Rationally Designed Fluorescent pH Sensors for Measurements in Extremely Alkaline Media

We investigated the pK_a increasing effects on meso-phenol BODIPY to achieve fluorescent pH sensors working in extreme alkalinity. We demonstrated the effects of substituents on meso, α, γ and position of the hydroxyl group on pK_a. By considering these parameters, we developed pH sensors working above 11.0 in aqueous medium (ethanol/water 1:1). One of these sensors had a pK_a of 12.0 and enabled the measurement of pH up to 13.6. This highly photostable and bright sensor compound could be synthesized easily from cheap starting materials.