Novel Templating Route Using Pt Infiltrated Block Copolymer Microparticles for Catalytic Pt Functionalized Macroporous WO₃ Nanofibers and Its Application in Breath Pattern Recognition

We propose a new route for transferring catalysts onto macroporous metal oxide nanofibers (NFs) using metallic nanoparticles (NPs) infiltrated block copolymer microparticles as sacrificial templates. Pt decorated polystyrene-<i>b</i>-poly­(4-vinylpyridine) (PS-<i>b</i>-P4VP) copolymer microparticles (Pt-BCP MPs), produced from oil-in-water emulsions, were uniformly dispersed within electrospun PVP/W precursor composite NFs. The macropore-loaded WO₃ NFs (macroporous Pt-WO₃ NFs), which are additionally functionalized by Pt NPs (10 nm), were achieved by decomposition of polymeric components and oxidization of W precursor after high-temperature calcination. In particular, macropores with the similar size distribution (50–300 nm) with BCP MPs were also formed on interior and exterior of WO₃ NFs. Chemical sensing performance of macroporous Pt-WO₃ NFs was investigated for pattern recognition of simulated breath gas components at highly humid ambient (95% RH). The result revealed that superior hydrogen sulfide sensitivity (<i>R</i>_air/<i>R</i>_gas = 834.2 ± 20.1 at 5 ppm) and noticeable selectivity were achieved. In addition, H₂S pattern recognition against other chemical components (acetone, toluene, and methyl mercaptan) was clearly identified without any overlapping of each pattern. This work demonstrates the potential application of BCP-templated maroporous Pt-WO₃ NFs in exhaled breath analysis for noninvasive monitoring of physical conditions

Metal Organic Framework-Templated Chemiresistor: Sensing Type Transition from P‑to‑N Using Hollow Metal Oxide Polyhedron via Galvanic Replacement

Facile synthesis of porous nanobuilding blocks with high surface area and uniform catalyst functionalization has always been regarded as an essential requirement for the development of highly sensitive and selective chemical sensors. Metal–organic frameworks (MOFs) are considered as one of the most ideal templates due to their ability to encapsulate ultrasmall catalytic nanoparticles (NPs) in microporous MOF structures in addition to easy removal of the sacrificial MOF scaffold by calcination. Here, we introduce a MOFs derived n-type SnO₂ (n-SnO₂) sensing layer with hollow polyhedron structures, obtained from p–n transition of MOF-templated p-type Co₃O₄ (p-Co₃O₄) hollow cubes during galvanic replacement reaction (GRR). In addition, the Pd NPs encapsulated in MOF and residual Co₃O₄ clusters partially remained after GRR led to uniform functionalization of efficient cocatalysts (PdO NPs and p-Co₃O₄ islands) on the porous and hollow polyhedron SnO₂ structures. Due to high gas accessibility through the meso- and macrosized pores in MOF-templated oxides and effective modulation of electron depletion layer assisted by the creation of numerous p–n junctions, the GRR-treated SnO₂ structures exhibited 21.9-fold higher acetone response (<i>R</i>_air/<i>R</i>_gas = 22.8 @ 5 ppm acetone, 90%RH) compared to MOF-templated p-Co₃O₄ hollow structures. To the best of our knowledge, the selectivity and response amplitudes reported here for the detection of acetone are superior to those MOF derived metal oxide sensing layers reported so far. Our results demonstrate that highly active MOF-derived sensing layers can be achieved via p–n semiconducting phase transition, driven by a simple and versatile GRR process combined with MOF templating route

Innovative Nanosensor for Disease Diagnosis

ConspectusAs a futuristic diagnosis platform, breath analysis is gaining much attention because it is a noninvasive, simple, and low cost diagnostic method. Very promising clinical applications have been demonstrated for diagnostic purposes by correlation analysis between exhaled breath components and specific diseases. In addition, diverse breath molecules, which serve as biomarkers for specific diseases, are precisely identified by statistical pattern recognition studies. To further improve the accuracy of breath analysis as a diagnostic tool, breath sampling, biomarker sensing, and data analysis should be optimized. In particular, development of high performance breath sensors, which can detect biomarkers at the ppb-level in exhaled breath, is one of the most critical challenges. Due to the presence of numerous interfering gas species in exhaled breath, selective detection of specific biomarkers is also important.This Account focuses on chemiresistive type breath sensors with exceptionally high sensitivity and selectivity that were developed by combining hollow protein templated nanocatalysts with electrospun metal oxide nanostructures. Nanostructures with high surface areas are advantageous in achieving high sensitivity because the sensing signal is dominated by the surface reaction between the sensing layers and the target biomarkers. Furthermore, macroscale pores between one-dimensional (1D) nanostructures can facilitate fast gas diffusion into the sensing layers. To further enhance the selectivity, catalytic functionalization of the 1D metal oxide nanostructure is essential. However, the majority of conventional techniques for catalytic functionalization have failed to achieve a high degree of dispersion of nanoscale catalysts due to aggregation on the surface of the metal oxide, which severely deteriorates the sensing properties by lowering catalytic activity. This issue has led to extensive studies on monolithically dispersed nanoscale particles on metal oxides to maximize the catalytic performances.As a pioneering technique, a bioinspired templating route using apoferritin, that is, a hollow protein cage, has been proposed to obtain nanoscale (∼2 nm) catalyst particles with high dispersity. Nanocatalysts encapsulated by a protein shell were first used in chemiresistive type breath sensors for catalyst functionalization on 1D metal oxide structures. We discuss the robustness and versatility of the apoferrtin templating route for creating highly dispersive catalytic NPs including single components (Au, Pt, Pd, Rh, Ag, Ru, Cu, and La) and bimetallic catalysts (PtY and PtCo), as well as the core–shell structure of Au–Pd (Au-core@Pd-shell). The use of these catalysts is essential to establish high performance sensors arrays for the pattern recognition of biomarkers. In addition, novel multicomponent catalysts provide unprecedented sensitivity and selectivity. With this in mind, we discuss diverse synthetic routes for nanocatalysts using apoferritin and the formation of various catalyst–1D metal oxide composite nanostructures. Furthermore, we discuss detection capability of a simulated biomarker gas using the breath sensor arrays and principal component analysis. Finally, future prospects with the portable breath analysis platform are presented by demonstrating the potential feasibility of real-time and on-site breath analysis using chemiresistive sensors

Abnormal Optoelectric Properties of Two-Dimensional Protonic Ruthenium Oxide with a Hexagonal Structure

Two-dimensional structures can potentially lead to not only modulation of electron transport but also the variations of optical property. Protonic ruthenium oxide, a two-dimensional atomic sheet material, has been synthesized, and its optoelectric properties have been investigated. The results indicate that protonic ruthenium oxide is an excellent candidate for use as a flexible, transparent conducting material. A hydrated-ruthenium-oxide sheet has been first prepared via the chemical exfoliation of sodium intercalated ruthenium oxide (NaRuO₂) and, subsequently, converted into a protonic ruthenium oxide sheet using thermal treatment. A thermally activated transport mechanism is dominant in hydrated ruthenium oxide but diminishes in protonic ruthenium oxide; this resulted in a high electrical conductivity of ∼200 S/cm of the protonic sheet. Because of the unique interband and intraband structure, protonic ruthenium oxide has a small optical absorption coefficient of ∼1.62%/L. Consequently, such high conductivity and low absorption coefficient of protonic ruthenium oxide results in excellent transparent conducting properties

Metal–Organic Framework Templated Catalysts: Dual Sensitization of PdO–ZnO Composite on Hollow SnO₂ Nanotubes for Selective Acetone Sensors

Metal–organic framework (MOF)-derived synergistic catalysts were easily functionalized on hollow SnO₂ nanotubes (NTs) via electrospinning and subsequent calcination. Nanoscale Pd NPs (∼2 nm) loaded Zn-based zeolite imidazole framework (Pd@ZIF-8, ∼80 nm) was used as a new catalyst-loading platform for the effective functionalization of a PdO@ZnO complex catalyst onto the thin wall of one-dimensional metal oxide NTs. The well-dispersed nanoscale PdO catalysts (3–4 nm) and multiheterojunctions (PdO/ZnO and ZnO/SnO₂) on hollow structures are essential for the development of high-performance gas sensors. As a result, the PdO@ZnO dual catalysts-loaded hollow SnO₂ NTs (PdO@ZnO–SnO₂ NTs) exhibited high acetone response (<i>R</i>_air/<i>R</i>_gas = 5.06 at 400 °C @ 1 ppm), superior acetone selectivity against other interfering gases, and fast response (20 s) and recovery (64 s) time under highly humid atmosphere (95% RH). In this work, the advantages of hollow SnO₂ NT structures with high surface area and open porosity were clearly demonstrated by the comparison to SnO₂ nanofibers (NFs). Moreover, the sensor arrays composed of SnO₂ NFs, SnO₂ NTs, PdO@ZnO–SnO₂ NFs, and PdO@ZnO–SnO₂ NTs successfully identified the patterns of the exhaled breath of normal people and simulated diabetics by using a principal component analysis

Metal–Organic Framework-Templated PdO-Co₃O₄ Nanocubes Functionalized by SWCNTs: Improved NO₂ Reaction Kinetics on Flexible Heating Film

Detection and control of air quality are major concerns in recent years for environmental monitoring and healthcare. In this work, we developed an integrated sensor architecture comprised of nanostructured composite sensing layers and a flexible heating substrate for portable and real-time detection of nitrogen dioxide (NO₂). As sensing layers, PdO-infiltrated Co₃O₄ hollow nanocubes (PdO-Co₃O₄ HNCs) were prepared by calcination of Pd-embedded Co-based metal–organic framework polyhedron particles. Single-walled carbon nanotubes (SWCNTs) were functionalized with PdO-Co₃O₄ HNCs to control conductivity of sensing layers. As a flexible heating substrate, the Ni mesh electrode covered with a 40 nm thick Au layer (i.e., Ni­(core)/Au­(shell) mesh) was embedded in a colorless polyimide (cPI) film. As a result, SWCNT-functionalized PdO-Co₃O₄ HNCs sensor exhibited improved NO₂ detection property at 100 °C, with high sensitivity (<i>S</i>) of 44.11% at 20 ppm and a low detection limit of 1 ppm. The accelerated reaction and recovery kinetics toward NO₂ of SWCNT-functionalized PdO-Co₃O₄ HNCs were achieved by generating heat on the Ni­(core)/Au­(shell) mesh-embedded cPI substrate. The SWCNT-functionalized porous metal oxide sensing layers integrated on the mechanically stable Ni­(core)/Au­(shell) mesh heating substrate can be envisioned as an essential sensing platform for realization of low-temperature operation wearable chemical sensor

Sub-Parts-per-Million Hydrogen Sulfide Colorimetric Sensor: Lead Acetate Anchored Nanofibers toward Halitosis Diagnosis

Lead­(II) acetate [Pb­(Ac)₂] reacts with hydrogen sulfide to form colored brownish precipitates of lead sulfide. Thus far, in order to detect leakage of H₂S gas in industrial sectors, Pb­(Ac)₂ has been used as an indicator in the form of test papers with a detection limit only as low as 5 ppm. Diagnosis of halitosis by exhaled breath needs sensors able to detect down to 1 ppm of H₂S gas. In this work, high surface area and porous Pb­(Ac)₂ anchored nanofibers (NFs) that overcome limitations of the conventional Pb­(Ac)₂-based H₂S sensor are successfully achieved. First, lead­(II) acetate, which melts at 75 °C, and polyacrylonitrile (PAN) polymer are mixed and stirred in dimethylformamide (DMF) solvent at 85 °C, enabling uniform dispersion of fine liquid droplets in the electrospinning solution. During the subsequent electrospinning, Pb­(Ac)₂ anchored NFs are obtained, providing an ideal nanostructure with high thermal stability against particle aggregation, numerous reactions sites, and enhanced diffusion of H₂S into the three-dimensional (3D)-networked NF web. This newly obtained sensing material can detect down to 400 ppb of H₂S at a relative humidity of 90%, exhibiting high potential feasibility as a high-performance colorimetric sensor platform for diagnosis of halitosis

Selective Detection of Acetone and Hydrogen Sulfide for the Diagnosis of Diabetes and Halitosis Using SnO₂ Nanofibers Functionalized with Reduced Graphene Oxide Nanosheets

Sensitive detection of acetone and hydrogen sulfide levels in exhaled human breath, serving as breath markers for some diseases such as diabetes and halitosis, may offer useful information for early diagnosis of these diseases. Exhaled breath analyzers using semiconductor metal oxide (SMO) gas sensors have attracted much attention because they offer low cost fabrication, miniaturization, and integration into portable devices for noninvasive medical diagnosis. However, SMO gas sensors often display cross sensitivity to interfering species. Therefore, selective real-time detection of specific disease markers is a major challenge that must be overcome to ensure reliable breath analysis. In this work, we report on highly sensitive and selective acetone and hydrogen sulfide detection achieved by sensitizing electrospun SnO₂ nanofibers with reduced graphene oxide (RGO) nanosheets. SnO₂ nanofibers mixed with a small amount (0.01 wt %) of RGO nanosheets exhibited sensitive response to hydrogen sulfide (<i>R</i>_air/<i>R</i>_gas = 34 at 5 ppm) at 200 °C, whereas sensitive acetone detection (<i>R</i>_air/<i>R</i>_gas = 10 at 5 ppm) was achieved by increasing the RGO loading to 5 wt % and raising the operation temperature to 350 °C. The detection limit of these sensors is predicted to be as low as 1 ppm for hydrogen sulfide and 100 ppb for acetone, respectively. These concentrations are much lower than in the exhaled breath of healthy people. This demonstrates that optimization of the RGO loading and the operation temperature of RGO–SnO₂ nanocomposite gas sensors enables highly sensitive and selective detection of breath markers for the diagnosis of diabetes and halitosis

Heterogeneous Sensitization of Metal–Organic Framework Driven Metal@Metal Oxide Complex Catalysts on an Oxide Nanofiber Scaffold Toward Superior Gas Sensors

We report on the heterogeneous sensitization of metal–organic framework (MOF)-driven metal-embedded metal oxide (M@MO) complex catalysts onto semiconductor metal oxide (SMO) nanofibers (NFs) via electrospinning for markedly enhanced chemical gas sensing. ZIF-8-derived Pd-loaded ZnO nanocubes (Pd@ZnO) were sensitized on both the interior and the exterior of WO₃ NFs, resulting in the formation of multiheterojunction Pd–ZnO and ZnO–WO₃ interfaces. The Pd@ZnO loaded WO₃ NFs were found to exhibit unparalleled toluene sensitivity (<i>R</i>_air<i>/R</i>_gas = 4.37 to 100 ppb), fast gas response speed (∼20 s) and superior cross-selectivity against other interfering gases. These results demonstrate that MOF-derived M@MO complex catalysts can be functionalized within an electrospun nanofiber scaffold, thereby creating multiheterojunctions, essential for improving catalytic sensor sensitization

Elaborate Manipulation for Sub-10 nm Hollow Catalyst Sensitized Heterogeneous Oxide Nanofibers for Room Temperature Chemical Sensors

Room-temperature (RT) operation sensors are constantly in increasing demand because of their low power consumption, simple operation, and long lifetime. However, critical challenges such as low sensing performance, vulnerability under highly humid state, and poor recyclability hinder their commercialization. In this work, sub-10 nm hollow, bimetallic Pt–Ag nanoparticles (NPs) were successfully formed by galvanic replacement reaction in bioinspired hollow protein templates and sensitized on the multidimensional SnO₂–WO₃ heterojunction nanofibers (HNFs). Formation of hollow, bimetallic NPs resulted in the double-side catalytic effect, rendering both surface and inner side chemical reactions. Subsequently, SnO₂–WO₃ HNFs were synthesized by incorporating 2D WO₃ nanosheets (NSs) with 0D SnO₂ sphere by <i>c</i>-axis growth inhibition effect and fluid dynamics of liquid Sn during calcination. Hierarchically assembled HNFs effectively modulate surface depletion layer of 2D WO₃ NSs by electron transfers from WO₃ to SnO₂ stemming from creation of heterojunction. Careful combination of bimetallic catalyst NPs with HNFs provided an extreme recyclability under exhaled breath (95 RH%) with outstanding H₂S sensitivity. Such sensing platform clearly distinguished between the breath of healthy people and simulated halitosis patients