A ‘turn-on’ iridium(III) complex probe for visualizing mitochondrial SO2 fluctuation in cancer cells and tumor models

Mitochondrial sulfur dioxide (SO2) plays critical biological roles in physiological processes. Notably, abnormal endogenous SO2 levels serve as potential biomarkers for early diagnosis of tumors. Therefore, visual detection of SO2 in cancer cells and tumor models is a subject of significant interest. A ‘turn-on’ iridium(III) complex probe (Ir-CHO) was designed and synthsized, utilizing 2-phenylpyridine (ppy) as the main ligand and 2-(pyridin-2-yl)imidazo[1,2-a]pyridine-3-carbaldehyde (PIP-CHO) as the ancillary ligand, for identifying SO2 in living Hela cells and tumors through bioimaging. The probe exhibits some outstanding properties, including high phosphorescence quantum yield (31.1%), microsecond lifetime (0.61 µs), relatively low detection limit (0.68 μM), rapid response (<1 min), excellent photostability (>60 min), high selectivity and specificity, low cytotoxicity, and mitochondria-targeted ability (PCC=0.805). Based on these properties, the probe was successfully applied to visualize endogenous and exogenous SO2 in living cells and effectively distinguish normal mice and tumor mice with negligible systemic toxicity. All the results suggest that this new probe Ir-CHO could monitor SO2 derivatives in real-time within tumor microenvironment research, highlighting its promising diagnostic potential for future clinical and biomedical applications.