Highly Flexible Dielectric Films from Solution Processable Covalent Organic Frameworks.

Covalent organic frameworks (COFs) are known to be a promising class of materials for a wide range of applications, yet their poor solution processability limits their utility in many areas. Here we report a pore engineering method using hydrophilic side chains to improve the processability of hydrazone and ß-ketoenamine-linked COFs and the production of flexible, crystalline films. Mechanical measurements of the free-standing COF films of COF-PEO-3 (hydrazone-linked) and TFP-PEO-3 (ß-ketoenamine-linked), revealed a Young's modulus of 391.7 MPa and 1034.7 MPa, respectively. The solubility and excellent mechanical properties enabled the use of these COFs in dielectric devices. Specifically, the TFP-PEO-3 film-based dielectric capacitors display simultaneously high dielectric constant and breakdown strength, resulting in a discharged energy density of 11.22 J cm-3 . This work offers a general approach for producing solution processable COFs and mechanically flexible COF-based films, which hold great potential for use in energy storage and flexible electronics applications.

Carrier gas triggered controlled biolistic delivery of DNA and protein therapeutics from metal-organic frameworks.

The efficacy and specificity of protein, DNA, and RNA-based drugs make them popular in the clinic; however, these drugs are often delivered via injection, requiring skilled medical personnel, and producing biohazardous waste. Here, we report an approach that allows for their controlled delivery, affording either a burst or slow release without altering the formulation. We show that when encapsulated within zeolitic-imidazolate framework eight (ZIF-8), the biomolecules are stable in powder formulations and can be inoculated with a low-cost, gas-powered "MOF-Jet" into living animal and plant tissues. Additionally, their release profiles can be modulated through judicious selection of the carrier gas used in the MOF-Jet. Our in vitro and in vivo studies reveal that when CO2 is used, it creates a transient and weakly acidic local environment that causes a near-instantaneous release of the biomolecules through an immediate dissolution of ZIF-8. Conversely, when air is used, ZIF-8 biodegrades slowly, releasing the biomolecules over a week. This is the first example of controlled-biolistic delivery of biomolecules using ZIF-8, which provides a powerful tool for fundamental and applied science research.

Supramolecular Reinforcement of a Large-Pore 2D Covalent Organic Framework.

Two-dimensional covalent organic frameworks (2D-COFs) are a class of crystalline porous organic polymers that consist of covalently linked, two-dimensional sheets that can stack together through noncovalent interactions. Here we report the synthesis of a novel COF, called PyCOFamide, which has an experimentally observed pore size that is greater than 6 nm in diameter. This is among the largest pore size reported to date for a 2D-COF. PyCOFamide exhibits permanent porosity and high crystallinity as evidenced by the nitrogen adsorption, powder X-ray diffraction, and high-resolution transmission electron microscopy. We show that the pore size of PyCOFamide is large enough to accommodate fluorescent proteins such as Superfolder green fluorescent protein and mNeonGreen. This work demonstrates the utility of noncovalent structural reinforcement in 2D-COFs to produce larger and persistent pore sizes than previously possible.

Stabilization of supramolecular membrane protein-lipid bilayer assemblies through immobilization in a crystalline exoskeleton.

Artificial native-like lipid bilayer systems constructed from phospholipids assembling into unilamellar liposomes allow the reconstitution of detergent-solubilized transmembrane proteins into supramolecular lipid-protein assemblies called proteoliposomes, which mimic cellular membranes. Stabilization of these complexes remains challenging because of their chemical composition, the hydrophobicity and structural instability of membrane proteins, and the lability of interactions between protein, detergent, and lipids within micelles and lipid bilayers. In this work we demonstrate that metastable lipid, protein-detergent, and protein-lipid supramolecular complexes can be successfully generated and immobilized within zeolitic-imidazole framework (ZIF) to enhance their stability against chemical and physical stressors. Upon immobilization in ZIF bio-composites, blank liposomes, and model transmembrane metal transporters in detergent micelles or embedded in proteoliposomes resist elevated temperatures, exposure to chemical denaturants, aging, and mechanical stresses. Extensive morphological and functional characterization of the assemblies upon exfoliation reveal that all these complexes encapsulated within the framework maintain their native morphology, structure, and activity, which is otherwise lost rapidly without immobilization.

Balancing Self-Healing and Shape Stability in Dynamic Covalent Photoresins for Stereolithography 3D Printing.

Dynamic covalent bonds impart new properties to 3D printable materials that help to establish 3D printing as an accessible and efficient manufacturing technique. Here, we studied the effect of a thermally reversible Diels-Alder cross-linker on the shape stability of photoprintable resins and their self-healing properties. Resins containing different concentrations of dynamic covalent cross-links in a polyacrylate network showed that the content of dynamic cross-links plays a key role in balancing shape stability with self-healing ability. The shape stability of the printed objects was evaluated by measuring the dimensional changes after thermal treatment. The self-healing efficiency of the 3D printed resins was characterized with a scratch test and tensile testing. A dynamic covalent cross-link concentration of 1.8 mol % was enough to provide 99% self-healing efficiency without disrupting the shape stability of the printed objects. Our work shows the potential of dynamic covalent bonds in broadening the availability of 3D printable materials that are compatible with vat photopolymerization.

The Importance of Evaluating the Lot-to-Lot Batch Consistency of Commercial Multi-Walled Carbon Nanotube Products.

The biological response of multi-walled carbon nanotubes (MWNTs) is related to their physicochemical properties and a thorough MWNT characterization should accompany an assessment of their biological activity, including their potential toxicity. Beyond characterizing the physicochemical properties of MWNTs from different sources or manufacturers, it is also important to characterize different production lots of the same MWNT product from the same vendor (i.e., lot-to-lot batch consistency). Herein, we present a comprehensive physicochemical characterization of two lots of commercial pristine MWNTs (pMWNTs) and carboxylated MWNTs (cMWNTs) used to study the response of mammalian macrophages to MWNTs. There were many similarities between the physicochemical properties of the two lots of cMWNTs and neither significantly diminished the 24-h proliferation of RAW 264.7 macrophages up to the highest concentration tested (200 µg cMWNTs/mL). Conversely, several physicochemical properties of the two lots of pMWNTs were different; notably, the newer lot of pMWNTs displayed less oxidative stability, a higher defect density, and a smaller amount of surface oxygen species relative to the original lot. Furthermore, a 72-h half maximal inhibitory concentration (IC-50) of ~90 µg pMWNTs/mL was determined for RAW 264.7 cells with the new lot of pMWNTs. These results demonstrate that subtle physicochemical differences can lead to significantly dissimilar cellular responses, and that production-lot consistency must be considered when assessing the toxicity of MWNTs.

2D-Covalent Organic Frameworks with Interlayer Hydrogen Bonding Oriented through Designed Nonplanarity.

We report the synthesis and characterization of a new class of 2D-covalent organic frameworks, called COFamides, whose layers are held together by amide hydrogen bonds. To accomplish this, we have designed monomers with a nonplanar structure that arises from steric crowding, forcing the amide side groups out of plane with the COF sheets orienting the hydrogen bonds between the layers. The presence of these hydrogen bonds provides significant structural stabilization as demonstrated by comparison to control structures that lack hydrogen bonding capability, resulting in lower surface area and crystallinity. We have characterized both azine and imine-linked versions of these COFs, named COFamide-1 and -2, respectively, for their surface areas, pore sizes, and crystallinity. In addition to these more conventional characterization methods, we also used variable temperature infrared spectroscopy methods and van der Waals density functional calculations to directly observe the presence of hydrogen bonding.

Hierarchical Porous Carbon Arising from Metal-Organic Framework-Encapsulated Bacteria and Its Energy Storage Potential.

Hierarchical porous carbons (HPCs) hold great promise in energy-related applications owing to their excellent chemical stability and well-developed porous structures. Attention has been drawn toward developing new synthetic strategies and precursor materials that permit greater control over composition, size, morphology, and pore structure. There is a growing trend of employing metal-organic frameworks (MOFs) as HPC precursors as their highly customizable characteristics favor new HPC syntheses. In this article, we report a biomimetically grown bacterial-templated MOF synthesis where the bacteria not only facilitate the formation of MOF nanocrystals but also provide morphology and porosity control. The resultant HPCs show improved electrochemical capacity behavior compared to pristine MOF-derived HPCs. Considering the broad availability of bacteria and ease of their production, in addition to significantly improved MOF growth efficiency on bacterial templates, we believe that the bacterial-templated MOF is a promising strategy to produce a new generation of HPCs.

Supramolecular design in 2D covalent organic frameworks.

2D covalent organic frameworks (COFs) are a class of porous polymers with highly crystalline structures and tunable function. The structure of a 2D-COF consists of two dimensional sheets held together through covalent bonds which are then stacked together through non-covalent forces. Since their first report, the synthesis of new COFs has relied mostly on imparting functionality to the monomer structures through covalent modification, or through the use of new thermodynamically controlled covalent bond forming methods. This tutorial review will discuss recent efforts to use supramolecular design to leverage the non-covalent forces between COF monomers and sheets to improve their properties and function. The importance of supramolecular interactions in COFs to their mechanisms of formation and overall structure will also be covered.