Visual monitoring of the lysosomal pH changes during autophagy with a red-emission fluorescent probe

Autophagy play crucial roles in maintaining normal intracellular homeostasis. Molecular probes capable of monitoring lysosomal pH changes during autophagy are still highly required yet challenging to develop. Here a lysosome-targeting pH fluorescent probe RML is presented by introducing a methylcarbitol unit as the lysosome-targeting group to the rhodamine B, which is highly sensitive to pH changes. RML exhibits remarkable pH-dependent behavior at 583 nm with the fluorescent enhancement more than 148-fold. The pKa value is determined as 4.96, and the linear response with pH changes from 4.50-5.70, which is favorable for lysosomal pH imaging. We also confirm that RML diffuses selectively into lysosomes using confocal fluorescent microscopy. Using RML, we have successfully visualized the autophagy by monitoring the lysosomal pH changes.