EVALUATING THE SAFETY OF MOTHERS’ MILK Worldwide breastfeeding rates have grown in concert with the use of nanoparticles in consumer products. Ag nanoparticles, in particular, are now found in household products, personal care products, and dietary supplements due to their antibacterial properties. However, animal studies suggest that Ag nanoparticles can cause liver damage, reproductive and developmental toxicity, and neurotoxicity after oral intake or inhalation. Rodent studies have shown that some nanoparticles can be distributed to pups through breast milk after mothers are exposed, potentially damaging the mammary gland as they pass through. It has not been known whether Ag nanoparticles can also be distributed through breast milk and what effects they might have on mothers and offspring. To investigate these questions, Morishita et al. (DOI: 10.1021/acsnano.6b01782) examined the effects of the size, composition, and timing of nanoparticle exposure on distribution in mouse breast milk, the time course of nanoparticle concentration in breast milk, and the effects on dams and pups. The researchers found that both Ag and Au nanoparticles injected into dams were distributed to breast milk, with smaller nanoparticles and those distributed early in the lactation period more readily entering the milk supply. Orally administered Ag nanoparticles also entered breast milk and were subsequently distributed to the brains of nursing pups. Although those that ended up in the pups’ brains were retained longer than those in their livers and lungs, a battery of behavioral tests in adulthood found no differences between exposed and unexposed offspring. The authors suggest that more studies are necessary to evaluate the safety of these widespread nanoparticles.
depletion (STED) fluorescence nanoscopy, the researchers found that the incomplete Gag lattice of immature virions was easily discernible from the condensed distribution of mature protein subunits. Employing a photodegradable protease inhibitor, the researchers were able to coordinate protease activation, inducing proteolysis of Gag and GagPol and conversion of the capsid to a mature structure. This strategy, the authors note, overcomes previous barriers to studying this process in HIV virions, including phototoxicity and fluorophore bleaching. They add that these findings set the stage for studying Gag assembly and processing at the plasma membrane of living cells as well as using STED nanoscopy and opto-chemical switches for studying other biological processes.
BRINGING TRANSMEMBRANE PORES INTO THE FOLD A variety of protein channels dot the surfaces of lipid membranes in cells, performing functions as diverse as recognizing substrates to transporting ions or large biomolecules between cellular compartments. Researchers have tried to mimic this functionality with synthetic pores that could act as drug-delivery systems, antimicrobial agents, or biosensors, among other roles. Although biological pores span a large range of variability in size and conductance, synthetic pores thus far have been limited in their design and have displayed relatively low conductance. Those made from DNA, a stable and readily available building block, exemplify these shortcomings: only three pore types have been constructed from DNA, with conductances ranging from 0.1 to 1.6 nS. Pushing synthetic pore design to new limits, Göpfrich et al. (DOI: 10.1021/acsnano.6b03759) report a funnel-shaped porin created from DNA origami that approaches the dimensions of the nuclear pore complex, increasing the pore area and conductance to 10-fold greater than other previously reported artificial pores. Assembled from a 7249-base scaffold and 179 single-stranded staples, the porin has 19 cholesterol anchors that facilitate insertion into a lipid membrane. Confocal fluorescent imaging shows effective attachment to the membranes of giant unilamellar vesicles, some of which were stable for tens of minutes, allowing the researchers to measure ionic conductance. All-atom and coarse-grained simulations add further insight into the workings of these synthetic pores. The authors suggest that DNA origami could be a viable option for customized pores for a variety of cross-disciplinary applications.
WATCHING THE MOMENT THAT HIV MATURES For HIV virions to mature and to become infective, their structure needs to be completely remodeled. This process, which happens concurrently with viral budding from the host cell, involves protease cleavage of thousands of copies of the Gag protein, responsible for the matrix, capsid, nucleocapsid, and p6 domains, and cleavage of more than a hundred copies of the GagPol protein, responsible for a subunit of protease, as well as reverse transcriptase and integrase. Both virion release and maturation are highly asynchronous, making it difficult to study the time course of these events. A study by Hanne et al. (DOI: 10.1021/acsnano.6b03850) investigated maturation in HIV virions, synchronizing activation of protease in multiple particles to enable time-resolved in situ observation of this phenomenon. Using stimulated emission © 2016 American Chemical Society
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in each curve. The researchers used lithography and transfer printing to place GaAs nanoribbons on prestrained soft substrates made of polydimethylsiloxane. When the prestrain of the soft substrate is released, the nanoribbons buckle. Inside each wave of this configuration, part is under compression, part is under tension, and part is unstrained. This unusual construct enabled the researchers to investigate the delicately strained nanoribbons’ photonic response with μ-Raman and μ-photoluminescent spectroscopy. Their investigations show that the band gap of this material could vary up to 1% in each portion of a single wave, shifting in a continuous and periodic way along a nanoribbon’s longitudinal direction. The authors suggest that this strategy could be used in other brittle optoelectronic semiconductors, broadening their use in photonic applications.
PROPOSED ANODES GIVE SILICON ROOM TO GROW Lithium-ion batteries have become ubiquitous in portable personal electronics and grid storage; however, next-generation electronics call for even greater energy-storage needs. Replacing state-of-the-art graphite anodes with silicon-based anodes offers significantly higher theoretical capacity, but silicon comes with a host of drawbacks. For example, the electrode cycle life is limited due to cracking and pulverization caused by silicon’s large volume changes during the charge and discharge processes, which can be as high as 311%. Silicon anodes also form an unstable solid−electrolyte interphase (SEI) on their surfaces, which eventually accumulates to block Li-ion transport and cause poor Coulombic efficiency in the battery. To overcome these hurdles, Zhu et al. (DOI: 10.1021/ acsnano.6b04522) propose an anode design in which siliconbased anodes are protected from both the dangers of expansion and the SEI. Using a multistep protocol, the researchers enclosed silicon nanoparticles inside carbon and graphene nanofibers, leaving a bubble of ample space around each nanoparticle to allow room for expansion. Arranging these nanofibers into a netlike film, they used these constructs directly as anodes. Tests showed that they offered high current densities and reversible capacities over more than a thousand cycles. Their excellent cyclic stability appears to be due both to the silicon’s expansion space, which mitigates both chemical and mechanical degradation during charge and discharge, and to a stable SEI layer forming on the nanofibers instead of the silicon. The authors suggest that this design could be a promising prospect in the field of energy storage.
JUMPING DROPLETS: TAKING A CLOSER LOOK When droplets coalesce on a superhydrophobic surface, the resulting droplet can jump off the surface. This phenomenon has received significant attention due to its potential role in a variety of applications, including anti-icing, self-cleaning, energy harvesting, and condensation heat transfer. However, researchers have lacked a good method to characterize jumping droplets’ three-dimensional path fully. A study by Cha et al. (DOI: 10.1021/acsnano.6b03859) reports a method called focal plane shift imaging (FPSI) that uses a single camera to provide three-dimensional (3D) droplet jumping information through the use of focal plane manipulation. The researchers tested their technique on three superhydrophobic surfaces with a wide range of length scales from 10 nm to 1 μm, forming droplets with radii ranging from 3 to 160 μm. Taking advantage of the temporal lag before droplets coalesce and jump, they initially used a camera to observe droplets prior to coalescence. Then, they shifted the imaging focal plane above the droplets and observed the departing droplets as they moved through the shifted plane. Using information that they gathered to calculate the initial center-ofmass of the droplets prior to coalescence and the time taken for the departed droplet to pass through the shifted focal plane, they could determine a jumping droplet’s 3D trajectory. Using this technique, they were able to study the effects of coalescing droplet mismatch, multidrop coalescence, and multihop coalescence on droplet jumping speed and the source of departure angle. The authors suggest that this work could improve the performance of applications utilizing droplet motion.
TO MEASURE STRAIN−PHOTONIC COUPLING, BUCKLE IN For some optoelectronic semiconductors, strain offers a way to control band structure: introducing or releasing compressive or tensile strain can predictably increase or decrease the band gap. This quality can be useful for tuning the band gap for desired applications. However, some optoelectronic materials, such as GaAs, are too brittle to take advantage of this useful characteristic. A study by Wang et al. (DOI: 10.1021/acsnano.6b03434) reports a way around this difficulty, designing GaAs nanoribbons with a buckled, wavy configuration that introduces periodic strain 8125
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applications such as phototransistors, solar cells, light-emitting devices (LEDs), and lasers. However, their low quantum efficiencies have prevented their widespread implementation in favor of high-gain materials such as quantum dots, quantum wells, light-emitting organic materials, and direct band gap semiconductors. Recent reports have shown that optical absorption can be enhanced in TMD materials through Fabry−Perot (F−P) interference, a phenomenon that can be observed between two parallel reflecting interfaces with multiple internal reflections. Optimizing the thickness of a SiO2 spacer formed on a Si substrate can induce this effect. Although this type of interference has proven useful for facilitating the macroscopic visualization of two-dimensional materials and helping to characterize their numbers of layers, no significant effort has focused on using F−P interference to enhance optical device performance. A study by Jeong et al. (DOI: 10.1021/acsnano.6b03237) reports a technique that combines F−P interference with plasmonic excitation to increase optical gain in MoS2. Placing a MoS2 layer on a substrate consisting of a TiO2 dielectric nanofilm spacer on a metal film, the researchers were able to tune the photoluminescence intensity based on the thickness of the spacer and the type of metal (Au, Ag, Cu, or Al), which affected the F−P interference. For setups with thin spacers, roughness of the metal film led to field enhancement of the optical emission. The authors suggest that these findings could eventually lead to TMD incorporation in optical devices such as LEDs, lasers, and nanoantennas.
MOS2/GRAPHENE PHOTODETECTOR FLEXES ITS MUSCLES Flexible optoelectronic components play key roles in a variety of recently reported devices, including flexible photodetectors, light-emitting diodes, optical filters, optical interconnect, photovoltaic devices, and biomedical sensors. However, the properties of these components are limited by the high stiffness of bulk semiconductors. For example, current flexible photodetectors, which are constructed primarily from micron-thick semiconductor membranes, tend to have drawbacks including small photoactive areas, low responsivities, high operating voltages, and lack of transparency. To overcome these shortcomings, De Fazio et al. (DOI: 10.1021/acsnano.6b05109) developed a design for flexible photodetectors. Using chemical vapor deposition, the researchers grew large-area single layers of MoS2 and graphene. A layer of MoS2, which acts as a light absorber in this device, was then sandwiched between a layer of graphene, which acts as a conductive channel for photocurrent flow, and a flexible polyethylene terephthalate substrate. The MoS2−graphene stack was gated with a polymer electrolyte. Tests showed that the external and internal responsivities of this device were on par with graphene/MoS2 photodetectors on rigid substrates at the same optical power level and at least 2 orders of magnitude higher than bulk-semiconductor flexible membranes. In addition, the devices have significantly larger photoactive areas, greater than 80% transparency, operation voltages of less than 1 V, and stable operation upon multiple bending cycles at a bending radius of 1.4 cm. The authors suggest that these characteristics could make this flexible MoS2/graphene photodetector an attractive candidate for wearable devices and a variety of other applications.
SHEDDING LIGHT ON ELECTRODEPOSITED ELECTROLUMINESCENT JUNCTIONS Nanoscale electroluminescent (EL) structures have a wide variety of potential applications, including in chemical sensors, information processing, and biological systems. In recent years, several intriguing architectures have been reported for nanostructured EL devices, including those based on single nanowires, nanowire arrays interfaced to films, crossed nanowire p-n junctions, and core−shell heterostructures. However, few studies have focused on electrodeposited EL nanostructures. A study by Qiao et al. (DOI: 10.1021/acsnano.6b04022) describes a method that uses electrodeposition to create linear arrays of nanoscale metal−semiconductor−metal electroluminescent junctions, which they term “transverse nanowire electroluminescent junctions” (tn-ELJs). The researchers first use photolithography to pattern a nickel film on glass, then place a trench adjacent to the edge of this film. CdSe nanowires are then electrodeposited within the trench using the nickel edges as working electrodes. Next, a gold electrical contact is electro-
MAKING BIG GAINS IN MOS2 OPTICAL GAIN The direct band gap in monolayer transition metal dichalcogenides (TMD) such as MoS2 made them ideal for optoelectronic 8126
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deposited onto each CdSe nanowire. The trench is then filled with gold. After a gold film is evaporated onto the entire device, the gold-coated photoresist layer is removed by lift-off. Tests show that the external quantum efficiency (EQE) and threshold voltage (Vth) of EL produced by these devices depends on the width of the CdSe nanowires. Both EQE and Vth increase with nanowire width, and thus, the resulting electrical resistance of these devices. The tn-ELJs produced EL emissions broadly in wavelengths ranging from 600 to 960 nm. The authors suggest that future designs for these devices could be optimized to obtain higher EL quantum efficiencies.
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