WIPING AWAY BARRIERS TO TRANSPARENT AND CONDUCTING ELECTRODES Flexible optoelectronic devices currently in development, such as flexible solar cells, touch panels, and displays, require transparent conducting electrodes with high mechanical flexibility, conductivity, and optical transparency. Materials such as metal nanowire networks, carbon-based materials, conducting polymers, and mesh- or grid-patterned metals have been investigated for this purpose. The latter materials have shown the most promise, having innate high conductivities through their interconnectivity, superior uniformity and reliability due to their patterns, and outstanding mechanical flexibility because of their small line widths. However, manufacturing these materials remains challenging due to complicated photolithography or etching steps, high facility cost, and low throughputs. In a recent study, Yu et al. (DOI: 10.1021/acsnano.7b00229) demonstrated that more facile synthesis is possible through a novel method based on nanomolding and area-selective delamination. The researchers created a mold out of polyurethane acrylate (PUA) that inversely replicates the surface geometry of the desired metal mesh structure and attached polyethylene terephthalate as a supporting substrate. Silver was then deposited on the PUA surface, filling the molded trenches and covering the square-shaped mesa regions. By swelling the PUA in ethanol, the researchers weakened the adhesion force between the PUA and the Ag films, enabling the squares on the mesas to be wiped away with soft rubbing or removed with ultrasonification and leaving the embedded metal mesh behind. The mesh displayed high transmittance and low sheet resistance, which the researchers took advantage of in a transparent heater and flexible touch panel. The authors suggest that this method could substantially reduce the cost and complexity of creating flexible transparent electrodes.
eradicating tumors without substantially damaging healthy tissues. Although various nanomaterials have been employed for PTT applications, including gold nanoparticles, carbon nanotubes, and graphene oxide nanosheets, recent research has focused on Fe3O4 magnetic nanoparticles (MNs). These MNs offer several advantages over other nanoparticles for PTT, including the ability to biodegrade safely and good contrast for magnetic resonance imaging (MRI). However, they are easily recognized by the immune system and rapidly cleared from circulation. One way to prevent immune clearance is by preparing cell membrane-coated nanoparticles through ultrasonic treatments or mechanical extrusion. However, both these methods have drawbacks that limit their ease and effectiveness. In a recent study, Rao et al. (DOI: 10.1021/acsnano.7b00133) developed a method to facilitate the synthesis of cell membrane-coated nanoparticles. The researchers infused red blood cell (RBC) membranes and Fe3O4 nanoparticles into a microfluidic chip with two inlets that lead to a mixing channel and an electroporation zone. Electric pulses promoted the entry of the MNs into the membranes, which were then collected from the chip’s outlet. Tests in mouse models of breast cancer showed that these RBC membrane-capped MNs were well retained in the animals’ blood, accumulated in tumors, and could be visualized using an MRI. Laser irradiation led to almost complete tumor inhibition. The authors suggest that this synthesis method offers promise for future clinical applications.
PUTTING SURFACE-ENHANCED RAMAN SCATTERING HOT SPOTS INSIDE CELLS Surface-enhanced Raman scattering (SERS), which uses an enhanced electromagnetic field to amplify Raman signals, has been exploited as a powerful analytical technique for chemical analysis since the 1970s. The magnetic field enhancement central to this technique often occurs at gaps between metal nanoparticles known as hot spots. In recent decades, SERS hotspots have been studied for bioanalysis due to their
MAKING CANCER-FIGHTING WOLVES IN SHEEP’S CLOTHING Although surgery, radiation, and chemotherapy have made tremendous advances in treating cancer, these interventions often do not offer cures and can have severe side effects. Photothermal therapy (PTT), in which photosensitive nanoparticles convert light to heat to kill cancer cells, could aid in © 2017 American Chemical Society
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gave the best results in these active layers, increasing the power conversion efficiency of the resulting BHJ OSC by ∼15%. The authors suggest that this enhanced performance is due to area matching the WSe2 flakes. They add that this strategy could be a viable way to boost the photovoltaic performances of other OSCs significantly.
numerous advantages over other imaging techniques, such as single-molecule sensitivity, the ability for multiplexed detection, and resistance to photobleaching. Although this method has been used to investigate analytes inside living cells by treating cells with SERS substrates, these studies have had numerous challenges, including difficulties collecting a strong signal from the desired analytes and distinguishing the “fingerprint” of that analyte from those of other competing species. To improve the sensitivity and specificity of SERS in living cells, Zhou et al. (DOI: 10.1021/acsnano.7b00531) developed a method that creates hot spots through target-triggered nanoparticle dimerization. The researchers tethered singlestranded DNA asymmetrically onto gold nanoparticles. They also attached alkyne- and nitrile-terminated molecules to act as reporters to create strong and sharp single peaks in the cellular Raman-silent region. When the DNA sequences encountered complementary target microRNAs (miRNAs), they bound with the miRNAs to form dimers with gaps able to create a SERS hot spot. Experiments showed that this technique could be used successfully to quantify the amount of target miRNA and to provide multiplexed imaging for two different miRNAs. The authors suggest that this technique opens up a new avenue for investigating specific biomolecules in situ.
MIMICKING NATURE’S HIERARCHY WITH PROTEIN−NANOPARTICLE CONSTRUCTS Complex, self-assembled structures exist throughout nature, particularly in living organisms. These structures are formed through multiple levels of hierarchical organization with the potential to evolve through dynamic reorganization during cellular processes. Numerous efforts have focused on mimicking these assemblies, including using DNA, proteins, and combinations of the two to create ordered, discrete structures. Although many of the resulting assemblies have structural complexity similar to that of biological systems, their components are rigidly set in place, preventing the dynamic behavior of their natural counterparts. Striving to replicate the dynamic behavior of biological systems, Mout et al. (DOI: 10.1021/acsnano.6b07258) used recombinant proteins and nanoparticles that were coengineered to create multilayered, hierarchical assemblies. The researchers synthesized green fluorescent protein (GFP) bearing a genetically incorporated glutamic acid peptide chain as well as gold nanoparticles decorated with arginine-terminated ligands. These two components self-assembled through carboxylate−guanidinium interactions to form hierarchical nanostructures guided by electrostatic forces. Microscopy showed three distinct layers of organization. The first layer consisted of particles surrounded by multiple proteins, which formed “corona-like” structures. These further evolved to produce a second layer of “granule-like” structures, which, in turn, assembled together to produce spherical superstructures. These structures continued to evolve with changes in the concentration of electrolytes in the solution, showcasing their dynamic nature. The authors suggest that similar structures could be used for applications including therapeutic delivery, catalysis, and photosynthetic energy harvesting.
SIZE MATTERS FOR WSE2 FLAKES IN ORGANIC SOLAR CELLS Organic solar cells (OSCs) with bulk heterojunction (BHJ) structures rely on an active layer composed of p-type polymer donors and n-type fullerene acceptors. Although BHJ OSCs have received increasing attention due to their low cost, lightweight, and versatility for large-scale fabrication on flexible substrates, their performance lags behind silicon and other inorganic solar cells. One option for improving performance is incorporating a third component into the active layer, such as WSe2, to broaden the absorption bandwidth, to improve energy or charge transfer, or to improve the photogenerated charge dissociation. However, the effects of WSe2 flake size on OSC BHJ performance were not known. To investigate this question, Kakavelakis et al. (DOI: 10.1021/acsnano.7b00323) used liquid-phase exfoliation combined with sedimentation-based separation to produce uniform WSe2 flakes in three different sizes, with areas of ∼70, ∼240, and ∼720 nm2 and thicknesses of ∼2.6, ∼6.1, and ∼8.5 nm, respectively. These three sizes were specifically chosen because their areas are about one-third, the same, and three times larger than the average PC71BM fullerene domain size in an active layer composed of a PTB7: PC71BM blend. The researchers found that the flakes of ∼240 nm2 area and ∼6.1 nm thickness 3426
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A NEW TWIST ON ASSEMBLING CHIRAL PLASMONIC NANOCHAINS Researchers have investigated plasmonic nanostructures with chiroptical properties for a wide range of applications. These materials can be formed through bottom-up techniques in which achiral inorganic nanoparticles self-assemble in the presence of chiral organic nanoparticles. These methods often rely on chiral templates built through the association of biomacromolecules. Gold nanorods (Au NRs) have been studied extensively as the plasmonic base for materials built through bottom-up methods due to their outstanding physical qualities. However, small biomolecules, such as amino acids and oligopeptides, are rarely used with nanorods due to their size mismatch. In a recent study, Lu et al. (DOI: 10.1021/acsnano.6b07697) developed a method to build chiral plasmonic nanochains using Au NRs with L- and D-glutathione (GSH) as templates. Their method relies on cetyltrimethylammonium bromide (CTAB), a cationic surfactant that is often used during synthesis of Au NRs. Above its critical micelle concentration (cmc), CTAB forms a tightly packed bilayer on Au NR surfaces through hydrophobic interactions of its alkyl chains. This property provides a confined space that can expedite peptide folding and self-association processes. The researchers took advantage of this phenomenon to promote the self-association of glutathione on the tips of Au NRs, where it selectively replaces CTAB. At concentrations above CTABs’ cmc, the Au NRs formed end-toend cross-linked structures that gave them chirality mirroring that of the attached GSH. Below the cmc, the NRs formed simple chains without chirality. The authors suggest that using polypeptides or proteins as chiral templates could provide even stronger chiroptical signals and longer range chiral nanostructures.
electron−phonon coupling. However, connecting this buckling to its electronic properties has been a challenge because of the difficulties in observing germanene’s 1 × 1 superstructure. In a recent study, Zhuang et al. (DOI: 10.1021/ acsnano.7b00687) used low-temperature scanning tunneling microscopy to investigate the 1 × 1 superstructure of germanium layers grown on Au (111). This technique revealed a regular honeycomb arrangement with atomic resolution, showing an unexpected rectangular superstructure that supports a 2D continuous nature. Combining these findings with first-principles calculations, the researchers assigned a √7 × √7 reconstruction to this buckled material. In situ Raman spectroscopy revealed distinctive vibrational phonon modes, indicating coupling between the Dirac fermions and lattice vibrations. Significant enhancement in this electron−phonon coupling was correlated with tensile strain. The authors suggest that lattice mismatch could be used to modulate the electronic properties of germanene for tailored use in applications.
THE ODD COUPLE: EXPLORING MODE COUPLING IN PLASMONIC HETERODIMERS Collective oscillations of the conduction electrons in metal nanostructures, known as localized surface plasmon resonances (LSPR), can be focused using optical nanoantennas. The electromagnetic properties of nanoantennas are governed by their eigenmodes, which range from dipoles to higher order multipoles. In nanoantennas formed by pairs or multimers of nanoparticles, these models couple to each other and hybridize. This phenomenon has been well studied in systems where two spherical nanoparticles made of the same material are separated by a nanogap, generating an intense, confined near field in the gap region. This electromagnetic “hot spot” can enable large nanoscale fluorescence enhancement and surface-enhanced Raman scattering down to the single molecular level. Heterogeneous pairs, composed of different shapes, sizes, and materials, have only recently attracted attention. However, studies to date have focused primarily on the far-field properties of sphere and disc dimers. In a recent study, Flauraud et al. (DOI: 10.1021/ acsnano.6b08589) focused instead on the modal responses of a wide range of other nanoparticle geometries and pairs of different materials. The researchers used a precise multilayer electron beam lithography fabrication process to produce nanorods, nanodiscs, and nanosquares of Ag, Au, and Al. Combining various pairs, the researchers investigated mode energy detuning and coupling with electron energy loss spectroscopy, findings they supported with full wave analysis numerical simulations. Their work revealed both the influence of individual members of each pair as well as the mode hybridization of these plasmonic heterodimers. The authors suggest that these findings could eventually be used to develop
BUCKLING DOWN TO STUDY ELECTRON−PHONON COUPLING The exotic properties of two-dimensional (2D) materials with Dirac fermions have spurred interest in both their fundamental study and their potential use in applications. Germanene, a germanium analog of graphene, has attracted particular focus due to its intrinsic energy gap of 24 meV in addition to a Dirac cone in its electronic band structure. Its Fermi level can likely be tuned through electronic gating while keeping its ultrahigh charge mobility, giving this material potential for incorporation in high-speed and low-energy consumption field-effect transistors. When placed on different substrates, germanene is thought to adopt complicated buckled structures that are influenced by the underlying substrates, which provide another way to tailor this material’s electronic properties through 3427
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plasmonic heterostructures with tailored responses that go beyond the possibilities offered by homodimers.
UNDERSTANDING TRANSPORT IN CARBON-BASED MOLECULAR JUNCTIONS The field of molecular electronics centers on using molecules as building blocks for electronic circuitry. One of its core principles is using variations in molecular structure to control charge transport, which could potentially lead to electronic functions that are not available using conventional semiconductors. A key factor that governs charge transport is the length of the molecular component (d), between conducting contacts. The dependence of molecular junction current on d, as well as temperature and bias, can be used to help determine the transport mechanism. In a recent study, Najarian and McCreery (DOI: 10.1021/ acsnano.7b00597) used these clues to determine the transport mechanism in molecular junctions between conducting sp2hybridized carbon electrodes in four different aromatic oligomers (fluorene, anthraquinone, nitroazobenzene, and bisthienyl benzyme). Each of these molecular structures exhibited an unusual, nonlinear log current density (J) versus bias voltage (V) dependence that did not match the characteristics of conventional coherent tunneling or activated hopping mechanisms. The observed current densities for each of these four molecules for d = 7−10 nm had no correlation with occupied (HOMO) or unoccupied (LUMO) molecular orbital energies. By combining these findings with ultraviolet−visible absorption spectroscopy of molecular layers bonded to carbon electrodes, which revealed the internal energy levels of these chemisorbed films, the researchers conclude that transport in these molecular junctions is governed by a multistep tunneling mechanism through a barrier defined by the HOMO−LUMO gap and not by charge transport at the electrode interfaces. The authors suggest that these findings could provide useful guidance for the rational design of molecular electronic devices with tailored electronic properties.
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