Humidity Sensor Research Articles

High-performance flexible humidity sensor based on B-TiO2/ SnS2 nanoflowers for non-contact sensing and respiration detection

The development of flexible humidity sensors is essential for the advancement of wearable devices and electronic skin. However, the process of preparing these sensors is typically complex, and they often suffer from poor stability and a limited sensing range, which fails to meet the requirements for practical applications. Here, we prepared SnS2 with nanoflower morphology by hydrothermal method, and successfully prepared a high-performance flexible humidity sensor with a high response range (20-95 %) by compositing it with B-TiO2 nanoparticles with oxygen defects after sodium borohydride reduction and coating it on a PET flexible substrate. This sensor demonstrates an exceptional response (753.2 at 90 % RH) in high humidity environments and exhibits rapid response (13 s) and recovery times (19 s). Moreover, the sensor accurately detects human breathing patterns and frequencies, making it suitable for non-contact sensing applications. This research offers valuable insights into the simplified preparation and advancement of flexible humidity sensors.

Development of a novel humidity sensor for breath pattern recognition using large dislocation hollow core fiber infused with nano-carbon quantum water gel microfiber

This study presents a humidity sensor based on a large-core-offset hollow core optical fiber (LD-HCF) combined with polyvinyl alcohol carbon quantum dots (PVA-CQDs) nanocomposite microfiber for monitoring human respiratory. PVA-CQDs was introduced into the hollow core structure of LD-HCF to develope the respiratory sensor. The sensor has been integrated into a respiratory monitoring mask, exposed to ultraviolet (UV) light and demonstrated with a humidity sensitivity of 0.1721 nm/%RH as well as response and recovery times of 0.232 s and 0.252 s, respectively. The mask provides the real-time monitoring of human respiratory rates during various activities, including walking, standing, coughing, running, apnea 1 time and apnea 3 times. The typical respiratory patterns have been classified with a remarkable accuracy of 97.62 % using a WOA-1D CNN-LSTM neural network. The respiratory monitoring device is proposed by optical fiber, offering a promising potential in the real-time monitoring for healthcare process.

Engineered nano-porous Cr₂O₃/TUD-1 nanocomposites for efficient and reliable humidity sensor

Engineered nano-porous Cr₂O₃/TUD-1 nanocomposites for efficient and reliable humidity sensor

Fabrication of a novel sensor based on ionic porous polymer microspheres for humidity monitoring

A novel humidity sensor based on ionic porous polymer (IPP) microspheres was prepared in this work. The chemical structure and morphology of IPP microspheres were carefully characterized. The intrinsic hydrophilic ionic units of IPP, –N+R– and Br–, could enhance the adsorption of water molecules and ionic conductivity. The aromatic groups in IPP can adjust its hydrophobicity and ensure stability. Meanwhile, the structure of microspheres provides a favorable space of channels for water molecules transport, which improves the response speed of sensing film further. The impedance of the sensor based on IPP microspheres changed more than three orders of magnitude in the relative humidity (RH) range of 11–95 % RH with a good linearity (R2=0.996) and a fast response speed (6 s/ 27 s). Finally, the mechanism was systematically studied by complex impedance spectrum and theoretical calculation. This study shows a new type of sensor based on ionic porous polymer microspheres, which will pave the way for IPP applications in humidity sensing.

Application of Metal Halide Perovskite in Internet of Things.

The Internet of Things (IoT) technology connects the real and network worlds by integrating sensors and internet technology, which has greatly changed people's lifestyles, showing its broad application prospects. However, traditional materials for the sensors and power components used in the IoT limit its development for high-precision detection, long-term endurance, and multi-scenario applications. Metal halide perovskite, with unique advantages such as excellent photoelectric properties, an adjustable bandgap, flexibility, and a mild process, exhibits enormous potential to meet the requirements for IoT development. This paper provides a comprehensive review of metal halide perovskite's application in sensors and energy supply modules within IoT systems. Advances in perovskite-based sensors, such as for gas, humidity, photoelectric, and optical sensors, are discussed. The application of indoor photovoltaics based on perovskite in IoT systems is also discussed. Lastly, the application prospects and challenges of perovskite-based devices in the IoT are summarized.

Flexible capacitive humidity sensor based on potassium ion-doped PVA/CAB double-layer sensing film

Flexible capacitive humidity sensor based on potassium ion-doped PVA/CAB double-layer sensing film

Early wildfire detection using different machine learning algorithms

Early detection of wildfires is essential for mitigating their impact on forests and surrounding areas. In this study, we propose a wireless sensor node system that combines multiple low-cost sensors with an artificial intelligence-based detection method for early wildfire detection. The system architecture includes temperature, humidity, and smoke sensors, as well as a wireless communication module. Four machine learning classifiers, including decision trees, random forests, support vector machines, and k-nearest neighbors, were evaluated for their effectiveness in predicting wildfire detection using a dataset collected in a forest area. The results showed that the random forest algorithm with optimum hyperparameters had the highest accuracy in classifying fire and non-fire samples (77.95% correctly classified). The proposed system provides an effective and cost-efficient solution for early wildfire detection in large forest areas.

Multi‐Structural and Biodegradable Humidity Sensors with Enhanced Surface Hydrophilicity

Multi‐Structural and Biodegradable Humidity Sensors with Enhanced Surface Hydrophilicity

Light-Modulated Humidity Sensing in Spiropyran Functionalized MoS2 Transistors.

The optically tuneable nature of hybrid organic/inorganic heterostructures tailored by interfacing photochromic molecules with 2D semiconductors (2DSs) can be exploited to endow multi-responsiveness to the exceptional physical properties of 2DSs. In this study, a spiropyran-molybdenum disulfide (MoS2) light-switchable bi-functional field-effect transistor is realized. The spiropyran-merocyanine reversible photo-isomerization has been employed to remotely control both the electron transport and wettability of the hybrid structure. This manipulation is instrumental for tuning the sensitivity in humidity sensing. The hybrid organic/inorganic heterostructure is subjected to humidity testing, demonstrating its ability to accurately monitor relative humidity (RH) across a range of 10%-75%. The electrical output shows good sensitivity of 1.0% · (%) RH-1. The light-controlled modulation of the sensitivity in chemical sensors can significantly improve their selectivity, versatility, and overall performance in chemical sensing.

Colorimetric polymer nanofilm-based time-temperature indicators for recording irreversible changes of temperatures in cold chain

Monitoring and recording subzero temperatures and humidity are essential activities for pharmaceutical, food-beverage, and cold storage industries, as product quality can be hampered during storage and transportation due to temperature disruptions. Traditional electronic subzero time-temperature indicators (TTIs) can be energy-inefficient, fragile, non-recyclable, and susceptible to data breaches and cyber-attacks. Hydrophilic colorimetric polymer nanofilms have been developed as colorimetric temperature and relative humidity (RH) sensors, however, the reversible color-changing behavior of these films significantly limited their application as TTIs, as cannot record temperature changes in the past. Herein, the first colorimetric polymer nanofilm-based TTI for recording an irreversible change of temperatures is reported. This device has shown quick color response in temperature ranges from 23 °C to -30 °C in fewer than 50 s. Remarkably, when the device experiences temperature disruption above a certain threshold time (tth), it shows irreversible color-changing behavior in response to the temperature change from subzero (-30 °C and -15 °C) to room temperature or above. It was demonstrated that tth, from minutes to days, of the TTI device can be precisely tuned by adjusting moisture absorber type and weight, interior RH, and storage temperature. Several field tests have demonstrated good versatility and applicability of the device.

Environmental monitoring system design based on STM32 platform

This study addresses the current societal demand for environmental monitoring by designing an environmental monitoring system based on the STM32 platform. This system assesses and monitors environmental conditions in real-time by tracking parameters such as CO, PM2.5, temperature, humidity, and light intensity. It holds significant value in preventing air pollution and improving indoor air quality. The system employs four types of sensors: the DHT11 digital temperature and humidity sensor, the BH1750FV light sensor, the GP2Y1010AUOF optical dust sensor, and the MQ-7 CO sensor to collect environmental data, which is then processed by the STM32F103C8T6 controller. This system is characterized by its real-time capabilities, high precision, and low power consumption, making it highly practical and valuable for widespread application. The paper provides a detailed discussion of sensor selection, measurement algorithms, and system design and implementation, offering valuable insights for research and applications in related fields.

Multifunctional pressure and humidity sensor modulated by electrostatic interactions and hydrogen bonds for wearable health monitoring

Breathing and urination, are vital physiological activities of the human body, continuous real-time monitoring of these physiological behaviors could offer timely feedback on an individual’s health status. However, current monitoring techniques predominantly rely on cumbersome and intricate medical apparatuses, posing challenges in adapting to the diverse requirements of multi-scenario detection. Consequently, there is a growing interest in developing wearable devices capable of monitoring breathing and urination. In this work, we developed a multifunctional sensor integrating humidity and pressure sensing modes using a simple dip-coating process. By introducing sodium carboxymethyl cellulose and conductive polyaniline hybrid intercalation between MXene layers, a stable conductive network is established through hydrogen bonds and electrostatic interactions among materials. The overall electromechanical properties of the composites will be well improved. And, the effects of different conductive filler ratios and the number of dipping times on the construction of conductive networks are investigated. The multifunctional sensor exhibited improved sensing characteristics, including detecting pressures up to 532 kPa and a sensitivity of 19.58 kPa−1. Furthermore, it also demonstrates good humidity-sensing capabilities. Tests on volunteers demonstrated the potential in the detection of breathing and urination. In addition, the sensors are capable of transmitting Morse code. This interesting application will offer the possibility of normal communication for people with speech impairments. Given its utility and sustainability, the sensor has potential for applications in wearable health monitoring, intelligent life and telemedicine.

Double-Fluorescent Response Spiropyran-Functionalized Graphitic Carbon Nitride Solvent Decoder and Nanofiber Mat-Based Visual Colorimetric Sensor for Humidity Detection within Grain Heaps.

Spiropyran-functionalized graphitic carbon nitride (MCH@g-C3N4) in acidic conditions was prepared for the first time. MCH@g-C3N4 exhibited dual-wavelength fluorescence emission with two distant bands and distinct solvatochromic behavior owing to the different molecular structures of spiropyran. A three-dimensional organic solvent decoder was fabricated for solvent identification based on the emission wavelength and fluorescence quantum yield of MCH@g-C3N4 in different polar solvents. The ratiometric fluorescence sensing of the water content in organic solvent was realized using water to induce the structural change of spiropyran. A solid-phase MCH@g-C3N4/polyvinylpyrrolidone (PVP) nanofiber mat-based sensor was prepared via cospinning MCH@g-C3N4 with hydrophilic PVP. A fluorescent humidity sensor was fabricated using a commercial syringe equipped with a stainless-steel needle (catcher) to obtain the air inside the grain heap, which enabled the accurate location of the sampling spot. The space between the syringe and piston was used as an enclosed sensing space. Thus, a simple, accurate, and intuitive visual colorimetric method for the internal humidity of a grain heap was realized by analyzing the recorded images.

All-printed MXene/WS2-based flexible humidity sensor for multi-scenario applications

As non-contact sensing, non-invasive medical diagnostics and environmental humidity monitoring technologies advance, the demand for high-performance humidity sensors is becoming increasingly apparent. Ti3C2Tx MXene is an attractive humidity-sensitive candidate due to its excellent chemical activity and hydrophilic surface. However, achieving fast responsiveness and high sensitivity of dynamic changes in humidity is still challenging. Herein, this work introduced a WS2-modified Ti3C2Tx MXene composite material as humidity sensing material with superior sensing performance. Then sodium carboxymethyl cellulose solution was added to the Ti3C2Tx MXene/WS2 composite material to adjust viscosity of the composite material, enabling the formulation of printable humidity-sensitive ink. Utilizing screen-printing technology, a mass-producible humidity sensor (CMXW2) was successfully fabricated. Experimental results show that the sensor has short response and recovery times of 4.65 s/1.33 s in the relative humidity of 11–97 % with a high humidity response of 1707 % and excellent mechanical properties. The efficacy of the humidity sensor for skin moisture monitoring, real-time respiration monitoring, and non-contact switch was also evaluated. The successful demonstration of multi-scenario applications using the CMXW2 humidity sensor opens up new possibilities and applications in the fields of smart healthcare and non-contact sensing.

Facile preparation of graphene oxide/Poly (N‐Isopropylacrylamide‐co‐acrylic acid) composite thin film and its quartz crystal microbalance humidity sensing property

AbstractThe composite microgels were synthesized from N‐Isopropylacrylamide (NIPAM) and acrylic acid (AA) monomers in the presence of graphene oxide (GO) using an in situ radical copolymerization method. The successful preparation of these composite microgels was investigated through Fourier transform infrared spectroscopy (FTIR), ultraviolet visible absorption spectroscopy (UV–vis), and Raman spectroscopy. Due to the hydrophilic properties of GO and the microgels containing oxygenated groups (OH, COOH, and CONH2), quartz crystal microbalance (QCM) sensors can be fabricated by spraying the GO/P(NIPAM‐co‐AA) dispersion onto QCM sensors as sensitive coating materials. The results indicate a notable enhancement in the performance of GO/P(NIPAM‐co‐AA) modified QCM humidity sensor, compared to QCM sensors modified with either GO or P(NIPAM‐co‐AA) microgels alone. This improvement is mainly evidenced by higher sensitivity and reduced moisture hysteresis. The humidity sensing mechanism is based on the combined effect of GO and P(NIPAM‐co‐AA) microgels, which synergistically enhance the sensor's performance. Additionally, the results from water contact angle measurements, laser scanning confocal microscopy (LSCM), and scanning electron microscope (SEM) show that GO/P(NIPAM‐co‐AA) exhibits greater roughness and stronger hydrophilicity than either GO or P(NIPAM‐co‐AA) microgels alone. These properties make GO/P(NIPAM‐co‐AA) an effective moisture‐sensitive material for QCM sensors.

Relative humidity sensor based on a nano-film deposited LPFG with hydrophilic polymer composite coating

A novel high-performance relative humidity (RH) sensor is proposed by using a hydrophilic polymer composite film-coated nano-film deposition deposited long-period fiber grating (LPFG). A high refractive index (RI) TiO2 nano-film is deposited on the LPFG as an inner layer. The film thickness is selected to maximize the surrounding refractive index (SRI) sensitivity of the grating when the SRI is approaching the cladding index. A composite film made of polyvinyl alcohol (PVA) and polyethylene glycol (PEG) is used as the an external humidity-to-RI transducer. The concentration and proportion of the PVA/PEG solutions are appropriately chosen to tune the refractive index (RI) RI of the polymer coating to the transition range during the RH varying from 60 % to 95 %. For the LPFG coated with two layers film, the a RH sensitivity up to 3.3 nm/% is achieved. The variation in RH is found to have a linear response. The proposed device offers a reliable alternative for higher RH environment sensing.

Observation of tapered multicore fibers based on Nb2CTx for humidity sensing: Experiment and DFT simulation investigations

Recent advancements in the area of human healthcare monitoring and medical diagnosis have sparked a revived interest in humidity sensors. Nb2CTx, characterized by its multilayered structure, large specific surface area, and plentiful hydrophilic groups, actively absorbs a sufficient quantity of water molecules. In this work, the sensing capability and mechanism of water molecules are investigated under different proportions of functional group distribution on the surface of Nb2CTx MXene employing plane-wave based density functional theory calculations, and the provided models can offer guiding principles for the experimental preparation of the highly sensitive humidity sensor based on Nb2CTx. To verify the effectiveness of the model, Nb2CTx material is coated on a Tapered Three-Core Fiber (TTCF), and the high evanescent field characteristics of the sensor are utilized to further improve its performance. As the relative humidity (RH) increased from 35 % to 75 %, the transmitted spectra exhibited a redshift with a sensitivity of 22pm/% RH. Within the relative humidity range of 75 % to 95 %, the Nb2CTx coating heightened water molecule absorption, resulting in a more pronounced redshift in the transmitted spectra. This phase manifests a maximum humidity sensitivity of 299pm/% RH. Noteworthy advantages of this sensor include its uncomplicated structure, cost-effectiveness, and robust stability, rendering it highly suitable a wide variety of potential applications in fields such as biology, chemical and health processing.

A monitoring platform based on electrical impedance and AI techniques to enhance the resilience of the built environment

Structural Health Monitoring (SHM) and early warning systems (EWSs) play a pivotal role in enhancing seismic resilience for both buildings and occupants. This paper introduces a monitoring platform that collects electrical impedance data from scaled concrete beams undergoing load and accelerated degradation tests. Artificial Intelligence (AI) algorithms are employed for predictive analysis, scrutinizing historical impedance data, and forecasting future trends. These algorithms adapt to environmental parameters, becoming valuable tools in data-driven decision-making processes. In particular, the study investigates concrete specimens in different test conditions, utilizing a distributed sensor network based on electrical impedance as well as temperature and relative humidity sensors. Real-time data are transmitted to a cloud infrastructure during accelerated degradation tests (both in water and in chloride-rich solution) and in room conditions. An AI-based forecasting approach using Prophet is proposed, ingesting electrical impedance and temperature data, and tested to predict electrical impedance corresponding to approximately 10 % of the time series balancing responsiveness with predictive accuracy, crucial for effective EWS operations and management requirements. The performance of the tested models is evaluated employing metrics such as Mean Average Error (MAE), Root Mean Square Error (RMSE), Mean Absolute Percentage Error (MAPE), and correlation. The proposed approach surpasses statistical methods and deep learning techniques, reporting a MAPE always lower than 3.20 % and a correlation higher than 81.65 % (in wet-dry cycles in water these values are 0.65 Ω and 91.85 %, respectively). This proves to be a promising step towards transparent SHM, which integrates AI models facilitating self-monitoring and early maintenance prediction, thus enhancing the resilience of the built environment.

Modulating Contact Electrification With Metal‐Organic Frameworks in Flexible Triboelectric Nanogenerators for Kinetic Energy Harvesting and Self‐Powered Humidity Sensing Applications

AbstractHerein, we present a novel method for fabricating a triboelectric nanogenerator using Polyacrylonitrile (PAN) on both sides as triboelectric pairs, incorporating metal‐organic frameworks (MOFs) such as ZIF‐8, ZIF‐67, MIL‐100, and HKUST‐1 during the electrospinning process. The triboelectric properties of the MOF‐incorporated fibers are thus tailored and positioned within the triboelectric series for the first time. The resulting triboelectric polarity of the composite fiber is linked to the optical bandgap energy of the PAN and the MOF/PAN composite, facilitating electron transfer between materials of different work functions and leading to enhanced output in the developed triboelectric devices. Fascinatingly, the appropriate choice of MOF filler also displayed the potential for reversing the triboelectric polarity of PAN nanofiber. Consequently, incorporating ZIF‐8 and MIL‐100 into PAN nanofibers led notably to contrasting trends in triboelectric polarity, with the pair generating an open‐circuit output voltage of 100 V, short‐circuit current of 1.35 μA, and a power density of 18.4 mW/m2 respectively. The fabricated device demonstrated effectiveness for mechanical energy harvesting applications and also as a self‐powered humidity sensor, displaying rapid response to changes in ambient humidity levels with a maximum sensitivity of 2.14 V/%RH, for relative humidity range between 50 and 90% during the humidifying cycle.

Transparent, Flexible Nanoporous Sensors with Humidity and Pressure Sensing Capabilities for Sports Health Status Monitoring

Transparent, Flexible Nanoporous Sensors with Humidity and Pressure Sensing Capabilities for Sports Health Status Monitoring