Ultrafast 2D Nanosheet Assembly via Spontaneous Spreading Phenomenon.

In 2D materials, a key engineering challenge is the mass production of large-area thin films without sacrificing their uniform 2D nature and unique properties. Here, it is demonstrated that a simple fluid phenomenon of water/alcohol solvents can become a sophisticated tool for self-assembly and designing organized structures of 2D nanosheets on a water surface. In situ, surface characterizations show that water/alcohol droplets of 2D nanosheets with cationic surfactants exhibit spontaneous spreading of large uniform monolayers within 10 s. Facile transfer of the monolayers onto solid or flexible substrates results in high-quality mono- and multilayer films with high coverages (>95%) and homogeneous electronic/optical properties. This spontaneous spreading is quite general and can be applied to various 2D nanosheets, including metal oxides, graphene oxide, h-BN, MoS2, and transition metal carbides, enabling on-demand smart manufacture of large-size (>4 inchϕ) 2D nanofilms and free-standing membranes.

The Development of Aptamer-Coupled Microelectrode Fiber Sensors (apta-µFS) for Highly Selective Neurochemical Sensing.

The selective and sensitive sensing of neurochemicals is essential to decipher in-brain chemistry underlying brain pathophysiology. The recent development of flexible and multifunctional polymer-based fibers has been shown useful in recording and modulating neural activities, primarily electrical ones. In this study, we were able to realize fiber-based neurochemical sensing with high sensitivity and selectivity. We achieved a generalizable method to couple aptamers, a type of synthetic receptors on the carbon composites within fibers, as microsensors for highly selective neurochemical detection. Such an aptamer-coupled microelectrode fiber sensor (apta-µFS) enables simple, label-free, and sensitive dopamine (DA) detection down to 5 nM with ultrahigh specificity across major interferents. We succeeded in monitoring DA selectively within the living brain using our apta-µFS. We further showed the proof-of-concept of using microelectronic fiber-based toolsets to target neural pathways across electrical and chemical modalities. In summary, such fiber-based toolsets hold great potential to advance multimodal mechanistic understanding of brain pathophysiology.

Useful High-Entropy Source on Spinel Oxides for Gas Detection.

This study aimed to identify a useful high-entropy source for gas detection by spinel oxides that are composed of five cations in nearly equal molar amounts and free of impurities. The sensor responses of the spinel oxides [1# (CoCrFeMnNi)3O4, 2# (CoCrFeMnZn)3O4, 3# (CoCrFeNiZn)3O4, 4# (CoCrMnNiZn)3O4, 5# (CoFeMnNiZn)3O4, and 6# (CrFeMnNiZn)3O4] were evaluated for the test gases (7 ppm NO2, 5000 ppm H2, 3 ppm NH3, and 3 ppm H2S). In response to NO2, 1# and 2# showed p-type behavior while 3-6# showed n-type semiconductor behavior. There are three p-type and one n-type AO structural compositions in AB2O4[AO·B2O3] type spinel, and 1# showed a stable AO composition because cation migration from site B to site A is unlikely. Therefore, it was assumed that 1# exhibited p-type behavior. The p-type behavior of 2# was influenced by Cr oxide ions that were present at the B site and the stable p-type behavior of zinc oxide at the A site. The spinel oxides 3# to 6# exhibited n-type behavior with the other cationic oxides rather than the dominant p-type behavior exhibited by the Zn oxide ions that are stable at the A site. In contrast, the sensor response to the reducing gases H2, NH3, and H2S showed p-type semiconductor behavior, with a particularly selective response to H2S. The sensor responses of the five-element spinel oxides in this study tended to be higher than that of the two-element Ni ferrites and three-element Ni-Zn ferrites reported previously. Additionally, the susceptibility to sulfurization was evaluated using the thermodynamic equilibrium theory for the AO and B2O3 compositions. The oxides of Cr, Fe, and Mn ions in the B2O3 composition did not respond to H2S because they were not sulfurized. The increase in the sensor response due to sulfurization was attributed to the decrease in the depletion layer owing to electron sensitization, as the top surface of the p-type semiconductors, ZnO and NiO, transformed to n-type semiconductors, ZnS and NiS, respectively. High-entropy oxides prepared using the hydrothermal method with an equimolar combination of five cations from six elements (Cr, Mn, Fe, Co, Ni, and Zn) can be used as a guideline for the design of high-sensitivity spinel-type composite oxide gas sensors.

Bio-inspired Incrustation Interfacial Polymerization of Dopamine and Cross-linking with Gelatin toward Robust, Biodegradable Three-Dimensional Hydrogels.

In nature, laccase enzymatically catalyzes the reaction of phenolic compounds with oxygen to produce hardened surfaces known as cuticles on insects and plants. Inspired by this natural process, the present work investigated a robust, biodegradable hydrogel synthesized from dopamine and gelatin. This gel is obtained by the oxidation of dopamine dissolved in water, after which the resulting quinone compound automatically undergoes self-polymerization. The oxidized dopamine subsequently undergoes Schiff base and Michael addition reactions with gelatin, such that the exposed gelatin surface cross-links to generate a continuous hardened hydrogel film. Because gelatin transitions between sol and gel states with changes in temperature, two- and three-dimensional structures could be obtained from the gel state. This bio-inspired interfacial cross-linking reaction provides a simple means of forming complex morphologies and represents a promising technique for bio-applications.

Synthesis of unused-wood-derived C-Fe-N catalysts for oxygen reduction reaction by heteroatom doping during hydrothermal carbonization and subsequent carbonization in nitrogen atmosphere.

There is an urgent need to develop renewable sources of energy and use existing resources in an efficient manner. In this study, in order to improve the utilization of unused biomass and develop green processes and sustainable technologies for energy production and storage, unused Douglas fir sawdust (SD) was transformed into catalysts for the oxygen reduction reaction. Fe and N were doped into SD during hydrothermal carbonization, and the N- and Fe-doped wood-derived carbon (Fe/N/SD) was carbonized in a nitrogen atmosphere. After the catalyst had been calcined at 800°C, its showed the highest current density (-5.86 mAcm-2 at 0.5 V versus reversible hydrogen electrode or RHE) and Eonset value (0.913 V versus RHE). Furthermore, its current density was higher than that of Pt/C (20 wt% Pt) (-5.66 mA cm-2 @0.5 V versus RHE). Finally, after 50 000 s, the current density of sample Fe/N/SD (2 : 10 : 10) remained at 79.3% of the initial value. Thus, the synthesized catalysts, which can be produced readily at a low cost, are suitable for use in various types of energy generation and storage devices, such as fuel cells and air batteries. This article is part of the theme issue 'Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 2)'.

Electrochemical Sensor Based on Reduced Graphene Oxide/Double-Walled Carbon Nanotubes/Octahedral Fe3O4/Chitosan Composite for Glyphosate Detection.

In this work, reduced graphene oxide/double-walled carbon nanotubes/octahedral-Fe3O4/chitosan composite material modified screen-printed gold electrodes (rGO/DWCNTs/Oct-Fe3O4/Cs/SPAuE) under inhibition of urease enzyme was developed for the determination of glyphosate (GLY). The electrochemical behaviors of GLY on these electrodes were evaluated by square wave voltammetry (SWV). With the electroactive surface area is 1.7 times higher than that of bare SPAuE, the rGO/DWCNTs/Oct-Fe3O4/Cs/SPAuE for detection of GLY shows a low detection limit (LOD) of ~ 0.08 ppb in a large concentration range of 0.1-1000 ppb. Moreover, it is also successfully applied to the determination of GLY in river water samples with recoveries and relative standard deviations (RSDs) from 98.7% to 106.9% and from 0.79% to 0.87%, respectively. The developed composite will probably provide an universal electrochemical sensing platform that is very promising for environmental monitoring.

Totally transparent hydrogel-based subdural electrode with patterned salt bridge.

A totally transparent subdural electrode was developed by embedding a conductive poly (vinyl alcohol) (PVA)-filled microchannel made of poly(dimethylsiloxane) (PDMS) into an another PVA hydrogel substrate. Tight bonding between the PVA substrate and the PDMS microchannel (salt bridge) was achieved by mechanical interlocking utilizing the microprotrusions formed on the microchannel. This simple method of bonding without the use of any additives such as silane molecules or nanofibers is very suitable for constructing biomedical devices. The salt bridge electrode (total thickness, ca. 1.5 mm) was sufficiently soft, and showed superior shape conformability that makes it an excellent choice as a subdural electrode used on the brain surface. In vivo measurement proved that the salt bridge electrode makes close contact to the exposed porcine brain and can record brain wave signals of frequencies 1 ~ 15 Hz. In addition, the high transparency of the electrode provided a clear view of the brain surface that would assist the effective surgical operation and optogenetic research.

Bioimaging using bipolar electrochemical microscopy with improved spatial resolution.

In this study, we developed bipolar electrochemical microscopy (BEM) using a closed bipolar electrode (cBPE) array with an electrochemiluminescence (ECL) detecting system. Because cBPEs are not directly connected to a detector, high spatio-temporal resolution imaging can be achieved by fabricating a microelectrode array in which each electrode point is arranged in a short interval. A cBPE array with individual cBPEs arranged in 41 µm intervals was successfully fabricated by depositing gold in the pores of a track-etched membrane using electroless plating. Using BEM with the cBPE array, which has a higher density of electrode points than the conventional multi-electrode array, we effectively demonstrated the imaging of [Fe(CN)6]3- diffusion and the respiratory activity of MCF-7 spheroids with high spatio-temporal resolution.

Shape-Controlled Syntheses of Magnetite Microparticles and Their Magnetorheology.

Magnetic microspheres in a concentrated suspension can be self-assembled to form chain structures under a magnetic field, resulting in an enhanced viscosity and elasticity of the suspension (i.e., the magnetorheological (MR) effect). Recently, interest has been raised about the relationship between nonspherical particles, such as octahedral particles and the MR effect. However, experimental studies have not made much progress toward clarifying this issue due to the difficulty associated with synthesizing microparticles with well-defined shapes and sizes. Here, we presented a method for the shape-controlled synthesis of magnetite (Fe3O4) microparticles and investigated the MR effects of two suspensions prepared from the two shape-controlled samples of Fe3O4 microparticles. Our method, which was based on the polyol method, enabled the preparation of spherical and octahedral Fe3O4 microparticles with similar sizes and magnetic properties, through a reduction of α-FeOOH in a mixed solvent of ethylene glycol (a polyol) and water. The water played an important role in both the phase transition (α-FeOOH to Fe3O4) and the shape control. No substantial difference in the MR effect was observed between an octahedral-particle-based suspension and a spherical-particle-based one. Therefore, in this study, the shape of the microparticles did not strongly influence the MR effect, i.e., the properties of the chain structures.

Electrochemicolor Imaging Using an LSI-Based Device for Multiplexed Cell Assays.

Multiplexed bioimaging systems have triggered the development of effective assays, contributing new biological information. Although electrochemical imaging is beneficial for quantitative analysis in real time, monitoring multiple cell functions is difficult. We have developed a novel electrochemical imaging system, herein, using a large-scale integration (LSI)-based amperometric device for detecting multiple biomolecules simultaneously. This system is designated as an electrochemicolor imaging system in which the current signals from two different types of biomolecules are depicted as a multicolor electrochemical image. The mode-selectable function of the 400-electrode device enables the imaging system and two different potentials can be independently applied to the selected electrodes. The imaging system is successfully applied for detecting multiple cell functions of the embryonic stem (ES) cell and the rat pheochromocytoma (PC12) cell aggregates. To the best of our knowledge, this is the first time that a real-time electrochemical mapping technique for multiple electroactive species, simultaneously, has been reported. The imaging system is a promising bioanalytical method for exploring complex biological phenomena.

Reversible Shape Transformation of Ultrathin Polydopamine-Stabilized Droplet.

Here we report on the flattening of water droplets using an ultrathin membrane of autopolymerized polydopamine at the air/water interface. This has only been previously reported with the use of synthetic or extracted peptides, two-dimensional designed synthetic peptide thin films with thicknesses of several tens of nanometers. However, in the previous study, the shape of the water droplet was changed irreversibly and the phenomenon was observed only at the air/water interface. In the present study, an ultrathin polydopamine membrane-stabilized droplet induced the flattening of a water droplet at the air/liquid and liquid/liquid interfaces because a polydopamine membrane was spontaneously formed at these interfaces. Furthermore, a reversible transformation of the droplet to flat and dome shape droplets were discovered at the liquid/liquid interface. These are a completely new system because the polydopamine membrane is dynamically synthesized at the interface and the formation speed of the polydopamine membrane overcomes the flattening time scale. These results will provide new insight into physical control of the interfacial shapes of droplets.

Biomimetic Bubble-Repellent Tubes: Microdimple Arrays Enhance Repellency of Bubbles Inside of Tubes.

The adhesion of bubbles underwater remains the greatest cause of malfunctions in applications such as microfluidics, medical devices, and heat exchangers. Recently, the combination of oxidization and peeling the top layer of self-organized honeycomb films with an adhesive tape resulted in the formation of an ultra-bubble-repellent and pillared polymer surface structure. However, the fabrication of honeycomb films on the inner surface of tubes and the formation of structured hydrophilic textures by peeling the top layer of honeycomb films still remain problematic. In this report, a simple fabrication technique for producing a honeycomb-patterned polymer film on the interior of a tube by dip-coating a polymer solution and blowing humid air in the tube is described. Furthermore, an ultra-bubble-repellent dimple-arrayed structure was fabricated by applying ultrasonication to the honeycomb structure formed on the interior surface of the tubes.

Electrochemical Imaging of Dopamine Release from Three-Dimensional-Cultured PC12 Cells Using Large-Scale Integration-Based Amperometric Sensors.

In the present study, we used a large-scale integration (LSI)-based amperometric sensor array system, designated Bio-LSI, to image dopamine release from three-dimensional (3D)-cultured PC12 cells (PC12 spheroids). The Bio-LSI device consists of 400 sensor electrodes with a pitch of 250 µm for rapid electrochemical imaging of large areas. PC12 spheroids were stimulated with K(+) to release dopamine. Poststimulation dopamine release from the PC12 spheroids was electrochemically imaged using the Bio-LSI device. Bio-LSI clearly showed the effects of the dopaminergic drugs l-3,4-dihydroxyphenylalanine (L-DOPA) and reserpine on K(+)-stimulated dopamine release from PC12 spheroids. Our results demonstrate that dopamine release from PC12 spheroids can be monitored using the device, suggesting that the Bio-LSI is a promising tool for use in evaluating 3D-cultured dopaminergic cells and the effects of dopaminergic drugs. To the best of our knowledge, this report is the first to describe electrochemical imaging of dopamine release by PC12 spheroids using LSI-based amperometric sensors.

Hydrothermal Synthesis of Yttria-Stabilized Zirconia Nanocrystals with Controlled Yttria Content.

In this study, we demonstrate for the first time the hydrothermal synthesis of yttria-stabilized zirconia (YSZ) nanocrystals with controlled yttria content (x = 3-12 mol %; xYSZ) with negligible aggregation from aqueous solution. The nanocrystals were grown via the hydrothermal treatment of basic Zr(IV) and Y(III) carbonate complex aqueous solutions in the presence of a cationic ligand, N(CH3)4(+). The nanocrystals were characterized in detail by dynamic light scattering, ζ-potential measurement, X-ray diffraction, specific surface area measurement based on the Brunauer-Emmett-Teller theory, transmission electron microscopy, energy dispersive X-ray spectroscopy, and Raman spectroscopy. Shorter reaction times and higher Y2O3 content produce aqueous solutions with higher transparencies containing nanocrystals with sizes of 10 nm or less. Nanocrystals with the target composition were obtained by hydrothermal reaction for longer than 3 h, regardless of the Y2O3 content. The main phase is tetragonal for (3-6)YSZ and cubic with disordered oxygen vacancies for (8-12)YSZ. The characteristics of the nanocrystalline material synthesized are consistent with those of bulk YSZ crystals, indicating the growth of high-quality nanocrystals.

Plasma Interaction with Organic Molecules in Liquid as Fundamental Processes in Plasma Medicine.

Investigation of plasma-organic materials interaction in aqueous solution with atmospheric pressure plasmas have been carried out. Degradation of methylene blue (MB) in aqueous solution via atmospheric pressure He plasma exposure through gas/liquid interface have been investigated. The optical emission spectrum shows considerable emissions of He lines and the emission of O, OH and N radicals attributed to dissociation of water (H2O) and air has been confirmed. Structure variation of MB in solution treated with the atmospheric-pressure He plasma has been measured by Fourier transform infrared spectroscopy (FT-IR). The results obtained from FT-IR analysis show degradation of MB in solution treated with the atmospheric-pressure He plasma. The pH effect of MB degradation was investigated using controlled pH solutions by an ultraviolet-visible (UV-Vis) spectroscopy and FT-IR. The results show no effect of MB degradation on pH. The results exhibit that the atmospheric pressure plasmas exposure has made it possible to degrade organic materials in solution due to irradiated radicals from plasma through plasma/liquid interface.

Surface capping-assisted hydrothermal growth of gadolinium-doped CeO2 nanocrystals dispersible in aqueous solutions.

Nanocrystals of 20 mol % Gd(3+)-doped CeO2 dispersible in basic aqueous solutions were grown via hydrothermal treatment of anionic Ce(IV) and Gd(III) carbonate complexes at 125-150 °C for 6-24 h with N(CH3)4(+) as a capping agent. The nanocrystals were characterized in detail using dynamic light scattering (DLS), ζ-potential measurements, X-ray diffraction (XRD), specific surface area measurements based on the Brunauer-Emmett-Teller theory (SSA(BET)), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy, and Raman spectroscopy. DLS analysis revealed that the highly transparent product solution consisted of nanocrystals approximately 10-20 nm of hydrodynamic diameter with a very narrow size distribution, while the ζ-potential analysis results strongly suggested that the N(CH3)4(+) capped negatively charged sites on the nanocrystals' surface and provided sufficient repulsive steric effect to prevent agglomeration. Moreover, the crystallite size (d(XRD)) estimated from the XRD patterns and the equivalent particle size (d(BET)) estimated from the SSA(BET) data were in the range between 5-6 and 4-4.5 nm, respectively, and nearly constant independent of reaction time, indicating suppressed Ostwald ripening due to capping. Good agreement between the values obtained from the d(XRD) and d(BET) analyses with the size of the primary nanocrystals observed in the TEM image also confirmed that the primary nanocrystals were single crystals and nearly free from aggregation. Furthermore, the gadolinium content in the as-prepared nanocrystals was determined to be very close to 20 mol % and remained unchanged after HCl treatment, indicating successful doping of stoichiometric amount of Gd(3+) into CeO2 lattices. Finally, the Raman analysis suggested that only a slightly Gd(3+)-rich phase was present in the nanocrystals grown for shorter reaction times. By increasing the reaction time, even at 125 °C, the Gd(3+) was homogeneously distributed into the CeO2 lattices via solid state diffusion.

Understanding the role of solvents in bottom-up synthesis of multi-element hydroxides.

Here we report a comparative study on the bottom-up synthesis of multi-element hydroxides composed of Mg, Al, Fe and Zn cations to understand the role of solvents. Two common solvents, water and ethylene glycol, a typical polyol, are used. The polyol-derived MgAlFeZn-OH are nanosheets with homogeneous elemental distribution, while the hydrothermal-derived MgAlFeZn-OH are mixtures of plate-like hydroxide layers and rod-like spinel oxides. The coordinating properties and the high viscosity of the ethylene glycol provide the possibility to mediate the hydrolysis rates and to control the particle growth. The high specific surface area of the polyol-derived multi-element hydroxide nanosheets (352.4 m2 g-1) guarantees them as excellent adsorbents for adsorbing anionic dyes in aqueous solution.

Scanning electrochemical microscopy for determining oxygen consumption rates of cells in hydrogel fibers fabricated using an extrusion 3D bioprinter.

Three-dimensional (3D)-cultured cells have attracted the attention of researchers in tissue engineering- and drug screening-related fields. Among them, 3D cellular fibers have attracted significant attention because they can be stacked to prepare more complex tissues and organs. Cellular fibers are widely fabricated using extrusion 3D bioprinters. For these applications, it is necessary to evaluate cellular activities, such as the oxygen consumption rate (OCR), which is one of the major metabolic activities. We previously reported the use of scanning electrochemical microscopy (SECM) to evaluate the OCRs of cell spheroids. However, the SECM approach has not yet been applied to hydrogel fibers prepared using the bioprinters. To the best of our knowledge, this is the first study to evaluate the OCR of cellular fibers printed by extrusion 3D bioprinters. First, the diffusion theory was discussed to address this issue. Next, diffusion models were simulated to compare realistic models with this theory. Finally, the OCRs of MCF-7 cells in the printed hydrogel fibers were evaluated as a proof of concept. Our proposed approach could potentially be used to evaluate the OCRs of tissue-engineered fibers for organ transplantation and drug screening using in-vitro models.

Progress on Separation and Hydrothermal Carbonization of Rice Husk Toward Environmental Applications.

Owing to the increasing global demand for carbon resources, pressure on finite materials, including petroleum and inorganic resources, is expected to increase in the future. Efficient utilization of waste resources has become crucial for sustainable resource acquisition for creating the next generation of industries. Rice husks, which are abundant worldwide as agricultural waste, are a rich carbon source with a high silica content and have the potential to be an effective raw material for energy-related and environmental purification materials such as battery, catalyst, and adsorbent. Converting these into valuable resources often requires separation and carbonization; however, these processes incur significant energy losses, which may offset the benefits of using biomass resources in the process steps. This review summarizes and discusses the high value of RHs, which are abundant as agricultural waste. Technologies for separating and converting RHs into valuable resources by hydrothermal carbonization are summarized based on the energy efficiency of the process.

Frustoconical porous microneedle for electroosmotic transdermal drug delivery.

A truncated cone-shaped porous microneedle (PMN) made of poly-glycidyl methacrylate was studied as a minimally invasive tool for transdermal drug delivery. The transdermal electrical resistance of a pig skin was evaluated during the indentation of the PMNs, revealing that the frustoconical PMN (300 µm height) significantly reduced the resistance of the skin by expanding the stratum corneum without penetrating into the skin. A thin film of poly (2-acrylamido-2-methylpropanesulfonic acid) (PAMPS) was grafted onto the inner wall of the microchannels of the frustoconical PMN to generate electroosmotic flow (EOF) upon current application in the direction of injection of the drug into the skin. Owing to the synergy of the expansion of the stratum corneum and the EOF-promotion, the PAMPS-modified frustoconical PMN effectively enhances the penetration of larger (over 500 Da) molecules, such as dextran (∼10 kDa).