Humidity-Sensing Properties of a BiOCl-Coated Quartz Crystal Microbalance

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.

Toward Continuous Breath Monitoring on a Mobile Phone Using a Frugal Conducting Cloth-Based Smart Mask

A frugal humidity sensor that can detect changes in the humidity of exhaled breath of individuals has been fabricated. The sensor comprises a humidity-sensitive conducting polymer that is in situ formed on a cloth that acts as a substrate. Interdigitated silver electrodes were screen-printed on the modified cloth, and conducting threads connected the electrodes to the measurement circuit. The sensor's response to changing humidity was measured as a voltage drop across the sensor using a microcontroller. The sensor was capable of discerning between fast, normal, and slow breathing based on the response time. A response time of ∼1.3 s was observed for fast breathing. An Android-based mobile application was designed to collect sensor data via Bluetooth for analysis. A time series classification algorithm was implemented to analyze patterns in breathing. The sensor was later stitched onto a face mask, transforming it into a smart mask that can monitor changes in the breathing pattern at work, play, and sleep.

Comparative Study of Gravimetric Humidity Sensor Platforms Based on CMUT and QCM

Humidity sensors with comprehensive performance are of great interest for industrial and environmental applications. Most sensors, however, have to compromise on at least one characteristic such as sensitivity, response speed, and linearity. This paper reports a gravimetric humidity sensor based on a capacitive micromachined ultrasonic transducer (CMUT) with exceptional all-around performance, and presents a side-by-side comparative investigation of two types of gravimetric humidity sensors for a better understanding of their characteristics and sensing mechanisms. For these purposes, a circular CMUT and a quartz crystal microbalance (QCM) with a resonance frequency of 10 MHz were designed and fabricated. Poly(vinyl alcohol) (PVA) was employed as the humidity sensing layer for its hydrophilicity and ease of film formation. The electrical properties of the sensors, including the electrical input impedances and quality factors, were characterized by a vector network analyzer. The relative humidity (RH) sensing performance of the sensors was evaluated and compared from RH levels of 11% to 97%. Both sensors exhibited good repeatability and low hysteresis. The unique microscale resonant structure of the CMUT humidity sensor contributed to a high sensitivity of 2.01 kHz/%RH, short response and recovery times of 8 s and 3 s, respectively, and excellent linearity (R² = 0.973), which were far superior to their QCM counterparts. The underlying mechanism was revealed and discussed.

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.

Respiratory Monitoring by Ultrafast Humidity Sensors with Nanomaterials: A Review

Respiratory monitoring is a fundamental method to understand the physiological and psychological relationships between respiration and the human body. In this review, we overview recent developments on ultrafast humidity sensors with functional nanomaterials for monitoring human respiration. Key advances in design and materials have resulted in humidity sensors with response and recovery times reaching 8 ms. In addition, these sensors are particularly beneficial for respiratory monitoring by being portable and noninvasive. We systematically classify the reported sensors according to four types of output signals: impedance, light, frequency, and voltage. Design strategies for preparing ultrafast humidity sensors using nanomaterials are discussed with regard to physical parameters such as the nanomaterial film thickness, porosity, and hydrophilicity. We also summarize other applications that require ultrafast humidity sensors for physiological studies. This review provides key guidelines and directions for preparing and applying such sensors in practical applications.

Analysis of the Uniformization of the QCM Mass Sensitivity Distribution through a Dot Multiring Electrode Structure

In this paper, to improve the uniformity of the quartz crystal microbalance (QCM) mass sensitivity distribution, the effect of the size of dot ring electrode and dot double-ring electrode on the mass sensitivity distribution is analyzed theoretically from the perspective of the electrode width ratio, the width of the partially electroded region, and the electrode thickness. Within a certain range of electrode thickness, there is an optimum electrode width ratio to obtain a relatively uniform distribution. As long as the width of the partially electroded region is not too large or too small, it has no significant influence on the uniformity of the mass sensitivity of the optimum electrode size. The dot triple-ring electrode is proposed and the comparison of the uniformity between the three electrode structures indicates that more division of the electrode region can greatly improve the uniformity of the mass sensitivity distribution. This research provides a new direction for the uniformization of QCM mass sensitivity distribution.

Method for QCM Resonator Device Equivalent Circuit Parameter Extraction and Electrode Quality Assessment

Quartz crystal microbalance (QCM) resonators are used in a wide range of sensors. Current QCM resonators achieve a simultaneous measurement of multiple physical quantities by analyzing lumped-element equivalent parameters, which are obtained via the introduction of external devices. This introduction of external devices will probably increase measurement error. To realize the measurement of multiple physical quantities while eliminating the measurement error caused by external devices, this paper proposes a measurement method for the lumped-element equivalent parameters of QCM resonators without the need for extra external devices. Accordingly, a numerical method for solving nonlinear equations with fewer data points required and a higher accuracy was adopted. A standard crystal resonator parameter extraction experiment is described. The extracted parameters were consistent with the nominal parameters, which confirms the accuracy of this method. Furthermore, six QCM resonator device samples with different electrode diameters and materials were produced and used in the parameter measurement experiment. The linear relationship between the electrode material conductivity and motional resistance R₁ is discussed. The ability of this method to characterize the electrode material and to detect the rust status of the electrode is also demonstrated. These abilities support the potential utility of the proposed method for an electrode quality assessment of piezoelectric devices.