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Programming artificially a sequence of organic-metal oxide multilayers (superlattices) by using atomic layered depositions (ALD) is a fascinating and challenging issue in material chemistry. However, the complex chemical reactions between ALD precursors and organic layer surfaces have limited their applications for various material combinations. Here we demonstrate the impact of interfacial molecular compatibility on the formation of organic-metal oxide superlattices using ALD. The effects of both organic and inorganic compositions on the metal oxide layer formation processes onto self-assembled monolayers (SAM) were examined by using scanning transmission electron microscopy (STEM), in-situ quartz crystal microbalance (QCM) measurements, and Fourier-transformed infrared spectroscopy (FT-IR). These series of experiments reveal that the terminal group of organic SAM molecules must satisfy two conflicting requirements, the first of which is to promptly react with ALD precursors and the second is not to bind strongly to the bottom metal oxide layers to avoid undesired SAM conformations. OH-terminated phosphate aliphatic molecules, which we have synthesized, were identified as one of the best candidates for such a purpose. Molecular compatibility between metal oxide precursors and the -OHs must be properly considered to form superlattices. In addition, it is also important to form densely packed and all-trans-like SAMs to maximize the surface density of reactive -OHs on the SAMs. Based on these design strategies for organic-metal oxide superlattices, we have successfully fabricated various superlattices composed of metal oxides (Al-, Hf-, Mg-, Sn-, Ti-, Zr oxides) and their multilayered structures.

Humidity and moisture effects, frequently called water poisoning, in surroundings are inevitable for various molecular sensing devices, strongly affecting their sensing characteristics. Here, we demonstrate a water-selective nanostructured dehumidifier composed of ZnO/TiO

Breath odor sensing-based individual authentication was conducted for the first time using an artificial olfactory sensor system. Using a 16-channel chemiresistive sensor array and machine learning, a mean accuracy of97% was successfully achieved. The impact of the number of sensors on the accuracy and reproducibility was also demonstrated.

Click reactions (e.g., Huisgen cycloaddition) on metal oxide nanostructures offer a versatile and robust surface molecular modification for various applications because they form strong covalent bonds in a wide range of molecular substrates. This study reports a rational strategy to maximize the conversion rate of surface click reactions on single-crystalline ZnO nanowires by monitoring the reaction progress. p-Polarized multiple-angle incidence resolution spectrometry (pMAIRS) and Fourier-transformed infrared (FT-IR) spectroscopy were employed to monitor the reaction progress of an azide-terminated self-assembled monolayer (SAM) on single-crystalline ZnO nanowires. Although various reaction parameters including the concentration of Cu(I) catalysts, triazolyl ligands, solvents, and target alkynes were systematically examined for the surface click reactions, 10-30% of terminal azide on the nanowire surface remained unreacted. Temperature-dependent FT-IR measurements revealed that such unreacted residual azides deteriorate the thermal stability of the nanowire molecular layer. To overcome this observed conversion limitation of click reactions on nanostructure surfaces, we considered the steric hindrance around the closely packed SAM reaction points, then experimented with dispersing the azide moiety into a methyl-terminated SAM. The mixed-SAM method significantly improved the azide conversion rate to almost 100%. This reaction method enables the construction of spatially patterned molecular surface modifications on metal oxide nanowire arrays without detrimental unreacted azide groups.

Seeded crystal growths of nanostructures within confined spaces offer an interesting approach to design chemical reaction spaces with tailored inner surface properties. However, such crystal growth within confined spaces tends to be inherently difficult as the length increases as a result of confinement effects. Here, we demonstrate a space-confined seeded growth of ZnO nanowires within meter-long microtubes of 100 μm inner diameter with the aspect ratio of up to 10 000, which had been unattainable to previous methods of seeded crystal growths. ZnO nanowires could be grown via seeded hydrothermal crystal growth for relatively short microtubes below the length of 40 mm, while any ZnO nanostructures were not observable at all for longer microtubes above 60 mm with the aspect ratio of 600. Microstructural and mass spectrometric analysis revealed that a conventional seed layer formation using zinc acetate is unfeasible within the confined space of long microtubes as a result of the formation of detrimental residual Zn complex compounds. To overcome this space-confined issue, a flow-assisted seed layer formation is proposed. This flow-assisted method enables growth of spatially uniform ZnO nanowires via removing residual compounds even for 1 m long microtubes with the aspect ratio of up to 10 000. Finally, the applicably of ZnO-nanowire-decorated long microtubes for liquid-phase separations was demonstrated.

Chemically stable and electrically conductive metal oxide nanofilms are promising as robust electrodes for chemical/biosensors and for photoelectrochemical applications, which require harsh conditi...

Electrical molecular nanosensors require two conflicting surface characteristics, which are catalytic activity for molecular selectivity and thermal stability for long-term data collection. Here, we show that a simple surface treatment using strong acids creates two such conflicting surface properties of WO3 hydrate nanowires, which enhance the electrical molecular sensing of aldehydes (nonanal, a biomarker). Mass-spectrometric measurements reveal that the surface treatment using strong acids substantially promotes both the oxidization of nonanal and the desorption of products, nonanoic acid, from the surfaces at a temperature of 50 °C, which is lower than the 300 °C required for untreated surfaces. Spectroscopic and structural measurements combined with numerical simulations identify two different adsorption structures of carbonyl groups on the surface, where molecules directly bound to coordinatively unsaturated surface tungsten preferentially proceed to the catalytic oxidization reaction and the subsequent desorption process. Furthermore, we confirm the thermal durability (over 10 years) of the catalytic activity on acid-treated WO3 hydrate nanowire surfaces up to 300 °C, which enables us to demonstrate the long-term stable sensor operations with the sensitivity (4 orders of magnitude) remaining for years.

The surface cation composition of nanoscale metal oxides critically determines the properties of various functional chemical processes including inhomogeneous catalysts and molecular sensors. Here we employ a gradual modulation of cation composition on a ZnO/(Cu1−xZnx)O heterostructured nanowire surface to study the effect of surface cation composition (Cu/Zn) on the adsorption and chemical transformation behaviors of volatile carbonyl compounds (nonanal: biomarker). Controlling cation diffusion at the ZnO(core)/CuO(shell) nanowire interface allows us to continuously manipulate the surface Cu/Zn ratio of ZnO/(Cu1−xZnx)O heterostructured nanowires, while keeping the nanowire morphology. We found that surface exposed copper significantly suppresses the adsorption of nonanal, which is not consistent with our initial expectation since the Lewis acidity of Cu2+ is strong enough and comparable to that of Zn2+. In addition, an increase of the Cu/Zn ratio on the nanowire surface suppresses the aldol condensation reaction of nonanal. Surface spectroscopic analysis and theoretical simulations reveal that the nonanal molecules adsorbed at surface Cu2+ sites are not activated, and a coordination-saturated in-plane square geometry of surface Cu2+ is responsible for the observed weak molecular adsorption behaviors. This inactive surface Cu2+ well explains the mechanism of suppressed surface aldol condensation reactions by preventing the neighboring of activated nonanal molecules. We apply this tailored cation composition surface for electrical molecular sensing of nonanal and successfully demonstrate the improvements of durability and recovery time as a consequence of controlled surface molecular behaviors., Unexpected features of surface Cu2+ on ZnO/(Cu1−xZnx)O nanowires for molecular transformation and electrical sensing of carbonyl compounds were found.

Here, we demonstrate the impact of tungsten precursor concentration on anisotropic crystal growth of hydrothermal tungsten oxide (WO3) nanowires. When varying the tungsten precursor concentration, ...

A facile methodology to create zinc titanate nanotubes via reaction-byproduct etching is demonstrated. The chemical etching by the sol-gel reaction byproduct—hydrochloric acid (HCl) allows us to cr...

While halide perovskite electronics are rapidly developing, they are greatly limited by the inferior charge transport and poor stability. In this work, effective surface charge transfer doping of vapor-liquid-solid (VLS)-grown single-crystalline cesium lead bromide perovskite (CsPbBr3) nanowires (NWs) via molybdenum trioxide (MoO3) surface functionalization is achieved. Once fabricated into NW devices, due to the efficient interfacial charge transfer and reduced impurity scattering, a 15× increase in the field-effect hole mobility (μh) from 1.5 to 23.3 cm2/(V s) is accomplished after depositing the 10 nm thick MoO3 shell. This enhanced mobility is already better than any mobility value reported for perovskite field-effect transistors (FETs) to date. The photodetection performance of these CsPbBr3/MoO3 core-shell NWs is also investigated to yield a superior responsivity (R) up to 2.36 × 103 A/W and an external quantum efficiency (EQE) of over 5.48 × 105% toward the 532 nm regime. Importantly, the MoO3 shell can provide excellent surface passivation to the CsPbBr3 NW core that minimizes the diffusion of detrimental water and oxygen molecules, improving the air stability of CsPbBr3/MoO3 core-shell NW devices. All these findings evidently demonstrate the surface doping as an enabling technology to realize high-mobility and air-stable low-dimensional halide perovskite devices.

Surface molecular transformations on nanoscale metal oxides are inherently complex, and directing those reaction pathways is still challenging but important for designing their various applications, including molecular sensing, catalysts, and others. Here, a rational strategy to direct a reaction pathway of volatile carbonyl compounds (nonanal: biomarker) on single-crystalline ZnO nanowire surfaces via molecular modification is demonstrated. The introduction of a methylphosphonic acid modification on the ZnO nanowire surface significantly alters the surface reaction pathway of nonanal via suppressing the detrimental aldol condensation reaction. This is directed by intentionally decreasing the probability of two neighboring molecular activations on the nanowire surface. Spectrometric measurements reveal the correlation between the suppression of the aldol condensation surface reaction and the improvement in the sensor performance. This tailored surface reaction pathway effectively reduces the operating temperature from 200 to 100 °C while maintaining the sensitivity. This is because the aldol condensation product ((E)-2-heptyl-2-undecenal) requires a higher temperature to desorb from the surface. Thus, the proposed facile strategy offers an interesting approach not only for the rational design of metal oxide sensors for numerous volatile carbonyl compounds but also for tailoring various surface reaction pathways on complex nanoscale metal oxides.

Tailoring the elemental doping of inorganic nanowires remains an important challenge due to complex dopant incorporation pathways. Here we report that the face-selectivity of tungstate ions controls growth direction and dopant incorporation of hydrothermal zinc oxide nanowires. The introduction of tungstate ions on nanowire surface during synthesis unexpectedly enhances nucleation at sidewall $$\left\{ {10\bar 10} \right\}$$ 10 1 ¯ 0 planes, while dopant incorporation occurs only on (0001) planes. This conflicting face-selective behavior leads to inhomogeneous dopant distribution. Density functional theory calculations reveal that the face-selective behavior can be interpreted in terms of the effect of coordination structure of the tungstate ions on each zinc oxide crystal plane. In addition, we demonstrate a rational strategy to control the morphology and the elemental doping of tungsten-doped zinc oxide nanowires.

Emerging interactive sensor electronics requires metal oxide electrodes that possess long-term atmospheric stability and electrical conductivity to function under harsh conditions (e.g., high tempe...

Here, we discuss a misunderstanding regarding chemical capping, which has intrinsically hindered the extension of the length of hexagonal (h)-WO3 nanowires in previous studies. Although divalent sulfate ions (SO42−) have been strongly believed to be efficient capping ions for directing anisotropic h-WO3 nanowire growth, we have found that the presence of SO42− is highly detrimental to the anisotropic crystal growth of the h-WO3 nanowires, and a monovalent sulfur oxoanion (HSO4−) rather than SO42− only substantially promotes the anisotropic h-WO3 nanowire growth. Ab initio electronic structure simulations revealed that the monovalent sulfur oxoanions were preferentially able to cap the sidewall plane (100) of the h-WO3 nanowires due to the lower hydration energy when compared with SO42−. Based on this capping strategy, using the monovalent sulfur oxoanion (CH3SO3−), which cannot generate divalent sulfur oxoanions, we have successfully fabricated ultra-long h-WO3 nanowires up to the millimeter range (1.2 mm) for a wider range of precursor concentrations. We have demonstrated the feasibility of these millimeter-long h-WO3 nanowires for the electrical sensing of molecules (lung cancer biomarker: nonanal) on flexible substrates, which can be operated at room temperature with mechanical flexibility with bending cycles up to 104 times due to the enhanced textile effect.

We propose a rational strategy to fabricate thermally robust, highly integrated molecular and gas sensors utilizing a lateral SnO

We demonstrate the facile, rational synthesis of monodispersedly sized zinc oxide (ZnO) nanowires from randomly sized seeds by hydrothermal growth. Uniformly shaped nanowire tips constructed in ammonia-dominated alkaline conditions serve as a foundation for the subsequent formation of the monodisperse nanowires. By precisely controlling the sharp tip formation and the nucleation, our method substantially narrows the distribution of ZnO nanowire diameters from σ = 13.5 nm down to σ = 1.3 nm and controls their diameter by a completely bottom-up method, even initiating from randomly sized seeds. The proposed concept of sharp tip based monodisperse nanowires growth can be applied to the growth of diverse metal oxide nanowires and thus paves the way for bottom-up grown metal oxide nanowires-integrated nanodevices with a reliable performance.

Herein, we explore the hidden molecular recognition abilities of ZnO nanowires uniformly grown on the inner surface of an open tubular fused silica capillary

Polymer-carbon nanocomposite sensor is a promising molecular sensing device for electronic nose (e-nose) due to its printability, variety of polymer materials, and low operation temperature; however, the lack of stability in an air environment has been an inevitable issue. Here, we demonstrate a design concept for realizing long-term stability in a polyethylene glycol (PEG)-carbon black (CB) nanocomposite sensor by understanding the underlying phenomena that cause sensor degradation. Comparison of the sensing properties and infrared spectroscopy on the same device revealed that the oxidation-induced consumption of PEG is a crucial factor for the sensor degradation. According to the mechanism, we introduced an antioxidizing agent (i.e., ascorbic acid) into the PEG-CB nanocomposite sensor to suppress the PEG oxidation and successfully demonstrated the long-term stability of sensing properties under an air environment for 30 days, which had been difficult in conventional polymer-carbon nanocomposite sensors.

Understanding the formation process of self-assembled monolayers (SAMs) of organophosphonic acids on ZnO surfaces is essential to designing their various applications, including solar cells, heterogeneous catalysts, and molecular sensors. Here, we report the significant effect of surface dissociation on SAM formation of organophosphonic acids on single-crystalline ZnO nanowire surfaces using infrared spectroscopy. When employing the most conventional solvent-methanol (relative permittivity εr = 32.6), the production of undesired byproducts (layered zinc compounds) on the surface was identified by infrared spectral data and microscopy. On the other hand, a well-defined SAM structure with a tridentate coordination of phosphonic acids on the surface was confirmed when employing toluene (εr = 2.379) or tert-butyl alcohol (εr = 11.22–11.50). The observation of layered zinc compounds as byproducts highlights that the degree of Zn2+ dissociation from the ZnO solid surface into a solvent significantly affects the surface coordination of phosphonic acids during the SAM formation process. Although the ZnO nanowire surface (m-plane) is hydrophilic, the present results suggest that a weaker solvent polarity is preferred to form well-defined phosphonic acid SAMs on ZnO nanowire surfaces without detrimental surface byproducts.

We present a method named NPFimg, which automatically identifies multivariate chemo-/biomarker features of analytes in chromatography-mass spectrometry (MS) data by combining image processing and machine learning. NPFimg processes a two-dimensional MS map (m/z vs retention time) to discriminate analytes and identify and visualize the marker features. Our approach allows us to comprehensively characterize the signals in MS data without the conventional peak picking process, which suffers from false peak detections. The feasibility of marker identification is successfully demonstrated in case studies of aroma odor and human breath on gas chromatography-mass spectrometry (GC-MS) even at the parts per billion level. Comparison with the widely used XCMS shows the excellent reliability of NPFimg, in that it has lower error rates of signal acquisition and marker identification. In addition, we show the potential applicability of NPFimg to the untargeted metabolomics of human breath. While this study shows the limited applications, NPFimg is potentially applicable to data processing in diverse metabolomics/chemometrics using GC-MS and liquid chromatography-MS. NPFimg is available as open source on GitHub (http://github.com/poomcj/NPFimg) under the MIT license.

Emerging interactive electronics for the Internet of Things era inherently require the long-term stability of semiconductor devices exposed to air. Nanostructured metal oxides are promising options for such atmospherically stable semiconductor devices owing to their inherent stability in air. Among various oxide nanostructures, ZnO nanowires have been the most intensively studied for electrical and optical device applications. Here, we demonstrate a strategy for achieving the atmospheric electrical stability of ZnO nanowire devices. Although the chemically active oxygen and water in air are strong candidates for affecting the electrical stability of nanoscale metal oxides, we found that the ppm-level redox-inactive CO2 in air critically determines the atmospheric electrical stability of hydrothermally grown single-crystalline ZnO nanowires. A series of analyses using atmosphere-controlled electrical characterization of single nanowire devices, Fourier transform infrared spectroscopy, scanning transmission electron microscopy, and X-ray photoelectron spectroscopy consistently revealed that atmospheric CO2 reacts substantially with the ZnO nanowire surfaces, even at room temperature, to form an electrically insulative zinc carbonate thin layer. The formation of this layer essentially limits the atmospheric electrical stability of the ZnO nanowire devices. Based on this surface carbonation mechanism, we propose a strategy to suppress the detrimental surface reaction, which is based on (1) reducing the density of surface hydroxyl groups and (2) improving the nanowire crystallinity by thermal pretreatment. This approach improves the atmospheric electrical stability to at least 40 days in air.

Here, we show the effect of water–organic (acetone, tert-butyl alcohol, and isopropanol) cosolvents on nucleation and anisotropic crystal growth of solution-synthesized ZnO nanowires. The addition of organic solution does not alter the face-selective crystal growth nature but significantly promotes the crystal growth of both length and diameter of the nanowires. Systematic investigations reveal that a variation of the relative dielectric constant in the cosolvent can rigorously explain the observed effect of the water–organic cosolvent on the ZnO nanowire growth via the degree of supersaturation for the nucleation. The difference between acetone, tert-butyl alcohol, and isopropanol on the cosolvent effect can be interpreted in terms of a local solvent-sorting effect.

Indole is a signal molecule derived from the conversion of tryptophan, and it is present in bacterial respiratory gas. Besides influencing bacterial growth, indole exhibits effects on human health, including a positive effect on inflammation and protection against pathogens. However, a high fecal indole concentration (FIC) can suggest an unbalanced gut flora or the presence of certain pathogens. To analyze the indole produced by bacteria, its collection and detection is required. Traditional methods usually require centrifugation of liquid bacterial culture medium and subsequent extraction of indole from the medium or partial purification of indole from fecal samples (e.g., by distillation or extraction). In this study, we demonstrate the possibility of identifying gas contents directly from bacteria, and we distinguish the difference in species and their genetics without the need to centrifuge or extract. Using an absorbent sheet placed above a liquid culture, we were able to collect gas content directly from bacteria. Gas chromatography-mass spectrometry (GC-MS) was used for the analysis. The GC-MS results showed a clear peak attributed to indole for wild-type Escherichia coli cells (MG1655 and MC4100 strains), whereas the indole peak was absent in the chromatograms of cells where proteins, part of the indole production pathway from tryptophan (TnaA and TnaB), were not expressed (by using tnaAB-deleted cells). The indole observed was measured to be present in a low nmol-range. This method can distinguish whether the bacterial genome contains the tnaAB gene or not and can be used to collect gas compounds from bacterial cultures quickly and easily. This method is useful for other goals and future research, such as for measurements in restrooms, for food-handling facilities, and for various applications in medical settings.

An impurity doping in semiconductors is an important irreversible process of manipulating the electrical properties of advanced electron devices. Here, we report an unusual reversible dopant activation/deactivation phenomenon, which emerges at an interface between indium tin oxide (ITO) and single-crystalline oxide channel. We found that the interface electrical resistance between ITO electrodes and single-crystalline oxide nanowire channel can be repeatedly switched between a metallic state and a near-insulative state by applying thermal treatments in air or vacuum. Interestingly, this electrical switching phenomenon disappears when the oxide nanowire changes from the single-crystalline structure to the lithography-defined polycrystalline structure. Atmosphere-controlled annealing experiments reveal that atmospheric oxygen induces repeatable change in the interfacial electrical resistance. Systematic investigations on metal cation species and channel crystallinity demonstrate that the observed electrical switching is related to an interface-specific reversible Sn-dopant activation/deactivation of ITO electrode in contact with a single-crystalline oxide channel.

2D metal phosphide loop-sheet heterostructures are controllably synthesized by edge-topological regulation, where Ni2 P nanosheets are edge-confined by the N-doped carbon loop, containing ultrafine NiFeP nanocrystals (denoted as NiFeP@NC/Ni2 P). This loop-sheet feature with lifted-edges prevents the stacking of nanosheets and induces accessible open channels for catalytic site exposure and gas bubble release. Importantly, these NiFeP@NC/Ni2 P hybrids exhibit a remarkable oxygen evolution activity with an overpotential of 223 mV at 20 mA cm-2 and a Tafel slope of 46.1 mV dec-1 , constituting the record-high performance among reported metal phosphide electrocatalysts. The NiFeP@NC/Ni2 P hybrids are also employed as both anode and cathode to achieve an alkaline electrolyzer for overall water splitting, delivering a current density of 10 mA cm-2 with a voltage of 1.57 V, comparable to that of the commercial Pt/C||RuO2 couple (1.56 V). Moreover, a photovoltaic-electrolysis coupling system can as well be effectively established for robust overall water splitting. Evidently, this ingenious protocol would expand the toolbox for designing efficient 2D nanomaterials for practical applications.

Indole is a signal molecule derived from the conversion of tryptophan, and it is present in bacterial respiratory gas. Besides influencing bacterial growth, indole exhibits effects on human health, including a positive effect on inflammation and protection against pathogens. However, a high fecal indole concentration (FIC) can suggest an unbalanced gut flora or the presence of certain pathogens. To analyze the indole produced by bacteria, its collection and detection is required. Traditional methods usually require centrifugation of liquid bacterial culture medium and subsequent extraction of indole from the medium or partial purification of indole from fecal samples (e.g., by distillation or extraction). In this study, we demonstrate the possibility of identifying gas contents directly from bacteria, and we distinguish the difference in species and their genetics without the need to centrifuge or extract. Using an absorbent sheet placed above a liquid culture, we were able to collect gas content directly from bacteria. Gas chromatography-mass spectrometry (GC-MS) was used for the analysis. The GC-MS results showed a clear peak attributed to indole for wild-type Escherichia coli cells (MG1655 and MC4100 strains), whereas the indole peak was absent in the chromatograms of cells where proteins, part of the indole production pathway from tryptophan (TnaA and TnaB), were not expressed (by using tnaAB-deleted cells). The indole observed was measured to be present in a low nmol-range. This method can distinguish whether the bacterial genome contains the tnaAB gene or not and can be used to collect gas compounds from bacterial cultures quickly and easily. This method is useful for other goals and future research, such as for measurements in restrooms, for food-handling facilities, and for various applications in medical settings.

Here, we discuss a misunderstanding regarding chemical capping, which has intrinsically hindered the extension of the length of hexagonal (h)-WO

Correction for ‘Breath odor-based individual authentication by an artificial olfactory sensor system and machine learning’ by Chaiyanut Jirayupat et al., Chem. Commun., 2022, DOI: https://doi.org/10.1039/D1CC06384G.

A three-dimensionally insulated Pt(II) bis(dithiobenzoate) complex with a [1]rotaxane structure was synthesized using permethylated cyclodextrins. We have confirmed macrocyclic insulation is necessary to yield the Pt(II) bis(dithiobenzoate) complex with high selectivity. Furthermore, the formed Pt(II) complex showed highly stable and reversible two-step redox behavior in cyclic voltammetry due to the three-dimensional insulation.

Emerging interactive electronics for the Internet of Things era inherently require the long-term stability of semiconductor devices exposed to air. Nanostructured metal oxides are promising options for such atmospherically stable semiconductor devices owing to their inherent stability in air. Among various oxide nanostructures, ZnO nanowires have been the most intensively studied for electrical and optical device applications. Here, we demonstrate a strategy for achieving the atmospheric electrical stability of ZnO nanowire devices. Although the chemically active oxygen and water in air are strong candidates for affecting the electrical stability of nanoscale metal oxides, we found that the ppm-level redox-inactive CO

Metal-oxide nanowires have demonstrated excellent capability in the electrical detection of various molecules based on their material robustness in liquid and air environments. Although the surface structure of the nanowires essentially determines their interaction with adsorbed molecules, understanding the correlation between an oxide nanowire surface and an adsorbed molecule is still a major challenge. Herein, we propose a rational methodology to obtain this information for low-density molecules adsorbed on metal oxide nanowire surfaces by employing infrared p-polarized multiple-angle incidence resolution spectroscopy and temperature-programmed desorption/gas chromatography-mass spectrometry. As a model system, we studied the surface chemical transformation of an aldehyde (nonanal, a cancer biomarker in breath) on single-crystalline ZnO nanowires. We found that a slight surface reconstruction, induced by the thermal pretreatment, determines the surface chemical reactivity of nonanal. The present results show that the observed surface reaction trend can be interpreted in terms of the density of Zn ions exposed on the nanowire surface and of their corresponding spatial arrangement on the surface, which promotes the reaction between neighboring adsorbed molecules. The proposed methodology will support a better understanding of complex molecular transformations on various nanostructured metal-oxide surfaces.

In general, the electrical conductivities of n-type semiconducting metal oxide nanostructures increase with the decrease in the oxygen partial pressure during crystal growth owing to the increased number of crystal imperfections including oxygen vacancies. In this paper, we report an unusual oxygen partial pressure dependence of the electrical conductivity of single-crystalline SnO2 nanowires grown by a vapor–liquid–solid (VLS) process. The electrical conductivity of a single SnO2 nanowire, measured using the four-probe method, substantially decreases by 2 orders of magnitude when the oxygen partial pressure for the crystal growth is reduced from 10–3 to 10–4 Pa. This contradicts the conventional trend of n-type SnO2 semiconductors. Spatially resolved single-nanowire electrical transport measurements, microstructure analysis, plane-view electron energy-loss spectroscopy, and molecular dynamics simulations reveal that the observed unusual oxygen partial pressure dependence of the electrical transport is at...

A crystal growth of hydrothermal ZnO nanowires essentially requires a concentration control within so-called “concentration window”, where the anisotropic crystal growth of ZnO nanowires preferentially occurs. Although understanding what exactly determines the width of “concentration window” is important to tailor the anisotropic crystal growth process, the fundamental knowledge as to “concentration window” is still scarce. Here we report the effect of ammonia addition on the width of “concentration window” using conventional hydrothermal ZnO nanowire growth. We found that the ammonia addition substantially narrows the width of “concentration window”. Within the narrow range of zinc complex concentration, we found a significant increase of growth rate (up to 2000 nm/h) of ZnO nanowires. The narrowed “concentration window” and the resultant increased growth rate by the ammonia addition can be understood in terms of synchronized effects of both (1) a reduction of zinc hydroxide complex (precursor) concentration and (2) a fast rate limiting process of ligand exchange between different zinc complexes. Thus, the present knowldege as to “concentration window” will accelerate further tailoring an anisotropic crystal growth of hydrothermal ZnO nanowires.

During the past two decades, one–dimensional (1D) metal–oxide nanowire (NW)-based molecular sensors have been witnessed as promising candidates to electrically detect volatile organic compounds (VOCs) due to their high surface to volume ratio, single crystallinity, and well-defined crystal orientations. Furthermore, these unique physical/chemical features allow the integrated sensor electronics to work with a long-term stability, ultra-low power consumption, and miniature device size, which promote the fast development of “trillion sensor electronics” for Internet of things (IoT) applications. This review gives a comprehensive overview of the recent studies and achievements in 1D metal–oxide nanowire synthesis, sensor device fabrication, sensing material functionalization, and sensing mechanisms. In addition, some critical issues that impede the practical application of the 1D metal–oxide nanowire-based sensor electronics, including selectivity, long-term stability, and low power consumption, will be highlighted. Finally, we give a prospective account of the remaining issues toward the laboratory-to-market transformation of the 1D nanostructure-based sensor electronics.

We report the synthesis of an insulated π-conjugated polymer containing 2,2'-bipyridine moieties as metal coordination sites. Metal coordination to the polymer enabled easy and reversible tuning of the luminescent color without changes to the main chain skeleton. The permethylated α-cyclodextrin (PM α-CD)-based insulation structure allowed the metalated polymers to demonstrate efficient emission even in the solid state, with identical spectral shapes to the dilute solutions. In addition, the coordination ability of the metal-free polymer was maintained in the solid state, resulting in reversible changes in the luminescent color in response to the metal ions. The synthesized polymer is expected to be suitable for application in recyclable luminescent sensors to distinguish different metal ions.

We describe herein the synthesis of a Ni(II) bis(dithiobenzoate)-type complex three-dimensionally insulated by a [1]rotaxane structure to reveal the importance of the insulation. Under cyclic voltammetry conditions, the complex showed a stable and reversible redox behavior in contrast to a non-insulated reference complex, clearly demonstrating the effectiveness of the rotaxane-type insulation as a new method of kinetic metal complex stabilization.

The formal hydroacylation reaction of allenes has been developed employing carboxylic anhydrides as acyl sources and hydrosilanes as reducing reagents in the presence of a commercially available palladium complex as a catalyst. The reaction affords α,β-unsaturated ketones regio- and stereoselectively. The similar catalyst system is also effective for the reduction of carboxylic anhydrides to the corresponding aldehydes employing hydrosilanes.

A bridging monomer with terpyridine moieties at both ends of the insulated conjugated chain was successfully synthesized. The synthesis necessitated a new strategy, in which FeII was employed as a removable water-solubilizing substituent. The monomer was able to coordinate with FeII, CoII, NiII, or RuII to give highly insulated conjugated metallopolymers. The high-coverage insulation suppressed the recombination of charge carriers relative to an uninsulated reference and provided higher charge mobility along the chain than the less insulated metallopolymer.

The formal hydroacylation reaction of allenes has been developed employing carboxylic anhydrides as acyl sources and hydrosilanes as reducing reagents in the presence of a commercially available palladium complex as a catalyst. The reaction affords α,β-unsaturated ketones regio- and stereoselectively. The similar catalyst system is also effective for the reduction of carboxylic anhydrides to the corresponding aldehydes employing hydrosilanes.