THE SWEET TASTE OF BIOMIMETIC SUCCESS The taste receptors on human tongues can recognize five basic tastes: sweet, umami, bitter, salty, and sour. Of these, sweet and umami are particularly important, with umami guiding humans to consume important amino acids and sweet doing the same for various sugars that are key for energy production. Over the past decade, several groups have attempted to develop detection tools for these two tastes using molecular biology and electrochemistry. Although electronic tongues have had some success, they have not previously been selective enough to mimic humans’ taste system truly and could not recognize unknown compounds. In their study, Ahn et al. (DOI: 10.1021/acsnano.6b02547) developed a duplex bioelectronic tongue (DBT) that can accurately detect both umami and sweet tastes using graphene field-effect transistors. The researchers expressed the monomeric components of the heteromeric umami and sweet taste receptors on the surfaces of human cells and then harvested these proteins by destabilizing the membranes, creating nanovesicles. They then attached these nanovesicles to two channels on a micropatterned graphene field-effect transistor. By observing changes in the current, the researchers could detect the presence of target tastants. This system successfully registered not only individual umami and sweet tastants but also complex mixtures such as tomato juice and green tea. The system’s response was appropriately stronger for stronger tastes, such as the sweetness of artificial sugar. The authors suggest that this platform overcomes the limitation of previously developed artificial taste sensors, giving it promise in the food and beverage industry.
In a recent study, Lakshmanan et al. (DOI: 10.1021/ acsnano.6b03364) demonstrate another advantage to GVs: their capability for biochemical and genetic engineering. Working with those isolated from the cyanobacterium Anabaena flosaquae, the researchers modified a GV shell protein, known as gas vesicle protein C (GvpC), by removing native GvpC with urea and replacing it genetic variants. This method produced GVs with a range of collapse pressure, allowing for multiplexed imaging, and enabled the researchers to tune the harmonic ultrasound signals of these GVs, improving their ability to be distinguished from background tissues. Further genetic engineering adjusted other characteristics, including surface charge, targeting specificity, and the ability to carry tags for multimodal imaging. The authors suggest that these versatile contrast agents could find use both within and outside biology and medicine.
RIVETING NEW GRAPHENE RESEARCH Graphene’s extraordinary mechanical, electronic, and optical properties give it promise for a variety of applications in flexible electronics, high-frequency transistors, logic devices, and other fields. Most large-area graphene is currently grown on transition metal substrates using chemical vapor deposition, then transferred to other substrates using a stronger polymer overlayer. However, this method leaves unavoidable polymer contaminants that diminish graphene’s electrical properties. The resulting large-area monolayer graphene is also fragile, cracking easily. To ameliorate these problems, researchers recently developed rebar graphene, in which carbon nanotubes interconnect in an in-plane network to reinforce monolayer graphene, allowing for polymerfree transfer and a material strong enough to float on water. However, anchoring transition metal nanoparticles on rebar graphene, a prerequisite to some applications, remains a challenge. In a recent study, Li et al. (DOI: 10.1021/acsnano.6b03080) report a method to attach Fe nanoparticles to rebar graphene by encapsulating them in carbon nano-onions, a combined material that they call “rivet graphene” because the nanoparticles mimic the rivet joints in metals. The researchers first synthesized rebar graphene, then spin-coated Fe3O4 nanoparticles on the surface. After annealing, these nanoparticles became covered with layers of graphene and adhered to the rebar graphene surface through covalent bonding. Besides requiring no polymer overlayer and being strong enough to float on water, this material exhibited high optical transparency, excellent electric conductivity, and
NEW ULTRASOUND CONTRAST AGENTS BUBBLING TO THE SURFACE Ultrasound’s high spatiotemporal resolution, safety, cost, and ease of use have made it one of the most widely used biomedical imaging modalities. Besides visualizing anatomy and physiology, ultrasound can also be used to image blood flow, to locate certain molecular targets, and to resolve structures beyond its normal wavelength limit via superlocalization through the use of contrast agents. However, the “microbubble” contrast agents currently in use, composed of micron-sized bubbles of gas stabilized by a biocompatible shell, are limited as molecular reporters because of their size and physical instability. As an alternative, researchers recently reported the use of gas vesicles (GVs) as a new class of ultrasound contrast agents. These gas-filled, protein-shelled nanostructures, derived from bacteria and archaea, are significantly smaller and more physically stable than microbubbles. © 2016 American Chemical Society
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NO BONES ABOUT IT: NANOCOMPOSITE MEMBRANES ENHANCE BONE REGENERATION Native bone and periosteum exhibit a natural electrical potential that plays important roles in maintaining bone volume and quality. When a bone becomes damaged, rehabilitating this electrical potential by the migration of periosteum-like tissue to the defect site is critical for bone regeneration. However, large defects tend to fill with fibrous and epithelial tissue instead, slowing bone healing. Finding a way to restore the bone electrical microenvironment could speed bone healing, even in large defects. To test this idea, Zhang et al. (DOI: 10.1021/acsnano.6b02247) developed a nanocomposite membrane with an inherent electrical potential, a material that can be applied to bone defects to influence regeneration. The researchers synthesized this membrane by modifying BaTiO3 (BTO) nanoparticles with polydopamine to prevent aggregation, then using a solution-casting method to incorporate these particles into a poly(vinylidene fluoridetrifluoroethylene) matrix. Tests show that these flexible nanocomposite membranes exhibit ferroelectric behavior, with surface potential increasing with an increasing fraction of BTO nanoparticles. The researchers achieved the highest surface potential of −76.8 mV with 5 vol % BTO nanoparticles, which mimics the endogenous potential of bone. To test the potential for clinical application, the researchers used these membranes to cover freshly formed bone defects in rats. New bone formation was observed after 4 weeks, with complete healing after 12 weeks. In contrast, in animals implanted with membranes without BTO nanoparticles, healing was still incomplete at 12 weeks. The authors suggest that these nanocomposite membranes might provide an innovative strategy for bone regenerative therapies.
good hole/electron mobility, even under 2.8% tensile/ compressive strain. As proof of principle, the researchers constructed a rivet graphene transistor. The authors suggest that this material will provide additional applications of nanocarbon-based films in transparent and flexible electronics.
GOLD NANOPARTICLE WORK THAT REALLY BLOSSOMS A wealth of research has been devoted to assembling composites incorporating metal nanoparticles. In particular, gold nanoparticles (AuNPs) have been assembled into molecules, clusters, and crystals with different symmetries and interparticle spacings. One route to controlling the interaction between AuNPs is functionalizing their surfaces with single-stranded DNA (ssDNA), which enables precisely engineering their arrangement through sequence-specific hybridization of complementary linker strands. Coating AuNPs with DNA has already been used to create colloidal crystals. However, to generate arbitrary lattices and more complex metallic nanostructures, researchers will need to gain additional control through anisotropic functionalization. DNA origami, in which a long DNA “scaffold” strand is folded and held into an engineered shape using shorter “staple” strands, holds promise for achieving this goal. In a recent study, Schreiber et al. (DOI: 10.1021/ acsnano.6b03076) harness this technology to create AuNPs surrounded by DNA origami “nanoflowers”. Through complementary base pairing, the scaffold strand and staples fold into 32 radial petals, two layers of 16 each, around the spherical nanoparticle. By selectively functionalizing the outer ends of some petals with ssDNA linkers, the researchers were able to use these daisy-like structures to create two-dimensional lattices. Varying the number of functionalization sites and attachment angles led to different lattice types, including chains, square lattices, and hexagonal lattices. Further investigation showed that these lattices were true composites, with interactions between the AuNPs entirely mediated by the DNA, which will not fold into its designated form without the AuNPs. The authors suggest that these hybrid materials could be used to create new plasmonic structures or, eventually, three-dimensional crystals.
HEXAGONAL BORON NITRIDE’S AMAZING TECHNICOLOR PHOTONS Hexagonal boron nitride (hBN) has recently attracted attention as a platform for room-temperature quantum photonics due to the discovery of room-temperature quantum emitters, realization of subdiffraction focusing and gliding, super-resolution imaging, and tunable phonon polariton propagation. The optical properties of bulk hBN have been studied thoroughly. However, little is known about those of its two-dimensional (2D) counterpart. Although traditional three-dimensonal semiconductors have color centers with similar spectral properties in bulk and nanostructured forms, those of van der Waals crystals, such as hBN, can have significantly different electronic and optical properties. The nanostructured forms of these materials can exhibit phenomenon such as spin valley splitting or strong exciton−phonon interactions at room temperature, which pose major challenges for engineering and control of single color centers. 7228
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In a recent study, Tran et al. (DOI: 10.1021/acsnano.6b03602) show that it is possible to engineer robust single quantum emitters in 2D hBN. After subjecting 2D hBN either to annealing or to electron beam radiation, confocal photoluminescence imaging revealed new color centers. These color centers were highly stable, withstanding aggressive annealing treatments in oxidizing and reducing environments such as hightemperature hydrogen, oxygen, and ammonia. These emitters could be classified into two general groups based on their zero phonon line energy and phonon sideband spectral shapes. Further investigation showed unambiguously that these emitters were point defects exhibiting a broad range of multicolor single photon emissions across the visible and the near-infrared spectral ranges at room temperature. The authors suggest that these findings could open up possibilities for employing quantum emitters in 2D materials for emerging applications in nanophotonics and nanoscale sensing devices.
the range of applications of graphene in energy-harvesting devices.
FOR SPATIAL CONTROL OF GENE EXPRESSION, HEPARIN IS HIP For decades, researchers have been investigating the possibility of using targeted systemic gene delivery to treat a variety of intractable diseases, including cancer. However, for this method to become a practical treatment modality, there must be a way to target specific cells in vivo spatially with minimal alteration of gene expression in healthy cells. Several different lipid- and polymer-based materials have been investigated as nanosized vehicles to carry plasmid DNA into cells. The liver and lungs typically operate as “first pass” organs, clearing the majority of nanoplexes quickly from the circulation. However, targeting other specific tissues after systemic delivery of these vectors remains a challenge. In a recent study, Chertok et al. (DOI: 10.1021/ acsnano.6b01199) detail a strategy for spatial control of genes target tissues by masking nanocarriers with heparin and using ultrasound-targeted microbubble destruction to enable them to enter cells. The researchers synthesized liposomes carrying green fluorescent protein (GFP)-encoded plasmid DNA, some with a polyethylene glycol (PEG) coating and others engrafted with heparin. After systemic delivery, tests showed that the PEG nanoplexes primarily accumulated in the liver and expressed GFP there, but the heparin-engrafted ones had negligible expression in all organs. In contrast, when the heparin-engrafted liposomes were administered to tumor-bearing mice with microbubbles and ultrasound at the tumor sites, which enabled the microbubbles to collapse and to increase permeability of cell membranes, the researchers saw a 10-fold increase in enhancement of gene expression in tumors relative to controls. The authors suggest that this strategy provides a way for targeted delivery of DNA using nonviral carriers.
POWER GENERATION FROM GRAPHENE ON POLYTETRAFLUOROETHYLENE: NOT A DROP IN THE BUCKET Graphene’s unique collection of useful properties, including ultrahigh electron mobility, high transparency, high mechanical elasticity, high thermal stability, and chemical inertness, make it a promising material for a variety of energy-related applications. Researchers recently reported another feature of this interesting material: the ability to produce electricity when ion-containing water moves over it through the triboelectric effect. A single moving droplet of seawater or ionic solution results in a power output of about 19 nW, thought to be the result of the pseudocapacitive effect between graphene and the single droplet of liquid. Because graphene is a one-atomic-layer thin, twodimensional material, a strong charge interaction exists between graphene and the substrate supporting it. Consequently, the choice of substrate could significantly influence this electric power output. In a recent study, Kwak et al. (DOI: 10.1021/acsnano.6b03032) take advantage of this effect by designing a graphene−substrate combination that produces a power output about 100 times larger than that reported in previous research. After placing graphene strips synthesized through chemical vapor deposition onto a polytetrafluoroethylene (PTFE) substrate, the researchers tested the electrical output from rolling a droplet of NaCl solution across the surface. Results show a maximum output power of 1.9 μW. This output increased with the size and speed of the water droplet. The researchers attribute this effect to a greater potential difference between the leading and trailing edge of the droplet. As proof of principle, the researchers created a device that produces a continuous voltage output from rain. The authors suggest that this graphene/PTFE structure expands
NO LINKER, NO PROBLEM FOR REVERSIBLE SELF-ASSEMBLY Nanoparticles that can self-assemble into functional structures have the potential to form a variety of useful new materials. Making this assembly reversible is key to many applications. However, creating nanoparticles with reversible self-assembly 7229
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remains challenging. Most designs rely on linkers such as polypeptide, polyrotaxane, spiropyran, and DNA to connect nanoparticles, but these molecules suffer from drawbacks including high cost, a tedious preparation process, and low stability and adaptability under some chemical and biological environments. These approaches also do not control aggregation, with many producing only random aggregates of nanoparticles. In a recent study, Hu et al. (DOI: 10.1021/acsnano.6b03396) overcame these difficulties by developing a linker-free approach that uses Janus metal-organosilica nanoparticles. The researchers used a sol−gel approach to craft Au nanoparticles capped with hexadecyltrimethylammonium bromide (CTAB) and partially coated with an organosilica layer, leaving some of the Au surface exposed. When these nanoparticles were in a CTAB solution, they remained dispersed due to steric repulsion. However, when the nanoparticles were placed in an ethanol solution, CTAB molecules on their surfaces were removed, allowing the Au surfaces to come together through van der Waals attraction to form dimers. Creating nanoparticles with more Au surface exposed enabled the researchers to use this method to form trimers and tetramers. These attractions were reversible upon returning the nanoparticles back to the CTAB solution, with multiple cycles possible. The researchers demonstrate that these assemblies can be useful as hotspots for surface-enhanced Raman scattering. They suggest that this reversible self-assembly strategy overcomes drawbacks of existing approaches, opening the door to new material designs.
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