Polymer membranes with molecular-sized channels that assemble themselves

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(PhysOrg.com) -- Many futurists envision a world in which polymer membranes with molecular-sized channels are used to capture carbon, produce solar-based fuels, or desalinating sea water, among many other functions. This will require methods by which such membranes can be readily fabricated in bulk quantities. A technique representing a significant first step down that road has now been successfully demonstrated.

Researchers with the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley have developed a solution-based method for inducing the self-assembly offlexible polymermembranes with highly aligned subnanometer channels. Fully compatible with commercial membrane-fabrication processes, this new technique is believed to be the first example of organicnanotubesfabricated into a functional membrane over macroscopic distances.

"We've used nanotube-forming cyclic peptides and block co-polymers to demonstrate a directed co-assembly technique for fabricating subnanometer porous membranes over macroscopic distances,"says Ting Xu, a polymer scientist who led this project."This technique should enable us to generate porous thin films in the future where the size and shape of the channels can be tailored by the molecular structure of the organic nanotubes."

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Schematic drawing depicts process by which a polymer is tethered to cyclic peptides (8CP)then blended with block copolymers (BCPs) to make a membrane permeated with subnanometer channels in the form of organic nanotubes.

Xu, who holds joint appointments with Berkeley Lab's Materials Sciences Division and the University of California Berkeley's Departments of Materials Sciences and Engineering, and Chemistry, is the lead author of a paper describing this work, which has been published in the journalACS Nano. The paper is titled"Subnanometer PorousThin Filmsby the Co-assembly of Nanotube Subunits andBlock Copolymers."Co-authoring the paper with Xu were Nana Zhao, Feng Ren, Rami Hourani, Ming Tsang Lee, Jessica Shu, Samuel Mao, and Brett Helms, who is with the Molecular Foundry, a DOE nanoscience center hosted at Berkeley Lab.

Channeled membranes are one of nature's most clever and important inventions. Membranes perforated with subnanometer channels line the exterior and interior of a biological cell, controlling– by virtue of size– the transport of essential molecules and ions into, through, and out of the cell. This same approach holds enormous potential for a wide range of human technologies, but the challenge has been finding a cost-effective means of orienting vertically-aligned subnanometer channels over macroscopic distances on flexible substrates.

"Obtaining molecular level control over the pore size, shape, and surface chemistry of channels in polymer membranes has been investigated across many disciplines but has remained a critical bottleneck,"Xu says."Composite films have been fabricated using pre-formed carbon nanotubes and the field is making rapid progess, however, it still presents a challenge to orient pre-formed nanotubes normal to the film surface over macroscopic distances."

For their subnanometer channels, Xu and her research group used the organic nanotubes naturally formed by cyclic peptides - polypeptide protein chains that connect at either end to make a circle. Unlike pre-formed carbon nanotubes, these organic nanotubes are"reversible,"which means their size and orientation can be easily modified during the fabrication process. For the membrane, Xu and her collaborators used block copolymers - long sequences or"blocks"of one type of monomer molecule bound to blocks of another type of monomer molecule. Just as cyclic peptides self-assemble into nanotubes, block copolymers self-assemble into well-defined arrays of nanostructures over macroscopic distances. A polymer covalently linked to the cyclic peptide was used as a"mediator"to bind together these two self-assembling systems

"The polymer conjugate is the key,"Xu says."It controls the interface between thecyclic peptidesand the block copolymers and synchronizes their self-assembly. The result is that nanotube channels only grow within the framework of the polymer membrane. When you can make everything work together this way, the process really becomes very simple."

Xu and her colleagues were able to fabricate subnanometer porous membranes measuring several centimeters across and featuring high-density arrays of channels. The channels were tested via gas transport measurements of carbon dioxide and neopentane. These tests confirmed that permeance was higher for the smaller carbon dioxide molecules than for the larger molecules of neopentane. The next step will be to use this technique to make thicker membranes.

"Theoretically, there are no size limitations for our technique so there should be no problem in making membranes over large area,"Xu says."We're excited because we believe this demonstrates the feasibility of synchronizing multiple self-assembly processes by tailoring secondary interactions between individual components. Our work opens a new avenue to achieving hierarchical structures in a multicomponent system simultaneously, which in turn should help overcome the bottleneck to achieving functional materials using a bottom-up approach."

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A greener path for the production of a vital chemical

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(PhysOrg.com) -- Nanoparticles of gold and palladium (Au-Pd) could lead to a more efficient and environmentally friendly way of producing benzyl benzoate, a chemical compound used widely in the food, pharmaceutical and chemical industries whose applications include a fixative for fragrances, a food additive and a solvent for chemical reactions.

The most common method of producing benzyl benzoate is to react benzoic acid with benzyl alcohol. It can also be generated from benzaldehyde. All three starting materials are derived from toluene, a component ofcrude oil. The manufacture of benzyl alcohol and benzaldehyde requires the use of halogens and acidic solvents, whereas benzoic acid is produced via a more environmentally friendly liquid phase cobalt-catalyzed reaction.

A research team led by Graham Hutchings, professor of chemistry at Cardiff University in Wales in the United Kingdom, and Christopher Kiely, professor of materials science and engineering at Lehigh, has found a way of producing benzyl benzoate directly from toluene in a solvent-free, single-step process using Au-Pd nanoparticles to catalyze the reaction.

“By optimizing the Au-Pd ratio in the nanoparticle, as well as the reaction conditions, we were able to achieve conversion rates of over 95 percent with no conversion to carbon dioxide,” says Hutchings.

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A high resolution electron micrograph of a gold-palladium nanoparticle on a titanium oxide support.

Shining a light on particle size and catalytic activity

The researchers reported their finding Jan. 14 inSciencemagazine in an article titled“Solvent-Free Oxidation of Primary Carbon-Hydrogen Bonds in Toluene Using Au-Pd AlloyNanoparticles.” The article was coauthored by Hutchings and Kiely and 10 other researchers, including Ramchandra Tiruvalam, a Lehigh Ph.D. candidate working with Kiely.

Rather than making the catalysts by conventional support impregnation techniques, the researchers chose a preparation route that involved the sol-immobilization of Au-Pd colloids using amorphous carbon and titanium oxide supports. This technique offers much greater control over particle size and composition than do conventional methods.

Transmission electron microscopy (TEM) studies carried out by Tiruvalam revealed that the average particle sizes were very similar, 3.3 nanometers on carbon and 3.5nm on titanium oxide.

“Despite having a very similar particle-size distribution, the Au-Pd/carbon samples were found to have approximately double the catalytic activity of the Au-Pd/titanium oxide samples,” says Kiely, who directs the Nanocharacterization Laboratory in Lehigh’s Center for Advanced Materials and Nanotechnology.

“This suggests that simple metal surface area considerations are not dominating the catalytic activity.”

Achieving stability and reusability

Using Lehigh’s aberration-corrected TEM, Tiruvalam was able to show that the particles were indeed Au-Pd alloy particles, that those on the titanium oxide were highly faceted and tended to form a flat interface with the support, and that those on the carbon were much more rounded.

“The difference in catalytic activity may be related to differences in the number of low coordination number edge and corner sites available,” explains Kiely.“The more rounded‘rougher’ particles on the carbon support have significantly more of these sites than the flatter particles on thetitanium oxidesupport.”

In a final set of experiments, the researchers were able to demonstrate that the Au-Pd/carbon catalysts showed no loss of activity after use and that there was little change in particle shape and size after extended reaction periods.

“It is clear that these highly active catalysts are both stable and reusable,” says Kiely.

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New 'frozen smoke' material: One ounce could carpet three football fields

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Scientists are reporting the development of a new, ultra-light form of"frozen smoke"-- renowned as the world's lightest solid material -- with amazing strength and an incredibly large surface area.

The new so-called"multiwalledcarbon nanotube(MCNT) aerogel"could be used in sensors to detect pollutants and toxic substances, chemical reactors, and electronics components. A report about the material appears inACS Nano.

Lei Zhai and colleagues explain thataerogelsmade fromsilicon dioxide(the main ingredient in sand) and other material already are used as thermal insulation in windows and buildings, tennis rackets, sponges to clean up oil spills, and other products.

Aerogels are solid but so light that they have been compared to frozen smoke. However, only a few scientists have succeeded in making aerogels from carbon nanotubes, wisps of carbon so small that almost 50,000 would fit across the width of a human hair.

The report describes a process for making MCNT aerogels and tests to determine their properties. MCNT aerogels infused with a plastic material are flexible, for instance, like a spring that can be stretched thousands of times. If the nanotubes in a one-ounce cube were unraveled and placed side-to-side and end-to-end, they would carpet three football fields.

The MCNT aerogels also are excellent conductors of electricity, making them ideal for sensing applications, such as sensing as little as 0.003527 ounce of a material resting in the palm of one hand, the report indicates.

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Novel technique selects molecules according to their chemical properties and dimensions

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Separating molecules is an important part of many manufacturing and testing processes, including pharmaceutical production and some biomedical tests. One way of carrying out such separation is by using nanofilters -- materials with holes of a precisely controlled tiny diameter, to allow molecules up to that size to pass through while blocking any that are larger. But a new system devised by researchers at MIT could add an important new capability: a way to selectively filter out molecules of the same size that have different chemical properties.

Karen Gleason, an MIT professor of chemical engineering and associate dean of engineering for research, and postdoctoral fellow Ayse Asatekin described the process in a paper published this month in the journalNano Letters.

This is“a fundamentally different way” of separatingmolecules, Gleason says.“People usually think of size as being the defining factor,” but by making the pores in the filter small enough so that there is a significant chemical interaction between the pore walls and the molecules passing through them, it becomes possible to discriminate according to other characteristics, she explains. In this case, the selection was based on the molecules’ affinity for water. Because the walls of the pores were hydrophobic (water repelling), other hydrophobic molecules were more easily drawn to the pores and propelled through them than were other, less hydrophobic molecules.

In living organisms, cell walls routinely perform this kind of chemical separation, letting certain specific kinds of molecules— for example, nutrients, enzymes or signaling molecules— pass freely through pores in a cell membrane, while blocking all others. But this is the first time, Asatekin says, that such chemical separation has been demonstrated in a synthetic membrane.

Many biological molecules that are similar in size yet have very different functions or properties, so the ability to separate them efficiently could be important. In this initial proof-of-concept demonstration, the molecules selected were two dyes, chosen because of their similar size and ease of detection. Using a polycarbonate membrane (a type of plastic) treated with a vapor-deposited layer of another polymer, the researchers were able to separate the two dyes very effectively, with more than 200 times more of one type passing through than the other. The coating process they used not only adds the capability for discriminating between molecules based on their differing affinities for water, but by coating the insides of tube-like pores in the material it also provides a way of creating extremely small pores of uniform size— much smaller than can be produced by conventional methods.

Joerg Lahann, an associate professor of chemical engineering at the University of Michigan who was not involved in this work, says that the team’s ability to produce tiny, uniform pores smaller than 10 nanometers (billionths of a meter) across is itself a significant accomplishment that solves a major problem in existing nanoseparation technology.

To test how the system works, the team tried making two different kinds of pores— some that were uniformly sized tubes, others that had a narrow bottleneck at one point and then widened out. The uniform cylinders were much more effective, demonstrating that the key factor is the interaction of the molecules with the wall of the pore over its entire length, which in this case was about 4,000 times the width.

In pharmaceutical manufacturing, many processes involve chemical reactions in which both the reactants and the chemical being produced are very similar in molecular size, so being able to separate the two efficiently could be a significant advance in allowing large-throughput processing instead of small-batch production as is done currently, Asatekin says.

In addition to possible applications in drug manufacturing, such membranes could be important for the detection of biologically significant molecules. For example, the U.S. military, which funded this research through the Institute for Soldier Nanotechnology, is interested in their possible use in detectors that could identify a chemical marker the body produces when an inflammatory response is triggered, which could be a way of quickly revealing that the body had been exposed to a toxin even without knowing what the toxin was.

As a next step, Asatekin and Gleason plan to try the technique to separate biomolecules that are of real relevance to biological processes, to demonstrate that it works for materials that would be of interest for actual applications.

Professor Mathias Ulbricht, chair of technical chemistry at the University of Duisburg-Essen in Germany, calls this a“powerful experimental demonstration” of a new technique that he says holds great promise for practical applications.

“This study opens a new avenue for truly‘tailored’ nanoporous membranes with different selectivities than those of traditional membranes,” he says.“More experimental work toward preparation of membranes with varied structure and other separation experiments are to be done. However, I am optimistic that the promising prospects can be demonstrated practically in such follow-up studies.”

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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.

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Fabrication of mosaic nanofilters for molecular transport, separation of macromolecules

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A team led by Dr. Sherif El-Safty, Exploratory Materials Research Laboratory for Energy and Environment, National Institute for Materials Science (NIMS; Japan), fabricated tight mosaic cage silica nanotubes (NTs) inside anodic alumina membranes (AAM) as a promising candidate nanofilter for high-speed (within several seconds) size-exclusion separation of high concentration macromolecules.

To date, separation of proteins into relatively homogeneous groups and sizes has been very important in biopharmaceuticals and medicines. From the practical viewpoint, the requirements for these applications include easy scaling-up, fast separation, suitability for high production volumes, and low cost. Technically, the design of extremely robust filter membranes without formation of air gaps among membrane nanochannels is a remaining challenge, as pore gaps not only reduce the potential of size-exclusion nanofiltration systems, but also limit the long-term storage stability of NTs, making storage difficult even for a month.

For practical control of mosaic nanofilter membranes, a general approach based on densely engineered three-dimensional (3D) mesocage structures insidesilicaNTs was adopted. In this design, multifunctionalsurface coatingof the pore channels of the AAM facilitated production of extremely robust constructed sequences of membranes as“real nanofilters” without“detachment pores” (air gaps) between the fabricated nanotubes inside the AAM. The approach used by the NIMS team is ideal for constructing tubular-structured architectures inside membranes with vertical alignment, open surfaces of top-bottom ends, multidirectional (3D) pore connectivity, and stability, which are promising for application to nanofilter systems.

The key to this development was the fact that the nanofilter system efficiently separatesmacromoleculessuch as proteins of various sizes over a wide, adjustable range of concentrations. Although conventional processes require as much as 12 hours or more, this technique provides a rapid filtration process that achieves filtration in seconds, despite the blocking effect of the proteins during the filtration process.

The intrinsic properties of the NIMS design (shelf-life or long-term stability, separation efficiency, reusability) are important advantages in comparison with the conventional protein nanofilter techniques used to date. Such advantages will be key to the development of a fabrication approach with the potential to become the optimal method for the design of nanofilters for filtration and molecular transport of multiple species.

The results of this research demonstrated that the NIMS approach offers a time- and cost-efficient alternative tool to current macromolecule analysis methods. This development also offers new insights into control design of devices in the fields of electronics, sensors, and other nanotechnologies.

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Researchers 'recalculate' efficiency paradigm for thin film solar panels

In recent years, developers have been investigating light-harvesting thin film solar panels made from nanotechnology -- and promoting efficiency metrics to make the technology marketable. Now a Tel Aviv University researcher is providing new evidence to challenge recent"charge"measurements for increasing solar panel efficiency.

Offering a less expensive, smaller solution than traditional panels, Prof. Eran Rabani of Tel Aviv University's School of Chemistry at the Raymond and Beverly Sackler Faculty of Exact Sciences puts a lid on some current hype that promises to increase efficiencies in thin film panels. His research, published recently in the journalsNano Lettersand Chemical Physics Letters, may bring the development of new solar energy technologies more down to earth.

Prof. Rabani combines a new theoretical approach withcomputer simulations."Our theory shows that current predictions to increase efficiencies won't work. The increase in efficiencies cannot be achieved yet through Multiexciton Generation, a process by which several charge carriers (electrons and holes) are generated from one photon,"he says.

Inefficient as"charged"

But both new and existing theories bode well for the development of other strategies in future solar energy technology, he points out. Newer approaches published in journals such as Science may provide means for increasing the efficiencies ofsolar technology, and perhaps would also be useful in storage of solar energy, Prof. Rabani and his team of researchers believe.

A chemical physicist, Prof. Rabani investigates how to separate charges from the sun efficiently. In 2004, physicists suggested that more than one electron-hole pair could be pulled from one photon in a complicated process insemiconductor nanocrystals. If this were possible, the charge would be doubled, and so the solar energy efficiency would increase."We've shown that this idea doesn't work,"Prof. Rabani says.

One step closer to marketing the sun

The development of more efficient and less expensive devices to make use ofsolar energyis one of the greatest challenges in science today. Billions of dollars are being spent to find the best methods to collect electron"charges"from the sun.

Typically, one photon from the sun absorbed in a thin film solar panel can excite one electron-hole pair, which is then converted to electricity. Currently there are claims that if more electron-hole pairs can be excited after the photon is absorbed, a larger fraction of the photon energy can successfully be converted into electricity, thus increasing device efficiency.

The theory that Prof. Rabani developed with his Israeli colleagues shows why this process is not as efficient as originally conceived. It's bad news for panel producers looking to create more efficientsolar panels, but good news for researchers who are now free to look to the next realistic step for developing a technology that works.

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Graphene grains make atom-thick patchwork 'quilts'

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(PhysOrg.com) -- A quick look at new Cornell research hints at colorful patchwork quilts, but they are actually pictures of graphene -- one atom-thick sheets of carbon stitched together at tilted interfaces. Researchers have unveiled striking, atomic-resolution details of what graphene"quilts"look like at the boundaries between patches, and have uncovered key insights into graphene's electrical and mechanical properties.

The multidisciplinary Cornell collaboration, publishing online Jan. 5 in the journalNature, focuses on graphene -- a one atom-thick sheet of carbon atoms bonded in a crystal lattice like a honeycomb or chicken wire -- because of its electrical properties and potential to improve anything from solar cells to cell phone screens. But it doesn't grow in perfect sheets; rather, it develops in pieces that resemble patchwork quilts, where the honeycomb lattice meets up imperfectly and creates five- or seven-member carbon rings, rather than the perfect six. Where these"patches"meet are called grain boundaries, and scientists had wondered whether these boundaries would allow the special properties of a perfect graphene crystal to transfer to the much larger quilt-like structures.

To study the material, the researchers grew graphene membranes on a copper substrate (a method devised by another group) but then conceived a novel way to peel them off as free-standing, atom-thick films. Then, with diffraction imaging electron microscopy, they imaged the graphene by seeing how electrons bounced off at certain angles, and using a color to represent that angle. By overlaying different colors according to how the electrons bounced, they created an easy, efficient method of imaging the graphene grain boundaries according to their orientation. And as a bonus, their pictures took an artistic turn, reminding the scientists of patchwork quilts.

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Another graphene sheet with different lattice orientations.

"You don't want to look at the whole quilt by counting each thread,"said David Muller, professor of applied and engineering physics and co-director of the Kavli Institute at Cornell for Nanoscale Science, who conducted the work with Paul McEuen, professor of physics and director of the Kavli Institute; and Kavli member Jiwoong Park, assistant professor of chemistry and chemical biology."You want to stand back and see what it looks like on the bed. And so we developed a method that filters out the crystal information in a way that you don't have to count every atom."

This new method could apply to other two-dimensional materials and sheds new light on the previously mysterious way that graphene was stitched together at grain boundaries.

Further analysis revealed that growing larger grains (bigger patches) didn't improve the electrical conductivity of the graphene, as was previously thought by materials scientists. Rather, it is impurities that sneak into the sheets that make the electrical properties fluctuate. This insight will lead scientists closer to the best ways to grow and use graphene.

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