Abstract: Organochlorine pesticides (OCPs), typical persistent organic pollutants (POPs), are toxic, bioaccumulative and refractory, threatening ecological safety and human health. Trace OCPs in complex water matrices are difficult to enrich owing to low selectivity of commercial adsorbents and low efficiency of traditional solvent-consuming pretreatment techniques. In this work, frosted glass slides were adopted as supporting substrates to immobilize post-modified chlorine-functionalized covalent organic frameworks (Cl-COF) for micro-solid-phase extraction (μ-SPE) of OCPs. Chlorine atoms were successfully introduced into two-dimensional covalent organic frameworks via a cycloaddition reaction to synthesize Cl-COF with abundant halogen active sites. FT-IR, XRD, TGA, XPS, nitrogen adsorption-desorption and water contact angle measurements were used for systematic material characterization. FT-IR spectra confirmed the successful Schiff-base polymerization and grafting of chlorine groups. XRD patterns demonstrated that post-modification did not damage the crystalline framework of the COF, and the prepared material exhibited outstanding resistance to acids, alkalis and organic solvents, with its thermal stability improved from 317 ℃to 363 ℃. Although the specific surface area and pore size slightly decreased after functionalization, abundant adsorption sites were still reserved for target capture. XPS analysis revealed a surface chlorine atomic ratio of 9.77%, further verifying the successful fabrication of Cl-COF. Besides, the water contact angle increased from 99.54° to 127.10°, indicating remarkably enhanced hydrophobicity of the modified material. Systematic characterizations reveal that the obtained Cl-COF possesses a large specific surface area, excellent chemical stability and hydrophobicity, and exhibits prominent extraction and adsorption performance toward OCPs. Mechanism investigations verify that the synergistic effects of hydrophobic interaction, halogen bonding between chlorine sites and OCP molecules, as well as interlayer π-π stacking interaction dominate the extraction and enrichment of trace OCPs on Cl-COF. The as-fabricated Cl-COF coating was combined with gas chromatography equipped with an electron capture detector (GC-ECD) to establish a novel μ-SPE-GC-ECD analytical method. Methodological validation was carried out to evaluate the analytical performance of the established method. The results demonstrated that the proposed method achieves a wide linear range covering 0.5-1β000 ng/L for the five OCPs, with a low limit of detection of 0.20 ng/L and high enrichment factors ranging from 891 to 1β308. Satisfactory recoveries of 89.2%-104.0% and 89.0%-105.5% were obtained in two batches of real water sample spiked tests, and the relative standard deviations (RSDs) of triple parallel tests ranged from 1.3% to 9.4%, proving good accuracy, precision and anti-interference ability for complex aqueous matrices. Moreover, the composite material adheres firmly to the glass slide substrate without peeling off under repeated elution and washing procedures, and no obvious decay in extraction capacity is observed after 30 consecutive adsorption-desorption cycles, demonstrating its superior structural stability and outstanding regeneration reusability. Compared with previously reported μ-SPE methods for OCPs determination, this proposed strategy exhibits better comprehensive extraction performance. This study provides a feasible technical support for routine safety supervision and rapid detection of OCPs in aquatic environments.

Key words: covalent organic frameworks (COFs), micro-solid-phase extraction (μ-SPE), organochlorine pesticides (OCPs), post-synthetic modification strategy