Porous organic framework as coating for stir bar sorptive extraction of carbamate pesticides from corn and potato samples

Highlights

• Three POFs were prepared with monomers containing different numbers of benzene rings.

• POF-c coated stir bar exhibited superior extraction efficiency and kinetics.

• Hydrophobic, π-π interaction and hydrogen bonding helped adsorbing CMPs on POFs.

• Adsorption mechanism was verified by FT-IR, XPS and molecular dynamics simulation.

• A method of SBSE-HPLC-VWD was proposed for CMPs analysis in corn and potatoes.

Abstract

Three porous organic frameworks (POFs) were synthesized by the reaction between phloroglucinol and 1,4-phthalaldehyde, 4,4′-biphenyldialdehyde or tris-(4-formylphenyl) amine; the products are named as POF-a, POF-b and POF-c, respectively. They were used to prepare POFs coated stir bars respectively for the extraction of four carbamate pesticides (CMPs). POF-c coated stir bar exhibited better adsorption performance than POF-a/b coated stir bar and commercial stir bars, probably due to the stronger conjugated structure and hydrophobicity of POF-c, and resultant hydrophobic, π-π and hydrogen bonding interactions between them. The adsorption mechanism for target CMPs was verified by characterization techniques and molecular dynamics simulation. A method of POF-c coated stir bar sorptive extraction-high performance liquid chromatography-variable wavelength ultraviolet detector was developed for the analysis of four CMPs in corn and potato samples. Under the optimal conditions, LODs of the method were between 0.017 and 0.048 μg/L, and the linear range for four CMPs was 0.1/0.2–200 μg/L.

Introduction

Pesticide residues analysis have always been one of the food safety issues that humans are concerned about. The development of fast and accurate methods to detect pesticide residues in food is currently the focus of attention by relevant institutions and researchers. Due to the low concentration of pesticide residues in food and the complex matrices of food, it is necessary to carry out appropriate sample preparation before the detection of pesticide residues, in order to separate and concentrate target pesticide residues from samples and reduce the sample matrix interference. As a powerful sample preparation method, stir bar sorptive extraction (SBSE) has many merits, such as easy phase separation, high extraction efficiency (EE), acceptable stability and low consumption of organic solvents, and has been widely used for analysis of different analytes in various samples. The stir bar used in SBSE technology consists of two parts: substrates and coatings. The frequently-used stir bar substrates include “dumbbell-shaped” glass capillary stir bars, stainless steel wires and poly(ether-ether-ketone) tubes. Various coating materials can be coated on them by sol–gel, adhesion, in-situ growth, solvent exchange, electrochemical polymerization and other methods (He, Wang, Zhang, Zang, Chen, & Hu, 2021).

The coating material is one of the important factors that affect the extraction performance of SBSE and its application. Polydimethylsiloxane (PDMS), ethylene glycol-silicone (EG-Silicone) and polyethersulfone coated stir bars have been commercialized at present. However, these coatings have poor selectivity and are more suitable for non-polar/weakly polar substances, and the extraction performance is still insufficient in many cases. In recent years, there has been growing interest in developing new coatings with good performance for extraction of analytes with different polarity by SBSE and, therefore, extending the applicability of this technique. Different materials including carbon-based materials (e.g., graphene, graphene oxide), metal organic frameworks and porous organic frameworks (POFs) have been explored as new SBSE coatings for extraction of various analytes with different polarity in samples with complex matrices. Among them, POFs have the advantages of low skeleton density, simple synthesis, good stability, rich spatial topology and large specific surface area. The skeleton or pore surface can also be modified with a variety of functional chemical substances or functional groups. As a new type of porous material, POFs were widely used in solid phase extraction, solid phase microextraction and magnetic solid phase extraction. However, there are few applications of POFs in SBSE at present. Covalent triazine framework (CTF) (Zhong, He, Liao, Chen, Wang, & Hu, 2016) was first introduced into SBSE technique as coating through sol–gel technology. Compared with commercial coatings (PDMS and EG-Silicone), CTF coating displayed faster extraction kinetics and higher EE for phenolic compounds. After that, several other POFs were also employed as stir bar coatings through sol–gel and adhesion method, and they exhibited superior extraction performance for different target analytes (Han et al., 2021, Wang et al., 2020, Wang et al., 2020, Zheng et al., 2020). The large conjugated system and abundant adsorption sites of POFs make it have good adsorption affinity for organic pollutants. Hydrogen bonding, hydrophobic interaction, π-π interaction and electrostatic interaction would contribute to the adsorption of organic pollutants by POFs. To explore the interactions between POFs and target carbamates, Wang et al. (Wang et al., 2019) compared EE of carbamates and that of chlorophenols with similar log P and less hydrogen-bonding sites, polycyclic aromatic hydrocarbons with larger conjugated systems and no hydrogen-bonding sites obtained on the same POFs. However, it is an indirect way, and the extraction mechanism cannot be intuitively verified.

The utilization of suitable characterization and calculation methods to explore adsorption mechanism is of great significance to the development of solid phase-based extraction techniques. Some spectral characterizations and theoretical calculation have been used to clarify hydrogen bonding, halogen bonding and π-π interaction between coatings and target analytes. Cui et al (Cui, Ren, Yang, & Yan, 2019) adopted a series of spectral characterizations to study the π-π interaction between microporous organic networks (MONs) and tetrabromobisphenol A (TBBPA). X-ray photoelectron spectrometry (XPS) characterization indicates that after MONs adsorbed TBBPA, the CC peak shifted to a low binding energy, and the CC content increased from 66 % to 75 %. Fluorescence spectroscopy analysis showed that the fluorescence of TBBPA was gradually quenched after MONs adsorbed TBBPA. The changes of characteristic peaks in Fourier transform infrared spectra (FT-IR) and redshifts in UV–vis absorption spectra also confirmed the π-π interaction between MONs and TBBPA. Zhuang et al. (Zhuang, Chen, Liu, & Wang, 2020) utilized density functional theory (DFT) to speculate the adsorption mechanism between magnetic COFs and diclofenac or sulfamethazine. A molecular force-field based method named Adsorption Locator was used. The results showed that CH⋯π interaction played a major role in the adsorption process, while hydrogen bonding could not be proved using this method. The independent gradient model and Hirshfeld surface analysis in Multiwfn were used to explore the weak interaction between kaolinite/montmorillonite and 2-phosphonobutane-1,2,4-tricarboxylic acid, the calculation demonstrated that the O atom of the carboxyl group is the main electrophilic reaction site, and the high adsorption is mainly due to hydrogen bonding (Zhu, Khan, Wang, Bano, & Xia, 2021). DFT has also been used to predict and analyze the adsorption behavior of TAPT-DMTA-COF for polybrominated diphenyl ethers (PBDEs) (Liu et al., 2021). PBDEs can easily enter the macrocyclic cavity of TAPT-DMTA-COF molecules through halogen bonding and a small amount of PBDEs was adsorbed on the surface of TAPT-DMTA-COF through π-π interaction. The binding energy of each optimized system and the structure of target analytes were calculated and analyzed. The results showed that hydrogen bonding, halogen bonding (N-Br or O-Br), π-π interaction and hydrophobic interaction played a synergistic effect in the adsorption process and the main interaction was halogen bonding.

Carbamate pesticides (CMPs) have a widespread use in animal-breeding, agriculture and forestry to protect plants from insects, weeds and diseases because of the strong selectivity, easy decomposition, high efficiency and low residual toxicity. CMPs are highly water-soluble, which allows them to be absorbed by the roots and leaves of plants (Georgiadis et al., 2018). Therefore, their residues may be widely distributed in crops and aquatic systems through runoff and soil leaching into the ground and surface water (Santaladchaiyakit, Srijaranai, & Burakham, 2012). Since CMPs are inhibitors of acetylcholinesterase and are suspected to be carcinogens and mutagens, some of them are extremely toxic to the central nervous system, which will be harmful to the environment and human health (Voulvoulis, Arpon, & Giakoumis, 2017). Most countries and organizations, such as the European Union Health, Safety Executive Committee and the US Environmental Protection Agency, have established maximum residue limits (MRL) for CMPs in various products. In Chinese National food safety standard (GB 2763-2021), the MRLs of carbaryl (CAR) were 5.0, 3.0 and 1.0 mg/kg in grapes, cucumbers and watermelons; the MRLs of desmedipham (DES) and methiocarb (MET) were 0.01 and 0.1 mg/kg in potatoes and corn; the MRLs of isoprocarb (ISO) were 0.5 mg/kg in cucumbers. It is imperative to develop sensitive, reliable and simple methods to analyze low-concentration CMPs residues in environmental and food samples to ensure environmental safety and food quality.

The aim of this work was to explore the feasibility of POFs as stir bar coatings in the extraction of target CMPs, study the adsorption mechanism of POFs towards target CMPs, and develop sensitive method based on POF coated SBSE-HPLC-variable wavelength ultraviolet detector (VWD) for analysis of trace CMPs in corn and potato samples. For this purpose, a simple, catalyst-free solvothermal method was used to synthesize three POFs with three aldehyde monomers containing different conjugated structures (1,4-phthalaldehyde, 4,4′-biphenyldicarboxaldehyde and tris (4-formylphenyl) amine) and phloroglucinol as monomers. In order to further explore the interactions, four common CMPs, including carbaryl (CAR), isoprocarb (ISO), methiocarb (MET) and desmedipham (DES), with different structures and properties (Table S1) were selected as target analytes. With three synthesized POFs as the coating component, three kinds of POFs coated stir bar were prepared by adhesion method, and their extraction performance for four CMPs was evaluated. Among them, POF-c (phloroglucinol and tris-(4-formylphenyl) amine as monomers) coated stir bar with the best extraction performance was selected to establish the SBSE-HPLC-VWD method for the determination of four CMPs in corn and potato samples. Under the optimal conditions, LODs of the method were between 0.017 and 0.048 μg/L, and the linear range for four CMPs was 0.1/0.2–200 μg/L. Based on the characterization and theoretical calculation, target CMPs are speculated to be adsorbed onto POFs mainly through hydrogen bonding, π-π interaction and hydrophobic interaction. This study provides not only a sensitive method with good potential for target CMPs analysis in food samples, but also an effective strategy for the design of coating composition for specific targets based on the theoretical calculation and investigation of adsorption mechanism, instead of repeated attempts on multiple sorbents and analytes.