MOFs for the Sensitive Detection of Ammonia: Deployment of fcu-MOF Thin Films as Effective Chemical Capacitive Sensors

Novel magnetic amine functionalized carbon nanotube/metal-organic framework nanocomposites: From green ultrasound-assisted synthesis to detailed selective pollutant removal modelling from binary systems

Herein, magnetic amine functionalized carbon nanotube (NH₂-CNT/Fe₂O₃)-zeolitic imidazolate framework-8 (ZIF-8) nanocomposites (NH₂-CNT/Fe₂O₃/ZIF-8: NCFZ) with different amounts of NH₂-CNT/Fe₂O₃ (5, 10, and 15 wt% denoted as NCFZ-5, NCFZ-10, and NCFZ-15) were synthesized. The synthesized nanomaterials including ZIF-8, Fe₂O₃, CNT/Fe₂O₃, NH₂-CNT/Fe₂O₃, CNT/Fe₂O₃/ZIF-8, NCFZ-5, NCFZ-10, and NCFZ-15 were characterized using BET, TEM, XRD, SEM, FTIR, VSM and zeta potential. The synthesized nanomaterials were applied for selective removing cationic dyes (MG: Malachite Green and RhB: Rhodamine B) from a binary system. Response surface methodology (RSM) and artificial neural networks (ANN) were used for optimizing dye removal. The BET data showed that the surface area of nanocomposite (NH₂-CNT/Fe₂O₃/ZIF-8: 1659 m²/g) was higher than that of pure ZIF-8 (1485 m²/g). Contaminant removal obeyed the Freundlich isotherm and pseudo-second order kinetic models. The optimum adsorption condition predicted by RSM was pH = 6, dye concentration = 25 mg/L, Dosage = 0.004 g and at time = 145 min. The outputs of ANN model well overlapped with the experimental data. The binary system dye removal data indicated the synthesized nanocomposite with recycling and regeneration ability could be used for treating wastewater.

A Highly Proton-Conductive 3D Ionic Cadmium-Organic Framework for Ammonia and Amines Impedance Sensing

Lately, the progressive study of metal-organic frameworks (MOFs) for the detection of ammonia and amines has made infusive achievements. Nevertheless, the investigation of proton-conductive MOFs used to detect the low concentrations of ammonia and amine gases at different relative humidities (RHs) at room temperature is relatively restricted. Herein, by solvothermal reaction of Cd(NO₃)₂ with 2-methyl-1 H-imidazole-4,5-dicarboxylic acid (H₃MIDC), a three-dimensional ionic MOF {Na[Cd(MIDC)]} _n (1) bearing ordered one-dimensional channels was successfully synthesized. Our research indicates that the uncoordination carboxylate sites are beneficial to proton transfer and the recognition of ammonia and amine compounds. The optimized proton conductivity of 1 reaches a high value of 1.04 × 10^-3 S·cm^-1 (100 °C, 98% RH). The room temperature sensing properties of ammonia and amine gases were explored under 68, 85, and 98% RHs, respectively. Satisfactorily, the detection limits of MOF 1 toward ammonia, methylamine, dimethylamine, trimethylamine, and ethylamine are 0.05, 0.1, 0.5, 1, and 4 ppm, respectively, which is one of the best room-temperature sensors for ammonia among previous sensors based on proton-conductive MOFs. The proton conducting and sensing mechanisms were highlighted as well.

Multifunctional drug carrier on the basis of 3d–4f Fe/La-MOFs for drug delivery and dual-mode imaging

Multifunctional drug carriers for simultaneous imaging and drug delivery have emerged as an important new direction for the treatment of cancer.

Detection of NO2 Down to One ppb Using Ion-in-Conjugation-Inspired Polymer

Nitrogen dioxide (NO₂ ) emission has severe impact on human health and the ecological environment and effective monitoring of NO₂ requires the detection limit (limit of detection) of several parts-per-billion (ppb). All organic semiconductor-based NO₂ sensors fail to reach such a level. In this work, using an ion-in-conjugation inspired-polymer (poly(3,3'-diaminobenzidine-squarine, noted as PDBS) as the sensory material, NO₂ can be detected as low as 1 ppb, which is the lowest among all reported organic NO₂ sensors. In addition, the sensor has high sensitivity, good reversibility, and long-time stability with a period longer than 120 d. Theoretical calculations reveal that PDBS offers unreacted amine and zwitterionic groups, which can offer both the H-bonding and ion-dipole interaction to NO₂ . The moderate binding energies (≈0.6 eV) offer high sensitivity, selectivity as well as good reversibility. The results demonstrate that the ion-in-conjugation can be employed to greatly improve sensitivity and selectivity in organic gas sensors by inducing both H-bonding and ion-dipole attraction.

Ce-doped UiO-67 nanocrystals with improved adsorption property for removal of organic dyes

In this study, we report the synthesis of Ce-doped UiO-67 nanocrystals via one-step hydrothermal method and their potential use for waste water treatment to remove organic dyes. The as-prepared samples were characterized by using SEM, TEM, FT-IR, XRD, XPS and TG techniques and the results demonstrate the formation of the framework structure of the Ce-doped UiO-67. The adsorption study of the material shows that the Ce-doped UiO-67 nanoparticles preferentially adsorb the cationic dyes such as rhodamine B and malachite green rather than the anionic dyes such as methyl orange. Adsorption capacities are 754.4, 589.2 and 191.6 mg g⁻¹ for rhodamine B, malachite green and methyl orange respectively. Based on its zeta potential and adsorption isotherm, it is understood that Ce doping increases its electrostatic interactions, and as a consequence, improves the adsorption capacity for cationic dyes. In addition, it is found that a pseudo-second-order kinetics and the Langmuir isothermal model were suitable for describing the adsorption behavior of cationic dyes onto the Ce-doped UiO-67.

The Ce-doped UiO-67 nanocrystals were successfully synthesized via a one-step hydrothermal method. Ce doping increases the negative charge on the surface of the material, thus the adsorbent exhibits high adsorption capacity to cationic dyes.

Phthalocyanine Photoregeneration for Low Power Consumption Chemiresistors

It is well-known that the applicability of phthalocyanine chemiresistors suffers from long recovery time after NO₂ exposure. This circumstance enforces the necessity to operate the sensors at elevated temperatures (150-200 °C), which shortens the sensor lifetime and increases its power consumption (regardless, a typical measurement period is longer than 15 min). In this paper, we propose a new method for fast and effective recovery by UV-vis illumination at a low temperature (55 °C). The method is based on short illumination following short NO₂ exposure. To support and optimize the method, we investigated the effects of light in the wavelength and intensity ranges of 375-850 nm and 0.2-0.8 mW/mm², respectively, on the rate of NO₂ desorption from the phthalocyanine sensitive layer during the recovery period. This investigation was carried out for a set of phthalocyanine materials (ZnPc, CuPc, H₂Pc, PbPc, and FePc) operating at slightly elevated temperatures (55-100 °C) and was further supported by the analysis of UV-vis and FTIR spectral changes. We found out that the light with the wavelength shorter than 550 nm significantly accelerates the NO₂ desorption from ZnPc, CuPc, and FePc, and allows bringing the measurement period under 2 min and decreasing the sensor power consumption by 75%. Possible mechanisms of the light-stimulated desorption are discussed.

Construction of Anti-Ultraviolet "Shielding Clothes" on Poly( p-phenylene benzobisoxazole) Fibers: Metal Organic Framework-Mediated Absorption Strategy

A metal-organic framework (MOF)-mediated adsorption strategy is first developed for improving the anti-ultraviolet (UV) properties of poly( p-phenylene benzobisoxazole) (PBO) fibers. In this work, UIO-66 was successfully anchored onto the surface of PBO fibers by one-step microwave-assisted heating method. The experimental results showed an obviously enhanced surface energy (91.1%), roughness (268.4%), interfacial shear strength (49.0%), and anti-UV properties (66.7%) compared to pristine PBO fibers. The anti-UV dye (tartrazine) was further immobilized onto the surface of PBO fibers via an adsorption strategy mediated by UIO-66. Interestingly, the PBO@tartrazine fibers demonstrated superior anti-UV performance (further up to 81.5%) compared to PBO@UIO-66 fibers. The extraordinary anti-UV properties of PBO@tartrazine fibers could be rationally ascribed to the synergistic effects of UIO-66 and tartrazine molecules. Considering the diversities and functionalities of MOFs and targeted materials, our work indicates that the MOF-mediated adsorption strategy would promisingly endow PBO fibers with other desired performance and applications such as solar-thermal transition and self-healing abilities.

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

We present a comparative study of two types of sensor with different transduction techniques but coated with the same sensing material to determine the effect of the transduction mechanism on the sensing performance of sensing a target analyte. For this purpose, interdigitated electrode (IDE)-based capacitors and quartz crystal microbalance (QCM)-based resonators were coated with a zeolitic⁻imidazolate framework (ZIF-8) metal⁻organic framework thin films as the sensing material and applied to the sensing of the volatile organic compound acetone. Cyclic immersion in methanolic precursor solutions technique was used for depositing the ZIF-8 thin films. The sensors were exposed to various acetone concentrations ranging from 5.3 to 26.5 vol % in N₂ and characterized/compared for their sensitivity, hysteresis, long-term and short-term stability, selectivity, detection limit, and effect of temperature. Furthermore, the IDE substrates were used for resistive transduction and compared using capacitive transduction.

Stabilizing Metastable Polymorphs of Metal-Organic Frameworks via Encapsulation of Graphene Oxide and Mechanistic Studies

Polymorphic transition from a metastable phase to a stable phase often occurs in metal-organic frameworks (MOFs) under the action of external stimuli. However, these transitions sometimes result in deteriorating their special performances and can even lead to serious safety problems. Therefore, developing a simple and efficient strategy for enhancing the stabilities of metastable MOF polymorphs is very imperative and meaningful. Herein, we propose a simple graphene oxide (GO)-encapsulating strategy for improving the stabilities of metastable MOF polymorphs. To illustrate this strategy, we designed and synthesized two polymorphic MOFs [MOF(ATA-a) and MOF(ATA-b)] as examples, which are based on energetic 5-amino-1 H-tetrazole as ligands. Single-crystal X-ray diffraction showed that these two polymorphs have a same chemical composition [Zn₂(ATA)₃(ATA)_2/2] _n, but different space groups, space systems, and different stacking modes of the neighboring ligands. As expected, the metastable polymorph [MOF(ATA-a)] underwent a complete polymorphic transition at room temperature to form its stable polymorph [MOF(ATA-b)]. Using the proposed strategy, we successfully encapsulated a small amount of GO in the metastable polymorph [GO⊂MOF(ATA-a)]. The resultant composite exhibited better chemical stability, extremely higher thermal stability, and larger Brunauer-Emmett-Teller surface area compared to both its precursor and the physically mixed analogue. Remarkably, its onset decomposition temperature ( T_d) was as high as 377.4 °C, which is even higher than that of 1,3,5-triamino-2,4,6-trinitrobenzene ( T_d = 321 °C), making it a potential heat-resistant explosive. The mechanism of stabilization was investigated in detail using various analytical techniques. This work may not only provide new insights into the stabilization of functional MOF polymorphs but also open up a new field for the application of GO.

Flexible Metal-Organic Framework-Based Mixed-Matrix Membranes: A New Platform for H2 S Sensors

Metal-organic framework (MOF)-polymer mixed-matrix membranes (MMMs) have shown great potential and superior performance in gas separations. However, their sensing application has not been fully established yet. Herein, a rare example of using flexible MOF-based MMMs as a fluorescent turn-on sensor for the detection of hydrogen sulfide (H₂ S) is reported. These MOF-based MMMs are readily prepared by mixing a highly stable aluminum-based nano-MOF (Al-MIL-53-NO₂ ) into poly(vinylidene fluoride) with high loadings up to 70%. Unlike the intrinsic fragility and poor processability of pure-MOF membranes, these MMMs exhibit desirable flexibility and processability that are more suitable for practical sensing applications. The uniform distribution of Al-MIL-53-NO₂ particles combined with the permanent pores of MOFs enable these MMMs to show good water permeation flux and consequently have a full contact between the analyte and MOFs. The developed MMM sensor (70% MOF loading) thus shows a highly remarkable detection selectivity and sensitivity for H₂ S with an exceptionally low detection limit around 92.31 × 10^-9 m, three orders of magnitude lower than the reported powder-form MOFs. This work demonstrates that it is feasible to develop flexible luminescent MOF-based MMMs as a novel platform for chemical sensing applications.

Mixed-Valence Cobalt(II/III) Metal-Organic Framework for Ammonia Sensing with Naked-Eye Color Switching

The construction of colorimetric sensing materials with high selectivity, low detection limits, and great stability provides a significant way for facile device implementation of an ammonia (NH₃) sensor. Herein, with excellent alkaline stability and exposed N sites in molecule as well as with naked-eye color switching nature generated from changeable cobalt (Co) valence, a three-dimensional mixed-valence cobalt(II/III) metal-organic framework (FJU-56) with tris-(4-tetrazolyl-phenyl)amine (H₃L) ligand was synthesized for colorimetric sensing toward ammonia. The activated FJU-56 demonstrates a limit of detection of 1.38 ppm for ammonia sensing, with high selectivity in ammonia and water competitive adsorption, and shows outstanding stability and reversibility in the cyclic test. The NH₃ or water molecules binding to the exposed N sites with the hydrogen-bond are observed by single-crystal X-ray diffraction, determining that the attachment of guest molecules to the FJU-56 framework changes the valence of Co ions with a naked-eye color switching response, which provides an ocular demonstration for ammonia capture and a valuable insight into ammonia sensing.

Improving the Efficiency of Mustard Gas Simulant Detoxification by Tuning the Singlet Oxygen Quantum Yield in Metal-Organic Frameworks and Their Corresponding Thin Films

The photocatalytically driven partial oxidation of a mustard gas simulant, 2-chloroethyl ethyl sulfide (CEES), was studied using the perylene-based metal-organic framework (MOF) UMCM-313 and compared to the activities of the Zr-based MOFs: PCN-222/MOF-545 and NU-1000. The rates of CEES oxidation positively correlated with the singlet oxygen quantum yield of the MOF linkers, porphyrin (PCN-222/MOF-545) < pyrene (NU-1000) < perylene (UMCM-313). Subsequently, thin films of UMCM-313 and NU-1000 were solvothermally grown on a conductive glass substrate to minimize catalyst loading and prevent light scattering by suspended MOF particles. Using a conductive glass support, the initial turnover frequencies of the MOFs in the photocatalytic reaction improved by 10-fold.

Impact of Oxygen Functional Groups on Reduced Graphene Oxide-Based Sensors for Ammonia and Toluene Detection at Room Temperature

The chemically reduced graphene oxide (rGO) was prepared by the reduction of graphene oxide by hydrazine hydrate. By varying the reduction time (10 min, 1 h, and 15 h), oxygen functional groups on rGO were tremendously controlled and they were named RG1, RG2, and RG3, respectively. Here, we investigate the impact of oxygen functional groups on the detection of ammonia and toluene at room temperature. Their effect on sensing mechanism was analyzed by first-principles calculation-based density functional theory. The sensing material was fabricated, and the effect of reduction time shown improved the recovery of ammonia and toluene sensing at room temperature. Structural, morphological, and electrical characterizations were performed on both RG1 and RG3. The sensor response toward toluene vapor of 300 ppm was found to vary 4.4, 2.5, and 3.8% for RG1, RG2, and RG3, respectively. Though RG1 shows higher sensing response with poor recovery, RG3 exhibited complete desorption of toluene after the sensing process with response and recovery times of approximately 40 and 75 s, respectively. The complete recovery of toluene molecules on RG3 is due to the generation of new sites after the reduction of oxygen functionalities on its surface. It could be suggested that these sites provided anchor to ammonia and toluene molecules and good recovery under N₂ purge. Both theoretical and experimental studies revealed that tuning the oxygen functional groups on rGO could play a vital role in the detection of volatile organic compounds (VOCs) on rGO sheets and was discussed in detail. This study could provoke knowledge about rGO-based sensor dependency with oxygen functional groups and shed light on effective monitoring of VOCs under ambient conditions for air quality monitoring applications.

MIL-Ti metal-organic frameworks (MOFs) nanomaterials as superior adsorbents: Synthesis and ultrasound-aided dye adsorption from multicomponent wastewater systems

Herein, 1,4-benzenedicarboxylate (BDC) and 2-amino-1,4-benzenedicarboxylate (NH₂-BDC) as organic linkers and tetraisopropyl orthotitanate as a metal source were used to synthesize several metal-organic frameworks (MOFs) nanomaterials. Five Materials Institut Lavoisiers (MILs) as MOFs include MIL-125(Ti), NH₂-MIL-125(Ti) and three MILs with different organic linkers molar ratios (BDC/NH₂-BDC: 75/25, 50/50 and 25/75 denoted as MIL-X1, MIL-X2 and MIL-X3, respectively). The synthesized nanomaterials were used for ultrasound-aided adsorption of cationic dyes (Basic Red 46 (BR46), Basic Blue 41 (BB41) and Methylene Blue (MB)) from single and multicomponent (binary) systems. The BET, XRD, FTIR, SEM, TEM, TGA and zeta potential were used for characterizing the MILs. Dye removal followed pseudo-second order kinetics with constant rate of 0.20833, 0.00481 and 0.00051 mg/g min for BR46, BB41 and MB, respectively. In addition dye adsorption obeyed the Langmuir isotherm model and the experimental dye adsorption capacity for BR46, BB41 and MB was 1296, 1257 and 862 mg/g, respectively. The synthesized MIL showed high reusability and stability over three cycles. The adsorption thermodynamics data presented that dye removal was a spontaneous, endothermic and physical reaction. The free Gibbs energy for dye removal by the NH₂-MIL-125(Ti) at 308K was -19.424, -15.721 and -17.413 kJ/mol for BR46, BB41 and MB, respectively.

Potential Utility of Metal-Organic Framework-Based Platform for Sensing Pesticides

The progress in modern agricultural practices could not have been realized without the large-scale contribution of assorted pesticides (e.g., organophosphates and nonorganophosphates). Precise tracking of these chemicals has become very important for safeguarding the environment and food resources owing to their very high toxicity. Hence, the development of sensitive and convenient sensors for the on-site detection of pesticides is imperative to overcome practical limitations encountered in conventional methodologies, which require skilled manpower at the expense of high cost and low portability. In this regard, the role of novel, advanced functional materials such as metal-organic frameworks (MOFs) has drawn great interest as an alternative for conventional sensory systems because of their numerous advantages over other nanomaterials. This review was organized to address the recent advances in applications of MOFs for sensing various pesticides because of their tailorable optical and electrical characteristics. It also provides in-depth comparison of the performance of MOFs with other nanomaterial sensing platforms. Further, we discuss the present challenges (e.g., potential bias due to instability under certain conditions, variations in the diffusion rate of the pesticide, chemical interferences, and the precise measurement of luminesce quenching) in developing robust and sensitive sensors by using tailored porosity, functionalities, and better framework stability.

Superhydrophobic Polymerized n-Octadecylsilane Surface for BTEX Sensing and Stable Toluene/Water Selective Detection Based on QCM Sensor

The present study reports a facile and low-cost route to produce a superhydrophobic polymerized n-octadecylsilane surface with micronano hierarchical structure on the surface of quartz crystal microbalance (QCM). The surface is used as a novel functional sensing material to detect benzene, toluene, ethylbenzene, and xylene (BTEX) vapor on the basis of QCM platform. The composites were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, and contact angle measurements. The type of solvent used to dissolve N-octadecyltrichlorosilane has a big impact on the morphology, wettability, and sensing performance of the polymer material. Further systematic studies suggest that surface wettability (contact angle) and molecular polarity of the detected analytes are effective factors in selective detection toward BTEX using resonator-type gas sensors. Gas sensing results toward toluene in different relative humidities show that the new-style sensor has stable toluene/water selective detection performance and that the disturbance of water is negligible. Besides, the limit of detection toward toluene of the sensor is lower than the odor threshold value.

A vapochromic strategy for ammonia sensing based on a bipyridinium constructed porous framework

A vapochromic strategy has been developed for ammonia sensing through the competitive complexation of ammonia and an enhanced charge transfer effect.

A semiconductor and fluorescence dual-mode room-temperature ammonia sensor achieved by decorating hydroquinone into a metal–organic framework

A semiconductor and fluorescence dual-mode MOF sensor exhibits extraordinary sensitivity, fast and single response, and good reusability for quantitative detection of ammonia vapor or liquid at room temperature.

Electronic metal–organic framework sensors

This review provides an overview on the different types of electronic MOF sensors used for the detection of molecules in the gas/vapour phase and how to assess their performances.

Isostructural lanthanide-based metal–organic frameworks: structure, photoluminescence and magnetic properties

Highly stable 3D Ln-MOFs were constructed. The Eu-MOF shows good luminescence properties while the Dy-MOF shows a typical single-molecule magnetic behavior with a slow relaxation of magnetization.

Water as a modulator in the synthesis of surface-mounted metal–organic framework films of type HKUST-1

By using water as modulator, the growth of thin nanoporous SURMOF films, prepared in a layer-by-layer fashion, can be improved.

Detection of gaseous amines with a fluorescent film based on a perylene bisimide-functionalized copolymer

A fluorescent copolymer containing PBI units and hydroxyl-ethyl structures was developed for the fast and sensitive detection of gaseous amines