Rational design of iridium complex with double-locked mitochondria-targeting for dynamic viscosity monitoring and precision two-photon tumor imaging

Perceiving the microenvironment around early-stage tumor tissue using mitochondrial viscosity as a nexus is crucial for tumorigenesis tracking and tumor detection. However, the high dependence of most current viscosity-activated mitochondrial probes on mitochondrial membrane potential (MMP) as well as short luminescence lifetimes and poor tissue penetration depths have severely hindered their widespread application. Herein, we report a novel viscosity-activated two-photon phosphorescent probe (Ir-visc) employing a double-locking mitochondrial targeting strategy that incorporates electrostatic force and probe-protein molecular docking, enabling real-time monitoring of dynamic changes in mitochondrial viscosity during cancer cell damage. The probe exhibits remarkable 93-fold enhancement of phosphorescence in the presence of highly viscous media and enabling consistent and accurate detection of mitochondrial viscosity, even in damaged mitochondria through its dual-locking strategy. Furthermore, Ir-visc has viscosity-activated two-photon absorption (TPA) characteristics, which makes it easily distinguishable from cellular autofluorescence while enhancing imaging depth and sensitivity. Notably, the probe Ir-visc selectively labels cancer cells and demonstrates excellent capability for real-time monitoring of viscosity changes in mitochondria. These findings suggest that the Ir (Ⅲ) complex probe offers a versatile strategy for studying mitochondrial viscosity and improving cancer diagnosis.