Mitochondria-targeted phosphorescent iridium (III) probes for turn-on imaging of selenocysteine dynamics during...

Selenocysteine (Sec), the 21st genetically encoded amino acid, plays a crucial role in cellular redox regulation and selenium metabolism. Abnormal Sec fluctuations are closely associated with inflammatory oxidative stress and the progression of inflammatory bowel disease (IBD). However, efficient tools for real-time visualization of Sec fluctuations in living systems remain highly limited. Herein, two novel mitochondria-targeted iridium(III) complex probes, Irppy-DNP and Irpiq-DNP, were successfully designed and developed based on a thiolysis-triggered phosphorescence activation strategy for Sec-responsive "turn-on" sensing. Upon reaction with Sec to generate the corresponding hydroxylated products (Irppy-OH and Irpiq-OH), the two probes exhibited distinct photophysical responses, characterized by increased photoluminescence quantum yields (2.3 → 34.3% and 1.3 → 26.9%), prolonged excited-state lifetimes (0.23 → 0.42 μs and 0.62 → 0.70 μs), and reduced nonradiative decay rates (k_nr) (4.25 × 10⁶ → 1.56 × 10⁶ s^-1 and 1.59 × 10⁶ → 1.04 × 10⁶ s^-1), respectively. In addition, the two probes exhibited excellent analytical performance, including favorable photostability (60 min), fast response time (15/20 min), low detection limit (0.79/0.70 nM), excellent selectivity and anti-interference capability, low cytotoxicity (>70% cell viability), and visualized endogenous and exogenous Sec in living cells. Especially, the probes showed high selectivity toward Sec over Cys, Hcy, and GSH, benefiting from the greater nucleophilicity of Sec-derived selenolate at physiological pH (7.4) and its efficient thiolysis of the dinitrophenyl ether moiety. Given the key role of Sec-mediated redox regulation in IBD-associated mitochondrial ROS elevation, mitochondrial targeting is essential for monitoring pathological processes, with both probes exhibiting efficient organelle localization (PCC = 0.783/0.803). Notably, Irpiq-DNP enabled longitudinal imaging of Sec fluctuations in dextran sulfate sodium (DSS)-induced colitis mice, where the luminescence intensity closely correlated with disease progression as well as therapeutic response to 5-aminosalicylic acid (5-ASA) treatment. This work provides a promising phosphorescent platform for monitoring Sec fluctuations during inflammatory processes and therapeutic intervention in IBD.