A naphthalimide-based fluorescent probe NACl with a large Stokes shift has been successfully developed for highly sensitive, selective and ultrafast detection of hypochlorite (ClO−). This probe utilizes an oxidative deoximation reaction to provide high selectivity and optical dynamic range for detection of ClO− in aqueous solution. Compared to similar systems involving reactive oxygen species (ROS), including superoxide, tert-butyl hydroperoxide, hydroxyl radical, alkylperoxyl radical, and H2O2, NACl provides 138-fold fluorescence enhancement. Its outstanding response time of less than 3s and detection limit of 2.88nM establish the potential value of NACl for interrogating the pathology and physiology of intracellular endogenous ClO−.