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-C3N4)　for　assessing　the　antioxidant　capacity　of　biological　fluids.　Dopamine　(DA)　can　self-polymerize　on　the　surface　of　g-C3N4　to　produce　a　polydopamine　(PDA)　uniform　coating　layer　in　which　the　fluorescence　of　g-C3N4　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-C3N4.　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.　©　2018　Elsevier　B.V.