Polysaccharide based metal organic frameworks (polysaccharide–MOF): A review

Coordination Chemistry Reviews 396 (2019) 1–21

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Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr

Review

Polysaccharide based metal organic frameworks (polysaccharide–MOF): A review Shamraja S. Nadar, Leena Vaidya, Shefali Maurya, Virendra K. Rathod ⇑ Department of Chemical Engineering, Institute of Chemical Technology, Matunga (E) Mumbai 400019, India

a r t i c l e

i n f o

Article history: Received 2 April 2019 Accepted 9 May 2019 Available online 12 June 2019 Keywords: Polysaccharides MOF Biological Flexible Bio-applications

a b s t r a c t The intriguing features and physico-chemical properties of metal organic frameworks (MOFs) have captivated numerous researchers for its application in an array of fields as a hybrid material. This highly ordered porous material is constructed by self-assembling of different organic ligands and metal ions. However, organic building blocks of MOFs are derived from non-renewable petroleum feedstock which limits their application in biological sector due to inherent toxicity and non-biodegradability. Recently, polysaccharide based metal organic framework (polysaccharide–MOF) has emerged as a selfassembled highly ordered functional nanostructure. This fascinating class of MOFs confers biological compatibility and flexibility to emerging hybrid material due to incorporation of naturally occurring polysaccharide. Also, it has shown some outstanding and promising biological properties such as biocompatibility, biosafety, and bioavailability. In this review, the novel properties (such as structural flexibility, tailored porosity, and chemico-thermal stability) offered by specifically designed polysaccharide–MOF have been discussed. Further, an overview of highly crystalline nature and functional MOF derived from different polysaccharides such as cyclodextrin, cellulose, chitin, chitosan, etc. have been explained with recent state-of-the-art examples. In the end, the outlook and possible challenges regarding polysaccharide–MOFs are illustrated. The exceptional properties of polysaccharide–MOF are providing a new avenue to explore various applications in biological sciences. Ó 2019 Elsevier B.V. All rights reserved.

Contents 1. 2. 3.

4. 5. 6. 7. 8.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Design of polysaccharide ligand in MOF. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Properties of polysaccharide–MOF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 3.1. Porosity of polysaccharide–MOFs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 3.2. Structural flexibility and stability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3.3. Biocompatibility and cytotoxicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Cyclodextrin based MOF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Cellulose based MOF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Chitin and chitosan based MOF. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Other polysaccharides based MOF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Future scope and summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Abbreviations: 3D, three dimensional; BC, bacterial cellulose; Bio–MOFs, biological metal–organic frameworks; BTC, benzene-1,3,5-tricarboxylic acid; C3N4, carbon nitride nanosheets; CA, cellulose aerogels; CD, cyclodextrin; CMC, carboxymethyl cellulose; CMFP, carboxymethylation method to functionalize the filter paper; CNF, cellulose nanofiber; CS, chitosan; DA, dopamine; DMF, dimethylformamide; DOX, doxorubicin; GO, graphene oxide; HKUST, Hong Kong University of Science and Technology; IBU, ibuprofen; MOFs, metal organic frameworks; MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide; NO, nitric oxide; PMs, particulate matters; PNP, p-nitrophenol; PSM, post synthetic modification; SEM, scanning electron microscope; SPE, solid-phase extraction; UiO, University of Oslo; VOC, volatile organic compound; XRD, X-ray diffraction; ZIF, zeolitic imidazolate framework. ⇑ Corresponding author. E-mail address: [email protected] (V.K. Rathod). https://doi.org/10.1016/j.ccr.2019.05.011 0010-8545/Ó 2019 Elsevier B.V. All rights reserved.

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Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

1. Introduction An evolving class of organic-inorganic hybrid metal-organic frameworks (MOFs) has been extensively explored as porous material over the past two decades. MOFs are constructed by selfassembling metal ions and organic ligands via strong coordination bonds. MOFs exhibit some interesting properties such as tailored topologies, high void volume, low density, high thermal and chemical stability. Also, the well-ordered pore poly-crystalline networks offer several chemically reactive sites in the form of metal centers as well as the ligands’ functional groups [1]. During the last few years, MOFs have been immensely used in various sectors such as gas separation/storage, water purification, chemical sensors, optics, drug delivery, contrast agents and bioreactors [2–4]. However, the vast majority of described MOFs are composed of metal ions and organic component derived from non-renewable petrochemical feedstocks. They are potential toxic chemicals which are unsuitable for biological applications [5]. Recently, green, renewable framework material composite, biological metal–organic frameworks (Bio–MOFs) have intrigued as a novel class of porous materials [6]. In Bio–MOFs, biomolecules such as amino acids, peptides, nucleobases, and saccharides are introduced as building units of MOFs produced from renewable raw materials which are usually non-toxic and biodegradable [7]. Compared with conventionally used ligands for MOF synthesis, biologically derived ligands intrinsically contain multiple co-ordination sites and contain a variety of functional groups which may be suitable for binding metal ions [8]. Also, it helps to control various interactions and chemical bonding, resulting in flexible structures. These bio-ligands also provide several advantages such as easy availability, ease of synthesis, tailor-made structure, multiple reactive sites and pre-existing chirality [9]. The incorporation of such ligands not only improves the biocompatibility of MOF but also makes functional composites. Thus, it is not surprising that Bio–MOFs have enthralled the scientific community and have shown a great promise in numerous fields such as gas storage, catalysis, sensing, separation and providing new opportunities for applications in bio-medicines [10,11]. Among the various available Bio–MOF, polysaccharide based MOF has received significant attention because of its biocompatibility and structural strength. Polysaccharides not only provide mechanical strength but also make MOF flexible without affecting the intrinsic properties of MOFs [12–14]. Additionally, metal ions, one of the main ingredients of MOFs, tend to form complexes with polysaccharides and interfere in the process of crystallization to improve the crystallites of MOF [15]. In this review, we highlight the design, structure, properties, and applications of polysaccharide derived MOFs. The sophisticated structural design of polysaccharide–MOF provides some added features such as tailored porosity, structural flexibility, mechanical stability and biocompatibility which have been further discussed in this review. More so, the current work and challenges of incorporating polysaccharideligands are discussed and have afforded insights and new knowledge into this exciting subdiscipline of research to design new generation Bio–MOFs. 2. Design of polysaccharide ligand in MOF A typical synthesis process of MOF includes metal ion and organic ligands which form co-ordination bonds resulting in a

highly crystalline structure with extremely high surface area. MOF performance and application is totally dependent on the counter parts of MOF. Different kinds of MOF with unique physiochemical properties have been synthesized with the rapid development in technology [16,17]. Recently, Bio–MOFs have emerged from the combination of MOF chemistry and bioscience. There is no clear definition for Bio–MOFs. However, there are two different perspectives regarding Bio–MOFs as reported in the literature [8]: (i) presence of at least one biomolecule as a ligand in construction of MOF (ii) high porosity of MOFs that are often employed in the sector of biology and medicine. Mostly, MOFs which comprise biomimetic building units are emphasized as Bio–MOF. The incorporation of these biomimetic materials imparts new features such as self-assembly, molecular recognition imprinting and structural functionality [18]. In some cases, they may exhibit novel properties which help to extend their applications. There are certain empirical strategies available to obtain polysaccharide based MOF (Fig. 1): (i) in synthesis process: polysaccharide as one of the organic ligands and (ii) post synthesis process: forming composites by establishing a covalent coupling with pre-existing MOF linkers via post synthetic modification (PSM). The use of polysaccharide as bio-ligands can provide biological compatibility, structural crystallinity, and functionality to Bio–MOFs. The detailed properties of polysaccharide–MOFs are discussed in the subsequent section.

3. Properties of polysaccharide–MOF MOFs are extensively studied crystalline materials due to their attractive properties. The variety of polysaccharide–MOFs was constructed with the organic and inorganic components along with polysaccharide as one of the counterparts of MOFs. Generally, the applications of polysaccharide–MOFs are based on their properties such as structure, morphology, ordered porosity (pore size and pore volume), stability (chemical and mechanical), pore functionality and crystallinity (Fig. 2). 3.1. Porosity of polysaccharide–MOFs Porosity and high surface area are the key advantages of polysaccharide–MOFs. The porous structure of MOFs provides an opportunity to encapsulate/entrap molecules by host–guest chemistry and extend their potential bio-based applications. The principle of host–guest chemistry in MOFs is specific molecular communication between the framework and the guest primarily happening in the interior or surface of porous host [8]. If the size and shape of the guest molecule match with the size and shape of the MOFs, the guest molecule can be confined within space of the host framework via various weak interactions, and sometimes even forms co-ordinate bonds. Indeed, the implementation of polysaccharide–MOFs is based on highly specific binding of molecules via host–guest chemistry. The cyclodextrin (CD) based MOFs are most widely used polysaccharide–MOF on account of their intrinsic uniform cavity (17 Å arising from CD) and presence of a large number of hydroxyl groups, which create spatially extended and ordered cage-like structure formation. This property makes it a potential candidate for adsorption and separation of various gases and other small organic molecules with its high selectivity [19]. At present, there are multiple reports on capturing various gases (e.g. CO2) [20], biomolecules (curcumin [21], Vitamin A [22], ferulic acid

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Fig. 1. The empirical strategies to prepare polysaccharide based MOF: (a) in situ synthetic process and (b) post synthetic process.

Fig. 2. Schematic representation of synthetic strategies and properties of polysaccharide–MOF.

[23]) and drugs (ibuprofen [24], fenbufen [25], doxorubicin [26], etc) which manifest host–guest chemistry. The structural topologies, different stabilities (in terms of thermal, chemical and mechanical) and porosities can be designed by changing either metal ion or organic cross-linker (polysaccharides). One of the research studies showed that the physical properties of cyclodextrin MOF (CD–MOFs) can be manipulated by using different metal ions. They observed that the prepared c-CD–MOF with different metal ions exhibited different pore sizes (c-KCD–MOF 24.13 Å, c-NaCD–MOF 32.86 Å and c-FeCD–MOF 30.19 Å) [27]. In another work, Wang and co-workers used different types of CD for the construction of MOF with podetium as an alkaline metal ion. They observed that different CD incorporated MOF resulted in varying shape and porosity of MOF. These variations are summarized in Table 1. Further, it was reported that these MOFs with tailored-morphological properties can be employed as an excellent candidate for multiple applications, especially in biomolecule encapsulation [28]. These porous materials are generally required for enantioselective separation and/or catalysis. The major challenge is associated with structural stability and porosity

Table 1 Properties of CD–MOF with respective incorporation of different cyclodextrin [23]. MOF

Structure

Small pore size

Big pore size

a-CD–MOF

left-handed helical chiral bowl-like structure body-centered cubic

7Å 5Å 4.2 Å

11 Å 6Å 7.8 Å

b-CD–MOF c-CD–MOF

of the host after removal of guest molecules. Also, it remains synthetically challenging to construct robust and porous Bio–MOFs for reversible adsorption and desorption of guest molecules without affecting their crystallinity especially in the gas storage and environmental remediation applications [29]. 3.2. Structural flexibility and stability Metal co-ordination geometry and degree of freedom of the ligands limits the porous adaptability in traditional MOF. Metal co-ordination geometry should change with respect to ligands. It is limited to configurations that are only accessible by single axial

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rotations. Polysaccharides based ligand can get access to substantial conformational space through low-energy torsions which eminently differs from the classical rigid aromatic based MOFs. This unique property of polysaccharide–MOFs differentiates them from other rigid inorganic porous materials [30]. Zhu and group combined functional Zn based MOFs with structural cellulose nanocrystal (CNC) aerogel to prepare a flexible and porous aerogel with hierarchical structure for the separation of chromium from water. The obtained hybrid aerogels were confirmed for their crystallinity using XRD technique. It was found that these aerogels had a uniform, structural flexible and integrity [31]. The flexibility of MOFs along with high shape selectivity has been reported by Stoddart and co-workers. They synthesized CD– MOF for the separation of acronym that stands for benzene, toluene, ethylbenzene, and xylenes (BTEX). The retention order of o- > m- > p-xylene separation was determined through adsorption isotherms and liquid-phase chromatographic measurements, with significant regioselectivity of ethyltoluene and cymene regioisomers during the liquid-phase chromatography. Along with separation of regioisomers, CD–MOF also facilitated purification of cumene from certain impurities and separation of industrial useful BTEX compounds. This study very well demonstrates the structural flexibility and selectivity of polysaccharide–MOFs which are noteworthy in industrial applications of these MOFs [32]. Similarly, Zhang and co-authors prepared cellulose nanofibrils/UiO-66-NH2 composite membrane for selective gas-separation of CO2/N2. The hybrid composite membrane exhibited 7-fold higher selectivity as compared to membrane without cellulose nanofibrils [33]. The flexibility of polysaccharide gives adaptability and dynamic response characteristics to MOF which makes MOF as powerful host in host–guest chemistry. These flexible polysaccharides based frameworks can be employed in real-world applications such as gas storage/delivery, separation, sensing, and catalysis. The polysaccharide based MOF provides high thermal as well as mechanical stability to MOF. In one of the studies conducted by Liu and co-worker, the crystallinity of CD–MOFs in the temperature range of 60–100 °C was analyzed using SEM and PXRD techniques. Authors determined that the crystalline structure of CD–MOF was decomposed at high temperature. However, the morphological structure of CD–MOFs remained same even at 100 °C after 24 h. Further, the same research group determined the stability of CD– MOFs under different humidity and in different organic solvents. The authors found that the CD–MOFs were more susceptible to humidity and lost the crystalline structure dramatically under the humidity higher than 75% RH for one day [34]. On the other hand, the CD–MOF was more stable in organic solvents such as methanol, ethanol, isopropanol, acetone, dichloromethane and N, N-dimethylformamide. The lower water stability of these materials can be improved by complexation or post-synthetic modification of the framework and cross linking [35]. Nevertheless, polysaccharide–MOFs are structurally flexible which makes them a potential candidate for numerous applications such as solid phase extraction, gas storage, separation, and bio-related applications. 3.3. Biocompatibility and cytotoxicity Besides crystallinity and porous nature, polysaccharide based MOFs are highly biocompatible and environmentally safe as it is derived from natural resources. Also, they are structurally compact with high density and having various functional groups to carry the drug. Additionally, they reduce the drug diffusion capacity in dissolution medium, promoting the greater control of drug release [36,37]. In one of the studies, Javanbakht and co-workers found that the hybrid CMC copper based MOF showed lower cytotoxicity to Caco-2 cells as compared to plain copper MOF. They observed that the presence of CMC helped to improve the cytotoxicity and

biocompatibility. Further, the presence of natural polysaccharide improved the bioavailability of encapsulated ingredient [26]. Xu and co-workers found that the ferulic acid loaded CD–MOF not only enhanced loading but also enhanced bioavailability by two folds as compared to the free ferulic acid suspension [23]. Considering the biocompatibility, antimicrobial nature and ease of preparation, this class of material can be employed in air purification, carrier for drugs and bio-ingredient encapsulation. It is evident from the literature that there are two different perspectives about polysaccharide–MOF. Some scientists believe that the polysaccharide–MOF should be constructed from polysaccharide as one of the building units of MOF which is serving as an organic ligand [10], whereas others perceive it as the polysaccharide based MOF composites wherein the polysaccharide is incorporated as an additional component in the MOF (pre or post synthesis) in order to get additional features such as flexibility, mechanical strength, chemical, and thermal stability [38]. The former emphasizes the polysaccharide–MOF composition as including one of the components derived from a polysaccharide. Regardless of the preparation methodologies, a polysaccharide– MOF has attracted wide attention because of their unique structure, rich supramolecular chemistry, and biocompatible properties. In this review, we have classified the polysaccharide–MOF based on the type of polysaccharide used in MOF preparation either as building blocks or as composite matrix. Over the last 5 years, several polysaccharide–MOFs have been synthesised and used across various sectors ranging from bioremediation to drug delivery system. The applications of these polysaccharide–MOFs are summarized in Table 2. The outline of recently developed polysaccharide based MOF and their applications have been briefly discussed in subsequent sections.

4. Cyclodextrin based MOF Cyclodextrin (CD) is a cyclic oligosaccharide consisting of glucopyranose monomers joined by 1,4-linkage which is obtained by enzymatic hydrolysis of starch. It is commonly categorised as a, b and c-CD depending on the number of glucopyranose units. Due to the chair conformation of subunits, secondary hydroxyl groups extend from the wider edge and the primary groups from the narrow edge resulting into cone like shape of CD molecules [39]. The cavity size of a, b and c-CDs increases with the increasing number of monomer units and form 0.49, 0.62, and 0.80 nm diameter, respectively. Chemically, the outer edge of cyclodextrin is hydrophilic, and the inner cavity is hydrophobic in nature [40]. Taking advantages of the cavity formed by hydrophilic and hydrophobic surface, CD has frequently been used as an effective host in host–guest chemistry where one or more ‘‘guest” molecules can be entrapped within host’s cavity [41]. Among these available CD, c-CD are most interesting bioligands in the construction of polysaccharide based MOFs. These c-CDs are linked by alkali metal ions (mostly potassium ions) to form a symmetrical cyclic oligosaccharide. The six cubes of c-CD are interconnected to four potassium ions which lead to the formation of extended three dimensional (3D) crystalline arrays [12]. The X-ray crystal structure of CD–MOF revealed that each potassium ion forms eight co-ordination bonds embracing two primary hydroxyl groups, two glycosidic rings and four secondary hydroxyl groups [42]. Generally, CD–MOFs are synthesized by mixing CD and alkaline metal cations for 12–14 h incubation at room temperature. In order to make synthetic process rapid, Lui et al. employed microwave assisted MOF methodology. They reported the facile synthesis of cyclodextrin based MOF within a minute under optimized microwave irradiation. The obtained environmentally benign materials are potentially befitting biological, environmental

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S.S. Nadar et al. / Coordination Chemistry Reviews 396 (2019) 1–21 Table 2 Summary of synthesis of polysaccharide–MOF and their applications. Polysaccharide

Metal ion

Polysaccharide–MOF

Application

Remark

Ref.

NH2–CD–MOF 1 CD–MOF 2 CD–MOF CD–MOF 2

CO2 adsorption

Functionalized CD–MOF was incorporated with different metal ion for CO2 adsorption. CO2 adsorption amount was 24 mg-CO2/g-CD–MOF at 30 °C Irreversible binding event with an enthalpy of 113.5 kJ mol 1 CO2 is observed

[19]

K+

FA/CD–MOF

Drug delivery

[110]

K+

c-CD–MOF/VAP

Drug delivery

K+

c -CD–MOF/ FBF

Drug delivery

c-kCD–MOF c-NaCD–MOF c-FeCD–MOF

Drug delivery

The solubility of FA was increased 1450-folds. Bioavailability was enhanced by 1.48-folds as compared to free FA. Encapsulation capacity of c-CD–MOF/VAP was 9.77 ± 0.24 (v/v) and elongated half-life of VAP by 1.6-folds Microwave assisted synthesis of c-CD–MOF/FBF with higher adsorption capability of 196 mg g 1 for FBF in 24 h Long controlled release of anti-inflammatory drug with a maximum of 62% released in 12 h.

PZ/CD–MOF-1. Ibuprofen/CD–MOF CAP /CD–MOF

Drug delivery Drug delivery Drug adsorption

The maximum drug payloads capacity of 23.2 wt% Rapid uptake in ibuprofen blood plasma during in vivo studies The adsorption capacity of CAP was 19.3% (w/w)

[111] [24] [34]

Sulfnamide adsorption Ferulic acid adsorption Formaldehyde adsorption

Rapid adsorption of SAs within 30 min Maximum adsorption of FA without affecting its bioactivity CD–MOF showed High adsorption capacity of 36.71 mg g 1 within 15 min Chemical stability was increased by 3-folds as compared to free curcumin at pH 11.5. The higher loading of 2.67 ± 0.46 wt%.

[56] [23] [28]

High thermal stability of sucralose was achieved

[52] [102] [104]

Cyclodextrin Separation and CO2 K+ Rb+ K+ Rb+

CO2 adsorption CO2 adsorption

[20] [45]

Drug Delivery

+

K Na+ Fe+ K+ K+ K+

Organic Molecule Adsorptions CD–MOF K+ K+ FA/CD–MOF K+ CD–MOF K+

Curcumin encapsulation

SF6/ c-CD–MOF

+

K

CD–MOF-Micro CD–MOF-Nano

Sulfur hexafluoride adsorption Sucralose encapsulation

Cu2+

Cu3BTC2 on chitin

Air filtration

Co2+

DOX embedded CS/Bio– MOF. Chitosan/Cu-BTTri membrane

Drug release

High surface areas up to 800 m2 g 1 with large pore volumes of 3.6 cm3 g 1 pH triggered drug released with efficiency 93%

Nitric oxide (NO) release

65-fold increase in NO generation

[105]

Cu2+

CMC/Cu-MOF@IBU

Drug delivery

[99]

Zn2+

CMC/MOF-5/GO

Drug delivery

Zn2+

CMFP/ZIFs

Dye adsorption

High stability of drug dosing for a long time and a controlled release in the intestinal tract and low toxicity against Caco-2 cells. GO modification with CMC and MOF-5 showed controlled DOX drug release at pH 7.4 The negatively charged dye was efficiently absorbed on CMFP/ZIF-8

Co2+

CP/CNF/ZIF-67

Antibacterial

[87]

Cu Co2+

AgNps@CFs@HKUST-1/CF ZIF-67@PAN

Zn2+

CFs@ZIF-8 filter

Antibacterial Air filtration and formaldehyde adsorption. Filtration and gas adsorption

CNF/ZIF-67 showed 11-folds increasement in the mechanical strength with higher antibacterial property against E. coli Composite was more effective against Gram-positive S. aureus Formaldehyde and PM2.5 removal with efficiency of 87.2% and 84% respectively Nitrogen adsorption was 200 times higher than pure cellulose based filters.

Zn2+

BC@Dopa-ZIF

Iodine adsorption

High iodine uptake capacity from vapor (1.87 ± 0.18 g I2/g) and aqueous I2/KI solution (1.31 ± 0.02 g I2/g).

[92]

Cu2+ Zn2+ Zr4+

Cellulose–MOF199 BC@ZIF-8 aerogel, BC@UiO-66 aerogel

Dye adsorption Metal ion adsorption

Methylene blue adsorption with maximum capacity of 1193 mg/g BC@ZIF-8 showed 81% of Pb+2 absorption

[65] [91]

Zn2+

ZIF@CA

Adsorption of heavy metals

Maximum removal capacity of Cr4+ was 90.8%.

[80]

ZIF-9@GEL ZIF-12@GEL UiO-66@CA UiO-66-NH2@CA PAF-1@CNF

Organic pollutant degradation Heavy metal removal

p-nitrophenol was 90% degraded and recyclability for three times

[81]

UiO-66@CA and UiO-66-NH2@CA showed adsorption capacities of 40.1 mg g 1 and 51.3 mg g 1, respectively for Pb2+ 1000 mg g 1 adsorption capacity shown by composite aerogel with 77.93% removal of BPA within 10 s

[79]

ZIF-8@CNF@Cellulose foam CNF/HKUST-1 Membrane

Gas and heavy metal adsorption Air purification

Cu

Cellulose

2+

Bacterial Cellulose (BC)

Cellulose Aerogels

2+

Co Zr

4+

Ni3+ Cellulose Nanofibers

[27]

Curcumin/CD–MOF

2+

CMC

[25]

+

K

Chitosan

[22]

Zn2+ 2+

Cu

Separation of bisphenol A

30 times higher N2 gas adsorption capacity and 80 times higher compression strength than native cellulose foam Blocking efficiency is 95% of PM2.5 with adsorption capacity of formaldehyde is 47.71 mg g 1

[21] [112]

[26] [83]

[88] [87] [84]

[113] [68] [73]

(continued on next page)

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Table 2 (continued) Polysaccharide

Metal ion

Polysaccharide–MOF

Application

Remark

Ref.

Zr4+

Cellulose nanofibrils/ UiO-66-NH2 Ag-MOFs@CNF@ZIF-8

Gas absorption

Showed CO2 permeability of 139 Barrer with CO2/N2 selectivity ratio of 46. Filtration efficiency of 94.3% for PM2.5

[33]

High functionality shown by hybrid nano-paper in VOC separation over pure filer paper and pure CNF paper

[62]

PCC fillers provided higher surface area for gas absorption

[114]

2+

Zn

Cellulose Paper

Cu2+

CNF@HKUST-1 nanopaper

Cu2+

Cellulose paper@MOF-5

Filtration and antibacterial activity Air filtration Gas absorption

and pharmaceutical applications [25]. Few other researchers developed CD–MOF with different alkali metal cations such as Rb+, Sr2+, and Cs+ in order to engineer the properties of MOF. The tori shape of CD induces supramolecular helicity in the MOF to form 3D frameworks [19]. Although several alkali metal ions have been employed for the formation of MOFs, they were incapable of producing a highly porous crystalline framework. To overcome these limitations and improvement in crystallinity of porous CD–MOFs, researchers demonstrated a synthetic methodology to prepare CD–MOF by co-crystallizing KOH and LiOH with c-CD. The different chemical representation of CD–MOF derived from different metal ions is shown in Fig. 3. This mix-metal MOF synthetic methodology was a promising method for obtaining porous framework [43,44]. The CDs offer interesting inclusion properties due to the presence of intrinsic cavity in CD based MOFs. This property makes it a potential candidate for gas storage and separation of small organic molecules. Considering this, Yan and co-workers synthesised c-CD–MOF by using the traditional method and employed it for CO2 fixation due to the high affinity over other gases. The two possible mechanisms of binding CO2 reversibly within

[67]

CD–MOF was well explained by Gassensmith and co-workers. They proposed two mechanisms (i) the binding of CO2 to vacant coordination sites on metal atoms which gives high selectivity and (ii) weak polar functional groups binding CO2 in a physisorptive manner by means of dipole interactions. The CD–MOF showed strong affinity (nearly 3000-fold affinity) for CO2 over CH4 at low pressures [20]. Wu and group determined adsorption enthalpy by calorimetric methodology (by isosteric heat of adsorption techniques) which provided a quite accurate model. This method essentially differentiates a free energy curve with respect to temperature (Fig. 4a). At near-zero surface coverage, the enthalpies of CO2 adsorption were 113.5 and 65.4 kJ mol 1 CO2 due to the most reactive primary OH groups and less reactive secondary OH groups, respectively (Fig. 4b and c) [45]. In summary, CD–MOF can be implemented as one of the robust materials which shows high adsorption capacity and energetic characteristics for CO2 adsorption and separation. Bioactive compounds derived from vegetables and fruits are becoming popular due to multifarious biological effects and increased awareness regarding health concerns [46,47]. However, these phytochemicals undergo a hydrolytic degradation due to

Fig. 3. The chemical structure of c-cyclodextrin. Crystallization of c-cyclodextrin in the presence of RbOH or KOH (Copyright Ó 2014 The American Chemical Society and Division of Chemical Education, Inc.) [44].

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Fig. 4. (a) CO2 adsorption isotherms and (b-c) corresponding calorimetric traces (at 25 °C) for the first (black) and second (red) CO2 adsorption on the same CD-MOF (Copyright Ó 2013 The American Chemical Society and Division of Chemical Education, Inc.) [45].

instability in neutral and alkaline conditions which ultimately reduces its shelf life [48]. Here, the initial efforts have been taken to incorporate/encapsulate bioactive ingredient into MOF [49]. However, lack of biocompatibility, unfriendly synthetic methods and inherent potential toxicity of MOF are limiting their use in the food sector. In this context, Bio–MOF appeared as an efficient strategy for encapsulation due to attractive bio-related properties. Taking advantage of the cavities and presence of large amount of functional hydroxy groups, a CD–MOF can be developed as encapsulating carrier for various biomolecules [50,51]. Michida and coworkers synthesized cyclodextrin based MOFs with the inclusion of ferulic acid (Fig. 5). Authors explained that ferulic acid was adsorbed inside the pores of CD–MOF by forming an inclusion complex [23]. Similarly, Moussa and colleagues developed a strategy for the assimilation of curcumin within CD–MOFs without affecting physicochemical properties of MOF (Fig. 6). They suggested that the hydrogen bonding between CD–MOF and hydroxyl group of the curcumin helped in entrapping curcumin within CD– MOFs. The stability of curcumin in CD–MOF was evaluated in terms of half-life. The encapsulated curcumin exhibited half-life of 56 h, which was forty folds higher as compared to native curcumin. These outcomes showed that the shelf life of curcumin was noticeably augmented due to unique complex formation between curcumin, K+ ions and CD [21]. In similar way, Zhang and co-workers encapsulated Vitamin A palmitate within CD– MOF which exhibited 1.6 fold extended shelf life [22]. In another study, CD–MOF was used to prevent thermal decomposition of sucralose (an artificial sweetener) by CD–MOFs. The highly porous framework crystals offer a confined microenvironment and shielding around sucralose which could have delayed decomposition at elevated temperatures [52]. It is worth to note that the polysaccharide–MOF can be used for protecting bioactive molecules from oxidation. In a nut-shell, encapsulation properties and shielding effect of CD–MOFs holds promising opportunity in the field of food and pharmaceutical industries. Also, CD based MOF was employed as solid-phase extraction (SPE) medium for the enrichment of molecules. In a typical procedure, the sorbents are used for the separation and preconcentra-

Fig. 5. Schematic representation of crystal growth of FA/CDMOF (Ó2015 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim, reprinted with permission) [23].

tion of different analytes from large reaction volumes. It has received widespread attention due to its simplicity, high enrichment factor, good recovery, use of the small amount of organic solvents and easily automate the whole process [53]. Sulfonamides (SAs) are synthetic (nonantibiotic) antimicrobial agents commonly used in poultry breeding. The long-term exposure of SAs can cause toxic effects [54,55]. For the first proof of principle studies, Li and co-workers used CD–MOF as a media for sulphonamides in the process of SPE. CD–MOF showed high selectivity and affinity towards SA which resulted in the satisfactory recovery of SA

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Fig. 6. Entrapment of curcumin within CD–MOF (Copyright Ó2016 Elsevier B.V. All rights reserved, reprinted with permission) [21].

(76–102%) within 30 min under optimized conditions [56]. Further, the application of CD–MOF is extended in the sector of air purification. Formaldehyde is one of the major indoor pollutants which causes adverse effects in the gastrointestinal tract and exhibits respiratory disorder [57]. Wang and group found that the formaldehyde was selectively adsorbed with rapid speed and high capacity with the help of CD–MOF. The selective adsorption was attributed to the host–guest interaction within cavities and hydrogen bonding of hydroxyl group from CD–MOF, and formaldehyde. The different types of CD–MOF derived from different CD were implemented. Among different CD–MOFs, c-CD–MOF showed excellent adsorption capacity which was ten folds higher than that of activated carbon due to synergic effect of various interactions and host–guest chemistry. These results indicate that the CD– MOF is suitable for the effective extraction, separation, and adsorption of different organic molecules from complicated matrices [28]. In biomedical domain, the drug carrying system is important to deliver drug to the target area under physiological conditions. CD– MOF has been successfully used as a carrier for drugs as it possesses large surface areas, high biocompatibility and exhibits low toxicity [58]. Few researchers analysed preliminary safety profile of c-CD–MOFs and implemented them in HepG2 (human hepatoma) and Caco-2 (human epithelial colorectal adenocarcinoma) cells. The toxicity study demonstrated that CD–MOF did not induce any toxicity up to 2000 lg mL 1 concentration in these cells [27]. Further, Bernini and co-workers studied different bio-compatible MOFs derived from different metals as a drug delivery carrier. In this study, the authors tested MIL-53(Fe), MIL-100(Fe), MIL-101 (Cr) and CD–MOF for their adsorption capacity and determined the thermodynamic parameter for the model drug ibuprofen (IBU). Amongst the tested MOFs, CD–MOF was found to be the potential drug delivery system due to the presence of cations in the pores that reinforce the interactions with the IBU molecules and help in controlled release and improved pharmaco-kinetics as compared to other MOFs. Remarkably, CD–MOF showed outstanding drug carrying capacity due to the presence of strong electrostatic interactions. Also, the controlled drug release was attributed to the presence of polar groups and metal sites [59]. This study was further extended by Hartlieb and group wherein they studied the effect of c-CD–MOF in the bioavailability of IBU. The testing was conducted on mice and compared with IBU in the presence and absence of c-CD–MOF. From these animal studies, it was observed that the IBU/CD–MOF exhibited rapid uptake and a longer half-life in blood plasma samples. Also, it showed intrinsically less hygroscopic as compared to pure form. The drug carrying capacity is dependent on physiochemical properties of CD–MOFs which was tuned by varying operating conditions, c-CD to KOH ratios, reactant and surfactant concentrations. It was found that the most of drug molecules containing COO– groups showed relatively high adsorption within CD–MOF, while low adsorption

capacity was observed for drugs with nitrogen-containing heterocyclic rings [60]. In one of the studies, the binding mechanism of drugs within CD–MOF was explained with the help of computer based molecular docking. In the docking model, the authors illustrated that drugs readily form hydrogen bonds and electrostatic interaction between different groups present on drug and CD– MOFs [25]. In summary, CD–MOF is the suitable candidate for the development of formulation of poorly water soluble drugs. Additionally, they improve stability, bioavailability, shelf life and protect the API. This makes CD–MOF, a promising new tool for drug delivery. One of the biggest hurdles in the application of CD–MOF materials is their stability under aqueous and humid conditions. To enhance the water stability, Hartlieb and co-workers came up with the novel strategy of post synthetic modification (PSM) of parent CD–MOF. The authors functionalized CD–MOF with an amino group through Mitsunobu reaction. The single-crystal superstructure reveals the inclusion of functional unit within CD resulting in hydrophobic nature of MOF and improves the stability in an aqueous environment. The prepared functionalized CD–MOF was also used for capturing CO2. The authors observed that the functionalization of MOF not only improved the stability under humid condition but also increased the affinity for CO2 under low pressure [60]. A bottom-up approach to the preparation of nano- and microsized cubic gel particles with distinct shapes and sizes from CD– MOFs that can act as latent drug carriers and cell-support material was illustrated by Sada and co-workers. Cubic gel particle was successfully prepared by authors where c-CD–MOF-1 transformed into gel particle through internal cross-linking of c-CD by bifunctional epoxide (Fig. 7). c-CD–MOF-1 retained the fundamental basic cubic shape even after the removal of co-ordinated metal ions and additionally stabilized with CTAB. Water stability experiment showed the excellent stability of cross linked CD–MOF than that of unmodified CD–MOF as it was dissolved within a minute [42]. Another strategy reveals that the co-ordination bond between the CDs and the metal ions using functionalized CDs strengthens the strong co-ordination supramolecular network containing CDs. For example, 2D cyclodextrin-metal co-ordination polymer was formed in which each functionalized c-CD ligand is co-ordinated to eight Cu2+ ions by eight sulfurylpropionate arms. This leads to the formation of 2-fold-interpenetrated layers running along the ab plane. PLATON calculations approximated the probable guest accessible volume per unit cell volume. It showed that 41% free volume was filled with highly disordered water molecules [61]. In the recent study, Li and team developed hybrid materials through the incorporation of hydrophobic fullerene (C60) within c-CD–MOF through a facile co-incubation process. In this process, the relatively smaller C60 (0.7 nm in diameter) was allowed to access the open pores of a c-CD–MOF and formed a highly hydrophobic c-CD–MOF/C60 composite. The water-stability of

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Fig. 7. (a) Schematic representation of synthesis of cubic gel particles of CD–MOF (b) TEM and (c) SEM image of CD–MOF (Copyright Ó 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) [42].

composite before and after C60 incorporation was assessed using confocal microscopy. Surprisingly, c-CD–MOF/C60 crystals retain their shape for over 24 h, while, the bare c-CD–MOF crystals were completely dissolved in a few seconds in aqueous environments. Further, doxorubicin, as an anticancer drug, was entrapped within the composite and drug release profile was studied. It showed a slow drug release profile with 49.2, 78.5 and 92.2% release within 3, 6 and 18 h, respectively [62]. In the development of greener hybrid materials, undoubtedly, CD–MOF is a promising candidate for a wide range of applications. The combination of CD and MOF together can pave the way towards novel superstructure and unexplored applications. 5. Cellulose based MOF The integration of MOFs with functional materials is an outstanding approach to produce advanced materials having appropriate and engineered properties. Cellulose, an abundant natural material is a promising candidate to form composites with MOFs. Also, it is a low cost, renewable, biodegradable, material having good processability and easy recyclability [63]. Kim and coauthors explained about interactions between cellulosic material and MOFs in their recent review article. The presence of hydroxyl groups of cellulose undergoes chemical modifications that help in the incorporation of MOF on the surface of cellulose. The presence of cellulose provides some additional features such as low density, hydrophilicity, biodegradability and significantly higher surface area compared to pure MOF. Also, the formation of flexible bonds makes the composite mechanically robust for various environmental remediation applications [38]. The pollution of air and water is a serious global concern with respect to environment as well as human health. Hence, major steps have been taken to reduce pollution. Most of the current environmental remediation solutions are either chemically or physically driven depending on adsorbent materials. However,

they are limited due to the costly raw materials, tedious preparation method and low efficiency [64]. In this regard, Rickhoff and co-workers prepared biomimetic composite material composed of cellulose and HKUST-1. Then, it was used for the adsorption of textile dye methylene blue which is an aqueous organic pollutant. The biocomposite exhibited excellent adsorption capacity (1193 mg g 1) which followed Langmuir–Freundlich isotherm adsorption model. In another example, the zeolitic imidazolate framework (ZIF-8/cellulose) composite materials have been employed for the removal of toxic gases and particulate matters (PMs, diameter 2.5 lm) to reduce the air pollution [65]. Cellulose nanofiber (CNF) as a green building block for reinforcing has attracted ever-increasing interest because of some exceptional properties such as high mechanical strength and good biocompatibility [66]. Considering this, Ma and co-workers prepared a green cellulose based air filter (Ag-MOF@CNF@ZIF-8) and reinforced the prepared air filter with CNF. The mechanism of air filtration is shown in Fig. 8. The prepared air filters showed a filtration efficiency of 94.3% for PM2.5. The high filtration efficiency was due to the presence of MOFs which could significantly enhance the specific surface area and improved interactions between the CNF and PMs. Additionally, these air filters exhibited excellent antibacterial activity because of the presence of active metal (silver and zinc) ions in MOFs and the strong interfacial bonding with CNF. Thus, multifunctional cellulose–MOF incorporated air filter can be used as a stand-alone to remove PM2.5, adsorbing toxic gases and achieve a healthier indoor living environment and health security fields [67]. Besides the antimicrobial properties, mechanical strength is an important factor that needs to be considered in the case of air filters. For improving mechanical properties, the same group employed low density porous cellulose based ZIF-8 foams for multiple environmental remediation application. In the synthetic procedure, zinc ions were adsorbed and bound to cellulosic surface via ionic interaction and hydrogen bonding. It formed the nucleation sites for the generation of ZIF-8 crystals. Further,

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Fig. 8. Schematic representation of filtration mechanism by Ag-MOFs@CNF@ZIF-8 filter Copyright Ó2018 Elsevier B.V. All rights reserved, reprinted with permission) [67].

cellulose nanofibers were employed to cross-link ZIF-8 which augmented mechanical properties i.e. robustness and compression strength (80% higher than native cellulose, Fig. 9) as compared to the native cellulose foam. The hybrid composite foam also showed outstanding adsorption ability in the removal of rhodamine B dyes (24.6 mg g 1), Cr4+ ions (35.6 mg g 1) and organic DMF solvents (45.2 g g 1) [68]. The separation of conventional cellulose based MOF is complicated due to its high dispersion and low density. Considering this hurdle, Wang and group prepared novel cellulose nanocrystal MOF composite material containing functionalized magnetic nanoparticle. In the synthetic procedure, cellulose coated magnetic nanoparticles were mixed with components of Zn-based MOF to form magnetically active cellulose nanocrystal (CNC)/MOF composite (Fig. 10). This novel material was utilized for the removal of Pb2+ from water. According to the adsorption data, the composite achieved maximum adsorption capacity (558.66 mg g 1) within 30 min and followed pseudo-secondorder kinetics. Authors further regenerated the composite with acid (mostly diluted HCl) and recycled up to 5 cycles without affecting its adsorption capacity [69]. Particulate matter (PM) is a key indicator of air pollution brought into the air by a variety of natural and human activities. As it can be suspended over a long time and travel over long distances in the atmosphere, it can cause a wide range of diseases that

Fig. 9. (a) Photograph of ZIF-8@CNF@cellulose foam, (b) The adsorption process of rhodamine B on ZIF-8@CNF@cellulose foam (c) Compression performance of ZIF8@CNF@cellulose (Copyright Ó2018 Elsevier B.V. All rights reserved, reprinted with permission) [68].

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Fig. 10. Schematic of fabrication of a magnetic cellulose nanocrystal/MOF composite for removal of Pb2+ (Copyright Ó 2017 The American Chemical Society and Division of Chemical Education, Inc.) [69].

lead to a significant reduction of human life [70]. To control the PMs and volatile organic compounds (VOCs), significant efforts have been taken in this field. Usually, in an air purifier, a highefficiency particulate air (HEPA) filter and an activated carbon layer are used to remove PMs, while the activated carbon layer removes VOCs [71]. However, they have several drawbacks such as large size unit, large pressure drop and high cost [72]. Therefore, there is a need to develop a novel approach for mitigating PMs and VOCs. Zhao and co-workers prepared hierarchical porous CNFs stringed HKUST based air purifier (Fig. 11) for formaldehyde and PM2.5 which is the most lethal indoor pollutant. The cellulose nanofiber

coated HKUST-1 membrane was used as a window screen for air purification. The porous structure of membrane effectively blocked above 95% of PM2.5. Also, the ordered micro-porous cavities with open copper active sites could efficiently adsorb formaldehyde in the nominal air flow rate which was enough for the indoor activity [73]. In another example, Zhou and co-workers fabricated cellulose nanofibers with MOF in the presence of polyvinylpyrrolidone (PVP) via template-assisted synthesis method in order to remove VOC from air. In this process, PVP on account of its amphiphilic nature acts as an interfacial stabilizer between the CNFs and the MOF layers. The prepared multi-layered CNF@MOF showed great

Fig. 11. Schematic representation of preparation of porous CNFs/HKUST-1/stainless steel screen (Copyright Ó2018 Elsevier B.V. All rights reserved, reprinted with permission) [73].

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processing stability in the construction of efficient air filters. This composite was further implemented for the removal of VOC and was compared for its performance with normal filter paper and pure CNF paper. The normal filter paper and pure CNF paper showed an extremely weak performance for VOC separation and remained functional up to 5 h, whereas, the hybrid paper comprising MOF particles loaded on CNFs showed improved functionality up to 12 h for VOC separation [74]. In the recent times, Zhang and co-authors prepared nanofibrous MOF filters for the removal of PM2.5 and PM10 particulate matters from the air with the efficiencies of 88.33 and 89.67%, respectively, in the hazy environment and it remained unchanged over 48 h of continuous filtration. Such kind of membrane had an immense potential for indoor air purification even during hazy weather. UiO-66, containing triangular pores of 60 nm, is an attractive alternative for membrane composites in the separation of carbon dioxide and methane. The affinity for CO2 can be potentially enhanced by appropriate functionalization. However, low polymer–MOF stability, interface defect, and gas leakage issues are prevalent [75]. In this context, Xiong-Fei and co-workers prepared membrane composite by a combination of functionalized UiO-66 and CNF. The amide group from MOF and carboxyl group from CNF can be anchored easily through acid-base interaction. The MOF/CNF hybrid membrane composite showed CO2 permeability of 139 Barrer with high CO2/N2 selectivity under optimized separation condition [33]. The chemical surface modification of membrane component is considered as critical aspect governing the CO2 transport behavior. This discovery spurred significant research interest in the separation of CO2 from various gases. These studies unveiled the role of polysaccharide based MOFs in the separation of CO2. The combination of two emerging materials i.e. cellulose and MOFs into multifunctional aerogel (cellulose aerogel, CA) can be one of the potential approaches for environmental remediation application [76]. There are two different synthetic approaches to prepare hybrid CA containing MOFs: (i) Pre-formed MOFs are mixed with CNCs to form functional hybrid CA [31]. (ii) Biomineralization wherein MOFs directly grow on the CNCs in one step [77]. The resultant hybrid CA has a high porosity and good adsorption capacity [78]. Recently, Lei and group fabricated UiO-66 and UiO66-NH2 onto flexible cellulose (as a template) to get highly porous aerogel. The authors prepared it by soaking CA in metal ion precursor and organic ligand and then, allowed to grow MOF crystals on CA. The performance of hybrid aerogel was checked by determining the adsorption capacity of Pb2+ metal ions from water. The UiO-66@CA and UiO-66-NH2@CA showed adsorption capacities of 40.1 and 51.3 mg g 1, respectively. Moreover, the maximum decomposition temperature of composite was higher than the native component which suggests that it could not form secondary pollutant during heavy metal removal process [79]. Similarly, Bo and co-authors loaded ZIF-8 onto cellulose aerogel to remove Cr4+ from water. The prepared 30% ZIF-8@CA composite showed excellent physical and surface properties, with high surface area as compared to individual components. The heavy metal removal ability of hybrid aerogel was reported to be 90.8%, and adsorption capacity reached up to as 41.8 mg g 1 [80]. In another example, Ren and co-workers loaded ZIF-9 and ZIF-12 on cellulose aerogels to prepare hybrid aerogels. Further, these were employed as metal catalysts for the activation of peroxymonosulfate (PMS) to aid in degradation of p-nitrophenol (PNP). The degradation mechanism was explored by using electron paramagnetic response method. The results showed that the PMS was activated by hydrogel and released sulfate and hydroxy radicals (Fig. 12). These free SO4 radicals played an important role in the degradation of organic pollutant PNPs. The hybrid aerogels could almost completely degrade PNP within an hour. One of the major advantages of hybrid aerogel is easy recyclability. In the recyclability test, the hybrid aerogels

Fig. 12. Schematic representation of mechanism of ZIF@GEL/PMS system via free radical generation (Copyright Ó2018 Elsevier B.V. All rights reserved, reprinted with permission) [81].

showed outstanding degradation performance even after 3 cycles. [81]. These studies demonstrated the catalytic potential of CA for advanced oxidation processes. Filter paper is typically composed of cellulose fibers containing hydroxyl groups which can be combined with MOFs to produce paper based hybrid materials [82]. However, the hydrogen bonding between cellulose fibers can reduce active hydroxyl groups, thus lowering the binding efficiency of MOFs onto cellulose fiber. Hence, few researchers have targeted surface modification of cellulose filter paper to increase the binding efficiency of MOFs. The two different strategies used for surface modification are (i) carboxymethylation and (ii) inorganic filler. Park and group used carboxymethylation method to functionalize the filter paper (CMFP) by base catalyst sodium chloroacetate (Fig. 13). In this process, CMFP was immersed in a methanol solution of MOF counter parts. The surface carboxylate group initiated the co-ordination bonding with metal ions and facilitated the growth of MOF which resulted in the uniform layer formation of ZIF-67/ZIF-8 onto cellulosic paper (referred as CMFP/ZIF). Further, MOF filter paper was employed for selective capturing of the anionic methyl orange dye through an ionic interaction between positively charged ZIFs. In the end, the prepared CMFP/ZIF was recycled up to three cycles and regenerated via washing process. The CMFP/ZIF exhibited excellent dye capture ability even after three recycling [83]. In another approach, Su and colleagues used an inorganic filler (precipitated calcium carbonate) to modify the surface hydroxyl groups of cellulose filter paper. The presence of calcium carbonate can cleave cellulose fiber bonding and expose the hydroxyl groups, thus boosting chemical modifications. Then, the exposed hydroxyl groups of cellulose fiber were bound with MOF-5 via ester bonding. In the end, prepared cellulose paper@MOF-5 was used to separate and adsorb different gases (N2, CH4, H2, CO2, etc.) [84]. Although the functionalization of cellulosic material improved the binding of MOF on its surface, the process of chemical modification is quite complicated which limits its large scale application. Cellulose paper has been demonstrated as a suitable alternative to medical packaging material due to its good biocompatibility and biodegradability [85]. However, its poor mechanical properties and lack of antibacterial properties have restricted its application in this field. Currently, the antibacterial modification property is imparted on paper based materials by surface grafting of antibacterial functional groups and loading metal nanoparticles on the material. Wang and co-authors reported a novel strategy of fabricating the sustainable cellulose based composite via coating and cross-linking method. They have prepared copper based

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Fig. 13. (a) Schematic representation of synthesis of ZIF-8/67 doped carboxymethylation of filter paper (CMFP/ZIF-8/67) (b) Elemental mapping images of CMFP/ZIF (Copyright Ó2017, Royal Society of Chemistry, reprinted with permission) [83].

MOFs/cellulose fiber composite via in situ green technique and tested for antimicrobial activity against Gram-positive S. aureus and Gram-negative E. coli. The composite was found to be more effective against S. aureus as against E. coli. The cytotoxicity against these microbes was attributed to the presence of Cu2+ ions in MOF. These ions were speculated to alter transmembrane potential thereby affecting growth and reproduction of the bacteria [86]. In another approach, Qian and co-workers prepared biodegradable composite with good mechanical and anti-microbial properties. It was carried out in two steps (Fig. 14): (i) functionalization of cellulose nanofibers with sodium carboxylate and cross-linked with epichlorohydrin, (ii) grafting pre-synthesized ZIF-67 MOF onto

cellulose materials. Mechanical properties of hybrid cellulose paper were determined in terms of stress–strain curves (tensile strength, elastic modulus), folding endurance (flexibility) and tear index (tear resistance). It was observed that all the mechanical properties improved many folds after deposition of ZIF-67 onto cellulose paper. The antibacterial behavior of composite was studied in terms of zone inhibition test. The inhibitory zone was observed for composite against E. coli, while, cellulose paper did not show any inhibition zone [87]. In order to obtain higher antimicrobial activity, Duan and co-helpers used cellulose fibers as a matrix for the immobilization of silver coated HKUST via microwave assisted reduction. In prepared hybrid cellulose fiber

Fig. 14. Schematic illustration of the fabrication process of CP/CNF/ZIF-67 (Copyright Ó2018 Elsevier B.V. All rights reserved, reprinted with permission) [87].

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material, the HKUST acted as host/stabilizer for formed silver nanoparticles which improved the stability and dispersity. The augmented antimicrobial activity was mainly attributed to three reasons: (i) presence of uniformly dispersed silver nanoparticles, (ii) adsorption properties of HKUST on bacteria and (iii) intrinsic antimicrobial properties of HKUST due to the presence of Cu2+ [88]. This sustainable and biodegradable MOF doped paper based material would be a promising candidate in various applications such as food and medicine packaging, health and medical services. The functionalization and chemical modification of native CNF is a complicated process. Hence, few researchers came up with the alternative class of cellulose, i.e. bacterial cellulose (BC) which is an excellent host and platform for immobilizing guest materials [89,90]. Also, it is a low cost, low density, and commercially available porous material. In a facile method reported by Xiaoting Ma and co-workers, the authors have prepared BC aerogel composite with two different MOFs; ZIF-8 and UiO-66 (Fig. 15). BC template not only improved porosity, mechanical flexibility of aerogel composite but also significantly prevented aggregation of MOFs. Owing to the intrinsic characteristics of MOF and BC, obtained sponge like aerogel exhibited high hierarchical porosity (3.6 nm pore size), large surface area (636.1 m2 g 1) and high mass transfer efficiency. Further, this sponge was employed for the removal of heavy metals (Pb2+ and Cd2+). At equilibrium condition, the sponge displayed

adsorption capacity of nearly 390 and 220 mg g 1, respectively for Pb2+ and Cd2+ [91]. In another example, Au-Duong and coworkers produced BC form Acetobacter xylinus which has highly hydrated and randomly webbed nanofiber structure. The excellent mechanical and chemical stability of BC pellicle makes a promising platform for deposition of MOFs. When Zn+ ions were soaked with BC, it created complex interaction near cellulose surface which induced MOF formation on the surface (Fig. 16). The authors evaluated its application for iodine capturing. At room temperature, the capture capacity of iodine was found to be 1.87 g g 1 of ZIF8-BC. This composite was reused for six cycles and regenerated by ethanol followed by heating at 150 °C after each cycle. The composite retained around 87% capacity after 6th cycle [92]. The interaction of polar and hydrophilic cellulose material with nonpolar and hydrophobic materials (MOFs) is inadequate which limits the proper binding of MOF on the surface of cellulose. In order to improve the adhesive properties of cellulose and ensure the growth of MOF crystals, the hydrophobicity of cellulosic material should be increased by surface modification. Zhu and coauthors reported the chemical attachment of MIL-100 (derived from ferrous) onto hydrazide modified cotton fibers. They noticed that the deposition of MOFs on cellulosic materials depends on the degree of surface functionalization. The hybrid aerogel was evaluated for the removal of heavy metals (Cr4+) from water. The hybrid

Fig. 15. (a) Schematic representation of BC@ZIF-8 composite aerogel sponges (b) Photograph of the composite aerogels (Copyright Ó2019 Elsevier B.V. All rights reserved, reprinted with permission) [91].

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Fig. 16. (a) The preparation of metal–organic framework-bacterial cellulose BC@DopaZIF nanocomposite (b) FE-SEM images of nanocomposite (Copyright Ó 2017 The American Chemical Society and Division of Chemical Education, Inc.) [92].

Fig. 17. (a) Synthetic procedure of MOF 5 around cellulose based fibers (b) SEM images of MOF-5@cellulose fibers with necklace like morphology (Copyright Ó 2019 The American Chemical Society and Division of Chemical Education, Inc.) [93].

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aerogel showed high efficiency (85% in 24 h) for the removal of Cr4+ from water [31]. The surface modification of MOF/cellulose with hydrophilic coating is another approach to improve the integration of polysaccharides and MOFs. Mirkovic and co-workers used dopamine (DA) to improve the hydrophilicity of MOF. Here, the cotton was dipped in DA solution and immersed in the MOF-5 mother solution of zinc and terephthalic acid. The DA was polymerized on the surface of cotton under alkaline conditions which provided nucleation site and facilitated the attachment of MOF-5 crystal seeds via metal-catecholate co-ordination complex formation. The grown MOF-5 crystals formed stable necklace like morphology around the fibers (Fig. 17). The obtained hybrid cotton presented exceptional stability under humid conditions because of the strong co-ordination binding between crystals and fibers [93]. Further, the application of polysaccharide–MOF composites was extended in the field of controlled drug delivery and biomedical applications. The controlled-drug release system is an efficient technique in the pharmacology [94,95]. The major benefits of cellulose based MOF in drug delivery systems are the biocompatibility, inertness, good mechanical strength and the ability to achieve a high drug loading [96]. There are three main strategies for controlled drug release: (i) drug-matrix- interaction-controlled drug release, (ii) coating-controlled drug release, and (iii) cationtriggered drug release [97,98]. Although MOF based carries having a good safety and easy preparation method, the controlled drug release system is needed to avoid the burst release. The pHsensitive biopolymer (carboxymethyl cellulose, CMC) has been applied by Javanbakht and group. They prepared a controlled drug delivery system based on Cu-MOF. The drug carrier was synthesised using copper salt and terephthalic acid in the presence of IBU under biocompatible condition (Fig. 18). The various interactions such as hydrogen bonds, p-p interaction, co-ordination bonds and ionic interactions can help to hold drugs within MOF based delivery system. Further, this drug carrier was protected with pH-sensitive carboxymethyl cellulose which facilitated controlled drug release. In drug release studies, 70% of drug was released in

8 h in a controlled manner. It is worth to note that the pHsensitive CMC offered the diffusion barrier during drug release [99]. The same research group further introduced green synthesis of bio-nanocarrier for drug delivery system. The authors used one-pot synthesis process, wherein graphene oxide (GO) was modified by the addition of zinc based MOF-5 and CMC. The synthesized bio-nanocomposite was further used for encapsulation of doxorubicin (DOX) as the model drug. In atomic force microscopic image, it was seen that the prepared carrier was of spongy nature with the thickness of 80 nm. The cytotoxicity of prepared material was calculated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetra zolium bromide (MTT) assay which showed higher cytotoxicity to cancer cell line (K562) because of positive charge of nanocarriers. The drug release studies were carried out at pH 7.4 and pH 5 under normal physiological conditions. The controlled drug release was observed at pH 5 as compared to pH 7. This result showed that these carries were favourable to target the low pH intracellular organelles which subsequently boosts the cytotoxicity to melamine cells [26]. The further development of biocompatible hybrid material as a delivery system is needed for effective dose delivery to the targets and avoid the intrinsic drawbacks of conventional therapeutic drugs. Over the years, molecular versatility and excellent features of cellulose–MOF have been demonstrated with their applicability in the sectors ranging from environmental remediation to drug delivery. Also, the predictable structure and flexibility of MOF material can be applied in several commercial fields. However, further research is needed to explore the applications of cellulose– MOF. 6. Chitin and chitosan based MOF Chitin, poly (b-(1-4)-N-acetyl-D-glucosamine), is a second largely available natural polysaccharide which is chemically and mechanically stable under harsh conditions. Structurally, it possesses a high content of hydroxy and acetamido groups which

Fig. 18. Schematic of encapsulating Cu–MOF@IBU with carboxymethylcellulose and IBU release from CMC/Cu–MOF@IBU (Copyright Ó2018 Elsevier B.V. All rights reserved, reprinted with permission) [99].

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makes it a suitable adsorbent in various applications. Three dimensional fibrous network of chitin can be a good matrix for MOF deposition through electrostatic interaction and hydrogen bonding [100,101]. Wisser and group prepared biological chitin–MOF composites of copper as a metal ion and benzene-1,3,5-tricarboxylic acid (BTC) as an organic cross-linker. The composite showed a remarkably high surface area up to 800 m2 g 1 with pore volumes

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of 3.6 cm3 g 1. The prepared chitin–MOF composite material was tested for the filtration of industrial chemicals. To test the suitability of composite, ammonia was passed through the column of composite materials and break-through curves were recorded. It was seen that chitin–MOF composite exhibited 40% higher ammonia adsorption capacity as compared to pure copper based MOF. The augmented gas adsorption was due to the highly porous network

Fig. 19. Schematic diagram of the DOX@CS/Bio–MOF formation and its drug release behaviour (Copyright Ó 2019 The American Chemical Society and Division of Chemical Education, Inc.) [104].

Fig. 20. (a) Schematic illustration of the fabrication of ZIF-8/C3N4 heterostructure containing hybrid aerogels by sol-gel process, (b) Photographs of the contaminated congo red (50 ppm) aqueous solution and (c) Visible light triggered the regeneration and continuous enhanced removal of CR (Copyright Ó 2017 The American Chemical Society and Division of Chemical Education, Inc.) [108].

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of chitin which acted as transport pores [102]. This approach can help to build air filtration assembly and extend their application in the segment of pollution remediation. Chitosan, the biopolymer is derived from N-deacetylation of chitin. This polymer is highly biodegradable, water soluble and nontoxic in nature. Chitosan has a cationic character because of its primary amino groups. The presence of amino groups is responsible for properties such as controlled drug release, mucoadhesion, in situ gelation and transfection [103]. In the preparation of drug carrier system, the stability of MOF under acidic conditions is a major concern. Considering this problem, Abazari and co-workers designed more stable MOF/chitosan (CS) hybrid materials to be used as a drug delivery system. The authors have synthesized CS/Bio–MOF by solvothermal method followed by chitosan capping. The drug, DOX, was loaded onto the synthesized CS/Bio–MOF by adding it in the aqueous solution of composite. The presence of monodispersed chitosan led to the formation of pH-responsive and target-selective drug carrier. The pH-dependent release mechanism of DOX is shown in Fig. 19. It was found that the release behavior of DOX in the acidic solution was significantly faster than the basic conditions. Moreover, the MTT assay and the trypan blue test were done to confirm its biological compatibility. The fluorescence microscopy studies showed that CS/Bio–MOF carrier significantly improved cellular drug uptake [104]. In a similar way, Neufeld and co-workers prepared MOF/chitosan hybrid materials and further employed it to accelerate formation of the therapeutically active molecule nitric oxide (NO) from S-nitrosothiols [105]. The three dimensional networks of chitinaceous material could provide hierarchical porosity as well as robustness to MOFs. The drug carrying efficiency and gas adsorption ability of the composite indicates their high potential application in medical science and filtration units. The integration of functional chitosan/chitin might support the development of future MOF-based biomaterials. 7. Other polysaccharides based MOF Agar, economically affordable biomass, is a family of linear galactan polysaccharides obtained from the cellular walls of red seaweeds, Rhodophyceae, typically used as a solid substrate for microbiological culture. [106,107]. So far, the regeneration of MOF composite after the application is difficult. Additionally, the leaching of active/guest material is another challenge. Taking advantage of modified aerogel, Zhang and co-workers proposed a new and striking approach in which carbon nitride nanosheets (C3N4, as a photocatalyst) were encapsulated with ZIF which was further integrated into agar aerogel (Fig. 20). The external framework around C3N4 ensures the photocatalytic activity and agglomeration. This advanced hybrid nano-formulation was used for adsorption of congo red (CR) dye. At equilibrium condition, the CR uptake capacity was reported to be 287.35 mg g 1. The combined photo-catalytically active MOF can be easily regenerated under the visible light. Impressively, under the visible light, hybrid aerogel removed the adsorbed dye within 50 min due to photocatalysis. This study showed that the integration of photocatalytic and MOFs based hybrid aerogel system can be an effective system for water purification [108]. This high performance hybrid aerogel can be economical and sustainable solution for environmental remediation due to ease of regeneration. Liang and co-authors introduced dextran for the biomimetic mineralization of MOFs. The abundant hydroxyl groups on dextran helped to form co-ordination interaction with Zn2+ ions and provided local nucleation sites for ZIF-8. The morphological features of dextran–MOF can be tuned by controlling biologically induced mineralization process (Fig. 21). Notably, biopolymer can also trigger the formation of MOF and form the coating around the

Fig. 21. (a) Schematic illustration of biomimetic mineralization of ZIF-8 particles, (b) a SEM images of ZIF-8 particles on cotton fibers. (c-f) Energy dispersive X-ray spectroscopy (EDS) elemental mappings of ZIF-8 on cotton fibers. (Copyright Ó2017, Royal Society of Chemistry, reprinted with permission) [109].

bio-fiber without need of any prior functionalization. Further, the dextran encapsulation in the ZIF-8 particles was confirmed by confocal laser scanning microscopy [109]. This facile and versatile biomimetic technique can lead to a new path for engineering the new Bio–MOF composite materials. In a nutshell, this crystalline and porous material offers a high degree of structural and functional tunability through varying organic (polysaccharide) and inorganic (metal ion) counter parts. The ultimate goal in the incorporation of different polysaccharide should be to bridge green synthesis of MOF with green application to accomplish the greatest possible environmental impact of these hybrid materials. 8. Future scope and summary Polysaccharide based MOF has become an emerging class of hybrid materials. The integration of saccharides within the MOF structures not only improves the structural robustness/flexibility but also improves physico-chemical properties of these materials. Additionally, polysaccharide–MOFs are biocompatible and provide

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mechanical strength to the material. In an engineered architecture of MOF, the selection of ligand plays a significant role in manipulating size, shape of a cavity and the construction of functional MOFs. It is completely dependent on the selection of appropriate polysaccharides based on charge, geometries, subunits, and functional groups and bio-active sites. These properties of polysaccharide–MOF can help to extend its application in the wide sectors such as catalysis, drug delivery, environmental remediation, air filters and so on. Although considerable progress has been made in the preparation techniques of polysaccharide–MOF, numerous challenges are yet to be addressed with respect to its stability and applications. These challenges include (i) difficulty in synthesizing ordered crystalline materials and to predict its global topologies due to low symmetry of bio-ligands. (ii) Complications in maintaining porosity and the open active site in polysaccharide–MOF. (iii) Achieving the stability under aqueous (relatively humid) condition. (iv) Restricted bio-based application on account of its rapid destruction under biological environments of phosphate due to strong co-ordination equilibrium between metal ions and organic ligands. (v) Ineffective biodistribution in vitro and/or in vivo in real biomedical applications due to machinability, dissolvability and low dispersion. The aforementioned challenges can be overcome by pre or post synthetic modification (PSM) of polysaccharide based MOF. PSM has become a very useful strategy in systematic functionalization of MOFs by modifying the linker, metal node and surface environment for the purpose of increasing the structural stability (i.e. chemical, thermal and water stability) but also introducing desired properties (i.e. endow catalytic active group, creating catalytic site, modification of surface, engineering porosity and pores). There are three possible PSM strategies proposed: (i) Pre-synthetic chemical modification of linker (such as functionalization of polysaccharide cross-linker) (ii) Post-synthetic modification of polysaccharide–MOF (such as covalent modifications, dative and post-synthetic deprotection) (iii) Post-synthetic elimination and installation (such as exchange of metal ions or ligands). These PSM strategies can be carried out in a single step or combination of multi-step reaction. The desired structural and functional polysaccharide–MOF can be achieved by the integration of different PSM tools. The selection of specific PSM for polysaccharide– MOF would be dependent on various structural factors including part or whole of the framework, interior or surface of the crystals, aperture, shape, size of the pores, framework rigidity and flexibility. Though there are limited reports in the literature on PSM of polysaccharide–MOF; it has an immense potential to solve those quite complex challenges in their applications. The functionalized polysaccharide–MOFs can be a great platform for immobilization of various enzymes and noble metal catalysts in synthetic chemistry. Additionally, the presence of multiple interactions with the guest molecule makes it an exceptional matrix for accumulation/encapsulation of molecular and protein banks. We believe that polysaccharide–MOFs have a promising future in bio-electrochemistry, bio-nanotechnology, nanomedicine, and material sciences. There are many questions that need to be addressed, but still, it can be anticipated that polysaccharide–MOFs can become an asset in the field of smart materials.

Acknowledgments The authors would like to acknowledge the University Grants Commission (UGC) of India for financial assistance in the research work.

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