Intrinsic peroxidase activity of carbon dots derived from expired drugs and its application in automatic detection of total antioxidant capacity

In the post-COVID-19 pandemic, the stockpiling of medicines by most households could pose a pollution risk as they may expire over time. In this study, an expired amoxicillin act as carbon source to prepare peroxide-like functional carbon dots nanozymes doped Fe (Fe-CDs). Importantly, a high-value treatment technique for expired drugs was proposed. To obtain outstanding catalytic performance, compared with other commonly doped metals, Fe-CDs catalyzed the oxidation of TMB by H2O2 with the highest efficiency. In particular, the oxidation product of TMB (oxTMB) would be alleviated in the presence of antioxidants. Significantly, antioxidants could briefly cause the fading of oxTMB and the fading level was correlated with various concentrations of antioxidants. Importantly, a sequential injection strategy was introduced to control the fading process caused by antioxidants to improve the accuracy. Under the optimum condition, the limit of detection (LOD) was 0.13 mg/L (for ascorbic acid) with the linear concentration range from 0.5 to 15 mg·L−1. Finally, the total antioxidant capacity (TAC) in human serum samples were evaluated successfully.