Quartz Crystal Microbalance Humidity Sensor Research Articles

High-sensitivity QCM humidity sensor based on chitosan/carboxymethylated multiwalled carbon nanotubes composite for non-contact respiratory monitoring

Respiratory humidity is an important indicator that can reflect respiratory disorders and is easily accessible in daily life, thus attracting attention in non-contact home respiratory monitoring systems. In this work, a high-sensitivity quartz crystal microbalance (QCM) humidity sensor based on a chitosan/carboxymethylated multiwalled carbon nanotubes composite coating is developed with a response time of 36 s and a recovery time of 38 s. The humidity variations from 11 to 97 % can be detected while the wet hysteresis is 0.95 % RH. The sensor also exhibits good repeatability and stability. The physicochemical characterizations of the materials reveal the mechanism of the rapid humidity response, i.e., compared to the physically blended CS with MWCNT, the crosslinking CS-MWCNT formed the new intercalation by stronger hydrogen and amide bonding, which leads to the homogeneous coverage of CS on MWCNT, exposing more active sites and facilitating the binding rate of water molecules. Combined with respiration monitoring, the sensor is able to accurately monitor human respiration rate and depth in real time, effectively predicting and differentiating between different types of obstructive sleep apnea syndromes, providing a fast and reliable solution for daily health monitoring.

Nanodiamond/Ti3C2 MXene-coated quartz crystal microbalance humidity sensor with high sensitivity and high quality factor

Nanodiamond/Ti3C2 MXene-coated quartz crystal microbalance humidity sensor with high sensitivity and high quality factor

High-sensitivity, fast-response quartz crystal microbalance humidity sensors coated with nanodiamond/halloysite nanotube composites

This paper develops a composite mixture of halloysite nanotubes (HNTs) and nanodiamond (ND) as a humidity-sensitive material for a quartz crystal microbalance (QCM) to achieve a balance between rapid response/recovery times and high sensitivity. The ND-HNTs composite-integrated QCM sensor exhibited rapid response/recovery times of 2/1 and 4/2 s when transitioning from an environmental humidity of 55 % RH to both low (11.3 % RH) and high (97.3 % RH) humidity levels. Throughout the humidity range of 11.3 %∼97.3 % RH, the sensor demonstrated a sensitivity of up to 98.14 Hz/%RH and low humidity hysteresis of around 1.95 % RH, good repeatability and stability. Furthermore, a detailed analysis of the response mechanism for the ND-HNTs composite-integrated QCM humidity sensor was conducted by integrating a bimodal exponential kinetic adsorption model. This paper introduces a new approach for fabricating QCM humidity sensors that possess both heightened sensitivity and rapid response/recovery times.

Ultralow-hysteresis quartz crystal microbalance humidity sensor based on electrospinned cellulose acetate/konjac glucomannan network-like film

There are numerous and significant uses for accurate and reliable humidity detection in day-to-day living. In this study, a QCM humidity sensor based on cellulose acetate/konjac glucomannan (CA/KGM) film was constructed. The elemental composition, morphology and nanostructure of the film were characterized using scanning electron microscopy (SEM), water contact angle tester, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR). The CA/KGM QCM humidity sensor exhibited excellent stability, selectivity and repeatability with low humidity hysteresis and short response/recovery time (12.3 s/0.1 s). The experimental results show that the introduction of CA in KGM can dramatically improve the mechanical properties and stability of the composite membrane. The humidity sensing mechanism of the sensor was systematically investigated, and water droplet counting test, fingertip moisture detection and human breath detection were performed. The sensor offers a wide range of potential applications.

Nanochitin/MXene Composite Coated on Quartz Crystal Microbalance for Humidity Sensing.

MXenes, as a typical graphene-like material, excels in the realm of humidity sensing owing to its two-dimensional layer structure, high electrical conductivity, tunable chemical properties, hydrophilicity, and large specific surface area. This study proposed a quartz crystal microbalance (QCM) humidity sensor using a nanochitin/Ti3C2Tx MXene composite as a humidity-sensing material. The morphology, nanostructure, and elemental composition of nanochitin, Ti3C2Tx MXene, and nanochitin/Ti3C2Tx MXene composite materials were characterized using transmission electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction. Compared to the pure Ti3C2Tx MXene-coated QCM humidity sensor, the nanochitin/Ti3C2Tx MXene-coated QCM humidity sensor exhibited a higher sensitivity (20.54 Hz/%RH) in the humidity range of 11.3% to 97.3%. The nanochitin/Ti3C2Tx Mxene-coated QCM humidity sensor also demonstrated low humidity hysteresis (2.12%RH), very fast response/recovery times (4.4/4.1 s), a high quality factor (37 k), and excellent repeatability and sustained stability over time. Eventually, a bimodal exponential kinetics adsorption model was utilized for the analysis of the response mechanism of the nanochitin/Ti3C2Tx MXene composite material-based QCM humidity sensor. This study provides new ideas for optimizing the moisture-sensitive performance of MXene-based QCM humidity sensors.

Synthesis of Guanine/Vermiculite Two-Dimensional Nanocomposites for Wireless Humidity Sensing in Nut Storage Environment.

Two-dimensional (2D) nanostructures have the advantages of high specific surface area, easy surface functionalization, abundant active sites, and good compatibility with device integration and can be assembled into three-dimensional structures, which are key to the development of high-performance gas sensors. In this study, 2D vermiculite (VMT) nanosheets and guanine (G), two renewable resources with unique chemical structures, were organically combined to fully use the specificity of their molecular structures and functional activities. Driven by the regulation of 2D VMT nanosheets, guanine/vermiculite (G/VMT)-based 2D nanocomposites with controllable pore structure, multiple binding sites, and unobstructed mass transfer were designed and synthesized. The G/VMT nanocomposite material was used as a quartz crystal microbalance (QCM) electrode-sensitive film material to build a QCM-based humidity sensor. G/VMT-based QCM humidity sensor had good logarithmic linear relation (0.9971), high sensitivity (24.49 Hz/% relative humidity), low hysteresis (1.75% RH), fast response/recovery time (39/6 s), and good stability. Furthermore, with a QCM sensor and a specially designed wireless circuit, a wireless humidity detection system transmitting via Wi-Fi allows real-time monitoring of nut storage. This study presents an environmentally friendly, high-performance, miniature 2D nanocomposite sensor strategy for real-time monitoring.

Highly Sensitive QCM Humidity Sensor Based on Fullerenol-Assisted Dispersion of Nafion/MWCNTs Composites

Water seepage of building walls is a common problem in our lives. However, the lack of early seepage detection methods often makes the problem worse. To develop effective early wall seepage detection methods, an ultra-sensitive quartz crystal microbalance (QCM) humidity sensor based on fullerenol (C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sub> -OH)-assisted dispersion of Nafion/multi-walled carbon nanotubes (MWCNTs) composites is proposed. The prepared sensor shows almost 7 times higher frequency shift than pure Nafion-based sensor, which reaches -10814 Hz. This is mainly attributed to the uniform dispersion of MWCNTs in the Nafion with the assistance of C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sub> -OH to form a reticular structure that generates more voids, which greatly improves the diffusion ability of water molecules in the composite materials and thus further enhances humidity sensing properties of the sensor. The high-performance QCM humidity sensor is expected to be used for early detection of water seepage in building walls by monitoring air humidity changes near the wall surface.

α‐Co(OH)2‐Based Quartz Crystal Microbalance Humidity Sensor for Real‐Time Monitoring of Respiration

Respiration is an important physiological process of the human body and can assess an individual's health status. Humidity detection is an effective method for real‐time breath monitoring. Herein, the α‐Co(OH)2‐based quartz crystal microbalance humidity sensor (Co‐QCM sensor) is prepared via the hydrothermal method. The Co‐QCM sensor is characterized by scanning electron microscopy, Fourier‐transform infrared, and X‐ray diffraction. The α‐Co(OH)2 with hierarchical nanostructure and hydroxy functional groups leads to good humidity‐sensing performance. The Co‐QCM sensor exhibits high sensitivity (23.1 Hz/% relative humidity), fast response and recovery times (within 3 and 4 s, respectively), high linearity (R2 = 0.994), and long‐term stability (≈30 days) in the humidity range of 4–73.3%. In addition, different human breathing signals, including free breathing, shallow breathing, and deep breathing, can be detected and distinguished by this humidity sensor, showing the potential application in human respiratory detection.

A QCM Humidity Sensor Based on MoS₂ Nanoflower-Modified Chitosan Composite With High Quality Factor

This article presented a quartz crystal microbalance (QCM) humidity sensor covered with chitosan/molybdenum disulfide (CS/MoS2) composite film. The prepared sensor possesses promising humidity sensing performance with high sensitivity [46.06 Hz/% relative humidity (RH)], short response/recovery time (15/3 s), and small humidity hysteresis (2.5% RH). Most notably, the MoS2 nanoflowers that play a supporting role in the CS/MoS2 composite significantly reduce the swelling effect of CS in humid environments, enabling the CS/MoS2-based QCM humidity sensor to maintain a high <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> -factor even under high humidity conditions, thereby considerably improving the stability of the sensor.

Humidity sensing properties and respiratory behavior detection based on chitosan-halloysite nanotubes film coated QCM sensor combined with support vector machine

This work demonstrated an accurate and reliable quartz crystal microbalance (QCM) humidity sensor based on the chitosan-halloysite nanotubes (CS-HNTs) composite that can be used to monitor respiration lines. The hydrophilic polymer chitosan (CS) with halloysite nanotubes (HNTs) was characterized using SEM, TEM, XRD, XPS, FT-IR, and WCA investigations to analyze the microscopic morphology, molecular structure, chemical bond, and hydrophilicity. The experimental results showed that the QCM humidity sensor of CS-HNTs film demonstrated high sensitivity (69.1 Hz/% RH), superior humidity hysteresis (<2.8% RH), excellent long-term stability, repeatability, and outstanding selectivity. The subtle changes in fingertip movements and exhaled air humidity in different modes were detected by the QCM humidity sensor. A respiratory behavior monitoring model based on support vector machines was constructed to accurately determine five breathing modes and apply it to human breath detection and health medical diagnosis.

Nanodiamond/cellulose nanocrystals composite-based acoustic humidity sensor

This paper fabricated a high-performance nanodiamond (ND)/cellulose nanocrystal (CNC) composite-based acoustic humidity sensor. First, the effects of changes in both mass and viscosity of humidity-sensitive films on the resonance displacement, vibration modes, and impedance characteristics of the quartz crystal microbalance (QCM) were discussed using the finite element simulation software COMSOL Multiphysics. Then, the surface morphology and elemental composition of ND/CNC composites were characterized by TEM and EDS. Next, the humidity-sensing properties of ND/CNC composites, including response sensitivity, humidity hysteresis, dynamic characteristics, and stability, were tested at a relative humidity range from 11.3 % to 97.3 % RH combined with QCM transducers. In addition, the humidity-sensing performance of the QCM sensors with different ratios of composites was compared. The best ND/CNC-based QCM humidity sensors exhibited high sensitivity (54.1 Hz/%RH), low humidity hysteresis (3.2%RH), and fast response/recovery times. Then the adsorption process and sensitivity mechanism of water molecules on ND/CNC composites were analyzed using Langmuir isothermal adsorption model. Finally, the potential application of ND/CNC composite-based QCM humidity sensor was illustrated with respiration monitoring as an example.

Impedance Analysis of Chitin Nanofibers Integrated Bulk Acoustic Wave Humidity Sensor with Asymmetric Electrode Configuration.

This paper fabricated a high-performance chitin nanofibers (ChNFs)-integrated bulk acoustic wave (BAW) humidity sensor with an asymmetric electrode configuration. The ChNFs were successfully prepared from crab shells and used as moisture-sensitive materials to compare the performance of quartz crystal microbalance (QCM) humidity sensors with symmetric and asymmetric electrode structures. The QCM humidity sensor with a smaller electrode area exhibited high sensitivity of 58.84 Hz/%RH, competitive response/recovery time of 30/3.5 s, and low humidity hysteresis of 2.5% RH. However, it is necessary to choose a suitable electrode diameter to balance the stability and sensitivity because the impedance analysis result showed that the reduction of the electrode diameter leads to a sharp decrease in the Q value (stability). Next, the possible humidity-sensitive mechanism of the ChNFs-integrated asymmetric n-m electrode QCM humidity sensor was discussed in detail. Finally, the reasons for the highest sensitivity of the asymmetric n-m electrode QCM humidity sensors having a smaller electrode diameter were analyzed in detail in terms of both mass sensitivity and fringing field effect. This work not only demonstrates that the chitin nanofiber is an excellent potential material for moisture detection, but also provides a new perspective for designing high-performance QCM humidity sensors.

All-inorganic lead-free Cs2XCl6 (X = Hf, Zr, Te) perovskites for humidity detection

Lead halide perovskites are recently regarded as promising sensing materials owing to their sensitive surface characteristics and solution processability. However, the toxicity issue and intrinsic instability restrict their applications. Herein, lead-free Cs2XCl6 (X = Hf, Zr, Te) perovskites are employed to manufacture an environmentally friendly humidity sensor. With the assistance of quartz crystal microbalance (QCM), real-time and low-cost humidity sensors are performed. The as-prepared Cs2XCl6 (X = Hf, Zr, Te) humidity sensors exhibit ultrafast response/recovery behaviors (2.8 s/2.2 s, 3.8 s/2 s and 3.7 s/2.1 s) and considerable long-term stability (over 30 days). Furthermore, the bimodal exponential model is adopted to reveal the kinetics reactions of water molecules on the Cs2XCl6 (X = Hf, Zr, Te) coated QCM humidity sensors. All results suggest that the Cs2XCl6 (X = Hf, Zr, Te) coated QCM humidity sensors could be promising real-time humidity detection techniques.

A versatile gas transmission device with precise humidity control for QCM humidity sensor characterizations

Previously, several high-sensitivity and inexpensive humidity sensors with rapid response times have been reported. However, obtaining reliable humidity characteristics of highly sensitive devices, such as quartz crystal microbalance (QCM) sensors, requires the development of new, affordable gas transmission setups. With that in mind, we developed a novel, comprehensive gas transmission system that combined elements from two commonly used systems (saturated salts and gas mixing), which facilitated automatic gas humidity changes, reliable gas flows in closed measurement chambers, and limited the influence of the external atmosphere/pollutants. The gas transmission device performance was tested using a commercial digital hygrometer and highly sensitive QCM coated with graphene oxide for six different humidity sets in range 5.3–82.6% RH. The dynamic repeatability tests proved the superb stability of the system, as QCM frequency deviation did not exceed 2% of frequency shifts among ten switching cycles (one hour each). Therefore, this system allowed a thorough investigation of humidity sensors properties in a controlled environment with a high precision.

A lead-free K2CuBr3 microwires-based humidity sensor realized via QCM for real-time breath monitoring

Halide perovskites have recently emerged as potential humidity-detection materials due to their excellent chemical and physical properties. However, the toxicity of Pb and environmental instability of the lead halide perovskites restrict the application in humidity detection. Herein, a lead-free K2CuBr3 microwires-based quartz crystal microbalance (QCM) humidity sensor was investigated, and the application in human respiratory monitoring was developed. Coupling the high humidity sensitivity of K2CuBr3 with the high mass change sensitivity of QCM, the lead-free K2CuBr3 coated QCM humidity sensor exhibited excellent logarithmic linearity response (R2 = 0.98626, 11–95% RH). In addition, the sensor demonstrated excellent repeatability, fast response/recover time (10.63 s/4.31 s), and considerable stability. Moreover, the Fourier transform infrared spectra (FTIR) and Langmuir adsorption model were applied to reveal the corresponding humidity sensing mechanism. The results indicate the lead-free K2CuBr3-based QCM sensor reported in this work could be a promising candidate for real-time breath monitoring.

Sb-doped WO3 based QCM humidity sensor with self-recovery ability for real-time monitoring of respiration and wound

Humidity sensing is a promising method for monitoring respiration and diagnosing diseases. However, the fabrication of a semiconductor-based humidity sensor with fast response/recovery times and high sensitivity even under wet conditions remains challenging. In this study, a quartz crystal microbalance (QCM) humidity sensor was fabricated using Sb-doped WO3 nanocrystals via a simple solvothermal method. Sb doping increased the number of oxygen vacancies and led to a narrow band gap. It also restricted the further growth of WO3 and provided more adsorption sites, which resulted in a small particle size and large specific surface area. The device displayed a wide range of relative humidity (0–85%) with fast response and recovery times (10 and 1.6 s, respectively) and good long-term stability. After 100 cycles of wet treatment, the response of the sensor recovered to 92.2%, showing good self-recovery ability. The Sb/WO3-based QCM sensors were applied to the sensitive monitoring not only of respiratory humidity but also of moisture changes of a wound; the latter had never previously been performed using a QCM. This study therefore provides valuable insights for the development of integrated wound-microenvironment monitoring.

Quartz Crystal Microbalance Humidity Sensors Based on Structured Graphene Oxide Membranes with Magnesium Ions: Design, Mechanism and Performance.

The application of graphene oxide (GO)-based membranes combined with a quartz crystal microbalance (QCM) as a humidity sensor has attracted great interest over the past few years. Understanding the influence of the structure of the GO membrane (GOM) on the adsorption/desorption of water molecules and the transport mechanism of water molecules in the membrane is crucial for development of applications using GOM-based humidity sensors. In this paper, by investigating the effects of oxygen-containing groups, flake size and interlayer spacing on the performance of humidity sensing, it was found that humidity-sensing performance could be improved by rational membrane-structure design and the introduction of magnesium ions, which can expand the interlayer spacing. Therefore, a novel HGO&GO&Mg2+ structure prepared by uniformly doping magnesium ions into GO&HGO thin composite membranes was designed for humidity sensing from 11.3% RH to 97.3% RH. The corresponding sensor exhibits a greatly improved humidity sensitivity (~34.3 Hz/%RH) compared with the original pure GO-based QCM sensor (~4.0 Hz/%RH). In addition, the sensor exhibits rapid response/recovery times (7 s/6 s), low hysteresis (~3.2%), excellent repeatability and good stability. This research is conducive to understanding the mechanism of GOM-based humidity sensors. Owing to its good humidity-sensing properties, the HGO&GO&Mg2+ membrane-based QCM humidity sensor is a good candidate for humidity sensing.

A Quartz Crystal Microbalance (QCM) Humidity Sensor Based on a Pencil-Drawn Method With High Quality Factor

In this work, we, for the first time, propose a quartz crystal microbalance (QCM) humidity sensor that is prepared by a one-step method based on pencil drawing. The scanning electron microscopy (SEM), atomic force microscope (AFM), and Raman spectra confirm some graphite flakes, and a layer of sensing film is formed, demonstrating the successful preparation of the QCM sensor. A series of experiments were performed by means of oscillating circuit and impedance analysis at room temperature. The impedance analysis method demonstrates that the QCM humidity sensor exhibits an excellent stability with a quality factor ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> ) that only drops from 47 923 to 43 849 in the whole humidity range, which is better than ever reported QCM humidity sensors. In addition, the sensor shows a good long-term stability with a maximal frequency deviation of less than 5% in a long-term test. Furthermore, the sensor displays a rapid response time ~8s and a recovery time less than 5 s. The underlying reasons for these excellent properties of the proposed sensor are analyzed and discussed. This research demonstrates that the pencil-drawn method is a simple, green, and cost-effective way for constructing a QCM humidity sensor with high performance.

Fabrication of quartz crystal microbalance humidity sensors based on super-hydrophilic cellulose nanocrystals

Natural cellulose nanocrystals (CNCs) possess outstanding properties, such as biodegradability, hydrophilicity, large specific surface areas, and are low in cost. A highly stable and sensitive humidity sensor based on a quartz crystal microbalance (QCM) was prepared using CNCs as the sensing film. To investigate the impact of CNCs loading on the sensor performance, a series of humidity-sensitive performance tests, including linearity, response and recovery times, repeatability, hysteresis characteristics, selectivity, and long-term stability tests, was conducted. The experimental results indicated that the humidity sensor with CNCs loading of 2 μg (QCM-2) exhibited excellent logarithmic linearity, high sensitivity (32.35 Hz/% relative humidity (RH)), excellent reversible behavior, and long-term stability at a RH of 11–84%. The contact angle of QCM-2 was 12.5°, revealing its suitable hydrophilicity. Thus, we demonstrate that CNCs are great prospects for potential application in humidity sensors with high performance and low cost.

High Sensitivity and High Stability QCM Humidity Sensors Based on Polydopamine Coated Cellulose Nanocrystals/Graphene Oxide Nanocomposite.

In this paper, a high sensitivity and high stability quartz crystal microbalance (QCM) humidity sensor using polydopamine (PDA) coated cellulose nanocrystal (CNC)/graphene oxide (GO) (PDA@CNC/GO) nanocomposite as sensitive material is demonstrated. The PDA@CNC was prepared by the self-polymerization action on the surface of CNC, and it acted as filler material to form functional nanocomposite with GO. The material characteristics of PDA@CNC, CNC/GO and PDA@CNC/GO were analyzed by transmission electron microscope (TEM) and Fourier transform infrared spectroscopy (FTIR), respectively. The experimental results show that the introduction of PDA@CNC into GO film not only effectively enhanced the sensitivity of GO-based nanocomposite-coated QCM sensor but also significantly maintained high stability in the entire humidity range. The PDA@CNC/GO30-coated QCM humidity sensor exhibited a superior response sensitivity up to 54.66 Hz/% relative humidity (RH), while the change rate of dynamic resistance of the sensor in the humidity range of 11.3–97.3% RH is only 14% that is much smaller than that of CNC/GO-coated QCM. Besides, the effect of the PDA@CNC content on the sensitivity and stability of GO-based nanocomposite-coated QCM humidity was also studied. Moreover, other performances of PDA@CNC/GO-coated QCM humidity sensor, including humidity hysteresis, fast response and recovery and long-term stability, were systematically investigated. This work suggests that PDA@CNC/GO nanocomposite is a promising candidate material for realizing high sensitivity and high stability QCM humidity sensor in the entire humidity detection range.