The present application discloses a sensor for determining a measurand correlated with a concentration of at least one analyte belonging to the class of reactive oxygen species in a measuring fluid, the sensor including a sensor element having an indicator substance, wherein the indicator substance is oxidized into an oxidized form of the indicator substance by the at least one analyte, a means for generating a flow of current in the sensor element that causes a reduction of the oxidized form of the indicator substance and thereby regeneration of the indicator substance, an optical measuring sensor to detect measuring radiation influenced by the oxidized form of the indicator substance and to generate an electrical measuring signal using the influenced measuring radiation, and a sensor switch connected to the optical measuring sensor to receive the measuring signal and to ascertain a measured value of the measurand using the measuring signal.