A two-photon endoplasmic reticulum-targeting fluorescent probe for the imaging of pH in living cells and zebrafish

The pH changes of the endoplasmic reticulum (ER) are closely related to many diseases. Herein, we present a new pH-sensitive fluorescent probe (Na-pH) for the observation of ER acidification in living cells andER stress in vivo. Na-pH consists of 1,8-naphthalimide as a fluorescence-on signal unit, 4-methylbenzenesulfonamide moiety for ER localization, and piperazine group as a pH-responsive site. Na-pH shows a sensitive fluorescence enhancement under acidic conditions, allowing it to monitor ER acidification. Meanwhile, Na-pH has a negligible response to common species, and possesses low cytotoxicity. In biological experiments, Na-pH can be used for tracking acidification during ER stress. In addition, Na-pH was also applied for two-photon imaging of pH in living tissues and zebrafish. We expect that Na-pH has great potential as a practical tool for real-time monitoring of ER acidification, which helps to reveal the pathogenesis of ER stress-related diseases.