Polarity-Sensitive fluorescent probes based on triphenylamine for fluorescence lifetime imaging of lipid droplets

Non-alcoholic fatty liver disease (NAFLD) is a disease closely associated with metabolic abnormalities. Lipid droplets (LDs) serve as organelles that store intracellular neutral lipids and maintain cellular energy homeostasis. Their abnormalities can cause metabolic disorders and disease, which is also one of the distinctive characteristics of NAFLD patients. However, the correlation between the polarity of LDs and NAFLD is easily overlooked. To monitor the polarity changes in LDs in order to assess the progression of NAFLD, triphenylamine was used as the electron donor (D), pyridine as the electron acceptor (A) and thiazolo[5,4-d]thiazole (TTz) as pi bridge in this study. The structure was modified by introducing different substituents at the triphenylamine to obtain a series of D-pi-A structural polar-responsive asymmetric thiazolo[5,4-d]thiazole (aTTz) fluorescent probes with different push-pull electron effects and steric hindrance. The fluorescent probes, which exhibit distinct fluorescence emission spectra in solutions with varying polarities, demonstrate excellent polarity-sensitive properties, and the displacement of the maximum emission wavelength varies from 125 to 150 nm. Meanwhile, the fluorescent probes exhibited low dark toxicity of cells and can specifically image lipid droplets, with a localization coefficient of more than 0.84 when imaging, and can be applied to the fluorescence imaging of C. elegans. Furthermore, the polar response properties of the fluorescent probes were used to distinguish normal liver tissue and nonalcoholic fatty liver tissue by fluorescence lifetime microscopic imaging (FLIM), thus providing a molecular tool for the diagnosis of NAFLD.