Thermoplastic Membranes Incorporating Semiconductive Metal–Organic Frameworks: An Advance on Flexible X-ray Detectors

Semiconductive metal–organic frameworks (MOFs) have emerged in applications such as chemical sensors, electrocatalysts, energy storage materials, and electronic devices. However, examples of semiconductive MOFs within flexible electronics have not been reported. We present flexible X-ray detectors prepared by thermoplastic dispersal of a semiconductive MOF ( SCU-13 ) through a commercially available polymer, poly(vinylidene fluoride). The flexible detectors exhibit efficient X-ray-to-electric current conversion with enhanced charge-carrier mobility and low trap density compared to pelleted devices. A high X-ray detection sensitivity of 65.86 μCGy_air⁻¹ cm⁻² was achieved, which outperforms other pelleted devices and commercial flexible X-ray detectors. We demonstrate that the MOF-based flexible detectors can be operated at multiple bending angles without a deterioration in detection performance. As a proof-of-concept, an X-ray phase contrast under bending conditions was constructed using a 5×5 pixelated MOF-based imager.     

Sequential Transformation of Zirconium(IV)‐MOFs into Heterobimetallic MOFs Bearing Magnetic Anisotropic Cobalt(II) Centers

Heterometallic metal–organic frameworks (MOFs) allow the precise placement of various metals at atomic precision within a porous framework. This new level of control by MOFs promises fascinating advances in basic science and application. However, the rational design and synthesis of heterometallic MOFs remains a challenge due to the complexity of the heterometallic systems. Herein, we show that bimetallic MOFs with MX₂(INA)₄ moieties (INA=isonicotinate; M=Co²⁺ or Fe²⁺; X=OH^?, Cl^?, Br^?, I^?, NCS^?, or NCSe^?) can be generated by the sequential modification of a Zr‐based MOF. This multi‐step modification not only replaced the linear organic linker with a square planar MX₂(INA)₄ unit, but also altered the symmetry, unit cell, and topology of the parent structure. Single‐crystal to single‐crystal transformation is realized so that snapshots for transition process were captured by successive single‐crystal X‐ray diffraction. Furthermore, the installation of Co(NCS)₂(INA)₄ endows field‐induced slow magnetic relaxation property to the diamagnetic Zr‐MOF.     

A Metal–Organic Framework Containing Unusual Eight‐Connected Zr–Oxo Secondary Building Units and Orthogonal Carboxylic Acids for Ultra‐sensitive Metal Detection

Two metal–organic frameworks (MOFs) with Zr–oxo secondary building units (SBUs) were prepared by using p,p′‐terphenyldicarboxylate (TPDC) bridging ligands pre‐functionalized with orthogonal succinic acid (MOF‐ 1 ) and maleic acid groups (MOF‐ 2 ). Single‐crystal X‐ray structure analysis of MOF‐ 1 provides the first direct evidence for eight‐connected SBUs in UiO‐type MOFs. In contrast, MOF‐ 2 contains twelve‐connected SBUs as seen in the traditional UiO MOF topology. These structural assignments were confirmed by extended X‐ray absorption fine structure (EXAFS) analysis. The highly porous MOF‐ 1 is an excellent fluorescence sensor for metal ions with the detection limit of <0.5 ppb for Mn²⁺and three to four orders of magnitude greater sensitivity for metal ions than previously reported luminescent MOFs.     

Formation of Ultrathin,Continuous Metal–Organic Framework Membranes on Flexible Polymer Substrates

Metal–organic framework (MOF) materials have an enormous potential in separation applications, but to realize their potential as semipermeable membranes they need to be assembled into thin continuous macroscopic films for fabrication into devices. By using a facile immersion technique, we prepared ultrathin, continuous zeolitic imidazolate framework (ZIF‐8) membranes on titania‐functionalized porous polymeric supports. The coherent ZIF‐8 layer was surprisingly flexible and adhered well to the support, and the composite membrane could sustain bending and elongation. The membranes exhibited molecular sieving behavior, close to the theoretical permeability of ZIF‐8, with hydrogen permeance up to 201×10^?7 mol m^?2 s^?1 Pa^?1 and an ideal H₂/CO₂ selectivity of 7:1. This approach offers significant opportunities to exploit the unique properties of MOFs in the fabrication of separation and sensing devices.     

A Dual‐Ligand Porous Coordination Polymer Chemiresistor with Modulated Conductivity and Porosity

Single‐ligand‐based electronically conductive porous coordination polymers/metal–organic frameworks (EC‐PCPs/MOFs) fail to meet the requirements of numerous electronic applications owing to their limited tunability in terms of both conductivity and topology. In this study, a new 2D π‐conjugated EC‐MOF containing copper units with mixed trigonal ligands was developed: Cu₃(HHTP)(THQ) (HHTP=2,3,6,7,10,11‐hexahydrotriphenylene, THQ=tetrahydroxy‐1,4‐quinone). The modulated conductivity (σ≈2.53×10^?5 S cm^?1 with an activation energy of 0.30 eV) and high porosity (ca. 441.2 m² g^?1) of the Cu₃(HHTP)(THQ) semiconductive nanowires provided an appropriate resistance baseline and highly accessible areas for the development of an excellent chemiresistive gas sensor.     

Copper‐based Metal‐organic Framework for Non‐enzymatic Electrochemical Detection of Glucose

A non‐enzymatic electrochemical glucose sensor based on a Cu‐based metal‐organic framework (Cu‐MOF) modified electrode was developed. The Cu‐MOF was prepared by a simple ionothermal synthesis, and the characterizations of the Cu‐MOF were studied by Fourier transform infrared spectroscopy (FT‐IR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), single‐crystal X‐ray powder diffraction (SCXRD), and X‐ray powder diffraction (XRD). Electrochemical behaviors of the Cu‐MOF modified electrode to glucose were measured by differential pulse voltammetry (DPV). The electrochemical results showed that the Cu‐MOF modified electrode exhibited an excellent electro‐catalytic oxidation towards glucose in the range of 0.06 μM to 5 mM with a sensitivity of 89 μA/mM cm² and a detection limit of 10.5 nM. Moreover, the fabricated sensor showed a high selectivity to the oxidation of glucose in coexistence with other interferences. The sensor was satisfactorily applied to the determination of glucose in urine samples. With the significant electrochemical performances, MOFs may provide a suitable platform in the construction of kinds of electrochemical sensors and/or biosensors and hold a great promise for sensing applications.     

Porous carbon derived from a metal–organic framework as an efficient adsorbent for the solid‐phase extraction of phthalate esters

A porous carbon designated as MOF‐5‐C was prepared by directly carbonizing a metal–organic framework (MOF‐5). The morphology and microstructure of MOF‐5‐C were characterized by scanning electron microscopy, N₂ adsorption, and powder X‐ray diffraction. The MOF‐5‐C retained the original porous structures of MOF‐5, and showed a high Brunauer–Emmett–Teller surface area (1808 m² g^?1) and large pore volume (3.05 cm³ g^?1). To evaluate its adsorption performance, the MOF‐5‐C was used as an adsorbent for the solid‐phase extraction of four phthalate esters from bottled water, peach juice, and soft drink samples followed by high‐performance liquid chromatographic analysis. Several parameters that could affect the extraction efficiencies were investigated. Under the optimum conditions, a good linearity was achieved in the concentration range of 0.1–50.0 ng mL^?1 for bottled water sample and 0.2–50.0 ng mL^?1 for peach juice and soft drink samples. The limits of detection of the method (S/N = 3) were 0.02 ng mL^?1 for bottled water sample, and 0.04–0.05 ng mL^?1 for peach juice and soft drink samples. The results indicated that the MOF‐5‐C exhibited an excellent adsorption capability for trace levels of phthalate esters, and it could be a promising adsorbent for the preconcentration of other organic compounds.     

Tetrazine Chromophore‐Based Metal–Organic Frameworks with Unusual Configurations: Synthetic,Structural, Theoretical,Fluorescent, and Nonlinear Optical Studies

Three unusual three‐dimensional (3D) tetrazine chromophore‐based metal–organic frameworks (MOFs) {(Et₄N)[WS₄Cu₃(CN)₂(4,4′‐pytz)_0.5]}_n ( 1 ), {[MoS₄Cu₄(CN)₂(4,4′‐pytz)₂] ? CH₂Cl₂}_n ( 2 ), and {[WS₄Cu₃(4,4′‐pytz)₃] ? [N(CN)₂]}_n ( 3 ; 4,4′‐pytz=3,6‐bis(4‐pyridyl)tetrazine) have been synthesized and characterized by using FTIR and UV/Vis spectroscopy, elemental analysis, powder X‐ray diffraction, gel permeation chromatography, steady‐state fluorescence, and thermogravimetric analysis; their identities were confirmed by single‐crystal X‐ray diffraction studies. MOF 1 possesses the first five‐connected M/S/Cu (M=Mo, W) framework with an unusual 3D (4⁴?6⁶) topology constructed from T‐shaped [WS₄Cu₃]⁺ clusters as nodes and single CN^?/4,4′‐pytz bridges as linkers. MOF 2 features a novel 3D MOF structure with (4²⁰?6⁸) topology, in which the bridging 4,4′‐pytz ligands exhibit unique distorted arch structures. MOF 3 displays the first 3D MOF structure based on flywheel‐shaped [WS₄Cu₃]⁺ clusters with a non‐interpenetrating honeycomb‐like framework and a heavily distorted “ACS” topology. Steady‐state fluorescence studies of 1 – 3 reveal significant fluorescence emissions. The nonlinear optical (NLO) properties of 1 – 3 were investigated by using a Z‐scan technique with 5 ns pulses at λ=532 nm. The Z‐scan experimental results show that the π‐delocalizable tetrazine‐based 4,4′‐pytz ligands contribute to the strong third‐order NLO properties exhibited by 1 – 3 . Time‐dependent density functional theory studies afforded insight into the electronic transitions and spectral characterization of these functionalized NLO molecular materials.     

Two‐dimensional π‐conjugated metal‐organic framework with high electrical conductivity for electrochemical sensing

A two‐dimensional π‐conjugated metal‐organic framework (MOF) with long‐range delocalized electrons has been prepared and applied as modified electrode material without further post‐modification. The MOF (Cu₃(HHTP)₂) is composed of Cu(II) centers and a redox‐active linker (2,3,6,7,10,11‐hexahydroxytriphenylene, HHTP). Compared to most MOFs, Cu₃(HHTP)₂ displays higher electrical conductivity and charge storage capacity owing to the collective effect of metal ions and aromatic ligands with π–π conjugation. In order to confirm the superior properties of this material, the electrochemical detection of dopamine (DA) was conducted and the satisfactory results were obtained. The currents increase linearly with the concentration of DA in the range 5.0 × 10^?8 to 2.0 × 10^?4 M with a detection limit of 5.1 nM. Furthermore, Cu₃(HHTP)₂ presents high selectivity and applicability in serum samples for electrochemical DA sensing. Overall, this material has excellent potential as a promising platform for establishing an MOF‐based electrochemical sensor.     

Design and Synthesis of a Water‐Stable Anionic Uranium‐Based Metal–Organic Framework (MOF) with Ultra Large Pores

Ionic metal–organic frameworks (MOFs) are a subclass of porous materials that have the ability to incorporate different charged species in confined nanospace by ion‐exchange. To date, however, very few examples combining mesoporosity and water stability have been realized in ionic MOF chemistry. Herein, we report the rational design and synthesis of a water‐stable anionic mesoporous MOF based on uranium and featuring tbo‐type topology. The resulting tbo MOF exhibits exceptionally large open cavities (3.9 nm) exceeding those of all known anionic MOFs. By supercritical CO₂ activation, a record‐high Brunauer‐Emmett‐Teller (BET) surface area (2100 m² g^?1) for actinide‐based MOFs has been obtained. Most importantly, however, this new uranium‐based MOF is water‐stable and able to absorb positively charged ions selectively over negatively charged ones, enabling the efficient separation of organic dyes and biomolecules.     

A Calixarene‐Based Metal–Organic Framework for Highly Selective NO2 Detection

A calixarene‐based metal–organic framework (Zr‐cal, [Zr₆O₄(OH)₄(FA)₆]₂(cal)₃], FA=formate, cal=1,3‐alt‐25,26,27,28‐tetrakis[(carboxy)methoxy]calixarene) was synthesized and characterized by single‐crystal X‐ray diffraction. The three‐dimensional framework is a 4,6‐connected network of gar topology and exhibits two equal but nonintersecting three‐dimensional pore systems. It has a specific BET surface area of 670 m² g^?1, and the calixarene cavities are accessible through the pore systems. The exposed calixarenes can be used for the visual detection and encapsulation of NO₂ through the formation of deeply colored charge–transfer complexes inside the MOF. The highly selective complexation was analyzed by UV/Vis and IR spectroscopy, and the stability of the material was confirmed by powder X‐ray diffraction and ¹H NMR spectroscopy. Finally, the MOF was used as a sensor material in a home‐made sensor cell and showed high sensitivity for NO₂.     

Amorphous Metal–Organic Framework‐Dominated Nanocomposites with Both Compositional and Structural Heterogeneity for Oxygen Evolution

Amorphous metal–organic frameworks (aMOFs) are an emerging family of attractive materials with great application potential, however aMOFs are usually prepared under harsh conditions and aMOFs with complex compositions and structures are rarely reported. In this work, an aMOF‐dominated nanocomposite (aMOF‐NC) with both structural and compositional complexity has been synthesized using a facile approach. A ligand‐competition amorphization mechanism is proposed based on experimental and density functional theory calculation results. The aMOF‐NC possesses a core–shell nanorod@nanosheet architecture, including a Fe‐rich Fe‐Co‐aMOF core and a Co‐rich Fe‐Co‐aMOF shell in the core–shell structured nanorod, and amorphous Co(OH)₂ nanosheets as the outer layer. Benefiting from the structural and compositional heterogeneity, the aMOF‐NC demonstrates an excellent oxygen evolution reaction activity with a low overpotential of 249 mV at 10.0 mA cm^?2 and Tafel slope of 39.5 mV dec^?1.     

A Titanium(IV)‐Based Metal–Organic Framework Featuring Defect‐Rich Ti‐O Sheets as an Oxidative Desulfurization Catalyst

While titanium‐based metal–organic frameworks (MOFs) have been widely studied for their (photo)catalytic potential, only a few Ti^IV MOFs have been reported owing to the high reactivity of the employed titanium precursors. The synthesis of COK‐47 is now presented, the first Ti carboxylate MOF based on sheets of Ti^IVO₆ octahedra, which can be synthesized with a range of different linkers. COK‐47 can be synthesized as an inherently defective nanoparticulate material, rendering it a highly efficient catalyst for the oxidation of thiophenes. Its structure was determined by continuous rotation electron diffraction and studied in depth by X‐ray total scattering, EXAFS, and solid‐state NMR. Furthermore, its photoactivity was investigated by electron paramagnetic resonance and demonstrated by catalytic photodegradation of rhodamine 6G.     

Air‐Free Synthesis of a Ferrous Metal‐Organic Framework Featuring HKUST‐1 Structure and its Mössbauer Spectrum

Single crystals of the Fe^II metal‐organic framework (MOF) with 1,3,5‐benzenetricarboxylate (BTC) as a linker were solvothermally obtained under air‐free conditions. X‐ray diffraction analysis of the crystals demonstrated a structure for Fe^II‐MOF analogous to that of [Cu₃(BTC)₂] (HKUST‐1). Unlike HKUST‐1, however, the Fe^II‐MOF did not retain permanent porosity after exchange of guest molecules. The Mössbauer spectrum of the Fe^II‐MOF was recorded at 80 K in zero field yielding an apparent quadrupole splitting of ΔE_Q = 2.43 mm · s^–1, and an isomer shift of δ = 1.20 mm · s^–1, consistent with high‐spin central iron(II) atoms. Air exposure of the Fe^II‐MOF was found to result in oxidation of the metal atoms to afford Fe^III. These results demonstrate that Fe^II‐based MOFs can be prepared in similar fashion to the [Cu₃(BTC)₂], but that they lack permanent porosity when degassed.     

Formation of Ultrathin,Continuous Metal–Organic Framework Membranes on Flexible Polymer Substrates

Metal–organic framework (MOF) materials have an enormous potential in separation applications, but to realize their potential as semipermeable membranes they need to be assembled into thin continuous macroscopic films for fabrication into devices. By using a facile immersion technique, we prepared ultrathin, continuous zeolitic imidazolate framework (ZIF‐8) membranes on titania‐functionalized porous polymeric supports. The coherent ZIF‐8 layer was surprisingly flexible and adhered well to the support, and the composite membrane could sustain bending and elongation. The membranes exhibited molecular sieving behavior, close to the theoretical permeability of ZIF‐8, with hydrogen permeance up to 201×10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ and an ideal H₂/CO₂ selectivity of 7:1. This approach offers significant opportunities to exploit the unique properties of MOFs in the fabrication of separation and sensing devices.     

Cooperative Cluster Metalation and Ligand Migration in Zirconium Metal–Organic Frameworks

Cooperative cluster metalation and ligand migration were performed on a Zr‐MOF, leading to the isolation of unique bimetallic MOFs based on decanuclear Zr₆M₄ (M=Ni, Co) clusters. The M²⁺ reacts with the μ₃‐OH and terminal H₂O ligands on an 8‐connected [Zr₆O₄(OH)₈(H₂O)₄] cluster to form a bimetallic [Zr₆M₄O₈(OH)₈(H₂O)₈] cluster. Along with the metalation of Zr₆ cluster, ligand migration is observed in which a Zr–carboxylate bond dissociates to form a M–carboxylate bond. Single‐crystal to single‐crystal transformation is realized so that snapshots for cooperative cluster metalation and ligand migration processes are captured by successive single‐crystal X‐ray structures. In³⁺ was metalated into the same Zr‐MOF which showed excellent catalytic activity in the acetaldehyde cyclotrimerization reaction. This work not only provides a powerful tool to functionalize Zr‐MOFs with other metals, but also structurally elucidates the formation mechanism of the resulting heterometallic MOFs.     

Cu3(hexaiminotriphenylene)2: An Electrically Conductive 2D Metal–Organic Framework for Chemiresistive Sensing

The utility of metal–organic frameworks (MOFs) as functional materials in electronic devices has been limited to date by a lack of MOFs that display high electrical conductivity. Here, we report the synthesis of a new electrically conductive 2D MOF, Cu₃(HITP)₂ (HITP=2,3,6,7,10,11‐hexaiminotriphenylene), which displays a bulk conductivity of 0.2 S cm^?1 (pellet, two‐point‐probe). Devices synthesized by simple drop casting of Cu₃(HITP)₂ dispersions function as reversible chemiresistive sensors, capable of detecting sub‐ppm levels of ammonia vapor. Comparison with the isostructural 2D MOF Ni₃(HITP)₂ shows that the copper sites are critical for ammonia sensing, indicating that rational design/synthesis can be used to tune the functional properties of conductive MOFs.     

Construction of Flexible‐on‐Rigid Hybrid‐Phase Metal–Organic Frameworks for Controllable Multi‐Drug Delivery

Multi‐component MOFs contain multiple sets of unique and hierarchical pores, with different functions for different applications, distributed in their inter‐linked domains. Herein, we report the construction of a class of precisely aligned flexible‐on‐rigid hybrid‐phase MOFs with a unique rods‐on‐octahedron morphology. We demonstrated that hybrid‐phase MOFs can be constructed based on two prerequisites: the partially matched topology at the interface of the two frameworks, and the structural flexibility of MOFs with acs topology, which can compensate for the differences in lattice parameters. Furthermore, we achieved domain selective loading of multiple guest molecules into the hybrid‐phase MOF, as observed by scanning transmission electron microscopy–energy‐dispersive X‐ray spectrometry elemental mapping. Most importantly, we successfully applied the constructed hybrid‐phase MOF to develop a dual‐drug delivery system with controllable loading ratio and release kinetics.     

Efficient Visible‐Light‐Driven Carbon Dioxide Reduction by a Single‐Atom Implanted Metal–Organic Framework

Modular optimization of metal–organic frameworks (MOFs) was realized by incorporation of coordinatively unsaturated single atoms in a MOF matrix. The newly developed MOF can selectively capture and photoreduce CO₂ with high efficiency under visible‐light irradiation. Mechanistic investigation reveals that the presence of single Co atoms in the MOF can greatly boost the electron–hole separation efficiency in porphyrin units. Directional migration of photogenerated excitons from porphyrin to catalytic Co centers was witnessed, thereby achieving supply of long‐lived electrons for the reduction of CO₂ molecules adsorbed on Co centers. As a direct result, porphyrin MOF comprising atomically dispersed catalytic centers exhibits significantly enhanced photocatalytic conversion of CO₂, which is equivalent to a 3.13‐fold improvement in CO evolution rate (200.6 μmol g^?1 h^?1) and a 5.93‐fold enhancement in CH₄ generation rate (36.67 μmol g^?1 h^?1) compared to the parent MOF.     

Microwave‐Assisted Synthesis of HKUST‐1 and Functionalized HKUST‐1‐@H3PW12O40: Selective Adsorption of Heavy Metal Ions in Water Analyzed with Synchrotron Radiation

A simple, rapid and efficient synthesis of the metal‐organic framework (MOF) HKUST‐1 [Cu₃(1,3,5‐benzene‐tri‐carboxilic‐acid)₂] by microwave irradiation is described, which afforded a homogeneous and highly selective material. The unusually short time to complete the synthesis by microwave irradiation is mainly attributable to rapid nucleation rather than to crystal growth rate. Using this method, HKUST‐1‐MW (MW=microwave) could be prepared within 20 min, whereas by hydrothermal synthesis, involving conventional heating, the preparation time is 8 h. Work efficiency was improved by the good performance of the obtained HKUST‐1‐MW which exhibited good selective adsorption of heavy metal ions, as well as a remarkably high adsorption affinity and adsorption capacity, but no adsorption of Hg²⁺ under the same experimental conditions. Of particular importance is the preservation of the structure after metal‐ion adsorption, which remained virtually intact, with only a few changes in X‐ray diffraction intensity and a moderate decline in surface area. Synthesis of the polyoxometalate‐containing HKUST‐1‐MW@H₃PW₁₂O₄₀ afforded a MOF with enhanced stability in water, due to the introduced Keggin‐type phosphotungstate, which systematically occluded in the cavities constituting the walls between the mesopores. Different Cu/W ratios were investigated according to the extrusion rate of cooper ions concentration, without significant structural changes after adsorption. The MOFs obtained feature particle sizes between 10–20 μm and their structures were determined using synchrotron‐based X‐ray diffraction. The results of this study can be considered important for potentially wider future applications of MOFs, especially to attend environmental issues.