Dual Site-Controlled and Lysosome-Targeted Intramolecular Charge Transfer–Photoinduced Electron Transfer–Fluorescence Resonance Energy Transfer Fluorescent Probe for Monitoring pH Changes in Living Cells

Acidic pH is a critical physiological factor for controlling the activities and functions of lysosome. Herein, we report a novel dual site-controlled and lysosome-targeted intramolecular charge transfer-photoinduced electron transfer-Fluorescence resonance energy transfer (ICT-PET-FRET) fluorescent probe (CN-pH), which was essentially the combination of a turn-on pH probe (CN-1) and a turn-off pH probe (CN-2) by a nonconjugated linker. Coumarin and naphthalimide fluorophores were selected as donor and acceptor to construct the FRET platform. Hydroxyl group and morpholine were simultaneously employed as the two pH sensing sites and controlled the fluorescence of coumarin and naphthalimide units by ICT and PET, respectively. The sensing mechanism of CN-pH to pH was essentially an integration of ICT, PET, and FRET processes. Meanwhile, the morpholine also can serve as a lysosome-targeted group. By combining the two data analysis approaches of the ratios of the two emission intensities (R) and the reverse ratio R' (R' = 1/R), the fluorescent ratio of CN-pH can show proportional relationship to pH values in a very broad range from pH 4.0 to 8.0 with high sensitivity. The probe has been successfully applied for the fluorescence imaging of the lysosomal pH values, as well as ratiometrically visualizing chloroquine-stimulated changes of intracellular pH in living cells. These features demonstrate that the probe can afford practical application in biological systems.