Two water-soluble two-photon fluorescence probes for ratiometric imaging endogenous SO2 derivatives in mitochondria

Herein, two low molecular weight, ratiometric fluorescence probes, bearing α,β-unsaturated benzoindolium moiety (EIM and EIS), are designed for monitoring endogenously generated SO2 derivatives in mitochondria using two-photon microscopy (TPM). The two probes show not only over 100nm blue-shift in emission with two well-resolved bands upon addition of HSO3−/SO32−, but also rapid response, high selectivity and sensitivity for monitoring and quantifying HSO3−/SO32− over other bio-species in living cells. Further studies indicated that probes EIM and EIS can highly discern HSO3−/SO32− with two different nucleophilic addition mechanisms, respectively, just by structural fine-tuning, which were confirmed by both theoretical calculation and experimental technique. In a word, probes EIM/EIS with the NIR absorption property, the large stocks shift, high selectivity and signal ratio, are promising molecular tool for bisulfite/sulfite biology.