Fluorescent Covalent Organic Framework Research Articles

Fluorescent Detection of 2,4-Dichlorophenoxyacetic Acid in Food Samples Based on Covalent Organic Frameworks and MnO2 Nanosheets

2,4-Dichlorophenoxyacetic acid (2,4-D) is a typical chlorinated aromatic herbicide which is widely used because of its low cost. Herein, a new fluorescent assay for 2,4-D is developed based on covalent organic frameworks (COFs) and MnO2 nanosheets. The fluorescence of COFs is quenched effectively by MnO2 nanosheets. Ascorbic acid 2-phosphate (AAP) is hydrolyzed by alkaline phosphatase (ALP) to produce ascorbic acid (AA), which reduces MnO2 nanosheets into Mn2+, so the quenched fluorescence recovers. Additionally, 2,4-D can inhibit the catalytic activity of ALP. Therefore, the addition of 2,4-D quenches the fluorescence of COF-MnO2 system again. Under the optimum conditions, this assay displays a wide linear range of 2,4-D from 1 to 150,000 ng/mL with a limit of detection of 0.36 ng/mL. Then, this method is successfully used to measure 2,4-D in rice, millet, and cucumber samples with satisfactory recoveries. Compared with other probes for 2,4-D detection, both of COFs and MnO2 nanosheets are stable, cost-effective, and easily prepared. The sensitivity and linear range of 2,4-D in this strategy are superior to most reported works. This is the first time that a novel sensing platform using fluorescent COFs and MnO2 nanosheets is developed for 2,4-D.

Regenerable and stable sp2 carbon-conjugated covalent organic frameworks for selective detection and extraction of uranium

Uranium is a key element in the nuclear industry, but its unintended leakage has caused health and environmental concerns. Here we report a sp2 carbon-conjugated fluorescent covalent organic framework (COF) named TFPT-BTAN-AO with excellent chemical, thermal and radiation stability is synthesized by integrating triazine-based building blocks with amidoxime-substituted linkers. TFPT-BTAN-AO shows an exceptional UO22+ adsorption capacity of 427 mg g−1 attributable to the abundant selective uranium-binding groups on the highly accessible pore walls of open 1D channels. In addition, it has an ultra-fast response time (2 s) and an ultra-low detection limit of 6.7 nM UO22+ suitable for on-site and real-time monitoring of UO22+, allowing not only extraction but also monitoring the quality of the extracted water. This study demonstrates great potential of fluorescent COFs for radionuclide detection and extraction. By rational designing target ligands, this strategy can be extended to the detection and extraction of other contaminants.

Fluorescent self-propelled covalent organic framework as a microsensor for nitro explosive detection

Covalent-organic-frameworks (COFs) are an emerging class of porous materials with fascinating structural features desired for a wide spectrum of applications. However, the potential sensing applications of COFs are hindered by their hydrophobic nature, as well as limited detection speed and sensing sensitivity. We report a fluorescent self-propelled COF as a microsensor for highly efficient nitro explosive detection. Together with MnO2 microsphere catalysts and magnetic Fe3O4 nanoparticles, the fluorescent COF is built in biodegradable polymeric spheres. The resulting microsensors can exhibit magnetically guided autonomous motion in aqueous solution. Fluorescence quenching of the microsensors is observed upon their navigation in solutions with trace amounts of explosives. Continuous mixing induced by the bubble propulsion of microsensors greatly shortens the sensing time to several minutes. We anticipate the presented strategy would provide implications for sensing applications of COFs.

A novel fluorescent covalent organic framework containing boric acid groups for selective capture and sensing of cis-diol molecules.

Owing to specific formation of five-membered or six-membered cyclic esters between boric acid groups and cis-diol molecules, boric acid bearing fluorescent materials can not only selectively capture but also specifically identify cis-diol substances. In this work, a novel covalent organic framework containing boric acid groups (COF-BA) was prepared through post-modification via the aza-Diels-Alder cycloaddition reaction. COF-BA with good stability, a permanent pore structure, a high specific surface area (606 m2 g-1) and a uniform pore size (2.59 nm) exhibited unique selectivity toward the cis-diol guest molecule 1,2-dihydroxyanthracene-9,10-dione (1,2-Doa) with a high adsorption capacity of 177.95 mg g-1. However, as for the isomers of 1,2-Doa (1,4-dihydroxyanthracene-9,10-dione and 2,6-dihydroxyanthracene-9,10-dione), the corresponding uptake capacities are distinctively decreased to 40.86 mg g-1 and 3.05 mg g-1, respectively. It is worth noting that the COF-BA can be recovered and recycled. Moreover, because the formation of the quinoline enhanced the conjugation effect of the COF skeleton, it was unexpectedly found that COF-BA possessed an intrinsic fluorescence property and could be used as an optical sensor for 1,2-Doa.

A trifluoromethyl-grafted ultra-stable fluorescent covalent organic framework for adsorption and detection of pesticides

A fluorine-rich ultra-stable fluorescent covalent organic framework for adsorption and detection of pesticides was designed and prepared via the Povarov reaction in a flask.

A highly fluorescent covalent organic framework as a hydrogen chloride sensor: roles of Schiff base bonding and π-stacking

In this paper we report the extremely crystalline structures, high thermal stabilities, and strong fluorescence emissions of covalent organic frameworks based on linked carbazole units.

A triarylamine-based fluorescent covalent organic framework for efficient detection and removal of Mercury(II) ion

A novel covalent organic framework (COF) polymer has been designed and synthesized by Suzuki polymerization of two monomers based on triarylamine derivatives, and the polymer displays nanosphere morphology due to mini-emulsion reaction system. On the basis of the above polymer, we further fabricated a Schiff base covalent organic framework polymer fluorescent sensor for efficient detection and removal of mercury(II) ions. The material underwent a fluorescence and colour change upon the touching of mercury(II) ions. Thus, taking advantage of the material to detect the presence of mercury(II) ions is quite convenient. Furthermore, the material is capable of efficiently adsorbing mercury(II) ions from aqueous solution. The fluorescence sensing device was successfully fabricated by immobilizing the polymer probe on a macroporous sponge, which was more convenient to detect and remove mercury(II) ions relative to powdery polymer probe.

Q-graphene-scaffolded covalent organic frameworks as fluorescent probes and sorbents for the fluorimetry and removal of copper ions

Metal-free fluorescent covalent organic frameworks (COFs) were synthesized initially with Q-Graphene (QG) scaffolds by the one-step covalent reactions of melamine-aldehyde and phenol-aldehyde poly-condensations using paraformaldehyde. It was discovered that onion-like hollow QG, which consists of multi-layer graphene and different carbon allotropes having a high proportion of folded edges and surface defects, could endow the scaffolded COFs with enhanced green fluorescence and environmental stability. Unexpectedly, they could exhibit the powerful absorption for Cu2+ ions resulting in the specific quenching of fluorescence. A fluorimetric strategy with QG-scaffolded COFs was thereby developed to probe Cu2+ ions separately in blood and wastewater with the linear concentration ranges of 0.0010–10.0 μM (limit of detection of 0.50 nM) and 0.0032–32.0 μM (limit of detection of 2.4 nM), respectively, promising the potential applications for the field-applicable monitoring of Cu2+ ions in the clinical and environmental analysis fields. In addition, the prepared COFs sorbents were employed to absorb Cu2+ ions in wastewater showing high removal efficiency. More importantly, such an one-pot fabrication route with hollow QG scaffolds may be tailorable extensively for the preparation of a variety of metal-free multifunctional COFs with enhanced fluorescence, water solubility, environmental stability, and metal removal capability.

Ultrathin two-dimensional covalent organic framework nanoprobe for interference-resistant two-photon fluorescence bioimaging.

The complex environment of living organisms significantly challenges the selectivity of classic small-molecule fluorescent probes for bioimaging. Due to their predesigned topological structure and engineered internal pore surface, covalent organic frameworks (COFs) have the ability to filter out coexisting interference components and help to achieve accurate biosensing. Herein, we propose an effective interference-resistant strategy by creating a COF-based hybrid probe that combines the respective advantages of COFs and small-molecule probes. As a proof of concept, a two-photon fluorescent COF nanoprobe, namely TpASH-NPHS, is developed for targeting hydrogen sulfide (H2S) as a model analyte. TpASH-NPHS exhibits limited cytotoxicity, excellent photostability and long-term bioimaging capability. More importantly, compared with the small-molecule probe, TpASH-NPHS achieves accurate detection without the interference from intracellular enzymes. This allows us to monitor the levels of endogenous H2S in a mouse model of cirrhosis.

Thioether-Based Fluorescent Covalent Organic Framework for Selective Detection and Facile Removal of Mercury(II).

Heavy metal ions are highly toxic and widely spread as environmental pollutants. New strategies are being developed to simultaneously detect and remove these toxic ions. Herein, we take the intrinsic advantage of covalent organic frameworks (COFs) and develop fluorescent COFs for sensing applications. As a proof-of-concept, a thioether-functionalized COF material, COF-LZU8, was "bottom-up" integrated with multifunctionality for the selective detection and facile removal of mercury(II): the π-conjugated framework as the signal transducer, the evenly and densely distributed thioether groups as the Hg(2+) receptor, the regular pores facilitating the real-time detection and mass transfer, together with the robust COF structure for recycle use. The excellent sensing performance of COF-LZU8 was achieved in terms of high sensitivity, excellent selectivity, easy visibility, and real-time response. Meanwhile, the efficient removal of Hg(2+) from water and the recycling of COF-LZU8 offers the possibility for practical applications. In addition, X-ray photoelectron spectroscopy and solid-state NMR investigations verified the strong and selective interaction between Hg(2+) and the thioether groups of COF-LZU8. This research not only demonstrates the utilization of fluorescent COFs for both sensing and removal of metal ions but also highlights the facile construction of functionalized COFs for environmental applications.