookyung Jung, Mikhail N. Slipchenko, Donald E. Bergstrom, Ji-Xin Cheng. Label-free imaging of semiconducting and metallic carbon nanotubes in cells and mice using transient absorption microscopy. Nature Nanotechnology, 2011; DOI: 10.1038/nnano.2011.210

Note: If no author is given, the source is cited instead.

New biosensor benefits from melding of carbon nanotubes, DNA

 Purdue University scientists have developed a method for stacking synthetic DNA and carbon nanotubes onto a biosensor electrode, a development that may lead to more accurate measurements for research related to diabetes and other diseases.

Standard sensors employ metal electrodes coated with enzymes that react with compounds and produce an electrical signal that can be measured. But the inefficiency of those sensors leads to imperfect measurements.

Carbon nanotubes, cylindrically shaped carbon molecules known to have excellent thermal and electrical properties, have been seen as a possibility for improving sensor performance. The problem is that the materials are not fully compatible with water, which limits their application in biological fluids.

Marshall Porterfield, a professor of agricultural and biological engineering and biomedical engineering, and Jong Hyun Choi, an assistant professor of mechanical engineering, have found a solution. Their findings, reported in the journal The Analyst, describe a sensor that essentially builds itself.

"In the future, we will be able to create a DNA sequence that is complementary to the carbon nanotubes and is compatible with specific biosensor enzymes for the many different compounds we want to measure," Porterfield said. "It will be a self-assembling platform for biosensors at the biomolecular level."

Choi developed a synthetic DNA that will attach to the surface of the carbon nanotubes and make them more water-soluble.

"Once the carbon nanotubes are in a solution, you only have to place the electrode into the solution and charge it. The carbon nanotubes will then coat the surface," Choi said.

The electrode coated with carbon nanotubes will attract the enzymes to finish the sensor's assembly.

The sensor described in the findings was designed for glucose. But Porterfield said it could be easily adapted for various compounds.

"You could mass produce these sensors for diabetes, for example, for insulin management for diabetic patients," Porterfield said.

Porterfield said it may one day be possible to develop other sensors using this technology that could lead to more personalized medicines that could test in real time the effectiveness of drugs on their targets as with cancer patients.

Porterfield said he would continue to develop biosensors to detect different compounds.

The National Institutes of Health and the Office of Naval Research funded the research.

Story Source:

The above story is reprinted from materials provided by Purdue University. The original article was written by Brian Wallheimer.

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

Journal Reference:

Jin Shi, Tae-Gon Cha, Jonathan C. Claussen, Alfred R. Diggs, Jong Hyun Choi, D. Marshall Porterfield. Microbiosensors based on DNA modified single-walled carbon nanotube and Pt black nanocomposites. The Analyst, 2011; 136 (23): 4916 DOI: 10.1039/c1an15179g

Note: If no author is given, the source is cited instead.

Why carbon nanotubes spell trouble for cells

It's been long known that asbestos spells trouble for human cells. Scientists have seen cells stabbed with spiky, long asbestos fibers, and the image is gory: Part of the fiber is protruding from the cell, like a quivering arrow that's found its mark.

But scientists had been unable to understand why cells would be interested in asbestos fibers and other materials at the nanoscale that are too long to be fully ingested. Now a group of researchers at Brown University explains what happens. Through molecular simulations and experiments, the team reports in Nature Nanotechnology that certain nanomaterials, such as carbon nanotubes, enter cells tip-first and almost always at a 90-degree angle. The orientation ends up fooling the cell; by taking in the rounded tip first, the cell mistakes the particle for a sphere, rather than a long cylinder. By the time the cell realizes the material is too long to be fully ingested, it's too late.

"It's as if we would eat a lollipop that's longer than us," said Huajian Gao, professor of engineering at Brown and the paper's corresponding author. "It would get stuck."

The research is important because nanomaterials like carbon nanotubes have promise in medicine, such as acting as vehicles to transport drugs to specific cells or to specific locations in the human body. If scientists can fully understand how nanomaterials interact with cells, then they can conceivably design products that help cells rather than harm them.

"If we can fully understand (nanomaterial-cell dynamics), we can make other tubes that can control how cells interact with nanomaterials and not be toxic," Gao said. "We ultimately want to stop the attraction between the nanotip and the cell."

Like asbestos fibers, commercially available carbon nanotubes and gold nanowires have rounded tips that often range from 10 to 100 nanometers in diameter. Size is important here; the diameter fits well within the cell's parameters for what it can handle. Brushing up against the nanotube, special proteins called receptors on the cell spring into action, clustering and bending the membrane wall to wrap the cell around the nanotube tip in a sequence that the authors call "tip recognition." As this occurs, the nanotube is tipped to a 90-degree angle, which reduces the amount of energy needed for the cell to engulf the particle.

Once the engulfing — endocytosis — begins, there is no turning back. Within minutes, the cell senses it can't fully engulf the nanostructure and essentially dials 911. "At this stage, it's too late," Gao said. "It's in trouble and calls for help, triggering an immune response that can cause repeated inflammation."

The team hypothesized the interaction using coarse-grained molecular dynamic simulations and capped multiwalled carbon nanotubes. In experiments involving nanotubes and gold nanowires and mouse liver cells and human mesothelial cells, the nanomaterials entered the cells tip-first and at a 90-degree angle about 90 percent of the time, the researchers report.

"We thought the tube was going to lie on the cell membrane to obtain more binding sites. However, our simulations revealed the tube steadily rotating to a high-entry degree, with its tip being fully wrapped," said Xinghua Shi, first author on the paper who earned his doctorate at Brown and is at the Chinese Academy of Sciences in Beijing. "It is counter-intuitive and is mainly due to the bending energy release as the membrane is wrapping the tube."

The team would like to study whether nanotubes without rounded tips — or less rigid nanomaterials such as nanoribbons — pose the same dilemma for cells.

"Interestingly, if the rounded tip of a carbon nanotube is cut off (meaning the tube is open and hollow), the tube lies on the cell membrane, instead of entering the cell at a high-degree-angle," Shi said.

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Researchers use carbon nanotubes to make solar cells affordable, flexible

Researchers from Northwestern University have developed a carbon-based material that could revolutionize the way solar power is harvested. The new solar cell material -- a transparent conductor made of carbon nanotubes -- provides an alternative to current technology, which is mechanically brittle and reliant on a relatively rare mineral.

Due to Earth's abundance of carbon, carbon nanotubes have the potential to boost the long-term viability of solar power by providing a cost-efficient option as demand for the technology increases. In addition, the material's mechanical flexibility could allow solar cells to be integrated into fabrics and clothing, enabling portable energy supplies that could impact everything from personal electronics to military operations.

The research, headed by Mark C. Hersam, professor of materials science and engineering and professor of chemistry, and Tobin J. Marks, Vladimir N. Ipatieff Professor of Catalytic Chemistry and professor of materials science and engineering, is featured on the cover of the October 2011 issue of Advanced Energy Materials, a new journal that specializes in science about materials used in energy applications.

Solar cells are composed of several layers, including a transparent conductor layer that allows light to pass into the cell and electricity to pass out; for both these actions to occur, the conductor must be both electrically conductive and also optically transparent. Few materials concurrently possess both of these properties.

Currently, indium tin oxide is the dominant material used in transparent conductor applications, but the material has two potential limitations. Indium tin oxide is mechanically brittle, which precludes its use in applications that require mechanical flexibility. In addition, Indium tin oxide relies on the relatively rare element indium, so the projected increased demand for solar cells could push the price of indium to problematically high levels.

"If solar technology really becomes widespread, as everyone hopes it will, we will likely have a crisis in the supply of indium," Hersam said. "There's a great desire to identify materials -- especially earth-abundant elements like carbon -- that can take indium's place in solar technology."

Hersam and Marks' team has created an alternative to indium tin oxide using single-walled carbon nanotubes, tiny, hollow cylinders of carbon just one nanometer in diameter.

The researchers have gone further to determine the type of nanotube that is most effective in transparent conductors. Nanotubes' properties vary depending on their diameter and their chiral angle, the angle that describes the arrangement of carbon atoms along the length of the nanotube. These properties determine two types of nanotubes: metallic and semiconducting.

Metallic nanotubes, the researchers found, are 50 times more effective than semiconducting ones when used as transparent conductors in organic solar cells.

"We have now identified precisely the type of carbon nanotube that should be used in this application," Hersam said.

Because carbon nanotubes are flexible, as opposed to the brittle indium tin oxide, the researchers' findings could pave the way for many new applications in solar cells. For example, the military could incorporate the flexible solar cells into tent material to provide solar power directly to soldiers in the field, or the cells could be integrated into clothing, backpacks, or purses for wearable electronics.

"With this mechanically flexible technology, it's much easier to imagine integrating solar technology into everyday life, rather than carrying around a large, inflexible solar cell," Hersam said.

Researchers are now examining other layers of the solar cell to explore also replacing these with carbon-based nanomaterials.

Besides Hersam and Marks, other authors include Timothy P. Tyler, Ryan E. Brock, and Hunter J. Karmel. This work was supported by the Argonne Northwestern-Northwestern Solar Energy Research Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

Story Source:

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

Journal Reference:

Timothy P. Tyler, Ryan E. Brock, Hunter J. Karmel, Tobin J. Marks, Mark C. Hersam. Organic Solar Cell Characterization: Electronically Monodisperse Single-Walled Carbon Nanotube Thin Films as Transparent Conducting Anodes in Organic Photovoltaic Devices (Adv. Energy Mater. 5/2011). Advanced Energy Materials, 2011; 1 (5): 701 DOI: 10.1002/aenm.201190021

New material synthesized: Graphene nanoribbons inside of carbon nanotubes

Physicists from Umea University (Sweden) and Finland have found an efficient way to synthesize graphene nanoribbons directly inside of single-walled carbon nanotubes.

The result was recently published in the journal Nano Letters.

Graphene, a one atom thin flake of plain carbon, has a wide range of unusual and highly interesting properties. As a conductor of electricity it performs as well as copper. As a conductor of heat it outperforms all other known materials. There are possibilities to achieve strong variations of the graphene properties by making graphene in the form of belts with various widths, so called nanoribbons. These nanoribbons are now the real focus of attention in physics and an extremely promising material for electronics, solar cells and many other things. However, it is has not been easy to make such ribbons.

Associate professor Alexandr Talyzin and his research group at the Department of Physics, Umea University, have together with colleagues from Professor Esko Kauppinen´s group, Aalto University in Finland, discovered a way to use the hollow space inside carbon nanotubes as a one-dimensional chemical reactor to make encapsulated graphene. An intriguing property of this space is that chemical reactions occur differently here compared to under bulk three-dimensional conditions.

"We used coronene and perylene, which are large organic molecules, as building blocks to produce long and narrow graphene nanoribbons inside the tubes. The idea of using these molecules as building blocks for graphene synthesis was based on our previous study," says Talyzin.

This study revealed that coronene molecules can react with each other at certain conditions to form dimers, trimers and longer molecules in a bulk powder form. The result suggested that coronene molecules can possibly be used for synthesis of graphene but need to be somehow aligned in one plane for the required reaction. The inner space of single-walled carbon nanotubes seemed to be an ideal place to force molecules into the edge-to-edge geometry required for the polymerization reaction.

In the new study, the researchers show that this is possible. When the first samples were observed by electron microscopy by Ilya Anoshkin at Aalto University, exciting results were revealed: all nanotubes were filled inside with graphene nanoribbons.

"The success of the experiments also depended a lot on the choice of nanotubes. Nanotubes of suitable diameter and in high quality were provided by our co-authors from Aalto University," says Talyzin.

Later the researchers found that the shape of encapsulated graphene nanoribbons can be modified by using different kinds of aromatic hydrocarbons. The properties of nanoribbons are very different depending on their shape and width. For example, nanoribbons can be either metallic or semiconducting depending on their width and type. Interestingly, carbon nanotubes can also be metallic, semiconducting (depending on their diameter) or insulating when chemically modified.

"This creates an enormous potential for a wide range of applications. We can prepare hybrids that combine graphene and nanotubes in all possible combinations in the future," says Talyzin.

For example, metallic nanoribbons inside insulating nanotubes are very thin insulated wires. They might be used directly inside carbon nanotubes to produce light thus making nano-lamps. Semiconducting nanoribbons can possibly be used for transistors or solar cell applications and metallic-metallic combination is in fact a new kind of coaxial nano-cable, macroscopic cables of this kind are used e.g. for transmitting radio signals.

The new method of hybrid synthesis is very simple, easily scalable and allows obtaining almost 100 percent filling of tubes with nanoribbons. The theoretical simulations, performed by Arkady Krasheninnikov in Finland, also show that the graphene nanoribbons keep their unique properties inside the nanotubes while protected from the environment by encapsulation and aligned within bundles of single-walled nanotubes.

"The new material seems very promising, but we have a lot of inter-disciplinary work ahead of us in the field of physics and chemistry. To synthesize the material is just a beginning. Now we want to learn its electric, magnetic and chemical properties and how to use the hybrids for practical applications," says Talyzin.

Story Source:

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

Journal Reference:

Alexandr V. Talyzin, Ilya V. Anoshkin, Arkady V. Krasheninnikov, Risto M. Nieminen, Albert G. Nasibulin, Hua Jiang, Esko I. Kauppinen. Synthesis of Graphene Nanoribbons Encapsulated in Single-Walled Carbon Nanotubes. Nano Letters, 2011; 110902093500003 DOI: 10.1021/nl2024678

DNA strands that select nanotubes are first step to a practical 'quantum wire'

DNA, a molecule famous for storing the genetic blueprints for all living things, can do other things as well. In a new paper, researchers at the National Institute of Standards and Technology (NIST) describe how tailored single strands of DNA can be used to purify the highly desired "armchair" form of carbon nanotubes. Armchair-form single wall carbon nanotubes are needed to make "quantum wires" for low-loss, long distance electricity transmission and wiring.

Single-wall carbon nanotubes are usually about a nanometer in diameter, but they can be millions of nanometers in length. It's as if you took a one-atom-thick sheet of carbon atoms, arranged in a hexagonal pattern, and curled it into a cylinder, like rolling up a piece of chicken wire. If you've tried the latter, you know that there are many possibilities, depending on how carefully you match up the edges, from neat, perfectly matched rows of hexagons ringing the cylinder, to rows that wrap in spirals at various angles -- "chiralities" in chemist-speak.

Chirality plays an important role in nanotube properties. Most behave like semiconductors, but a few are metals. One special chiral form -- the so-called "armchair carbon nanotube"* -- behaves like a pure metal and is the ideal quantum wire, according to NIST researcher Xiaomin Tu.

Armchair carbon nanotubes could revolutionize electric power systems, large and small, Tu says. Wires made from them are predicted to conduct electricity 10 times better than copper, with far less loss, at a sixth the weight. But researchers face two obstacles: producing totally pure starting samples of armchair nanotubes, and "cloning" them for mass production. The first challenge, as the authors note, has been "an elusive goal."

Separating one particular chirality of nanotube from all others starts with coating them to get them to disperse in solution, as, left to themselves, they'll clump together in a dark mass. A variety of materials have been used as dispersants, including polymers, proteins and DNA. The NIST trick is to select a DNA strand that has a particular affinity for the desired type of nanotube. In earlier work, team leader Ming Zheng and colleagues demonstrated DNA strands that could select for one of the semiconductor forms of carbon nanotubes, an easier target. In this new paper, the group describes how they methodically stepped through simple mutations of the semiconductor-friendly DNA to "evolve" a pattern that preferred the metallic armchair nanotubes instead.

"We believe that what happens is that, with the right nanotube, the DNA wraps helically around the tube," explains Constantine Khripin, "and the DNA nucleotide bases can connect with each other in a way similar to how they bond in double-stranded DNA." According to Zheng, "The DNA forms this tight barrel around the nanotube. I love this idea because it's kind of a lock and key. The armchair nanotube is a key that fits inside this DNA structure -- you have this kind of molecular recognition."

Once the target nanotubes are enveloped with the DNA, standard chemistry techniques such as chromatography can be used to separate them from the mix with high efficiency.

"Now that we have these pure nanotube samples," says team member Angela Hight Walker, "we can probe the underlying physics of these materials to further understand their unique properties. As an example, some optical features once thought to be indicative of metallic carbon nanotubes are not present in these armchair samples."

* From the distinctive shape of the edge of the cylinder.

Story Source:

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

Journal References:

Xiaomin Tu, Angela R. Hight Walker, Constantine Y. Khripin, Ming Zheng. Evolution of DNA Sequences Toward Recognition of Metallic Armchair Carbon Nanotubes. Journal of the American Chemical Society, 2011; : 110728080027017 DOI: 10.1021/ja205407qXiaomin Tu, Suresh Manohar, Anand Jagota, Ming Zheng. DNA sequence motifs for structure-specific recognition and separation of carbon nanotubes. Nature, 2009; 460 (7252): 250 DOI: 10.1038/nature08116

Researchers graft olfactory receptors onto nanotubes

Penn researchers have helped develop a nanotech device that combines carbon nanotubes with olfactory receptor proteins, the cell components in the nose that detect odors.

Because olfactory receptors belong to a larger class of proteins that are involved in passing signals through the cell membrane, these devices could have applications beyond odor sensing, such as pharmaceutical research.

The research was led by professor A. T. Charlie Johnson, postdoctoral fellow Brett R. Goldsmith and graduate student Mitchell T. Lerner of the Department of Physics and Astronomy in the School of Arts and Sciences, along with assistant professor Bohdana M. Discher and postdoctoral fellow Joseph J. Mitala Jr. of the Department of Biophysics and Biochemistry at Penn's Perelman School of Medicine. They collaborated with researchers from the Monell Chemical Senses Center, the University of Miami, the University of Illinois, Princeton University and two private companies, Nanosense Inc. and Evolved Machines Inc.

Their work was published in the journal ACS Nano.

The Penn team worked with olfactory receptors derived from mice, but all olfactory receptors are part of a class of proteins known as G Protein Coupled Receptors, or GPCRs. These receptors sit on the outer membrane of cells, where certain chemicals in the environment can bind to them. The binding action is the first step in a chemical cascade that leads to a cellular response; in the case of an olfactory receptor, this cascade leads to the perception of a smell.

The Penn team succeeded in building an interface between this complicated protein and a carbon nanotube transistor, allowing them to convert the chemical signals the receptor normally produces to electrical signals, which could be incorporated in any number of tools and gadgets.

"Our nanotech devices are read-out elements; they eavesdrop on what the olfactory receptors are doing, specifically what molecules are bound to them," Johnson said.

As the particular GPCR the team worked with was an olfactory receptor, the test case for their nanotube device was to function as sensor for airborne chemicals.

"If there's something in the atmosphere that wants to bind to this molecule, the signal we get through the nanotube is about what fraction of the time is something bound or not. That means we can get a contiguous read out that's indicative of the concentration of the molecule in the air," Johnson said.

While one could imagine scaling up these nanotube devices into a synthetic nose -- making one for each of the approximately 350 olfactory GPCRs in a human nose, or the 1,000 found in a dog's -- Johnson thinks that medical applications are much closer to being realized.

"GPCRs are common drug targets," he said. "Since they are known to be very important in cell-environment interactions, they're very important in respect to disease pathology. In that respect, we now have a pathway into interrogating what those GPCRs actually respond to. You can imagine building a chip with many of these devices, each with different GPCRs, and exposing them all at once to various drugs to see which is effective at triggering a response."

Figuring out what kinds of drugs bind most effectively to GPCRs is important because pathogens often attack through those receptors as well. The better a harmless chemical attaches to a relevant GPCR, the better it is at blocking the disease.

The Penn team also made a technical advancement in stabilizing GPCRs for future research.

"In the past, if you take a protein out of a cell and put it onto a device, it might last for a day. But here, we embedded it in a nanoscale artificial cell membrane, which is called a nanodisc," Johnson said. "When we did that, they lasted for two and half months, instead of a day."

Increasing the lifespans of such devices could be beneficial to two scientific fields with increasing overlap, as the as evidenced by the large, interdisciplinary research team involved in the study.

"The big picture is integrating nanotechnology with biology, " Johnson said. "These complicated molecular machines are the prime method of communication between the interior of the cell and the exterior, and now we're incorporating their functionality with our nanotech devices."

In addition to Johnson, Goldsmith, Lerner, Discher and Mitala, the research was conducted by Jesusa Josue and Joseph G. Brand of Monell; Alan Gelperin of Monell and Princeton; Ana Castro and Charles W. Luetje of the University of Miami; Timothy H. Bayburt and Stephen G. Sligar of the University of Illinois, Urbana; Samuel M. Khamis of Adamant Technologies, Ryan A. Jones of Nanosense Inc.; and Paul A. Rhodes of Nanosense Inc. and Evolved Machines Inc.

The research was supported by the Defense Advanced Research Projects Agency's RealNose project, Penn's Nano/Bio Interface Center, the National Science Foundation and the Department of Defense.

Story Source:

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

Journal Reference:

Brett R. Goldsmith, Joseph J. Mitala, Jesusa Josue, Ana Castro, Mitchell B. Lerner, Timothy H. Bayburt, Samuel M. Khamis, Ryan A. Jones, Joseph G. Brand, Stephen G. Sligar, Charles W. Luetje, Alan Gelperin, Paul A. Rhodes, Bohdana M. Discher, A. T. Charlie Johnson. Biomimetic Chemical Sensors Using Nanoelectronic Readout of Olfactory Receptor Proteins. ACS Nano, 2011; 5 (7): 5408 DOI: 10.1021/nn200489j

.

Supramolecules get time to shine: Technique reveals interactions between nanotubes, photoluminescent materials

What looks like a spongy ball wrapped in strands of yarn -- but a lot smaller -- could be key to unlocking better methods for catalysis, artificial photosynthesis or splitting water into hydrogen, according to Rice University chemists who have created a platform to analyze interactions between carbon nanotubes and a wide range of photoluminescent materials.

The microscopic particles assembled in the lab of Angel Martí, an assistant professor of chemistry and bioengineering, combine single-walled carbon nanotubes with porous silicate materials that can absorb various molecules -- in this case, a ruthenium complex.

Martí, graduate student and lead author Avishek Saha and their colleagues reported their results July 12 in the Royal Society of Chemistry journal Chemical Science.

The ability to immobilize individual carbon nanotubes on a solid surface is interesting enough, but combining supramolecular systems with nanomaterials to produce hybrids is unique, they said.

"This can be used as a general platform to study the interaction of not only ruthenium complexes, but most photoactive molecules can be encapsulated within these porous silicates in a very simple way without chemical modification, without anything," Marti said.

Saha endured trial and error at every step in bringing the new particles to fruition, first figuring out the best way to keep long, single-walled carbon nanotubes produced by the Rice-born HiPco process from aggregating into bundles while allowing them to adhere to the particles.

The solution suggested by co-author Matteo Pasquali, a Rice professor in chemical and biomolecular engineering and in chemistry, involved dissolving the bundles in chlorosulfonic acid, which added protons -- and thus a positive charge -- to each nanotube.

That was the key to making nanotubes attractive to the three types of silicate particles tested: a commercial version of MCM-41, a mesoporous material used as a molecular sieve; another version of MCM-41 synthesized at Rice by Saha, and microporous Zeolyte-Y.

"We don't fully understand the mechanism, but the truth is they have a very strong affinity to silicon oxide networks," said Marti, describing the nanotube-wrapped particles. "Once they're protonated, they just bind."

But that wasn't enough to create a proper platform because protonated nanoparticles are no longer photoluminescent, a quality the researchers required to "see" such tiny structures under a spectroscope. "Protonated nanotubes are cool, but we want to have pristine nanotubes," Martí said.

"We were stuck there for a while. We tried a lot of things," he said. Acetone, ammonia, chloroform and other substances would deprotonate the nanotubes, but would also release them from the silicate sponges and allow them to clump. But vinylpyrrolidone (VP) did the trick by giving the nanotubes a polymer-like coating while returning them to their pristine states.

"This becomes interesting not only from the standpoint of getting individualized nanotubes on top of a surface, but also because we got fluorescence of nanotubes not from a solution, but from a solid material," Martí said.

The experiment went one critical step further when the researchers introduced ruthenium molecules to the mix. The silicates absorbed the ruthenium molecules, putting them into close proximity with an array of nanotubes. Under a spectroscope, the ruthenium complexes would photoluminesce, but they saw something unexpected in the interaction.

"Basically, we found out that if you put a photoactive species (ruthenium) there and excite it with light, two different processes happen. If it has carbon nanotubes close by, it will transfer an electron to the nanotubes. There's a charge transfer, and we knew that would happen," Martí said. "What we didn't expect when we analyzed the spectrum was seeing two different species of ruthenium complexes, one with a very short photoluminescence lifetime and one very long."

The researchers theorized that ruthenium in the center of the sponge was too far from the nanotubes to transfer electrons, so it retained its standard luminescence.

The research leads to some interesting possibilities for materials science, Saha said. "MCM itself has many applications (as a mesoporous sieve in fuel refineries, for instance), and carbon nanotubes are wonderful materials that many people are interested in. We're just combining these two into a hybrid material that might have the virtues of both."

While pore sizes in zeolites are locked by their crystalline structure at 0.7 nanometers, pores in MCM can be customized, as Saha has done, to absorb specific materials. "There are many things we can do to tune the system that we haven't explored," he said; combining metal molecules or even quantum dots with MCM and nanotubes might lead to interesting results.

Martí said putting charged nanotubes on the surface of a solid also opens the door to use them as catalysts in solar-energy conversion. "You need that driving force, that charge separation, for artificial photosynthesis," he said.

Co-authors of the paper are Rice graduate students Saunab Ghosh and Natnael Behabtu.

The Welch Foundation supported the research.

Story Source:

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

Journal Reference:

Avishek Saha, Saunab Ghosh, Natnael Behabtu, Matteo Pasquali, Angel A. Mart. Single-walled carbon nanotubes shell decorating porous silicate materials: A general platform for studying the interaction of carbon nanotubes with photoactive molecules. Chemical Science, 2011; DOI: 10.1039/C1SC00323B

'Amplified' nanotubes may power the future

Rice University scientists have achieved a pivotal breakthrough in the development of a cable that will make an efficient electric grid of the future possible. Armchair quantum wire (AQW) will be a weave of metallic nanotubes that can carry electricity with negligible loss over long distances. It will be an ideal replacement for the nation's copper-based grid, which leaks electricity at an estimated 5 percent per 100 miles of transmission, said Rice chemist Andrew R. Barron, author of a paper about the latest step forward published online by the American Chemical Society journal Nano Letters.

A prime technical hurdle in the development of this "miracle cable," Barron said, is the manufacture of massive amounts of metallic single-walled carbon nanotubes, dubbed armchairs for their unique shape. Armchairs are best for carrying current, but can't yet be made alone. They grow in batches with other kinds of nanotubes and have to be separated out, which is a difficult process given that a human hair is 50,000 times larger than a single nanotube.

Barron's lab demonstrated a way to take small batches of individual nanotubes and make them dramatically longer. Ideally, long armchair nanotubes could be cut, re-seeded with catalyst and re-grown indefinitely.

The paper was written by graduate student and first author Alvin Orbaek, undergraduate student Andrew Owens and Barron, the Charles W. Duncan Jr.-Welch Professor of Chemistry and a professor of materials science.

Amplification of nanotubes was seen as a key step toward the practical manufacture of AQW by the late Rice professor, nanotechnology pioneer and Nobel laureate Richard Smalley, who worked closely with Barron and Rice chemist James Tour, 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, to lay out a path for its development.

Barron charged Orbaek with the task of following through when he joined the lab five years ago. "When I first heard about Rice University, it was because of Rick Smalley and carbon nanotubes," said Orbaek, a native of Ireland. "He had a large global presence with regard to nanotechnology, and that reached me.

"So I was delighted to come here and find I'd be working on nanotube growth that was related to Smalley's work."

Orbaek said he hasn't strayed far from Barron's original direction, which involved chemically attaching an iron/cobalt catalyst to the ends of nanotubes and then fine-tuning the temperature and environment in which amplification could occur.

"My group, with Smalley and Tour's group, demonstrated you could do this -- but in the first demonstration, we got only one tube to grow out of hundreds or thousands," Barron said. Subsequent experiments raised the yield, but tube growth was minimal. In other attempts, the catalyst would literally eat -- or "etch" -- the nanotubes, he said.

Refining the process has taken years, but the payoff is clear because up to 90 percent of the nanotubes in a batch can now be amplified to significant lengths, Barron said. The latest experiments focused on single-walled carbon nanotubes of various chiralities, but the researchers feel the results would be as great, and probably even better, with a batch of pristine armchairs.

The key was finding the right balance of temperatures, pressures, reaction times and catalyst ratios to promote growth and retard etching, Barron said. While initial growth took place at 1,000 degrees Celsius, the researchers found the amplification step required lowering the temperature by 200 degrees, in addition to adjusting the chemistry to maximize the yield.

"What we're getting to is that sweet spot where most of the nanotubes grow and none of them etch," Barron said.

Wade Adams, director of Rice's Richard E. Smalley Institute for Nanoscale Science and Technology and principal investigator on the AQW project, compared the technique to making sourdough bread. "You make a little batch of pure metallics and then amplify that tremendously to make a large amount. This is an important increment in developing the science to make AQW.

Adams noted eight Rice professors and dozens of their students are working on aspects of AQW. "We know how to spin nanotubes into fibers, and their properties are improving rapidly too," he said. "All this now has to come together in a grand program to turn quantum wires into a product that will carry vast amounts of electricity around the world."

Barron and his team are continuing to fine-tune their process and hope that by summer's end they can begin amplifying armchair nanotubes with the goal of making large quantities of pure metallics. "We're always learning more about the mechanisms by which nanotubes grow," said Orbaek, who sees the end game as the development of a single furnace to grow nanotubes from scratch, cap them with new catalyst, amplify them and put out a steady stream of fiber for cables.

"What we've done is a baby step," he said. "But it verifies that, in the big picture, armchair quantum wire is technically feasible."

Orbaek said he is thrilled to play a role in achieving amplification, which Smalley recognized as necessary to his dream of an efficient energy grid that would catalyze solutions to many of the world's problems.

"I'd love to meet him now to say, 'Hey, man, you were right,'" he said.

The Robert A. Welch Foundation and the Air Force Office of Scientific Research funded the research.

Story Source:

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

Journal Reference:

Alvin W. Orbaek, Andrew C. Owens, Andrew R. Barron. Increasing the Efficiency of Single Walled Carbon Nanotube Amplification by Fe–Co Catalysts Through the Optimization of CH4/H2Partial Pressures. Nano Letters, 2011; 11 (7): 2871 DOI: 10.1021/nl201315j

New way to store sun's heat: Modified carbon nanotubes can store solar energy indefinitely, then be recharged by exposure to the sun

 A novel application of carbon nanotubes, developed by MIT researchers, shows promise as an innovative approach to storing solar energy for use whenever it's needed.

Storing the sun's heat in chemical form -- rather than converting it to electricity or storing the heat itself in a heavily insulated container -- has significant advantages, since in principle the chemical material can be stored for long periods of time without losing any of its stored energy. The problem with that approach has been that until now the chemicals needed to perform this conversion and storage either degraded within a few cycles, or included the element ruthenium, which is rare and expensive.

Last year, MIT associate professor Jeffrey Grossman and four co-authors figured out exactly how fulvalene diruthenium -- known to scientists as the best chemical for reversibly storing solar energy, since it did not degrade -- was able to accomplish this feat. Grossman said at the time that better understanding this process could make it easier to search for other compounds, made of abundant and inexpensive materials, which could be used in the same way.

Now, he and postdoc Alexie Kolpak have succeeded n doing just that. A paper describing their new findings has just been published online in the journal Nano Letters, and will appear in print in a forthcoming issue.

The new material found by Grossman and Kolpak is made using carbon nanotubes, tiny tubular structures of pure carbon, in combination with a compound called azobenzene. The resulting molecules, produced using nanoscale templates to shape and constrain their physical structure, gain "new properties that aren't available" in the separate materials, says Grossman, the Carl Richard Soderberg Associate Professor of Power Engineering.

Not only is this new chemical system less expensive than the earlier ruthenium-containing compound, but it also is vastly more efficient at storing energy in a given amount of space -- about 10,000 times higher in volumetric energy density, Kolpak says -- making its energy density comparable to lithium-ion batteries. By using nanofabrication methods, "you can control [the molecules'] interactions, increasing the amount of energy they can store and the length of time for which they can store it -- and most importantly, you can control both independently," she says.

Thermo-chemical storage of solar energy uses a molecule whose structure changes when exposed to sunlight, and can remain stable in that form indefinitely. Then, when nudged by a stimulus -- a catalyst, a small temperature change, a flash of light -- it can quickly snap back to its other form, releasing its stored energy in a burst of heat. Grossman describes it as creating a rechargeable heat battery with a long shelf life, like a conventional battery.

One of the great advantages of the new approach to harnessing solar energy, Grossman says, is that it simplifies the process by combining energy harvesting and storage into a single step. "You've got a material that both converts and stores energy," he says. "It's robust, it doesn't degrade, and it's cheap." One limitation, however, is that while this process is useful for heating applications, to produce electricity would require another conversion step, using thermoelectric devices or producing steam to run a generator.

While the new work shows the energy-storage capability of a specific type of molecule -- azobenzene-functionalized carbon nanotubes -- Grossman says the way the material was designed involves "a general concept that can be applied to many new materials." Many of these have already been synthesized by other researchers for different applications, and would simply need to have their properties fine-tuned for solar thermal storage.

The key to controlling solar thermal storage is an energy barrier separating the two stable states the molecule can adopt; the detailed understanding of that barrier was central to Grossman's earlier research on fulvalene dirunthenium, accounting for its long-term stability. Too low a barrier, and the molecule would return too easily to its "uncharged" state, failing to store energy for long periods; if the barrier were too high, it would not be able to easily release its energy when needed. "The barrier has to be optimized," Grossman says.

Already, the team is "very actively looking at a range of new materials," he says. While they have already identified the one very promising material described in this paper, he says, "I see this as the tip of the iceberg. We're pretty jazzed up about it."

Yosuke Kanai, assistant professor of chemistry at the University of North Carolina at Chapel Hill, says "the idea of reversibly storing solar energy in chemical bonds is gaining a lot of attention these days. The novelty of this work is how these authors have shown that the energy density can be significantly increased by using carbon nanotubes as nanoscale templates. This innovative idea also opens up an interesting avenue for tailoring already-known photoactive molecules for solar thermal fuels and storage in general."

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Massachusetts Institute of Technology. The original article was written by David L. Chandler, MIT News Office.

Journal Reference:

Alexie M. Kolpak, Jeffrey C. Grossman. Azobenzene-Functionalized Carbon Nanotubes As High-Energy Density Solar Thermal Fuels. Nano Letters, 2011; : 110705085331088 DOI: 10.1021/nl201357n

Red light from carbon nanotubes

To the human eye, carbon nanotubes usually appear as a black powder. They can hardly be forced to emit light, as they are excellent electrical conductors and capture the energy from other luminescent chemical species placed nearby. The researchers from the Institute of Physical Chemistry of the Polish Academy of Sciences in Warsaw contributed recently to the development of a relatively simple method allowing the nanotubes exposed to UV to emit red light.

The researchers involved in the international FINELUMEN project, coordinated by Dr. Nicola Armaroli from Italy's Istituto per la Sintesi Organica e la Fotoreattivita, Consiglio Nazionale delle Ricerche (CNR-ISOF) in Bolonia, have developed an efficient method to fabricate a new photonic material: carbon nanotubes coated with chemicals that are capable of displaying red light. "We take part at the project as a research group specializing in studies on lanthanide compounds. We decided to combine their high luminescent properties with excellent mechanical and electrical characteristics of nanotubes," says Prof. Marek Pietraszkiewicz from Warsaw's Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS).

Carbon nanotubes can be envisaged as a graphite sheet rolled-up into a seamless cylinder. The surface area of each nanotube is relatively high and allows to attach many other molecules, including those capable to emit light. "Attachment of light-emitting complexes directly to the nanotube is, however, not favourable, because the latter, as a black absorber, would highly quench the luminescence," explains Valentina Utochnikova, a PhD student at the IPC PAS. To reduce undesired effect of light absorption, the nanotubes are first subject to a thermal reaction at temperature 140-160 oC in a solution of ionic liquid modified with a terminal azido function. The reaction yields nanotubes coated with molecules acting as anchors-links. On one side the anchors are attached to the surface of the nanotube, and on the other they can attach molecules capable of displaying visible light. The free terminal of each link bears a positive charge.

So prepared nanotubes are subsequently transferred into another solution containing a negatively charged lanthanide complex -- tetrakis-(4,4,4-trifluoro-1-(2-naphtyl-1,3-butanedionato)europium. "Lanthanide compounds contain elements from the VI group of the periodic table and are very attractive for photonics, as they are characterised by a high luminescence quantum yield and a high colour purity of the emitted light," stresses Utochnikova.

After dissolving in solution, negatively charged europium complexes are spontaneously caught by positively charged free terminals of anchors attached to nanotubes due to electrostatic interaction. As a result, each nanotube is durably coated with molecules capable to emit visible light. Upon completion of the reaction, the modified nanotubes are washed and dried. The final product is a sooty powder. If the powder is, however, exposed to UV irradiation, the lanthanide complexes anchored to nanotubes start immediately to emit red light.

The concept of how to modify the nanotubes and the reagents -- ionic liquid and lanthanide complex for carbon nanotube coating -- has been developed in Prof. Pietraszkiewicz's research group at the IPC PAS, whereas the modification of nanotubes and spectral studies have been performed by research groups from the University of Namur, Belgium, and CNR-ISOF from Bolonia, Italy. It is essential that chemical reactions leading to fabrication of new light-emitting nanotubes turned out to be significantly simpler than those used so far.

The photonic material received can be used, among others, to detect molecules including those of biological importance. The identification would then take place by analysing of how the luminescence of nanotubes changes upon deposition of molecules under study thereon. Good charge conductivity combined with high luminescence properties make new nanotubes an attractive material also for OLED-based technologies.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by Institute of Physical Chemistry of the Polish Academy of Sciences, via AlphaGalileo.

Splitsville for boron nitride nanotubes

For Hollywood celebrities, the term "splitsville" usually means "check your prenup." For scientists wanting to mass-produce high quality nanoribbons from boron nitride nanotubes, "splitsville" could mean "happily ever after."

Scientists with the Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley, working with scientists at Rice University, have developed a technique in which boron nitride nanotubes are stuffed with atoms of potassium until the tubes split open along a longitudinal seam. This creates defect-free boron nitride nanoribbons of uniform lengths and thickness. Boron nitride nanoribbons are projected to display a variety of intriguing magnetic and electronic properties that hold enormous potential for future devices.

Nanoribbons are two-dimensional single crystals (meaning only a single atom in thickness) that can measure multiple microns in length, but only a few hundred or less nanometers in width. Graphene nanoribbons, which are made from pure carbon, carry electrons at much faster speeds than silicon, and can be used to cover wide areas and a broad assortment of shapes. Boron nitride nanoribbons offer similar advantages plus an additional array of electronic, optical and magnetic properties.

"There has been a significant amount of theoretical work indicating that, depending on the ribbon edges, boron nitride nanoribbons may exhibit ferromagnetism or anti-ferromagnetism, as well as spin-polarized transport which is either metallic or semi-conducting," says physicist Alex Zettl, one of the world's foremost researchers into nanoscale systems and devices who holds joint appointments with Berkeley Lab's Materials Sciences Division (MSD) and the Physics Department at UC Berkeley, where he is the director of the Center of Integrated Nanomechanical Systems (COINS).

"The unique properties of boron nitride nanoribbons are of great fundamental scientific interest and also have implications for applications in technologies that include spintronics and optoelectronics," Zettl says. "However, the facile, scalable synthesis of high quality boron nitride nanoribbons has been a significant challenge."

Zettl and members of his research group met this challenge using the chemical process known as "intercalation," whereby atoms or molecules of one type are inserted between atoms and molecules of another type. James Tour at Rice University and his research group had demonstrated that the intercalation of potassium atoms into carbon nanotubes promotes a longitudinal splitting of the tubes. This prompted Zettl and Tour to collaborate on a study that used the same approach on boron nitride nanotubes, which are very similar in structure to nanotubes made from carbon.

Zettl and Tour reported the results of this study in the journal Nano Letters.  Co-authoring the paper were Kris Erickson, Ashley Gibb, Michael Rousseas and Nasim Alem, who are all members of Zettl's research group, and Alexander Sinitskii, a member of Tour's research group.

"The likely mechanism for the splitting of both carbon and boron nitride nanotubes is that potassium islands grow from an initial starting point of intercalation," Zettl says. "This island growth continues until enough circumferential strain results in a breakage of the chemical bonds of the intercalated nanotube. The potassium then begins bonding to the bare ribbon edge, inducing further splitting."

Alex Zettl holds joint appointments with Berkeley Lab and UC Berkeley where he directs the Center of Integrated Nanomechanical Systems.

This synthesis technique yields boron nitride nanoribbons of uniform widths that can be as narrow as 20 nanometers. The ribbons are also at least one micron in length, with minimal defects within the plane or along the edges. Zettl says the high quality of the edges points to the splitting process being orderly rather than random. This orderliness could explain why a high proportion of the boron nitride nanoribbons display the coveted zigzag or armchair-shaped edges, rather than other edge orientations.

Edges are critical determinants of a nanoribbon's properties because the electrons along the edge of one ribbon edge can interact with the electrons along the edge of another ribbon, resulting in the type of energy gap that is crucial for making devices. For example, zigzagged edges in graphene nanoribbons have been shown to be capable of carrying a magnetic current, which makes them candidates for spintronics, the computing technology based on the spin rather than the charge of electrons.

Kris Erickson, who was the lead author on the Nano Letters paper, says that, "Given the significant dependence upon boron nitride nanoribbon edges for imbuing particular electronic and magnetic properties, the high likelihood of synthesizing ribbons with zigzag and armchair edges makes our technique particularly suitable for addressing theoretical predictions and realizing proposed applications."

Erickson also says it should be possible to functionalize the edges of the boron nitride nanoribbons, as these edges are terminated with chemically reactive potassium atoms following synthesis and with reactive hydrogen atoms following exposure to water or ethanol.

"The potassium-terminated edge could easily be replaced with a species other than hydrogen," Erickson says. "Different chemicals could be used for quenching to impart other terminations, and, furthermore, hydrogen could be replaced after quenching by either utilizing established boron nitride functionalization routes, or by devising new routes unique to the highly reactive nanoribbon edge."

Zettl and his research group are now investigating alternative syntheses using different boron nitride nanotube precursors to increase yields and improve the purification process. They are also attempting to functionalize the edges of their nanoribbons and they are in the process of determining if the various predicted edge states for these nanoribbons can be studied.

"What we really need most right now is a better source of boron nitride nanotubes," Zettl says.

This work was supported by the U.S. Department of Energy's Office of Science, with additional support from the National Science Foundation through the Center of Integrated Nanomechanical Systems (COINS), the Office of Naval Research, and the Air Force Research Laboratory.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by DOE/Lawrence Berkeley National Laboratory.

Journal Reference:

Kris J. Erickson, Ashley L. Gibb, Alexander Sinitskii, Michael Rousseas, Nasim Alem, James M. Tour, Alex K. Zettl. Longitudinal Splitting of Boron Nitride Nanotubes for the Facile Synthesis of High Quality Boron Nitride Nanoribbons. Nano Letters, 2011; : 110524125655048 DOI: 10.1021/nl2014857

Better viewing through fluorescent nanotubes when peering into innards of a mouse

ScienceDaily (May 28, 2011) — Developing drugs to combat or cure human disease often involves a phase of testing with mice, so being able to peer clearly into a living mouse's innards has real value.

But with the fluorescent dyes currently used to image the interior of laboratory mice, the view becomes so murky several millimeters under the skin that researchers might have more success divining the future from the rodent's entrails than they do extracting usable data.

Now Stanford researchers have developed an improved imaging method using fluorescent carbon nanotubes that allows them to see centimeters deep into a mouse with far more clarity than conventional dyes provide. For a creature the size of a mouse, a few centimeters makes a great difference.

"We have already used similar carbon nanotubes to deliver drugs to treat cancer in laboratory testing in mice, but you would like to know where your delivery went, right?" said Hongjie Dai, a professor of chemistry. "With the fluorescent nanotubes, we can do drug delivery and imaging simultaneously -- in real time -- to evaluate the accuracy of a drug in hitting its target."

Researchers inject the single-walled carbon nanotubes into a mouse and can watch as the tubes are delivered to internal organs by the bloodstream.

The nanotubes fluoresce brightly in response to the light of a laser directed at the mouse, while a camera attuned to the nanotubes' near infrared wavelengths records the images.

By attaching the nanotubes to a medication, researchers can see how the drug is progressing through the mouse's body.

Dai is the one of the authors of a paper describing the research published online this month in Proceedings of the National Academy of Sciences.

The key to the nanotubes' usefulness is that they shine in a different portion of the near infrared spectrum than most dyes.

Biological tissues -- whether mouse or human -- naturally fluoresce at wavelengths below 900 nanometers, which is in the same range as the available biocompatible organic fluorescent dyes. That results in undesirable background fluorescence, which muddles the images when dyes are used. But the nanotubes used by Dai's group fluoresce at wavelengths between 1,000 and 1,400 nanometers. At those wavelengths there is barely any natural tissue fluorescence, so background "noise" is minimal.

The nanotubes usefulness is further boosted because tissue scatters less light in the longer wavelength region of the near-infrared, reducing image smearing as light moves or travels through the body, another advantage over fluorophores emitting below 900nm.

"The nanotubes fluoresce naturally, but they emit in a very oddball region," Dai said. "There are not many things -- living or inert -- that emit in this region, which is why it has not been explored very much for biological imaging."

By selecting single-walled carbon nanotubes (SWNTS) with different chiralities diameters and other properties, Dai and his team can fine-tune the wavelength at which the nanotubes fluoresce.

The nanotubes are imaged immediately upon injection into the bloodstream of mice.

Dai and graduate students Sarah Sherlock and Kevin Welsher, who are also coauthors of the PNAS paper, observed the fluorescent nanotubes passing through the lungs and kidneys within seconds after injection. The spleen and liver lit up a few seconds later.

The group also did some "post-production" work on digital video footage of the circulating nanotubes to further enhance the image quality using a process called "principal component analysis."

"In the raw imaging, the spleen, pancreas and kidney might appear as one generalized signal," Sherlock said. "But this process picks up the subtleties in signal variation and resolves what at first appears to be one signal into the distinct organs."

"You can really see things that are deep inside or blocked by other organs such as the pancreas," Dai said.

There are some other imaging methods that can produce deep tissue images, such as magnetic resonance imaging (MRI) and computer tomography (CT) scans. But fluorescence imaging is widely used in research and requires simpler machinery.

Dai said that the fluorescent nanotubes are not capable of reaching the depth of CT or MRI scans, but nanotubes are a step forward in broadening the potential uses of fluorescence as an imaging system beyond the surface and near-surface applications it has been restricted to up until now.

Since nanotube fluorescence was discovered about ten years ago, researchers have been trying to make the fluorescence brighter, Dai said. Still, he has been a little surprised at just how well they now work in animals.

"I did not imagine they could really be used in animals to get deep images like these," he said. "When you look at images like this, you get a sense that the body almost has some transparency to it."

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Stanford University, via EurekAlert!, a service of AAAS.

Journal Reference:

K. Welsher, S. P. Sherlock, H. Dai. Deep-tissue anatomical imaging of mice using carbon nanotube fluorophores in the second near-infrared window. Proceedings of the National Academy of Sciences, 2011; DOI: 10.1073/pnas.1014501108

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

New desalination process developed using carbon nanotubes

A faster, better and cheaper desalination process enhanced by carbon nanotubes has been developed by NJIT Professor Somenath Mitra. The process creates a unique new architecture for the membrane distillation process by immobilizing carbon nanotubes in the membrane pores. Conventional approaches to desalination are thermal distillation and reverse osmosis.

"Unfortunately the current membrane distillation method is too expensive for use in countries and municipalities that need potable water," said Mitra. "Generally only industry, where waste heat is freely available, uses this process. However, we're hoping our new work will have far-reaching consequences bringing good, clean water to the people who need it."

The process is outlined by Mitra and his research team in the current issue of the American Chemistry Society's Applied Materials & Interfaces. Doctoral students Ken Gethard and Ornthida Sae-Khow worked on the project. Mitra is chairman of the department of chemistry and environmental science.

Membrane distillation is a water purification process in which heated salt water passes through a tube-like membrane, called a hollow fiber. "Think of your intestines," said Mitra. "It's designed in such a way that nutrition passes through but not the waste." Using a similar structure, membrane distillation allows only water vapor to pass through the walls of the hollow tube, but not the liquid. When the system works, potable water emerges from the net flux of water vapor which moves from the warm to the cool side. At the same time, saline or salt water passes as body waste would through the fiber.

Membrane distillation offers several advantages. It's a clean, non-toxic technology and can be carried out at 60-90oC. This temperature is significantly lower than conventional distillation which uses higher temperatures. Reverse osmosis uses relatively high pressure.

Nevertheless, membrane distillation is not trouble free. It is costly and getting the membrane to work properly and efficiently can be difficult. "The biggest challenge," said Mitra, "is finding appropriate membranes that encourage high water vapor flux but prevent salt from passing through."

Mitra's new method creates a better membrane by immobilizing carbon nanotubes in the pores. The novel architecture not only increases vapor permeation but also prevents liquid water from clogging the membrane pores. Test outcomes show dramatic increases in both reductions in salt and water production. "That's a remarkable accomplishment and one we are proud to publish," said Mitra.

Another advantage is that the new process can facilitate membrane distillation at a relatively lower temperature, higher flow rate and higher salt concentration. Compared to a plain membrane, this new distillation process demonstrates the same level of salt reduction at a 20°C lower temperature, and at a flow rate six times greater.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by New Jersey Institute of Technology.

Journal Reference:

Ken Gethard, Ornthida Sae-Khow, Somenath Mitra. Water Desalination Using Carbon-Nanotube-Enhanced Membrane Distillation. ACS Applied Materials & Interfaces, 2011; 3 (2): 110 DOI: 10.1021/am100981s

Metallic molecules to nanotubes: Ruthenium complexes dissolve nanotubes, add functionality

A lab at Rice University has stepped forward with an efficient method to disperse nanotubes in a way that preserves their unique properties -- and adds more.

The new technique allows inorganic metal complexes with different functionalities to remain in close contact with single-walled carbon nanotubes while keeping them separated in a solution.

That separation is critical to manufacturers who want to spin fiber from nanotubes, or mix them into composite materials for strength or to take advantage of their electrical properties. For starters, the ability to functionalize the nanotubes at the same time may advance imaging sensors, catalysis and solar-activated hydrogen fuel cells.

Better yet, a batch of nanotubes can apparently stay dispersed in water for weeks on end.

Keeping carbon nanotubes from clumping in aqueous solutions and combining them with molecules that add novel abilities have been flies in the ointment for scientists exploring the use of these highly versatile materials.

They've tried attaching organic molecules to the nanotubes' surfaces to add functionality as well as solubility. But while these techniques can separate nanotubes from one another, they take a toll on the nanotubes' electronic, thermal and mechanical properties.

Angel Marti, a Rice assistant professor of chemistry and bioengineering and a Norman Hackerman-Welch Young Investigator, and his students reported this month in the Royal Society of Chemistry journal Chemical Communications that ruthenium polypyridyl complexes are highly effective at dispersing nanotubes in water efficiently and for long periods. Ruthenium is a rare metallic element.

One key is having just the right molecule for the job. Marti and his team created ruthenium complexes by combining the element with ligands, stable molecules that bind to metal ions. The resulting molecular complex is part hydrophobic (the ligands) and part hydrophilic (the ruthenium). The ligands strongly bind to nanotubes while the attached ruthenium molecules interact with water to maintain the tubes in solution and keep them apart from one another.

Another key turned out to be moderation.

Originally, Marti said, he and co-authors Disha Jain and Avishek Saha weren't out to solve a problem that has boggled chemists for decades, but their willingness to "do something crazy" paid off big-time. Jain is a former postdoctoral researcher in Marti's lab, and Saha is a graduate student.

The researchers were eyeing ruthenium complexes as part of a study to track amyloid deposits associated with Alzheimer's disease. "We started to wonder what would happen if we modified the metal complex so it could bind to a nanotube," Marti said. "That would provide solubility, individualization, dispersion and functionality."

It did, but not at first. "Avishek put this together with purified single-walled carbon nanotubes (created via Rice's HiPco process) and sonicated. Absolutely nothing happened. The nanotubes didn't get into solution -- they just clumped at the bottom.

"That was very weird, but that's how science works -- some things you think are good ideas never work."

Saha removed the liquid and left the clumped nanotubes at the bottom of the centrifuge tube. "So I said, 'Well, why don't you do something crazy. Just add water to that, and with the little bit of ruthenium that might remain there, try to do the reaction.' He did that, and the solution turned black."

A low concentration of ruthenium did the trick. "We found out that 0.05 percent of the ruthenium complex is the optimum concentration to dissolve nanotubes," Marti said. Further experimentation showed that simple ruthenium complexes alone did not work. The molecule requires its hydrophobic ligand tail, which seeks to minimize its exposure to water by binding with nanotubes. "That's the same thing nanotubes want to do, so it's a favorable relationship," he said.

Marti also found the nanotubes' natural fluorescence unaffected by the ruthenium complexes. "Even though they've been purified, which can introduce defects, they still exhibit very good fluorescence," he said.

He said that certain ruthenium complexes have the ability to stay in an excited state for a long time -- about 600 nanoseconds, or 100 times longer than normal organic molecules. "It means the probability that it will transfer an electron is high. That's convenient for energy transfer applications, which are important for imaging," he said.

That nanotubes stay suspended for a long time should catch the eye of manufacturers who use them in bulk. "They should stay separated for weeks without problems," Marti said. "We have solutions that have been sitting for months without any signs of crashing."

The Welch Foundation supported the research.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Rice University.

Journal Reference:

Disha Jain, Avishek Saha, Angel A. Mart. Non-covalent ruthenium polypyridyl complexes–carbon nanotubes composites: an alternative for functional dissolution of carbon nanotubes in solution. Chemical Communications, 2011; 47 (8): 2246 DOI: 10.1039/C0CC05295G