An indole-fused quinacridone-based fluorescent probe with high photostability and specificity for cellular lipid droplets

Lipid droplets (LDs) are critical organelles that participate in numerous biological processes in eukaryotic cells. However, the unsatisfying specificity to LDs and poor photostability of previous fluorescent probes (e.g. BODIPY 493/503, Nile Red) have substantially limited the in-depth investigations into LDs, particularly for the small and nascent LDs. Herein, inspired by the outstanding photophysical properties of the classical pigment quinacridone (QA), QA was chosen as an excellent π-skeleton to constitute superior fluorescent probe. The indole cores and ether chains were successfully incorporated into its π-skeleton to construct a novel fluorescent probe IQA1. Compared to conventional fluorescent probes, IQA1 not only demonstrates innate superb photostability, but also exhibits superior specificity to LDs. The long ether chains effectively reversed the insolubility of QA in lipids, endowing IQA1 with appropriate lipophilicity to traverse the cell membrane and target LDs. These advantages are conductive to the application of IQA1 in various LDs fluorescence imaging, e.g. 3D confocal imaging of the quantification and dimensions of LDs in the ferroptosis HeLa cells and the oleic acid stimulated HeLa cells, time-lapse structured illumination microscopy (SIM) imaging to observe the dynamics of LDs at nanoscale (178 nm). This study thus not only provides a fascinating fluorescent probe IQA1 for LDs advanced imaging, but also explores a new area for quinacridone pigment application.