Synthesis of fluorescent dyes based on the electron-withdrawing core of malononitrile and construction of viscosity-sensitive AIE probe with large Stokes shift for lipid droplets imaging

Viscosity is an important parameter of the cell microenvironment and is closely related to the occurrence of many diseases in vivo. Lipid droplets, as organelles that store and metabolize lipids, play a crucial role in various physiological processes related to cell metabolism. Monitoring cell viscosity and lipid droplets status is of great significance in studying cell function and disease pathogenesis. In this study, a series of novel fluorescent dyes were successfully synthesized by using malononitrile derivatives as fluorophores and modified by different substituents. The photophysical data of these dyes were summarized and the effects of different substituents on the optical properties of the dyes were studied. In addition, these dyes showed significant fluorescence enhancement effects in high-viscosity environments. Based on this, we further developed a novel viscosity-sensitive fluorescent probe DCN-LD that can be used for lipid droplets imaging. DCN-LD not only showed high viscosity sensitivity and good fluorescence stability but also exhibited AIE properties, large Stokes shifts (151 nm), and was capable of labeling intracellular lipid droplets. Finally, DCN-LD was successfully applied to monitor the changes in intracellular lipid droplets and changes in intracellular viscosity.