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

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.