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PACKAGING FOOD IN “CRAB-ON-A-TREE” Plastics have long been the go-to packaging materials for food due to their versatile capacity to protect their contents from mechanical, microbial, and chemical damage. Because of their low cost and good moldability, poly(ethylene terephthalate) (PET) and polyethylene are often used for this purpose. However, these materials are mostly permeable to oxygen, which degrades food quality over time. To improve plastics’ gas barrier, various coatings have been tested, including metals, metal oxides, and chlorinated polymers. However, these additions have various drawbacks, including environmental or health concerns and blocking visible light or electromagnetic signals that can be useful for tracking purposes. Thus, nextgeneration barrier materials that are renewable, biofriendly, easily processable, and transparent to visible light and radio frequencies will be necessary. Toward those goals, Kim et al. (DOI: 10.1021/acsnano.8b08522) developed a coating for commercial PET film that is based on chitin nanowhiskers (CNWs) and cellulose nanofibers (CNFs). The researchers synthesized both nanostructures, with negatively charged CNFs being considerably longer than positively charged CNWs. Using a sprayassisted layer-by-layer assembly method, they found that the CNFs provided a rigid framework with the smaller CNWs filling holes and voids. Together, this barrier reduced the oxygen transmission rate to as low as 0.5 mL m−2 day−1. The coating retained its high performance under wet and humid conditions, bending stress, aging, freezing, and exposure to ethanol, and experienced only slight performance loss when heated in a microwave oven. In addition, the film is light, radio frequency transparent, and inhibits bacterial growth. The authors suggest that it has significant promise for food packaging applications.
nonabsorbable membranes require a second surgery for removal that can damage healing tissues and absorbable membranes have limited control over degradation rates, poor mechanical strength, and the inability to maintain space. Hasani-Sadrabadi et al. (DOI: 10.1021/acsnano.8b09623) overcome these disadvantages, reporting a periodontal membrane based on electrospun poly(ε-caprolactone) (PCL) with a polydopamine (PDA) coating. The researchers formed some of these membranes into meshes for morphological patterning. By incorporating other polymers, the researchers were able to tune the membranes’ degradation time and mechanical properties. Tests showed that the PDA coating encouraged cells to adhere and to produce more biomineralization without affecting their viability. In vivo experiments in rat models of periodontal disease showed significantly higher levels of bone regeneration in animals with the PCL−PDA membranes compared to untreated animals and those with PCL membranes alone. The authors suggest that these membranes could be effective tools to help heal periodontal disease in patients.
TEXTILE SENSORS TO DYE FOR Wearable electronics in the form of textiles (e-textiles) have significant potential for applications in healthcare, sportswear, fitness, space, and the military. These garments have considerable requirements: in addition to the need for electronics and sensing capabilities, they must also be breathable, washable, flexible, wearable, and produced using environmentally friendly manufacturing processes. Although several techniques have been investigated for producing metalbased electroconductive yarns for this purpose, their manufacturing processes have thus far proven to be expensive, unscalable, and environmentally unfriendly. Seeking a better way to make e-textiles, Afroj et al. (DOI: 10.1021/acsnano.9b00319) developed a method to “dye” commercially available cotton yarn by coating it with reduced graphene oxide (rGO) flakes. The researchers first engineered graphene flakes, testing various reducing agents and surface functionalizations to optimize their conductivity and dispersibility. They then used a commercial batch dyeing technique, known as exhaust dyeing, to coat the yarn with rGO flakes quickly, a technique that can efficiently dye tons of yarn in
HELPING PERIODONTAL TISSUE HEAL Periodontal disease affects up to half the world’s population, in developed and developing countries alike. Periodontitis, the most serious form, can lead to progressive loss of periodontal attachment and surrounding bone loss, as well as early loss of teeth, if untreated. Various repair strategies, including guided tissue regeneration (GTR), have shown inconsistent results. To improve outcomes for GTR, some studies have tested the use of periodontal membranes to prevent the junctional epithelium from migrating and to encourage periodontal ligament and bone tissue to repopulate the tooth root surface. Commercial membranes have a host of disadvantages: © 2019 American Chemical Society
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Published: April 23, 2019 3746
DOI: 10.1021/acsnano.9b02653 ACS Nano 2019, 13, 3746−3749
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Cite This: ACS Nano 2019, 13, 3746−3749
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minutes. This conductive yarn retained its properties after multiple washes and bending. Knitting the yarn into a fabric, the researchers demonstrate that it can serve as the base of a flexible temperature sensor. They suggest that this sensor could be integrated with a self-powered radio-frequency identification tag or low-powered Bluetooth to send data wirelessly to a device. This highly scalable production process, they add, could be an important step toward realizing practical e-textiles for dozens of applications.
DYNAMIC PLASMONIC PIXELS SEE THE LIGHT Liquid crystal displays (LCDs) are a mainstay for watches, calculators, and other electronics, presenting information by controlling the amplitude, phase, and polarization of light. However, LCDs come with some drawbacks, such as relatively slow switching speeds that can result in undesirable motionblur artifacts. One emerging alternative is displays based on plasmonic nanomaterials. Although several studies have used the wealth of plasmonic materials and geometric architectures to create vibrant static images, dynamic plasmonic color displays have been more challenging to realize. Although several strategies have been applied to achieve this goal, these approaches still suffer from drawbacks including slow switching times, small amplitude changes, or narrow wavelength tunability. Trying a different strategy, Greybush et al. (DOI: 10.1021/ acsnano.9b00905) report spatial, spectral, and temporal control of light using dynamic plasmonic pixels formed by aligning plasmonic nanorods with electric fields. The researchers placed polymer-coated gold nanorods suspended in a nonpolar organic solvent between two transparent conducting electrodes. Applying a voltage across the electrodes caused these nanorods to align, changing their collective optical properties. The researchers were able to modify the colors they produced by changing their dimensions or their chemical compositions, for example, blue-shifting the light by coating the gold nanorods with a silver shell. Nanorod concentration affected both luminance and chromaticity values. The researchers demonstrated these effects by creating a seven-segment numerical indicator. They suggest that these dynamic plasmonic pixels, controlled by electric fields, could offer a platform for engineering high-performance optical devices.
MAKING MICELLAR ARRAYS WITH OPTICAL TWEEZERS In recent years, solution-phase self-assembly of block copolymers has been used to produce a variety of formulations and morphologies of soft-matter nanostructures. This bottomup method holds significant promise for developing materials for applications in optoelectronics, biomedicine, catalysis, and other fields. However, applying block copolymer micelles has been limited by difficulties in controlling the transfer of these structures into the solid state. In particular, it will be critical to develop methods that enable fast and efficient manipulation of these materials into large-scale arrays with controlled density and placement. For better control of these soft nanostructures, Gould et al. (DOI: 10.1021/acsnano.9b00342) looked to optical tweezers, a technique that uses light to manipulate dielectric objects. The researchers used this method on cylindrical and larger dandelion-shaped hybrid block copolymer micelles with a polyferrocenylsilane core, a material with a high refractive index suitable for optical manipulation. The researchers used total internal reflection microscopy to visualize these dyefunctionalized micelles. By creating an optical trap in close proximity to particles, the researchers were able to pull individual micelles into the trap and to hold them stable in three dimensions. Translating the stage enabled them to control the trapped micelles’ positions and deposit them onto substrates, creating customized arrays. By creating a program that automatically identifies, traps, and deposits multiple assemblies simultaneously, the researchers were able to hasten this process dramatically, shortening its time scale from hours to minutes. The authors suggest that this technique could be the key for creating micelle-based devices with a wide variety of potential applications.
GIVING BOROPHENE SYNTHESIS THE GOLDEN TOUCH Recently, the discovery of two-dimensional boron polymorphs, known collectively as borophene, has added to the intriguing body of boron chemistry. Theory suggests that these materials 3747
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lithography, the researchers found that it can act as an easily removable spacer that protects films from the solvents used to process SU-8, a commonly used photoresist. The researchers demonstrate this technique on a highly emissive perovskite film with potential applications for a perovskite LED display. Tests showed that this photolithography process only slightly affected the photoluminescence quantum yield of the film, and the resulting features are sufficiently flat to be incorporated into monolithic electronic devices. This technique proved to be viable for single-color patterns, with a variation capable of crafting multicolor displays. The authors suggest that their method could enable a wealth of potential applications for this class of materials.
could have useful characteristics, including a tensile strength comparable to graphene and a relatively high superconducting transition temperature. Because they do not have a bulk layered counterpart and thus cannot be isolated through exfoliation methods, it is critical to understand their growth. Borophenes were initially synthesized on Ag(111), but theoretical modeling suggests that stable growth is possible on other surfaces, including Au. Testing this idea, Kiraly et al. (DOI: 10.1021/acsnano.8b09339) performed an ultrahigh vacuum synthesis and characterization of borophene on Au(111). Their studies show that low-temperature deposition from an atomic source results in boron clusters confined to the surface. However, at high substrate temperatures, these boron clusters are no longer present, and the Au(111) surface switches from a herringbone reconstruction to a trigonal network with nanoscale borophene islands. Microscopy, X-ray photoelectron spectroscopy, and theoretical modeling studies suggest that the reason for this shift lies in the location for the majority of the boron: dissolved into the bulk of the gold substrate. As the boron dose increases, these islands gradually grow and coalesce into borophene sheets. Time-of-flight secondary ion mass spectrometry further confirmed that at elevated temperatures, boron diffuses deeply into the gold subsurface region. Further theoretical modeling and experimental evidence suggests that the nature of this borophene layer is metallic. This growth method, the authors say, could produce materials with applications in two-dimensional plasmonics, superconductivity, interconnects, electrodes, and transparent conductors.
TAKING ADVANTAGE OF THE LIQUID−SOLID TRANSITION Liquid behaviors of material precursors have served as powerful tools to control the placement of anisotropic microstructures by aligning them in the direction of shear stress using dynamic fluid channels. However, with static liquid droplets, researchers can take advantage of surface tension to form specific shapes with minimal surface locally and to confine and to guide the assembly process of nonvolatile components as the droplets dry. Although both processes are useful, it has been impossible to take advantage of the benefits of both dynamic and static liquid behaviors in the same compound due to their conflicting states. Huang et al. (DOI: 10.1021/acsnano.9b00551) show that this contradictory combination is possible with the aid of hybrid hydrogels. The researchers synthesized these hydrogels by polymerizing acrylamide with phenylboronic acid/catechol complexes as cross-linkers in the presence of cellulose nanocrystals (CNCs). With uniaxial stretching, the scientists generated dynamic liquid behaviors that aligned the embedded CNCs. This alignment is preserved in the relaxed hydrogel networks due to the hydrogels relaxing much faster than the CNCs after unloading the external force. The drying process compounded this alignment, with the surface tension of the hydrogels further enhancing the orientation index of the CNCs. These anisotropic structures demonstrated highly tunable birefringence. The researchers show that the final xerogels can be shaped into fibers, films, or even complex three-dimensional structures without any external molds. They suggest that these materials could find diverse uses such as a yarn to prepare tough but flexible textiles with hidden patterns, or lightweight engineering materials with extreme load-bearing capabilities.
HYBRID PEROVSKITES: MAKING TOP-DOWN PATTERNING A REALITY Lead-halide perovskites’ excellent optoelectronic properties and ability to be incorporated into devices using solution processing have given them promise in a range of applications, including solar cells, light-emitting diodes (LEDs), photodiodes, light-emitting field-effect transistors, and lasers. Using them in commercial products such as multicolor displays or camera modules requires patterning perovskites into arrays of individual devices rather than bulk films. Top-down lithography, particularly photolithography or deep UV/electronbeam lithography, is the preferred fabrication technique in industry due to the ability to pattern large areas quickly, economically, and with high reproducibility. However, these techniques have largely been considered off limits due to hybrid perovskites’ incompatibility with solvents used during the lithography processes. Harwell et al. (DOI: 10.1021/acsnano.8b09592) found a way around these limitations by developing a technique that protects hybrid perovskite films from solvents with a commercially available resist. Although poly(methyl methacrylate) (PMMA) is an ineffective photoresist for top-down 3748
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GAINING CONTROL OVER QUANTUM DOT LIGANDS Quantum dots and other colloidal nanocrystals have attracted increasing attention both for fundamental study and for potential applications, including in transistors, solar cells, quantum computers, and medical imaging. Although early work focused on controlled synthesis of the shape, size, and composition of the inorganic core and surface chemistry, it has become increasingly clear that the ligand shell is pivotal for applying these materials; the surface of the quantum dot is key to electronic coupling between quantum dots to make conductive films, and ligand−shell vacancies can be the source of electronic defects that degrade performance. However, ligand binding and exchange isotherms on nanocrystal surfaces remain underexplored. To fill this deficit, Bronstein et al. (DOI: 10.1021/ acsnano.9b00191) examined in situ ligand exchange isotherms using simple linear absorption spectroscopy. Using PbS quantum dots, the researchers performed a solution-phase Xtype ligand exchange to replace the native oleate ligands with cinnamic acid derivatives to form cinnamate passivated PbS quantum dots. They found that the ligand dipole moment significantly impacts the ligand-exchange isotherms. Specifically, ligands with large electron withdrawing character create a sharper transition from an oleate-dominated ligand shell to a cinnamate-dominated one. To understand this phenomenon, the scientists ran simulations in a two-dimensional lattice model. Results showed that ligands with larger ligand−ligand coupling energy had sharper isotherms, indicating an order− disorder phase transition. The researchers used these findings to develop quantum dots with anisotropic Janus ligand shells. They suggest that finding further ways to control the electrostatics, morphology, and symmetry of the ligand shell could have considerable influence over their emergent properties.
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