Post-esterification preparation of carboxyl groups enriched magnetic microporous organic network for the efficient enrichment and determination of quaternary ammonium alkaloids.

BACKGROUND: Microporous organic networks (MONs) are an emerging class of porous materials characterized by extremely large surface area, tunable topology, and superior stabilities, which have received increasing attention in sample pretreatment. However, their superhydrophobic nature has largely limited their application scope. The synthesis of functionalized MONs has proved to be a feasible strategy to broaden their utility. Nevertheless, this approach remains constrained by the limited variety and availability of functionalized monomers. Therefore, developing novel post-synthetic modification strategies is crucial for expanding the functionality and applications of MONs, particularly in trace analyte enrichment.

RESULTS: Herein, we present a novel post-esterification strategy to synthesize a new carboxyl groups enriched magnetic MON (MMON-2COOH) for the highly efficient magnetic solid-phase extraction (MSPE) of four quaternary ammonium alkaloids (QAAs) from complex urine samples. The resulting MMON-2COOH not only retains the porous structure, excellent solvent and thermal stability, and rapid magnetic responsiveness of its precursor (hydroxyl-functionalized magnetic MON) but also incorporates conjugated networks and abundant carboxyl groups. This enables efficient extraction of QAAs through multiple interactions, including π-π stacking, hydrogen bonding, and electrostatic attraction. After optimization of extraction factors, an analytical method integrating MMON-2COOH-based extraction with HPLC-UV detection was established, achieving sensitive and selective determination of QAAs with low limits of detection of 0.05-0.5 μg L-1 and limits of quantification of 0.5-1.0 μg L-1, wide linear range of 0.5-1000 μg L-1, negligible matrix effect of 0.88-1.09, rapid extraction of 2 min, and less adsorbent consumption of 2 mg.

SIGNIFICANCE: This is the first example of post-esterification preparation of carboxyl groups functionalized MON. The results demonstrate that MMON-2COOH is a highly effective adsorbent, and the developed method is suitable for accurate determination of QAAs in biological samples. This work highlights the significant potential of post-esterification as a versatile approach for designing high-performance adsorbents in sample pretreatment.