One-pot synthesis of protein-embedded metal-organic frameworks with enhanced biological activities

Nanoformulation approach for improved stability and efficiency of lactoperoxidase

Lactoperoxidase is a glycosylated protein with a molecular mass of 78 kDa, which being excreted in several mammalian secretions. Lactoperoxidase is included in many biological processes and well-known to have biocidal actions, attending as active antibiotics and antiviral agents. This wide-spectrum of biocidal activities mediates via a definite inhibitory system named lactoperoxidase system which acts a potent role in the innate immune response since its activity is not restricted by the antimicrobial effect, but might act a significant role in the hydrolysis of many toxins like aflatoxin. Hence with the current progresses in technology, nanoparticles can offer chances as an active candidate that might be utilized for stabilizing and potentiating the activity of LPO for use in several applications. Due to the variability functions of LPO, this enzyme considers an active target to be encapsulated or coated to NPs for developing novel nanocombinations with controlled surface characteristics. The development of approaches which might enhance conformational stabilization for several weeks of LPO via nanoformulation could improve the biopharmaceutical applicability of this bioactive ingredient. Nanoformulation of LPO enhances novel functions that can be useful in many biotechnological applications like food industry, cosmetic and pharmaceutical applications or to deliver and encapsulate bioactive components.

Peptide-Mediated Synthesis of Zeolitic Imidazolate Framework-8 with Controllable Morphology and Size

Peptides with a sequence of Nap-I_x-GPLGLAG-R₄-NH₂ (x = 2, 4, and 6, shorted as I₂R₄, I₄R₄, and I₆R₄) were used as capping agents for the synthesis of zeolitic imidazolate framework-8 (ZIF-8) in water. Peptide addition can significantly inhibit the growth of ZIF-8 crystals. The shape and size of ZIF-8 crystals was related closely to the number of isoleucine (Ile, I) residues as well as concentration of the peptide. The shape of ZIF-8 crystals changes from rhomboid dodecahedron to truncated rhombic dodecahedron to cube with the decreasing number of isoleucine residues from six to two. At a peptide concentration of 1.0 mM, the morphology of ZIF-8 crystals was cubic, truncated rhombic dodecahedron, and typical rhombic dodecahedron in the cases of I₂R₄, I₄R₄, and I₆R₄, respectively. Also, the particle size can be regulated from ca. 1.7 μm to <100 nm by controlling the peptide concentration from 0 to 2.0 mM. This work develops a simple and green method for the synthesis of ZIF-8 crystals with controllable shape and size in water, which shows high potential for biomedical and biological applications.

Bioengineering of nano metal-organic frameworks for cancer immunotherapy

Immunotherapy techniques, such as immune checkpoint inhibitors, chimeric antigen receptor (CAR) T cell therapies and cancer vaccines, have been burgeoning with great success, particularly for specific cancer types. However, side effects with fatal risks, dysfunction in tumor microenvironment and low immune response rates remain the bottlenecks in immunotherapy. Nano metal-organic frameworks (nMOFs), with an accurate structure and a narrow size distribution, are emerging as a solution to these problems. In addition to their function of temporospatial delivery, a large library of their compositions, together with flexibility in chemical interaction and inherent immune efficacy, offers opportunities for various designs of nMOFs for immunotherapy. In this review, we overview state-of-the-art research on nMOFs-based immunotherapies as well as their combination with other therapies. We demonstrate that nMOFs are predominantly customized for vaccine delivery or tumor-microenvironment modulation. Finally, a prospect of nMOFs in cancer immunotherapy will be discussed.

Engineering of a Core-Shell Nanoplatform to Overcome Multidrug Resistance via ATP Deprivation

Inhibiting the function of P-glycoprotein (P-gp) transporter, which causes drug efflux through adenosine triphosphate (ATP)-dependent manner, has become an effective strategy to conquer multidrug resistance (MDR) of cancer cells. However, there remains challenges for effective co-delivery, sequential release of P-gp modulator and chemotherapeutic agent. In this work, a novel type of core-shell nanoparticle is reported. It can independently encapsulate a high amount (about 683 µg mg^-1 ) of chemotherapeutic agent doxorubicin (DOX) in the mesoporous polydopamine (MPDA) core and glucose oxidase (GOx) in the zeolite imidazolate frameworks-8 (ZIF-8) shell, namely MPDA@ZIF-8/DOX+GOx. The fast release of GOx triggered by acid-sensitive degradation of the ZIF-8 shell consumes glucose to starve cancer cells for ATP deprivation and effective suppress ATP-dependent drug efflux in advance, and then effectively facilitates the accumulation of DOX in MCF-7/ADR cancer cells. Experiments in vitro and in vivo demonstrate that the fabricated nanosystem can dramatically improve anticancer effects for MDR through sequential release property and exhibit excellent biocompatibility. Overall, this work reveals new insights in the use of GOx for MDR treatment.

Integrating Target-Triggered Aptamer-Capped HRP@Metal-Organic Frameworks with a Colorimeter Readout for On-Site Sensitive Detection of Antibiotics

Colorimetric analytical strategies exhibit great promise in developing on-site detection methods for antibiotics, while substantial recent research efforts remain problematic due to dissatisfactory sensitivity. Taking this into account, we develop a novel colorimetric sensor for in-field detection of antibiotics by using aptamer (Apt)-capped and horseradish peroxidise (HRP)-embedded zeolitic metal azolate framework-7 (MAF-7) (Apt/HRP@MAF-7) as target recognition and signal transduction, respectively. With the substrate 3,3',5,5'-tetramethylbenzidine (TMB)-impregnated chip attached on the lid, the assay can be conveniently operated in a tube and reliably quantified by a handheld colorimeter. Hydrophilic MAF-7 can not only prevent HRP aggregation but also enhance HRP activity, which would benefit its detection sensitivity. Besides, the catalytic activity of HRP@MAF-7 can be sealed through assembling with Apt and controllably released based on the bioresponsivity via forming target-Apt complexes. Consequently, a significant color signal can be observed owing to the oxidation of colorless TMB to its blue-green oxidized form oxTMB. As a proof-of-concept, portable detection of streptomycin was favorably achieved with excellent sensitivity, which is superior to most reported methods and commercial kits. The developed strategy affords a new design pattern for developing on-site antibiotics assays and immensely extends the application of enzyme embedded metal-organic framework composites.

Metal-Organic Frameworks for Enzyme Immobilization: Beyond Host Matrix Materials

Enzyme immobilization in metal-organic frameworks (MOFs) as a promising strategy is attracting the interest of scientists from different disciplines with the expansion of MOFs' development. Different from other traditional host materials, their unique strengths of high surface areas, large yet adjustable pore sizes, functionalizable pore walls, and diverse architectures make MOFs an ideal platform to investigate hosted enzymes, which is critical to the industrial and commercial process. In addition to the protective function of MOFs, the extensive roles of MOFs in the enzyme immobilization are being well-explored by making full use of their remarkable properties like well-defined structure, high porosity, and tunable functionality. Such development shifts the focus from the exploration of immobilization strategies toward functionalization. Meanwhile, this would undoubtedly contribute to a better understanding of enzymes in regards to the structural transformation after being hosted in a confinement environment, particularly to the orientation and conformation change as well as the interplay between enzyme and matrix MOFs. In this Outlook, we target a comprehensive review of the role diversities of the host matrix MOF based on the current enzyme immobilization research, along with proposing an outlook toward the future development of this field, including the representatives of potential techniques and methodologies being capable of studying the hosted enzymes.

Insights into Nitrogenase Bioelectrocatalysis for Green Ammonia Production

There is a growing interest in using ammonia as a liquid carrier of hydrogen for energy applications. Currently, ammonia is produced industrially by the Haber-Bosch process, which requires high temperature and high pressure. In contrast, bacteria have naturally evolved an enzyme known as nitrogenase, that is capable of producing ammonia and hydrogen at ambient temperature and pressure. Therefore, nitrogenases are attractive as a potentially more efficient means to produce ammonia via harnessing the unique properties of this enzyme. In recent years, exciting progress has been made in bioelectrocatalysis using nitrogenases to produce ammonia. Here, the prospects for developing biological ammonia production are outlined, key advances in bioelectrocatalysis by nitrogenases are highlighted, and possible solutions to the obstacles faced in realising this goal are discussed.

Size-Tunable Metal-Organic Framework-Coated Magnetic Nanoparticles for Enzyme Encapsulation and Large-Substrate Biocatalysis

Immobilizing enzymes on nanoparticles (NPs) enhances the cost-efficiency of biocatalysis; however, when the substrates are large, it becomes difficult to separate the enzyme@NP from the products while avoiding leaching or damage of enzymes in the reaction medium. Metal-organic framework (MOF)-coated magnetic NPs (MNPs) offer efficient magnetic separation and enhanced enzyme protection; however, the involved enzymes/substrates have to be smaller than the MOF apertures. A potential solution to these challenges is coprecipitating metal/ligand with enzymes on the MNP surface, which can partially bury (protect) the enzyme below the composite surface while exposing the rest of the enzyme to the reaction medium for catalysis of larger substrates. Here, to prove this concept, we show that using Ca²⁺ and terephthalic acid (BDC), large-substrate enzymes can be encapsulated in CaBDC-MOF layers coated on MNPs via an enzyme-friendly, aqueous-phase one-pot synthesis. Interestingly, we found that using MNPs as the nuclei of crystallization, the composite size can be tuned so that nanoscale composites were formed under low Ca²⁺/BDC concentrations, while microscale composites were formed under high Ca²⁺/BDC concentrations. While the microscale composites showed significantly enhanced reusability against a non-structured large substrate, the nanoscale composites displayed enhanced catalytic efficiency against a rigid, crystalline-like large substrate, starch, likely due to the improved diffusivity of the nanoscale composites. To our best knowledge, this is the first report on aqueous-phase one-pot synthesis of size-tunable enzyme@MOF/MNP composites for large-substrate biocatalysis. Our platform can be applied to immobilize other large-substrate enzymes with enhanced reusability and tunable sizes.

A novel signal amplification strategy electrochemical immunosensor for ultra-sensitive determination of p53 protein

In this work, we fabricated a novel sandwich-type electrochemical immunosensor for quantitative and ultra-sensitive determination of tumor suppressor protein p53 by signal amplification strategy. Conductive polymers poly (3, 4-ethylenedioxythiophene): polystyrenesulfonate (PEDOT:PSS) has significantly effect on enhancing charge transfer and markedly increases the sensitivity of electrochemical immunosensing. Gold nanoparticles (AuNPs) as high conductivity nanocarriers were also used to capture monoclonal antibodies (Ab₁) due to their large specific surface areas. In addition, pH responsive zeolitic imidazolate framework (ZIF-8) was used to load the redox probe 2, 3-diaminophenazine (DAP) and the secondary antibodies (Ab₂) to form a sensitive-type ZIF-8-DAP-Ab₂ immunoprobe. After the sandwich-type immunoassay with the free p53 protein, with the release of probe DAP after the electrochemical signal amplificated by PEDOT:PSS and AuNPs, the ultra-sensitive and quantitative determination of p53 protein was realized with working range of 1-120 ng mL^-1 and low detection limit of 0.09 ng mL^-1. Besides, the fabricated electrochemical immunosensor exhibited good recovery, high sensitivity, reliability, and selectivity.

Metal-Organic Frameworks and Metal-Organic Cages - A Perspective

The fields of metal-organic cages (MOCs) and metal-organic frameworks (MOFs) are both highly topical and continue to develop at a rapid pace. Despite clear synergies between the two fields, overlap is rarely observed. This article discusses the peculiarities and similarities of MOCs and MOFs in terms of synthetic strategies and approaches to system characterisation. The stability of both classes of material is compared, particularly in relation to their applications in guest storage and catalysis. Lastly, suggestions are made for opportunities for each field to learn and develop in partnership with the other.

A metal-organic framework based inner ear delivery system for the treatment of noise-induced hearing loss

Noise-induced hearing loss (NIHL) is associated with both acute and chronic noise exposure. The application of steroid hormones is the first-line treatment for NIHL. However, a high dose of steroid hormone in the body is necessary to maintain its efficacy and causes side effects, such as headache and osteoporosis. In this work, we prepared a zeolitic imidazolate framework (ZIF)-based system for steroid hormone delivery in the inner ear. Methylprednisolone (MP), a typical steroid hormone, was encapsulated into ZIF-90 nanoparticles (NPs) using one-pot synthesis method. The obtained MP@ZIF-90 NPs are negatively charged and 120 nm in size and showed good biocompatibility and stability at a pH value of 7.4. After intraperitoneal injection, ZIF-90 could efficiently protect drugs during peripheral blood circulation, enter the inner ear via the blood labyrinthine barrier (BLB) and slowly release the drugs. Auditory brainstem response (ABR) tests indicated that MP@ZIF-90 exhibits better protection of mice from noise than those using the free MP and ZIF-8 with encapsulated MP (MP@ZIF-8). More importantly, MP@ZIF-90 showed no defects to the inner ear after being treated for noise and low nephrotoxicity during therapy, which demonstrates the biocompatibility of this material. We believe the ZIF-90 based delivery system is an efficient strategy for inner ear therapy of NIHL.

Facile Synthesis of Enzyme-Embedded Metal-Organic Frameworks for Size-Selective Biocatalysis in Organic Solvent

In situ immobilization of enzyme into metal-organic frameworks (MOFs) is performed through a one-step and facile method. Candida antarctica lipase B (CalB) is directly embedded in zeolitic imidazolate framework (ZIF)-8 by simply mixing an aqueous solution of 2-methylimidazole and zinc nitrate hexahydrate [Zn(NO₃)₂⋅6H₂O] containing CalB at room temperature. Due to the intrinsic micropores of ZIF-8, the obtained CalB@ZIF composite is successfully applied in size-selective transesterification reaction in organic solvent. CalB@ZIF not only shows much higher catalytic activity but also exhibits higher thermal stability than free CalB. Besides, the robust ZIF-8 shell also offers the hybrid composites excellent reusability.

Tunable Polymeric Scaffolds for Enzyme Immobilization

The number of methodologies for the immobilization of enzymes using polymeric supports is continuously growing due to the developments in the fields of biotechnology, polymer chemistry, and nanotechnology in the last years. Despite being excellent catalysts, enzymes are very sensitive molecules and can undergo denaturation beyond their natural environment. For overcoming this issue, polymer chemistry offers a wealth of opportunities for the successful combination of enzymes with versatile natural or synthetic polymers. The fabrication of functional, stable, and robust biocatalytic hybrid materials (nanoparticles, capsules, hydrogels, or films) has been proven advantageous for several applications such as biomedicine, organic synthesis, biosensing, and bioremediation. In this review, supported with recent examples of enzyme-protein hybrids, we provide an overview of the methods used to combine both macromolecules, as well as the future directions and the main challenges that are currently being tackled in this field.

Review of the Biomolecular Modification of the Metal-Organ-Framework

Metal-organ frameworks (MOFs), as a kind of novel artificial material, have been widely studied in the field of chemistry. MOFs are capable of high loading capacities, controlled release, plasticity, and biosafety because of their porous structure and have been gradually functionalized as a drug carrier. Recently, a completely new strategy of combining biomolecules, such as oligonucleotides, polypeptides, and nucleic acids, with MOF nanoparticles was proposed. The synthetic bio-MOFs conferred strong protection and endowed the MOFs with particular biological functions. Biomolecular modification of MOFs to form bridges for communication between different subjects has received increased attention. This review will focus on bio-MOFs modification methods and discuss the advantages, applications, prospects, and challenges of using MOFs in the field of biomolecule delivery.

Fluorescence-Scattering Dual-Signal Response of Carbon Dots@ZIF-90 for Phosphate Ratiometric Detection

Ratiometric fluorescence has drawn extensive attention owing to its self-calibration property. However, it is difficult to obtain appropriate fluorescent materials that can be excited under one excitation and possess well-resolved signals simultaneously. In this work, with the optical properties of the fluorescence of carbon dots (CDs) and the second-order scattering (SOS) of ZIF-90 (zeolitic imidazole frameworks-90) nanoparticles, the synthesized CDs@ZIF-90 can be applied to phosphate (PO₄^3-) ratiometric detection. The fluorescence of CDs is greatly suppressed through encapsulating CDs into ZIF-90. Nevertheless, the SOS is quite obvious due to the high scattering intensity of large size ZIF-90. The competitive coordination between PO₄^3- and the metal node of ZIF-90 decomposes CDs@ZIF-90, leading to the restoration of fluorescence and the diminution of SOS. On the basis of the PO₄^3--induced ZIF-90 decomposition and CD release, a novel method for PO₄^3- ratiometric detection is developed through the dual-signal response of the fluorescence scattering. Under the optimal condition, the method shows a linear range from 1.0 to 50.0 μmol L^-1 with a detection limit of 0.23 μmol L^-1. Furthermore, the probes are employed to assess PO₄^3- in practical aqueous samples successfully. Compared with the traditional approach, which only records fluorescence signals, the method reported here provides a new strategy to design ratiometric sensors by fluorescence and scattering.

Biomineralized metal-organic framework nanoparticles enable a primer exchange reaction-based DNA machine to work in living cells for imaging and gene therapy

Sensitive tumor imaging and precise tumor therapy play critical roles in the cancer combat. Herein, we build a DNA machine based on a primer exchange reaction (PER) for mRNA imaging and gene therapy. By using zeolitic imidazolate framework-8 nanoparticles (ZIF-8 NPs) to co-deliver the components including a primer, hairpin and strand displacing polymerase to the living cells, the PER-based DNA machine can be initiated by intracellular survivin mRNA and continuously produce Bcl-2 antisense DNA (ASD), which enables the DNA machine not only to image survivin mRNA but also to implement gene therapy. The results demonstrate that ZIF-8 NPs can protect the polymerases and nucleic acid probes from protease attack and nuclease degradation. After internalization, pH-responsive ZIF-8 NPs can efficiently release cargos from endo-lysosomes due to the protonation effect. The intracellular PER-based DNA machine has been demonstrated to be able to sensitively image survivin mRNA expression levels and selectively kill the cancer cells and has no effect on the normal cells. The PER-based DNA machine may provide a promising platform for early stage tumor diagnosis and more precise tumor therapy.

Modulation of metal-azolate frameworks for the tunable release of encapsulated glycosaminoglycans

Glycosaminoglycans (GAGs) are biomacromolecules necessary for the regulation of different biological functions. In medicine, GAGs are important commercial therapeutics widely used for the treatment of thrombosis, inflammation, osteoarthritis and wound healing. However, protocols for the encapsulation of GAGs in MOFs carriers are not yet available. Here, we successfully encapsulated GAG-based clinical drugs (heparin, hyaluronic acid, chondroitin sulfate, dermatan sulfate) and two new biotherapeutics in preclinical stage (GM-1111 and HepSYL proteoglycan) in three different pH-responsive metal-azolate frameworks (ZIF-8, ZIF-90, and MAF-7). The resultant GAG@MOF biocomposites present significant differences in terms of crystallinity, particle size, and spatial distribution of the cargo, which influences the drug-release kinetics upon applying an acidic stimulus. For a selected system, heparin@MOF, the released therapeutic retained its antithrombotic activity while the MOF shell effectively protects the drug from heparin lyase. By using different MOF shells, the present approach enables the preparation of GAG-based biocomposites with tunable properties such as encapsulation efficiency, protection and release.

A capillary-based fluorimetric platform for the evaluation of glucose in blood using gold nanoclusters and glucose oxidase in the ZIF-8 matrix

A capillary-based fluorimetric analysis method was developed for probing glucose (Glu) in blood using Glu oxidase-anchored gold nanoclusters (GOD-AuNCs) and the ZIF-8 matrix. AuNCs were attached with GOD to be further encapsulated into the ZIF-8 matrix through the protein-mediated formation route. The resulting GOD-AuNCs@ZIF-8 nanocomposites could present the AuNC-improved catalysis of GOD and ZIF-8-improved environmental stability. The ZIF-8-enhanced fluorescence intensity of AuNCs could also be expected. Moreover, a capillary-based fluorometric platform was constructed for sensing Glu by coating the capillaries first with GOD-AuNCs and then the ZIF-8 matrix. Herein, Glu was introduced through the self-driven sampling to trigger the GOD-catalyzed production of hydrogen peroxide, which could induce the fluorescence quenching rationally depending on the Glu concentrations. The developed fluorimetric method could allow for the rapid and simple detection of Glu with the concentrations linearly ranging from 5.0 μM to 2.5 mM. Besides, the feasibility of practical applications was demonstrated by the evaluation of Glu in blood showing the recoveries of 96.2%-103.4%. Importantly, the proposed design of the capillary-based fluorimetric platform by the synergetic combination of catalysis-specific recognition and fluorescence signaling may open a new door toward extensive applications in the biological sensing, catalysis, and imaging fields.

Metal-Organic Frameworks: A Potential Platform for Enzyme Immobilization and Related Applications

Enzymes, as natural catalysts with remarkable catalytic activity and high region-selectivities, hold great promise in industrial catalysis. However, applications of enzymatic transformation are hampered by the fragility of enzymes in harsh conditions. Recently, metal-organic frameworks (MOFs), due to their high stability and available structural properties, have emerged as a promising platform for enzyme immobilization. Synthetic strategies of enzyme-MOF composites mainly including surface immobilization, covalent linkage, pore entrapment and in situ synthesis. Compared with free enzymes, most immobilized enzymes exhibit enhanced resistance against solvents and high temperatures. Besides, MOFs serving as matrixes for enzyme immobilization show extraordinary superiority in many aspects compared with other supporting materials. The advantages of using MOFs to support enzymes are discussed. To obtain a high enzyme loading capacity and to reduce the diffusion resistance of reactants and products during the reaction, the mesoporous MOFs have been designed and constructed. This review also covers the applications of enzyme-MOF composites in bio-sensing and detection, bio-catalysis, and cancer therapy, which is concerned with interdisciplinary nano-chemistry, material science and medical chemistry. Finally, some perspectives on reservation or enhancement of bio-catalytic activity of enzyme-MOF composites and the future of enzyme immobilization strategies are discussed.

Anode-Driven Controlled Release of Cathodic Fuel via pH Response for Smart Enzymatic Biofuel Cell

Enzymatic biofuel cells (EBFCs) with or without a membrane to separate the anodic and cathodic compartments generally suffered from high internal resistance or interactive interference, both of which restricted the improvement of their performance. Herein, a smart membrane-less EBFC was engineered based on anode-driven controlled release of cathodic acceptor via pH-responsive metal-organic framework ([Fe(CN)6]3-@ZIF-8) nanocarriers. The glucose anodic oxidation would produce gluconic acid accompanied by the change in pH value from neutral to the acidic case, which could drive the degradation of [Fe(CN)6]3-@ZIF-8 nanocarriers and further realize the controlled release of cathodic acceptor [Fe(CN)6]3-. More importantly, compared with controlled EBFC with or without membrane, the power output of the as-proposed EBFC enhanced at least 700 times due to the seamless electronic communication. Therefore, the ingenious strategy not only realized the successful engineering of the membrane-less EBFC but also provided an appealing idea for constructing smart devices.

Catalytically Active Hollow Fiber Membranes with Enzyme-Embedded Metal-Organic Framework Coating

Metal-organic frameworks (MOFs) are suitable enzyme immobilization matrices. Reported here is the in situ biomineralization of glucose oxidase (GOD) into MOF crystals (ZIF-8) by interfacial crystallization. This method is effective for the selective coating of porous polyethersulfone microfiltration hollow fibers on the shell side in a straightforward one-step process. MOF layers with a thickness of 8 μm were synthesized, and fluorescence microscopy and a colorimetric protein assay revealed the successful inclusion of GOD into the ZIF-8 layer with an enzyme concentration of 29±3 μg cm^-2 . Enzymatic activity tests revealed that 50 % of the enzyme activity is preserved. Continuous enzymatic reactions, by the permeation of β-d-glucose through the GOD@ZIF-8 membranes, showed a 50 % increased activity compared to batch experiments, emphasizing the importance of the convective transport of educts and products to and from the enzymatic active centers.

Layer-by-Layer Decorated Nanoscale ZIF-8 with High Curcumin Loading Effectively Inactivates Gram-Negative and Gram-Positive Bacteria

Bacteria-mediated infectious diseases have become a health-care challenge globally since the development of antibiotic resistance. Reactive oxygen species produced by photosensitizers have great potential in fighting bacterial infections, especially against Gram-negative bacteria that are hard to kill by regular methods owing to their formidable defensive membrane structures under the premise of avoiding overuse of antibiotics. In this work, a small molecular photosensitizer, curcumin (CCM), was used as a model and encapsulated into zeolitic imidazolate framework-8 (ZIF-8). Then the ZIF-8 loaded with CCM (CCM@ZIF-8) was decorated with biocompatible polymers hyaluronic acid (HA) and chitosan (CS) by the layer-by-layer self-assembly technique to yeild in an antibacterial CCM@ZIF-8@HA@CS nanoparticle with a high local positive charge density and is capable of binding the surface of bacteria by electrostatic interactions. The CCM drug loading capability of the nanoparticle was found to be as high as 10.89%. Upon exposure to blue light (72 J/cm²) for 10 min, the minimum inhibitory concentration and minimum bactericidal concentration of CCM@ZIF-8@HA@CS against Gram-positive bacteria (G⁽⁺⁾) Staphylococcus aureus (S. aureus) and Gram-negative bacteria (G^(-)) Escherichia coli (E. coli) were the same, which were as low as 0.625 and 2.5 μg/mL, respectively, showing highly effective antibacterial activities. After treatment with CCM@ZIF-8@HA@CS under blue-light irradiation, the membranes of S. aureus and E. coli folded and cracked. Importantly, the antibacterial agent showed good biocompatibility in the cytotoxicity test using L929 cells and hemolysis test using rabbit blood cells under blue-light irradiation. Therefore, this CCM@ZIF-8@HA@CS nanocomposite is expected to find application in the treatment of superficial traumatic and refractory chronic infections caused by G⁽⁺⁾ and G^(-).

Coupling Two Sequential Biocatalysts with Close Proximity into Metal-Organic Frameworks for Enhanced Cascade Catalysis

The encapsulation of multiple enzyme/nanoenzyme systems within mental-organic frameworks (MOFs) shows great promise for a myriad of practical applications. Herein, two sequential biocatalysts, oxidase and hemin, were coupled together with close proximity using a bifunctional polymer, poly(1-vinylimidazole) (PVI), and encapsulated into MOFs. As a demonstration of the power of such a protocol, glucose oxidase&PVI-hemin encapsulated in ZIF-8 showed significant enhancement of bioactivity for a cascade reaction compared to its counterpart without PVI. For the colorimetric assay of glucose, it showed a low limit of detection of 0.4 μM (S/N = 3), high selectivity, and excellent stability. Because there are numerous biocatalysts that can readily be coupled and encapsulated into MOFs, a myriad of interesting properties can be simply realized by encapsulating different sequential biocatalysts.

Catalytic Activity, Stability, and Loading Trends of Alcohol Dehydrogenase Enzyme Encapsulated in a Metal-Organic Framework

Recently, it has been shown that enzyme encapsulation inside metal-organic frameworks (MOFs) can increase enzyme activity and serve as protection from adverse environmental conditions. Little is understood about how the enzymes move into and are held inside the MOFs although it is believed that intermolecular forces between the MOF and the enzyme cause it to be held in place. If this process can be better understood, it can have dramatic implications on the cost-effectiveness and implementation of enzyme-MOF complexes. This is of specific importance in the medical sector for protein therapy and the industrial sector where enzyme use is expected to increase. Herein, we synthesized alcohol dehydrogenase (ADH) and PCN-333 to study encapsulation, stability, and enzyme activity to expand the knowledge of our field and offer a potential improvement to a synthetic route for biofuel synthesis. From this, we found a correlation between the concentration of a buffer and the loading of an enzyme, with surprising loading trends. We conclude that the buffer solution decreases interactions between the enzyme and MOF, supporting conventional theory and allowing it to penetrate deeper into the structure causing higher enzyme loading while allowing for excellent stability over time at various pH values and temperatures and after multiple reactions. We also observe new trends such as a rebounding effect in loading and "out-of-bounds" reactions.

The Progress and Prospect of Zeolitic Imidazolate Frameworks in Cancer Therapy, Antibacterial Activity, and Biomineralization

The progressive development of zeolitic imidazolate frameworks (ZIFs), as a subfamily of metal-organic frameworks (MOFs), and their unique features, including tunable pore size, large surface area, high thermal stability, and biodegradability/biocompatibility, have made them attractive in the field of biomedicine, especially for drug delivery and biomineralization applications. The high porosity of ZIFs gives them the opportunity for encapsulating a high amount of therapeutic drugs, proteins, imaging cargos, or a combination of them to construct advanced multifunctional drug delivery systems (DDSs) with combined therapeutic and imaging capabilities. This review summarizes recent strategies on the design and fabrication of ZIF-based nansystems and their exploration in the biomedical field. First, recent developments for the adjustment of particle size, functionality, and morphology of ZIFs are discussed, which are important for achieving optimized therapeutic/theranostic nanosystems. Second, recent trends on the application of ZIF nanocarriers for the loading of diverse cargos, including anticancer medicines, antibiotic drugs, enzymes, proteins, photosensitizers, as well as imaging and photothermal agents, are investigated in order to understand how multifunctional DDSs can be designed based on the ZIF nanoparticles to treat different diseases, such as cancer and infection. Finally, prospects on the future research direction and applications of ZIF-based nanomedicines are discussed.

In situ encapsulation of horseradish peroxidase in zeolitic imidazolate framework-8 enables catalyzing luminol reaction under near-neutral conditions for sensitive chemiluminescence determination of cholesterol

HRP@ZIF-8 nanocomposite was prepared by in situ encapsulation of horseradish peroxidase (HRP) in the frame of zeolitic imidazolate framework-8 (ZIF-8) with a simple one-pot method. The HRP@ZIF-8 nanocomposite displays outstanding thermal stability and efficiently catalyzes the chemiluminescence (CL) reaction of luminol with hydrogen peroxide (H₂O₂) under near-neutral pH condition (pH 7-8). This CL system has a good response to H₂O₂ with a linear range of 0.1-100.0 μmol L^-1. The limit of detection (LOD) is 0.06 μmol L^-1 H₂O₂. By marriage with cholesterol oxidase, cholesterol is determined with a linear range from 0.1 to 100.0 μmol L^-1 and a LOD of 0.04 μmol L^-1. The relative standard deviations (RSD) are 1.7% and 2.5%, respectively, in 11 repeated measurements of 50.0 μmol L^-1 solutions of H₂O₂ and cholesterol, indicating excellent precision of the method. The method shows good selectivity and has been applied to the determination of total cholesterol in real serum samples. No significant difference has been observed between the results obtained by this method and the cholesterol oxidase-peroxidase coupling method. Graphical abstract Schematic presentation of in situ one-pot synthesis of horseradish peroxidase@zeolitic imidazolate framework-8 (HRP@ZIF-8) nanocomposite and chemiluminescence determination of cholesterol with HRP@ZIF-8 catalyzing luminol-H₂O₂ system.

Enzyme Immobilization on Graphite Oxide (GO) Surface via One-Pot Synthesis of GO/Metal-Organic Framework Composites for Large-Substrate Biocatalysis

Although enzyme immobilization has improved many areas, biocatalysis involving large-size substrates is still challenging for immobilization platform design because of the protein damage under the often "harsh" reaction conditions required for these reactions. Our recent efforts indicate the potential of using Metal-Organic Frameworks (MOFs) to partially confine enzymes on the surface of MOF-based composites while offering sufficient substrate contact. Still, improvements are required to expand the feasible pH range and the efficiency of contacting substrates. In this contribution, we discovered that Zeolitic Imidazolate Framework (ZIF) and a new calcium-carboxylate based MOF (CaBDC) can both be coprecipitated with a model large-substrate enzyme, lysozyme (lys), to anchor the enzyme on the surface of graphite oxide (GO). We observed lys activity against its native substrate, bacterial cell walls, indicating lys was confined on composite surface. Remarkably, lys@GO/CaBDC displayed a stronger catalytic efficiency at pH 6.2 as compared to pH 7.4, indicating CaBDC is a good candidate for biocatalysis under acidic conditions as compared to ZIFs which disassemble under pH < 7. Furthermore, to understand the regions of lys being exposed to the reaction medium, we carried out a site-directed spin labeling (SDSL) electron paramagnetic resonance (EPR) spectroscopy study. Our data showed a preferential orientation of lys in GO/ZIF composite, whereas a random orientation in GO/CaBDC. This is the first report on immobilizing solution-state large-substrate enzymes on GO surface using two different MOFs via one-pot synthesis. These platforms can be generalized to other large-substrate enzymes to carry out catalysis under the optimal buffer/pH conditions. The orientation of enzyme at the molecular level on composite surfaces is critical for guiding the rational design of new composites.

Metal-Organic Frameworks: A New Platform for Enzyme Immobilization

Metal-organic frameworks (MOFs) with attractive properties such as high surface area, tunable porosity, designable functionality and excellent stability, have aroused great interest from researchers as the matrices for enzyme immobilization. Recently, several efficient strategies including surface immobilization, post-synthetic infiltration and in situ encapsulation have been explored. MOF-immobilized enzymes, named enzymes@MOFs, show remarkably enhanced stability and recyclability, accelerating cell-free biocatalysis in diverse applications. This concept will impart the typical strategies for enzyme immobilization with MOFs, and their potential applications.

Continuous-Flow Synthesis of ZIF-8 Biocomposites with Tunable Particle Size

Zeolitic imidazolate framework (ZIF) biocomposites show the capacity to protect and deliver biotherapeutics. To date, the progress in this research area is based on laboratory batch methods. Now, the first continuous flow synthetic method is presented for the encapsulation of a model protein (bovine serum albumin, BSA) and a clinical therapeutic (α1-antitrypsin, AAT) in ZIF-8. The in situ kinetics of nucleation, growth, and crystallization of BSA@ZIF-8 were studied by small-angle X-ray scattering. By controlling the injection time of ethanol, the particle growth could be quenched by ethanol-induced crystallization from amorphous particles to ZIF-8 crystals. The particle size of the biocomposite was tuned in the 40-100 nm range by varying residence time prior to introduction of ethanol. As a proof-of-concept, this procedure was used for the encapsulation of AAT in ZIF-8. Upon release of the biotherapeutic from the composite, the trypsin inhibitor function of AAT was preserved.