Selective extraction of quercetin using a Cu(II)-immobilized molecularly imprinted covalent organic framework.

In this study, a Cu(II)-immobilized molecularly imprinted covalent organic framework material (MI-COF@Cu²⁺) was fabricated and used for the selective extraction of quercetin (QUE) in onion and apple samples. The synergistic combination of in situ imprinting and Cu²⁺ coordination endows MI-COF@Cu²⁺ with excellent adsorption performance toward QUE. The imprinting factor of MI-COF@Cu²⁺ for QUE was 6.57, with a saturation adsorption capacity of 15.1 mg g⁻¹ and equilibrium attainment within 2 min. MI‑COF@Cu²⁺ was employed as an adsorbent for filter‑based solid‑phase microextraction (F‑SPME) and coupled with high‑performance liquid chromatography (HPLC) for the sensitive detection of QUE. The limit of detection (LOD) was 0.038 mg L⁻¹, and the limit of quantification (LOQ) was 0.129 mg L⁻¹, with inter-day and intra-day relative standard deviations (RSDs) below 4%. The method successfully detected QUE in onion and apple, with spiked recoveries ranging from 89.24% to 100.88% and RSDs below 5%. This study provides novel insights into the design of metal ion-immobilized molecularly imprinted covalent organic framework materials and expands their application potential for the separation and analysis of bioactive components in food matrices.