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Review Article
DOI: 10.1016/j.jmrt.2018.12.001
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Available online 11 January 2019
Multifunctional metal–organic frameworks-based biocatalytic platforms: recent developments and future prospects
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Muhammad Bilala,
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bilaluaf@hotmail.com

Corresponding authors.
, Muhammad Adeelb, Tahir Rasheedb, Hafiz M.N. Iqbalc,
Corresponding author
hafiz.iqbal@itesm.mx

Corresponding authors.
a School of Life Science and Food Engineering, Huaiyin Institute of Technology, Huaian 223003, China
b School of Chemistry & Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China
c Tecnologico de Monterrey, School of Engineering and Sciences, Campus Monterrey, Ave. Eugenio Garza Sada 2501, Monterrey, N.L., CP 64849, Mexico
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Received 15 August 2018, Accepted 04 December 2018
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Table 1. List of recently reported various MOF–enzyme biocatalytic platforms with improved catalytic properties and their application.
Abstract

In recent years, metal–organic frameworks (MOFs) have received accelerating research attention as a versatile carrier and promising bio-immobilization support materials for enzyme immobilization. This is particularly due to their extraordinary structural properties and multi-functionalities, such as surface area, high porosity, tunable topography, crystallinity, electronic and optical properties, thermal/chemical stability and multiple affinities (hydrophobicity and hydrophilicity). Excellent biocatalytic performance, improved stability and repeatability, high loading ability, and greater accessibility to catalytic sites are the key attributes associated with the use of novel MOF–enzyme bio-composites. This review discusses the recent developments in the use of MOFs as immobilization support materials as a platform to engineer different kinds of enzymes with requisite functionalities for biocatalysis applications in different sectors of the modern world. The second part of the review mainly focuses on MOFs-assisted immobilization strategies including surface immobilization, covalent binding, cage inclusion and in situ MOF formation and enzyme immobilization to develop enzyme–MOF bio-catalytic system. The characteristic properties rendering MOFs as interesting matrices for bio-immobilization are also presented following applications of MOFs-immobilized bio-catalyst for catalysis, sensing and detection, and protein digestion. Lastly, the review is wrapped up with conclusions and an outlook in terms of upcoming challenges and prospects for their scale-up applications.

Keywords:
Metal–organic frameworks
Biocatalytic system
Support materials
Immobilization strategies
Catalytic performance
Biosensing and detection
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1Introduction

Metal–organic frameworks (MOFs) are crystalline and highly porous materials composed of organic bridging ligands (act as linkers) and a three-dimensional (3D) network of metal ions (secondary building units) [1]. MOFs are highly tunable materials owing to their structural and functional diversity. MOFs have unique characteristics and functionalities such as structural diversity, high surface area, high porosity, crystallinity, adjustable topography, ordered structure, electronic transduction, optical and multiple affinity (hydrophobicity and hydrophilicity) [2–6]. Overview of MOF synthesis, properties, and applications is shown in Fig. 1. Many factors control the structure and crystal of MOFs. Firstly, the microemulsion is a spontaneous mixture formed in the presence of a surfactant and co-surfactant (low molecular weight alcohol), hydrophobic part and water. If water act as the dispersed phase and organic material as the continuous phase, it formed spontaneous mixture reverse micro-emulsion. At the start, the material present in the aqueous phase causes collision between droplets and responsible for crystallization. The nature of these droplets can be tuned by changing the composition of the continuous phase and surfactant resulting in the synthesis of MOFs with adjustable size and structure. Secondly, the temperature influences the relative nucleation and rates of crystal growth. MOFs with different crystal sizes and structures can be fabricated by controlling the temperature. Thirdly, the addition of additives with essential characteristics generates specific crystal size. For instance, the capping agent stops the crystal growth, and blocking agents slow down the crystal growth. In the same way, the size and morphology of MOFs can also be controlled by modulators. In short, specific processing conditions such as temperature, pressure, temperature source and time have great influence on the crystal growth of MOFs [4,7]. The other fundamental properties including hybrid nature, enantioselectivity, and linker functionality have facilitated their use in the field of catalysis [8]. The catalytic nature of MOFs-based materials can be applicable for ring opening reactions, oxidation, aldol condensation and epoxidation reactions [9,10]. Moreover, the large specific surface area of MOFs ranging from 1000 to 10,000m2/g enables it to replace porous zeolites and carbonaceous materials [11]. Due to the exceptional properties, MOFs find potential applications as a luminescent material, gas storage, separation, a capacitor electrode, drug delivery, magnetism, sensing and catalysis [3,12–16]. In recent years, several attempts have been made for the development of MOFs with the best architecture to overcome the poor thermodynamic stability and weak mechanical strength that restrict their practical applications [17].

Fig. 1.

A schematic overview of MOF synthesis, properties, and applications.

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There are two main components of MOFs: (1) organic linker and (2) inorganic connector. Organic linkers are employed for the synthesis of MOFs with tunable functional groups. Transition metals with a different geometry such as linear, trigonal, tetrahedral and square planar are generally used as an inorganic connector for the synthesis of MOFs. In the past, the conventional electric heater is used with double temperature ranges from solvothermal to non-solvothermal. Many precipitation reactions through solvent evaporation or recrystallization have been reported [18]. By tuning the reaction conditions, it is possible to grow a simple molecular crystal from a clear solution by increasing the critical nucleation concentration. Mostly, MOFs are formed by the adjustment of concentration gradient through slow diffusion of reactants followed by evaporation of solvent and layering of solutions. A temperature gradient of slow cooling is used to get large crystals of MOFs to examine the structural properties. On the other hand, the abrupt change in temperature and prolonged reaction time disintegrate the MOFs structure [19]. The MOF synthesis routes can be categorized as sonochemical methods, solvothermal methods, electrochemical methods, direct precipitation and microwave-assisted synthesis [20,21], and are greatly dependent on the type of metal, organic linker and required properties and applications.

Earlier synthesis of MOFs as an alternate to zeolites contains single metal nodes to enhance their porosity. On the other hand, pre-designed linkers are used to maintain their integrity during the fabrication process of MOFs. Previously, the synthesis of MOFs is dominated by the synthetic modification of linkers and design of the organic ligands [22]. A number of MOFs have been synthesized with tremendous advantages such as (1) adjustable pore size, (2) ordered structure, (3) tunable diameter, (4) resistance toward change in their morphologies, (5) convenient and economical processing conditions, (6) facile sample collection, (7) resistance toward metal aggregation, (8) resistance toward architecture disintegration and (9) attachment of other substances inside the pores or on the surface [23–25]. These advantages open new directions for MOFs materials as supercapacitors, electrocatalysts, and sodium and lithium-ion batteries [26,27]. The high-performance MOFs material has been greatly developed by the calcination-thermolysis process, but they still have some unresolved problems. Moreover, the synthesis of highly functionalized and controllable MOFs is still under process [28].

From the enzyme immobilization view point, several materials with different geometries have been employed though using different strategies and reviewed elsewhere [29–38]. However, each of them has their own advantages and limitations. In earlier studies, many immobilized enzymes have been developed for different applications such as dye degradation and detoxification, silver removal, dehairing, biofuel production and fine chemicals [39–52]. However, difficulty in recycling, instability and marginal shelf-life suppress most of the above-mentioned applications at large scale. In this context, MOFs is an excellent candidate for bio-immobilization as they have a great number of functionalities, as discussed above [53–55]. The problems mentioned above can be addressed by the use of the solid substrate for immobilization. In this context, MOFs are the latest support for enzyme immobilization as it can enhance loading of biomolecules and increase their efficiency. A great number of applications have been explored, in the last decade, using MOFs as a substrate for the immobilization of enzymes, DNA, proteins, and drugs. The availability of multiple approaches such as diffusion toward pores, in situ entrapment and surface functionalization to the developed enzyme–MOFs system also broadened its applications, recycling, and stability on MOFs support. In the same way, it also opens a new way of easy washing, separation, and purification of the enzyme. The present review discusses the various immobilization strategies used to develop enzyme–MOF bio-catalytic system. Several features that render MOFs as multipurpose support matrices for bio-immobilization are also discussed. Also, the potential applications of MOFs-immobilized bio-catalyst for catalysis, sensing and detection, and protein digestion along with an outlook in terms of upcoming challenges and prospects are presented.

2Strategies for enzyme immobilization on MOFs

Till today, several strategies have been developed to immobilize numerous enzymes of interests using newly engineered pristine or functionalized MOFs. Enzyme–MOFs are fabricated to keep active enzymes with the highest loading and negligible leaking. These are typically developed by identical immobilization approaches reported for other supporting material including biopolymers, mesoporous oxides, nanoparticles, nanotubes, and layered double hydroxides, either by surface adsorption, covalent linkages, cage inclusion/encapsulation or in situ synthesis [56–59]. Fig. 2 illustrates a schematic representation of different immobilization methods to immobilize/encapsulate enzymes using MOFs. Notably, in situ synthesis with particular reference to MOF building blocks is preferred due to the moderate synthesis conditions which additionally prevent the enzyme degradation [59].

Fig. 2.

A schematic representation of different immobilization methods to immobilize/encapsulate enzymes using MOFs.

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2.1Surface immobilization

Surface immobilization is commonly employed for developing bioelectrodes depending on the physical attachment of biocatalysts on the surface of the material through electrostatic, hydrophobic or van der Waals interactions. A comparatively greater surface area of MOFs offers higher loading yield of enzyme molecules through surface immobilization. Additionally, MOF functionalization enables the establishment of stable linkages between MOF and the enzyme. The pendant groups of the organic linker can be activated through surface functionalization, or enzymes are covalently coupled to design MOF–enzyme systems [58]. Table 1 summarizes a comprehensive survey on the MOF–enzyme platforms, improved catalytic performance and their application in various fields. Ma et al. [72] constructed a series of zeolitic imidazole frameworks (ZIFs) of different functional groups and surface areas and examined for the immobilization of GDH and methylene green (MG) by surface adsorption. They evaluated their GDH and showed that the highest adsorption capacities of ZIF-68 and ZIF-70 for GDH enzyme was typically ascribed to their high surface area and robust hydrophobic nature. In addition to physical linkages, the chemical interactions also participated in the adsorption of GDH onto ZIF-70 as evidenced by FT-IR spectroscopy analysis. Fig. 3 illustrates a schematic representation of ZIF-70-based electrochemical biosensor development [72]. This study signifies the hybrid character and tunability of the functional groups of MOFs. In an alternative strategy, enzyme molecules first covalently conjugated to some small molecules (e.g., dyes), which can diffuse/penetrate through the micropores of MOFs, while the enzyme molecules remain adsorbed on the outer surface [58,59]. Based on this technique, Liu et al. [73] carried out the immobilization of trypsin via covalently bonding with the dye 4-chloro-7-nitrobenzofurazan (NBD). The resultant trypsin-assisted bioreactor revealed the good catalytic efficacy for protein digestion. In another study, the fluorescein isothiocyanate (FITC)-trypsin biocomposite was rapidly (2-min) prepared using a microwave-assisted procedure with an effective loading capacity of 55.2μg of enzyme per mg of MOF material. The newly-developed biocatalytic system presented outstanding proteolytic activity (protein digestion efficiency) in several successive batch cycles [74].

Table 1.

List of recently reported various MOF–enzyme biocatalytic platforms with improved catalytic properties and their application.

MOF  Enzyme  Type of immobilization  Performance  Application  Reference 
MIL-100(Fe)), HKUST-1 (Cu3(BTC)2  Porcine pancreatic lipase  Encapsulation  Improved thermal, pH and operational stability of PPL compared to the soluble enzyme
Retained 90.4% of its initial activity after 5 successive catalytic rounds
Exhibited 80.0% yield after 8 reuses 
Synthesis of benzyl cinnamate by enzymatic esterification of the cinnamic acid  Nobakht et al. [60] 
CA@ZIF-8 composite  Carbonic anhydrase  Surface adsorption  Outstanding thermostability and tolerance against denaturants
Good reusability compared with free enzyme
Yields of the CaCO3 obtained using CA@ZIF-8 composites were 22-folds compared to free derivatives. 
Sequestration of CO2 in carbonate minerals  Ren et al. [61] 
Fe-BTC MOF  Laccase and lipase  In situ or post-synthesis methodology  Efficient encapsulation capacity (≥98% for laccase and ≥87% for lipase)
Fully enzyme activity retention and no leaching after an initial release of only 10% of the enzyme molecules
About 97% of immobilized lipase activity can be maintained than free enzyme 
Biomolecules immobilization  Gascón et al. [62] 
ZIF-8  HRP  Encapsulation  Modulation of the enzyme activity
Protection of the enzyme from thermal denaturation 
Biomolecules immobilization  Tadepalli et al. [63] 
La-MOF-NH2
Fe-MOF-NH2, Zr-MOFNH2 
Acetylcholinesterase  Encapsulation  –  Biosensors for the detection of methyl parathion  Dong et al. [64] 
ZIF-8  Cytochrome c  In situ encapsulation  Increased apparent substrate affinity
∼128% increased enzymatic activity
1.4-fold increased sensitivity 
Electrochemical detection of H2O2  Zhang et al. [65] 
Enzyme/ZIF-8 composites  Cytochrome c, horseradish peroxidase, and Candida Antarctica lipase B  Encapsulation  The stability of the encapsulated enzyme was greatly increased
Preserved almost 100% of activity toward protein denaturing solvents including dimethyl sulfoxide, dimethylformamide, methanol, and ethanol 
  Wu et al. [66] 
MIL-53(Al), NH2-MIL-53(Al) and Mg-MOF-74  β-Glucosidase laccase  In situ immobilization  Higher enzyme loading (over 85%) and lower enzyme leaching (around 5%)
Enzyme functionality in a non-aqueous (N, N-dimethylformamide) media 
Biomolecules immobilization  Gascón et al. [67] 
ZIF-8  Glucose oxidase  Co-precipitation  Maintained the enzymatic activity of GOx
High electrocatalytic activity for the oxygen reduction reaction 
Electrochemical detection of glucose  Wang et al. [5] 
Fe-MOF  Alcohol dehydrogenase, lipase, and glucose oxidase  Co-precipitation  Retention of activity for at least 1 week
Good catalytic activity 
Gascón et al. [68] 
UiO-66-NH2  Soybean epoxide Hydrolase  Covalent linkage  Increased pH stability, thermostability, and tolerance to organic solvents
Retained around 97.5% of its initial activity after storage at 4°C for 4 weeks
Enhanced enzyme–substrate affinity and catalytic efficiency 
Biosynthesis of enantiopure (R)-1,2-octanediol in a deep eutectic solvent  Cao et al. [69] 
(NH2-MOF) (NH2-MIL53(Al) and NH2-MIL101(Cr))  Glucose oxidase  Covalent attachment  Effective preservation of 46% activity for MIL53(Al)@GOx and 44% for MIL101(Cr)@GOx, of the total activity per unit mass in contrast with free enzyme.
High selectivity for glucose 
Biomolecules immobilization  Tudisco et al. [70] 
ZIF-8, HKUST-1, Eu-BDC, Tb-BDC and MIL-88A  HRP, urease  Encapsulation  88% catalytic activity vs. 20% in the free form of HRP
Bioactivity retention after being treated at 80°C and boiling in dimethylformamide (153°C) 
Biostorage, chemical processing and pharmaceutics  Liang et al. [53] 
ZIF-8  HRP, glucose oxidase  Multienzyme encapsulation  High catalytic efficiency and selectivity
Enhanced stability due to the protecting effect of the framework
Retained 20% activity up to 4 reusability cycles 
Selective glucose detection  Wu et al. [71] 
Fig. 3.

Schematic representation of the development of ZIF-70-based electrochemical biosensor.

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Reprinted from Ma et al. [72], with permission from American Chemical Society. Copyright (2013) American Chemical Society.
2.2Covalent binding

Although certain enzymes have been effectively attached on the surface of MOFs by physical immobilization, adsorbed enzymes held via only weak interactions between MOFs and enzyme molecules reveal poor stability [75]. Generally, enzymes can be immobilized on the surface of support matrices by covalent attachment. Multipoint covalent coupling is postulated to be one of the robust chemical bonds used to conjugate biomolecules/enzymes. It is worth noting that the creation of multiple covalent bonds between the support carrier and enzyme diminishes conformational flexibility and fixes the enzyme in a way that circumvents protein denaturation or unfolding [76]. MOF surfaces present a great variety of functional groups (e.g., amino, carboxyl, hydroxyl groups, etc.) that can be functionalized to couple with reactive groups of the enzyme. For instance, Deng et al. [11] showed that MOFs might incorporate a wide array of different functionalities on linking groups by mixing the linker. They designed multifaceted MOFs from 1,4-benzenedicarboxylate and its derivatives that comprise up to eight separate functionalities in one phase. Cao et al. [69] reported the commendable immobilization of soybean epoxide hydrolase by precipitation and cross-linking approach onto the outer surface of UiO-66-NH2 MOF. The synthesized SEH@UiO-66-NH2 bioconjugate showed high enzyme loading, increased substrate affinity and preserved about 97.5% of its original activity after for 4 weeks at 4°C.

2.3Cage inclusion/encapsulation

The cage inclusion method implicates the encapsulation of enzyme molecules within the cages of MOFs by diffusion. In comparison to adsorption, encapsulated enzymes may show considerably enhanced stability, even under harsh or biologically incompatible environments [58]. Furthermore, the enzymes confinement in 3D microenvironments is identified to prevent protein unfolding and thereby to increase stability [77]. The encapsulation/entrapment of enzymes also diminished the enzymes leaching from the carrier support, which is considered as one of the major limitations of other immobilization strategies [78,79]. However, the accessibility to the encapsulated enzymes and thus the mass-transfer efficiency of analytes may be strongly hindered due to a dense host milieu [78,79]. Due to their high porosity, MOFs should be very interesting host matrices although encapsulation is not feasible for large enzymes due to size limitations. Feng et al. [80] developed an array of ultra-large mesoporous stable MOFs and utilized for the encapsulation of three enzyme including HRP, Cyt c, and MP-11). Maximum loading was observed to be 22.7, 77.0 and 478mmol/g for HRP, Cyt c, and MP-11, respectively. Concerning HRP and Cyt c, MP-11 could penetrate each meso-cage resulting in a much higher molar concentration. Moreover, the low Km of the immobilized enzymes indicated a greater affinity for their substrates and thus requiring a lower concentration of substrate to attain maximum velocity (Vmax). The MOF-encapsulated immobilized enzymes were reusable for several rounds owing to the no leaching from the carrier. The carrier-encapsulated HRP displayed higher catalytic potentiality in volatile organic solvents than that of the free derivative. The Tb-mesoMOF-encapsulated myoglobin has shown superior catalytic performance for the oxidation of small substrates combined with excellent reusability and stability [81].

2.4In situ MOF formation and enzyme immobilization

In this technique, nucleation and growth of MOF formation and enzyme immobilization occur concurrently in one step. The MOF building blocks and enzyme are mixed in a solution that results in the crystallization of MOF particles with embedded enzymes at the particle surface or as a core–shell material. Similar to the encapsulation method, this protocol also offers a 3D microenvironment for the biomolecules with negligible leaching but restricted to MOFs capable of prepared only under mild biocompatible conditions, in particular, aqueous solutions and lower temperature to prevent the enzyme from denaturation [59]. On the other hand, “in situ MOF synthesis” approach represents the notable advantages of encapsulation of a greater number of enzymes owing to the highly uniform and ordered nano sizes, larger specific areas, and tunable pore sizes of MOFs [23–25,67,68]. This is why ZIFs, which are formed easily under mild environment, have been widely pursued this purpose [59]. Wu et al. [71] reported an interesting one-step and facile synthetic approach containing a multi-enzyme system using ZIF-8 nanocomposite. To this end, two enzymes including GOx and HRP were co-immobilized with the ZIF-8 precursors yielding GOx&HRP@ZIF-8 bioconjugates that exhibited high selectivity, elevated stability and superior catalytic performance owing to the shielding impact of the developed framework. Further, the prospect of catalyzing enzymatic cascade reactions within the particle is the exceptional characteristic of this approach, demonstrating the significance of restricting bi-enzyme systems in close proximity. Hou et al. [82] reported an “indirect” in situ MOF fabrication strategy for the immobilization of functional biomolecules. The resulting enzyme–MOF composites were found to be highly microporous, robust, and provided a straightforward way to encapsulate enzymes for size-selective and recyclable biocatalytic system.

3Application perspective of MOFs-immobilized bio-catalyst

Due to the high selectivity nature of enzymes, the applications of immobilized enzyme@MOFs materials are mainly for catalysis, biosensing and detection. Immobilized enzymes are separated by the pores of MOFs, which avoids their aggregation and facilitates a high conversion rate. Also, the physical confinement of the immobilized enzymes by the cavity wall of MOFs prevents the occurrence of protein denaturation.

3.1Biosensing and detection

Enzyme-immobilized sensing and detection devices so-called biosensors have received great interest in the arena of glucose monitoring, quality control, food safety, and diagnosis of cancer [83]. Glucose biosensors are particularly attractive accounting for approximately 85% of the entire biosensor market due to the great need for monitoring the blood glucose levels of millions of daily diabetic patients [2]. Generally, the design of biosensor allows rapid and one-step analyses at the point of concern for detecting an analyte and necessitates the biosensors enzymes to be functional and stable in unfavorable environmental conditions. The employment of native enzymes in industrial applications is often impeded by their marginal operational stabilization, difficult separation and lack of repeatability. In this context, MOFs has recently emerged as potential and new types of immobilization matrices in protecting the biocatalysts against perturbations [6]. This potentiality of MOFs as promising host candidates might be ascribed to the possibility of fine-tuning the pore size and easy functionalization of the pore walls, which consequently, favoring specific interactions between biomolecules/enzyme and pore walls of MOFs [84]. Ma et al. [72] synthesized and utilized a series of novel zeolitic imidazolate frameworks (ZIFs) including ZIF-7, ZIF-8, ZIF-67, ZIF-68, and ZIF-70 with distinct surface areas, pore sizes, and functional groups as support matrices to develop integrated dehydrogenase-assisted biosensors for the detection of glucose. In their work, ZIFs were found to function as suitable support materials for the co-immobilization of methylene green (MG) and glucose dehydrogenase (GDH) onto the surface of electrode resulting in the rapid construction of an integrated electrochemical biosensor. Amongst all the synthesized ZIFs, ZIF-70 demonstrated excellent adsorption performance toward MG, as well as, GDH and is therefore selected as the matrix of choice for glucose biosensing. Notably, an MG/ZIF composite was drop-coated on a glassy carbon electrode followed by GDH coating to fabricate the biosensor. In addition to high sensitivity toward glucose with a linearity range of 0.1–2mM, ZIF-based biosensor also showed a potential selectivity for glucose than other endogenous electroactive species in the cerebral system.

Nanocomposites developed from the combination of mesoporous iron(III) trimesate MIL-100(Fe) and platinum nanoparticles (Pt-NPs) were used as carrier support for immobilizing glucose oxidase (GOx). The resulting GOx–MIL-100(Fe)–PtNP bioelectrode presents remarkable electrocatalytic activity for glucose monitoring owing to physicochemical properties of Pt-NPs and exceptional characteristics of MIL-100(Fe) including biocompatibility and high specific surface area and pore volume. Under optimized operating conditions, the newly synthesized biosensor displayed a high sensitivity (71mAM−1cm−2), stability and a low limit of detection (5mM) in shorter response time (less than 5s). Poor sensitivity and high response time values by replacing the iron with aluminum or chromium in MIL-100 indicated the involvement of iron centers of MIL-100(Fe) in a synergistic effect which certainly increases the catalytic oxidation of glucose and biosensor performance [83]. He et al. [85] designed for the first time a smart porous material, Cu-hemin MOFs by combing Cu2+ with hemin to incorporate GOD for electrochemical detection of glucose (Fig. 4). The ball-flower like resulting smart nanocomposites comprises a huge number of mesopores along with a larger surface area that greatly facilitated the loading of GOD molecules in the pores of Cu-hemin MOFs to retain their bioactivity. The GOD/Cu-hemin MOFs nanocomposites not only catalyzed the reduction of oxygen via Cu-hemin MOFs but also efficiently catalyzed glucose oxidation via GOD. Accordingly, the as-prepared GOD/Cu-hemin MOFs-based electrochemical glucose sensor exhibited good performance, i.e. excellent sensitivity and selectivity, wide linear range (9.10–36.0mM) and low limit of detection (2.73μM). In short, the proposed method provides a simple, efficient and practical electrochemical sensing platform based on MOFs and biomolecules.

Fig. 4.

SEM images of GOD/Cu-hemin MOFs nanocomposites at (A) low- and (B, C) high-magnification. (D) UV-vis spectra and (E) fluorescence spectra of GOD (a), hemin (b), Cu-hemin MOFs (c) and GOD/Cuhemin MOFs nanocomposites (d). (F) N2 adsorption/desorption isotherms measured at 77K for the Cu-hemin MOFs (black curve) and GOD/Cu-hemin MOFs nanocomposites (red curve).

(0.41MB).
Reprinted from He et al. [85], an open-access journal licensed under a Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/). Copyright (2016) the authors(s).
3.2MOFs as host for biomimetic catalysis

Enzymes are promising (bio)catalysts due to their abundance; non-toxicity; great catalytic efficiency; regio-, stereo-, and chemoselectivity, environmentally-friendliness. Notwithstanding these useful traits, the costly preparation and purification of heterogeneous catalysts from enzymes is a major drawback [86]. To address these issues, considerable research attention has been projected to design highly stabilized artificial enzymes as a reproducible and cost-efficient alternative [87]. Biomimetic catalysis is an important area in biomimetic chemistry that pertains to chemical catalysis mimicking certain key features of enzymes [88]. By mimicking the structural features and mechanisms of enzymes, biomimetic strategies can be employed to design catalysts with enhanced robustness, high throughput rate, and great efficiency and specificity [89]. Biomimetic catalysts can have the ability to survive under unfavorable reaction conditions (i.e., strong acid or base, high temperatures) and in various organic solvents including ethanol, methanol, DMF, and DCM with better stability than that of their enzyme equivalents. Besides, often these catalysts present catalytic selectivity usually not detected in the case of native enzymes [90]. Among the several analogs of materials widely attempted for biomimetic catalyses, such as nanoparticles, porous aluminosilicates, mesoporous silicas, polymers, and organic macrocycles [88,89], aluminosilicates, porous polymers, and silicas have been used as carrier support to develop biomimetic catalysts. Nevertheless, these porous materials are comprised of either organic or inorganic compounds or exhibit inherent limitations. Explicitly, organic compounds are typically amorphous lacking crystalline structures, whereas inorganic compounds are lacking structural flexibility. In this context, well-designed MOFs by combining the useful properties of both organic and inorganic materials into one system by choosing appropriate organic building blocks and inorganic metals or by post-synthetic modifications can largely expand the repertoire of porous materials. Moreover, several exceptional properties, such as higher specific surface area, porosity, and structural versatility, MOFs hold a unique position for the development of biomimetic catalysts as compared to other porous materials [91]. Composite material has been successfully prepared for the first time using an amino-containing metal–organic framework (MOF) as a new type of host matrix material to anchor hemin and simulate the peptidic microenvironment of the native peroxidase.

Qin et al. [92] anchored hemin molecule into a new type of immobilization host material, amino-containing MOF (MIL-101(Al)-NH2) for the first time. In the presence of H2O2, the resulting fabricated Hemin@MIL-101 demonstrated peroxidase-like activity through the catalytic oxidation of the substrate 3,3,5,5-tetramethylbenzidine (TMB). Afterward, a highly selective and sensitive method was established for the detection of glucose using as-prepared mimetic peroxidase. By selective encapsulation of catalytically active metalloporphyrins in HKUST-1 nanoscale cages by a “ship-in-a-bottle” fashion, Larsen et al. [93] synthesized a class of biomimetic catalysts, MOMzyme-1. The peroxidase activity of the resulting Fe4SP@HKUST-1 was assessed toward mono-oxygenation of organic substrates using 2,2′-azinodi(3-ethylbenzthiazoline)-6-sulfonate (ABTS) as a redox indicator. Maximum yield in this biomimetic system was found to be comparable to its MP-11 and Fe4SP counterpart in solution. Therefore, the MOMzyme-1 might manifest a new paradigm for heme biomimetic catalysis by combining the activity of a catalyst along with the stability and reusability of heterogeneous catalytic systems within a single material. In another study, Liang et al. [53] immobilized biomacromolecules, i.e. proteins, HRP and DNA into ZIF-8 MOF using a ‘one pot’ biomimetic mineralization approach (Fig. 5). The immobilization or encapsulation efficiency of most of the tested proteins was reached 90% after recovery and washing. The MOF-loaded all biomolecules demonstrated excellent catalytic activity under extreme environments (e.g., boiling DMF at 153°C) indicating the usefulness of MOFs as protecting hosts for different biomolecules.

Fig. 5.

Schematic illustration of biomimetically mineralized MOF. (a) Schematic of a sea urchin; a hard porous protective shell that is biomineralized by soft biological tissue; (b) schematic of a MOF biocomposite showing a biomacromolecule (for example, protein, enzyme or DNA), encapsulated within the porous, crystalline shell.

(0.22MB).
Adapted from Liang et al. [53], an open-access journal licensed under a Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/). Copyright (2015) Macmillan Publishers Limited.
3.3Enzyme–MOF for digestion of protein

Trypsin is a commonly-used proteolytic enzyme that catalyzes protein digestion and transforms into peptides for proteomics research and industrial production. However, its practical applicability is often hampered due to the long running time (18–24h) and self-digestion in the reaction mixture [94]. Surface immobilization of trypsin on MOFs overcomes the self-digestion and improves their recycling capacity [6]. For example, a trypsin enzyme was immobilized onto a simple nanoporous MOF bioreactor prepared via a low energy 30min vortex as driving force. Lack of any chemical modifications on the support surface eliminates the use of volatile organic wastes and saves time. The digestion efficiency of trypsin-FITC@MOF bioreactor was found to be exceptionally high even in repeated uses, over the use of free enzyme-assisted digestion, or nanoparticle immobilized enzyme reactors described so far [74]. Shih et al. [95] successfully immobilized the proteolytic enzyme onto MOFs (MIL-101(Cr), MIL-88B(Cr), and MIL-88B-NH2(Cr)) activated with dicyclohexylcarbodiimide (DCC) through the formation of peptide linkages. The trypsin–MOFs composite was applied for the digestion of BSA with the assistance of ultra-sonication for 2min (Fig. 6) [95]. Notably, the BSA digestion efficiency of trypsin–MOF was comparable to that of traditional in-solution digestion (free trypsin), representing that the MOF-assisted immobilization did not affect substrate approachability or activity of the enzyme. Also, the trypsin–MOF conjugate was found to be reusable and recoverable.

Fig. 6.

Protein digestion through trypsin–MOF.

(0.09MB).
Reprinted from Shih et al. [95], with permission from John Wiley and Sons. Copyright (2012) John Wiley and Sons.
4Useful enzyme properties obtained after immobilization on MOFs

As a biocompatible support carrier for enzyme immobilization, the MOF-immobilized biocatalysts showed improved attributes in terms of substrate accessibility, catalytic activity, stability, or recyclability. These features are particularly evident in a biologically incompatible environment that leads to the denaturation of native enzymes [96]. The improvements usually obtained after enzyme immobilization on MOF are briefly summarized in this section.

4.1Enzyme catalytic performance

In recent years, several reports revealed that immobilization increased bio-catalytic performance of enzymes on MOF in comparison to free enzymes. Lykourinou et al. [81] reported the successful immobilization of microperoxidase-11, for the first time, into a mesoporous MOF comprising of nanoscopic cages. The resulting MP-11@Tb-meso MOF demonstrated superior catalytic activity than its mesoporous silica material MCM-41. In another study, Lyu et al. [97] embedded cytochrome c directly on the surface region of MOFs by a co-precipitation method. In contrast to the native counterpart, the ZIF-8- Cyt c embedded enzyme demonstrated a 10-fold improvement in peroxidase activity suggesting a rapid, convenient, and highly sensitive monitoring of volatile organic peroxides in solution. Similarly, the catalytic activity of Cu3(btc)2-based hierarchically porous MOF-immobilized lipase was greatly improved than that of its free derivative. Importantly, the immobilized MOF–lipase conjugate exhibited 17-times higher reaction rate in comparison to catalyze by a free enzyme [98]. The increased catalytic activity and conversion rates of the MOF-conjugated enzyme are typically attributed to the confinement of single enzymes by MOF cavities, active involvement of functional groups present on the MOF backbones in catalysis, and unusual size selectivity of MOF immobilized enzymes [96].

4.2Enzyme stabilization

In contrast to pristine enzymes which generally get denatured at extreme pH and temperature, or chemical denaturants, the carrier-supported biocatalysts exhibit greater tolerance to these denaturing conditions. Numerous reports have shown that MOF–enzyme bioconjugates displayed higher catalytic stability and thermal tolerance stability than their free counterparts. Recently, Nadar and Rathod [99] scrutinized the thermal stability of zeolite imidazolate framework-8 (ZIF-8)-MOF encapsulated lipase at different temperatures ranging from 55 to 75°C. In comparison to free enzyme, ZIF-8-lipase showed 3.2-folds increment in half-life, and deactivation rate constants (kd) of the immobilized enzyme were significantly lower than free lipase at each tested temperature point. Furthermore, it retained 54% of initial activity after seven consecutive reusability batches and preserved 90% of original activity after 25 days of storage. The improved temperature stability might be ascribed to confinement of enzymes molecule in three-dimensional biocompatible microenvironments leading to increased stabilization by preventing from protein unfolding or denaturation. Similarly, the robustness and one-step self-fabricated glucoamylase–MOF exhibited 6-folds increment in thermal steadiness at various temperatures (60–80°C) than that of free state of the enzyme. After six successive cycles of reuse and 25 days of storage, the glucoamylase embedded MOF maintained up to 57% and 91% of its original activity, respectively. However, after immobilization, Vmax value was reduced combined with a higher Km value as compared to free enzyme [100]. Liu et al. [101] immobilized GOX and uricase enzymes on self-assembled highly stable hierarchical micro/mesoporous metal–organic frameworks (HP-MOFs). The newly fabricated HP-MOFs were found to be quite stable over a wide-working pH range varying from 2.0 to 11 and fulfilled the catalysis conditions of most enzymes. Maximum adsorption capability of synthesized MOFs for GOX and uricase was 208 and 225mgg−1, respectively. The MOFs-incorporated enzymes presented improved temperature stability as compared to the native form, and exhibited outstanding recyclability owing to the suitable mesoporous size and strong affinity of HP-MOFs. Also, they constructed two multi-enzyme biosensors by embedding GOX and uricase with horseradish peroxidase (HRP) on HP-MOFs for the colorimetric detection of glucose and uric acid, respectively. These sensors presented the advantages of facile synthesis, wide applicability, high catalytic activity, good sensitivity, selectivity, and recyclability for biosensing glucose and uric acid. Apart from thermal resistance, the stability of the enzymes against chemical denaturants is also the most important aspect for enzyme applicability. Wu et al. [66] encapsulated different enzymes, i.e. cytochrome c, HRP, and Candida Antarctica lipase B in MOFs via co-precipitation process to construct highly stabilized enzyme–MOF composites in protein denaturing solvents (Fig. 7). After immobilization on MOFs, the stability of MOF-immobilized enzyme at varying temperatures was significantly enhanced. Remarkably, the enzyme–MOF conjugate did not lose any activity and retained almost 100% of activity following exposure to different protein denaturing organic solvents such as ethanol, methanol, dimethyl sulfoxide, and dimethylformamide indicating the extraordinary protective influence of MOF shell at adverse environments. On the other hand, free enzymes lost more than 80% of their original activities under the identical conditions due to serious alteration of protein configuration.

Fig. 7.

Scheme of the green synthesis of enzyme–MOF composites exhibiting tolerance for denaturing solvents and heat.

(0.18MB).
Reprinted from Wu et al. [66], an open-access paper distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/). Copyright (2017) the authors(s).
4.3Enzyme recovery and reusability

One of the paramount advantages of the immobilization is to facilitate the efficient recovery and repeated uses of enzymes which are considered important factors in reducing bioprocess costs. Also, carrier-supported biocatalysts exhibit a considerably lower amount of impurities that result from product contamination in comparison to pristine enzyme forms [75]. In a recent study conducted by Nadar and Rathod [99], the lipase@ZIF-8 preserved 54% of its initial activity after repeatedly use for continuous seven cycles. Laccase adsorbed on a new type of bimodal micro-mesoporous Zr-MOF was reusable 10 times while retaining almost 50% its residual activity. The catalytic performances of trypsin-MIL-88B composite developed by covalent anchoring of enzymes to MOF support showed an insignificant loss in activity after 4 repeated cycles. Besides in addition to covalent interactions between MOF matrices and enzymes, weak linkages also greatly augmented the recycling perspective of the MOF-embedded enzymes [95]. Similarly, mesoporous MOF, PCN-333-embedded HRP retained at least 80% activity after 5 reusability cycles, whereas more than 70% of initial activity was observed to be lost in the first round by SBA-15 immobilized HRP [80]. Despite good recoverability, the nanometer size of enzyme–MOF conjugates, and their good solvents dispersity hinder the reusability of these composites in industrial-scale enzymatic catalysis. In this respect, the magnetic zeolitic imidazolate framework 8 (mZIF-8@GOx) were synthesized by Hou et al. [82] and utilized embedding GOX enzyme to further improve its recyclability potential. The resultant mZIF-8@GOx composite presented remarkable reusability for 12 batches with no noticeable activity reduction in first 5 rounds and retaining approximately 88.7% residual activity after 12 repeated uses.

5Conclusions and prospects

Metal–organic frameworks have shown great promise for applications in the field of chemistry and biological sciences due to their extraordinary structural properties with tunable functionalities such as porosity, thermal stability and high surface area. A variety of synthesis methods have been developed and extensively used to design different MOFs derivative due to their ease in synthesis, cost-effectiveness, and control of the shape, size, and functionalization of the pores. Current literature survey revealed MOFs as a multifunctional and emerging class of novel support materials or carries to develop robust, stable, recoverable, and cost-efficient, biocatalytic systems. Though the employment of MOF-assisted biocatalysis has been extended to diagnostics and therapeutics arenas, the applications of MOF–enzyme biocatalytic supports for real-time practical purposes is still at its infancy stage. Immobilization of enzyme molecules into/onto the MOFs exhibit several benefits, such as substantial enzyme loading, catalytic efficacy, stability, and recyclability. However, further developments in the fabrication of new MOFs together with an extensive understanding of MOF–enzyme relationships are the key challenges that need to be addressed with sophisticated and state-of-the-art scientific analysis. Also, scientific literature lacking systematic research efforts devoted to design MOF-based biocatalytic supports with capacity in hosting numerous enzymes to mediate multiple reactions in a single use.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgments

The authors are grateful to their representative universities/institutes for providing literature services.

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Hafiz M.N. Iqbal is a full-time research professor at the School of Engineering and Sciences, Tecnologico de Monterrey, Mexico. He completed his Ph.D. in Biomedical Sciences with specialization in Applied Biotechnology and Materials Science at the University of Westminster, London, UK. Dr. Iqbal does research in Bioengineering, Biomedical Engineering, and Environmental Engineering. Dr. Iqbal had guest edited special issues and served as an Editorial Board member for several peer-reviewed journals. Dr. Iqbal has published more than 180 scientific contributions in the form of Research, Reviews, Book Chapters, and Editorials.

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