A stable core-shell metal-organic framework@covalent organic framework composite as solid-phase extraction adsorbent for selective enrichment and determination of flavonoids.

Hydrophobization and stability is crucial for the practical application of most metal-organic frameworks (MOFs) in extraction technique. In this study, a stable core-shell MOF@COF composite (NH-MIL-101(Fe)@TAPB-FPBA-COF) was successfully prepared by Schiff base reaction and applied to solid-phase extraction (SPE) of hydrophobic flavonoids. Notably, the TAPB-FPBA-COF shell acts as a hydrophobic "shield", which not only improves the hydrophobicity and stability of hydrophilic NH-MIL-101(Fe), but also makes the extraction efficiency of flavonoids from MOF@COF composite significantly higher than that of pure NH-MIL-101(Fe) and TAPB-FPBA-COF. In addition, a sensitive analytical method with excellent linearities (0.1-500 ng mL, R ≥ 0.9967), low limits of detection (0.02-0.04 ng mL for water; 0.04-0.07 ng mL for grape juice; 0.06-0.08 ng mL for honey), good repeatability (intra-day/inter-day precision are 1.86-5.37%/1.82-7.79%, respectively) and only 5 mg of adsorbent per cartridge was established by optimizing the SPE process combined with high performance liquid chromatography with ultraviolet-visible detector (HPLC-UV). Meanwhile, selectivity study and comparative experiments with the commercial C18 adsorbent showed that the MOF@COF adsorbent exhibited satisfactory extraction efficiency for flavonoids due to multiple interactions such as hydrogen bonding, hydrophobic, and π-π interactions. Finally, the good recoveries in grape juice (84.5-102.5%) and honey (87.5-104.6%) samples further validated the applicability of the proposed method in complex samples.