Metal–Organic Framework Disintegrants: Enzyme Preparation Platforms with Boosted Activity

Crystal Phase Transition-Driven Integration of Enzymes into 2D Metal-Organic Frameworks

In situ encapsulation of enzymes within a metal-organic framework (MOF) represents a promising technique for engineering robust biocatalysts. However, the success of enzyme encapsulation is often constrained by intricate interfacial interactions between enzyme surfaces and MOF precursors, limiting the versatility of this MOF method. Herein, we introduce a phase transition strategy for encapsulating enzymes within a Zn-HHTP framework, demonstrating its effectiveness across a wide range of enzymes irrespective of their surface chemistry. In this approach, enzyme molecules are preloaded in a zinc oxide (ZnO) template through a simple yet efficient coprecipitation process, followed by a ZnO-to-Zn-HHTP MOF crystal phase transition in the presence of ligand precursors, resulting in the formation of a quasi-mesoporous hybrid Zn-HHTP MOF inside, for which the original enzymes are preserved. The long-range ordered quasi-mesopore channels enhance substrate accessibility to the immobilized enzymes, endowing enzyme@Zn-HHTP with higher catalytic activity compared to enzymes immobilized within the well-known MOF, ZIF-8, which has narrow apertures. Additionally, the resultant enzyme@Zn-HHTP exhibits exceptional structural stability across a broad pH range (3-14), and Zn-HHTP can provide robust protection against enzyme denaturation by heat, organic solvents, and proteases. This work offers a facile and reliable phase transition strategy for synthesizing active and robust MOF biocatalysts, advancing biocatalysis across various fields.

Dual-functional carboxymethyl chitosan-coated silver nanoparticles for bacterial detection integrated with spectral processing and sterilization

The development of a dual-functional nanomaterial integrating microbial detection and sterilization capabilities are of great significance for advancing food safety applications. In this work, we synthesized carboxymethyl chitosan coated with spherical silver nanoparticles (CMCS@Ag NPs). CMCS@Ag NPs exhibited improved stability of surface-enhanced Raman scattering (SERS) signals compared to single silver nanoparticles in the detection of three strains including E. coli, S. aureus and S. putrefaciens. Through deuterium isotope labelling, metabolically active bacterial cells produced characteristic carbon-deuterium (C-D) vibrational signatures detectable by Raman spectroscopy. The spectral overlap between C-D and C-H stretching modes showed concentration-dependent linear responses, achieving a detection sensitivity of 0.64 CFU/mL for E. coli. Meanwhile, different combinations of algorithm were used to pretreatment with the raw Raman spectra, among that adaptive iterative reweighting punishment least square method and moving average smoothing (air-PLS + MAS) sensitively obtain better preprocessing accuracy of the three strains. Principal component analysis (PCA) was used to extract characteristic peaks from pretreatment spectra to successfully discriminate the three microorganisms. Furthermore, CMCS@Ag NPs incubated with phosphate-buffered saline (pH ∼ 5.8) exhibited potent antibacterial activity, achieving 99 % reduction in viability for E. coli, S. aureus and S. putrefaciens at a concentration of 5×107 CFU/mL within 45 min. This work presents a versatile nanomaterial-based strategy for simultaneous pathogen detection and inhibition.

Metal-organic framework-based tunable platform for the immobilization of lipase with enhanced activity in non-aqueous systems

Nowadays, metal-organic frameworks (MOFs) have been emerged as an efficient platform for enzyme immobilization due to their high porosity, tunability, and chemical versatility. In this study, a series of hybrid lipase@NKMOF-101-M (M = Mg, Mn, Zn, Co, or Ni) biocatalysts were constructed through a facile in situ encapsulation method, and the encapsulation and immobilization of lipase in MOFs were carefully validated. The catalytic activity of lipase@NKMOF-101-Mn was 2-fold higher than that of lipase@ZIF-8 and 3-fold higher than that of lipase@MCM-41 due to its excellent dispersibility and hydrophobicity in hexane. The reduced Km value demonstrated a superior affinity of lipase@NKMOF-101s toward to the substrate in non-aqueous reaction system. Moreover, the effects of MOF particle size, metal ions, and enzyme distribution on the catalytic performance of the immobilized lipase were systematically investigated. The results demonstrated that as the particle size of lipase@NKMOF-101s decreased, the apparent enzyme activity increased dramatically. Metal ions in MOFs exhibited activation effect toward to enzyme activity and an approximate 12-fold increase in activity was achieved when transesterification was performed using lipase@NKMOF-101-Mn compared with free lipase. Notably, lipase@NKMOF-101-Co and lipase@NKMOF-101-Ni exhibited substrate selectivity owing to the specific distribution of the lipase in the MOF carriers. Lipase@NKMOF-101s can maintain >80 % of its initial activity even after 5 recycles and a long-term storage (30 days). Consequently, NKMOF-101 is a tunable and sustainable platform for the construction of enzyme@MOFs biocatalysts with superior catalytic performance.

Immobilized Multi-Enzyme/Nanozyme Biomimetic Cascade Catalysis for Biosensing Applications

Multiple enzyme-induced cascade catalysis has an indispensable role in the process of complex life activities, and is widely used to construct robust biosensors for analyzing various targets. The immobilized multi-enzyme cascade catalysis system is a novel biomimetic catalysis strategy that immobilizes various enzymes with different functions in stable carriers to simulate the synergistic catalysis of multiple enzymes in biological systems, which enables high stability of enzymes and efficiency enzymatic cascade catalysis. Nanozymes, a type of nanomaterial with intrinsic enzyme-like characteristics and excellent stabilities, are also widely applied instead of enzymes to construct immobilized cascade systems, achieving better catalytic performance and reaction stability. Due to good stability, reusability, and remarkably high efficiency, the immobilized multi-enzyme/nanozyme biomimetic cascade catalysis systems show distinct advantages in promoting signal transduction and amplification, thereby attracting vast research interest in biosensing applications. This review focuses on the research progress of the immobilized multi-enzyme/nanozyme biomimetic cascade catalysis systems in recent years. The construction approaches, factors affecting the efficiency, and applications for sensitive biosensing are discussed in detail. Further, their challenges and outlooks for future study are also provided.

Biocompatible Metal-Organic Framework-Based Fabric Composite as an Efficient Personal Protective Equipment for Particulate Matter-Induced Pulmonary Injury

Efficient personal protection has emerged as a crucial approach for reducing pulmonary injury induced by particulate matter (PM). However, current personal protective equipments usually lack essential biosafety concerns and fail to own adsorbing/antioxidant/antibacterial function together, making it a challenge to develop an integrated platform with the above characteristics. Herein, a facile oxygen-free hydrothermal strategy is proposed to synthesize new copper-based metal-organic frameworks, Cu-HHTPs, (HHTP: 2,3,6,7,10,11-hexahydroxytriphenylene), with great adsorbing/antioxidant/antibacterial activity and high biosafety. The Cu-HHTPs can serve as an efficient additive incorporated with various fabrics including cellulose acetate (CA) membrane to achieve novel fabric composites, such as CA@Cu-HHTPs, with ideal scavenging outcome for the main components of PM. Evidenced by the animal experiments, CA@Cu-HHTPs can highly mitigate PM-induced adverse effects via adsorbing PM, scavenging ROS, and killing bacteria, leading to a significant reduction in lung permeability, inflammation and oxidative stress, and pulmonary infection. Last but not least, a two-week exposure of CA@Cu-HHTPs exhibits no obvious damage toward the animals by examining their long-term toxicity. Collectively, this study not only highlights the potential of Cu-HHTPs as attractive additives for the preparation of fabric composites, but also lays out a new concept toward the development of new-generation multifunctional personal protective equipment against PM.

Host molecules inside metal-organic frameworks: host@MOF and guest@host@MOF (Matrjoschka) materials

The controllable encapsulation of host molecules (such as porphyrin, phthalocyanine, crown ether, calixarene or cucurbituril organic macrocycles, cages, metal-organic polyhedrons and enzymes) into the pores of metal-organic frameworks (MOFs) to form host-in-host (host@MOF) materials has attracted increasing research interest in various fields. These host@MOF materials combine the merits of MOFs as a host matrix and functional host molecules to exhibit synergistic functionalities for the formation of guest@host@MOF materials in sorption and separation, ion capture, catalysis, proton/ion conduction and biosensors. (This guest@host@MOF construction is reminiscent of Russian (Matrjoschka) dolls which are nested dolls of decreasing size placed one inside another.) In this tutorial review, the advantages of MOFs as a host matrix are presented; the encapsulation approaches and general important considerations for the preparation of host@MOF materials are introduced. The state-of-the-art examples of these materials based on different host molecules are shown, and representative applications and general characterization of these materials are discussed. This review will guide researchers attempting to design functional host@MOF and guest@host@MOF materials for various applications.

Probe on the pivotal role of green rust in improving the enzymatic activity and facilitating the formation of 1O2 in Fenton-like process for degradation of 4-chlorophenol

Iron-based materials have demonstrated significant efficacy in catalyzing hydrogen peroxide (H2O2) for the removal of antibiotics from aquatic environments. Green rust (GR), a hybrid valence state iron-based catalyst, was synthesized. By exploiting the catalytic properties of glucose oxidase (GOx) to generate H2O2 from glucose (Glu), a GR-GOx/Glu system for the removal of recalcitrant organic compound 4-chlorophenol (4-CP) was constructed. In terms of pollutant degradation efficiency, an increase of 30% was observed compared to Fe2+/H2O2 system. Utilizing density functional theory (DFT), we calculated the electrostatic potential energy and charge density distribution, demonstrating the existence of active electron transfer between GR and flavin adenine dinucleotide (FAD), which subsequently enhanced the activity of GOx. The enhancement was pivotal for the sustained generation of preferred oxidative species and rapid degradation of pollutants within the system. Furthermore, the acids and H2O2 generated during enzyme catalysis not only neutralized the alkalinity released by the Fenton-like reaction but also promoted the conversion of superoxide radical (·O2-) to singlet oxygen (1O2). Overall, this study elucidates the fundamental motivation underlying the enhancement of enzymatic activity and highlights the critical role of 1O2 within the system, providing valuable insights into the potential mechanisms by which metal hydroxides catalyze the H2O2 process.

Unlocking of Hidden Mesopores for Enzyme Encapsulation by Dynamic Linkers in Stable Metal-Organic Frameworks

Mesoporous metal-organic frameworks (MOFs) are promising supports for the immobilization of enzymes, yet their applications are often limited by small pore apertures that constrain the size of encapsulated enzymes to below 5 nm. In this study, we introduced labile linkers (4,4',4''-(2,4,6-boroxintriyl)-tribenzoate, TBTB) with dynamic boroxine bonds into mesoporous PCN-333, resulting in PCN-333-TBTB with enhanced enzyme loading and protection capabilities. The selective breaking of B-O bonds creates defects in PCN-333, which effectively expands both window and cavity sizes, thereby unlocking hidden mesopores for enzyme encapsulation. Consequently, this strategy not only increases the adsorption kinetics of small enzymes (<5 nm) such as cytochrome c (Cyt C) and horseradish peroxidase (HRP), but also enables the immobilization of various large-sized enzymes (>5 nm), such as glycoenzymes. The glycoenzymes@PCN-333-TBTB platform was successfully applied to synthesize thirteen complex oligosaccharides and polysaccharides, demonstrating high activity and enhanced enzyme stability. The dynamic linker-mediated enzyme encapsulation strategy enables the immobilization of enzymes exceeding the inherent pore size of MOFs, thus broadening the scope of enzymatic catalytic reactions achievable with MOF materials.

Reticular Chemistry for Enhancing Bioentity Stability and Functional Performance

Addressing the fragility of bioentities that results in instability and compromised performance during storage and applications, reticular chemistry, specifically through metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), offers versatile platforms for stabilization and enhancement of bioentities. These highly porous frameworks facilitate efficient loading and mass transfer, offer confined environments and selective permeability for stabilization and protection, and enable finely tunable biointerfacial interactions and microenvironments for function optimization, significantly broadening the applications of various bioentities, including enzymes, nucleic acids, cells, etc. This Perspective outlines strategies for integrating bioentities with reticular frameworks, highlighting new design ideas for existing issues within these strategies. It emphasizes the crucial roles of these frameworks for bioentities in enhancing stability, boosting activity, imparting non-native functions, and synergizing bioentity systems. Concluding with a discussion of the challenges and prospects in the design, characterization, and practical applications of these biocomposites, this Perspective aims to inspire further development of high-performance biocomposites in this promising field.

Enhanced Interfacial Electron Transfer in Photocatalyst-Natural Enzyme Coupled Artificial Photosynthesis System: Tuning Strategies and Molecular Simulations

Laccase is capable of catalyzing a vast array of reactions, but its low redox potential limits its potential applications. The use of photocatalytic materials offers a solution to this problem by converting absorbed visible light into electrons to facilitate enzyme catalysis. Herein, MIL-53(Fe) and NH2-MIL-53(Fe) serve as both light absorbers and enzyme immobilization carriers, and laccase is employed for solar-driven chemical conversion. Electron spin resonance spectroscopy results confirm that visible light irradiation causes rapid transfer of photogenerated electrons from MOF excitation to T1 Cu(II) of laccase, significantly increasing the degradation rate constant of tetracycline (TC) from 0.0062 to 0.0127 min-1. Conversely, there is only minimal or no electron transfer between MOF and laccase in the physical mixture state. Theoretical calculations demonstrate that the immobilization of laccase's active site and its covalent binding to the metal-organic framework surface augment the coupled system's activity, reducing the active site accessible from 27.8 to 18.1 Å. The constructed photo-enzyme coupled system successfully combines enzyme catalysis' selectivity with photocatalysis's high reactivity, providing a promising solution for solar energy use.

Self-Assembly-Activated Engineered Magnetic Biohybrids Loaded with Phosphotriesterase for Sustainable Decontamination and Detection of Organophosphorus Pesticides

Phosphotriesterase (PTE) biodegradation of organophosphorus pesticides (OPs) is an efficient and environmentally friendly method. However, the instability and nonreusability of free PTE become the key factors restricting its practical application. In this study, a novel cross-linked magnetic hybrid nanoflower (CLMNF) was prepared. Molecular dynamics (MD) simulations were performed to further investigate the enhanced catalytic efficiency of the enzymes. The recovery rate of enzyme activity was 298% due to the large specific surface area and metal ion activation effect. More importantly, the immobilization scheme greatly improved the stability and reuse performance of the catalyst and simplified the recovery operation. CLMNFs retained 90.32% relative activity after 5 consecutive cycles and maintained 84.8% relative activity after 30 days at 25 °C. It has a good practical application prospect in the degradation and detection of OPs. Consequently, the immobilized enzyme as a biocatalyst has the characteristics of high efficiency, stability, safety, and easy separation, establishing the key step in a biodetoxification system to control organophosphorus contamination in food and the environment.

Nanopore-regulated in situ polymerization for synthesis of homogeneous heparan sulfate with low dispersity

The biological activities of heparan sulfate (HS) are intimately related to their molecular weights, degree and pattern of sulfation and homogeneity. The existing methods for synthesizing homogeneous sugar chains of low dispersity involve multiple steps and require stepwise isolation and purification processes. Here, we designed a mesoporous metal-organic capsule for the encapsulation of glycosyltransferase and obtained a microreactor capable of enzymatically catalyzing polymerization reactions to prepare homogeneous heparosan of low dispersity, the precursor of HS and heparin. Since the sugar chain extension occurs in the pores of the microreactor, low molecular weight heparosan can be synthesized through space-restricted catalysis. Moreover, the glycosylation co-product, uridine diphosphate (UDP), can be chelated with the exposed metal sites of the metal-organic capsule, which inhibits trans-cleavage to reduce the molecular weight dispersity. This microreactor offers the advantages of efficiency, reusability, and obviates the need for stepwise isolation and purification processes. Using the synthesized heparosan, we further successfully prepared homogeneous 6-O-sulfated HS of low dispersity with a molecular weight of approximately 6 kDa and a polydispersity index (PDI) of 1.032. Notably, the HS generated exhibited minimal anticoagulant activity, and its binding affinity to fibroblast growth factor 1 was comparable to that of low molecular weight heparins.

Confinement-Induced Biocatalytic Activity Enhancement of Light- and Thermoresponsive Polymer@Enzyme@MOF Composites

Metal-organic frameworks (MOFs) are favorable hosting materials for fixing enzymes to construct enzyme@MOF composites and to expand the applications of biocatalysts. However, the rigid structure of MOFs without tunable hollow voids and a confinement effect often limits their catalytic activities. Taking advantage of the smart soft polymers to overcome the limitation, herein, a protection protocol to encapsulate the enzyme in zeolitic imidazolate framework-8 (ZIF-8) was developed using a glutathione-sensitive liposome (L) as a soft template. Glucose oxidase (GOx) and horseradish peroxidase (HRP) were first anchored on a light- and thermoresponsive porous poly(styrene-maleic anhydride-N,N-dimethylaminoethyl methacrylate-spiropyran) membrane (PSMDSP) to produce PSMDSP@GOx-HRP, which could provide a confinement effect by switching the UV irradiation or varying the temperature. Afterward, embedding PSMDSP@GOx-HRP in L and encapsulating PSMDSP@GOx-HRP@L into hollow ZIF-8 (HZIF-8) to form PSMDSP@GOx-HRP@HZIF-8 composites were performed, which proceeded during the crystallization of the framework following the removal of L by adding glutathione. Impressively, the biocatalytic activity of the composites was 4.45-fold higher than that of the free enzyme under UV irradiation at 47 °C, which could benefit from the confinement effect of PSMDSP and the conformational freedom of the enzyme in HZIF-8. The proposed composites contributed to the protection of the enzyme against harsh conditions and exhibited superior stability. Furthermore, a colorimetric assay based on the composites for the detection of serum glucose was established with a linearity range of 0.05-5.0 mM, and the calculated LOD value was 0.001 mM in a cascade reaction system. This work provides a universal design idea and a versatile technique to immobilize enzymes on soft polymer membranes that can be encapsulated in porous rigid MOF-hosts. It also holds potential for the development of smart polymer@enzyme@HMOFs biocatalysts with a tunable confinement effect and high catalytic performance.

Designable immobilization of D-allulose 3-epimerase on bimetallic organic frameworks based on metal ion compatibility for enhanced D-allulose production

D-allulose, a low-calorie rare sugar catalyzed by D-allulose 3-epimerase (DAE), is highly sought after for its potential health benefits. However, poor reusability and stability of DAE limited its popularization in industrial applications. Although metal-organic frameworks (MOFs) offer a promising enzyme platform for enzyme immobilization, developing customized strategies for MOF immobilization of enzymes remains challenging. In this study, we introduce a designable strategy involving the construction of bimetal-organic frameworks (ZnCo-MOF) based on metal ions compatibility. The DAE@MOFs materials were prepared and characterized, and the immobilization of DAE and the enzymatic characteristics of the MOF-immobilized DAE were subsequently evaluated. Remarkably, DAE@ZnCo-MOF exhibited superior recyclability which could maintain 95 % relative activity after 8 consecutive cycles. The storage stability is significantly improved compared to the free form, with a relative activity of 116 % remaining after 30 days. Molecular docking was also employed to investigate the interaction between DAE and the components of MOFs synthesis. The results demonstrate that the DAE@ZnCo-MOF exhibited enhanced catalytic efficiency and increased stability. This study introduces a viable and adaptable MOF-based immobilization strategy for enzymes, which holds the potential to expand the implementation of enzyme biocatalysts in a multitude of disciplines.

A Synergetic Pore Compartmentalization and Hydrophobization Strategy for Synchronously Boosting the Stability and Activity of Enzyme

Spatial immobilization of fragile enzymes using a nanocarrier is an efficient means to design heterogeneous biocatalysts, presenting superior stability and recyclability to pristine enzymes. An immobilized enzyme, however, usually compromises its catalytic activity because of inevasible mass transfer issues and the unfavorable conformation changes in a confined environment. Here, we describe a synergetic metal-organic framework pore-engineering strategy to trap lipase (an important hydrolase), which confers lipase-boosted stability and activity simultaneously. The hierarchically porous NU-1003, featuring interconnected mesopore and micropore channels, is precisely modified by chain-adjustable fatty acids on its mesopore channel, into which lipase is trapped. The interconnected pore structure ensures efficient communication between trapped lipase and exterior media, while the fatty acid-mediated hydrophobic pore can activate the opening conformation of lipase by interfacial interaction. Such dual pore compartmentalization and hydrophobization activation effects render the catalytic center of trapped lipase highly accessible, resulting in 1.57-fold and 2.46-fold activities as native lipase on ester hydrolysis and enantioselective catalysis. In addition, the feasibility of these heterogeneous biocatalysts for kinetic resolution of enantiomer is also validated, showing much higher efficiency than native lipase.

Co-Immobilization of ADH and GDH on Metal-Organic-Framework: An Effective Biocatalyst for Asymmetric Reduction of Ketones

Chiral alcohols are not only important building blocks of various bioactive natural compounds and pharmaceuticals, but can serve as synthetic precursors for other valuable organic chemicals, thus the synthesis of these products is of great importance. Bio-catalysis represents one effective way to obtain these molecules, however, the weak stability and high cost of enzymes often hinder its broad application. In this work, we designed a biological nanoreactor by embedding alcohol dehydrogenase (ADH) and glucose dehydrogenase (GDH) in metal-organic-framework ZIF-8. The biocatalyst ADH&GDH@ZIF-8 could be applied to the asymmetric reduction of a series of ketones to give chiral alcohols in high yields (up to 99 %) and with excellent enantioselectivities (>99 %). In addition, the heterogeneous biocatalyst could be recycled and reused at least four times with slight activity decline. Moreover, E. coli containing ADH and GDH was immobilized by ZIF-8 to form biocatalyst E. coli@ZIF-8, which also exhibits good catalytic behaviours. Finally, the chiral alcohols are further converted to marketed drugs (R)-Fendiline, (S)-Rivastigmine and NPS R-568 respectively.

Textural Precursor Compositions Harvested for Independent Signal Generators: Scaling Micron-Sized Flower-Like Metal-Organic Frameworks as Amplifying Units for Dual-Mode Glycoprotein Assay

In this work, a micron-sized flower-like metal-organic frameworks (MOFs)-based boronate-affinity sandwich-type immunoassay was fabricated for the dual-mode glycoprotein assay. For proof of concept, the flower-like MOFs were synthesized from transition Cu nodes and tetrakis (4-carboxyphenyl) porphyrin (TCPP) ligands by spontaneous standing assembly. In addition, the specificity toward glycoprotein involved the antigen recognition as well as covalent bonding via the boronate-glycan affinity, and the immediate signal responses were initiated by textural decomposition of the flower-like MOFs. Intriguingly, Cu nodes, of which the valence state is dominant by CuI species, can endow the Fenton-like catalytic reaction of the fluorogenic substrate for generating fluorescence signals. For benefits, TCPP ligands, in which each TCPP molecule has four guest donors, can provide multiple valences for the assembly of cyclodextrin-capped gold nanoparticles via host-guest interaction for colorimetry output. Albeit important, the scaling micrometer patterns for the flower-like MOFs carrying numerous Cu nodes and TCPP ligands can also function as amplifying units, signifying the output signal. The detection limit of the dual-mode glycoprotein assay can reach 10.5 nM for the fluorescence mode and 18.7 nM for the colorimetry mode, respectively. Furthermore, the merits of harvesting different signal generators toward the multimodal readout patterns can allow the mutual verification and make the analytical results more reliable. Collectively, our proposed assay may offer a new idea in combining the inherent textural merits from MOFs for dual signal generators, which can also emphasize accurate detection capability for glycoprotein assay.

Enhancing Chymotrypsin Activity and Stability of Capillary Immobilized Enzyme Microreactors Using Zeolitic Imidazolate Frameworks as Encapsulation Materials

Open-tubular immobilized enzyme microreactors (OT-IMERs) are some of the most widely used enzyme reaction devices due to the advantages of simple preparation and fast sample processing. However, the traditional approaches for OT-IMERs preparation had some defects such as limited enzyme loading amount, susceptibility to complex sample interference, and less stability. Here, we report a strategy for the preparation of highly active and stable OT-IMERs, in which the single-stranded DNA-enzyme composites were immobilized in capillaries and then encapsulated in situ in the capillaries via zeolitic imidazolate frameworks (ZIF-L). The phosphate groups of the DNA adjusted the surface potential of the enzyme to negative values, which could attract cations, such as Zn2+, to promote the formation of ZIF-L for enzyme encapsulation. Using chymotrypsin (ChT) as a model enzyme, the prepared ChT@ZIF-L-IMER has higher activity and better affinity than the free enzyme and ChT-IMER. Moreover, the thermal stability, pH stability, and organic solvent stability of ChT@ZIF-L-IMER were much higher than those of free enzyme and ChT-IMER. Furthermore, the activity of ChT@ZIF-L-IMER was much higher than that of ChT-IMER after ten consecutive reactions. To demonstrate the versatility of this preparation method, we replaced ChT with glucose oxidase (GOx). The stability of GOx@ZIF-L-IMER was also experimentally demonstrated to be superior to that of GOx and GOx-IMER. Finally, ChT@ZIF-L-IMER was used for proteolytic digestion analysis. The results showed that ChT@ZIF-L-IMER had a short digestion time and high digestive efficiency compared with the free enzyme. The present study broadened the synthesis method of OT-IMERs, effectively integrating the advantages of metal-organic frameworks and IMER, and the prepared OT-IMERs significantly improved enzyme stability. All of the results indicated that the IMER prepared by this method had a broad application prospect in capillary electrophoresis-based high-performance enzyme analysis.

Chitosan-based GOx@Co-MOF composite hydrogel: A promising strategy for enhanced antibacterial and wound healing effects

A novel composite hydrogel was synthesized via Schiff base reaction between chitosan and di-functional poly(ethylene glycol) (DF-PEG), incorporating glucose oxidase (GOx) and cobalt metal-organic frameworks (Co-MOF). The resulting CS/PEG/GOx@Co-MOF composite hydrogel was characterized using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and energy-dispersive X-ray spectroscopy (EDS). The results confirmed successful integration and uniform distribution of Co-MOF within the hydrogel matrix. Functionally, the hydrogel exploits the catalytic decomposition of glucose by GOx to generate gluconic acid and hydrogen peroxide (H2O2), while Co-MOF gradually releases metal ions and protects GOx. This synergy enhanced the antibacterial activity of the composite hydrogel against both Gram-positive (S. aureus) and Gram-negative bacteria (E. coli), outperforming conventional chitosan-based hydrogels. The potential of the composite hydrogel in treating wound infections was evaluated through antibacterial and wound healing experiments. Overall, CS/PEG/GOx@Co-MOF hydrogel holds great promise for the treatment of wound infections, paving the way for further research and potential clinical applications.

Boosted Enzyme Activity via Encapsulation within Metal-Organic Frameworks with Pores Matching Enzyme Size and Shape

A novel and versatile approach called "physical imprinting" is introduced to modulate enzyme conformation using mesoporous materials, addressing challenges in achieving improved enzyme activity and stability. Metal-organic frameworks with tailored mesopores, precisely matching enzyme size and shape, are synthesized. Remarkably, enzymes encapsulated within these customized mesopores exhibit over 1670% relative activity compared to free enzymes, maintaining outstanding efficiency even under harsh conditions such as heat, exposure to organic solvents, wide-ranging pH extremes from acidic to alkaline, and exposure to a digestion cocktail. After 18 consecutive cycles of use, the immobilized enzymes retain 80% of their initial activity. Additionally, the encapsulated enzymes exhibit a substantial increase in catalytic efficiency, with a 14.1-fold enhancement in kcat/KM compared to native enzymes. This enhancement is among the highest reported for immobilized enzymes. The improved enzyme activity and stability are corroborated by solid-state UV-vis, electron paramagnetic resonance, Fourier-transform infrared spectroscopy, and solid-state NMR spectroscopy. The findings not only offer valuable insights into the crucial role of size and shape complementarity within confined microenvironments but also establish a new pathway for developing solid carriers capable of enhancing enzyme activity and stability.

Two 2D Metal-Organic Frameworks Based on Purine Carboxylic Acid Ligands for Photocatalytic Oxidation of Sulfides and CO2 Chemical Fixation

Two two-dimensional (2D) layered metal-organic frameworks (MOFs), namely, {[Yb(L)(H2O)2NO3]·2H2O}n (Yb-MOF) and [Er(L)(H2O)3Cl]n (Er-MOF) (H2L = 5-((6H-purin-6-yl)amino)isophthalic acid), were constructed by a solvothermal method and characterized. The catalytic performance study showed that the Yb-MOF could efficiently catalyze the oxidation of sulfides to sulfoxides under 15 W light-emitting diode (LED) blue light irradiation. Electron paramagnetic resonance spectroscopy and free-radical trapping experiments demonstrated that the photocatalytic reaction process involved •O2-, and the corresponding mechanism was proposed. Moreover, Er-MOF exhibited good catalytic efficiency and excellent substrate tolerance in the cycloaddition reaction of CO2, and the reaction conditions were mild. After 5 cycles, the catalytic activities of two MOFs did not significantly decrease, and the framework structures remained unchanged. Therefore, the Yb-MOF and Er-MOF were considered efficient and stable heterogeneous catalysts.

Trypsin Encapsulation in the Zeolitic Imidazolate Framework for Low-Molecular Weight Protein Analysis with High Selectivity and Efficiency

Low-molecular weight proteins (LWPs) are important sources of biological information in biomarkers, signaling molecules, and pathology. However, the separation and analysis of LWPs in complex biological samples are challenging, mainly due to their low abundance and the complex sample pretreatment procedure. Herein, trypsin modified by poly(acrylic acid) (PAA) was encapsulated by a zeolitic imidazolate framework (ZIF-L). Mesopores were formed on the ZIF-L with the introduction of PAA. An alternative strategy for separation and pretreatment of LWPs was developed based on the prepared ZIF-L-encapsulated trypsin with adjustable pore size. The mesoporous structure of the prepared materials selectively excluded high-molecular weight proteins from the reaction system, allowing LWPs to enter the pores and react with the internal trypsin, resulting in an improved separation efficiency. The hydrophobicity of the ZIF-L simplified the digestion process by inducing significant structural changes in substrate proteins. In addition, the enzymatic activity was significantly enhanced by the developed encapsulation method that maintained the enzyme conformation, allowed low mass transfer resistance, and possessed a high enzyme-to-substrate ratio. As a result, the ZIF-L-encapsulated trypsin can achieve highly selective separation, valid denaturation, and efficient digestion of LWPs in a short time by simply mixing with substrate proteins, greatly simplifying the separation and pretreatment process of the traditional hydrolysis method. The prepared materials and the developed strategy demonstrated an excellent size-selective assay performance in model protein mixtures, showing great potential in the application of proteomics analysis.

An acid-responsive DNA hydrogel-mediated cascaded enzymatic nucleic acid amplification system for the sensitive imaging of alkaline phosphatase in living cells

Alkaline phosphatase (ALP) is a class of hydrolase that catalyzes the dephosphorylation of phosphorylated species in biological tissues, playing an important role in many physiological and pathological processes. Sensitive imaging of ALP activity in living cells is contributory to the research on these processes. Herein, we propose an acid-responsive DNA hydrogel to deliver a cascaded enzymatic nucleic acid amplification system into cells for the sensitive imaging of intracellular ALP activity. The DNA hydrogel is formed by two kinds of Y-shaped DNA monomers and acid-responsive cytosine-rich linkers. The amplification system contained Bst DNA polymerase (Bst DP), Nt.BbvCI endonuclease, a Recognition Probe (RP, containing a DNAzyme sequence, a Nt.BbvCI recognition sequence, and a phosphate group at the 3'-end), and a Signal Probe (SP, containing a cleavage site for DNAzyme, Cy3 and BHQ2 at the two ends). The amplification system was trapped into the DNA hydrogel and taken up by cells, and the cytosine-rich linkers folded into a quadruplex i-motif in the acidic lysosomes, leading to the collapse of the hydrogel and releasing the amplification system. The phosphate groups on RPs were recognized and removed by the target ALP, triggering a polymerization-nicking cycle to produce large numbers of DNAzyme sequences, which then cleaved multiple SPs, restoring Cy3 fluorescence to indicate the ALP activity. This strategy achieved sensitive imaging of ALP in living HeLa, MCF-7, and NCM460 cells, and realized the sensitive detection of ALP in vitro with a detection limit of 2.0 × 10-5 U mL-1, providing a potential tool for the research of ALP-related physiological and pathological processes.

Engineered polymer nanoparticles as artificial chaperones facilitating the selective refolding of denatured enzymes

Molecular chaperones assist in protein refolding by selectively binding to proteins in their nonnative states. Despite progress in creating artificial chaperones, these designs often have a limited range of substrates they can work with. In this paper, we present molecularly imprinted flexible polymer nanoparticles (nanoMIPs) designed as customizable biomimetic chaperones. We used model proteins such as cytochrome c, laccase, and lipase to screen polymeric monomers and identify the most effective formulations, offering tunable charge and hydrophobic properties. Utilizing a dispersed phase imprinting approach, we employed magnetic beads modified with destabilized whole-protein as solid-phase templates. This process involves medium exchange facilitated by magnetic pulldowns, resulting in the synthesis of nanoMIPs featuring imprinted sites that effectively mimic chaperone cavities. These nanoMIPs were able to selectively refold denatured enzymes, achieving up to 86.7% recovery of their activity, significantly outperforming control samples. Mechanistic studies confirmed that nanoMIPs preferentially bind denatured rather than native enzymes, mimicking natural chaperone interactions. Multifaceted analyses support the functionality of nanoMIPs, which emulate the protective roles of chaperones by selectively engaging with denatured proteins to inhibit aggregation and facilitate refolding. This approach shows promise for widespread use in protein recovery within biocatalysis and biomedicine.

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.

Encapsulation of an enzyme-immobilized smart polymer membrane in a metal-organic framework for enhancement of catalytic performance

Encapsulation of enzymes within porous materials has shown great promise for protecting enzymes from denaturation, increasing their tolerance to harsh environments and promoting their industrialization. However, controlling the conformational freedom of the encapsulated enzymes to enhance their catalytic performance remains a great challenge. To address this issue, herein, following immobilization of GOx and HRP on a thermo-responsive porous poly(styrene-maleic-anhydride-N-isopropylacrylamide) (PSMN) membrane, a GOx-HRP@PSMN@HZIF-8 composite was fabricated by encapsulating GOx-HRP@PSMN in hollow ZIF-8 (HZIF-8) with liposome (L) as the sacrificial template. The improved conformational freedom for enzymes arising from the hollow cavity formed in ZIF-8 through the removal of L enhanced the mass transfer and dramatically promoted the catalytic activity of the composite. Interestingly, at high temperature, the coiled PN moiety in PSMN provided the confinement effect for GOx-HRP, which also significantly boosted the catalytic performance of the composites. Compared to the maximum catalytic reaction rates (Vmax) of GOx-HRP@PSMN@LZIF-8, the free enzyme and GOx-HRP@ZIF-8, the Vmax of the GOx-HRP@PSMN@HZIF-8 composite exhibited an impressive 17.8-fold, 10.8-fold and 6.0-fold enhancement at 37 °C, respectively. The proposed composites successfully demonstrated their potential as catalytic platforms for the colorimetric detection of glucose in a cascade reaction. This study paves a new way for overcoming the current limitations of immobilizing enzymes in porous materials and the use of smart polymers for the potential fabrication of enzyme@polymer@MOF composites with tunable conformational freedom and confinement effect.

Covalent Organic Frameworks Based Photoenzymatic Nano-reactor for Asymmetric Dynamic Kinetic Resolution of Secondary Amines

Bio-catalysis represents a highly efficient and stereoselective method for the synthesis of valuable chiral compounds, however, the poor stability and limited reaction types of free enzymes restrict their wide application in industrial production. In this work, to overcome these problems, a multifunctional photoenzymatic nanoreactor CALB@COF-Ir was developed through the encapsulation of Candida antarctica lipase B (CALB) in a photosensitive covalent organic framework COF-Ir. This bio-nanocluster serves as efficient catalysts in asymmetric dynamic kinetic resolution (DKR) of secondary amines to give a series of chiral amines in high yields (up to 99 %) and enantioselectivities (up to 99 % ee). The well-designed COF-Ir not only acts as safety cover to prevent CALB from deactivation but promotes racemization of secondary amines via photo-induced hydrogen atom transfer (HAT) process. Photoelectric characterization and TDDFT calculation revealed that (ppy)2Ir units in COF-Ir play crucial role in this photocatalytic system which enhance its photo-redox properties through facilitating the separation between photoelectrons (e-) and holes (h+). Furthermore, the heterogeneous photoenzymatic nanoreactor could be recycled for five rounds with slight decline of catalytic reactivity.

Boosting Enzyme Activity in Enzyme Metal-Organic Framework Composites

Enzymes, as highly efficient biocatalysts, excel in catalyzing diverse reactions with exceptional activity and selective properties under mild conditions. Nonetheless, their broad applications are hindered by their inherent fragility, including low thermal stability, limited pH tolerance, and sensitivity to organic solvents and denaturants. Encapsulating enzymes within metal-organic frameworks (MOFs) can protect them from denaturation in these harsh environments. However, this often leads to a compromised enzyme activity. In recent years, extensive research efforts have been dedicated to enhancing enzymatic activity within MOFs, leading to the development of new enzyme-MOF composites that not only preserve their catalytic potential but also outperform their free counterparts. This Review provides a comprehensive review on recent developments in enzyme-MOF composites with a specific emphasis on their enhanced enzymatic activity compared to free enzymes.

Electrostatically Mediated In Situ Polymerization for Enzyme Immobilization and Activation

Enzyme immobilization in nanoparticles is of interest for boosting their catalytic applications, yet rational approaches to designs achieving both high enzyme loading and activation remain a challenge. Herein, we report an electrostatically mediated in situ polymerization strategy that simultaneously realizes enzyme immobilization and activation. This was achieved by copolymerizing cationic monomers with a cross-linker in the presence of the enzyme lipase (anionic) as the template, which produces enzyme-loaded nanogels. The effects of different control factors such as pH, lipase dosage, and cross-linker fraction on nanogel formation are investigated systematically, and optimal conditions for enzyme loading and activation have been determined. A central finding is that the cationic polymer network of the nanogel creates a favorable environment that not only protects the enzyme but also boosts enzymatic activity nearly 2-fold as compared to free lipase. The nanogels improve the stability of the lipase to tolerate a broader working range of pH (5.5-8.5) and temperature (25-70 °C) and allow recycling such that after six cycles of reaction, 70% of the initial activity is conserved. The established fabrication strategy can be applied generally to different cationic monomers, and most of these nanogels exhibit adequate immobilization and activation of lipase. Our study confirms that in situ polymerization based on electrostatic interaction provides a facile and robust strategy for enzyme immobilization and activation. The wide variety of ionic monomers, therefore, features great potential for developing functional platforms toward satisfying enzyme immobilization and demanding applications.

Metal-pyrimidine nanocubes immobilized enzymes with pH-switchable multienzyme-like activity for broad-pH-responsive sensing assay for organophosphorus pesticides

Peroxidase (POD)-like can only function in acidic environments and the pH mismatch restricts the application of enzyme-nanozyme cascade catalytic sensing platforms in the broad-pH-responsive assay for organophosphorus pesticides (OPs). Herein, the metal-pyrimidine nanocubes (MPNCs) with intrinsic pH-switchable POD-like and catalase (CAT)-like properties were synthesized via the coordination of pyrimidin-2-ol with Cu2+. Meanwhile, acetylcholinesterase (AChE) and choline oxidase (CHO) were simultaneously encapsulated in MPNCs to construct an enzyme-nanozyme cascade catalytic platform (AChE/CHO@MPNCs). AChE/CHO@MPNCs could catalyze the hydrolysis of acetylcholine to choline, which was subsequently converted to H2O2. The POD-like activity of MPNCs was dominant under acidic conditions, while the CAT-like activity prevailed under neutral and alkaline conditions, which could catalyze H2O2 to •OH and O2, respectively, then oxidizing dopamine (DA) to polydopamine quantum dots (PDA QDs) with different fluorescence characteristics. Consequently, OPs could be detected in a linear range from 0.05 to 1000 nM with a LOD of 0.015 nM in acidic environments and a linear range from 0.05 to 500 nM with a LOD of 0.023 nM in alkaline environments. Overall, our work expands the horizon of constructing enzyme@MOFs composites with high catalytic activity. Meanwhile, the intrinsic pH-switchable multienzyme-like property opens avenues to construct sensing platforms with broad-pH-responsive for OPs and other analytes detection.

DNA flexible chain modified MOFs as a versatile platform for chemoenzymatic cascade reactions in glucose catalysis

Glucose oxidase (GOD) is widely used in the pharmaceutical industry, fermentation products and glucose biosensors for its essential role in catalyzing the conversion of glucose to gluconic acid and hydrogen peroxide (H2O2). As H2O2 is the by-product and will have a toxic effect on glucose oxidase, so introducing another enzyme that could consume H2O2 to form an enzymatic cascade reaction is a practical solution. However, this decision will lead to extra expenses and complex condition optimization such as the specific mass ratio, temperature and pH to improve the activity, stability and recyclability. Herein, we describe a mild and versatile strategy by anchoring GOD on carboxyl-activated MOF (Cu-TCPP(Fe)) through DNA-directed immobilization (DDI) technology. Robust MOF nanosheets were utilized as not only the carrier for the immobilization of GOD, but also a peroxidase-like catalyst for the decomposition of H2O2 to reduce its harmful impacts. In this work, the immobilized GOD retained 55.78% of its initial activity after being used for 7 times. More than 60% of the immobilized enzyme's catalytic activity was still maintained after 96 h of being stored at 50 ℃. This study provides a new idea for preparing immobilized enzymes with enhanced stability, fast diffusion and high activity, which can be used in fields such as biocatalysis and biotechnology.

Integration of Enzyme and Covalent Organic Frameworks: From Rational Design to Applications

Manufacturing is undergoing profound transformations, among which green biomanufacturing with low energy consumption, high efficiency, and sustainability is becoming one of the major trends. However, enzymes, as the "core chip" of biomanufacturing, are often handicapped in their application by their high cost, low operational stability, and nonreusability. Immobilization of enzymes is a technology that binds or restricts enzymes in a certain area with solid materials, allows them to still carry out their unique catalytic reaction, and allows them to be recycled and reused. Compared with free enzymes, immobilized enzymes boast numerous advantages such as enhanced storage stability, ease of separation, reusability, and controlled operation. Currently, commonly used supports for enzyme immobilization (e.g., mesoporous silica, sol-gel hydrogels, and porous polymer) can effectively improve enzyme stability and reduce product inhibition. However, they still face drawbacks such as potential leaching or conformational change during immobilization and poor machining performance. Especially, most enzyme carrier solid materials possess disordered structures, inevitably introducing deficiencies such as low loading capacity, hindered mass transfer, and unclear structure-property relationships. Additionally, it remains a notable challenge to meticulously design immobilization systems tailored to the specific characteristics of enzyme/reaction. Therefore, there is a significant demand for reliable solid materials to overcome the above challenges. Crystalline porous materials, particularly covalent organic frameworks (COFs), have garnered significant interest as a promising platform for immobilizing enzymes due to their unique properties, such as their crystalline nature, high porosity, accessible active sites, versatile synthetic conditions, and tunable structure. COFs create a stabilizing microenvironment that protects enzymes from denaturation and significantly enhances reusability. Nevertheless, some challenges still remain, including difficulties in loading large enzymes, reduced enzyme activities, and the limited functionality of carriers. Therefore, it is essential to develop innovative carriers and novel strategies to broaden the methods of immobilizing enzymes, enabling their application across a more diverse array of fields.The integration of enzymes with advanced porous materials for intensified performance and diverse applications is still in its infancy, and our group has done a series of pioneering works. This Account presents a comprehensive overview of recent research progress made by our group, including (i) the development of innovative enzyme immobilization strategies utilizing COFs to make the assembly and integration of enzymes and carriers more effective; (ii) rational design and construction of functional carriers for enzyme immobilization using COFs; and (iii) extensions of immobilized enzyme applications based on COFs from industrial catalysis to biomedicine and chiral separation. The integration of enzymes with functional crystalline materials offers mutual benefits and results in a performance that surpasses what either component can achieve individually. Additionally, immobilized enzymes exhibit enhanced functionality and intriguing characteristics that differ from those of free enzymes. Consistent with our research philosophy centered on integration, platform development, and engineering application, this Account addresses the critical challenges associated with enzyme immobilization using COFs while extending the applications of COFs and proposing future design principles for biomanufacturing and enzyme industry.

Biomimetic Metal-Pyrimidine Nanoflowers: Enzyme Immobilization Platforms with Boosted Activity

For the enzyme immobilization platform, enhancing enzyme activity retention while improving enzyme stability remains a challenge for sensitive sensing analysis. Herein, an in situ biomimetic immobilized enzyme carrier (metal-pyrimidine nanoflowers, MPNFs) synthesized by the coordination of DNA base derivative (2-aminopyrimidine) with Zn2+ in the aqueous phase at room temperature is developed. The biocompatibility of 2-aminopyrimidine and the hydrophilicity and green synthetic conditions of MPNFs allows the immobilized enzymes to retain above 91.2% catalytic activity. On this basis, a cascade catalytic platform is constructed by simultaneously immobilizing acetylcholinesterase (AChE), choline oxidase (CHO), and horseradish peroxidase (HRP) in MPNFs (AChE/CHO/HRP@MPNFs) for organophosphorus pesticides (OPs) colorimetric biosensing detection. The assay could specifically detect parathion-methyl within 13 min with a wider linear range (0.1-1000.0 nm) and a lower limit of detection (LOD) (0.032 nm). The remarkable stability of the immobilized enzymes is also achieved under harsh environments, room temperature storage, and recycling. Furthermore, a portable and cost-effective biosensing platform is developed by integrating AChE/CHO/HRP@MPNFs with a smartphone-assisted paper device for the on-site detection of OPs. Overall, the high catalytic activity retention and the enhanced detection performance demonstrate that MPNF is a robust carrier in enzyme immobilization and holds great promise in biosensing and other field applications.

De novo artificial synthesis of hexoses from carbon dioxide

Developing artificial "CO2-sugar" platforms is meaningful for addressing challenges posed by land scarcity and climate change to the supply of dietary sugar. However, upcycling CO2 into complex polyoxygenated carbohydrates involves several major challenges, including achieving enantioselective and thermodynamically driven transformation and expanding product repertoires while reducing energy consumption. We present a versatile chemoenzymatic roadmap based on aldol condensation, iso/epimerization, and dephosphorylation reactions for asymmetric CO2 and H2 assembly into sugars with perfect stereocontrol. In particular, we developed a minimum ATP consumption and the shortest pathway for bottom-up biosynthesis of the fundamental precursor, fructose-6-phosphate, which is valuable for synthesizing structure-diverse sugars and derivatives. Engineering bottleneck-associated enzyme catalysts aided in the thermodynamically driven synthesis of several energy-dense and functional hexoses, such as glucose and D-allulose, featuring higher titer (63 mmol L-1) and CO2-product conversion rates (25 mmol C L-1 h-1) compared to established in vitro CO2-fixing pathways. This chemical-biological platform demonstrated greater carbon conversion yield than the conventional "CO2-bioresource-sugar" process and could be easily extended to precisely synthesize other high-order sugars from CO2.

Opportunities and Challenges of Metal-Organic Framework Micro/Nano Reactors for Cascade Reactions

Building bridges among different types of catalysts to construct cascades is a highly worthwhile pursuit, such as chemo-, bio-, and chemo-bio cascade reactions. Cascade reactions can improve the reaction efficiency and selectivity while reducing steps of separation and purification, thereby promoting the development of "green chemistry". However, compatibility issues in cascade reactions pose significant constraints on the development of this field, particularly concerning the compatibility of diverse catalyst types, reaction conditions, and reaction rates. Metal-organic framework micro/nano reactors (MOF-MNRs) are porous crystalline materials formed by the self-assembly coordination of metal sites and organic ligands, possessing a periodic network structure. Due to the uniform pore size with the capability of controlling selective transfer of substances as well as protecting active substances and the organic-inorganic parts providing reactive microenvironment, MOF-MNRs have attracted significant attention in cascade reactions in recent years. In this Perspective, we first discuss how to address compatibility issues in cascade reactions using MOF-MNRs, including structural design and synthetic strategies. Then we summarize the research progress on MOF-MNRs in various cascade reactions. Finally, we analyze the challenges facing MOF-MNRs and potential breakthrough directions and opportunities for the future.

Enhanced Bioactivity of Enzyme/MOF Biocomposite via Host Framework Engineering

This study reports the successful development of a sustainable synthesis protocol for a phase-pure metal azolate framework (MAF-6) and its application in enzyme immobilization. An esterase@MAF-6 biocomposite was synthesized, and its catalytic performance was compared with that of esterase@ZIF-8 and esterase@ZIF-90 in transesterification reactions. Esterase@MAF-6, with its large pore aperture, showed superior enzymatic performance compared to esterase@ZIF-8 and esterase@ZIF-90 in catalyzing transesterification reactions using both n-propanol and benzyl alcohol as reactants. The hydrophobic nature of the MAF-6 platform was shown to activate the immobilized esterase into its open-lid conformation, which exhibited a 1.5- and 4-times enzymatic activity as compared to free esterase in catalyzing transesterification reaction using n-propanol and benzyl alcohol, respectively. The present work offers insights into the potential of MAF-6 as a promising matrix for enzyme immobilization and highlights the need to explore MOF matrices with expanded pore apertures to broaden their practical applications in biocatalysis.

Improving the Bioactivity and Stability of Embedded Enzymes by Covalent Organic Frameworks

De novo embedding enzymes within reticular chemistry materials have shown the enhancement of physical and chemical stability for versatile catalytic reactions. Compared to metal-organic frameworks (MOFs), covalent organic frameworks (COFs) are usually considered to be the more superior host of enzymes because of their large channels with low diffusion barriers, outstanding chemical/thermal stability, and metal-free nature. However, detailed investigations on the comparison of COFs and MOFs in enhancing biocatalytic performance have not been explored. Here, we de novo encapsulated enzymes within two COFs via a mechanochemical strategy, which avoided the extreme synthetic conditions of COFs and highly maintained the biological activities of the embedded enzymes. The enzymes@COFs biocomposites exhibited a much higher activity (3.4-14.7 times higher) and enhanced stability than those in MOFs (ZIF-8, ZIF-67, HKUST-1, MIL-53, and CaBDC), and the rate parameter (kcat/Km) of enzyme@COFs was 41.3 times higher than that of enzyme@ZIF-8. Further explorations showed that the conformation of enzymes inside MOFs was disrupted, owing to the harmful interfacial interactions between enzymes and metal ions as confirmed by ATR-FTIR, fluorescence spectroscopy, and XPS data. In contrast, enzymes that were embedded in metal-free COFs highly preserved the natural conformation of free enzymes. This study provides a better understanding of the interfacial interactions between reticular supports and enzymes, which paves a new road for optimizing the bioactivities of immobilized enzymes.

Manganese-Based Immunostimulatory Metal-Organic Framework Activates the cGAS-STING Pathway for Cancer Metalloimmunotherapy

Metal-organic frameworks (MOFs) show tremendous promise for drug delivery due to their structural and functional versatility. However, MOFs are usually used as biologically inert carriers in most cases. The creation of intrinsically immunostimulatory MOFs remains challenging. In this study, a facile and green synthesis method is proposed for the preparation of a manganese ion (Mn2+)-based immunostimulatory MOF (ISAMn-MOF) for cancer metalloimmunotherapy. ISAMn-MOF significantly facilitates the activation of cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) related genes and signaling pathways in bone-marrow-derived dendritic cells (BMDCs). BMDCs treated with ISAMn-MOF secrete 4-fold higher type I interferon and 2- to 16-fold higher proinflammatory cytokines than those treated with equivalent MnCl2. ISAMn-MOF alone or its combination with immune checkpoint antibodies significantly suppresses tumor growth and metastasis and prolongs mouse survival. Mechanistic studies indicate that ISAMn-MOF treatment facilitates the infiltration of stimulatory immune cells in tumors and lymphoid organs. This study provides insight into the design of bioactive MOFs for improved cancer metalloimmunotherapy.

Impact of Crystallinity on Enzyme Orientation and Dynamics upon Biomineralization in Metal-Organic Frameworks

Aqueous-phase co-crystallization (also known as biomimetic mineralization or biomineralization) is a unique way to encapsulate large enzymes, enzyme clusters, and enzymes with large substrates in metal-organic frameworks (MOFs), broadening the application of MOFs as enzyme carriers. The crystallinity of resultant enzyme@MOF biocomposites, however, can be low, raising a concern about how MOF crystal packing quality affects enzyme performance upon encapsulation. The challenges to overcome this concern are (1) the limited database of enzyme performance upon biomineralization in different aqueous MOFs and (2) the difficulty in probing enzyme restriction and motion in the resultant MOF scaffolds, which are related to the local crystal packing quality/density, under the interference of the MOF backgrounds. We have discovered several new aqueous MOFs for enzyme biomineralization with varied crystallinity [Jordahl, D.; Armstrong, Z.; Li, Q.; Gao, R.; Liu, W.; Johnson, K.; Brown, W.; Scheiwiller, A.; Feng, L.; Ugrinov, A.; Mao, H.; Chen, B.; Quadir, M.; Pan, Y.; Li, H.; Yang, Z. Expanding the Library of Metal-Organic Frameworks (MOFs) for Enzyme Biomineralization. ACS Appl. Mater. Interfaces 2022, 14 (46), 51619-51629, DOI: 10.1021/acsami.2c12998]. Here, we address the second challenge by probing enzyme dynamics/restriction in these MOFs at the residue level via site-directed spin labeling (SDSL)-electron paramagnetic resonance (EPR) spectroscopy, a unique approach to determine protein backbone motions regardless of the background complexity. We encapsulated a model large-substrate enzyme, lysozyme, in eight newly discovered MOFs, which possess various degrees of crystallization, via aqueous-phase co-crystallization. Through the EPR study and simulations, we found rough connections between (a) enzyme mobility/dynamics and MOF crystal properties (packing quality and density) and (b) enzyme areas exposed above each MOF and their catalytic performance. This work suggests that protein SDSL and EPR can serve as an indicator of MOF crystal packing quality/density when biomineralized in MOFs. The method can be generalized to probing the dynamics of other enzymes on other solid surfaces/interfaces and guide the rational design of solid platforms (ca. MOFs) to customize enzyme immobilization.

Hierarchical Porous Metal-Organic Framework as Biocatalytic Microreactor for Enzymatic Biofuel Cell-Based Self-Powered Biosensing of MicroRNA Integrated with Cascade Signal Amplification

Enzymatic biofuel cells have become powerful tools in biosensing, which however generally suffer from the limited loading efficiency as well as low catalytic activity and poor stability of bioenzymes. Herein, the hierarchical porous metal-organic frameworks (MOFs) are synthesized using tannic acid (TA) for structural etching, which realizes co-encapsulation of glucose dehydrogenase (GDH) and nicotinamide adenine dinucleotide (NAD+ ) cofactor in zeolitic imidazolate framework (ZIF-L) and are further used as the biocatalytic microreactors to modify bioanode. In this work, the TA-controlled etching can not only expand the pore size of microreactors, but also achieve the reorientation of enzymes in their lower surface energy form, therefore enhancing the biocatalysis of cofactor-dependent enzyme. Meanwhile, the topological DNA tetrahedron is assembled on the microreactors, which acts as the microRNA-responsive "lock" to perform the cascade signal amplification of exonuclease III-assisted target recycling on bioanode and hybridization chain reaction (HCR) on biocathode. The proposed self-powered biosensor has achieved a detection limit as low as 2 aM (6 copies miRNA-21 in a 5 µL of sample), which is further successfully applied to identify cancer cells and clinical serums of breast cancer patients based on the different levels of miRNA-21, holding great potential in accurate disease identification and clinical diagnosis.

Harnessing Self-Repairing and Crystallization Processes for Effective Enzyme Encapsulation in Covalent Organic Frameworks

Immobilization of fragile enzymes in crystalline porous materials offers new opportunities to expand the applications of biocatalysts. However, limited by the pore size and/or harsh synthesis conditions of the porous hosts, enzymes often suffer from dimension limitation or denaturation during the immobilization process. Taking advantage of the dynamic covalent chemistry feature of covalent organic frameworks (COFs), herein, we report a preprotection strategy to encapsulate enzymes in COFs during the self-repairing and crystallization process. Enzymes were first loaded in the low-crystalline polymer networks with mesopores formed at the initial growth stage, which could offer effective protection for enzymes from the harsh reaction conditions, and subsequently the encapsulation proceeded during the self-repairing and crystallization of the disordered polymer into the crystalline framework. Impressively, the biological activity of the enzymes can be well-maintained after encapsulation, and the obtained enzyme@COFs also show superior stability. Furthermore, the preprotection strategy circumvents the size limitation for enzymes, and its versatility was verified by enzymes with different sizes and surface charges, as well as a two-enzyme cascade system. This study offers a universal design idea to encapsulate enzymes in robust porous supports and holds promise for developing high-performance immobilized biocatalysts.

Bioinspired Framework Catalysts: From Enzyme Immobilization to Biomimetic Catalysis

Enzymatic catalysis has fueled considerable interest from chemists due to its high efficiency and selectivity. However, the structural complexity and vulnerability hamper the application potentials of enzymes. Driven by the practical demand for chemical conversion, there is a long-sought quest for bioinspired catalysts reproducing and even surpassing the functions of natural enzymes. As nanoporous materials with high surface areas and crystallinity, metal-organic frameworks (MOFs) represent an exquisite case of how natural enzymes and their active sites are integrated into porous solids, affording bioinspired heterogeneous catalysts with superior stability and customizable structures. In this review, we comprehensively summarize the advances of bioinspired MOFs for catalysis, discuss the design principle of various MOF-based catalysts, such as MOF-enzyme composites and MOFs embedded with active sites, and explore the utility of these catalysts in different reactions. The advantages of MOFs as enzyme mimetics are also highlighted, including confinement, templating effects, and functionality, in comparison with homogeneous supramolecular catalysts. A perspective is provided to discuss potential solutions addressing current challenges in MOF catalysis.

Promoting sensitive colorimetric detection of hydroquinone and Hg2+ via ZIF-8 dispersion enhanced oxidase-mimicking activity of MnO2 nanozyme

Reducing the agglomeration and improving the dispersibility in water of two-dimensional (2D) nanozymes is one of the effective ways to improve their enzyme-like activity. In this work, we propose a method by constructing zeolitic imidazolate framework-8 (ZIF-8)-dispersed 2D manganese-based nanozymes to achieve the specific regulated improvement of oxidase-mimicking activity. By in-situ growth of manganese oxides nanosheets of MnO₂(1), MnO₂(2) and Mn₃O₄ on the surface of ZIF-8, the corresponding nanocomposites of ZIF-8 @MnO₂(1), ZIF-8 @MnO₂(2), and ZIF-8 @Mn₃O₄ were prepared at room temperature. The Michaelis-Menton constant measurements indicated that ZIF-8 @MnO₂(1) exhibits best substrate affinity and fastest reaction rate for 3,3',5,5'-tetramethylbenzidine (TMB). The ZIF-8 @MnO₂(1)-TMB system was exploited to detection of trace hydroquinone (HQ) based on the reducibility of phenolic hydroxyl groups. In addition, by employing the fact that the cysteine (Cys) with the excellent antioxidant capacity can bind the Hg²⁺ based on the formation of "S-Hg²⁺" bonds, the ZIF-8 @MnO₂(1)-TMB-Cys system was applied to detection of Hg²⁺ with high sensitivity and selectivity. Our findings not only provide a better understanding of the relationship between dispersion of nanozyme and enzyme-like activity, but also provide a general method for the detection of environmental pollutants using nanozymes.

A highly robust cluster-based indium(III)-organic framework with efficient catalytic activity in cycloaddition of CO2 and Knoevenagel condensation

The efficient catalytic performance displayed by MOFs is decided by an appropriate charge/radius ratio of defect metal sites, large enough solvent-accessible channels and Lewis base sites capable of polarizing substrate molecules. Herein, the solvothermal self-assembly led to a highly robust nanochannel-based framework of {[In₄(CPDD)₂(μ₃-OH)₂(DMF)(H₂O)₂]·2DMF·5H₂O}_n (NUC-66) with a 56.8% void volume, which is a combination of a tetranuclear cluster [In₄(μ₃-OH)₂(COO)₁₀(DMF)(H₂O)₂] (abbreviated as {In₄}) and a conjugated tetracyclic pentacarboxylic acid ligand of 4,4'-(4-(4-carboxyphenyl)pyridine-2,6-diyl)diisophthalic acid (H₅CPDD). To the best of our knowledge, NUC-66 is a rarely reported {In₄}-based 3D framework with embedded hierarchical triangular-microporous (2.9 Å) and hexagonal-nanoporous (12.0 Å) channels, which are shaped by six rows of {In₄} clusters. After solvent exchange and vacuum drying, the surface of nanochannels in desolvated NUC-66a is modified by unsaturated In³⁺ ions, N_pyridine atoms and μ₃-OH groups, all of which display polarization ability towards polar molecules due to their Lewis acidity or basicity. The catalytic experiments performed showed that NUC-66a had high catalytic activity in the cycloaddition reactions of epoxides with CO₂ under mild conditions, which should be ascribed to its structural advantages including nanoscale channels, rich bifunctional active sites, large surface areas and chemical stability. Moreover, NUC-66a, as a heterogeneous catalyst, could greatly accelerate the Knoevenagel condensation reactions of aldehydes and malononitrile. Hence, this work confirms that the development of rigid nanoporous cluster-based MOFs built on metal ions with a high charge and large radius ratio will be more likely to realize practical applications, such as catalysis, adsorption and separation of gas, etc.

Recent advances in MOF-bio-interface: a review

Metal-organic frameworks (MOFs), as a class of promising material with adjustable function and controllable structure, have been widely used in the food industry, chemical industry, biological medicine, and sensors. Biomacromolecules and living systems play a critical role in the world. However, the insufficiency in stability, recyclability, and efficiency, significantly impedes their further utilization in slightly harsh conditions. MOF-bio-interface engineering effectively address the above-mentioned shortages of biomacromolecules and living systems, and thereby attracting considerable attentions. Herein, we systematically review the achievements in the area of MOF-bio-interface. In particular, we summarize the interface between MOFs and proteins (enzymes and non-enzymatic proteins), polysaccharides, DNA, cells, microbes, and viruses. Meanwhile, we discuss the limitations of this approach and propose future research directions. We expect that this review could provide new insights and inspire new research efforts towards life science and material science.

Bioelectrocatalysis for CO2 reduction: recent advances and challenges to develop a sustainable system for CO2 utilization

Activation and turning CO₂ into value added products is a promising orientation to address environmental issues caused by CO₂ emission. Currently, electrocatalysis has a potent well-established role for CO₂ reduction with fast electron transfer rate; but it is challenged by the poor selectivity and low faradic efficiency. On the other side, biocatalysis, including enzymes and microbes, has been also employed for CO₂ conversion to target C_n products with remarkably high selectivity; however, low solubility of CO₂ in the liquid reaction phase seriously affects the catalytic efficiency. Therefore, a new synergistic role in bioelectrocatalysis for CO₂ reduction is emerging thanks to its outstanding selectivity, high faradic efficiency, and desirable valuable C_n products under mild condition that are surveyed in this review. Herein, we comprehensively discuss the results already obtained for the integration craft of enzymatic-electrocatalysis and microbial-electrocatalysis technologies. In addition, the intrinsic nature of the combination is highly dependent on the electron transfer. Thus, both direct electron transfer and mediated electron transfer routes are modeled and concluded. We also explore the biocompatibility and synergistic effects of electrode materials, which emerge in combination with tuned enzymes and microbes to improve catalytic performance. The system by integrating solar energy driven photo-electrochemical technics with bio-catalysis is further discussed. We finally highlight the significant findings and perspectives that have provided strong foundations for the remarkable development of green and sustainable bioelectrocatalysis for CO₂ reduction, and that offer a blueprint for C_n valuable products originate from CO₂ under efficient and mild conditions.

Carrier Variety Used in Immobilization of His6-OPH Extends Its Application Areas

Organophosphorus hydrolase, containing a genetically introduced hexahistidine sequence (His6-OPH), attracts the attention of researchers by its promiscuous activity in hydrolytic reactions with various substrates, such as organophosphorus pesticides and chemical warfare agents, mycotoxins, and N-acyl homoserine lactones. The application of various carrier materials (metal-organic frameworks, polypeptides, bacterial cellulose, polyhydroxybutyrate, succinylated gelatin, etc.) for the immobilization and stabilization of His6-OPH by various methods, enables creation of biocatalysts with various properties and potential uses, in particular, as antidotes, recognition elements of biosensors, in fibers with chemical and biological protection, dressings with antimicrobial properties, highly porous sorbents for the degradation of toxicants, including in flow systems, etc. The use of computer modeling methods in the development of immobilized His6-OPH samples provides in silico prediction of emerging interactions between the enzyme and immobilizing polymer, which may have negative effects on the catalytic properties of the enzyme, and selection of the best options for experiments in vitro and in vivo. This review is aimed at analysis of known developments with immobilized His6-OPH, which allows to recognize existing recent trends in this field of research, as well as to identify the reasons limiting the use of a number of polymer molecules for the immobilization of this enzyme.

Expanding the "Library" of Metal-Organic Frameworks for Enzyme Biomineralization

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 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 multiphase 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 pH values. The stability under weakly basic conditions depended upon 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 enzyme stability, functionality, and optimal pH.