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Super-resolution microscope builds 3D images by mapping negative space

Scientists at The University of Texas at Austin have demonstrated a methaod for making three-dimensional images of structures in biological material under natural conditions at a much higher resolution than other existing methods. The method may help shed light on how cells communicate with one another and provide important insights for engineers working to develop artificial organs such as skin or heart tissue.


The scientists, led by physicist Ernst-Ludwig Florin, used their method, called thermal noise imaging, to capture nanometer-scale images of networks of collagen fibrils, which form part of the connective tissue found in the skin of animals. A nanometer is a billionth of a meter or about one-hundred-thousandth of the width of a human hair. Examining collagen fibrils at this scale allowed the scientists to measure for the first time key properties that affect skin’s elasticity, something that could lead to improved designs for artificial skin or tissues.

Taking crisp 3-D images of nanoscale structures in biological samples is extremely difficult, in part because they tend to be soft and bathed in liquid. This means that tiny fluctuations in heat cause structures to move back and forth, an effect known as Brownian motion.


To overcome the blurriness that this creates, other super-resolution imaging techniques often "fix" biological samples by adding chemicals that stiffen various structures, in which case, materials lose their natural mechanical properties. Scientists can sometimes overcome blurriness without fixing the samples if, for example, they focus on rigid structures stuck to a glass surface, but that severely limits the kinds of structures and configurations they can study.

Florin and his team took a different approach. To make an image, they add nanospheres – nanometer-sized beads that reflect laser light – to their biological samples under natural conditions, shine a laser on the sample and compile superfast snapshots of the nanospheres viewed through a light microscope.


The scientists explain that the method, thermal noise imaging, works something like this analogy: Imagine you needed to take a three-dimensional image of a room in total darkness. If you were to throw a glowing rubber ball into the room and use a camera to collect a series of high-speed images of the ball as it bounces around, you would see that as the ball moves around the room, it isn’t able to move through solid objects such as tables and chairs. Combining millions of images taken so fast that they don't blur, you would be able to build a picture of where there are objects (wherever the ball couldn’t go) and where there aren’t objects (where it could go).

In thermal noise imaging, the equivalent of the rubber ball is a nanosphere that moves around in a sample by natural Brownian motion – the same unruly force that has bedeviled other microscopy methods.

The original concept for the thermal noise imaging technique was published and patented in 2001, but technical challenges prevented it from being developed into a fully functioning process until now.

The tool allowed the researchers to measure for the first time the mechanical properties of collagen fibrils in a network. Collagen is a biopolymer that forms scaffolds for cells in the skin and contributes to the skin’s elasticity. Scientists are still not sure how a collagen network’s architecture results in its elasticity, an important question that must be answered for the rational design of artificial skin.

The paper's first author is Tobias Bartsch, a former graduate student at UT Austin and currently a postdoctoral associate at The Rockefeller University. Other co-authors are Martin Kochanczyk, Emanuel Lissek and Janina Lange.





3-D graphene has promise

for bio applications

Flakes of graphene welded together into solid materials may be suitable for bone implants, according to a study led by Rice University scientists.

The Rice lab of materials scientist Pulickel Ajayan and colleagues in Texas, Brazil and India used spark plasma sintering to weld flakes of graphene oxide into porous solids that compare favorably with the mechanical properties and biocompatibility of titanium, a standard bone-replacement material.

The researchers believe their technique will give them the ability to create highly complex shapes out of graphene in minutes using graphite molds, which they believe would be easier to process than specialty metals.

“We started thinking about this for bone implants because graphene is one of the most intriguing materials with many possibilities and it’s generally biocompatible,” said Rice postdoctoral research associate Chandra Sekhar Tiwary, co-lead author of the paper with Dibyendu Chakravarty of the International Advanced Research Center for Powder Metallurgy and New Materials in Hyderabad, India. “Four things are important: its mechanical properties, density, porosity and biocompatibility.”

Tiwary said spark plasma sintering is being used in industry to make complex parts, generally with ceramics. “The technique uses a high pulse current that welds the flakes together instantly. You only need high voltage, not high pressure or temperatures,” he said. The material they made is nearly 50 percent porous, with a density half that of graphite and a quarter of titanium metal. But it has enough compressive strength — 40 megapascals — to qualify it for bone implants, he said. The strength of the bonds between sheets keeps it from disintegrating in water.

The researchers controlled the density of the material by altering the voltage that delivers the highly localized blast of heat that makes the nanoscale welds. Though the experiments were carried out at room temperature, the researchers made graphene solids of various density by raising these sintering temperatures from 200 to 400 degrees Celsius. Samples made at local temperatures of 300 C proved best, Tiwary said. “The nice thing about two-dimensional materials is that they give you a lot of surface area to connect. With graphene, you just need to overcome a small activation barrier to make very strong welds,” he said.

The researchers measured the load-bearing capacity of thin sheets of two- to five-layer bonded graphene by repeatedly stressing them with a picoindenter attached to a scanning electron microscope and found they were stable up to 70 micronewtons. Colleagues successfully cultured cells on the material to show its biocompatibility.

As a bonus, the researchers also discovered the sintering process has the ability to reduce graphene oxide flakes to pure bilayer graphene, which makes them stronger and more stable than graphene monolayers or graphene oxide.

“This example demonstrates the possible use of unconventional materials in conventional technologies,” Ajayan said. “But these transitions can only be made if materials such as 2-D graphene layers can be scalably made into 3-D solids with appropriate density and strength”.

“Engineering junctions and strong interfaces between nanoscale building blocks is the biggest challenge in achieving such goals, but in this case, spark plasma sintering seems to be effective in joining graphene sheets to produce strong 3-D solids,” he said.

The research was supported by the Department of Defense, the U.S. Air Force Office of Scientific Research and its Multidisciplinary University Research Initiative, the Sao Paulo Research Foundation, the Center for Computational Engineering and Sciences at Unicamp, Brazil, and the Government of India Department of Science and Technology.



Scientists Trick

Solid Into Acting as Liquid

Scientists at the University of Central Florida have discovered how to get a solid material to act like a liquid without actually turning it into liquid, potentially opening a new world of possibilities for the electronic, optics and computing industries.


When chemistry graduate student Demetrius A. Vazquez-Molina took COF-5, a nano sponge-like, non-flammable manmade material and pressed it into pellets the size of a pinkie nail, he noticed something odd when he looked at its X-ray diffraction pattern. The material’s internal crystal structure arranged in a strange pattern. He took the lab results to his chemistry professor Fernando Uribe-Romo, who suggested he turn the pellets on their side and run the X-ray analysis again.


The result: The crystal structures within the material fell into precise patterns that allow for lithium ions to flow easily – like in a liquid.


The findings, published in the Journal of the American Chemical Society earlier this summer, are significant because a liquid is necessary for some electronics and other energy uses. But using current liquid materials sometimes is problematic.


For example, take lithium-ion batteries. They are among the best batteries on the market, charging everything from phones to hover boards. But they tend to be big and bulky because a liquid must be used within the battery to transfer lithium ions from one side of the battery to the other. This process stores and disperses energy. That reaction creates heat, which has resulted in cell phones exploding, hover boards bursting into flames, and even the grounding of some airplanes a few years ago that relied on lithium batteries for some of its functions.


But if a nontoxic solid could be used instead of a flammable liquid, industries could really change, Uribe-Romo said.


“We need to do a lot more testing, but this has a lot of promise,” he said. “If we could eliminate the need for liquid and use another material that was not flammable, would require less space and less packaging, that could really change things. That would mean less weight and potentially smaller batteries.”


Smaller, nontoxic and nonflammable materials could also mean smaller electronics and the ability to speed up the transfer of information via optics. And that could mean innovations to communication devices, computing power and even energy storage.

“This is really exciting for me,” said Vazquez-Molina who was a pre-med student before taking one of Uribe-Romo’s classes. “I liked chemistry, but until Professor Romo’s class I was getting bored. In his class I learned how to break all the (chemistry) rules. I really fell in love with chemistry then, because it is so intellectually stimulating.”


Uribe-Romo has his high school teacher in Mexico to thank for his passion for chemistry. After finishing his bachelor’s degree at Instituto Tecnológico y de Estudios Superiores de Monterrey in Mexico, Uribe-Romo earned a Ph.D. at the University of California at Los Angeles. He was a postdoctoral associate at Cornell University before joining UCF as an assistant professor in 2013.





Low-cost and

defect-free graphene

Graphene is one of the most promising new materials. However, researchers across the globe are still looking for a way to produce defect-free graphene at low costs. Chemists at FAU have now succeeded in producing defect-free graphene directly from graphite for the first time.


Graphene is two dimensional and consists of a single layer of carbon atoms. It is particularly good at conducting electricity and heat, transparent and flexible yet strong. Graphene’s unique properties make it suitable for use in a wide range of pioneering technologies, such as in transparent electrodes for flexible displays.


However, the semi-conductor industry will only be able to use graphene successfully once properties such as the size, area and number of defects – which influence its conductivity – can be improved during synthesis. A team of FAU researchers led by Dr. Andreas Hirsch from the Chair of Organic Chemistry II has recently made a crucial break-through in this area. With the help of the additive benzonitrile, they have found a way of producing defect-free graphene directly from a solution. Their method enables the graphene – which is of a higher quality than ever achieved before – to be cut without causing defects and also allows specific electronic properties to be set through the number of charge carriers. Furthermore, their technique is both low-cost and efficient.


A common way of synthesising graphene is through chemical exfoliation of graphite. In this process, metal ions are embedded in graphite, which is made of carbon, resulting in what is known as an intercalation compound. The individual layers of carbon – the graphene – are separated using solvents. The stabilised graphene then has to be separated from the solvent and reoxidised. However, defects in the individual layers of carbon, such as hydration and oxidation of carbon atoms in the lattice, can occur during this process. FAU researchers have now found a solution to this problem. By adding the solvent benzonitrile, the graphene can be removed without any additional functional groups forming – and it remains defect-free.


The method devised by FAU researchers has another advantage: the reduced benzonitrile molecule formed during the reaction turns red as long as it does not come into contact with oxygen or water. This change in colour allows the number of charge carriers in the system to be determined easily through absorption measurements. This could previously only be done by measuring voltage and means that graphene and battery researchers now have a new way of measuring the charge state.



DOI: 10.1038/ncomms12411





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