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1. Expanding the “library” of Metal-Organic Frameworks (MOFs) for enzyme biomineralization

   Jordahl, Drew Jordahl; Armstrong, Zoe; Li, Qiaobin; Gao, Runxiang; Liu, Wei; Johnson, Kelley; Brown, William; Scheiwiller, Allison; Feng, Li; Ugrinov, Angel; et al (October 2022, ACS applied materials interfaces)

   Farha, Omar (Ed.)

   Metal-Organic Frameworks (MOFs) are advanced platforms for enzyme immobilization. Enzymes can be entrapped via either diffusion (into pre-formed MOFs) or co-crystallization. Enzyme co-crystallization with specific metals/ligands in the aqueous phase, also known as biomineralization, minimizes the enzyme loss as compared to organic phase co-crystallization, removes the size limitation on enzymes and substrates, and can potentially broaden the application of enzyme@MOF composites. However, not all enzymes are stable/functional in the presence of excess metal ions and/or ligands currently available for co-crystallization. Furthermore, most current biomineralization-based MOFs have limited (acid-) pH stability, making it necessary to explore other metal-ligand combinations that can also immobilize enzymes. Here, we report our discovery on the combination of five metal ions and two ligands that can form biocomposites with two model enzymes differing in size and hydrophobicity in the aqueous phase under ambient conditions. Surprisingly, most of the formed composites are single- or multi- phase crystals even though the reaction phase is aqueous, with the rest as amorphous powders. All 20 enzyme@MOF composites showed good to excellent reusability, and were stable under weakly acidic pHs. The stability under weakly basic conditions depended on the selection of enzyme and metal-ligand combinations, yet for both enzymes, 3-4 MOFs offered decent stability under basic conditions. This work initiates the expansion of the current “library” of metal-ligand selection for encapsulating/biomineralizing large enzymes/enzyme clusters, leading to customized encapsulation of enzymes according to enzymes stability, functionality, and optimal pH. 

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2. A Protocol for Custom Biomineralization of Enzymes in Metal–Organic Frameworks (MOFs)

   https://doi.org/10.21769/BioProtoc.4930  

   Armstrong, Zoe; Jordahl, Drew; MacRae, Austin; Li, Qiaobi; Lenertz, Mary; Shen, Patrick; Botserovska, Anastasiia; Feng, Li; Ugrinov, Angel; Yang, Zhongyu (January 2024, BIO-PROTOCOL)

   Enzyme immobilization offers a number of advantages that improve biocatalysis; however, finding a proper way to immobilize enzymes is often a challenging task. Implanting enzymes in metal–organic frameworks (MOFs) via co-crystallization, also known as biomineralization, provides enhanced reusability and stability with minimal perturbation and substrate selectivity to the enzyme. Currently, there are limited metal–ligand combinations with a proper protocol guiding the experimental procedures. We have recently explored 10 combinations that allow custom immobilization of enzymes according to enzyme stability and activity in different metals/ligands. Here, as a follow-up of that work, we present a protocol for how to carry out custom immobilization of enzymes using the available combinations of metal ions and ligands. Detailed procedures to prepare metal ions, ligands, and enzymes for their co-crystallization, together with characterization and assessment, are discussed. Precautions for each experimental step and result analysis are highlighted as well. This protocol is important for enzyme immobilization in various research and industrial fields. 

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3. A general Ca-MOM platform with enhanced acid-base stability for enzyme biocatalysis

   https://doi.org/https://doi.org/10.1016/j.checat.2021.03.001  

   Pan, Y.; Li, Q.; Li, H.; Farmakes, J.; Ugrinov, A.; Zhu, X.; Lai, Z.; Chen, B.; Yang, Z. (April 2021, Chem catalysis)

   null (Ed.)

   Co-precipitation of enzymes in metal-organic frameworks is a unique enzyme-immobilization strategy but is challenged by weak acid-base stability. To overcome this drawback, we discovered that Ca2+ can co-precipitate with carboxylate ligands and enzymes under ambient aqueous conditions and form enzyme@metal-organic material composites stable under a wide range of pHs (3.7–9.5). We proved this strategy on four enzymes with varied isoelectric points, molecular weights, and substrate sizes—lysozyme, lipase, glucose oxidase (GOx), and horseradish peroxidase (HRP)—as well as the cluster of HRP and GOx. Interestingly, the catalytic efficiency of the studied enzymes was found to depend on the ligand, probing the origins of which resulted in a correlation among enzyme backbone dynamics, ligand selection, and catalytic efficiency. Our approach resolved the long-lasting stability issue of aqueous-phase co-precipitation and can be generalized to biocatalysis with other enzymes to benefit both research and industry. 

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4. Rational design of stable functional metal–organic frameworks

   https://doi.org/10.1039/d3mh00541k  

   Chen, Zhijie; Kirlikovali, Kent O.; Shi, Le; Farha, Omar K. (July 2023, Materials Horizons)

   Functional porous metal–organic frameworks (MOFs) have been explored for a number of potential applications in catalysis, chemical sensing, water capture, gas storage, and separation. MOFs are among the most promising candidates to address challenges facing our society related to energy and environment, but the successful implementation of functional porous MOF materials are contingent on their stability; therefore, the rational design of stable MOFs plays an important role towards the development of functional porous MOFs. In this Focus article, we summarize progress in the rational design and synthesis of stable MOFs with controllable pores and functionalities. The implementation of reticular chemistry allows for the rational top-down design of stable porous MOFs with targeted topological networks and pore structures from the pre-selected building blocks. We highlight the reticular synthesis and applications of stable MOFs: (1) MOFs based on high valent metal ions ( e.g. , Al 3+ , Cr 3+ , Fe 3+ , Ti 4+ and Zr 4+ ) and carboxylate ligands; (2) MOFs based on low valent metal ions ( e.g. , Ni 2+ , Cu 2+ , and Zn 2+ ) and azolate linkers. We envision that the synthetic strategies, including modulated synthesis and post-synthetic modification, can potentially be extended to other more complex systems like metal-phosphonate framework materials. 

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5. Structure-Property Relationship Studies of Electrically Conductive Metal-Organic Framework Materials

   Zhang, Shiyu (May 2025, Clemson University, Tiger Prints https://open.clemson.edu/all_dissertations/3631)

   ABSTRACT Electricity is vital for modern life. With the rapid development of industries, society, and the ever-increasing population worldwide, the demand for electricity and advanced modern electronic devices with multifunctionality has increased tremendously. Therefore, advanced electronic materials that meet the requirements of the modern electronics and energy industry are needed. Electrically conductive metal-organic frameworks (EC-MOFs), composed of metal clusters acting as nodes and organic ligands as linkers connected by coordination bonds, are among the most promising materials due to their tunable structures, properties, and diverse functions. However, the lack of comprehensive understanding of their structure-property relationship hinders the development of EC-MOFs. My dissertation focuses on the design, construction, and structure-property relationship studies of EC-MOFs. Chapter 1 describes how MOFs' structural features and compositions affect the electronic properties and demonstrates conductive mechanisms and the corresponding design strategies to develop EC-MOFs. Chapter 2 describes the synthesis, characterization, electronic and optical properties, and structure-property relationships of four novel alkali metal (Na, K, Rb, and Cs) and an electron-rich tetrathiafulvalene tetracarboxylate (TTFTC) ligand-based 3D-MOFs where the ligands form continuous π-stacks and become partially oxidized by air forming TTFTC•+ radical cations. Depending on the π–π distance and TTFTC•+ population, the MOFs enjoyed different degrees of intervalence charge transfer (IVCT) interaction, which commensurately affected their band gaps and electrical conductivity. Chapter 3 depicts a novel design strategy to enhance the bulk electrical conductivity of 2D graphitic MOFs by promoting simultaneous in-plane and out-of-plane charge transport. To this end, we have synthesized the first π-intercalated two-dimensional graphitic MOF (iGMOF1) through an elegant bottom-up synthesis method. This novel structure featured hexagonal Cu-HATP (hexaaminotriphenylene, HATP) coordination network and built-in alternating π-donor/acceptor stacks of electron-rich HATP ligands and non-coordinatively intercalated π-acceptors hexacyano-triphenylene (HCTP) molecules, which simultaneously facilitated both in-plane and out-of-plane to endow iGMOF1 with higher bulk electrical conductivity compared to pristine Cu3(HATP)2. Chapter 4 describes design, synthesis, and structure-property relationship studies of a novel 2D Cu3(HOTP)(HHTQ) MOF based on a π-donor 2,3,6,7,10,11-hexahydrotriphenylene (HOTP) ligand and a π-acceptor 2,3,7,8,12,13-Hexahydroxytricycloquinazoline (HHTQ) ligand. The resulting framework featured continuous out-of-plane π-D/A stacks and in-plane hexagonal π-D-A sheets, promoting through-space and through-bond charge transport, respectively, which gave rise to a bulk electrical conductivity than pristine Cu3(HOTP)2 and Cu3(HHTQ)2. Finally, Chapter 5 describes the transformation of a collapse-prone, electrically insulating MOF-74 analog into structurally more stable, porous, and electrically conductive MOF/PEDOT composites. 

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