Specific Two-Photon Imaging of Live Cellular and Deep-Tissue Lipid Droplets by Lipophilic AIEgens at Ultralow Concentration

Lipid droplets are highly associated with obesity, diabetes, inflammatory disorders, and cancer. A reliable two-photon dye for specific lipid droplets imaging in live cells and live tissues at ultralow concentration has rarely been reported. In this work, four new aggregation-induced emission luminogens (AIEgens) based on the naphthalene core were designed and synthesized for specific two-photon lipid droplet staining. The new molecules, namely, NAP AIEgens, exhibit large Stokes shift (>110 nm), high solid-state fluorescence quantum yield (up to 30%), good two-photon absorption cross section (45–100 GM at 860 nm), high biocompatibility, and good photostability. They could specifically stain lipid droplets at ultralow concentration (50 nM) in a short time of 15 min. Such ultralow concentration is the lowest value for lipid droplets staining in live cells reported so far. In vitro and ex vivo two-photon imaging of lipid droplets in live cells and live mice liver tissues were successfully demonstrated. In addition, selective visualization of lipid droplets in live mice liver tissues could be achieved at a depth of about 70 μm. These excellent properties render them as promising candidates for investigating lipid droplet-associated physiological and pathological processes in live biological samples.