Application of Ferric–Graphene Quantum Dot Complex for Evaluation and Imaging of Antioxidants in Foods Based on Fluorescence Turn-Off–On Strategy

Antioxidants have drawn much interest owing to their capacity to shield the human body from reactive oxygen species (ROS). Therefore, it is essential to develop a quick and easy assay for the evaluation of antioxidants and for imaging their distribution in food. Herein, we describe a fluorescence measurement platform for assessing and visualizing antioxidant capacity. Our method is based on using the composite of graphene quantum dots (GQDs) with Fe³⁺ (Fe³⁺-GQDs) as a reagent for evaluating and imaging the antioxidant capacity in foods using a fluorescence turn-off-on strategy. The fluorescence of GQDs was found to be selectively quenched with Fe³⁺ at pH 3.5. Upon addition of an antioxidant, Fe³⁺ is reduced to Fe²⁺, and the fluorescence of GQDs is regained. Next, we investigated the fluorescence intensity after the reaction of Fe³⁺-GQDs with seven typical antioxidants, and it showed excellent sensitivity down to 0.60 µM of antioxidant. Next, using Fe³⁺-GQDs as a reagent, we developed a paper-based fluorescence imaging method for antioxidants in foods. Furthermore, we analyzed the distribution of antioxidant capacity on cucumber and carrot slices (tips, central parts, and shoulders). Next, the antioxidant capacity of cucumber and carrot slice extracts was measured, and the results were consistent with the fluorescence imaging results of the intact slices.