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High sensitivity and fast response piezo-humidity sensing have been realized from Ga-doped ZnO nanowire (NW) nanogenerator (NG) as self-powered/active gas sensor. The piezoelectric output generated by Ga-doped ZnO NW NG can not only act as a power source for driving the device, but also as a sensing signal for detecting humidity. Upon exposure to 80% relative humidity (RH) at room temperature, the piezoelectric output of Ga-doped ZnO NWs decreases from 0.56 V (in 45% RH) to 0.12 V, and the sensitivity is up to 358, higher than that of undoped ZnO NWs. In addition, the response time of Ga-doped ZnO NWs is 5 s, much shorter than that of undoped ZnO NWs. Such high performance can be ascribed to that Ga 3+ ions can provide many donor defects and produce high local charge density/strong electrostatic field. The present results demonstrate a feasible approach for realizing high performance self-powered/active humidity sensor. [ABSTRACT FROM AUTHOR]

Optical sensors for environmental humidity have been constructed from poly (N-isopropylacrylamide)- co -acrylic acid (pNIPAm- co -AAc) microgels. The devices were constructed by first depositing a monolithic layer of pNIPAm- co -AAc microgels on a Au-coated glass substrate followed by the addition of another Au layer on top. The resultant assembly showed visual color, and exhibited multipeak reflectance spectra. We found that the thickness of the device's microgel layer depended on environmental humidity, which corresponded to a change in the device's optical properties. Specifically, at low humidity the microgel layer was collapsed, while it absorbed water from the atmosphere (and swelled) as the humidity increased. Additionally, we investigated how the deposition of the hygroscopic polymer poly (diallyldimethylammonium chloride) (pDADMAC) onto the microgel layer (prior to final Au layer deposition) influenced the devices humidity response. We found that the devices were more sensitive to humidity as the number of pDADMAC layers in the device increased. Finally, we evaluated the device performance at various temperatures, and found that the sensitivity was enhanced at low temperature, although the response was more linear at elevated temperature. [ABSTRACT FROM AUTHOR]

This paper reports characterization and humidity sensing studies of pure zinc oxide (ZnO) andAl2O3-doped ZnO nanomaterials prepared by solid-state reaction route. Pellet samples of ZnO-Al2O3 nanocrystalline powders with 3, 5, 10, 15, and 20 wt% of Al2O3 in ZnO were prepared. Pellet samples of pure ZnO were also prepared. Pellets were annealed at temperatures of 400 °C to 700 °C. Prepared powders were given pellet shape by applying pressure of 260 MPa. When samples were exposed to humidity it was found that as relative humidity (RH) increased, resistance of pellets decreased for the range of humidity from 10% to 90% RH range. Sample with 15 wt% of Al2O3 in ZnO and annealed at 700 °C showed the best results with sensitivity of 14.98 M $\Omega $ /%RH. For this sensing element repeatability over different cyclic operations was within ±2% of the measured values of sensitivity after six months. Response and recovery time of sensing element of 15% Al2O3 in ZnO were found to be 85 and 286 s, respectively. This sensing element manifested lower hysteresis, less effect of ageing and high reproducibility for annealing temperature 700 °C. X-ray diffraction pattern of this sensing element showed peaks of cubic gahnite and hexagonal corundum. Scherrer’s formula gave crystallite size for the sensing elements of pure ZnO and Al2O3-doped ZnO in 18–96- and 23–98-nm range, respectively. Average grain size measured from scanning electron micrograph for pure ZnO was 107 nm and for Al2O3-doped ZnO (AZ-15) 114 nm suggesting agglomeration of the crystallites in the sensing element to form larger grains. [ABSTRACT FROM PUBLISHER]

LiCl–Pebax 2533 composite nanofibers with different amounts of LiCl·H 2 O have been successfully synthesized by an electrospinning method. Humidity sensing properties, including the impedance versus relative humidity (RH), humidity hysteresis, and response and recovery time of these composites have been investigated. These results indicated that LiCl-added material possessed higher humidity sensitivity and LiCl–Pebax 2533 composite with LiCl·H 2 O content at 44.4 wt% displayed the best humidity sensing property. The impedance of the sensor changed nearly four orders of magnitude when the relative humidity changed from 11% RH to 95% RH and the hysteresis can be neglected. Furthermore, the response and recovery time of the sensor was 30 s and 80 s, respectively. Humidity sensing mechanism was analyzed by complex impedance spectra and the corresponding equivalent circuit. [ABSTRACT FROM AUTHOR]

The design and development of a new fibre optic sensor system for the optical detection of leakages in sewerage tunnels is reported. The system developed overcomes the disadvantages of the usually employed camera based inspection systems which are relatively complex and, in addition, require cleaning of the structures to be monitored beforehand. The sensor concept created combines a Fibre Bragg Grating (FBG)-based humidity sensor and a swellable polymeric fibre optic sensor. Both sensors are located along the sewerage tunnel so that they can response immediately to any leakages that may occur. The swellable polymeric fibre optic sensor shows a response of 34.2 dB in the presence of water, a performance which is superior to that seen form other swellable polymeric fibre optic sensors reported so far. Furthermore, the resistance of both sensors to highly alkaline environments (pH 13.4), an important feature of such sensors was verified. Consequently, when compared to the use of conventional inspection techniques, the novel fibre optic sensor system provides a robust, relatively low-cost and continuous monitoring system well suited to use in sewerage tunnels. [ABSTRACT FROM AUTHOR]

[Display omitted] • Sustainable bacterial cellulose reinforced buckypaper (BC-BP) was fabricated. • The BC-BP showed excellent electrical conductivity and flexibility simultaneously. • The BC-BP exhibited excellent EMI SE, TC and Joule heating performances. • The BC-BP achieved the combination of EMI shielding and humidity sensing. • This work provides a new strategy for preparing multifunctional wearable devices. There is currently enormous and growing demand for sustainable, high-performance and multifunctional electromagnetic interference (EMI) shielding materials for wearable devices. In this work, sustainable resources bacterial cellulose (BC) was selected as reinforced filler of carbon nanotubes (CNT) buckypaper (BP) due to its excellent mechanical strength. A new kind of multifunctional BC reinforced BP (BC-BP) was fabricated through the facile vacuum assisted filtration method. The obtained BC-BP with interpenetrating structure showed outstanding mechanical properties and high electrical conductivity simultaneously. The BC-BP with 50 wt% BC (50%BC) exhibited the tensile strength of 55.78 MPa, the fracture strain of 11.67%, and the electrical conductivity of 16.67 S/cm. The corresponding EMI shielding efficiency (EMI SE) and the specific EMI SE value (SSE/t) of the 50%BC with the thickness of 36 μm was 24.01 dB and 9074 dB⋅cm2/g in the C-band range, respectively. Moreover, the 50%BC also exhibited excellent thermal conductivity (TC) of 7.39 W/mK, Joule heating (124 °C at 6 V), and humidity sensing performance of high sensitivity (10.306%/% RH) in the wide relative humidity range (5–90% RH). In addition to EMI shielding, BC-BP also showed its potential application in the management and monitoring of temperature and humidity. This work provides a new strategy to prepare sustainable, high-performance and multifunctional BC-based EMI shielding material for wearable devices. [ABSTRACT FROM AUTHOR]

[Display omitted] • A neuron-like composite film based on Nb 2 CT x /sodium alginate is developed by electrospinning. • The electrospun film has the dual functions of power generation and humidity sensing. • A prototype of self-powered humidity sensing system is constructed for visual measuring. Existing wearable sensing electronics generally require to connect a battery for the power supply. Despite many years of effort, it still remains a great challenge to develop self-powered sensing systems without external power supplies. Herein, we develop a neuron-like MXene Nb 2 CT x /sodium alginate (Nb 2 CT x /SA) composite films prepared by an electrospinning approach. The Nb 2 CT x /SA composite film not only can be used to fabricate a high-performance electronic humidity sensor, but also to construct a moisture-triggered nanogenerator with a sustainable voltage output. Importantly, the humidity sensor is demonstrated to be portable and compatible for applications in respiration assessment, noncontact monitoring and intelligent alarm. More importantly, by simply integrating the Nb 2 CT x /SA film-based sensor/nanogenerator with a capacitor and light-emitting diode (LED), a prototype of self-powered sensing system capable of indicating different relative humidity (i.e., low, medium or high) through the LED's brightness without external power supplies is demonstrated. This work not only demonstrates the design and fabrication of a smart self-powered humidity sensing system, but also opens a new avenue for development of next-generation intelligent wearable electronics. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fast humidity sensors based on PoPD films were fabricated. • The H 3 PO 4 doped PoPD films show the highest response, the fastest response/recovery behavior, excellent repeatability and good stability. • The H 3 PO 4 doped PoPD films demonstrate great potential applications on humidity detections. Micro/nanostructured poly(o-phenylenediamine) (PoPD) thin films were synthesized by a templateless method in the presence of inorganic/organic acids as dopants, including hydrochloric acid (HCl), phosphate acid (H 3 PO 4), camphorsulfonic acid (C 4 H 6 O 6), and sulfuric acid (H 2 SO 4). It was found that the morphology of products was dependent on the dopant species seriously at the same experimental conditions. The fibrous, big and long microbelts, narrow and short microbelts, wide and short microbelts, and nanorods were obtained correspondingly. The as-synthesized one-dimensional micro/nano-belt/rod structures of PoPD were firstly used for the fabrication of resistor-type humidity sensors. All the samples showed humidity sensing ability. However, the performance of the sensitivity was obviously different with changing the doping acids. Compared with other acids, PoPD doped with phosphate acid has the highest sensitivity, the quickest adsorption–desorption time, and the longest lifetime in a wide range of humidity. Between the two relative humidity (RH) of 11% and 97%, the response and recovery time on phosphate acid doped PoPD were 5 and 21 s, respectively. Both the response time and the recovery time are very fast and show great potential in practical applications. [ABSTRACT FROM AUTHOR]

Restricted by the rigid chain structure, chitosan-based materials are very fragile and easy to break up. Herein, a stable and positively charged chitosan/Ti 3 C 2 T x suspension was successfully constructed by adjusting mixing sequence, pH and ratio of chitosan to Ti 3 C 2 T x to prepare a multifunctional membrane (P-CM) with self-standing ability, good flexibility, biocompatibility, excellent photothermal antibacterial properties, and sensitive humidity sensing properties. In the first stage, protonated chitosan acted as sugar coatings was able to be encapsulated on the surface of Ti 3 C 2 T x nanosheets by electrostatic self-assembly, in turn achieving charge reversal of the nanoparticles. In the second stage, chitosan acted as molecular needles to suture the membrane formed by chitosan/Ti 3 C 2 T x particles via vacuum filtration (VAF) with the commercial flexible poly(vinylidene fluoride) (PVDF) membrane. The as-prepared P-CM membrane simultaneously solves the problem of insufficient flexibility of a single component chitosan membrane and the poor bonding between pure Ti 3 C 2 T x membranes and substrate membrane. [Display omitted] • Positive charged stable suspensions were constructed using chitosan as sugar coatings. • Chitosan as molecular needles improved interfacial bonding. • Multifunctional chitosan-based membranes with real time humidity sensing were prepared. • Photothermal effect enhanced antibacterial activity of chitosan. [ABSTRACT FROM AUTHOR]

Vertically aligned uniform WO3 nanorod film has been successfully synthesized by using chemical vapor deposition (CVD) technique without any catalyst. X-ray diffraction (XRD), Raman spectrum, field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images indicate that the as-prepared WO3 nanorod film is monoclinic phase and consists of densely-aligned single crystalline nanorods with diameters approximately 30-110 nm and lengths around 1 µm. A room temperature photoluminescence-type humidity sensing device based on this WO3 nanorod film integrated with Si supported substrate has been directly established to investigate their humidity sensing properties, which presents its high response, excellent linearity, quick response/recovery performance and reliable repeatability toward a very wide humidity range. Further comparison with the WO3 control sample without oxygen vacancy or defect which has poor response demonstrates that oxygen vacancies in the structure play a pivotal role in the high response humidity sensing application. [ABSTRACT FROM AUTHOR]

Ordered silicon nanopillar array (Si-NPA) was fabricated by nanosphere lithography. The size of silicon nanopillars can be easily controlled by an etching process. The period and density of nanopillar arrays are determined by the initial diameter of polystyrene (PS) spheres. It was studied as a sensing material to detect humidity. Room temperature current sensitivity of Si-NPA sensor was investigated at a relative humidity (RH) ranging from 50 to 70%. As a result, the measured current showed there was a significant increase at 70% RH. The response and recovery time was about 10 s and 15 s. These excellent sensing characteristics indicate that Si-NPA might be a practical sensing material. [ABSTRACT FROM AUTHOR]

An on-chip optical humidity sensor using Nano-anatase TiO2 coating is presented here. The coating material was prepared so that the result is in solution form, making the fabrication process quick and simple. Then, the solution was effortlessly spin-coated on an SU8 straight channel waveguide. Investigating the sensitivity and performance (response time) of the device revealed a great linearity in the wide range (35% to 98%) of relative humidity (RH). In addition, a variation of more than 14 dB in transmitted optical power was observed, with a response time of only ~0.7 s. The effect of coating concentration and UV treatment was examined on the performance and repeatability of the sensor. Interesting observations were found, and the attributed mechanisms were described. In addition, the proposed sensor was extensively compared with other state-of-the-art proposed counterparts from the literature and remarkable advantages were found. Since a high sensitivity of ~0.21 dB/%RH and high dynamic performances were demonstrated, this sensor is proposed for use in biomedical applications.

In the present work, Cu 2 MgSnS 4 (CMTS) films are deposited by spray pyrolysis technique. The impact of magnesium concentration during the synthesis of CMTS films is studied. Structural, morphological, optical, electrochemical and electrical properties are analyzed using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), UV–visible spectroscopy, electrochemical impedance spectroscopy (EIS) and I-V characterizations. CMTS films grown with various magnesium concentrations, [Mg] = 3, 5, 7 and 9 mM, show kesterite structure with a preferential orientation along (112) plane. XRD patterns show the appearance of Cu 2 S and Sn 2 S 3 secondary phases for the majority of Mg concentrations. Contrary, MgS appears only for the highest Mg concentration. The best crystalline quality and highest crystallite size are obtained for [Mg] = 5 mM sample. Surface morphology of CMTS films reveals homogenous and continuous surface for [Mg] = 5 mM. The film thickness varies in very sensitive way with Mg concentration. Optical properties of CMTS reveal a high absorption coefficient in the visible range. A band gap narrowing from 1.56 eV to 1.50 eV with the increase of Mg concentration till 5 mM is noticed, whereas a band gap widening takes place for further increase of Mg concentration. The optimum value of 1.50 eV is obtained at [Mg] = 5 mM which is suitable for solar cell applications. Electrical analysis exhibits a good electrical conductivity at [Mg] = 5 mM and the I–V characteristic indicates an ohmic behavior and high current value of 10−2 A at 4 V under illumination. These properties make CMTS a potential candidate for solar cell applications as well as humidity sensing. • CMTS films are deposited on glass substrates by Spray pyrolysis using ethanol as solvent. • The effect of magnesium concentration on structural, morphological, optical and electrochemical properties is investigated. • The optimum of grown CMTS films is obtained at [Mg] = 5 mM for solar cell application. • A band gap energy in the range of 156–1.89 eV depending on [Mg] concentration is obtained. • Humidity sensing measurements was investigated through experiments and an impedance analysis at [Mg] = 7 mM. [ABSTRACT FROM AUTHOR]

• A fast-manufactured optical fiber temperature and humidity sensor with compact size was proposed. • The temperature and humidity sensitivities of the proposed sensor were −0.02539 dB/%RH and 11.14 pm/℃. • The measurement humidity sensitivity increased by nearly four times after the addition of nano-ZnO particles to the PVA. [Display omitted] A peanut-shaped optical fiber sensor cascaded with a fiber Bragg grating (FBG) was designed to measure temperature and humidity. A mixed polyvinyl alcohol/nano-zinc oxide (PVA/nano-ZnO) solution was electrospinning into nanofibers, and these nanofibers were then attached to the peanut-shaped structure surface to form a nanofiber film. Humidity changes caused changes in both the refractive index and the thickness of the PVA/nano-ZnO composite nanofiber film that subsequently caused changes in the transmitted light loss. Temperature changes caused a shift in the center wavelength of the FBG. Experimental results showed that the humidity and temperature sensitivities of the proposed sensor coated with the PVA/nano-ZnO composite film were −0.02539 dB/%RH and 11.14 pm/℃, respectively. Compared with an optical fiber sensor using a PVA film alone, the humidity sensitivity of the proposed sensor was increased by nearly four times after the addition of nano-ZnO. [ABSTRACT FROM AUTHOR]

Abstract: The present communication reports the synthesis of polypyrrole-grafted-cellulose (PPy-g-Cellu) copolymer by a chemical method at ambient condition. The representative samples of PPy-g-Cellu were characterized using infra red spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscope (SEM), UV–vis spectroscopy (UV–vis) and themogravimetry (TG) techniques. The prepared copolymer was crystalline in nature with improved thermal stability. Further, electrical properties reveals the formation of polypyrole grafted cellulose copolymer with 107 fold improved electrical conductivity and 102 folds hydrophillcity than cellulose and polypyrrole, respectively. The electrical resistance of a PPy-g-Cellu film casted on a glass substrate was measured as function of relative humidity in the range of 5–95% to explore its use for humidity sensing purposes. Finally, PPy-g-Cellu offers a promising perspective as a humidity sensing material with response time 25s, recovery time 20s and stability for 40 days. [Copyright &y& Elsevier]

Abstract: This work is focused on the fabrication of a human breathing sensor based on the in situ synthesis of silver nanoparticles (Ag-NPs) inside a polymeric coating previously deposited on an optical fiber core by means of the Layer-by-Layer self-assembly. The Ag-NPs were created using a synthesis protocol consisting of a loading step of the Ag+ cations into the polymeric film and a further reduction step using dimethylamine borane (DMAB). The morphology and distribution of the Ag-NPs inside the polymeric coating have been studied using atomic force microscopy (AFM). Furthermore, UV–VIS spectroscopy and energy dispersive X-ray (EDX) were also used to confirm the synthesis of the Ag-NPs within the resultant coating. The amount of Ag-NPs increases when the number of loading/reduction cycles is higher. Therefore the incorporation of the Ag-NPs affects the refractive index of the overlay promoting the observation of a resonant attenuation band in the infrared region (NIR), known as Lossy Mode Resonance (LMR), which can be used as a sensing signal to monitor the human breathing. The quality of the device has been experimentally tested with good sensitivity (0.455nm per RH%) and fast response time (692ms and 839ms for rise/fall). [Copyright &y& Elsevier]

A novel photonic crystal fiber (PCF) interferometer RH (relative humidity) sensor is proposed and demonstrated in this paper. The sensor is made from a piece of PCF spliced to standard fibers in transmission mode, which has the advantage that it does not require the use of any hygroscopic thin film or layer. in addition, the all-silica nature of the sensing element means the sensor is insensitive to temperature. The sensor has a humidity sensitivity of 60.3 pm/%RH over the range of 60-80% RH and 188.3 pm/%RH in the region from 80 to 95% RH. A response time around ~1 s is observed for a ~2 cm long section of photonic crystal fiber interferometer (PCFI). The sensor exhibits a very low cross temperature sensitivity of around 0.5 pm/°C. The proposed sensor has potential for high humidity monitoring applications, and temperature compensation is not necessary if such a sensor is operated in normal surrounding atmosphere. [ABSTRACT FROM AUTHOR]

Highlights: [•] Ag nanodendrites can be obtained by a simple template method via reduction process. [•] An optical humidity Ag nanodendrites-based sensor is constructed at room temperature. [•] Ag nanodendrites show high sensitivity and good repeatability for humidity sensing. [ABSTRACT FROM AUTHOR]

Abstract: ZnO colloid crystals were coated on quartz crystal microbalance (QCM) by the self-assembly method. Field emission scanning electron microscopy and X-ray diffraction were used to analyze the morphology and crystal structure of ZnO colloid crystals. The sensing behavior was examined by measuring the resonance frequency shifts of QCM. The device exhibits excellent humidity sensing properties in the whole humidity range from 11% to 95%, such as good linearity, fast response time and recovery time, excellent reliability, and long-term stability. We also discussed the adsorption and desorption of water molecules on ZnO colloid crystals at different humidity conditions. [Copyright &y& Elsevier]

We propose a novel non-hygroscopic humidity sensor based on a photonic bandgap fiber (PBF) and a digital wavelength modulation spectroscopy (WMS). PBF is employed as the sensing element due to the large overlap of transmitted light with humid air inside the core of the PBF. High sensitivity detection is achieved by using WMS–this shifts the detection bandwidth to a higher frequency. We use the ratio between second harmonic (2f) and the first harmonic (f) of the WMS signals at the water vapor absorption peak around 1368.59 nm for detection of relative humidity. In addition, a PBF fiber reference cell is introduced to lock the DFB laser source at 1368.59 nm. With the WMS and reference cell, various disturbances, such as laser power fluctuations, attenuation variations of the sensing element and fiber can be suppressed. We experimentally implement the scheme with a PBF sensing head of 5 cm and a PBF reference cell of 7 cm. High humidity resolution of \sim0.2\% Relative humidity (RH) over the range from 0 to 95% RH is achieved. [ABSTRACT FROM PUBLISHER]

Abstract: In this work, a study about the optical response of electrospun nanofiber (ENF) coatings for their use in evanescent optical fiber sensors is presented. Several types of ENF mats composed of poly(acrylic acid) (PAA) were developed with different ENF diameters and densities. These ENF mats were deposited onto an optical fiber core in order to fabricate humidity evanescent optical fiber sensors. The devices were exposed to relative humidity (RH) variations from 30% RH to 95%RH. The transfer functions of the devices (transmitted optical power versus relative humidity) presented two well-differenced behaviors depending on the ENF diameter and the ENF mat density. The devices with lower ENF diameters and higher mat density showed an increase in the transmitted optical power when RH increased. On the contrary, the devices with higher ENF diameters and lower mat density showed a decrease in the transmitted optical power when RH increased. In addition to this, sensors with thinner ENF overlays, showed a higher sensitivity. In order to study the response time of these devices, the ENFs sensors were submitted to human breathing cycles and presented a response time around 340ms (exhalation). In spite of the high RH conditions of this experiment, the devices showed a recovery time around 210ms and a negligible hysteresis or drift with respect to the initial condition (inhalation). [Copyright &y& Elsevier]

Abstract: In this paper, the influence on humidity sensitive characteristics of micro/nano-polyaniline (PANI) thin films with different doping degree of (1S)-(+)-10-camphorsulfonic acid (D-CSA) is investigated. The PANI is synthesized by an in situ chemical oxidative polymerization of aniline in the presence of ammonium peroxydisulfate (APS) acidic at ice–water bath for 10h, and characterized by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM), respectively. Then, the composite films are formed between interdigital electrodes on a silicon (Si) substrate via self-assembly method. The humidity sensitive characteristics of the PANI films are measured. The experimental results show that the conductivity of PANI increases with increasing the molar ratios of [D-CSA]/[Ani]. Besides, the humidity sensitivity of the PANI films enhances with increasing the molar ratios of [D-CSA]/[Ani] and decreasing the layer number of PANI films, respectively. The PANI films with [D-CSA]/[Ani] molar ratio of 1.0 ([D-CSA]/[Ani]=1:1) has higher sensitivity. The results indicate that the doping degree of D-CSA actually influences on the humidity sensitive characteristics of PANI thin films, and may be referencing significance for the fabrication of better humidity sensor. [Copyright &y& Elsevier]

To date, multifunctional sensors have aroused widespread concerns owing to their vital roles in the healthcare area. However, there are still significant challenges in the fabrication of functionalized integrated devices. In this work, hydrophobic-hydrophilic patterns are constructed on polyester-spandex-blended knitted fabric surface by the chemical click method, enabling accurate deposition of functionalized materials for sensitive and stable motion and humidity sensing. Representatively, a conductive silver nanowire (Ag NW) network was deliberately deposited on only the designated hydrophilic fabric surface to realize accurate, repeatable, and stable motion sensing. Such a Ag NWs sensor recorded a low electrical resistance (below 60 Ω), stable resistance cycling response (over 2000 cycles), and fast response time to humidity (0.46 s) during the sensing evaluation. In addition to experimental sensing, real human motions, such as mouth-opening and joint-flexing (wrist and neck), could also be detected using the same sensor. Similar promising outputs were also obtained over the humidity sensor fabricated over the same chemical click method, except the sensing material was replaced with polydopamine-modified carboxylated carbon nanotubes. The resultant sensor exhibits excellent sensitivity to not only experimentally adjusted environment humidity but also to the moisture content of breath and skin during daily activities. On top of all these, both sensors were fabricated over highly flexible fabric that offers high wearability, promising great application potential in the field of healthcare monitoring.

In the field of humidity sensors, a major challenge is how to improve the sensing performance of existing materials. Based on our previous work on Na 0.5 Bi 0.5 TiO 3 , a facile strategy of tuning the Bi content in the material was proposed to improve its sensing performance. Na 0.5 Bi x TiO 3 (x = 0.3, 0.35, 0.4, 0.45) nanocomposites were synthesized by a hydrothermal method. Humidity sensing properties of these nanocomposites were investigated in the relative humidity range of 11% to 95%. Our results show that, compared to the sensor based on nominally pure sample (Na 0.5 Bi 0.5 TiO 3 ), the sensor based on Na 0.5 Bi 0.35 TiO 3 exhibits boosted sensing performance of excellent linear humidity response in the humidity range of 11-75% relative humidity, lower hysteresis value, and faster response/recovery time. The improvement of the sensing performance was argued to be the reason that the proper reduction in Bi content leads to a minimum value of oxygen-vacancy concentrations, thereby weakening the chemical adsorption but enhancing the physical adsorption. These results indicate that the proper underdose of the Bi content in Na 0.5 Bi 0.5 TiO 3 can greatly boost the sensing performance.