Hydrogen Separation Performance of UiO-66-NH2 Membranes Grown via Liquid-Phase Epitaxy Layer-by-Layer Deposition and One-Pot Synthesis.

The quality assurance of hydrogen fuel for mobile applications is assessed by the guidelines and directives given in the European and international standards. However, the presence of impurities in the hydrogen fuel, in particular nitrogen, water, and oxygen, is experienced in several refueling stations. Within this work, metal-organic framework (MOF)-based membranes are investigated as a fine-purification stage of the hydrogen fuel. Three H2/N2 concentrations have been used to analyze the separation factor of UiO-66-NH2 membranes prepared using the layer-by-layer (LBL) and the one-pot (OP) synthesis methods. It is shown that the separation factor for an equimolar ratio is 14.4% higher for the LBL sample compared to the OP membrane, suggesting a higher orientation and continuity of the LBL surface-supported metal-organic framework (SURMOF). Using an equimolar ratio of H2/N2, it is shown that selective separation of hydrogen over nitrogen occurs with a separation factor of 3.02 and 2.64 for the SURMOF and MOF membrane, respectively. To the best of our knowledge, this is the highest reported performance for a single-phase UiO-66-NH2 membrane. For higher hydrogen concentrations, the separation factor decreases due to reduced interactions between pore walls and N2 molecules.

Reversible Diels-Alder and Michael Addition Reactions Enable the Facile Postsynthetic Modification of Metal-Organic Frameworks.

Functionalization of metal-organic frameworks (MOFs) is critical in exploring their structural and chemical diversity for numerous potential applications. Herein, we report multiple approaches for the tandem postsynthetic modification (PSM) of various MOFs derived from Zr(IV), Al(III), and Zn(II). Our current work is based on our efforts to develop a wide range of MOF platforms with a dynamic functional nature that can be chemically switched via thermally triggered reversible Diels-Alder (DA) and hetero-Diels-Alder (HDA) ligations. Furan-tagged MOFs (furan-UiO-66-Zr) were conjugated with maleimide groups bearing dienophiles to prepare MOFs with a chemically switchable nature. As HDA pairs, phosphoryl dithioester-based moieties and cyclopentadiene (Cp)-grafted MOF (Cp-MIL-53-Al) were utilized to demonstrate the cleavage and rebonding of the linkages as a function of temperature. In addition to these strategies, the Michael addition reaction was also applied for the tandem PSM of IRMOF-3-Zn. Maleimide groups were postsynthetically introduced in the MOF lattice, which were further ligated with cysteine-based biomolecules via the thiol-maleimide Michael addition reaction. On the basis of the versatility of the herein presented chemistry, we expect that these approaches will help in designing a variety of sophisticated functional MOF materials addressing diverse applications.

Introducing electrical conductivity to metal-organic framework thin films by templated polymerization of methyl propiolate.

We herein present a case study on the templated, Pd-catalyzed polymerization reaction of methyl propiolate in the confined pore space of three different surface anchored metal-organic framework (SURMOF) systems in order to introduce electrical conductivity to MOF thin films and provide predictions for potential device integrations. To gain comprehensive insight into the influence of the template on polymerization, we chose Cu(bpdc), Cu2(bdc)2(dabco) and HKUST-1 because of their different types of pore channels, 1D, quasi-1D and 3D, and their free pore volumes. Well-defined MOF thin films were prepared using layer-by-layer deposition, which allows for the application of several characterization techniques not applicable for conventional powder MOFs. With SEM, AFM, XRD, MALDI-ToF/MS, ToF-SIMS and QCM, we were able to investigate the behaviour of the polymer formation. For lower dimensional pore channels, we find a depot-like release of monomeric units leading to top-layer formation determined by desorption kinetics, whereas for the 3D channels, quick release of an excess amount of monomers was observed and polymerization proceeds perfectly. Despite polymerization issues, control over the maximum chain lengths and the molecular weight distribution was achieved depending on the dimensionality of the pore systems. For the HKUST-1 system, polymerization was optimized and we were able to measure the electrical conductivity introduced by the conjugated polymer inside the channels.

Conductive Metal-Organic Framework Thin Film Hybrids by Electropolymerization of Monosubstituted Acetylenes.

1-Hexyne monomers were potentiostatically electropolymerized upon confinement in 1D channels of a surface-mounted metal-organic framework Cu(BDC) (SURMOF-2). A layer-by-layer deposition method allowed for SURMOF depostition on substrates with prepatterned electrodes, making it possible to characterize electrical conductivity in situ, i.e., without having to delaminate the conductive polymer thin film. Successful polymerization was evidenced by mass spectroscopy, and the electrical measurements demonstrated an increase of the electrical conductivity of the MOF material by 8 orders of magnitude. Extensive DFT calculations revealed that the final conductivity is limited by electron hopping between the conductive oligomers.

Liquid-Phase Quasi-Epitaxial Growth of Highly Stable, Monolithic UiO-66-NH2 MOF thin Films on Solid Substrates.

Invited for this month's cover is the group of Dr. Tawheed Hashem from the Karlsruhe Institute of Technology. The cover picture shows a successful synthesis of high quality, monolithic UiO-66-NH2 MOF thin films on diverse solid substrates via a low-temperature liquid phase epitaxy method. The achievement of continuous MOF-coatings with low defect densities and pronounced stability against high temperatures and hot water was proven. The new type of coatings clearly outperforms other reported types of MOF thin films. Read the full text of their Communication at https://doi.org/10.1002/open.201900324.

Liquid-Phase Quasi-Epitaxial Growth of Highly Stable, Monolithic UiO-66-NH2 MOF thin Films on Solid Substrates.

High quality, monolithic UiO-66-NH2 thin films on diverse solid substrates have been prepared via a low temperature liquid phase epitaxy method. The achievement of continuous films with low defect densities and great stability against high temperatures and hot water is proven, clearly outperforming other reported types of MOF thin films.

Siloxane-functionalised surface patterns as templates for the ordered deposition of thin lamellar objects.

A novel method is demonstrated for ordered deposition of thin lamellar objects from a liquid environment onto solid substrates by solid/fluid/solid-driven organisation. Surface functionalisation forms a template pattern that accumulates the lamellar objects by site-selective wetting of the target area without the need for a physical fluid containment. Contrary to conventional handling methods, no mechanical contact occurs, which facilitates the ordered deposition without wrinkles or ruptures. An additive and a subtractive process for the creation of such templates are presented. The subtractive process starts with the complete silanisation of the substrate in the vapour phase followed by site-selective oxygen plasma treatment of the siloxane film. The additive process uses microcontact printing to transfer the target pattern. Both processes are characterised by optical inspection of the wetting contours and it is found that site-selective plasma treatment shows a better pattern fidelity. The patterns obtained by site-selective plasma treatment are also subject to ToF-SIMS analysis and show good chemical contrast between hydrophilic and hydrophobic areas. The ordered deposition of lamellar objects by this new method is demonstrated for 60 nm thick ultramicrotome sections of epoxide resin on pre-patterned glass substrates.

Mobility of charge carriers in self-assembled monolayers.

We present a new approach to study charge transport within 2D layers of organic semi-conductors (OSCs) using atomic force microscopy (AFM)-based lithography applied to self-assembled monolayers (SAMs), fabricated from appropriate organothiols. The extent of lateral charge transport was investigated by insulating pre-defined patches within OSC-based SAMs with regions of insulating SAM made from large band gap alkanethiolates. The new method is demonstrated using a phenyl-linked anthracenethiolate (PAT), 4-(anthracene-2-ylethynyl)benzyl thiolate. I-V characteristics of differently shaped PAT-islands were measured using the AFM tip as a top electrode. We were able to determine a relationship between island size and electrical conductivity, and from this dependence, we could obtain information on the lateral charge transport and charge carrier mobility within the thin OSC layers. Our study demonstrates that AFM nanografting of appropriately functionalized OSC molecules provides a suitable method to determine intrinsic mobilities of charge carriers in OSC thin films. In particular, this method is rather insensitive with regard to influence of grain boundaries and other defects, which hamper the application of conventional methods for the determination of mobilities in macroscopic samples.

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

Thin films of nanoporous metal-organic frameworks, MOFs, are widely used for various purposes, ranging from molecular separation to electronic and optical applications. Ultrathin MOF films prepared in a controlled layer-by-layer fashion, also referred to as surface-mounted MOFs or SURMOFs, have attracted particular attention. While many aspects of SURMOF synthesis have been investigated and optimized, the impact of growth modulators, which are well established in bulk MOF synthesis, has so far received only a little attention in the context of layer-by-layer approaches. Here, we investigate the impact of water as a growth modulator during the synthesis of thin MOF films of type HKUST-1. We find that water has a tremendous impact on the crystallinity and on the defect-density of the prepared MOF films. The use of the optimized water concentrations allows considerable improvement in the SURMOF crystallinity and at the same time reduces their defect density. This study shows that water is an important modulator in MOF thin film growth and can be used to tune the material from a low defect to high defect state.

Electrochemically-Driven Insertion of Biological Nanodiscs into Solid State Membrane Pores as a Basis for "Pore-In-Pore" Membranes.

Nanoporous membranes are of increasing interest for many applications, such as molecular filters, biosensors, nanofluidic logic and energy conversion devices. To meet high-quality standards, e.g., in molecular separation processes, membranes with well-defined pores in terms of pore diameter and chemical properties are required. However, the preparation of membranes with narrow pore diameter distributions is still challenging. In the work presented here, we demonstrate a strategy, a "pore-in-pore" approach, where the conical pores of a solid state membrane produced by a multi-step top-down lithography procedure are used as a template to insert precisely-formed biomolecular nanodiscs with exactly defined inner and outer diameters. These nanodiscs, which are the building blocks of tobacco mosaic virus-deduced particles, consist of coat proteins, which self-assemble under defined experimental conditions with a stabilizing short RNA. We demonstrate that the insertion of the nanodiscs can be driven either by diffusion due to a concentration gradient or by applying an electric field along the cross-section of the solid state membrane. It is found that the electrophoresis-driven insertion is significantly more effective than the insertion via the concentration gradient.

Surface-Anchored Metal-Organic Frameworks as Versatile Resists for Gas-Assisted E-Beam Lithography: Fabrication of Sub-10 Nanometer Structures.

We demonstrate that surface-anchored metal-organic frameworks (SURMOFs) are extraordinary well-suited as resists for high-resolution focused electron beam induced processing (FEBIP) techniques. The combination of such powerful lithographic protocols with the huge versatility of MOF materials are investigated in respect to their potential in nanostructures fabrication. The applied FEBIP methods rely on the local decomposition of Fe(CO)5 and Co(CO)3NO as precursors, either by the direct impact of the focused electron beam (electron beam induced deposition, EBID) or through the interaction of the precursor molecules with preirradiated/activated SURMOF areas (electron beam induced surface activation, EBISA). We demonstrate the huge potential of the approach for two different types of MOFs (HKUST-1 and Zn-DPDCPP). Our "surface science" approach to FEBIP, yields well-defined deposits with each investigated precursor/SURMOF combination. Local Auger electron spectroscopy reveals clean iron deposits from Fe(CO)5; deposits from Co(CO)3NO contain cobalt, nitrogen, and oxygen. EBISA experiments were successful with Fe(CO)5. Remarkably EBISA with Co(CO)3NO does not result in deposit formation on both resists, making the process chemically selective. Most importantly we demonstrate the fabrication of "nested-L" test structures with Fe(CO)5 on HKUST-1 with extremely narrow line widths of partially less than 8 nm, due to reduced electron proximity effects within the MOF-based resists. Considering that the actual diameter of the electron beam was larger than 6 nm, we see a huge potential for significant reduction of the structure sizes. In addition, the role and high potential of loading and transport of the precursor molecules within the porous SURMOF materials is discussed.

Light-Induced Step-Growth Polymerization of AB-Type Photo-Monomers at Ambient Temperature.

We introduce two AB-type monomers able to undergo a facile catalyst-free photoinduced polycycloaddition of photocaged dienes, enabling rapid Diels-Alder ligations under UV-irradiation (λmax = 350 nm) at ambient temperature, closely adhering to Carother's equation established by a careful kinetic study (17800 g mol-1 < Mw < 24700 g mol-1). The resulting macromolecules were in-depth analyzed via size exclusion chromatography (SEC) and nuclear magnetic resonance (NMR) spectroscopy. Additionally, SEC hyphenated to high resolution-electrospray ionization-mass spectrometry (HR-ESI-MS) enabled the careful mapping of the end group structure of the generated polymers. Furthermore, we demonstrate that both monomer systems can be readily copolymerized. The study thus demonstrates that Diels-Alder ligation resting upon photocaged dienes is a powerful tool for accessing step-growth polymers.

Chemically reprogrammable metal organic frameworks (MOFs) based on Diels-Alder chemistry.

We present the reversible Diels-Alder functionalization of metal organic frameworks (MOFs). Cyclopentadiene (Cp) functional MOFs are ligated with dienophiles to fabricate functional MOFs with a reprogrammable chemical nature. Our strategy thus constitutes an unprecedented concept for chemically dynamic MOFs able to be recoded.

Two-in-One: λ-Orthogonal Photochemistry on a Radical Photoinitiating System.

An alkyne functional radical photoinitiator, 2-(4-(2-hydroxy-2-methylpropanoyl)phenoxy)ethyl hex-5-ynoate, and evidence that both reactive moieties - the alkyne and the photoinitiator terminus - can be independently addressed with light of disparate wavelength (λ-orthogonality) are introduced. The alkyne functionality is subjected to a visible light (420 nm) induced copper-catalyzed Huisgen reaction, which is employed for the selective functionalization of the initiator with a poly(ethylene glycol) (PEG) chain. This reaction proceeds completely λ-orthogonal in the presence of the UV-reactive photoinitiating moiety. Conversely, it is demonstrated that the alkyne functionality of the photoinitiator is quantitatively orthogonal to UV irradiation emitted by the pulsing action of an excimer laser (351 nm, pulsed-laser polymerization, PLP) and the generated radical species. In turn, the PEGylated initiator can readily be employed as a macrophotoinitiator during PLP. The introduced λ-orthogonally addressable dual functional initiator can be used in a wide range of applications, including surface lithography and post-synthetic modification of photocured materials.

Novel roles for well-known players: from tobacco mosaic virus pests to enzymatically active assemblies.

The rod-shaped nanoparticles of the widespread plant pathogen tobacco mosaic virus (TMV) have been a matter of intense debates and cutting-edge research for more than a hundred years. During the late 19th century, their behavior in filtration tests applied to the agent causing the 'plant mosaic disease' eventually led to the discrimination of viruses from bacteria. Thereafter, they promoted the development of biophysical cornerstone techniques such as electron microscopy and ultracentrifugation. Since the 1950s, the robust, helically arranged nucleoprotein complexes consisting of a single RNA and more than 2100 identical coat protein subunits have enabled molecular studies which have pioneered the understanding of viral replication and self-assembly, and elucidated major aspects of virus-host interplay, which can lead to agronomically relevant diseases. However, during the last decades, TMV has acquired a new reputation as a well-defined high-yield nanotemplate with multivalent protein surfaces, allowing for an ordered high-density presentation of multiple active molecules or synthetic compounds. Amino acid side chains exposed on the viral coat may be tailored genetically or biochemically to meet the demands for selective conjugation reactions, or to directly engineer novel functionality on TMV-derived nanosticks. The natural TMV size (length: 300 nm) in combination with functional ligands such as peptides, enzymes, dyes, drugs or inorganic materials is advantageous for applications ranging from biomedical imaging and therapy approaches over surface enlargement of battery electrodes to the immobilization of enzymes. TMV building blocks are also amenable to external control of in vitro assembly and re-organization into technically expedient new shapes or arrays, which bears a unique potential for the development of 'smart' functional 3D structures. Among those, materials designed for enzyme-based biodetection layouts, which are routinely applied, e.g., for monitoring blood sugar concentrations, might profit particularly from the presence of TMV rods: Their surfaces were recently shown to stabilize enzymatic activities upon repeated consecutive uses and over several weeks. This review gives the reader a ride through strikingly diverse achievements obtained with TMV-based particles, compares them to the progress with related viruses, and focuses on latest results revealing special advantages for enzyme-based biosensing formats, which might be of high interest for diagnostics employing 'systems-on-a-chip'.

Peptide-equipped tobacco mosaic virus templates for selective and controllable biomineral deposition.

The coating of regular-shaped, readily available nanorod biotemplates with inorganic compounds has attracted increasing interest during recent years. The goal is an effective, bioinspired fabrication of fiber-reinforced composites and robust, miniaturized technical devices. Major challenges in the synthesis of applicable mineralized nanorods lie in selectivity and adjustability of the inorganic material deposited on the biological, rod-shaped backbones, with respect to thickness and surface profile of the resulting coating, as well as the avoidance of aggregation into extended superstructures. Nanotubular tobacco mosaic virus (TMV) templates have proved particularly suitable towards this goal: Their multivalent protein coating can be modified by high-surface-density conjugation of peptides, inducing and governing silica deposition from precursor solutions in vitro. In this study, TMV has been equipped with mineralization-directing peptides designed to yield silica coatings in a reliable and predictable manner via precipitation from tetraethoxysilane (TEOS) precursors. Three peptide groups were compared regarding their influence on silica polymerization: (i) two peptide variants with alternating basic and acidic residues, i.e. lysine-aspartic acid (KD) x motifs expected to act as charge-relay systems promoting TEOS hydrolysis and silica polymerization; (ii) a tetrahistidine-exposing polypeptide (CA4H4) known to induce silicification due to the positive charge of its clustered imidazole side chains; and (iii) two peptides with high ZnO binding affinity. Differential effects on the mineralization of the TMV surface were demonstrated, where a (KD) x charge-relay peptide (designed in this study) led to the most reproducible and selective silica deposition. A homogenous coating of the biotemplate and tight control of shell thickness were achieved.

Electric transport properties of surface-anchored metal-organic frameworks and the effect of ferrocene loading.

Understanding of the electric transport through surface-anchored metal-organic frameworks (SURMOFs) is important both from a fundamental perspective as well as with regards to possible future applications in electronic devices. To address this mostly unexplored subject, we integrated a series of representative SURMOF thin films, formed by copper nodes and trimesic acid and known as HKUST-1, in a mercury-drop-based tunneling junction. Although the transport properties of these SURMOFs are analogous to those of hybrid metal-organic molecular wires, manifested by a very low value of the tunneling decay constant (ß ≈ 0.006 Å(-1)), they are at the same time found to be consistent with a linear increase of resistance with film thickness. Upon loading of SURMOF pores with ferrocene (Fc), a noticeable increase in transport current was observed. A transport model and ab initio electronic structure calculations were used to reveal a hopping transport mechanism and to relate the changes upon Fc loading to those of the electronic and vibrational structures of the SURMOF films.

Hierarchically functionalized magnetic core/multishell particles and their postsynthetic conversion to polymer capsules.

The controlled synthesis of hierarchically functionalized core/multishell particles is highly desirable for applications in medicine, catalysis, and separation. Here, we describe the synthesis of hierarchically structured metal-organic framework multishells around magnetic core particles (magMOFs) via layer-by-layer (LbL) synthesis. The LbL deposition enables the design of multishell systems, where each MOF shell can be modified to install different functions. Here, we used this approach to create controlled release capsules, in which the inner shell serves as a reservoir and the outer shell serves as a membrane after postsynthetic conversion of the MOF structure to a polymer network. These capsules enable the controlled release of loaded dye molecules, depending on the surrounding media.

A photolithographic approach to spatially resolved cross-linked nanolayers.

The preparation of cross-linked nanosheets with 1-2 nm thickness and predefined shape was achieved by lithographic immobilization of trimethacryloyl thioalkanoates onto the surface of Si wafers, which were functionalized with 2-(phenacylthio)acetamido groups via a photoinduced reaction. Subsequent cross-linking via free radical polymerization as well as a phototriggered Diels-Alder reaction under mild conditions on the surface led to the desired nanosheets. Electrospray ionization mass spectrometry (ESI-MS), X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), as well as infrared reflection-absorption spectroscopy (IRRAS) confirmed the success of individual surface-modification and cross-linking reactions. The thickness and lateral size of the cross-linked structures were determined by atomic force microscopy (AFM) for samples prepared on Si wafers functionalized with a self-assembled monolayer of 1H,1H,2H,2H-perfluorodecyl groups bearing circular pores obtained via a polymer blend lithographic approach, which led to the cross-linking reactions occurring in circular nanoareas (diameter of 50-640 nm) yielding an average thickness of 1.2 nm (radical cross-linking), 1.8 nm (radical cross-linking in the presence of 2,2,2-trifluoroethyl methacrylate as a comonomer), and 1.1 nm (photochemical cross-linking) of the nanosheets.

Modified TMV Particles as Beneficial Scaffolds to Present Sensor Enzymes.

Tobacco mosaic virus (TMV) is a robust nanotubular nucleoprotein scaffold increasingly employed for the high density presentation of functional molecules such as peptides, fluorescent dyes, and antibodies. We report on its use as advantageous carrier for sensor enzymes. A TMV mutant with a cysteine residue exposed on every coat protein (CP) subunit (TMVCys) enabled the coupling of bifunctional maleimide-polyethylene glycol (PEG)-biotin linkers (TMVCys/Bio). Its surface was equipped with two streptavidin [SA]-conjugated enzymes: glucose oxidase ([SA]-GOx) and horseradish peroxidase ([SA]-HRP). At least 50% of the CPs were decorated with a linker molecule, and all thereof with active enzymes. Upon use as adapter scaffolds in conventional "high-binding" microtiter plates, TMV sticks allowed the immobilization of up to 45-fold higher catalytic activities than control samples with the same input of enzymes. Moreover, they increased storage stability and reusability in relation to enzymes applied directly to microtiter plate wells. The functionalized TMV adsorbed to solid supports showed a homogeneous distribution of the conjugated enzymes and structural integrity of the nanorods upon transmission electron and atomic force microscopy. The high surface-increase and steric accessibility of the viral scaffolds in combination with the biochemical environment provided by the plant viral coat may explain the beneficial effects. TMV can, thus, serve as a favorable multivalent nanoscale platform for the ordered presentation of bioactive proteins.