GETTING TATTED UP WITH GRAPHENE ELECTRONIC SENSORS Electronic tattoos (E-tattoos) have recently been developed as alternatives to more traditional wearable biometric sensors. These hair-thin and skin-soft temporary tattoos have potential applications ranging from continuous recording of oxygen saturation to skin temperature or hydration to reading biomarkers in sweat. A major advantage of E-tattoos is the intimate sensor−skin integration, which enables these devices to conform to skin’s natural microscopic roughness to enlarge the contact area with a dry electrode. This proximity, in turn, lowers the contact impedance, leading to enhanced signal-to-noise ratios and less susceptibility to disruption due to motion. Although gold has been a popular choice thus far for E-tattoo dry electrodes and interconnects, its price is prohibitive for disposable devices, and its lack of optical transparency makes it obviously visible. Seeking a better option, Kabiri Ameri et al. (DOI: 10.1021/ acsnano.7b02182) looked to graphene, using this two-dimensional material to craft E-tattoos that are only submicrons thick but demonstrate high electrical and mechanical performance. The researchers fabricated these devices by growing graphene on copper foil, then transferring it to tattoo paper. Supported by the paper, they used a programmable mechanical cutter plotter to carve out filamentary serpentine shapes. When these devices were transferred to hairless skin, the researchers demonstrated in proof-of-concept experiments that they could be used as electrophysiological sensors, resistance temperature detectors, and skin hydration sensors. These E-tattoos were durable, lasting for up to 96 h before developing cracks when coated with a liquid bandage. The authors suggest that these graphene-based Etattoos could be applied in numerous biosensing applications.
communication, images, sensors, and medical treatments. Giving them a transient, dissolvable nature could expand this range of applications while improving their environmental friendliness. However, making traditional lasers into transient devices is challenging due to difficulties in finding suitable materials for their precise optical resonator cavities with meticulous fabrications and special components. Shi et al. (DOI: 10.1021/acsnano.7b00201) designed lasers made of environmentally friendly materials that are able to dissolve in water and can be easily reproduced and recycled. Rather than crafting traditional lasers, the researchers looked to random lasers that achieve light amplification by random light scattering between nanoparticles, creating closed-loop pathways that increase with the pumping energy density to enable lasing action to occur. Toward that end, they fabricated their lasers using TiO2 and ZnO nanoparticles embedded in poly(vinyl alcohol) (PVA), a water-soluble polymer. Tests reveal excellent emission characteristics in this system, which gradually diminish over 40 min once these devices have been immersed in water. Further experiments show that the nanoparticles can be collected and refabricated into new devices without loss of performance. The authors suggest that these transient random lasers could be used in applications ranging from biodegradable substrates to one-off medical devices.
AND FOR OUR NEXT TRICK: LEVITATED PLASMONIC NANOANTENNAS Plasmonic nanoantennas have recently drawn increasing attention for both their interesting fundamental properties and their potential for applications such as enhancing Raman scattering cross sections. Most efforts in this area have focused on dry conditions. However, a growing number of researchers have focused on solutions, a field known as plasmofluidics. An important feature of plasmonic nanoantennas in general is their near-field sensitivity, which requires subwavelength separation between a nanoantenna and the surface to be affected. Several techniques have been employed to accomplish this objective,
A LASER TO DISSOLVE OR TO RECYCLE To minimize impact on the environment, researchers have recently developed many electrical and optical devices that are nontoxic and dissolve naturally or are recyclable. These devices have included inductors, capacitors, resistors, memristors, transistors, memory, generators, wireless control systems, lightemitting diodes, and waveguides. Lasers play important roles in © 2017 American Chemical Society
Published: August 22, 2017 7556
DOI: 10.1021/acsnano.7b05667 ACS Nano 2017, 11, 7556−7559
In Nano
www.acsnano.org
ACS Nano
In Nano
lead to defect-free directed self-assembly and achieve selective functionalization of the substrate.
including self-assembly, lithographic nanofabrication, random distribution of emitters, chemical synthesis, and use of scanning probe technology for nanopositioning. Tuna et al. (DOI: 10.1021/acsnano.7b03310) detail a method for manipulating a plasmonic nanoantenna close to an aqueous interface: using an electrostatic trap created between a glass nanopipette and a substrate to levitate the plasmonic nanoantenna in water. The researchers embedded an individual semiconductor CdSe/CdS core/thick-shell quantum dot at the upper interface of a thin polymer film on the substrate. They then used a charged glass nanopipette to trap a gold nanoparticle in solution. When the embedded quantum dot was excited with a laser beam, the fluorescent signal was significantly enhanced, confirming a near-field interaction. Finite-difference timedomain simulations suggested a drop in the fluorescence signal as a function of the axial separation of the gold nanoparticle, results that were confirmed experimentally. The authors suggest that this approach might be used in other areas where plasmonic effects are employed at liquid−solid interfaces.
EASING THE WAY FOR RNA INTERFERENCE RNA interference (RNAi), a phenomenon in which short pieces of RNA can be used to silence genes selectively, has the potential to treat a variety of diseases such as viral infections, cancer, and autoimmune diseases that cannot currently be treated with existing drug classes. Although this technology holds enormous promise, progress of RNAi therapeutics has been hindered by challenges in delivery and a lack of understanding of the link between physicochemical properties of delivery vehicles, biological barriers, and silencing activity. Several delivery strategies have relied on lipid formulations, particularly those that incorporate cationic lipids, which promote efficient encapsulation of anionic short interfering RNA (siRNA), cytoplasmic delivery, and endosomal escape. However, these formulations are plagued with challenges including lower transfection efficiency, colloidal instability, and toxicity. In a review article, Rietwyk and Peer (DOI: 10.1021/ acsnano.7b04734) detail the use of next-generation ionizable cationic lipids for nucleic acid delivery. These lipids are charged at mildly acidic pH and can be complexed with siRNAs to generate stable nanoparticles. The net charge of these particles under physiological pH is neutral to mildly cationic, which avoids the robust immune response triggered by permanently charged cationic lipids. These ionizable cationic lipids can be incorporated into lipid nanoparticles through microfluidic mixing or in lipoid formulations, which can be actively targeted to cells with antibodies, aptamers, proteins, and natural ligands. However, problems remain in scaling up lipid nanoparticle formulations and avoiding immune responses. The authors suggest that future research could aid in the rational design of safe, specific, and efficient lipid-based carriers for RNAi therapeutics.
TAKING LITHOGRAPHY TO THE NEXT LEVEL The ability to create increasingly smaller features using photolithography has enabled increased computing power at lower cost. However, this progress has been stymied as photolithographic technologies have matured and reached their physical limitations. The most advanced technique currently cannot scale below about 40 nm half pitch, limiting potential applications. To help break through this limit, researchers are exploring a variety of multiple patterning techniques, including block copolymer (BCP) lithography. This method uses chemical incompatibility between blocks to encourage separation into period nanostructures or lamellae, which can have length scales in the sub-10 nm regime. By prepatterning the substrate surface, BCP films can be forced to adopt well-aligned structures suitable for lithography. Lane et al. (DOI: 10.1021/acsnano.7b02698) adopt this methodology to form well-resolved 5 nm half-pitch features in thin films with high etch selectivity. The researchers created these materials by thermally annealing a poly(5-vinyl-1,3-benzodioxide-block-pentamethyldisilylstyrene) (PVBD-b-PDSS) film and subjecting it to a CO2-based reactive ion etching process, selectively removing the PVBD block. The resulting fingerprint patterns were then transferred into an underlying chromium hard mask and carbon layer. The researchers were able to create the 5 nm features with directed self-assembly of the BCP using guidelines patterned on the substrate with nanoimprint lithography (NIL). Coating the substrate with a neutral brush polymer avoided the formation of a wetting layer but also reduced the chemical contrast between the NIL guidelines and the substrate. The authors suggest that future research can help
SLIP AND SLIDE: DEMONSTRATING CONTACT SIZE EFFECTS IN STRUCTURAL LUBRICITY Nearly three decades ago, investigators introduced the concept of structural superlubricity, which describes virtually frictionless 7557
DOI: 10.1021/acsnano.7b05667 ACS Nano 2017, 11, 7556−7559
ACS Nano
In Nano
using face-to-face assembly in solution with the addition of NaCl. At higher ionic strengths, these chains contract, bringing the plates into closer contact; at lower ionic strengths, this process is reversible, leading the nanoplates to disassemble. When Janus particles with a nanoplate-attracting and a nanoplate-repelling side were added to the mix, they served as “chain stoppers,” shortening the chain length. Higher ionic strengths led some Janus particles to accommodate more than one chain on their attractive sides, forming two-dimensional instead of just onedimensional structures. The researchers were able to enrich the structural possibilities further by incorporating spheres with multiple attractive patches, creating I-, V-, Y-, or X-shaped bonding. The authors suggest that these highly anisometric particles have the potential to expand the number of structures and properties available to the nanoparticle self-assembly toolbox.
sliding that results from an atomic lattice mismatch between two atomically flat surfaces that prevents interlocking. Because most contacts consist of dissimilar materials with misaligned orientation, structural superlubicity should be common. However, experiments have suggested that this phenomenon is more exotic than widespread. Besides being thwarted by dirt molecules between surfaces and relaxations at the interface, theory suggests that contact size of particles on a surface is a fundamental limit, with this threshold mainly dependent on an interplay between lateral contact compliance and interface interaction energies. Testing this concept experimentally, Dietzel et al. (DOI: 10.1021/acsnano.7b02240) measured the sliding force of differently sized Sb particles on MoS2 and highly oriented pyrolytic graphene (HOPG), two well-known lubricants that can be cleaved to create atomically flat surfaces. The researchers measured the interfacial friction between these surfaces and Sb nanoparticles grown on them by measuring the force used to push the Sb nanoparticles using the sharp tip of an atomic force microscope probe. Their tests showed that Sb nanoparticles on MoS2 showed superlubric sliding as long as their contact area was under 15 000 nm2; however, constant shear stress behavior prevailed for larger particles. In contrast, Sb nanoparticles on HOPG showed superlubricity over the entire size range. Density functional theory simulations showed that the chemical interaction energies for Sb on MoS2 are significantly stronger than Sb on HOPG, explaining these differences. The authors suggest that these effects must be considered when designing low friction contacts that take advantage of structural superlubricity.
A SPIN ON THERMOELECTRIC GENERATORS: CARBON NANOTUBE YARN Thermoelectric (TE) materials, which harvest electrical energy from temperature differences, have attracted increasing attention for applications in next-generation power generators. A trend toward more flexible power-conversion devices in general has led to a search for alternatives to the brittle inorganic materials from which thermoelectric generators (TEGs) are typically composed. Organic polymers such as poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) have been investigated as potential options due to their high electrical conductivity. However, TE materials based on PEDOT:PSS are sensitive to humidity in ambient conditions, precluding their practical use. Another possible candidate is carbon nanotubes, which have high electrical conductivity and thermopower, excellent mechanical properties, and easily tunable carrier type and carrier concentration with chemical doping. Relying on these characteristics, Choi et al. (DOI: 10.1021/ acsnano.7b01771) constructed flexible thermoelectric generators (TEGs) using yarn woven from carbon nanotubes. The researchers wound the synthesized yarn onto a flexible supporting unit and alternately doped sides of the yarn on this unit into n- and p-types using polyethylenimine and FeCl3 solutions, respectively, leaving sections between the doped regions undoped. Tests showed that this material exhibited high electrical conductivity due to its highly aligned structure. Furthermore, it was able to perform as a complete TEG unit without metal electrodes because the conductive yarn between the doped regions served as electrodes. A flexible TEG based on 60 pairs of n- and p-doped regions had power densities higher than any other TEG based on flexible materials. A device using yarn four times as long could power a red light-emitting diode. The authors suggest that this system has potential for flexible or wearable power-conversion devices.
BEAUTIFUL PLATING THROUGH SELF-ASSEMBLY Nature often incorporates anisometry in nanoscale building blocks, imbuing the resulting assemblies with exceptional characteristics. For example, collagen is made of staggered tropocollagen units that are thin and long, enabling the fibers of this protein to undergo multiple modes of tensile deformation that enable it to have outstanding energy absorption. Synthetic nanofiber assemblies can also benefit from anisometry, with carbon nanotube assemblies and ultrathin metal nanowire systems displaying excellent mechanical, electrical, and optical properties. Taking advantage of this quality, Luo et al. (DOI: 10.1021/ acsnano.7b02059) detail the self-assembly of ultra-anisometric silver nanoplates, which stack to form assemblies using a mechanism analogous to molecular step-growth polymerization in ionic solutions. These nanoplates, with nanometer thickness but micrometer edge length, stack into chains of up to 100 μm 7558
DOI: 10.1021/acsnano.7b05667 ACS Nano 2017, 11, 7556−7559
ACS Nano
In Nano
PERFECTING DIRECTED SELF-ASSEMBLY OF BLOCK COPOLYMER FILMS The relatively recent development of directed self-assembly (DSA) of block copolymer (BCP) thin films with density multiplication has offered a promising option to improve resolution and throughput in nanolithography. This technique involves using a set of periodic, sparse chemical patterns on a substrate to direct the assembly of BCP films on top, resulting in feature sizes that are difficult to achieve from nanolithography of the BCP alone. However, controlling defect density of the resulting BCP films remains a challenge. Low-density multiplication factors and thinner films lead to dramatically fewer defects; however, these features undermine the promise of enhanced resolution and formation of robust masks for pattern transfer. To get around these issues, Wan et al. (DOI: 10.1021/ acsnano.7b03284) developed a technique that produces nearly perfect DSA of thick BCP films with high-density multiplication factors. This concept, which they name self-registered selfassembly (SRSA), involves first laying down a sparse, conventional chemical pattern on a substrate. On top, the researchers cast a thin film consisting of a BCP blended with a small fraction of end-functionalized polymer as chemical markers. During annealing, these markers graft to the substrate onto the first chemical pattern. After this film is washed away, leaving a new chemical pattern of self-registered markers, a thicker BCP film is cast over the pattern. Microscopy shows that this SRSA technique can create virtually defect-free patterns while maintaining robust BCP thickness. The authors suggest that this method could be extended to a variety of BCP materials and morphologies, leading to defect-free films with high resolution.
7559
DOI: 10.1021/acsnano.7b05667 ACS Nano 2017, 11, 7556−7559
