Covalent Organic Frameworks for the Purification of Recombinant Enzymes and Heterogeneous Biocatalysis

Isolation of Biomolecules Using MXenes

Biomolecule isolation is a crucial process in diverse biomedical and biochemical applications, including diagnostics, therapeutics, research, and manufacturing. Recently, MXenes, a novel class of two-dimensional nanomaterials, have emerged as promising adsorbents for this purpose due to their unique physicochemical properties. These biocompatible and antibacterial nanomaterials feature a high aspect ratio, excellent conductivity, and versatile surface chemistry. This timely review explores the potential of MXenes for isolating a wide range of biomolecules, such as proteins, nucleic acids, and small molecules, while highlighting key future research trends and innovative applications poised to transform the field. This review provides an in-depth discussion of various synthesis methods and functionalization techniques that enhance the specificity and efficiency of MXenes in biomolecule isolation. In addition, the mechanisms by which MXenes interact with biomolecules are elucidated, offering insights into their selective adsorption and customized separation capabilities. This review also addresses recent advancements, identifies existing challenges, and examines emerging trends that may drive the next wave of innovation in this rapidly evolving area.

Construction of Cyclodextrin-Based Covalent Organic Frameworks for Efficient Encapsulation of Menthol

The application of flavoring ingredients like menthol in the food industry is hindered by their high volatility and poor thermal stability, which lead to significant losses during processing and storage. Encapsulation of flavors into porous materials to obtain inclusion complexes (ICs) has proved to be an efficient strategy. In the present study, we synthesized a series of relatively food-safe three-dimensional anionic cyclodextrin-based covalent organic frameworks (CD-COFs) with spiroborate linkages using a facile microwave-assisted method. The high surface area and newly formed cavities of COFs significantly enhanced the encapsulation efficiency of menthol compared to native CD materials. Our findings revealed that γ-CD-COF-Li, with Li+ as the counterion, achieved superior encapsulation efficiency of 25.9 %, outperforming γ-CD-COF-Na, γ-CD-COF-K and α-CD-COF-Li under the same conditions. Thermal stability measurements show that the menthol/γ-CD-COF-Li-ICs effectively stabilize menthol against heat evaporation at elevated temperatures due to the strengthened interaction between menthol and γ-CD-COF-Li. The promising results of this research suggest that rapid advancements in COF technology will provide new opportunities for enhancing the stability of flavoring ingredients in the food industry.

Highly Enantioselective Transportation Across Liquid Membranes Mediated by Porous Covalent Organic Frameworks

Chiral liquid membrane separation is crucial in pharmaceuticals and chemical synthesis for its simplicity and stability, yet designing membrane carriers that enable efficient enantioseparation remains a challenge. Here, we demonstrated for the first time that chiral porous materials can act as mobile carriers of bulk liquid membranes (BLMs) to enhance enantioselective transport and separation. We design and prepare three 2D chiral covalent organic frameworks (CCOFs) by imine condensations of a chiral dialdehyde with triamines containing ethyl, fluorine and/or isopropyl groups. These isostructural CCOFs feature ABC stacking, excellent water, acid and base tolerance, and chiral amine groups in 1D porous channels, promoting efficient enantioselective transportation of amino acid enantiomers. Among them, the CCOF with both -F and -iPr groups showing superior transport performance. Exfoliating the CCOF into chiral nanosheets creates flexible layers with accessible active sites, enabling nanosheet-mediated liquid membranes to separate chiral drug enantiomers, a feat unattainable with the pristine CCOF. This work establishes CCOFs as a promising platform for chiral BLM separations and will guide the design of high-performance BLMs using porous materials for enantioselective separation.

Enhancing photocatalytic performance of covalent organic frameworks via ionic polarization

Covalent organic frameworks have emerged as a thriving family in the realm of photocatalysis recently, yet with concerns about their high exciton dissociation energy and sluggish charge transfer. Herein, a strategy to enhance the built-in electric field of series β-keto-enamine-based covalent organic frameworks by ionic polarization method is proposed. The ionic polarization is achieved through a distinctive post-synthetic quaternization reaction which can endow the covalent organic frameworks with separated charge centers comprising cationic skeleton and iodide counter-anions. The stronger built-in electric field generated between their cationic framework and iodide anions promotes charge transfer and exciton dissociation efficiency. Moreover, the introduced iodide anions not only serve as reaction centers with lowered H* formation energy barrier, but also act as electron extractant suppressing the recombination of electron-hole pairs. Therefore, the photocatalytic performance of the covalent organic frameworks shows notable improvement, among which the CH3I-TpPa-1 can deliver an high H2 production rate up to 9.21 mmol g-1 h-1 without any co-catalysts, representing a 42-fold increase compared to TpPa-1, being comparable to or possibly exceeding the current covalent organic framework photocatalysts with the addition of Pt co-catalysts.

Self-Assembly of Super-Uniform Covalent Organic Framework Colloidal Particles into Multi-Dimensional Ordered Superstructures

Precise self-assembly of colloidal particles is crucial for understanding their aggregation properties and preparing macroscopic functional devices. It is currently very challenging to synthesize and self-assemble super-uniform covalent organic framework (COF) colloidal particles into well-organized multidimensional superstructures. Here, simple and versatile strategies are proposed for synthesis of super-uniform COF colloidal particles and self-assembly of them into 1D supraparticles, 2D ordered mono/multilayers, and 3D COF films. For this purpose, several self-assembly techniques are developed, including emulsion solvent evaporation, air-liquid interfacial self-assembly, and drop-casting. These strategies enable the superstructural self-assembly of particles of varying sizes and species without any additional surfactants or chemical modifications. The assembled superstructures maintain the porosity and high specific surface area of their building blocks. The feasibility of the strategies is examined with different types of COFs. This research provides a new approach for the controllable synthesis of super-uniform COF colloidal particles capable of self-assembling into multidimensional superstructures with long-range order. These discoveries hold great promise for the design of emerging multifunctional COF superstructures.

Fast Production of Covalent Organic Frameworks for Covalent Enzyme Immobilization with Boosted Enzymatic Catalysis by Solar-Driven Photothermal Effect

Developing new enzyme-immobilization systems to stabilize their dynamic structures and meanwhile enhance their catalytic activity is of great significance but very challenging. Herein, we design and fabricate a class of robust mesoporous covalent organic frameworks (COFs) via Michael addition-elimination reaction. It is found that highly crystalline COFs can be produced in 10 min, which is attributed to the promoting effect of the intramolecular hydrogen bond activation. The COFs rich in hydroxyl groups can be facilely post-modified by epibromohydrin to covalently immobilize enzymes with both high loading and activity. Furthermore, we create a solar-driven photothermal-promoted strategy by introducing photoactive azo groups to COF carriers, which can boost the enzyme catalytic performance (lipase) with much higher conversion of various racemic substrates and chiral resolution upon solar light irradiation. The heterogeneous biocatalysts also demonstrate exceptional reusability and stability. This work provides a green and energy-efficient approach to facilitate the scale application of enzyme-immobilized biocatalysts.

Developing Covalent Organic Framework Biocatalysts through Enzyme Encapsulation

Utilizing covalent organic frameworks (COFs) as porous supports to encapsulate enzyme represents an advanced strategy for constructing COFs biocatalysts, which has inspired numerous interests across various applications. As the structural advantages including ultrastable covalent-bonded linkage, tailorable pore structure, and metal-free biocompatibility, the resultant enzyme-COFs biocatalysts showcase functional enhancement in catalytic activity, chemical stability, long-term durability, and recyclability. This Concept describes the recent advances in the methodological strategies for engineering the COFs biocatalysts, with specific emphasis on the pore entrapment and in situ encapsulation strategies. The structural advantages of the COFs hybrid biocatalysts for organic synthesis, environment- and energy-associated applications are also canvassed. Additionally, the remaining challenges and the forward-looking directions in this field are also discussed. We believe that this Concept can offer useful methodological guidance for developing active and robust COFs biocatalysts.

Precise Regulation in Chain-Edge Structural Microenvironments of 1D Covalent Organic Frameworks for Photocatalysis

Covalent organic frameworks (COFs) constitute a promising research topic for photocatalytic reactions, but the rules and conformational relationships of 1D COFs are poorly defined. Herein, the chain edge structure is designed by precise modulation at the atomic level, and the 1D COFs bonded by C, O, and S elements is directionally prepared for oxygen-tolerant photoinduced electron transfer-atom transfer radical polymerization (PET-ATRP) reactions. It is demonstrated that heteroatom-type chain edge structures (─O─, ─S─) lead to a decrease in intra-plane conjugation, which restricts the effective transport of photogenerated electrons along the direction of the 1D strip. In contrast, the all-carbon type chain edge structure (─C─) with higher intra-plane conjugation not only reduces the energy loss of photoexcited electrons but also enhances the carrier density, which exhibits the optimal photopolymerization performance. This work offers valuable guidance in the exploitation of 1D COFs for high photocatalytic performance. This work offers valuable guidance in the exploitation of 1D COFs for high photocatalytic performance.

Enzyme Immobilization using Covalent Organic Frameworks: From Synthetic Strategy to COFs Functional Role

Enzymes, a class of biocatalysts, exhibit remarkable catalytic efficiency, specificity, and selectivity, governing many reactions that are essential for various cascades within living cells. The immobilization of structurally flexible enzymes on appropriate supports holds significant importance in facilitating biomimetic transformations in extracellular environments. Covalent organic frameworks (COFs) have emerged as ideal candidates for enzyme immobilization due to high surface tunability, diverse chemical/structural designs, exceptional stability, and metal-free nature. Various immobilization techniques have been proposed to fabricate COF-enzyme biocomposites, offering significant enhancements in activity and reusability for COF-immobilized enzymes as well as new insights into developing advanced enzyme-based applications. In this review, we provide a comprehensive overview of state-of-the-art strategies for immobilizing enzymes within COFs by focusing on their applicability and versatility. These strategies are systematically summarized and compared by categorizing them into postsynthesis immobilization and in situ immobilization, where their respective strengths and limitations are thoroughly discussed. Combined with an overview of critical emerging applications, we further elucidate the multifaceted roles of COFs in enzyme immobilization and subsequent applications, highlighting the advanced biofunctionality achievable through COFs.

Two-Dimensional Covalent Organic Frameworks with Pentagonal Pores

The pore shapes of two-dimensional covalent organic frameworks (2D COFs) significantly limit their practical applications in separation and catalysis. Although various 2D COFs with polygonal pores have been well developed, constructing COFs with pentagonal pores remains an enormous challenge. In this work, we developed one kind of pentagonal COFs with the mcm topological structure for the first time, through the rational combination of C4 and C2 symmetric building blocks. The resulting pentagonal COFs exhibit high crystallinity, excellent porosity, and strong robustness. Moreover, the inbuilt porphyrin units render these COFs as efficient electrocatalytic catalysts toward oxygen reduction reaction with a half-wave potential of up to 0.81 V, which ranks as one of the highest values among COFs-based electrocatalysts. This work not only verified the possibility of constructing 2D COFs with pentagonal pores but also developed a strategy for the construction of functional 2D COFs for interesting applications.

Enzyme-Immobilized Porous Crystals for Environmental Applications

Developing efficient technologies to eliminate or degrade contaminants is paramount for environmental protection. Biocatalytic decontamination offers distinct advantages in terms of selectivity and efficiency; however, it still remains challenging when applied in complex environmental matrices. The main challenge originates from the instability and difficult-to-separate attributes of fragile enzymes, which also results in issues of compromised activity, poor reusability, low cost-effectiveness, etc. One viable solution to harness biocatalysis in complex environments is known as enzyme immobilization, where a flexible enzyme is tightly fixed in a solid carrier. In the case where a reticular crystal is utilized as the support, it is feasible to engineer next-generation biohybrid catalysts functional in complicated environmental media. This can be interpreted by three aspects: (1) the highly crystalline skeleton can shield the immobilized enzyme against external stressors. (2) The porous network ensures the high accessibility of the interior enzyme for catalytic decontamination. And (3) the adjustable and unambiguous structure of the reticular framework favors in-depth understanding of the interfacial interaction between the framework and enzyme, which can in turn guide us in designing highly active biocomposites. This Review aims to introduce this emerging biocatalysis technology for environmental decontamination involving pollutant degradation and greenhouse gas (carbon dioxide) conversion, with emphasis on the enzyme immobilization protocols and diverse catalysis principles including single enzyme catalysis, catalysis involving enzyme cascades, and photoenzyme-coupled catalysis. Additionally, the remaining challenges and forward-looking directions in this field are discussed. We believe that this Review may offer a useful biocatalytic technology to contribute to environmental decontamination in a green and sustainable manner and will inspire more researchers at the intersection of the environment science, biochemistry, and materials science communities to co-solve environmental problems.

Covalent Organic Frameworks as Potential Drug Carriers and Chemotherapeutic Agents for Ovarian Cancers

Anticancer drugs are often associated with limitations such as poor stability in aqueous solutions, limited cell membrane permeability, nonspecific targeting, and irregular drug release when taken orally. One possible solution to these problems is the use of nanocarriers of drug molecules, particularly those with targeting ability, stimuli-responsive properties, and high drug loading capacity. These nanocarriers can improve drug stability, increase cellular uptake, allow specific targeting of cancer cells, and provide controlled drug release. While improving the therapeutic efficacy of cancer drugs, contemporary researchers also aim to reduce their associated side effects, such that cancer patients are offered with a more effective and targeted treatment strategy. Herein, a set of nine porous covalent organic frameworks (COFs) were tested as drug delivery nanocarriers. Among these, paclitaxel loaded in COF-3 was most effective against the proliferation of ovarian cancer cells. This study highlights the emerging potential of COFs in the field of therapeutic drug delivery. Due to their biocompatibility, these porous COFs provide a viable substrate for controlled drug release, making them attractive candidates for improving drug delivery systems. This work also demonstrates the potential of COFs as efficient drug delivery agents, thereby opening up new opportunities in the field of sarcoma therapy.

One-Pot Synthesis of a Mixed-Valent Copper(I/II)-Coordinated Covalent Organic Framework Induced by γ-Ray Radiation

Metal-anchored covalent organic frameworks (COFs), as a class of significant derivatives of COFs, are widely used as heterogeneous catalysts in diverse chemical reactions. However, they are typically synthesized via post-treatment strategies, which often lead to the decline of COF crystallinity, decrease of porous properties, instability in catalytic performances, generation of additional chemical waste, and consumption of excess time and energy. In this work, we demonstrate an approach to construct a metal-functionalized COF via a one-pot method induced by γ-ray radiation. Specifically, copper-coordinated COF was in situ synthesized by irradiating a mixture of monomers and copper salt under ambient conditions. Interestingly, the initial Cu2+ ions were reduced to Cu+ ions by the radiation-generated reducing species, affording a unique mixed-valent copper(I/II)-coordinated COF. Additionally, the copper-coordinated COF displayed enhanced crystallinity and porous properties compared to those of the parent COF, displaying an opposite trend to the postsynthetic method. Notably, the introduced copper on the COF skeleton endowed the parent COF with catalytic ability. The resulting copper-coordinated COF exhibited remarkable catalytic performances in the reduction of 4-nitrophenol to 4-aminophenol and maintained almost unchanged catalytic performance after five catalytic cycles.

Selective Removal of Denatured Proteins Using MOF Nanopores

Here we present, for the first time, the selective adsorption of denatured proteins using a metal-organic framework (MOF), demonstrating promising potential for protein purification. Typical proteins, such as lysozyme and carbonic anhydrase B, enter the pores of MIL-101 through their narrow apertures when they are denatured to an unfolded state. Selective adsorption is achieved by finely tuning two key features: the sizes of the aperture and cage of the MOF nanopores, which are responsible for sorting unfolded polypeptide chains and inhibiting the translocation of the native form into the pores, respectively. By leveraging this selective adsorption, we successfully purified a mixture of native and denatured proteins by adding MOF to the mixture, achieving a native purity of over 99%.

Characterization of a recombinant Aspergillus niger GZUF36 lipase immobilized by ionic liquid modification strategy

Enzyme immobilized on magnetic nanomaterials is a promising biocatalyst with efficient recovery under applied magnets. In this study, a recombinant extracellular lipase from Aspergillus niger GZUF36 (PEXANL1) expressed in Pichia pastoris GS115 was immobilized on ionic liquid-modified magnetic nano ferric oxide (Fe3O4@SiO2@ILs) via electrostatic and hydrophobic interaction. The morphology, structure, and properties of Fe3O4@SiO2@ILs and immobilized PEXANL1 were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, x-ray diffraction, vibration sample magnetometer, and zeta potential analysis. Under optimized conditions, the immobilization efficiency and activity recovery of immobilized PEXANL1 were 52 ± 2% and 122 ± 2%, respectively. The enzymatic properties of immobilized PEXANL1 were also investigated. The results showed that immobilized PEXANL1 achieved the maximum activity at pH 5.0 and 45 °C, and the lipolytic activity of immobilized PEXANL1 was more than twice that of PEXANL1. Compared to PEXANL1, immobilized PEXANL1 exhibited enhanced tolerance to temperature, metal ions, surfactants, and organic solvents. The operation stability experiments revealed that immobilized PEXANL1 maintained 86 ± 3% of its activity after 6 reaction cycles. The enhanced catalytic performance in enzyme immobilization on Fe3O4@SiO2@ILs made nanobiocatalysts a compelling choice for bio-industrial applications. Furthermore, Fe3O4@SiO2@ILs could also benefit various industrial enzymes and their practical uses. KEY POINTS: • Immobilized PEXANL1 was confirmed by SEM, FT-IR, and XRD. • The specific activity of immobilized PEXANL1 was more than twice that of PEXANL1. • Immobilized PEXANL1 had improved properties with good operational stability.