A coumarin-based fluorescent probe: Small but multi-signal

There remains a challenge for elegantly designing structurally simple but multi-signal fluorescence probes to visualize simultaneously various intracellular redox species and microenvironmental parameters. Herein we present such a design in the small probe COU-Mito featuring the graft of a strong electron-withdrawing 4-pyridiniumylacrylonitrile unit to an electron-donating diethylamino coumarin. The introduction of the cyano group destroys the coplanarity of the total push-pull π-conjugation system, thus the probe exhibits very weak background red fluorescence but a turn-on response to increased viscosity which restricts the rotation of the molecular rotor between the push and pull parts around a single bond. More importantly, the introduction of the cyano group together with the 4-pyridinium moiety improves significantly the electrophilicity of the carbon-carbon double bond in the probe that allows selective oxidative cleavage and nucleophilic addition on the double bond by peroxynitrite (ONOO - ) and sulfur dioxide (SO 2 ), respectively. Hence the probe, whilst superficially simple, is in fact a multifunctional probe capable of simultaneously discriminating viscosity, ONOO - and SO 2 in a fashion of three sets of distinct signals: red for viscosity, green for ONOO - , and blue for SO 2 . The probe was successfully employed to visualize the change of mitochondrial viscosity and the fluctuation of mitochondrial ONOO - and SO 2 levels in living cells and zebrafish, and to probe into their roles in the hepatotoxicity induced by acetaminophen, a widely used analgesic and antipyretic drug. • Imaging of viscosity, ONOO - and SO 2 in red, green and blue channels, respectively. • Excellent sensitivity and fast response toward ONOO - . • Imaging of mitochondrial ONOO - , SO 2 and viscosity in living cells and zebrafish. • Visualizing the toxicity mechanism of acetaminophen.