Iron-Mineralization-Induced Mesoporous Metal-Organic Frameworks Enable High-Efficiency Synergistic Catalysis of Natural/Nanomimic Enzymes

Calcium-based MOFs as scaffolds for shielding immobilized lipase and enhancing its stability

The enzyme immobilization technology has become a key tool in the field of enzyme applications; however, improving the activity recovery and stability of the immobilized enzymes is still challenging. Herein, we employed a magnetic carboxymethyl cellulose (MCMC) nanocomposite modified with ionic liquids (ILs) for covalent immobilization of lipase, and used Ca-based metal-organic frameworks (MOFs) as the support skeleton and protective layer for immobilized enzymes. The ILs contained long side chains (eight CH2 units), which not only enhanced the hydrophobicity of the carrier and its hydrophobic interaction with the enzymes, but also provided a certain buffering effect when the enzyme molecules were subjected to compression. Compared to free lipase, the obtained CaBPDC@PPL-IL-MCMC exhibited higher specific activity and enhanced stability. In addition, the biocatalyst could be easily separated using a magnetic field, which is beneficial for its reusability. After 10 cycles, the residual activity of CaBPDC@PPL-IL-MCMC could reach up to 86.9%. These features highlight the good application prospects of the present immobilization method.

The Promise and Potential of Metal-Organic Frameworks and Covalent Organic Frameworks in Vaccine Nanotechnology

The immune system's complexity and ongoing evolutionary struggle against deleterious pathogens underscore the value of vaccination technologies, which have been bolstering human immunity for over two centuries. Despite noteworthy advancements over these 200 years, three areas remain recalcitrant to improvement owing to the environmental instability of the biomolecules used in vaccines─the challenges of formulating them into controlled release systems, their need for constant refrigeration to avoid loss of efficacy, and the requirement that they be delivered via needle owing to gastrointestinal incompatibility. Nanotechnology, particularly metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), has emerged as a promising avenue for confronting these challenges, presenting a new frontier in vaccine development. Although these materials have been widely explored in the context of drug delivery, imaging, and cancer immunotherapy, their role in immunology and vaccine-related applications is a recent yet rapidly developing field. This review seeks to elucidate the prospective use of MOFs and COFs for biomaterial stabilization, eliminating the necessity for cold chains, enhancing antigen potency as adjuvants, and potentializing needle-free delivery of vaccines. It provides an expansive and critical viewpoint on this rapidly evolving field of research and emphasizes the vital contribution of chemists in driving further advancements.

Co-Immobilization of Laccase and Mediator into Fe-Doped ZIF-8 Significantly Enhances the Degradation of Organic Pollutants

Co-immobilization of laccase and mediator 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) for wastewater treatment could simultaneously achieve the reusability of laccase and avoid secondary pollution caused by the toxic ABTS. Herein, Fe-induced mineralization was proposed to co-immobilize laccase and ABTS into a metal-organic framework (ZIF-8) within 30 min. Immobilized laccase (Lac@ZIF-8-Fe) prepared at a 1:1 mass ratio of Fe2+ to Zn2+ exhibited enhanced catalytic efficiency (2.6 times), thermal stability, acid tolerance, and reusability compared to free laccase. ABTS was then co-immobilized to form Lac+ABTS@ZIF-8-Fe (ABTS = 261.7 mg/g). Lac@ZIF-8-Fe exhibited significantly enhanced bisphenol A (BPA) removal performance over free laccase due to the local substrate enrichment effect and improved enzyme stability. Moreover, the Lac+ABTS@ZIF-8-Fe exhibited higher BPA removal efficiency than the free laccase+ABTS system, implying the presence of a proximity effect in Lac+ABTS@ZIF-8-Fe. In the successive malachite green (MG) removal, the MG degradation efficiency by Lac@ZIF-8-Fe was maintained at 96.6% at the fifth reuse with only an extra addition of 0.09 mM ABTS in each cycle. As for Lac+ABTS@ZIF-8-Fe, 58.5% of MG was degraded at the fifth cycle without an extra addition of ABTS. Taken together, this research has provided a novel strategy for the design of a co-immobilized laccase and ABTS system for the degradation of organic pollutants.

Metal-Organic Framework for the Immobilization of Oxidoreductase Enzymes: Scopes and Perspectives

Oxidoreductases are a wide class of enzymes that can catalyze biological oxidation and reduction reactions. Nowadays, oxidoreductases play a vital part in most bioenergetic metabolic pathways, which have important applications in biodegradation, bioremediation, environmental applications, as well as biosensors. However, free oxidoreductases are not stable and hard to be recycled. In addition, cofactors are needed in most oxidoreductases catalyze reactions, which are so expensive and unstable that it hinders their industrial applications. Enzyme immobilization is a feasible strategy that can overcome these problems. Recently, metal-organic frameworks (MOFs) have shown great potential as support materials for immobilizing enzymes due to their unique properties, such as high surface-area-to-volume ratio, chemical stability, functional designability, and tunable pore size. This review discussed the application of MOFs and their composites as immobilized carriers of oxidoreductase, as well as the application of MOFs as catalysts and immobilized carriers in redox reactions in the perspective of the function of MOFs materials. The paper also focuses on the potential of MOF carrier-based oxidoreductase immobilization for designing an enzyme cascade reaction system.

Laccase encapsulation immobilized in mesoporous ZIF-8 for enhancement bisphenol A degradation

Endocrine disruptors (EDCs) such as bisphenol A (BPA) have many adverse effects on environment and human health. Laccase encapsulation immobilized in mesoporous ZIF-8 was prepared for efficient degradation of BPA. The ZIF-8 (PA) with highly ordered mesopores was synthesized using trimethylacetic acid (PA) as a template agent. On account of the improvement of skeletal stability by cross-linking agent glutaraldehyde, ZIF-8 (PA) realized laccase (FL) immobilization within the mesopores through encapsulation strategy. By replacing the template agent, the effect of pore size on the composite activity and immobilization efficiency by SEM characterization and kinetic analysis were investigated. Based on the physical protection of ZIF-8(PA) on laccase, as well as electrostatic interactions between substances and changes in surface functional groups (e.g. -OH, etc.), multifaceted enhancement including activity, stability, storability were engendered. FL@ZIF-8(PA) could maintain high activity in complex systems at pH 3-11, 10-70 °C or in organic solvent containing system, which exhibited an obvious improvement compared to free laccase and other reported immobilized laccase. Combined with TGA, FT-IR and Zeta potential analysis, the intrinsic mechanism was elaborated in detail. On this basis, FL@ZIF-8(PA) achieved efficient removal of BPA even under adverse conditions (removal rates all above 55% and up to 90.28%), and was suitable for a wide range of initial BPA concentrations. Combined with the DFT calculations on the adsorption energy and differential charge, the mesoporous could not only improve the enrichment performance of BPA on ZIFs, but also enhance the interaction stability. Finally, FL@ZIF-8(PA) was successfully applied to the degradation of BPA in coal industry wastewater. This work provides a new and ultra-high performances material for the organic pollution treatment in wastewater.

GOx-encapsulated iron-phenolic networks power catalytic cascade to eradicate bacterial biofilms

Bacterial biofilms, especially ones caused by multi-drug resistant strains, are increasingly posing a significant threat to human health. Inspired by nature, we report the fabrication of glucose oxidase-loaded iron-phenolic networks that can power the cascade reaction to generate free radicals to eradicate bacterial biofilms. A soft template, sodium deoxycholate, is employed to guarantee glucose oxidase activity during encapsulation, yielding the porous nanocomplexes after removing the template. The porous nature of nanocomplexes, characterized via transmission electron microscopy, N₂ adsorption isotherms, and thermogravimetric analysis, facilitates the diffusion of substrates and products during the cascade reaction and protects glucose oxidase from protease attack. Our optimized nanocomplexes (Fe-GA/GOx) could efficiently kill drug-resistant ESKAPE pathogens, including the clinically isolated strains and eradicate their biofilms. In this regard, Fe-GA/GOx could induce over 90% of the biomass of Klebsiella pneumoniae and Staphylococcus aureus biofilms. In the murine peritonitis infection model induced by Staphylococcus aureus and pneumonia model induced by Klebsiella pneumoniae, our Fe-GA/GOx nanocomplexes could efficiently eradicate the bacteria (over 3-log reduction in colony-forming units) and alleviate the inflammatory response without notable side effects on normal tissues. Therefore, our strategy may provide an efficient alternative treatment to combat bacterial biofilms and address the emergence of drug resistance.

Hierarchically Structured CA@ZIF-8 Biohybrids for Carbon Dioxide Mineralization

Carbonic anhydrase (CA) is a powerful biocatalyst for carbon dioxide (CO₂) mineralization, of which immobilization is usually used for maintaining its catalytic activity against harsh external stimuli. However, the incorporated materials for CA immobilization would commonly increase the internal diffusion resistance during the catalytic process, thereby decreasing the catalytic efficiency. In our study, poly-L-glutamic acid (PLGA) as the structure regulator was used to induce the synthesis of CA@zeolitic imidazolate framework-8 (CA@ZIF-8) biohybrids. The introduction of PLGA that could coordinate with Zn²⁺ interfered the crystallization of ZIF-8, thereby changing the morphological structure of CA@ZIF-8 biohybrids. With the increase of PLGA amount from 0 to 60 mg, PLGA(x)-CA@ZIF-8 biohybrids were gradually transformed from a dodecahedron structure to a 3D lamellar nano-flower structure, which caused elevated exposed surface area. Accordingly, the loading ratio was increased from 34.6 to 49.8 mg g_cat^-1, while the catalytic activity was elevated from 20.6 to 23.4%. The CO₂ conversion rate was enhanced by nearly two folds compared to PLGA(0)-CA@ZIF-8 under the optimized condition. The final CaCO₃ yield could reach 5.6 mg mg_cat^-1, whereas the reaction system could remain above 80% of the initial reaction activity after 8 cycles.

Gentle one-step co-precipitation to synthesize bimetallic CoCu-MOF immobilized laccase for boosting enzyme stability and Congo red removal

Laccase has received extensive attention in pollutant degradation due to its high efficiency and environmental friendliness, but free laccase has poor stability, easy inactivation, and difficulty in recycling, which limited its application. It was a smart strategy to construct a synergistic system for the efficient adsorption and degradation of pollutants by enzyme immobilization to improve the stability and recyclability of the enzyme. In this study, the materials were synthesized by a one-step co-precipitation method. With Cu-MOF as the main body, Co²⁺ was introduced to construct bimetallic CoCu-MOF as the protective carrier of the enzyme. The enzyme-carrying capacity and enzyme activity of Lac@CoCu-MOF were 2-fold and 3.5-fold higher than those of Lac@Cu-MOF, respectively. Lac@MOF composites had a good protective effect on enzyme in various interfering environments. At pH = 7, free laccase was completely inactivated and Lac@CoCu-MOF maintained 51.76% enzyme activity. In addition, the removal rate of Congo red by Lac@CoCu-MOF reached 90 % in 1 h at pH = 4 % and 95 % in 5 h at pH = 7, and the final TOC mineralization rate reached 86.05 %. After six cycles, the degradation rate of Lac@CoCu-MOF remained above 75 %. Therefore, Lac@CoCu-MOF was constructed with the advantages of enzyme immobilization (enhanced stability and easy operation), material adsorption, and biocatalysis (fast diffusion and high activity), which has great guiding significance for the industrial application of enzyme.

Confining enzymes in porous organic frameworks: from synthetic strategy and characterization to healthcare applications

Enzymes are a class of natural catalysts with high efficiency, specificity, and selectivity unmatched by their synthetic counterparts and dictate a myriad of reactions that constitute various cascades in living cells. The development of suitable supports is significant for the immobilization of structurally flexible enzymes, enabling biomimetic transformation in the extracellular environment. Accordingly, porous organic frameworks, including metal organic frameworks (MOFs), covalent organic frameworks (COFs) and hydrogen-bonded organic frameworks (HOFs), have emerged as ideal supports for the immobilization of enzymes because of their structural features including ultrahigh surface area, tailorable porosity, and versatile framework compositions. Specially, organic framework-encased enzymes have shown significant enhancement in stability and reusability, and their tailorable pore opening provides a gatekeeper-like effect for guest sieving, which is beneficial for mimicking intracellular biocatalysis processes. This immobilization technique brings new insight into the development of next-generation enzyme materials and shows huge potential in healthcare applications, such as biomarker diagnosis, biostorage, and cancer and antibacterial therapies. In this review, we describe the state-of-the-art strategies for the structural immobilization of enzymes using the well-explored MOFs and burgeoning COFs and HOFs as scaffolds, with special emphasis on how these porous framework-confined technologies can provide a favorable microenvironment for mimicking natural biocatalysis. Subsequently, advanced characterization techniques for enzyme conformation, the effect of the confined microenvironment on the activity of enzymes, and the emerging healthcare applications will be surveyed.

Macro-Meso-Microporous Metal-Organic Frameworks: Template-Assisted Spray Drying Synthesis and Enhanced Catalysis

Hierarchically porous metal-organic frameworks (HP-MOFs) are promising in many applications. However, most previous studies focus on HP-MOFs with two kinds of pore structures. Herein, a strategy for efficient construction of HP-MOFs possessing macro-meso-micropores using template-assisted spray drying followed by etching process is proposed. Taking ZIF-8 as an example, using polystyrene (PS) templates, the complete HP-ZIF-8 with all the three categories of pores can be easily fabricated. The close arrangement of intrinsic microporous nanosized ZIF-8 (N-ZIF-8) in the spray drying process results in the creation of mesopores, while the macropores are further generated after the removal of PS templates. The structures of macropores and mesopores can be easily adjusted by altering the size and proportion of PS and the size of N-ZIF-8, respectively. Furthermore, this method is extended to the preparation of HP-HKUST-1. As a proof-of-concept, HP-ZIF-8 displays excellent catalytic properties in Knoevenagel reaction owing to its unique pore features. Compared with conventional microsized ZIF-8 (M-ZIF-8) with similar size, HP-ZIF-8 achieves the significantly increased conversion of benzaldehyde from 55% to 100% within 3 h, and shows better recycling performance than N-ZIF-8.

Recent Advances in Emerging Metal- and Covalent-Organic Frameworks for Enzyme Encapsulation

Enzyme catalysis enables complex biotransformation to be imitated. This biomimetic approach allows for the application of enzymes in a variety of catalytic processes. Nevertheless, enzymes need to be shielded by a support material under challenging catalytic conditions due to their intricate and delicate structures. Specifically, metal-organic frameworks and covalent-organic frameworks (MOFs and COFs) are increasingly popular for use as enzyme-carrier platforms because of their excellent tunability in structural design as well as remarkable surface modification. These porous organic framework capsules that host enzymes not only protect the enzymes against harsh catalytic conditions but also facilitate the selective diffusion of guest molecules through the carrier. This review summarizes recent progress in MOF-enzyme and COF-enzyme composites and highlights the pore structures tuned for enzyme encapsulation. Furthermore, the critical issues associated with interactions between enzymes and pore apertures on MOF- and COF-enzyme composites are emphasized, and perspectives regarding the development of high-quality MOF and COF capsules are presented.

"Four-in-One" Nanozyme and Natural Enzyme Symbiotic System of Cu2-x Se-GOx for Cervical Cancer Therapy

Cervical cancer, as a common malignant tumor of the reproductive system, seriously threatens women's life and health, and is difficult to be cured by traditional treatments, such as surgery, chemotherapy and radiotherapy. Fortunately, tumor microenvironment (TME)-activated catalytic therapy with high efficiency and reduced off-target toxicity has emerged as a novel treatment model. Herein, we designed a "four-in-one" nanozyme and natural enzyme symbiotic system of Cu_2-x Se-GOx for TME-triggered cascaded catalytic enhanced cancer treatment. In response to unique TME, Cu_2-x Se with catalase activity could effectively catalyze over-expressed H₂ O₂ in cancer cells into O₂ . Subsequently, the glucose oxidase (GOx) could deplete intracellular glucose with the assistance of O₂ ; this not only achieves starvation therapy, but also regenerates H₂ O₂ to boost the generation of highly cytotoxic ^. OH due to the peroxidase activity of Cu_2-x Se. Moreover, although the free-radical scavenger glutathione (GSH) is overexpressed in tumor cells, Cu_2-x Se with glutathione oxidase activity could effectively consume GSH for enhanced ROS production. Thus, the "four-in-one" nanozyme@natural enzyme symbiotic system of Cu_2-x Se-GOx could induce significant ROS accumulation at the tumor regions, thus providing a potential approach for the treatment of cervical cancer.

Pyrrole-Based Conjugated Microporous Polymers as Efficient Heterogeneous Catalysts for Knoevenagel Condensation

Conjugated microporous polymers (CMPs) with robust architectures, facilely tunable pore sizes and large specific surface areas have emerged as an important class of porous materials due to their demonstrated prospects in various fields, e.g. gas storage/separation and heterogeneous catalysis. Herein, two new pyrrole-based CMPs with large specific surface areas and good stabilities were successfully prepared by one-step oxidative self-polycondensation of 1,2,4,5-tetra (pyrrol-2-ly)benzene or 1,3,5-tri (pyrrol-2-ly)benzene, respectively. Interestingly, both CMPs showed very high catalytic activity toward Knoevenagel condensation reaction, which was attributed to the inherent pore channels, high specific surface areas and abundant nitrogen sites within CMPs. Additionally, both CMPs displayed excellent recyclability with negligible degradation after 10 cycles. This work provides new possibilities into designing novel nitrogen-rich high-performance heterogeneous catalysts.

Oligonucleotide-Functionalized Enzymes Chemisorbing on Magnetic Layered Double Hydroxides: A Multimodal Catalytic Platform with Boosted Activity for Ultrasensitive Glucose Detection

A reasonable design of multifarious chemo- and biocatalytic functions within individual nano/microunits is urgently desired for high-performance cascade reactions but has heretofore remained elusive. Herein, glucose oxidase was functionalized with oligonucleotides and steadily chemisorbed on magnetic layered double hydroxides (mLDHs) to construct a multimodal catalytic platform for realizing divergent reactions with heterogeneous and biocatalytic steps. The flowerlike mLDHs served both as an enzyme support and a peroxidase mimic cooperating with enzymes for tandem catalysis. Oligo-DNA connected the enzymes to mLDHs like a bridge, and a stepwise ligand-exchange-assisted coordination mechanism was proposed to explain the robust interaction between DNA and mLDHs. More importantly, DNA significantly improved the bioactivity of the whole system. The acceleration mechanism was attributed to the diffusion tunnels for the substrate/product and enhanced substrates binding on mLDHs. The multimodal catalytic platform was applied for colorimetric and electrochemical sensing of glucose with a low limit of detection and high selectivity. The practical analysis capability of the ultrasensitive sensor was evaluated by detecting glucose in human serum and sweat, showing reliable results, satisfactory recovery, and excellent stability. The strategy of combining mLDHs and enzymes for cascade catalysis provides a universal approach to prepare chemo-enzyme hybrids with high performance, which holds great promise for applications in biosensors and industrial catalysis.