Ligand as Buffer for Improving Chemical Stability of Coordination Polymers

Chemical stability is one of the key concerns in coordination polymers (CPs). However, technologies to protect CPs against acidic or alkaline aqueous environments have yet to be implemented. Herein we demonstrate an approach for improving the pH stability by utilizing the ligand salt as buffering site to modify the unsaturated coordination sites of CPs. For the selective one-dimensional CP Eu-d-DBTA (d-H₂DBTA = d-O,O'-dibenzoyltartaric acid) with a pH stability range of 6-8, the introduction of the ligand salt Na-d-DBTA extends the pH stability interval from 3 to 11. Crystallographic structure data reveal the formation of a Eu/Na-d-DBTA dynamic structure with Na-d-DBTA buffer sites on the Eu-O cluster of the Eu-d-DBTA skeleton. Benefiting from the dynamic single-crystal-to-single-crystal transformation, the buffer sites protect the skeleton from the impact of the acidic or alkaline aqueous environment. In addition, Eu/Na-d-DBTA produces stable photoluminescence properties and selective responses toward l-tryptophan (l-Trp) and further toward l-lysine (l-Lys) over the whole buffer capacity range of 3-11. Noticeably, other Ln/Na-d-DBTA CPs and star metal-organic frameworks also exhibit pH stability improvement when the ligand-as-buffer technology is used, which is significant for developing advanced inorganic-organic hybrid materials with superior functionality.

The Chemistry and Applications of Metal-Organic Frameworks (MOFs) as Industrial Enzyme Immobilization Systems

Enzymatic biocatalysis is a sustainable technology. Enzymes are versatile and highly efficient biocatalysts, and have been widely employed due to their biodegradable nature. However, because the three-dimensional structure of these enzymes is predominantly maintained by weaker non-covalent interactions, external conditions, such as temperature and pH variations, as well as the presence of chemical compounds, can modify or even neutralize their biological activity. The enablement of this category of processes is the result of the several advances in the areas of molecular biology and biotechnology achieved over the past two decades. In this scenario, metal–organic frameworks (MOFs) are highlighted as efficient supports for enzyme immobilization. They can be used to ‘house’ a specific enzyme, providing it with protection from environmental influences. This review discusses MOFs as structures; emphasizes their synthesis strategies, properties, and applications; explores the existing methods of using immobilization processes of various enzymes; and lists their possible chemical modifications and combinations with other compounds to formulate the ideal supports for a given application.

Iron(II) Immobilized within a Metal-Organic Framework Mixed-Matrix Membrane as a H2O2 Turn-On Sensor

H2O2 detection is closely relevant to human health; however, most of the H2O2 probes suffer from low accuracy and sensitivity because of the aggregating nature of solid sensors. In contrast, a mixed-matrix membrane (MMM) with high processability and flexibility is a suitable H2O2 probe to overcome these drawbacks. Herein, we fabricated MOF-based MMMs by using a robust UiO-66-(COOH)2 with carboxylate-chelating moieties, which were utilized for binding Fe (II) metal centers. The Fe (II)-immobilized MOF-MMM involved in a Fenton reaction when treated with H2O2, exhibiting a fluorescence turn-on property. Compared to the bulk-state MOF powder, the MOF-MMM sensor showed much-improved sensitivity (detection limit down to 0.0215 μM) because of the uniform dispersion of the probe and a sufficient contact with the analyte. This MOF-MMM sensor combinedly exhibited a turn-on fluorescence response and outstanding sensing properties with flexibility and processability, providing a novel platform suitable for practical sensing applications.

A Comprehensive Review on the Use of Metal–Organic Frameworks (MOFs) Coupled with Enzymes as Biosensors

Several studies have shown the development of electrochemical biosensors based on enzymes immobilized in metal–organic frameworks (MOFs). Although enzymes have unique properties, such as efficiency, selectivity, and environmental sustainability, when immobilized, these properties are improved, presenting significant potential for several biotechnological applications. Using MOFs as matrices for enzyme immobilization has been considered a promising strategy due to their many advantages compared to other supporting materials, such as larger surface areas, higher porosity rates, and better stability. Biosensors are analytical tools that use a bioactive element and a transducer for the detection/quantification of biochemical substances in the most varied applications and areas, in particular, food, agriculture, pharmaceutical, and medical. This review will present novel insights on the construction of biosensors with materials based on MOFs. Herein, we have been highlighted the use of MOF for biosensing for biomedical, food safety, and environmental monitoring areas. Additionally, different methods by which immobilizations are performed in MOFs and their main advantages and disadvantages are presented.

Visual Hg(II) sensing in aqueous solution via a new 2,5-Bis(4-pyridyl)thiazolo[5,4-d]thiazole-based fluorescence coordination polymer

A new fluorescence coordination polymer [Zn(Py₂TTz)(5-OH-IPA)]_n (1) (Py₂TTz = 2,5-bis(4-pyridyl)thiazolo[5,4-d]thiazole, 5-OH-IPA = 5-hydroxyisophthalic acid dianion) was synthesized, which exhibited the characteristics of fluorescence quenching and bathochromic shift toward Hg(II) in aqueous solution at pH 7.00. Mechanism study showed that the interactions between Hg(II) ions and Py₂TTz ligands in 1 were responsible for the fluorescence emission change. Thanks to the specific interactions between 1 and Hg(II), excellent selectivity was achieved both in aqueous solution and in solid test paper. The detection limit of 1 for Hg(II) sensing was 125.76 nmol L^-1 and a linear rang was 1.00-10.00 μmol L^-1. More importantly, satisfactory recovery and accuracy of 1 for Hg(II) sensing were also obtained in buffer-free real water samples.

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...

Recent advances and future challenges in the synthesis of 2,4,6-triarylpyridines

2,4,6-Triarylpyridines are key building blocks to access functional molecules that are used in the design of advanced materials, metal-organic frameworks, supramolecules, reactive chemical intermediates and drugs. A number of synthetic protocols to construct this heterocyclic scaffold have been developed to date, the most recent of which (2015-present) are included and discussed in the present review. An emphasis has been placed on the utility of each synthetic approach in view of the scope of aryl/hetaryl substituents, limitations and an outlook of each method to be used in applied sciences.

Dual-Functional Mesoporous Copper(II) Metal-Organic Frameworks for the Remediation of Organic Dyes

By using tritopic and ditopic organic linkers derived from the same 2,4,6-triphenylpyridine core, copper(II) metal-organic frameworks with different three-dimensional structures have been successfully synthesized under ambient conditions. The crystalline framework, ^PTB MOF ([Cu₃ (PTB)₂ (H₂ O)₃ ]_n , where H₃ PTB=4,4',4''-(pyridine-2,4,6-triyl)tribenzoic acid, was observed to be mesoporous in nature and exhibited dual functionality in the removal of organic dyes. While cationic dyes such as methylene blue and malachite green, which are of different sizes, were adsorbed by ^PTB MOF; anionic dyes such as tartrazine could be effectively degraded in a photo-Fenton-like reaction catalyzed by the MOFs under irradiation with visible light.

Regiocontrolled synthesis of 2,4,6-triarylpyridines from methyl ketones, electron-deficient acetylenes and ammonium acetate

A novel one-pot two-step approach for the synthesis of 2,4,6-triarylpyridines via t-BuOK/DMSO-promoted C-vinylation of a variety of methyl ketones with electron-deficient acetylenes (alkynones) followed by a cyclization of the in situ generated unsaturated 1,5-dicarbonyl species with ammonium acetate has been developed. This approach possesses competitive advantages such as high regioselectivity, available starting materials and the absence of transition-metal catalysts, oxidants and undesirable byproducts. A wide synthetic utility of the developed approach was demonstrated by the synthesis of trisubstituted, tetrasubstituted and fused pyridines.

One-Pot Synthesis of 1,5-Diketones under a Transition-Metal-Free Condition: Application in the Synthesis of 2,4,6-Triaryl Pyridine Derivatives

We developed a facile and green one-pot synthetic method for substituted 1,3,5-triaryl-1,5-diketones by Claisen-Schmidt condensation following Michael addition reaction of aryl ketones and aryl aldehydes under a transition-metal-free condition. This convenient one-pot synthetic strategy has several advantages, including being transition-metal-free, having no extra additives or reagents, having a broad substrate scope, having a high isolated yield, having a minimum amount of base employment, having a shorter reaction time, use of cheap starting materials, cost-effectiveness, and being environment friendly. Some of the chemical structures of 1,5-diketones were confirmed by X-ray single-crystal diffraction analysis. The application of 1,5-diketones was demonstrated in the preparation of 2,4,6-triaryl pyridine derivatives under a catalyst-free system using ammonium acetate as a nitrogen source.

Ficin encapsulated in mesoporous metal-organic frameworks with enhanced peroxidase-like activity and colorimetric detection of glucose

Ficin has been reported to possess peroxidase activity, but its applications in some respects have been limited because of its relatively low activity. Herein, a mesoporous metal-organic framework, PCN-333(Fe), was synthesized, which was selected to encapsulate ficin to form ficin@PCN-333(Fe). Compared with ficin, the peroxidase-like activity of ficin@PCN-333(Fe) toward 3,3',5,5'-tetramethylbenzidine (TMB) oxidation was about 3 times increase in the presence of H₂O₂, and followed classical Michaelis-Menten model. The kinetic parameters showed that stronger affinity and higher catalytic constant (K_cat) of ficin@PCN-333(Fe) to both TMB and H₂O₂ compared with ficin, and K_cat of ficin@PCN-333(Fe) was increased by 3.65 folds and 3.59 folds for TMB and H₂O₂, respectively. Taking advantages of higher catalytic property of ficin@PCN-333(Fe), we developed a colorimetric method with high sensitivity and selectivity to detect glucose, which displayed a good linear response toward glucose in the range of 0.5-180 μM with a limit of detection of 97 nM. Furthermore, ficin@PCN-333(Fe) has been proven to successfully detect glucose in human serum, implying its great potentialities and wide applications as peroxidase mimics.

Tunable photoluminescence in flexible carboxylate ligand-based coordination polymers with interesting topologies and Fe3+ sensitivity

Complexes 1–5, showing different interesting topological networks from 1D ladder to 3D frameworks, exhibit selective and sensitive fluorescence quenching of Fe³⁺ ion.

Two-dimensional layered lanthanide diphosphonates: synthesis, structures and sensing properties toward Fe3+ and Cr2O72−

Four new lanthanide phosphonates with two-dimensional layered crystal structures have been synthesized and investigated for the optical sensing of Fe³⁺ and Cr₂O₇²⁻ ions.

Monofunctionalized 1,3,5,7-TetraarylazaBODIPYs and Their Application in the Synthesis of AzaBODIPY Based Conjugates

A series of monofunctionalized 1,3,5,7-tetraarylazaBODIPYs containing functional groups such as p-hydroxymethyl phenyl, p-hydroxyphenyl, p-cyanophenyl, p-nitrophenyl, and p-formylphenyl groups at the 1-position of the azaBODIPY core were synthesized by mixed condensation of two different nitrochalcones in n-butanol in the presence of CH₃COONH₄ at reflux followed by complexation with BF₃·OEt₂. The mixed condensation of nitrochalcones resulted in the formation of three different dipyrromethenes, which was treated with BF₃·OEt₂ to afford the desired monofunctionalized tetraarylazaBODIPYs in 30-36% yields. To demonstrate the application of monofunctionalized tetraarylazaBODIPYs, we reacted monoformyl functionalized tetraarylazaBODIPY with excess pyrrole to afford mono-dipyrromethanyl-substituted tetraarylazaBODIPY, which was used as a key precursor to prepare novel covalently linked azaBODIPY-based conjugates. The mono-dipyrromethanyl azaBODIPY was in situ oxidized with 2,3-dichloro-5,6-dicyanobenzoquinone and either reacted with BF₃·OEt₂ to afford azaBODIPY-BODIPY conjugates or reacted with metal salt such as Pd(acac)₂ to afford azaBODIPY-Pd(II)dipyrrin conjugates. Alternately, dipyrromethanyl-substituted azaBODIPY was condensed with dipyrromethane dicarbinol or 16-oxatripyrrane under mild acid catalyzed conditions followed by oxidation and chromatographic purification to afford azaBODIPY-porphyrin or azaBODIPY-oxasmaragdyrin conjugates, respectively. The photophysical studies on conjugates revealed that azaBODIPY is a good energy acceptor and invoked the possibility of energy transfer from the donor to acceptor in covalently linked conjugates.

Tunable Light Emission and Multiresponsive Luminescent Sensitivities in Aqueous Solutions of Two Series of Lanthanide Metal-Organic Frameworks Based on Structurally Related Ligands

Two series of lanthanide metal-organic frameworks (Ln-MOFs) from two structurally related flexible carboxylate-based ligands were solvothermally synthesized. H₃L2 with additional -CH₂- group provides more flexibility and different coordination modes and conformations compared with H₃L1. As a result, 2-Ln MOFs are modulated from two-dimensional kgd of 1-Ln to three-dimensional rtl topological frameworks and further achieve enhanced chemical stability. The Eu- and Tb-MOFs exhibit strong fluorescent emission at the solid state because of the antenna effect of the ligands. Interestingly, the emissions can be tuned by simply doping Eu³⁺ and Tb³⁺ of different concentrations within the Eu _xTb_{1- x} MOFs. Notably, 2-Ln MOFs realize nearly white light emission by means of a trichromatic approach (red of Eu(III), green of Tb(III), and blue of the H₃L2 ligand). Furthermore, 2-Ln MOFs also exhibit water stability and demonstrate high selective and sensitive sensing activities toward Fe(III) and Cr(VI) in aqueous solutions. The results further highlight the importance of the ligand flexibility on tuning MOF structures with improved structural stability and ion-sensing properties.

Three layer-structured cadmium coordination polymers based on flexible 5-(4-pyridyl)-methoxylisophthalic acid: rapid synthesis and luminescence sensing

Three layered Cd-CPs, synthesized by a MW-assisted solvothermal method within two or three minutes, serve as fluorescent sensors for Fe³⁺ ions.