A pH-optode is described based on 1, 3, 5, 7-tetramethyl-8-(4-dimethylamino)-4-difluorobora-3a, 4a-diaza-(s)-indacene (1) immobilized in a hydrogel matrix. Its uncharged form is non-fluorescent, whereas the protonated form displays strong emission. The quenching of the emission can be mechanistically explained by a PET (photoinduced electron transfer) from the dimethylamino group to the fluorophore. The perpendicular configuration at the C(8)-aryl bond contributes to the efficiency of the on/off-switching behavior. Incorporation of the dye into a hydrogel matrix results in a transparent sensor membrane which can be used to measure the pH in the range of 1-3. The sensor shows high signal intensity, good reproducibility, and the response characteristic is stable over hours of operation even at low pH-values.