MOSX: SEMICONDUCTING WHEN WET Producing hydrogen through water splitting could offer a ready source of carbon-neutral, clean fuel. Thus, far, most research on hydrogen-evolution catalysis has been conducted using liquid electrolytes. However, gas-phase water splitting could overcome many drawbacks using liquid systems, including efficiencylimiting bubble formation, freezing at low temperatures, the need for liquid pumping, corrosion, and catalyst poisoning. Gas-phase water splitting is also predicted to require less energy. However, only a few gas-phase water splitting systems have been reported to date. Daeneke et al. (DOI: 10.1021/acsnano.7b01632) report the use of MoSx-based systems for hydrogen evolution using vaporphase water. The researchers synthesized partially hydrogenated MoSx, which characterization showed to have a stoichiometry close to MoS3 2/3. Further tests showed that this material readily shed moisture in dry atmospheres and regained it in ambient, suggesting that surface H2O molecules are bound through relatively weak physisorption processes. This bound water significantly changed the electronic properties of the material, with resistance rapidly increasing upon drying and decreasing upon exposure to moisture. The researchers took advantage of these properties to fashion the material into a relative humidity sensor. In addition, by combining 90% MoSx with 10% TiO2 and making it into a catalytic ink, the researchers showed that it could readily split water into H2 and O2 without the use of an electrolyte or any external power source. The researchers suggest that these experiments could eventually lead to an efficient, electrolyteless method to split water photocatalytically, offering an alternative to current water-splitting technologies.
found in nature. The researchers created double-layered DNA origami tiles with DNA anchor strands extending from both sides. They then attached gold nanoparticles hybridized with DNA strands complementary to these anchors. Mixing these two components together resulted in linear stacks of 3 + 3 and 3 + 4 arrangements of same-size nanoparticles. The versatility of this technique also enabled the researchers to alternate three particles of one size and three of another as well as 3 + 2 stacks with nanoparticles of different compositions. These varied assemblies differed significantly in their plasmonic and electronic transport properties. The authors suggest that this strategy offers a general method for creating a variety of layered, pillared heterostructures with controllable layered morphology and different nanoparticle configurations that could eventually be used to create nanoparticle-based functional materials.
CONTACTS MATTER FOR BLACK PHOSPHORUS FIELD-EFFECT TRANSISTORS Black phosphorus (BP), a layered allotrope of phosphorus made by heating white phosphorus under high pressure, has been rediscovered as an elemental two-dimensional material with interesting optical and electrical properties that give it promise in potential next-generation optoelectronics and electronics. However, field-effect transistors (FETs) made with single-layer or few-layer BP usually possess small Schottky barriers, causing increased contact resistance that significantly limits the performance and scalability of these devices. Recent studies have suggested several ways of getting around this shortcoming, including choosing a metal with the appropriate work function. While Pd has commonly been used to achieve low contact resistances for p-type nanomaterial devices, such as carbon nanotube FETs, it has not been known whether it is possible to improve the work function of this metal further for BP FETs. To address this question, Ma et al. (DOI: 10.1021/ acsnano.7b02858) adsorbed hydrogen to Pd contacts on BP FETs, a method previously shown to increase Pd’s work function. By exposing BP FETs with Pd contacts to 5% H2 for as little as 2 min, the contact resistance became as low as ∼1.05 Ω·mm. Adsorption of H2 also changed the Pd contacts’ behavior from Schottky behavior to ohmic behavior and nearly doubled the transconductance from 0.499 to 0.937 μS/μm. Because of the Pd contacts’ sensitivity to H2, the researchers found that BP FETs with these contacts could be used as
ASSEMBLING NANOPARTICLES INTO PILLARS OF STRENGTH Assembling nanoparticles into larger scale structures not only takes advantage of their intrinsic optical, electrical, and magnetic functional properties but also taps into their cooperative and synergistic effects. Thus far, such assemblies have been investigated in a variety of fields, including plasmonics, photonics, sensing, catalysis, and biomedicine. Several approaches have been used to guide and to organize such assemblies through modulation of their interactions, packing properties, and geometrical features. However, controlling the overall shape and texture of the assembled material remains difficult. Tian et al. (DOI: 10.1021/acsnano.7b02671) used a strategy similar to those of the sequential superbiomolecular assemblies © 2017 American Chemical Society
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THE FORCE IS WITH YOU FOR VISUALIZING DRUG−CELL INTERACTIONS Chemotherapy remains a first-line therapy for a variety of cancers. Different chemotherapeutic drugs have diverse modes of action, including damaging DNA and affecting cell-cycle checkpoints and arrest, DNA repair, and altered expression of oncogenes or regulatory proteins. Understanding these responses could aid in the design of more effective chemotherapeutic drugs and other treatment strategies. However, elucidating the molecular, cellular, and physiological processes of chemotherapeutics using current methods, including cell viability assays, cell cycle analysis, oncogene or regulatory protein expression, and drug-target protein binding detection, have a number of drawbacks. These drawbacks include long processing times, expensive equipment, high reagent consumption, and difficulty in observing results in a label-free and noninvasive manner. Wu et al. (DOI: 10.1021/acsnano.7b02376) detail a method of measuring cellular response to chemotherapy drugs by measuring the nanomechanical traction force or the force generated when cells probe, push, or pull on the surrounding extracellular matrix. Using HeLa cells as a model system, the researchers treated them with paclitaxel, a chemotherapy often used for ovarian carcinoma treatment. This drug is known to work by binding microtubules. Compared to cells treated with a paclitaxel analog with reduced microtubule-binding ability, the researchers found that the paclitaxel-treated cells generated less force on ultrasoft polydimethylsiloxane embedded with a single layer of fluorescent beads as position markers. This effect was evident minutes after treatment, in contrast to traditional tests of drug−cell interaction that can take hours. The authors suggest that this method could serve as a quick and sensitive test to provide insights in drug screening.
hydrogen gas sensors, whereas those with Au contacts had almost no response to H2. The authors suggest that these results show the importance of contact engineering for improving BP FET performance.
EYEING A TOPICAL TREATMENT FOR BACTERIAL KERATITIS Most cases of infectious keratitis, a type of eye infection that inflames the cornea, are caused by bacteria introduced through improper use of contact lenses, trauma, cornea surgery, and immunodeficiency. In extreme conditions, this infection can destroy the corneal stroma in just a few days, leading to a loss of visual acuity. This condition is typically treated through broad-spectrum antibiotics or more aggressively by topical steroids, corneal collagen cross-linking, and amniotic membrane grating to the conjunctival flap. However, each of these treatments has serious drawbacks, suggesting the need to develop new and innovative treatment modalities. Jian et al. (DOI: 10.1021/acsnano.7b01023) showcase an innovative method of creating carbon quantum dots (CQDs) from spermidine, a biogenic polyamine that has shown antibiotic activity in previous studies. Through simple dry heating, the researchers created CQDs in a one-step method. Tests showed that this material showed inhibition against four strains of nonmultidrug-resistant bacteria (E. coli, S. aureus, P. aeuruginosa, and S. enteritidis) as well as multidrug resistant S. aureus (MRSA). Tests suggest that the high density of spermidine and/or its pyrolytic products contained in the CQDs as well as their extremely high positive charge appear to interact with bacterial cell membranes, causing severe disruption. In vivo tests in rabbit eyes show high biocompatibility and effective infection clearing, enhanced by the CQDs’ ability to open the tight junctions between cornea cells. The authors suggest that these CQDs show significant potential as alternatives to current treatments for bacterial keratitis.
LIVING ON THE EDGE: NITRILE DOPANTS ON GRAPHENE NANORIBBONS Graphene nanoribbons (GNRs) maintain many of graphene’s structural and charge mobility properties while also overcoming its intrinsic lack of a band gap. Chemical doping during synthesis can expand the potential of these nanostructures by modifying their band alignments, changing their band gaps, modifying their density of states, and/or generating highly reflective electron scatterers. Although the most common approach to doping GNRs has been chemically substituting carbon atoms with heteroatoms in the organic precursor, surface synthesis of GNRs provides an additional way to dope these structures while building them with atomic precision. 6508
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In a recent study, Carbonell-Sanromà et al. (DOI: 10.1021/ acsnano.7b03522) explore the effects of doping 7-armchair graphene nanoribbons (AGNRs) with nitrile groups, which have strong electron-accepting behavior, at the edges during surface synthesis. The researchers produced AGNRs with nitrile groups in their bay regions through an on-surface reaction using cyano-substituted dibromo bianthracene precursors. Highresolution scanning tunneling microscopy using CO-functionalized tips showed the loss of nearly 50% of these functional groups during the reaction process as well as on-surface formation of pyridine rings. Using a combination of scanning tunneling spectroscopy and density functional theory calculations, the researchers show that the strong electron-accepting quality of the nitrile groups induces a charge redistribution over each entire nanoribbon that results in sizable dipoles at the nitrile sites pointing toward the ribbon backbone. These dipoles make the nanoribbons more electronegative, downshifting the bands. The authors suggest that this work showcases the potential of using functional groups to modify the physicochemical properties of GNRs.
understanding of this effect could lead to SMM-based devices with engineered local gates for tuning and controlling the properties of quantum electronics and spintronics.
MEASURING A CROWD WITH DNA ORIGAMI Many biomolecules are able to respond to subtle cues in their local environments, such as a force or binding of a cofactor, by changing their conformational states. For example, molecular crowding can drive changes in the conformations of heatshock protein 90 and the enzyme phosphoglycerate kinase. Being able to engineer nanodevices that can also change conformation with high sensitivity in response to similar local conditions could offer a way to probe crowding forces at a molecular scale. Seeking to mimic this functionality, Hudoba et al. (DOI: 10.1021/acsnano.6b07097) developed a DNA origami device that has two stable states separated by a transition state, triggered by a weak energy barrier. The device, which the researchers named NanoDyn due to its nanodynamic behavior, is composed of two barrel-shaped components held together by six scaffold linkers in either a closed or open position. Using transmission electron microscopy and single-molecule Förster resonance energy transfer, the researchers monitored the equilibrium and dynamic properties of this device. Their results showed that the transition between the stable states of this device could readily be triggered by molecular crowding. Using poly(ethylene glycol) as the crowding molecule, experiments showed that NanoDyn was sensitive to subpiconewton depletion forces with a resolution down to 40 fN on a second time scale. The authors suggest that using this device to probe depletion forces quantitatively could enable insight into the physical processes inside molecularly crowded environments, such as inside cells.
UNDERSTANDING THE ATTRACTION OF SINGLE-MOLECULE MAGNETS Molecular spintronics, or the field of characterizing, manipulating, and reading out the molecular spin states of nanosystems down to the single molecule level, could eventually lead to significant advances in biomedical and nanoelectronic applications. In particular, single-molecule magnets (SMMs) offer a way to realize nanometer-scale structures with stable spin orientations. These devices are formed by an inner magnet core with organic ligands, which can be tailored to bind to surfaces or into molecular junctions, as a surrounding shell. Singlemolecule magnets bring together the model properties of macroscale magnets with the quantum features of nanoscale objects. Studies have shown that SMMs in contact with nanostructures, such as carbon nanotubes (CNTs), can strongly modify their transport properties, sometimes leading to effects such as giant magnetoresistance. However, the theoretical underpinnings of this effect are not well understood. To help explain this phenomenon, Krainov et al. (DOI: 10.1021/acsnano.7b02014) used experimental results and a theoretical model to address the gate-controlled spin-valve effect in CNTs with side-attached SMMs composed of terbium(III) bis-phthalocyanine (TbPc2). Their findings suggest that the giant magnetoresistance in this model results from the interplay of Fano resistance induced by the sideattached SMMs combined with a Coulomb interaction between electrons inside the nanotube, leading to a Coulomb blockade of the linear transport for the antiparallel orientation of molecule spins at all values of gate voltage. Because the sign and the strength of the molecular spin−spin interactions can vary with the gate voltage, the authors suggest that better
HYDROGEN EVOLUTION THROUGH SYNTHETIC BIOLOGY Hydrogen could eventually offer a source of renewable energy to reduce the carbon footprint. One particularly “green” way to 6509
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produce hydrogen fuel is by combining inorganic structures with biological structures capable of water splitting, light harvesting, or proton reduction. With the advent of synthetic biology, an interdisciplinary branch of biology and engineering that focuses on building artificial biological systems, it is possible to design and to produce these key biological structures completely cell-free. Taking advantage of this possibility, Wang et al. (DOI: 10.1021/acsnano.7b01142) combined synthetic biology and nanotechnology to assemble cell-free expressed transmembrane proton pumps and TiO2 semiconductor nanoparticles noncovalently to function as efficient nanophotocatalysts for H2 evolution. Using artificial nanodisc membranes previously shown to be effective stand-ins for natural cell membranes, the researchers expressed bacteriorhodopsin proton pumps from the extremophile organism Halobacterium salinarum. After purifying these proton pumps, the researchers then assembled them on the surface of semiconductor TiO2 nanoclusters decorated with Pt cocatalyst dots. Tests showed that when these nano−bio catalysts were exposed to monochromatic green or white light, this system efficiently produced H2 with a high turnover rate in water at neutral pH and room temperature, with methanol as a sacrificial electron donor. The researchers suggest that this approach of combining synthetically produced biological components with nanoparticles could be used to develop other efficient photocatalytic architectures as well as artificial systems ranging from metabolic pathways to signaling networks.
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