Rhodamine-based fluorescent probe for direct bio-imaging of lysosomal pH changes

Intracellular pH plays a pivotal role in various biological processes. In eukaryotic cells, lysosomes contain numerous enzymes and proteins exhibiting a variety of activities and functions at acidic pH (4.5–5.5), and abnormal variation in the lysosomal pH causes defects in lysosomal function. Thus, it is important to investigate lysosomal pH in living cells to understand its physiological and pathological processes. In this work, we designed a one-step synthesized rhodamine derivative (RM) with morpholine as a lysosomes tracker, to detect lysosomal pH changes with high sensitivity, high selectivity, high photostability and low cytotoxicity. The probe RM shows a 140-fold fluorescence enhancement over a pH range from 7.4 to 4.5 with a pKa value of 5.23. Importantly, RM can detect the chloroquine-induced lysosomal pH increase and monitor the dexamethasone-induced lysosomal pH changes during apoptosis in live cells. All these features demonstrate its value of practical application in biological systems.