A Mitochondrial-Targeting Near-Infrared Fluorescent Probe for Visualizing and Monitoring Viscosity in Live Cells and Tissues

The intracellular viscosity is closely related to many functional disorders and diseases. Especially, abnormal mitochondrial viscosity changes are one of the distinct indications in metabolite diffusion as well as mitochondrial metabolism. In this work, we report a novel fluorescent probe (NI-VIS), which uses quinoline as an acceptor group and employs a TICT mechanism (twisted intramolecular charge transfer) to detect viscosity. NI-VIS features a good mitochondrion targeting ability and near-infrared emission. NI-VIS possesses a highly sensitive response toward viscosity changes in aqueous environments. As the viscosity of a DPBS-glycerol system increased from 1.0 to 999 cP, NI-VIS exhibited a hundred-fold enhancement in fluorescence. We demonstrated that after the treatment with ionophores, NI-VIS could identify the variation of mitochondrial viscosity in HeLa cells. The probe also recognized the decrease of mitochondria viscosity during starvation-induced mitophagy. More importantly, NI-VIS was successfully applied to visualize the viscosity variation in cirrhotic liver tissues. Our trial with zebrafish suggested this probe could map the microviscosity in vivo. These findings reveal that NI-VIS can serve as a powerful tool to monitor viscosity of biological samples and shows broad potential applications in the biomedical field.