Mitochondria-dependent benzothiadiazole-based molecule probe for quantitatively intracellular pH imaging

In this work, we reported two new pH-sensitive benzothiadiazole-pyridine compounds 1 and 2, which distinctly showed the red-shifted absorption and “turn-off” fluorescence responses to increased pH in a broad range (pH = 2–12). Because of excllent mitochondria-immobilized preformance, suitable fluorescence detection window and linear response behavior in the range of pH = 5–8, compound 1 was suitable to explore as a imaging detector of intracelluar pH in live cell. More intereastingly, different from “turn-off” fluorescence response to increased pH in buffer solution, probe 1 displayed a “turn-on” fluorescence in live cell imaging experiment. This opposite performance between buffer solution and live cell are so unique that no such literature has been ever mentioned in our best knowledge. The further study revealed that this unique performance was dependent on the mitochondrial function under different physiological pH conditions. Herein, on the basis of special “turn-on” fluorescence response, intracellular pH was successfully imaged in a quantitatively ratiometric method by using 1 in conjunction with MitoTracker Red (MTR).