Metal–Organic Frameworks for Resonant-Gravimetric Detection of Trace-Level Xylene Molecules

Tailoring crystal facets of metal-organic frameworks to enhance sensing performance for aromatic vapors detection

It is well known that metals and metal oxides with different crystal facets exhibit varying sensitivity in gas sensors, but this strategy is rarely used in metal-organic frameworks (MOFs). Herein, we proved for the first time that Cu metal-organic with high energy crystal facets (Cu-MOF-74-300) shows a much higher sensitivity than the low energy crystal facets (Cu-MOF-74-110), with a up to 2 times response more than Cu-MOF-74-110 and ultra-low limit of detection (LOD) of 68 ppb to toluene vapors. In addition, this strategy was further demonstrated on MOF-14 and HKUST-1, which are also Cu-centered and exhibit clear recognition effects on benzene and xylene, respectively. Furthermore, the Cu-MOF-74-300 shows high selectivity (92 %) and excellent stability, with a minor reduction in response of less than 1.9 % after 6 months. Additionally, the sensitive mechanism is explored by DFT and GCMC methods. The simulations reveal that Cu-MOF-74 includes two adsorption sites, one is Cu site with adsorption enthalpy (ΔH) of -59.04 kJ/mol, another is the benzene ring site (ΔH is -67.50 kJ/mol) in the ligand. The difference in charge densities reveal that the Cu2+ and benzene ring on the surface of Cu-MOF-74-300 enables synergistic sensing, leading to more electron transfer in toluene than that of Cu-MOF-74-110 (only benzene ring site). Finally, based on the temperature-varying experiments, the experimental adsorption energy (-51.35 kJ/mol) was obtained by calculations. Combined quasi-in-situ XPS, implying Cu is the critical role to the improvement of Cu-MOF-74-300 sensitivity to toluene.

Solid-State Gas Sensors with Ni-Based Sensing Materials for Highly Selective Detecting NOx

Precise monitoring of NOx concentrations in nitric oxide delivery systems is crucial to ensure the safety and well-being of patients undergoing inhaled nitric oxide (iNO) therapy for pulmonary arterial hypertension. Currently, NOx sensing in commercialized iNO instruments predominantly relies on chemiluminescence sensors, which not only drives up costs but also limits their portability. Herein, we developed solid-state gas sensors utilizing Ni-based sensing materials for effectively tracking the levels of NO and NO2 in the NO delivery system. These sensors comprised of NiO-SE or (NiFe2O4 + 30 wt.% Fe2O3)-SE vs. Mn-based RE demonstrated high selectivity toward 100 ppm NO under the interference of 10 ppm NO2 or 3 ppm NO2 under the interference of 100 ppm NO, respectively. Meanwhile, excellent stability, repeatability, and humidity resistance were also verified for the proposed sensors. Sensing mechanisms were thoroughly investigated through assessments of adsorption capabilities and electrochemical reactivity. It turns out that the superior electrochemical reactivity of NiO toward NO, alongside the NO2 favorable adsorption characteristics of (NiFe2O4 + 30 wt.% Fe2O3), is the primary reason for the high selectivity to NOx. These findings indicate a bright future for the application of these NOx sensors in innovative iNO treatment technologies.

A Low-Cost Method for Producing User-Specified Concentrations of VOCs

Monitoring volatile organic compounds is critical to mitigate the risks they pose to human health and the environment. Developing technologies for detection of VOCs requires methods to produce the desired concentrations for benchmarking. Herein, we present a simple, inexpensive, and flexible platform capable of producing user-specified concentrations of a gas-phase analyte for the purpose of fine-tuning and benchmarking VOC detection technologies. This technology, the gas-phase dilution apparatus (GPDA), is built around two mass flow controllers that mix precise flows of the analyte and dilution gas. We used a custom APPI-MS configuration as well as a commercial photoionization detector to detect benzene and toluene. These two detection methods were employed to assess the linear output of concentrations over a combined range of 1-20 000 ppbV which yielded average R2 values of 0.9980 and 0.9988 for benzene and toluene, respectively. Additionally, output stability was assessed at 10 ppbV, 1 ppmV, and 5 ppmV of benzene and toluene. Six measurements were averaged over the course of 30 min, and RSDs were below 2% for all three concentrations of both compounds. These results suggest that GPDA is capable of producing precise and repeatable concentrations of gas-phase analytes.

Ultrafast and Parts-per-Billion-Level MEMS Gas Sensors by Hetero-Interface Engineering of 2D/2D Cu-TCPP@ZnIn2S4 with Enriched Surface Sulfur Vacancies

The primary challenge for resonant-gravimetric gas sensors is the synchronous improvement of the sensitivity and response time, which is restricted by low adsorption capacity and slow mass transfer in the sensing process and remains a great challenge. In this study, a novel 2D/2D Cu-TCPP@ZnIn2S4 composite is successfully constructed, in which Cu-TCPP MOF is used as a core substrate for the growth of 2D ultrathin ZnIn2S4 nanosheets with well-defined {0001} crystalline facets. The Cu-TCPP@ZnIn2S4 sensor exhibited high sensitivity (1.5 Hz@50 and 2.3 Hz@100 ppb), limit of detection (LOD: 50 ppb), and ultrafast (9 s @500 ppb) detection of triethylamine (TEA), which is the lowest LOD and the fastest sensor among the reported TEA sensors at room temperature, tackling the bottleneck for the ultrafast detection of the resonant-gravimetric sensor. These above results provide an innovative and easily achievable pathway for the synthesis of heterogeneous structure sensing materials.

Thermodynamically and Kinetically Enhanced Benzene Vapor Sensor Based on the Cu-TCPP-Cu MOF with Extremely Low Limit of Detection

As a carcinogenic and highly neurotoxic hazardous gas, benzene vapor is particularly difficult to be distinguished in BTEX (benzene, toluene, ethylbenzene, xylene) atmosphere and be detected in low concentrations due to its chemical inertness. Herein, we develop a depth-related pore structure in Cu-TCPP-Cu to thermodynamically and kinetically enhance the adsorption of benzene vapor and realize the detection of ultralow-temperature benzene gas. We find that the in-plane π electronic nature and proper pore sizes in Cu-TCPP-Cu can selectively induce the adsorption and diffusion of BTEX. Interestingly, the theoretical calculations (including density functional theory (DFT) and grand canonical Monte Carlo (GCMC) simulations) exhibit that benzene molecules are preferred to adsorb and array as a consecutive arrangement mode in the Cu-TCPP-Cu pore, while the TEX (toluene, ethylbenzene, xylene) dominate the jumping arrangement model. The differences in distribution behaviors can allow adsorption and diffusion of more benzene molecules within limited room. Furthermore, the optimal pore-depth range (60-65 nm) of Cu-TCPP-Cu allows more exposure of active sites and hinders the gas-blocking process. The optimized sensor exhibits ultrahigh sensitivity to benzene vapor (155 Hz/μg@1 ppm), fast response time (less than 10 s), extremely low limit of detection (65 ppb), and excellent selectivity (83%). Our research thus provides a fundamental understanding to design and optimize two-dimensional metal-organic framework (MOF)-based gas sensors.

Advances and Applications of Metal-Organic Frameworks (MOFs) in Emerging Technologies: A Comprehensive Review

Metal-organic frameworks (MOFs) that are the wonder material of the 21st century consist of metal ions/clusters coordinated to organic ligands to form one- or more-dimensional porous structures with unprecedented chemical and structural tunability, exceptional thermal stability, ultrahigh porosity, and a large surface area, making them an ideal candidate for numerous potential applications. In this work, the recent progress in the design and synthetic approaches of MOFs and explore their potential applications in the fields of gas storage and separation, catalysis, magnetism, drug delivery, chemical/biosensing, supercapacitors, rechargeable batteries and self-powered wearable sensors based on piezoelectric and triboelectric nanogenerators are summarized. Lastly, this work identifies present challenges and outlines future opportunities in this field, which can provide valuable references.

Microgravimetric Modeling-A New Method for Extracting Adsorption Parameters of Functionalized MIL-101(Cr)

As a volatile air pollutant, formaldehyde can enter people's living environment through interior decoration, furniture and paint, causing serious harm to human health. Therefore, it is necessary to develop a sensor for the real-time detection of formaldehyde in low concentrations. According to the chemical interaction between amino groups and formaldehyde, a MIL-101(Cr) aminated-material-based formaldehyde cantilever sensor was prepared, of which ethylenediamine- functionalized MIL-101(Cr) named ED-MIL-101(Cr)) showed the best gas sensing performance. Using quasi-in situ infrared spectroscopy, ED-MIL-101(Cr) was found bound to formaldehyde through a Schiff base. The adsorption enthalpy of formaldehyde-bound ED-MIL-101(Cr) was -52.6 kJ/mol, which corresponds to weak chemical adsorption, so the material showed good selectivity. In addition, ED-MIL-101(Cr) has the most active sites, so its response value to formaldehyde is larger and it takes longer to reach saturation adsorption than bare MIL-101(Cr). Through the research on the gas sensing performance of functionalized MIL-101(Cr) material, we found that it has a strong application potential in the field of formaldehyde monitoring, and the material performance can be quantitatively and accurately evaluated through combining calculation and experimentation for understanding the gas sensing mechanism.

Mesoporous-Structure MOF-14-Based QCM p-Xylene Gas Sensor

In this work, a facile synthesis method was adopted to synthesize MOF-14 with mesoporous structure. The physical properties of the samples were characterized by PXRD, FESEM, TEM and FT-IR spectrometry. By coating the mesoporous-structure MOF-14 on the surface of a quartz crystal microbalance (QCM), the fabricated gravimetric sensor exhibits high sensitivity to p-toluene vapor even at trace levels. Additionally, the limit of detection (LOD) of the sensor obtained experimentally is lower than 100 ppb, and the theoretical detection limit is 57 ppb. Furthermore, good gas selectivity and fast response (15 s) and recovery (20 s) abilities are also illustrated along with high sensitivity. These sensing data indicate the excellent performance of the fabricated mesoporous-structure MOF-14-based p-xylene QCM sensor. On the basis of temperature-varying experiments, an adsorption enthalpy of -59.88 kJ/mol was obtained, implying the existence of moderate and reversible chemisorption between MOF-14 and p-xylene molecules. This is the crucial factor that endows MOF-14 with exceptional p-xylene-sensing abilities. This work has proved that MOF materials such as MOF-14 are promising in gravimetric-type gas-sensing applications and worthy of future study.

New Application of Quartz Crystal Microbalance: A Minimalist Strategy to Extract Adsorption Enthalpy

The capture and separation of CO₂ is an important means to solve the problem of global warming. MOFs (metal-organic frameworks) are considered ideal candidates for capturing CO₂, where the adsorption enthalpy is a crucial indicator for the screening of materials. For this purpose, we propose a new minimalist solution using QCM (quartz crystal microbalance) to extract the CO₂ adsorption enthalpy on MOFs. Three kinds of MOFs with different properties, sizes and morphologies were employed to study the adsorption enthalpy of CO₂ using a QCM platform and a commercial gas sorption analyzer. A Gaussian simulation calculation and previously data reported were used for comparison. It was found that the measuring errors were between 5.4% and 6.8%, proving the reliability and versatility of our new method. This low-cost, easy-to-use, and high-accuracy method will provide a rapid screening solution for CO₂ adsorption materials, and it has potential in the evaluation of the adsorption of other gases.

Interpenetrated metal-organic frameworks with enhanced photoluminescence for selective recognition of m-xylene from xylene isomers

Two novel luminescent metal-organic frameworks (MOFs), [Zn₃(TCA)₂(BPB)₂]_n (DZU-101, where H₃TCA = 4,4',4''-tricarboxyltriphenylamine and BPB = 1,4-bis(pyrid-4-yl)benzene) and [Zn₃(TCA)₂(BPB)DMA]_n (DZU-102), based on the same ligands and metal ions were synthesized by regulating the amount of water in the solvothermal reaction system. Structural analyses show that the two MOFs have pillar-layered frameworks with Zn₃ clusters connected by the TCA^3- and BPB ligands. Interestingly, DZU-102 possessed a two-fold interpenetrated framework distinct from the individual network of DZU-101. As a result, DZU-102 showed a visual fluorescence color change from chartreuse to azure in m-xylene, while the fluorescence color was turquoise in p-/o-xylene with no change. Furthermore, compared with p/o-xylene, the fluorescence emission peak of DZU-102 in m-xylene suspension produced an obvious blue shift. Moreover, selective fluorescence sensing experiments were also carried out, which demonstrated that the degree of peak shift was related to the concentration of m-xylene, indicating the potential application of DZU-102 in fluorescence sensing of m-xylene from xylene isomers and further revealed the application of structural interpenetration for luminescence tuning of MOFs.

Metal-Organic Framework Based Gas Sensors

The ever-increasing concerns over indoor/outdoor air quality, industrial gas leakage, food freshness, and medical diagnosis require miniaturized gas sensors with excellent sensitivity, selectivity, stability, low power consumption, cost-effectiveness, and long lifetime. Metal-organic frameworks (MOFs), featuring structural diversity, large specific surface area, controllable pore size/geometry, and host-guest interactions, hold great promises for fabricating various MOF-based devices for diverse applications including gas sensing. Tremendous progress has been made in the past decade on the fabrication of MOF-based sensors with elevated sensitivity and selectivity toward various analytes due to their preconcentrating and molecule-sieving effects. Although several reviews have recently summarized different aspects of this field, a comprehensive review focusing on MOF-based gas sensors is absent. In this review, the latest advance of MOF-based gas sensors relying on different transduction mechanisms, for example, chemiresistive, capacitive/impedimetric, field-effect transistor or Kelvin probe-based, mass-sensitive, and optical ones are comprehensively summarized. The latest progress for making large-area MOF films essential to the mass-production of relevant gas sensors is also included. The structural and compositional features of MOFs are intentionally correlated with the sensing performance. Challenges and opportunities for the further development and practical applications of MOF-based gas sensors are also given.

A photoprogrammable electronic nose with switchable selectivity for VOCs using MOF films

Advanced analytical applications require smart materials and sensor systems that are able to adapt or be configured to specific tasks. Based on reversible photochemistry in nanoporous materials, we present a sensor array with a selectivity that is reversibly controlled by light irradiation. The active material of the sensor array, or electronic nose (e-nose), is based on metal-organic frameworks (MOFs) with photoresponsive fluorinated azobenzene groups that can be optically switched between their trans and cis state. By irradiation with light of different wavelengths, the trans-cis ratio can be modulated. Here we use four trans-cis values as defined states and employ a four-channel quartz-crystal microbalance for gravimetrically monitoring the molecular uptake by the MOF films. We apply the photoprogrammable e-nose to the sensing of different volatile organic compounds (VOCs) and analyze the sensor array data with simple machine-learning algorithms. When the sensor array is in a state with all sensors either in the same trans- or cis-rich state, cross-sensitivity between the analytes occurs and the classification accuracy is not ideal. Remarkably, the VOC molecules between which the sensor array shows cross-sensitivity vary by switching the entire sensor array from trans to cis. By selectively programming the e-nose with light of different colors, each sensor exhibits a different isomer ratio and thus a different VOC affinity, based on the polarity difference between the trans- and cis-azobenzenes. In such photoprogrammed state, the cross-sensitivity is reduced and the selectivity is enhanced, so that the e-nose can perfectly identify the tested VOCs. This work demonstrates for the first time the potential of photoswitchable and thus optically configurable materials as active sensing material in an e-nose for intelligent molecular sensing. The concept is not limited to QCM-based azobenzene-MOF sensors and can also be applied to diverse sensing materials and photoswitches.

Anatase porous titania nanosheets for resonant-gravimetric detection of ppb-level NO2 at room-temperature

The trace-level detection of harmful NO2 gas at room-temperature is very important for environmental protection and public health. This paper reports the resonant-gravimetric detection of ppb-level NO2 at room-temperature using two-dimensional porous TiO2 nanosheets (PTNSs) as highly active sensing materials. They are synthesized by a facile high-temperature calcination approach based on a graphene oxide self-sacrificial template. The PTNS sample prepared at 500 °C (TiO2-500 °C) show an anatase structure, while the sample prepared at 800 °C (TiO2-800 °C) contains an impurity rutile phase. By loading pure anatase PTNSs onto resonant microcantilevers, the sensors exhibit high sensitivity to NO2 gas with a limit of detection as low as 15 ppb. Compared with the TiO2-800 °C sample, the much higher sensitivity of the TiO2-500 °C sample can be attributed to the bigger adsorption enthalpy (-ΔH°) of pure anatase TiO2 to NO2 gas molecules (21.7 and 57.8 kJ mol-1, respectively). Density functional theory calculations further demonstrate that the existence of the rutile impurity phase in the TiO2-800 °C sample results in its significantly decreased adsorption activity to NO2. This work approves the great application potential of anatase PTNSs for the highly sensitive resonant-gravimetric detection of NO2 gas at room-temperature.

Integrated Resonant Micro/Nano Gravimetric Sensors for Bio/Chemical Detection in Air and Liquid

Resonant micro/nanoelectromechanical systems (MEMS/NEMS) with on-chip integrated excitation and readout components, exhibit exquisite gravimetric sensitivities which have greatly advanced the bio/chemical sensor technologies in the past two decades. This paper reviews the development of integrated MEMS/NEMS resonators for bio/chemical sensing applications mainly in air and liquid. Different vibrational modes (bending, torsional, in-plane, and extensional modes) have been exploited to enhance the quality (Q) factors and mass sensing performance in viscous media. Such resonant mass sensors have shown great potential in detecting many kinds of trace analytes in gas and liquid phases, such as chemical vapors, volatile organic compounds, pollutant gases, bacteria, biomarkers, and DNA. The integrated MEMS/NEMS mass sensors will continuously push the detection limit of trace bio/chemical molecules and bring a better understanding of gas/nanomaterial interaction and molecular binding mechanisms.

Inorganic-Diverse Nanostructured Materials for Volatile Organic Compound Sensing

Environmental pollution related to volatile organic compounds (VOCs) has become a global issue which attracts intensive work towards their controlling and monitoring. To this direction various regulations and research towards VOCs detection have been laid down and conducted by many countries. Distinct devices are proposed to monitor the VOCs pollution. Among them, chemiresistor devices comprised of inorganic-semiconducting materials with diverse nanostructures are most attractive because they are cost-effective and eco-friendly. These diverse nanostructured materials-based devices are usually made up of nanoparticles, nanowires/rods, nanocrystals, nanotubes, nanocages, nanocubes, nanocomposites, etc. They can be employed in monitoring the VOCs present in the reliable sources. This review outlines the device-based VOC detection using diverse semiconducting-nanostructured materials and covers more than 340 references that have been published since 2016.

Surface Engineering of Metal-Organic Framework Prepared on Film Bulk Acoustic Resonator for Vapor Detection

Gravimetric resonators based on micro/nanoelectromechanical systems (M/NEMS) are potential candidates in developing smaller, less expensive, and higher-performance gas sensors. Metal-organic frameworks (MOFs) with high surface areas have recently come into focus as advanced nanoporous sensitive materials in microgravimetric gas sensors. The surface of MOFs on those sensors is critical in offering water stability and varying absorption behaviors. However, the influences of the surface on sensing performance are less explored and the strategy to tune surface properties of MOFs mounted on gravimetric resonators is still rare. In this paper, a straightforward strategy to engineer surface properties of MOFs, specifically Cu₃(benzenetricarboxylate)₂ (known as HKUST-1), is reported and the surface hydrophilicity/hydrophobicity of HKUST-1 is tuned by chemical vapor deposition combined with monolayer self-assembly. It was found that the hybrid inorganic and organic surface engineering strategy not only preserves the absorption capacity of inner MOFs but also significantly enhances the sensor's stability toward water.

Microporous Triptycene-Based Affinity Materials on Quartz Crystal Microbalances for Tracing of Illicit Compounds

Triptycene-based organic molecules of intrinsic microporosity (OMIMs) with extended functionalized π-surfaces are excellent materials for gas sorption and separation. In this study, the affinities of triptycene-based OMIM affinity materials on 195 MHz high-fundamental-frequency quartz crystal microbalances (HFF-QCMs) for hazardous and illicit compounds such as piperonal and (-)-norephedrine were determined. Both new and existing porous triptycene-based affinity materials were investigated, resulting in very high sensitivities and selectivities that could be applied for sensing purposes. Remarkable results were found for safrole - a starting material for illicit compounds such as ecstasy. A systematic approach highlights the effects of different size of π-surfaces of these affinity materials, allowing a classification of the properties that might be optimal for the design of future OMIM-based affinity materials.

Functional metal–organic frameworks as effective sensors of gases and volatile compounds

This review summarizes the recent advances of metal organic framework (MOF) based sensing of gases and volatile compounds.

Sensing organic analytes by metal–organic frameworks: a new way of considering the topic

In this review, our goal is comparison of advantageous and disadvantageous of MOFs about signal-transduction in different instrumental methods for detection of different categories of organic analytes.

In situ construction of metal–organic framework (MOF) UiO-66 film on Parylene-patterned resonant microcantilever for trace organophosphorus molecules detection

UiO-66 film is directly grown on the sensing area of a resonant microcantilever for toxic organophosphorus molecules detection.

Metal-organic framework and Tenax-TA as optimal sorbent mixture for concurrent GC-MS analysis of C1 to C5 carbonyl compounds

We report a multi adsorbent-based method using combinations of metal-organic frameworks (MOFs) and a commercial sorbent Tenax-TA for sampling and thermal desorption (TD) gas chromatography-mass spectrometry (GC-MS) quantification of mixtures of six (C1 to C5) aldehydes. The feasibility of this approach was demonstrated along with the optical analytical conditions for maximum recovery. Optimal TD conditions for adsorption and desorption of aldehydes using MOF-5 (Zn-based MOF)+ Tenax-TA were determined as -25 °C and 150 °C, respectively (purge volume: 100 ml). These conditions yielded good linearity (R² = 0.997), precision, and high sensitivity. Analysis of the aldehyde mixtures yielded slightly smaller R² values than the analysis of single species. Additionally, the performance of MOF-5+ Tenax-TA was compared with other combinations comprising of Cu-based MOF-199 and Zr-based MOF of UiO-66 topology. The results of the theoretical modelling analyses propose simultaneous interaction of the C=O group of aldehydes with open metal sites of the studied MOFs and van der Waals interaction of hydrocarbon "tail" of aldehydes with linkers of MOFs. The combined interactions significantly increased the enthalpy (eV/molecule) of formaldehyde adsorption on MOF. Our findings unravel a potential way to extend the application of GC-based detection toward concurrent analysis of organic molecules of variable sizes.