NEEDLING TUMORS AWAY Cancer is typically treated with surgery, chemotherapy, radiation, or a combination of these three modalities. However, these treatments sometimes have limited efficacy and are associated with toxicity and side effects. One alternative that is gaining increasing attention is photothermal therapy (PTT), in which plasmonic nanoparticles localized to tumors convert focused light into heat, irreversibly damaging cells and subsequently destroying tumors. Although this modality has shown promise, it has drawbacks that must be addressed. For example, because repeated treatments are typically necessary, multiple injections of nanoparticles can have adverse and uncomfortable side effects. To avoid this issue while capitalizing on the treatment’s positive effects, Chen et al. (DOI: 10.1021/acsnano.5b05043) developed a near-infrared light-activatable microneedle system that delivers PTT and encapsulated cancer drugs at the same time. This system is made of embeddable polycaprolactone microneedles in a dissolvable poly(vinyl alcohol)/polyvinylpyrrolidone support array patch. When pressed into the skin, the supporting patch dissolves in the interstitial fluid, leaving behind microneedles embedded in the tumor. Nanoparticles in these microneedles convert infrared light from a laser into heat, sharply increasing temperature in the tumor. This heat damages tumor cells while also melting the microneedles, releasing doxorubicin, a cancer-fighting drug. By switching the laser on for short cycles, the researchers show that this system can be used for repeat treatments after a single microneedle delivery. These treatments successfully eradicated tumors in a mouse superficial tumor model. The authors suggest that this method could eventually fight superficial tumors effectively, conveniently, and tolerably in human patients.
HFBI membranes have high buckling strength, 66.9 mN/m. Because the surface tension of water is only 42 mN/m, the researchers suggest that this protein’s buckling strength is responsible for the droplets’ flattened dome conformations, changing the water’s natural shape at the air/water interface.
MINERAL PRECIPITATION UNDER THE MICROSCOPE Precipitation of dissolved salts in aqueous solutions is involved in a variety of bio-, geo-, and soil-mineralization, and is the most common way of producing crystals in nature. When recapitulating this phenomenon synthetically, having fundamental understanding of crystallization from solution is necessary to control and to optimize crystal structure, morphology, and size of the products. Particularly important in this process is crystal nucleation and subsequent growth. Although gaining greater understanding of this process is pivotal for many useful applications, it has been stymied by a lack of experimental methods that can study processes that occur in liquid with high spatial and temporal resolution. In a new study, Yuk et al. (DOI: 10.1021/acsnano.5b04064) developed a method to study mineral precipitation in real time with in situ graphene liquid cell electron microscopy. Using an electron beam to induce radiolysis in water with dissolved sodium persulfate salts, the researchers were able to view the precipitation of the mineral thernadite (Na2SO4) within the graphene liquid cells using high-resolution transmission electron microscopy. Their findings show that this mineral nucleates on the graphene surfaces in two-dimensional island structures. These mineral grains then grow by grain boundary migration and grain rotation. The authors suggest that this method provides direct observation of the dynamics of crystal growth from ionic solutions that faithfully mimic mineral precipitation processes occurring in nature, such as mineral nucleation and mineral grain growth in highly supersaturated solutions, and can be applied to a variety of other naturally occurring minerals.
EXPLAINING FLAT TOP DROPS Many proteins self-organize into intricate structures. One example is the Trichoderma reesei hydrophobin HFBI, a small amphiphilic protein produced in the cell walls of fungi that self-organizes at phase interfaces, such as air/water or water/solid interfaces. When HFBI self-organizes at air/water interfaces, it forms twodimensional structures that cause liquid droplets to form an unusual shape with a flattened top. The mechanism behind this unusual conformation was not yet known. In a new study, Yamasaki et al. (DOI: 10.1021/acsnano. 5b04049) used atomic force microscopy (AFM) to investigate this phenomenon. Images taken of water droplets containing dissolved HFBI showed increased flattening of their tops over time. Using a fluorescent stain on HFBI, microscopy studies showed organized membranes on the flat tops of the domical drops. After transferring this membrane to a highly oriented pyrolytic graphite substrate for further imaging, the researchers found that these membranes are composed of an array of honeycomb structures. Further AFM studies show that the © 2016 American Chemical Society
Published: January 26, 2016 3
DOI: 10.1021/acsnano.6b00076 ACS Nano 2016, 10, 3−5
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THE REVERSE SANDWICH: BETTER UNDERSTANDING AN ANTI-MOS2 Transition metal dichalcogenides, low-dimensional materials with high anisotropy, have attracted enormous attention for their potential in fields including electronics, sensing, energy generation, and storage. Receiving significantly less, but growing, interest are other metal chalcogenides, particularly those containing group 13 elements such as thallium. Unlike transition metal dichalcogenides, which contain a layer of metal sandwiched between layers of chalcogens, compounds such as Tl2S contain the chalcogen sandwiched between metal layers. Consequently, the electronic structures of this type of compound are to differ substantially from those of transition metal dichalcogenides such as MoS2. To explore its electrochemical and electrocatalytic behavior, Chia et al. (DOI: 10.1021/acsnano.5b05157) performed a variety of experiments on Tl2S. Cyclic voltammetric scans revealed that this material exhibits four distinct redox signals at ca. +0.4 V, −0.5 V, −1.0 V, and −1.5 V vs Ag/AgCl in both directions, suggesting that this material has innate electroactivity. Further investigation showed that Tl2S possesses slow electron transfer abilities with a rate as low as 6.3 × 10−5 cm s−1, a rate significantly slower than MoS2. When the researchers evaluated its potential as an electrocatalyst for the hydrogen evolution reaction, results showed an efficiency higher than that of a bare glassy carbon electrode. However, Tl2S’s poor conductivity still leaves it far less efficient than MoS2. Further investigation into this material’s electronic properties shows an unusually narrow band dispersion around the Fermi level. The authors suggest that this material’s unusual properties could eventually lend themselves to a variety of useful applications.
random orientations. However, after placing pads of poly(dimethylsiloxane) (PDMS) onto the films before annealing, the researchers found that the pores instead became vertically aligned and physically continuous through the thickness of the film. Further investigation showed dramatic differences in transport properties between open and confined films, with confined films showing a 44-fold increase in room-temperature ion conductivity compared to open films. The authors suggest that these films could eventually find applications in wastewater treatment, water softening, and separation of small organic contaminants.
SHEDDING LIGHT ON NANOSTRUCTURED SE−TE FILMS Photolithography typically uses a photomask to create desired configurations on a surface. Another way to direct photo-driven processes and to produce target structures is to manipulate the polarization of the light. Recently, investigators discovered that semiconducting Se−Te films grown photoelectrochemically using lineally polarized light produced highly anisotropic, nanoscale lamellar patterns that had orientations correlated with the electric field vector of the incident light. These structures were formed without any physical or chemical templating agent or photomask. To understand how to control this phenomenon, Carim et al. (DOI: 10.1021/acsnano.5b05119) grew Se−Te films using light from two different sources with different polarizations simultaneously. Using two non-orthogonally polarized samewavelength sources, the researchers generated films in which the long axes of the lamellae were oriented parallel to the intensity-weighted average polarization orientation. Computer simulations faithfully replicated the experimental morphologies and quantitatively agreed with the experimental pattern orientations by considering only the fundamental light−material interactions during growth. In other experiments, depositing with light from two orthogonally polarized same-wavelength as well as different-wavelength sources produced structures that had two intersecting sets of orthogonally oriented lamellae in which the heights of the two sets could be varied by adjusting the relative source intensities. Simulations suggested that the lamellae preferentially absorbed light polarized with the electric field vector along their long axes. The authors suggest that the resulting morphology of the produced patterns results from a combination of all light inputs despite different polarizations. Tailoring these inputs, they add, provides control that could lead to novel and useful ways to generate complex, three-dimensional patterns.
SOFT SOLUTION FOR CONTINUOUS VERTICALLY ALIGNED NANOPORES Membrane separation processes, such as ultrafiltration and nanofiltration, are critically important for a variety of fields, including food processing, antibiotics manufacturing, and water purification. The ideal membranes for these applications have pores of approximately the same diameter and are aligned or oriented parallel to the macroscopic transport direction. However, stateof-the-art polymer membranes are still far from this ideal. Ultrafiltration membranes with pore sizes between 10 and 100 nm tend to have highly tortuous interconnected pores with large variations in pore diameters. Although continuous vertically aligned nanopores have been realized in films fabricated with block copolymers for nanofiltration, pore sizes lower than 5 nm have yet to be achieved. To achieve this goal with smaller pore sizes, Feng et al. (DOI: 10.1021/acsnano.5b06130) developed a method that uses soft confinement of porous films using an elastomeric polymer pad. The researchers first created thin films of the amphiphilic liquid crystal monomer Na-GA3C11, composed of wedge-shaped molecules that assemble to form hexagonally packed nanopores with an estimated diameter of 1 nm. After thermal annealing and crosslinking, microscopy studies showed that these nanopores adopted
SILICON AT GRAPHENE EDGES GOES THE DISTANCE Dopants in graphene, such as N, Si, and Fe, can play several fundamentally interesting and potentially useful roles. Consequently, they have been well studied within the graphene lattice. Dopants have been directly imaged using aberration 4
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efficiency values were shown in holey silicon with a neck size of 24 nm. Capitalizing on these findings could eventually aid in developing efficient new silicon-based thermoelectric devices.
corrected transmission electron microscopy (AC-TEM), and the nature of chemical bonding has been explored with electron energy-loss spectroscopy (EELS) in conjunction with scanning transmission electron microscopy with an Ångström-scale electron probe. Studying dopants at the graphene edge, however, has proven challenging because edges are rapidly degraded by chemical etching under the electron beam, leading to high mobility of edge dopant atoms. To avoid this problem, Chen et al. (DOI: 10.1021/acsnano. 5b06050) developed an imaging method in which an in situ heating holder is used to image samples at 800 °C, since chemical etching of graphene edges under the electron beam has been shown to be significantly reduced at elevated temperatures. Using this method, the researchers detailed Si-edge interactions and measured Si−C bond lengths in various edge configurations, including armchair, zigzag, reconstructed zigzag, and extended Klein (EK). Their findings show that pentagonal bonding structures at the armchair edge are most common. Substitutional and edge bonded Si atoms were fairly stable along zigzag edges, and groups of Si atoms along this edge type can form an extended Si Klein edge. Compared to dopants in the bulk lattice, Si along edges appeared to reduce out-of-plane distortion and form elongated bonds. The authors report that these findings could inform future catalytic and electronic applications.
MECHANICALLY TUNABLE DIELECTRIC METASURFACES? NOT A STRETCH Researchers have devoted increasing attention to developing new ways to manipulate light for next-generation information processing. Instead of bulky conventional components based on geometrical optics, several recent studies have focused on optical metasurfaces made of planar thin film layers of subwavelength resonant elements, which can reduce device size. Within this realm, investigators have shown that dielectric resonators, rather than metallic ones, can boost the efficiency of light manipulation. However, apart from efficiency, the ability to tune the responses of these metasurfaces is important to ensure that their functionality can be harnessed for real-world applications. In a new study, Gutruf et al. (DOI: 10.1021/acsnano.5b05954) developed a mechanically tunable metasurface using an array of uniform TiO2 cylindrical dielectric resonators embedded in a polydimethylsiloxane (PDMS) elastomeric matrix. Deformation of the soft PDMS enables tuning the period of the array without altering the shape of the hard TiO2 resonators. Under uniaxial strain, the embedded array shows a resonance shift toward longer wavelengths for excitation perpendicular to the strain direction and toward shorter wavelengths for excitation polarized along the strain direction. With only 6% applied strain, the measured resonance peak shifts 5.08% to red and 0.96% to blue from the base resonance wavelength under different polarizations, spectral behavior well predicted with mechanical and electromagnetic finite element modeling. Further analysis with a Lagrangian model suggests that strain modulates the near-field mutual interaction between resonant dielectric elements. The authors suggest that this work demonstrates an approach for tuning the resonance in alldielectric low-loss metasurfaces, offering a building block for high-efficiency optical devices.
HOLEY GRAIL FOR THERMOELECTRIC TRANSPORT Thermoelectric power generation from solar, automobile, and industrial heat sources has received renewed attention recently due to ever-expanding research on nanostructured materials that could play a useful role for this application. Silicon, in particular, has been considered for this purpose. However, its overall thermoelectric performance is poor due to its high thermal conductivity. Recent findings suggest that nanostructuring silicon could overcome this drawback by damping phonon transport without causing severe electron scattering. For example, precisely roughening silicon nanowires suppressed their thermal conductivity. However, because controlling roughness on a large scale remains a challenge, researchers have turned to holey silicon as an alternative. This singlecrystalline material containing an array of periodic holes several tens of nanometers in diameter also shows significant thermal conductivity reduction. However, the characteristics necessary to optimize this benefit remain largely unknown. In a new study, Lim et al. (DOI: 10.1021/acsnano.5b05385) tested holey silicon samples with tailored neck size, doping level, and ribbon geometry using a custom microdevice fabricated by combining block copolymer lithography with conventional lithography. This microdevice was capable of measuring the Seebeck coefficient, electrical conductivity, and thermal conductivity simultaneously for the same holey silicon microribbon. Results showed that neck size was a critical factor in performance. At 300 K, thermal conductivity as low as 1.8 ± 0.2 W/mK was found in holey silicon with the lowest neck size evaluated, 16 nm. Optimized heat-to-electricity conversion 5
DOI: 10.1021/acsnano.6b00076 ACS Nano 2016, 10, 3−5
