Imidazole-fused benzothiadiazole-based red-emissive fluorescence probe for lysosomal pH imaging in living cells

Intracellular pH is a key physiological factor for controlling the activities and functions of cells and lysosome is a vital subcellular organelle. Thus, developing a novel lysosome-targeting fluorescence probe for selective and sensitive detection of lysosomal pH in living cells is very important. In this work, we synthesized a series of fluorescence probes based on imidazole-fused benzothiadiazole. The optical properties of these probes were easily adjusted by modifying the substituents with different electron-withdrawing/donating ability in imidazole moiety. All of them showed acid-response and decreased fluorescence intensity during pH values changing from 4.0 to 8.5. The introduction of morpholine group allowed them to specifically respond to the changes of lysosomal pH. Among them, probe MIBTAA possessed a suitable pK_a value (5.3) and showed good linear response to pH (R² = 0.9918) with red emission when pH changed from 4.4 to 5.6. The probe was successfully applied for monitoring pH variation in living cells induced by proton-pump inhibitor Baf-A1 and chloroquine, indicating its great potential for pH imaging in biological applications.