Optimizing the framework of indolium hemicyanine to detect sulfur dioxide targeting mitochondria

Endogenous sulfur dioxide (SO2) is mainly produced by the enzymatic reaction of sulfur-containing amino acids in mitochondria, which has unique biological activity in inflammatory reaction, regulating blood pressure and maintaining the homeostasis of biological sulfur. It is more and more common to detect monitor SO2 levels by fluorescence probe. In recent years, the indolium hemicyanine skeleton based on the D-π-A structure has been widely used in the development of fluorescent sensors for the detection of SO2. However, subtle changes in the chemical structure of indolium may cause significant differences in SO2 sensing behavior. In this article, we designed and synthesized two probes with different lipophilicities to further study the relationship between the structure and optical properties of hemicyanine dyes. On the basis of previous studies, the structure of indolium hemicyanine skeleton was optimized by introducing -OH group, so that MC-1 and MC-2 had the best response to SO32- in pure PBS system. In addition, the lipophilicity of MC-2 was better than that of MC-1, which enabled it to respond quickly to SO32- and better target mitochondria for SO2 detection. Most importantly, the low detection limits of MC-1 and MC-2 conducive to the detection of endogenous SO2. This work provided an idea for developing SO2 fluorescent sensors with excellent water solubility and low detection limit.