Rhodamine-derived ratiometric fluorescent probes for high-sensitivity detection and real-time imaging of mitochondrial pH and viscosity in HeLa cells and <i>Drosophila melanogaster</i>

The spirolactam on/off switch attached to rhodamine dye is known to be a highly selective and sensitive fluorescent probe, yet few studies have explored extending the π-conjugation system within its skeleton for pH detection in live cells. An extended π-conjugated rhodamine section should enable ratiometric pH detection in the near-infrared region. In this study, we synthesized probes A and B by coupling a rhodamine derivative with 7-nitrobenzofurazan and 7-(diethylamino)-2-oxo-3,8<i>a</i>-dihydro-2<i>H</i>-chromene-3-carbaldehyde sections, respectively. Probe A exhibits emission <i>via</i> a Förster resonance energy transfer (FRET) mechanism. Under excitation at 370 nm, the conjugated 7-nitrobenzofurazan in probe A exhibits fluorescence at 465 nm in the ring-closed state, while fluorescence at 660 nm appears in the ring-open state due to increased conjugation in the rhodamine moiety. Excitation of probe B at 325 nm resulted in reduced emission around 350 nm and a significantly enhanced response at 525 nm. Probe A was evaluated for mitochondrial pH detection through ratiometric fluorescence emission measurements. Additional tests in living HeLa cells, including responses to stimuli such as carbonyl cyanide-4(trifluoromethoxy)phenylhydrazone (FCCP), hydrogen peroxide (H₂O₂), <i>N</i>-acetyl cysteine (NAC), mitophagy induced by nutrient deprivation, and hypoxia triggered by cobalt chloride (CoCl₂) treatment, as well as pH changes in fruit fly larvae, further validated its applicability for ratiometric measurement of mitochondrial pH variations. Probe A's emission was dependent on the pH level under basic conditions, but under acidic conditions, the change in conformation upon ring opening resulted in the emission also being affected by viscosity.