Development of a Series of Practical Fluorescent Chemical Tools to Measure pH Values in Living Samples

In biological systems, the pH in intracellular organelles or tissues is strictly regulated, and differences of pH are deeply related to key biological events such as protein degradation, intracellular trafficking, renal failure, and cancer. Ratiometric fluorescence imaging is useful for determination of precise pH values, but existing fluorescence probes have substantial limitations, such as inappropriate pK_a for imaging in the physiological pH range, inadequate photobleaching resistance, and insufficiently long excitation and emission wavelengths. Here we report a versatile scaffold for ratiometric fluorescence pH probes, based on asymmetric rhodamine. To demonstrate its usefulness for biological applications, we employed it to develop two probes. (1) SiRpH5 has suitable pK_a and water solubility for imaging in acidic intracellular compartments; by using transferrin tagged with SiRpH5, we achieved time-lapse imaging of pH in endocytic compartments during protein trafficking for the first time. (2) Me-pEPPR is a near-infrared (NIR) probe; by using dextrin tagged with Me-pEPPR, we were able to image extracellular pH of renal tubules and tumors in situ. These chemical tools should be useful for studying the influence of intra- and extracellular pH on biological processes, as well as for in vivo imaging.