Guest-Responsive Metal–Organic Frameworks as Scaffolds for

Article Cite This: Acc. Chem. Res. 2017, 50, 2457-2469

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Guest-Responsive Metal−Organic Frameworks as Scaffolds for Separation and Sensing Applications Avishek Karmakar,† Partha Samanta,† Aamod V. Desai,† and Sujit K. Ghosh*,†,‡ †

Department of Chemistry, Indian Institute of Science Education and Research (IISER) Pune, Dr. Homi Bhabha Road, Pashan, Pune-411 008, India ‡ Centre for Research in Energy & Sustainable Materials, IISER Pune, Pune-411 008, India CONSPECTUS: Metal−organic frameworks (MOFs) have evolved to be next-generation utility materials because of their serviceability in a wide variety of applications. Built from organic ligands with multiple binding sites in conjunction with metal ions/clusters, these materials have found profound advantages over their other congeners in the domain of porous materials. The plethora of applications that these materials encompass has motivated material chemists to develop such novel materials, and the catalogue of MOFs is thus ever-escalating. One key feature that MOFs possess is their responsiveness toward incoming guest molecules, resulting in changes in their physical and chemical properties. Such uniqueness generally arises owing to the influenceable ligands and/or metal units that govern the formation of these ordered architectures. The suitable host−guest interactions play an important role in determining the specific responses of these materials and thus find important applications in sensing, catalysis, separation, conduction, etc. In this Account, we focus on the two most relevant applications based on the host−guest interactions that are carried out in our lab, viz., separation and sensing of small molecules. Separation of liquid-phase aromatic hydrocarbons by less energy-intensive adsorption processes has gained attention recently. Because of their tailored structures and functionalized pore surfaces, MOFs have become vital candidates in molecular separation. Prefunctionalization of MOFs by astute choice of ligands and/or metal centers results in targeted separation processes in which the molecular sieving effect plays a crucial role. In this view, separation of C6 and C8 liquid aromatic hydrocarbons, which are essential feedstock in various chemical industries, is one area of research that requires significant attention because of the gruesome separation techniques adopted in such industries. Also, from the environmental perspective, separation of oil/water mixtures demands significant attention because of the hazards of marine oil spillage. We have achieved successful separation of such by careful impregnation of hydrophobic moieties inside the nanochannels of MOFs, resulting in unprecedented efficiency in oil/water separation. Also, recognition of small molecules using optical methods (fluorescence, UV, etc.) has been extended to achieve sensing of various neutral species and anions that are important from environmental point of view. Incorporation of secondary functional groups has been utilized to sense nitroaromatic compounds (NACs) and other small molecules such as H2S, NO, and aromatic phenols. We have also utilized the postfunctionalization strategy via ion exchange to fabricate MOFs for sensing of environmentally toxic and perilous anionic species such as CN− and oxoanions. Our current endeavors to explore the applicability of MOFs in these two significant areas have widened the scope of research, and attempts to fabricate MOFs for real-time applications are underway. delivery, photonics, etc.4−7 On the basis of the choice of the linker and/or metal node and the scope of tailorability from aspects of structural engineering, one can design suitable MOFs that can result in achieving targeted functionality in a very precise manner. MOFs have been the foremost candidates for storage and separation applications because of their large pore apertures and functionalized pore surfaces, which result in suitable interactions with incoming guest molecules. Size/shape exclusion plays a crucial role in governing the adsorption

1. INTRODUCTION The advent of metal−organic frameworks (MOFs) in the field of crystalline porous materials has led to gigantic leaps forward from the perspective of applications based on these newfangled utility materials.1−3 The advantages MOFs have over other contemporary materials are tunable synthesis, predesigned architectures, and suitable host−guest interactions, which enable them to be sculpted into desirable materials for diverse applications. The emergence of MOFs in the scientific world has attracted scientists to explore the fascinating properties associated with these porous architectures and even scrutinize their applicability in gas storage, separation, catalysis, conduction, sensing, drug © 2017 American Chemical Society

Received: March 28, 2017 Published: September 5, 2017 2457

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Accounts of Chemical Research process, which is also often aided by the unique structural flexibility, resulting in an outstanding molecular sieving effect.8−10 Additionally, the tunable structure of MOFs allows scope for prefunctionalization of the pore surface, resulting in chemical separations that would have otherwise been difficult to achieve using conventional methods of separation being used by chemical industries. The incoming guest molecules can also bring about changes in the physical and/or chemical properties of the MOF, which may lead to a unique fingerprinting effect. This forms the genesis of designing MOFs as an exclusive class of chemosensory materials, where the incoming analyte molecule actuates suitable signal transduction that can be monitored with various analytical methods.11−13 Luminescent MOFs (LMOFs) constitute important candidates for such chemosensing applications in the MOF regime, although systematic exploration of their functional behavior and exhaustive structure−property correlation require further endeavors. An overwhelming number of reports based on rational design of MOFs for selective and sensitive detection of target analytes have emerged in the past decade. This includes sensing of small molecules, gases, solvents, ionic species, etc., which in turn leads to the scope of fabrication of MOFs for identification of molecular species relevant to environmental applications. The sensing phenomenon in MOFs is governed by numerous factors such as (i) easy transport of the analyte molecules within the functionalized pores of the MOF, (ii) host− guest interactions, and (iii) chemical interactions/bonding of the analyte molecule with the secondary functional groups of the MOF.14 The ability of MOFs to respond to single or multiple stimuli can thus lead to hybridization and fabrication into materials that can represent a new class of smart chemosensors for diverse applications. In this Account, we focus on our research work based on two potent applications of MOFs, (a) chemical separation and (b) sensing of small molecules, which conjointly have the potential to address issues related to the environment (Figure 1). We demonstrate the effect of pore functionalization by judicious choice of the linker and metal center in tackling gruesome chemical separation challenges (viz., separation of C6−C8 aromatic solvents) now being faced in industry. Because of the similarity in their physical properties, separation of C6−C8

aromatic solvents is one of the most challenging tasks in industry, but it can be overcome using MOFs by virtue of their pore functionality. Also, in the subsequent sections we will highlight the utility of MOFs for sensing of anions, nitroaromatic compounds, neurotransmitters, and oxoanion pollutants, which illustrates the versatility of these ordered porous architectures in addressing key energy and environmental applications.

2. CHEMICAL SEPARATION BY MOFS TO COUNTER INDUSTRIAL AND ENVIRONMENTAL CHALLENGES Chemical separation is one of the vastly explored applications that have been studied extensively by researchers around the globe. Separation of liquid-phase hydrocarbon mixtures (C6−C8 aromatics) by less-energy-demanding processes remains an exigent problem from the industrial standpoint.15 Since most aromatic hydrocarbons are essential feedstocks and are subsequently converted into various commercial products, there is a necessity to inculcate low-cost and energy-effective processes for separation in petrochemical industries. 2.1. Rational Utilization of Pore Functionalization in Separation of C6 Cyclic Analogues

Among the most pertinent issues faced by chemical industries, separation of benzene (Bz) and cyclohexane (Cy) is considered to be one of the most thought-provoking processes.16,17 Since cyclohexane remains as the major side product during the hydrogenation of benzene, there is a need to separate it from the mixtures for further use. Also, similarity in their kinetic diameters and boiling points restricts the use of conventional methods like fractional distillation in their separation processes. MOFs have been effective in Bz/Cy separation via a less energy-demanding adsorption process utilizing rational pore functionalization by strategic choice of the building blocks.18 In this view, the electron-deficient diaminotriazine (DAT) core appended to a monocarboxylic acid linker was strategically employed to construct DAT-MOF-1, a functionalized MOF with molecular formula [{Cu(L)2}·xG]n (G = guest). Structural analysis revealed a supramolecular H-bonded three-dimensional (3D) framework with pore apertures of 6.71 Å × 7.08 Å along the crystallographic a axis.19 Since the pore walls were decorated with electron-deficient triazine rings, DAT-MOF-1 showed excellent Bz/Cy separation properties. At 298 K, it showed an uptake amount of 1.5 mol kg−1 for Bz but only a meager 0.2 mol kg−1 for Cy (Figure 2). This could be attributed to the differential interactions of Bz and Cy with the framework of DAT-MOF-1. The efficiency of the separation process was even evaluated by ideal adsorbed solution theory (IAST) calculations. The selectivity of adsorption (Sads) for equimolar Bz/Cy mixtures was evaluated to be in excess of 200. A dynamic 3D MOF based on an electron-deficient anhydridecore-based ditopic linker and Zn2+ metal ion with molecular formula [Zn2L2(DMF)2]n was constructed.20 The pristine MOF underwent structural changes upon removal of the coordinated N,N′-dimethylformamide (DMF) molecules, resulting in the formation of a new desolvated phase. The desolvated phase of the compound showed a distinct Bz/Cy separation at 298 K. The benzene uptake amount was about 1 molecule per formula unit, whereas the cyclohexane uptake was negligible. This was attributed to π−π interactions between the electron-rich benzene and electron-deficient anhydride-based framework, resulting in selective sorption. Additionally, an inherent dynamic character of the MOF was also responsible for the selective adsorption process.

Figure 1. Schematic representation showing the strategies adopted for utilizing MOFs for two major applications: separation and sensing. 2458

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Figure 2. (a) Representation of the DAT-core-based ligand. (b) Porous view of DAT-MOF-1 along the a axis. (c) Adsorption profile of benzene and cyclohexane at 298 K using DAT-MOF-1. (d) IAST results for Bz/Cy mixtures. Reproduced from ref 19. Copyright 2015 Royal Society of Chemistry.

Figure 3. (a) Adsorption profile showing Bz/Cy separation by Mn-MOF-74. (b) Comparison of the uptake amounts of Bz and Cy in M-MOF-74. (c) Optimized distance of benzene in M-MOF-74 using DFT calculations. Reproduced from ref 21. Copyright 2016 Royal Society of Chemistry.

Furthermore, we screened the MOF series M-MOF-74 (M = Mg, Mn, Fe, Co, Ni, Cu, Zn), which exhibited a clear distinction in the adsorption profiles for Bz and Cy.21 This was attributed to the formation of a Lewis base adduct of benzene with the open metal sites (OMSs) in these MOFs. The uptake amounts of benzene followed the order Mn-MOF-74 > Ni-MOF-74 > MgMOF-74 > Cu-MOF-74 > Zn-MOF-74 > Co-MOF-74 > FeMOF-74 (Figure 3), which is in accordance with the ionization

potentials of the respective metal ions. From IAST calculations, a selectivity factor in excess of 105 was observed, which marks these MOFs as among the best candidates in the list of porous materials for Bz/Cy separation. 2.2. Framework-Flexibility Driven Separation of C8 Aromatic Hydrocarbons

Separation of C8 aromatic species, especially the xylene isomers (p-, o-, and m-xylene), is among the major challenges faced in 2459

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Figure 4. Framework flexibility resulting in selective adsorption of p-xylene and styrene molecules in DynaMOF-100. Adapted from refs 24 and 25. Copyright 2014 Nature Publishing Group and 2015 American Chemical Society, respectively.

Figure 5. Schematic illustration of p-xylene separation over its isomers in a dynamic framework. Reproduced from ref 24. Copyright 2014 Nature Publishing Group.

majority of ethylbenzene remains in the unreacted form, and petrochemical industries use energy-intensive vacuum distillation columns to separate the two. DynaMOF-100 showed greater selectivity than MIL-47(V) and MIL-53(Al) by 1−2 orders of magnitude and superior performance over MAF-X8 and BaX zeolite.22 This selectivity was attributed to the sieving effect of DynaMOF-100 due to the specific non-covalent interactions between the host MOF and styrene molecules, which were evidenced from the crystal structure.

chemical industry. The use of fractional crystallization for the separation of p-xylene is affected by the formation of eutectic mixtures with other xylene components. As a result, very low yields of p-xylene are obtained, rendering the common industrial processes impractical. p-Xylene is the essential raw material for the production of terephthalic acid, which later is converted into the thermoplastic polymer poly(ethylene terephthalate). Therefore, obtaining pure phases of the xylene isomers by less energydemanding and low-cost processes is desired.22,23 DynaMOF-100, a porous 2D framework based on a flexible linker and Zn2+ with molecular formula {[Zn4O(L)3(DMF)2]· xG}n, was constructed.24 The MOF underwent a drastic structural transformation and resulted in a nonporous phase. The guest-free phase showed selective uptake of p-xylene over other xylene isomers (Figure 4), which was explained by the restricted limiting allowance principle exhibited by the MOF due to the flexibility adapted in the present case (Figure 5). Similarly, selectivity of styrene over ethylbenzene was achieved by DynaMOF-100 in another report.25 Ethylbenzene, which is formed by alkylation of benzene, is converted to styrene, an important industrial monomer, by dehydrogenation. However, a

2.3. Implementation of Bulk Hydrophobicity in MOFs To Address Marine Oil Spillage via Oil/Water Separation

Marine oil spillage has become one of the most pressing environmental concerns, especially given the magnitude of the hazards posed to the aquatic biota by such incidents. The problem has been compounded by the lack of feasible methods for tackling the spread of spilled oil without causing widespread damage to the local ecosystem.26 By the use of a highly fluorous ligand, 4,4′-{[3,5-bis(trifluoromethyl)phenyl]azanediyl}benzoic acid (H2L), as the backbone, the ultrahydrophobic MOF [{Cu4L4(DMF)4}(DMF)3]n (UHMOF-100) was synthesized.25 2460

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Figure 6. (a) Packing view and (b) single pore unit of UHMOF-100 showing −CF3 groups. (c) Water contact angle on the surface of UHMOF-100. (d) Photographs showing (top) the membrane base (white) and MOF-coated membrane (green) and (bottom) retention of hydrophobicity in the MOFcoated membrane. (e) Bar diagrams representing the absorption capacities of UHMOF-100 toward various crude oil components. Reproduced from ref 27. Copyright 2016 Wiley-VCH.

Figure 7. (a) Guest-induced structural transformation in [{Zn(L)(MeOH)2(NO3)2·xG]n. (b) Tunable luminescence response upon exchange with different anions. (c) Corresponding fluorescence spectra. Reproduced from ref 32. Copyright 2013 Wiley-VCH.

As anticipated, the MOF had a rich decoration of −CF3 moieties protruding toward the pores, making the channels sufficiently hydrophobic. The water contact angle was measured to be 176°, along with an oil contact angle of ∼0°, which made it the first example of an ultrahydrophobic MOF. Encouraged by these results, we fabricated a MOF-coated polymer-based membrane

to test the viability of the compound for real-time applications. The MOF-coated membrane was tested for its absorption capacities toward several constituents of marine oil, such as hexadecane, biodiesel, crude oil, toluene, and CCl4 (Figure 6). The absorption performance was found to be retained even after 10 cycles. The development of similar solid materials for oil/ 2461

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Figure 8. Schematic illustration of guest-driven structural transformation and tunable luminescence behavior upon anion exchange. Reproduced from ref 33. Copyright 2014 American Chemical Society.

Figure 9. (a) Cationic MOF showing free SO42− anions. (b) Visual changes upon exchange of SO42− anions with Cr2O72− and MnO4− anions. (c, d) Changes in UV−vis spectra upon exchange with (c) Cr2O72− and (d) MnO4− anions. Reproduced from ref 34. Copyright 2016 Wiley-VCH.

3.1. Anion Exchange as an Efficient Contrivance for Sensing of Anions

water separation is currently underway in our lab, with emphasis on environmentally friendly substrates.

Anion exchange in MOFs is one of the most efficient ways to formulate anion sensors based on interactions of the anions with the host framework. Since a majority of the anion exchange processes are governed by thermodynamic parameters, they generally lead to selective anion exchange, allowing a scope to design suitable anion exchangers based on size, shape, and charge of a particular anion. Ionic MOFs (i-MOFs) are perhaps the best manifestations of substrates that can undergo anion exchange depending upon the overall charge of the framework and the porosity involved therein.30 Assigning luminescence properties to such frameworks can lead to monitoring of anion exchange phenomena by suitable optical output.29 We developed a cationic framework based on a neutral, flexible N-donor ligand L (formed by Schiff base condensation of 4,4′ethylenedianiline and 2-pyridinecarboxaldehyde) and Zn2+ cation, resulting in the formation of [{Zn(L)(MeOH)2(NO3)2· xG]n.32 The as-made framework underwent a guest-driven structural transformation to form a new stable phase. This phase

3. RECOGNITION OF ANIONS BY SMART OPTICAL SENSORS BASED ON METAL−LIGAND SUPRAMOLECULAR ARCHITECTURES Anion recognition by metal−ligand supramolecular assemblies (MOFs and coordination polymers (CPs)) is a vastly researched topic recently. Since anions play key roles in a majority of chemical and biological processes, selective sensing of anions is vital for environmental and biomedical applications. MOFs/CPs represent a unique class of materials for anion sensing because of their versatile building blocks (ligands and metal ions) and wide variety of tunable architectures. Since anions are of different sizes and shapes and interact differentially with the frameworks of MOFs and CPs, they induce significant changes in the photophysical properties, thereby resulting in sensing applications.28,29 2462

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Figure 10. (a) Single crystal-to-single crystal (SC−SC) transformation upon inclusion of ClO4− anions in [{Zn(L)(OH2)2}(NO3)2·xG]n. (b) FT-IR spectra affirming anion exchange. (c) Circular dichroism spectra of the parent and anion-exchanged compounds. Reproduced from ref 35. Copyright 2014 Wiley-VCH.

Figure 11. (a) Packing diagram of ZIF-90 with free −CHO groups. (b) Schematic representation of nucleophilic attack by CN− ion. (c) Comparison of the fluorescence responses of M-ZIF-90 with CN− and other anions. (d) Percent fluorescence change upon addition of binary combinations of anions to M-ZIF-90. Reproduced from ref 36. Copyright 2016 Wiley-VCH.

yields were observed. Likewise, another MOF based on [(E)-N′[1-(pyridin-4-yl)ethyidene]hydrazinecarbohydrazide (L) and a Zn(II) metal salt underwent a 3D → 2D structural transformation under ambient conditions, resulting in the formation of a new stable phase.31 This phase underwent anion exchange

exhibited differential luminescent properties in response to different anions (Figure 7). For strongly coordinating anions such as SCN− and N3−, a decrease in the quantum yield in the solid phase was observed, whereas for noncoordinating anions such as ClO4− and N(CN)2−, comparatively higher quantum 2463

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Figure 12. (a) Sensing of CN− ions in the aqueous phase with bio-MOF-1. (b) Confocal images showing changes in luminescence: (left) bio-MOF-1; (middle) bio-MOF-1⊃DAAC; (right) CN−-treated bio-MOF-1⊃DAAC). (c, d) Fluorescence response upon addition of CN− and other competing analytes. (e) Fluorescence response after recycling of the MOF. (f) Cell viability results with the dye-incorporated MOF and the cyanide−dye adduct. Reproduced from ref 37. Copyright 2017 Royal Society of Chemistry.

underwent nucleophilic attack by the CN− ions, resulting in quenching of the fluorescence signal of the M-ZIF-90 compound in the aqueous phase (Figure 11). None of the other competing anions led to spectral changes of the M-ZIF-90 compound (Figure 11c,d). The selectivity was even maintained for binary combinations of cyanide and competing anions. This report was the first example of a MOF-based sensor for highly toxic CN− ions. Another postsynthetic modification strategy was employed in a hydrolytically stable anionic metal−organic framework, bioMOF-1.37 The MOF with molecular formula [Zn8(ad)4(BPDC)6·2Me2NH2]·G (G = DMF, water) has free dimethylammonium (DMA) cations located inside the porous framework, which were exchanged with diaminoacridinium (DAAC) cations, resulting in changes in the photophysical properties of the compound. The DAAC molecules, which have an electrophilic center at C9, are attacked exclusively by CN− ions upon addition of the tetrabutylammonium (TBA) salt of CN− in aqueous medium. The DAAC−CN adduct, being rendered neutral, escapes the reaction flask of the MOF and is replaced by the free TBA cations. The dye molecule (DAAC− CN) is highly fluorescent, inspiring a turn-on fluorescence response, and this response was realized selectively even in binary combinations of different anions and even over multiple cycles (Figure 12).

behavior in the presence of weakly coordinating anions such as ClO4− and BF4−, resulting in fluorescence quenching (Figure 8).This could be attributed to the differential interaction of the anions with the framework and intraligand charge transfer occurring in the MOF. We recently came up with a strategy to develop MOFs that can trap and sense oxoanions.34 A MOF based on Ni2+ and the neutral N-donor ligand tris[4-(1Himidazol-1-yl)phenyl]amine was constructed, which had free SO42− lying inside the porous framework,. These noninteracting anions could be exchanged with oxoanions such as Cr2O72− and MnO4−, as evidenced by IR and UV−vis experiments (Figure 9). This work proved to be an efficient way to trap and sense environmentally perilous oxoanionic pollutants. In another work, the neutral N-donor ligand 4,4′-(ethane-1,2diyl)bis(N-(pyridin-2-ylmethylene)aniline was utilized to fabricate a cationic framework with molecular formula [{Zn(L)(OH2)2}(NO3)2·xG]n.35 The parent compound was bulk-phasehomochiral, which was induced by the overall helical structure of the compound. The homochirality was maintained even after exchange with different foreign anions (Figure 10). The ClO4−exchanged compound showed a turn-on response behavior, indicating that such predesigned cationic MOFs could be effectively used not only in enantioselective catalysis and separation but also as anion sensors. 3.2. Sensing of Highly Toxic Cyanide Ion by Postfunctionalized MOFs

4. SENSING OF NITROAROMATIC COMPOUNDS AND BIOLOGICALLY PERTINENT NEUROTRANSMITTERS Apart from the anions, LMOFs have also been investigated as signaling agents to sense the presence of relevant neutral toxic species.38 In the past few years, we have demonstrated selective recognition of 2,4,6-trinitrophenol (TNP) and neurotransmitters such as nitric oxide (NO) and H2S using suitably functionalized MOFs. Selective sensing of TNP has been achieved via non-covalent interactions between the analyte and the pendant recognition sites on the MOF pore surface. Unlike the case of TNP detection, sensing of neurotransmitters has been observed as a consequence of the irreversible chemical reaction

Of all the known toxic anions occurring in nature, cyanide is a top-tier pollutant along with the heavy-metal oxoanions. Because of the detrimental effects of cyanide on living animals, it is imperative to monitor and control the concentration of CN− ions in aqueous systems. In this view, a fluorescent, hydrolytically stable porous MOF, i.e., zeolitic imidazolate framework 90 (ZIF-90), was utilized for selective sensing of CN− ions.36 The highly reactive free aldehyde (−CHO) groups in ZIF-90 were utilized for a postsynthetic modification reaction in which they were converted into dicyanovinyl (DCV) groups by a simple room-temperature reaction. The postsynthetically modified MOF, M-ZIF-90, 2464

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Figure 13. (a) Change in fluorescence intensity of the MOF upon addition of TNP. (b) Response in fluorescence corresponding to different nitro analytes. (c) Fluorescence changes upon sequential addition of other analytes and TNP. (d) Spectral overlap of MOF emission and the absorption spectra of nitro analytes. Reproduced from ref 39. Copyright 2013 Wiley-VCH.

Figure 14. (a) Structural diagram of UiO-67@N showing recognition sites. (b) Quenching effect caused by TNP upon addition to UiO-67@N. (c) Changes in fluorescence intensity of UiO-67@N upon addition of various other nitro analytes. (d) Stern−Volmer plots for different analytes. Reproduced from ref 40. Copyright 2014 Royal Society of Chemistry.

acid), for selective detection of TNP.39 Almost 78% fluorescence quenching was observed for TNP with high selectivity, and notably, this was the first report of selective TNP sensing in the MOF regime (Figure 13). This remarkable selectivity of the MOF toward TNP molecules has been attributed to the electrostatic interactions as well as the electron and energy transfer mechanism. Given the well-documented water stability of UiO-series MOFs, we employed the Zr(IV)-based UiO-67@N having 2phenylpyridine-5,4′-dicarboxylic acid as the ligand (Figure 14).40 The selective detection of TNP via fluorescence quenching was

of NO or H2S with the probe, an approach commonly referred to as chemodosimetry. 4.1. Sensing of 2,4,6-Trinitrophenol by MOFs with Appended Lewis Basic Sites

TNP is acidic in nature with one highly acidic proton (−OH group), and this −OH group can interact with Lewis basic sites to impart selectivity for TNP molecules. In an early report, we synthesized a 3D porous Cd(II)-based LMOF, [{Cd(NDC)0.5(PCA)}·G]x (G = guest molecule; NDC = 2,6naphthalenedicarboxylic acid; PCA = 4-pyridinecarboxylic 2465

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Figure 15. (a) Schematic representation of the preparation of UiO-66@N3. (b) Fluorescence response of UiO-66@N3 toward various analytes, showing turn-on response in the presence of Na2S. Reproduced from ref 42. Copyright 2014 Nature Publishing Group.

attributed to the pore surface functionalized with tertiary pyridyl groups. We further exploited MOFs grafted with primary amine groups for selective TNP detection. The Zn(II)-based luminescent bio-MOF-1 was employed for selective and fast recognition of TNP.41 High selectivity was observed because of the presence of free primary amine groups, and high quenching efficiency was found as a consequence of suitable energy and electron transfer between the probe and analytes. 4.2. Hydrolytically Stable MOFs as Unique Fingerprinting Materials for Chemosensing of Neurotransmitters

Gasotransmitters such as NO, CO, H2S, etc. are well-known for their function to transmit chemical signals. Azide (−N3) or nitro (−NO2) groups can easily be reduced to the corresponding amine (−NH2) derivatives by H2S, thus changing the electron density of the system, which is directly reflected in the fluorescence property. Inspired by this, we utilized UiO-66@ N3 (Figure 15a) and UiO-66@NO2, which are nonfluorescent because of the presence of the electron-deficient core. A fluorescence turn-on response was observed for UiO-66@N3 dispersed in HEPES buffer (pH 7.4) in the presence of Na2S as a result of the formation of UiO-66@NH2 (Figure 15).42 The selectivity was checked in the presence of concurrent competitive analytes, and we found UiO-66@N3 to be selective toward H2S even in the presence of other analytes in binary mixtures. The electron-deficient-core-based nonluminescent UiO-66@NO2 turned into luminescent UiO-66@NH2 upon addition of Na2S to the MOF dispersion. Fluorescence enhancement of UiO-66@ NO2 was observed with significant selectivity in the presence of other competitive analytes.43 Nitric oxide is the most toxic neurotransmitter, ahead of CO and H2S. We recently reported the MOF UiO-66@NH2 functionalized with −NH2 groups for selective and fast recognition of NO with deamination in the MOF system. Strong fluorescence of UiO-66@NH2 due to the presence of the free −NH2 groups was quenched very rapidly when NO was added to the MOF dispersed in HEPES buffer medium (Figure 16). Furthermore, the probe had selective response even in the concurrent presence of competitive analytes.44

Figure 16. (a) Porous view of UiO-66@NH2. (b) Fluorescence quenching of the compound upon gradual addition of NO. Reproduced from ref 44. Copyright 2015 Royal Society of Chemistry.

N-donor linker having the molecular formula [{CuL2(NO3)2}·oxylene·DMF]n was synthesized (Figure 17).45 Upon desolvation, the compound transformed into a 2D nonporous phase that showed selective uptake of benzene over cyclohexane (Figure 17d). The parent phase of the compound was exchanged with SCN− ion, and the exchange process was monitored through a naked-eye color change from blue to green (Figure 17b). The presence of SCN− anion in human blood plasma is biologically significant, and designing a visual colorimetric sensor as well as receptor for such an important anion deserves special mention.

5. KILLING TWO BIRDS WITH ONE STONE: CONJUNCTION OF SEPARATION AND SENSING APPLICATIONS BY PREFUNCTIONALIZED MOFS MOFs with the potential to render multiple functionalities are always desired, as divergent chemical problems can be resolved within the same entity. Construction of such frameworks depends on an appropriate choice of both the metal ions and the linker moieties. A Cu(II)-based MOF with an amide-based

6. CONCLUSION AND FUTURE OUTLOOK In this Account, we have summarized our recent contributions toward the fields of separation and sensing based on guest recognition within the functional nanospace of MOFs. Identification of targeted guests by MOFs is a vital aspect to 2466

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Figure 17. (a) Porous views of (left) [{CuL2(NO3)2}·o-xylene·DMF]n and (right) the desolvated phase of the same. (b) Visual changes upon exchange of NO3 ̅ with SCN ̅ anions. (c) Selective adsorption of CO2 over N2 at low temperatures. (d) selective adsorption of benzene over cyclohexane at 298 K. Reproduced from ref 45. Copyright 2015 Wiley-VCH.

fabricate smart MOFs that can tackle pressing energy and environmental issues. The key theme of this Account has been to direct the reader’s attention toward the essence of pre- and postfunctionalization of MOFs to address important chemical problems. Guest-responsive building blocks have been utilized to achieve industrially relevant liquid hydrocarbon separation in MOFs. Also, a challenging environmental issue, oil/water separation to confront marine oil spillage, has been shown to be effective by judicious choice of the ligand used in the construction of MOFs. In view of the recent threat of environmental pollution, mostly water pollution due to toxic and hazardous chemicals, guest-responsive MOFs have been shown to be among the best candidates for sensing of such perilous agents by strategic interfusion of ion exchange, incorporation of secondary functional groups, and grafting of recognition sites into the MOFs by postfunctionalization. Furthermore, the pore spaces of the MOFs have also been shown to be responsive toward biologically relevant neurotransmitters, thus making these materials important for biomedical applications. Currently, extensive research is being dedicated in our lab toward the development of MOFs that have the capability to respond to multiple stimuli and can perform better in terms of practical usage. Functional water-stable MOFs that can sense perilous entities under real-time conditions need to be developed for usage of MOFs in environmental applications. Also, more challenging separations that are relevant in chemical industries have to be targeted, which would add toward the applicability of such functional materials.

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Sujit K. Ghosh: 0000-0002-1672-4009 Notes

The authors declare no competing financial interest. Biographies Avishek Karmakar completed his B.S. and M.S. degrees from the University of Calcutta and University of Delhi in 2009 and 2011, respectively. He then joined the research group of Dr. Sujit K. Ghosh, where he is currently working as a senior research fellow. His research principally focuses on the applications of porous materials, especially MOFs, for sensing and separation applications. Partha Samanta earned his B.S. from the University of Calcutta in 2011 and then went to pursue his M.S. from the Indian Institute of Technology (IIT) Kharagpur. He joined the research group of Dr. Sujit K. Ghosh in 2013, and since then he has been working on the applications of porous organic materials for a wide variety of applications. Aamod V. Desai received his B.S. from the University of Pune in 2011 and joined IISER Pune as an integrated M.S./Ph.D student under the supervision of Sujit K. Ghosh. His research area encompass around nitrogen rich MOFs for sensing applications. Sujit K. Ghosh completed his Ph.D. at IIT Kanpur in 2006, after which he went to Kyoto University in Japan as a JSPS and CREST Postdoctoral Fellow (Host: Prof. Susumu Kitagawa). He is presently an Associate Professor in the Department of Chemistry at IISER Pune. Currently, his research is primarily focused upon functional MOFs aimed at energy and environmental applications.

AUTHOR INFORMATION

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Corresponding Author

ACKNOWLEDGMENTS A.K. and A.V.D. acknowledge IISER Pune for fellowships. P.S is grateful to UGC (India) for fellowship. S.K.G. acknowledges DST-SERB (Project EMR/2016/000410) for funding.

*E-mail: [email protected]. ORCID

Aamod V. Desai: 0000-0001-7219-3428 2467

DOI: 10.1021/acs.accounts.7b00151 Acc. Chem. Res. 2017, 50, 2457−2469

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Accounts of Chemical Research

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