GIVING CRISPR/CAS9-RIBONUCLEOPROTEIN THE GOLDEN TOUCH The CRISPR/Cas9 system, derived from bacteria, has proven to be a useful tool for genome editing and transcriptional control. Its success in the lab at correcting disease-causing mutations in cells and animal models suggests enormous therapeutic promise for eventually curing human genetic diseases. However, the genedelivery strategies used thus far to insert this system into cells lead to a permanent placement of the CRISPR genes, causing unwanted gene editing over time. Although delivering the Cas9 protein along with a guide RNA offers a transient way to edit genes, the methods to deliver this combination system are generally impractical for in vivo applications. Seeking a better delivery system, Mout et al. (DOI: 10.1021/ acsnano.6b07600) grouped the Cas9 protein and guide RNA onto gold nanoparticles for co-delivery. The researchers engineered Cas9 protein with a glutamate peptide tag on the N-terminus to attach to cationic arginine gold nanoparticles. After also attaching guide RNAs to the nanoparticles, the researchers tested these assemblies’ ability to deliver Cas9 protein directly to the cytoplasm. Tests confirmed efficient delivery in HeLa cells as well as several other cell lines, including human embryonic kidney cells and mouse macrophages. The entry mechanism appears to be through a cholesterol-mediated membrane-fusion-like process. Additional experiments showed editing efficiency of up to 30% for multiple gene targets using targeted guide RNAs. The authors suggest that this system could provide a starting point for delivering the CRISPR/Cas9 system for therapeutics.
is inefficient and discontinuous due to their quick fouling with oil. Thus, more innovations will be necessary to advance this field. In a recent study, Dou et al. (DOI: 10.1021/ acsnano.6b07918) report an oil/water filter that is inspired by the crossflow filtration of gills in suspension-feeding fishes. In this filter, oil/water mixtures flow parallel to the membrane surface and through a slanted gradient membrane, with larger pores at the bottom and smaller pores at the top. The surface of this porous membrane is covered with ultrathin hydrophilic Co3O4 nanosheets. Water passes easily through the larger pores but tends to coat the surface of the smaller pores, helping oil to slide easily over the top into a collection bin. Because of the hydrophilic layer, the membrane has excellent antifouling properties. The authors suggest that this bioinspired crossflow membrane, when attached to a moving surface such as a ship’s prow, could offer a promising solution for large-scale oil spill cleanup.
SINGLE-MOLECULE REACTIONS: READY FOR THEIR CLOSE-UP Until recently, available technology has limited nearly all observations of reaction kinetics to ensemble-averaging analytical techniques. Although helpful in supporting proposed mechanisms, these techniques cannot provide confirmation or rule out alternate mechanisms that may be responsible for the same macroscale observations. Only direct observation at the single-molecule level of the reactants transforming into products over time via metastable intermediates can provide definitive information about reaction mechanisms. Newer methods, such as noncontact atomic force microscopy and aberration-corrected high-resolution transmission electron microscopy, have begun to provide insights into reactions of individual molecules adsorbed on surfaces. However, recording a “movie” of intermolecular reactions has remained a challenge.
OIL, GOING WITH THE CROSSFLOW A variety of technologies have been used to clean up oil spills, including skimmers, sorbents, controlled burning, chemical dispersion, and bioremediation. However, each of these methods suffers from low efficiency or secondary pollution. More recent efforts have focused on developing superwetting materials by manipulating their microstructures and surface chemistry for contrasting wettability toward water and oil. To this end, superhydrophilic and underwater superoleophobic membranes have been evaluated for oil/water separation. However, the perpendicular gravity-driven approach that these membranes use © 2017 American Chemical Society
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In a recent study, Chamberlain et al. (DOI: 10.1021/ acsnano.6b08228) detail an approach, termed ChemTEM, that is capable of creating the equivalent of a stop-frame video to follow chemical transformations at the single-molecule level. Using the electron beam of a transmission electron microscope (TEM), the researchers stimulated polycondensation of individual perchlorocoronene (PCC) molecules on a graphene substrate. By tuning the amount of introduced energy while taking images, the researchers were able to visualize the transformation of these disc-shaped molecules into arynes. Similar experiments with multiple PCC molecules enclosed into single-walled nanotubes, which predisposed them to intermolecular reactions, showed the molecules eventually transforming into polymeric ribbon-like products. The authors demonstrate the versatility of this technique using a second disc-shaped molecule, octathio[8]circulene. Together, these results suggest that ChemTEM has the potential to reveal atomistic mechanisms of previously unknown processes.
SHINING A NEW LIGHT ON ANTIREFLECTIVE COATINGS Many applications rely on broadband antireflective coatings (ARCs), including corrective lenses, telescopes, and flat-panel displays. Despite a variety of different fabrication methods, tuning ARCs’ refractive indices can be labor intensive, costly, and challenging. In addition, creating graded-index ARCs on surfaces with low refractive indices currently requires the initial deposition of coatings with high refractive indices, a step that is necessary because only a few materials suitable for these coatings have refractive indices less than 1.5. In a recent study, Berman et al. (DOI: 10.1021/ acsnano.6b08361) detail an approach for synthesizing coatings with finely controlled refractive indices suitable for single-layer and graded-index ARCs. Their method is based on sequential infiltration synthesis (SIS), diffusion-controlled penetration, and subsequent chemisorption of inorganic precursor molecules inside polymer templates. The researchers exposed two different block copolymer templates to vapor phase trimethylaluminum in several cycles, enabling the growth of multiple layers of Al2O3. The polymer templates were then removed by oxidative thermal annealing. Tests showed that even though these porous films had low refractive indices that were tunable by the number of cycles, their thickness was too low to provide antireflective properties in the spectral range above 200 nm. To increase thickness, the researchers swelled the polymers in ethanol before performing SIS. The resulting films were suitable for ARCs in individual layers or as graded-index ARCs when placed in stacked layers with different refractive indices. The authors suggest that this technique can be applied to a broad range of materials and could be a cost-effective alternative to other methods for creating broadband ARCs.
A GRAPHITIC MONOLAYER THAT IS THRICE AS NICE The extraordinary qualities of two-dimensional (2D) materials have stimulated research into both their properties and the possibility of synthesizing new such materials. For example, 2D hexagonal boron nitride (h-BN), a wide band gap insulator, has properties distinctly different from those of bulk BN. Similarly, graphene, atom-thick sheets of carbon, has extraordinary charge carrier mobility and processability, yet no band gap. These two materials can be combined into graphenic BCN, a ternary network with atoms of boron, carbon, and nitrogen in a 2D sheet. Although experimental efforts have successfully led to BCN phases that are not low-dimensional or modified graphene, they have thus far failed at creating true graphenic BCN. In a recent study, Beniwal et al. (DOI: 10.1021/ acsnano.6b08136) report the successful synthesis of 2D hexagonal graphenic BCN monolayers (h-BCN). To create this material, the researchers relied on bis-BN cyclohexane (B2N2C2H12) as a precursor. In ultrahigh vacuum, they exposed atomically flat Ir(111) substrates to bis-BN vapor, causing graphenic BCN to form through a thermal dehydrogenation of the precursor. Increasing the substrate temperature to between 400 and 1000 °C during exposure led to improved ordering in the layer that forms on top. A lattice mismatch between the substrate and this layer led to film buckling and moiré patterns. First-principles calculations predict a direct electronic band gap between that of graphene and h-BN. The authors suggest that hBCN monolayers are exciting candidates for useful 2D electronic materials. 2310
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flexible, nine amino acid peptide bradykinin on Cu(110) and Cu(100) surfaces as their model systems, this technique enabled the researchers to investigate the resulting conformations of this polypeptide using high-resolution STM. Their findings show that on the Cu(110) surface, isolated adsorbates adopt either one of two major classes of conformation or elements from both. On the Cu(100) surface, only a single conformation consisting of a folded dimer was possible. The authors suggest that folding at surfaces may be a promising path to sequence-programmable functional nanostructures.
DIAMOND IN THE ROUGH: ENGINEERING PROTEIN CRYSTALS INSIDE CELLS Several known proteins are spontaneously crystallized in cells, a phenomenon that has attracted attention in the fields of structural biology and nanotechnology. These protein crystals are porous materials with highly ordered three-dimensional (3D) arrangements of protein molecules, providing a wide range of pore sizes and large surface areas. Consequently, they have potential in a variety of applications in molecular separation, gas storage, and structural analysis. The solvent channels of these 3D protein crystals have been studied as promising scaffolds for preparation of inorganic materials, heterogeneous catalysis, and drug-delivery systems. For many of these applications, the ability to apply protein-engineering technologies to protein crystallization will be key. In a recent study, Abe et al. (DOI: 10.1021/acsnano.6b06099) demonstrate a strategy for extending the porous networks in polyhedra crystals (PhC), a type of protein crystal produced by cypovirus in infected insect cells, in crystals grown in living cells. The wild type form of PhC (WTPhC) has solvent channels with low porosity. By deleting selected amino acid groups located on the intermolecular contact region of the monomer units that form these crystals, the researchers perturb hydrogen bonds, leading to highly porous networks while retaining the original crystal lattice structures. These porous crystals readily retained fluorescent dyes both when purified outside cells and while still contained inside cells, whereas WTPhC could not. These results suggest that it is possible to engineer PhC in living cells for a variety of promising applications.
TURNING UP THE HEAT ON AMORPHOUS SILICON NANOSTRUCTURES Amorphous Si (a-Si) nanostructures play important roles in a variety of electronic and optoelectronic devices. Thermal management is important for their performance, reliability, and lifetime. Thermal transport behavior in a-Si and other amorphous materials has traditionally been described by the “amorphous limit”, a concept that dates back to Einstein’s work in 1911 and refers to the lowest thermal conductivity (κ) that can be reached. Although this topic has been well researched, recent measurements for a-Si suggest that κ might be higher than the amorphous limit of ∼1 W·m−1 K−1. In a-Si films greater than 1 μm thick, κ measurement can be higher than 3 W·m−1 K−1, with extra thermal conductivity believed to be contributed by propagating vibrational modes known as “propagons”. However, precisely determining κ in a-Si has been challenging. In a recent study, Kwon et al. (DOI: 10.1021/ acsnano.6b07836) used a-Si nanotubes and suspended a-Si films to measure in-plane thermal conductivity precisely within a wide range of thickness between 5 nm and 1.7 μm. Their measurements for in-plane thermal conductivity were unexpectedly high: ∼1.5, ∼3.0, and ∼5.3 W·m−1 K−1 for thicknesses of ∼5 nm, ∼100 nm, and ∼1.7 μm, respectively. These values were significantly higher than those for cross-plane measurements on the same materials. In addition, these measurements suggest that propagons contribute significantly to κ of a-Si films with thicknesses down to 5 nm, much smaller than the lower bound of 100 nm suggested by previous research, which primarily used cross-plane measurements. The authors suggest that these results could shed light on the design and performance of numerous aSi-based devices.
ABOVE THE FOLD: PEPTIDE NANOSTRUCTURES ON SURFACES Each protein, composed of a flexible sequence of the 20 native amino acids, has just a specific, functional conformation as well as a folding pathway to reach it. Within this conformation, covalent and various noncovalent bonds provide stability and an adaptable structure, enabling highly specific induced-fit binding. Inspired by nature, researchers have sought to replicate this phenomenon with self-assembly of complex molecular architectures, a promising technology for surface functionalization. Such structures, which are fabricated by vacuum sublimation onto defined surfaces, have the advantage of enabling investigation by scanning tunneling microscopy (STM) at sub-nanometer resolution. However, self-assembly by molecular folding in this type of environment has never been reported. Because of the size and functionality necessary for folding, such molecules are not volatile and cannot be applied to surfaces through vacuum sublimation. In a recent study, Rauschenbach et al. (DOI: 10.1021/ acsnano.6b06145) used a technique called soft-landing electrospray ion beam deposition (ES-IBD), which gently deposits large, nonvolatile molecules on surfaces in a vacuum. Using the 2311
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LOW THRESHOLD, BIG OPTICAL GAIN FOR CDSE/CDTE NANOPLATELETS The unique properties of colloidal cadmium chalcogenide nanoplatelets (NPLs), including uniform one-dimensional quantum confinement, high luminescence quantum yield, and giant oscillator strength, can be further improved in NPL heterostructures with type-I or type-II band alignment, such as CdSe/CdTe core/crown NPLs. In these type-II heterostructures, both the conduction band and valence band of CdSe are lower than those of CdTe, causing excitons to form long-lived charge transfer states across the interface between CdSe and CdTe. The resulting long exciton lifetime, combined with their large absorption cross-section, could make these materials promising for low-threshold multiple exciton lasing. To investigate this possibility, Li et al. (DOI: 10.1021/ acsnano.6b08674) studied the energetics, spatial distribution, and dynamics of up to six exciton states in these materials. Using excitation fluence and wavelength-dependent ultrafast transient absorption spectroscopy, the researchers identified three different types of excitons: XCT, with an electron in the CdSe core bound to a hole in the CdTe crown; XCdTe, localized to the CdTe crown; and XCdSe, localized to the CdSe core. Each of these exciton levels can accommodate up to two excitons, sequentially filling at high excitation energy and flux to generate the six exciton states. Further investigation showed that optical gain can be achieved in tri- (two XCT and one XCdTe) or four- (two XCT and two XCdTe) exciton states, with over 40-fold lower optical gain threshold compared to single-exciton gain threshold in typeII core/shell quantum dots. Together, these findings confirm the potential of these materials for low-threshold lasing.
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