A super-sensitive ratiometric fluorescent probe for monitoring intracellular subtle pH fluctuation

A new type of xanthene-based fluorescent probe capable of intracellular subtle pH fluctuation sensing was developed. The probe underwent reversible deprotonation-protontation processes and therefore generated two states with distinct contrast in fluorescence emission features. In the physiological range (pH 7.0-8.0), the probe markedly presented 31.2% fluorescence signal change upon pH fluctuation of 0.1 pH unit, enabling sensitivity infinitely superior to previous fluorescent pH probes. The preliminary biological experiments revealed the large dynamic range of the probe for intracellular pH sensing within the range of pH 6.0-8.0. Owing to its sensitive responses to pH fluctuation in the physiological pH range, the probe definitely demonstrated its ability for “distinguishing” tumor from normal cells based on the difference in microenvironment pH level between the former and the latter. The figure of merits of the probe, including pKa (7.45) in the physiological pH range, excellent photostability, extraordinary large dynamic range for pH sensing, as well as the preliminary bioimaging results regarding differentiating tumor from normal cells are indicative of its potential for providing the microenvironment information regarding the interfacial features at the perimeter of tumor tissues by presenting high-contrast fluorescence images.