Application of Inhibitor‐Loaded Halloysite Nanotubes in Active Anti‐Corrosive Coatings

Halloysite particles are aluminum‐silicate hollow cylinders with a length of 0.5–1 µm, an outer diameter of ca. 50 nm and a lumen of 15 nm. These nanotubes are used for loading and sustained release of corrosion inhibitors. The inhibitor is kept inside the particles infinitely long under dry conditions. Here, halloysite nanotubes filled with anticorrosive agents are embedded into a SiO_x–ZrO_x hybrid film. An aluminum plate is dip‐coated and immersed into 0.1 M sodium chloride aqueous solution for corrosion tests. A defect in the sol–gel coating induces pitting corrosion on the metal accompanied by a strong anodic activity. The inhibitor is released within one hour from halloysite nanotubes at corrosion spots and suppresses the corrosion process. The anodic activity is successfully restrained and the protection remains for a long time period of immersion in NaCl water solution. The self‐healing effect of the sol–gel coating doped with inhibitor‐loaded halloysite nanotubes is demonstrated in situ via scanning vibrating electrode technique measurements.     

Carbon‐Nanotube–PDMS Composite Coatings on Optical Fibers for All‐Optical Ultrasound Imaging

Polymer–carbon nanotube composite coatings have properties that are desirable for a wide range of applications. However, fabrication of these coatings onto submillimeter structures with the efficient use of nanotubes has been challenging. Polydimethylsiloxane (PDMS)–carbon nanotube composite coatings are of particular interest for optical ultrasound transmission, which shows promise for biomedical imaging and therapeutic applications. In this study, methods for fabricating composite coatings comprising PDMS and multiwalled carbon nanotubes (MWCNTs) with submicrometer thickness are developed and used to coat the distal ends of optical fibers. These methods include creating a MWCNT organogel using two solvents, dip coating of this organogel, and subsequent overcoating with PDMS. These coated fibers are used as all‐optical ultrasound transmitters that achieve high ultrasound pressures (up to 21.5 MPa peak‐to‐peak) and broad frequency bandwidths (up to 39.8 MHz). Their clinical potential is demonstrated with all‐optical pulse‐echo ultrasound imaging of an aorta. The fabrication methods in this paper allow for the creation of thin, uniform carbon nanotube composites on miniature or temperature‐sensitive surfaces, to enable a wide range of advanced sensing capabilities.     

Highly Efficient Coaxial TiO2‐PtPd Tubular Nanomachines for Photocatalytic Water Purification with Multiple Locomotion Strategies

Titania is a promising photocatalyst for water purification or production of solar fuels. However, due to its large band gap, titania is photoactive solely under UV light, which accounts for less than 5% of the solar spectrum. In this work, TiO₂‐based hybrid 1D nanostructures with photocatalytic activities extended to visible light region are designed and fabricated. Highly efficient coaxial TiO₂‐PtPd‐Ni nanotubes (NTs) are fabricated by a template‐assisted electrochemical synthesis route for water remediation under UV light, visible light, and natural sunlight. These coaxial hybrid nanotubes display a 100% degradation of organic pollutant rhodamine B in only 50 min (k‐value 0.071 min^?1) and 30 min under visible light and natural sunlight, respectively. For comparison, TiO₂ nanotubes doped with Pd nanoparticles are also fabricated and they show inferior photocatalytic properties and degrading stability over time. The multicomponent design enables to actuate the hybrid NTs by using two different external energy sources, i.e., magnetic and acoustic fields. Self‐propelled, autonomous actuation in the presence of H₂O₂ is also realized. These versatile actuation modes have the potential to enable the reported photocatalytic nanomachines to work efficiently under complex environments and to be easily collected for reuse.     

A Facile and Scalable Strategy for Fabrication of Superior Bifunctional Freestanding Air Electrodes for Flexible Zinc–Air Batteries

The large‐scale fabrication of efficient and inexpensive bifunctional catalysts is highly desirable but very challenging for oxygen reduction reaction and oxygen evolution reaction (ORR–OER) in metal–air batteries. Here, a facile and scalable approach for the fabrication of hierarchically porous air electrode consisting of cobalt nanoparticles embedded in bamboo‐like nitrogen‐rich carbon nanotubes (Co/N@CNTs), which are in situ grown onto the surface of carbon nanotube macrofilm (CNMF) through a catalytic growth of crosslinked carbon nanotubes is reported. The resulting hybrid macrofilm (Co/N@CNTs@CNMF) can be directly used as a freestanding air electrode without adding any binder or addivities. More importantly, when incorporated in a zinc–air battery (ZAB), the Co/N@CNTs@CNMF electrode demonstrates drastically enhanced ORR and OER activity while maintaining excellent durability during cycling. Further, when it is used to assemble an all‐solid‐state ZAB, the cell also displays excellent mechanical flexibility, implying promising perspectives as power sources for wearable electronics.     

Programmable Arrays of “Micro‐Bubble” Constructs via Self‐Encapsulation

The fabrication of ordered arrays of self‐encapsulated “micro‐bubble” material constructs based on the capillary‐driven collapse of flexible silk fibroin sheets during propagation of the diffusion front of the encapsulated material is demonstrated. The individual micro‐bubbles of different shapes are composed of a sacrificial material encapsulated within the ultrathin silk coating, which covers and seals the inner material during dissolution of supporting layer. The array of microscopic rectangular multi‐layer silk sheets on supporting polymer layers can be selectively dissolved along the edges to initiate their self‐encapsulation. The resulting micro‐bubble morphology, shape, and arrangements can be readily pre‐programmed by controlling the geometry of the silk sheets, such as thickness, dimension, and aspect ratio. These micro‐bubble constructs can be utilized for encapsulation of various materials as well as nanoparticles in a single or multi compartmental manner. These biocompatible and biodegradable micro‐bubble constructs present a promising platform for one‐shot spatial and controllable loading and locking material arrays with addressable abilities.     

Dual‐Activity Controlled Asymmetric Synthesis of Superconducting Lead Hemispheres**

A binary surfactant mixture of cetyltrimethylammonium bromide and polyvinyl pyrrolidone is used as the tailoring agent in the fabrication of lead micro/nanostructures. Electron microscopy studies indicate that the morphologies of the products can be efficiently controlled in this simple one‐step synthetic procedure. Intriguingly, well‐defined asymmetric functional colloids, Pb hemispheres, are obtained for the first time, and a dual‐activity‐controlled growth process is proposed to explain their formation. The magnetization measurements show that the as‐prepared samples are superconducting with the same transition temperature as bulk Pb. These findings prove the unique morphology tailoring efficacy of mixed surfactants, which could be used to obtain more variform structures or architectures in the fabrication of advanced materials.     

Multifunctional Nanobiomaterials for Neural Interfaces

Neural electrodes are designed to interface with the nervous system and provide control signals for neural prostheses. However, robust and reliable chronic recording and stimulation remains a challenge for neural electrodes. Here, a novel method for the fabrication of soft, low impedance, high charge density, and controlled releasing nanobiomaterials that can be used for the surface modification of neural microelectrodes to stabilize the electrode/tissue interface is reported. The fabrication process includes electrospinning of anti‐inflammatory drug‐incorporated biodegradable nanofibers, encapsulation of these nanofibers by an alginate hydrogel layer, followed by electrochemical polymerization of conducting polymers around the electrospun drug‐loaded nanofibers to form nanotubes and within the alginate hydrogel scaffold to form cloud‐like nanostructures. The three‐dimensional conducting polymer nanostructures significantly decrease the electrode impedance and increase the charge capacity density. Dexamethasone release profiles show that the alginate hydrogel coating slows down the release of the drug, significantly reducing the burst effect. These multifunctional materials are expected to be of interest for a variety of electrode/tissue interfaces in biomedical devices.     

Micro-“factory” for self-assembled peptide nanostructures

This study describes an integrated micro “factory” for the preparation of biological self-assembled peptide nanotubes and nanoparticles on a polymer chip, yielding controlled growth conditions. Self-assembled peptides constitute attractive building blocks for the fabrication of biological nanostructures due to the mild conditions of their synthesis process. This biological material can form nanostructures in a rapid way and the synthesis method is less expensive as compared to that of carbon nanotubes or silicon nanowires. The present article thus reports on the on-chip fabrication of self-assembled peptide nanostructures by means of a microfluidic device that is able to resist the harsh conditions imposed by the solvent used during the nanostructure synthesis. This on-chip fabrication was found to be simple, rapid, and convenient.     

Lessons Learned from Engineering Biologically Active Hybrid Nano/Micro Devices

Engineering devices based upon the interfacing of biological with inorganic systems have led to fascinating research results and present important implications for next‐generation technologies. The development of cell‐ and protein‐based micro/nano systems has demonstrated that several key factors must be considered when establishing fabrication rules. These include material interface properties, preserving biological viability, as well as self‐assembly as a device‐fabrication methodology, to name a few. Here, we present two proposed devices that have been developed through the application of these principles. They include muscle‐powered microfabricated devices, as well as protein‐functionalized polymeric vesicles based on protein‐coupling reactions. These systems have successfully bridged the gap between biological and conventional engineering to yield exciting prospects, as well as important lessons and questions for the development of cell‐/protein‐based hybrids.     

Magnetic,Multilayered Nanotubes of Low Aspect Ratios for Liquid Suspensions

This work presents a synthesis route for low‐aspect‐ratio nanotubes consisting of a layer of magnetite (Fe₃O₄) sandwiched between SiO₂ layers. In this template‐based strategy, self‐ordered porous alumina membranes are combined with the atomic layer deposition of SiO₂ and Fe₂O₃. An optimized electrochemical setup yields nanoporous Al₂O₃ membranes on 4‐inch Al substrates, which serve as templates for the large‐scale fabrication of nanotubes. A selective chemical etching step releases the magnetic tubes for suspension in a carrier fluid and permits recycling of the underlying aluminum foils for the fabrication of subsequent nanotube batches. The nanotubes consisting of an iron oxide layer protected by a silica shell are magnetically characterized in suspensions as well as in dried form on a substrate. High‐resolution transmission electron imaging reveals a polycrystalline, magnetite spinel structure of iron oxide, with the proper stoichiometry proven by the presence of the Verwey transition. Furthermore, field‐dependent viscosity measurements show an enhancement of the magnetoviscosity, thus demonstrating the technological potential of nanotube suspensions as a new class of ferrofluidic solutions. Owing to the tubular shape being closed at one end, these nanoparticles might additionally function as magnetic containers for targeted drug‐delivery or as chemical nanoreactors.     

Bench‐Top Fabrication of Hierarchically Structured High‐Surface‐Area Electrodes

Fabrication of hierarchical materials, with highly optimized features from the millimeter to the nanometer scale, is crucial for applications in diverse areas including biosensing, energy storage, photovoltaics, and tissue engineering. In the past, complex material architectures have been achieved using a combination of top‐down and bottom‐up fabrication approaches. A remaining challenge, however, is the rapid, inexpensive, and simple fabrication of such materials systems using bench‐top prototyping methods. To address this challenge, the properties of hierarchically structured electrodes are developed and investigated by combining three bench‐top techniques: top‐down electrode patterning using vinyl masks created by a computer‐aided design (CAD)‐driven cutter, thin film micro/nanostructuring using a shrinkable polymer substrate, and tunable electrodeposition of conductive materials. By combining these methods, controllable electrode arrays are created with features in three distinct length scales: 40 μm to 1 mm, 50 nm to 10 μm, and 20 nm to 2 μm. The electrical and electrochemical properties of these electrodes are analyzed and it is demonstrated that they are excellent candidates for next generation low‐cost electrochemical and electronic devices.     

Nano/Microrobots Meet Electrochemistry

Artificial autonomous self‐propelled nano and microrobots are an important part of contemporary technology. They are typically self‐powered, taking chemical energy from their environment and converting it to motion. They can move in complex environments and channels, deliver cargo, perform nanosurgery, act as chemotaxis and perform sense‐and‐act actions. The electrochemistry is closely interwoven within this field. In the case of self‐electrophoretically driven nano/microrobots, electrochemical mechanism has been the basis of power, which translates chemical energy to motion. Electrochemistry is also a major tool for the fabrication of these micro and nanodevices. Electrochemistry and electric fields can be used for the directing of nanorobots and for detection of their positions. Ultimately, nano and microrobots can dramatically improve performances of electrochemical sensors and biosensors, as well as of the energy generating devices. Here, all aspects in the fundamentals and applications of electrochemistry in the realm of nano‐ and microrobots are reviewed.     

Conductivity of SU‐8 Thin Films through Atomic Force Microscopy Nano‐Patterning

Processing flexibility and good mechanical properties are the two major reasons for SU‐8 extensive applicability in the micro‐fabrication of devices. In order to expand its usability down to the nanoscale, conductivity of ultra‐thin SU‐8 layers as well as its patterning by AFM are explored. By performing local electrical measurements outstanding insulating properties and a dielectric strength 100 times larger than that of SiO₂ are shown. It is also demonstrated that the resist can be nano‐patterned using AFM, obtaining minimum dimensions below 40nm and that it can be combined with parallel lithographic methods like UV‐lithography. The concurrence of excellent insulating properties and nanometer‐scale patternability enables a valuable new approach for the fabrication of nanodevices. As a proof of principle, nano‐electrode arrays for electrochemical measurements which show radial diffusion and no overlap between different diffusion layers are fabricated. This indicates the potential of the developed technique for the nanofabrication of devices.     

Diatom Frustule‐Inspired Metamaterial Absorbers: The Effect of Hierarchical Pattern Arrays

Diatoms are photosynthetic algae that exist ubiquitously throughout the planet in water environments. Over the preceding decades, the diatom exoskeletons, termed frustules, featuring abundant micro‐ and nanopores, have served as the source material and inspiration for myriad research efforts. In this work, it is demonstrated that frustule‐inspired hierarchical nanostructure designs may be utilized in the fabrication of metamaterial absorbers, thereby realizing a broadband infrared (IR) absorber with excellent performance in terms of absorption. In an effort to investigate the origin of this absorption characteristic, numerical models are developed to study these structures, revealing that the hierarchical organization of the constituent nanoparticulate metamaterial unit cells introduce an additional resonance mode to the device, broadening the absorption spectrum. It is further demonstrated that the resonant peaks shift linearly as a function of inter‐unit‐cell spacing in the metamaterial, which is attributed to the induced collective dipole mode by the nanoparticles. Ultimately, the work herein represents an innovative perspective in terms of the design and fabrication of IR absorbers inspired by naturally occurring biomaterials, offering the potential to lead to advances in metamaterial absorber technology.     

Bioinspired Superwettability Micro/Nanoarchitectures: Fabrications and Applications

Biological systems have evolved over billions of years to develop wetting strategies for advantageous structure–property–performance relations that are crucial for their survival. The discovery of these intriguing relationships has inspired tremendous efforts to investigate the micro/nanoscale features of naturally occurring structures with superwettability. Researchers have since developed new methods and techniques to construct artificial materials that mimic natural structures and functionalities. Here, a brief review of natural hierarchical architectures with liquid repellent properties is presented, and the critical underlying mechanism is summarized with an emphasis on the micro/nanoscopic architectures. The state‐of‐the‐art micro/nanofabrication techniques for creating bioinspired hierarchical superwettability structures that are categorized by random and exquisite features are also reviewed, followed by an overview of their emerging applications, with special attention to biomedical‐related fields. The development of fabrication techniques enhances capabilities relative to those of living systems, paving the way toward advanced structural materials with superior functions and unprecedented characteristics for potential applications.     

Hierarchical Carbon Nanotubes with a Thick Microporous Wall and Inner Channel as Efficient Scaffolds for Lithium–Sulfur Batteries

The proposal herein is based on an efficient sulfur host, namely hierarchical microporous–mesoporous carbonaceous nanotubes (denoted as HMMCNT) that feature a thick microporous wall and inner hollow channel. The electrochemical performance of the composite (HMMCNT‐S) is studied systematically at different discharge cut‐off voltages and at varying sulfur content. The cycling behavior in different voltage windows is compared and the highest specific capacity is shown for HMMCNT‐S‐50 in the range of 1.4–2.8 V. These results imply that better energy densities can be achieved by controlling the discharge cut‐off voltage. Moreover, we show that when the sulfur loading is 50% (HMMCNT‐S‐50), the cycling and rate performance is better than that of the composite loaded with 40% sulfur (HMMCNT‐S–40). Benefiting from the attractive hierarchical micro/mesoporous configuration, the obtained hybrid structure not only promotes electron and ion transfer during the charge/discharge process, but also efficiently impedes polysulfide dissolution. More specifically, the electrode can deliver a specific capacity of 558 mA h g^‐1 even after 150 cycles at a high rate of 1600 mA g^‐1 with a decay rate of only 0.13% per cycle. Considering the beneficial structure of these carbon nanotubes, it is very feasible that these structures may also be used in other research fields, including in catalysis, as supercapacitors, in drug‐delivery applications, for absorption, and so on.     

Cover Picture: Bilayer Organic–Inorganic Gate Dielectrics for High‐Performance,Low‐Voltage,Single‐Walled Carbon Nanotube Thin‐Film Transistors,Complementary Logic Gates,and p–n Diodes on Plastic Substrates (Adv. Funct. Mater. 18/2006)

Low‐voltage, hysteresis‐free, flexible thin‐film‐type electronic systems based on networks of single‐walled carbon nanotubes and bilayer organic–inorganic nanodielectrics are detailed in work by Rogers and co‐workers reported on p. 2355. The cover image shows a schematic array of such thin‐film transistors (TFTs) on a plastic substrate. The structure of the bilayer nanodielectric, which consists of a film of HfO₂ formed by atomic layer deposition and an ultrathin layer of epoxy formed by spin‐casting, is also illustrated schematically. High‐capacitance bilayer dielectrics based on atomic‐layer‐deposited HfO₂ and spin‐cast epoxy are used with networks of single‐walled carbon nanotubes (SWNTs) to enable low‐voltage, hysteresis‐free, and high‐performance thin‐film transistors (TFTs) on silicon and flexible plastic substrates. These HfO₂–epoxy dielectrics exhibit excellent properties including mechanical flexibility, large capacitance (up to ca. 330 nF cm^–2), and low leakage current (ca. 10^–8 A cm^–2); their low‐temperature (ca. 150 °C) deposition makes them compatible with a range of plastic substrates. Analysis and measurements of these dielectrics as gate insulators in SWNT TFTs illustrate several attractive characteristics for this application. Their compatibility with polymers used for charge‐transfer doping of SWNTs is also demonstrated through the fabrication of n‐channel SWNT TFTs, low‐voltage p–n diodes, and complementary logic gates.     

Self‐Sharpening Teeth: Self‐Sharpening Mechanism of the Sea Urchin Tooth (Adv. Funct. Mater. 4/2011)

The sea urchin tooth is a mosaic of calcite crystals shaped precisely into plates and fibers, cemented together by a robust calcitic polycrystalline matrix. The tooth is formed continuously at one end, while it grinds and wears at the opposite end, the sharp tip. Remarkably, these teeth enable the sea urchin to scrape and bore holes into rock, yet the teeth remain sharp rather than dull with use. Here we describe the detailed structure of the tooth of the California purple sea urchin Strongylocentrotus purpuratus, and focus on the self‐sharpening mechanism. Using high‐resolution X‐ray photoelectron emission spectromicroscopy (X‐PEEM), scanning electron microscopy (SEM), EDX analysis, nanoindentation, and X‐ray micro‐tomography, we deduce that the sea urchin tooth self‐sharpens by fracturing at discontinuities in the material. These are organic layers surrounding plates and fibers that behave as the “fault lines” in the tooth structure, as shown by nanoindentation. Shedding of tooth components at these discontinuities exposes the robust central part of the tooth, aptly termed “the stone”, which becomes the grinding tip. The precise design and position of the plates and fibers determines the profile of the tooth tip, so as the tooth wears it maintains a tip that is continually renewed and remains sharp. This strategy may be used for the top‐down or bottom‐up fabrication of lamellar materials, to be used for mechanical functions at the nano‐ and micrometer scale.     

Self‐Sharpening Mechanism of the Sea Urchin Tooth

The sea urchin tooth is a mosaic of calcite crystals shaped precisely into plates and fibers, cemented together by a robust calcitic polycrystalline matrix. The tooth is formed continuously at one end, while it grinds and wears at the opposite end, the sharp tip. Remarkably, these teeth enable the sea urchin to scrape and bore holes into rock, yet the teeth remain sharp rather than dull with use. Here we describe the detailed structure of the tooth of the California purple sea urchin Strongylocentrotus purpuratus, and focus on the self‐sharpening mechanism. Using high‐resolution X‐ray photoelectron emission spectromicroscopy (X‐PEEM), scanning electron microscopy (SEM), EDX analysis, nanoindentation, and X‐ray micro‐tomography, we deduce that the sea urchin tooth self‐sharpens by fracturing at discontinuities in the material. These are organic layers surrounding plates and fibers that behave as the “fault lines” in the tooth structure, as shown by nanoindentation. Shedding of tooth components at these discontinuities exposes the robust central part of the tooth, aptly termed “the stone”, which becomes the grinding tip. The precise design and position of the plates and fibers determines the profile of the tooth tip, so as the tooth wears it maintains a tip that is continually renewed and remains sharp. This strategy may be used for the top‐down or bottom‐up fabrication of lamellar materials, to be used for mechanical functions at the nano‐ and micrometer scale.     

Single‐Crystalline LiMn2O4 Nanotubes Synthesized Via Template‐Engaged Reaction as Cathodes for High‐Power Lithium Ion Batteries

Single‐crystalline nanotubes of spinel LiMn₂O₄ with a diameter of about 600 nm, a wall thickness of about 200 nm and a length of 1–4 μm have been synthesized via a template‐engaged reaction using β‐MnO₂ nanotubes as a self‐sacrifice template. In this fabrication, a minimal structural reorganization can be responsible for the chemical transformation from [001]‐oriented β‐MnO₂ template to [110]‐oriented LiMn₂O₄. Galvanostatic charge/discharge measurements indicate that the nanotubes exhibit superior high‐rate capabilities and good cycling stability. About 70% of its initial capacity can be retained after 1500 cycles at 5 C rate. Importantly, the tubular nanostructures and the single‐crystalline nature of the most LiMn₂O₄ nanotubes are also well preserved after prolonged charge/discharge cycling at a relatively high current density, indicating good structural stability of the single‐crystalline nanotubes during lithium intercalation/deintercalation process. As is confirmed from Raman spectra analyses, no evident microstructural changes occur upon long‐term cycling. These results reveal that single‐crystalline nanotubes of LiMn₂O₄ will be one of the most promising cathode materials for high‐power lithium ion batteries.