SELF-ASSEMBLY SHOWDOWN AT THE NANOCORRAL
length scales above 10 nm. Imaging techniques able to capture structures in the nucleus in a wide range of length scales will be necessary to elucidate DNA packing in cells. In a recent study, Hémonnot et al. (DOI: 10.1021/ acsnano.6b05034) explore X-ray nanodiffraction in combination with visible light phase contrast imaging to follow DNA compaction and decompaction during different stages in the cell cycle in intact, freeze-dried cells. After assessing the extent of radiation damage caused by this technique, the researchers recorded individual X-ray diffraction patterns at a resolution of 350 × 430 nm2. Then, using a generalized Porod’s law, the researchers were able to derive information on structure morphology and compactness as well as the size and aggregation state of the probed material. From the obtained Porod exponents and constants, their method identified particularly dense and aggregated regions within the nucleus as the nucleoli and surrounding heterochromatin. Changing values indicated a pronounced decompaction of the DNA between the initial expansion phases, which reversed during the synthesis phase. The authors suggest that combining this method with other imaging techniques, such as ptychography, holography, or live fluorescence microscopy, could reveal even more details on the nanostructure within cellular components.
Researchers are increasingly focusing on supramolecular selfassembly as a route to synthesizing a range of two-dimensional (2D) crystals. Developing better understanding of the parameters involved in the self-assembly process is key to producing high-quality crystals of a desired morphology. Thus, far, gaining complete understanding and control of 2D crystal formation has been hampered by the time scales and complexity of molecular recognition and assembly processes. Consequently, studies targeting kinetic and thermodynamic parameters are rare. In a recent study, Verstraete et al. (DOI: 10.1021/ acsnano.6b05954) detail a way to study 2D supramolecular self-assembly by confining the assembling molecules into nanocorrals. The researchers created these corrals by covalently modifying highly oriented pyrolytic graphite (HOPG) with aryl radicals and then using the tip of a scanning tunneling microscope to remove these species from a defined area, a process they refer to as “nanoshaving.” They studied the self-assembly of 10,12-pentacosadiynoic acid (PCDA) in 1-phenyloctane (1-PO) on these surfaces either ex situ (after the nanoshaving process was complete) or in situ (while the corral creation process took place). Their findings show that when nanoshaving was performed in situ, the PCDA molecules tended to form a single, large domain of lamellae that was directionally influenced by the gradual graphite surface exposure. In contrast, self-assembly on the ex situ-prepared surface resulted in multiple domains similar to unconfined HOPG. Further experiments showed that nanocorral size and shape could also affect the organization of lamellar domains or their likelihood of forming. The authors suggest that this approach could be used on other surfaces and molecules to help elucidate thermodynamic and kinetic parameters involved in crystal formation.
SEAMLESS SUPERCAPACITORS WITH WO3/WS2 NANOWIRES Supercapacitors are one of the most promising energy storage technologies due to numerous advantages, including high specific capacitance, long cyclic stability, large rate capability, high power density, and fast charging and discharging. Twodimensional (2D) transition-metal dichalcogenides (TMDs) have been explored as potential capacitive materials for these devices due to their intrinsically layered structures and large surface areas. However, most 2D TMDs do not have electrical conductivities sufficient for this purpose. Some research has explored incorporating 2D TMDs with other functional nanomaterials to improve performance, but these combined materials have still suffered from limited cyclic stability and capacitance losses due to poor structural integrity at the interface between the different components. To solve this problem, Choudhary et al. (DOI: 10.1021/ acsnano.6b06111) developed a method that creates core−shell supercapacitor nanowires composed of one-dimensional WO3 wrapped with 2D WS2. To produce these structures, the
FOLLOWING DNA’S CELL-CYCLE DANCE WITH X-RAY NANODIFFRACTION DNA’s double helix structure was resolved with X-ray diffraction in the 1950s. The existence of nucleosomes, 10 nm fibers consisting of DNA coiled around a core of bead-like histones, was shown in the 1970s. In contrast, researchers have less detailed knowledge about DNA structure formation on © 2016 American Chemical Society
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researchers oxidized W foil, which formed vertically aligned, single-crystalline WO3 nanowires. Then, sulfurizing these nanowires in a chemical vapor deposition furnace under sulfur environment converted their outer surfaces to 2D WS2. Consequently, the outer and inner components of these core−shell nanowires are seamlessly connected, avoiding issues caused by poor interfaces. Tests showed no loss of initial capacitance even after 30,000 cycles, stable performance at a variety of voltages, a fast charge−discharge process, and good conductivity. In addition, these supercapacitor nanowires maintained over 98% of their initial capacitance even after folding at a 90° angle 100 times, displaying bendability and mechanical robustness. The authors suggest that this material design principle could be extended to other systems, leading to a variety of energy storage devices customized for emerging flexible and wearable technologies.
MICROHONEYCOMB MONOLITHS: EASY AS ONE, TWO, TREE Materials scientists have long been interested in honeycomb structures for their geometrical merits, which enable a high strength-to-weight ratio. Just over a decade ago, researchers reported the first monolithic microhoneycomb, crafted from silica gel using a technique known as unidirectional freezedrying (UDF); until then, all artificial honeycombs with a similar or smaller size were in the form of films or powders. However, nature has synthesized monolithic microhoneycombs for millions of years in the form of xylem in natural trees. Cellulose, the major component in tree xylem, is thought to play a major role in forming these structures. Taking advantage of this material, Pan et al. (DOI: 10.1021/ acsnano.6b05808) used cellulose as the starter to assemble xylem-like monoliths (XMs). Using a chemical method to disintegrate bundles of cellulose into cellulose nanofibers, the researchers created viscous aqueous cellulose dispersions, which they then subjected to UDF. This process resulted in cylindrical monoliths with aligned, straight, polygonal-shaped channels composed of interlaced cellulose fibers. By adding other components to the dispersion before UDF, the researchers created a variety of composite XMs. These include those incorporating several types of polyurethane as well as an “artificial wood” that includes hemicelluloses and lignin. Of particular interest is a XM that the researchers crafted with graphene oxide, which they reduced with a mild heat treatment. These reduced graphene oxide (rGO) XMs showed promise as strain sensors when combined with a homemade pressure drop device. The authors suggest that this general design principle could be used to create a variety of functional XMs for a wide range of applications.
CHEMICAL SENSING THAT LEAVES AN ENCRYPTED FINGERPRINT Sensors for chemical detection are important for a variety of fields, including law enforcement, manufacturing, and homeland security. Most of these devices are designed for online sensing to give a rapid response in the presence of an analyte. However, there is also a significant need for offline sensors, which detect and analyze the occurrence of a chemical event after the fact. In a recent study, Borodinov et al. (DOI: 10.1021/ acsnano.6b06044) detail a system they developed to meet these requirements based on gradient-grafted nanofoam films. The researchers crafted these films by depositing a film made of polymer-possessing epoxy groups on a Si wafer and then crosslinking it through a reaction of the epoxy groups to create an insoluble coating that is able to swell. The film was then grafted in a gradient with different polymers capable of reacting with the remaining unreacted epoxy groups. Finally, the resulting film was submerged in a solvent that caused it to swell and then freeze-dried under reduced pressure. Tests show that when macromolecules in the film attract a sufficient amount of the target chemical, they become mobile, causing the foam to shrink. The obtained record of the chemical event is encrypted both by the distinctive conditions used to fabricate the film and the level of the film response upon chemical vapor exposure, factors that are impossible to replicate without prior knowledge. Growing these films on microring resonators enables optical interrogation of the refractive index and optical absorption at different locations. The authors suggest that this system could offer a unique method for offline chemical sensing.
A (MAGNETICALLY) ATTRACTIVE WAY TO DETECT ANTIBODIES Autoantibodies, or antibodies that bind to self-antigens, are associated with multiple types of pathologies, including 10624
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these two components is only about 6%. To find the source of this discrepancy, the researchers used transient absorption spectroscopy to observe the evolution of the spectral signatures for excited states between 10−13 and 10−3 s. Results showed charge generation proceeding on an ultrafast time scale, which the authors attribute to rapid hole delocalization along the polymer backbone. This high local mobility appears to exacerbate nongeminate, bimolecular charge recombination that results in the formation of nonradiative triplet excitons. The authors say that if this energy loss can be successfully reduced, then breaking the 12% ηPCE barrier can be achieved in future organic solar cells.
autoimmune disease, immunodeficiency, infectious disease, and multiple types of cancer. Being able to detect these immune components in serum readily could improve diagnoses and prognostic predictions as well as offer a facile way to track progression. Following the success of DNA microarrays to analyze gene expression, significant effort has focused on developing protein microarrays for autoantibody detection. However, such systems have multiple challenges, including the need for high-binding capacity and sensitivity since protein samples cannot be amplified like DNA; the costly and laborious processes necessary to produce protein libraries for microarray immobilization; and the potential for protein degradation during microarray production processes. To avoid these issues, Lee et al. (DOI: 10.1021/ acsnano.6b03786) developed a novel method for detecting autoantibodies by attaching peptide libraries to giant magnetoresistive (GMR) nanosensors. These sensors measure changes in resistance induced by a stray field from magnetic nanoparticles that become bound to the surface. After synthesizing peptides associated with various autoantibody targets to the GMR nanosensor in situ, the researchers exposed these microarrays to autoantibodies that were labeled with secondary antibodies attached to magnetic nanoparticles. Tests showed that this system was capable of highly sensitive and specific detection of antibodies with a resolution of a single posttranslationally modified amino acid. Additionally, by disrupting autoantibody binding with a glycine-HCl solution, the same sensor could be reused. The authors suggest that this system could eventually find use in rapid and portable systems that test for autoantibodies as the point of care.
CO PUTS A CHARGING RING ON IT Over the past several decades, scanning tunneling microscopy (STM) has proven itself as a critical tool for atomic-level imaging and manipulation, leading to numerous scientific discoveries. The long-range electrostatic interaction between probe tip and sample has also shown other interesting and useful effects. For example, the potential from a probe tip locally top gates graphene due to insufficient screening of the electric field in the STM junction at low carrier densities, a phenomenon referred to as tip-induced band bending because of its effect on electronic structure. The probe potential of STM can also be used to change the charge state of an adsorbate or impurity, which researchers have shown in semiconductors and graphene. In a recent study, Wyrick et al. (DOI: 10.1021/ acsnano.6b05823) take advantage of this latter phenomenon in converse by using adatoms on the probe tip to change the induced potential on a graphene surface. In initial experiments using Co atoms scattered on a graphene surface, the researchers mapped the differential tunneling conductance from an STM probe around these impurities. Their findings show that distinct charging rings surrounded these adatoms due to Co ionization by the tip potential. With vertical atom manipulation, the researchers added Co atoms to the probe tip. Experimental measurements of the resulting charging rings and density functional theory modeling showed that these additions resulted in locally weaker induced potential in graphene below the tip. The authors suggest that these findings might eventually be used to custom engineer tip potentials by attaching selected atoms and using charging rings to determine changes to the induced potential.
LOW ENERGY LOSS, BUT HIGH RECOMBINATION Although donor−acceptor organic solar cells often show high quantum yields for charge collection, their power conversion efficiencies (ηPCEs) have been limited to around 12%. This barrier is partly due to significant energetic losses incurred either during the formation of charges or charge transfer (CT) states or due to recombination of charges or CT states. To increase ηPCEs, it will be critical to increase the energy of the charge transfer state while limiting pathways to recombination. In a recent study, Menke et al. (DOI: 10.1021/ acsnano.6b06211) investigate the dynamics of charge generation and recombination in a PIPCP:PC61BM blend. This system exhibits low electronic disorder, high carrier mobilities, and a low driving energy for initial charge separation. While each of these characteristics should translate into excellent performance, the ηPCE of donor−acceptor solar cells based on 10625
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A HOLOGRAM THAT ROY G. BIV WOULD APPROVE Unlike images formed by lenses, holograms contain the complete amplitude and phase information on light scattered from the original object. Although conventional holography requires large optical components for wavefront shaping, recent research has produced optical metasurfaces made of subwavelength nanoantenna arrays that can manipulate the wavefront of light with well-controlled amplitude, phase, and polarization. The meta-holograms produced using these advances have enhanced space-bandwidth product, high resolution, and a wide field of view. However, most have only a single color or primary colors. In a recent study, Wan et al. (DOI: 10.1021/acsnano.6b05453) report a method to reconstruct both two- and three-dimensional full-color holographic images using subwavelength nanoslits. Using aluminum film due to its optical response covering the entire visible spectrum, the researchers created metasurface holograms using a wavelength-multiplexed encoding method in which various orientation angles controlled the amplitude of light and different lengths controlled the phase. These images reconstructed not only the three primary colors (red, green, and blue) but also their secondary colors (cyan, magenta, yellow, and white) with high resolution and low noise. Using this technology, the researchers were able to generate twodimensional holographic images of objects such as letters and an apple as well as three-dimensional images of a three-turn hollow helix, some in microscopic sizes of
