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• 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]

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]

Abstract: Chemoresistive H2S sensors using various oxide nanoparticles were prepared and heat-treated at 600°C in an effort to define halitosis in human breath. WO3, ZnO, and SnO2 were tested as sensing materials, among which WO3 showed the highest stability to H2S. XPS analysis showed a metal sulfate peak on the surface of ZnO and SnO2, which causes a recovery problem after 2ppm H2S exposure. On the contrary, the WO3 sensor showed a stable signal in long-term operation in the presence of H2S gas. WO3 gas sensors decorated with various catalytic metals were fabricated to investigate their sensing properties in the 0.2–5ppm H2S range with 80% relative humidity (RH). A 0.03wt% Au-doped WO3 sensor exhibited excellent H2S sensitivity (R air/R gas =12.40at 2ppm) toward H2S, whereas pure WO3 showed a sensitivity of 4.85 with negligible interference from the volatile organic compounds (VOCs). [Copyright &y& Elsevier]

In this work, we report a remarkably improved toluene response and superior cross-sensitivity against H2S molecules by combining Pd catalysts and highly porous WO3 nanofibers (NFs). We functionalized Pd catalysts inside and/or outside of WO3 NFs synthesized by electrospinning, which is a facile and versatile process for producing webs of metal oxide NFs. Pd-embedded WO3 NFs were obtained by the electrospinning of a Pd and W composite precursor/poly(methyl methacrylate) (PMMA) solution followed by calcination at 700°C. Pd nanoparticles (NPs) (6–10nm) synthesized by a polyol method were decorated on the WO3 NFs (Pd-NPs/WO3 NFs) by the attachment of the Pd NPs on as-prepared WO3 NFs. The gas sensing characteristics of pure WO3, Pd-embedded WO3, Pd-NPs/WO3, and Pd-NPs/Pd-embedded WO3 NFs were tested using H2S and toluene gases in a highly humid atmosphere (RH 90%), which is similar to human exhaled breath. The results showed that the Pd-NPs/Pd-embedded WO3 NFs whose inner and outer layers were decorated by Pd catalysts, exhibited a high toluene response (R air/R gas =5.5 at 1ppm) and remarkable selectivity against H2S (R air/R gas =1.36 at 1ppm) at 350°C, whereas pristine WO3 NFs showed superior H2S response (R air/R gas =11.1 at 1ppm) along with a negligible response toward toluene (R air/R gas =1.27 at 1ppm). The highly porous WO3 NFs decorated with Pd catalysts, exhibited potential feasibility, i.e., a low limit of detection (LOD) of 20ppb (R air/R gas =1.32) at 350°C, for application in VOCs sensors, particularly for diagnoses of lung cancer. [ABSTRACT FROM AUTHOR]

[Display omitted] • Noble selector molecules composed of asymmetric N,N- substituted croconamides with strong binding affinity with anions. • The unique design of croconamide-based selectors providing versatile utility for chemiresistive anion sensing platforms. • Practical application of croconamide-based selectors for real-time and rapid screening of various oxoanions. Croconamides are a new class of dual-hydrogen bond donors that have attracted interest in anion recognition and sensing applications due to their strong H-bond donation capability. In this work, we report chemiresistive sensors for the detection of anions based on poly(4-vinylpyridine) (P4VP)-wrapped single-walled carbon nanotubes (SWCNTs) functionalized with croconamide-based selectors. Model structures of asymmetric N,N -substituted croconamide selectors with N,N -substitutional groups of alkyl cationic pyridinium and electron-withdrawing moieties such as aniline (6) and 3,5-bis(trifluoromethyl)phenyl (7) groups were synthesized and their binding affinities for various anions were characterized using UV–vis and 1H NMR titrations. Switchable binding of oxoanions was observed with selector 7 , wherein HSO 4 – formed hydrogen bonding interactions and acetate (AcO–) induced deprotonation of the NH protons of the croconamide. Large sensitivity transitions were obtained toward AcO– for both P4VP- 6 -SWCNT (S = 95.68 ± 2.11 %) and P4VP- 7 -SWCNT (S = 140.91 ± 3.68 %) at 83.33 mM, which was attributed to deprotonation of the croconamide selectors upon addition of AcO–. Three serial sensors were assembled in a sensor array that rapid and selective screening of AcO– was demonstrated against other oxoanions such as HSO 4 – and H 2 PO 4 – in real-time. [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]

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]

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]

To achieve highly sensitive chemiresistive gas sensors, facile and effective synthesis for catalytic decoration is essential on an electrically conductive scaffold containing a large surface area and high porosity. Herein, we report a new synthetic strategy for the functionalization of one-dimensional carbon nanofibers (CNFs) using dual-catalytic nanoparticles (NPs) of both oxygen-incorporated MoS 2 (MoS x O y) and Pt. The functionalized CNFs were synthesized through an electrospinning process followed by heat treatment in a reducing ambient to ensure the decoration of ultra-small MoS x O y and Pt NPs along the CNFs (Pt-MoS x O y @CNFs). As a result, Pt-MoS x O y @CNFs exhibited an improved sensing response of 39.6% toward 5 ppm of NO 2 as compared to the responses of pristine CNFs (7.3%) and MoS x O y @CNFs (13.7%) at room temperature. At the optimized operating temperature of 150 °C, the Pt-MoS x O y @CNFs exhibited a 5.8-fold higher response than that of the pristine CNFs and also presented fully reversible and selective sensing properties. The enhanced NO 2 sensing properties were primarily attributed to the chemical sensitization of Pt NPs toward NO 2 and a p-n heterojunction between the CNFs and MoS x O y NPs. This study demonstrates the uniform distribution of multiple nano-sized catalysts on CNFs, paving the way for establishing multiplexed sensor arrays and application in electronic nose systems. • Simultaneous synthesis of nano-sized dual catalysts on CNFs through an electrospinning and heat treatment processes. • A study on the NO 2 sensing performances of surface functionalized CNFs with Pt and MoS x O y nanoparticles. • Demonstration of real-time NO 2 detection using CNFs-based chemiresistive gas sensor with reversibility and selectivity. [ABSTRACT FROM AUTHOR]

We report on facile synthesis of SnO2 nanofibers (NFs) functionalized by Pt nanoparticles (NPs) with various loading contents (0.05–0.4wt%) and their gas response toward H2, NO2, NH3, CO, and SO2 gases as a function of the operation temperature (100–450°C) and gas concentration (0.125–2.5ppm). Mesoporous SnO2 NFs with large surface-to-volume ratio, which facilitates sensitive gas response, were synthesized by electrospinning followed by subsequent calcination at 500°C for 2h. Colloidal Pt NPs synthesized by simple polyol method were immobilized on SnO2 NFs as a catalytic sensitizer to H2 oxidation. The gas sensing properties and response kinetics were significantly affected by the Pt NPs and the operation temperature. It was found that 0.1wt% Pt-loaded SnO2 sensor showed the highest H2 response (I gas/I air =16.6 at 2.5ppm) at 300°C. A remarkably high NO2 response (I air/I gas =57.0 at 2.5ppm) was achieved in the case of pristine SnO2 sensor at 150°C. This work demonstrates that sensitive and selective detection of H2 and NO2 gases with negligible cross-response to interfering gases such as NH3, CO, and SO2 can be obtained by decoration with Pt NPs on metal oxide NFs in conjunction with manipulation of the operation temperature. [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]

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]

• 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]

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]