p-methylaminophenol sulfate, also known as 4-(methylamino)phenol sulfate, can be used to prepare a hypochlorous acid fluorescent probe. Hypochlorous acid is an important reactive oxygen species produced by Cl- and H2O2 under the action of Chemicalbook myeloperoxidase (myeloperoxidase) in biological systems. Although hypochlorous acid is extremely low in biological systems, it plays a key role in immune defense.