A ‘turn-on’ iridium( <scp>iii</scp> ) complex probe for visualizing mitochondrial SO ₂ 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 synthesized, utilizing 2-phenylpyridine (ppy) as the main ligand and 2-(pyridin-2-yl)imidazo[1,2-<i>a</i>]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 mitochondrial targeting 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 between 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.