Highly Crystalline Covalent Organic Frameworks Act as a Dual-Functional Fluorescent-Sensing Platform for Myricetin and Water, and Adsorbents for Myricetin

Ionic Covalent Organic Framework as a Dual Functional Sensor for Temperature and Humidity

Visual sensing of humidity and temperature by solids plays an important role in the everyday life and in industrial processes. Due to their hydrophobic nature, most covalent organic framework (COF) sensors often exhibit poor optical response when exposed to moisture. To overcome this challenge, the optical response is set out to improve, to moisture by incorporating H-bonding ionic functionalities into the COF network. A highly sensitive COF, consisting of guanidinium and diformylpyridine linkers (TG-DFP), capable of detecting changes in temperature and moisture content is fabricated. The hydrophilic nature of the framework enables enhanced water uptake, allowing the trapped water molecules to form a large number of hydrogen bonds. Despite the presence of non-emissive building blocks, the H-bonds restrict internal bond rotation within the COF, leading to reversible fluorescence and solid-state optical hydrochromism in response to relative humidity and temperature.

Construction of MOF@COF-derived composites for ratiometric fluorescence detection of water with ultralow background

A ratiometric sensor with ultralow background is highly desired due to its low environmental influence and high sensitivity. Herein, inspired by the solubility difference of carboxylate in aqueous and organic solvents, we prepared a core-shell structure porous zirconia-covalent organic framework (COF) composite through thermal hydrolysis of UiO-66-COF precursors in organic alkali solution. The ligand 2-aminoterephthalic acids (H2BDC-NH2) of UiO-66 were transformed into 2-aminoterephthalate salts (ATA salts) that existed in zirconium-oxo clusters building units. The composites emitted only yellow emission (597 nm) from the COF in organic solvent due to the insolubility of ATA salts that induce aggregation-caused quenching (ACQ) and the protection of the COF shell. Contrarily, when water was added into mixture, the ATA salts were released into solution and its fluorescence recovered at 446 nm, while the fluorescence of COF was quenched due to the blockage of the intramolecular charge transfer (ICT) process by water. Thus, a high-sensitivity ratiometric fluorescence method is obtained with ultralow background signal and fast response (less than 1 min) for sensing water in organic solvent. We believe that the proposed ratiometric fluorescence sensor based on the zirconia-COF composite will provide the guidance for detection with wide applications.

Design and preparation of fluorescent covalent organic frameworks for biological sensing

Covalent organic frameworks (COFs) are a new class of functional solids featuring several fantastic structural characteristics, including a great diversity of building units and cross-linking patterns, precise integration of building blocks, and adjustable topology of porous architecture. In addition to the above features, some COF samples are constructed with high-density conjugated fragments, which have unique potential advantages in fluorescence imaging, and thus may have great potential applications in bioimaging. Herein, this article summarizes the recent progress in the design and preparation of fluorescent covalent organic frameworks. We investigate the systemic correlation between the structural qualities of COF networks and biological sensors. Finally, the significant advantages, major challenges, and future opportunities of fluorescent covalent organic frameworks are discussed for the development of next-generation porous materials for sensing applications.

(3,3)-Connected Triazine-Based Covalent Organic Frameworks for Efficient CO2 Separation over N2 and Dye Adsorption

Covalent organic frameworks (COFs) are a promising class of adsorption and separation materials that can meet the needs of ecological sustainability, such as the removal of carbon dioxide and organic dyes. The two synthesized (3,3)-connected triazine-based COFs demonstrate high specific surface area and good thermal and chemical stability. COFZ1 shows good CO2 adsorption selectivities for different CO2 and N2 volume percentage systems at 273 K and 1 bar, with an ideal adsorbed solution theory (IAST) CO2 selectivity (i.e., separation factor) of 35.09 for the simulated flue gas component and a CO2 adsorption capacity of 24.21 cm3 g-1. In the aqueous dye solutions, both COFs present good adsorption performance for the selected dyes, and the maximum adsorption capacities of COFZ1 for methylene blue (MB) and gentian violet (GV) reach 510 and 564 mg g-1, respectively. Each of the two COFs shows a high anti-interference performance and excellent recyclability. The adsorption capacities of two COFs for RhB (Rhodamine B), MB, and GV hardly vary with pH values and salt concentrations. The adsorption behaviors of the two COFs for dyes follow Langmuir isothermal adsorption and quasi-secondary kinetic adsorption, approaching monolayer adsorption and chemisorption.

Fluorescent covalent organic frameworks for environmental pollutant detection sensors and enrichment sorbents: a mini-review

Covalent organic frameworks (COFs) are a class of porous crystalline materials based on organic building blocks containing light elements, such as C, H, O, N, and B, interconnected by covalent bonds. Because of their regular crystal structure, high porosity, stable mechanical structure, satisfactory specific surface area, easy functionalization, and high tunability, they have important applications in several fields. Currently, most of the established methods based on COFs can only be used for individual detection or adsorption of the target. Impressively, fluorescent COFs as a special member of the COF family are able to achieve highly selective and sensitive detection of target pollutants by fluorescence enhancement or quenching. The construction of a dual-functional platform for detection and adsorption based on fluorescent COFs can enable the simultaneous realization of visual monitoring and adsorption of target pollutants. Therefore, this paper reviews the research progress of fluorescent COFs as fluorescence sensors and adsorbents. First, the fluorescent COFs were classified according to the different bonding modes between the building blocks, and then the applications of fluorescent COF-based detection and adsorption bifunctional materials for various environmental contaminants were highlighted. Finally, the challenges and future application prospects of fluorescent COFs are discussed.

An 'on-off-on' fluorescent switch based on a luminous covalent organic framework for the rapid and selective detection of glyphosate

Glyphosate, the most used herbicide in the world, has a residue problem that cannot be ignored. However, glyphosate itself does not have fluorescence emission and lacks the conditions for fluorescence detection. In this work, a rapid and selective fluorescence detection method of glyphosate was designed by an 'on-off-on' fluorescent switch based on a luminous covalent organic framework (L-COF). Only the fixed concentration of Fe3+ as an intermediate could trigger the fluorescent switch and no incubation step was required. The proposed method showed good accuracy with a correlation coefficient of 0.9978. The method's limits of detection and quantitation were 0.88 and 2.93 μmol/L, which were lower than the maximum allowable residue limits in some regulations. Environmental water samples and tomatoes were selected as actual samples to verify the application in a complex matrix. A satisfactory mean recovery from 87% to 106% was gained. Furthermore, Fe3+ could induce fluorescence quenching of L-COF through the photo-induced electron transfer (PET) effect, while the addition of glyphosate could block the PET effect to achieve detection. These results demonstrated the proposed method had abilities to detect glyphosate and broaden the application of L-COF.

Phenylboronic Ester-Bridged Chitosan/Myricetin Nanomicelle for Penetrating the Endothelial Barrier and Regulating Macrophage Polarization and Inflammation against Ischemic Diseases

The brain and liver are more susceptible to ischemia and reperfusion (IR) injury (IRI), which triggers the reactive oxygen species (ROS) burst and inflammatory cascade and results in severe neuronal damage or hepatic injury. Moreover, the damaged endothelial barrier contributes to proinflammatory activity and limits the delivery of therapeutic agents such as some macromolecules and nanomedicine despite the integrity being disrupted after IRI. Herein, we constructed a phenylboronic-decorated chitosan-based nanoplatform to deliver myricetin, a multifunctional polyphenol molecule for the treatment of cerebral and hepatic ischemia. The chitosan-based nanostructures are widely studied cationic carriers for endothelium penetration such as the blood-brain barrier (BBB) and sinusoidal endothelial barrier (SEB). The phenylboronic ester was chosen as the ROS-responsive bridging segment for conjugation and selective release of myricetin molecules, which meanwhile scavenged the overexpressed ROS in the inflammatory environment. The released myricetin molecules fulfill a variety of roles including antioxidation through multiple phenolic hydroxyl groups, inhibition of the inflammatory cascade by regulation of the macrophage polarization from M1 to M2, and endothelial injury repairment. Taken together, our present study provides valuable insight into the development of efficient antioxidant and anti-inflammatory platforms for potential application against ischemic disease.

Efficient selective removal of uremic toxin precursor by olefin-linked covalent organic frameworks for nephropathy treatment

Indoxyl sulfate is a protein-bound uremic toxin synthesized from indole that cannot be efficiently removed by the hemodialysis method and thus becomes a key risk factor for the progression of chronic kidney disease. Here, we develop a non-dialysis treatment strategy to fabricate an ultramicroporous olefin-linked covalent organic framework with high crystallinity in a green and scalable fashion for selectively removing the indoxyl sulfate precursor (i.e., indole) from the intestine. Various analyses show that the resulting material exhibits excellent gastrointestinal fluid stability, high adsorption efficiency, and good biocompatibility. Notably, it realizes the efficient and selective removal of indole from the intestine and significantly attenuates serum indoxyl sulfate level in vivo. More importantly, the selective removal efficacy of indole is substantially higher than that of the commercial adsorbent AST-120 used in the clinic. The present study opens up a new avenue to eliminate indoxyl sulfate by a non-dialysis strategy and further expands the in vivo applications of covalent organic frameworks.

Highly photoluminescent nitrogen-doped carbon quantum dots as a green fluorescence probe for determination of myricetin

Highly photoluminescent N-doped carbon quantum dots (N-CDs) which the quantum yield reached 63% were prepared through hydrothermal treatment. The obtained N-CDs displayed a uniform distribution of particle size, superior stability in high-salt conditions, and excellent sensitivity. A green fluorescence probe based on N-CDs was constructed for ultrasensitive determination of myricetin in vine tea on account of the static quenching. The N-CDs presented excellent linear fluorescence response in the concentration range of 0.2-40 μM and 56-112 μM and with a low detection limit of 56 nM. Additionally, the practicability of the probe was verified in spiked vine tea sample, and the satisfactory recoveries of myricetin varied from 98.8% to 101.2%, with relative standard deviations in the range of 1.52%-3.48%. It is the first time to employ N-CDs without any material modification as a fluorescence sensor to detect myricetin, which is a promising approach to expand the path for myricetin screening.

Boric Acid Functional Fluorescent Covalent-Organic Framework for Sensitive and Selective Visualization of CH3Hg. ACS APPLIED MATERIALS & INTERFACES 2023;15:9524-9532. [PMID: 36757912 DOI: 10.1021/acsami.2c23302]

Methylmercury (CH3Hg+) recognition remains a challenging and imperative task due to its high toxicity and wide existence in the ecosystem. Herein, a novel fluorescent covalent-organic framework containing a boric acid functional group (COF-BA) was prepared by a postmodification strategy for CH3Hg+ detection. COF-BA served as a sensing platform for CH3Hg+ with fluorescence static quenching accompanied by fluorescence color changing from intense blue to colorless, and the detection limit was determined as 1.68 μM in a relatively narrow concentration range. COF-BA also exhibited superior selectivity toward CH3Hg+ detection. Furthermore, the spiked and recovery test in real water samples showed its efficient detection practicality. The detection mechanism of COF-BA toward CH3Hg+ was investigated. The recognitive boric acid group in COF-BA was first replaced by CH3Hg+. Then, the quinoline structure that served to limit the rotation of the imine bond was disrupted, leading to dramatic fluorescence quenching. The boric acid functional COF fluorescent probe can be a promising sensing platform for the detection of methylmercury and also provides new ideas for the construction of new fluorescent COF materials.

Simple Way to Fabricate Emissive Boron-Containing Covalent Organic Frameworks

Highly fluorescent covalent organic frameworks (COFs) are rarely obtained because of the π-π stacked layers with aggregation-caused quenching behavior. Unarguably, highly fluorescent COFs with tunable emission colors are even more rarely achieved. Herein, a general strategy to modify the classical COF material (named COF-1) by different fluorescent molecules via N → B interaction was developed. In this method, the boron-containing COF-1 acted as a porous and crystalline matrix as well as a reaction partner of Lewis acid; after interacting with fluorescent molecules with the anchoring group of pyridine (Lewis base), COF-1 takes a gorgeous transfiguration from a non-emissive powder into a highly fluorescent COF material with tunable emission colors. This disclosed method endowed the typical COFs with new emissive life and is speculated with the general research concept for all boron-containing COFs. Benefiting from the prominent fluorescent emission in the aggregation state, sensitive probes toward amines are achieved.

Improving the Reaction-to-Fire Properties of Thermoplastic Polyurethane by New Phosphazene-Triazinyl-Based Covalent Organic Framework

In this study, an approach to simultaneously improve fire resistance and mechanical performance of thermoplastic polyurethane (TPU) was introduced through the penetration of a conjugated network containing nitrogen and phosphorus elements. For this purpose, a Bg-HCCP COF was synthesized through a solvothermal method from benzoguanamine (Bg) and hexachlorophosphazene (HCCP) monomers. Then, it was combined with TPU using the wet mixing method. The TPU/Bg-HCCP composites showed better mechanical strength than the untreated sample. The fire safety of TPU/Bg-HCCP composites was greatly improved by increasing the Bg-HCCP contents. The reduction of the peak heat release rate and the total heat release for the TPU/Bg-HCCP composite with 3 wt % Bg-HCCP were about 44.8 and 60.4%, respectively. Besides, the results showed that adding Bg-HCCP to TPU significantly improved the suppression of smoke generation so that 3% by weight of the fire retardant reduced the total smoke released by 53.1%. It also decreased the peak of the carbon monoxide production rate by 26.5%. Generally, our research provides a promising strategy for constructing flame-retardant composites with high performance.

Tale of COF-on-MOF Composites with Structural Regulation and Stepwise Luminescence Enhancement

Integrating metal-organic framework (MOF)-covalent organic framework (COF) allows versatile engineering of hybrid materials with properties superior to pristine components, especially COFs suffered from aggregation-caused quenching (ACQ), unlocking more possibilities to improve the luminescence of COFs. In this work, we prepared various MOF@COF composites with different COF layer thicknesses, in which stable UiO-66-NH₂ served as the inner substrate and 1,3,5-benzenetricarboxaldehyde (BT), and 3,3'-dihydroxybenzidine (DH) were used to construct a COF layer. In addition to the conventional preparation method, we increased the ratio of BT and DH to be 1:2.5, and impressively, the morphologies of acquired UC (1:2.5) materials were quite different from the previous reticular structure and gradually extended from the spherical structure to the prickly structure with the increase of COF monomers. Remarkably, all of the UC materials possessed better luminescence properties than individual COF due to the limited COF layers. Meanwhile, UC-1 materials with an optimal COF layer displayed the strongest emission. In comparison with a single COF, the quantum yields of UC-1 and UC-1 (1:2.5) were increased nearly 7 times and 5 times, respectively. Moreover, the fluorescence of UC-1 materials was progressively enhanced via selective F^- sensing. This work is expected to shed light on the potential hybridization of MOF-COF with structural adjustment, morphological design, and luminescence enhancement.

Determination of water in organic solvents and raw food products by fluorescence quenching of a crystalline vinyl-functionalized COF

Covalent organic frameworks (COFs) with good chemical stability, flexible chemical functionalization, tunable pore sizes, and high specific surface areas have been increasingly employed in the field of fluorescence sensing. In this work, a crystalline vinyl-functionalized COF TzDa-V was facilely prepared through a room-temperature synthetic method via condensation reaction between 4,4',4″-(1,3,5-triazine-2,4,6-triyl)trianiline (Tz) and 2,5-diallyloxyterephthalaldehyde (Da-V). The intermolecular charge transfer (ICT) effect endowed the TzDa-V with fluorescence characteristic, and it was sensitive to trace water and can be quenched due to the disruption of ICT process by water. On this base, the prepared COF TzDa-V with excellent chemical/thermal stability was applied to sensing of trace water in common organic solvents such as DMF, acetone, THF, and ethyl acetate with rapid response (less than 10 s), satisfactory sensing range (0.5-18% water in DMF, 0.5-15% water in acetone, 0.5-16% water in THF, 0.5-5% in ethyl acetate, v/v), and high sensitivity. The limits of detection for water in DMF, acetone, THF, and ethyl acetate were 0.0497%, 0.0590%, 0.0502%, and 0.0766% (v/v), respectively. The proposed probe was successfully used for the detection of trace water in food products such as salt and sugar. The COF TzDa-V would be a good candidate for application in water sensing.

Detection of Trace Water Based on Electro-oxidation of Molybdenum Disulfide Nanomaterials to Form Molybdenum Oxysulfide

Molybdenum disulfide nanomaterials nowadays are very popular in electrocatalysis field due to their outstanding catalytic performance toward many electrochemical reactions. However, the electrochemical oxidation reaction of molybdenum disulfide nanomaterials in the range of positive potential has not been studied thoroughly. Herein, we have investigated electro-oxidation of molybdenum disulfide nanomaterials and put forward a new reaction mechanism: molybdenum disulfide nanomaterials are electro-oxidized with water to form molybdenum oxysulfide (MoOS2) and hydrogen ions, leading to the release of hydrogen on the counter electrode. Various characterization methods such as contact angle measurement, scanning electron microscope (SEM), transmission electron microscope (TEM) with energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES) spectroscopy, time-of-flight secondary ion mass spectrometry (ToF-SIMS), and gas chromatography (GC) were applied to attest the doping of oxygen and the generation of hydrogen. Based on this reaction, we constructed a novel ultrasensitive electrochemical sensor for detecting trace water with the minimum detectable content of 0.0010% (v/v) in various organic solvents and ionic liquids, which is comparable to the Karl Fischer titration, but with much simpler reagent.

Excited-state intramolecular proton transfer based covalent organic framework for fluorescence anions sensing

An azine linked covalent organic framework, ACOF, has been constructed via hydrazine hydrate and aldehyde group building unit with hydroxyl group in situ under the solvothermal condition. ACOF possesses good...

Synthesis of Three Dimensional Cs-γ-CD-MOFs and the Adsorption of Myricetin

A new Cs-γ-CD-MOF material obtained in colourless large crystals and with a three-dimensional porous structure containing coordinated by cesium ions and γ-cyclodextrin was synthesised by an improved vapour diffusion method. The chemical formula of the Cs-γ-CD-MOF material was C24H34CsO20, with the I4 space group. Compared with the traditional solvent vapour diffusion method (7 days), this method is advantageous for rapid crystal formation (1 day). Simultaneously, the drug adsortion capacity of γ-CD and Cs-γ-CD-MOFs for myricetin was compared and the results indicated that Cs-γ-CD-MOFs (280.05 mg g−1) have a higher drug adsorption capacity than γ-CD (142.92 mg g−1). Finally, the energy and conformation of the Cs-γ-CD-MOF material for adsorbing the drug myricetin were obtained through molecular docking.