Fluorescent Covalent Organic Framework Research Articles

A novel fluorescent covalent organic framework for the selective detection of fluoride ion

A novel fluorescent covalent organic framework for the selective detection of fluoride ion

Fluorescent difluoroboron covalent organic frameworks via N, O-bidentate ligation

The multi-dentate chelate ligands containing N and O atoms have good chelating ability to boron trifluoride and their complexes show good fluorescence properties. In this work, a fluorescent covalent organic framework (COF) with good crystallinity and porosity, 50% B-COF, was successfully synthesized by coordination of starting COFs 50% COF-OH with boron trifluoride diethyl etherate via N, O-bidentate ligation. Imine bonds are highly polarized. The coordination of nitrogen and boron changes the electron cloud distribution of imine bonds, causing the non-fluorescent 50% COF-OH to be transformed into fluorescent 50% B-COF. 50% B-COF exhibited red fluorescence under ultraviolet excitation. This work provides a new strategy for the design of fluorescent COFs.

Glutathione-functionalized highly crystalline fluorescent covalent organic framework as a fluorescence-sensing and adsorption double platform for cationic dyes

Considering the high toxicity of cationic dyes to the environment and humans, the development of materials which possessed the function both of detection and adsorption of cationic dyes are of great significance. In this study, a novel glutathione (GSH)-functionalized highly crystalline fluorescent covalent organic framework (denoted as COF-TPDD-COOH), which can act as a fluorescence-sensing and adsorption double platform for cationic dyes, was fabricated reported for the first time. To effectively postsynthetically modification and form COF-TPDD-COOH, a fluorescent COF (namely COF-TPDD) with a high crystallinity was fabricated by optimizing a series of conditions affecting crystallinity. The as-prepared COF-TPDD-COOH exhibited good dispersion and outstanding fluorescence and adsorption performances, which yielded remarkable performances in the detection and enrichment of cationic dyes (malachite green, methylene blue and crystal violet). Low detection limits (LODs, 0.003–0.007 μg·mL−1) and high adsorption capacities (88.02–128.64 mg·g−1) were obtained for three cationic dyes. The introduction of GSH increased the possibility of COF-TPDD-COOH binding with cationic dyes, which further increased the enrichment effect of cationic dyes. After adsorption, the cationic dyes could more easily interact with COF-TPDD-COOH and capture the protons needed for fluorescence, thus destroying the excited-state intramolecular proton transfer effect and selectively quench the fluorescence. A sensing platform based on COF-TPDD-COOH was developed and applied for the detection and adsorption of three cationic dyes from real samples.

Fluorescent determination of cysteine and homocysteine via adjustable synthesis of flower-shaped covalent organic frameworks

Detection of cysteine (Cys) and homocysteine (Hcy) is very important for the early diagnosis and prevention of related diseases. Herein, we propose a facile, green, room-temperature controllable approach for the synthesis of fluorescent covalent organic frameworks (COFs) with high specific surface areas, high crystallinity, good morphology, excellent chemical and thermal stability by using 2,5-dimethoxyterephthaldehyde and 1,3,5-tris(4-aminophenyl)benzene as precursors. Interestingly, we found that the size and morphology of COFs could be controlled by the amount of acetic acid. With increasing numbers of acetic acid (HAc) molecules, the morphology of COFs gradually changed from spherical to flower-shaped, and the particle size gradually decreased from micrometer to nanometer. Furthermore, we were surprised to find that the reaction time influenced the uniform dispersion and crystallinity of flower-shaped COFs. With increasing the reaction time, the boundary around COFs became blurred, hierarchical flower-shaped COFs gradually appeared, and the flower-shaped gradually became clear and uniform. On this basis, the flower-shaped COFs were further used for highly sensitive determination of Cys and Hcy, with detection limits as low as 11.4 nM and 6.7 nM, respectively. The possible quenching mechanism was deduced by the Dmol3 package provided by Materials Studio and DFT calculations of Gaussian 09 software. This strategy could provide us a new insight into the controllable preparation of flower-shaped fluorescent COFs and broaden the application range of COFs for biomarker detection.

Construction of Two-Dimensional Fluorescent Covalent Organic Framework Nanosheets for the Detection and Removal of Nitrophenols.

In this work, we synthesized a two-dimensional fluorescent covalent-organic framework (TFPB-TTA COF) nanosheet by selecting and designing reactive monomers to realize the dual-functional processing of nitrophenols. The staggered benzene ring, triazine structure, and imine bond (C═N) of the TFPB-TTA COF can capture free nitrophenols through hydrogen bonding and conjugation interaction, and then, the photoinduced electron transfer and fluorescence resonance energy transfer (FRET) between the TFPB-TTA COF and nitrophenols affects the fluorescence emission of the TFPB-TTA COF, realizing the fluorescence sensing of nitrophenols. The large Ksv values and the low detection limit suggest that the TFPB-TTA COF can serve as sensitive and selective fluorescence sensors for nitrophenol detection in an aqueous system. At the same time, the strong interaction combined with the porous network structure of the TFPB-TTA COF facilitates the efficient adsorption and removal of nitrophenols. Especially for 2,4,6-trinitrophenol, the maximum adsorption capacity can reach 1045.53 mg/g with good recyclability and high structural stability of the TFPB-TTA COF. This work proposed a simple synthetic method for the construction of a fluorescent COF platform for the sensitive determination and efficient adsorption of nitrophenols.

A double responsive fluorescent platform for sensing heavy metal ions based on a dual-emitting fluorescent covalent organic framework hydrogel film.

A new luminescent hybrid material with dual-emission centers (Eu@Dye@TpDq hydrogel) based on a covalent organic framework (COF) has been successfully prepared. First, we designed a dye (coumarin) modified COF (TpDq) hybrid fluorescent material (Dye@TpDq), and then the material was formed into a hydrogel film, with an Eu3+ ion being introduced as another luminescent center into the material during the cross-linking process. As a dual-emission fluorescent platform, it exhibits a double-response towards different heavy metal ions: a ratiometric identification response to Cu(II), and a turn-off fluorescence response to Cr(VI). This research not only introduces a novel synthesis of Eu3+-functionalized double emitting COF-based hydrogels, but also inspires the development of COF-based multifunctional analytical tools.

A facile imine-linked covalent organic framework doped with a carbon dot composite for the detection and removal of Hg2+ in surface water.

Hg2+ is one of the most toxic chemical species in the water environment, and thus developing a new fluorescent covalent organic framework for both the detection and removal of Hg2+ is highly desirable. Herein, a fluorescent composite, termed TpPa-1 COF@CDs, was synthesized by inverse emulsion polymerization method using an imine covalent organic framework as the supporting material and carbon dots as the fluorescent sensor element. The crystallinity, porosity, rich functional receptors (hydroxyl and amino groups), thermal stability and fluorescent properties of TpPa-1 COF@CDs were characterized. The results showed that TpPa-1 COF@CDs exhibited a good detection and removal performance for Hg2+, which was evidenced by its high sensitivity (LOD = 0.75 μg L−1), superior selectivity, large adsorption capacity (235 mg g−1), fast adsorption rate (30 min equilibrium time) and good regeneration (at least five cycles). More importantly, the simple functional monomer, short reaction time and metal-free raw material made TpPa-1 COF@CDs reliable, cost effective and eco-friendly. This research demonstrated the facile construction of a functional covalent organic framework composite for water environmental remediation technologies of metal pollution.

Designed synthesis of an sp2 carbon-conjugated fluorescent covalent organic framework for selective detection of Fe3.

A new fully conjugated covalent organic framework material (COFTFPPy-ThDAN) has been synthesized via the Knoevenagel condensation reaction using 1,3,6,8-tetra(4-formylphenyl)pyrene (TFPPy) as the chromogenic unit and 2,2'-([2,2'-bithiophene]-5,5'-diyl)diacetonitrile (ThDAN) as the linker. The COFTFPPy-ThDAN has been successfully prepared under the optimized conditions and showed excellent crystallinity and chemical stability. Especially, it was able to maintain its crystallinity even when immersed under harsh conditions such as HCl (3 M) and NaOH (3 M). Meanwhile, COFTFPPy-ThDAN exhibited good fluorescence properties, which could remain relatively stable under different pH conditions. Moreover, COFTFPPy-ThDAN was further employed for the detection of Fe3+ with high sensitivity and selectivity with a limit of detection (LOD) of 1.26 μM. COFTFPPy-ThDAN with high stability and fluorescence is a promising material for chemical sensing.

Application of a novel hydroxyl functionalized fluorescent covalent organic framework for turn-off ultrasensitive Zn2+ ion detection.

In this research, a novel hydroxyl functionalized covalent organic framework (COF) with fluorescence properties was rationally designed based on the reaction between 2,5-dihydroxy-terephthalic-dihydrazide (DHTPz) and 1,3,5-benzenetricarboxaldehyde (Bt) for Zn2+ detection. The prepared DHTPz-Bt exhibited strong fluorescence, while the apparent quenching of fluorescence was observed after the introduction of Zn2+. Meanwhile, DHTPz-Bt exhibited high sensitivity and promising selectivity during the detection of Zn2+. Additionally, the recognition process was revealed to be attributed to the coordination between the hydroxyl groups on the phenyls of DHTPz-Bt and Zn2+ ions, as verified by using Fourier-transform infrared spectra and X-ray photoelectron spectra. This work demonstrates the great potential of fluorescent probes by rationally introducing metal ligands, which will lead to a suite of new COF materials for metal ion sensing in a practical manner.

A conveniently synthesized redox-active fluorescent covalent organic framework for selective detection and adsorption of uranium

Uranium is a key element in the nuclear industry and also a global environmental contaminant with combined highly toxic and radioactive. Currently, the materials based on post-modification of amidoxime have been developed for uranium detection and adsorption. However, the affinity of amidoxime group for vanadium is stronger than that of uranium, which is the main challenge hindering the practical application of amidoxime-based adsorbents. Herein, we synthesized a fluorescent covalent organic framework (TFPPy-BDOH) through integrating biphenyl diamine and pyrene unit into the π-conjugated framework. TFPPy-BDOH has an excellent selectivity to uranium due to the synergistic effect of nitrogen atom in the imine bond and hydroxyl groups in conjugated framework. It can achieve ultra-fast fluorescence response time (2 s) and ultra-low detection limit (8.8 nM), which may be attributed to its intrinsic regular porous channel structures and excellent hydrophilicity. More excitingly, TFPPy-BDOH can chemically reduce soluble U (VI) to insoluble U (IV), and release the binding site to adsorb additional U (VI), achieving high adsorption capacity of 982.6 ± 49.1 mg g−1. Therefore, TFPPy-BDOH can overcome the challenges faced by current amidoxime-based adsorbents, making it as a potential adsorbent in practical applications.

Chemiluminescence resonance energy transfer determination of uric acid with fluorescent covalent organic framework as energy acceptor

A simple and feasible strategy was developed for the preparation of fluorescent covalent organic frameworks (COFs) TpPa-1@FL. The TpPa-1–1@FL was prepared via a self-assembly strategy by soaking non-fluorescent COFs TpPa-1 into strong fluorescent fluorescein (FL) solution. A chemiluminescence resonance energy transfer (CRET) system was constructed by the combination strong fluorescent TpPa-1@FL with TCPO-hydrogen peroxide (H2O2) reaction. The chemiluminescence (CL) signal of the system was further improved by the addition of bovine serum albumin (BSA). The CRET system can determine H2O2 with a linear range response from 5.0 µmol/L to 20.0 mmol/L and a detection limit of 1.1 µmol/L. The CRET system was further exploited for indirect detection of uric acid with coupling of uricase. A good linear relationship was obtained for uric acid in the 10.0–400.0 µmol/L concentration range with a detection limit of 3.8 µmol/L. The practicability of this method was assessed by the determination of uric acid in real samples of human serum and urine.

Rational Synthesis of Imine-Linked Fluorescent Covalent Organic Frameworks with Different pKa for pH Sensing In Vitro and In Vivo.

Precise modulation of pH in living cells plays a vital role in the study of many diseases, such as cancer and rheumatoid arthritis. Here, a series of imine-linked covalent organic frameworks (COFs) were rationally designed and developed for pH sensing in tumor cells and zebrafish. Four monomers were chosen to synthesize COFs (COF1-COF4) with different pKa by a simple orthogonal combination through condensation reaction. The as-obtained COFs exhibited a sensitive pH-dependent fluorescence response compared to their building blocks. Among them, COF2 possessed a high crystallinity, excellent fluorescence, and suitable pKa for biosensing. For bioimaging applications, COF2 was modified with poly-d-lysine (PDL) to improve its biocompatibility and endocytosis efficiency. After that, PDL-modified COF2 (PDL@COF2) was used as a novel fluorescence probe with a superior linear pH response over the range from 5.0 to 8.0 due to its fully reversible protonation and deprotonation. The fluorescent PDL@COF2 was further employed as a good candidate for pH imaging in tumor cells and zebrafish. The as-constructed environment-sensitive fluorescent COFs have greatly expanded the applications of COFs in the biological area.

Benzobisthiazole-bridged white fluorescent emitting covalent organic framework for simultaneous mercury detection and removal

Mercury (Hg2+), a heavy metal ion, is considered one of the most toxic metals on earth and is widespread as an environmental pollutant. Various promising adsorbents have been developed with accessible chelating sites and high affinity for heavy metal ions. Herein, we utilized the advantages of covalent organic frameworks (COFs) and designed benzobisthiazole-bridged (BBT) fluorescent COFs (FCOFs) for Hg2+ sensing applications. The BBT-FCOF nanosheets has a π-π transition, effective electron transfer, and enhanced delocalization within the π orbitals of the BBT ring. It emits a white fluorescence enabling real-time detection of Hg2+ metal ions. The excellent sensing property of BBT-FCOF nanosheets was attributed to the high sensitivity towards Hg2+; this was confirmed by the x-ray photoelectron spectroscopy binding energy of sulfur and nitrogen atom in BBT-FCOF. The limit of detection of Hg2+ was 30 ppb, which is very low compared to other published methods. ICP-mass measurement also proved that BBT-FCOF nanosheets removed almost 99% of Hg and BBT-FCOF distribution coefficient (Kd) was calculated to be 5 × 105 g−1. The recyclability test proved that HCl effectively removed the adsorbed Hg2+ metal ions from BBT-FCOF nanosheets, making it a useful tool for real-time Hg2+ detection and removal from waste water.

Study on the fluorescent covalent organic framework for selective “turn-off”recognition and detection of Fe3+ ions

Covalent organic framework (TT-COF) based on 2,5-dihydroxyterephthalaldehyde and 1,3,5-tris(4-aminophenyl)benzene was prepared. TT-COF displays obvious fluorescence quenching response in ethanol towards Fe3+ with high selectivity accompanied by fluorescence red-shift. The fluorescence intensity of TT-COF at 425 nm shows a linear relationship with Fe3+ concentration with a low detection limit of 8.4 × 10−5 mol/L. The high fluorescence selectivity can be attributed to the synergetic effects of unique pore structure and the coordination of Fe3+ ions with N and O atoms of TT-COF. The coordination between TT-COF and Fe3+ ion was confirmed by reversible fluorescence change upon the addition of ethylenediaminetetraacetic acid disodium salt (EDTA), XPS spectra, and DFT calculation. The fluorescence quenching is attributed to the intramolecular charge transfer (ICT) and the absorption competition quenching (ACQ) mechanism. This work indicating the fluorescent COF as a fluorescence sensor has promising application in recognition and detection of specific metal ions.

Fluorescent core-shell SiO2@vertical covalent organic frameworks nanosheets for sensing application

Fluorescent covalent organic frameworks (COFs) are attracting more and more attentions. However, it is very difficult to obtain COFs with strong fluorescence due to fluorescence quenching caused by interlaminar accumulation and intramolecular rotation. Here, the core-shell SiO2@COFTFP-TAPB (1,3,5-tri(4-aminophenyl)-benzene: TAPB, 1,3,5-triformylphloroglucinol: TFP) was prepared by assembling vertical COFTFP-TAPB through amine-aldehyde dehydration condensation reaction on -NH2-SiO2 nanospheres (NPs) for the first time. The 2D COFTFP-TAPB nanosheets with thickness of 25 nm and good crystallinity vertically stand on the surface of -NH2-SiO2 NPs based on the imine-linkage. Such core-shell structure results in larger BET surface area, effectively overcomes the interlaminar accumulation, and improves optical properties of COFTFP-TAPB. The core-shell SiO2@COFTFP-TAPB exhibits multiemission from the enol-imine form and keto-enamine form, which is sensitive to pH. After loading with glucose oxidase and urease, it could be used to detect glucose and urea successfully. The detection ranges are 0.23 μM-14 mM and 1.50 μM-90 mM, and the detection limits reaches 0.07 μM and 5.14 μM, respectively. The work provides a feasible idea to overcome the interlaminar accumulation and reaggregation of COFs during application.

AIEgens-based fluorescent covalent organic framework in construction of chemiluminescence resonance energy transfer system for serum uric acid detection.

A covalent organic framework (COF) with aggregation-induced emission (AIE) property was successfully synthesized through in situ marriage of a commonly used AIE molecule tetraphenylethylene (TPE) with Schiff base network (SNW-1) through a simple one-pot method. The TPE@SNW-1 was characterized with different techniques of Fourier transform infrared spectroscopy, X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and nitrogen adsorption/desorption experiments. The fluorescence of the TPE@SNW-1 strongly depends on the composition of tetrahydrofuran-water binary system. The AIE property of TPE@SNW-1 was directly supported with particle size distribution by dynamic light scattering technique. With the TPE@SNW-1 as an energy acceptor, a chemiluminescence resonance energy transfer (CRET) system was constructed with bis(2,4,6-trichlorophenyl) oxalate (TCPO)-hydrogen peroxide (H2O2) reaction as an energy donor. The chemiluminescence (CL) signal displays a good linear relationship with concentration of H2O2 in the 5.0-1000.0 μmol·L-1 range, and a detection limit of 2.34 μmol L-1. The system was further exploited to determine uric acid based on the fact that equal stoichiometric amount of H2O2 can be concurrently generated under the catalysis of uricase. The procedure exhibits a linear response to uric acid concentration in the range 10.0-150.0 μmol·L-1 and a detection limit of 4.94 μmol·L-1. The practicability of the method was demonstrated in the determination of uric acid in human serum samples.

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

Selective detection of active ingredients in complex samples has always been a crucial challenge because there are many disturbing compounds, especially structural analogues that interfere with the detection. In this work, a fluorescent covalent organic framework (named COF-TD), which can be used for the selective fluorescence detection and enrichment of myricetin from complex samples, was reported for the first time. The highly crystalline COF-TD with bright blue fluorescence was formed through a solution polymerization method by the condensation reaction between 4,4',4″-(1,3,5-triazine-2,4,6-triyl)trianiline and 2,5-dihydroxy-1,4-benzenedicarboxaldehyde. Due to spatial size selectivity, multisites hydrogen bonding, and π-π interaction, myricetin can quench the fluorescence of COF-TD with an inner filter effect (IFE) and static quenching mechanisms as well as can be enriched on COF-TD. Myricetin can observably eliminate the interference of other compounds and selectively quench the fluorescence of COF-TD with a limit of detection (LOD) of 0.30 μg·mL-1. The high adsorption ability of COF-TD (Q = 124.6 mg·g-1) to myricetin was also obtained. Finally, a sensing platform based on COF-TD for myricetin was successfully developed and applied for the detection of myricetin from vine teas. In addition, COF-TD also showed good water sensing ability and could be used effectively to detect water content in organic solvent (1-18% water in acetone, 0.5-5% water in acetonitrile, 1-4.5% water in ethyl acetate, v/v). To the best of our knowledge, this is the first report where COF-TD was used to detect water in a relatively wide concentration range. In all, this work provided dual-functional fluorescent COFs with the properties of an adsorbent, opening up new methodologies for the simple, selective, and enrichment detection method for myricetin.

A novel microporous phosphazene-triazinyl based covalent organic framework: Preparation, characterization and fluorescence properties

Covalent organic frameworks(COFs) attracted more and more attention due to the unique advantages, such as flexible topological connectivity, regular periodic pore structures, excellent stability and tunable functionality. Recently, several covalent organic frameworks(COFs) were developed for fluorescence detection of metal ions, explosives, VOCs, and so on. In this work, a novel fluorescent COFs(HM-COF) based on phosphazene and triazinyl was prepared by polymerization of hexachlorocyclo-triphosphazene(HCCP) and melamine(MA) successfully. The BET surface area and pore size of HM-COF were calculated as 305.7 m2 g-1 and 12.6 Å, respectively. HM-COF can emit strong fluorescence under ultraviolet lamp. An obvious fluorescent quenching of HM-COF was observed by adding Fe3+ aqueous solution, which demonstrated that HM-COF can act as a good fluorescent sensor for detecting Fe3+ cations. Furthermore, HM-COF can also be used for the qualitative detection of nitro aromatic compounds.

Flexible and Semi-flexible Amide-Hydrazide Decorated Fluorescent Covalent Organic Frameworks as On-Off pH Responsive Proton Scavengers.

One of the desired ways to improve robustness and crystallinity in the covalently linked reticular frameworks is through the incorporation of varied and key functionalities in these promising materials. In this work, we have strategically constructed a series of triazine-based amide-hydrazide linked COFs (CON, CONN, and CONNCO) by combining the flexible tri(4-formylphenoxy)cyanurate with semi-flexible 4-amino-N-(4-aminophenyl)benzamide, semi-flexible 4-aminobenzo-hydrazide, or flexible 4-amino-N-(4-aminobenzoyl)benzo-hydrazide linkages, respectively. The incorporation of an amide or a hydrazide functionality is the key to improving the crystallinity of the framework through strong intra- and interlayer H-bonding. The structural characterization of these COFs has been done with the help of numerous analytical methods. All three COFs exhibit good thermal and chemical stability in acid and base verified by PXRD and N2 sorption studies. Their intra- and interlayer H-bonding control the rotation in eclipsed layers, which follows emissive nature. Their stability, linkage functionality, and wettability in water have been judiciously used in fluorescent on-off pH responsive proton scavenging. The protonation-deprotonation of the imine N and N-H bond of the amide or hydrazide linkage adorned in the pore walls of three COFs plays a vital role for such an outcome.

A recyclable fluorescent covalent organic framework for exclusive detection and removal of mercury(II)

Here, by virtue of the natural merits of covalent organic frameworks (COFs), we report the rational design of a novel reaction-based recyclable fluorescent COF for both sensing and removal applications. The design concept is demonstrated by constructing allyl together with hydroxy groups functionalized COF material, AH-COF, for the exclusive detection and effective removal of mercury (II): the densely and well-distributed allyl cooperated with hydroxy groups as the Hg2+ reaction receptor sites via forming unique mercury-containing product based on the specific mercury-promoted reaction, and the π-conjugation structure as the signal sensor. The satisfactory detection performance of AH-COF was obtained in aspects of excellent uniqueness, high sensitivity, facile visualization, and real-time response. More fascinatingly, AH-COF can be easily recycled in the presence of NaBH4. Moreover, the effective Hg2+ removal from water and the easy recycling of AH-COF provides the potential prospect for actual applications. Furthermore, the solid-state NMR investigations overwhelmingly certified the privileged reaction between Hg2+ and the allyl in cooperation with hydroxy groups of AH-COF. This study not only shows the use of fluorescent COFs for both exclusive detection and facile removal of Hg2+, but also emphasizes a novel design concept for rational structure construction of functionalized COFs for Hg2+ remediation.