BETTER SAMPLE PREP FOR ELECTRON MICROSCOPY Electron microscopy (EM) has attracted increasing interest in recent years due to improved instrumentation, including direct electron-detection cameras and better image-processing software systems. However, standard methods to prepare samples for EM suffer from a major drawback: Blotting, which occurs twice during the preparation process, removes the vast majority of the entire multimicroliter sample and uncontrollably and preferentially removes some sample subclasses. Thus, researchers must start with a large sample volume and may end up with a nonrepresentative sample population on the EM grid. In a new study, Arnold et al. (DOI: 10.1021/acsnano.6b01328) developed a novel method for sample preparation for negative stain EM or tetrahalose embedding for cryo-EM that retains the majority of the sample, enabling analysis of nanoliter sample volumes with little loss. In this method, the researchers use instrumentation initially developed for singlecell lysis, with a microcapillary connected to a high-precision pump. Using this system to draw in 3−5 nL of sample, the researchers then immersed the microcapillary tip into a reservoir of negative stain or tetrahalose, where the contained sample becomes conditioned by a diffusive exchange of salt and heavy metal ions for negative stain EM or sugar molecules for cryoEM. This small-volume, conditioned sample is then deposited on the EM grid, ready for analysis. The authors suggest that this method enables new strategies for biological experiments, such as analyzing lysates of single cells by visual proteomics.
core−shell red QDS and ZnO, and then an Al electrode through a shadow mask to create multilayer stacks with an emissive layer, a charge transfer layer, and a metal electrode. Contact and removal of selected regions of this stack left pixilated patterns in a desired geometry. By adding a thin film of a fluoropolymer, leaving a low-energy coating, the researchers were able to retrieve these multilayer stacks from the donor and enable release onto a target substrate without damage. Repeating this procedure enabled the delivery of red and green/blue QDs onto a single target substrate. The authors suggest that this strategy enables the co-integration of several optoelectronic and electronic devices with heterogeneous energy band diagrams onto the same substrate in a practical and high-throughput manner.
NANOSTRUCTURES ON A ROLL Roll-to-roll nanoimprint lithography (R2R-NIL) is a highresolution patterning technique that can produce nanostructures in a cost-effective way over large, flexible substrates. In one variation of this technique, known as UV-NIL, the pattern imprinted in the resist by a stamp is fixed simultaneously by ultraviolet light. The requirements for a resist in this method include good substrate adhesion during quick curing, low surface energy and low enough viscosity for fast spreading and filling of stamp cavities, the right elastic properties to accommodate use ranging from a flexible imprinting tool or soft nanopatterned film, and residue-free printing for later use as a mask. In a new study, Leitgeb et al. (DOI: 10.1021/acsnano.5b07411) developed two new R2R-UV-NIL resist materials: JRcure, which has highly tunable viscous, mechanical, and surface properties that allow it to be used as a flexible imprint resist or hardened into a polymer stamp material, and JRlif t, a water-soluble resist that enables residue-free printing. For the JRcure system, the researchers combined urethane acrylate oligomers for adjusting stiffness, acrylate monomers as reactive thinners, mono- or trifunctional thiol monomers for tuning the elasticity and accelerating cross-linking, various antiadhesion additives, and a photoinitiator to start the radical polymerization reaction. JRlif t consists of a non-cross-linking monoacrylate material with ultralow viscosity, thiol monomers for curing under atmosphere, a surfactant additive for adjusting surface energy, and a photoinitiator. Using the JRcure system, the researchers fabricated flexible films with multiscale structures that provided superhydrophobic properties. With the JRlif t system, the researchers created metallic patterns with only 200 nm line width.
TRANSFER PRINTING SEES THE LIGHT FOR QUANTUM DOT LIGHT-EMITTING DIODES Light-emitting diodes (LEDs) that incorporate solutionprocessed films of quantum dots (QDs) have recently attracted increased interest due to rapid progress in their performance, with efficiency now comparable to that of organic LEDs. Energy band alignment between charge transport layers and QDs in these devices is key to their operation. However, achieving complex, multilayer assemblies using solution-processing methods has been challenging due to the need to avoid redissolving existing layers while depositing overcoats. In a new study, Kim et al. (DOI: 10.1021/acsnano.5b06387) detail a transfer printing method to achieve this goal, using a thin film of fluoropolymer as a low-energy protective coating on the top of preformed, layered stacks that are optimized to match these materials’ energy bands. Starting with a donor silicon wafer functionalized with a surface layer to minimize adhesion, the researchers spin-casted CdSe/CdS/ZnS © 2016 American Chemical Society
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ELECTRIFYING ASSEMBLY OF TETHERED GOLD NANORODS Organizing functional nanoparticles into one-, two-, or threedimensional hierarchical structures can provide these materials with different and enhanced properties that could be useful for a variety of applications. Thus far, researchers have made significant progress in controlling the assembly of nanoparticles using templates, external fields, biomolecular recognition, directional chemical binding, and other means. Recently, strong two-dimensional confinement, through means such as anodic aluminum oxide channels or carbon nanotubes, has proven useful in creating one-dimensional nanoparticle assemblies with structures such as linear chains, zig-zags, and double-helices. However, researchers have yet to use this method to assemble structures of anisotropic nanoparticles such as nanorods, despite the strong promise of these materials in a bevy of applications including sensors, catalysis, photoelectric devices, and data storage. In a new study, Wang et al. (DOI: 10.1021/acsnano.6b00487) move forward in this arena by using an electrical field to control assembly of polystyrene-tethered gold nanorods (AuNRs@PS) under cylindrical confinement. The researchers filled anodic aluminum oxide channels with solutions of AuNRs@PS using capillary force. After evaporating the solvent, the researchers tested various strengths and directions of electric fields as well as different channel sizes and ligand lengths to determine their effect on the assembly of this material. Using this method, they were able to generate a variety of hybrid assemblies, including single-, double-, triple-, or quadruple-helix; linear; and hexagonally packed structures. Correspondingly, these different assemblies displayed different surface plasmonic properties. The researchers suggest that this method could be used to tune the plasmonic properties of AuNR assemblies to make them suited for applications including photoelectric devices, biosensors, and data-storage devices.
The authors suggest that these materials could have myriad applications in R2R-UV-NIL.
DOING THE WAVE TO INVESTIGATE CARRIER MOBILITY IN NANOWIRES For decades, researchers have used surface acoustic waves (SAWs) as a way to investigate conductivity in bulk and nanoscale semiconductor structures. In conventional SAWspectroscopy, conductivity is derived from the attenuation of the SAWs by shunt currents of the mobile carriers. Consequently, one drawback of this technique is that its sensitivity can significantly decrease for submicrometer objects, such as semiconductor nanowires (NWs). However, optically active materials provide a way around this problem, with large electric fields accompanying the SAWs, efficiently dissociating electron− hole pairs and suppressing their radiative recombination. Using this advantage, researchers can model underlying spatiotemporal carrier dynamics (STCDs) induced by SAWs with high fidelity. Although the first SAW experiments have recently been performed on individual semiconductor NWs, no detailed and complete studies on the SAW-induced SCTDs have been reported, mainly due to a lack of information on transport mobilities of both electrons and holes. In a new study, Kinzel et al. (DOI: 10.1021/acsnano.5b07639) use piezoelectric SAWs to derive the transport mobilities of both species of charge carriers in the GaAs core of individual GaAs/ AlGaAs core−shell semiconductor NWs. The researchers determined these figures of merit by combining the results of SAW-spectroscopy experiments that determined the time-dependent interbank optical recombination to numerical simulations. Because SAW-spectroscopy probes carrier transport at low carrier densities, their results represent the native material limit of these NWs. Additionally, their results suggest hole localization within long wurtzite-rich segments and electrons in zinc blende regions reflected at the interface to a wurtzite-rich segment. The authors suggest that SAW-spectroscopy offers numerous advantages for studying charge carriers in other semiconductor NWs.
TETRABLOCK TERPOLYMERS HAVE SEVERAL DIFFERENT BALLS IN THE AIR Block copolymers, macromolecules with discrete sequences of chemically distinct repeating units that can segregate into periodic nanoscale structures with different domain geometries, have been investigated intensely since their discovery decades ago. Since the development of the first simple AB diblock copolymer, researchers have developed various other block copolymer architectures, including triblock terpolymers and higher functionality multiblocks. Self-consistent-field theory (SCFT) and other methods have suggested a multitude of potential phases in these more complex materials, including various sphere-like phases. However, it is unclear which phases are indeed possible. In a new study, Chanpuriya et al. (DOI: 10.1021/ acsnano.6b00495) investigate this question using a series of poly(styrene)-b-poly(isoprene)-b-poly(ethylene oxide) (SIS′O) tetrablock terpolymers. With SCFT calculations suggesting a 4886
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intercalator binding, removing these molecules and returning the origami structures to their original conformations. The authors suggest that this strategy could be useful for developing new types of dynamic DNA origami and for structural DNA technology in general.
rich array of sphere-forming morphologies with variations in the molecular symmetry parameter (τ), a metric calculated from the block degree of polymerization and the volume fraction of O, the researchers investigated eight SIS′O samples with different τ. Evidence from small-angle X-ray scattering and transmission electron microscopy revealed the presence of nine different spherical phases including hexagonal, face centered cubic, hexagonal close packed, body centered cubic, rhombohedral, liquid-like packing, dodecagonal quasicrystal, and Frank-Kasper σ and A15 phases. At temperatures close to the order−disorder transition, these materials formed equilibrium morphologies mediated by facile chain exchange between micelles. As temperature decreased below this critical value, chain exchange arrested and nonequilibrium ordered structures formed. The authors suggest that these results emphasize the importance of combining theory, computation, and experiments to discover new forms of self-assembled soft matter.
PROTEIN NANOFIBRIL GROWTH ON A ONE-WAY TRACK Many natural proteins form nanofibrils, including pathogenic amyloids such as Aβ(1-42), amylin, and α-synuclein, as well as many common food proteins. Developing a detailed understanding of how such nanofibrillar structures form is crucial for a variety of applications, such as treating related diseases or creating biocompatible fibrillar hydrogels that could give structure to food or mimic the extracellular matrix in supporting and instructing cell growth for regenerative medicine. Researchers recently developed a group of biomimetic protein polymers that are able to self-assemble into long fibrils and hydrogels to aid in the basic study of this phenomenon. These materials have a triblock structure composed of a central silk-like domain, responsible for forming fibrils, flanked by two lateral hydrophilic collagen-like domains that form random coils and provide water solubility. Although these protein polymers have been extensively studied on the macroscopic level, with characteristics like gelation time, strength, and recovery after breaking now known, the mechanism of their assembly on the nanoscale was unclear. In a new study, Beun et al. (DOI: 10.1021/acsnano.6b01017) address this question using atomic force microscopy and stochastic optical reconstruction microscopy. Experiments with labeled monomers showed that self-assembly takes place on only one side of the growing fibrils by the irreversible addition of protein polymer subunits. When monomers ran out in solution, these “living ends” remained active indefinitely, restarting growth in the presence of new monomers. Taking advantage of these characteristics, the researchers were able to create patterned protein nanofibrils by varying monomer feed conditions. The authors suggest that these findings could open up new ways to influence the architecture of fibrillar networks.
WORKING THE KINKS OUT OF DNA ORIGAMI The programmable and complementary nature of DNA strands and their facile functionality has made DNA origami a valuable research tool and spurred many emerging applications. Recently, researchers have developed dynamic origami structures that can change their conformation in response to environmental cues or external signals. However, these structures have significant drawbacks. For example, they typically operate in only two states, such as open and closed, with no progressive intermediate conformations between. Additionally, the open state is often not well-defined because the hinge is composed of soft, singlestranded DNA, leading to large conformation fluctuations. Seeing a solution to these shortcomings, Chen et al. (DOI: 10.1021/acsnano.6b01339) developed an alternative approach for dynamic DNA origami that involves noncovalent bonding of DNA intercalating molecules, such as ethidium bromide and meso-tetra(N-methyl-4-pyridyl)porphine. These intercalators partially unwind the DNA, changing its shape. Taking advantage of helicity mismatches in two structures, a hierarchically assembled origami nanoribbon made from rectangular DNA tiles and a monomeric origami shaft with a DNA “flag” on each end, the researchers were able to modify their shapes with intercalators, removing kinks that caused the buckled or twisted structures to flatten. With increasing concentrations of intercalators, these structures developed opposite conformations to the original ones, buckling or twisting in the opposite direction. Adding excess short DNA strands competed with
SIRNA DELIVERY: IT’S A WRAP! Small interfering RNA (siRNA) is an attractive tool for gene suppression and knockdown, useful for both basic research and potential therapy for a variety of diseases. Although this tool has shown enormous promise, implementing its widespread use has been challenging due to a lack of targeting modalities, limited loading efficiency of transfection agents, internalization barriers, and biocompatibility issues. Developing safe and effective delivery vehicles will be key to ensuring its broad clinical application. In a new study, Esteban-Fernández de Á vila et al. (DOI: 10.1021/acsnano.6b01415) detail a new method to deliver siRNA inside cells by ferrying it on self-propelled nanowires. Using green fluorescent protein-targeted siRNA (siGFP) for their proof-of-principle experiments, the researchers attached 4887
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these molecules to gold nanowires (AuNWs) by wrapping them with DNA structures made by rolling circle amplification (RCA). These RCA strands were specifically designed with multiple binding regions to bind the target siRNA. Because the AuNWs were functionalized with cysteamine, the DNA strands holding bound siRNAs readily attached. Using ultrasound, the researchers were able to steer the nanowires toward two different GFP-expressing cell lines in vitro, which were then penetrated by the AuNWs. Various analytic methods showed effective knockdown of this protein, with up to 94% silencing after a few minutes treatment. Compared to static modified nanowires, the ultrasound-powered AuNWs improved the silencing response up to 13-fold. The authors suggest that this platform could find use in the fields of cancer biology, drug development, functional genomics, and nanomedicine.
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