Photonic experiments are of key importance in life sciences but light-induced side effects are serious confounding factors. Here we introduce roNaV2, an engineered voltage-gated Na+ channel harboring a selenocysteine in its inactivation motif, as a non-photonic, sensitive, gateable, and reversible sensor for membrane-delimited reactive species. roNaV2 allows for the assessment of chemical modification induced in fluorescence microscopy settings with high sensitivity and time resolution and it demonstrates the usefulness of ion channels as highly sensitive reporters of membrane processes.