Tuning covalent organic frameworks polarity and active sites with quaternary ammonium groups for enhanced adsorption and detection of trace sulfonamide antibiotics in milk

DI-COF incorporating diammonium salt exhibited higher polarity, a larger pore size, and more accessible interaction sites, resulting in superior adsorption efficiency toward SAs compared to the other COFs (Dp-COF containing carboxyl groups, TpBD-COF lacking charged groups and EB-COF containing a single quaternary ammonium group). And this presented DI-COF-based DSPE HPLC method for five SAs achieved low detection limits (0.4–2.7 ng mL −1 ), satisfactory recoveries (83.81 %–110.50 %), a wide linear range (1–500 ng mL −1 ), and excellent reusability. • Four COFs with varied polarity, active sites and pore size were successfully synthesized. • Ionic monomers enabled fine-tuning of COF polarity and porosity. • COFs were applied as adsorbents for sulfonamide antibiotic residues. • The most polar COF with largest pores showed superior adsorption of SAs. • HPLC-based method detected five SAs in milk with high accuracy and recovery. Rationally introducing specific functional components into an adsorbent can endow it with the desired extraction performance for target analytes. Herein, we report the fabrication of a covalent organic framework (COF) with enhanced polarity for the extraction and detection of sulfonamide antibiotics (SAs), using the diammonium salt 1,1′-bis(4-aminophenyl)-[4,4′-bipyridine]-1,1′-diium chloride (DI) as a linker and active site, and 1,3,5-triformylphloroglucinol (Tp) as the aldehyde building block. To evaluate the influence of cationic adsorption sites on SA extraction, three additional COFs were synthesized: one using Tp with ethidium bromide (containing a single quaternary ammonium group), one with benzidine (lacking charged groups), and another with 3,5-diaminobenzoic acid (containing one carboxyl group). Among these, the COF incorporating DI—referred to as DI-COF—exhibited higher polarity, a larger pore size, and more accessible interaction sites, resulting in superior adsorption efficiency toward SAs. The adsorption performance of DI-COF remained stable over five cycles, demonstrating excellent repeatability. Adsorption isotherm, kinetic, and thermodynamic studies revealed that the adsorption followed a pseudo-second-order kinetic model and fitted the Langmuir isotherm, indicating a spontaneous monolayer chemisorption process. Capitalizing on these properties, a high-performance liquid chromatography–based extraction method using DI-COF was developed for the enrichment and detection of five SAs in milk samples. Under optimized conditions, the method achieved low detection limits (0.4–2.7 ng mL −1 ), satisfactory recoveries (83.81 %–110.50 %), a wide linear range (1–500 ng mL −1 ), and excellent reusability (at least five times). The plausible adsorption mechanisms were further investigated using Fourier transform infrared spectroscopy, matrix-assisted laser desorption ionization mass spectrometry, inductively coupled plasma mass spectrometry, and X-ray photoelectron spectroscopy.