Abstract: | Glutathione peroxidase (GPx) is a selenoenzyme that protects cells against oxidative damage. Although the formation of a seleninic acid (‐SeO2H) by this enzyme during oxidative stress has been proposed, a selenic acid has not been identified in cells. Herein, we report that the formation of a seleninic acid can be monitored in living cells by using a redox‐active ebselen analogue with a naphthalimide fluorophore. The probe reacts with H2O2 to generate the highly fluorescent seleninic acid. The electron withdrawing nature of the ‐SeO2H moiety and strong Se???O interactions, which prevent the photoinduced electron transfer, are responsible for the fluorescence. |