Carboxylate-Containing Two-Photon Probe for the Simultaneous Detection of Extra- and Intracellular pH Values in Colon Cancer Tissue

Acidified extracellular pH (pHe) is directly related to various disorders such as tumor invasion and the resistance to drugs. In this study, we developed two-photon excitable emission ratiometric probes (XBH1-3) for the in situ measurement of pHe. These probes, based on benzimidazole and polar solubilizing groups, exhibited a strong two-photon-induced fluorescence and sensitive blue-to-green emission color changes with pK(a) values of 5.1-5.7. XBH1, containing a carboxylic acid, stained the extracellular region in neutral media; it entered the cell under acidic media, thereby allowing a precise measurement of the extra- and intra-cellular pH values in the acidified tissue. XBH2, containing the sulfonate peripheral unit, facilitated the monitoring of the pHe value only. Ratiometric two-photon microscopy imaging revealed that XBH1 can directly monitor the pH values both inside and outside the cells in colon cancer tissue; there is also the morphological aspect. This could be useful for cancer analyses and drug development.