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GOING LONG: HALOGEN FUNCTIONALIZATION IN NANORIBBON GROWTH Graphene has attracted enormous attention over the past decade due to its extraordinary electronic and optical properties, yet its lack of a band gap significantly limits its use in electronic devices. Researchers have devised several approaches to open band gaps, such as geometrical confinement of graphene sheets through topdown approaches. However, this tactic only produces well-defined band gaps for structures with atomically precise geometries. The bottom-up on-surface synthesis of graphene nanoribbons (GNRs) has yielded structures with atomically precise width and edge geometries with width-dependent band structures. However, the mechanisms that limit the length of these structures, which are critical to their use in devices, are not well understood. To investigate what drives the growth of GNRs, Di Giovannantonio et al. (DOI: 10.1021/acsnano.7b07077) used scanning tunneling microscopy, fast X-ray photoelectron spectroscopy, and temperature-programmed desorption to study the roles of different halogens present in GNR precursor materials. The researchers compared armchair GNRs produced using the precursor 3′6′-dibromo-1,1′:2′1″-terphenyl (DBTP) with those that used the precursor 3′,6′-diiodo-1,1′:2′,1″-terphenyl (DITP). Experiments showed that GNRs grown from DITP were significantly longer than those grown from DBPT. Further investigation showed that this length discrepancy appears to stem from a larger separation between the polymerization and the cyclodehydrogenation temperatures in the iodine-containing compound. This difference limits the passivation of growing chains by atomic hydrogen. Kinetic curves that the researchers extracted from fast X-ray photoelectron spectroscopy data are consistent with these results and further elucidate the sequence of reactions occurring as a function of the temperature. The authors suggest that these findings could eventually be used for the rational design of GNRs for electronic applications.
components but in the way those components are connected. One challenge in developing synthetic systems that mimic this quality is developing a general manufacturing technique that enables the production of functionally different fibers from a single type of building block. This type of modular approach would not only be convenient from a design perspective but would also enable the direct correlation between tiny changes in how the subunits interact with a macroscopic change in the resulting material’s qualities. Toward this end, Pfeifer et al. (DOI: 10.1021/acsnano.7b06012) used DNA origami to create reconfigurable modules with two quasi-independent domains and four possible interfaces. Using specific recognition patterns, these modules are capable of facial and lateral growth. Using this strategy, the researchers created more than 15 homo-oligomeric filaments, all displaying exactly the same chemical content but differing both in the patterns of subunits and the types and numbers of interactions between them. The elasticity of these filaments could be regulated by both the components themselves and by how they interact. The flexibility of each building block changed, depending on the pattern of switchable DNA motifs, whereas association between the interdomain interfaces could be adjusted to vary periodicity and persistence length. Together, the authors suggest, these features successfully mimic the qualities that lend elasticity to natural protein fibers. They add that this design principle could eventually be used to create synthetic structures with bending strengths comparable to or even superior to their natural counterparts.
A CHARGED WAY TO FILTER RESULTS Most current membrane separation technologies use differences in solute size relative to membrane pores to separate compounds. However, being able to combine this size-based selectivity with the capability to separate solutes using other characteristics could greatly expand the use of membrane separations in chemical and
NOT A STRETCH: DNA ORIGAMI WITH CUSTOMIZED ELASTICITY The elasticity of natural protein fibers, such as actin filaments, microtubules, and bacterial flagella, is encoded not only in their © 2018 American Chemical Society
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the researchers avoided this problem by sandwiching stacks of MXenes with carbon nanotubes. These assemblies were spraycoated onto latex films and connected to electrodes, creating highly stretchable strain sensors. Tests showed that these sensors could be used to detect both tiny and large deformations with a detection limit of 0.1% strain. They also displayed high sensitivity, a tunable sensing range, and were stable for more than 5000 stretching cycles. The authors suggest that this system could offer a promising platform for monitoring in health and human motion applications.
pharmaceutical manufacturing. This chemical selectivity is intrinsic in biological pores such as porins, proton channels, and ion channels. Each of these structures is hydrophobic, only slightly larger than their target, and lined with functional groups that reversibly but selectively interact with the target. Sadeghi et al. (DOI: 10.1021/acsnano.7b07596) used these design principles to create membranes that separate organic molecules of similar size but have different electrostatic charges. The researchers formed micelles through the self-assembly of a random amphiphilic copolymer, poly(trifluoroethyl methacrylate-random-methacrylic acid) in methanol. They spread this solution as a thin film onto a commercial porous membrane. After evaporating the solvent briefly, they immersed the film into water to freeze the packed micelle array before the micelles merged. The resulting film had pores in the interstices between the micelles with sizes as low as 1−3 nm. Carboxylate groups on the micelles imbued these pores with negative charges. In filtration tests, these membranes readily separated anionic and neutral small organic molecules of similar sizes. In single-solute diffusion experiments, a neutral solute permeated through the membrane up to 263 times faster than the negatively charged compound. This selectivity was further enhanced in solute mixtures. The authors suggest that the carboxylate groups on these membranes could be functionalized, offering the opportunity to customize functionality even further.
WATCHING ATHEROSCLEROSIS IN ACTION Atherosclerosis, the gradual buildup of arterial plaque over time, contributes significantly to cardiovascular disease. Most types of atherosclerosis can be prevented with lifestyle modifications, making detection and monitoring of plaque development key in fighting cardiovascular disease. However, current imaging technologies have significant drawbacks. For example, noninvasive modalities such as coronary computed tomography angiography cannot be used to track the biological development of atherosclerosis, and plaque build-up must be fairly advanced for accurate visualization. Invasive techniques such as intravascular ultrasound carry significant risks, making them impractical for general monitoring over time. Seeking a noninvasive imaging modality that could track atherosclerosis from its biological beginnings, Wei et al. (DOI: 10.1021/acsnano.7b07720) harnessed the power of platelets, which have been implicated in atherosclerosis at multiple stages of development. The researchers created platelet membranecoated nanoparticles (PNPs) loaded with a far-red fluorescent dye. In vitro tests show that these particles readily bind to cells and tissues that play a role in atherosclerosis, including foam cells, collagen, and activated endothelium. In contrast, nanoparticles
FEELING THE STRAIN WITH MXENE/CARBON NANOTUBE SENSORS Stretchable and wearable sensors have a variety of applications, including as epidermal sensors for health monitoring systems. Toward this end, a large body of research has been devoted to developing large-area, high-performing, and stretchable sensing devices. However, the metal and semiconductor materials in conventional strain gauge sensors can only detect a narrow range of strain and exhibit low gauge factors due to their rigid nature. Although some nanostructures offer promising characteristics, efforts centered on these materials have not yet been able to realize a single type of sensor able to achieve low strain detection, high stretchability, ultrahigh sensitivity, tunable sensing ranges, and thin device dimensions. In a step closer to this goal, Cai et al. (DOI: 10.1021/ acsnano.7b06251) developed a sensor based on MXenes, a class of two-dimensional transition-metal carbides and carbonitrides with metallic conductivity and excellent mechanical properties. These materials typically delaminate into thin flakes that are difficult to assemble without losing their useful properties; however, 4
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methicillin-resistant Staphylococcus aureus, and Escherichia coli) showed that these nanozymes were highly selective toward biofilms but not toward mammalian cells. They had no significant toxicity toward fibroblasts, suggesting high biocompatibility. These qualities were highlighted in coculture models in which biofilms readily fluoresced after treatment with the nanozymes, but fibroblasts showed minimal fluorescence. The authors suggest that these pH-responsive nanozymes offer a viable strategy for penetrating and imaging a broad spectrum of biofilm-generating bacteria, circumventing the need for microbe-specific probes.
coated with polyethylene glycol or red blood cell membranes had no affinity for these tissues. Platelet membrane-coated nanoparticles also bound both to areas of overt plaque formation in a mouse model of atherosclerosis as well as asymptomatic areas in which the underlying biological processes of atherogenesis had already begun. The researchers demonstrated live imaging of plaques by loading PNPs with a commercial magnetic resonance imaging contrast agent. The authors suggest that this technique could eventually be used for the prevention and management of cardiovascular disease.
IMAGING BIOFILMS WITH NANOZYMES Biofilms are three-dimensional bacterial communities of microbes living in an extracellular polymeric substance (EPS) matrix, which protects them from exogenous agents. About 80% of bacterial infections are associated with biofilm formation on living tissue. These infections tend to be diagnosed only after they have become systemic or caused significant harm, underlining the importance of effective imaging. Current imaging techniques for bacteria use agents that are generally unable to penetrate the dense and amphiphilic EPS matrix; some imaging agents, such as fluorescent dyes conjugated to biorecognition elements, also have a high rate of false responses due to phenotypic mutations of biofilm-residing microbes. To address these difficulties, Gupta et al. (DOI: 10.1021/ acsnano.7b07496) developed a method to image biofilm-associated infections using charge-switchable “nanozymes”. The researchers functionalized 2 nm gold nanoparticles with pH-responsive sulfonamide, which become protonated and develop adhesive qualities under the weakly acidic conditions of a typical biofilm. Encapsulated in these particles is a bioorthogonal catalyst, which provides a sensitive readout mechanism. Tests on biofilms generated by three different types of bacteria (Enterobacter cloacae,
TRUE CALLING: USING CELL PHONES TO DIAGNOSE AND TO MONITOR EBOLA Ebola virus disease (EVD) causes periodic epidemics of hemorrhagic illness that are widespread and result in many deaths. Thus, there is an urgent need for point-of-care diagnostic tools for survivors to understand and to limit the spread of this disease. Current serological diagnosis of Ebola typically relies on enzymelinked immunosorbent assays (ELISA), which take hours to perform in suitably equipped laboratories. A promising alternative to ELISA is lateral flow immunochromatography, which tests for immune markers of disease using color changes on a strip. Coupling this method with smart phones as readout devices could offer ways of analyzing, storing, transmitting, and sharings results readily in the field. Capitalizing on these technologies, Brangel et al. (DOI: 10.1021/acsnano.7b07021) developed immunochromatographic strips that test for Ebola-specific antibodies in serum and a custom smartphone app that gleaned results from test strips while also quantifying and geotagging the findings. The test strips had one of two configurations: either a single test line plotted with one type of recombinant viral protein or three test lines plotted with three different recombinant viral proteins. Gold nanoparticles conjugated with antihuman IgG antibodies provided a color shift upon a positive result. Tests of the monoplex in human survivors and controls showed 100% sensitivity and 98% specificity compared to ELISA. The multiplex test had a sensitivity and specificity of 100% compared to ELISA. The researchers also developed a multiplex test for different strains of Ebola. The authors suggest that these tests hold great potential as a field tool for diagnosis, vaccine development, and therapeutic evaluation.
STATE-OF-THE-ART NANOTECHNOLOGIES FOR CANCER CARE Various nanotechnologies have been explored over the past few decades, with tremendous growth and success from a technological perspective. From the first approval of a liposomal formulation of a chemotherapeutic agent in 1995 to more than 80 currently active clinical trials exploring cancer nanotechnologies, this field continues to advance. However, its progress is limited by an incomplete understanding of human cancer biology. In a review article, Hartshorn et al. (DOI: 10.1021/acsnano.7b05108) explore several evolving areas in cancer nanotechnology that 5
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have had successes but also come with key clinical and biological challenges that need to be addressed to improve patient outcomes. For example, researchers are exploring harnessing the intrinsic immune-activating properties of nanoparticles to use these particles as immune adjuvants, to deliver pro-inflammatory and proimmune molecules, or as tools to develop tumor-targeting T-cells. Nanomaterials are also being explored as tools to distinguish cancerous tissue during resection rather than using traditional visual and tactile cues. Stimuli-sensitive nanostructures can assemble or disassemble spurred by stimuli in the unique microenvironment of tumors, enabling better diagnostics and targeted therapies. In addition, more targeted therapies, and even therapies with the ability to cross the blood−brain barrier, may be developed through functionalizing nanoparticles with molecules that target specific receptors. Nanoparticles can also be used as local emitters of radiation or as radiosensitizers for improving more traditional radiation therapy. The authors suggest that as each of these areas continues to advance, patients will have even better options for nanotechnology-based cancer care.
varying nanostructures in a fused silica glass, making formbirefringent patterns that integrate two different functional geometric phase lenses into a dynamic phase lens. Experiments show that this meta-lens can be combined with two independent detectors to retrieve the polarization state of the incident beam simultaneously with very limited form factor. The authors suggest that these findings provide extra degrees of freedom to control spin photonics and note that such a broadband spin Hall meta-lens could eventually find applications in imaging, sensing, and multifunctional spin photonics devices.
PHOTONIC SPIN HALL, READY FOR ITS CLOSE-UP Much like the spin of electrons is split in an electric field, a phenomenon known as the spin Hall effect, photons can also undergo spin-dependent splitting. The advent of metasurfaces, which greatly enhance the spin−orbital interaction, has enabled a wealth of research into the photonic spin Hall effect (PSHE). These investigations suggest that applications of PSHE, such as metrology and spin-based photonic devices, may be possible. However, major research efforts have been restricted to singledimension modulation of light, including transverse or longitudinal spin-dependent splitting. Accomplishing multidimensional spindependent splitting in a single device has remained a challenge. Toward that end, Zhou et al. (DOI: 10.1021/acsnano.7b07379) designed a spin Hall meta-lens that can manipulate photonic spindependent splitting induced by spin−orbital coupling in transverse and longitudinal directions simultaneously, with low dispersion and more than 90% diffraction efficiency. The researchers formed this device through femtosecond laser writing of spatially 6
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