Ratiometric fluorescent probes for pH mapping in cellular organelles

The intracellular pH (pHi) in organelles, including mitochondria, endoplasmic reticulum, lysosomes, and nuclei, differs from the cytoplasmic pH, and thus maintaining the pH of these organelles is crucial for cellular homeostasis. Alterations in the intracellular pH (ΔpHi) in organelles lead to the disruption of cell proliferation, ion transportation, cellular homeostasis, and even cell death. Hence, accurately mapping the pH of organelles is crucial. Accordingly, the development of fluorescence imaging probes for targeting specific organelles and monitoring their dynamics at the molecular level has become the forefront of research in the last three decades. Among them, ratiometric fluorescent probes minimize the interference from the excitation wavelength of light, auto-fluorescence from probe concentration, environmental fluctuations, and instrument sensitivity through self-correction compared to monochromatic fluorescent probes, which are known as turn-on/off fluorescent probes. Small-molecular ratiometric fluorescent probes for detecting ΔpHi are challenging yet demanding. To date, sixty-two ratiometric pH probes have been reported for monitoring internal pH alterations in cellular organelles. However, a critical review on organelle-specific ratiometric probes for pH mapping is still lacking. Thus, in the present review, we report the most recent advances in ratiometric pH probes and the previous data on the role of mapping the ΔpHi of cellular organelles. The development strategy, including ratiometric fluorescence with one reference signal (RFRS) and ratiometric fluorescence with two reversible signals (RFRvS), is systematically illustrated. Finally, we emphasize the major challenges in developing ratiometric probes that merit further research in the future.