A Fe2+-activated carbon dots nanozyme with high oxidase-like activity at neutral pH for antioxidants detection and discrimination

In this study, we developed an innovative stimuli-responsive oxidase-mimicking nanozyme system based on carbon dots (CDs) and ferrous ions (Fe 2+ ) for rapid detection and effective discrimination of antioxidants under neutral conditions. The uniqueness of this system lies in its exhibition of oxidase-like activity activated by Fe 2+ , which acts as both a switch and a regulator of catalytic performance. Importantly, the Fe 2+ -triggered activation shows instantaneous response and reaches steady-state within 240 s. Another key feature of this system is its exceptional oxidase-like activity at neutral pH, overcoming the limitation of most oxidase mimics that require acidic conditions. Moreover, it shows remarkable cold tolerance, retaining over 70% catalytic activity even at 0 °C. Mechanistic studies reveal that Fe 2+ coordinates with functional groups on CDs, enhancing Fe 2+ /Fe 3+ conversion efficiency and accelerating electron transfer, which greatly promotes the generation of reactive oxygen species (ROS) and subsequent TMB oxidation. Additionally, the generated Fe 3+ further catalyze TMB oxidation, synergistically enhancing the colorimetric reaction. Capitalizing on these attributes, we developed a colorimetric sensing platform for rapid and sensitive detection of antioxidants including ascorbic acid (AA), glutathione (GSH), and cysteine (Cys). Based on their differential inhibitory effects, we further constructed a sensor array that enables effective discrimination among these antioxidants. This work not only opens new avenues for designing nanozymes with stimulus-responsive properties, but also expands the toolbox for detecting and identifying antioxidants in complex samples under neutral conditions.