Humidity Sensitive Material Research Articles

High sensitivity chipless RFID humidity sensor tags are based on SnO2/G nanomaterials

In this study, a chipless RFID humidity sensor tag based on SnO2/G (Graphene) nanomaterial with recognition and sensing functions is proposed. Chip-less RFID sensors are usually composed of tag ID and sensing materials. In this paper, similar nano-spherical SnO2 powder was generated by hydrothermal method and doped with G to get SnO2/G mixture. For chip-free RFID tags, a microstrip coupled line resonator is proposed, which consists of a coupled line segment and three line segments of different shapes. Among them, u-shaped resonators coated with humidity sensitive materials for humidity sensing, and the other two resonators can code four different states. Through testing the sensitivity, hysteresis, repeatability and response characteristics of humidity sensor, it is proved that humidity sensitive material has excellent humidity sensing performance. At 11%-98%RH, the frequency offset is up to 257 MHz, achieving a sensitivity of 2.95MHz/%RH. HFSS simulation and experimental results verify that the proposed integration of humidity sensor and RFID technology has great application potential.

Printing assembly of flexible devices with oxidation stable MXene for high performance humidity sensing applications

Transition metal carbides and carbonitrides MXenes have developed to be attractive nanomaterials for humidity-sensitive materials due to abundant hydrophilic active sites on their chemically active surfaces. Nevertheless, the low stability exhibited by MXenes in moist air or water limits their long-term storage and wide applications. This work proposes a composite of sodium ascorbate-decorated MXene (SA-MXene) by combining ascorbate ions with Ti3C2Tx MXene synthesized through a minimally intensive layer delamination (MILD) process. The synthesizing and decoration of MXene with SA to obtain SA-MXene had been shown to be a viable method for avoiding Ti3C2Tx MXene materials from oxidizing. The as-prepared SA-MXene was formulated as screen-printing ink with high stability and oxidation resistance despite being stored at ambient temperature after 30 days. Furthermore, a flexible humidity sensor was assembled by screen-printing technology with SA-MXene ink as the humidity-sensitive layer on the copper interdigital electrode prepared by the electrodeless deposition (ELD) process. Consequently, the SA-MXene sensor showed remarkable potential in humidity sensing with an ultrahigh relative capacitance response (131.4% to 97% RH) and rapid response time across a wide detection range. This work represents a promising strategy of developing SA-MXene to fabricate high sensitivity sensors derived from MXene.

Proton transport over nanoparticle surface in insulating nanoparticle film-based humidity sensor

We study proton transport on the surface of insulating nanoparticles for humidity sensors. We use this approach to reveal proton transfer mechanisms in humidity-sensitive materials. Hydrophilic and hydrophobic ligand-terminated silica nanoparticle films are adopted for evaluating the temperature dependence of the ion conductivity. According to the activation energy of the conductivity, we explain that Grotthuss (H+ transfer) and vehicular (H3O+ transfer) mechanisms are mainly dominant on hydrophilic (−OH terminated) and hydrophobic (acrylate terminated) surfaces of the nanoparticles, respectively. This investigation gives us a clue to understanding the proton transfer mechanism in solution-processed flexible humidity-sensitive nanomaterial films.

A Novel Differential Capacitive Humidity Sensor on SIW Re-Entrant Cavity Microwave Resonators With PEDOT:PSS Film

This work presents a differential humidity sensor by longitudinally stacking the sensing and referencing substrate integrated waveguide (SIW) re-entrant cavity resonators (RECRs) for relative humidity (RH) detection. Three different technologies, such as air-filled substrate, metal grid holes (METGH) and sensitive material, are integrated to enhance the humidity sensing response. Firstly, introduction of air-filled substrate and METGH can effectively improve the flowability of the humid air, and can reduce the response time. Secondly, PEDOT:PSS is used as a humidity sensitive (HS) material and deposited in the highly concentrated E-field distribution area of sensing SIW RECR, and the modulation of the dielectric constant of HS material by humid air can achieve high humidity sensitivity. Furthermore, the proposed differential structure itself can suppress common-mode perturbations to prevent temperature cross sensitivity. Several different sensors are fabricated and a series of experiments are performed to investigate the dependence of humidity sensing response on the device structure and the deposited amount of HS material. When a specific amount of HS material is deposited in the HS area, excellent humidity sensing performances such as high sensitivity, low hysteresis and fast response can be successfully achieved. Among them, the sensor <inline-formula> <tex-math notation="LaTeX">$\text{B}_{{1}}$ </tex-math></inline-formula> (300-uL HS material and with METGH) demonstrates the best humidity sensing performance in the range of 32.8&#x0025; to 97.3&#x0025; RH, and shows high sensitivity of 209.3 kHz/&#x0025;RH and low hysteresis of 0.592&#x0025; RH, and the corresponding response time is reduced to 1/6 of that without METGH. Moreover, the sensor <inline-formula> <tex-math notation="LaTeX">$\text{B}_{{1}}$ </tex-math></inline-formula> has excellent suppression on the temperature variation.

A wearable, flexible sensor for real-time, home monitoring of sleep apnea

SummaryA flexible sensor that can be attached to the body to collect vital data wirelessly enables real-time human healthcare management. One potential application for home-use healthcare devices is monitoring of sleep conditions to diagnose sleep apnea syndrome. Such data are not readily gathered using conventional tools, owing to the bulk and cost of instrumentation. In order to monitor respiration at home, it is necessary to improve sensing performance and long-term stability of the sensors without sacrificing wearability and comfortability. To build a platform for wireless home-use respiration monitoring, this study develops a mask-borne flexible humidity sensor using ZnIn2S4 nanosheets as a humidity-sensitive material with high sensitivity and stability for more than 150 h. As proof-of-concept, long-term wireless respiration monitoring is demonstrated during sleep to identify symptoms of sleep apnea in wearers.

High-Sensitivity and Environmentally Friendly Humidity Sensors Deposited with Recyclable Green Microspheres for Wireless Monitoring.

The reliable, high-sensitive, wireless, and affordable requirements for humidity sensors are needed in high-precision measurement fields. Quartz crystal microbalance (QCM) based on the piezoelectric effect can accurately detect the mass changes at the nanogram level. However, water-capture materials deposited on the surface of QCM generally show disadvantages in either cost, sensitivity, or recyclability. Herein, novel QCM-based humidity sensors (NQHSs) are developed by uniformly depositing green microspheres (GMs) of natural polymers prepared by the chemical synthesis of the emulsification/inner gel method on QCM as humidity-sensitive materials. The NQHSs demonstrate high accuracy and sensitivity (27.1 Hz/% RH) owing to the various hydrophilic groups and porous nano-3D deposition structure. Compared with the devices deposited with a smooth film, the frequency of the NQHSs shows almost no changes during the cyclic test and exhibits long-term stability. The NQHSs have been successfully applied to non-contact sensing human activities and remote real-time humidity monitoring via Bluetooth transmission. In addition, the deposited humidity-sensitive GMs and QCM substrate are fully recycled and reused (72% of the original value). This work has provided an innovative idea to construct environmental-friendly, high-sensitivity, and wireless humidity sensors.

PVA/Tween 20 thin-film-based fiber optic humidity sensor with enhanced sensing performance.

A fiber optic humidity sensor based on polyvinyl alcohol (PVA)/Tween 20 film has been fabricated by modulating the intensity of light transmitted in optical fiber. PVA/Tween 20 film was used as the cladding and humidity-sensitive material of optical fiber. The logarithmic of output light intensity exhibited a linear increase with the increase of humidity (22%-82%RH). With the addition of Tween 20 in the formation of film, average sensitivity increased by 13-fold. Fast equilibrium on adsorption and desorption of water molecules were also achieved on the film. The response and recovery times were determined to be 11 s and 9 s, respectively. Moreover, the sensor possesses good repeatability. The sensing mechanism was probably based on the swelling of PVA after adsorbing water molecules, which affected scattering of evanescent waves in the cladding. The output light intensity varied with the decay of evanescent waves.

Surface plasmon resonance optical fiber sensor for relative humidity detection without temperature crosstalk

A new method to overcome temperature crosstalk in humidity detection is proposed in this manuscript, which is realized by integrated surface plasmon resonance (SPR) and Mach-Zehnder (MZ) interference in the same optical fiber detection structure. The proposed sensor is made by photonic crystal fiber and multimode fiber, SPR excited by half coated gold film on one side of photonic crystal fiber, MZ interference is made by photonic crystal fiber without gold film coating and multimode fiber. Graphene quantum dots and polyvinyl alcohol (GQDs-PVA) are adopted as relative humidity sensitive materials. By data demodulation, the sensitivity of proposed sensor to relative humidity is 23 pm/%RH. Compared with other RH sensors, the proposed sensor in this manuscript is high integrated, and overcomes the problems of complex data demodulation, which provides a novel idea for the sensor to overcome crosstalk of other factors.

Upconversion Luminescent Humidity Sensors Based on Lanthanide-Doped MOFs

Lanthanide-doped metal-organic frameworks (Y/Yb/Er-MOF) were synthesized by a low-cost solvothermal method. The obtained Y/Yb/Er-MOF shows the cooperative upconversion luminescence of Yb3+ and upconversion luminescence of Er3+ (Yb3+-sensitized) irradiated by a continuous wave 980 nm laser. In order to explore the potential application of Y/Yb/Er-MOF in relative humidity (RH) sensors, the RH responsiveness of Y/Yb/Er-MOF was investigated by measuring the intensity changes of upconversion luminescence. The Y/Yb/Er-MOF possesses two luminescence centers, in which Yb3+ forms emission at 500 nm through the cooperative luminescence effect, and Er3+ achieves 660 nm emission through excited state absorption and successive energy transfer from Yb3+. Hence, the ratio meter luminescence sensor for RH is constructed based on Y/Yb/Er-MOF. The results show that the response of Y/Yb/Er-MOF to RH presents a linear relationship in the range of 11–95%. The cycle stability of Y/Yb/Er-MOF responses to RH was investigated with the intensity changes of upconversion luminescence, and the recovery ratio was more than 93% each time. Therefore, the Y/Yb/Er-MOF is a humidity-sensitive material with great potential for applications such as humidity sensors.

A Facile and Flexible Humidity Sensor Based on Porous PDMS/AgNWs and GO for Environmental Humidity and Respiratory Detection

AbstractHumidity sensors, benefit from their superior humidity‐sensitive components, are able to respond to the environment moisture changes. Based on this feature, the humidity sensor plays an increasingly vital role in biological detection. To improve the performance of humidity sensors, a large number of optimizations have been made from several aspects, such as material synthesis and structural design. In this study, a novel humidity sensor is designed to increase its specific surface area to carry more humidity‐sensitive and conductive materials. A porous polydimethylsiloxane (PDMS) substrate is prepared in an economical friendly approach by using the thermal decomposition of sodium bicarbonate (NaHCO3). Meanwhile, high‐quality nano silvers (AgNWs) are applied as the intermediate conductive layer connecting the substrate and detective layer. graphene oxide (GO) is used as the humidity sensitive material in the sensor. It is remarkable that this sensor can detect the humidity with a wide range of 8.6 RH% to 93 RH% in a quick response with high sensitivity. Since this humidity sensor showed powerful functions, such as monitoring respiration and detecting environmental humidity, it will have broad potentials for real‐time clinical and domestic healthcare.

Highly Sensitive Humidity Sensor Based on a GO/Co-MOF-74 Coated Long Period Fiber Grating

A humidity sensor based on graphene oxide (GO)/Co-MOF-74 coated long period fiber grating is experimentally demonstrated. The long period fiber grating is prepared by CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser direct writing method, with GO/Co-MOF-74 composite humidity sensitive material coated around the grating region. Combination of GO and Co-MOF-74 with porous structure can absorb water molecules effectively and improve the sensitivity. A high humidity sensitivity of 0.204 nm/%RH can be realized in the RH range of 30-50% as well as a value of 0.16 dB/%RH in the range of 50-90%, both with a linearity of more than 0.98. The proposed highly sensitive humidity sensor also shows a good performance in the stability and repeatability.

Dual-Parameter Fiber-Optics Relative Humidity and Displacement Sensor Based on the Topological Ring Structures

The dual-parameter sensors based on topological ring structure are proposed and experimentally demonstrated, which has obvious advantages in relative humidity (RH) and displacement measurement. The topological ring structures are composed of two separated optical microfiber couplers (OMCs) as independent sensing units. The two OMCs are connected by two 30 cm length single-mode fibers (SMFs). The first sensor is formed by introducing OMC into the loop resonator (LR). The second sensor is constructed by introducing OMC into the Sagnac interferometer (SI). The LR-OMC and SI-OMC structures are made by splicing the two ports of OMC with two ports on one side of the different central axis. The coupling coefficient and effective refractive index (ERI) of OMC vary with RH and displacement. The reason for the wavelength shift caused by RH is that water molecules trigger the change ERI of the OMC evanescent. Furthermore, the RH measurement is realized without any humidity-sensitive material. Here, we present a topological ring structure that can measure two parameters discretely, eliminating the problem of RH and displacement crosstalk. The LR-OMC structure and SI-OMC structure have different RH and displacement sensitivity. The proposed topology sensors could find applications in the multi-parameter RH and displacement sensing due to their simple fabrication and low cross-sensitivity.

Humidity sensing and temperature response performance of polymer gel cold-spliced optical fiber Fabry-Perot interferometer

An extrinsic Fabry-Perot Interferometric (EFPI) fiber humidity probe has been proposed. This composite EFPI fiber humidity sensor combines the characteristics of various fiber structures, such as the cascaded spliced fibers, coating film on the fiber end-face and in-line fixed Fabry-Perot cavity in capillary. The polymethyl methacrylate (PMMA) was filled in between the air gap between single mode fiber and muti-modes fiber as the humidity sensitive material to establish the EFPI optical fiber humidity sensor. The best sensitivity has been experimentally demonstrated with 0.4172 nm/%RH in the relative humidity range of 10%RH-70%RH, where the corresponding fitting coefficient is 0.99939, and the response time is less than 14 min. Furthermore, the wavelength temperature sensitivity based on wavelength modulation is 0.06074 nm/℃, and the power responded sensitivity is 0.056 dBm/℃ in the temperature range of 20℃-65℃. The experimental results indicate that the proposed fiber probe can simultaneously measure humidity and temperature, or in other way, it will significantly improve the accurate measurement for humidity by analyzing and compensating the influencing factors from the environmental temperature.

Optical fiber humidity sensor based on the vernier effect of the Fabry-Perot interferometer coated with PVA

In this paper, a relative humidity (RH) sensor based on vernier effect of two parallel Fabry-Perot interferometers is proposed and experimentally demonstrated. The polyvinyl alcohol (PVA) is chosen as the humidity sensitive material. The sensor consists of a reference cavity formed by single mode fiber – fiber capillary - single mode fiber (SMF-FC-SMF) structure and a sensing cavity formed by SMF-FC-coated structure. The sensor shows the high sensitivity of 1.78 nm/%RH in the range of 55%RH-85%RH. We measured the stability of the sensor at 45%RH and 75%RH, and the response time between 55%RH and 75%RH is 12 s. In addition, it also has the advantages of low temperature disturbance, simple principle, and easy manufacture.

Ringed Electrode Configuration Enhances the Sensitivity of QCM Humidity Sensor Based on Lignin Through Fringing Field Effect

This paper presents a new, effective, and facile method to enhance the humidity sensitivity of lignin-based QCM sensor through fringing field effect. The biodegradable and renewable lignin was deposited on the surface of metal electrode as humidity-sensitive material. The microscopic morphology, hydrophilic groups, and water contact angle of the lignin sample were characterized. Then, the humidity-sensing performances of the lignin-based QCM sensors with symmetric and ringed electrode configurations were investigated through the oscillating circuit method. The experimental results showed that the humidity sensitivity of the ringed electrode QCM (61 Hz/%RH) is higher than that of the symmetric electrode QCM in the range of 11.3%-97.3% RH. Moreover, the ringed electrode QCM based on lignin exhibited low humidity hysteresis (~6.2% RH), good logarithmic relationship, rapid response/recovery time (28/5 s), excellent repeatability, and long-term stability. In addition, the Langmuir adsorption isotherm model and the Gibbs free energy were used to analyze the adsorption of water molecules on the surface of the lignin film. Finally, the finite element method (FEM) was used to analyze the fringing field distributions of the QCMs with symmetric and ringed electrode. The sensitivity enhancement mechanism of the lignin-based QCM humidity sensor with ringed electrode configuration was discussed through the equivalent electronic circuit analysis and simulation methods. This work demonstrates that the lignin is a potential candidate material for humidity detection, and it is an excellent method to improve the humidity sensitivity of QCM sensors through optimizing the electrode structure configuration using fringing field effect.

Wearable breath monitoring based on a flexible fiber-optic humidity sensor

Breath, as an important health monitoring indicator, provides valuable diagnostic information for cardiovascular disease and pulmonary function. Humidity can act as a bridge between breath and sensing signals. Current monitoring methods depend on humidity-sensitive material characteristics. In this work, an all fiber-optic flexible humidity sensor for wearable breath monitoring is reported. An eccentric fiber Bragg grating (EFBG) is inscribed in a single mode fiber to excite a stable core mode and sensitive cladding modes. The core mode is shown to maintain stable spectral features under a high-humidity atmosphere and can be used to calibrate the wavelength and power of the system. Importantly, the interface evanescent field of the cladding mode is highly sensitive to the ambient refractive index (RI) and even humidity-induced RI variation. Without combining any sensitized material, EFBG can directly perceive humidity fluctuations during breath with fast response (92 ms) and recovery times (100 ms). Different breathing patterns can be recognized, and breathing frequency can be extracted by sensor responses. The EFBG humidity sensor demonstrates great reproducibility, fast response, high flexibility, excellent robustness, and self-compensation capability, showing promising potential for wearable breath monitoring.

Supercontraction of spider dragline silk for humidity sensing.

The spider dragline silk (SDS) has a supercontraction characteristic, which may cause the axial length of the SDS to shrink up to 50% when the SDS is wet or the relative humidity is higher than 58% RH. In this manuscript, we employ the supercontraction characteristic of the SDS to measure relative humidity. We connect two sections of a single-mode fiber (SMF) and a section of multimode fiber (MMF) with a sandwich structure to fabricate a single-mode-multimode-single-mode (SMS) interferometer. Then we fix the SDS on two SMFs to configure a bow-shaped sensing unit. The increase of environmental humidity will cause the supercontraction of the SDS, which will cause the change of the SDS length. The excellent mechanical properties of the SDS will generate a strong pulling force and change the bending of the arch, whose interference spectrum will shift correspondingly. In this way, we may perform relative humidity sensing. In the relative humidity range of 58% RH to 100% RH, the average sensitivity is as high as 6.213 nm/% RH, higher than most fiber-based humidity sensors. Compared with the traditional sensing structure with humidity-sensitive materials, the proposed sensor improves the sensitivity with environmental friendliness. The results suggest that the SDS can be used for high-sensitivity humidity sensors, and its degradability and biocompatibility also have a vast development space in biochemical sensors.

Simulation analysis and experimental verification for sensitivity of IDE-QCM humidity sensors

• A facile and effective approach to improve the sensitivity of piezoelectric quartz crystal humidity sensors was presented. • Both IDE and QCM are used as sensitive components to improve humidity sensitivity. • The superposition relationship of the frequency change in IDE-PQC and QCM was proved using equivalent circuit method. • The sensitivity of the IDE-QCM humidity sensor reaches 262.21 Hz/%RH in the humidity range of 11.3–97.3 % RH. This paper proposes a new, simple, and effective approach that uses both an interdigital electrode (IDE) and a quartz crystal microbalance (QCM) as sensitive components to improve humidity sensitivity. First, a simulation analysis of the equivalent electronic circuit was used to verify that the frequency response of the IDE-QCM humidity sensor is the sum of that of the QCM and interdigital electrode-piezoelectric quartz crystal (IDE-PQC) sensors. Then, cellulose nanocrystals (CNCs) were used as humidity-sensitive materials deposited on IDE and QCMs, respectively, to fabricate several PQC sensors. The humidity-sensing performances of all sensors were tested at room temperature, including the dynamic response, sensitivity, response/recovery time, repeatability, and long-term stability. The experimental results prove the superposition relationship and can be used to enhance the humidity sensitivity. The maximum sensitivity of the IDE-QCM sensor reaches 262.21 Hz/%RH in the humidity range of 11.3 %–97.3 %, which is higher than that of all frequency-modulated piezoelectric quartz crystal humidity sensors. Moreover, the humidity hysteresis (∼ 8.1 %RH) and response/recovery time (68/4 s) did not significantly increase compared with the IDE-PQC and QCM sensors. All the sensors exhibited good repeatability and long-term stability. Finally, the humidity-sensing mechanism of the CNC-based IDE-QCM sensor is discussed in detail. This work demonstrates that using both IDE and QCM as sensitive elements is a simple and effective method to improve the humidity sensitivity of frequency-modulated piezoelectric quartz crystal sensors. This superposition relationship of capacitance-inductance frequency modulation to enhance sensitivity also provides an excellent reference for the design of other high-performance sensors.

Dual-core fiber based in-line Michelson interferometer for humidity sensing

A high-sensitive optical fiber humidity sensor based on in-line Michelson interferometer (MI) is proposed and demonstrated. The MI is formed by an asymmetric dual-core fiber (DCF) spliced with a short section of multimode fiber (MMF). The MMF serves as fiber coupler, while the DCF creates a strong intermodal interference to strengthen the interaction between light and ambient moisture. The sensor presents a humidity sensitivity of −0.199 dB/%RH in a wide relative humidity (RH) range of 20–90%RH without any humidity-sensitive material. An experiment based on human breathing shows that the response time and recovery time are 562 ms and 308 ms, respectively. In addition, the performance of stability and temperature response are tested, the standard measurement error is 0.21 %RH and the temperature sensitivity is 0.0398 nm/°C. Experimental results indicate the sensor has a good practical prospect.

High Sensitivity Humidity Detection Based on Functional GO/MWCNTs Hybrid Nano-Materials Coated Titled Fiber Bragg Grating.

A high performance humidity sensor using tilted fiber Bragg grating (TFBG) and functional graphene oxide (GO)/multi-walled carbon nanotubes (MWCNTs) hybrid nano-materials was proposed. The humidity-sensitive material with three-dimensional (3D) structure was synthesized by the MWCNTs and GOs. Comparing with traditional two dimensional (2D) GOs film, water molecules could be absorbed effectively due to the larger ripples and more holes in GO/MWCNTs layers. The water molecule will fill the entire space in the 3D structure instead of air, which further enhances the absorption efficiency of the hybrid nanomaterial. TFBG as a compact and robust surrounding complex dielectric constant sensing platform was utilized. The mode coupling coefficient or the amplitude of TFBG cladding mode will vary sharply with the imaginary part of permittivity of the hybrid nanomaterial, realizing the high performance RH sensing. In the experiments, we successfully demonstrated that this 3D structural nanomaterial composed by the MWCNTs and GOs has significant advantages for expanding the range of humidity detection (range from 30% to 90%) and enhancing the detection sensitivity (0.377 dB/% RH is twice more than humidity sensor with 2D GO film). The TFBG-based RH sensor also exhibits good repeatability and stability. Our proposed humidity sensor has potential application in environmental and healthy monitoring fields.