The application of metal-organic frameworks (MOFs) as a sensing layer has been attracting great interest over the last decade, due to their uniform properties in terms of high porosity and tunability, which provides a large surface area and/or centers for trapping/binding a targeted analyte. Here we report the fabrication of a highly sensitive humidity sensor that is based on composite thin films of HKUST-1 MOF and carbon nanotubes (CNT). The composite sensing films were fabricated by spin coating technique on a quartz-crystal microbalance (QCM) and a comparison of their shift in resonance frequencies to adsorbed water vapor (5–75% relative humidity) is presented. Through optimization of the CNT and HKUST-1 composition, we could demonstrate a 230% increase in sensitivity compared to plain HKUST-1 film. The optimized CNT-HKUST-1 composite thin films are stable, reliable, and have an average sensitivity of about 2.5 × 10−5 (Δf/f) per percent of relative humidity, which is up to ten times better than previously reported QCM-based humidity sensors. The approach presented here is facile and paves a promising path towards enhancing the sensitivity of MOF-based sensors.

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The quest for highly sensitive QCM humidity sensors: the coating of CNT/MOF composite sensing films as case study

In this paper, a new approach to enhance the humidity sensitivity of quartz crystal microbalance (QCM) humidity sensor was proposed. By the asymmetric treatment of QCM electrode, the as-fabricated QCM humidity sensor produces an additional frequency shifts associated with the change of dielectric constant of sensitive material besides mass sensitivity. Renewable cellulose nanocrystals (CNCs) as humidity sensing material was deposited on the sensing electrode of QCM. The material characteristics of the synthesized CNCs were examined by TEM, AFM, FTIR and XRD, respectively. The humidity sensing performances of CNCs based QCM humidity sensor, including humidity sensitivity, stability, humidity hysteresis and dynamic response and recovery, were comprehensively studied by using an oscillating circuit method. The results proved that the humidity sensitivity of CNCs based QCM humidity sensor can be effectively enhanced by the asymmetric treatment of QCM electrode structure. Further, the underlying sensitivity enhancement mechanism was analyzed and discussed using an equivalent circuit analysis method. This work highlights that QCM transducer with an asymmetric sensitive electrode structure is a promising approach to improve the humidity sensitivity, and CNCs is also a promising material for humidity sensing.

Facile fabrication of high sensitivity cellulose nanocrystals based QCM humidity sensors with asymmetric electrode structure

This paper described a quartz crystal microbalance (QCM) humidity sensor based on fullerene (C60)/graphene oxide (GO) nanocomposites. Humidity sensing properties such as the quality factor, frequency response property, response/recovery time, and humidity hysteresis were measured and quantitatively compared with a GO based QCM humidity sensor. The experiments provided evidences that the GO film was viscous even at low relative humidity, which had a large impact on the measurable result of a GO coated QCM humidity sensor. As a result, the proposed sensor showed better performances at quality factor and dynamic response/recovery behavior than the GO based QCM humidity sensor. Based on the admittance analysis and frequency response analysis of the sensors, it was speculated that the introduction of C60 molecules into GO reduced the aggregation of the GO sheets and formed some hydrophobic isolation layers between the GO sheets, which could inhibit the water molecules permeation and maintain the mechanical stiffness of the C60/GO film.

A QCM humidity sensor based on fullerene/graphene oxide nanocomposites with high quality factor

Respiratory monitoring is crucial for the assessment of human health, and can be realized by detecting the humidity of exhaled gases. In this work, a facile method for the in situ fabrication of stable humidity sensors based on gel polymer electrolytes was developed. The obtained humidity sensors demonstrate ultrafast response/recovery times (0.19/0.30 s), which are very promising for monitoring respiration. Reliable signals for different respiratory states were obtained via humidity sensors. In addition, the ultrafast-response mechanism of the sensors was investigated using complex impedance spectroscopy and a quartz crystal microbalance (QCM).

Design Strategy for Ultrafast-Response Humidity Sensors Based on Gel Polymer Electrolytes and Application for Detecting Respiration

This paper demonstrates three kinds of TiO2 with hollow ball, nanosphere, and nanoflower nanostructures coated quartz crystal microbalance (QCM) humidity sensors, which were synthesized by metal organic framework (MOF)-derived, hydrothermal and solvothermal route. The compositional and morphological characteristics of three kinds of TiO2 nanostructures were characterized by means of X-ray diffraction, specific surface area analysis, scanning electron microscopy, and water contact angle measurement. We found that the surface structure plays a crucial role in the humidity-sensing response properties of QCM sensors. The humidity sensing experiment was performed at room temperature. The MOF-derived hollow ball-like TiO2 coated QCM sensor exhibited excellent humidity sensing characteristics over a wide RH range (0%–97%RH), such as high sensitivity (33.8 Hz/%RH), good repeatability, and short dynamic response/recovery times (5 s/2 s). In addition, Langmuir adsorption model was developed to explain the humidity-sensing properties of the TiO2-based QCM sensor. This paper proves that MOF-derived hollow-ball TiO2 is a potential candidate nanomaterial for humidity sensor fabrication.

Humidity Sensing Properties of Metal Organic Framework-Derived Hollow Ball-Like TiO 2 Coated QCM Sensor

A novel quartz crystal microbalance (QCM) humidity sensor using nanodiamond (ND) as sensing films has been presented in this paper. For evaluating the performance of ND-coated QCM humidity sensor, a series of humidity-sensing experiments were carried out by using both oscillating circuit and impedance analysis methods. Based on the large specific surface area and high mechanical modulus, the ND-coated sensor exhibited large humidity-sensing response and good stability. In addition, the resultant sensors displayed low hysteresis, good repeatability, and short response time. The influence of the depositing amount of ND on the sensor's performance has also been discussed. The result indicated that the stability of QCM humidity sensor showed an extreme decrease when the deposition amount of ND on QCM is large enough. The possible reason for this anomalous phenomenon is analyzed and discussed. This study suggests that ND could be a promising candidate material for developing high-performance QCM humidity sensor.

Weiwei Ling

Papers

Quartz Crystal Microbalance Humidity Sensors Based on Nanodiamond Sensing Films