An oxidized, porous graphitic carbon nitride nanosheets (CNNs) was successfully obtained via a bottom-up approach. The inner filter effect (IFE) of Fe (III) tetrakis (1-methyl-4-pyridyl) porphyrin pentachlorideporphyrin pentachloride (FeTMPyP) on the CNNs results in fluorescence quenching of the CNNs due to the overlap of FeTMPyP absorbance band and CNNs emission band. It is interesting that the quenched fluorescence could be "turned on" in response to the participation of H O , which caused by decomposition of FeMPyP. In this study, for the first time, a porous fluorescence probe based on CNNs and FeTMPyP was designed and an excellent H O detection performance with a large detection range of 0.1 ~ 100 μM and a detection limit of 0.07 μM was achieved. Furthermore, the proposed method was successfully used for H O detection in RAW 264.7 cells.