A Luminescent Microporous Metal−Organic Framework for the Recognition and Sensing of Anions

Designed Eu(III)-functionalized nanoscale MOF probe based on fluorescence resonance energy transfer for the reversible sensing of trace Malachite green

An ingenious nanoscale fluorescent sensor derived from Eu³⁺-postfunctionalized MIL-53 (Al) (Eu³⁺@MIL-53 [Al]) was fabricated though a simple and effective approach. Malachite green (MG) effectively turned off the luminescence of Eu³⁺@MIL-53 (Al) via fluorescence resonance energy transfer (FRET), thus enabling MG sensing. The developed probe exhibited instantaneous reusability after being cleaned with deionized water. The fluorescence intensity, quenching efficiency, and crystal structure of the recoverable sensor after five recycling processes were unchanged compared with those of the original sample. Moreover, the potential mechanism of MG detection was revealed in detail. This work represents the first attempt to determine MG in aquaculture water and products by using metal-organic frameworks (MOFs). The Eu³⁺@MIL-53 (Al) probe proved to be a remarkable fluorescence probe for MG with high selectivity, sensitivity, and excellent regeneration capability. It provides a promising functional platform for the recognition of illegal MG addition to aquaculture water and products.

Metal-Organic Frameworks for Liquid Phase Applications

In the last two decades, metal-organic frameworks (MOFs) have attracted overwhelming attention. With readily tunable structures and functionalities, MOFs offer an unprecedentedly vast degree of design flexibility from enormous number of inorganic and organic building blocks or via postsynthetic modification to produce functional nanoporous materials. A large extent of experimental and computational studies of MOFs have been focused on gas phase applications, particularly the storage of low-carbon footprint energy carriers and the separation of CO2-containing gas mixtures. With progressive success in the synthesis of water- and solvent-resistant MOFs over the past several years, the increasingly active exploration of MOFs has been witnessed for widespread liquid phase applications such as liquid fuel purification, aromatics separation, water treatment, solvent recovery, chemical sensing, chiral separation, drug delivery, biomolecule encapsulation and separation. At this juncture, the recent experimental and computational studies are summarized herein for these multifaceted liquid phase applications to demonstrate the rapid advance in this burgeoning field. The challenges and opportunities moving from laboratory scale towards practical applications are discussed.

Designed Tb(III)-Functionalized MOF-808 as Visible Fluorescent Probes for Monitoring Bilirubin and Identifying Fingerprints

Abnormal bilirubin (BR) level is a sign of several fatal diseases, so it is of great significance and challenge to develop a facile and effective family routine strategy for BR sensing. Herein, novel water-stable Tb³⁺@MOF-808 has been synthesized using a coordinated postsynthetic modification strategy and designed as a convenient and efficient fluorescence probe. The fabricated fluorescent probe exhibits a remarkable fluorescence quenching effect with the successive addition of BR, which displays fascinating features, such as fast response time, high sensitivity, and excellent selectivity. The quenching mechanism between the fluorescent probe and BR was also illustrated in detail. Importantly, the devised fluorescent probe successfully achieved the determination of BR in serum and urine, which has also been successfully used in the design of portable BR test paper. The developed monitoring platform for BR levels in vivo provides promising application potential for the prevention and early diagnosis of fatal diseases. Additionally, a molecular logic gate device that performs intelligent fluorescent sensing of BR was constructed. More interestingly, Tb³⁺@MOF-808 is used for development of latent fingerprints on different guest surfaces. The lines of the fluorescent fingerprints are clear and coherent, the details are obvious, and even sweat pores can be observed by naked eyes, which provides new means for tracking the criminal clue and handling cases efficiently.

Cd-Based Metal-Organic Framework for Selective Turn-On Fluorescent DMSO Residual Sensing

Dimethyl sulfoxide (DMSO) is a universally used solvent in various synthetic reactions, and trace amounts of DMSO residual are often seen on the surface of chemical product. It is difficult to quickly determine whether the residual DMSO is washed completely. This work reports a Cd^II metal-organic framework (MOF) SXU-4 which can detect trace amounts of DMSO in various solvents. Fluorescence experiments reveal its turn-on fluorescence effect toward DMSO with high selectivity and sensitivity, indicating that it can be used as an effective luminescent probe for rapid chemical product purity detection by testing the washing solution. Crystallographically characterized DMSO loaded SXU-4 (DMSO@SXU-4), in combination with computational results uncover that the enhanced DMSO-MOF conjugation through multiple DMSO-MOF supramolecule interactions and charge rearrangement are the main causes of fluorescence intensification.

Controlling the Stacking Modes of Metal-Organic Framework Nanosheets through Host-Guest Noncovalent Interactions

The tuning of metal-organic framework (MOF) nanosheet stacking modes from molecular level was rarely explored although it significantly affected the properties and applications of nanosheets. Here, the different stacking modes of Zr-1, 3, 5-(4-carboxylphenyl)-benzene framework nanosheets were synthesized through the induction of different host-guest noncovalent interactions. The solvents of methyl benzene and ethyl acetate induced twisted stacking of nanosheets with the specific rotation angles of 12°, 18°, 24° and 6°, 18°, 24°, 30°, respectively, which was in agreement with theoretical calculations. Meanwhile, the alkanes were likely to vertically enter the pores of Zr-BTB nanosheets because of steric hindrance and hydrophobic interactions, resulting in the untwisted stacking of nanosheets. The untwisted ordered nanopores showed the excellent gas chromatographic separations of benzene derivative isomers, which was better than twisted nanosheets stacking and commercial columns. This work uncovers a rational strategy to control the stacking of two-dimensional MOF nanosheets.

Phenoxo-bridged dinuclear mixed valence cobalt(III/II) complexes with reduced Schiff base ligands: synthesis, characterization, band gap measurements and fabrication of Schottky barrier diodes

Two homometallic class-I dinuclear mixed valence cobalt complexes, [(N3)CoIIIL1(μ-C6H4(NO2)CO2)CoII(N3)] (1) and [(N3)CoIIIL2(μ-C6H4(NO2)CO2)CoII(N3)] (2), have been synthesized using multisite N2O4 coordination ligands, H2L1 {where H2L1 = (2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(6-methoxyphenol) and H2L2 = (2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(6-ethoxyphenol)}. Each complex has been structurally characterized by single crystal X-ray diffraction and spectral analysis. Both the cobalt centers in these dinuclear complexes adopt a distorted-octahedral geometry, where the cobalt(iii) center resides at the inner N2O2 cavity and the cobalt(ii) center resides at the outer O4 cavity of the reduced Schiff base. Both of them show good electrical conductivity, which has been rationalized by band gap measurements. The band gap in the solid state has been determined by experimental and DFT calculations and it confirms that each of the two complexes behaves as a semiconductor. The space-charge-limited current (SCLC) theory is employed to evaluate the charge transport parameters such as effective carrier mobility and transit time for both complexes. The difference in the conductivity values of the complexes may be correlated with the strengths of extended supramolecular interactions in the complexes. Bader's quantum theory of atoms-in-molecules (QTAIM) is applied extensively to get quantitative and qualitative insights into the physical nature of weak non-covalent interactions. In addition, the non-covalent interaction reduced density gradient (NCI-RDG) methods well support the presence of such non-covalent intermolecular interactions.

Alkali and alkaline earth coordination polymers constructed from benzene-1,2,4,5-tetracarboxylic acid and flexible dicarboxylate acid ligands: syntheses, structures and spectroscopic and thermal properties

Two new metal coordination complexes, namely, poly[aqua(μ₆-benzene-1,2,4,5-tetracarboxylic acid-κ⁸O¹:O¹,O²:O^2':O⁴:O⁴,O⁵:O^5')(μ-but-2-enedioato-κ²O¹:O⁴)potassium(I)], [K₂(C₄H₂O₄)(C₁₀H₆O₈)(H₂O)₂]_n or [K₂(fum)(H₄btec)(H₂O)₂]_n, (1), and poly[aqua(μ₈-2,5-dicarboxybenzene-1,4-dicarboxylato-κ¹²O¹:O^1',O²:O²,O^2':O^2':O⁴:O^4',O⁵:O⁵,O^5':O^5')(μ-ethanedioato-κ⁴O¹,O²:O^1',O^2')strontium(II)], [Sr₂(C₂O₄)(C₁₀H₄O₈)(H₂O)₂]_n or [Sr₂(ox)(H₂btec)(H₂O)₂]_n, (2) (H₄btec = benzene-1,2,4,5-tetracarboxylic acid, H₂btec = 2,5-dicarboxybenzene-1,4-dicarboxylate, fum = fumarate and ox = oxalate), have been obtained under hydrothermal conditions by reacting the different alkali and alkaline earth metal salts with H₄btec, fumaric acid (H₂fum) and oxalic acid (H₂ox). Complexes (1) and (2) were structurally characterized by single-crystal X-ray diffraction, IR and UV-Vis spectroscopy, powder X-ray diffraction (PXRD) and thermogravimetic analysis-differential scanning calorimetry (TGA-DSC). Complex (1) displays a two-dimensional (2D) layer with the K⁺ ion in a distorted pentagonal bipyramidal geometry and exhibits a uninodal 6-connected hxl/Shubnikov plane net (3,6) with {3⁶.4⁶.5³} topology. Complex (2) displays a three-dimensional (3D) network structure, in which the Sr²⁺ ion is in a distorted monocapped square antiprism geometry. The framework possess a binodal (5,8)-connected net with the Schläfli symbol {3².4¹⁰.5⁸.6⁴.7⁴}{3².4⁶.5²}₂. The 3D Hirshfeld surfaces and 2D fingerprint plots show that the main interactions are the O...H/H...O intermolecular interactions. Moreover, the thermal decompositions of (1) and (2) in the temperature range 303-1273 K revealed that they both decompose in three steps and transform to the corresponding metal oxide.

Luminescent metal–organic frameworks as chemical sensors based on “mechanism–response”: a review

The comprehensive review systematically summarizes the recent developments in the study of LMOFs as chemical sensors based on “mechanism–response”.

Synthesis of a bimetallic metal–organic framework catalyst via selective detection and adsorption of Fe3+ for enhanced bio-based catalysis

A MOF with good chemical resistance exhibits a sensitive fluorescence response and absorbance for Fe3+; Tb-HODA⊃Fe3+ enhances the catalytic efficiency for the dehydration of fructose/glucose into 5-hydroxymethylfurfural.

Porous and Nonporous Coordination Polymers Induced by Pseudohalide Ions for Luminescence and Gas Sorption

The three-dimensional (3D) coordination polymers [Cd(tpmd)(NCX)₂]_n [X = O (1), S (2), and BH₃ (3); tpmd = N,N,N',N'-tetrakis(pyridin-4-yl)methanediamine] have been determined to display their network structures through coordinated anionic ligands. Polymers 1 and 2 show nonporous structures, whereas polymer 3 shows a porous coordination framework. On the basis of the Cd(II) network structures, the 3D coordination polymer [Zn(tpmd)(NCBH₃)₂]_n·nMeOH (4) was self-assembled. In the cases of polymers 1 and 2, pseudohalide ions acted to form nonporous network structures; however, in polymers 3 and 4, NCBH₃^- helps to construct porous network structures. Polymers 1-4 show strong ultraviolet luminescence emissions, depending on the pseudohalide ions present, compared to the tpmd ligands. Interestingly, coordination polymers 3 and 4 that possess NCBH₃^- ions exhibit high porosities and gas sorption properties. The polymers appeared to absorb N₂, H₂, CO₂, and CH₄. In the case of polymer 4, the structure is almost identical with that of polymer 3, except for the Cd(II) ion. However, polymer 4 has a larger void volume and higher gas absorption ability for N₂ gas than polymer 3. For the sorption of gases, polymers 3 and 4 showed similar behaviors.

Adsorption of Lead Ions by a Green AC/HKUST-1 Nanocomposite

A new nanocomposite consisting of activated carbon (AC) from the Cortaderia selloana flower and copper-based metal-organic framework (HKUST-1) was synthesized through a single-step solvothermal method and applied for the removal of lead ions from aqueous solution through adsorption. The nanocomposite, AC/HKUST-1, was characterized by Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Fourier Transform Infrared (FTIR), and Energy-Dispersive X-ray Spectroscopy (EDX) methods. The SEM images of both HKUST-1 and AC/HKUST-1 contain octahedral crystals. Different factors affecting adsorption processes, such as solution pH, contact time, adsorbent dose, and initial metal pollution concentration, were studied. The adsorption isotherm was evaluated with Freundlich and Langmuir models, and the latter was fitted with the experimental data on adsorption of lead ion. The adsorption capacity was 249.4 mg g^-1 for 15 min at pH 6.1, which is an excellent result rivalling previously reported lead adsorbents considering the conditions. These nanocomposites show considerable potential for use as a functional material in the ink formulation of lead sensors.

Reusable colorimetric and fluorescent chemosensors based on 1,8-naphthalimide derivatives for fluoride ion detection

Two 1, 8-naphthalimide derivatives, 2-(2-ethylhexyl)-6-(2-(4-(5-phenyl-1,3,4-oxadiazol-2-yl)benzylidene)hydrazinyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (named as NAOZ) and 2-(2-ethylhexyl)-6-(2-(4-(5-phenyl-1,3,4-thiadiazol-2-yl)benzylidene)hydrazinyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (named as NATZ), containing hydrazone and thiadiazole/oxadiazole, were successfully synthesized. Compounds NAOZ and NATZ were further served as efficient reversible colorimetric and fluorescent chemosensors for fluoride ion over a wide range of other anions (Cl^-, Br^-, I^-, NO₃^-, ClO₄^-, HSO₄^-, BF₄^-, and PF₆^- as their (C₄H₉)₄N⁺ salts) because fluoride ion can deprotonate the hydrazone moiety, leading to the observed long-wavelength color change. Interestingly, CO₂ can recover the charge transfer (CT) absorption and photoluminescence intensities of compound NAOZ/NATZ + F^-, because CO₂ can react with a small amount of water to form acids, which can provide protons.

Broad-band luminescence involving fluconazole antifungal drug in a lead-free bismuth iodide perovskite: Combined experimental and computational insights

The synthesis and characterization of a lead-free perovskite-type material, (C₁₃H₁₄N₆F₂O)₂ Bi₂I₁₀ is reported. It exhibits a zero-dimensional (0D) Bi₂I₁₀^4- octahedral unit, surrounded by a flexible tripodal antifungal ligand (H₂Fluconazole)²⁺. The several intermolecular interactions of the independent cation and the bismuth iodide octahedra were tested via the Hirshfeld surface analysis. The detailed interpretation of the vibrational modes was carried out. The band gap (Eg) of 2.10 eV agrees with the theoretical values. Upon photoexcitation, the crystals exhibit a broadband green emission peaked at 534 nm, which originates from electronic transitions within the inorganic cluster [Bi₂I₁₀]^4-. The theoretical calculations were carried out using DFT and TD-DFT methods to appraise the molecular geometry, vibrational spectra, electronic absorption spectra, frontier molecular orbitals (FOMs) and global reactivity descriptors. Calculations reveal that the energy gap (Eg) and other chemical reactivity descriptors are primarily linked to the inorganic anion and the triazolium rings (A and B) of the organic cation reflecting their importance in the activity and the antioxidant ability of the molecule.

Electronic Structure Modeling of Metal-Organic Frameworks

Owing to their molecular building blocks, yet highly crystalline nature, metal-organic frameworks (MOFs) sit at the interface between molecule and material. Their diverse structures and compositions enable them to be useful materials as catalysts in heterogeneous reactions, electrical conductors in energy storage and transfer applications, chromophores in photoenabled chemical transformations, and beyond. In all cases, density functional theory (DFT) and higher-level methods for electronic structure determination provide valuable quantitative information about the electronic properties that underpin the functions of these frameworks. However, there are only two general modeling approaches in conventional electronic structure software packages: those that treat materials as extended, periodic solids, and those that treat materials as discrete molecules. Each approach has features and benefits; both have been widely employed to understand the emergent chemistry that arises from the formation of the metal-organic interface. This Review canvases these approaches to date, with emphasis placed on the application of electronic structure theory to explore reactivity and electron transfer using periodic, molecular, and embedded models. This includes (i) computational chemistry considerations such as how functional, k-grid, and other model variables are selected to enable insights into MOF properties, (ii) extended solid models that treat MOFs as materials rather than molecules, (iii) the mechanics of cluster extraction and subsequent chemistry enabled by these molecular models, (iv) catalytic studies using both solids and clusters thereof, and (v) embedded, mixed-method approaches, which simulate a fraction of the material using one level of theory and the remainder of the material using another dissimilar theoretical implementation.

Flexible Luminescent MOF: Trapping of Less Stable Conformation of Rotational Isomers, In Situ Guest-Responsive Turn-Off and Turn-On Luminescence and Mechanistic Study

Flexible and dynamic Cu^I metal-organic framework [Cu(I)-MOF (1)] with well-defined nanoporous channel built with flexible terpyridine ligand offers a scaffold for the inclusion of different classes of guest molecules through a single-crystal-to-single-crystal (SCSC) transformation in the vapor phase at ambient conditions with visual color change. Thus, Cu(I)-MOF (1) offers a potential platform for molecular recognition and undergoes guest-responsive structural dynamism that can be triggered by interfacial interactions. Despite having the stable conformation of the rotational isomers, it selectively encapsulates the less stable conformation (eclipsed and gauche) into its cavity from their vapor phases in the SCSC process. All of the guest-exchanged processes are reversible. It shows selectivity toward less polar guest in a class. The intermediate of all of the guest-exchanged processes appeared as a black material (H₂O@Cu(I)-MOF) (1z) prior to the encapsulation of each guest that happens through the SCSC manner followed by encapsulation of the guests replacing H₂O in situ at ambient conditions through SCSC transformation. This confirms that the process is a two-step process leading to a common intermediate. The MOF loses its luminescence behavior with H₂O removing lattice solvents in situ and appears as a black material, and it regains its luminescence property with the guests replacing H₂O. Thus, the MOF displays both luminescence "turn-off" and "turn-on" before and after incorporation of the guests, respectively, leading to a common turn-off mechanism. A fluorescence titration experiment shows selectivity toward aniline among benzene and its derivatives.

Three coordination polymers with regulated coordination interactions as fluorescent sensors for monitoring purine metabolite uric acid

A facile optical sensor for uric acid (UA), an early pathological signature for the metabolic function of humans, was developed based on water-stable coordination polymers (CPs). Herein, three new isostructural fluorescent CPs, [Ln(TCPB)(DMF)3]n (Ln = La, CP 1; Ce, CP 2 and Pr, CP 3; H3TCPB = 1,3,5-tris(1-(2-carboxyphenyl)-1H-pyrazol-3-yl)benzene), with various metal ions were solvothermally synthesized. Significantly, by regulating the metal-organic coordination interactions, the fabricated CP 3 can quantitatively recognize UA with higher sensitivity compared with CP 1 and CP 2. The mechanism for the sensing properties further demonstrates the best performance of CP 3 and the excellent selectivity for UA monitoring. This work represents the strategy of designing fluorescent CP sensors to determine UA and provides a convenient approach for developing analysis platforms for the assessment of related disease progress and human health monitoring.

Anilate Tethered Neutral Tetrahedral Pd(II) Cages Exhibiting Selective Encapsulation of Xylenes and Mesitylene

The design of porous materials for the recognition of multiple hydrocarbons is highly desirable for the energy-efficient separation and recognition of chemical feedstock. Herein, three new iso-structural porous discrete metal-organic cages of formula {[Pd₃ (NiPr)₃ PO]₄ (R-AN)₆ } (R-AN=anilate linkers) for the selective recognition of substituted aromatic hydrocarbons are reported. The tetrahedral cages 1, 2, and 3 containing anilate, chloranilate, and bromanilate linkers exhibited selective encapsulation of mesitylene, o-xylene, and p-xylene, respectively, over other analogous aromatic hydrocarbons. These selective encapsulations were driven by the variations in the portal diameters present at each of these cages and their interactions with the hydrocarbon guests. These observations are supported by mass spectrometry, NMR studies, and theoretical binding-energy calculations.

Redox-Based Electrochemical Affinity Sensor for Detection of Aqueous Pertechnetate Anion

Rapid, selective, and in situ detection of pertechnetate (TcO₄^-) in multicomponent matrices consisting of interfering anions such as the ubiquitous NO₃^- and Cl^- or the isostructural CrO₄^2- is challenging. Present sensors lack the selectivities to exclude these interferences or the sensitivities to meet detection limits that are lower than the drinking water standards across the globe. This work presents an affinity-based electrochemical sensor for TcO₄^- detection that relies on selective reductive precipitation of aqueous TcO₄^- induced by a 1,4-benzenedimethanethiol capture probe immobilized on an electrode platform. This results in a direct decrease in the electron transfer current, the magnitude of the decrease being proportional to the amount of TcO₄^- added. Using this approach, a detection limit of 1 × 10^-10 M was achieved, which is lower than the drinking water standard of 5.2 × 10^-10 M set by United States Environmental Protection Agency. The proposed approach shows selectivity to the TcO₄^- anion, allowing detection of TcO₄^- from a multicomponent groundwater sample obtained from a well at the Hanford site in Washington (well 299-W19-36) that also contained NO₃^-, Cl^-, and CrO₄^2-, without discernably affecting the detection limits.

Transforming surface-modified metal organic framework powder into room temperature porous liquids via an electrical balance strategy

The design of MOF-based room temperature porous liquids via an electrical balance strategy.

Luminescent sensing of nitroaromatics by crystalline porous materials

Designing strategies for the syntheses of targeted luminescent MOFs, nanoparticle/MOF composites and COFs described and their application in sensing nitroaromatic compounds and explosives discussed.

Rare-earth metal–organic frameworks: from structure to applications

In the past 30 years, rare-earth metal–organic frameworks (MOFs) have been gaining attention owing to their diverse chemical structures, and tunable properties.

Water-stable LnIII-based coordination polymers displaying slow magnetic relaxation and luminescence sensing properties

Six Ln-CPs were synthesized: Dy-CP shows slow magnetic relaxation, and Eu-CP and Tb-CP exhibit recyclable and multi-responsive sensing for Fe³⁺, MnO₄⁻, Cr^VI-anions (CrO₄²⁻, Cr₂O₇²⁻) and TNP in an aqueous system.

Synthesis, structure, and photoluminescence properties of coordination polymers of 4,4′,4′′,4′′′-tetrakiscarboxyphenylsilane and 3,5-bis(1′,2′,4′-triazol-1′-yl)pyridine

Three coordination polymers based on 4,4′,4′′,4′′′-tetrakiscarboxyphenylsilane and 3,5-bis(1′,2′,4′-triazol-1′-yl)pyridine ligands were synthesized. Luminescence studies revealed that [Cd₃(Htcps)₂(btap)(H₂O)₆] exhibits a luminescence quenching response towards Fe³⁺ and Cu²⁺.

Eu-based coordination polymer microrods for low-loss optical waveguiding application

Lanthanide-based coordination polymers (CPs) have received great attention due to their tuneable structures and excellent luminescence properties. However, limited by the stability and micro/nanoscale morphology, a very small number of lanthanide-based CPs have been used for photonic applications. Herein, we present the synthesis of Eu-based CPs (compound 2) with highly regular one-dimensional (1D) microrod morphology by in situ structure transformation from compound 1. Moreover, the Eu-based CP microrods have an excellent stability and a high photoluminescence quantum yield (PLQY), and the distance-dependent PL spectra also exhibited typical characteristics of photoactive waveguides with a low optical-loss coefficient (0.00894 dB μm^-1) during propagation. These intriguing behaviors not only extend the research field of optical waveguides through lanthanide-based CPs, but also provide an opportunity for further application in optical communication systems.

A Zn(ii) metal-organic framework based on bimetallic paddle wheels as a luminescence indicator for carcinogenic organic pollutants: phthalate esters

Based on the multifunctional ligand 3-(1H-1,2,4-triazol-1-yl)isophthalic acid (H2TIA), a three-dimensional coordination polymer, namely {[Zn(TIA)]·DMA} n (Zn-1) was synthesized solvothermally. Single-crystal X-ray diffraction analyses confirmed that Zn-1 is a 3D framework composed of binuclear Zn2 paddle wheels with one-dimensional channels long the a direction. Further topological analyses revealed that MOF Zn-1 existed as a (3,6)-connected rtl binodal net {4·62}2{42·610·83}. Furthermore, the luminescence explorations indicate that complex Zn-1 is the first MOF for luminescent probing of phthalate esters (carcinogenic organic pollutants) with a high quenching-efficiency constant and low fluorescence-detection limit.

Large and tunable magnetocaloric effect in gadolinium-organic framework: tuning by solvent exchange

Magnetic properties of three variants of MOF-76(Gd), {[Gd(BTC)(H2O)]·G}n (BTC = benzene-1,3,5-tricarboxylate, G = guest molecules) were investigated by static susceptibility, isothermal magnetization and specific heat capacity measurements. In the study we used as synthesized MOF-76(Gd)-DMF (1) (G = DMF = dimethylformamide), containing DMF molecules in the cavity system, compound MOF-76(Gd) (2), activated complex without solvents in the cavities and water exchanged sample MOF-76(Gd)-H2O (3). A pronounced change in the magnetic entropy was found near the critical temperature for all three compounds. It was shown, that magnetic entropy change depends on the solvatation of the MOF. The highest value entropy change, ΔSMpk(T) was observed for compound 2 (ΔSMpk(T) = 42 J kg-1 K-1 at 1.8 K for ΔH = 5 T). The ΔSMpk(T) for the compounds 1, 2 and 3 reached 81.8, 88.4 and 100% of the theoretical values, respectively. This suggests that in compound 3 Gd3+···Gd3+ antiferromagnetic interactions are decoupled gradually, and higher fields promote a larger decoupling between the individual spin centers. The observed entropy changes of compounds were comparable with other magnetic refrigerants proposed for low-temperature applications. To study the magnetothermal effect of 2 (the sample with largest -ΔSMpk), the temperature-dependent heat capacities (C) at different fields were measured. The value of magnetic entropy S obtained from heat capacities (39.5 J kg-1 K-1 at 1.8 K for an applied magnetic field change of 5 T) was in good agreement with that derived from the magnetization data (42 J kg-1 K-1 at 1.8 K).

Fluorescent "Turn-on" Sensing Based on Metal-Organic Frameworks (MOFs)

Metal-organic frameworks (MOFs) have evolved as an exciting class of materials in the domain of porous materials. The unique features of these materials arise from the combined properties of metal ions/clusters and organic struts which form the building blocks of these fascinating architectures. Among other multifarious applications, MOFs have shown tremendous applications as sensory materials for a wide variety of species. The signal transduction induced mechanism in these confined nanospaces generate optical output in response to a particular analyte which can be detected by wide variety of detection techniques. Fluorometric methods of sensing is one of widely studied method over past few decades. MOF-based fluorometric detection is a key research theme developed over the past few years. In this review, we give a brief overview of the recent developments of MOFs as "turn-on" sensors for a wide range of analytes (viz. cations, anions, volatile organic compounds (VOCs), etc.).