Sulfur dioxide (SO2) and its derivatives sulfite and bisulfite play important roles in biological systems. However, in vivo detection of sulfite/bisulfite remains challenging. In this study, we developed a dinuclear Ir(III) complex (Ir4) as a two-photon phosphorescent probe for sulfite and bisulfite. Ir4 selectively and rapidly responded, with high sensitivity, to sulfite/bisulfite over other bio-related ions and molecules. One-photon and two-photon microscopy images revealed that Ir4 preferentially targeted mitochondria and was capable of imaging biological sulfite/bisulfite levels in vitro and in vivo. In situ sulfite generation in Caenorhabditis elegans was visualized by two-photon excitation real-time imaging. Finally, Ir4 was employed to monitor sulfite distribution in rat brain and other tissues. This study is the first report of the direct visualization of SO2 derivatives in vivo. These results provide new insights into the biological importance of SO2.
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