Bicontinuous Nanoporous Frameworks: Caged Longevity for Enzymes

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.

3D-Continuous Nanoporous Covalent Framework Membrane Nanoreactors with Quantitatively Loaded Ultrafine Pd Nanocatalysts

The confinement effect of catalytic nanoreactors containing metal catalysts within nanometer-sized volumes has attracted significant attention for their potential to enhance reaction rate and selectivity. Nevertheless, unregulated catalyst loading, aggregation, leaching, and limited reusability remain obstacles to achieving an efficient nanoreactor. A robust and durable catalytic membrane nanoreactor prepared by incorporating palladium nanocatalysts within a 3D-continuous nanoporous covalent framework membrane is presented. The reduction of palladium precursor occurs on the pore surface within 3D nanochannels, producing ultrafine palladium nanoparticles (Pd NPs) with their number density adjustable by varying metal precursor concentrations. The precise catalyst loading enables controlling the catalytic activity of the reactor while preventing excess metal usage. The facile preparation of Pd NP-loaded free-standing membrane materials allows hydrodechlorination in both batch and continuous flow modes. In batch mode, the catalytic activity is proportional to the loaded Pd amount and membrane area, while the membrane retains its activity upon repeated use. In continuous mode, the conversion remains above 95% for over 100 h, with the reactant solution passing through a single 50 µm-thick Pd-loaded membrane. The efficient nanoporous film-type catalytic nanoreactor may find applications in catalytic reactions for small chemical devices as well as in conventional chemistry and processes.

Ultrathin Nanostructured Films of Hyaluronic Acid and Functionalized β-Cyclodextrin Polymer Suppress Bacterial Infection and Capsular Formation of Medical Silicone Implants

A type of ultrathin films has been developed for suppressing capsule formation induced by medical silicone implants and hence reducing the inflammation response to such formation and the differentiation to myofibroblasts. The films were each fabricated from hyaluronic acid (HA) and modified β-cyclodextrin (Mod-β-CyD) polymer which was synthesized with a cyclodextrin with partially substituted quaternary amine. Ultrathin films comprising HA and Mod-β-CyD or poly(allylamine hydrochloride) (PAH) were fabricated by using a layer-by-layer dipping method. The electrostatic interactions produced from the functional groups of Mod-β-CyD and HA influenced the surface morphology, wettability, and bio-functional activity of the film. Notably, medical silicone implants coated with PAH/HA and Mod-β-CyD multilayers under a low pH condition exhibited excellent biocompatibility and antibiofilm and anti-inflammation properties. Implantation of these nanoscale film-coated silicones showed a reduced capsular thickness as well as reduced TGFβ-SMAD signaling, myofibroblast differentiation, biofilm formation, and inflammatory response levels. We expect our novel coating system to be considered a strong candidate for use in various medical implant applications in order to decrease implant-induced capsule formation.

Mesoporous Materials-Based Electrochemical Biosensors from Enzymatic to Nonenzymatic

Mesoporous materials have drawn more and more attention in the field of biosensors due to their high surface areas, large pore volumes, tunable pore sizes, as well as abundant frameworks. In this review, the progress on mesoporous materials-based biosensors from enzymatic to nonenzymatic are highlighted. First, recent advances on the application of mesoporous materials as supports to stabilize enzymes in enzymatic biosensing technology are summarized. Special emphasis is placed on the effect of pore size, pore structure, and surface functional groups of the support on the immobilization efficiency of enzymes and the biosensing performance. Then, the development of a nonenzymatic strategy that uses the intrinsic property of mesoporous materials (carbon, silica, metals, and composites) to mimic the behavior of enzymes for electrochemical sensing of some biomolecules is discussed. Finally, the challenges and perspective on the future development of biosensors based on mesoporous materials are proposed.

Facile Synthesis of Robust and Pore-Size-Tunable Nanoporous Covalent Framework Membrane by Simultaneous Gelation and Phase Separation of Covalent Network/Poly(methyl methacrylate) Mixture

We report a facile route toward the preparation of organic-solvent-resistant and three-dimensionally continuous nanoporous covalent framework membrane. The membrane was prepared from the blend of linear poly(methyl methacrylate) and the cross-linked polyurea-based organic network, followed by selective removal of the linear polymer part. The pore morphologies, porosity, and solvent permeation properties of the membrane could be simply modified by the initial composition of the poly(methyl methacrylate) added to a sol of the organic network. The pore was three-dimensionally continuous with pore size ranging from 5 nm to tens of nanometers. Despite the broad pore size distribution, ultrafiltration of sub-10 nm solutes was realized with a molecular size cutoff near 5 nm thanks to the bicontinuous pore structure of the membrane. The nanoporous structure exhibited long-term resistance to organic solvents as well as thermal stability and mechanical strength. The separation performance remained unchanged in organic-rich medium for a prolonged time. Our strategy provides a synthetic route to a structurally robust, three-dimensionally continuous nanoporous polymeric membrane for potential application that necessitates the use of organic solvent.

Enzymatic Cascade Catalysis in a Nanofiltration Membrane: Engineering the Microenvironment by Synergism of Separation and Reaction

Microenvironment plays a significant role in enzymatic catalysis, which directly influences enzyme activity and stability. It is important to regulate the enzyme microenvironment, especially for the liquid with unfavored properties (e.g., pH and dissolved oxygen). In this work, we propose a methodology that can regulate pH and substrate concentration for enzymatic catalysis by a biocatalytic membrane, which is composed of glucose oxidase (GOx) and horseradish peroxidase (HRP) co-immobilized in a polyamide nanofiltration (NF) membrane (i.e., beneath the separation layer). By virtue of the selective separation function of NF membrane and in situ production of organic acid/electron donor with GOx, a synergism effect of separation and reaction in the liquid/solid interface was manipulated for engineering the microenvironment of HRP to enhance its activity and stability for micropollutant removal in water. The outcome of this work not only provides a new methodology to precisely control enzymatic reaction but also offers a smart membrane system to efficiently and steadily remove the micropollutants in portable water.

Tuning Pore Heterogeneity in Covalent Organic Frameworks for Enhanced Enzyme Accessibility and Resistance against Denaturants

Achieving high-performance biocomposites requires knowledge of the compatability between the immobilized enzyme and its host material. The modular nature of covalent organic frameworks (COFs), as a host, allows their pore geometries and chemical functionalities to be fine-tuned independently, permitting comparative studies between the individual parameters and the performances of the resultant biocomposites. This research demonstrates that dual pores in COFs have profound consequences on the catalytic activity and denaturation of infiltrated enzymes. This approach enforces a constant pore environment by rational building-block design, which enables it to be unequivocally determined that pore heterogeneity is responsible for rate enhancements of up to threefold per enzyme molecule. More so, the enzyme is more tolerant to detrimental by-products when occupying the larger pore in a dual-pore COF compared to a corresponding uniform porous COF. Kinetic studies highlight that pore heterogeneity facilitates mass transfer of both reagents and products. This unparalleled versatility of these materials allows many different aspects to be designed on demand, lending credence to their prospect as next-generation host materials for various enzyme biocomposites catalysts.

Biobased, Internally pH-Sensitive Materials: Immobilized Yellow Fluorescent Protein as an Optical Sensor for Spatiotemporal Mapping of pH Inside Porous Matrices

The pH is fundamental to biological function and its measurement therefore crucial across all biosciences. Unlike homogenous bulk solution, solids often feature internal pH gradients due to partition effects and confined biochemical reactions. Thus, a full spatiotemporal mapping for pH characterization in solid materials with biological systems embedded in them is essential. In here, therefore, a fully biocompatible methodology for real-time optical sensing of pH within porous materials is presented. A genetically encoded ratiometric pH sensor, the enhanced superfolder yellow fluorescent protein (sYFP), is used to functionalize the internal surface of different materials, including natural and synthetic organic polymers as well as silica frameworks. By using controlled, tailor-made immobilization, sYFP is homogenously distributed within these materials and so enables, via self-referenced imaging analysis, pH measurements in high accuracy and with useful spatiotemporal resolution. Evolution of internal pH is monitored in consequence of a proton-releasing enzymatic reaction, the hydrolysis of penicillin by a penicillin acylase, taking place in solution or confined to the solid surface of the porous matrix. Unlike optochemical pH sensors, which often interfere with biological function, labeling with sYFP enables pH sensing without altering the immobilized enzyme's properties in any of the materials used. Fast response of sYFP to pH change permits evaluation of biochemical kinetics within the solid materials. Thus, pH sensing based on immobilized sYFP represents a broadly applicable technique to the study of biology confined to the internally heterogeneous environment of solid matrices.

Pore Environment Control and Enhanced Performance of Enzymes Infiltrated in Covalent Organic Frameworks

In the drive toward green and sustainable methodologies for chemicals manufacturing, biocatalysts are predicted to have much to offer in the years to come. That being said, their practical applications are often hampered by a lack of long-term operational stability, limited operating range, and a low recyclability for the enzymes utilized. Herein, we show how covalent organic frameworks (COFs) possess all the necessary requirements needed to serve as ideal host materials for enzymes. The resultant biocomposites of this study have shown the ability boost the stability and robustness of the enzyme in question, namely lipase PS, while also displaying activities far outperforming the free enzyme and biocomposites made from other types of porous materials, such as mesoporous silica and metal-organic frameworks, exemplified in the kinetic resolution of the alcohol assays performed. The ability to easily tune the pore environment of a COF using monomers bearing specific functional groups can improve its compatibility with a given enzyme. As a result, the orientation of the enzyme active site can be modulated through designed interactions between both components, thus improving the enzymatic activity of the biocomposites. Moreover, in comparison with their amorphous analogues, the well-defined COF pore channels not only make the accommodated enzymes more accessible to the reagents but also serve as stronger shields to safeguard the enzymes from deactivation, as evidenced by superior activities and tolerance to harsh environments. The amenability of COFs, along with our increasing understanding of the design rules for stabilizing enzymes in an accessible fashion, gives great promise for providing "off the shelf" biocatalysts for synthetic transformations.

Nanoporous polysulfones with in situ PEGylated surfaces by a simple swelling strategy using paired solvents

Strong and ductile nanoporous polymers are obtained from inexpensive PSF-b-PEG block copolymers using a facile and nondestructive swelling method.