Selective Luminescence Turn-On-Based Sensing of Phosphate in the Presence of Other Interfering Anions Using a Heterobimetallic (3d-4d) MOF with an Acidic Pocket

A luminescent metal-organic framework with the molecular formula [YMn_1.5(C₇N₁H₃O₅)₃(H₂O)₆]·11H₂O, 1 {where C₇N₁H₃O₅ = chelidamate}, was synthesized by a hydrothermal method by employing chelidamic acid as an organic ligand and Y(III) and Mn(II) as metal ions. A two-dimensional heterobimetallic structure with phenolic hydroxyl-functionalized pockets was revealed by single-crystal X-ray diffraction analysis of compound 1. PXRD, TGA, IR, BET analysis, and UV-vis spectroscopy were used for the thorough characterization of compound 1. Upon excitation at 280 nm, compound 1 shows bright blue emission, which was utilized for the selective and sensitive turn-on detection of the PO₄^3- ion. Based on Bronsted-Lowry acid-base interactions, the photoluminescence of compound 1 was enhanced in the presence of very low concentrations of the aforementioned anion. The mechanism behind the detection of the phosphate ion has been explored in detail. It was seen that the PO₄^3- anion entered the hydroxyl-functionalized pockets of compound 1 and stabilized the aromatic portion of compound 1 via molecular-level interactions through acid-base interactions. These molecular-level interactions are responsible for the enhancement of the photoluminescence intensity of compound 1 after the incorporation of phosphate ions by reducing the nonradiative transitions. These phenomena were also confirmed by time-correlated single photon counting (TCSPC) measurement, which shows that the excited-state lifetime increased with the increase in addition of phosphate anions. The calculated limit of detection (LOD) of 1 was 19.55 ppb for phosphate (PO₄^3-), which was significantly lesser than the recommended level for the PO₄^3-anion toward the human body. The luminescence enhancement coefficient, K_SV, value was also much higher than those of other reported metal-organic frameworks.

Desolvation-Degree-Induced Structural Dynamics in a Rigid Cerium-Organic Framework Exhibiting Tandem Purification of Ethylene from Acetylene and Ethane

Due to the industrial requirements for high production and high quality of ethylene, efficient purification of ethylene from acetylene and ethane is of prime importance but challenging. Dynamic metal-organic frameworks (MOFs) have demonstrated intriguing structural dynamics and diverse applications recently. Among them, although a few flexible ones have exhibited interesting ethylene purification capability, rigid ones were yet barely investigated for such purpose. In this regard, a cerium(III)-based MOF was solvothermally synthesized, which is rigid and assembled from rod molecular building blocks associated with coordinative N,N-dimethylformamide (DMF) molecules. After liberating different degrees of DMF ligands via heating under vacuum or acetone exchange, both partially desolvated compounds of Ce-MOF-1 and Ce-MOF-2 were concertedly isolated in a fashion of single-crystal to single-crystal transformation. Although both newly generated materials crystallize in the same space group, they exhibit dissimilar unit cell parameters and slightly distinct ultramicropore sizes and pore microenvironments, thanks to the discrepancy in the desolvation degree. Consequently, Ce-MOF-1 and Ce-MOF-2 individually demonstrate C₂H₂- and C₂H₆-selective adsorption behavior, resulting in the potential tandem separation of C₂H₄ from C₂H₂ and C₂H₆ mixtures. The above results were successfully supported by not only single gas adsorption isotherms but also grand canonical Monte Carlo (GCMC) calculation studies and dynamic breakthrough experiments. The present work may pave the way for rigid MOFs aiming at advancing applications via solid-state structural dynamics.

A Red-Emitting COF Ionic Exchanged With Green-Emitting Tb(III) Complex Anion: Synthesis, Characterization, Ratiometric Emission Sensing, and Removal of Picric Acid

Picric acid (PA) is an important chemical product which has been widely used in dye manufacturing, antiseptics, and pharmaceuticals. Owing to PA’s extreme electron-deficient structure, its natural degradation is hard, leading to accumulation in the environment and finally threatening the ecosystem and human health. In this case, PA detection and removal becomes more and more important, concerning environmental protection and human health. In this study, an ionic covalent organic framework (I-COF) was synthesized and modified with a luminescent Tb(III) emitter (Tb(DPA)₃^3-, DPA = pyridine-2,6-dicarboxylic acid), via ionic exchange. The resulting composite material (Tb-COF) was fully characterized by geometric analysis, IR, XRD, porosity analysis, SEM/TEM, and elemental analysis. It was found that Tb(DPA)₃^3- was loaded into the hexagonal cage in an I-COF host with an ionic exchange ratio of 41%. The as-synthesized Tb-COF showed weak Tb(III) emission and strong red COF emission, after adding PA, Tb(III) emission was increased whereas COF emission weakened greatly, showing sensing behavior. Linear working curves were observed with good selectivity. The sensing mechanism was revealed as follows. PA molecules replaced the [Tb(PDA)₃]^3- component trapped in Tb-COF, releasing free luminescent [Tb(PDA)₃]^3-. After incorporating PA in the hexagonal cage, the COF emission was quenched. This sensing mechanism ensured a good selectivity over competing species, including cations, anions, and nitrocompounds. The adsorption and removal performance of I-COF for PA were investigated as well.

Remodelling a shp: Transmetalation in a Rare-Earth Cluster-Based Metal-Organic Framework

Postsynthetic modification of metal-organic frameworks (MOFs) is an important strategy for accessing MOF analogues that cannot be easily synthesized de novo. In this work, the rare-earth (RE) cluster-based MOF Y-CU-10 with shp topology was modified through transmetalation using a series of RE ions, including La(III), Nd(III), Eu(III), Tb(III), Er(III), Tm(III), and Yb(III). In all cases, metal exchange higher than 70% was observed, with reproducible results. All transmetalated materials were fully characterized and compared to the parent MOF Y-CU-10 with regard to crystallinity, surface area, and morphology. Additionally, single-crystal X-ray diffraction measurements were performed to provide further evidence of transmetalation occurring in the nonanuclear cluster nodes of the MOF.

Coordination networks for the recognition of oxo-anions

Crystalline coordination networks with an infinite extended framework, also known as coordination polymers (CPs) or metal-organic frameworks (MOFs), have attracted significant attention owing to their tremendous performance in an extensive range of applications. The unique structural features and high stability of coordination networks are responsible for their utilization and potential in diverse fields especially in the area of sensing using luminescent CPs/MOFs. The recognition of toxic oxo-anions present in water is a crucial and first step towards environmental remediation, mainly in the case of water pollution. Accordingly, the utilization of luminescent coordination networks has received significant interest, particularly for the recognition of various toxic oxo-anions, which are considered a threat to aquatic life and the environment. In this frontier article, we summarize recent reports on luminescent CPs/MOFs, their utilization for the sensing of oxo-anions and the chemistry of the interaction of oxo-anions with CPs/MOFs, which is responsible for tuning their optical signals.

Linker-Eliminated Nano Metal-Organic Framework Fluorescent Probe for Highly Selective and Sensitive Phosphate Ratiometric Detection in Water and Body Fluids

Phosphate is an important anion in both the aquatic environment and biological systems. The search for a selective and sensitive phosphate ratiometric fluorescent probe to quantify the phosphate level in water samples and body fluids is of great significance for the protection of the ecological environment and human health. Here, a porphyrin-based nano metal-organic framework (NMOF), PCN-224, was successfully exploited as a simple but highly sensitive and selective single-component ratiometric fluorescent probe with accurate composition and measurable structure for the quantitative determination of phosphate, based on the interesting double-emission fluorescence of the porphyrin ligand itself. Compared with other zirconium-based NMOF probes for phosphate, the reduced number of connections for ZrO clusters with the ligand in PCN-224 obtained by a linker-elimination strategy simultaneously provides more active recognition sites for phosphate, which effectively improves the sensitivity of the zirconium-based NMOF probes. The detection limit of the probe is only 54 nM. Additionally, the accuracy of the ratiometric detection based on this probe was further proved by the detection of phosphate in human serum and drinking water.

Cost-effective 17O enrichment and NMR spectroscopy of mixed-metal terephthalate metal-organic frameworks

17O solid-state NMR spectroscopy is employed to investigate the cation disorder in metal-organic frameworks containing two different types of metal cations. Although NMR offers exquisite sensitivity to the local, atomic-scale structure, making it an ideal tool for the characterisation of disordered materials, the low natural abundance of 17O (0.037%) necessitates expensive isotopic enrichment to acquire spectra on a reasonable timescale. Using dry gel conversion and a novel steaming method we show that cost-effective and atom-efficient enrichment of MOFs is possible, and that high-resolution 17O NMR spectra are sensitive both to the structural forms of the MOF and the presence of guest molecules. For mixed-metal forms of MIL-53, NMR can also provide information on the final composition of the materials (notably different to that of the initial starting material) and the preference for cation clustering/ordering within the MOFs. For Al, Ga MIL-53, the distribution of cations results in a mixed-pore form upon exposure to water, unlike the different structures seen for the corresponding end members. This work shows that as good levels of enrichment can be achieved at reasonable cost, 17O NMR spectroscopy should be an invaluable tool for the study of these important functional materials.

Single-crystal-to-single-crystal (SCSC) transformation and dissolution–recrystallization structural transformation (DRST) among three new copper(ii) coordination polymers

Three new copper(ii) complexes with single-crystal-to-single-crystal (SCSC) transformation and dissolution–recrystallization structural transformation (DRST) have been synthesized and fully characterized.

Luminescent lanthanide metal–organic frameworks for chemical sensing and toxic anion detection

A co-doped lanthanide metal–organic framework (LnMOF) consisting of anion affinity sites exhibits self-calibrating, high distinguishable and stable photoluminescent signals for detecting toxic anions.