Phenoxazine-conjugated-benzoeindolium as a novel mitochondria-targeted fluorescent probe for turn-on detection of sulfur dioxide and its derivatives in vivo

As a vital important signaling molecule, sulfur dioxide (SO2) was closely related to steady state equilibrium in the body. In this study, a new turn-on fluorescent probe (PhB) has been designed and synthesized by combining phenoxazine and benzoeindolium moieties, which can adequately detect SO2 and its derivatives (HSO3-/SO32-) with high sensitivity and selectivity in PBS-DMSO (8:2, v/v, pH = 7.4) solution. Only by adding SO2 and its derivatives (HSO3-/SO32-) can lead to the dramatical enhancement of fluorescence emission at 470 nm, whose sensing mechanism was demonstrated to be the nucleophilic addition of the electron deficit carbon–carbon double bonds with SO2 and its derivatives (HSO3-/SO32-). PhB displayed a low detection limit (0.10 µM) and fast response time (200 s). Meanwhile, the probe featured the fluorescence sensing ability in a wide pH range (5.5–8.0), meaning that PhB showed an excellent light-up fluorescence recognition of SO2 and its derivatives (HSO3-/SO32-) under the physiological conditions. Furthermore, PhB possessed good cell membrane permeability and low cytotoxicity, which was successfully employed to monitor the level of HSO3- in the mitochondria of HCT116 cells and zebrafish.