C60 could form a new kind of gel

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(PhysOrg.com) -- C₆₀, the spherical carbon molecule also known as a buckminsterfullerene, has intrigued scientists for its unique properties and potential applications in nanotechnology and electronics. Now scientists have found that C₆₀may have another unusual property: it may take the form of a one-component gel under certain conditions. To date, all known gels consist of two components: an evenly distributed substance (a colloid) and a substance that dissolves the colloid (a solvent).

Scientists have previously discovered that C₆₀can take the form of different phases of matter, including solids and liquids. Here, chemists Patrick Royall from the University of Bristol and Stephen Williams from the Australian National University found that C₆₀can theoretically exist in a dense liquid phase containing clusters, which bind together to form a gel structure, specifically a"spinodal"gel. The gel is made entirely of carbon.

In their study, the scientists performedcomputer simulationsshowing that C₆₀can form a gel at moderately high temperatures and very high quench rates. The simulations showed that C₆₀gels form in about 10 nanoseconds and are stable atroom temperaturefor at least 100 nanoseconds, which is the maximum time that the simulations were run. Although the gel showed some coarsening, the scientists predict that it would remain stable for more than 100 nanoseconds. Eventually, however, the gel would separate into a crystal and a gas.

As far as experimentally demonstrating the C₆₀gel, the scientists predict that it will be a challenge, largely due to the extremely high quench rates required, which are not currently experimentally feasible. However, they may investigate creative ways to lower the quench rate, or try to use larger fullerenes such as C₅₄₀, which may also become a carbon gel. In any case, the potential existence of a one-componentgelcould lead to an overall better understanding of the nature of gels.

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Introducing youths to big ideas about a nano-sized world through video games

(PhysOrg.com) -- Working on a science fair project with his lab partner Nikki, Harold Biggums finds himself transformed into a tiny superhero and flung into the midst of an alien plot to take over the world— a plot that he and Nikki can foil only by defying gravity, walking on water and charging across electric fields.

This narrative dilemma is the basic storyline forGeckoman!, an online video gamedeveloped by Northeastern University researchers at the Center for High-rate Nanomanufacturing (CHN), which seeks to educate middle-school students about nanoscience and technology.

CHN director Ahmed Busnaina and associate director Jacqueline Isaacs led an interdisciplinary team of educators and game designers to develop the game, which is available in English and Spanish.

“Geckoman! is both engaging and challenging, and along the way, students pick up a lot of nanoscience fundamentals,” said Busnaina, the William Lincoln Smith Professor of Mechanical and Industrial Engineering at Northeastern.

“We had excellent teachers working with us to develop four lesson plans that guide student learning,” said Isaacs, a professor in the Department of Mechanical and Industrial Engineering. “The results of student play tests indicate that students are learning new concepts.”

Game players follow Harold on an adventurous journey, after he has been shrunk to the nanoscale following an explosion in his laboratory. Players must navigate Harold through various levels across three different worlds, while also collecting scattered notebook pages that provide nanoscience tips to help him progress.

The game was created with funding help from the National Science Foundation; and 15 Days LLC, a company founded by Northeastern alumni and faculty, collaborated with CHN faculty on design. Staff members at Boston’s Museum of Science helped match the game content to national and Massachusetts K-12 science standards.

How did the game get its name? Early in the game development process, the team worked on incorporating a key concept in nanoscience— the“van der Waals” adhesion force, which dominates other forces at the nanoscale. In fact, it is this force that enables geckos to run up walls; the pads of their feet have millions of nanoscale extensions. The game developers decided that Harold would have to become Geckoman, enabling him to move with greater ease between all the unusual surfaces he must navigate in addressing the game’s multi-level challenges. 

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Researchers develop new technology for cheaper, more efficient solar cells

The sun provides more than enough energy for all our needs, if only we could harness it cheaply and efficiently. Solar energy could provide a clean alternative to fossil fuels, but the high cost of solar cells has been a major barrier to their widespread use.

Stanford researchers have found that adding a single layer oforganic moleculesto a solar cell can increase its efficiency three-fold and could lead to cheaper, more efficient solar panels. Their results were published online inACS Nanoon Feb. 7.

Professor of chemical engineering Stacey Bent first became interested in a new kind ofsolar technologytwo years ago. These solar cells used tiny particles of semiconductors called"quantum dots."Quantum dot solar cells are cheaper to produce than traditional ones, as they can be made using simple chemical reactions. But despite their promise, they lagged well behind existing solar cells in efficiency.

"I wondered if we could use our knowledge of chemistry to improve their efficiency,"
Bent said. If she could do that, the reduced cost of these solar cells could lead to mass adoption of the technology.

Bent will discuss her research on Sunday, Feb. 20, at the annual meeting of the American Association for the Advancement of Science in Washington, D.C.

In principle, quantum dot cells can reach much higher efficiency, Bent said, because of a fundamental limitation of traditional solar cells.

Solar cells work by using energy from the sun to excite electrons. The excited electrons jump from a lower energy level to a higher one, leaving behind a"hole"where the electron used to be. Solar cells use a semiconductor to pull an electron in one direction, and another material to pull the hole in the other direction. This flow of electron and hole in different directions leads to an electric current.

But it takes a certain minimum energy to fully separate the electron and the hole. The amount of energy required is specific to different materials and affects what color, or wavelength, of light the material best absorbs. Silicon is commonly used to make solar cells because the energy required to excite its electrons corresponds closely to the wavelength of visible light.

But solar cells made of a single material have a maximum efficiency of about 31 percent, a limitation of the fixed energy level they can absorb.

Quantum dot solar cells do not share this limitation and can in theory be far more efficient. The energy levels of electrons in quantum dot semiconductors depends on their size– the smaller the quantum dot, the larger the energy needed to excite electrons to the next level.

So quantum dots can be tuned to absorb a certain wavelength of light just by changing their size. And they can be used to build more complex solar cells that have more than one size of quantum dot, allowing them to absorb multiple wavelengths of light.

Because of these advantages, Bent and her students have been investigating ways to improve the efficiency of quantum dot solar cells, along with associate Professor Michael McGehee of the department of Materials Science and Engineering.

The researchers coated a titanium dioxide semiconductor in their quantum dot solar cell with a very thin single layer of organic molecules. These molecules were self-assembling, meaning that their interactions caused them to pack together in an ordered way. The quantum dots were present at the interface of this organic layer and the semiconductor. Bent's students tried several different organic molecules in an attempt to learn which ones would most increase the efficiency of the solar cells.

But she found that the exact molecule didn't matter– just having a single organic layer less than a nanometer thick was enough to triple the efficiency of the solar cells."We were surprised, we thought it would be very sensitive to what we put down,"said Bent.

But she said the result made sense in hindsight, and the researchers came up with a new model– it's the length of the molecule, and not its exact nature, that matters. Molecules that are too long don't allow the quantum dots to interact well with the semiconductor.

Bent's theory is that once the sun's energy creates an electron and a hole, the thin organic layer helps keep them apart, preventing them from recombining and being wasted. The group has yet to optimize the solar cells, and they have currently achieved an efficiency of, at most, 0.4 percent. But the group can tune several aspects of the cell, and once they do, the three-fold increase caused by the organic layer would be even more significant.

Bent said the cadmium sulfide quantum dots she is currently using are not ideal for solar cells, and the group will try different materials. She said she would also try other molecules for the organic layer, and could change the design of the solar cell to try to absorb more light and produce more electrical charge. Once Bent has found a way to increase the efficiency of quantum dotsolar cells, she said she hopes their lower cost will lead to wider acceptance of solarenergy.

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A nano-Solution to global water problem: Nanomembranes could filter bacteria

(PhysOrg.com) -- New nanomaterials research from the University at Buffalo could lead to new solutions for an age-old public health problem: how to separate bacteria from drinking water.

To the naked eye, bothwater moleculesand germs are invisible -- objects so tiny they are measured by the nanometer, a unit of length about 100,000 times thinner than the width of a human hair.

But at the microscopic level, the two actually differ greatly in size. A single water molecule is less than a nanometer wide, while some of the most diminutive bacteria are a couple hundred.

Working with a special kind of polymer called a block copolymer, a UB research team has synthesized a new kind of nanomembrane containing pores about 55nanometersin diameter -- large enough for water to slip through easily, but too small for bacteria.

The pore size is the largest anyone has achieved to date using block copolymers, which possess special properties that ensure pores will be evenly spaced, said Javid Rzayev, the UB chemist who led the study. The findings were published online on Jan. 31 inNano Lettersand will appear in the journal's print edition later this year, with UB chemistry graduate student Justin Bolton as lead author.

"These materials present new opportunities for use as filtration membranes,"said Rzayev, an assistant professor of chemistry."Commercial membranes have limitations as far as pore density or uniformity of the pore size. The membranes prepared from block copolymers have a very dense distribution of pores, and the pores are uniform."

"There's a lot of research in this area, but what our research team was able to accomplish is to expand the range of available pores to 50 nanometers in diameter, which was previously unattainable by block-copolymer-based methods,"Rzayev continued."Making pores bigger increases the flow of water, which will translate into cost and time savings. At the same time, 50 to 100 nm diameter pores are small enough not to allow any bacteria through. So, that is a sweet spot for this kind of application."

The new nanomembrane owes its special qualities to the polymers that scientists used to create it. Block copolymers are made up of two polymers that repel one another but are"stitched"together at one end to form the single copolymer.

When many block copolymers are mixed together, their mutual repulsion leads them to assemble in a regular, alternating pattern. The result of that process, called self-assembly, is a solid nanomembrane comprising two different kinds of polymers.

To create evenly spacedporesin the material, Rzayev and colleagues simply removed one of the polymers. The pores' relatively large size was due to the unique architecture of the originalblock copolymers, which were made from bottle-brush molecules that resemble round hair brushes, with molecular"bristles"protruding all the way around a molecular backbone.

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Researchers discover new way to design metal nanoparticle catalysts

Tiny metal nanoparticles are used as catalysts in many reactions, from refining chemicals to producing polymers and biofuels. How well these nanoparticles perform as catalysts for these reactions depend on which of their crystal faces are exposed.

But previous attempts to design thesenanoparticlesby changing their shape have failed because the structures are unstable and will revert back to their equilibrium shape.

Now, researchers at Northwestern University's Institute for Catalysis in Energy Processing have discovered a new strategy for fabricatingmetal nanoparticlesin catalysts that promises to enhance the selectivity and yield for a wide range of structure-sensitive catalytic reactions. The team, led by Laurence D. Marks, professor of materials science and engineering at the McCormick School of Engineering and Applied Science, discovered that they could design nanoparticles by designing the particle's support structure.

"Instead of trying to engineer the nanoparticles, we've engineered the substrate that the nanoparticle sits on,"Marks said."That changes what faces are exposed."Their results were published in February in the journalNano Letters.

This solution was a bit of a discovery: the team created the nanoparticle samples, discovered that they didn't change their shape (as the laws of thermodynamics caused previously designed nanoparticles to do), then set out figuring how it worked. It turns out that epitaxy— the relationship between the position of the atoms in the nanoparticle and the position of the atoms on the substrate— was more important to design than previously thought.

The team is currently testing the nanoparticles in a catalytic reactor, and early results look promising, Marks says. The nanoparticles appear to be stable enough to survive the rigors of long-term use as catalysts.

"It opens the door to designing better catalysts,"Marks said."This method could be used with a variety of different metal nanoparticles. It's a new strategy, and it could have a very big impact."

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Metallic molecules to nanotubes: Spread out!

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(PhysOrg.com) -- 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 inorganicmetal complexeswith 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 theirelectrical properties. For starters, the ability to functionalize the nanotubes at the same time may advance imaging sensors, catalysis and solar-activatedhydrogen 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 journalChemical Communicationsthat 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 tometal 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 normalorganic 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.

Thatnanotubesstay 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."

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Simpler way of making proteins could lead to new nanomedicine agents

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Researchers have developed a simple method of making short protein chains with spiral structures that can also dissolve in water, two desirable traits not often found together. Such structures could have applications as building blocks for self-assembling nanostructures and as agents for drug and gene delivery.

Led by Jianjun Cheng, a professor of materials science and engineering at the University of Illinois, the research team will publish its findings in the Feb. 22 edition of the journalNature Communications.

Materials scientists have been interested in designing largepolymer moleculesthat could be used as building blocks for self-assembling structures. The challenge has been that the molecules generally adopt a globular, spherical shape, limiting their ability to form orderly aggregates. However, polypeptides– chains of amino acids such as proteins– can form helical structures. Short polypeptide chains that adopt a spiral shape act like cylindrical rods.

"If you have two rigid rods, one positive and one negative, right next to each other, they're going to stick to each other. If you have a way to put the charge on the surface then they can pack together in a close, compact way, so they form a three-dimensional structure,"Cheng said.

However, it is difficult to make helical polypeptides that are water-soluble so they can be used in solution. Polypeptides gain their solubility from side chains– molecular structures that stem from each amino acid link in the polypeptide chain. Amino acids with positive or negative charges in their side chains are needed to make a polypeptide disperse in water.

The problem arises when chains with charged side chains form helical structures. The charges cause a strong repulsion between the side chains, which destabilizes the helical conformation. This causes water-soluble polypeptides to form random coil structures instead of the desired helices.

In exploring solutions to the riddle of helical, water-soluble polypeptides, researchers have tried several complicated methods. For example, scientists have attempted grafting highly water-soluble chemicals to the side chains to increase the polypeptides' overall solubility, or creating helices with charges only on one side.

"You can achieve the helical structure and the solubility but you have to design the helical structure in a very special way. The peptide design needs a very specific sequence. Then you're very limited in the type of polypeptide you can build, and it's not easy to design or handle these polypeptides,"Cheng said.

In contrast, Cheng's group developed a very straightforward solution. Since the close proximity of the charges causes the repulsion that disrupts the helix, the researchers simply elongated the side chains, moving the charges farther from the backbone and giving them more freedom to keep their distance from one another.

The researchers observed that as they increased the length of the side chains with charges on the end, the polypeptides' propensity for forming helices also increased.

"It's such a simple idea– move the charge away from the backbone,"Cheng said."It's not difficult at all to make the longer side chains, and it has amazing properties for winding up helical structures simply by pushing the distance between the charge and the backbone."

The group found that not only do polypeptides with long side chains form helices, they display remarkable stability even when compared to non-charged helices. The helices seem immune to temperature, pH, and other denaturing agents that would unwind most polypeptides.

This may explain why amino acids with large hydrophobic side chains are not found in nature. Such immutability would preclude dynamic winding and unwinding of protein structures, which is essential to many biological processes. However, rigid stability is a desirable trait for the types of applications Cheng's group explores: nanostructures for drug andgene delivery, particularly targeting cancerous tumors and stem cells.

"We want to test the correlation of the lengths of the helices and the circulation in the body to see what's the impact of the shape and the charge and the side chains for clearance in the body,"Cheng said."Recent studies show that the aspect ratio of the nanostructures– spherical structures versus tubes– has a huge impact on their penetration of tumor tissues and circulation half-lives in the body."

Cheng plans to create a library of short helical polypeptides of varying backbone lengths, side chain lengths and types of charge. He hopes to simplify the chemistry even further and make the materials widely accessible. His lab already has demonstrated that helical structures can be effective gene delivery and membrane transduction agents, and building the library of soluble helical molecules will allow further investigation of tailoring suchnanostructuresfor specific biomedical applications.

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