A luminescent metal-organic framework with pre-designed functionalized ligands as an efficient fluorescence sensing for Fe3+ ions

Imine-Decorated Copper-Based Metal-Organic Framework for the Photodegradation of Methylene Blue

A low cost imine-decorated linker, 4,4'-(hydrazine-1,2-diylidenedimethylylidene)dibenzoic acid was utilized for the preparation of copper-based metal-organic framework (MOF) denoted as Cu-L via a solvothermal technique. The synthesized MOF material has been fully characterized by different analytical techniques such as Fourier-transform infrared (FT-IR) spectroscopy, powder X-Ray diffraction (PXRD), scanning electron microscopy (SEM), energy dispersive X-Ray spectroscopy (EDX), nitrogen adsorption-desorption isotherm analysis, and thermogravimetric analysis (TGA). It has been found that the coordination of Cu2+ with L considerably reduced the band gap of the L of nearly about 1 eV, which is approximately 26% decline in total. Notably, a narrow band gap of the photocatalyst is an essential requirement for the proficient photodegradation of organic contaminants. An excellent optical properties and narrow band gap of (2.8 eV) of Cu-L ensure their suitability as a photocatalyst for the degradation of methylene blue (MB) dye. In the presence of Cu-L photocatalyst, 84.22% degradation of MB dye was observed after 150 min under sunlight exposure. It is the first time that imine-functionalized MOF was utilized for the degradation of MB dye under sunlight irradiation. For understanding the photodegradation of MB dye by the Cu-L photocatalyst, all the plausible mechanistic studies have been carried out in detail. Both theoretical (with the help of density functional theory (DFT) calculations) as well as experimental studies have been conducted to justify the possible mechanisms for the photodegradation of MB dye by Cu-L. The current work may open a new opportunity to construct a cheap MOF-based photocatalysts for fast degradation of dye contaminants.

Amine/hydrazone functionalized Cd(II)/Zn(II) metal-organic framework for ultrafast sensitive detection of hazardous 2,4,6-trinitrophenol in water

Amine/hydrazone functionalized dual ligand Cd(II)/Zn(II) based metal-organic frameworks (MOFs) denoted as CdMOF- and ZnMOF-NH₂, respectively were synthesized via a simple conventional high-yield reflux method using low-cost and readily available starting materials, i.e., a Schiff base linker, 4-pyridylcarboxaldehydeisonicotinoylhydrazone (L₁) and 2-aminoterephthalic acid (H₂ata) linker. Crystallographic and thermogravimetric studies confirmed the formation of MOFs with good crystallinity and thermal stability. Photoluminescence studies point out that both MOFs can be used efficiently for fast sensing of 2,4,6-trinitrophenol (TNP) in water with noticeable turn-off quenching response. Their limits of detection (LODs) for TNP were 7 ppb and 10 ppb, respectively with enhanced selectivity toward TNP (over other nitro explosives) as verified by competitive nitro explosive tests. Density functional theory calculations and spectral overlap were used to assess the sensing mechanism. These MOF-based fluorescent sensing systems for TNP are demonstrated to have easy recoverability and high sensitivity.

Highly fluorescent nickel based metal organic framework for enhanced sensing of Fe3+ and Cr2O72- ions

Heavy metal contamination has sparked widespread concern among the populace. The significant issues necessitate the creation of high-performance fluorescent pigments that can identify harmful elements in water. The present study deals with metal organic framework [MOF] based on nickel [Ni-BDC MOF]. The Ni-BDC MOF was prepared by facile solvothermal method using nickel nitrate hexahydrate and terephthalic acid ligand as precursors. The MOF was characterized by various techniques in order to examine the crystal, morphological, structural, composition, thermal and optical properties. The detailed characterizations revealed that the synthesized Ni-BDC MOF are well-crystalline with high purity and possessing 3D rhombohedral microcrystals with rough surface. The MOF demonstrate good luminescence performance and excellent water stability. According to the Stern Volmer plot, the tests set up under optimized conditions demonstrate a linear correlation between the fluorescence intensity and concentration of both ions, i.e. Fe³⁺, and Cr₂O₇^2- ions. The linear range and detection limit for Fe³⁺ and Cr₂O₇^2- were found to be 0-1.4 nM and 0.159 nM, and 0-1 nM and 0.120 nM, respectively. The mechanisms for the selective detection of cations and anions were also explored. The recyclability for the prepared MOF was checked up to five cycles which showed excellent stability with just a slight reduction in efficiency. The constructed sensor was also used to assess the presence of Fe³⁺ and Cr₂O₇^2- ions in actual water samples. The results of the different experiments revealed that the prepared MOF is a good material for detecting Fe³⁺ and Cr₂O₇^2- ions.

A Luminescent Cu(II)-MOF with Lewis Basic Schiff Base Sites for the Highly Selective and Sensitive Detection of Fe3+ Ions and Nitrobenzene

A Schiff base functionalized Cu(II)-based metal-organic framework (MOF) denoted as Cu-L, was developed via a solvothermal method using low-cost starting material, i.e., Schiff base linker, 4,4'-(hydrazine-1,2-diylidenedimethylylidene)dibenzoic acid (L). Good crystallinity and thermal stability of synthesized Cu-L was confirmed by the crystallographic and thermogravimetric studies. An excellent photoluminescent properties of Cu-L ensure their suitability for the ultrafast detection of Fe³⁺ ions and nitrobenzene via a turn-off quenching response. The remarkable sensitivity of Cu-L towards Fe³⁺ ions and nitrobenzene was certified by the low limit of detection (LOD) of 47 ppb and 0.004 ppm, respectively. With incorporated free azine groups, this MOF could selectively capture Fe³⁺ ions and nitrobenzene in aqueous solution. The plausible mechanistic pathway for the quenching in the fluorescence intensity of the Cu-L in the presence of Fe³⁺ ions and nitrobenzene have been explained in detail through the density functional theory calculations, photo-induced electron transfer (PET), fluorescence resonance energy transfer (FRET), and competitive energy adsorption. This present study open a new avenue to synthesize novel crystalline MOF-based sensing materials from cheap Schiff base linkers for fast sensing of toxic pollutants.

Schiff Base-functionalized Metal-organic Frameworks for Selective Sensing of Chromate and Dichromate in Water

In this research, Zn- or Cd-based metal-organic frameworks (coded ZnMOF-1 and CdMOF-1) containing benzene-1,4-dicarboxylic acid (H₂bdc) and pyridyl-based Schiff base (4-pyridylcarboxaldehydeisonicotinoylhydrazone (L)) dual ligands were successfully assembled via a conventional solvothermal method. The photoluminescence quenching response of ZnMOF-1 and CdMOF-1 and their sensing sensitivity and selectivity towards various inorganic anions were evaluated in aqueous media. Crystallographic and thermogravimetric studies confirm the formation of both MOFs with good crystallinity and thermal stability. Photoluminescence studies also verify the selectivity of ZnMOF-1 and CdMOF-1 for efficient sensing of inorganic oxyanions (like chromate/dichromate: CrO₄^2- and Cr₂O₇^2-). Further, it was noted that only chromate/dichromate (CrO₄^2-/Cr₂O₇^2-) anions showed a significant turn-off quenching effect while other anions (like F^-, Br^-, I^-, Cl^-, ClO₄^-, SCN^-, SO₄^2-, NO₃^-, and NO₂^-) have a low/negligible effect on the photoluminescence intensity of both MOFs. The limit of detection (LOD) of chromate/dichromate by ZnMOF-1 and CdMOF-1 was 9.79/10.94 µM and 2.68/1.48 µM, respectively. A probable mechanism for turn-off quenching response towards chromate and dichromate anions could be attributed to the spectral overlap of both excitation and emission spectra of ZnMOF-1/CdMOF-1 with the absorption spectra of chromate/dichromate anions. As a result, the energy transfer from ZnMOF-1 or CdMOF-1 to the target chromate and dichromate anions decreased fluorescence intensity (i.e., fluorescence quenching effect).

Metal-organic frameworks (MOFs) based nanofiber architectures for the removal of heavy metal ions

Environmental heavy metal ions (HMIs) accumulate in living organisms and cause various diseases. Metal-organic frameworks (MOFs) have proven to be promising and effective materials for removing heavy metal ions from contaminated water because of their high porosity, remarkable physical and chemical properties, and high specific surface area. MOFs are self-assembling metal ions or clusters with organic linkers. Metals are used as dowel pins to build two-dimensional or three-dimensional frameworks, and organic linkers serve as carriers. Modern research has mainly focused on designing MOFs-based materials with improved adsorption and separation properties. In this review, for the first time, an in-depth look at the use of MOFs nanofiber materials for HMIs removal applications is provided. This review will focus on the synthesis, properties, and recent advances and provide an understanding of the opportunities and challenges that will arise in the synthesis of future MOFs-nanofiber composites in this area. MOFs decorated on nanofibers possess rapid adsorption kinetics, a high adsorption capacity, excellent selectivity, and good reusability. In addition, the substantial adsorption capacities are mainly due to interactions between the target ions and functional binding groups on the MOFs-nanofiber composites and the highly ordered porous structure.

Anions-induced two stable isostructural Cd(II) LMOFs based on benzotriazole with highly selective detection of Fe3+ ion

Two new title LMOFs, [Cd9(BTA)6(5-tbuip)6O2]n (LMOF-1), [Cd3(BTA)2(5-tbuip)2]n (LMOF-2) (BTA=1H-Benzotriazole, 5-tbuip=5-tert-Butylisophthalic Acid) have been synthesized by solvothermal method. The different anions of cadium salts lead to isostructural two LMOFs. Fe3+ could...

Design of turn-on luminescent sensor based on nanostructured molecularly imprinted polymer-coated zirconium metal-organic framework for selective detection of chloramphenicol residues in milk and honey

Herein, an optical sensor based on nanostructured molecularly imprinted polymer (MIP) coated on a luminescent zirconium metal-organic framework (MIP/Zr-LMOF) is introduced, and its performance is investigated for the fluorescent determination of chloramphenicol (CAP) antibiotic residues in milk and honey. To fabricate the sensor, the surface of Zr-LMOF is modified with MIP in the presence of CAP template, resulting in the introduction of recognition sites for antibiotic molecules. The porous structure of Zr-LMOF with specific binding sites for CAP recognition benefiting from coated MIP leads to selective and sensitive detection of antibiotic. The probe yields a linear range for detection of CAP in trace concentrations (0.16-161.56 µg.L^-1) and provides a detection limit of 0.013 µg.L^-1. Acceptable recoveries are achieved for antibiotic in real samples, which are consistent with that obtained from liquid chromatography-tandem mass spectrometry (LC-MS/MS), confirm the favorable performance of sensor for accurate determination of CAP in practical applications.

Designing an "Off-On" Fluorescence Sensor Based on Cluster-Based CaII-Metal-Organic Frameworks for Detection of l-Cysteine in Biological Fluids

Recently, luminescent metal-organic framework (MOF) materials have attracted considerable attention in fluorescence sensing. In this essay, we prepared a new cluster-based Ca^II-MOFs {[Ca_1.5(μ₈-HL₁)(DMF)₂]·DMF}_n (1) with good water dispersibility, excellent photoluminescence properties (FL quantum yield of 20.37%) and great fluorescence stability. Further, it was employed to design as an "off-on" fluorescence sensor for sensitive detection of l-cysteine. This proposed strategy was that fluorescence of Ca^II-MOFs 1 was quenched for providing a low fluorescence background by the introduction of Pb²⁺ forming the Ca^II-MOFs 1/Pb²⁺ hybrid system. The quenching effect could be ascribed to the static quenching mechanism because of the formation of ground-state complexes and coordination interactions between the free carboxyl of H₄L₁ ligands of Ca^II-MOFs 1 and Pb²⁺. Then, with the addition of l-cysteine into the Ca^II-MOFs 1/Pb²⁺ hybrid system, the fluorescence signal was immediately restored. This result was because the Pb²⁺ was gradually released from the hybrid system by chelation interactions between the -SH groups of l-cysteine and Pb²⁺. This method received a relative wide linear range varying from 0.05 to 40 μM and a low detection limit of 15 nM for detection of l-cysteine. This proposed strategy was also successfully applied to detect l-cysteine in human serum samples with satisfactory recoveries from 95.9 to 101.5%.