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Porous and hollow nanostructures functionalized with nanoscopic catalysts have attracted considerable interest in a wide range of applications depending on surface reactions. In this study, we present an efficient and versatile synthetic strategy of porous metal oxide hollow frameworks (HFs) decorated with noble metal nanoparticles (NPs). The key lies in the use of porous and bicontinuous polystyrene- block -poly(4-vinylpyridine) (PS- b -P4VP) microparticles as templates, which feature pores on the order of several hundred nanometers in size. Homogeneous Pt loading on the surface of the porous microparticles is achieved through the preferential adsorption of Pt precursors onto the P4VP block via electrostatic interactions. Subsequent SnO 2 thin film deposition and calcination ultimately yield honeycomb-like SnO 2 HFs with macropores and uniform distribution of Pt NPs. The Pt NP-loaded SnO 2 HFs exhibit exceptional formaldehyde (HCHO) sensing capabilities, characterized by remarkable sensitivity (31.4 at 5 ppm), rapid response time (4.7 s at 5 ppm), ultra-low detection limit of 10 ppb, and remarkable cross-selectivity. These outstanding features result from their high surface area, gas permeability within porous hollow structures, and well-dispersed Pt NPs, which catalytically activate the sensor. Our study provides great promise for advancing surface catalytic reactions due to their exceptional surface area and gas permeability, paving the way for future applications. [Display omitted] • We present an innovative synthetic strategy for the fabrication of Pt/SnO 2 hollow frameworks. • The key is the use of Pt-functionalized porous block copolymer microparticles as a sacrificial template. • The Pt/SnO 2 HFs demonstrate remarkable sensitivity (31.4 at 5 ppm), rapid response time (4.7 s at 5 ppm). • Our nanoscopic catalyst-loaded porous metal oxide hollow frameworks serve as a versatile catalytic platform. [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]

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]

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]

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]