Abstract:

Pesticides are widely used in agriculture to increase grain yields and prevent crop diseases and insect pests. However, pesticides pose a serious threat to ecosystems and human health owing to their high toxicity and persistence. Therefore, it is imperative to establish an efficient and sensitive detection method for pesticides in water samples. Rapid and accurate detection of trace pesticides in environmental water samples has been a challenge because of complex matrix effects and trace concentrations. Appropriate sample pretreatment is a critical step for the effective extraction of analytes and removal of interferences, and the development and design of novel and stable nanomaterial adsorbents is key to continuous innovation in sample pretreatment technology. In recent years, carboxylated multiwalled carbon nanotubes (MWCNTs-COOH) and layered double hydroxide (LDHs) have been widely used as new adsorbent materials for various pretreatment technologies because of their large specific surface area, good stability, and easy functionalization. Based on this background, MWCNTs-COOH and LDHs were combined to obtain a new efficient composite adsorbent, so that the synergistic effect of the individual components could be exploited in entirety. In this study, a zeolitic metal organic framework ZIF-67/multiwalled carbon nanotube (ZIF-67/MWCNTs) composite was prepared by a simple one-step method, and a cobalt-nickel double metal hydroxide/multiwalled carbon nanotube (CoNi-LDH/MWCNTs) hybrid material with a three-dimensional cage-like structure was synthesized by a solvothermal method using ZIF-67/MWCNTs as templates. The cage-like structure of the CoNi-LDH/MWCNTs composite, which is different from the traditional layered bimetallic hydroxide, could accelerate mass transfer. Given the excellent properties of the CoNi-LDH/MWCNTs composite, it was used as a solid-phase microextraction (SPME) coating for the efficient enrichment of six pesticides (chlorothalonil, tebuconazole, chlorpyrifos, butralin, deltamethrin, and pyridaben) and combined with high performance liquid chromatography-ultraviolet (HPLC-UV) detection for the determination of the six pesticides in real water samples. The prepared materials were characterized by scanning electron microscopy, electron dispersion spectroscopy, infrared spectroscopy, X-ray powder diffraction, and N₂ adsorption/desorption. The results confirmed that the CoNi-LDH/MWCNTs composite was successfully synthesized, and that its surface area and pore volume were 281.4 m²/g and 0.49 cm³/g, respectively. An orthogonal array design was used to optimize the extraction conditions of SPME, including the extraction time, extraction temperature, stirring rate, salt effect, and desorption time. The optimal extraction conditions were as follows: extraction temperature, 40 ℃; extraction time, 30 min; stirring rate, 500 r/min; desorption time, 6 min; and salt (NaCl) mass concentration, 150 mg/L.

Under optimal conditions, the method had a wide linear range (chlorothalonil: 0.015-200 μg/L, tebuconazole: 0.140-200 μg/L, chlorpyrifos: 0.250-200 μg/L, butralin: 0.077-200 μg/L, deltamethrin: 1.445-200 μg/L, pyridaben: 0.964-200 μg/L), low detection limit (0.004-0.434 μg/L), and good reproducibility. The relative standard deviations (RSDs) of single fiber and fiber-to-fiber were in the range of 0.5% to 5.7% and 0.5% to 4.8%, respectively. The spiked recoveries at two levels of 10.0 μg/L and 50.0 μg/L were in the range of 83.9%-108.2%, with RSDs less than 5.3%. Compared with other coated fibers (MWCNTs-COOH, ZIF-67, ZIF-67/MWCNTs, and silicone sealant), the CoNi-LDH/MWCNTs-coated fibers showed a better enrichment effect for pesticides, which was attributed to their high specific surface area and π-π interactions, hydrophobic interactions, cation-π interactions, and hydrogen bonding interactions between the CoNi-LDH/MWCNTs coating and the target analytes, which can enhance their ability to extract pesticides. The stability test on the SPME fibers revealed that after 128 cycles, the extraction efficiency of the CoNi-LDH/MWCNTs-coated fibers for the six pesticides decreased only slightly (< 10%), implying that the coated fibers had good stability and reusability. Therefore, this method can be used to detect pesticide residues in environmental water samples with high selectivity, sensitivity, and accuracy.

Key words: cobalt-nickel layered double hydroxide (CoNi-LDH), multi-walled carbon nanotubes (MWCNTs), solid-phase microextraction (SPME), high performance liquid chromatography (HPLC), pesticides