A ratiometric fluorescent probe based on FRET mechanism for pH and its cell imaging

pH balance is a prerequisite for maintaining cell function optimally and performs an important role in diverse physiological activities. Even subtle changes in cellular pH can influence many cell behaviors. More specifically, for mitochondria, its unique functions under physiological conditions depend largely on its pH values. Therefore, an effective analytical method needs to be developed to detect the alteration of intracellular pH values, especial in mitochondria. In this work, a ratiometric pH fluorescent probe (DPM) based on fluorescence resonance energy transfer (FRET) mechanism was developed by constructing coumarin and hemicyanine fluorophore. DPM could respond to pH changes by the FRET-off and FRET-on process depending on the cyclization and ring-opening reaction. DPM exhibited high energy transfer efficiency (89.1 %) and responded to pH ranges of 6.0-8.0 rapidly and selectively (pKa value of 7.6 in solution and 7.0 in cells). Moreover, DPM exhibited nearly nontoxicity to living cells and excellent photostability. DPM showed great pH-responsive feature in cells and achieved detection of mitochondrial acidification. The present work not only provided a new structure for pH fluorescence probe and simple synthetic method for the FRET-based probe but also found that the probe could target mitochondria in living cells.