FRET-based colorimetric and ratiometric sensor for visualizing pH change and application for bioimaging in living cells, bacteria and zebrafish

Abstract Acid-alkaline balance plays a crucial role in all biological processes. Accordingly, monitoring pH changes will help us to understand the functional status of these physiological and pathological processes. Though fluorescent probes may be a useful tool for detecting pH changes, and there are many limitations to currently available probes, such as background interference, potential cytotoxicity, and poor cell permeability, which call for a solution urgently. In this work, a rhodamine-derived colorimetric and ratiometric sensor (Rh-HN) was fabricated for monitoring pH change via the mechanism of fluorescence resonance energy transfer (FRET). Rh-HN has been shown to possess several advantages over other probes, such as high sensitivity, outstanding permeability, and low toxicity. Besides, the fluorescence intensity ratio (F526/F592) of Rh-HN displays a pH-sensitive response from 2.0 to 7.5 (pKa=5.05) and linear response from pH 3.8 to 6.4, which was desirable for mapping pH change in the biological systems. Besides, the results indicated that Rh-HN generated a pH-dependent response regulated by switchable forms between closed and opened spirolactam ring. Overall, Rh-HN has accomplished sensing and mapping of pH in living cells, bacteria, and zebrafish. Those results demonstrated that the great potential of Rh-HN in sensing and visualizing pH in the living biosystem.