HIGHER ORDER ASSEMBLY: ARRANGING VIRUS-LIKE PARTICLES INTO CUBES Self-assembly remains an active area of research for creating functional constructs from nanoscale building blocks. Nature is an inspiring source for the design of these materials. For example, bacterial compartments called carboxysomes, assembled from nanoscale protein subunits, encapsulate a series of enzymes for segregating chemical reactions, leading to improved catalytic efficiencies. Viral capsids, also composed of nanoscale protein subunits, have also been used to build virus-like particles (VLPs) devoid of the viral genome for cargo encapsulation. However, these two concepts have not been merged to create hierarchically assembled compartments to facilitate multistep catalysis. Uchida et al. (DOI: 10.1021/acsnano.7b06049) bring this idea to fruition by assembling VLPs loaded with enzymes in threedimensional arrays. Using VLPs derived from bacteriophage P22, the researchers created surface modifications by including a small peptide repeat on the C-terminus of each P22 subunit. This alteration generated sterically repulsive interactions between the assembled VLPs. When these modified VLPs were mixed with a positively charged dendrimer in an ionic solution, the VLPs assembled into superlattices with face-centered cubic structures. By loading up some of these modified VLPs with ketoisovalerate decarboxylase and others with alcohol dehydrogenase A, both involved in a two-step reaction that produces isobutanol, the researchers generated nanoreactors that effectively catalyzed this reaction. The authors suggest that these findings represent a step toward creating hierarchies of materials across length scales with functions that arise from the interaction between individual building blocks.
started with model building blocks with a Au nanorod core and a Ag shell. Thiolated polystyrene was used as a capping agent to assemble nanocrystals at the air−water interface through a twostage drying-mediated self-assembly process. After drying, the polystyrene brushes collapsed to form nanosheets with the core− shell particles embedded. By using gentle etchants to remove the Ag shell while retaining the Au seed, these assemblies could be reversibly transformed into second-generation sheet-like nanoassemblies. Additional reactions could transform the secondgeneration products into third-generation ones with building blocks of different shapes, such as Au nanobipyramidal cores and Ag shells or a sphere-like Au core and a Ag shell nanocube, or could incorporate different elements altogether. This shape transformation process was so robust that it could be achieved in a freestanding suspended system and three-dimensional origami. The authors suggest that this strategy could be used to design a rich library of switchable metamaterials and devices.
MULTIPLEX BIOASSAY WITH A SINGLE DROPLET Droplet-based microfluidic systems hold decided advantages over conventional microfluidic devices with fluid channels: efficient biological and chemical analyses with small amounts of samples and simple reagents and no complex channel networks, pumps, or tubing systems. Droplet-based systems have thus far used a variety of approaches to control samples on substrates, such as surface energy gradients, electrowetting, electromagnetic force, light, and acoustic waves. However, these methods typically require additives to enhance the droplets’ response to applied forces, making these systems unusable for many analytical applications. Although superhydrophobic surfaces have been explored as an alternative to manipulation methods that rely on additives, these systems have not demonstrated the full range of manipulations necessary for droplet analysis, such as transporting, mixing, and dispensing. Han et al. (DOI: 10.1021/acsnano.7b05826) sought a better way to utilize extremely water-repellent surfaces in droplet-based microfluidics. The researchers made a stretchable superamphiphobic surface by spray coating a double layer of adhesive and fluorinated silica nanoparticles on a thin and flexible polydimethylsiloxane (PDMS) sheet. They then patterned regions of superhydrophilicity on the PDMS by treating it with oxygen plasma and a shadow mask, adding reagents in these regions for colorimetric assays. By manipulating droplets over the
SELF-ASSEMBLED, BUT NOT STUCK Self-assembly has emerged as a useful approach for fabricating nanocrystals using building blocks such as molecules, DNA, or polymers. Thus far, this strategy has been used to achieve large-scale syntheses of superlattices, freestanding sheets or membranes, binary systems, orientation control, and origami. However, each of these assemblies represents a single generation of “frozen” structures in which static building blocks and their resulting assemblies are not transformable into new shapes or compositions. Changing this paradigm, Shi et al. (DOI: 10.1021/ acsnano.7b08334) demonstrate that it is possible to transform self-assembled nanosheets into other types of structures through the unique properties of their building blocks. The researchers © 2018 American Chemical Society
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DOI: 10.1021/acsnano.8b01195 ACS Nano 2018, 12, 904−907
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from the radiative recombination of sp-band electrons and d-band holes, the main resonance originates from both radiative interband and intraband transitions. The authors suggest that the photoluminescence of plasmonic nanostructures might be used as a quantitative tool to probe plasmon-generated hot carriers and their energetic relaxation, including electron-transfer pathways in heterogeneous photocatalysis.
superhydrophobic regions onto each superhydrophilic region, the researchers were able to test various types of droplet samples for analytes such as glucose, uric acid, and lactate. As proof of principle, the researchers used the system to determine glucose concentrations in the plasma of healthy and diabetic mice. The authors suggest that this system could find broad applications in the biomedical field.
GIVING TWO-DIMENSIONAL NANOSTRUCTURES A NEW TWIST Two-dimensional (2D) transition-metal dichalcogenides (TMDs) have attracted increasing attention due to their interesting properties, including direct band gaps, conductivity, flexibility, transparency, and large surface area. MoS2, in particular, has potentially useful semiconducting characteristics such as a band gap energy that is dependent on layer thickness and transitions for an indirect-to-direct band gap in few-layer MoS2. One proposed application of 2D MoS2 is in the emerging field of valleytronics, which uses the wave quantum number of an electron in a crystalline material to encode data by controlling the photon angular momentum through circular polarized light. Although chiral MoS2 would facilitate this use, no chiral 2D TMDs have been reported. Purcell-Milton et al. (DOI: 10.1021/acsnano.7b06691) demonstrate that producing chiral 2D MoS2 is possible through a facile method. The researchers prepared this material by liquid exfoliation in water, a common method for producing 2D flakes of this material from bulk MoS2. However, they performed this technique in the presence of either of two chiral ligands: cystein and penicillamine. They then used a variety of analytical techniques to characterize the resulting materials, including UV−vis absorption spectroscopy, circular dichroism (CD) spectroscopy, diffuse reflectance infrared Fourier transform, X-ray photoelectron spectroscopy, Raman spectroscopy, transmission electron microscopy, and scanning transmission electron microscopy. They also performed theoretical modeling. Their findings suggest that the chiral ligands caused preferential folding of the MoS2 sheets, imbuing them with chirality. The authors suggest that chiral MoS2’s high aspect ratio planar morphology could be a boon for developing chiroptical sensors, materials, and devices for valleytronics as well as having applications in nanobiotechnology, nanomedicine, and nanotoxicology.
SEEING THE LIGHT WITH HOT CARRIERS The surface plasmon resonance of metal nanoparticles can be engineered to absorb photons at desired wavelengths. The resulting photon excitation produces hot carriers generated through plasmon decay. Hot carriers created either through interband excitation or by plasmon excitation and decay are typically viewed as intermediate states in energy relaxation pathways, and plasmonic modes are considered to be the main photoluminescence channels for enhancing emission through an antenna effect. However, this view has been heavily debated, with arguments that emissions from metal nanoparticles actually arise from electronic Raman scattering rather than from the radiative recombination of hot carriers. To help resolve this controversy, Cai et al. (DOI: 10.1021/ acsnano.7b07402) used a combination of experiments and modeling. The researchers collected the photoluminescence spectra and quantum yields for 80 individual Au nanorods exposed to five different excitation wavelengths. They then created simulated photoluminescence spectra using finite difference time domain simulations. Their findings suggest that the photoluminescence of Au nanorods can be viewed as a Purcelleffect-enhanced spontaneous emission of hot carriers. The simulated spectra captured both a main peak following the scattering line shape and a weaker, shorter-wavelength close to the excitation energy. Whereas the latter emission appears to stem
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GIVING GENE EDITING A MORE SPECIFIC TARGET The CRISPR/Cas9 system has arisen in recent years as a way to edit genomes in a precise and sequence-dependent manner with high specificity, efficiency, and simplicity. Consequently, researchers have used it to induce point mutations, gene deletions and insertions, chromosomal translocations, and other DNA manipulations. To improve the clinical potential of this tool, many studies have explored using nonviral vectors to deliver CRISPR/Cas9 in vivo, such as plasmid, mRNA, or ribonucleoprotein carriers. Although these systems have shown success, a significant problem that still remains for CRISPR/Cas9 is indiscriminate delivery, which could cause offtarget effects. To solve this issue, Luo et al. (DOI: 10.1021/acsnano.7b07874) added a tissue-specific promoter to a CRISPR/Cas9 plasmid for specific in vivo gene editing in target cells. The researchers replaced the original chicken β-actin promoter in two plasmids, one marked with a fluorescent tag, with the human CD68 promoter, which drives specific gene expression in monocytes and macrophages. The researchers then encapsulated these plasmids in cationic lipid-assisted PEG-b-PLGA nanoparticles. When this system was delivered systemically in mouse models, Cas9 was specifically expressed in monocytes and macrophages but not in other cell types. As proof of principle, the researchers used this system to disrupt the Ntn1 gene in the macrophages of a mouse model of type 2 diabetes, which increased glucose sensitivity while minimizing off-target effects. The authors suggest that this strategy provides a promising avenue for using CRISPR/Cas9 in targeted cells and tissues.
abundant surface functionality without mutual interference. Tests showed that the charge carrier transport behavior of the channels was always hindered when a gas molecule, such as acetone, ethanol, ammonia, or propanal, was adsorbed. The sensitivity of this device was below 1000 ppb for these gases, with calculations suggesting a subppb limit of detection for acetone and ammonia. When compared with other types of gas sensors constructed with two-dimensional materials, Ti3C2Tx had the lowest noise level. The authors suggest that this study could open the door for other MXenes to be employed as highly sensitive sensors.
GOING BIG, GOING BLUE: LARGE-SCALE WSE2 ON SAPPHIRE Two-dimensional (2D) semiconducting transition-metal dichalcogenides (TMDs) have a variety of characteristics that are potentially useful to the electronics industry. As monolayers, these materials enable atomic level scaling and excellent electrostatic gate control. Consequently, researchers have developed several different methods to synthesize TMDs as mono- or fewlayer constructs. The powder vapor method has benefits including ease of preparation and setup, making it the fastest and most convenient route for obtaining materials for proof-ofconcept demonstrations. However, its limited scalability and lack of precursor control make it unusable for large-area commercial applications. Metal−organic chemical vapor deposition (MOCVD) can produce wafer-scale films. However, the carbon-containing metal−organic precursors used to make these materials often leave particulate accumulation on the surface and carbon between the TMD and the substrate. Seeking a better method, Lin et al. (DOI: 10.1021/acsnano. 7b07059) developed a way to grow large-area WSe2 by tweaking several different aspects of typical MOCVD methods. The researchers discovered that using hydrogen selenide rather than dimethyl selenium as the selenium precursor led to conformal WSe2 growth over a sapphire surface. In addition, annealing the sapphire substrate and having a high growth temperature was necessary for epitaxial growth. Field-effect transistors constructed from this resulting transfer-free epitaxial WSe2/sapphire exhibit ambipolar behavior with excellent on/off ratios, high current density, and good field-effect transistor mobility at room temperature. The authors suggest that these findings are applicable to other TMDs and could help to guide and to stimulate
HIGH SIGNAL, LOW NOISE VOLATILE ORGANIC COMPOUND DETECTION Detection of extremely low levels of volatile organic compounds (VOCs) is important for a variety of health-related applications, such as therapeutic diagnosis by breath analysis. For example, ammonia present in breath at 50−2000 parts per billion (ppb) can signal the presence of peptic ulcers caused by Helicobacter pylori or end-stage renal failure, and acetone at the 300−1800 ppb level can signal diabetes. Although highly reactive gases such as H2S and NO2 can be sensitively detected with relative ease, finding materials that can detect VOCs at low levels at room temperature has been more challenging. Such a sensor needs to have both low electrical noise, induced by high conductivity, and high signal, induced by strong and abundant analyte adsorption sites. However, these characteristics are mutual trade-offs. To get around this conundrum, Kim et al. (DOI: 10.1021/ acsnano.7b07460) developed a sensor based on a Ti3C2Tx MXene film for channels. Because MXenes possess metal conductivity while the outer surface is fully covered with functional groups, it is possible to achieve both metallic conductivity and 906
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research in synthesis and transport of other 2D epitaxial layers for electronic applications.
THIN SURFACE, WIDE ABSORPTION Plasmonic devices rely on the capacity to concentrate and to deliver electromagnetic energy at the nanoscale. Graphene’s ability to confine plasmons spatially is extremely desirable for this purpose, but it also limits the capacity to access and to tailor these modes. Researchers have solved this problem in the past by using local probes or making use of subwavelength periodic arryas of patterned graphene structures and gratings. These periodic arrays and gratings have also been proposed as metasurfaces, enabling the manipulation of radiation with a single atomic layer in the THz regime. Galiffi et al. (DOI: 10.1021/acsnano.7b07951) take this work a step further by developing a singular graphene metasurface composed of a graphene sheet periodically doped along one spatial dimension to form a subwavelength grating with vanishing Fermi level at the minimum grating points. Starting with a simple, flat, vertical slab of plasmonic material, the researchers used transformation optics to design a singular plasmonic graphene metasurface with broadband spectral absorption. Although conventional gratings only present a series of discrete absorption peaks, analytical techniques showed that gratings on the graphene surface can strongly couple incident radiation into extremely confined surface plasmon modes, achieving absorption levels higher than 70% of the theoretical maximum over a frequency band whose width is 185% of the central frequency. The authors suggest that because previous studies have shown that the conductivity can be modulated at GHz frequencies, these new findings might lead to the development of efficient, highspeed, broadband switching by this atomically thin layer.
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