A water-soluble, ER-targeting fluorescent probe for the simultaneous ratiometric detection of SO32-/HSO3- and viscosity in live cells.

Sulfur dioxide derivatives (SO32-/HSO3-) serve as critical environmental pollutants and endogenous signaling molecules, with dysregulated levels contributing to disease pathogenesis. We report a water-soluble fluorescent probe, (E)-5-(4-(7-(diethylamino)-2-oxo-2H-chromene-3‑carbonyl)piperazine-1‑carbonyl)-2-(4-(diethylamino)styryl)-1,3,3-trimethyl-3H-indol-1-ium (CIS), based on a tunable FRET/ICT (Förster resonance energy transfer/ Intramolecular charge transfer) scaffold that enables single-wavelength excitation, ratiometric detection of SO32-/HSO3- with 99 % energy-transfer efficiency, a 13 nM detection limit, and a sub-2-min response. CIS also functions as a viscosity reporter via a TICT (Twisted intramolecular charge transfer) mechanism. With excellent selectivity, endoplasmic reticulum targeting specificity, and low cytotoxicity, CIS enabled successfully real-time visualization of both exogenous and endogenous SO32-/HSO3- fluctuations in live cells. This dual-functional probe provides a powerful tool for investigating SO2-related pathophysiology and cellular microenvironment dynamics.