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HARNESSING THE SUN’S HEAT WITH TRANSPARENT AEROGELS Heat at intermediate temperatures (120−220 °C) is important for a variety of applications, such as water or space heating, steam generation, and sterilization. The sun is a promising “green” source to generate this thermal energy; however, state-of-the-art solar-thermal approaches toward capturing sunlight effectively to achieve this temperature range suffer from several drawbacks, including the requirement of costly optical concentrators, selective surfaces, and vacuum enclosures to concentrate the dilute solar flux and suppress heat losses. Finding ways to generate heat in this range from unconcentrated sunlight remains a difficult technical challenge. Zhao et al. (DOI: 10.1021/acsnano.9b02976) accomplished this goal with non-evacuated transparent aerogels, specifically tailoring the nanostructure of the aerogels to transmit sunlight while suppressing conduction. Whereas silica aerogels typically scatter electromagnetic radiation at visible and UV wavelengths because of scattering centers formed by silica particles and pores, those used by Zhao et al. were engineered to have small and uniform scattering centers to maximize light transmittance. The researchers designed a solar-thermal receiver by using multiple pieces of this aerogel to cover a blackbody absorber comprising a thin copper sheet coated with nonselective high temperature black paint. Laboratory tests showed that this device reached a stagnation temperature as high as 265 °C. On a sunny winter day, it reached a stagnation temperature of 220 °C, even in cold ambient temperatures. The authors suggest that this device can effectively achieve these intermediate temperatures without the drawbacks of other solar-thermal systems, making it versatile for applications including industrial process heat, solar fuel production, desalination, and solar cooling.
trials, they feature low overall response rates of less than 30% when used as monotherapies. To help boost these numbers, some researchers have investigated combining checkpoint inhibitors with cancer vaccines, photothermal immunotherapy, and other strategies. Le et al. (DOI: 10.1021/acsnano.9b02071) united these multiple approaches, creating a nanoadjuvant tumor vaccine that self-assembles in situ and becomes activated after exposure to near-infrared radiation. The researchers loaded polydopamine nanoparticles with the adjuvant imiquimod, then modified their surfaces with anti-PDL1 antibody. The presence of these antibodies increased the binding of the nanoparticles to CT26 cancer cells overexpressing PDL1. Near-infrared radiation increased the anticancer effect through a photothermal immune response and the release of the adjuvant. Treating mice with this combined strategy not only ablated primary tumors but also completely prevented growth of secondary tumors at distant sites. T-cells isolated from splenocytes showed active killing of CT26 cells. The authors suggest that this all-in-one approach could offer a promising way to incite immunity in individual patients with an off-the-shelf method, avoiding the complex processing of patient-specific tumor vaccines as well as the need for chemotherapy.
FOR PRINTABLE ENERGY STORAGE, ADD A PINCH OF SALT Graphene powders hold significant promise for energy storage applications. Several wet chemistry methods exist to generate graphitic powders from graphite precursors, producing tens of kilotons per year at relatively low cost. However, these graphitic products contain ample structural defects and chemical impurities and possess nonuniform layer thickness, qualities that stymie their use in graphene ink dispersions. In addition, their production processes require copious amounts of H2SO4 and KMnO4, plus large quantities of organic solvents, which raise environmental and safety concerns. One alternative to these processes is an emerging technology in which graphene forms over three-dimensional particulated substrates. However, this method has its own drawbacks, including the high temperatures required for synthesis, as well as the large amounts of HF necessary to etch away the growth templates.
ONE−TWO PUNCH: IMMUNE CHECKPOINT INHIBITOR PLUS VACCINE Immunotherapeutic approaches for cancer can potentially help patients avoid the collateral damage of conventional chemotherapy, surgery, and radiotherapy, fighting this disease by harnessing the power of the immune system. One promising immunotherapeutic approach makes use of immune checkpoint inhibitors that block the interaction of PD1 (programmed cell death 1) with PDL1 (programmed cell death ligand 1). However, although these agents have had success in clinical © 2019 American Chemical Society
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Published: July 23, 2019 7366
DOI: 10.1021/acsnano.9b05315 ACS Nano 2019, 13, 7366−7369
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Cite This: ACS Nano 2019, 13, 7366−7369
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Seeking a better way to synthesize graphene powders, Wei et al. (DOI: 10.1021/acsnano.9b03157) report a scalable, safe, efficient, and green way to produce nitrogen-doped graphene (NG) nanosheets using salt as the growth template. The researchers used plasma-enhanced chemical vapor deposition to synthesize graphene directly on commercially available NaCl powder. The resulting hollow cubic cages were obtained by simply washing away the salt cores with water. An ultrasonic treatment transformed these cages into two-dimensional NG nanosheets. By mixing these nanosheets with graphene oxide solutions, the scientists produced composite inks that could be used as components in supercapacitors and Li−S batteries. The authors suggest that this approach could be used to incorporate graphene inks into a variety of energy storage technologies.
STAY STRONG: RECOVERING FRACTURE STRENGTH IN SILICON CARBIDE NANOWIRES The extraordinary properties of many wires give them potential as building blocks of nanotechnology and nanodevices. These materials often exhibit high yield strength, bending strength, and superplasticity with local elongation. Nanowires’ mechanical properties are important for the reliability and optimum design of the devices in which they are used. However, nanomechanical testing of these materials is challenging because of the difficulties in manipulating materials at this scale. Consequently, overall mechanical properties and underlying deformation mechanisms of many nanowire types are unknown. Cui et al. (DOI: 10.1021/acsnano.9b02658) add to this knowledge base for SiC nanowiresmaterials with exceptional properties including low density, high strength, high thermal conductivity, stability at high temperatures, high resistance to shocks, low thermal expansion, a wide bandgap, and chemical inertness. Amorphous SiC is a brittle material, which fractures under large stress. However, nanomechanical testing on mismatched fractured brittle SiC nanowires has not been reported. The researchers manipulated SiC nanowires with a weasel hair taken from a Chinese writing brush and fixed it on a push-to-pull device using a conductive silver epoxy. The fracture strength was determined to be 8.8 GPa. However, when the broken surfaces were placed in close proximity, self-healing took place. The fracture strength of the healed surface was 5.6 GPa, 63.6% of that of pristine nanowires. Simulations suggested that healing occurred by reorganization of Si−C bonds forming Si− C and Si−Si bonds. The authors suggest that these findings provide insight into avoid catastrophic failure of these materials in harsh and extreme environments.
TURNING GRAPHENE NANORIBBONS INTO QUANTUM DOTS Graphene quantum dots have attracted interest both from fundamental and applied perspectives, with potential for use as sensitive charge sensing devices, spin qubits, and in valley degeneracy-based electronics. Because monolayer graphene has no band gap, an energy gap needs to be opened in graphene to create gate-defined quantum dots. The most straightforward way to accomplish this goal is to pattern narrow graphene nanoribbons used as tunnel junctions with lithography and etching techniques. However, this patterning can be a challenge in such small dimensions. An alternative to this approach is doping graphene with chemical solutions such as AuCl3. However, this technique has a low spatial resolution. Seeking a better way to create single quantum dot-like transport in graphene nanoribbons, Wang et al. (DOI: 10.1021/ acsnano.9b02935) doped graphene with hydrogen silsesquioxane (HSQ) at an extremely high spatial resolution of a few nanometers. In graphene, hydrogen and oxygen atoms are mild n-dopants and strong p-dopants, respectively. The researchers used HSQ as both an etching mask and surface dopant, using different electron beam strengths to break Si−H and Si−O bonds selectively and to pattern H and O onto the graphene surface. Using this method, the researchers were able to design and to achieve single quantum dot-like transport. In addition, they defined geometric design rules, discovering that the length of the doped graphene nanoribbon must be less than two times the average hopping length to accomplish this objective. The authors suggest that this method might be used with other twodimensional materials and for other applications, such as p−n junctions and tunnel field-effect transistors.
ANALOG TUNING FOR COLLOIDAL GOLD NANOCRYSTAL ASSEMBLIES Both the electronic configuration of atoms and their interatomic distance determine the optical properties of metals. Whereas these characteristics are fixed for bulk metals, they are variable for colloidal metallic nanocrystals, which consist of inorganic metal cores and organic or inorganic ligand shells. By carefully choosing the ligands, the optical response of these materials can 7367
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be tuned from a dielectric to a more metallic response to light. These surface ligands can be exchanged for alternative ligand chemistries, a phenomenon explored in previous research. However, these studies have investigated properties after a complete exchange of one ligand type for another, which varies properties in a “digital” fashion rather than a more continuous “analog” tuning. Chen et al. (DOI: 10.1021/acsnano.9b02818) investigate the latter, using an unconventional ligand hybridization strategy to achieve a range of ratios of multiple ligand types. The researchers synthesized colloidal gold nanocrystals capped with a dense shell of long-chain oleylamine ligands. They then exchanged increasing amounts of these ligands with shorter-chain thiocyanate by varying reaction times between 0 and 7 min. With longer reaction times, the nanocrystals were gradually brought into greater proximity, touching and fusing with different degrees of necking. These physical changes altered the optical properties of the resulting films, with transmittance, reflectance, permittivity, and the DC electrical resistivity continuously tuned from dielectric to metallic characteristics. The researchers used this strategy to create ultrathin optical absorbers. They suggest that this strategy could be used with a variety of different colloidal nanocrystals with different properties, leading to customized materials for various applications.
technique, the researchers processed these supercapacitors in various shapes ranging from one-dimensional strips to twodimensional shapes akin to paper art to three-dimensional paper carving boxes. These devices demonstrated excellent performance with good stability and rate capability. For example, the three-dimensional device could power a Christmas-like lightemitting diode light strip. The authors suggest that these supercapacitors hold promise for a variety of portable, stretchable, and wearable electronics.
UNDERSTANDING MEMORY DISTANCE FOR NANOSCALE FRICTION Rate and state friction laws can successfully describe friction for a variety of materials. Such laws include a memory distance, Dc, which is the distance required for a population of frictional contacts to renew itself through slip, counteracting the effects of static friction increasing with contact time during slow or static contact. Although memory distance has been defined for various frictional scenarios, the physical meaning of the effective contact time remains vague. In addition, memory distance for individual asperities at the nanoscale has not been well explored. Tian et al. (DOI: 10.1021/acsnano.8b09714) seek to understand memory distance at the nanoscale, using atomic force microscopy to study interfacial chemical bond-induced kinetic friction for single silica−silica nanocontacts. The researchers found a logarithmic trend of decreasing friction with sliding velocity at low velocities and a transition to increasing friction with velocity at higher velocities. The researchers used these findings to develop an “activationpassivation loop” model, which accounts for activation and passivation of chemical reaction sites and the formation of new chemical bonds from dangling bonds during sliding. In this model, Dc is defined as the average sliding distance that accrues before an activated reaction site becomes passivated. The model, which matches experimental friction data well, suggests that Dc is sensitive to surface chemistry and nearly independent of sliding velocity. The authors suggest that these findings help establish physically based rate and state friction relations for nanoscale contacts with reactive bonding mechanisms and may be useful for developing models to help explain rate and state friction behavior for macroscale multiasperity contacts.
FROM MACRO- TO MICRO-SUPERCAPACITORS WITH A LASER FOCUS The burgeoning field of flexible and wearable electronics necessitates energy supply devices with arbitrary size and desired energy densities. One way to meet this need is through macroscopic and microscopic supercapacitors. However, both have inherent benefits and drawbacks. For example, reported macro-supercapacitors have higher energy and power densities but greater bulk and less flexibility than their microscale counterparts. Micro-supercapacitors are smaller and more flexible, but have low areal energy density. Because of the particular demands of each of these devices, no synthesis method exists that can span the entire size range. Thus, an important goal for next-generation electronics is creating an editable supercapacitor with high energy density that can span the macro-to-micro scale. Gao et al. (DOI: 10.1021/acsnano.9b02176) used a laser direct writing (LDW) strategy to craft multiscale supercapacitors. The researchers used carbon paper to support ion liquid-mediated reduced graphene oxide and to serve as the electrode material. They chose qualitative filter paper as the interlayer to improve the mechanical robustness of the supercapacitor while also preventing breakdown of the device during laser processing. Carbon paste was used as a cross-linking agent between the electrode and interlayer. Using the LDW 7368
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MORPHING AMINO ACIDS INTO MUSIC Although materials and music have been intimately connected throughout human history, today’s technologies enable blurring of boundaries between materials and sounds, translating physical or other qualities into audible representations. For example, recent work has created sonic representations of spider webs and whole proteins. These efforts provide an alternate way of understanding these materials as well as a new route toward constructing new materials. In that vein, Yu et al. (DOI: 10.1021/acsnano.9b02180) report a self-consistent method to translate amino acid sequences into audible sound. To do this, they used the normal mode vibrations of all 20 natural amino acids to compute audible representations for each one. They turned the characteristic frequency spectrum and sound associated with the amino acids into a musical scale of 20 notes, which most closely matched the C-minor scale in conventional Western music. Each note associated with the amino acids was assigned to a specific key on a piano roll, allowing the researchers to map the sequence of amino acids in proteins into a musical score. Musical rhythm was computed using the secondary structure of proteins. They then used this representation in the musical space to train a neural network, generating protein designs using artificial intelligence. The final result was multihour audible representations of natural proteins and protein-based musical compositions generated solely by artificial intelligence. The authors suggest that this approach could be used to understand sequence patterns, variations, and mutations, as well as to offer insight into protein folding and the effects of mutations through sound.
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