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Strategies for improving the functionality of zeolitic imidazolate frameworks

Strategies for improving the functionality of zeolitic imidazolate frameworks

Tailoring nanoarchitectures for functional applications

   Zeolitic imidazole frameworks (ZIFs), a subclass of metal-organic frameworks (MOFs) built with tetrahedral metal ions and imidazolates, offer permanent porosity and high thermal and chemical stability. While ZIFs possess some attractive physical and chemical properties, it remains important to enhance their functionality for practical applications. Here, we provide an overview of the extensive strategies that have been developed to improve the functionality of ZIFs, including linker modifications and functional ZIF hybridization via the encapsulation of guest species (e.g., metal and metal oxide NPs and biomolecules).

Figure 1. An illustration showing the functionality of ZIF-derived materials.

Encapsulation of biomolecules in ZIFs

   
The coupling of MOFs with enzymes either by physical adsorption or covalent binding of the enzyme molecules to the presynthesized host can impart molecularly specific functions to the enzymes.[1] In the case of using a MOF with microporous windows for encapsulating an enzyme with a larger dimension than its normal window or cage size, the enzyme has to be encapsulated by constructing the MOF in the presence of the enzyme, otherwise change in the conformation of the protein can allow it to travel through the nanopores. However, such a phenomenon is not yet known for ZIF materials. To date, there are two main methods that have been used to encapsulate enzymes in MOFs, namely, de novo encapsulation (including coprecipitation and biomineralization) and postsynthetic encapsulations (mainly used for channel-type and cage-type MOFs).

   
Highly functional ZIF-enzyme composites have been synthesized by employing a de novo approach using ZIFs with a pore size smaller than the size of the mixture of two or more enzymes. For instance, ZIF-90, with a pore window of ~0.35 nm, could be used to selectively encapsulate the catalase enzyme, a hydrogen peroxidase, with an average size of ~10 nm (Figure 2). This unique concept of size-selective encapsulation has been shown to increase the resistance of embedded enzymes to inhibitors or proteases, such as proteinase K.[2] The true embedding of the enzyme, catalase in this case, can be demonstrated by comparing its gel-electrophoresis behavior with that of a fluorescent-labeled catalase. Despite the lower rate of H2O2 degradation of CAT@ZIF-90 in the presence of protease because of the encapsulation of the enzyme in ZIF-90 and due to the mass-transport limitations or nonoptimized interfaces between ZIF-90 and catalase, the de novo aqueous embedding method can lead to a functional composite with more active enzyme than an embedding method that uses conventional ZIF-90 synthesis in ethanol, an agent that may denature catalase or other embedded enzymes. This type of size-selective encapsulation to protect an enzyme from digestion or inhibition may offer a novel tool to immobilize and impart new functions to biomolecules (DNA, RNA, proteins) for biomedical applications.

Figure 2. Schematic illustration of the selective encapsulation of catalase in ZIF-90 with high functional activity for hydrogen peroxide decomposition.

Hybridization of ZIFs with polymeric matrix

   Even though ZIF particles have shown promising properties for gas and liquid separations, they are often present as dry powders, which are unfavorable for industrial-scale applications. To improve the practical applications of ZIF for gas and liquid separations, they can be incorporated as additives into polymeric matrices to form mixed matrix membranes (MMMs). Despite the advantages exhibited by ZIF-8, there are several challenges for synthesizing ZIF-8-based MMMs, including the following: (i) achieving good ZIF-8 particle dispersion in the polymer matrix without significant agglomeration and (ii) avoiding the formation of nonselective interfacial voids between the polymer and ZIF-8 particles. To overcome these issues, we have recently developed a drying-free, water-based process for the fabrication of a ZIF-8/polyvinyl alcohol (PVA) MMM with outstanding performance for water/ethanol separation.[3] The proposed water-phase synthesis is highly advantageous, as water molecules can ensure the good distribution of ZIF-8 nanoparticles (NPs) in the PVA matrix. In contrast to conventional methods that involve the drying and re-dispersion of ZIF-8 and result in cracking and phase-separated MMMs (Path A in Figure 3), the new approach does not require a drying step and generates a transparent, crack-free MMM with well-dispersed ZIF-8 NPs (Path B in Figure 3). When employed for the dehydration of an ethanol/water mixture (90:10 w/w) at 25 °C, the PVA/ZIF-8 MMM with 39 wt% ZIF-8 loading showed a significantly larger permeability flux and separation factor values of 2.07 x 106 Barrer and 4725, respectively, compared with a pure PVA membrane (0.75 x 106 Barrer and 543, respectively). Furthermore, the separation performance of these PVA/ZIF-8 MMMs exceeded that of many previous ZIF-based MMMs. The significant enhancement of the separation performance was attributed to the large increase in the fractional free volume and the absence of interfacial voids as a result of the good adherence of PVA to ZIF-8.

Figure 3. Schematic illustration of the preparation of PVA/ZIF-8 MMMs from ZIF-8 suspensions with and without drying.

References
1. Vasiliki Lykourinou, Yao Chen, Xi-Sen Wang, Le Meng, Tran Hoang, Li-June Ming,* Ronald L. Musselman, and Shengqian Ma* (2011). Immobilization of MP-11 into a Mesoporous Metal–Organic Framework, MP-11@mesoMOF: A New Platform for Enzymatic Catalysis. Journal of the American Chemical Society, 133(27), 10382-10385. DOI:10.1021/ja2038003.
2. Fa-Kuen Shieh,* Shao-Chun Wang, Chia-I Yen, Chang-Cheng Wu, Saikat Dutta, Lien-Yang Chou, Joseph V. Morabito, Pan Hu, Ming-Hua Hsu, Kevin C.-W. Wu,* and Chia-Kuang Tsung* (2015). Imparting Functionality to Biocatalysts via Embedding Enzymes into Nanoporous Materials by a de Novo Approach: Size-Selective Sheltering of Catalase in Metal–Organic Framework Microcrystals. Journal of the American Chemical Society, 137(13), 4276-4279. DOI:10.1021/ja513058h.
3. Yu-Heng Deng, Jung-Tsai Chen, Chia-Hao Chang, Kuo-Sung Liao, Kuo-Lun Tung,* William E. Price, Yusuke Yamauchi, and Kevin C.-W. Wu* (2016). A Drying-Free, Water-Based Process for Fabricating Mixed-Matrix Membranes with Outstanding Pervaporation Performance. Angewandte Chemie International Edition, 55(41), 12793-12796. DOI:10.1002/anie.201607014.

Kevin C.-W. Wu
Department of Chemical Engineering
kevinwu@ntu.edu.tw