IT IS A WRAP: SILKWORM COCOON-LIKE COMPOSITE FOR LITHIUM-ION BATTERIES Silicon is a promising alternative anode material for the next generation of Li-ion batteries. However, the large volume changes that occur during Li insertion cause cracking of Si, the generation of uneven electrolyte interphase film on its surface, and electrical disconnection between Si and the current collector, all leading to quick capacity fade. Researchers have investigated numerous strategies to mitigate these issues, including incorporating Si nanoparticles into yolk−shell composites that enable expansion. However, the etching process necessary to generate these composites typically uses hydrogen fluoride, which is both environmentally unfriendly and can cause a significant reduction in Si content. To avoid these drawbacks, Du et al. (DOI: 10.1021/ acsnano.7b03830) report a strategy to prepare yolk−shell composites that results in porous Si nanorods (pSi NRs) encased in silkworm cocoon-like N-doped carbon (NC) shells. Their protocol starts with a simple magnesiothermic reduction method using mesoporous SiO2 nanorods as a template and Si source to create −Si NRs. The researchers then generated a sacrificial layer of Al2O3 on the surface of these NRs and further coated them with NC using chemical vapor deposition. After using dilute HCl to etch away the Al2O3 layer, the result was pSi NRs wrapped with NC with an internal void between. Experiments showed excellent cycling behavior, rate performance, and other electrochemical characteristics. Transmission electron microscopy and finite element analysis confirmed that the internal void enabled the pSi NRs to expand without destroying the NC nanoshells. The authors suggest that this composite shows great promise for mass production as a highperformance Li-ion battery anode.
To attack ROS in TBI, Yoo et al. (DOI: 10.1021/ acsnano.7b03426) developed a highly versatile, reproducible, and scalable method to synthesize core-cross-linked nanoparticles from polysorbate 80, a nonionic and biologically friendly surfactant that is widely used in pharmaceutical, cosmetic, and food applications. Using thiol-ene and thiolMichael chemistry, the researchers generated nanoparticles with a ROS reactive thioether cross-linked core stabilized in aqueous solution by hydroxy-functional oligoethylene oxide segments. Experiments both in solution and in vitro astrocyte models show that these nanoparticles can act as potent ROS sponges, reducing ROS levels by as much as 140-fold. In controlled cortical impact mouse models, the nanoparticles effectively controlled secondary spread of injury and neuroinflammation with no overt toxicity and were cleared through the liver and kidneys within 24 h. The authors suggest that these findings support further development of these nanoparticles as a therapy for TBI.
A NEW TWIST ON AMYLOID FIBRILLATION Several diseases are associated with undesired fibrillation of amyloid protein, including Alzheimer’s disease, prion encephalopathies, type II diabetes, and cystic fibrosis. This fibrillation arises from a misfolding process in which the proteins undergo a three-dimensional structural conversion from their typically soluble native forms into insoluble fibrillar aggregates. Understanding why these transformations take place and various possible conformations of amyloid proteins could lead to new ways to treat or to prevent these diseases. One way to study this phenomenon is through short amyloid fragments, which can exhibit polymorphic β-sheet structures when structural modifications are applied, just like the proteins’ full-size iterations. Using this strategy, Wang et al. (DOI: 10.1021/acsnano. 7b02325) determined the effects of various amino acid sequence changes on the self-assembly behavior and fibril morphology of amyloidogenic peptides derived from islet amyloid polypeptide (IAPP). This 37-amino-acid hormone, cosecreted with insulin to regulate glucose levels, has been implicated in type II diabetes through the deposition of amyloid plaques. After tailoring the terminal residues through substitution, elongation, and N-terminal capping/uncapping, the researchers used microscopy to assess how the altered fibrils compared to the native form. Dramatic changes included the
STOPPING BRAIN INJURIES’ SECONDARY SPREAD Traumatic brain injury (TBI) is a leading cause of death and disability in children and young adults. However, it is typically not the initial insult that causes the entirety of TBI’s damage; rather, reperfusion injury, delayed cortical edema, blood−brain barrier breakdown, and local electrolyte imbalance combine to cause secondary injury from a release of reactive oxygen species (ROS) into surrounding healthy brain tissue. In addition, neuroinflammation from reactive astrocytes and activated microglia that are recruited to damaged tissue releases additional ROS. Together, these circumstances render ROS a critical therapeutic target for TBI. © 2017 American Chemical Society
Published: September 26, 2017 8533
DOI: 10.1021/acsnano.7b06547 ACS Nano 2017, 11, 8533−8536
In Nano
www.acsnano.org
ACS Nano
In Nano
transformation of amyloid fibrils from twisted ribbons into untwisted structures triggered by the substitution of the Cterminal serine with threonine. After confirming this effect with subsequent substitutions with alanine and valine, the researchers ascribed it to restriction of intersheet torsional strain through increased hydrophobic interactions and hydrogen bonding. These insights, the authors suggest, could help inform structural interpretation of amyloidogenesis and could eventually lead to structure-specific imaging agents and aggregation inhibitors.
FORGING A BOND WITH SINGLE HYDROGEN ATOMS As scanning probe microscopy has improved over time, so has the ability to induce and to visualize chemical reactions at the atomic scale. In particular, noncontact atomic force microscopy (NC-AFM) has been used to produce reactions through mechanical force. Several recent studies have investigated this tool for force-induced atomic-scale switching, quantitative force measurements to induce the diffusion of single atoms and molecules, and molecular conformers and tautomerization. However, few reports have detailed the use of this tool for direct observation of mechanically induced covalent bonding of two different atoms. Huff et al. (DOI: 10.1021/acsnano.7b04238) confirm the utility of NC-AFM for this purpose, reporting the mechanically induced formation of a silicon−hydrogen covalent bond with this tool. Silicon dangling bonds (DBs) on H−Si(100) surfaces have recently been established as promising building blocks for complementary metal-oxide-semiconductor technology due to behavior similar to a single-atom quantum dot. To create a single DB, the researchers positioned a scanning tunneling microscope (STM) tip above a single hydrogen atom, then applied a voltage pulse for a few milliseconds. This localized electronic excitation caused the hydrogen to desorb selectively and often to transfer to the tip, demonstrated through a unique signature in frequency shift curves. Tips functionalized with a single hydrogen atom also significantly enhanced the STM contrast. When this adsorbed hydrogen atom was brought very close to a silicon DB, it passivated the DB through the mechanical formation of a silicon−hydrogen covalent bond. The authors suggest that this manipulation technique could be used in the emerging technology of on-surface DB-based nanoelectronic devices.
RECOGNIZING SIGN LANGUAGE WITH AN IONIC SENSOR Human muscle movement, joint motion, and bodily motion all generate large strain or stress change that could feasibly be monitored by wearable sensors, devices that provide an opportunity for disease diagnosis, health observation, motion control, and human−computer interaction. Whereas several advances in this area have focused on detecting tiny strain or stress changes for health monitoring, few have been made in the area of large-scale, complex, changeable human motions, such as wrist and finger movements. Seeking a way to accommodate these motions, Zhao et al. (DOI: 10.1021/acsnano.7b02767) developed an ionic sensor, an effort that could lead to devices for recognizing sign language. Their sensor incorporated a layer of two-dimensional, holey reduced graphene oxide as an inner electrode, a layer of Ag nanowires as the outer electrode, and carbon nanotubes as smart spacers that act as electrical conducting paths and mechanical bridging ligands between these electrodes. A layer of thermoplastic polyurethane (TPU) in the middle helps the device flex, and an ionic liquid was added as the electrolyte. When the sensor is flat, anions and cations in the ionic liquid uniformly distribute throughout the TPU membrane. However, when it becomes deformed, uneven distribution results in a voltage signal, a phenomenon that serves not only as an indicator but also as a driver that self-powers the device. Smart gloves that incorporate these sensors were able to generate unique potentials for signs for numbers and phrases such as “I love you” and “Hi, how are you?” The authors suggest that this sensor could provide opportunities for gesture recognition and human−computer interaction.
GOING WITH THE GRAIN BOUNDARY ON CURVED SURFACES Studies have long demonstrated that formation of defects can change a material’s macroscopic properties. In some instances, these degrade performance; however, in others, they can impart useful properties. Although the synthesis of two-dimensional (2D) crystals has attracted significant recent attention due to their diverse chemical makeups and electronic, mechanical, catalytic, and optical properties, their propensity to develop random defects during typical growth on a flat surface presents a challenge. One way around this conundrum is to develop methods to grow these 2D crystals such that defects develop deterministically rather than by chance. Yu et al. (DOI: 10.1021/acsnano.7b03681) detail the growth of 2D crystals on conical surfaces using theory, modeling, and experimental evidence. With calculations and modeling using the phase-field method, the researchers show that it is possible to track the crystal misorientation on a curved surface and to 8534
DOI: 10.1021/acsnano.7b06547 ACS Nano 2017, 11, 8533−8536
ACS Nano
In Nano
STITCHING TOGETHER TRANSITION-METAL DICHALCOGENIDES Two-dimensional materials other than graphene have rapidly attracted interest in the last several years, with transition-metal dichalcogenides (TMDs) as a particular focus due to their promising physical and chemical properties. Members of this broad class of materials each consist of sandwiches of metal and chalcogen atoms that assume one of three different atomic configurations in monolayer form, known as 1H, 1T, and 1T′. Some studies have reported using various techniques to induce some of these materials to transform from one phase to another or patterning them to create phase heterostructures. However, synthesizing in-plane heterostructures of monolayer TMDs of different atomic configurations has not yet been explored. To investigate this possibility, Naylor et al. (DOI: 10.1021/ acsnano.7b03828) combined semiconductor 1H-MoS2 and semimetal 1T′-MoTe2. Using chemical vapor deposition, the researchers grew single-crystal 1H-MoS2 flakes, then switched furnace conditions to grow 1T′-MoTe2, which nucleated at 1HMoS2 edges to form a stitched heterophase interface. The chemical composition and atomic structure of these heterostructures was confirmed by X-ray photoelectron spectroscopy and tip-enhanced Raman spectroscopy. Combining the observed Raman modes and density functional perturbation theory results suggested that Te was substituted for some S atoms in 1H-MoS2, findings supported by aberration-corrected scanning transmission electron microscopy. These findings helped explain the buckling in the 1H-MoS2 seen in atomic force microscopy images. Electrical measurements of a single heterostructure flake showed that 1H-MoS2 remained semiconducting even with Te substitutions and interfacial buckling. The authors suggest that this study motivates further work toward combining relatively unexplored 1T′ TMDs with 1H TMDs to create new in-plane TMD heterostructures.
detect the formation of grain boundaries (GBs). The GBs form as the crystal grain wraps around the cone and meets itself, except in cones with “magic” measurements, which accommodate the crystal orientation exactly as it grows without creating a GB. A GB also tends to form at the base of the cone as the crystal spreads onto a flat surface. Experiments with 2D crystal WS2 match these simulations, confirming their utility. The authors suggest that these findings offer guidance for engineering GBs in 2D materials based on their substrates, potentially providing a way to control material electronic and magnetic properties for devices and circuits.
NANOREACTORS FOR TRANSFORMING HGCL2 TO HG2CL2 The cavities inside carbon nanotubes (CNTs) can readily house various guest molecules and materials, which are then protected by mechanically and chemically stable shells. These shells do not just serve as a container; rather, several studies have shown that CNTs can act as effective nanoreactors, providing confined spaces for chemical reactions. The restricted nature of these spaces encourages reactions that can be difficult or impossible to produce in open space conditions. In some cases, the CNTs themselves contribute to the reaction by raising or lowering the activation barrier with their electronic cloud. Such participation could be particularly useful for the controlled synthesis of inorganic nanocrystals. Fedoseeva et al. (DOI: 10.1021/acsnano.7b04361) detail this very scenario, taking advantage of single-walled CNTs to convert environmentally hazardous HgCl2 to Hg2Cl2. In this reaction, pre-opened and purified single-walled CNTs and HgCl2 powder were mixed in ampules and heated above the melting temperature of HgCl2, providing an opportunity for the guest molecule to migrate into the CNTs. Various methods, including X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure spectroscopy, all showed that the resulting compound inside the CNTs had transformed to Hg2Cl2 nanocrystals. Further investigation indicated that the CNTs remained intact after the reaction except for p-doping by the encapsulated Hg2Cl2, suggesting that they serve as the electron-donating catalyst in this redox process. The authors suggest that this reaction could be useful for extracting Hg(II) from contaminated water and flue gases and could open a route to synthesizing other functional nanomaterials.
DETERMINING ELASTIC PROPERTIES IN LIQUID AND AIR? NOT A STRETCH Atomic force microscopy (AFM) has produced numerous advances in providing high-resolution maps of material properties. For example, this tool has been used to identify the surface structure of thin-film block copolymers, to measure the mechanical response of novel materials and devices, and to strengthen the relationship between cell mechanics, physiology, 8535
DOI: 10.1021/acsnano.7b06547 ACS Nano 2017, 11, 8533−8536
ACS Nano
In Nano
and disease. However, none of the techniques used in these advances have combined subnanometer resolution, quantitative accuracy, fast data acquisition speed, operation in air and liquid, and broad applicability for use on both soft matter to inorganic crystalline surfaces. Seeing a way to unite these attributes in a single technique, Amo et al. (DOI: 10.1021/acsnano.7b04381) looked to bimodal force microscopy. This method combines amplitude and frequency modulation feedback to enable fast, accurate, and subnanometer-scale Young’s modulus mapping on a wide variety of materials in both air and liquid. The researchers first developed a theoretical approach to convert observed results into Young’s modulus and deformation values, then tested the validity of their theoretical equations with a numerical simulator. Their findings show that bimodal force microscopy can provide accurate Young’s modulus measurements of surfaces ranging from 1 MPa to 100 GPa. As proof of principle, they demonstrated the versatility of this technique to measure metal−organic frameworks and mica in air, as well as protein membranes in liquid, with each measurement taking ∼30 s or less. The authors suggest that bimodal force microscopy fulfills several long-standing goals in microscopy and could have broad future applications.
8536
DOI: 10.1021/acsnano.7b06547 ACS Nano 2017, 11, 8533−8536
