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Research summaries for 2017

Transparent electrodes: carbon nanomaterials find their silver lining

Spraying graphene oxide and carbon nanotubes onto silver nanowire films enhances the flexibility and durability of see-through electrodes. Conventional transparent electrodes, such as indium tin oxide, are made from rigid crystals that are unsuitable for today's increasingly wearable electronic displays. Now, a team led by Dae-Eun Kim at Yonsei University and Young-Jei Oh at KIST, Korea, has detailed a way to optimize conductivity and mechanical properties in promising replacements based on carbon-metal composites. A multifaceted series of experiments revealed that carbon nanotubes added spring-like stress resistance when deposited on transparent silver nanowire films, and a finishing coating of reduced graphene oxide helped lock the nanowires in place and provided lubrication against friction. As a demonstration, the researchers turned the new three-layer electrode into a transparent heater that could selectively remove frost in patterned regions, even after repeated bending.

NPG Asia Mater Research Summary. Published online 13 October 2017

Stretchable electronics: Snake-like skin now in fashion

A wearable sensor can track full-range human motion and sniff out toxic gases using polymer nanowires with features similar to snakeskin. The device, which was developed by Pooi See Lee and co-workers at Nanyang Technological University, is based on polyaniline, a conducting polymer with natural flexibility and chemical sensitivity. Polyaniline often becomes unworkably rigid when deposited as a thin film, but the Singapore-based team avoided this issue by growing a vertical forest of polymer nanowires on a thin silicone support. Scanning electron microscopy revealed that when stretched, the nanowires formed scale-like islands and cracks that alter the resistance of the thin film. The researchers demonstrated that these electrical changes were sensitive enough to use for forehead sensors of breathing and eye movement and as a ‘smelling skin’ for detecting methane–an odorless and explosive gas.

NPG Asia Mater Research Summary. Published online 13 October 2017

Biomaterials: Stitching cells together

Nanomaterials can speed up the rate at which wounds heal and tissue regenerates. Songlin Peng from the Jinan University Second College of Medicine, China, and colleagues review the development of artificial materials that achieve this aim by mimicking the hierarchical architecture of the extracellular matrix. Cells can proliferate and migrate by secreting proteins that signal to adjacent cells. These proteins – known as growth factors – bind to specific receptors on the target cell. But it is difficult to harness this simple concept in clinical applications such as wound healing because the instability of growth factors limits their safety and cost effectiveness. Peng and co-workers review recent progress in the use of functionalized microporous scaffolds functionalized with growth factor encapsulated nanoparticles. They also outline its advantages over alternative approaches employing polymers, lipids and mesoporous silica-based nanoparticles.

NPG Asia Mater Research Summary. Published online 06 October 2017

Graphene: Silver halide sends a shock to tunable phototransistors

Adding silver chloride nanoparticles to graphene films creates an internal amplification effect that is ideal for see-through solar sensors. Graphene is transparent and light sensitive, but it generates only a low photocurrent in its natural state. One way to boost its responsiveness is with semiconductors that absorb photons and inject ‘hot’ carriers into graphene to create an electronic gain. Jinyao Tang from the University of Hong Kong and colleagues have now discovered how to overcome the semiconductor-limited responsiveness of these hybrids. The team's material uses graphene as a broadband light absorber that reverses the usual flow of hot electrons into a reservoir of silver chloride nanoparticles undergoing electrochemical redox reactions. This setup enables light amplification at smaller wavelengths than typical optical band gaps, with color selections controlled through a simple insulating coating.

NPG Asia Mater Research Summary. Published online 06 October 2017

Metal–organic frameworks: Fluorine lends a degree of flexibility

Adding chemistry's most electronegative atom to porous polymer networks may lead to gas-specific filters capable of flexible ‘breathing’. The tunable activity of metal–organic frameworks towards specific guest molecules, such as drugs or pollutants, has led to a flurry of recent research reports. Shin–ichiro Noro and Takayoshi Nakamura from Hokkaido University in Japan review these findings to show the potential of fluorinated building blocks in porous polymers. The hydrophobic nature and strong bond polarity of organic–fluorine units induce preferential adsorption sites and hydrophobic character useful for oil spill clean-ups. Alternatively, combining inorganic fluorine anions with neutral molecules produces weaker network bonds that easily slip and deform in certain directions. Framework flexibilities and other properties including luminescent emission for sensors are adjustable through appropriate organic or inorganic fluorine units.

NPG Asia Mater Research Summary. Published online 29 September 2017

Drug delivery: whirlpool cleanups put a spin on potency

Particles sorting due to vortex patterns inside microfluidic devices can enhance drug release from polymer agents and aid microvesicles recovery from whole blood. When liquids flowing along a straight tube encounter a bend, the change in pressure generates secondary rotational fluid motion known as Dean vortices. Han Wei Hou at Singapore's Nanyang Technological University and colleagues have exploited this effect to realize high-throughput micro- and nanoparticles separation based on small size differences. The team's technology uses spiral microchannels to sort and enrich smaller microparticles up to 1,000 times. Trials with drug-loaded copolymer microparticles revealed sub-micron delivery agents greatly improved tumor cell growth inhibition due to their quicker release properties. In addition, this approach enabled single-step isolation of membrane-based microvesicles from whole blood directly, within minutes instead of hours with typical centrifugations.

NPG Asia Mater Research Summary. Published online 29 September 2017

Flexible electronics: Spinning wearable heaters

A quick method to make a large-area, stretchable and transparent electrode for wearable electronics has been developed by a team in Korea. Materials that are both transparent and conducting are useful for many optoelectronic applications. Indium tin oxide is a common choice, but it lacks the flexibility required for wearable technologies. Jang-Ung Park and co-workers from the Ulsan National Institute of Science and Technology spun long metal nanofibers to create a wearable heater that can operate wirelessly. They form a conductive random network of silver nanofibers as a random network on a flexible and stretchable substrate. The high thermal conductivity of silver means a current passing through the nanofiber can quickly attain a temperature of 250 degrees Celsius on a polyimide sheet.

NPG Asia Mater Research Summary. Published online 08 September 2017

Memelements: Devices with memory in their DNA

A DNA-based nanowire has the electronic properties required for a novel computer known as a memcomputer, show a team from Taiwan and the USA. Memristors and memcapacitors are devices whose resistance and capacitance depend on their previous electronic state. They are the building blocks of a so-called memcomputer, which processes information in a similar way to the human brain. By using DNA origami and self-assembly, Chia-Ching Chang from the National Chiao Tung University and co-workers incorporated nickel ions into DNA and demonstrated electronic memory effects. Specifically, they created a 2-micrometer-long DNA molecule chelated with nickel ions and showed that this nanowire has both memcapacitor and memristor properties. Simulations of the device indicated that these useful characteristics are a result of voltage-driven changes to the nickel ion redox species.

NPG Asia Mater Research Summary. Published online 01 September 2017

Image sensors: Perovskite stacks outperform silicon

The light-capturing efficiencies of full-color sensors could be tripled by switching to tiny pixels made from stacked perovskite crystals. Vertically stacked layers of red-, green-, and blue-detecting silicon pixels convert light into charges more efficiently than the lateral arrangement used in typical digital-camera sensors. They also provide desirable long-pass filtering. Maksym Kovalenko from ETH Zürich in Switzerland and colleagues now report a route to low-cost stacked RGB sensors using solution-grown methylammonium lead halides containing bromide, chloride or iodide ions. The direct band gap of these perovskites gives them sharply defined color absorptions that outperform silicon stacks by several orders of magnitude. Imaging experiments with a single-pixel prototype produced accurate color reproductions at high photoresponse speeds and gave promising prospects for miniaturization onto transistor devices.

NPG Asia Mater Research Summary. Published online 01 September 2017

Graphene: making seawater drinkable with custom membranes

Water desalination plants stand to markedly reduce their energy consumption once nanoporous graphene-based membranes are optimized. Purifying brackish water with reverse osmosis works most efficiently when membranes are as thin, selective and strong as possible. Mady Elbahri from Aalto University in Finland reviews efforts to achieve these goals using graphene monolayers perforated with nanoscale pores. Recent roll-to-roll fabrications of graphene onto polymer supports show that inexpensive, large-scale production of these ultrathin membranes is feasible. It remains challenging, however, to tailor the nanochannels and selectivity of single-atom-thick material. An alternative may be to switch to multilayered graphene oxide membranes. These substances can be engineered with different molecular cross-linking agents between each carbon layer, opening room to insert negatively charged functional groups that target and electrostatically repel sodium ions.

NPG Asia Mater Research Summary. Published online 25 August 2017

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