Divergent Adsorption-Dependent Luminescence of Amino-Functionalized Lanthanide Metal-Organic Frameworks for Highly Sensitive NO2 Sensors

Electrochemical Anion Sensing Using Conductive Metal-Organic Framework Nanocrystals with Confined Pores

Anion sensing technology is motivated by the widespread and critical roles played by anions in biological systems and the environment. Electrochemical approaches comprise a major portion of this field but so far have relied on redox-active molecules appended to electrodes that often lack the ability to produce mixtures of distinct signatures from mixtures of different anions. Here, nanocrystalline films of the conductive metal-organic framework (MOF) Cr(1,2,3-triazolate)2 are used to differentiate anions based on size, which consequently affect the reversible oxidation of the MOF. During framework oxidation, the intercalation of larger charge-balancing anions (e.g., ClO4-, PF6-, and OTf-) gives rise to redox potentials shifted anodically by hundreds of mV due to the additional work of solvent reorganization and anion desolvation. Smaller anions (e.g., BF4-) may enter partially solvated, while larger ansions (e.g., OTf-) intercalate with complete desolvation. As a proof-of-concept, we leverage this "nanoconfinement" approach to report an electrochemical ClO4- sensor in aqueous media that is recyclable, reusable, and sensitive to sub-100-nM concentrations. Taken together, these results exemplify an unusual combination of distinct external versus internal surface chemistry in MOF nanocrystals and the interfacial chemistry they enable as a novel supramolecular approach for redox voltammetric anion sensing.

Informative Training Data for Efficient Property Prediction in Metal-Organic Frameworks by Active Learning

In recent data-driven approaches to material discovery, scenarios where target quantities are expensive to compute and measure are often overlooked. In such cases, it becomes imperative to construct a training set that includes the most diverse, representative, and informative samples. Here, a novel regression tree-based active learning algorithm is employed for such a purpose. It is applied to predict the band gap and adsorption properties of metal-organic frameworks (MOFs), a novel class of materials that results from the virtually infinite combinations of their building units. Simpler and low dimensional descriptors, such as those based on stoichiometric and geometric properties, are used to compute the feature space for this model owing to their ability to better represent MOFs in the low data regime. The partitions given by a regression tree constructed on the labeled part of the data set are used to select new samples to be added to the training set, thereby limiting its size while maximizing the prediction quality. Tests on the QMOF, hMOF, and dMOF data sets reveal that our method constructs small training data sets to learn regression models that predict the target properties more efficiently than existing active learning approaches, and with lower variance. Specifically, our active learning approach is highly beneficial when labels are unevenly distributed in the descriptor space and when the label distribution is imbalanced, which is often the case for real world data. The regions defined by the tree help in revealing patterns in the data, thereby offering a unique tool to efficiently analyze complex structure-property relationships in materials and accelerate materials discovery.

Controllable Structural Transformation of Non-Porous Propyl-Malonate Hexakis[60]fullerene by Chloroform Uptake/Release

The design of dynamic structures with high recognition host-guest materials capable to host selectively small volatile molecules is an emergent field of research with both fundamental and applied implications. The challenge of exploring novel materials with advanced functionalities has led to the development of dynamic crystalline structures promoted by soft interactions. Here, a new pure organic dynamic framework based on hexakis[60]fullerene that are held together by weak van der Waals interactions is described. This crystalline structure is capable of absorbing and releasing chloroform, through internal structural reorganization. This research provides new insight into the design of organic molecular crystals for selective adsorption applications.

Double-emitting lanthanide metal-organic frameworks composed of Eu/Tb doping and ratiometric fluorescence detection of nitrofurazone

Lanthanide metal-organic frameworks (LnMOFs) have substantial potential in luminescence due to their unique antenna effect. Nevertheless, the single emission is susceptible to pseudo-signals caused by external environmental conditions, which significantly threaten the accurate measurement of the concentration. In this case, we prepared a dual-emission fluorescent probe {EuxTb1-x(NH2-BDC)3(DMF)4·2DMF}∞ (NH2-BDC = Diaminoterephthalic acid, DMF = N,N-dimethylformamide). The stable dual-emission signal provides a superior signal output for detecting nitrofurazone (NFZ), which is detected by the probe with excellent fluorescence for 0-10 μM NFZ. In the investigation of the detection mechanism, it is speculated that NFZ incorporates with probe to generate a novel complex. Furthermore, The UV absorption curves of the novel complexes and NFZ overlap extensively with those of the probe. The addition of NFZ attenuates the characteristic luminescence of Eu and Tb by competing for the absorption of the excitation light of the probe. The probe has exhibits rapid response, excellent sensitivity, visual detection and a meagre detection limit (LOD = 0.013 μM) for the detection of NFZ. This work not only broadens the application of LnMOFs in the field of ratiometric detection but also provides a favorable fluorescent probe for the quantitative detection of NFZ.

Synergistic binding sites in a metal-organic framework for the optical sensing of nitrogen dioxide

Luminescent metal-organic frameworks are an emerging class of optical sensors, able to capture and detect toxic gases. Herein, we report the incorporation of synergistic binding sites in MOF-808 through post-synthetic modification with copper for optical sensing of NO₂ at remarkably low concentrations. Computational modelling and advanced synchrotron characterization tools are applied to elucidate the atomic structure of the copper sites. The excellent performance of Cu-MOF-808 is explained by the synergistic effect between the hydroxo/aquo-terminated Zr₆O₈ clusters and the copper-hydroxo single sites, where NO₂ is adsorbed through combined dispersive- and metal-bonding interactions.

Multicolor luminescent material based on interaction between TiNbO5- nanosheets and lanthanide ions for visualization of pH change in inorganic gel electrolyte

Layered nanosheet materials showing a drastic luminescence change in response to changes in proton concentration (pH) were prepared by sandwiching Eu³⁺ and Tb³⁺ cations with anionic TiNbO₅^- nanosheets using electrostatic interaction. Each trivalent lanthanide ion showed a different response to pH change: a strong red emission from Eu³⁺ was observed at low proton concentrations (pH: 13) and a green emission from Tb³⁺ was dominant at high proton concentrations (pH: 1). The photoluminescence intensity was determined by the balance between the photocatalytic activity of TiNbO₅^- nanosheets and energy transfer from the host layer to the guest lanthanide ions. Moreover, the trivalent lanthanide/TiNbO₅^- nanosheet hybrid formed a gel-like solid in aqueous solution, which functioned as an inorganic gel electrolyte when mixed with Na₂SO₄. The multicolor luminescence (red-yellow-green) of the lanthanide/TiNbO₅^- nanosheet hybrid enabled direct visualization of the diffusion of protons in an inorganic gel electrolyte during water electrolysis.

Optical Oxygen Sensors Show Reversible Cross-Talk and/or Degradation in the Presence of Nitrogen Dioxide

A variety of luminescent dyes including the most common indicators for optical oxygen sensors were investigated in regard to their stability and photophysical properties in the presence of nitrogen dioxide. The dyes were immobilized in polystyrene and subjected to NO₂ concentrations from 40 to 5500 ppm. The majority of dyes show fast degradation of optical properties due to the reaction with NO₂. The class of phosphorescent metalloporphyrins shows the highest resistance against nitrogen dioxide. Among them, palladium(II) and platinum(II) complexes of octasubstituted sulfonylated benzoporphyrins are identified as the most stable dyes with almost no decomposition in the presence of NO₂. The phosphorescence of these dyes is reversibly quenched by nitrogen dioxide. Immobilized in various polymeric matrices, the sulfonylated Pt(II) benzoporphyrin demonstrates about one order of magnitude more efficient quenching by NO₂ than by molecular oxygen. Our study demonstrates that virtually all commercially available and reported optical oxygen sensors are likely to show either irreversible decomposition in the presence of nitrogen dioxide or reversible luminescence quenching. They should be used with extreme caution if NO₂ is present in relatively high concentrations or it may be generated from other species such as nitric oxide. As an important consequence of nearly anoxic systems, production of nitrogen dioxide or nitric oxide may be therefore erroneously interpreted as an increase in oxygen concentration.

Amino-Functionalized Single-Lanthanide Metal-Organic Framework as a Ratiometric Fluorescent Sensor for Quantitative Visual Detection of Fluoride Ions

Excessive content of fluoride ions (F^-) in water will lead to water pollution and endanger human health, so the research on the method of low-cost, rapid, and efficient detection of F^- is of particular significance. In this work, an amino-functionalized ligand with an appropriate triplet energy excited state, 2'-amino-[1,1':4',1″-terphenyl]-3,3″,5,5″-tetracarboxylic acid (H₄TPTC-NH₂), was selected to construct a luminescent single-lanthanide metal-organic framework, EuTPTC-NH₂, with uncoordinated amino groups for the detection of F^-. Based on host-guest interactions, that is, hydrogen bonds formed between the free amino groups and F^- ions, EuTPTC-NH₂ was developed as a ratiometric fluorescence probe for F^- detection with good anti-interference ability, low detection limit, high water stability, and selectivity. It was found that EuTPTC-NH₂ has an excellent linear response to F^- in the concentration range of 0-80 μM with high sensitivity and a low detection limit of 11.26 μM. A hydrogel membrane based on the combination of EuTPTC-NH₂ and agarose was also prepared for the quantitative visual detection of F^- in water.

Recent Progress in Metal-Organic Framework Based Fluorescent Sensors for Hazardous Materials Detection

Population growth and industrial development have exacerbated environmental pollution of both land and aquatic environments with toxic and harmful materials. Luminescence-based chemical sensors crafted for specific hazardous substances operate on host-guest interactions, leading to the detection of target molecules down to the nanomolar range. Particularly, the luminescence-based sensors constructed on the basis of metal-organic frameworks (MOFs) are of increasing interest, as they can not only compensate for the shortcomings of traditional detection techniques, but also can provide more sensitive detection for analytes. Recent years have seen MOFs-based fluorescent sensors show outstanding advantages in the field of hazardous substance identification and detection. Here, we critically discuss the application of MOFs for the detection of a broad scope of hazardous substances, including hazardous gases, heavy metal ions, radioactive ions, antibiotics, pesticides, nitro-explosives, and some harmful solvents as well as luminous and sensing mechanisms of MOF-based fluorescent sensors. The outlook and several crucial issues of this area are also discussed, with the expectation that it may help arouse widespread attention on exploring fluorescent MOFs (LMOFs) in potential sensing applications.

Synthesis, Crystal Structure, and Luminescence Properties of the Iso-Reticular Series of Lanthanide Coordination Polymers Synthesized from Hexa-Lanthanide Molecular Precursors

Microwave-assisted reactions in DMSO, between a hexa-lanthanide octahedral complex ([Ln₆(μ₆-O)(μ₃-OH)₈(NO₃)₆(H₂O)₁₂·2NO₃·2H₂O] with Ln = Nd-Yb plus Y) and either 3-halogenobenzoic acid (hereafter symbolized by 3-xbH with x = f or c for fluoro or chloro, respectively) or 4-halogenobenzoic acid (hereafter symbolized by 4-xbH with x = f, c, or b for fluoro, chloro, or bromo, respectively), lead to 1D lanthanide coordination polymers. These coordination polymers are almost iso-reticular. The crystal structure is described on the basis of the coordination polymer with chemical formula [Tb(4-fb)₃(DMSO)(H₂O)₂·DMSO]_∞ obtained from 4-fluorobenzoic acid (4-fbH) and the Tb³⁺-based octahedral complex: It crystallizes in the triclinic system, space group P1̅ (n°2), with the following cell parameters: a = 9.8561(9) Å, b = 10.5636(9) Å, c = 15.1288(15) Å, α = 100.840(3)°, β = 95.552(3)°, γ = 110.482(3)°, V = 1426.4(3) ų, and Z = 2. It can be described on the basis of 1D molecular chains. Luminescence properties of the Tb and Eu derivatives have been measured and compared vs the halogeno-function and its position (meta or para). Some molecular alloys have also been prepared to estimate the strength of the intermetallic energy transfers. To confirm that the hexa-nuclear complexes (and not the halogenated ligand) have a structuring effect for the formation of the straight chain-like molecular motif, another coordination polymer with chemical formula [Tb(4-npa)₃DMSO·DMSO·H₂O]_∞ where 4-npaH symbolizes 4-nitro-phenyl-acetic acid has been prepared. It crystallizes in the triclinic system, space group P1̅ (n°2) with the following cell parameters: a = 7.8784(8) Å, b = 14.8719(16) Å, c = 15.2753(17) Å, α = 73.612(4)°, β = 86.406(4)°, γ = 83.104(4)°, V = 1703.8(3) ų, and Z = 2. Its crystal structure can be described on the basis of a molecular motif that is similar to the one observed in the five previous crystal structures which confirms the structuring effect of the hexa-nuclear complexes.

N-doped three-dimensional needle-like CoS2 bridge connection Co3O4 core-shell structure as high-efficiency room temperature NO2 gas sensor

The N-doped three-dimensional (3D) needle bridge connection core-shell structure N-CoS₂@Co₃O₄ synthesized in this work was prepared by simple hydrothermal and high-temperature vulcanization methods. The optimized N-CoS₂@Co₃O₄^-2 composite response to NO₂ is 62.3-100 ppm, a response time of 1.3 s, the recovery time of 17.98 s, the detection limit of 5 ppb and stability of as long as 10 weeks at room temperature (RT). Its excellent NO₂ sensing performance is attributed to the unique porous and bridge connection core-shell structure of the N-CoS₂@Co₃O₄^-2 with high specific surface area, interconnected internal channels, abundant exposed S edge active sites, and high catalytic performance promoted by N-doping. This simple manufacturing method of high-performance sensing materials paves the way for the design of N-doped bridge connection core-shell structures.

2-Dimensional rare earth metal–organic frameworks based on a hexanuclear secondary building unit as efficient detectors for vapours of nitroaromatics and volatile organic compounds

A new family of microporous 2-dimensional rare earth metal organic frameworks based on a hexanuclear secondary building unit with capability to selectively detect vapours of volatile organic compounds and nitroaromatic explosives is reported.

Tb3+ luminescence cholate hydrogel-based multi-functionalized platform for Hg2+ and NO2 detection

In this paper, Tb3+ luminescence cholate hydrogel (Tb3+/hydrogel) was selected as a multi-functionalized platform, and PS-BD@Tb3+/hydrogel or PS-BS@Tb3+/hydrogel systems were fabricated respectively for selective detecting of Hg2+ in water and...

Metal-Organic-Frameworks: Low Temperature Gas Sensing and Air Quality Monitoring

As an emerging class of hybrid nanoporous materials, metal-organic frameworks (MOFs) have attracted significant attention as promising multifunctional building blocks for the development of highly sensitive and selective gas sensors due to their unique properties, such as large surface area, highly diversified structures, functionalizable sites and specific adsorption affinities. Here, we provide a review of recent advances in the design and fabrication of MOF nanomaterials for the low-temperature detection of different gases for air quality and environmental monitoring applications. The impact of key structural parameters including surface morphologies, metal nodes, organic linkers and functional groups on the sensing performance of state-of-the-art sensing technologies are discussed. This review is concluded by summarising achievements and current challenges, providing a future perspective for the development of the next generation of MOF-based nanostructured materials for low-temperature detection of gas molecules in real-world environments.

Tunable Proton Conductivity and Color in a Nonporous Coordination Polymer via Lattice Accommodation to Small Molecules

Nonporous coordination polymers (npCPs) able to accommodate molecules through internal lattice reorganization are uncommon materials with applications in sensing and selective gas adsorption. Proton conduction, extensively studied in the analogue metal-organic frameworks under high-humidity conditions, is however largely unexplored in spite of the opportunities provided by the particular sensitivity of npCPs to lattice perturbations. Here, AC admittance spectroscopy is used to unveil the mechanism behind charge transport in the nonporous 1·2CH3 CN. The conductance in the crystals is found to be of protonic origin. A vehicle mechanism is triggered by the dynamics of the weakly coupled acetonitrile molecules in the lattice that can be maintained by a combination of thermal cycles, even at low humidity levels. An analogue 1·pyrrole npCP is formed by in situ exchange of these weakly bound acetonitrile molecules by pyrrole. The color and conduction properties are determined by the molecules weakly bonded in the lattice. This is the first example of acetonitrile-mediated proton transport in an npCP showing distinct optical response to different molecules. These findings open the door to the design of switchable protonic conductors and capacitive sensors working at low humidity levels and with selectivity to different molecules.

Strongly Bound Excitons in Metal-Organic Framework MOF-5: A Many-Body Perturbation Theory Study

During the past years, one of the most iconic metal-organic frameworks (MOFs), MOF-5, has been characterized as a semiconductor by theory and experiments. Here we employ the GW many-body perturbation theory in conjunction with the Bethe-Salpeter equation to compute the electronic structure and optical properties of this MOF. The GW calculations show that MOF-5 is a wide-band-gap insulator with a fundamental gap of ∼8 eV. The strong excitonic effects, arising from highly localized states and low screening, result in an optical gap of 4.5 eV and in an optical absorption spectrum in excellent agreement with experiments. The origin of the incorrect conclusion reported by past studies and the implication of this result are also discussed.

Multidimensional Ln-Aminophthalate Photoluminescent Coordination Polymers

The development of straightforward reproducible methods for the preparation of new photoluminescent coordination polymers (CPs) is an important goal in luminescence and chemical sensing fields. Isophthalic acid derivatives have been reported for a wide range of applications, and in addition to their relatively low cost, have encouraged its use in the preparation of novel lanthanide-based coordination polymers (LnCPs). Considering that the photoluminescent properties of these CPs are highly dependent on the existence of water molecules in the crystal structure, our research efforts are now focused on the preparation of CP with the lowest water content possible, while considering a green chemistry approach. One- and two-dimensional (1D and 2D) LnCPs were prepared from 5-aminoisophthalic acid and Sm³⁺/Tb³⁺ using hydrothermal and/or microwave-assisted synthesis. The unprecedented LnCPs were characterized by single-crystal X-ray diffraction (SCRXD), powder X-ray diffraction (PXRD), Fourier transform infrared (FT-IR) spectroscopy and scanning electron microscopy (SEM), and their photoluminescence (PL) properties were studied in the solid state, at room temperature, using the CPs as powders and encapsulated in poly(methyl methacrylate (PMMA) films, envisaging the potential preparation of devices for sensing. The materials revealed interesting PL properties that depend on the dimensionality, metal ion, co-ligand used and water content.

A Fluorescent Metal-Organic Framework for Food Real-Time Visual Monitoring

Due to increasing food-safety issues, exploiting efficient approaches for food quality assessment and instrumentation has attracted concerns worldwide. Herein, a smart evaluation system based on a fluorescent metal-organic framework (MOF) is developed for real-time visual monitoring of food freshness. Via post-synthetic modification, a ratiometric fluorescent MOF probe is constructed by covalently coupling fluorescein 5-isothiocyanate (5-FITC) with NH₂ -rich lanthanide MOF. The probes exhibit a dual-emissive-responsive to biogenic amine, resulting in an increase in FITC emission along with a decrease in Eu³⁺ emission accompanied by a clear distinguishable color transition from orange red to green. After doping the probes on a flexible substrate, the obtained MOF composite film can be integrated with a smartphone-based portable platform easily. It is proved that this smart evaluation system can be used for on-site inspection of the freshness of raw fish samples. This work develops a fluorescent MOF-based smart evaluation system as a novel platform for application in food monitoring, which not only has enormous economic value but also holds great public health significance.

Highly Luminescent Europium-Based Heteroleptic Coordination Polymers with Phenantroline and Glutarate Ligands

Isostructural lanthanide-based coordination polymers with general chemical formula [Ln(phen)(glu)(NO₃)]_∞ with Ln = La-Tm (except Ce and Pm) have been synthesized by hydrothermal methods (H₂glu stands for glutaric acid and phen stands for 1,10-phenantroline). They crystallize in the monoclinic system with the P2₁/c (no. 14) space group. The crystal structure has been solved on the basis of the La derivative. It can be described as the superimposition of molecular chains of dimeric La(phen)(NO₃)-La(phen)(NO₃) units bridged by glutarate ligands. Luminescent properties have been explored and show that the Eu derivative exhibits the highest luminance observed for Eu-based coordination polymers (85 to 105 cd·m^-2). Effects of the dilution of the Eu³⁺ and Tb³⁺ luminescent ions by Gd³⁺ optically inactive ions are unexpected and to the best of our knowledge unprecedented. This could be related to the different intermetallic energy-transfer mechanisms in competition and to the nonisotropic distribution of the lanthanide ions in these molecular alloys. The investigation of molecular alloys with general chemical formula [Eu_1-xTb_x(phen)(glu)(NO₃)]_∞ with 0 ≤ x ≤ 1 highlights a very sizable and constant Eu³⁺ luminescence whatever the x value, which further confirms the existence of very strong intermetallic energy transfers in this family of compounds. It is also noticeable that some coordination polymers based on weakly emissive lanthanide ions exhibit very well defined emission spectra.

The construction of a novel luminescent lanthanide framework for the selective sensing of Cu2+ and 4-nitrophenol in water

It is challenging to develop highly stable lanthanide luminescent sensors for detecting heavy metal ions and nitroaromatics in view of the human health and environmental security. To this end, two water stable Ln-MOFs with the chemical constitution of {[Ln(HL)]·3DMF·3H2O}n (Ln = Eu, LZG-Eu and Ln = Tb, LZG-Tb) have been developed solvothermally using a multidentate ligand (H4L) with the central phenyl backbone bisubstituted by 2,6-pyridine-dicarboxylic acid at the para-position, H4L = 1,4-bis(2',2'',6',6''-tetracarboxy-1,4':4,4''-pyridyl)benzene. Single crystal analysis demonstrates that two novel Ln-MOFs feature 4,4,4-connected nets with an unprecedented topology symbol of {42·6·83}2{42·62·82}{42·84} and contain two kinds of one-dimensional channels. Powder X-ray diffraction as well as the luminescence determination results indicate that they retain their crystallinity and structural integrity in harsh acidic and basic conditions with pH in the range of 4-11. Moreover, they are highly luminescent, which makes them excellent chemical sensors for detecting Cu2+ and 4-NP (4-nitrophenol) with high selectivity and sensitivity in aqueous media such as deionized water, tap water, and river water based on distinct quenching effects. To the best of our knowledge, their detection limits are lower than those documented so far. In addition, the quenching efficiency of 4-NP was retained in the presence of interfering ions even after the compounds were used for five cycles, which makes them attractive, reliable, visual, and recyclable luminescent Ln-MOF sensor materials for 4-NP. The recognition mechanism for Cu2+ could be attributed to the dissociation of the main framework induced by Cu2+ and the subsequent formation of a Cu2+ coordination species and that for 4-NP is considered to be multi-quenching mechanisms dominated by competition absorption.

Luminescence response mode and chemical sensing mechanism for lanthanide-functionalized metal–organic framework hybrids

This comprehensive review systematically summarizes the luminescence response mode and chemical sensing mechanism for lanthanide-functionalized MOF hybrids (abbreviated as LnFMOFH).

Functional metal–organic frameworks as effective sensors of gases and volatile compounds

This review summarizes the recent advances of metal organic framework (MOF) based sensing of gases and volatile compounds.