Far-Red Fluorophore for Monitoring Viscosity In Vitro, In Vivo, and Ex Vivo and Dynamic Tracking of Lipid Droplets in Live Systems

Viscosity is an important factor closely related to various life activities. On the other hand, lipid droplets (LDs) are intracellular dynamic organelles involved in the storage of energy and protein homeostasis. Real-time monitoring of viscosity variation in vitro and in vivo and dynamic tracking of LDs are quite necessary in biochemical research. In this contribution, a viscosity-responsive far-red fluorophore, named Cin-Rho, was synthesized, which exhibited good biocompatibility, good photostability, and a large Stokes shift (148 nm). Upon the increase of viscosity, Cin-Rho enhanced the fluorescence intensity, verifying the excellent far-red fluorescence response to viscosity. Cin-Rho has been successfully utilized for monitoring viscosity variation in live cells, zebra fish, and inflammatory mice. Furthermore, ex vivo experiments confirmed that tumor tissues and adipose tissues treated with Cin-Rho exhibited enhanced fluorescence compared to that of normal tissues, confirming the higher viscosity. Moreover, Cin-Rho could accumulate in LDs and track the LD migration and fusion processes. Collectively, this research shows that Cin-Rho could be used as a powerful nanoplatform to monitor the viscosity change in vivo and ex vivo and explore the dynamic behavior of LDs.