A Sensitive Fluorescent Probe with Large Stokes Shift for Real-Time Tracking Lysosomal pH Changes in Live Cells

The lysosome is the typical acidic organelle involved in many biological processes, the fluctuation of its pH influences its normal digestive function and even the fate of the cells. To conveniently detect and visualize lysosomal pH changes during heatstroke, we designed and synthesized a lysosomal-targeted ratiometric pH-sensitive fluorescent probe TCF-LysopH. TCF-LysopH showed an ideal pK(a) value (pK(a)=6.12) in the lysosomal pH range during heatstroke (5.00-7.00), rapid response for the changes of pH in real-time, reversible pH switching ability, good selectivity, low cytotoxicity, and excellent lysosome-targeting ability. The large Stokes shift of 110 nm allows us to apply the probe for fluorescence imaging in MCF-7 cells. The probe has much better photostability (only less than 20 % decrease in intensity for 15 min irradiation) than LysoTracker Green. The fluorescence intensity became higher with the increase of pH, which is consistent with the tendency of the lysosomal pH change during heatstroke. Importantly, TCF-LysopH was applied to measure the change of lysosomal pH during heatstroke in live cells, the results prove once again that the pH of the lysosome is increased during heatstroke. Thus, TCF-LysopH was successfully developed as a practical and useful tool to provide a better understanding of the complicated mechanisms between lysosome pH change and heatstroke.