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Lithium chloride (LiCl)-loaded 3D cubic mesoporous silica KIT-6 (LiCl-KIT-6) composites were prepared as resistance-type humidity-sensitive materials. The structures of the resultant composites were characterized by X-ray diffraction, N 2 adsorption–desorption, and transmission electron microscopy. The humidity sensors based on LiCl-KIT-6 composites were fabricated and the sensing properties were investigated. Results indicate the uniform pore distribution of KIT-6 contributes to the rapid hydrone transport in mesoporous channels. The impedance of optimized 3 wt% LiCl-KIT-6 sensor changed by more than three orders of magnitude over the relative humidity range of 11–95%. The response and recovery times of the sensor were 15 s and 28 s, respectively. Finally, the complex impedance method was used to analyze the humidity sensing mechanism of the sensor based on LiCl-KIT-6. [ABSTRACT FROM AUTHOR]

[Display omitted] Application of metal organic frameworks (MOFs) on sensors is of great interest for researchers. Film forming ability of the sensing material is very important for both the preparation process and sensing properties of the devices. Humidity sensors based on UIO-66 derived polyelectrolyte films were well prepared by in situ thiol-ene click cross-linking polymerization in this work. The hydrophilicity of the sensing film could be controlled by the feed ratios. The optimized humidity sensor shows a fast response to RHs change (Res/Rec time is 3.1 s/1.5 s, respectively) with ∼1.2% RH of humidity hysteresis. The water molecules adsorption behavior of the film and the sensing mechanism were also be investigated. The humidity sensor with good water and thermal stability and repeatability was applied in breath monitoring, which can well distinguish different breath states. [ABSTRACT FROM AUTHOR]

Low humidity detection has been a great challenge in the field of humidity sensing. In recent years, polyelectrolytes have been proven to be promising materials for highly sensitive humidity sensors. In this work, cross-linked poly(ionic liquid)s (PILs) were prepared for low humidity detection. The strong hydrophilicity of PILs contributes to humidity sensing while the cross-linked structure guarantees the stability of the sensing film. The optimized sensor shows high response (48.0) in 5–35% RH, with short response/recovery time (1 s/10 s) and a small humidity hysteresis (0.2% RH). The possible sensing mechanism of the sensors was discussed. • Cross-linked poly(ionic liquid)s were used as low humidity sensitive materials. • The optimized sensor shows high response in 5–35% RH. • The optimized sensor has short recovery time and small humidity hysteresis. • The introduction of glycerol could reduce the impedance of the sensor. [ABSTRACT FROM AUTHOR]

[Display omitted] • Au-loaded mesoporous silica/gelatin hydrogel was used the excellent humidity sensing film. • The prepared sensor could detect NH 3 by utilizing the outstanding ability to adsorb water molecules. • An egg storage scenario was simulated, and the water loss in eggs was monitored by the humidity sensor. • The sensing mechanism and contribution of each component in the sensing process were analyzed. In this paper, a composite hydrogel film (Au/Me-SiO 2 /G) based on Au nanoparticles, mesoporous silica and gelatin was prepared, which can be used for low relative humidity (RH) detection. Au-1%/Me-SiO 2 /G exhibits a response of 423% in the range of 0.4–30% RH, a short response time of 28 s, a linearity of R2 = 0.997, and good long-term stability. With the outstanding ability to adsorb water molecules, the sensor can detect NH 3 at room temperature and low humidity environment. The response of the sensor to NH 3 significantly increases with the humidity. At 30% RH, the initial impedance value of the sensor is as low as ∼6.6 MΩ, and the response to 50 ppm NH 3 is 78%. In addition, an egg storage scenario was simulated, and the water loss in eggs was monitored simultaneously by the prepared humidity sensor and a commercial sensor. The results demonstrate the practical application detection ability of the prepared humidity sensor. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • In this work, the obtained Zn 2 SnO 4 -RGO hybrids exhibited outstanding sensing performance for detecting oxidizing gases (NO 2 and O 3) with very low cross sensitivities to reducing gases, such as C 2 H 5 OH, CH 3 COCH 3 and CO. • The Zn 2 SnO 4 -RGO based sensors exhibited high response values of up to 3.50 for 500 ppb NO 2 , which is higher than that for detection of 500 ppb O 3 (1.78) at 30 °C under 50% relative humidity (RH). • The surface reaction between Zn 2 SnO 4 -RGO hybrids and NO 2 can be concluded from Operando diffuse reflectance infrared Fourier transformed spectroscopy (Operando DRIFT). Abstract Highly selective oxidizing gas sensors are of great importance for environmental pollution monitoring. In this work, a hybrid material containing Zn 2 SnO 4 nanoparticles (NPs) and immobilized reduced graphene oxide (Zn 2 SnO 4 -RGO) was developed as a high performance gas sensing material for the detection of ppb-levels of oxidizing gases (NO 2 and O 3). The structural, morphological and compositional properties of the Zn 2 SnO 4 -RGO hybrids were systematically characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), which demonstrated the successful anchoring of Zn 2 SnO 4 NPs on RGO nanosheets. The obtained Zn 2 SnO 4 -RGO hybrids exhibited outstanding sensing performance for detecting oxidizing gases (NO 2 and O 3) with very low cross sensitivities to reducing gases, such as C 2 H 5 OH, CH 3 COCH 3 and CO. The Zn 2 SnO 4 -RGO based sensors exhibited high response values of up to 3.50 for 500 ppb NO 2 , which is higher than that for detection of 500 ppb O 3 (1.78) at 30 °C under 50% relative humidity (RH). Moreover, the NO 2 sensing performances of Zn 2 SnO 4 -RGO-based sensors were investigated under various RH. In all cases, the sensors based on RGO and Zn 2 SnO 4 -RGO hybrids presented p-type behavior. The sensors based on Zn 2 SnO 4 -RGO hybrids also exhibited high response values of up to 3.62 for 1 ppm NO 2 at 50 °C in 80% RH, which is much higher than that of pure RGO (1.31). The excellent sensing performances are mainly ascribed to the synergetic effect of Zn 2 SnO 4 NPs and RGO. Furthermore, the surface reaction between Zn 2 SnO 4 -RGO hybrids and NO 2 can be concluded from Operando diffuse reflectance infrared Fourier transformed spectroscopy (Operando DRIFT). [ABSTRACT FROM AUTHOR]

Mesoporous silicas loading hydrophilic materials composites were widely used in humidity sensors during the last decade. However, the physical mixing process used for loading the hydrophilic materials in the channel of mesoporous silicas is hard to guarantee their uniform dispersion, and there is no strong force to bind the host and guest materials together. To solve above problems, novel vinyl functionalized mesoporous silicas (SBA-15-vinyl) were chemically modified with hydrophilic sodium p -styrenesulfonate (SSS) with free radical polymerization. The chemical and porous structures of the obtained hybrid materials (SBA-15-PSSx) were characterized by fourier transform infrared spectroscopy (FT-IR), elemental analyses (EA), X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and N 2 adsorption-desorption. The humidity sensing properties of the sensors based on SBA-15-vinyl and SBA-15-PSS indicate the introduction of PSS could effectively enhance the sensing properties of SBA-15-vinyl. The impedance of the optimized sensor changed by more than three orders of magnitude over the relative humidity (RH) range of 11%–95%, with rapid response to RH change. [ABSTRACT FROM AUTHOR]

Self-supported polymer films were fabricated by the thiol-ene click cross-linking reaction in this work. The cross-linked frameworks were used to guarantee the stability the films. The hydrophilic monomer with different contents were introduced into the self-supported thick films and uniformly dispersed in the polymer frameworks. The interdigitated electrodes were screen printed with silver paste on the self-supported films to obtain humidity sensors. Herein, the self-supported film acts as both the sensor substrate and the humidity-sensing material. The humidity sensing performances of the obtained sensors were systematically researched. By optimizing the ratio of hydrophilic monomer in the polymer films, the sensitivities of the sensors were enhanced from 1.58 to 103.75 (in the humidity from 11% RH to 95% RH), the response under low RH environment was significantly improved with good linearity. The response/recovery times of the sensor within the humidity range are measured to be 12.5 s and hundreds of seconds. The prepared flexible sensors possess certain mechanical properties, and exhibit good stability under bending and long-term tests up to 60 days. The applications of the as-prepared sensor in the detection of non-contact human skin humidity, human respiration and relative humidity of green plant were explored. The sensor shows excellent sensing performance in various applications and indicates the potential application in wearable and multifunctional devices. [Display omitted] • Self-supported polymer films were fabricated by the thiol-ene click cross-linking reaction. • Flexible and stable humidity sensors were fabricated by screen printing electrodes on the self-supported films. • The roles of the self-supported film are the substrate of the sensor, as both the humidity-sensing material. • The humidity sensing properties and the multifunctional applications of the obtained sensors were systematically researched. [ABSTRACT FROM AUTHOR]

Low humidity detection has always been a challenge in the field of humidity detection. Humidity sensors base on ionic liquid modified metal organic frameworks (MOFs) derived polymer films were prepared in this work. The ionic liquid ligand was uniformly introduced in the frameworks of UIO-66 to enhance the hydrophilicity of sensing materials, the caused defects sites are beneficial for adsorption and desorption, and the MOFs based functional films were easily obtained by in situ photoinduced thiol-ene click reaction. The obtained humidity sensor exhibits a linear response in low relative humidity (RH) range (5% RH - 30% RH) with a tiny humidity hysteresis (~0.2% RH) and short response time to humidity change (response/recovery time is 0.6 s/1.7 s, respectively). ● Humidity sensors based on ionic liquid modified UIO-66 derived polyelectrolyte films were prepared. ● The sensors were prepared facilely in situ on the electrodes by thiol-ene click polymerization in batches. ● The sensor presents linear response under low humidity (5% RH - 30% RH) with a small hysteresis and fast response speed. ● The introduction of ILs and the resulting defect sites are the reasons for effective low humidity detection. [ABSTRACT FROM AUTHOR]

[Display omitted] • Humidity sensors based on glycidyl trimethyl ammonium chloride (EPTAC) modified cellulose paper via a facile solution method were fabricated. • The paper is used for the purpose of the humidity sensing material and the sensor substrate. • The as-fabricated paper-based humidity sensor is provided with outstanding humidity sensing properties and good stability. • The humidity sensor was applied to multifunctional applications including human breath rate, non-contact switch and skin humidity. It is of great significance to exploit a simple, cost-effective and environmentally friendly preparation method of multifunctional humidity sensors. However, most humidity sensors need complex manufacturing processes, high cost and narrow usage range. Herein, humidity sensors based on glycidyl trimethyl ammonium chloride (EPTAC) modified cellulose paper via a facile solution method were fabricated, among which the paper is used for the purpose of the humidity sensing material and the sensor substrate. The sensitivity of the obtained sensor was improved by the modification of EPTAC, the response time was decreased to 25 s, which is equivalent to the first-rate paper-based humidity sensors. In addition, the paper-based humidity sensor is provided with well flexibility and biocompatibility, and it exhibits multifunctional applications in respiratory monitoring, non-contact switch and skin humidity monitoring. The low cost and facile preparation technique in this work could provide a useful strategy for developing multifunctional humidity sensor. [ABSTRACT FROM AUTHOR]

• The porous ionogel humidity sensitive material was successfully fabricated with the aid of PAA as the template and humidity sensing component. • The humidity sensor based on the porous ionogel shows rapid response (1.2/0.6 s) to humidity change. • Various molecular weights PAA were applied to investigate PAA's function in pore construction and humidity sensing. • The ultrafast-response humidity sensor can accurately record the human breath in real time, which is benefit for respiration data calculation. Electronic devices that can accurately record and monitor human respiration in real-time are of great interest for researchers. Ultrafast-response ionogel humidity sensors that can follow the human respiration were realized by an in-situ vinyl-thiol click reaction in this work. Polyacrylic acid (PAA) was utilized both as the template to construct porous films and for preparing humidity sensing component. The obtained optimal sensor owns a rapid response to humidity change (response and recovery time is 1.2 s and 0.6 s, respectively) and a small humidity hysteresis (∼2.2% RH). The water adsorption capability and sensing mechanism were investigated systematically. The ionogel humidity sensor with good stability and repeatability was applied in breath monitoring, which has a great potential application in diagnostic breath analysis. [ABSTRACT FROM AUTHOR]

• An ultrafast-response humidity sensor based on gel polymer electrolytes was fabricated. • The humidity sensors were in situ prepared facilely on the substrate by thiol-ene click chemistry reaction in batches. • The sensitive materials with the three-dimensional network structure guarantee the stability under high humidity. • A novel mechanism is proposed and studied in detail by QCM sensors and Cole-Cole plots. • The humidity sensor was successfully applied to human breath monitoring due to the particularly rapid response. Respiratory monitoring is crucial for the assessment of human health, and can be realized by detecting the humidity of exhaled gases. In this work, a facile method for the in situ fabrication of stable humidity sensors based on gel polymer electrolytes was developed. The obtained humidity sensors demonstrate ultrafast response/recovery times (0.19/0.30 s), which are very promising for monitoring respiration. Reliable signals for different respiratory states were obtained via humidity sensors. In addition, the ultrafast-response mechanism of the sensors was investigated using complex impedance spectroscopy and a quartz crystal microbalance (QCM). [ABSTRACT FROM AUTHOR]