Rapidly covalent immobilization of β-ketoenamine-linked covalent organic framework on fibers for efficient solid-phase microextraction of phthalic acid esters

Thiocarbamide conversion-based nitrogen-rich covalent organic framework coatings for electro-enhanced solid-phase microextraction of bisphenols

Design and preparation of fiber coatings with excellent electrochemical performance and high polarity is significant for efficient extraction of polar targets in electro-enhanced solid-phase microextraction (EE-SPME). In this work, a combination strategy for structure regulation of covalent organic framework (COF) was proposed to fabricate a nitrogen-rich thiocarbamide linked COF coating (Thiocarbamide-TZ-DHTP) via molecular design and post-synthetic thiocarbamide conversion. The prepared COF coating possesses a large number of O, N, and S functional groups, which not only endow the coating with higher polarity but also significantly enhance its electrochemical performance. The COF coating was used for EE-SPME of polar bisphenols (BPs), demonstrating excellent enrichment efficiency and durability. Subsequently, coupled with gas chromatography-tandem mass spectrometry (GC-MS/MS), a sensitive method was developed for determination of trace BPs. The established method possess wide linear ranges (2.0-800.0 ng L-1), good correlation coefficients (0.9985-0.9994) and low detection limits (0.1-2.0 ng L-1). Moreover, the established method had been successfully applied to detection of trace BPs in tea beverage with satisfactory recoveries (81.6 % to 118.6 %). This research provides a feasible pathway for preparing COF coating with excellent electrochemical performance and high polarity for EE-SPME.

Supramolecular Materials as Solid-Phase Microextraction Coatings in Environmental Analysis

Solid-phase microextraction (SPME) has been widely proposed for the extraction, clean-up, and preconcentration of analytes of environmental concern. Enrichment capabilities, preconcentration efficiency, sample throughput, and selectivity in extracting target compounds greatly depend on the materials used as SPME coatings. Supramolecular materials have emerged as promising porous coatings to be used for the extraction of target compounds due to their unique selectivity, three-dimensional framework, flexible design, and possibility to promote the interaction between the analytes and the coating by means of multiple oriented functional groups. The present review will cover the state of the art of the last 5 years related to SPME coatings based on metal organic frameworks (MOFs), covalent organic frameworks (COFs), and supramolecular macrocycles used for environmental applications.

Point-of-Use SERS Approach for Efficient Determination and Removal of Phthalic Acid Esters Based on a Metal-Organic Framework-Coated Melamine Sponge

Phthalic acid esters (PAEs) are ubiquitous environmental contaminants, and their real-time monitoring and removal remain challenging. Herein, a point-of-use (POU) device integrating adsorption, surface-enhanced Raman spectroscopy (SERS), and removal strategy was developed and realized ultrafast on-site determination of PAEs and cleanup of them from water. A piece of flexible melamine sponge (MS) was coated with gold nanostars (AuNSs) and metal-organic frameworks (MOFs), thus obtaining SERS activity and adsorption capacity. Based on this multifunctional AuNSs@MOFs/MS composite, clear trends were observed between SERS signal intensity and concentration of di(2-ethylhexyl)phthalate (DEHP) and dibutyl phthalate (DBP). The method detection limits of DEHP and DBP were calculated to be 0.75 × 10-7 and 0.67 × 10-7 M in water, respectively, proving good sensitivity. Furthermore, this POU device exhibited satisfactory adsorption capacity (∼82.3 g/g for DBP and ∼90.0 g/g for DEHP), high adsorption efficiency (equilibrium in 100 s), and good regeneration capability (removal efficiency >70% after 5 cycles). The applicability of this device was verified by its good determination and removal performance in real environmental water matrices. The whole process could be completed within 5 min. This approach provides a new POU alternative for real-time monitoring and removal of PAEs in water.

Mechanochemical synthesis ionic covalent organic frameworks/cotton composites for pipette tip solid-phase extraction of domoic acid in seafood

Pipette tip solid-phase extraction (PT-SPE) as a miniaturized solid-phase extraction technique have a wide range of applications in the field of sample pretreatment. In this study, ionic covalent organic frameworks@cotton (iCOF@cotton) were facilely synthesized by mechanochemical grinding method only in half an hour, and used as the adsorbents of PT-SPE. The synthesized iCOF@cotton not only had high specific surface area, suitable pore structure and cationic charge groups of iCOF that can extract polar targets quickly, but also reduced the problem of high back pressure of PT-SPE by the addition of cotton, thus accelerating extraction time. Combined with high performance liquid chromatographic tandem mass spectrometry (HPLC-MS/MS), an efficient and sensitive method was established for detection of domoic acid (DA, a toxin produced by algae). Under the optimal conditions, the proposed analysis method displayed excellent analytical performance, including broad range of linearity (10-1000 pg mL-1), low limit of detection (LOD, 5 pg mL-1), high correlation coefficient (0.9993), satisfactory precision (RSDs ≤6.4 %). In addition, the developed method was applied to the detection of DA in marine samples, and detected trace DA (18.6 pg mL-1) with satisfactory recovery (85.7%-107.2 %). The above results indicated that the prepared iCOF@cotton have great potential as the adsorbents for PT-SPE.

Recent progress of covalent organic frameworks as attractive materials for solid-phase microextraction: A review

Solid-phase microextraction (SPME) is a green, environmentally friendly, and efficient technique for sample pre-treatment. Covalent organic frameworks (COFs), a class of porous materials formed by covalent bonds, have gained prominence owing to their remarkable attributes, including large specific surface area, tunable pore size, and robust thermal/chemical stability. These characteristics have made COFs highly appealing as potential coatings for SPME fiber over the past decades. In this review, various methods used to prepare SPME coatings based on COFs are presented. These methods encompass physical adhesion, sol-gel processes, in situ growth, and chemical cross-linking strategies. In addition, the applications of COF-based SPME coating fibers for the preconcentration of various targets in environmental, food, and biological samples are summarized. Moreover, not only their advantages but also the challenges they pose in practical applications are highlighted. By shedding light on these aspects, this review aims to contribute to the continued development and utilization of COF materials in the field of sample pretreatment.

Hierarchical covalent organic framework hollow nanofibers-bonded stainless steel fiber for efficient solid phase microextraction

The solid phase microextraction (SPME) technique has been widely applied in the detection of trace compounds in food, environment, and medicine due to its advantages of easy quantification, simple operation, and greenness. Herein, a templating strategy with SiO2 nanofibers (SiO2 NFs) is reported to synthesize hierarchical covalent organic framework hollow nanofibers (COF HNFs)-coated stainless steel fiber for SPME application with dramatically enhanced enrichment performance for trace analytes. The construction of hierarchical porosity inside the microextraction coatings can not only increase the specific surface area of COF extraction materials for obtaining more abundant adsorption sites but also greatly improve the accessibility of internal COF micropores. Moreover, the thicknesses of the microextraction COF coatings can be facilely tailored by adjusting the amount of SiO2 NFs pre-assembled on the SPME fibers. On the headspace solid phase microextraction (HS-SPME) of antimicrobial residues, the developed COF TpBD-Me2 HNFs-12 fibers achieve enrichment factors of 2026 and 1823 for thymol and carvacrol respectively, which are significantly higher than those obtained from the counterpart COF TpBD-Me2-bonded fiber (8.5-8.2 times) and commercial CAR/PDMS fiber (3.3-4.4 times). Furthermore, the developed method was demonstrated to have wide linearity (0.1-50 μg L-1), low limits of detection (0.010 μg L-1), good thermal stability and excellent reusability (>60 recycles), demonstrating great application potential in the extraction of trace organic pollutants. The strategy developed in this work is applicable to preparing a variety of topological COF (e.g., TpBD, TpPa-1) HNFs-bonded fibers.

Rapid and selective removal of aristolochic acid I in natural products by vinylene-linked iCOF resins

Rapid, efficient, and selective removal of toxicants such as aristolochic acid I (AAI) from complex natural product systems is of great significance for the safe use of herbal medicines or medicine-food plants. Addressing this challenge, we develop a high-performance separation approach based on ionic covalent organic frameworks (iCOFs) to separate and remove AAI. Two vinylene-linked iCOFs (NKCOF-46-Br- and NKCOF-55-Br-) with high crystallinity are fabricated in a green and scalable fashion via a melt polymerization synthesis method. The resulting materials exhibit a uniform morphology, high stability, fast equilibrium time, and superior affinity and selectivity for AAI. Compared to conventional separation media, NKCOF-46-Br- and NKCOF-55-Br- achieve the record high adsorption capacities of 246.0 mg g-1 and 178.4 mg g-1, respectively. Various investigations reveal that the positively charged framework and favorable pore microenvironment of iCOFs contribute to their high selectivity and adsorption efficiency. Moreover, the iCOFs exhibit excellent biocompatibility by in vivo toxicity assays. This study paves a new avenue for the rapid, selective and efficient removal of toxicants from complex natural systems.

Efficient decontamination of organic pollutants from wastewater by covalent organic framework-based materials

Covalent organic frameworks (COFs), assembling through covalent bonds, are a rising class of porous materials. Nowadays, various COFs are widely applied in organic pollutants decontamination due to the outstanding capabilities of large surface area, multiple functional groups, porous structure, excellent absorptivity, flexible design and so on. This review concentrates on the applications of COFs in different decontamination technologies such as solid-phase extraction, membrane filtration and sieving, adsorption, and catalysis reaction. The factors influencing water chemistry, such as pH, temperature, salt concentration and natural organic matter, are summarized in terms of their impact on decontamination performance and the extraction mechanisms for the diverse analytes. The interaction mechanisms between COFs and organic pollutants were hydrogen bonding, π-π stacking, hydrophilic, hydrophobic, and electrostatic interactions. Furthermore, a perspective on current obstacles and upcoming developments of COFs for organic pollutant removal has been provided. Due to their adaptable and versatile design as well as elaborate and diverse functionalization, COFs possess significant possibility in ameliorating environmental pollution.

Three-dimensional rose-like zinc oxide fiber coating for simultaneous extraction of polychlorinated biphenyls and polycyclic aromatic hydrocarbons by headspace solid phase microextraction

The three-dimensional (3D) rose-like zinc oxide (ZnO) material was prepared by a simple one-step CTAB-assisted hydrothermal strategy and used as a headspace solid-phase microextraction (HS-SPME) coating. Polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) were analyzed by gas chromatography with flame ionization detector (GC-FID), and conclusively applied to ultrasensitive detection in lake and river water. Compared with one-dimensional (1D) pencil-like ZnO, the layer-by-layer petal-like structure could fully expose mass adsorption sites on the surface, which could significantly improve the adsorption. The enrichment factors with 7535-8595 for PCBs and 3855-7320 for PAHs were achieved. The established method provided a satisfactory linear range (0.005-30 ng·mL-1), coefficient (R2 > 0.9978), ultra-low limit detection (1-3 pg·mL-1), and long service life (≥ 150 times). The recoveries of 83.42-120.86 % were obtained in the real detection application of lake and river water. This work demonstrated that 3D rose-like ZnO with low cost, simple synthesis, fast extraction ability and high enrichment performance was an ideal coating material, which was hoped to enrich other compounds with similar structures with PCBs and PAHs.

Covalent organic framework-based magnetic solid-phase extraction coupled with gas chromatography-tandem mass spectrometry for the determination of trace phthalate esters in liquid foods

Covalent organic framework-coated magnetite particles (Fe3O4@COF) were synthesized and applied as the adsorbent to the selective capture of phthalate esters (PAEs) in liquid foods. Combined with the magnetic solid-phase extraction (MSPE) technology, a gas chromatography-tandem mass spectrometry (GC-MS/MS) method was employed for the separation and quantification of PAEs. Following optimization of the magnetic extraction and elution parameters, the developed analytical method offered a satisfactory linear range (0.1-5 μg L-1) with determination coefficients ranging from 0.9934 to 0.9975 for the five different PAEs studied. The limits of detection (LOD) were in the range 1.9-12.8 ng L-1. The recoveries ranged from 70.0 to 119.8% with a relative standard deviation (RSD) less than 9.7%. Density functional theory (DFT) calculations established that the dominant adsorption mechanism used by the COF to bind PAEs involved π-π stacking interactions. Results encourage the wider use of COF-based adsorbents and MSPE methods in the analytical determination of PAEs in foods.

Recent Progress of Covalent Organic Frameworks Applied in Electrochemical Sensors

As an emerging porous crystalline organic material, the covalent organic frameworks (COFs) are given more and more attention in many fields, such as gas storage and separation, catalysis, energy storage and conversion, luminescent devices, drug delivery, pollutant adsorption and removal, analysis and detection due to their special advantages of high crystallinity, flexible designability, controllable porosities and topologies, intrinsic chemical and thermal stability. In recent years, the COFs are applied in analytical chemistry, for instance, chromatography, solid-phase microextraction, luminescent and colorimetric sensing, surface-enhanced Raman scattering and electroanalytical chemistry. The COFs decorated electrodes show high performance for detecting trace substances with remarkable selectivity and sensitivity, such as heavy metal ions, glucose, hydrogen peroxide, drugs, antibiotics, explosives, phenolic compounds, pesticides, disease metabolites and so on. This review mainly summarized the application of COF based electrochemical sensor according to different target analytes.

Carboxylated mesoporous carbon hollow spheres for the efficient solid-phase microextraction of aromatic amines

An efficient and stable fiber coating is of great importance for solid-phase microextraction (SPME). In this study, carboxylated mesoporous carbon hollow spheres (MCHS-COOH) were developed as an efficient SPME coating of polar aromatic amines (AAs) for the first time. The MCHS-COOH coating material with high specific surface area (1182.32 m² g^-1), large pore size (10.14 nm), and rich oxygen-containing groups was fabricated via a facile H₂O₂ post-treatment. The as-prepared MCHS-COOH-coated fiber exhibited fast adsorption rate and excellent extraction properties, mainly due to its π-π interactions, hollow structure, and abundant affinity sites (carboxyl groups). Subsequently, coupled with gas chromatography-tandem mass spectrometry (GC-MS/MS), a sensitive method with low limits of detection (0.08-2.0 ng L^-1), a wide linear range (0.3-500.0 ng L^-1), and good repeatability (2.0-8.8%, n = 6) was developed for the analysis of AAs. The developed method was validated against three river water samples, with satisfactory relative recoveries being obtained. The above results demonstrated that the prepared MCHS-COOH-coated fiber exhibited good adsorption capacity, suggesting a promising application to monitor trace polar compounds in real environment.

Determination of trace bisphenols in milk based on Fe3O4@NH2-MIL-88(Fe)@TpPa magnetic solid-phase extraction coupled with HPLC

Herein, a covalent organic framework (COF) was grown on a magnetic metal-organic framework (MOF) by a solvothermal method for the efficient extraction of bisphenols (BPs). The magnetic solid-phase extraction (MSPE) of four bisphenols (bisphenol A, bisphenol B, bisphenol AF and bisphenol C) was carried out without adjusting the pH and salt concentration. When 30 mg Fe₃O₄@NH₂-MIL-88(Fe)@TpPa was used to adsorb for 25 min, 6 mL methanol was used to elute for 20 min, and the extract was detected by high-performance liquid chromatography (HPLC). The proposed method has a low detection limit of 0.011-0.036 ng mL^-1, a wide linear range of 0.05-100 ng mL^-1, and a correlation coefficient (R²) of 0.9980-0.9998. The intra-day and inter-day precisions are 0.74-2.54% and 1.68-3.72%, respectively. Bisphenol A was determined by applying the proposed method to the determination of actual milk samples. The standard addition experiment showed that the relative recovery of the four bisphenols was 85.70-119.7%. Pseudosecond-order, first-order, Langmuir and Freundlich models were applied to explore the adsorption characteristics of Fe₃O₄@NH₂-MIL-88(Fe)@TpPa. In general, the established Fe₃O₄@NH₂-MIL-88(Fe)@TpPa-MSPE-HPLC-UV method exhibits attractive sensitivity, simple manipulation, and excellent reusability, and it has excellent prospects for the detection of trace BPs in complex milk matrices.

The Preparation of Covalent Bonding COF-TpBD Coating in Arrayed Nanopores of Stainless Steel Fiber for Solid-Phase Microextraction of Polycyclic Aromatic Hydrocarbons in Water

Covalent organic framework (COF)-TpBD was grafted on the arrayed nanopores of stainless steel fiber (SSF) with (3-aminopropyl) triethoxysilane as the cross-linking agent. The prepared SSF bonded with COF-TpBD showed high thermal and chemical stability and excellent repeatability. The prepared SSF bonded with COF-TpBD was also used for the solid-phase microextraction (SPME) of seven kinds of polycyclic aromatic hydrocarbons (PAHs) in actual water samples, followed by gas chromatography with flame ionization detection (GC-FID) determination, which exhibited low limits of detection (LODs), good relative standard deviation (RSD) and high recoveries.

Covalent triazine-based frameworks for efficient solid-phase microextraction of phthalic acid esters from food-contacted plastics

Owing to various health threats associated with phthalic acid esters (PAEs), this category of endocrine-disrupting compounds has attracted more and more public scrutiny. However, the efficient preconcentration of PAEs from complex food-contacted plastics still remains challenging. Herein, three covalent triazine-based frameworks (CTFs) were constructed by facile Friedel-Crafts reactions of cyanuric chloride (CC), with triptycene (TPC), fluorene (FL) and 1,3,5-triphenylbenzene (TPB), respectively. Three CTFs were then employed as solid-phase microextraction (SPME) coatings for the extraction of PAEs. Benefiting from the large surface area and high pore volume, the newly-synthesized CC-TPC based SPME method exhibited large enrichment factors (978-2210), low limits of detection (0.027-0.10 ng g ^- 1), satisfactory linear ranges (0.09-20 ng g ^- 1), acceptable repeatabilities (4.3-9.6%) and high relative recoveries (92.0-104.6%).

Porous Hexagonal Boron Nitride as Solid-Phase Microextraction Coating Material for Extraction and Preconcentration of Polycyclic Aromatic Hydrocarbons from Soil Sample

Sample pretreatment plays important role in the analysis and detection of trace pollutants in complex matrices, such as environmental and biological samples. The adsorption materials of sample pretreatment receive considerable attention, which has a significant effect on the sensitivity and selectivity of the analytical method. In this work, the porous hexagonal boron nitride (h-BN) was utilized as a coating material of solid-phase microextraction (SPME) to extract and preconcentrate polycyclic aromatic hydrocarbons (PAHs) prior to separation and detection with GC-FID. Attributed to the multiple interactions including hydrophobicity, hydrogen bonding and strong π–π interaction, the h-BN coating showed excellent extraction performance for PAHs. Under the optimal conditions, the method showed the linear relationship in the range of 0.1–50 ng mL⁻¹ for acenaphthene, 0.05–50 ng mL⁻¹ for pyrene, and 0.02–50 ng mL⁻¹ for fluorene, phenanthrene and anthracene with a correlation coefficient (R²) not lower than 0.9910. The enrichment factors were achieved between 1526 and 4398 for PAHs with h-BN as SPME fiber coating. The detection limits were obtained in the range of 0.004–0.033 ng mL⁻¹, which corresponds to 0.08–0.66 ng g⁻¹ for soil. The method was successfully applied to analysis of real soil samples. The recoveries were determined between 78.0 and 120.0% for two soil samples. The results showed that h-BN material provided a promising alternative in sample pretreatment and analysis.