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Coatings of metal–organic frameworks (MOFs) have potential applications in surface modification for medical implants, tissue engineering, and drug delivery systems. Therefore, developing an applicable method for surface-mounted MOF engineering to fabricate protective coating for implant tissue engineering is a crucial issue. Besides, the coating process was desgined for drug infusion and effect opposing chemical and mechanical resistance. In the present review, we discuss the techniques of MOF coatings for medical application in both in vitro and in vivo in various systems such as in situ growth of MOFs, dip coating of MOFs, spin coating of MOFs, Layer-by-layer methods, spray coating of MOFs, gas phase deposition of MOFs, electrochemical deposition of MOFs. The current study investigates the modification in the implant surface to change the properties of the alloy surface by MOF to improve properties such as reduction of the biofilm adhesion, prevention of infection, improvement of drugs and ions rate release, and corrosion resistance. MOF coatings on the surface of alloys can be considered as an opportunity or a restriction. The presence of MOF coatings in the outer layer of alloys would significantly demonstrate the biological, chemical and mechanical effects. Additionally, the impact of MOF properties and specific interactions with the surface of alloys on the anti-microbial resistance, anti-corrosion, and self-healing of MOF coatings are reported. Thus, the importance of multifunctional methods to improve the adhesion of alloy surfaces, microbial and corrosion resistance and prospects are summarized.
The high diversity of topological networks and attractive properties of metal–organic frameworks (MOFs) led to the development of their design and synthesis. MOFs, also recognized as porous polymers, are essentially composed of two main segments: a metal ion and an organic unit called a linker that typically includes di-, tri-, or tetradentate ligands [1, 2]. The porous hybrid solids-derived tubes (1D), layers (2D), and even frameworks (3D) were called MOFs [3, 4]. Various transition metal ions lead to diverse coordination numbers and geometries like linear, square planar, tetrahedral, square-pyramidal, trigonal–bipyramidal, and octahedral shapes [5]. Especially in the field of medicine, MOF materials are attracting much attention due to their unusual structures and properties, such as massive porosity and easy tunability of their pore size and shape from microporous to mesoporous scale [6]. The high internal porosity of MOFs provides spaces for enclosing different materials such as large amounts of drugs and biological molecules. Bio-MOFs typically would use natural organic ligands such as amino acids, proteins, peptides, nucleobases, porphyrins, and saccharides as bridges. The biological ligands have various coordination paths to metal centers that lead to the different structures of bio-MOFs [7]. In addition, other natural small molecules can also be applied as biological ligands to coordinate with metals to fabricate bio-MOFs for medical and surgical applications [8, 9]. The hybrid organic–inorganic MOFs allows the appropriate and optimal fabrication of molecular building blocks possessing favorable functionality and porosity can be selected for a system suitable for medical and surgical applications. The ability and passion to prepare porous MOFs is emerged from their various applicability for bio-sensing [10], drug delivery [11], biological imaging [12], biocatalysts [13], disease diagnosis [14], implants [15], and therapy treatment [16] compared with traditional materials. In addition, researchers are currently exploring high multifunctional platforms to satisfy the growing demand for medical care, diagnostic care, and preventive healthcare. The preparation of thin MOF coatings as promising approaches to biomaterials is the surface-mounted strategy, which involves the direct fabrication of MOFs on the substrate surface. This method allows control of the MOF thickness and crystal orientation, as well as the integration of functional groups into the MOF structure.
The chemical composition and outstanding structure of MOFs include a variety of advantages such as the high porosity morphology, the regular micro and mesopores shape, the great specific surface area, flexible design ability, and simple diffusion of reagents via pores [17, 18]. There are several drawbacks in some samples of MOFs, like relatively low yields and poor reproducibility [19]. The poor chemical stability is one of the main drawbacks of MOFs which depends largely on the metal node and the strength of the chemical bond between the metal node and the bonder. In general, typical MOF substances are unstable in water and are also gradually degraded by water molecules due to the interference of coordination bonds [20]. Zr-based MOFs are one of the best chemically stable MOFs in organic solvents and water under an extensive pH range as well as high thermal stability (near 375 °C) [19]. MOF materials also have their benefits and drawbacks for specific applications. The solvothermal and hydrothermal techniques have typically achieved the best results in crystallinity and morphological properties compared to the other preparation methods. In the solvothermal process, the complete removal of all organic solvents is very difficult and also causes a risk to biomedical applications in vivo. But in the hydrothermal method, water is used, which is the friendliest solvent [21]. The multidentate organic linkers also bonded generally via carboxylic or heterocyclic nitrogen moieties. Therefore, the study of coordination between the metal ions and biological ligands such as amino acids, peptides, and proteins can give us more insight into the flexibility and collapse of pores [22, 23]. For medical treatment, there are diverse limitations of drugs after encapsulating in the frameworks of MOF structures, such as poor solubility, blood instability, and systemic toxicity [8]. These challenges potentially restrict the empirical usage of MOF in biomedical applications. As regards, MOF thin film coatings are of interest due to their wide range of applications in the medical and surgical field. The application of MOF in the medical field requires a complete and extensive analysis of cellular biocompatibility and nano-safety. Among various MOFs, a group of isostructural MOF-74 s have been known principally due to their high surface area and active metal sites that are candidate for medical applications. For instance, Mg-MOF-74 for CO2 adsorption [24], Fe-MOF-74 for drug delivery of ibuprofen anions [25], Mg-MOF-74 for drug delivery of combined ibuprofen and curcumin [26], the mixing Mg-MOF-74 with Zn-MOF-74 for drug delivery of curcumin [27] that were studied.
The present review mainly discusses the growth and medical usages of MOF films on substrate surfaces. We intend to fill this gap by examining the effects of various MOF coating for different medical uses ranging from drug delivery systems to surface coating of medical implants. This paper portrays insights concerning the flexible idea of coating MOFs for implant surface treatment, responsive surface, therapy, drug delivery, biological imaging, and bio-sensing (Fig. 1 ). The present paper gives a point-by-point audit of the various properties of MOF coating, including anti-corrosion and anti-microbial properties of MOF coating. This paper provides a brief review of multiple techniques of MOF coating on different substrates. In particular, it summarizes work focusing on peer growth and applying MOF coatings on substrate surfaces. Moreover, the future challenges and outlook of MOF coating are also discussed. This review may interest researchers attempting to fabricate other MOF coatings and those involved in expanding the medical coating applications of MOFs.
Schematic illustration of the characteristics and performance of MOF coatings, their application of biomaterials, and medical application
Biomedical surfaces play an important and influencing role in various medical applications, such as implantable devices and tissue engineering scaffolds. These surfaces are designed to interact with biological systems, and the characteristics of the substrate and coating have a significant effect on their performance [28]. They play an essential role in ensuring biocompatibility, promoting tissue integrity, preventing infection and increasing their performance. The design and properties of these surfaces directly affect their biocompatibility, which refers to a material's ability to interact with living tissues without causing adverse reactions. A well-designed biomedical surface can significantly improve patient outcomes and the overall success of medical procedures [29, 30]. The properties of both substrates and coatings on biomedical surfaces straightly affect their biocompatibility, functionality, and overall performance. By understanding these features and using their application to develop innovative medical devices, improving patient care and advancing the field of healthcare is necessary [31]. The substrate is the basic component of biomedical surfaces and its properties affect the performance of the surface. One of the essential specifications of the biocompatible substrate is that it does not cause immune responses or toxicity in the body. Mechanical strength is another important factor that is very important in the substrate, as it must withstand physiological forces and stress without compromising its integrity. In addition, the surface topography of the substrate affects cell adhesion, proliferation and differentiation. Also, the roughness or smoothness of the substrate can strengthen or hinder these cellular processes. Another key factor is porosity, particularly in tissue engineering applications, where it allows tissue ingrowth and nutrient exchange. But some of these properties can be improved or modified by applying coating [32, 33]. Coatings are often applied to enhance substrate performance for biomedical surface applications. Biocompatibility and strong adhesion of the coating to the substrate is very important and ensures that the coating does not cause adverse reactions in the body and ensures strong adhesion to the substrate to prevent peeling. Coatings can also be designed to have antimicrobial properties and reduce the risk of infection. In addition, coatings can act as reservoirs for controlled drug release, enabling localized treatment and improved therapeutic outcomes [34, 35]. Surface modification is a common strategy for preparing new chemical, mechanical, and geometrical properties of the material. The adhesion of bacteria to the surface starts a process that leads to the formation of a biofilm by bacterial colonization and, eventually infection [36]. The current methods for disinfection modifications mostly include anti-adhesive polymer coatings, but MOF coatings are highly regarded [37–39] (Fig. 2 a-c). In general, the surface modification of MOFs is widely applied for medical applications [40–42]. MOF coatings have been considered as an interesting biomedical surface and due to their unique properties in the field of biomedicine. They can be used as thin films on various substrates including medical implants to improve their performance and biocompatibility. MOF coatings have several unique properties in biomedical applications. They can be used as drug delivery systems, where the porous structure of MOFs can be used to entrap and release therapeutic agents in a controlled manner [43]. The large surface area of MOFs also allows effective adsorption of biomolecules such as proteins and enzymes, which can improve the biocompatibility and performance of implants. MOF coatings can be designed to have special properties such as anti-bacterial or anti-fouling capabilities that can help prevent infection and biofouling on biomedical surfaces. They can also be engineered to have stimuli-responsive properties, where the release of drugs or other molecules can be triggered by external stimuli, such as changes in pH or temperature [44]. The surface is coated by MOFs crystalline porous structure including a hybrid array of metal ions or clusters mounted to organic ligands. In general, there are several advantages of a MOF thin film compared to a metal oxide, such as porosity, selectivity, flexibility, surface area, tunability of coordination, synthesis, and processing. Besides, among the weak points of MOF thin films compared to other materials, we can mention techniques such as stability and low diffusion rate, complex and specialized processing, low mechanical strength, and relatively expensive cost. The ability to modify surface properties with MOFs to achieve improved corrosion resistance and biocompatibility while maintaining substrate material properties is a special advantage of surface-mounted MOF approaches.
Schematic illustration anti-adhesive activity of the hydrogel coating (a), and the bacterial cell damage upon contact with nano-pillars aligned on the surface (b), the anti-microbial peptide coated surface with bactericidal activity upon contact (c)
Bioactive and biocompatible coatings are a vital factor in various medical and biotechnological applications, such as implantable medical devices, drug delivery systems, and tissue engineering scaffolds. Due to their unique properties, these coatings can interact favorably with biological systems, leading to improved system integrity and minimizing adverse reactions. There are several technical challenges impeding the clinical applications of implant alloys. One of the critical challenges for the coating implant approach is maintaining the balance of obtaining efficient bactericidal ion concentrations without damaging the host tissue [45]. Furthermore, different architectural metal implants can be manufactured using a variation of accessible manufacturing technology [46]. Overall, there are several challenges, such as stability, biocompatibility, and controlled release of drugs, adhesion, and durability, cost-effectiveness associated with coating implants with MOFs. Overcoming challenges requires extensive research, collaboration between multidisciplinary teams, and the development of innovative techniques for MOF synthesis, characterization, and coating processes. Among the metallic implants, Ti-based alloys, Stainless steel, Co-based, and biodegradable Mg-based alloys are also highly available bone implant substances. The application of implants depends on various requirements the healing of the fractured bones, rectifying deformities, the improvement of the function of other parts of the human body, and the replacement of the damaged part of the anatomy [47]. For Ti alloys, extensive studies are yet to be carried out due to extremely challenging alloy processing through the melting casting route [48]. In addition, the low rate of destruction of traditional implants makes them unsuitable for temporary implant applications [49]. For Mg alloys, the challenge is that susceptibility to localized corrosion attack is when corrosion products cover their surface [50, 51]. The severe limitations of implant alloys lie in their inappropriate mechanical properties that lead to the stress-shielding severe problem, and the non-degradability of these materials. Sometimes these materials need a second surgery for implant elimination which leads to the release of toxic ions through the corrosion process [52]. The development of coatings with controlled development factor release rate is considered to simplify continuous bone conduction of Mg-based implants [53]. Therefore, the improvement and expansion of next-generation implant biomaterials is an urgent necessity. To develop bioactive and biocompatible coatings, there are several challenges, such as ensuring biocompatibility, stability in the long term, adhesion, exfoliation, controlled release, multiple functions, and biodegradability and absorption. Addressing these challenges requires interdisciplinary collaboration and advances in materials science, surface modification methods, and a deep understanding of biological interactions.
In general, the well-known MOFs are in powder form which are suitable for catalysts and sensor applications, but their usage as surface coatings is also of great interest. Then, thin or thick film depositions of MOFs have been explored due to their excellent surface area and abundant active sites due to their several applications of film MOFs in optical, medical, electronic, and energy devices [54–56]. It is noted as one of the efficient methods to design and fabricate MOF-coated implants using functional groups to make synergistic properties of alloy substrate and MOF coatings. It is believed that the unique MOF coating can significantly change the reactivity of the alloy surface [57]. Among the various thin film deposition techniques of MOFs, the liquid phase stepwise growth method makes it possible to prepare uniform and highly crystalline MOFs called surface-mounted MOFs (SURMOFs). This method is based on the consecutive immersion of functionalized substrate into the reagent solutions to coordinate them. Then the samples are washed with solvent to eliminate the uncoordinated precursor materials [58, 59]. Besides, the improvement of effective strategies to combine high-quality functional MOF coatings with alloy substrate is highly desired to achieve multifunctional applications with sequential multiple-drug delivery and implant integration [60]. Surface protective coatings can be found in several applications such as medicine and aerospace. Organic coatings are often designed to protect a metal substrate from corrosion [61]. Nowadays, multifunctional organic coatings are in demand to improve surface performance. Due to the insufficient properties of coatings, the development of layers with nanoporous materials is known as a hot research area. Amongst, the porous nanoparticles with functional groups and organic–inorganic hybrid compounds with polymer matrices have attracted high attention [62, 63]. MOFs have a variety of flexible substituents to organic ligands, and the powerful coordination ability of metal ions is a considerable potential that guarantees for surface treatments to improve biocompatibility of implants. One of the significant challenges in the widespread utilization of MOFs is controlling their growth or deposition as thin layers on substrates [64]. Fabrication of a robust mechanical barrier via surface coating is an easy and efficient approach for coating substrates. Hence, it is a big challenge to achieve a surface coating with a robust and reliable mechanical and chemical barrier [65–67]. Generally, the medical layers should be safe, biocompatible, and biodegradable with nontoxic effects on the living cells [68, 69]. Coating metal implant surfaces with MOF coatings combines the chemical advantages of metal alloys with the compatibility of MOFs and the release of ions in the implant system. The development of fabrication and modified approaches can further decrease the toxicity of MOF coating and improve the chemical and mechanical stability. Researchers studied the various protective coatings available for the protection of alloys [70, 71]. For battery applications, MOF nanocomposites, including cobalt and cerium presented considerable effect as either the sulfur host or the coating interlayer for Li–S batteries [72]. In general, the most crucial advantage of MOF coatings is their poor chemical stability. The development of fabrication can further reduce the toxicity of MOF coatings and improve the stability properties. Application of the various coating strategies can achieve surface coverage by MOF coating materials to ensure medical performance and can be extended to different types of substrate surfaces. Some of the positive effects of biological coatings, such as anti-microbial activity against pathogenic bacteria, antioxidants and anti-inflammatory are important and can be included in orthopedic implants for natural modification of the surroundings. Several limitations related to the surface treatment of MOFs such as stability, homogeneity, maintaining porosity, the introduction of impurities, scalability, and compatibility with intended applications, are considered.
MOFs are porous materials that, when used as coatings on surfaces for biomedical aims, thin-film layer of MOFs offers a unique set of properties that enhance their performance. Biocompatible MOF-coated surfaces are an emerging research area with potential applications in various fields, including biomedicine, drug delivery, and tissue engineering. Biocompatibility is the most fundamental characteristic of the MOF-coated surface's ability to interact with living tissues without causing adverse reactions such as inflammation, cytotoxicity, or immune response [73]. Among other important characteristics, the hydrophobicity and hydrophilicity of the surface play an essential task in the performance of materials, particularly implanted devices. All living organisms and cells, bacteria can take place at the interface between an implant and the human body. Thus, an implant material was directed by researchers toward modifying implant material surfaces to obtain favorable host-material interactions [74]. The charge and hydrophobicity are two crucial surface properties that affect the bacterium-surface interaction [75]. The surface energy of a substrate is generally higher than coating materials to attain adhesion. The surface energy substrates can improve to increase uniformity, adherence, and durability. The concepts of hydrophobicity and hydrophilicity depend on the level of the surface energy of the substrate [74]. In general, the hydrophilic surfaces are osteogenic and they can enhance the activation time of several signaling pathways that directly or indirectly contribute to bone formation [76]. The origin of many infections is related to complex interactions between the pathogen and the implant surface. Therefore, the designing and manufacturing an infection-resistant coating is very challenging, and this coating must provide optimal broad-spectrum anti-microbial activity, protection against biofilm formation, and biocompatibility [77–79]. The contact angle measurements indicate the wettability (hydrophobic or hydrophilic) of a surface. Besides, the surfaces with a contact angle above 65° will be studied hydrophobic and below 65° hydrophilic [80]. At hydrophobic surfaces, the driving force is generally hydrophobic interactions that occur between the surface and the outer hydrophobic shell of most proteins. In addition, the driving force is generally the electrostatic interactions between the surface and the proteins [81]. The biomacromolecules can lead to the growth of MOFs in aqueous solution to give a strong coating that offers protection from environments that are usually destructive. ZIF-8 chemistry has suitability for bio-mineralization and its hydrophobicity [82]. The hydrophilic surface is also helpful for bone cell adhesion and proliferation to improve bone growth. For instance, the Mg-MOF-74/MgF2 coating is prepared on the AZ31B Mg alloy surface, and this coating has progressed the anti-corrosion and hydrophilicity of Mg alloys [83].
Infections caused by pathogenic bacteria are a severe threat to human health that results in seriously affects for human health [84]. Among traditional materials, silver, copper, zinc and gold-based inorganic metal nanoparticles have been proven to be effective anti-bacterial agents with broad-spectrum anti-microbial activity [85, 86]. Coating methods can offer new opportunities for providing topical therapeutic agents. Besides, the coating materials do not form a strong bond with the bone. Numerous antibiotics or anti-microbial peptides have been loaded onto implant surfaces for anti-bacterial treatments [87, 88]. Bacterial acid-responsive MOF coating on the implant surface can be fabricated for releasing large amounts of metal ions to kill pathogenic bacteria in the early postoperative period without affecting ossification in the later period [39]. For example, Mg 2+ ions can induce an alkaline media to destroy pathogenic bacteria and are attributed to the regulation of inflammation [89]. Due to the emergence of new resistance mechanisms, anti-microbial resistance may disrupt the ability of the present medical care system. Therefore, the expansion of new bactericidal agents is necessary [90]. In general, bioactive MOFs act by releasing biologically active metal ions or ligands into the environment after the separation of metal–ligand bonds. MOFs should be made more stable using strong cross-links and post-surface modification to avoid toxicity to host tissue and microbes [91, 92]. Moreover, the anti-microbial activity of MOFs is related to the kind of metal in MOF structure, which can release metal ions gently as an excellent benefit compared to the metal/metal oxide NPs [93]. In general, anti-microbial agents can be bonded to the material surface via quaternary ammonium, imidazolium, or other groups [94, 95]. The Cu-SURMOF is used as marine anti-fouling coatings via the release of Cu 2+ against marina bacteria for an anti-fouling protective layer, which is responsible for unwanted bacterial colonization of underwater surfaces [96]. The iodine-modified ZIF-8 coated surface demonstrates anti-bacterial activity against Gram-negative Escherichia coli and Gram-positive Staphylococcus epidermidis and Staphylococcus aureus [97]. The ant-biofilm performances of MIL-88B(Fe) and its hydrophobic variant MIL-88B(Fe)PA were compared, MIL-88B (Fe) loaded with IMI binds well to polystyrene surfaces and hinders Salmonella biofilm [98]. In another study, the Naringin-loaded MOF NPs for coating mineralized collagen were fabricated with the MOF structure. The release kinetics of Naringin could be controlled to improve osseointegration and evade bacterial infection [99]. Thin polymer using a MOF templating method, which contained porphyrin molecules, was synthesized and demonstrated PDT effects against E. coli [100]. The synergistic anti-bacterial activity was investigated by using the shrill edges of the GO sheets and the action of Co 2+ ions released from GO/Co-MOF [101]. In additional work, NO was produced via the immobilization of MOFs on polymeric substrates with the deposition of Cu(II) CuBTC crystals onto the surface of carboxyl-functionalized cotton [102]. MOF thin films have shown promising anti-bacterial properties and have the potential to reduce bacterial infection due to metal ions or clusters coordinated organic ligands. Some MOF thin films can release anti-microbial agents such as silver ions, copper ions, or antibiotics, effectively killing bacteria and reducing infection. In addition, some MOF thin films can encapsulate and release antibiotics (or therapeutic agents( in a controlled manner, providing sustained and localized treatment for bacterial infections.
The usage of protective anti-corrosion coatings is one of the most applicable ways to progress the corrosion resistance of metals. Metal corrosion significantly limits the use and development of medical metal-based materials due to its economic and medical disadvantages [103–105]. Applying MOFs in polymer coatings as a nano-container for packing corrosion inhibitors. MOF structure based on Ce ions and 1,3,5-BTC on graphene oxide (GO) nanosheets was fabricated and applied to make an epoxy-based anti-corrosion coating [106]. In other work, MIL-53 (Al) nanoparticles were prepared. Then the nanoparticles were incorporated in hybrid (tetraethyl orthosilicate + γ-glycidyloxypropyl trimethoxysilane) sol–gel coating as nano-filler, and the fabricated nanocomposite was used on Al 2024 alloy for superior barrier and protection properties [107]. The thermal stability of the sol–gel layer improved with the integration of MIL-53 (Al) MOFs. Silica sol–gel matrix incorporates Cu-1,3,5-tricarboxylate (Cu-BTC MOF) treated with sulfur compounds electrodeposited on Ni–Fe binary alloy substrate and used as a shielding coating. Cu-BTC MOF loaded with 2-aminobenzothiazol was fabricated by the encapsulation technique and inhibitor release has also been investigated by cyclic voltammetry [108]. In addition, one of the green, easy, and cheap methods for surface adjustment uses PDA for a comprehensive class of inorganic and organic materials [109]. ZIF contains zinc as a metal ion and imidazole or its derivatives, some of which have apparent corrosion inhibition against mild corrosion of acidic steel as an organic linker. The zeolite-imidazole (ZIF-67) of MOFs was fabricated on the GO platform that constructs ZIF-67/GO NPs for supplying polymer-based anti-corrosion coatings with both self-healing and barrier efficiency [110]. The ZIF-67/GO@APS composite could protect the surface of steel via mixed anodic/cathodic type (O2 reduction/Fe oxidation) mechanisms, and the corrosion of the iron sample was reduced. ZIF-8 has been the most widely investigated due to its easy fabrication, controllable size, excellent stability, and appropriate pore size. ZIF-8 nanoparticles were synthesized, and subsequently, superhydrophobic composite coating was fabricated by combining the super-hydrophobicity of ZIF-8/EP coating and the excellent bonding property of epoxy resin coating [111]. In general, MOFs have an excellent matrix for encapsulating corrosion inhibitors. MOFs-based anti-corrosive materials were loaded with zinc gluconate (ZnG) corrosion inhibitors (Fig. 3 a), and next the ZnG@ZIF-8 composite anti-corrosive material are dispersed in an epoxy resin to fabricate film coatings and these coatings have excellent corrosion resistance compared with ZIF-8/EP coatings, EP coatings and ZnG + ZIF-8/EP coatings [112]. The Ce-based MOF loaded with benzotriazole with the controlled-release ability was fabricated for the simultaneous enhancement of the hydrophobic property and corrosion resistance of epoxy coatings [113] (Fig. 3 b). ZIF-8 with rhombic dodecahedral structure interrelate with the matrix of various polymers and progress their thermal/mechanical properties due to organic ligands like imidazole [114]. Epoxy coatings containing enclosed MOF corrosion inhibitors, corrosive environment penetrates quickly from cracks and attacks MOF encapsulated in corrosion inhibitors. 2-Mercaptobenzimidazole was built in ZIF-8 on GO nanosheets and then embedded into epoxy coating to synthesize M-ZIF-8/GO/EP composite coating with pH-responsive and self-healing performances [115]. In another study, the dopamine was grafted on the surface of the MOF-5 and was combined into waterborne epoxy coatings [116]. The results indicate that the corrosion protection performance of epoxy coating with dopamine-MOFs (0.5 wt%) was higher than others. The mesoporous silica nanoparticle-benzotriazole- ZIF-8 (HMSN-BTA@ZIF-8) as corrosion inhibitor-encapsulated nanocontainer was fabricated using ZIF-8 as self-sacrificial template [117]. The synthesized material shows prominent pH-triggered activities in acidic and alkaline situations. The fabricated HMSN-BTA@ZIF-8 can covalently interact with epoxy, resulting in outstanding anti-corrosion and impressive self-healing properties. The loaded capacity of 2-Mercaptobenzimidazole on M-ZIF-8/GO was 9.12%, and the release trend of 2-Mercaptobenzimidazole and Zn 2+ cations was synchronous (Fig. 3 c). In ZIF-7 MOF, the linking benzimidazole is spatially separated by a framework as an inhibitor. ZIF-7 MOF with acid-sensitive coordination bonds can deliver the loaded active corrosion inhibitors to the desired location and enable the delivery of a more considerable amount of inhibitor. The intelligent dynamic polymer coatings with ZIF-7 for self-healing corrosion protection in acid conditions were studied by electrochemical impedance spectroscopy [118] (Fig. 3 d).
