APF is an aromatic amino-fluorescein derivative and fluorescent probe for highly reactive radicals. It has low intrinsic fluorescence, however, upon oxidation by hydroxyl radical, hypochlorite ion, and certain peroxidase intermediates, it is converted to the highly fluorescent molecule fluorescein, facilitating the detection of highly reactive biological radicals. APF is not oxidized by nitric oxide (NO), hydrogen peroxide (H2O2), or other oxidants. Fluorescein displays excitation/emission maxima of 490/515 nm, respectively.