Nanoscale Rope: Complex Nanomaterials That Assemble Themselves

Berkeley Lab scientists have developed a nanoscale rope that braids itself, as seen in this atomic force microscopy image of the structure at a resolution of one-millionth of a meter.

Their work is the latest development in the push to develop self-assembling nanoscale materials that mimic the intricacy and functionality of nature's handiwork, but which are rugged enough to withstand harsh conditions such as heat and dryness.

Although still early in the development stage, their research could lead to new applications that combine the best of both worlds. Perhaps they'll be used as scaffolds to guide the construction of nanoscale wires and other structures. Or perhaps they'll be used to develop drug-delivery vehicles that target disease at the molecular scale, or to develop molecular sensors and sieve-like devices that separate molecules from one another.

Specifically, the scientists created the conditions for synthetic polymers called polypeptoids to assemble themselves into ever more complicated structures: first into sheets, then into stacks of sheets, which in turn roll up into double helices that resemble a rope measuring only 600 nanometers in diameter (a nanometer is a billionth of a meter).

"This hierarchichal self assembly is the hallmark of biological materials such as collagen, but designing synthetic structures that do this has been a major challenge," says Ron Zuckermann, who is the Facility Director of the Biological Nanostructures Facility in Berkeley Lab's Molecular Foundry.

In addition, unlike normal polymers, the scientists can control the atom-by-atom makeup of the ropy structures. They can also engineer helices of specific lengths and sequences. This"tunability" opens the door for the development of synthetic structures that mimic biological materials' ability to carry out incredible feats of precision, such as homing in on specific molecules.

"Nature uses exact length and sequence to develop highly functional structures. An antibody can recognize one form of a protein over another, and we're trying to mimic this," adds Zuckermann.

Zuckermann and colleagues conducted the research at The Molecular Foundry, which is one of the five DOE Nanoscale Science Research Centers premier national user facilities for interdisciplinary research at the nanoscale. Joining him were fellow Berkeley Lab scientists Hannah Murnen, Adrianne Rosales, Jonathan Jaworski, and Rachel Segalman. Their research was published in a recent issue of theJournal of the American Chemical Society.

The scientists worked with chains of bioinspired polymers called a peptoids. Peptoids are structures that mimic peptides, which nature uses to form proteins, the workhorses of biology. Instead of using peptides to build proteins, however, the scientists are striving to use peptoids to build synthetic structures that behave like proteins.

The team started with a block copolymer, which is a polymer composed of two or more different monomers.

"Simple block copolymers self assemble into nanoscale structures, but we wanted to see how the detailed sequence and functionality of bioinspired units could be used to make more complicated structures," says Rachel Segalman, a faculty scientist at Berkeley Lab and professor of Chemical and Biomolecular Engineering at University of California, Berkeley.

With this in mind, the peptoid pieces were robotically synthesized, processed, and then added to a solution that fosters self assembly.

The result was a variety of self-made shapes and structures, with the braided helices being the most intriguing. The hierarchical structure of the helix, and its ability to be manipulated atom-by-atom, means that it could be used as a template for mineralizing complex structures on a nanometer scale.

"The idea is to assemble structurally complex structures at the nanometer scale with minimal input," says Hannah Murnen. She adds that the scientists next hope is to capitalize on the fact that they have minute control over the structure's sequence, and explore how very small chemical changes alter the helical structure.

Says Zuckermann,"These braided helices are one of the first forays into making atomically defined block copolymers. The idea is to take something we normally think of as plastic, and enable it to adopt structures that are more complex and capable of higher function, such as molecular recognition, which is what proteins do really well."

X-ray diffraction experiments used to characterize the structures were conducted at beamlines 8.3.1 and 7.3.3 of Berkeley Lab's Advanced Light Source, a national user facility that generates intense x-rays to probe the fundamental properties of substances. This work was supported in part by the Office of Naval Research.

Source

Self-Assembling Structures Open Door to New Class of Materials

The helical"supermolecules" are made of tiny colloid balls instead of atoms or molecules. Similar methods could be used to make new materials with the functionality of complex colloidal molecules. The team publishes its findings in the Jan. 14 issue of the journalScience.

"We can now make a whole new class of smart materials, which opens the door to new functionality that we couldn't imagine before," said Steve Granick, Founder Professor of Engineering at the University of Illinois and a professor of materials science and engineering, chemistry, and physics.

Granick's team developed tiny latex spheres, dubbed"Janus spheres," which attract each other in water on one side, but repel each other on the other side. The dual nature is what gives the spheres their ability to form unusual structures, in a similar way to atoms and molecules.

In pure water, the particles disperse completely because their charged sides repel one another. However, when salt is added to the solution, the salt ions soften the repulsion so the spheres can approach sufficiently closely for their hydrophobic ends to attract. The attraction between those ends draws the spheres together into clusters.

At low salt concentrations, small clusters of only a few particles form. At higher levels, larger clusters form, eventually self-assembling into chains with an intricate helical structure.

"Just like atoms growing into molecules, these particles can grow into supracolloids," Granick said."Such pathways would be very conventional if we were talking about atoms and molecules reacting with each other chemically, but people haven't realized that particles can behave in this way also."

The team designed spheres with just the right amount of attraction between their hydrophobic halves so that they would stick to one another but still be dynamic enough to allow for motion, rearrangement, and cluster growth.

"The amount of stickiness really does matter a lot. You can end up with something that's disordered, just small clusters, or if the spheres are too sticky, you end up with a globular mess instead of these beautiful structures," said graduate student Jonathan Whitmer, a co-author of the paper.

One of the advantages of the team's supermolecules is that they are large enough to observe in real time using a microscope. The researchers were able to watch the Janus spheres come together and the clusters grow -- whether one sphere at a time or by merging with other small clusters -- and rearrange into different structural configurations the team calls isomers.

"We design these smart materials to fall into useful shapes that nature wouldn't choose," Granick said.

Surprisingly, theoretical calculations and computer simulations by Erik Luijten, Northwestern University professor of materials science and engineering and of engineering sciences and applied mathematics, and Whitmer, a student in his group, showed that the most common helical structures are not the most energetically favorable. Rather, the spheres come together in a way that is the most kinetically favorable -- that is, the first good fit that they encounter.

Next, the researchers hope to continue to explore the colloid properties with a view toward engineering more unnatural structures. Janus particles of differing sizes or shapes could open the door to building other supermolecules and to greater control over their formation.

"These particular particles have preferred structures, but now that we realize the general mechanism, we can apply it to other systems -- smaller particles, different interactions -- and try to engineer clusters that switch in shape," Granick said.

The team also included University of Illinois graduate students Qian Chen and Shan Jiang and research scientist Sung Chul Bae. The U.S. Department of Energy and the National Science Foundation supported this work.

Source

Chemists Develop Fully Biodegradable and Recyclable Synthetic Resin

Most plastic products for domestic or construction use consist of three-dimensional networks of cross-linked polymers. These are thermosetting plastics. A classic example is the Bakelite resin produced from the reaction of phenol with formaldehyde. This material is still used to bind wood fibers in pressed wood such as medium density fiberboard (MDF) and formica. Synthetic resins are widely used in the construction industry. The resin of urea / formaldehyde is used in Medium Density Overlay (MDO), a combination of concrete and plywood, used in concrete molds.

Completely biodegradable bioplastics

By selecting the right raw materials and process conditions for the cross-linking reaction the scientists, who work for the UvA'sHeterogeneous Catalysis and Sustainable Chemistryresearch group, were able to make a range of bio-plastics ranging from hard foam material to flexible thin sheet materials. These are non-toxic and biodegradable. The process requires no toxic ingredients and no harmful substances are released from combustion. Moreover, the raw materials are readily available at competitive prices on the world market.

The new plastic could replace polyurethane and polystyrene in the construction and packaging industries. This also applies to the epoxy resins used for panels such as MDF. The follow-up research will focus on new applications and process development and upscaling.

Source

No Left Turn: 'Superstreet' Traffic Design Improves Travel Time, Safety

Superstreets are surface roads, not freeways. It is defined as a thoroughfare where the left-hand turns from side streets are re-routed, as is traffic from side streets that needs to cross the thoroughfare. In both instances, drivers are first required to make a right turn and then make a U-turn around a broad median. While this may seem time-consuming, the study shows that it actually results in a significant time savings since drivers are not stuck waiting to make left-hand turns or for traffic from cross-streets to go across the thoroughfare.

"The study shows a 20 percent overall reduction in travel time compared to similar intersections that use conventional traffic designs," says Dr. Joe Hummer, professor of civil, construction and environmental engineering at NC State and one of the researchers who conducted the study."We also found that superstreet intersections experience an average of 46 percent fewer reported automobile collisions -- and 63 percent fewer collisions that result in personal injury."

The researchers assessed travel time at superstreet intersections as the amount of time it takes a vehicle to pass through an intersection from the moment it reaches the intersection -- whether traveling left, right or straight ahead. The travel-time data were collected from three superstreets located in eastern and central North Carolina, all of which have traffic signals. The superstreet collision data were collected from 13 superstreets located across North Carolina, none of which have traffic signals.

The superstreet concept has been around for over 20 years, but little research had been done to assess its effectiveness under real-world conditions. The NC State study is the largest analysis ever performed of the impact of superstreets in real traffic conditions.

A paper on the travel time research is being presented Jan. 24 at the Transportation Research Board Annual Meeting in Washington, D.C. The paper is co-authored by Hummer, former NC State graduate students Rebecca Haley and Sarah Ott, and three researchers from NC State's Institute for Transportation Research and Education: Robert Foyle, associate director; Christopher Cunningham, senior research associate; and Bastian Schroeder, research associate.

The collision research was part of an overarching report of the study submitted to the North Carolina Department of Transportation (NCDOT) last month, and is the subject of a forthcoming paper. The study was funded by NCDOT.

NC State's Department of Civil, Construction and Environmental Engineering is part of the university's College of Engineering.

Source

Coiled Nanowires May Hold Key to Stretchable Electronics

"In order to create stretchable electronics, you need to put electronics on a stretchable substrate, but electronic materials themselves tend to be rigid and fragile," says Dr. Yong Zhu, one of the researchers who created the new nanowire coils and an assistant professor of mechanical and aerospace engineering at NC State."Our idea was to create electronic materials that can be tailored into coils to improve their stretchability without harming the electric functionality of the materials."

Other researchers have experimented with"buckling" electronic materials into wavy shapes, which can stretch much like the bellows of an accordion. However, Zhu says, the maximum strains for wavy structures occur at localized positions -- the peaks and valleys -- on the waves. As soon as the failure strain is reached at one of the localized positions, the entire structure fails.

"An ideal shape to accommodate large deformation would lead to a uniform strain distribution along the entire length of the structure -- a coil spring is one such ideal shape," Zhu says."As a result, the wavy materials cannot come close to the coils' degree of stretchability." Zhu notes that the coil shape is energetically favorable only for one-dimensional structures, such as wires.

Zhu's team put a rubber substrate under strain and used very specific levels of ultraviolet radiation and ozone to change its mechanical properties, and then placed silicon nanowires on top of the substrate. The nanowires formed coils upon release of the strain. Other researchers have been able to create coils using freestanding nanowires, but have so far been unable to directly integrate those coils on a stretchable substrate.

While the new coils' mechanical properties allow them to be stretched an additional 104 percent beyond their original length, their electric performance cannot hold reliably to such a large range, possibly due to factors like contact resistance change or electrode failure, Zhu says."We are working to improve the reliability of the electrical performance when the coils are stretched to the limit of their mechanical stretchability, which is likely well beyond 100 percent, according to our analysis."

A paper describing the research was published online Dec. 28 byACS Nano. The paper is co-authored by Zhu, NC State Ph.D. student Feng Xu and Wei Lu, an assistant professor at the University of Michigan. The research was funded by the National Science Foundation.

NC State's Department of Mechanical and Aerospace Engineering is part of the university's College of Engineering.

Source

Recycled Haitian Concrete Can Be Safe, Strong and Less Expensive, Researchers Say

In a paper published in theBulletin of the American Ceramic Society, researchers Reginald R. DesRoches, Kimberly E. Kurtis and Joshua J. Gresham say that they have made new concrete, from recycled rubble and other indigenous raw materials using simple techniques, which meets or exceeds the minimum strength standards used in the United States.

Most of the damaged areas of Haiti are still in ruins. The trio says their work points to a successful and sustainable strategy for managing an unprecedented amount of waste, estimated to be 20 million cubic yards.

"The commodious piles of concrete rubble and construction debris form huge impediments to reconstruction and are often contaminated," says DesRoches, professor and Associated Chair of Civil and Environmental Engineering at Georgia Tech."There are political and economic dilemmas as well, but we have found we can turn one of the dilemmas -- the rubble -- into a solution via some fairly simple methods of recycling the rubble and debris into new concrete."

DesRoches, who was born in Haiti, traveled several times in 2010 to Port-au-Prince to gather samples of typical concrete rubble and additionally collect samples of two readily available sand types used as fine aggregates in some concrete preparation.

He and Gresham also studied the methods, tools and raw materials used by local laborers to make concrete mixes. DesRoches recalls they encountered no mixing trucks."Instead, all of the construction crews were manually batching smaller amounts of concrete. Unfortunately, they were mixing volumes of materials 'by eye,' an unreliable practice that probably caused much of the poor construction and building failure during the earthquake," he says.

Before leaving, DesRoches and Gresham manually cast an initial set of standard 3-inch by 6-inch concrete test blocks using mixes from several different construction sites.

They returned to Georgia Tech with their cast blocks, sand samples and notes, where they were joined by Kurtis, also a professor and Chair of the American Concrete Institute's Materials Science of Concrete Committee.

They quickly discovered that the concrete test samples cast in Haiti were of poor quality."The Haitian-made concrete had an average compressive strength of 1,300 pounds per square inch," says Kurtis."In comparison, concrete produced in the U.S. would be expected to have a minimum strength of 3,000 pounds per square inch.

They then manually crushed the samples with a hammer to provide course aggregate for a second round of tests. In this round, they made concrete samples from mixes that combined the course aggregate with one of the two types of sands they had collected. However, instead of"eye-balling" the amounts of materials, in this round of tests they carefully measured volumes using methods prescribed by the American Concrete Institute. The materials were still mixed by hand to replicate the conditions in Haiti.

Subsequent tests of samples made from each type of sand provided good news: The compressive strength of both of the types of new test blocks, still composed of Haitian materials, dramatically increased, showing an average over 3,000 pounds per square inch.

"Based upon these results, we now believe that Haitian concrete debris, even of inferior quality, can be effectively used as recycled course aggregate in new construction," says Kurtis."It can work effectively, even if mixed by hand. The key is having a consistent mix of materials that can be easily measured. We are confident are results can be scaled up mix procedure where quantities can be measured using common, inexpensive construction equipment."

DesRoches is pleased because recycling eliminates two hurdles to reconstruction."First, removing the remaining debris is nearly impossible because there are few, if any, safe landfill sites near Port-au-Prince, and the nation lacks the trucks and infrastructure to haul it away. It is better to use it than to move it."Second," DesRoches says,"Finding fresh aggregate is more difficult than getting rid of the debris. It is costly to find, mine and truck in."

The trio notes recycled concrete aggregate has been used worldwide for roadbeds, drainage, etc., and that many European Union countries commonly use 20 percent recycled aggregates in structural concrete. Published research by others has also demonstrated that the use of local-sourced recycled aggregate concrete production can be more sustainable.

Because of the urgency of quick and safe reconstruction, the researchers urge that recycling the debris quickly move from proof-of-concept to large scale testing."More work must be done to characterize the recycled materials, test additional performance parameters and gauge the safest ways to crush the rubble. Seismic behavior and building codes must be studied. But, these tests can and should be done dynamically, during reconstruction, because the benefits can be so immediate and significant," says DesRoches.

DesRoches, Kurtis and Gresham say they plan on sharing their research with Haitian government officials and nongovernmental organizations working on reconstruction projects. DesRoches is hopeful that a debris strategy and infrastructure will eventually emerge from the government once the disputed presidential elections in Haiti are resolved."Some think that many rebuilding projects have on hold for the past few months because of distraction from the elections. The next round of elections is this month, so it soon may be possible to accelerate reconstruction."

Source