Spherical Covalent Organic Frameworks Research Articles

Hollow spherical covalent organic framework supported gold nanoparticles for photocatalytic H2O2 production

Covalent organic frameworks (COFs) are porous crystalline materials with promising applications in photocatalysis, but their performance is greatly hampered by the fast recombination of photogenerated carriers. Herein, a hollow spherical COF was synthesized via the condensation polymerization of 1,3,5-tris(4-aminophenyl)benzene and 1,3,5-benzenetricarbaldehyde, and Au nanoparticles were in-situ deposited onto the COF through NaBH4 reduction to ameliorate the photocatalytic performance. The composite exhibits impressive photocatalytic H2O2-production performance with the highest H2O2-evolution rate of 6067 μmol g−1 h−1 under visible light irradiation, which is about 1.9 times that of the pristine COF. The enhanced photocatalytic activity was ascribed to improved light absorption, abundant active Au sites, and efficient transfer and separation of photogenerated carriers in space. This work provides an avenue for designing high-performance H2O2-production photocatalysts by decorating porous COFs with active noble-metal sites.

Facile room-temperature synthesis of a spherical mesoporous covalent organic framework for capillary electrochromatography

Herein, a spherical covalent organic framework COF TAPB-DMTP was facilely synthesized from 2,5-dimethoxyterephthalaldehyde (DMTP) and 1,3,5-tri-(4-aminophenyl)benzene (TAPB) as monomers. COF TAPB-DMTP with regular mesoporous and excellent mass transfer ability was first introduced into the capillary and immobilized on the inner wall of the capillary through a simple in situ growth method. Through various characterization results, COF TAPB-DMTP was successfully prepared and modified onto the capillary inner wall. The separation performance and potential of COF TAPB-DMTP modified capillary column was explored. The new developed COF modified column achieved a highly efficiency and selective separation between analytes with different properties, including halogeno benzenes, alkylbenzenes, phenols and sulfonamides. Satisfactory stability and reproducibility were observed on COF TAPB-DMTP modified columns. The intraday, interday and three batch columns relative standard deviations were all less than 1.85 % for the retention time. The separation performance of prepared column has no significant change after 90 continuous runs. Additionally, the COF TAPB-DMTP modified capillary column was successfully used for separation and detection of triazole antifungal drugs in human plasma, and the recoveries of three antifungal drugs (fluconazole, isavuconazole and posaconazole) in spiked samples were in the range of 98.6–100.8 %, 92.4–102.1 % and 99.9–107.5 %, respectively. This self-made column showed excellent application potential in chromatography separation science.

Large–size porous spherical 3D covalent organic framework for preconcentration of bisphenol F in water samples and orange juice

Large–size spherical sorbents with particle size of 10–50 μm are widely applied in separation fields, however it is still a great challenge to synthesize such large–size spherical covalent organic framework (COF). In this work, a type of large–size porous 3D COF was size–controablly synthesized via a two–step strategy, in which a large–size porous 3D spherical polymer was prepared first through a Pickering emulsion polymerization using nano silica as the stabilizer, and subsequently it was converted into porous spherical 3D COF by a solvothermal method. The as–prepared porous spherical COF (COF–320 as a model) showed size–controllable uniform spherical morphology within 15–45 μm, large specific surface area, fine crystalline structure, and good chemical stability. When used as the sorbent for dispersive solid–phase extraction (d–SPE) of bisphenol F (BPF), the porous spherical COF–320 (15 μm) displayed high adsorption capacity (Qmax = 335.6 mg/g), high enrichment factor (80 folds), and good reusability (at least five cycles). By coupling the d–SPE method to HPLC, a new analytical approach was developed and successfully applied to the determination of trace BPF in two water samples, an orange juice and a standard sample with recoveries of 96.0–102.2 % (RSD = 1.1–1.5 %), 95.7–97.4 % (RSD = 1.4–4.4 %) and 98.7 % (RSD = 2.3 %), respectively. The limit of detection (S/N = 3) and limit of quantification (S/N = 10) were 0.1 and 0.3 ng/mL, respectively. The new synthesis strategy opens a viable way to prepare large–size porous spherical COFs, and the developed analytical method can be potentially applied to sensitively detect the trace BPF in water samples and beverages.

Post-synthetically functionalized covalent organic frameworks for highly efficient recovery of gold from leaching liquor of electronic waste

Recovery of precious metals from e-waste was of great economic and environmental significance. Herein, the spherical covalent organic frameworks (COF-V) were constructed by the Schiff base reaction between 1, 3, 5-tris (4-aminophenyl) benzene and 2, 5-divinylterephthalaldehyde, followed by post-synthetic modification using mono-(6-mercapto-6-deoxy)-β-cyclodextrin (S-β-CD) and dithiothreitol (DTT) for recovery of gold from e-waste. Multi-farious microscopic and spectroscopic techniques were used to explore the micromorphology and elemental composition of all the materials. Meanwhile, the response surface method was used to optimize the experimental parameters to improve the adsorption performance of Au(III). The functionalized spherical COFs exhibited excellent Au(III) adsorption abilities with maximum adsorption capacities of 652.6 mg/g and 820.8 mg/g for COF-V-DTT and COF-V-S-β-CD, respectively. The impressive properties were due to the synergistic adsorption effect of the grafted hydroxyl and sulfhydryl groups in appropriately sized ordered pores. The spectral analysis demonstrated that Au(III) was firstly adsorbed on the adsorbents by coupling with the S atom, and then in situ reduced to Au(0) by hydroxyl groups. Moreover, two COFs materials also exhibited an excellent selectivity in the actual gold-containing waste liquid. Thus, this study would provide an efficient and economical way to recovery Au(III) from wastewater.

Facile synthesis of spherical covalent organic frameworks for enrichment and quantification of aryl organophosphate esters in mouse serum and tissues.

Here, an imine-linked-based spherical covalent organic framework (COF) was prepared at room temperature. The as-synthesized spherical COF served as an adsorbent in dispersive solid-phase extraction (dSPE), by its virtue of great surface area (1542.68 m2 /g), regular distribution of pore size (2.95nm), and excellent stability. Therefore, a simple and high-efficiency dispersive solid phase extraction method based on a spherical COF coupled with high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) was established to determine aryl organophosphate esters in biological samples. This approach displayed favorable linearity in the range of 10.0-1000.0ng/L (r>0.9989), a high signal enhancement factor (58.8-181.8 folds) with low limits of detection (0.3-3.3ng/L). Moreover, it could effectively eliminate complex matrix interference to accurately extract seven aryl organophosphate esters from mouse serum and tissue samples with spiked recoveries of 82.0%-117.4%. The as-synthesized spherical COF has been successfully applied in sample preparation. The dSPE-HPLC-MS/MS method based on a spherical COF has potential application to study the pollutants' metabolism in vivo.

Ultrasensitive detection and efficient removal of mercury ions based on covalent organic framework spheres with double active sites

In present work, a new spherical covalent organic framework (TFPB−APTU COF) with good photoelectric property and double active sites (secondary amine (−NH−) group and sulfur (S) atom) was prepared for ultrasensitive detection and efficient removal of mercury ions (Hg2+). The –NH− group and S atom can capture free Hg2+ by coordination and chelation interaction, and the related steric hindrance effect reduces the photocurrent signal of the TFPB−APTU COF, resulting in the highly sensitive photoelectrochemical analysis of Hg2+ with a wide linear response range (0.01−100000 nM) and low detection limit (0.006 nM). On the other hand, the developed TFPB−APTU COF has large removal capacity (2692 mg g−1), good regeneration capability, and high removal speed for Hg2+ removal based on the double active sites (−NH− group and S atom), large specific surface area and porous spherical structure. The developed TFPB−APTU COF spheres show great potential in monitoring and treatment of environmental pollution of Hg2+.

Size-controlled synthesis of unusual hydrogen-bonded imine-linked covalent organic framework for trypsin immobilization and drug delivery

The interlayer hydrogen-bonded, uniform, and spherical covalent organic frameworks (USCOFs) and non-hydrogen bonded, and spherical covalent organic polymers (USCOPs) were synthesized in a series of aromatic and nonaromatic solvents at room temperature. The presence of interlayer hydrogen bonds between USCOFs were investigated via electron density difference (EDD) and stacking energy calculation in various solvents by DFT methods. The in-situ mid-angle X-ray scattering measurements indicate that the (100)-plane at 0.33 Å continuously grows in USCOFs, which was absent in USCOPs. The crystalline USCOFs exhibit a high surface area of 658 m2g−1, with a pore size of 3 nm, whereas USCOPs exhibit a surface area of 16 m2 g−1 with a pore size of 19 nm. Therefore, USCOFs formation in aromatic solvents, involving solvation and reduced π–π stacking, which allow the defective intermediates to dissociate and react reversibly, leading to well-ordered molecular arrangements. These solvent characteristics are absent in nonaromatic solvents; therefore, they fail to produce well-ordered molecular arrangement, forming amorphous USCOPs. Finally, the as-synthesized USCOFs were used for a protein(trypsin)-loading and drug delivery application. In addition, the loaded protein retains its original activity. Notably, the conducted MTT assay and cellular uptake proved COF-DOX selectively killed the cancer cells rather than normal cells.

Solvothermal Synthesis of Uniform Covalent Organic Framework Microspheres Enabling High-Loading Palladium for Oxygen Reduction Reaction

Although the solvothermal synthesis has become a prevailing strategy for the preparation of diverse spherical covalent organic frameworks (COFs), synthesis of uniform COF microspheres has still been a grand challenge due to the uncontrollable kinetics. Herein, we introduced 2,4,6-trimethylbenzaldehyde (TBA) as a molecular regulator in the reaction system, where the size and uniformity of spherical COFs were controlled by dynamic reversibility of imine exchange between TBA and building units and depended on the addition amount, molecular structure, and synthesis temperature. The applicability of the synthetic strategy was demonstrated by varying different structural units. Moreover, imine linkages of COF microspheres function as active sites suitable for coordinating with Pd single atoms and atomic clusters to promote electron transfer and synergistically enhance electrocatalytic oxygen reduction reaction. This work addressed the problems of poor uniformity and untunable size of COF spheres previously faced by the solvothermal method, thereby providing guidance for the construction of spherical COF-based catalyst supports.

Spherical covalent organic framework and gold nanoparticles modified 3D-printed nanocarbon electrode for the sensor of acetaminophen

3D-printing technology has emerged as an advanced manufacturing method to prepare novel sensors using in various areas. In this work, 3D-printed electrodes (3DEs) were fabricated using a graphene/polylactic acid (PLA) filament. Then, the activated 3DE was modified with gold nanoparticles (AuNPs) and spherical covalent organic framework (COF) to build an original sensor for the high sensitivity detection of acetaminophen (APAP). The fabricated 3DEs were characterized using scanning electron microscopy (SEM), X-ray photoelectricity spectroscopy (XPS) and contact angle experiments. The electrochemical properties of the prepared 3DEs were explored by methods of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The electrochemical sensor can be used to determine APAP at concentrations ranging from 0.1 to 100 μmol/L, with an ultra-low LOD (0.076 μmol/L). Moreover, the prepared sensor has been further used to test APAP in real sample with high recovery. Compared with traditional detection methods, the constructed 3D-printed sensor shows high sensitivity and has the potential to determine APAP in practical sample.

Enzyme-immobilized spherical covalent organic frameworks as nanoreactors for heterogeneous biocatalysis

Uniform spherical COFs were prepared to immobilize horseradish peroxidase with high loading capacity. The HRP@COF exhibited good reusability and enhanced tolerance to extreme circumstances.

Online solid-phase extraction based on size-controllable spherical covalent organic framework for efficient determination of polybrominated diphenyl ethers in foods

An analytical method of microspheric brominated covalent organic framework (Br-COF)-online solid-phase extraction integrated with high-performance liquid chromatography (online SPE-HPLC) was proposed for efficiently enriching six polybrominated diphenyl ethers (PBDEs) in foods. The Br-COF microspheres were facilely prepared with uniformity and dispersion by a size-controllable synthesis at the room temperature. Attributed to multiple interactions of the halogen bonding, Van der Waals forces, hydrophobic interaction along with size-matching effect, Br-COF performed satisfactory extraction capacity for PBDEs compared with commercial adsorbents. Five primary influencing factors were optimized, including loading solvent, loading flow rate, elution solvent, elution flow rate and elution volume. Under the optimal parameters, the implement displayed excellent linear ranges (0.5–500 ng mL−1) and low detection limits (0.01–0.05 ng mL−1). The relative recoveries in six spiked food samples ranged from 87.8 to 119.7 % with relative standard deviations below 10 %. This research estabished a promising platform for quantitatively determining trace PBDEs in complex foods.

Fluoro-Functionalized Spherical Covalent Organic Frameworks as a Liquid Chromatographic Stationary Phase for the High-Resolution Separation of Organic Halides.

The development of novel stationary phases with specific functionality is of great importance in chromatographic separation. Herein, we fabricated fluoro-functionalized spherical covalent organic frameworks (SF-COFs) via a bottom-up strategy as stationary phases for high-performance liquid chromatography (HPLC). Benefiting from the significant monodispersity, narrow size distribution, and high fluorine content, the SF-COFs packed column showed high column efficiency and excellent resolution for the separation of the organic fluorides involving polyfluorobenzenes, polychlorobenzenes, polybromobenzenes, perfluoroalkyl methacrylates, and halogenated trifluorotoluenes, which cannot be separated on the fluorine-free spherical covalent organic frameworks packed column. Especially, the column efficiency of 20 100-38 500 plates/m was obtained for polyfluorobenzenes, and the relative standard deviations of the retention time for continuous 10 separations of polychlorobenzenes and polybromobenzenes were less than 0.98%. Furthermore, the prepared SF-COFs packed column showed overwhelming superiority in the separation of organic halides compared with commercial C18 and pentafluorophenyl (PFP) packed columns. In addition, the compounds with different hydrophobicity or aromatic ring structure were also successfully separated on the SF-COFs packed column. This work extended the application of spherical COFs and provided a new way to introduce specific functional groups into the COF-based stationary phase for HPLC.

Spherical covalent organic frameworks supported black phosphorus@Au nanocatalysts for nitrophenol hydrogenation in a high efficiently flow-through process

Exfoliated black phosphorus (BP) nanosheets possessing unique electron-reservoir and electron-donor properties could fast electron transfer, promising as a cocatalyst for catalysis. However, the catalytic process was limited by the morphology of BP nanosheets because the continuous pressure deriving from the effluent would dense the channel of stacked BP nanosheets. In this work, spherical covalent organic frameworks were synthesized and used as a platform to support BP nanosheets/Au nanocatalysts. The prepared heterostructure was applied to catalytic reduction of 4-nitrophenol under a flow-through catalytic process. An optimized permeation flux of 2500 L/m−2(−|-)h−1 (LMH)and a reduction efficiency of99 % was achieved. Furthermore, only 0.064 wt% of Au NPs was released after testing for 24 h. Our straightforward strategy to solve the issues suffering from the continuous treatment of effluent would facilitate the application of novel nanomaterials in environmental remediation.

The synthesis of size-controlled hollow spherical covalent organic frameworks and its application in photocatalysis and Suzuki coupling reactions

Here, we reported the self-templated preparation of an allyl-containing, internally hollow, spherical COFs. Various factors affecting the formation of hollow spherical COFa-2 were investigated using diverse spectroscopic and microscopic techniques. Detailed time-dependent studies revealed a continuous transition of COFa-2 catalyst from an initial solid spherical structure to a homogeneous hollow spherical system by an Ostwald maturation mechanism. In addition, the effects of precursors with different structures on the photoelectric properties and photocatalytic degradation of RhB of the prepared COFa-1, COFa-2 and COFa-3 were also studied. Results have shown that allyl can effectively improve the response of COFa-2 to visible light and the degradation ability of RhB. Furthermore, allyl-palladium (COFa-2@Pd2+) formed by the coordination of palladium ions with allyl can effectively catalyze the Suzuki coupling (TOF = 16 × 10-4) and the reduction reaction of p-nitrophenol (Kapp = 1.56 min−1). More importantly, the catalysts COFa-2 and COFa-2@Pd2+ showed high stability after being recycled 10 times.

On-site colorimetric detection of Salmonella typhimurium

Rapid qualitative and quantitative detection of Salmonella typhimurium (S. typhimurium) takes an important role in ensuring food safety. Herein, a colorimetric assay aptasensor for S. typhimurium utilizing intrinsic peroxidase-like activity of gold nanoparticles embedded spherical covalent organic framework and the affinity and specificity of S. typhimurium-aptamer has been explored. This aptasensor can capture the S. typhimurium via the selective binding effect of aptamer, and the catalytically active sites were shielded. As a result, the colorimetric signals of the 3,3′,5,5′-tetramethylbenzidine-H2O2 system were turned off. Under optimum conditions, the aptasensor gave a linear response over the range of 10 to 107 CFU/mL for S. typhimurium. The detection limit of 7 CFU/mL was obtained within 45 min and was effectively applied to detect S. typhimurium in milk and lake water samples with recoveries in the range from 96.4 to 101.0%. More importantly, combined with a self-developed smartphone-based image analysis system, the proposed aptasensor can be used for point-of-care testing applications.

Preparation of hollow spherical covalent organic frameworks via Oswald ripening under ambient conditions for immobilizing enzymes with improved catalytic performance

Preparation of hollow spherical covalent organic frameworks via Oswald ripening under ambient conditions for immobilizing enzymes with improved catalytic performance

In situ synthesis of a spherical covalent organic framework as a stationary phase for capillary electrochromatography

Covalent organic frameworks (COFs) are a novel type of crystalline porous organic polymer materials recently developed. It has several advantages in chromatographic separation field, such as high thermal stability, porosity, structural regularity, and large specific surface area. Here, a novel spherical COF 1,3,5-tris(4-aminophenyl)benzene (TAPB) and 2,5-bis(2-propyn-1-yloxy)-1,4-benzenedicarboxaldehyde (BPTA) was developed as an electrochromatographic stationary phase for capillary electrochromatography separation. The COF TAPB-BPTA modified capillary column was fabricated via a facile in situ growth method at room temperature. The characterization results of scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, and X-ray diffraction (XRD) confirmed that COF TAPB-BPTA were successfully modified onto the capillary inner surface. The electrochromatography separation performance of the COF TAPB-BPTA modified capillary was investigated. The prepared column demonstrated outstanding separation performance toward alkylbenzenes, phenols, and chlorobenzenes compounds. Furthermore, the baseline separations of non-steroidal anti-inflammatory drugs (NSAIDs) and parabens with good efficiency and high resolution were achieved. Also, the prepared column possessed satisfactory precision of the intra-day runs (n = 5), inter-day runs (n = 3), and parallel columns (n = 3), and the relative standard deviations (RSDs) of the retention times of tested alkylbenzenes were all less than 2.58%. Thus, this new COF-based stationary phase shows tremendous application potential in chromatographic separation field.

Construction of a Hollow Spherical Covalent Organic Framework with Olefin and Imine Dual Linkages Based on Orthogonal Reactions

Covalent organic frameworks (COFs) have widespread applications in many fields. However, successful attempts to construct COFs through the formation of two types of covalent bonds remain limited and challengeable. Here, we report the first example of a COF with olefin and imine dual linkages, which was enabled by orthogonal Schiff base reaction and the Knoevenagel condensation. The obtained COF-FD1 exhibits good stability and high selectivity in the fluorescence sensing of Fe3+ ions. Remarkably, this COF features a relatively rare hollow spherical morphology, and a time-dependent study of the continuous transformation from initial solid spherical to hollow spherical structures reveals an Ostwald ripening mechanism. This research not only demonstrates a new approach for the design and synthesis of COFs with dual linkages but also may bring about the possibility of constructing COFs with hollow structures.

Fabrication of cellulose derivative coated spherical covalent organic frameworks as chiral stationary phases for high-performance liquid chromatographic enantioseparation

Porous spherical silica-based chiral stationary phases (CSPs) have been commercially used in the field of chiral separation, however, the scope of their application is, to some extent, limited by the instability of silica towards mobile phase containing strong base or acid. As such, developing new matrix-based CSPs is one of the effective strategies to overcome this bottleneck in studies of chiral separation materials. In this work, we have demonstrated that stable spherical covalent organic frameworks (SCOFs) can be utilized as matrixes for the fabrication of new CSPs for the first time. Specifically, a porous imine-linked SCOF with good crystallinity, large surface area, and high chemical stability is synthesized at room temperature. Then, cellulose-tris (3,5-dimethylphenylcarbamate) (CDMPC), a typical cellulose derivative, is selected as a potential chiral selector and coated onto the robust SCOFs, giving rise to the fabrication of new CDMPC@SCOF CSPs. The as-synthesized stable SCOF-based CSPs are exploited for high-performance liquid chromatographic (HPLC) enantioseparation, showing high resolution abilities for the separation of racemic compounds such as metalaxyl, 1-(1-naphthalenyl)ethanol, epoxiconazol, trans-stilbene oxide, and so on. Moreover, the prepared SCOF-based CSPs exhibit more superior acid and base stability than those of the silica-based CSPs. Our work not only uncovers the great potential of SCOFs as matrixes for constructing novel CSPs, but also expands the application of COFs in the field of enantiomeric separation under harsh base and acid conditions.

Spherical covalent organic framework supported Cu/Ag bimetallic nanoparticles with highly catalytic activity for reduction of 4-nitrophenol

4-Nitrophenol (4-NP) is water pollutant with good stability, high toxicity and difficult degradation, which will cause great harm to the environment. Thus, the development of high efficiency and rapid catalysts for the degradation of 4-NP will be an urgent problem to be solved. Herein, we synthesized covalent organic framework supporting Cu/Ag bimetallic nanoparticles (Cu/Ag–COF), which had better catalytic activity for the reduction of 4-NP. The porous COF possess large surface area of approximately 737.8 ​m2/g, so it can provide abundant sites for loading metal nanoparticles. At the same time, COF could also improve the stability and catalytic effect of nanocatalyst under flexible condition. The prepared Cu/Ag–COF exhibited the higher catalytic reduction of 4-NP into 4-aminophenol (4-AP) under the condition that the molar ratio of NaBH4 to 4-NP was 147 and achieved an apparent rate constant of kCu/Ag–COF ​= ​0.0073 s−1. The apparent rate constant of Cu/Ag–COF was higher than that of Ag–COF (kAg-COF ​= ​0.0032 s−1) and Cu–COF (kCu-COF ​= ​0.0047 s−1), indicating that this system had a synergistic catalytic effect to improve the catalytic activity of metal nanoparticles. Moreover, Cu/Ag–COF still had good catalytic effect after eight cycles, which indicated that it had good stability and reusability. This work provides a novel strategy to fabricate COF supported nanoparticles for outstanding performance in treating wastewater containing 4-NP.