Probing fluctuations in sulfur dioxide and viscosity levels during mitochondrial dysfunction using a dual-response fluorescent probe with good water solubility.

Mitochondrial dysfunction is associated with increased viscosity and reactive oxygen species (ROS) levels. As an effective antioxidant, sulfur dioxide (SO 2 ) can actively scavenge excess ROS to regulate the redox state and protect cells from oxidative stress. However, few studies have evaluated the connection between viscosity and SO 2 during mitochondrial dysfunction. Herein, a water-soluble fluorescent probe (MBI) is designed and synthesized for dual-detecting SO 2 and viscosity. The probe rapidly detects SO 2 within 12 s based on Michael's addition reaction. Meanwhile, increasing viscosity further inhibits the intramolecular rotation, causing the probe to show a greatly enhanced fluorescence. Probe MBI possesses mitochondria targeting capability due to its quaternary ammonium salt. More importantly, probe MBI successfully supports SO 2 and viscosity imaging in living cells and can effectively monitor them during mitochondrial dysfunction and cell apoptosis.