Quaternized chitosan enhances laccase-mimicking catalytic activity and stability of copper-based metal-organic framework for sensitive colorimetric detection of quercetin in food.

Quercetin is an important flavonoid compound in food. The existing rapid colorimetric methods for quercetin detection face many limitations including poor stability and strong substrate dependence. Herein, a copper-based metal-organic framework (Cu-MOF) possessing inherent laccase-mimicking activity was synthesized, which was further modified with quaternized chitosan (QCS) to construct a robust laccase mimic (named QCS@Cu-MOF) with enhanced catalytic activity and stability. The increased surface hydrophilicity and positive charge density promote closer contact with quercetin, thereby boosting laccase-mimicking catalytic activity. Besides, the formation of a protective QCS coating layer shields Cu2+ active sites from environmental erosion, which significantly improves the stability of laccase-mimicking activity. Capitalizing on these enhancements, QCS@Cu-MOF could efficiently catalyze quercetin to generate an orange product with a characteristic absorption peak at 467 nm. The absorbance of the product was linearly correlated with quercetin quantity, enabling the establishment of a rapid colorimetric method for quercetin detection. This method demonstrated a wide detection range (3.78-108 μM) and a limit of detection as low as 1.13 μM, and the spiked recovery rates in food samples were consistent with HPLC results. This work not only developed a rapid colorimetric approach for quercetin detection, but also expanded the strategy for improving the catalytic properties of MOF nanozymes with QCS.