MnO nanozyme-based dual-mode colorimetric and fluorescence determination of antioxidant activity and HPLC - UV - MS/MS profiling of antioxidants.

MnO nanozyme-based strategy is for the first time exploited for dual-mode colorimetric and fluorescence determination of antioxidant activity and HPLC - UV - MS/MS profiling of antioxidants. MnO nanosheets (MnO NS) with outstanding oxidase-like property can trigger rhodamine B (RhB) chromogenic reaction, leading to design an accurate, selective, and sensitive dual-mode colorimetric and fluorescence method for total antioxidant capacity (TAC) determination. Additionally, antioxidants in complex extract can react with MnO NS catalytic reactive oxygen species (ROS) intermediates (•O and O), which deduced the HPLC peaks decreased or disappeared. And screened antioxidants can be identified by MS/MS analysis. As proof of concept, antioxidant levels of four flavonoids (quercetin, rutin, hesperidin, and nobiletin) with different substituent groups and six samples (peels, pulps, and juices for two citrus cultivars ChunJian tangerine and Debao navel orange) were successfully measured. Nineteen flavonoids with ROS scavenging activity from citrus samples have been screened out and characterized. Especially, polymethoxyflavones, nonactive in DPPH•/ABTS•-based assays, presented certain ROS scavenging activity. Together, this study provided a novel and efficient platform to accurately and selectively measure and screen physiological antioxidants in real complex samples, revealing its great promise and convenient applications in future.