A Comparative Study of Interdigitated Electrode and Quartz Crystal Microbalance Transduction Techniques for Metal⁻Organic Framework-Based Acetone Sensors

ZIF-8 decorated cellulose acetate mixed matrix membrane: An efficient approach for textile effluent treatment

The textile industry is the second largest water-intensive industry and generates enormous wastewater. The dyes and heavy metals present in the textile effluent, even at their lower concentrations, can cause an adverse effect on the environment and human health. Recently, mixed matrix membranes have gained massive attention due to membrane property enhancement caused by incorporating nanofillers/additives in the polymer matrix. This current study examines the efficacy of ZIF-8/CA membrane on dye removal and treatment of real-time textile industry effluent. Initially, ZIF-8 nanoparticles were synthesized using a probe sonicator. The XRD, FT-IR, and SEM analysis confirmed the formation of crystalline and hexagonal facet ZIF-8 nanoparticles. The ZIF-8 nanoparticles were dispersed into a cellulose acetate matrix, and a membrane was prepared using the "phase inversion method." The membrane was characterized using FT-IR and SEM analysis, which endorse incorporating ZIF-8 into the polymer matrix. Later, the efficacy of the ZIF-8/CA membrane was verified by dye removal studies. The dye removal studies on crystal violet, acid red 13, and reactive black 5 reveal that the membrane is ∼85% efficient in dye removal, and the studies were further extended to real-time textile effluent treatment. The studies on textile effluent prevail that ZIF-8/CA membrane is also proficient in removing chemical oxygen demand (COD) ∼70%, total organic carbon (TOC) ∼80%, and heavy metals such as lead, chromium, and cadmium from textile wastewater and proved to be efficient in treating the textile effluent.

Enhancing Solid-Phase Extraction of Tamoxifen and Its Metabolites from Human Plasma Using MOF-Integrated Polyacrylonitrile Composites: A Study on CuBTC and ZIF-8 Efficacy

This study investigates electrospun fibers of metal-organic frameworks (MOFs), particularly CuBTC and ZIF-8, in polyacrylonitrile (PAN) for the solid-phase extraction (SPE) of Tamoxifen (TAM) and its metabolites (NDTAM, ENDO, and 4OHT) from human blood plasma. The focus is on the isolation, pre-concentration, and extraction of the analytes, aiming to provide a more accessible and affordable breast cancer patient-monitoring technology. The unique physicochemical properties of MOFs, such as high porosity and surface area, combined with PAN's stability and low density, are leveraged to improve SPE efficiency. The study meticulously examines the interactions of these MOFs with the analytes under various conditions, including elution solvents and protein precipitators. Results reveal that ZIF-8/PAN composites outperform CuBTC/PAN and PAN alone, especially when methanol is used as the protein precipitator. This superior performance is attributed to the physicochemical compatibility between the analytes' properties, like solubility and polarity, and the MOFs' structural features, including pore flexibility, active site availability, surface polarity, and surface area. The findings underscore MOFs' potential in SPE applications and provide valuable insights into the selectivity and sensitivity of different MOFs towards specific analytes, advancing more efficient targeted extraction methods in biomedical analysis.

Comparison of Thin-Film Capacitor Geometries for the Detection of Volatile Organic Compounds Using a ZIF-8 Affinity Layer

Their chemical diversity, uniform pore sizes, and large internal surface areas make metal-organic frameworks (MOFs) highly suitable for volatile organic compound (VOC) adsorption. This work compares two geometries of capacitive VOC sensors that use the MOF material ZIF-8 as an affinity layer. When using a permeable top electrode (thickness < 25 nm), the metal-insulator-metal (MIM) sandwich configuration exhibits superior sensitivity, an improved detection limit, and a smaller footprint than the conventional interdigitated electrode layout. Moreover, the transduction of VOC adsorption in ZIF-8 via MIM capacitors is more sensitive to polar VOCs and provides better selectivity at high loadings than gravimetric and optical transductions.

MIL-101(Cr)@QCM and MIL-101(Cr)@IDE as Sorbent-Based Humidity Sensors for Indoor Air Monitoring

MIL-101(Cr) films were deposited on the quartz crystal microbalance and interdigitated electrode transductors as humidity sensors. Both devices combine high sensitivity with fast response/recovery times, good repeatability, long-term stability, favorable selectivity versus toluene alongside a dual mode behavior in the optimal domain of humidity for indoor air.

Metal-Organic Framework Coated Devices for Gas Sensing

The demand for monitoring chemical and physical information surrounding, air quality, and disease diagnosis has propelled the development of devices for gas sensing that are capable of translating external stimuli into detectable signals. Metal-organic frameworks (MOFs), possessing particular physiochemical properties with designability in topology, specific surface area, pore size and/or geometry, potential functionalization, and host-guest interactions, reveal excellent development promises for manufacturing a variety of MOF-coated sensing devices for multitudinous applications including gas sensing. The past years have witnessed tremendous progress on the preparation of MOF-coated gas sensors with superior sensing performance, especially high sensitivity and selectivity. Although limited reviews have summarized different transduction mechanisms and applications of MOF-coated sensors, reviews summarizing the latest progress of MOF-coated devices under different working principles would be a good complement. Herein, we summarize the latest advances of several classes of MOF-based devices for gas sensing, i.e., chemiresistive sensors, capacitors, field-effect transistors (FETs) or Kelvin probes (KPs), electrochemical, and quartz crystal microbalance (QCM)-based sensors. The surface chemistry and structural characteristics were carefully associated with the sensing behaviors of relevant MOF-coated sensors. Finally, challenges and future prospects for long-term development and potentially practical application of MOF-coated sensing devices are pointed out.

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.

PLL-Based Readout Circuit for SiC-MOS Capacitor Hydrogen Sensors in Industrial Environments

For proper operation in real industrial conditions, gas sensors require readout circuits which offer accuracy, noise robustness, energy efficiency and portability. We present an innovative, dedicated readout circuit with a phase locked loop (PLL) architecture for SiC-MOS capacitor sensors. A hydrogen detection system using this circuit is designed, simulated, implemented and tested. The PLL converts the MOS nonlinear small-signal capacitance (affected by hydrogen) into an output voltage proportional to the detected gas concentration. Thus, the MOS sensing element is part of the PLL’s voltage-controlled oscillator. This block effectively provides a small AC signal (around 70 mV at 1 MHz) for the sensor and acquires its response. The correct operation of the proposed readout circuit is validated by simulations and experiments. Hydrogen measurements are performed for concentrations up to 1600 ppm. The PLL output exhibited voltage variations close to those discernable from experimental C-V curves, acquired with a semiconductor characterization system, for all investigated MOS sensor samples.

Metal-organic frameworks for advanced transducer based gas sensors: review and perspectives

The development of gas sensing devices to detect environmentally toxic, hazardous, and volatile organic compounds (VOCs) has witnessed a surge of immense interest over the past few decades, motivated mainly by the significant progress in technological advancements in the gas sensing field. A great deal of research has been dedicated to developing robust, cost-effective, and miniaturized gas sensing platforms with high efficiency. Compared to conventional metal-oxide based gas sensing materials, metal-organic frameworks (MOFs) have garnered tremendous attention in a variety of fields, including the gas sensing field, due to their fascinating features such as high adsorption sites for gas molecules, high porosity, tunable morphologies, structural diversities, and ability of room temperature (RT) sensing. This review summarizes the current advancement in various pristine MOF materials and their composites for different electrical transducer-based gas sensing applications. The review begins with a discussion on the overview of gas sensors, the significance of MOFs, and their scope in the gas sensing field. Next, gas sensing applications are divided into four categories based on different advanced transducers: chemiresistive, capacitive, quartz crystal microbalance (QCM), and organic field-effect transistor (OFET) based gas sensors. Their fundamental concepts, gas sensing ability towards various gases, sensing mechanisms, and their advantages and disadvantages are discussed. Finally, this review is concluded with a summary, existing challenges, and future perspectives.

ZIF-8 Nanoparticles Based Electrochemical Sensor for Non-Enzymatic Creatinine Detection

There is a consistent demand for developing highly sensitive, stable, cost-effective, and easy-to-fabricate creatinine sensors as creatinine is a reliable indicator of kidney and muscle-related disorders. Herein, we reported a highly sensitive and selective non-enzymatic electrochemical creatinine sensor via modifying poly(3,4 ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) coated indium tin oxide (ITO) substrate by zeolitic imidazolate framework-8 nanoparticles (ZIF-8 NPs). The topography, crystallinity, and composition of the sensing electrode were characterized by field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The peroxidase-like activity of ZIF-8 nanoparticles enabled it to detect creatinine forming a zinc-creatinine composite. The electrochemical behavior and sensing performance were evaluated by amperometric and impedimetric analysis. The sensor obtained a sufficiently low limit of detection (LOD) of 30 µM in a clinically acceptable linear range (0.05 mM–2.5 mM). The interference study demonstrated high selectivity of the sensor for creatinine concerning other similar biomolecules. The sensing performance of the creatinine sensor was verified in the actual human serum, which showed excellent recovery rates. Hence, the magnificent performance of ZIF-8 based non-enzymatic creatinine sensor validated it as a responsible entity for other complicated renal markers detection.

ZnO@ZIF-8 Core-Shell Structure Gas Sensors with Excellent Selectivity to H2

As the energy crisis becomes worse, hydrogen as a clean energy source is more and more widely used in industrial production and people's daily life. However, there are hidden dangers in hydrogen storage and transportation, because of its flammable and explosive features. Gas detection is the key to solving this problem. High quality sensors with more practical and commercial value must be able to accurately detect target gases in the environment. Emerging porous metal-organic framework (MOF) materials can effectively improve the selectivity of sensors as a result of high surface area and coordinated pore structure. The application of MOFs for surface modification to improve the selectivity and sensitivity of metal oxides sensors to hydrogen has been widely investigated. However, the influence of MOF modified film thickness on the selectivity of hydrogen sensors is seldom studied. Moreover, the mechanism of the selectivity improvement of the sensors with MOF modified film is still unclear. In this paper, we prepared nano-sized ZnO particles by a homogeneous precipitation method. ZnO nanoparticle (NP) gas sensors were prepared by screen printing technology. Then a dense ZIF-8 film was grown on the surface of the gas sensor by hydrothermal synthesis. The morphology, the composition of the elements and the characters of the product were analyzed by X-ray diffraction analysis (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), Brunauer-Emmett-Teller (BET) and differential scanning calorimetry (DSC). It is found that the ZIF-8 film grown for 4 h cannot form a dense core-shell structure. The thickness of ZIF-8 reaches 130 nm at 20 h. Through the detection and analysis of hydrogen (1000 ppm), ethanol (100 ppm) and acetone (50 ppm) from 150 °C to 290 °C, it is found that the response of the ZnO@ZIF-8 sensors to hydrogen has been significantly improved, while the response to ethanol and acetone was decreased. By comparing the change of the response coefficient, when the thickness of ZIF-8 is 130 nm, the gas sensor has a significantly improved selectivity to hydrogen at 230 °C. The continuous increase of the thickness tends to inhibit selectivity. The mechanism of selectivity improvement of the sensors with different thickness of the ZIF-8 films is discussed.

GC-like Graphene-Coated Quartz Crystal Microbalance Sensor with Microcolumns

Many research groups have been interested in the quartz crystal microbalance (QCM)-based gas sensors due to their superb sensitivity originated from direct mass sensing at the ng level. Despite such high sensitivities observed from QCM sensors, their ability to identify gas compounds still needs to be enhanced. Herein, we report a highly facile method that utilizes microcolumns integrated on a QCM gas-responsive system with enhanced chemical selectivity for sensing and ability to identify volatile organic compound single gases. Graphene oxide (GO) flakes are coated on the QCM electrode to substantially increase the adsorption of gas molecules, and periodic polydimethylsiloxane microcolumns with micrometer-scale width and height were installed on the GO-coated QCM electrode. The observed frequency shifts upon sensing of various single gas molecules (such as ethanol, acetone, hexane, etc.) can be analyzed accurately using a simple exponential model. The QCM sensor system with and without the microcolumn both exhibited high detection response values above 50 ng/cm² for sensing of the gases. Notably, the QCM sensor equipped with the microcolumn features gas identification ability, which is observed as distinct diverging behavior of time constants upon detection of different gases caused by the difference in diffusional transfer of molecules through the microcolumns. For example, the difference in the calculated time constant between ethanol and acetone increased from 22.6 to 92.1 s after installation of the microcolumn. This approach provides an easy and efficient method for identification of single gases, and it may be applied in various advanced sensor systems to enhance their gas selectivity.

Volatile organic compounds gas sensor based on quartz crystal microbalance for fruit freshness detection: A review

In this review article, the state of the art of gas sensors based on quartz crystal microbalance (QCM) for fruit freshness detection is overviewed from the aspects of development history, working principle, selection and modification of sensitive materials, and volatile organic compounds detection of fruits. According to the characteristics of respiratory intensity at the stage of fruit ripening, fruits can be divided into respiration climacteric fruits and non-climacteric fruits. In recent years, research has mainly focused on respiration climacteric fruits, such as bananas and mangoes, etc., while related studies on non-climacteric fruits have been rarely reported, except for citrus fruits. The preparation methods and structure design of sensitive materials based on physical/chemical adsorption mechanisms are further discussed according to the odor components that affect the freshness of fruits, namely alkenes, esters, aldehydes and alcohols.

Realization of an Ultrasensitive and Highly Selective OFET NO2 Sensor: The Synergistic Combination of PDVT-10 Polymer and Porphyrin-MOF

Organic field-effect transistors (OFETs) are emerging as competitive candidates for gas sensing applications due to the ease of their fabrication process combined with the ability to readily fine-tune the properties of organic semiconductors. Nevertheless, some key challenges remain to be addressed, such as material degradation, low sensitivity, and poor selectivity toward toxic gases. Appropriately, a heterojunction combination of different sensing layers with multifunctional capabilities offers great potential to overcome these problems. Here, a novel and highly sensitive receptor layer is proposed encompassing a porous 3D metal-organic framework (MOF) based on isostructural-fluorinated MOFs acting as an NO₂ specific preconcentrator, on the surface of a stable and ultrathin PDVT-10 organic semiconductor on an OFET platform. Here, with this proposed combination we have unveiled an unprecedented 700% increase in sensitivity toward NO₂ analyte in contrast to the pristine PDVT-10. The resultant combination for this OFET device exhibits a remarkable lowest detection limit of 8.25 ppb, a sensitivity of 680 nA/ppb, and good stability over a period of 6 months under normal laboratory conditions. Further, a negligible response (4.232 nA/%RH) toward humidity in the range of 5%-90% relative humidity was demonstrated using this combination. Markedly, the obtained results support the use of the proposed novel strategy to achieve an excellent sensing performance with an OFET platform.

Realization of an Ultrasensitive and Highly Selective OFET NO2 Sensor: The Synergistic Combination of PDVT-10 Polymer and Porphyrin-MOF

Organic field-effect transistors (OFETs) are emerging as competitive candidates for gas sensing applications due to the ease of their fabrication process combined with the ability to readily fine-tune the properties of organic semiconductors. Nevertheless, some key challenges remain to be addressed, such as material degradation, low sensitivity, and poor selectivity toward toxic gases. Appropriately, a heterojunction combination of different sensing layers with multifunctional capabilities offers great potential to overcome these problems. Here, a novel and highly sensitive receptor layer is proposed encompassing a porous 3D metal-organic framework (MOF) based on isostructural-fluorinated MOFs acting as an NO₂ specific preconcentrator, on the surface of a stable and ultrathin PDVT-10 organic semiconductor on an OFET platform. Here, with this proposed combination we have unveiled an unprecedented 700% increase in sensitivity toward NO₂ analyte in contrast to the pristine PDVT-10. The resultant combination for this OFET device exhibits a remarkable lowest detection limit of 8.25 ppb, a sensitivity of 680 nA/ppb, and good stability over a period of 6 months under normal laboratory conditions. Further, a negligible response (4.232 nA/%RH) toward humidity in the range of 5%-90% relative humidity was demonstrated using this combination. Markedly, the obtained results support the use of the proposed novel strategy to achieve an excellent sensing performance with an OFET platform.

Methanol and Humidity Capacitive Sensors Based on Thin Films of MOF Nanoparticles

The successful development of modern gas sensing technologies requires high sensitivity and selectivity coupled to cost effectiveness, which implies the necessity to miniaturize devices while reducing the amount of sensing material. The appealing alternative of integrating nanoparticles of a porous metal-organic framework (MOF) onto capacitive sensors based on interdigitated electrode (IDE) chips is presented. We report the deposition of MIL-96(Al) MOF thin films via the Langmuir-Blodgett (LB) method on the IDE chips, which allowed the study of their gas/vapor sensing properties. First, sorption studies of several organic vapors like methanol, toluene, chloroform, etc. were conducted on bulk MOF. The sorption data revealed that MIL-96(Al) presents high affinity toward water and methanol. Later on, ordered LB monolayer films of MIL-96(Al) particles of ∼200 nm were successfully deposited onto IDE chips with homogeneous coverage of the surface in comparison to conventional thin film fabrication techniques such as drop-casting. The sensing tests showed that MOF LB films were selective for water and methanol, and short response/recovery times were achieved. Finally, chemical vapor deposition (CVD) of a porous thin film of Parylene C (thickness ∼250-300 nm) was performed on top of the MOF LB films to fabricate a thin selective layer. The sensing results showed an increase in the water selectivity and sensitivity, while those of methanol showed a huge decrease. These results prove the feasibility of the LB technique for the fabrication of ordered MOF thin films onto IDE chips using very small MOF quantities.

Investigation on Mass Sensitivity of N-M Type Electrode Quartz Crystal Microbalance

Mass sensitivity plays a crucial role in the practical application of quartz crystal microbalances (QCMs)-based quantitative analysis. n-m type QCMs have many applications, so it is necessary to clarify the relationship between the mass sensitivity and the electrode of the n-m type QCM. The performance of gold-plated films with different electrodes was studied by theoretical calculation and experiment. The results show that the mass sensitivity on the surface of the n electrode and the surface of the m electrode are essentially the same. Meanwhile, the mass sensitivity of n-m type QCMs varies with the diameter of the n and m electrodes. When the diameter of the n electrode is close to half the diameter of the m electrode, mass sensitivity is at maximum value. These results are important for the further designs and applications of n-m type QCMs.

A silver nanoparticle-anchored UiO-66(Zr) metal–organic framework (MOF)-based capacitive H2S gas sensor

Metal–organic frameworks anchored with metal oxide nanoparticles for the detection of H₂S gas with enhanced sensitivity.