Stable Lanthanide Metal-Organic Frameworks with Ratiometric Fluorescence Sensing for Amino Acids and Tunable Proton Conduction and Magnetic Properties

A novel ratiometric sensor for fluorimetric and visual dual-mode detection of Al3+ in environmental water based on the target-regulated formation of Eu MOFs

Herein, a novel ratiometric sensor for fluorimetric and smartphone-assisted visual detection of Al3+ in environmental water was developed based on the target-regulated formation of Eu metal-organic frameworks (Eu MOFs). By employing 2-[4-(2-hydroxyethyl) piperazin-1-yl] ethanesulfonic acid (Hepes), Eu3+ and tetracycline (TC) as raw materials, Eu MOFs with red emission were facilely synthesized through the coordination of Eu3+ with Hepes and TC. However, upon the introduction of Al3+, a higher affinity of TC towards Al3+ resulted in the formation of a TC-Al3+ complex with green fluorescence and inhibited the generation of Eu MOFs. This led to an increase in green fluorescence and a decrease in red fluorescence accompanied by the fluorescence color of the solution changing from red to green under the illumination of the UV lamp. Thus, a ratiometric sensor for fluorimetric and the smartphone-assisted visual detection of Al3+ was established. The ratiometric sensor exhibited high sensitivity for Al3+ detection with a detection limit of 0.14 μM for fluorescence detection and 1.21 μM for visual detection. Additionally, the proposed strategy was successfully applied to detect Al3+ in the environmental water samples with satisfactory results, indicating great application prospects for environmental monitoring.

On-Off Ratiometric Fluorescence Europium(III) Metal-Organic Framework for Quantitative Detection of the Inflammatory Marker Neopterin

Benefiting from the unique photoluminescence behavior of the lanthanide(III) ions and organic ligands, a lanthanide(III) metal-organic framework (Ln-MOF) material can simultaneously demonstrate photoluminescence of lanthanide(III) cations and organic molecules and endow its superior applications of fluorescence sensing behaviors. Herein, we present a europium(III) MOF material {[Eu2(BPTA)·(CH3COO)2·3DMA]·0.5DMA·3H2O}n (1) (where H4BPTA is 3,3',5,5'-biphenyltetracarboxylic acid) for photoluminescence performance of quantitatively sensing the inflammatory marker neopterin (Neo). The obtained 1 comprises Eu2(COO)4 paddlewheel secondary building units, which could be bridged by BPTA4- ligands to extend a 2D framework. The fluorescence titration indicates 1 can achieve simultaneous fluorescence behavior of Eu3+ ions and Neo via on-off ratiometric effects and thus could be exploited as the ratiometric fluorescence sensor matrix. Such a fluorescence phenomenon of 1 as a ratiometric sensor for quantitative detection of Neo via an on-off ratiometric effect is never observed in MOF chemistry. Moreover, naked-eye visible color variations of the fluorescence for 1 could be observed from red to blue with increasing concentrations of Neo, confirmed by fluorescent test strips as well as portable fluorescent hydrogels. And 1 also shows a low detection limit of 15.11 nM. A synergetic contribution of the competitive absorption, fluorescence resonance energy-transfer, and photoinduced electron-transfer mechanisms between Neo and the framework of 1 realizes the on-off ratiometric fluorescence behavior for Neo detection, supported by the UV-vis spectral overlap experiment and DFT calculations.

Near-infrared fluorescent probe with large Stokes shift for specific detection of lysine

A new near-infrared (NIR) fluorescent probe CL based on coumarin- dicyanoisophorone was synthesized. Addition of Lys to probe CL solution in DMF/H2O (9:1, v/v) medium resulted in noticeable enhancement in the intensity of the fluorescence emission at 702 nm, accompanying distinct color change from yellow to pink. While addition of other amino acids and biothiols (Gly, Hcy, GSH, Glu, Val, Tyr, Arg, Trp, Lys, His, Leu, Phe, Asp and Met) did not bring about substantial changes in both fluorescence emission and color. The detection limit was calculated to be 0.51 μM. Job's plot test revealed that probe CL and Lys formed a complex of 1:1 stoichiometry. Probe CL showed high stability and could be used to recognize Lys in a wide pH range of 4.0-10.0. The sensing mechanism was proposed and verified by 1H NMR spectral measurement. The dual-modal fluorescence turn-on and colorimetric NIR probe with an extremely large Stokes shift of 280 nm may be utilized for highly specific and practical sensing of Lys.

The construction of dual-emissive ratiometric fluorescent probes based on fluorescent nanoparticles for the detection of metal ions and small molecules

With the rapid development of fluorescent nanoparticles (FNPs), such as CDs, QDs, and MOFs, the construction of FNP-based probes has played a key role in improving chemical sensors. Ratiometric fluorescent probes exhibit distinct advantages, such as resistance to environmental interference and achieving visualization. Thus, FNP-based dual-emission ratiometric fluorescent probes (DRFPs) have rapidly developed in the field of metal ion and small molecule detection in the past few years. In this review, firstly we introduce the fluorescence sensing mechanisms; then, we focus on the strategies for the fabrication of DRFPs, including hybrid FNPs, single FNPs with intrinsic dual emission and target-induced new emission, and DRFPs based on auxiliary nanoparticles. In the section on hybrid FNPs, methods to assemble two types of FNPs, such as chemical bonding, electrostatic interaction, core satellite or core-shell structures, coordination, and encapsulation, are introduced. In the section on single FNPs with intrinsic dual emission, methods for the design of dual-emission CDs, QDs, and MOFs are discussed. Regarding target-induced new emission, sensitization, coordination, hydrogen bonding, and chemical reaction induced new emissions are discussed. Furthermore, in the section on DRFPs based on auxiliary nanoparticles, auxiliary nanomaterials with the inner filter effect and enzyme mimicking activity are discussed. Finally, the existing challenges and an outlook on the future of DRFP are presented. We sincerely hope that this review will contribute to the quick understanding and exploration of DRFPs by researchers.

Self-Assembly of Lanthanide-Aluminum Cluster-Organic Frameworks with Magnetocaloric Effect and Luminescence

The self-assembly of the lanthanide metal-organic frameworks presents a formidable challenge but profound significance. Compared with the metal-organic frameworks based on 4f-3d ions, the chemistry of 4f-3p metal-organic frameworks has not been fully explored so far. In this study, two lanthanide-aluminum-based clusters [Ln6Al(IN)10(μ3-OH)5(μ3-O)3(H2O)8]·xH2O (x = 2, Ln = Gd, abbreviated as Gd6Al; x = 2.5, Ln = Eu, abbreviated as Eu6Al; HIN = isonicotinic acid) have been meticulously designed and obtained by hydrothermal reaction at low pH. The crystallographic study revealed that both Gd6Al and Eu6Al clusters exhibit an unprecedented sandwiched metal-organic framework holding a highly ordered honeycomb network. To our knowledge, it is the first case of Ln-Al-based cluster-organic frameworks. Furthermore, magnetic investigation of Gd6Al manifests a decent magnetic entropy change of -ΔSmmax = 28.8 J kg-1 K-1 at 2 K for ΔH = 7.0 T. Significantly, the introduction of AlIII ions into the lanthanide metal-organic frameworks displays excellent solid-state luminescent capability with a lifetime of 371.6 μs and quantum yield of 6.64%. The construction and investigation of these two Ln-Al clusters represent great progress in the 4f-3p metal-organic framework.

Fluorescence sensing and device fabrication with luminescent metal-organic frameworks

Metal-organic frameworks (MOFs) are a novel class of hybrid porous multi-functional materials consisting of metal ions/clusters and organic ligands. MOFs have exclusive benefits due to their tunable structure and diverse properties. Luminescent MOFs (LMOFs) exhibit both porosity and light emission. They display abundant host and guest responses, making them conducive to sensing. Currently, LMOF sensing research is gaining more depth, with attention given to their device and practical applications. This work reviews recent advancements and device applications of LMOFs as chemical sensors toward ions, volatile organic compounds, biomolecules, and environmental toxins. Furthermore, the detection mechanism and the correlation between material properties and structure are elaborated. This analysis serves as a valuable reference for the preparation and efficient application of targeted LMOFs.

A ratiometric fluorescent paper sensor based on dye-embedded MOF for high-sensitive detection of arginine

Ratiometric fluorescent sensors can suppress the interference of factors unrelated to analysis due to their built-in self-calibration characteristics, which exhibit higher sensitivity and more obvious visual detection in the process of qualitative and quantitative analysis. Herein, we constructed a ratiometric fluorescence probe based on fluorescent/colorimetric dual-mode method for the determination of arginine by encapsulating rhodamine B in-situ into UiO-66-NH2 MOFs (UiO-66-NH2@RhB). The as-prepared probe showed dual-emission characteristics under a single excitation wavelength. The fluorescence intensity of UiO-66-NH2 was increased significantly by arginine, while the emission peak intensity of rhodamine B remained stable, resulting in a single-signal response with fixed reference. Furthermore, the practicality of the presented sensor was successfully validated by quantitative detection of arginine in human serum. More significantly, paper-based sensors for arginine detection were devised by using carboxymethyl cellulose modified filter papers. Under the irradiation of ultraviolet light, the paper-based sensors would produce obvious color variation from lightpink to bluish violet. This work provided a convenient and efficient method for on-site detection of arginine.

Assembly of dysprosium(III) cubanes in a metal-organic framework with an ecu topology and slow magnetic relaxation

A dysprosium(III) metal-organic framework constructed using dysprosium(III) cubanes as secondary building units has been reported to exhibit field-induced slow magnetic relaxation behavior and an unprecedented ecu topology, which is the first example of an 8-connected Ln-cubane-based framework material and a rare Dy4-MOF showing slow magnetic relaxation.

Two Stable Bifunctional Zinc Metal-Organic Frameworks with Luminescence Detection of Antibiotics and Proton Conduction

Functionalized crystalline solids based on metal-organic frameworks (MOFs) enable efficient luminescence detection and high proton conductivity, making them crucial in the realms of environmental monitoring and clean energy. Here, two structurally and functionally distinct zinc-based MOFs, [Zn(TTDPa)(bodca)]·H2O (1) and [Zn(TTDPb)(bodca)]·H2O (2), were successfully designed and synthesized using 3,6-di(pyridin-4-yl)thieno[3,2-b]thiophene (TTDPa) and 2,5-di(pyridin-4-yl)thieno[3,2-b]thiophene (TTDPb) as ligands, in the presence of bicyclo[2.2.2]octane-1,4-dicarboxylic acid (H2bodca). Both 1 and 2 display a three-dimensional (3D) structure with 5-fold interpenetration, and notably, 2 forms a larger one-dimensional pore measuring 17.16 × 10.81 Å2 in size. Fluorescence experiments demonstrate that 1 and 2 can function as luminescent sensors for nitrofurantoin (NFT) and nitrofurazone (NFZ) with low detection limits, remarkable selectivity, and good recyclability. A comprehensive analysis was conducted to investigate the differing sensing effects of compounds 1 and 2 and to explore potential sensing mechanisms. Additionally, at 328 K and 98% relative humidity, 1 and 2 exhibit proton conductivity values of 2.13 × 10-3 and 4.91 × 10-3 S cm-1, respectively, making them suitable proton-conducting materials. Hence, the integration of luminescent sensing and proton conductivity in monophasic 3D Zn-MOFs holds significant potential for application in intelligent multitasking devices.

A pH-Stable Tb-MOF as Luminescence Sensor for Highly Sensitive Detection of Amino Acids through Diverse Sensing Mechanism

A luminescent Tb-MOF with excellent stability and dual-emitting properties was constructed with an amide-functionalized tetracarboxylate ligand. Tb-MOFs were initially assembled on one-dimensional Tb3+ chains, then formed a two-dimensional double-decker layer through the synergistic linking of organic ligands and bridging formic acid anions, and further fabricated the final three-dimensional structure through the connection of the organic ligands. Powder X-ray diffraction experiments revealed that Tb-MOFs not only exhibited excellent stability in water but also maintained structural integrity in the pH range of 2-12. Importantly, this Tb-MOF provided the first example of a metal-organic framework (MOF)-based luminescence sensor that can simultaneously detect two acid amino acids (aspartic and glutamic acids) through a turn-off sensing mechanism and two basic amino acids (lysine and arginine acids) through unusual turn-on and turn-off-on sensing mechanisms. Moreover, high sensitivity, low detection limit, and excellent recyclability of this sensor endow Tb-MOFs with great potential as a highly efficient amino acid fluorescence sensor in chemical detection and biological environments.

Highly Stable Rare Earth Metal-Organic Frameworks for Fluorescence Recognition of Folic Acid, Proton Conduction, and Magnetic Refrigeration

Four new isostructural rare earth metal-organic frameworks (RE-MOFs) were synthesized and full characterized, namely, {[(CH)2NH2]3[RE2(BTDBA)2(HCOO)]·5H2O·2DMF}n (H4BTDBA = (4',4'''-(benzo[c][1,2,5]thiadiazole-4,7-diyl)bis([1,1'-biphenyl]-3,5-dicarboxylic acid); RE = Eu (JXUST-34), Gd (JXUST-35), Tb (JXUST-36), and Dy (JXUST-37)). The single-crystal structures analysis shows that JXUST-34-37 are chain-based three-dimensional structures. Importantly, JXUST-34 exhibits excellent water, organic solvents, and acid-base stability, which can be used as a fluorescence sensor for folic acid and Al3+ with detection limits of 0.02 mM and 0.05 μM, respectively. The presence of free [(CH)2NH2]+ cations in the channels can engage the proton carrier during proton conduction. JXUST-34-37 display good proton conductivity, and the conductivities vary with relative humidity and temperatures, among which JXUST-37 has the highest conductivity of 9.66 × 10-3 S·cm-1 at 60 °C and 98% RH. The magnetic studies show that the -ΔSm of JXUST-35 reaches 16.13 J kg-1 K-1 at 2 K and ΔH = 7 T. JXUST-34-37 show multifunctional properties of fluorescence sensing, high proton conductivity, and magnetic refrigeration, which provides a new clue for the development of fluorescent-responsive, magnetic-refrigerant, and proton-conductive RE-MOF materials.

Dual-Emitting Mixed-Lanthanide Metal-Organic Framework for Ratiometric and Quantitative Visual Detection of 2,6-Pyridine Dicarboxylic Acid

The detection of the major biomarker of Bacillus anthracis, 2,6-dipicolinic acid (DPA), has attracted great interest in recent years. In this work, mixed-lanthanide metal-organic frameworks (M'LnMOFs), TbxEu1-x-cppa (cppa = 5-(5-carboxypyridin-3-yl)isophthalic acid), with different Tb/Eu ratios, were solvothermally synthesized. The results reveal that ratiometric fluorescent probe [Tb0.533Eu0.467-(Hcppa)1.5(H2O)(DMF)]·3H2O is water and acid-base stable and exhibits excellent sensitivity (LOD = 2.286 μM), high selectivity, and fast response (<2 min) for the detection of DPA. Due to the blocked energy transfer from Tb3+ to Eu3+ and the inner filter effect upon the addition of DPA, the fluorescent probe shows a distinct color change from orange-red to green. Furthermore, the visual detection of DPA was realized by identifying the RGB values of MOF-based agarose hydrogel films via a smartphone, highlighting the practical application of the fluorescent probe for DPA detection under aqueous solution conditions.

Stable Europium(III) Metal-Organic Framework Demonstrating High Proton Conductivity and Fluorescence Detection of Tetracyclines

A europium(III) metal-organic framework (MOF), namely, {[[(CH₃)₂NH₂]₃Eu₂(DTTP-2OH)₂(HCOO)(H₂O)]·4H₂O}_n (Eu-MOF, H₄DTTP-2OH = 2',5'-dihydroxy-[1,1':4',1″-terphenyl]-3,3″,5,5″-tetracarboxylic acid) has been assembled through solvothermal method. The Eu-MOF is a three-dimensional (3D) (4,4,8)-connected topological framework with binuclear Eu(III) clusters as secondary building units, in which a richly ordered hydrogen bonding network formed among the free H₂O molecules, dimethylamine cations, and phenolic hydroxyl groups provides a potential pathway for proton conduction. The proton conductivity reaches the category of superionic conductors (σ > 10^-4 S cm^-1) at room temperature with a maximum conductivity of 1.91 × 10^-3 S cm^-1 at 60 °C and 98% RH. Moreover, it also can be used as a fluorescence sensor in aqueous solution with detection limits of 0.14 μM for tetracycline, 0.13 μM for oxytetracycline and 0.11 μM for doxycycline. These results pave new methods for constructing MOFs with high proton conductivity and responsive fluorescence.

Novel Lanthanide-Based Metal-Organic Framework Isomer as a Double Ratiometric Fluorescent Probe for Vanillymandelic Acid

The concentration of vanillymandelic acid (VMA) in urine is closely related with pheochromocytoma diagnosis. Thus, it is essential to develop more accurate and convenient fluorescence sensing strategies toward VMA. Until now, the design of double ratiometric detection methods for VMA was still in the unexplored stage. In this work, novel Ln³⁺-based metal-organic frameworks (QBA-Eu and QBA-Gd_0.875Eu_0.125) possessing dual emission peaks was fabricated successfully, which served as isomers of YNU-1 and exhibited more excellent water stability in fluorescence and structure than the ones of YNU-1. The formation of the complex between QBA ligands and VMA molecules via hydrogen bonds within QBA-Eu frameworks produced a new emission band centered at 450 nm and resulted in the decline of monomer emission intensity for QBA at 390 nm. Owing to the reduced energy gap [ΔE (S₁ - T₁)], the antenna effect was hampered and luminescence of Eu³⁺ ions also decreased. The developed double ratiometric (I_615nm/I_475nm, I_390nm/I_475nm) fluorescence sensors based on QBA-Eu and QBA-Gd_0.875Eu_0.125 possessed the advantages of fast response (4 min), low detection limits (0.58 and 0.51; 0.22 and 0.31 μM), and wide linear ranges (2-100 and 2-80 μM), which met the requirements of pheochromocytoma diagnosis. We also applied them to determine VMA in an artificial urine sample and diluted human urine sample and obtained satisfactory results. They will become prospective fluorescence sensing platforms for VMA.

Solvothermal synthesis and device fabrication of a Eu3+-based metal-organic framework as a turn-on and blue-shift fluorescence sensor toward Cr3+, Al3+ and Ga3

A novel three-dimensional Eu³⁺-based metal-organic framework with the formula {[(CH₃)₂NH₂][Eu(BTDI)]·H₂O·DMF}_n (JXUST-25) was prepared by solvothermal method based on Eu³⁺ and 5,5'-(benzothiadiazole-4,7-diyl)diisophthalic acid (H₄BTDI) with benzothiadiazole (BTD) luminescent groups. Due to the presence of Eu³⁺ and organic fluorescence ligand, JXUST-25 displays turn-on and blue-shift fluorescence toward Cr³⁺, Al³⁺ and Ga³⁺ with limits of detection (LOD) of 0.073, 0.006 and 0.030 ppm, respectively. Interestingly, the alkaline environment can change the fluorescence of JXUST-25 toward Cr³⁺/Al³⁺/Ga³⁺ and the addition of HCl solution realizes the reversible change of the fluorescence of JXUST-25 toward Cr³⁺/Al³⁺/Ga³⁺. It is noteworthy that the fluorescent test paper and light-emitting diode lamp based on JXUST-25 can effectively detect Cr³⁺, Al³⁺ and Ga³⁺ by the visual changes. In addition, the turn-on and blue-shift fluorescence between JXUST-25 and M³⁺ ions may be caused by the host-guest interaction and the absorbance caused enhancement mechanism.

Controllable Synthesis of TbIII Metal-Organic Frameworks with Reversible Luminescence Sensing for Benzaldehyde Vapor

Two novel lanthanide metal-organic frameworks (MOFs) with the formulas [Tb(bidc)(Hbidc)(H₂O)]_n (JXUST-20) and {[Tb₃(bidc)₄(HCOO)(DMF)]·solvents}_n (JXUST-21) were synthesized based on 2,1,3-benzothiadiazole-4,7-dicarboxylic acid (H₂BTDC) under solvothermal conditions. Interestingly, benzimidazole-4,7-dicarboxylic acid (H₂bidc) was formed in situ using H₂BTDC as the starting material. The self-assembly process of the targeted MOFs with different topological structures can be controlled by the solvents and concentration of the reactants. Luminescence experiments show that JXUST-20 and JXUST-21 exhibit strong yellow-green emission. JXUST-20 and JXUST-21 can selectively sense benzaldehyde (BzH) via a luminescence quenching effect with detection limits of 15.3 and 1.44 ppm, respectively. In order to expand the practical application of MOF materials, mixed-matrix membranes (MMMs) have been constructed by mixing targeted MOFs and poly(methyl methacrylate) in a N,N-dimethylformamide (DMF) solution, which can also be used for BzH vapor sensing. Therefore, the first case of MMMs derived from Tb^III MOFs has been developed for the reversible detection of BzH vapor, providing a simple and efficient platform for the future detection of volatile organic compounds.

A porous cobalt(II)-organic framework exhibiting high room temperature proton conductivity and field-induced slow magnetic relaxation

A two-dimensional (2D) cobalt(II) metal-organic framework (MOF) constructed by a ditopic organic ligand, formulated as {[Co(Hbic)(H₂O)]·4H₂O}_n (1) (H₂bic = 1H-benzimidazole-5-carboxylic acid), was hydrothermally synthesized and structurally characterized. Single-crystal X-ray diffraction shows that the distorted octahedral Co²⁺ ions, as coordination nodes, are bridged to form 2D honeycomb networks, which are further organized into a 3D supramolecular porous framework through multiple hydrogen bonds and interlayer π-π interactions. Dynamic crystallography experiments reveal the anisotropic thermal expansion behavior of the lattice, suggesting a flexible hydrogen-bonded 3D framework. Interestingly, hydrogen-bonded (H₂O)₄ tetramers were found to be located in porous channels, yielding 1D proton transport pathways. As a result, the compound exhibited a high room-temperature proton conductivity of 1.6 × 10^-4 S cm^-1 under a relative humidity of 95% through a Grotthuss mechanism. Magnetic investigations combined with theoretical calculations reveal giant easy-plane magnetic anisotropy of the distorted octahedral Co²⁺ ions with the experimental and computed D values being 87.1 and 109.3 cm^-1, respectively. In addition, the compound exhibits field-induced slow magnetic relaxation behavior at low temperatures with an effective energy barrier of U_eff = 45.2 cm^-1. Thus, the observed electrical and magnetic properties indicate a rare proton conducting SIM-MOF. The foregoing results provide a unique bifunctional cobalt(II) framework material and suggest a promising way to achieve magnetic and electrical properties using a supramolecular framework platform.

Supramolecular encapsulation of hexaaquacobalt(II) cations in a hydrogen-bonded framework for slow magnetic relaxation and high proton conduction

The supramolecular assembly of hexaaquacobalt(II) nitrate and a tetradentate carboxylate ligand resulted in the isolation of a cobalt hydrogen-bonded organic framework (HOF). Variable-temperature X-ray diffraction experiments reveal high thermal stability of the framework sustained by charge-assisted, multiple hydrogen bonding interactions with the co-former. Interestingly, the material shows field-induced slow relaxation of magnetization originating from the magnetically anisotropic Co²⁺ ions within the supramolecular framework, revealing a rare single-ion magnet (SIM) HOF. Additionally, the HOF also exhibits high proton conductivity above 100 °C due to the extensive H-bond networks and high content of water and carboxylate within the material. More importantly, these results not only observe the magnetic and electrical properties of an old molecule but also demonstrate a significant turn-on effect of multifunctionalities from non-functional synthons achieved in a supramolecular approach.