Microstructural Engineering and Architectural Design of Metal–Organic Framework Membranes

In the past decade, a huge development in rational design, synthesis, and application of molecular sieve membranes, which typically included zeolites, metal–organic frameworks (MOFs), and graphene oxides, has been witnessed. Owing to high flexibility in both pore apertures and functionality, MOFs in the form of membranes have offered unprecedented opportunities for energy‐efficient gas separations. Reports on the fabrication of well‐intergrown MOF membranes first appeared in 2009. Since then there has been tremendous growth in this area along with an exponential increase of MOF‐membrane‐related publications. In order to compete with other separation and purification technologies, like cryogenic distillation, pressure swing adsorption, and chemical absorption, separation performance (including permeability, selectivity, and long‐term stability) of molecular sieve membranes must be further improved in an attempt to reach an economically attractive region. Therefore, microstructural engineering and architectural design of MOF membranes at mesoscopic and microscopic levels become indispensable. This review summarizes some intriguing research that may potentially contribute to large‐scale applications of MOF membranes in the future.     

The Nano–Intestine Interaction: Understanding the Location‐Oriented Effects of Engineered Nanomaterials in the Intestine

Engineered nanomaterials (ENMs) are used in food additives, food packages, and therapeutic purposes owing to their useful properties, Therefore, human beings are orally exposed to exogenous nanomaterials frequently, which means the intestine is one of the primary targets of nanomaterials. Consequently, it is of great importance to understand the interaction between nanomaterials and the intestine. When nanomaterials enter into gut lumen, they inevitably interact with various components and thereby display different effects on the intestine based on their locations; these are known as location‐oriented effects (LOE). The intestinal LOE confer a new biological‐effect profile for nanomaterials, which is dependent on the involvement of the following biological processes: nano–mucus interaction, nano–intestinal epithelial cells (IECs) interaction, nano–immune interaction, and nano–microbiota interaction. A deep understanding of NM‐induced LOE will facilitate the design of safer NMs and the development of more efficient nanomedicine for intestine‐related diseases. Herein, recent progress in this field is reviewed in order to better understand the LOE of nanomaterials. The distant effects of nanomaterials coupling with microbiota are also highlighted. Investigation of the interaction of nanomaterials with the intestine will stimulate other new research areas beyond intestinal nanotoxicity.     

Super‐Stable,Highly Efficient,and Recyclable Fibrous Metal–Organic Framework Membranes for Precious Metal Recovery from Strong Acidic Solutions

Precious metals such as palladium (Pd) and platinum (Pt) are marvelous materials in the fields of electronic and catalysis, but they are tapering day by day. Zr(IV)‐based metal–organic frameworks (MOFs) are competent for their recovery, notably in harsh environments, while the general powder form limits their practical application. Porous MOF‐based membranes with ultraefficient metal ion permeation, strong stability, and high selectivity are, therefore, strikingly preferred. Herein, a set of polymeric fibrous membranes incorporated with the UiO‐66 series are fabricated; their adsorption/desorption capabilities toward Pd(II) and Pt(IV) are evaluated from strongly acidic solutions; and the MOF–polymer compatibilities are investigated. Polyurethane (PU)/UiO‐66‐NH₂ showed strong acid resistance and high chemical stability, which are attributable to strong π–π interactions between PU and MOF nanoparticles with a high configuration of energy. The as‐fabricated MOF membranes show extremely good adsorption/desorption performances without ruptures/coalitions of nanofibers or leak of MOF nanoparticles, and successfully display the efficacy in a gravity‐driven or even continuous‐flow system with good recycle performance and selectivity. The as‐fabricated MOF membranes set an example of potential MOF–polymer compatibility for practical applications.     

Water/Vapor Assisted Fabrication of Large-Area Superprotonic Conductive Covalent Organic Framework Membranes

Fabrication of large-area ionic covalent organic framework membranes (iCOMs) remains a grand challenge. Herein, the authors report the liquid water and water vapor-assisted fabrication of large-area superprotonic conductive iCOMs. A mixed monomer solution containing 1,3,5-triformylphloroglucinol (TFP) in 1,4-dioxane and p-diaminobenzenesulfonic acid (DABA) in water is first polymerized to obtain a pristine membrane which subsequently underwent crystallization process in mixed vapors containing water vapor. During the polymerization stage, water played a role of a diluting agent, weakening the Coulombic repulsion between sulfonic acid groups. During the crystallization stage, water vapor played a role of a structure-directing agent to facilitate the formation of highly crystalline, large-area iCOMs. The resulting membranes achieved a proton conductivity value of 0.76 S cm⁻¹ at 90 °C under 100% relative humidity, which is among the highest ever reported. Using liquid water and water vapor as versatile additives open a novel avenue to the fabrication of large-area membranes from covalent organic frameworks and other kinds of crystalline organic framework materials.     

Nanosheets of Nonlayered Aluminum Metal–Organic Frameworks through a Surfactant‐Assisted Method

During the last decade, the synthesis and application of metal–organic framework (MOF) nanosheets has received growing interest, showing unique performances for different technological applications. Despite the potential of this type of nanolamellar materials, the synthetic routes developed so far are restricted to MOFs possessing layered structures, limiting further development in this field. Here, a bottom‐up surfactant‐assisted synthetic approach is presented for the fabrication of nanosheets of various nonlayered MOFs, broadening the scope of MOF nanosheets application. Surfactant‐assisted preorganization of the metallic precursor prior to MOF synthesis enables the manufacture of nonlayered Al‐containing MOF lamellae. These MOF nanosheets are shown to exhibit a superior performance over other crystal morphologies for both chemical sensing and gas separation. As revealed by electron microscopy and diffraction, this superior performance arises from the shorter diffusion pathway in the MOF nanosheets, whose 1D channels are oriented along the shortest particle dimension.     

Bottom‐Up Fabrication of Semiconductive Metal–Organic Framework Ultrathin Films

Though generally considered insulating, recent progress on the discovery of conductive porous metal–organic frameworks (MOFs) offers new opportunities for their integration as electroactive components in electronic devices. Compared to classical semiconductors, these metal–organic hybrids combine the crystallinity of inorganic materials with easier chemical functionalization and processability. Still, future development depends on the ability to produce high‐quality films with fine control over their orientation, crystallinity, homogeneity, and thickness. Here self‐assembled monolayer substrate modification and bottom‐up techniques are used to produce preferentially oriented, ultrathin, conductive films of Cu‐CAT‐1. The approach permits to fabricate and study the electrical response of MOF‐based devices incorporating the thinnest MOF film reported thus far (10 nm thick).     

Optochemically Responsive 2D Nanosheets of a 3D Metal–Organic Framework Material

Outstanding functional tunability underpinning metal–organic framework (MOF) confers a versatile platform to contrive next‐generation chemical sensors, optoelectronics, energy harvesters, and converters. A rare exemplar of a porous 2D nanosheet material constructed from an extended 3D MOF structure is reported. A rapid supramolecular self‐assembly methodology at ambient conditions to synthesize readily exfoliatable MOF nanosheets, functionalized in situ by adopting the guest@MOF (host) strategy, is developed. Nanoscale confinement of light‐emitting molecules (as functional guest) inside the MOF pores generates unusual combination of optical, electronic, and chemical properties, arising from the strong host–guest coupling effects. Highly promising photonics‐based chemical sensing opened up by the new guest@MOF composite systems is shown. By harnessing host–guest optochemical interactions of functionalized MOF nanosheets, detection of an extensive range of volatile organic compounds and small molecules important for many practical applications has been accomplished.     

Kinetic‐Controlled Formation of Bimetallic Metal–Organic Framework Hybrid Structures

Heterometallic metal–organic frameworks (MOFs) are constructed from two or more kinds of metal ions, while still remaining their original topologies. Due to distinct reaction kinetics during MOF formation, partial distribution of different metals within a single MOF crystal can lead to sophisticated heterogeneous nanostructures. Here, this study reports an investigation of reaction kinetics for different metal ions in a bimetallic MOF system, the ZIF‐8/67 (M(2‐mIM)₂, M = Zn for ZIF‐8, and Co for ZIF‐67, 2‐mIM = 2‐methylimidazole), by in situ optical method. Distinct kinetics of the two metals forming single‐component MOFs are revealed, and when both Co and Zn ions are present in the starting solution, homogeneous distributions of the two metals are only achieved at high Co/Zn ratio, while at low Co/Zn ratio concentration gradient from Co‐rich cores to Zn‐rich shells is observed. Further, by adding the two metals in sequence, more sophisticated structures are achieved. Specifically, when Co²⁺ is added first, ZIF‐67@ZIF‐8/67 core–shell nanocrystals are achieved with tunable core/shell thickness ratio depending on the time intervals; while when Zn²⁺ is added first, only agglomerates of irregular shape form due to the weak nucleation ability of Zn²⁺.     

Metal–Organic Framework Encapsulation for the Preservation and Photothermal Enhancement of Enzyme Activity

Interfacing biomolecules with functional materials is a key strategy toward achieving externally‐triggered biological function. The rational integration of functional proteins, such as enzymes, with plasmonic nanostructures that exhibit unique optical properties such as photothermal effect provides a means to externally control the enzyme activity. However, due to the labile nature of enzymes, the photothermal effect of plasmonic nanostructures is mostly utilized for the enhancement of the biocatalytic activity of thermophilic enzymes. In order to extend and utilize the photothermal effect to a broader class of enzymes, a means to stabilize the immobilized active protein is essential. Inspired by biomineralization for the encapsulation of soft tissue within protective exteriors in nature, metal–organic framework is utilized to stabilize the enzyme. This strategy provides an effective route to enhance and externally modulate the biocatalytic activity of enzymes bound to functional nanostructures over a broad range of operating environments that are otherwise hostile to the biomolecules.     

Restricting Lattice Flexibility in Polycrystalline Metal–Organic Framework Membranes for Carbon Capture

Although polycrystalline metal‐organic framework (MOF) membranes offer several advantages over other nanoporous membranes, thus far they have not yielded good CO₂ separation performance, crucial for energy‐efficient carbon capture. ZIF‐8, one of the most popular MOFs, has a crystallographically determined pore aperture of 0.34 nm, ideal for CO₂/N₂ and CO₂/CH₄ separation; however, its flexible lattice restricts the corresponding separation selectivities to below 5. A novel postsynthetic rapid heat treatment (RHT), implemented in a few seconds at 360 °C, which drastically improves the carbon capture performance of the ZIF‐8 membranes, is reported. Lattice stiffening is confirmed by the appearance of a temperature‐activated transport, attributed to a stronger interaction of gas molecules with the pore aperture, with activation energy increasing with the molecular size (CH₄ > CO₂ > H₂). Unprecedented CO₂/CH₄, CO₂/N₂, and H₂/CH₄ selectivities exceeding 30, 30, and 175, respectively, and complete blockage of C₃H₆, are achieved. Spectroscopic and X‐ray diffraction studies confirm that while the coordination environment and crystallinity are unaffected, lattice distortion and strain are incorporated in the ZIF‐8 lattice, increasing the lattice stiffness. Overall, RHT treatment is a facile and versatile technique that can vastly improve the gas‐separation performance of the MOF membranes.     

Nano‐Biohybrids: In Vivo Synthesis of Metal–Organic Frameworks inside Living Plants

Plants have a complex passive fluid transport system capable of internalizing small molecules from the environment, and this system offers an ideal route for augmenting plants with functional nanomaterials. Current plant augmentation techniques use pre‐formed nanomaterials and permeabilizing agents or plant cuttings. A so far unexplored concept is the formation of the functional material, in situ, from precursors small enough to be passively internalized through the roots without harming the plants. Metal–organic frameworks are ideal for in situ synthesis as they are composed of metal ions coordinated with organic ligands and have recently been mineralized around single‐celled organisms in mild aqueous conditions. Herein, the synthesis of two types of metal‐organic frameworks, zinc(2‐methylimidazole)₂ and lanthanide₂(terephthalate)₃, are reported inside a variety of plants. In situ synchrotron experiments help elucidate the formation kinetics and crystal phases of the nano‐biohybrid plants. Plants augmented with luminescent metal‐organic frameworks are utilized for small molecule sensing, although other applications, such as pathogen sensing, proton conductive plants, improved CO₂ capture, bacteria‐free nitrogen fixation, drought and fungi‐resistance, and enhanced photosynthesis and photocatalysis, are foreseeable. Overall, the generation of functional materials inside of fully intact plants could lead to more complex nano‐biohybrid sensors and organisms augmented with superior performance characteristics.     

Flexible Metal–Organic Framework‐Based Mixed‐Matrix Membranes: A New Platform for H2S 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 (H₂S) 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 H₂S with an exceptionally low detection limit around 92.31 × 10^?9m , 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.     

Versatile and Switchable Responsive Properties of a Lanthanide‐Viologen Metal–Organic Framework

Metal–organic frameworks (MOFs) provide intriguing platforms for the design of responsive materials. It is challenging to mobilize as many components as possible of a MOF to collaboratively accomplish multiple responsive properties. Here, reversible photochromism, piezochromism, hydrochromism, ionochromism, and luminescence modulation of an ionic Eu(III) MOF is reported furnished by cationic electron‐deficient viologen units and exchangeable guest anions. Mechanistically, the extraordinarily versatile responsive properties are owed to electron transfer (ET), charge transfer (CT), and energy transfer, involving viologen as electron acceptor, anion as electron donor, luminescing Eu(III) as energy donor, and anion‐viologen CT complex or ET‐generated radical as energy acceptor (luminescence quencher). Moreover, guest anions and waters provide flexible handles to control the ET‐based responsive properties. Water release/reuptake or exchange with organic solvents can switch on/off the response to light, while reversible anion exchange can disenable or awaken the responses to pressure, light, and water release/reuptake. The impacts of water and anions on ET are justified by the high polarity and hydrogen‐bonding capability of water, the different electron donor strength of anions, and the strong I^?‐viologen CT interactions. The rich responsive behaviors have great implications for applications such as pressure sensors, iodide detection, and chemical logic gates.     

Atom‐by‐Atom Fabrication of Monolayer Molybdenum Membranes

Fabrication of materials in the monolayer regime to acquire fascinating physical properties has attracted enormous interest during the past decade, and remarkable success has been achieved for layered materials adopting weak interlayer van der Waals forces. However, the fabrication of monolayer metal membranes possessing strong intralayer bonding remains elusive. Here, suspended monolayer Mo membranes are fabricated from monolayer MoSe₂ films via selective electron beam (e‐beam) ionization of Se atoms by scanning transmission electron microscopy (STEM). The nucleation and subsequent growth of the Mo membranes are triggered by the formation and aggregation of Se vacancies as seen by atomic resolution sequential STEM imaging. Various novel structural defects and intriguing self‐healing characteristics are unveiled during the growth. In addition, the monolayer Mo membrane is highly robust under the e‐beam irradiation. It is likely that other metal membranes can be fabricated in a similar manner, and these pure metal‐based 2D materials add to the diversity of 2D materials and introduce profound novel physical properties.