Polymorphism and orientation control of copper-dicarboxylate metal-organic framework thin films through vapour- and liquid-phase growth.

Precise control over the crystalline phase and crystallographic orientation within thin films of metal-organic frameworks (MOFs) is highly desirable. Here, we report a comparison of the liquid- and vapour-phase film deposition of two copper-dicarboxylate MOFs starting from an oriented metal hydroxide precursor. X-ray diffraction revealed that the vapour- or liquid-phase reaction of the linker with this precursor results in different crystalline phases, morphologies, and orientations. Pole figure analysis showed that solution-based growth of the MOFs follows the axial texture of the metal hydroxide precursor, resulting in heteroepitaxy. In contrast, the vapour-phase method results in non-epitaxial growth with uniplanar texture only.

Metal-Organic Framework-Based Enzyme Biocomposites.

Because of their efficiency, selectivity, and environmental sustainability, there are significant opportunities for enzymes in chemical synthesis and biotechnology. However, as the three-dimensional active structure of enzymes is predominantly maintained by weaker noncovalent interactions, thermal, pH, and chemical stressors can modify or eliminate activity. Metal-organic frameworks (MOFs), which are extended porous network materials assembled by a bottom-up building block approach from metal-based nodes and organic linkers, can be used to afford protection to enzymes. The self-assembled structures of MOFs can be used to encase an enzyme in a process called encapsulation when the MOF is synthesized in the presence of the biomolecule. Alternatively, enzymes can be infiltrated into mesoporous MOF structures or surface bound via covalent or noncovalent processes. Integration of MOF materials and enzymes in this way affords protection and allows the enzyme to maintain activity in challenge conditions (e.g., denaturing agents, elevated temperature, non-native pH, and organic solvents). In addition to forming simple enzyme/MOF biocomposites, other materials can be introduced to the composites to improve recovery or facilitate advanced applications in sensing and fuel cell technology. This review canvasses enzyme protection via encapsulation, pore infiltration, and surface adsorption and summarizes strategies to form multicomponent composites. Also, given that enzyme/MOF biocomposites straddle materials chemistry and enzymology, this review provides an assessment of the characterization methodologies used for MOF-immobilized enzymes and identifies some key parameters to facilitate development of the field.

Direct X-ray and electron-beam lithography of halogenated zeolitic imidazolate frameworks.

Metal-organic frameworks (MOFs) offer disruptive potential in micro- and optoelectronics because of the unique properties of these microporous materials. Nanoscale patterning is a fundamental step in the implementation of MOFs in miniaturized solid-state devices. Conventional MOF patterning methods suffer from low resolution and poorly defined pattern edges. Here, we demonstrate the resist-free, direct X-ray and electron-beam lithography of MOFs. This process avoids etching damage and contamination and leaves the porosity and crystallinity of the patterned MOFs intact. The resulting high-quality patterns have excellent sub-50-nm resolution, and approach the mesopore regime. The compatibility of X-ray and electron-beam lithography with existing micro- and nanofabrication processes will facilitate the integration of MOFs in miniaturized devices.

Combining a Genetically Engineered Oxidase with Hydrogen-Bonded Organic Frameworks (HOFs) for Highly Efficient Biocomposites.

Enzymes incorporated into hydrogen-bonded organic frameworks (HOFs) via bottom-up synthesis are promising biocomposites for applications in catalysis and sensing. Here, we explored synthetic incorporation of d-amino acid oxidase (DAAO) with the metal-free tetraamidine/tetracarboxylate-based BioHOF-1 in water. N-terminal enzyme fusion with the positively charged module Zbasic2 strongly boosted the loading (2.5-fold; ≈500 mg enzyme gmaterial-1 ) and the specific activity (6.5-fold; 23 U mg-1 ). The DAAO@BioHOF-1 composites showed superior activity with respect to every reported carrier for the same enzyme and excellent stability during catalyst recycling. Further, extension to other enzymes, including cytochrome P450 BM3 (used in the production of high-value oxyfunctionalized compounds), points to the versatility of genetic engineering as a strategy for the preparation of biohybrid systems with unprecedented properties.

Crystallizing Sub 10 nm Covalent Organic Framework Thin Films via Interfacial-Residual Concomitance.

Synthesis of covalent organic framework (COF) thin films on different supports with high crystallinity and porosity is crucial for their potential applications. We have designed a new synchronized methodology, residual crystallization (RC), to synthesize sub 10 nm COF thin films. These residual crystallized COF thin films showcase high surface area, crystallinity, and conductivity at room temperature. We have used interfacial crystallization (IC) as a rate-controlling tool for simultaneous residual crystallization. We have also diversified the methodology of residual crystallization by utilizing two different crystallization pathways: fiber-to-film (F-F) and sphere-to-film (S-F). In both cases, we could obtain continuous COF thin films with high crystallinity and porosity grown on various substrates (the highest surface area of a TpAzo COF thin film being 2093 m2 g-1). Precise control over the crystallization allows the synthesis of macroscopic defect-free sub 10 nm COF thin films with a minimum thickness of ∼1.8 nm. We have synthesized two COF thin films (TpAzo and TpDPP) using F-F and S-F pathways on different supports such as borosilicate glass, FTO, silicon, Cu, metal, and ITO. Also, we have investigated the mechanism of the growth of these thin films on various substrates with different wettability. Further, a hydrophilic support (glass) was used to grow the thin films in situ for four-probe system device fabrication. All residual crystallized COF thin films exhibit outstanding conductivity values. We could obtain a conductivity of 3.7 × 10-2 mS cm-1 for the TpAzo film synthesized by S-F residual crystallization.

Can 3D electron diffraction provide accurate atomic structures of metal-organic frameworks?

Many framework materials such as metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) are synthesized as polycrystalline powders, which are too small for structure determination by single crystal X-ray diffraction (SCXRD). Here, we show that a three-dimensional (3D) electron diffraction method, namely continuous rotation electron diffraction (cRED), can be used for ab initio structure determination of such materials. As an example, we present the complete structural analysis of a biocomposite, denoted BSA@ZIF-CO3-1, in which Bovine Serum Albumin (BSA) was encapsulated in a zeolitic imidazolate framework (ZIF). Low electron dose was combined with ultrafast cRED data collection to minimize electron beam damage to the sample. We demonstrate that the atomic structure obtained by cRED is as reliable and accurate as that obtained by single crystal X-ray diffraction. The high accuracy and fast data collection open new opportunities for investigation of cooperative phenomena in framework structures at the atomic level.

On the completeness of three-dimensional electron diffraction data for structural analysis of metal-organic frameworks.

Three-dimensional electron diffraction (3DED) has been proven as an effective and accurate method for structure determination of nano-sized crystals. In the past decade, the crystal structures of various new complex metal-organic frameworks (MOFs) have been revealed by 3DED, which has been the key to understand their properties. However, due to the design of transmission electron microscopes (TEMs), one drawback of 3DED experiments is the limited tilt range of goniometers, which often leads to incomplete 3DED data, particularly when the crystal symmetry is low. This drawback can be overcome by high throughput data collection using continuous rotation electron diffraction (cRED), where data from a large number of crystals can be collected and merged. Here, we investigate the effects of improving completeness on structural analysis of MOFs. We use ZIF-EC1, a zeolitic imidazolate framework (ZIF), as an example. ZIF-EC1 crystallizes in a monoclinic system with a plate-like morphology. cRED data of ZIF-EC1 with different completeness and resolution were analyzed. The data completeness increased to 92.0% by merging ten datasets. Although the structures could be solved from individual datasets with a completeness as low as 44.5% and refined to a high precision (better than 0.04 Å), we demonstrate that a high data completeness could improve the structural model, especially on the electrostatic potential map. We further discuss the strategy adopted during data merging. We also show that ZIF-EC1 doped with cobalt can act as an efficient electrocatalyst for oxygen reduction reactions.

High-Throughput Electron Diffraction Reveals a Hidden Novel Metal-Organic Framework for Electrocatalysis.

Metal-organic frameworks (MOFs) are known for their versatile combination of inorganic building units and organic linkers, which offers immense opportunities in a wide range of applications. However, many MOFs are typically synthesized as multiphasic polycrystalline powders, which are challenging for studies by X-ray diffraction. Therefore, developing new structural characterization techniques is highly desired in order to accelerate discoveries of new materials. Here, we report a high-throughput approach for structural analysis of MOF nano- and sub-microcrystals by three-dimensional electron diffraction (3DED). A new zeolitic-imidazolate framework (ZIF), denoted ZIF-EC1, was first discovered in a trace amount during the study of a known ZIF-CO3 -1 material by 3DED. The structures of both ZIFs were solved and refined using 3DED data. ZIF-EC1 has a dense 3D framework structure, which is built by linking mono- and bi-nuclear Zn clusters and 2-methylimidazolates (mIm- ). With a composition of Zn3 (mIm)5 (OH), ZIF-EC1 exhibits high N and Zn densities. We show that the N-doped carbon material derived from ZIF-EC1 is a promising electrocatalyst for oxygen reduction reaction (ORR). The discovery of this new MOF and its conversion to an efficient electrocatalyst highlights the power of 3DED in developing new materials and their applications.

Continuous-Flow Synthesis of ZIF-8 Biocomposites with Tunable Particle Size.

Zeolitic imidazolate framework (ZIF) biocomposites show the capacity to protect and deliver biotherapeutics. To date, the progress in this research area is based on laboratory batch methods. Now, the first continuous flow synthetic method is presented for the encapsulation of a model protein (bovine serum albumin, BSA) and a clinical therapeutic (α1-antitrypsin, AAT) in ZIF-8. The in situ kinetics of nucleation, growth, and crystallization of BSA@ZIF-8 were studied by small-angle X-ray scattering. By controlling the injection time of ethanol, the particle growth could be quenched by ethanol-induced crystallization from amorphous particles to ZIF-8 crystals. The particle size of the biocomposite was tuned in the 40-100 nm range by varying residence time prior to introduction of ethanol. As a proof-of-concept, this procedure was used for the encapsulation of AAT in ZIF-8. Upon release of the biotherapeutic from the composite, the trypsin inhibitor function of AAT was preserved.

Enzyme Encapsulation in a Porous Hydrogen-Bonded Organic Framework.

Protection of biological assemblies is critical to applications in biotechnology, increasing the durability of enzymes in biocatalysis or potentially stabilizing biotherapeutics during transport and use. Here we show that a porous hydrogen-bonded organic framework (HOF) constructed from water-soluble tetra-amidinium (1·Cl4) and tetracarboxylate (2) building blocks can encapsulate and stabilize biomolecules to elevated temperature, proteolytic and denaturing agents, and extend the operable pH range for catalase activity. The HOF, which readily retains water within its framework structure, can also protect and retain the activity of enzymes such as alcohol oxidase, that are inactive when encapsulated within zeolitic imidazolate framework (ZIF) materials. Such HOF coatings could provide valid alternative materials to ZIFs: they are metal free, possess larger pore apertures, and are stable over a wider, more biologically relevant pH range.

Enhanced Activity of Enzymes Encapsulated in Hydrophilic Metal-Organic Frameworks.

Encapsulation of biomacromolecules in metal-organic frameworks (MOFs) can preserve biological functionality in harsh environments. Despite the success of this approach, termed biomimietic mineralization, limited consideration has been given to the chemistry of the MOF coating. Here, we show that enzymes encapsulated within hydrophilic MAF-7 or ZIF-90 retain enzymatic activity upon encapsulation and when exposed to high temperatures, denaturing or proteolytic agents, and organic solvents, whereas hydrophobic ZIF-8 affords inactive catalase and negligible protection to urease.

Encapsulation, Visualization and Expression of Genes with Biomimetically Mineralized Zeolitic Imidazolate Framework-8 (ZIF-8).

Recent work in biomolecule-metal-organic framework (MOF) composites has proven to be an effective strategy for the protection of proteins. However, for other biomacromolecules such as nucleic acids, the encapsulation into nano MOFs and the related characterizations are in their infancy. Herein, encapsulation of a complete gene-set in zeolitic imidazolate framework-8 (ZIF-8) MOFs and cellular expression of the gene delivered by the nano MOF composites are reported. Using a green fluorescent protein (GFP) plasmid (plGFP) as a proof-of-concept genetic macromolecule, successful transfection of mammalian cancer cells with plGFP for up to 4 days is shown. Cell transfection assays and soft X-ray cryo-tomography (cryo-SXT) demonstrate the feasibility of DNA@MOF biocomposites as intracellular gene delivery vehicles. Expression occurs over relatively prolonged time points where the cargo nucleic acid is released gradually in order to maintain sustained expression.

MOF-on-MOF: Oriented Growth of Multiple Layered Thin Films of Metal-Organic Frameworks.

The precise alignment of multiple layers of metal-organic framework (MOF) thin films, or MOF-on-MOF films, over macroscopic length scales is presented. The MOF-on-MOF films are fabricated by epitaxially matching the interface. The first MOF layer (Cu2 (BPDC)2 , BPDC=biphenyl-4,4'-dicarboxylate) is grown on an oriented Cu(OH)2 film by a "one-pot" approach. Aligned second (Cu2 (BDC)2 , BDC=benzene 1,4-dicarboxylate, or Cu2 (BPYDC)2 , BPYDC=2,2'-bipyridine-5,5'-dicarboxylate) MOF layers can be deposited using liquid-phase epitaxy. The co-orientation of the MOF films is confirmed by X-ray diffraction. Importantly, our strategy allows for the synthesis of aligned MOF films, for example, Cu2 (BPYDC)2 , that cannot be grown on a Cu(OH)2 surface. We show that aligned MOF films furnished with Ag nanoparticles show a unique anisotropic plasmon resonance. Our MOF-on-MOF approach expands the chemistry of heteroepitaxially oriented MOF films and provides a new toolbox for multifunctional porous coatings.

Centimetre-scale micropore alignment in oriented polycrystalline metal-organic framework films via heteroepitaxial growth.

The fabrication of oriented, crystalline films of metal-organic frameworks (MOFs) is a critical step toward their application to advanced technologies such as optics, microelectronics, microfluidics and sensing. However, the direct synthesis of MOF films with controlled crystalline orientation remains a significant challenge. Here we report a one-step approach, carried out under mild conditions, that exploits heteroepitaxial growth for the rapid fabrication of oriented polycrystalline MOF films on the centimetre scale. Our methodology employs crystalline copper hydroxide as a substrate and yields MOF films with oriented pore channels on scales that primarily depend on the dimensions of the substrate. To demonstrate that an anisotropic crystalline morphology can translate to a functional property, we assembled a centimetre-scale MOF film in the presence of a dye and showed that the optical response could be switched 'ON' or 'OFF' by simply rotating the film.

Metal-Organic Frameworks at the Biointerface: Synthetic Strategies and Applications.

Many living organisms are capable of producing inorganic materials of precisely controlled structure and morphology. This ubiquitous process is termed biomineralization and is observed in nature from the macroscale (e.g., formation of exoskeletons) down to the nanoscale (e.g., mineral storage and transportation in proteins). Extensive research efforts have pursued replicating this chemistry with the overarching aims of synthesizing new materials of unprecedented physical properties and understanding the complex mechanisms that occur at the biological-inorganic interface. Recently, we demonstrated that a class of porous materials termed metal-organic frameworks (MOFs) can spontaneously form on protein-based hydrogels via a process analogous to natural matrix-mediated biomineralization. Subsequently, this strategy was extended to functional biomacromolecules, including proteins and DNA, which have been shown to seed and accelerate crystallization of MOFs. Alternative strategies exploit co-precipitating agents such as polymers to induce MOF particle formation thus facilitating protein encapsulation within the porous crystals. In these examples the rigid molecular architecture of the MOF was found to form a protective coating around the biomacromolecule offering improved stability to external environments that would normally lead to its degradation. In this way, the MOF shell mimics the protective function of a biomineralized exoskeleton. Other methodologies have also been explored to encapsulate enzymes within MOF structures, including the fabrication of polycrystalline hollow MOF microcapsules that preserve the original enzyme functionality over several batch reaction cycles. The potential to design MOFs of varied pore size and chemical functionality has underpinned studies describing the postsynthesis infiltration of enzymes into MOF pore networks and bioconjugation strategies for the decoration of the MOF outer surface, respectively. These methods and configurations allow for customized biocomposites. MOF biocomposites have been extended from simple proteins to complex biological systems including viruses, living yeast cells, and bacteria. Indeed, a noteworthy result was that cells encapsulated within a crystalline MOF shell remain viable after exposure to a medium containing lytic enzymes. Furthermore, the cells can adsorb nutrients (glucose) through the MOF shell but cease reproducing until the MOF casing is removed, at which point normal cellular activity is fully restored. The field of MOF biocomposites is expansive and rapidly developing toward different applied research fields including protection and delivery of biopharmaceuticals, biosensing, biocatalysis, biobanking, and cell and virus manipulation. This Account describes the current progress of MOFs toward biotechnological applications highlighting the different strategies for the preparation of biocomposites, the developmental milestones, the challenges, and the potential impact of MOFs to the field.

An updated roadmap for the integration of metal-organic frameworks with electronic devices and chemical sensors.

Metal-organic frameworks (MOFs) are typically highlighted for their potential application in gas storage, separations and catalysis. In contrast, the unique prospects these porous and crystalline materials offer for application in electronic devices, although actively developed, are often underexposed. This review highlights the research aimed at the implementation of MOFs as an integral part of solid-state microelectronics. Manufacturing these devices will critically depend on the compatibility of MOFs with existing fabrication protocols and predominant standards. Therefore, it is important to focus in parallel on a fundamental understanding of the distinguishing properties of MOFs and eliminating fabrication-related obstacles for integration. The latter implies a shift from the microcrystalline powder synthesis in chemistry labs, towards film deposition and processing in a cleanroom environment. Both the fundamental and applied aspects of this two-pronged approach are discussed. Critical directions for future research are proposed in an updated high-level roadmap to stimulate the next steps towards MOF-based microelectronics within the community.

Correction: An updated roadmap for the integration of metal-organic frameworks with electronic devices and chemical sensors.

Correction for 'An updated roadmap for the integration of metal-organic frameworks with electronic devices and chemical sensors' by Ivo Stassen et al., Chem. Soc. Rev., 2017, DOI: 10.1039/c7cs00122c.

Chemical vapour deposition of zeolitic imidazolate framework thin films.

Integrating metal-organic frameworks (MOFs) in microelectronics has disruptive potential because of the unique properties of these microporous crystalline materials. Suitable film deposition methods are crucial to leverage MOFs in this field. Conventional solvent-based procedures, typically adapted from powder preparation routes, are incompatible with nanofabrication because of corrosion and contamination risks. We demonstrate a chemical vapour deposition process (MOF-CVD) that enables high-quality films of ZIF-8, a prototypical MOF material, with a uniform and controlled thickness, even on high-aspect-ratio features. Furthermore, we demonstrate how MOF-CVD enables previously inaccessible routes such as lift-off patterning and depositing MOF films on fragile features. The compatibility of MOF-CVD with existing infrastructure, both in research and production facilities, will greatly facilitate MOF integration in microelectronics. MOF-CVD is the first vapour-phase deposition method for any type of microporous crystalline network solid and marks a milestone in processing such materials.

An Enzyme-Coated Metal-Organic Framework Shell for Synthetically Adaptive Cell Survival.

A bioactive synthetic porous shell was engineered to enable cells to survive in an oligotrophic environment. Eukaryotic cells (yeast) were firstly coated with a ß-galactosidase (ß-gal), before crystallization of a metal-organic framework (MOF) film on the enzyme coating; thereby producing a bioactive porous synthetic shell. The ß-gal was an essential component of the bioactive shell as it generated nutrients (that is, glucose and galactose) required for cell viability in nutrient-deficient media (lactose-based). Additionally, the porous MOF coating carried out other vital functions, such as 1) shielding the cells from cytotoxic compounds and radiation, 2) protecting the non-native enzymes (ß-gal in this instance) from degradation and internalization, and 3) allowing for the diffusion of molecules essential for the survival of the cells. Indeed, this bioactive porous shell enabled the survival of cells in simulated extreme oligotrophic environments for more than 7 days, leading to a decrease in cell viability less than 30 %, versus a 99 % decrease for naked yeast. When returned to optimal growth conditions the bioactive porous exoskeleton could be removed and the cells regained full growth immediately. The construction of bioactive coatings represents a conceptually new and promising approach for the next-generation of cell-based research and application, and is an alternative to synthetic biology or genetic modification.

A Robust Metal-Organic Framework for Dynamic Light-Induced Swing Adsorption of Carbon Dioxide.

Adsorbents for CO2 capture need to demonstrate efficient release. Light-induced swing adsorption (LISA) is an attractive new method to release captured CO2 that utilizes solar energy rather than electricity. MOFs, which can be tailored for use in LISA owing to their chemical functionality, are often unstable in moist atmospheres, precluding their use. A MOF is used that can release large quantities of CO2 via LISA and is resistant to moisture across a large pH range. PCN-250 undergoes LISA, with UV flux regulating the CO2 desorption capacity. Furthermore, under UV light, the azo residues within PCN-250 have constrained, local, structural flexibility. This is dynamic, rapidly switching back to the native state. Reusability tests demonstrate a 7.3 % and 4.9 % loss in both adsorption and LISA capacity after exposure to water for five cycles. These minimal changes confirm the structural robustness of PCN-250 and its great potential for triggered release applications.