1 Introduction
Metal-organic frameworks (MOFs), also known as porous coordination polymers, have developed rapidly in recent years
[1]. MOFs are porous crystal materials with special spatial structure, which are connected by coordinated metal ions and organic ligands through coordination bonds.Therefore, most of the organic ligands used to synthesize MOFs have benzene rings and N-containing heterocycles as their rigid host structures, and the common ligands mainly include functional groups containing N, O, P and other atoms that can provide lone-pair electrons and have strong coordination
[2,3]. Due to the diversity of metal ions and organic ligands, a variety of MOFs with different molecular structures, topologies and pore structures have been synthesized in recent years
[4,5]. Due to the spatial structure and composition of MOFs materials, MOFs materials have good functional adjustability, for example, by reasonably selecting materials, adjusting the addition sequence, introducing functional groups and other methods, MOFs materials can be endowed with good functional selectivity to adjust the application performance of MOFs
[6⇓~8]. Based on the porous structure and adjustable function of MOFs, MOFs have been applied in various fields, such as gas adsorption and separation, energy storage, catalysis, sensing, drug carriers and water treatment
[9][10][11][12][13][14]. However, most of MOFs exist in the form of crystals and powders, and their rigidity and fragility limit their practical application. At the same time, the instability of MOFs in solution will lead to the decomposition of materials, and some MOFs with high crystallinity are also very fragile and difficult to process
[15][16]. Although MOFs itself has abundant pores (see Table 1), its structure is mostly microporous due to the aggregation of macromolecules in the crystalline state, which will affect the surface permeability of the material itself and limit the application of the porous structure of the material itself
[25]. Therefore, designing stable MOFs composites with good surface accessibility, flexibility, processability and high specific surface area is a way to improve the practical performance of MOFs.
Due To the porous, multifunctional and small size structure of MOFs, the study of combining MOFs with other materials to enhance their functionality has attracted great interest of researchers, and people try to combine MOFs with some functional materials with unique electrical, magnetic and catalytic properties.Uch as metal oxide nanoparticles, quantum dots, polymers, biological enzymes, conductive carbon materials and the like, so as to manufacture a composite material which has the unique structure of the MOFs and the excellent performance of the functional materials at the same time, and has physical and chemical properties which are not possessed by single-phase materials
[26⇓~28][29]. Hydrogel is a kind of cross-linked three-dimensional network gel with high water content and three-dimensional structure.Because it carries a large number of hydrophilic groups, such as carboxyl and hydroxyl, the Hydrogel matrix has a strong ability to absorb and retain water, and at the same time, various functionalities can be endowed to the hydrogel material by adjusting the composition and structure
[30]. The hydrogel material is rich in a large number of hydrophilic groups and has a three-dimensional network structure, so that the MOFs can be well attached to the network structure of the hydrogel, and the dispersion of the MOFs in the hydrogel matrix can be improved by utilizing the three-dimensional network structure
[31]. At the same time, the addition of MOFs can interact with other components to improve the mechanical strength, water absorption and total pore volume of the hydrogel matrix
[32⇓⇓~35]. Hydrogel materials and MOFs have strong functional adjustability, such as the addition of components and the introduction of functional groups, and the addition of MOFs in hydrogel materials can better combine the functionality of the two to prepare functional materials with high application performance, and MOFs can also control the crosslinking points in the process of hydrogel crosslinking
[36]. In addition, the incorporation of MOFs into the hydrogel matrix can effectively avoid the instability of MOFs in the solution environment, and the application of MOFs in the fields of drug delivery and wound healing can also be realized by utilizing the advantages of high porosity and biocompatibility of the hydrogel matrix, which is helpful to broaden the application fields of MOF
[37].
MOF-based hydrogels can not only take the advantages of both MOFs and hydrogel materials, but also avoid the application limitations of MOFs crystal powder.In this paper, the classification and application fields of MOF-based hydrogels in recent years are reviewed.It is hoped that this review will lead more researchers to this field and develop better functional materials.
2 Method for synthesizing MOF-based hydrogel
The synthesis process of MOF-based hydrogels is usually to add MOFs to the hydrogel matrix. The hydrogel matrix can be regarded as a continuous phase, and MOFs can be regarded as a dispersed phase. MOFs are uniformly dispersed in the hydrogel matrix to form a variety of structures and play a variety of functions. In general, MOF-based hydrogels can be synthesized by two methods: (1) direct synthesis
[38]; (2) In-situ growth method.
2.1 Direct synthesis method
In the direct synthesis method, MOFs crystals are prepared by adding an organic ligand to a metal salt solution through a subsequent process, and then a hydrogel matrix is prepared. In the sol state of the hydrogel, MOFs crystals are added to the matrix to modify the hydrogel, as shown in fig. 1. The MOFs are wrapped by gelation of the sol to generate a MOF-based hydrogel. However, the dispersion of MOFs in the MOF-based hydrogel is not easy to control, and some instruments are needed to improve the uniformity of dispersion.
2.2 In situ growth method
The in-situ growth method is to prepare a hydrogel matrix first, and then add a metal salt into the sol state of the gel or the aqueous solution of the gel, so that the metal ions are fully dispersed and attached to the hydrogel network structure, and then add an organic ligand into the system, as shown in fig. 2, to react for a period of time, so that the metal ions and the organic ligand are fully combined. In this method, MOFs are formed by in-situ growth in the pores of the hydrogel, and the prepared MOF-based hydrogel is uniformly dispersed.
In addition to the above two methods, researchers have also explored other methods, such as Chakraborty's use of saponite clay and other raw materials to prepare MOF-based hydrogels through the charge self-assembly of nanoscale metal-organic frameworks, and the anisotropic growth of layers generated by metal ions and organic ligands to synthesize MOF-based hydrogels
[39][40].
3 Classification of MOF-based Hydrogel
In recent years, MOFs have been used more and more because of its potential application value. However, most of the molded MOFs are in the state of powder crystallization, which greatly limits the advantages of the material itself and the processing of the material. As a new material with great potential, the final preparation and application of MOFs should be developed towards industrialization.Combining MOFs with flexible hydrogel materials can effectively avoid the processing and application problems of MOFs, and MOFs can also enhance some mechanical properties of hydrogels, which has great enlightenment for the final application of MOF.
3.1 Pure MOF hydrogel
Pure MOF hydrogel means that only MOF metal precursors and organic ligands are combined together through coordination, and no other substances are added, but the gel is formed by special expansion or lamellar growth. Chen et al. reported pure MOF hydrogels with luminescence properties and tunable color prepared from 5-borohydroisophthalic acid (5-bop) and lanthanide metal ions such as Tb
3+, Eu
3+ or Dy
3+, based on the fact that lanthanide metal ions can be used for fluorescence detection in chemical sensors
[41][42]. As shown in fig. 3, a single rare earth metal ion gel has a uniform color under sunlight, but has a fixed monochromatic light emission under the irradiation of light having a wavelength of 275 nm, so that the ratio of the lanthanide metal ions can be adjusted to obtain a hydrogel material with various color light emissions.
The unique nucleation and growth mechanism shows that the precursor first assembles to form nanoribbons, the introduction of boric acid groups into isophthalic acid can improve the anisotropic growth through steric hindrance and hydrogen bonding, and the metal ions with high coordination valence also enhance the anisotropic growth, and then the nanoribbons are entangled to form MOF gels. Nanobelts are formed by chemical bonds between ligands and metal ions, which are the basic characteristics of chemical gels. The nanoribbons are entangled through non-covalent interactions, similar to a physical gel. Therefore, the prepared MOF gel is a chemical/physical hybrid gel.
In addition to pure MOF hydrogels, there are some other MOF composite hydrogels, but the formation of MOFs nanoparticles in the gelation process of the composite hydrogel does not match the gelation process of the gel, so the so-called "gel" is still the product of the formation of MOFs nanoparticles and the non-interfering gelation process
[41]. Chen et al. utilized the method of preparing MOFs for the preparation of gels, similar to the preparation of low molecular weight gels, using anisotropic growth of metal ions and organic ligands to form flexible fibers or ribbons that may become entangled to form effective MOF hydrogels
[41]. As shown in fig. 3, this pure MOF hydrogel has a more homogeneous composition and a more regular structure than the other composite hydrogels in Table 2. At present, researchers have not studied pure MOF hydrogels in depth, and can not compare the performance of pure MOF hydrogels with other MOF-based composite hydrogels. We believe that the limitations of pure MOF-based hydrogels lie in the difficulty in selecting the types of metal ions and organic ligands.Moreover, the process of binding growth is not easy to control, and the application fields after successful preparation need to be explored, so it is a very meaningful idea to select functional metal ions as central ions and use metal ions to play a role.
3.2 MOF @ Bioorganic Macromolecular Hydrogel
With the development of science and technology, biomaterials derived from nature have gradually come into people's vision, and their own characteristics of recyclability and easy preparation can combine the advantages of low cost, sustainable production and environmental protection, so the combination of MOFs and bio-derived organic macromolecular materials is a promising research idea.
Carrageenan is a kind of hydrophilic colloid. Its chemical structure is calcium, potassium, sodium and ammonium salts of polysaccharide sulfate composed of galactose and anhydrogalactose
[43]. Javanbakht et al. prepared UIO-66 by hot solvent method to load tramadol (Tr) for various moderate and severe acute and chronic pain, and then coated k-carrageenan on the surface of MOF material to prepare a MOF-based bio-organic macromolecular hydrogel material k-Cr/Tr @ UIO-66, as shown in Figure 4
[44]. In vitro drug release and kinetics analysis showed that the composite material had good gastric acid resistance, effectively avoided the disintegration of MOFs in acidic environment, controlled the release of drugs and improved the dosage stability of drugs, and provided cell compatibility for k-Cr/Tr @ UIO-66 coated by carrageenan, which could be used as a new oral drug delivery carrier.
Gum Arabic (AG), also known as gum Arabic, is mainly composed of macromolecular polysaccharides and their calcium, magnesium and potassium salts, mainly including arabinose, galactose, glucuronic acid and so on
[45]. Ribeiro et al. reported a superabsorbent hydrogel based on gum Arabic and a cross-linkable inorganic material UIO-66. Acrylic acid (AA) ligand was introduced to UIO-66 for modification, and then combined with gum Arabic through covalent bonds to form a stable structure to form a composite hydrogel material AGMOF @ AA, which has super water absorption
[34]. The results of water absorption experiments showed that 1 G of HAGMA without MOFs could absorb 1014 times of water at pH = 9, while the HAGMA with MOFs could only absorb 305 times of its own dry weight at the same pH, which was due to the repulsion between the macromolecules in the hybrid material. However, compared with HAGMA, the elastic modulus of AGMOF @ AA increases by 535%, and the energy required for the fracture of the hybrid material is 5 times that of pure hydrogel. AGMOF @ AA can also be used to adsorb methylene blue (MB) in wastewater (768.03 mg/G) with excellent regeneration capability.
Chitosan (CS) is obtained by partial deacetylation of insoluble natural chitin, which is the only alkaline polysaccharide in nature. It is also a polycationic electrolyte polymer. It not only has good biocompatibility and biodegradability, but also has certain antibacterial properties
[46]. Ren et al. embedded carbonic anhydrase (CA) molecules into metal-organic framework (MOF) by co-precipitation (CA @ ZIF-8), and then encapsulated these CA @ ZIF-8 nanocomposites in polyvinyl alcohol (PVA) -chitosan (CS) hydrogel network to prepare CA @ ZIF-8-PVA-CS composite hydrogels with high activity, stability and reusability
[47]. The immobilization efficiency of CA is more than 70%, which indicates that it has high immobilization efficiency. The prepared PVA/CS/CA @ ZIF-8 composite membrane showed superior resistance to high temperature, denaturant and acid than free CA and CA @ ZIF-8. In addition, these membranes also exhibit excellent CO
2 trapping properties. Lu et al., inspired by the fact that hydrogels can hold chloride ions for a long time and MOF-Pt materials have highly stable hydrogen peroxide-like activity, constructed a long-acting chemiluminescence system N- (4-aminobutyl) -N-ethyl isomer-chitosan (ABEI/Co
2+/CS) Cl
- imaging sensor, with the detection mechanism shown in Figure 5, for the detection of organophosphorus pesticides (OPs) and D-amino acids (D-AAs)
[48]. The hydrogel with porous polymer network structure can immobilize metal ions, and adsorb the Co
2+ into the hydrogel framework through the coordination bond between hydroxyl and amine groups. The catalytic activity of the hydrogel can be maintained due to its high stability.
Agarose is a kind of polysaccharide extracted from plants (Rhodophyceae). Because of its special gel properties, agarose has been widely used in many fields. MOF @ agarose complex can also be synthesized by using agarose hydrogel as matrix
[49]. Gao et al. Doped the fluorescent dye rhodamine B (RhB) into a stable UiO-66-NH
2 with zirconium (Zr) as the coordinating metal ion, then dissolved the substance in hot water with dispersed agarose powder, and prepared agarose hydrogels based on MOFs after cooling
[50]. The hydrogel can be used to detect phosphate in human serum and has great potential in field clinical diagnosis. Zhang et al. integrated a highly dispersed and small-sized (14.95 nm) metal-organic framework (ZIF-8) into an agar aerogel by limiting the overgrowth and agglomeration of carbon nitride (C
3N
4) nanosheets, resulting in a flexible, highly efficient aerogel absorber with photocatalytic activity
[51]. The mixed aerogel without separation can maintain the small-size crystal structure of the zeolite imidazole salt framework (ZIF-8), and the adsorption capacity of the mixed aerogel for dye is 1.44 times that of the original ZIF-8. Most importantly, the introduction of C
3N
4 nanosheets not only regulates the growth of ZIF-8 crystals, but also enables the mixed aerogel to have stable reusability in five consecutive adsorption degradation cycles obtained by solar irradiation.
In addition, Zhu et al. employed an in-situ growth method to incorporate metal-organic frameworks (MOFs) materials into an alginate matrix, overcoming the challenge of processing MOFs particles into specially shaped structures for practical industrial applications
[52]. The MOF alginate complex was prepared by post-treating a metal ion-crosslinked alginate hydrogel with a MOF ligand solution. The MOF particles are well distributed and embedded in the surface and interior of the composite, as shown in Figure 6. The macroscopic shape of the composite material can be designed by controlling the shape of the corresponding hydrogel, thereby obtaining MOF alginate beads, fibers, membranes, and the like. Zhuang et al. used a novel one-step method to directly chelate Co
2+ with alginate and 2-methylimidazole compound to form MOF hydrogel (MAM), and compared this hydrogel with ZIF-67 @ alginate hydrogel (AMM) prepared by direct synthesis method for tetracycline adsorption.The results showed that the maximum adsorption capacity of tetracycline on MAM was 364. 89 mg/G, which was higher than that of AMM (302. 32 mg/G), and MAM still had good regeneration performance after 10 cycles
[53]. At the same time, inspired by brown algae, Park et al. Prepared a biomimetic phosphate MOF alginate hydrogel, and successfully prepared a MOF-based composite material for heavy metal ion adsorption by using the adsorption of carboxylic acid groups and phosphate groups of alginate on Li
+[54].
In addition to the above MOF-based hydrogels, there are some other types of bio-derived organic macromolecular hydrogels, such as MOF @ konjac glucomannan hydrogel, MOF @ gelatin hydrogel, MOF @ guar gum hydrogel and so on, which have attracted the attention of researchers. It is believed that more and more MOF @ bio-derived organic macromolecular hydrogels will be developed and applied well
[55][56][57].
3.3 MOF @ Biocompatible Hydrogel
With the development of medical technology, biocompatible hydrogels play an important role in the fields of drug delivery and wound healing, but they often have the disadvantages of instability and easy decomposition.Adding MOFs to biocompatible hydrogels can improve their mechanical properties.
Chen et al. Have reported in detail the synthesis of metal-organic framework nanoparticles coated with a stimuli-responsive nucleic acid-based polyacrylamide hydrogel cross-linked in a cage-like structure by nucleic acid duplexes containing an aptamer resistant to adenosine triphosphate as a carrier for doxorubicin
[58]. Fig. 7 shows the schematic and morphology of water-soluble MOF nanoparticles. As shown in fig. 7A, after the MOF material (UIO-68) was synthesized, the amino functional group on its surface was converted into an azide functional group to attach the modified nucleic acid. The anticancer drug doxorubicin was then loaded into UIO-68 nanoparticles, and the carrier was hybridized to form a double-stranded immobilized unit. Finally, these nanoparticles are further precipitated and suspended in a buffer solution containing functional acrylamide chains of DNA. As shown in figs. 7B and 7C, the UIO-68 nanoparticles have a double pyramid structure with a size of 280-350 nm. After hydrogel coating, the sharp angle of UIO-68 nanoparticles disappeared and the surface was smooth. Lin et al. utilized target-reactive DNA hydrogels with high loading and good biocompatibility to construct chemiluminescent biosensors for the specific detection of adenosine
[59]. They prepared DNA hydrogels with acrylate-modified adenosine aptamers (Acry-A-Apt), acrylate-modified single-stranded DNA (Acry-ssDNA), and heme aptamers (H-Apt), which are partially complementary to each other. Therefore, the DNA hydrogel dissolves only when adenosine and heme are present simultaneously. Gold nanoparticles were coated in the cavity of HKUST-1 (MOF with Cu
2+ as the coordinating metal ion) to obtain Au @ HKUST-1, which was coated in DNA hydrogel as a sensor for detecting adenosine. This DNA hydrogel system provides a simple, sensitive and biocompatible method for the detection of adenosine, which can be used in the field of biotechnology.
3.4 Other MOF-based composite hydrogels
In addition to the above composite hydrogel materials, other MOF-based composite hydrogels have been developed and applied.
3.4.1 MOF @ graphene hydrogel
Graphene materials are a new class of carbon materials, including graphene, graphene oxide (GO) and reduced graphene oxide (rGO), which have attracted great attention in the field of chemistry and materials in recent years
[60]. Graphene materials have a wide range of applications because of their three-dimensional graphene network structure, good conductivity and rich functional group structure
[61].
Mao et al. First reported a method to gradually construct stable, porous and multifunctional MOF @ reduced graphene oxide hydrogel composites in a short time (< 1 H), which was mainly divided into metal ion crosslinking process and chemical reduction process
[62]. They used the rich hydroxyl and carboxyl groups on graphene oxide sheets as binding sites to adsorb MOF particles, and then chemically reduced graphene oxide (GO) to remove oxygen-containing groups (carboxyl and hydroxyl groups), as shown in Figure 8. Due to the enhancement of π-π stacking, the self-assembly of GO nanosheets was promoted. Yan et al. Synthesized graphene oxide (GO) by modified Hummer method, then dissolved GO powder in water with the assistance of ultrasound to obtain GO solution, added ascorbic acid and sodium hydroxide to the solution and heated at 70 ℃ to obtain graphene oxide hydrogel.After the hydrogel is added into methanol containing zinc nitrate hexahydrate and 2-methylimidazole to react for a period of time, ZIF-8 grows in the network of the graphene oxide hydrogel, and finally the ZIF-8 @ graphene hydrogel is obtained
[63][64]. The composite hydrogel can be used as a catalyst to degrade cellulose under alkaline conditions. The 3D nanocomposite is stable in alkaline solution and has excellent catalytic activity for the conversion of cellulose in aqueous solution, and the formic acid generation rate reaches 93.66%.
3.4.2 MOF @ cellulose hydrogel
Cellulose nanocrystals (CNCs), cellulose nanofibers (CNFs), and bacterial cellulose (BC) are the most common types of nanocellulose produced on an industrial scale today. CNCs and CNFs have also been found to help unstable colloidal nanoparticles, such as carbon nanotubes, boron nitride, manganese dioxide, and others, to be better suspended in water
[65,66]. In addition, co-suspended nanoparticles can be processed into hybrid nanocomposites with a uniform distribution of components. Therefore, some researchers have combined cellulose with MOFs to prepare a composite material to take into account the advantages of MOFs and cellulose.
Javanbakht et al. First dissolved copper nitrate hydrate and terephthalic acid in N, N-dimethylformamide (DMF) solution and stirred to prepare the metal-organic framework material Cu-MOF.Then soaking the Cu-MOF in the anti-inflammatory drug IBU, centrifugally drying to prepare the drug-loaded Cu-MOF @ IBU, dissolving the drug-loaded Cu-MOF @ IBU in water, adding carboxymethyl cellulose (CMC) into the water to prepare a mixed solution, and dropwise adding the mixed solution into a ferric chloride solution to form gel balls
[67]. This is because multivalent cations, such as Al
3+ or Fe
3+, can interact with many of the carboxylate groups on the CMC macromolecule, forming a cross-linked network. The electrostatic interaction between the carboxylate groups of the CMC molecules and the ferric ions and the incorporation of metal coordination can facilitate the formation of cross-linked polymer networks (fig. 9)
[68]. Jin et al. Prepared a new type of carboxymethyl cellulose coated metal-organic framework material MOF-5 @ CMC microspheres for the removal of lead ions in aqueous solution, and the experiments showed that the material had good adsorption performance
[69]. Sultan et al. prepared ZIF-8 metal-organic framework materials in situ by using carboxyl adsorption Zn
2+ on nanocellulose (CNFs), and prepared gel materials by 3D printing technology, which can be used as drug carriers to encapsulate some drugs into the system
[70].
In addition to the MOF-based hydrogels mentioned above, there are other types of MOF-based hydrogels, such as MOF @ polyvinyl alcohol hydrogel and MOF @ thermoresponsive citrate hydrogel.However, in general, the research of MOF-based hydrogels is not too deep. At the same time, with the popularity of smart wearable trend, the demand for MOF-based hydrogels in the field of self-repair and sensing is increasing. It is believed that more and more excellent results will emerge in this field in the future
[71][72].
4 Application of MOF-based Hydrogel
MOF-based hydrogels combine the advantages of MOFs materials and hydrogel matrices, and also create composites with better properties. Most of the matrix of MOF-based hydrogels are derived from biological or biocompatible materials, so they have good application prospects in the fields of drug release and wound healing. At the same time, MOFs materials have their own coordination metal ions, which can endow the composite materials with catalytic properties. In addition, due to the porosity of MOFs and hydrogels, the composite material has more excellent layered porous structure and mechanical properties, and has broad prospects in the fields of sensing and water adsorption.
4.1 Sensing
Chemical sensing technology refers to the detection of specific substances in liquids or gases through special sensing materials, and the use of signals from these sensing materials to detect their presence or concentration
[73]. In recent years, with the development of MOFs materials, more and more attention has been paid to the advantages of high porosity, versatility and high specific surface area of MOFs. It is a new idea to combine MOFs with hydrogels in the field of biosensing.
Penicillin, a beta-lactam-based antibiotic, is a common anti-inflammatory drug, but its use is sometimes accompanied by poisoning and allergic reactions. Therefore, the detection of beta-lactamase in the blood of patients with penicillin can effectively avoid the risk of poisoning and allergy
[74]. Lian et al. prepared a novel MOF @ sodium alginate hydrogel composite (HM) for β-lactamase detection
[75]. HM has high sensitivity and selectivity for β-lactamases in aqueous solution and serum. As shown in fig. 10a, the luminescence signal of HM was positively correlated with the concentration of beta-lactamase. Fig. 10B shows the linear relationship between the intensity of the luminescence signal and the concentration of beta-lactamase. After the sensing test, the hydrogels containing different concentrations of beta-lactamase were irradiated with UV light, and fig. 10c shows that the color of HM under UV light changes from dark red to bright red as the concentration of beta-lactamase increases. This shows that the color of HM irradiated by ultraviolet light can directly reflect the resistance of patients to penicillin and diagnose whether they are allergic to penicillin.
The powder characteristics of MOFs themselves limit their application in the field of sensing. Zhao et al synthesized a hydrothermal stable luminescent MOF material EuNDC (HNDC = 1,4-naphthalene dicarboxylic acid) to detect explosive picric acid. They used biocompatible polyethylene glycol diacrylate (PEGDA) and alginate to construct a core-spun fiber structure to wrap EuNDC and apply it to the field of sensing
[76]. Alginate hydrogel can allow ions, small molecules and macromolecules to pass through, while PEGDA can only allow ions and small molecules to pass through, which is equivalent to forming a layered porous structure, which not only facilitates the detection of ions and small molecules, but also fixes MOF in the core of the hydrogel to avoid direct contact with the harsh environment. Lin et al. constructed a selective and highly sensitive chemiluminescence biosensor to detect the concentration of thrombin in solution. They first formed sodium alginate hydrogel through the interaction between sodium alginate and calcium ions to separate and enrich thrombin in solution.Then, a metal-organic framework nanosheet complex Cu-TCPP (Co) MOFs/Au-ssDNA (ssDNA: single-stranded DNA) is added to the hydrogel to emit a signal, wherein the metal-organic framework nanosheet Cu-TCPP (Co) MOFs can play a role in signal amplification
[77].
Current studies have shown that MOF-based hydrogels have good sensing properties, but for sensors, its specificity and high efficiency are inevitable problems. It is hoped that more MOF-based hydrogel materials with selectivity and high sensitivity can be developed in the future.
4.2 Catalyze
Catalysts can effectively improve the reaction rate and shorten the reaction time, which plays a very important role in all aspects of the chemical field. MOF-based hydrogels not only have metal ions with certain catalytic properties, but also can load some substances with catalytic properties like enzymes by porous structure.
Duan et al. designed and fabricated a multifunctional bio-hybrid hydrogel composed of Ag NPs @ MIL-100 (Fe) photocatalyst and guar gum (GG) by simple mixing and self-crosslinking
[57]. The mixed hydrogel integrates multiple functions of dye adsorption and degradation, oil-water separation, antibiosis and the like, and can be applied to a complex wastewater treatment process. The presence of silver nanoparticles (Ag NPs) not only enhanced the photocatalytic activity of MIL-100 (Fe), but also endowed Ag NPs @ MIL-100 (Fe)/GG hydrogels with antibacterial activity. In particular, the bio-based GG hydrogel scaffold has good injectability, plasticity and self-healing, and enables the composite hydrogel to have efficient oil-water separation capability (fig. 11).
Cheng et al. Introduced a method of preparing hierarchical microporous and mesoporous Zn-MOFs by template emulsification using hydrogel as template, which increased part of the pore volume of MOFs materials
[78]. Zinc-based hydrogels with a three-dimensional network were first prepared by the formation of hydrogen bonds between melamine and salicylic acid, in which zinc ions were well distributed. After the hydrogel template was removed, it was further combined with organic ligands to generate hierarchical Zn-MOFs containing micropores and mesopores. These novel MOFs can be used for the encapsulation of glucose oxidase and horseradish peroxidase. The K
cat/K
m of the immobilized enzyme (the K
cat/K
m can be used not only to measure the specificity of the enzyme to the substrate, but also to test whether the enzyme-catalyzed reaction has reached a constant state or equilibrium state) is 85.68 mmol·L
-1·s
-1, which is 7.7 times higher than that of free enzyme in solution and 2.7 times higher than that of enzyme adsorbed on traditional microporous MOFs. The results showed that the catalytic activity of the mesoporous conjugate was much higher than that of the microporous conjugate, because the mesoporous conjugate had larger pores.
For catalytic reactions, the stability and continuity of the reaction and the recyclability of the catalyst are still the focus of attention.
4.3 Water Treatment
In daily life and industrial production, various pollutants are discharged into the water environment, and the adsorption of pollutants in the water environment has become an urgent problem to be solved.
With the development of global agriculture, the application of herbicides has become a common practice. These herbicides will enter the surface water and groundwater through infiltration and natural drainage. Most of the herbicides contain toxic substances, which have a great impact on human beings and other organisms in the ecological environment
[79]. Tan et al. prepared a novel regenerable hydrogel composite containing alginate, MIL-100(Cr)-NH
2, and TiO2 nanoparticles for use as an adsorbent for the removal of chlorophenoxy acid (CPA) herbicides from water
[80]. In the composite material, the porous MIL-100(Cr)-NH
2 plays a vital role in the adsorption and extraction of CPA in water samples, and the alginate polymer mainly provides a hydrophilic surface for the composite material.It is easier to adsorb herbicides in water environment, and the addition of Fe
3O
4 nanoparticles is mainly to facilitate the use of magnets to suck the composite material out of water in the future, which is conducive to the recycling of adsorption materials.
In addition to the adsorption of herbicides, Maan et al. Reported the preparation of composite hydrogel (ZIF-8 PAM) by using polyacrylamide (PAM) hydrogel as matrix, adding zinc hydroxide to adsorb Zn
2+ on the hydrogel chain, and then adding organic ligands to synthesize ZIF-8 in situ
[81]. They found that the maximum adsorption capacity of this composite hydrogel for humic acid (HA) can reach 111.5 mg/g-ZIF-8, which is comparable to other adsorbents for HA removal and can effectively remove HA from water. Zhu et al. prepared HKUST-1 @ PVA-co-PE/PVA nanofiber hybrid hydrogel to improve its adsorption capacity and recycling convenience
[71]. The experimental results show that the hydrogel has good adsorption properties for cationic dyes such as methylene blue and rhodamine B, because the ionized HKUST-1 and PVA-co-PE are anions, which can attract cationic dyes through electrostatic interaction. The hydrogel has a three-dimensional structure, which is beneficial to the uniform dispersion and adsorption performance of the MOF. The introduction of PVA-co-PE nanofibers into the composite material can improve the mechanical strength of the PVA hydrogel, which is conducive to the recycling of the material.
In the research of MOF-based composite hydrogels in the field of adsorption, MOF-based hydrogels are usually dried into MOF-based aerogels by freeze-drying and other methods, which is conducive to enhancing the specific surface area of the composite and improving its adsorption performance. Zhu et al. embedded UIO-66 powder in CNC aerogel to prepare composite aerogel and used it to adsorb Cr (Ⅵ) in water environment. During the experiment, they compared the adsorption of Cr (Ⅵ) by aerogel, aerogel embedded with MOFs and pure MOFs powder (with the same mass of MOFs in composite aerogel)
[82]. The results show that composite aerogels containing the same mass of MOFs powder have the same or higher adsorption capacity in all cases, which proves that the good dispersion of MOFs particles in aerogels and the adsorption performance of MOFs particles themselves are not affected by the surrounding embedded cellulose matrix. We believe that the layered porous structure of aerogels greatly improves the adsorption performance of MOF composite aerogels, through the combination of the accessibility of macropores and the capillary effect of mesopores.Can quickly absorb water and make the MOF micropores quickly and fully contact with pollutants, thus effectively avoiding the problem that the conventional hydrogel has too high density and hinders the diffusion of substances.
The specific surface area of MOFs may be affected by macromolecules during the preparation of composite hydrogel materials by in-situ growth method, but as far as the current research results are concerned, when the combination of MOFs and hydrogel is used for adsorption applications, the adsorption capacity of composite hydrogel is usually higher than that of single MOFs powder and single hydrogel material.The composite material may have a layered porous structure mechanism to facilitate adsorption, and the preparation of the composite material can ensure that the MOFs are in full contact with the substance to be adsorbed, and the MOFs material can be recycled by removing the gel after adsorption. This kind of composite material does have some advantages in the field of adsorption, but the preparation and research of this kind of composite gel is still in its infancy, there are still many problems, which need more researchers to explore, but it can not conceal the advantages of this material itself.
4.4 Wound dressing
Wound dressings are essential for surgical wounds, which can cause bacterial adhesion and colonization and cause inflammation if not treated in time.However, with the development of medical level, people are no longer satisfied with the treatment effect of simple natural gauze, and some new biocompatible wound dressings with drug carriers have gradually come into people's vision. MOF-based hydrogels have good drug loading because of their high porosity, and the choice of biocompatible hydrogel matrix can also avoid the immune response of the wound, so MOF-based hydrogels have a good application prospect in the field of wound healing.
Bacterial adhesion and colonization can affect wound healing.Current hydrophilic wound dressings can release antibacterial drugs into the wound tissue fluid, but may make the wound too wet and lead to bacterial growth.Hydrophobic wound dressings can prevent the external environment from contaminating the wound, but it is difficult to encapsulate and release bactericides
[83]. Zhu et al. Prepared a porous polyvinyl alcohol (PVA) hydrogel wound dressing containing metal-organic framework material ZIF-8 by microfluidic-template-emulsion method
[84]. As shown in fig. 12c, the prepared hydrogel film, although hydrophilic in nature, has a repellent effect on human blood and body fluids, which not only reduces the adhesion of harmful microorganisms on the wound, but also enables the encapsulation and release of antibacterial ingredients from the hydrophilic polymer network to the injury site. In addition, hydrogel membranes not only protect MOFs from degradation by physiological fluids, but also slowly and continuously release metal ions. In addition to the antimicrobial agents they carry, the slowly released Zn
2+ can also accelerate wound healing by promoting cell migration, angiogenesis and collagen deposition. Xiao et al. embedded HKUST-1 nanoparticles into citrate hydrogels to promote wound healing by sustained release of Cu
2+[72].
Yu et al. Used coaxial capillary microfluidic spinning technology to prepare microfibers with sodium alginate hydrogel as the shell and Zn/Cu-MOF as the core. The size of hydrogel microfibers can be controlled by adjusting the release of microfluids
[85]. With the increase of the concentration of the Cu
2+ in the system, the hydrogel microfiber can effectively reduce the survival rate of Escherichia coli so as to prevent bacterial infection in the environment, and when the concentration of the Cu
2+ is 0.158 mmol·L
-1, the biocompatibility and antibacterial effect of the microfiber are better. Yao et al prepared ZIF-8 nanoparticles by biomimetic mineralization to load a new anti-inflammatory drug SP, and then combined SP-loaded ZIF-8 nanoparticles (SP @ ZIF-8) with PEG-TK to produce SP @ ZIF-8-PEG-TK nanoparticles, which were encapsulated in a hydrogel composed of sodium alginate and pectin to prepare a wound dressing
[86]. The SP @ ZIF8-PEG-TK nanoparticles can promote the proliferation of human dermal fibroblasts, and the dressing has good cell compatibility and good wound healing effect.
4.5 Drug carrier
In the treatment of some medical diseases such as cancer, how to deliver drugs to the right place is an inevitable problem, and how to make the drugs embedded in the carrier release quantitatively and uniformly at a constant rate and keep the concentration of drugs constant is also a major problem.
Javanbakht et al. immersed the zinc-based metal-organic framework MOF-5 into the anticancer drug 5-fluorouracil (5-FU) solution to make the MOF material as a drug carrier, and then encapsulated the encapsulated MOF-5 nanoparticles in pH-sensitive carboxymethyl cellulose (CMC) to develop a new CMC-coated 5-FU @ MOF-5 bio-nanocomposite hydrogel beads (CMC/5-FU @ MOF-5) as a drug delivery carrier that can pass through the digestive system
[87]. CMC is a kind of biocompatible polysaccharide, which can protect 5-FU @ MOF-5 from degradation under acidic conditions.The in vitro drug delivery test under simulated gastrointestinal tract (GIT) conditions showed that the drug release of CMC/5-FU @ MOF-5 was 70% within 8 H, and it had excellent sustained release ability. Karimzadeh et al. combined graphene oxide (GO) with biopolymers CMC and MOF-5 to enhance the solubility of the antibiotic drug tetracycline (TC) and control the release of the drug at specific sites
[88]. The loading and release experiments showed that TC was effectively loaded on and released from the CMC/MOF-5/GO nanocomposite, and the drug release rate reached 65% within 8 H. Therefore, CMC/MOF-5/GO can be used as a potential drug carrier for targeted delivery and controlled release oral delivery.
5 Conclusions and outlook
In this paper, the types and applications of MOF-based hydrogels are introduced in detail. MOF-based hydrogels combine the advantages of two single materials, which can not only have the catalytic performance, excellent framework structure, ultra-high porosity and large specific surface area of MOFs, but also have the mechanical properties, biocompatibility and versatility of hydrogel matrix.Moreover, the hydrogel matrix is used as a support to load the MOFs material, so that the decomposition tendency of the MOFs material in certain solution environments can be effectively reduced, the processing limitation of powdery crystallization of the MOFs in practical application is overcome, and the functionality of the hydrogel matrix can be expanded and improved by loading the MOFs material on the hydrogel substrate.
Based on the rich porosity, flexible structure and stability in solution environment of MOF-based hydrogels, researchers have explored a variety of applications of this composite material, including sensing, catalysis, water treatment, wound dressing, drug release and oil-water separation. However, relatively speaking, the application of this new composite material has not been developed in depth and systematically, and compared with other materials (such as graphene materials), it is still in its early stage. Therefore, more researchers need to focus on this composite material, continue to expand the application scope of MOF-based hydrogels, and explore a large-scale pipeline synthesis process suitable for industrialization.
With the development of the times, green and clean energy and materials have gradually come into people's vision. Although MOF-based hydrogels have great potential for development, the current focus should be on developing some green and economic methods to synthesize MOF-based hydrogel composites and apply them to industrialization. It is hoped that more and more researchers will enter this field to further promote the research and exploration of MOF-based hydrogel composites.
Tianjin Key Research and Development Project(20YFZCSN00130)
National Key Research and Development Project Foundation of China(2017YFB0309800)
National Key Research and Development Project Foundation of China(2016YFC0400503-02)
Xinjiang Autonomous Region Major Significant Project Foundation(2016A03006-3)
Tianjin Natural Science Foundation(18JCYBJC89600)
Science and Technology Guidance Project of China National Textile and Apparel Council(2017011)
Innovation Research Institute of Wolfberry Industry Co. LTD(ZNGQCX-B-2019006)
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