Nano-engineering enhances charge transport, promises more efficient future solar cells

Solar cells based on semiconducting composite plastics and carbon nanotubes is one of the most promising novel technology for producing inexpensive printed solar cells. Physicists at Umeå University have discovered that one can reduce the number of carbon nanotubes in the device by more than 100 times while maintaining exceptional ability to transport charges. This is achieved thanks to clever nano-engineering of the active layer inside the device. Their results are published as front page news in the journal Nanoscale.

Carbon nanotubes are more and more attractive for use in solar cells as a replacement for silicon. They can be mixed in a semiconducting polymer, and deposited from solution by simple and inexpensive methods to form thin and flexible solar cells. The hybrid material is easy to spread out over a large surface and the nanotubes have outstanding electrical conductivity, and they can effectively separate and transport electrical charges generated from solar energy.

Earlier this year, Dr. David Barbero and his research team at Umeå University, demonstrated for the first time that if carbon nanotubes are connected to each other in a controlled manner to form complex nanosized networks, one can achieve significantly higher charge transport and electricity than had previously been possible using the same materials. This means that the transport of electric charges occurs with a very little energy loss.

Previous studies have reported that there is a percolation threshold for the amount of carbon nanotubes necessary to transport efficiently electric charges in a device. Below this threshold, the device become completely inefficient and no current can be generated.

In this new study, Dr. Barbero and his team at Umeå University show that this threshold can be reduced by more than 100 times in a semiconducting polymer and still generate high currents and charge transport at very low nanotube loadings, thereby strongly reducing materials costs.

“Our results are important from a fundamental point of view, but also of practical importance. The purified semiconducting nanotubes, which are necessary for high-performance devices, are still quite expensive and time consuming to produce. Now, nano-carbon devices, such as carbon nanotube based solar cells, can be produced with a much smaller number of carbon nanotubes and therefore much reduced material costs,” says David Barbero.

The new results are expected to accelerate the development of next generation of solution processed thin film nano-carbon based solar cells, which are both more effective in generating power and less costly to produce in comparison with today’s solar cells.

Explore further: Nanotube composites increase the efficiency of next generation of solar cells

A “Perfect” Solar Absorber – The ‘How To’ Guide

The key to creating a material that would be ideal for converting solar energy to heat is tuning the material’s spectrum of absorption just right: It should absorb virtually all wavelengths of light that reach Earth’s surface from the sun—but not much of the rest of the spectrum, since that would increase the energy that is reradiated by the material, and thus lost to the conversion process.

Now researchers at MIT say they have accomplished the development of a material that comes very close to the “ideal” for solar absorption. The material is a two-dimensional metallic dielectric photonic crystal, and has the additional benefits of absorbing sunlight from a wide range of angles and withstanding extremely high temperatures. Perhaps most importantly, the material can also be made cheaply at large scales.

The creation of this material is described in a paper published in the journal Advanced Materials, co-authored by MIT postdoc Jeffrey Chou, professors Marin Soljacic, Nicholas Fang, Evelyn Wang, and Sang-Gook Kim, and five others.

This rendering shows the metallic dielectric photonic crystal that stores solar energy as heat. Credit: Jeffrey Chou

The material works as part of a solar-thermophotovoltaic (STPV) device: The sunlight’s energy is first converted to heat, which then causes the material to glow, emitting light that can, in turn, be converted to an electric current.

Some members of the team worked on an earlier STPV device that took the form of hollow cavities, explains Chou, of MIT’s Department of Mechanical Engineering, who is the paper’s lead author. “They were empty, there was air inside,” he says. “No one had tried putting a dielectric material inside, so we tried that and saw some interesting properties.”

When harnessing solar energy, “you want to trap it and keep it there,” Chou says; getting just the right spectrum of both absorption and emission is essential to efficient STPV performance.

Most of the sun’s energy reaches us within a specific band of wavelengths, Chou explains, ranging from the ultraviolet through visible light and into the near-infrared. “It’s a very specific window that you want to absorb in,” he says. “We built this structure, and found that it had a very good absorption spectrum, just what we wanted.”

In addition, the absorption characteristics can be controlled with great precision: The material is made from a collection of nanocavities, and “you can tune the absorption just by changing the size of the nanocavities,” Chou say

Another key characteristic of the new material, Chou says, is that it is well matched to existing manufacturing technology. “This is the first-ever device of this kind that can be fabricated with a method based on current … techniques, which means it’s able to be manufactured on silicon wafer scales,” Chou says—up to 12 inches on a side. Earlier lab demonstrations of similar systems could only produce devices a few centimeters on a side with expensive metal substrates, so were not suitable for scaling up to commercial production, he says.

In order to take maximum advantage of systems that concentrate sunlight using mirrors, the material must be capable of surviving unscathed under very high temperatures, Chou says. The new material has already demonstrated that it can endure a temperature of 1,000 degrees Celsius (1,832 degrees Fahrenheit) for a period of 24 hours without severe degradation.

And since the new material can absorb sunlight efficiently from a wide range of angles, Chou says, “we don’t really need solar trackers”—which would add greatly to the complexity and expense of a solar power system.

“This is the first device that is able to do all these things at the same time,” Chou says. “It has all these ideal properties.”

While the team has demonstrated working devices using a formulation that includes a relatively expensive metal, ruthenium, “we’re very flexible about materials,” Chou says. “In theory, you could use any metal that can survive these high temperatures.”

“This work shows the potential of both photonic engineering and materials science to advance solar energy harvesting,” says Paul Braun, a professor of materials science and engineering at the University of Illinois at Urbana-Champaign, who was not involved in this research. “In this paper, the authors demonstrated, in a system designed to withstand high temperatures, the engineering of the optical properties of a potential solar thermophotovoltaic absorber to match the sun’s spectrum. Of course much work remains to realize a practical solar cell, however, the work here is one of the most important steps in that process.”

The group is now working to optimize the system with alternative metals. Chou expects the system could be developed into a commercially viable product within five years. He is working with Kim on applications from this project.

Explore further: Nanotechnology leads to better, cheaper LEDs for phones and lighting

Healing Wounds with Antimicrobial Curcumin Nanoparticles

Despite significant progress in medical treatments of severe burn wounds, infection and subsequent sepsis persist as frequent causes of morbidity and mortality for burn victims. This is due not only to the extensive compromise of the protective barrier against microbial invasion, but also as a result of growing pathogen resistance to therapeutic options. Michael Berger. Copyright © Nanowerk

Innovative therapies are urgently needed that overcome mechanisms of pathogen resistance – not only for thermal injuries but in general – and are easily administered without concerning systemic side effects (read more: “Nanotechnology solutions to combat superbugs“). “Antimicrobial resistance continues to be a growing crisis, highlighted by the FDA’s Generating Antibiotic Incentives Now (GAIN) program, through which three new antibiotics with the indication for acute bacterial skin and skin structure infections were rapidly approved in unprecedented succession. All three however are systemically administered, and we have yet to see new topical antimicrobials emerge,” Dr Adam Friedman, Assistant Professor of Dermatology and Director of Dermatologic research at the Montefiore-Albert Einstein College of Medicine, tells Nanowerk.

“For me, this gap fuels innovation, serving as the inspiration for my research with broad-spectrum, multi-mechanistic antimicrobial nanomaterials.” In new work, Friedman and a team of researchers from Albert Einstein College of Medicine and Oregon State University have explored the use of curcumin nanoparticles for the treatment of infected burn wounds, an application that resulted in reduced bacterial load and enhancing wound healing. The findings are available as an Article in Press in Nanomedicine (“Curcumin-encapsulated nanoparticles as innovative antimicrobial and wound healing agent”).

Curcumin nanoparticles induce cellular damage of MRSA. High-power transmission electron microscopy demonstrated interaction of nanoparticles (arrows) with MRSA cells. (A) Untreated MRSA showed uniform cytoplasmic density and central cross wall surrounding a highly contrasting splitting system. (B) After 24 hours, cells incubated with control nanoparticles 5 mg/ml did not exhibit changes in cellular morphology compared to untreated control. (C) After 6 hours, cells incubated with curcumin nanoparticles 5 mg/ml exhibited distortion of cellular architecture and edema, followed by lysis and extrusion of cytoplasmic contents after 24 hours (D). All scale bars=500 nm. (© Elsevier)

Turmeric (Curcuma longa L.) is the shining star among the cornucopia of traditional medicinal plants. It has a long history of usage in traditional medicine in India and China. Ancient Indians have known the medicinal properties of turmeric – i.e. curcumin – for several millennia. In the scientific literature there is a large body of evidence showing that curcuminoids exhibit a broad spectrum of biological and pharmacological activities including anti-oxidant, anti-inflammatory, anti-bacterial, anti-fungal, anti-parasitic, anti-mutagen, anti-cancer and detox properties. Curcumin’s unique ability to work through so many different pathways with its extraordinary antioxidant and anti-inflammatory attributes can have a positive influence in combating almost every known disease (read more: “Nanotechnology-enhanced curcumin: Symbiosis of ancient wisdom with modern medical science” and, if you are really into details, this: “Nanotechnology-enhanced curcumin – literature and patent analysis“).

“There has been tremendous excitement regarding curcumin in multiple fields of medicine, most prominently in Oncology,” Friedman points out. “Here, for the first time, we demonstrated that curcumin nanoparticles were more effective at both accelerating thermal burn wound closure and clearing infection with Methicillin Resistant S. aureus (MRSA) as compared to curcumin in its bulk size.” Friedman and his team utilized an innovative sol-gel-based polymerization technique to create silane composite nanoparticles that incorporate curcumin within a highly structured porous lattice. The versatility of the resulting nanoformulation allows for loading of different active ingredients, with therapeutic efficacy when applied topically, intradermally, and intravenously. “While so much is known about curcumin’s therapeutic potential, there have been numerous limitations with respect to clinical translation resulting from its poor solubility, instability at physiology pH and unsightly yellow-orange color,” says Friedman. “Nanotechnology can and has overcome many of these impediments. At the nanoscale, the likelihood of curcumin interfacing with its intended target is much greater.”

To sum it up, this work nicely demonstrates that curcumin nanoparticle technology circumvents the difficulties inherent in curcumin administration, enabling delivery of this therapeutic substance. Unlike currently used treatments, curcumin nanoparticles are less likely to select for resistant bacterial strains or delay wound healing. “We believe our technology has the potential to serve as a novel topical agent for burn wound infection and possibly other cutaneous injuries,” Friedman concludes.

Scientists improve microscopic batteries with homebuilt imaging analysis

In a rare case of having their cake and eating it too, scientists from the National Institute of Standards and Technology (NIST) and other institutions have developed a toolset that allows them to explore the complex interior of tiny, multi-layered batteries they devised. It provides insight into the batteries’ performance without destroying them—resulting in both a useful probe for scientists and a potential power source for micromachines.

The microscopic lithium-ion batteries are created by taking a silicon wire a few micrometers long and covering it in successive layers of different materials. Instead of a cake, however, each finished battery looks more like a tiny tree.

The analogy becomes obvious when you see the batteries attached by their roots to silicon wafers and clustered together by the million into “nanoforests,” as the team dubs them.

But it’s the cake-like layers that enable the batteries to store and discharge electricity, and even be recharged. These talents could make them valuable for powering autonomous MEMS – microelectromechanical machines – which have potentially revolutionary applications in many fields.

With so many layers that can vary in thickness, morphology and other parameters, it’s crucial to know the best way to build each layer to enhance the battery’s performance, as the team found in previous research.** But conventional transmission electron microscopy (TEM) couldn’t provide all the details needed, so the team created a new technique that involved multimode scanning TEM (STEM) imaging. With STEM, electrons illuminate the battery, which scatters them at a wide range of angles. To see as much detail as possible, the team decided to use a set of electron detectors to collect electrons in a wide range of scattering angles, an arrangement that gave them plenty of structural information to assemble a clear picture of the battery’s interior, down to the nanoscale level.

                                   A STEM image of an individual battery. Credit: Oleshko/NIST

The promising toolset of electron microscopy techniques helped the researchers to home in on better ways to build the tiny batteries. “We had a lot of choices in what materials to deposit and in what thicknesses, and a lot of theories about what to do,” team member Vladimir Oleshko says. “But now, as a result of our analyses, we have direct evidence of the best approach.

     A colorized 3D side view of a same battery showing the metallized silicon core and its        

outer layers. Credit: Oleshko/NIST

“MEMS manufacturers could make use of the batteries themselves, a million of which can be fabricated on a square centimeter of a silicon wafer. But the same manufacturers also could benefit from the team’s analytical toolset. Oleshko points out that the young, rapidly emerging field of additive manufacturing, which creates devices by building up component materials layer by layer, often needs to analyze its creations in a noninvasive way. For that job, the team’s approach might take the cake.

Explore further: Toward making lithium-sulfur batteries a commercial reality for a bigger energy punch

More information: V.P. Oleshko, T. Lam, D. Ruzmetov, P. Haney, H.J. Lezec, A.V. Davydov, S.Krylyuk, J.Cumings and A.A. Talin. “Miniature all-solid-state heterostructure nanowire Li-ion batteries as a tool for engineering and structural diagnostics of nanoscale electrochemical processes.” Nanoscale, DOI: 0.1039/c4nr01666a, Aug. 15, 2014.

Journal reference: Nanoscale

Provided by National Institute of Standards and Technology

Prolonged Power for Mobile Devices: New Technology from U of Texas

Researchers from The University of Texas at Dallas have created technology that could be the first step toward wearable computers with self-contained power sources or, more immediately, a smartphone that doesn’t die after a few hours of heavy use.

This technology, published online in Nature Communications, taps into the power of a single electron to control energy consumption inside transistors, which are at the core of most modern electronic systems.

Researchers from the Erik Jonsson School of Engineering and Computer Science found that by adding a specific atomic thin film layer to a transistor, the layer acted as a filter for the energy that passed through it at room temperature. The signal that resulted from the device was six to seven times steeper than that of traditional devices. Steep devices use less voltage but still have a strong signal.

Dr. Jiyoung Kim (left) and Dr. Kyeongjae “K.J.” Cho examine a wafer used to make transistors. The two created new technology that could reduce energy consumption in mobile devices and computers.

“The whole semiconductor industry is looking for steep devices because they are key to having small, powerful, mobile devices with many functions that operate quickly without spending a lot of battery power,” said Dr. Jiyoung Kim, professor of materials science and engineering in the Jonsson School and an author of the paper. “Our device is one solution to make this happen.”

Tapping into the unique and subtle behavior of a single electron is the most energy-efficient way to transmit signals in electronic devices. Since the signal is so small, it can be easily diluted by thermal noises at room temperature. To see this quantum signal, engineers and scientists who build electronic devices typically use external cooling techniques to compensate for the thermal energy in the electron environment. The filter created by the UT Dallas researchers is one route to effectively filter out the thermal noise.

Dr. Kyeongjae “K.J.” Cho, professor of materials science and engineering and physics and an author of the paper, agreed that transistors made from this filtering technique could revolutionize the semiconductor industry.

“Having to cool the thermal spread in modern transistors limits how small consumer electronics can be made,” said Cho, who used advanced modeling techniques to explain the lab phenomena. “We devised a technique to cool the electrons internally—allowing reduction in operating voltage—so that we can create even smaller, more power efficient devices.”

Continuous Wave and Linear Imagers Academic & Industrial Applications

Each time a device such as a smartphone or a tablet computes it requires electrical power for operation. Reducing operating voltage would mean longer shelf lives for these products and others. Lower power devices could mean computers worn with or on top of clothing that would not require an outside power source, among other things.

To create this technology, researchers added a chromium oxide thin film onto the device. That layer, at room temperature of about 80 degrees Fahrenheit, filtered the cooler, stable electrons and provided stability to the device. Normally, that stability is achieved by cooling the entire electronic semiconductor device to cryogenic temperatures—about minus 321 degrees Fahrenheit.

Another innovation used to create this technology was a vertical layering system, which would be more practical as devices get smaller.

“One way to shrink the size of the device is by making it vertical, so the current flows from top to bottom instead of the traditional left to right,” said Kim, who added the thin layer to the device.

Lab test results showed that the device at room temperature had a signal strength of electrons similar to conventional devices at minus 378 degrees Fahrenheit. The signal maintained all other properties. Researchers will also try this technique on electrons that are manipulated through optoelectronic and spintronic—light and magnetic—means.

The next step is to extend this filtering system to semiconductors manufactured in Complementary Metal-Oxide Semiconductor (CMOS) technology.

“Electronics of the past were based on vacuum tubes,” Cho said. “Those devices were big and required a lot of power. Then the field went to bipolar transistors manufactured in CMOS technology. We are now again facing an energy crisis, and this is one solution to reduce energy as devices get smaller and smaller.”

Explore further: Team uses nanotechnology to help cool electrons with no external sources

Long Life Phosphors: Breakthrough ‘Blue’ LED’s’ for More Efficient Electronics (10X)

In a step that could lead to longer battery life in smartphones and lower power consumption for large-screen televisions, researchers at the University of Michigan have extended the lifetime of blue organic light emitting diodes by a factor of 10.

Blue OLEDs are one of a trio of colors used in OLED displays such as smartphone screens and high-end TVs. The improvement means that the efficiencies of blue OLEDs in these devices could jump from about 5 percent to 20 percent or better in the near future. OLEDs are the latest and greatest in television technology, allowing screens to be extremely thin and even curved, with little blurring of moving objects and a wider range of viewing angles. In these “RGB” displays, each pixel contains red, green and blue modules that shine at different relative brightness to produce any color desired. But not all OLEDs are created equal. Phosphorescent OLEDs, also known as PHOLEDs, produce light through a mechanism that is four times more efficient than fluorescent OLEDs. Green and red PHOLEDs are already used in these new TVs—as well as in Samsung and LG smartphones—but the blues are fluorescent. “Having a blue phosphorescent pixel is an important challenge, but they haven’t lived long enough,” said Stephen Forrest, the Paul G. Goebel Professor of Engineering.

He and his colleagues demonstrated the first PHOLED in 1998 and the first blue PHOLED in 2001.

Now, with their new results, Forrest and his team hope that is about to change. Efficient blues will make a significant dent in power consumption for large-screen TVs and extend battery life in smartphones.

The lifetime improvement will also help prevent blue from dimming relative to red and green over time.

“In a display, it would be very noticeable to your eye,” Forrest said.

Jaesang Lee, Electrical Engineering PhD Student, demonstrates use of blue PHOLEDs.

In collaboration with researchers at Universal Display Corp. in 2008, Forrest’s group proposed an explanation for why blue PHOLEDs’ lives are short. The team showed that the high energies required to produce blue light are more damaging when the brightness is increased to levels needed for displays or lighting.

This is because a concentration of energy on one molecule can combine with that on a neighbor, and the total energy is enough to break up one of the molecules. It’s less of a problem in green- and red-emitting PHOLEDs because it takes lower energies to make these colors of light.

“That early work showed why the blue PHOLED lifetime is short, but it didn’t provide a viable strategy for increasing the lifetime,” said Yifan Zhang, a recent graduate from Forrest’s group who is first author on the new study (“Ten-Fold Increase in the Lifetime of Blue Phosphorescent Organic Light Emitting Diodes”). “We tried to use this understanding to design a new type of blue PHOLED.”

The solution, demonstrated by Zhang and Jae Sang Lee, a current doctoral student in Forrest’s group, spreads out the light-producing energy so that molecules aren’t as likely to experience the bad synergy that destroys them.

The blue PHOLED consisted of a thin film of light-emitting material sandwiched between two conductive layers—one for electrons and one for holes, the positively charged spaces that represent the absence of an electron. Light is produced when electrons and holes meet on the light-emitting molecules.

If the light-emitting molecules are evenly distributed, the energetic electron-hole pairs tend to accumulate near the layer that conducts electrons, causing damaging energy transfers. Instead, the team arranged the molecules so that they were concentrated near the hole-conducting layer and sparser toward the electron conductor. This drew electrons further into the material, spreading out the energy.

The new distribution alone extended the lifetime of the blue PHOLED by three times. Then, the team split their design into two layers, halving the concentration of light-emitting molecules in each layer. This configuration increased the lifetime tenfold.

“Our university research programs are a strong element of our effort towards solving critical OLED issues and accelerating the growth of the display and lighting industries,” said Julie Brown, senior vice president and chief technical officer of Universal Display. “This exciting result by Professor Forrest and his team is an important step towards a full commercial phosphorescent RGB solution.”

Source: University of Michigan

Read more: Live long and phosphor: Blue LED breakthrough for efficient electronics http://www.nanowerk.com/nanotechnology-news/newsid=37521.php#ixzz3EWCm60rI
Follow us: @nanowerk on Twitter

High Density SO Quantum Dot Solar Cells: Potential for Conversion Efficiences Greater Than 50%

Kouichi Yamaguchi is internationally recognized for his pioneering research on the fabrication and applications of ‘semiconducting quantum dots’ (QDs). “We exploit the ‘self-organization’ of semiconducting nanocrystals by the ‘Stranski-Krasnov (SK) mode of crystal growth for producing ordered, highly dense, and highly uniform quantum dots,” explains Yamaguchi. “Our ‘bottom-up’ approach yields much better results than the conventional photolithographic or ‘top-down’ methods widely used for the fabrication of nano-structures.”

Notably, electrons in quantum dot structures are confined inside nanometer sized three dimension boxes. Novel applications of ‘quantum dots‘—including lasers, biological markers, qubits for quantum computing, and photovoltaic devices—arise from the unique opto-electronic properties of the QDs when irradiated with light or under external electromagnetic fields.

“Our main interest in QDs is for the fabrication of high efficiency solar cells,” says Yamaguchi. “Step by step we have pushed the limits of ‘self-organization’ based growth of QDs and succeeded in producing highly ordered, ultra-high densities of QDs.”

The realization of an unprecedented QDs density of 5 x 10¹¹ cm^-2 in 2011 was one of the major milestones in the development of ‘self-organization‘ based semiconducting QDs for solar cells by Yamaguchi and his colleagues at the University of Electro-Communications (UEC). “This density was one of the critical advances for achieving high efficiency quantum dot based photo-voltaic devices,” says Yamaguchi.

Specifically, Yamaguchi and his group used molecular beam epitaxy (MBE) to grow a layer of InAs QDs with a density of 5 x 10¹¹ cm^-2 on GaAsSb/GaAs (100) substrates. Importantly, the breakthrough that yielded this high density of highly ordered QDs was the discovery that InAs growth at a relatively low substrate temperature of 470 degrees Celsius on Sb-irradiated GaAs layers suppressed coalescence or ‘ripening’ of InAs QDs that was observed at higher temperatures. Thus the combination of the Sb surfactant effect and lower growth temperature yielded InAs QDs with an average height of 2.02.5 nm.

                                                         InAs QD density: 1.0×10¹² cm^-2

The potential for photovoltaic device applications was examined by sandwiching a single layer of InAs QDs in a pin-GaAs cell structure. The resulting external quantum efficiency of these solar cell structures in the 900 to 1150 nm wavelength range was higher than devices with the QD layer.

“Theoretical studies suggest QDs solar cells could yield conversion efficiencies over 50%,” explains Yamaguchi. “This is a very challenging target but we hope that our innovative approach will be an effective means of producing such QD based high performance solar cells. We have recently achieved InAs QDs with a density of 1 x 10¹² cm^-2.”

Enlarge

Variation of power conversion efficiency with quantum dot density (calculated results).

Explore further: Resonant energy transfer from quantum dots to graphene

Studying nanocrystals by passing them through tiny pores

Date: September 26, 2014

Source: University of Pennsylvania

Summary:

Researchers have now applied a cutting-edge technique for rapid gene sequencing toward measuring other nanoscopic structures. By passing nanoscale spheres and rods through a tiny hole in a membrane, the team was able to measure the electrical properties of those structures’ surfaces. Their findings suggest new ways of using this technique, known as ‘nanopore translocation,’ to analyze objects at the smallest scale.

An interdisciplinary team of University of Pennsylvania researchers has now applied a cutting-edge technique for rapid gene sequencing toward measuring other nanoscopic structures. By passing nanoscale spheres and rods through a tiny hole in a membrane, the team was able to measure the electrical properties of those structures’ surfaces.

Their findings suggest new ways of using this technique, known as “nanopore translocation,” to analyze objects at the smallest scale.

An illustration of a nanocrystal passing through a nanopore.

Credit: Image courtesy of University of Pennsylvania

The research was led by Marija Drndić, professor in the Department of Physics and Astronomy in Penn’s School of Arts & Sciences; Jennifer Lukes, associate professor in the Department of Mechanical Engineering and Applied Mechanics in Penn’s School of Engineering and Applied Science; and Christopher Murray, a Penn Integrates Knowledge Professor who has appointments in both schools through the departments of Chemistry and Materials Science and Engineering. Kimberly Venta, of Drndić’s lab, and Mehdi Bakhshi Zanjani, of Lukes’ lab, were co-lead authors on the paper, and Xingchen Ye and Gopinath Danda also contributed to the work.

It was published in Nano Letters.

For the past several years, Drndić’s lab has been exploring an approach to gene sequencing involving DNA translocation through a nanopore. The technique typically involves threading DNA, suspended in an ionic fluid, through a tiny hole in a thin membrane. Each of the four bases of a DNA sequence is expected to block different amounts of the aperture as they pass through, thus allowing a different number of ions to pass through along with them. In most nanopore sequencing, researchers attempt to identify bases by reading changes in the surrounding ion current as it passes through the nanopore.

This technique has its roots in a device known as a Coulter counter. Such devices have been used for decades to count and sort microscopic particles, like blood cells and bacteria. The principle is the same; particles with larger diameters block more of the aperture, reducing the electrical current measured by electrodes positioned above and below the aperture. This technique has been used on particles that are typically on the micro scale, however, whereas DNA bases are on the nano scale, a thousand times smaller.

Advances in nanotechnology have allowed researchers to make smaller and smaller pores, and early successes in using this technique with DNA suggested that it could also be applied to better measure other nanoscale structures. Spherical nanocrystals and oblong nanorods, for example, are thought to have potential uses in medicine, electronics and other fields, but their properties must be accurately measured before they can be fine,tuned for their ultimate applications.

To that end, the members of Drndić’s contingent drew upon their sequencing research involving silicon nitride nanopores, which can be customized to work at various sizes between the nano and micro scales.

“A great feature of solid-state nanopores is that we can change diameters at will,” Drndić said. “We can use an electron microscope to drill them in whatever size and shape we want, unlike pores in biological membranes, where we would need to find a new system each time.”

For their measurement targets, the team drew on the Murray lab’s expertise in making uniformly sized gold nanospheres and nanorods that are covered with ligands that give them an overall positive charge. The surface chemistry of these nanoparticles was an attractive match for the translocation technique, which relies on drawing charged objects through the pore.

“The degree of ligand coverage on the surface of nanoparticles greatly affects the nanoparticle function and quality,” said Murray. “That’s one reason we need to be able to measure them in more detail.”

The team first used the spherical nanoparticles to calibrate their measurement system.

“For spherical nanoparticles with charged ligands on their surface,” said Venta, “there is a well-known method for determining the surface charge density, and thus the surface ligand density. However, this method fails for non-spherical nanoparticles.”

To get around this limitation, the team enlisted modeling expertise from Lukes’ group.

“Based on the data obtained from the experiments and our computational models,” Zanjani said, “we can calculate the surface charge density of the nanorods based on their diameter. Conversely, if we know their surface charge density, we can extrapolate their diameter. The same method can also be used to characterize a variety of other nanoparticles with different sizes and shapes.”

In developing the model for understanding the relationship between these properties, the team also found something unexpected. As nanorods pass through the pore, they typically reduce the ionic current through the pores, as they decrease the amount of space ions can inhabit. However, sometimes an increase in ionic current through pores was recorded.

The team determined that this was another area where pore diameter was critical. On average, the pores they drilled were 20 nanometers in diameter, with some a few nanometers wider or narrower. Taking a closer look at these unusual, current-increasing measurements, they determined that, paradoxically, the narrowest pores were triggering them. This suggested that the mechanism had something to do with the proximity between the nanorod and the edge of the pore.

“There is something about the interaction between the rods and the pores that causes these ‘positive’ events,” said Lukes. “Even though there is less space for the ions to pass through, we think that the current increases because the charged surfaces of the rods and pores attract an even higher concentration of ions than would normally be there for larger pores.”

This phenomenon could potentially be exploited as different way of measuring particles passing through nanopores. Further research will provide a clearer picture of the diameter tolerances necessary for particles of different shapes. Other aspects of the pore, such as if it has a tapered, hourglass shape versus a smooth, cylindrical one, can also be investigated to see if they make a difference in the kind of signals that can be recorded.

“This kind of study would not have been possible without Penn’s Materials Science Research and Engineering Center,” Drndić said. “Drawing on physics, chemistry, materials science, mechanical engineering provides us with a unique opportunity to discover interesting phenomena while advancing their practical applications at the same time.”

The research was supported by the National Science Foundation through Penn’s Materials Research Science and Engineering Center and a Graduate Research Fellowship.

---

Story Source:

The above story is based on materials provided by University of Pennsylvania. Note: Materials may be edited for content and length.

Magnetic nanoemulsion measures blood glucose

A new type of glucose sensor that works using a magnetically polarizable nanoemulsion could help change the way blood sugar is measured. The new device does not rely on glucose oxidase enzymes, unlike conventional glucometers, but instead simply changes colour when it comes into contact with glucose.

A team of researchers, led by John Philip at the Indira Gandhi Centre for Atomic Research in India, made the new sensor using a magnetically polarizable oil-in-water nanoemulsion of droplets that have a radius of around 100 nm. They made the emulsion by mixing together ferrimagnetic nanoparticles of iron oxide (around 10 nm across) with oil, a surfactant and water.

With and without glucose

When the solution is exposed to glucose and a magnetic field applied, its colour simply changes.

“We stumbled on this effect quite by accident while working with magnetically polarizable nanoemulsions for fundamental physics studies,” explains Philip. “We then measured the colour (or diffracted light wavelength) of the nanoemulsion using a spectrograph and noticed that the shift (or change) in the diffracted wavelength (Δλmax) was quite high and that it varied linearly with glucose concentration.”

Bragg peaks at different glucose concentrations

To our surprise, the Δλmax value at 30 mM glucose concentrations was as high as around 69 nm in the system under study, but this shift could be even larger with more suitably tailored emulsions, says Philip. “Since the Δλmax varies linearly with glucose concentration, we realized that the emulsion itself could be used as a biosensor,” he tells nanotechweb.org.

The new device could help change the way diabetics monitor their blood sugar levels. Most existing glucometers are based on glucose oxidase enzyme platforms coupled to electromechanical systems in which the device response depends on enzyme activity or glucose mass transport. These techniques take a relatively long time to produce results and require quite complicated apparatus.

Label free and fast

“The novelty of our technique is that it is label (or enzyme) free and fast (it works within just milliseconds rather than minutes),” says Philip. “It also allows us to detect glucose concentrations visually without any electronic equipment.”

The device is also portable. “For qualitative glucose testing, you simply need to look at the colours in the nanoemulsion upon mixing with a fraction of blood or urine under a magnetic field that you might generate with a tiny magnet or solenoid. For quantitative sensing, all you would need is about 200 microlitres of nanoemulsion and a pocket sized fibre-optic spectrograph for testing your samples.”

How it works

So how does the sensor actually work? At a constant applied magnetic field, the nanoemulsion droplets form 1D chain-like structures that diffract light in the visible region of the electromagnetic spectrum, explains Philip. “The diffracted wavelength depends on the distance between the droplets. When glucose concentrations in a sample reach the 1–30 mM range, the diffracted wavelength shifts and since it varies linearly with glucose concentration, we can accurately determine this concentration using a calibration curve.”

Without an external magnetic field, the nanoemulsion droplets move about randomly (thanks to ordinary Brownian motion) but an applied magnetic field induces a dipole moment in each droplet, orienting it along the field direction. “Linear chain-like structures are formed along the field direction when the repulsive forces between the droplets exactly balance the attractive forces between them,” says Philip. “For perfectly aligned droplets spaced a distance d apart, the so-called first order Bragg condition is 2d = λmax/n, where λmax is the Bragg peak wavelength and n is the refractive index of water.”

As the droplets and the spaces between the droplets are about the same size as the wavelength of visible light, we see a Bragg peak in the visible wavelength range – which manifests itself as a colour change in these fluids that we can actually see with naked eye.”

The researchers are now busy trying to improve the sensitivity of their device. “We also need to work with companies that are interested in developing the sensor into a marketable product,” adds team member Vellaichamy Mahendran.

The current work is published in Applied Physics Letters.

Article by Belle Dumé

Graphene provides ultrasensitive generic biodetection

Graphene’s host of impressive attributes includes several properties that are ideal for sensing. Now researchers have successfully fabricated a generic biosensing platform based on functionalized graphene and used it to detect markers related to cancer risk for the first time. The sensitivity of detection outperforms other assay-based sensors allowing valuable early-stage detection of the disease.

“What you need to detect biomarkers is high surface to volume ratio – graphene as a material has an inherently high surface to volume ratio,” says Owen Guy, associate professor in the College of Engineering at Swansea University in the UK, who led this latest research. “That, and its superb electronic transport properties make it an excellent material for biosensing.”

Guy explains how he was studying silicon carbide (SiC) systems for electronics when someone at a conference pointed out that graphene could be grown on SiC. “In 2006 it was early days for graphene – a lot of people were mainly from a physics background so looking at fundamental properties, growing graphene, improving the quality and characterization,” says Guy. “We were doing lots with the medical school here in Swansea at the time so we thought of attaching antibodies and looking at sensing.”

Illustration of the generic graphene biosensor platform

Guy and his team, a collaboration between researchers at the Centre for Nanohealth, Swansea University, in Wales and the Research Institute of the Petroleum Industry in Iran, showed that they could attach antibodies to functionalized graphene. The antibodies would bind with a range of specific biomarkers, resulting in detectable changes in the current-voltage measurements.

This generic sensing platform demonstrated the first detection of cancer biomarkers in a system of this kind at a sensitivity of 0.1 ng/ml – five times the sensitivity of conventional immunoassay-based devices. The researchers have also used the platform for detecting cardiac and pregnancy biomarkers, detecting pregnancy biomarkers at concentrations as low as a pictogram per millilitre.

Functionalization and further challenges

Guy explains how Zari Tehrani, a senior researcher in the team at Swansea, spent a great deal of time developing and optimizing the functionalization process. While similar chemistry had been demonstrated before, Tehrani developed a process that was much faster, occurring over minutes instead of hours and with no need for harsh chemical conditions.

Another challenge was proving that the desired interactions with target antibodies had taken place. The researchers used quantum-dot-labelled antibodies to “see” that the antibodies had attached to the functionalized graphene.

Images of graphene channels exposed to quantum-dot labelled antibodies

“The breakthrough I suppose was when we saw the quantum-dot-labelled antibodies on the sensor – then we knew we had what we thought we had,” says Guy, adding, “I think Zari was already quite sure but you have to prove it.”

Few layers better than single layer

Guy and his team used “epitaxially” grown graphene, which can be produced over large areas on silicon carbide substrates and can be processed like a silicon wafer. “At the time people doing chemistry were using solution-based graphene, which results in small flakes that aren’t really suitable for higher-throughput fabrication of devices,” explains Guy.

“There are also many different types of graphene and different layer thicknesses – single layer, bilayer, multilayer graphene,” he adds. “A purist might say only single layer is really graphene but we were less concerned with single layers because few-layer graphene actually works better.”

He explains that the chemical functionalization effectively introduces defects into the graphene that disrupt the conduction. “With few-layer graphene the layer beneath may still conduct so it’s more tolerant,” he says.

Future work will focus on optimization and further studies to understand how the sensor responds to non-specific markers. “We are also looking at scaling up from single devices to wafer devices, transfer to different markers, and then multiplexing to test for different biomarkers at the same time,” says Guy.

The full details are available at 2D Materials 1, 025004.

Article by Anna Demming