Direct imaging of biological sulfur dioxide derivatives in vivo using a two-photon phosphorescent probe

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.