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Graphene-based Sensor Brings New Wrinkle to Wearables

human os iconThe capabilities of wearable sensors seem to be expanding every day. However, for the most part these sensors have measured just physical attributes, like heart rate.

Now researchers at the University of Michigan have developed a graphene-based wearable sensor capable of detecting airborne chemicals that serve as indicators of medical conditions. For instance, the sensor could detect acetone, which is a biomarker for diabetes. Or it could detect abnormal levels of nitric oxide and oxygen, which would be an indicator of conditions such as high blood pressure, anemia, or lung disease.

"With our platform technology, we can measure a variety of chemicals at the same time, or modify the device to target specific chemicals. There are limitless possibilities," said Zhaohui Zhong, an associate professor at the University of Michigan, in a press release.

The researchers had to take a novel approach to how the nanosensor detect chemicals. In research, which was published in the journal Nature Communications, the Michigan researchers developed a sensing mechanism based on detecting molecular dipoles.

This sensing mechanism stands in contrast to most other nanosensors, which are based on detecting a change in charge density due to a molecule binding to the sensor.

"Nanoelectronic sensors typically depend on detecting charge transfer between the sensor and a molecule in air or in solution," said Girish Kulkarni, a doctoral candidate and one of the researchers, in a press release. "Instead of detecting molecular charge, we use a technique called heterodyne mixing, in which we look at the interaction between the dipoles associated with these molecules and the nanosensor at high frequencies."

The researchers claim that the graphene made this sensing technique possible, resulting in extremely fast response times of tenths of a second as opposed to tens or hundreds of seconds in existing technology. In addition to fast response times the sensors are highly sensitive, capable of detecting molecules with a concentration of a few parts per billion.

With these graphene-based sensors, the researchers have been able to put an entire chromatography system  on a single chip that is able to operate with very little power. With this setup, a badge-side device could be worn on the body to give continuous monitoring of health conditions.

Used Cigarette Filters Could Enable Next Generation of Supercapacitors

A number of lab-engineered nanomaterials have thus far just fallen short of providing a suitable alternative material for supercapacitor electrodes. But the answer may be found in the nearest ashtray. That's right: the much-despised cigarette butt may, in fact, offer the solution.

A group of researchers in South Korea has discovered that the material that makes up used cigarette butts outperforms carbon, graphene and carbon nanotubes in energy storage. 

The researchers, who published their findings in the journal Nanotechnology, took used cigarette filters and found that under a nitrogen-containing atmosphere, they could be transformed into a porous carbon material that by itself has a pore structure that is perfect for energy storage in the electrodes of supercapacitors.

Supercapacitors, also known as ultracapacitors or electrochemical double-layer capacitors (EDLCs), have held out the promise that they could store as much energy as electrochemical batteries such as lithium-ion batteries, but charge up in a matter of seconds and offer high power density for quick bursts of a large amount of energy. Supercapacitors are currently used for this purpose in applications such as powering cranes or buses.

What’s perhaps most attractive about cigarette butts for supercapacitors is that there is a seemingly endless supply of the discarded filters. According to the press release accompanying the research, there are an estimated 5.6 trillion used cigarettes, or 766,571 metric tons, being deposited into the environment every year. And if, on the off chance that it were possible to completely deplete the resource by using the butts to create these supercapacitor electrodes, doing so would still raise cheers at the fact that a way to eliminate the blight of cigarette butts had been found.

"Our study has shown that used-cigarette filters can be transformed into a high-performing carbon-based material using a simple one step process, which simultaneously offers a green solution to meeting the energy demands of society,” said Jongheop Yi, a professor at Seoul National University, and a coauthor of the study, in the press release. "Numerous countries are developing strict regulations to avoid the trillions of toxic and non-biodegradable used-cigarette filters that are disposed of into the environment each year—our method is just one way of achieving this."

The new material possesses the key features that an electrode storage material for supercapacitors would ideally have. It has a large surface area, but perhaps more critically, it has the proper distribution of pore sizes that makes it capable of utilizing a large amount of electrolyte ions and it has quick transfer mobility. It compares favorably to graphene, which has looked very attractive for the electrodes of supercapacitors because of its transfer mobility, but doesn’t really stack up to activated carbon in terms of surface area.

"A high-performing supercapacitor material should have a large surface area, which can be achieved by incorporating a large number of small pores into the material," said Yi in the release. "A combination of different pore sizes ensures that the material has high power densities, which is an essential property in a supercapacitor for the fast charging and discharging."

The bottom line is that in tests, the used-cigarette filters were capable of storing more electrical energy than commercially available carbon. They also beat previously published results for graphene and carbon nanotubes.

Just conjecture, but it would seem that cigarette manufacturers might be wise to invest in this technology since it offers a way to make an extra buck on each cigarette sold.

Graphene Transforms Itself Into a Sphere for Drug Delivery

When one of the top research organizations in the world makes a list of potential applications for graphene in electronics and that list consists of one application—RF electronics—it might be time to look somewhere else.

Of course, researchers around the world already know this, and they've been exploring a variety of medical applications, such as graphene-based gene sequencing techniques, and other far-out things like a contact lens that would give the user infrared vision.

Now researchers at Monash University in Melbourne, Australia have made a surprise discovery with graphene oxide that could lead to the material's use in disease detection and drug delivery.

In research published in the journal ChemComm, the Australian researchers discovered that when graphene oxide is exposed to a certain pH level it transforms into liquid crystal droplets. Previously, researchers needed atomizers and other mechanical equipment to change graphene into a spherical form.

“To be able to spontaneously change the structure of graphene from single sheets to a spherical assembly is hugely significant. No one thought that was possible. We’ve proved it is,” said Monash's Rachel Tkacz in a press release. “Now we know that graphene-based assemblies can spontaneously change shape under certain conditions, we can apply this knowledge to see if it changes when exposed to toxins, potentially paving the way for new methods of" disease detection.

In addition to disease detection, the researchers believe that the spontaneous transformation of graphene oxide into liquid crystal droplets could lead to new approaches to drug delivery.

“Drug delivery systems tend to use magnetic particles which are very effective but they can’t always be used because these particles can be toxic in certain physiological conditions,” said Mainak Majumder, one of the researchers, in the release. “In contrast, graphene doesn’t contain any magnetic properties. This combined with the fact that we have proved it can be changed into liquid crystal simply and cheaply, strengthens the prospect that it may one day be used for a new kind of drug delivery system."

While this would appear pretty preliminary research, and one based on a serendipitous discovery, the Monash researchers have found industrial partners that are working with them to translate the discovery into commercial applications.

Layer of Nanospheres Enables a Pure Lithium Battery Anode

Yi Cui of Stanford University has been one of the leading researchers in applying nanomaterials to the improvement of lithium-ion (Li-ion) batteries.  He’s developed nanostructured silicon for the anodes of Li-ion batteries capable of 6000 cycles while maintaining 85 percent of its capacity. He’s even chucked the whole lithium approach and developed cathodes for rechargeable batteries using potassium or sodium ions in place of lithium that were capable of 40,000 cycles while maintaining 83 percent of their charge.

While these have all been important developments in creating the next generation of rechargeable batteries, his latest achievement may be his “pièce de résistance”. Cui and his colleagues at Stanford have developed a battery designed around a pure lithium anode.

"Of all the materials that one might use in an anode, lithium has the greatest potential. Some call it the Holy Grail," said Cui, in a press release. "It is very lightweight and it has the highest energy density. You get more power per volume and weight, leading to lighter, smaller batteries with more power."

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Carbyne's List of Amazing Properties Grows

Last year the strongest material in the world was revealed: carbyne. Carbyne is a chain of carbon atoms held together by either double or alternating single and triple atomic bonds. Whereas its carbon cousin graphene is two-dimensional, carbyne is one-dimensional.

While this may get some wondering when we can get that material in some of our beloved products that are more fragile than we would like, there’s just one catch, and it’s a big one: it’s nearly impossible to produce carbyne outside of computer models.

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Topological Insulators Offer Efficient Solution to Magnetic Memory

Since they were first theorized in 2005 and later experimentally produced in 2007, topological insulators, with their ability to insulate on the inside and conduct on the outside, present a tantalizing new class of materials for electronics applications. The hope has been that they can provide a simple way to manipulate an electron’s spin and further the field of “spintronics.”

While it has been proposed that materials like uranium and plutonium can be made to act as topological insulators, most topological insulators are made from alloys of bismuth.

In collaborative research led by researchers at Penn State and Cornell University, physicists have used the topological insulator bismuth selenide in combination with a standard ferromagnetic alloy material to create a material that is capable of controlling magnetic memory 10 times as efficiently as other combinations of materials.

"This is a really exciting development for the field because it is the first promising indication that we actually may be able to build a practical technology with these topological insulator materials, which many condensed-matter physicists have been studying with spintronics applications as the motivation," said Nitin Samarth, a professor of physics at Penn State, in a press release. "Our experiment takes advantage of the very special surface of bismuth selenide—a material that is a topological insulator—which inherently supports the flow of electrons with an oriented spin."

The Cornell side of the team discovered that at normal room temperatures, it is possible to use these spin-oriented electrons to efficiently control the direction of the magnetic polarity in the adjacent material.

"Our team's research has overcome one of the key challenges to developing a spintronics technology based on spin-orbit coupling -- the efficiency with which an ordinary charge current can be converted into a spin current," said Dan Ralph, the co-principal-investigator at Cornell University, in a press release.

The research, which was published in the journal Nature last week, set out to see if they could find an efficient way to reorient the magnetization of a magnetic material using the least amount of current and power. In their experiments, the researchers discovered that charge current flowing through a thin film of topological insulator can have a strong influence on the spin of electrons in the adjacent ferromagnetic material.

"The rapid progress shown in this field at Penn State and at laboratories around the world indicates that 'topological spintronics' shows great promise of becoming an attractive offshoot of more traditional approaches to spintronics technology,” Samarth added.

Spintastic Nanorods Rotate at 150,000 RPM

Recent research has demonstrated that triggering nanoparticles to spin after being injected into the bloodstream can provide some remarkable medical benefits. Just this week, we reported on work out of the University of Georgia in which nanorods that were stimulated to spin by rotating magnets provided greater efficacy for the blood-thinning drug recombinant tissue plasminogen activator, or t-PA, used in stroke treatment. Earlier this year, researchers at Penn State University got nanorods to spin in response to both magnets and ultrasonic waves to churn cancer cells into mush.

While these have been promising results, nobody really knew how fast these nanorods were spinning—that is until now. Researchers at the National Institute of Standards and Technology (NIST) in cooperation with the Penn State researchers discovered that these nanorods were spinning so quickly that they believe that their application may extend beyond medical uses.

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Nanoparticles Improve Stroke Treatment

Currently there is just one drug that has been approved for treatment of acute stroke—recombinant tissue plasminogen activator, or t-PA. Essentially it works by thinning blood clots. Researchers at the University of Georgia (UGA) announced last week that they have developed a magnetic nanoparticle that when combined with t-PA can make the drug significantly more effective.

The Georgia researchers injected magnetic nanorods into the bloodstream. When stimulated by rotating magnets the nanorods act as a kind of mixing tool that shakes up blood clots that have already been thinned by t-PA.

The injected nanorods "act like stirring bars to drive t-PA to the site of the clot," said Yiping Zhao, professor of physics at UGA, in a press release. "Our preliminary results show that the breakdown of clots can be enhanced up to twofold compared to treatment with t-PA alone."

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Mechanical Properties of Nanoantennas Explored for First Time

Researchers have been leveraging the field of plasmonics—which takes advantage of the surface plasmons that are generated when photons hit a metal structure—to enable arrays of nanoantennas to manipulate light in ways that make possible a number of optoelectronic applications including optoelectronic circuits and for producing hydrogen gas through artificial photosynthesis.

Recent experiments have shown that placing these plasmonic nanoantennas on top of glass pillars enhances their power for sensor applications, such as fluorescence enhancement in biochemical sensing. Now researchers at the University at Illinois have discovered that this high perch for the nanoantennas introduces mechanical properties into the system that can be tuned and manipulated.

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Nanoporous Silicon Oxide Is Back in the Race for Resistive Memory

Resistive random-access memory (RRAM) has promised a new generation of computer memory by decreasing the size of memory cells through the storage of data as resistance rather than charge.  With RRAM, a dielectric material is sandwiched between two electrodes so that when a voltage is applied ions are pulled from one of the electrodes, forming conductive filaments that lower the cell’s resistance.

Research has focused on finding the best material for the dielectric, and a wide variety of materials are being pursued, including paper.

Since 2010, James Tour and his colleagues at Rice University have been pursuing silicon dioxide as the dielectric material for a RRAM cell after they discovered conductive filament pathways can be formed in the material.  Now Tour and his colleagues have taken another step in making silicon oxide the basis for high-density, next-generation computer memory.

The Rice team has refined the production of the memory cells to make it possible to fabricate the devices at room temperature with conventional production methods.

Companies are looking to win at RRAM using a variety of materials, such as Panasonic’s use of tantalum oxide and others the development of hafnium oxide. But Tour and his colleagues recently published a paper in the journal Nano Letters that makes the case that nanoporous silicon oxide is superior to the other technologies.

“Our technology is the only one that satisfies every market requirement, both from a production and a performance standpoint, for nonvolatile memory,” Tour said in a press release. “It can be manufactured at room temperature, has an extremely low forming voltage, high on-off ratio, low power consumption, nine-bit capacity per cell, exceptional switching speeds and excellent cycling endurance.”

Tour comes to this conclusion after the latest version of silicon oxide produced by the Rice researchers exceeded previous versions in a number of important performance parameters. The Rice team used a nanoporous version of silicon dioxide that reduced the amount of voltage needed to create the conductive pathways down to less than two volts. This represents a 13-fold improvement over the team’s previous best. And it brings silicon oxide right back into the running against competing materials.

The researchers were also able to eliminate the need to fabricate so-called device edge structures. “That means we can take a sheet of porous silicon oxide and just drop down electrodes without having to fabricate edges,” Tour said. “When we made our initial announcement about silicon oxide in 2010, one of the first questions I got from industry was whether we could do this without fabricating edges. At the time we could not, but the change to porous silicon oxide finally allows us to do that.”

The advantages of switching to a nanoporous variety of silicon oxide didn’t end there. The new porous version allows the cell to endure 100 times as many write-erase cycles as the previous version. Additionally, the porous silicon oxide cell's capacity to hold up to nine bits is the highest number among oxide-based memories, the Rice team claims.

The research team reports that they have already received overtures from companies interested in licensing the technology.

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Nanoclast

IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.

 
Editor
Dexter Johnson
Madrid, Spain
 
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Rachel Courtland
Associate Editor, IEEE Spectrum
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