Lipid droplet formation and dynamics: tracking by time-resolved fluorescence imaging

Emerging studies have implicated that overabundant and enlarged lipid droplets (LDs) are hallmarks or causes of various human pathologies; however, the mechanisms of the biogenesis of LDs and their dynamics remain unclear. Toward a better understanding of LDs, we constructed a novel long-lived fluorescence micelle nanoprobe AI-Cz-M by encapsulating the aromatic-imide-based thermally activated delayed fluorescence (TADF) luminophore AI-Cz into an amphiphilic block copolymer DSPE-mPEG2000. The core cavity of the micelle provides an excellent hydrophobic environment for the accommodation of AI-Cz, which can efficiently isolate the TADF emitter from oxygen. AI-Cz-M dispersed well in aqueous solution and exhibited low cytotoxicity, high cytomembrane permeability and high selectivity toward LDs, which could visualize the formation process and dynamics of LDs in living tumor cells and adipocytes in real time and quantitatively. Most remarkably, delayed emission with a long lifetime was observed from AI-Cz-M in both oxygenic aqueous solution (299 mu s) and LDs in HeLa cells (162 mu s), which has been the longest average fluorescence lifetime obtained by time-resolved fluorescence imaging (TRFI) with TADF materials. AI-Cz-M showed great potential for TRFI to overcome the background autofluorescence drawback and improve the signal-to-noise ratio. Our results delineated a promising strategy for tracking LD formation and dynamics in biological samples.