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Oxidative stress, which involved in several inflammatory and degenerative diseases, has become interesting in most of biomedical disciplines and many types of clinical research. Antioxidants can protect biological systems from the harmful effects of oxidative stress. It can reflect the monitor effect to oxidative stress from another aspect by detecting the antioxidant capacity. Therefore, it is particularly significant to assay the antioxidant capacity of biological fluids. Herein, for the first time a fluorescence-tunable probe was developed based on polydopamine-quenched fluorescent graphitic carbon nitride (g-C 3 N 4 ) for assessing the antioxidant capacity of biological fluids. Dopamine (DA) can self-polymerize on the surface of g-C 3 N 4 to produce a polydopamine (PDA) uniform coating layer in which the fluorescence of g-C 3 N 4 is efficiently quenched by the PDA layer, showing the fluorescence “off” state. However, in the presence of antioxidants, the self-polymerized effect of DA can be effectively inhibited and leading to the fluorescence “on” state. Therefore, detecting antioxidants can be achieved by monitoring the fluorescent emission of g-C 3 N 4 . Moreover, endowed with high sensitivity and selectivity, the proposed sensing system was successfully applied to assessment antioxidant capacity in biological fluid and the results matched well with commonly used antioxidant detection method. [ABSTRACT FROM AUTHOR]