Sustainable Conversion of Coffee Ground Waste into Carbon Dots for Sensing Food Antioxidants

The total antioxidant capacity (TAC) of food products is a key parameter for assessing food quality and safety. In this work, iron-doped carbon dots (Fe-CDs) were successfully prepared using waste coffee grounds as a precursor with a satisfactory fluorescence quantum yield of 9.6%. The Fe-CDs exhibited exceptional peroxidase-like activity, which can oxidize colorless 3,3',5,5'-tetramethylbenzidine (TMB) to form blue oxTMB. Concurrently, oxTMB induced an inner filter effect, quenching the fluorescence of Fe-CDs. After being added to antioxidants such as glutathione, ascorbic acid, and L-cysteine, the generated reactive oxygen species (ROS) are consumed, thereby preventing the oxidation of TMB. The color of the mixed solution changed from dark to light blue, accompanied by the fluorescence recovery of Fe-CDs. Nevertheless, these three antioxidants possessed remarkable differences in ROS elimination capability, which resulted in different signal responses in absorption and fluorescence, and were successfully used for constructing the colorimetric/fluorescent dual-channel sensor array. Furthermore, the sensor array signals were processed using principal component analysis to achieve simultaneous detection of glutathione, ascorbic acid, and L-cysteine, and were able to effectively discriminate between mixtures and individual antioxidants. The constructed sensor array was successfully applied for the TAC detection in various foods (including vegetables, fruit, and beverages) and for the precise differentiation of antioxidants in milk samples. Overall, the prepared sensor array exhibited outstanding potential in detecting food quality.