Recent advances in covalent organic frameworks for separation and analysis of complex samples

Facile Preparation of Magnetic COF-on-COF for Rapid Adsorption and Determination of Sulforaphane from Cruciferous Vegetables

Sulforaphane (SFN) is a natural isothiocyanate compound widely abundant in cruciferous vegetables with multiple bioactive functions. However, traditional analytical methods for the extraction and determination of SFN are cumbersome, time-consuming, and low sensitivity with large amounts of organic solvents. Herein, novel magnetic COF-on-COFs (MB-COFs) were fabricated using Fe3O4 as a magnetic core and COFs-1 grown with COFs-2 as a shell, and they were used as efficient adsorbents of magnetic dispersive solid-phase extraction for rapid quantification of SFN in cruciferous vegetables by combining with HPLC-MS/MS. At the optimal ratio of COFs-1 to COFs-2, MB-COFs had a spherical cluster-like structure and a rough surface, with a sufficient magnetic response for rapid magnetic separation (1 min). Due to the introduction of Fe3O4 and COFs-2, MB-COFs exhibited outstanding extraction efficiencies for SFN (92.5-97.3%), which was about 18-72% higher than that of the bare COFs. Moreover, MB-COFs showed good adsorption capacity (Qm of 18.0 mg/g), rapid adsorption (5 min) and desorption (30 s) to SFN, and favorable reusability (≥7 cycles) by virtue of their unique hierarchical porous structure. The adsorption kinetic data were well fitted by the pseudo-second-order, Ritchie-second-order, intra-particle diffusion, and Elovich models, while the adsorption isotherm data were highly consistent with the Langmuir, Temkin, and Redlich-Peterson models. Finally, under the optimized conditions, the developed method showed a wide linear range (0.001-0.5 mg/L), high sensitivity (limits of quantification of 0.18-0.31 μg/L), satisfactory recoveries (82.2-96.2%) and precisions (1.8-7.9%), and a negligible matrix effect (0.82-0.97). Compared to previous methods, the proposed method is faster and more sensitive and significantly reduces the use of organic solvents, which can achieve the efficient detection of large-scale samples in practical scenarios. This work reveals the high practical potential of MB-COFs as adsorbents for efficient extraction and sensitive analysis of SFN in cruciferous vegetables.

Size-controllable synthesis of large-size spherical 3D covalent organic frameworks as efficient on-line solid-phase extraction sorbents coupled to HPLC

BACKGROUND

Covalent organic frameworks (COFs) have found promising applications in separation fields due to their large surface area and high adsorption capacity, but the exiting COFs can not be directly used as the packing materials of on-line solid-phase extraction (SPE) coupled to HPLC and HPLC because their nano/submicron size or irregular shapes might cause ultrahigh column back pressure and low column efficiency. To synthesize the large-size spherical COFs larger than 3 μm as sorbents might be able to address these problems, however it is still a great challenge till now.

RESULTS

In this work, two large-size spherical 3D COFs (COF-320 and COF-300) were size-controllably synthesized within 10-90 μm via a two-step strategy. These two spherical COFs showed large surface area, fine crystallinity, good chemical/mechanical stability, and good reproducibility. As an application case, when used as the on-line SPE sorbents coupled to HPLC, the large-size spherical COF-320 displayed high binding capacity for bisphenol F (Qmax of 452.49 mg/g), low column back pressure (6-8 psi at flow rate of 1 mL/min), and good reusability (at least 30 cycles). The developed on-line-SPE-HPLC-UV method presented good analytical performance with enrichment factor of 667 folds, linear range of 1.0-400 ng/mL, limit of detection (LOD, S/N = 3) of 0.3 ng/mL, limit of quantification (LOQ, S/N = 10) of 1.0 ng/mL, and recoveries of 100.3-103.2 % (RSDs of 2.0-3.5 %) and 95.2-97.0 % (RSDs of 4.3-5.6 %) for tap water and lake water samples, respectively.

SIGNIFICANCE

This is the first case to synthesize the large-size spherical COFs within 10-90 μm, and this work made it possible to directly use COFs as the filling materials of on-line SPE coupled to HPLC and HPLC. The developed analytical method can be potentially applied to the rapid and sensitive detection of trace bisphenol F in environmental water samples.

Three-dimensional fluorinated covalent organic frameworks coated capillary for the separation of fluoroquinolones by capillary electrochromatography

In this work, a three-dimensional fluorinated covalent organic frameworks (3D FCOFs) JUC-515 was synthesized from tetra(4-aminophenyl)methane (TAM) and 2,3,5,6-tetrafluoroterephthalol (TFA) by an ionic liquid method. JUC-515 was introduced into the capillary column and bonded to the inner wall of the capillary column by chemical bonding. Through a variety of characterization results, JUC-515 was successfully synthesized and introduced into the capillary column. The effects of buffer solution concentration, organic additive content and pH of the buffer solution on the separation of fluoroquinolones (FQs) were investigated in detail. The JUC-515-coated capillary column showed good resolution (>1.5) and reproducibility. The relative standard deviations (RSDs) of the retention time for intraday, interday, column-to-column and interbatch precision were less than 0.88%, 2.45%, 2.74% and 3.32%, respectively. The RSDs of the peak area for intraday, interday, column-to-column and interbatch precision were less than 3.79%, 4.31%, 3.33% and 5.62%, respectively. The JUC-515-coated capillary column could be used no less than 150 times. The results showed that the JUC-515-coated capillary column had good separation performance. In addition, by separating fluorinated β-phenylalanine analogs, β-phenylalanine and trifluoromethyl β-phenylalanine analogs, the separation mechanism based on fluorine interactions was discussed. In conclusion, JUC-515 had good potential as a stationary phase for capillary electrochromatography.

Covalent organic framework in situ grown on the metal-organic framework as fiber coating for solid-phase microextraction of polycyclic aromatic hydrocarbons in tea

A novel MIL-88-NH2@COF composite was produced by in situ growth of covalent organic framework (COF) on the metal-organic framework (MOF) surface. To obtain a coating fiber for solid-phase microextraction (SPME), the MIL-88-NH2@COF composite physically adhered to the stainless steel wire. Combined with gas chromatography-flame ionization detection (GC-FID), various analytes such as chlorophenols (CPs), phthalates (PAEs), and polycyclic aromatic hydrocarbons (PAHs) were extracted and determined to evaluate the extraction performance of MIL-88-NH2@COF coated fibers and explore their extraction mechanism. This composite exhibit excellent extraction performance and adsorption capacity for various analytes, especially for PAHs with enrichment factor up to 9858. The SPME-GC-FID method based on MIL-88-NH2@COF fiber was established for the determination of five PAHs after the main extraction conditions were optimized. Under optimal conditions, the proposed technique showed a wide linear range (1-150 ng mL-1) with a low limit of detection (0.019 ng mL-1) and a high coefficient of determination (R2 > 0.99). The developed SPME-GC-FID method was used to determine PAHs in green tea and black tea samples, with good recoveries of 51.70-103.64% and 68.56-103.64%, respectively. It is worth mentioning that this is the first time MIL-88-NH2@COF composites have been prepared and applied to SPME. The preparation method of the composite provides a new idea in adsorbent preparation, which will contribute to the field of SPME.

Covalent Organic Framework Nanosheets for Fluorescence Quantification of Peptide

Rapid and sensitive quantification of peptides plays an important role in clinical diagnosis. Fluorescence assay is one of the most promising peptide detection tools, but it relies on intrinsic fluorescence or additional derivatization, resulting in poor versatility. Covalent organic frameworks (COFs) have shown a good application prospect in the field of fluorescence detection, but their application scope is limited to heavy metal ions and some small polar organic molecules. Herein, we report the application of COFs nanosheet for fluorescence detection of peptides. Fluorescent sp² acrylonitrile-linked COFs nanosheets (TTAN-CON) were prepared by water-assisted ultrasonic exfoliation which performed with excellent fluorescence properties with Stokes shifts of 146 nm and fluorescence quantum yield of up to 24.45%. Compared to the bulk fluorescent COFs, exfoliated CONs films performed with better stability of fluorescence signal in solution. We found the fluorescence of TTAN-CON can be effectively quenched by hydrophobic peptides at a very rapid rate (less than 5 min per sample). TTAN-CON presented good sensitivity and selectivity for hydrophobic peptides detection via the static and dynamic joint quenching mechanism. TTAN-CON was further used to detect NLLGLIEAK and ProGRP_31-98, two target peptide fragments of lung cancer biomarker ProGRP. The fluorescence intensities of TTAN-CON were negative linearly correlated with the amounts of hydrophobic NLLGLIEAK over the range of 5-1000 ng/mL with the correlation coefficients over 0.99, and the limit of detection was 1.67 ng/mL, displaying higher sensitivity and convenience than traditional optical methods. What's more, the quantification of ProGRP_31-98 was achieved by the quantification of hydrophobic peptides in its enzyme hydrolysis products. We anticipate COFs nanosheets to be a universal fluorescence detection work-box for peptides biomarkers with clinical significance.

The covalent organic framework based nylon membrane extraction coupled with UHPLC-MS/MS for highly efficiency determination of hexabromocyclododecanes in environmental water

Hexabromocyclododecanes (HBCDs) have given their adverse effects on environment and human health, and highly sensitive analysis of HBCDs in water is urgent. In this study, a new method for the determination of trace HBCDs in water was established by covalent organic framework (COF) based nylon membrane extraction (ME) coupled with ultrahigh-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS). The COF had been self-assembled onto the nylon membrane in a gentle strategy to fabricate COF nylon membrane. Several important ME parameters including the dosage of COF, pH, eluent condition and salinity were systematically investigated. The limits of detection and quantification were 0.011-0.014 and 0.038-0.047 ng/L for three HBCDs, respectively. The linear ranges were from 0.04 to 20 ng/L, and the relative standard deviations were 5.7-17.8 % (intra-day) and 5.2-14.1 % (inter-day). In addition, density functional theory (DFT) calculations on adsorption energy proved that the introduction of halogen bond (XB) made a key contribution to high extraction efficiency and excellent selectivity of COF nylon membrane for HBCDs. The 500 mL of samples, including tap water and reservoir water, could be extracted only in 23 min. The established method presented highly sensitive for ultra-trace analysis of HBCDs in environmental water.

Covalent organic framework: A state-of-the-art review of electrochemical sensing applications

Covalent organic framework (COF), a kind of porous polymer with crystalline properties, is a periodic porous framework material with precise regulation at atomic level, which can be formed by the orderly connection of pre-designed organic construction units through covalent bonds. Compared with metal-organic frameworks, COFs exhibit unique performance, including tailor-made functions, stronger load ability, structural diversity, ordered porosity, intrinsic stability and excellent adsorption features, are more conducive to the expansion of electrochemical sensing applications and the universality of applications. In addition, COFs can accurately integrate organic structural units with atomic precision into ordered structures, so that the structural diversity and application of COFs can be greatly enriched by designing new construction units and adopting reasonable functional strategies. In this review, we mainly summarized state-of-the-art recent advances of the classification and synthesis strategy of COFs, the design of functionalized COF for electrochemical sensors and COFs-based electrochemical sensing. Then, an overview of the considerable recent advances made in applying outstanding COFs to establish electrochemical sensing platform, including electrochemical sensor based on voltammetry, amperometry, electrochemical impedance spectroscopy, electrochemiluminescence, photoelectrochemical sensor and others. Finally, we discussed the positive outlooks, critical challenges and bright directions of COFs-based electrochemical sensing in the field of disease diagnosis, environmental monitoring, food safety, drug analysis, etc.

Determination of trace phenoxy carboxylic acid herbicides in environmental water samples by covalent organic frameworks based solid phase extraction coupled with liquid chromatography-tandem mass spectrometry

The determination of traces levels of pesticide residue in water is crucial for monitoring water quality. In this study, covalent organic frameworks (COFs), namely TAPA-TFPB-COFs were prepared at room temperature (25 °C) and applied as adsorbents for the solid phase extraction (SPE) of phenoxy carboxylic acid herbicides (PCAs). The extraction was followed by analyzation using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Under the optimal conditions, ultrasensitive and specific analysis of PCAs in water samples was achieved. The method exhibited high sensitivity with low limits of detection (0.08-0.28 ng L^-1), good linearity in the range of 1.00 to 200 ng L^-1 and satisfactory repeatability (intra-day: 3.72-5.30%; inter-day: 2.02-4.04%). The method was successfully applied to the analyzation of trace PCAs in tap, well, and river water and the spiked recoveries were in the range of 81.1-112%. These results indicate that the SPE-LC-MS/MS technique with TAPA-TFPB-COFs as the SPE adsorbent is a promising technique for the detection of trace levels of PCAs in environmental water samples.

Magnetic guanidyl-functionalized covalent organic framework composite: a platform for specific capture and isolation of phosphopeptides and exosomes

A guanidine-functionalized (GF) covalent organic framework (COF) nanocomposite has been developed by a post-synthetic approach for specific capture and separation of phosphopeptides and exosomes. The abundant binding sites on COF can immobilize a large number of gold nanoparticles (AuNPs), which can be used to react with amino groups to graft polyethyleneimine (PEI). Finally, Fe₃O₄@COF@Au@PEI-GF is obtained through the reaction of PEI and guanidyl group for phosphopeptides and exosomes detection. This composite shows a low detection limit (0.02 fmol), size exclusion effect (β-casein digests:Albumin from bovine serum protein = 1:10,000), good reusability (10 cycles), and high selectivity (β-casein digests:Albumin from bovine serum digests = 1:10,000). For complex biological sample, 4 phosphopeptides can be successfully identified from human serum. Furthermore, for the first time, we used guanidyl-functionalized probe to capture exosomes in human serum, providing a new method for enriching exosomes. The above experiments showed that Fe₃O₄@COF@Au@PEI-GF not only effectively enrich phosphopeptides and remove macromolecular proteins, but also successfully separate and capture exosomes. This demonstrates the great potential of this composite for the specific enrichment of phosphopeptides and isolation of exosomes.

Metalated covalent organic frameworks: from synthetic strategies to diverse applications

Covalent organic frameworks (COFs) are a class of organic crystalline porous materials discovered in the early 21st century that have become an attractive class of emerging materials due to their high crystallinity, intrinsic porosity, structural regularity, diverse functionality, design flexibility, and outstanding stability. However, many chemical and physical properties strongly depend on the presence of metal ions in materials for advanced applications, but metal-free COFs do not have these properties and are therefore excluded from such applications. Metalated COFs formed by combining COFs with metal ions, while retaining the advantages of COFs, have additional intriguing properties and applications, and have attracted considerable attention over the past decade. This review presents all aspects of metalated COFs, from synthetic strategies to various applications, in the hope of promoting the continued development of this young field.

Fabrication of magnetic covalent organic framework for efficient extraction and determination of phthalate esters in milk samples

Using two monomers of 4,4″-diamino-p-terphenyl and 1,3,5-triformylphloroglucinol, a co-precipitation structured magnetic covalent organic framework adsorbent was fabricated. After that, a high efficient vortex-assisted magnetic solid-phase extraction method was developed prior to gas chromatography-tandem mass spectrometry analysis for the determination of phthalate esters in milk samples. The fabricated magnetic adsorbent was facilely fabricated, fully characterized, and exhibited high extraction efficiency, which can be attributed to its larger pore size as well as strong hydrophobic and π-π stacking interactions between adsorbent and phthalate esters. Key parameters affecting extraction efficiency were investigated. Under the optimized conditions, the proposed method possessed good linearity (3.0-1000 μg/L), high sensitivity (0.8-2.1 μg/L for limits of detection), and satisfactory recoveries (76.8%-99.2%). The relative standard deviations for intra-day was 3.1%-4.5% and inter-day was 3.3%-6.1%. This work is suitable for high efficient separation/preconcentration of phthalate esters in milk samples.

Synthesis of the Magnetically Nanoporous Organic Polymer Fe3O4@SiO2-NH2-COP and Its Application in the Determination of Sulfonamide Residues in Surface Water Surrounding a Cattle Farm

Efficient extractions of trace antibiotic residues in the environment are a key factor for accurate quantification of the residues. A new nanoporous material, namely, magnetically covalent organic polymer (MCOP, Fe₃O₄@SiO₂-NH₂-COP) was synthesized in this work and was used for magnetic solid-phase extraction (MSPE). The combination of MSPE with high-performance liquid chromatography separation together with ultraviolet detection (HPLC-UV) was established as an effective method for the determination of four sulfonamide (SA) residues in surface water surrounding a cattle farm. The synthesized magnetic material was characterized by SEM, TEM, FT-IR, magnetic properties measurement system (MPMS), and nitrogen gas porosimetry. The material possessed many attractive features, such as a unique microporous structure, a larger specific surface area (137.93 m²·g⁻¹) than bare Fe₃O₄ (24.84 m²·g⁻¹), high saturation magnetization (50.5 emu·g⁻¹), open adsorption sites, and high stability. The influencing parameters, including pH, the used amount of MCOPs, the type of eluent, adsorption solution, and desorption time, were optimized. Under the optimized conditions, the method conferred good linearity ranges (R² ≥ 0.9990), low detection limits (S/N = 3, LOD, 0.10–0.25 μg·L⁻¹), and satisfactory recoveries (79.7% to 92.2%). The enrichment factor (EF) for the four SAs was 34.13–38.86. The relative standard deviations of intraday (n = 5) and of interday (n = 3) were less than 4.8% and 8.9%, respectively. The equilibria between extraction and desorption for SAs could be reached within 150 s. The proposed method was sensitive and convenient for detecting SA residues in complex environmental matrices, and the successful application of the new MCOPs as an adsorbent was demonstrated.

The electrospun polyacrylonitrile/covalent organic framework nanofibers for efficient enrichment of trace sulfonamides residues in food samples

In this work, the electrospun polyacrylonitrile/covalent organic frameworks Tp-BD nanofibers (PAN/Tp-BD) were synthesized and applied as an adsorbent for thin film microextraction (TFME) of seven sulfonamides in animal derived food samples. The morphology, structure, porosity, and stability of the prepared nanofibers were investigated. The PAN/Tp-BD nanofibers exhibited good chemical stability, high flexibility, porous fibrous structure, and excellent extraction efficiency. Based on the PAN/Tp-BD nanofibers as the adsorbent, a thin film microextraction-high performance liquid chromatography (TFME-HPLC) method for the determination of seven sulfonamides (SAs) in food samples was developed. Under the optimal conditions, the TFME-HPLC exhibited the low limit of detection (0.10-0.18 ng·mL^-1), the low limit of quantitation (0.33-0.60 ng·mL^-1), the wide linear range (0.5-50 ng·mL^-1) with correlation coefficients between 0.994 and 0.998, and good enrichment factors between 39.7 to 170.1 towards 20 ng/mL SAs solution. The relative standard deviation (RSD) was lower than 11% in the interday and intraday analysis. Furthermore, the applicability of PAN/Tp-BD nanofibers was demonstrated for measuring trace SAs residues in the spiked food samples with recoveries ranging from 85.3% to 115.2%. The results demonstrated that the PAN/Tp-BD nanofibers have great potential for the efficient extraction of sulfonamides from complex food samples.

Novel Metal-Free Fluorescent Sensor Based on Molecularly Imprinted Polymer N-CDs@MIP for Highly Selective Detection of TNP

In this article, we designed a fluorometric sensor based on nitrogen-passivated carbon dots infused with a molecularly imprinted polymer (N-CDs@MIP) via a reverse microemulsion technique using 3-aminopropyltriethoxysilane as a functional monomer, tetraethoxysilane as a cross-linker, and 2,4,6-trinitrophenol (TNP) as a template. The synthesized probe was used for selective and sensitive detection of trace amounts of TNP. The infusion of N-CDs (QY-21.6 percent) with a molecularly imprinted polymer can increase the fluorescent sensor sensitivity to detect TNP. Removal of template molecules leads to the formation of a molecularly imprinted layer, and N-CDs@MIP fluorescence response was quenched by TNP. The developed fluorescence probe shows a fine linear range from 0.5 to 2.5 nM with a detection limit of 0.15 nM. The synthesized fluorescent probe was used to analyze TNP in regular tap and lake water samples.

Covalent organic framework DQTP modified pencil graphite electrode for simultaneous determination of bisphenol A and bisphenol S

The sensitivity and selectivity of electrochemical analysis are challenging due to the materials used for electrode modification as well as electrical conductivity, catalytic activity and recognition ability of the working electrode. In this work, a portable 3D-printed electrochemical electrode clamp was designed and applied in combination with the developed covalent organic framework (COF DQTP)-modified pencil graphite electrode (DQTP/PGE). The β-ketoenamine-linked COF DQTP synthesized by 1,3,5-triformylphloroglucinol (TP) and 2,6-diaminoanthraquinone (DQ) through solvothermal method is a porous crystalline with excellent conductivity and large periodic π-arrays, coupled with commercial available pencil graphite electrode to fabricate a disposable sensor for simultaneous determination of environmental endocrine disruptors bisphenol A and bisphenol S. The DQTP/PGE sensor exhibited high electrical conductivity and catalytic activity, and a good linearity was obtained in a range of 0.5-30 μM for two bisphenols with a detection limit of 0.15 μM (S/N = 3). Moreover, the sensor showed a reproducible and stable response over one month with negligible interference, and an accepted recovery with real food packaging samples.

Post-synthesis modification of covalent organic frameworks for ultrahigh enrichment of low-abundance glycopeptides from human saliva and serum

In this study, a novel type of covalent organic framework (COF) material rich in boronic acid sites was prepared through post-synthesis modification (TbBD@PEI@Au@4-MPBA). The surface of COF material had abundant carboxylic acid groups, which could bind a large amount of polyethyleneimine (PEI) through electrostatic interaction. At the same time, the amino groups on the PEI can be grafted with Au nanoparticles (Au NPs) in situ, and then 4-mercaptophenylboronic acid (4-MPBA) was modified by the reaction of Au and sulfhydryl groups. The massive grafting of boronic acid groups made the material's enrichment effect on glycopeptides expected. The results of experiments indicated that the composite material has high sensitivity (5 amol μL^-1) and selectivity (1:1000). In addition, the material has outstanding stability and reusability, with a load capacity of about 100 mg g^-1 and a recovery of 99.3 ± 2.2%. What's more, after enriched by TbBD@PEI@Au@4-MPBA, 56 endogenous glycopeptides from fresh human saliva were detected by MALDI-TOF MS, 56 unique glycopeptides corresponding to 31 glycoproteins from human saliva and 513 unique glycopeptides corresponding to 208 glycoproteins from serum of throat cancer patient were detected by nano-LC-MS/MS, respectively, which was expected to be applied to glycoproteomics research.

Covalent organic frameworks as multifunctional materials for chemical detection

Sensitive and selective detection of chemical and biological analytes is critical in various scientific and technological fields. As an emerging class of multifunctional materials, covalent organic frameworks (COFs) with their unique properties of chemical modularity, large surface area, high stability, low density, and tunable pore sizes and functionalities, which together define their programmable properties, show promise in advancing chemical detection. This review demonstrates the recent progress in chemical detection where COFs constitute an integral component of the achieved function. This review highlights how the unique properties of COFs can be harnessed to develop different types of chemical detection systems based on the principles of chromism, luminescence, electrical transduction, chromatography, spectrometry, and others to achieve highly sensitive and selective detection of various analytes, ranging from gases, volatiles, ions, to biomolecules. The key parameters of detection performance for target analytes are summarized, compared, and analyzed from the perspective of the detection mechanism and structure-property-performance correlations of COFs. Conclusions summarize the current accomplishments and analyze the challenges and limitations that exist for chemical detection under different mechanisms. Perspectives on how future directions of research can advance the COF-based chemical detection through innovation in novel COF design and synthesis, progress in device fabrication, and exploration of novel modes of detection are also discussed.

Facile synthesis of Ti4+-immobilized magnetic covalent organic frameworks for enhanced phosphopeptide enrichment

In this work, core-shell structured Ti⁴⁺-immobilized magnetic covalent organic frameworks (denoted as Fe₃O₄@TAPTDHTA-Ti⁴⁺ composites) were prepared for enhanced phosphopeptide enrichment by one-pot synthesis of COFs shell with inherent bifunctional groups on Fe₃O₄ NPs and further Ti⁴⁺ immobilization. The widely distributed bifunctional groups could provide abundant chelating sites for Ti⁴⁺ immobilizing. Combining with the high specific surface area and mesoporous structure, the Fe₃O₄@TAPTDHTA-Ti⁴⁺ composites exhibited excellent enrichment efficiency for phosphopeptides, such as low detection limit (0.05 fmol μL^-1), high selectivity (1:5000 of molar ratio of β-casein/bovine serum albumin (BSA) tryptic digests), high adsorption capacity (62.9 μg mg^-1) and strong size-exclusive effect (1:250:250 of molar ratio of β-casein tryptic digest/β-casein/BSA). In addition, this method was general for immobilizing other metal ions (Zr⁴⁺ and Fe³⁺). Notably, the Fe₃O₄@TAPTDHTA-Fe³⁺ composites exhibited controllable affinity towards mono-phosphopeptides and multi-phosphopeptides. Furthermore, the Fe₃O₄@TAPTDHTA-Ti⁴⁺ composites were successfully applied to selectively capture phosphopeptides from complex biological samples including the tryptic digest of nonfat milk, human serum and human saliva. More significantly, 3333 phosphopeptides derived from 1409 phosphoproteins with 3492 phosphorylation sites were clearly identified from the tryptic digest of HeLa cell lysate. In addition to providing a potential excellent enrichment probe for comprehensive phosphoproteomic analysis, this study also offers a new perspective for the functionalization of COFs.

Nanoscale covalent organic frameworks: from controlled synthesis to cancer therapy

Covalent organic frameworks (COFs), as a new type of crystalline porous materials, mainly consist of light-weight elements (H, B, C, N and O) linked by dynamic covalent bonds to form periodical structures of two or three dimensions. As an attribute of their low density, large surface area, and excellent adjustable pore size, COFs show great potential in many fields including energy storage and separation, catalysis, sensing, and biomedicine. However, compared with metal organic frameworks (MOFs), the relatively large size and irregular morphology of COFs affect their biocompatibility and bioavailability in vivo, thus impeding their further biomedical applications. This Review focuses on the controlled design strategies of nanoscale COFs (NCOFs), unique properties of NCOFs for biomedical applications, and recent progress in NCOFs for cancer therapy. In addition, current challenges for the biomedical use of NCOFs and perspectives for further improvements are presented.

A review on recent advances in the enrichment of glycopeptides and glycoproteins by liquid chromatographic methods: 2016-Present

Among various protein post-translational modifications (PTMs), glycosylation has received special attention due to its immense role in molecular interactions, cellular signal transduction, immune response, etc. Aberration in glycan moieties of a glycoprotein is associated with cancer, diabetes, and bacterial and viral infections. In biofluids (plasma, saliva, urine, milk, etc.), glycoproteins are low in abundance and are masked by the presence of high abundant proteins. Hence, prior to their identification using mass spectrometry methods, liquid chromatography (LC)-based approaches were widely used. A general enrichment strategy involves a protein digestion step, followed by LC-based enrichment and desorption of glycopeptides, and enzymatic excision of the glycans. The focus of this review article is to highlight the articles published since 2016 that dealt with different LC-based approaches for glycopeptide and glycoprotein enrichment. The preparation of stationary phases, their surface activation, and ligand immobilization strategies have been discussed in detail. Finally, the major developments and future trends in the field have been summarized.

Fabrication of a Magnetic Fluorinated Covalent Organic Framework for the Selective Capture of Benzoylurea Insecticide Residue in Beverages

Efficient capture of benzoylurea insecticide (BU) residue in food is a vital procedure for food safe monitoring. Herein, a core-shell structured magnetic fluorinated covalent organic framework with good magnetic responsiveness and abundant fluorine affinity sites was successfully synthesized, suitable for magnetic solid-phase extraction (MSPE) of BUs. Using a room-temperature synthesis strategy, the magnetic fluorinated covalent organic framework was fabricated by in situ polymerization of 1,3,5-tris(4-aminophenyl) triazine (TAPT) and 2,3,5,6-tetrafluoroterephthaldehyde (TFTA) on the surface of carboxylated Fe₃O₄ nanoparticles. The competitive adsorption experiment and molecular simulation verified that this magnetic fluorinated covalent organic framework possesses favorable adsorption affinity for BUs. This magnetic fluorinated covalent organic framework could be easily regenerated and reused at least eight times with no reduction of enrichment performance. Combining this magnetic fluorinated covalent organic framework-based MSPE with high-performance liquid chromatography-tandem mass spectrometry, a novel sensitive method for the analysis of BUs was developed. In yellow wine and fruit juice samples, good linear correlations were obtained for BUs in the range of 10-2000 and 20-4000 ng·L^-1, respectively. The limit of quantitation of the BUs ranged from 1.4 to 13.3 ng·L^-1 in the two beverage matrices. Desirable precision was achieved, with intraday and interday relative standard deviations lower than 11%.

Judicious design functionalized 3D-COF to enhance CO2 adsorption and separation

The effects of functional groups (including OH, OCH₃ , NH₂ , CH₂ NH₂ , COOH, SO₃ H, OCO(CH₂ )₂ COOH(E-COOH), and (CH₂ )₄ COOH(c-COOH)) in 3D covalent organic frameworks (3D-COFs) on CO₂ adsorption and separation are investigated by grand canonical Monte Carlo (GCMC) simulations and density functional theory calculations. The results indicate that interaction between CO₂ and the framework is the main factor for determining CO₂ uptakes at low pressure, while pore size becomes the decisive factor at high pressure. The binding energy of CO₂ with functionalized linker is correlated to CO₂ uptake at 0.3 bar and 298 K on 3D-COF-1, suggesting functional groups play a key role in CO₂ capture in microporous 3D-COFs. Moreover, CO₂ selectivity over CH₄ , N₂ , and H₂ can be significantly enhanced by functionalization, where CH₂ NH₂ , COOH, SO₃ H, and E-COOH enhance CO₂ adsorption more effectively at 1 bar. Among them, SO₃ H is the most promising functional group in 3D-COFs for CO₂ separation.

Diltiazem-imprinted porphyrinic covalent organic frameworks as solid-phase extractants and fluorescent sensors

Diltiazem, which is a calcium channel blocker, is involved in the formation of covalent organic frameworks (COFs) through the Schiff base reaction of tetrakis (4-aminophenyl)-porphine (TAPP) and dihydroxynaphthalene-dicarbaldehyde (DHNDC) and the next enol-to-keto tautomerization. The diltiazem-imprinted COFs (DICOFs) were optimally formed using Sc(OTf)₃ as the catalyst, TAPP/DHNDC/diltiazem in a molar ratio of 2/3/4, N-methylpyrrolidone/mesitylene (v/v = 3/5) as the porogen, and a 1-h reaction with a high imprinting factor of 10.5 compared to the nonimprinted counterparts (NICOFs). The optimized DICOF exhibited a more amorphous XRD pattern, a larger surface area (1650 vs. 930 m²/g), a larger pore volume (1.33 vs. 0.75 cm³/g), and a finer porous SEM feature than NICOF. The selectivity of NICOF toward diltiazem and diazepam at 250 nM (α = 1.03, RSD = 1.3%) was smaller than the selectivity of DICOF (α = 2.94, RSD = 1.6%). The diltiazem samples (5.0-300 ng mL^-1) dynamically quenched the fluorescence of 15 μg/mL DICOF in 50 mM phosphate buffer at pH 6.5 at 8.0 min equilibrium; thus, Stern-Volmer plots were linearly constructed for sensing diltiazem with an LOD of 3.4 ng mL^-1 and an LOQ of 10.2 ng mL^-1. According to the plots, 30 ng mL^-1 diltiazem solutions that were diluted from 30 mg-specified tablets had an average measured concentration of 29.5 ng mL^-1 (σ = 1.3% and n = 5). In addition to application as fluorescent sensors, DICOFs (30 mg) could be used as dispersive extractants to recover 95.2% of 0.6 ng mL^-1 diltiazem from 25 mL phosphate buffer with quadruplicate uses of 0.5 mL methanol/acetic acid (v/v = 9/1) as the eluent. Langmuir and pseudo-second-order models were fitted to the isothermal and kinetic sorption mechanisms, respectively. The maximum sorption capacity of DICOF was ten times larger than that of NICOF (156 vs. 15.2 mg/g). The interday recoveries of 0.6 ng mL^-1 spiked in 20-fold diluted human urine, and 60-fold diluted human serum were 93.2% and 90.6%, respectively.

Triazine-cored covalent organic framework for ultrasensitive detection of polybrominated diphenyl ethers from real samples: Experimental and DFT study

Food and environmental safety issues attributable to the polybrominated diphenyl ethers (PBDEs) are gaining increasing attention, and these urge us to establish a high-performance sample-handling technique. In this study, an outstanding adsorption performance with short adsorption time (10 min) was achieved for PBDEs using a novel synthesized dispersive solid-phase extraction adsorbent, a reticulated covalent organic framework with N/O functional groups (i.e., imine linkage, triazine, and methoxy) (TAPT-DMTA-COF). By conducting sufficient experimentation and theoretical simulation on adsorption mechanism, the halogen bond between electronegative N/O atoms of TAPT-DMTA-COF and the electropositive Br atoms of PBDEs were observed to play a more pivotal role than π-π, C-H…π interactions, and hydrophobic effects. Furthermore, the positive linear relation between calculated adsorption energy and Br content directly clarified that enrichment behavior of PBDEs can be attributed to halogen bonding. These data implied that integrated nanostructure (i.e., N/O functional groups and reticulated architecture) effectively enhanced adsorption capacity. In case of PBDE analysis, this approach achieved excellent results with low limits of detection (0.03-0.13 ng L^-1). Finally, the promising potential applications of aforementioned method were verified by spiking water, fish, and milk samples with PBDEs; good PBDEs recoveries were obtained.

Inulinase Immobilized Lectin Affinity Magnetic Nanoparticles for Inulin Hydrolysis

In this presented paper, concanavalin A-modified cysteine-functionalized Fe₃O₄/Ag core/shell magnetic nanoparticles were synthesized and used as a support material for inulinase enzyme, which has been intensively used for the preparation of high-fructose syrup by hydrolyzing inulin. Inulinase adsorption capacity of Con A-functionalized Ag-coated magnetic nanoparticles was optimized by changing medium pH, temperature, and initial inulinase concentration, and maximum inulinase adsorption capacity was found to be 655.32 mg/g nanoparticle by using 1.00 mg/mL of inulinase solution in pH 3.0 buffer system at 25 °C. Finally, efficient inulin degradation capacity of the inulinase immobilized magnetic nanoparticles was demonstrated by TLC studies and released fructose amount was determined as 0.533 mg/mL only within the 5 min of hydrolysis. This newly developed hydrolysis strategy holds considerable promise to produce high-fructose syrup in many industries.

CdS nanoparticle decorated triazine-based COFs with enhanced photocatalytic activity for highly effective degradation of emerging contaminants

COF-based photocatalysts containing C3N4 units decorated with CdS were constructed and employed as the mediator of peroxydisulfate activation for removing pollutants.

Insights into the structure-performance relationships of extraction materials in sample preparation for chromatography

Sample preparation is one of the most crucial steps in analytical processes. Commonly used methods, including solid-phase extraction, dispersive solid-phase extraction, dispersive magnetic solid-phase extraction, and solid-phase microextraction, greatly depend on the extraction materials. In recent decades, a vast number of materials have been studied and used in sample preparation for chromatography. Due to the unique structural properties, extraction materials significantly improve the performance of extraction devices. Endowing extraction materials with suitable structural properties can shorten the pretreatment process and improve the extraction efficiency and selectivity. To understand the structure-performance relationships of extraction materials, this review systematically summarizes the structural properties, including the pore size, pore shape, pore volume, accessibility of active sites, specific surface area, functional groups and physicochemical properties. The mechanisms by which the structural properties influence the extraction performance are also elucidated in detail. Finally, three principles for the design and synthesis of extraction materials are summarized. This review can provide systematic guidelines for synthesizing extraction materials and preparing extraction devices.

Covalent organic frameworks embedded membrane via acetic-acid-catalyzed interfacial polymerization for dyes separation: Enhanced permeability and selectivity

With the increasing demand of high water-quality, membrane filtration technologies are playing further important roles in water treatment owing to their small footprints, reduced use of chemicals and stable performances. However, the inherent permeability-selectivity trade-off is still a significant obstacle restricting the broad applications of membrane separation. Hydrophilic modification via doping nanoparticles into membranes is considered an effective solution to improve the permeability while maintaining selectivity. However, agglomeration of nanoparticles often results in inhomogeneity of the modified membranes. In this study, hybrid membranes with separated covalent organic framework (COF) particles that were uniformly embedded in the membrane surface pores were firstly fabricated via acetic-acid-catalyzed in situ synthesis. Owing to the ample hydrophilic chemical groups and tunable molecular transport channels in COFs, the modified membranes yielded almost twice higher water flux (about 200 L m^-2·h^-1·bar) than the pristine membranes with simultaneously enhanced rejection of water pollutants (i.e., dyes). In addition, the pure organic structure of COF improves the polymer-filler interaction of the mixed film, thereby reducing the risk of leakage. Therefore, the hybrid membranes also exhibited relatively high stability in long-term operations and different pH conditions, which makes them promising candidates in future membrane applications.

Electrochemical Sensors Based on Covalent Organic Frameworks: A Critical Review

The metal-free cousins of metal-organic frameworks, covalent organic frameworks (COFs), are a class of pre-designable crystalline polymers composed of light elements and connected by strong covalent bonds. COFs are being given more and more attention in the electrochemical sensor field due to their fascinating properties, such as highly tunable porosity, intrinsic chemical and thermal stability, structural diversity, large specific surface area, and unique adsorption characteristics. However, there are still some key issues regarding COFs that need to be urgently resolved before they can be effectively applied in electrochemical sensing. In this review, we summarized recent achievements in developing novel electrochemical sensors based on COFs, and discussed the key fundamental and challenging issues that need to be addressed, including the mechanisms underlying charge transport, methods to improve electrical conductivity, immobilization methods on different substrates, synthesis strategies for nanoscale COFs, and the application of COFs in different fields. Finally, the challenges and outlooks in this promising field are tentatively proposed.

Covalent Organic Framework Composites: Synthesis and Analytical Applications

In the recent years, composite materials containing covalent organic frameworks (COFs) have raised increasing interest for analytical applications. To date, various synthesis techniques have emerged that allow for the preparation of crystalline and porous COF composites with various materials. Herein, we summarize the most common methods used to gain access to crystalline COF composites with magnetic nanoparticles, other oxide materials, graphene and graphene oxide, and metal nanoparticles. Additionally, some examples of stainless steel, polymer, and metal-organic framework composites are presented. Thereafter, we discuss the use of these composites for chromatographic separation, environmental remediation, and sensing.

In Situ Growth of Cationic Covalent Organic Frameworks (COFs) for Mixed Matrix Membranes with Enhanced Performances

Covalent organic frameworks (COFs) are increasingly utilized as doping agents for the design of advanced ultrafiltration mixed matrix membranes, thanks to their prominent nanoporosity and excellent polymer compatibility. However, current strategies are largely limited in the complicated postaddition of neutral COF particulates. Herein, cationic COFs, namely, TpEB, with sizes down to ∼39 nm are in situ synthesized in polyacrylonitrile (PAN) solution as crystalline fillers for the production of highly permeable TpEB-PAN ultrafiltration membranes. After the condensation of monomer pairs, the growth of cationic TpEB crystallites is restrained due to the electrostatic interaction with negatively charged PAN chains, leading to the formation of a homogeneous TpEB-incorporated casting solution. During the subsequent nonsolvent-induced phase separation process, TpEB crystallites facilitate exchange between the solvent and the nonsolvent because of their hydrophilic and nanoporous nature, accelerating the rate of phase inversion to form a highly porous membrane surface. Thus-prepared TpEB-PAN membranes deliver a tight rejection of BSA with water permeance of up to 380 L m^-2 h^-1 bar^-1, which is 35.6% higher than that of the original PAN membranes prepared without TpEB. The TpEB-PAN membranes also exhibit enhanced operation stabilities and fouling resistances. This in situ growth strategy suggests a new avenue for the preparation of advanced mixed matrix membranes.

Nanoscale covalent organic frameworks as theranostic platforms for oncotherapy: synthesis, functionalization, and applications

Cancer nanomedicine is one of the most promising domains that has emerged in the continuing search for cancer diagnosis and treatment. The rapid development of nanomaterials and nanotechnology provide a vast array of materials for use in cancer nanomedicine. Among the various nanomaterials, covalent organic frameworks (COFs) are becoming an attractive class of upstarts owing to their high crystallinity, structural regularity, inherent porosity, extensive functionality, design flexibility, and good biocompatibility. In this comprehensive review, recent developments and key achievements of COFs are provided, including their structural design, synthesis methods, nanocrystallization, and functionalization strategies. Subsequently, a systematic overview of the potential oncotherapy applications achieved till date in the fast-growing field of COFs is provided with the aim to inspire further contributions and developments to this nascent but promising field. Finally, development opportunities, critical challenges, and some personal perspectives for COF-based cancer therapeutics are presented.

Facile synthesis of magnetic sulfonated covalent organic framework composites for simultaneous dispersive solid-phase extraction and determination of β-agonists and fluoroquinolones in food samples

In this work, an efficient method for the determination of β-agonists and fluoroquinolones was established, based on a mixed-mode sorbent of magnetic sulfonated covalent organic framework composites. By coupling with HPLC-MS/MS, the main factors that affect the extraction procedure were optimized. Under the optimal conditions, the proposed HPLC-MS/MS method was successfully utilized for the extraction of β-agonists and fluoroquinolones in milk and pork meat samples. The method showed good linearities (R² ≥ 0.9916), and low LOQs of 0.1-0.2 ng g^-1 for β-agonists and fluoroquinolones. The adsorption mechanism was investigated with the assistance of quantum chemistry calculation method, and it is worth noting that the sorbent relied mainly on the multiple adsorption mechanisms, including π-π stacking, hydrophobic, electrostatic attraction and hydrogen-bonding interactions. This work not only provides a simple method for the preparation of a mixed-mode sorbent, but also a routine analysis strategy for monitoring the illegal use of β-agonists and fluoroquinolones.

Solid-phase microextraction using a β-ketoenamine-linked covalent organic framework coating for efficient enrichment of synthetic musks in water samples

Covalent organic frameworks with tunable porous crystallinity and outstanding stability have recently exhibited fascinating pretreatment performance as solid-phase microextraction coatings. In this report, a β-ketoenamine-linked covalent organic framework (TpPa-1) was successfully constructed through a Schiff-base-type reaction between 1,3,5-triformylphloroglucinol (Tp) and para-phenylenediamine (Pa-1). A TpPa-1 coating was then fabricated on a stainless-steel fiber for capturing trace synthetic musks. This TpPa-1 coating exhibited strong interaction with synthetic musks because of its hydrophobicity and π-π affinity. This TpPa-1-based solid-phase microextraction methodology, coupled with gas chromatography-tandem mass spectrometry, provided high enrichment factors (1214-12 487), wide linearity (0.5-1000 ng L-1), low limits of detection (0.04-0.31 ng L-1), and acceptable reproducibility (relative standard deviation, <10%) for nine synthetic musks. Recoveries at three spiked levels in three types of water samples were between 76.2% and 118.7%. These results indicated the promising applicability of the TpPa-1 as a solid-phase microextraction fiber coating for reliably detecting trace concentrations of synthetic musks in the environment.

Facile Removal of Phytochromes and Efficient Recovery of Pesticides Using Heteropore Covalent Organic Framework-Based Magnetic Nanospheres and Electrospun Films

Nontargeted analysis of food safety requires selective removal of interference matrices and highly efficient recovery of chemical hazards. Porous materials such as covalent organic frameworks (COFs) show great promise in selective adsorption of matrix molecules via size selectivity. Considering the complexity of interference matrices, we prepared crystalline heteropore COFs whose two kinds of pores have comparable sizes to those of several common phytochromes, main interference matrices in vegetable sample analysis. By controlling the growth of COFs on the surface of Fe3O4 nanoparticles or by utilizing a facile co-electrospinning method, heteropore COF-based magnetic nanospheres or electrospun nanofiber films were prepared, respectively. Both the nanospheres and the films maintain the dual-pore structures of COFs and show good stability and excellent reusability. Via simple magnetic separation or immersion operation, respectively, they were successfully used for the complete removal of phytochromes and highly efficient recovery of 15 pesticides from the extracts of four vegetable samples, and the recoveries are in the range of 83.10-114.00 and 60.52-107.35%, respectively. Film-based immersion operation gives better sample pretreatment performance than the film-based filtration one. This work highlights the great application potentials of heteropore COFs in sample pretreatment for nontargeted analysis, thus opening up a new way to achieve high-performance sample preparation in many fields such as food safety analysis, environment monitoring, and so on.