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Electrospinning–etching strategy was employed to synthesize Pt-loaded SnO 2 hollow microbelts (Pt_SnO 2 HBLs) based chemiresistive-type sensor demonstrating high sensitivity and selectivity toward acetone gas. This strategy overcomes a major hurdle toward achieving ubiquitous gas diffusion in electrospun structures, and provides highly porous thin-walled HBLs with large surface area and small crystallites. [Display omitted] • In situ templating of SnO 2 with SiO 2 was employed to form Pt-loaded SiO 2 @SnO 2 core–shell microbelts. • Etching away of SiO 2 in the Pt-loaded SiO 2 @SnO 2 microbelts yielded Pt-loaded SnO 2 hollow microbelts. • The Pt-loaded SnO 2 hollow microbelts exhibited good response (R a /R g = 93.5) and selectivity toward 2 ppm acetone at 350 °C. Challenges for ubiquitous diffusion of gas analytes into semiconducting metal oxides (SMOs) based sensing layers have necessitated the introduction of unprecedented synthesis techniques for thin, porous and high-performance gas sensing materials. In this work, electrospinning and calcination were employed to prepare unprecedented SiO 2 –SnO 2 core–shell microbelts (hereinafter referred to as SiO 2 @SnO 2 BLs) followed by etching away of SiO 2 in NaOH solution (pH 12) to achieve hollow SnO 2 BLs (hereinafter referred to as SnO 2 HBLs) with mean crystal size of 14.01 nm, large BET surface area (143.5 m2‧g–1), high porosity (mean pore size of 5.7 nm), and shell thickness of 58.3 ± 11.4 nm. Sensitization of SnO 2 HBLs with apoferritin-templated platinum nanoparticles (Pt NPs) enhanced their detection capability toward acetone. Response (defined as R a /R g , where R a and R g are sensor resistances in air and target gas, respectively) of Pt(0.12 %)_SnO 2 HBLs toward 2 ppm acetone was up to 7.8 times higher compared to that of pristine SnO 2 HBLs, and exhibited faster response (9.2 s). The Pt(0.12 %)_SnO 2 HBLs based sensor indicated a promising long-term stability, and outstanding repeatability of response (R a /R g = 93.7 ± 0.89) toward 25 cycles of 2 ppm acetone exposure in a humid environment of 90 % relative humidity. This performance could be attributed to the unique morphology of HBLs, sensitization effects of catalytic Pt NPs, and enlargement of the electron-depleted layer resulting from a Schottky barrier between Pt NPs and SnO 2 grains. [ABSTRACT FROM AUTHOR]

• Novel all-carbon fiber-based NO 2 sensor was fabricated for reliable environmental monitoring. Both RGO fiber and CNT fiber were synthesized by wet-spinning process, which is facile and compatible with large-scale production. • Unique sensing architectures of 1D graphene fiber were achieved with nitrogen doping for NO 2 sensors. Highly conductive nitrogen-doped reduced graphene oxide (RGO) fiber was achieved with wrinkled surface morphology. • CNT fibers were firstly demonstrated as heating networks for reversible NO 2 reaction. The CNT fibers with high electrical conductivity and mechanical stability are suitable for heating element to control the operating temperature of sensor. All-carbon fiber-based chemiresistor is fabricated by assembling reduced graphene oxide (RGO) fiber and carbon nanotube (CNT) fiber as reversible NO 2 sensing layer and flexible heater, respectively. Both graphene oxide (GO) and CNT fibers were synthesized by wet-spinning technique facilitating lyotropic nematic liquid crystal (LC) property. Randomly entangled CNT fiber-based heater, which is embedded in one surface of colorless polyimide (cPI) film with thickness of ˜200 μm, exhibits high bending stability and heating property up to 100 °C. Single reduced graphene oxide (RGO) fiber obtained after heat treatment at 900 °C in H 2 /N 2 ambient was integrated on the CNT fiber-embedded cPI heater, thereby establishing a new type of all-carbon fiber sensing platform. As a result, accelerated NO 2 adsorption and desorption kinetics were achieved with RGO fiber at an elevated temperature. In particular, a 9.22-fold enhancement in desorption kinetic (k des = 8.85 × 10–3 s–1) was observed at 100 °C compared with the desorption kinetic (k des = 0.96 × 10–3 s–1) at 50 °C, which was attributed to the effective heating by CNT fiber networks. This work pioneered a research on the use of emerging carbonaceous fibers for potential application in wearable chemical detectors. [ABSTRACT FROM AUTHOR]

Semiconducting metal oxides (SMOs) based gas sensors are inherently hampered by requirement of high working temperature because of their wide bandgaps. Consequently, there are remain challenges to utilize SMOs based gas sensors with regard to continuous gas monitoring platform with low power consumption. To address the inherent limitation, nitridation (i.e., N doping) of WO 3 NFs to WO x N y NFs was herein introduced for realization of low-temperature NO 2 sensing. Importantly, nitridation was conducted at intervals of 50 °C from 400 to 600 °C to adjust the degree of phase transition from semiconducting WO 3 to conductive WON phases. To further optimize the conductivity and improve the sensing characteristics, i.e., reduction of optimal operating temperature and enhancement of sensitivity, WO x N y NFs were functionalized with platinum (Pt) catalytic nanoparticles (NPs) by physical mixing. Pt NPs functionalized WO x N y NFs nitrided at 550 °C exhibited improved normalized resistance change and superior selectivity against C 2 H 5 OH, C 7 H 8 , CH 4 , CO, NH 3 , and NO at 50 °C. The noble metal catalyst–metal oxynitride hybrid system is suggested as an effective NO 2 sensing platform for low-temperature chemical sensor. [ABSTRACT FROM AUTHOR]

Electrospun WO 3 nanofibers (NFs) with hierarchically interconnected porosity (HP_WO 3 NFs) are designed by using dual sacrificial templates including zero-dimensional (0D) polystyrene (PS) colloids and one-dimensional (1D) multi-walled carbon nanotubes (MWCNTs). Multi-dimensionally interconnected pore-loaded structures offer much enhanced surface area and abundant gas penetration pathway into the inner sensing layers, which are essential requirements for effective gas adsorption-desorption reactions, fast gas diffusion, and high gas response. Moreover, such porous WO 3 NFs are homogeneously decorated by Pt nanoparticles (Apo-Pt) encapsulated in a protein nanocage, i.e., apoferritin, which enables uniform catalyst sensitization in interior and exterior of highly gas accessible NFs. The HP_WO 3 NFs sensitized by Apo-Pt (Apo-Pt@HP_WO 3 NFs) exhibited highly selective and sensitive acetone response (R air /R gas  = 10.80 ± 0.06 @ 1 ppm) under high humidity atmosphere (90% RH). To investigate the potential suitability as exhaled breath analyzers, we tested three sensor arrays consisting of mesoporous WO 3 NFs templated by MWCNTs (MP_WO 3 NFs), HP_WO 3 NFs, and Apo-Pt@HP_WO 3 NFs. The result revealed that targeted acetone molecules were clearly classified against six different interfering molecules (H 2 S, C 7 H 8 , C 2 H 5 OH, CO, NH 3 and CH 4 ) through principle component analysis (PCA), confirming excellent acetone selectivity of the designed sensor arrays. [ABSTRACT FROM AUTHOR]

Functionalization of catalytic nanoparticles (NPs) on semiconductor metal oxide (SMO) sensing layer is an indispensable process to obtain improved sensitivity and selectivity for high performance chemical sensors. It is a critical challenge to achieve homogeneous distribution of nanoscale catalysts on SMO in consideration that gas sensing characteristics of SMO-based sensing layer are significantly influenced by the size and distribution of catalysts. Here, we propose a highly effective functionalization method to achieve well-distributed catalytic NPs onto one dimensional (1D) SMO nanofibers (NFs) using protein cage templates: apoferrtin. By simply replacing precursor in the apoferritin assisted method, not only precious catalyst such as Pt but also non-precious catalysts such as La and Cu were successfully synthesized in nanoscale ( i.e. , 3–5 nm). Furthermore, the apoferritin-encapsulated catalysts exhibited high dispersion property due to repulsive force between protein shells. For this reason, catalytic NPs were homogeneously decorated on ZnO NFs after electrospinning followed by calcination. Catalytic Pt NPs and Cu NPs functionalized ZnO NFs exhibited approximately 6.38-fold (R air /R gas = 13.07) and 2.95-fold (R air /R gas = 6.04) improved acetone response compared with the response (R air /R gas = 2.05) of pristine ZnO NFs. In the case of La NPs functionalized ZnO NFs, 9.31-fold improved nitrogen monoxide response (R air /R gas = 10.06) was achieved compared with the response of pristine ZnO NFs. The four catalyst-ZnO composite NFs successfully distinguished simulated breath components such as acetone, toluene, nitrogen monoxide, carbon monoxide, and ammonia with well-classified patterns by principal component analysis (PCA). This work demonstrated a robustness of synthetic and functionalization method using bio-inspired protein templates combined with electrospinning technique and a promising potential of using non-precious catalysts to establish diverse sensing material libraries that can be applicable to breath pattern recognition for diagnosis of diseases. [ABSTRACT FROM AUTHOR]

In this work, a novel catalytic synthesis and functionalization method using apoferritin is used to fabricate RuO 2 nanoparticles (NPs) loaded WO 3 nanofibers (NFs) for potential diagnosis of diabetes. Catalytic ruthenium (Ru) NPs with very small average diameters of 1.8 ± 0.9 nm were synthesized using apoferritin which is a hollow protein cage, and were easily functionalized on WO 3 NFs by introducing electrospinning solution with W precursor and polyvinylpyrrolidone (PVP). As-spun Ru NPs-loaded W precursor/PVP composite NFs were calcined at 600 °C for 1 h in air atmosphere to achieve RuO 2 -functionalized WO 3 NFs. The small size and uniform distribution of catalytic RuO 2 NPs were well maintained due to hollow nature of apoferritin cages after calcination. The chemo-resistive sensors using RuO 2 -functionalized WO 3 NFs showed significantly enhanced acetone (CH 3 COCH 3 ) sensing response (R air /R gas = 78.61–5 ppm), which was 7.4 times higher than the response (R air /R gas = 10.61–5 ppm) of pristine WO 3 NFs at highly humid atmosphere (95% RH). In addition, the RuO 2 -functionalized WO 3 NFs showed outstanding selectivity toward acetone gas in comparison with other gases such as hydrogen sulfide (H 2 S), toluene (C 6 H 5 CH 3 ), ethanol (C 2 H 5 OH), pentane (C 5 H 12 ), ammonia (NH 3 ), hydrogen (H 2 ), and water vapor (H 2 O) at 5 ppm. These results represent potential feasibility for the detection of acetone in exhaled breath for diagnosis of diabetes. [ABSTRACT FROM AUTHOR]

In this work, catalytic Rh 2 O 3 -functionalized WO 3 nanofibers (NFs) were synthesized via an electrospinning route and used as a highly selective acetone-sensing layer for potential diagnosis of diabetes. Catalytic rhodium nanoparticles (Rh NPs) with average diameters of 5.0 ± 0.52 nm, which were synthesized by the polyol process, were dispersed in water with W precursor and poly(vinylpyrrolidone) (PVP) for electrospinning. As-spun Rh NP-loaded W precursor/PVP composite NFs were calcined at 600 °C for 1 h in air atmosphere to achieve Rh 2 O 3 -decorated WO 3 NFs. Microstructure evolution and chemical composition of Rh 2 O 3 -decorated WO 3 NFs as a function of Rh-loading amounts, i.e., 0.01 wt%, 0.05 wt%, 0.10 wt%, and 0.15 wt%, were examined using energy-dispersive X-ray spectroscopy (EDS), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM). The mean size (30 nm) of the WO 3 crystallites in Rh 2 O 3 -decorated WO 3 NFs was much smaller than that (60 nm) of the WO 3 crystallites in pristine WO 3 NFs. The Rh 2 O 3 -decorated WO 3 NFs showed outstanding acetone (CH 3 COCH 3 ) sensing response ( R air / R gas = 41.2 to 5 ppm), which was 4.6 times higher than the response ( R air / R gas = 9.0 to 5 ppm) of pristine WO 3 NFs at highly humid atmosphere (95% RH). In addition, superior acetone cross-sensitivity of the Rh 2 O 3 -decorated WO 3 NFs was observed in other interfering gases such as pentane ( n -C 5 H 12 ), ammonia (NH 3 ), toluene (C 6 H 5 CH 3 ), carbon monoxide (CO), and ethanol (C 2 H 5 OH) at 5 ppm. These results are highly promising for the accurate and selective detection of acetone in exhaled breath for potential diagnosis of diabetes. [ABSTRACT FROM AUTHOR]

In this work, highly porous WO 3 nanotubes (NTs) were synthesized by facile layer-by-layer (LbL) self-assembly on polymeric nanofiber (NF) templates followed by calcination. Polymeric NFs using poly(methyl methacrylate) (PMMA) were prepared by electrospinning as sacrificial templates. Then, ionic polymers were coated on PMMA to modify the surface charge of PMMA NFs. Catalyst-loaded WO 3 NTs were synthesized by self-assembly of tungsten precursor and catalytic precursor on the surface of the polymeric PMMA NFs followed by calcination at 500 °C for 1 h. Gas sensing performances were evaluated in highly humid atmosphere (90% RH) using pristine WO 3 NTs, Pt-loaded WO 3 NTs (Pt-WO 3 NTs), and Pd-loaded WO 3 NTs (Pd-WO 3 NTs). Pristine WO 3 NTs exhibited a high NO response ( R gas / R air = 63.59 at 5 ppm) at 350 °C and cross-selectivity toward toluene ( R air / R gas = 1.05 at 5 ppm). On the other hand, Pt-WO 3 NTs and Pd-WO 3 NTs exhibited a high toluene response ( R air / R gas = 2.24 for the Pt-WO 3 NTs and R air / R gas = 2.35 for the Pd-WO 3 NTs at 5 ppm) at 400 °C and a negligible NO response ( R gas / R air = 1.25 for the Pt-WO 3 NTs and R gas / R air = 1.04 for the Pd-WO 3 NTs at 5 ppm) at 400 °C. These results demonstrated that LbL synthesis is a highly promising method for producing hollow semiconductor metal oxide NTs functionalized with various catalysts, which leads to potential application in exhaled breath analysis for asthma and lung cancer diagnosis. [ABSTRACT FROM AUTHOR]

• The non-noble metals are introduced on graphene-based fiber as a sensitizer for the high-performance chemiresistor. • Metal oxides on graphene fibers naturally are converted into metals at a relatively low temperature annealing (under 700 °C) due to catalytic effect of graphene. • The graphene fiber acts as a flexible conductive support as well as a catalyst for the deoxidation of metal oxide. • The metal/metal oxide/graphene fibers exhibited a 16 times higher response to NO 2 gas and recovery than metal oxide/graphene fiber without metal. • The proposed strategy opens a new avenue for the realization of high-performance devices, such as chemical catalysts, photoactive devices. [Display omitted] Noble metal/metal-oxide-based hybrid gas sensors exhibit a low operating temperature, remarkable sensitivity, and fast recovery. As additives, noble metals induce a catalytic sensitization effect, which promotes charge transfer from the metal oxide to the analyte molecules, the so-called spillover mechanism. This suggests that metal catalysts can improve gas sensing performance. Herein, for the first time, non-noble metals are introduced on hybrid metal oxide/graphene fibers as sensitizers to fabricate high-performance chemiresistive sensors. The formation of metal components can be effectively controlled by annealing the metal oxide on graphene. Remarkably, compared with the corresponding metal oxide/graphene fiber sensors without metal components, the metal/metal oxide/graphene fiber sensors exhibit over a 16-fold higher response to NO 2 gas as well as effective recovery characteristics. Specifically, the Cu/Cu 2 O/graphene and Ni/NiO/graphene fiber sensors operating at 150 °C exhibit sensitivities of 18.90 % and 0.82 %, respectively, for 5 ppm NO 2 gas. The proposed strategy to achieve flexible graphene fiber chemiresistors by decorating them with non-noble metal and metal oxide nanoparticles opens a new avenue for realizing high-performance devices, such as photovoltaic devices, photocatalysts, and chemical catalysts. [ABSTRACT FROM AUTHOR]

• Porosity control of nanofibers: The high porosity of the metal oxide nanofibers is the key to achieving the dual sensitization owing to their high gas permeability and numerous active sites. Both saponin and water-ethanol co-solvent play critical roles in modifying morphology of nanofibers into elliptical nanofibers during electrospinning. Additionally, saponin forms numerous pores in the nanofibers when being burnt out during calcination. These porous, elliptical nanofibers possess a larger surface area and porosity.Saponin and water-ethanol co-solvent enabled porous, elliptical nanofibers to emerge. • Bifunctional Ga doping: Ga dopants enhanced sensing performance via conductivity improvement and grain growth inhibition. • Functionalization of Pt catalysts : Spill-over effects and chemical sensitization of remarkably improved C 2 H 2 sensitivity. Herein, we demonstrate a sensitization strategy via solid-solution doping and catalytic metal nanoparticles (NPs) onto porous metal oxide nanofibers (NFs) for superior acetylene (C 2 H 2) sensing capabilities. The introduction of dopants and catalysts not only increases the inherent reactivity of metal oxides, but also induces the spillover of C 2 H 2 onto metal oxides, significantly improving the sensing properties. We elucidate that the high porosity of the metal oxide NFs, prepared by electrospinning and utilizing saponin as a sacrificial template, is the key to achieving high gas permeability and numerous active sites. Consequently, Ga-doped ZnO porous NFs functionalized with Pt catalysts exhibited the highest C 2 H 2 sensing responses among the current state-of-the-art C 2 H 2 sensors (R a /R g = 26.2 at 400 °C) with a fast response time (10.2 s), at a particularly low concentration of 5 ppm. Moreover, these NFs showed excellent selectivity against the interfering gas (H 2 , C 7 H 8 , CO, CH 4 , C 8 H 10 , CH 3 COCH 3 , HCHO and C 2 H 5 OH) and outstanding stability up to 20 sensing cycles to 0.4 ppm of C 2 H 2. These results provide an efficient strategy to tailor composites of metal oxide nanostructures, dopants, and catalysts for highly sensitive gas sensors. [ABSTRACT FROM AUTHOR]

The low response and sluggish reaction kinetics of graphene-based gas sensors remain as obstacles for practical applications as wearable chemical sensors. To overcome these limitations, we demonstrate a heterogeneous sensitization of nanocatalysts using Pt and ZnO nanoparticles (NPs) on porous reduced graphene oxides (Pt_ZnO/PRGO) driven by a metal-organic framework (MOF)-templated synthesis and subsequent pyrolysis. The pyrolysis of zeolitic imidazolate framework-8 layers grown on graphene oxide leads to the creation of porous carbon frameworks decorated with catalytic Pt and ZnO NPs, which possess increased reaction sites for the target gas. In addition, we investigated the effect of pyrolysis conditions to control the size of ZnO NPs with a 10 nm scale for enhanced catalytic sensitization. As a result, Pt_ZnO/PRGO exhibited improved NO 2 sensing properties with regard to response (43.28 % to 5 ppm), selectivity, and detection limit (0.1 ppm), as well as having reversible reaction kinetics. Potential applications of MOF-derived Pt_ZnO/PRGO in wearable chemical sensors was demonstrated, in which the material exhibited excellent mechanical stability and reliable response after 450 bending cycles. [ABSTRACT FROM AUTHOR]

• To control the porosity, the e-spraying and PS beads templating route are combined, resulting in highly porous structure. • Pt functionalized PH-SnO 2 spheres exhibited high response (R air /R gas = 44.83) toward 5 ppm of acetone. • Pt-PH-SnO 2 spheres exhibited exceptional selectivity toward acetone against interfering molecules. Tailoring of semiconducting metal oxides (SMOs) nanostructures with high porosity is of importance for enhanced gas sensing performance. Hierarchically-assembled SMOs possess high surface area but often suffer from low porosity. Here, bimodal pore-loaded hierarchical SnO 2 (PH-SnO 2) spheres were successfully synthesized via electrostatic spraying method (e-spraying) combined with colloidal templating route using polystyrene beads. The resulting porous PH-SnO 2 spheres were used as sensing layers for detection of acetone, which exhibited about 20 % enhanced response compared with hierarchical SnO 2 (H-SnO 2) spheres without pores. In addition, e-spraying is a fascinating technique for uniform catalytic functionalization through a simple dispersion of catalytic nanoparticle (NPs) in the e-spraying solution to improve the sensing performance. As a result, the Pt-functionalized PH-SnO 2 (Pt-PH-SnO 2) spheres showed dramatically improved acetone detection capability with a response (R air /R gas) of 44.83 at 5 ppm as compared to PH-SnO 2 spheres (R air /R gas = 6.61) as well as superior selectivity. In this work, the unique combination of e-spraying and PS templating route paves the way for robust and facile synthetic method for bimodal pore loaded 3D hierarchical SMOs, and demonstrates the feasibility for application in exhaled breath sensors. [ABSTRACT FROM AUTHOR]