Spiropyran isomerization triggering ESIPT for visualization of pH fluctuations during oxidative stress in living cells

Intracellular pH homeostasis is foundation of maintaining normal physiological functions. More and more evidences show that intracellular pH fluctuations were usually associated with many diseases (such as cancer, epilepsy and neurodegenerative diseases). It is very important to develop in situ real-time determination of pH. In recent years, it has been verified that pH can regulate the isomerization process of spiropyran. Thus, we report a pH fluorescent probe BSL, which is a closed loop spiropyran structure by coupling benzothiazole derivatives with indole salts. We utilizes the process of spiropyran isomerization as the trigger of excited state intramolecular proton transfer (ESIPT) effect, and adjust the process of spiropyran isomerization through pH, and then the molecular transformation from enol to ketone (enol: 525 nm, ketone: 677 nm) through the ESIPT effect. This process achieved accurate measurement of pH. The probe BSL showed sensitive and reversible fluorescence response to pH in vitro. Ultimately, BSL was successfully applied to detect pH fluctuations in cell oxidative stress model.