Lipophilic phosphorescent iridium(III) complexes as one- and two-photon selective bioprobes for lipid droplets imaging in living cells

Abstract

Lipid droplets (LDs) have been the focus of intense research in recent years due to their deep involvement in lipid metabolism and association with diverse diseases. Compared with conventional organic fluorescent LD probes, metal-based phosphorescent probes have some unique advantages. In this study, two lipophilic neutral phosphorescent Ir(III) complexes (Ir1: [Ir(mtq)2(acac)] and Ir2: [Ir(mtq)2(dpm)], where mtq is 4-methyl-2-(thiophen-2-yl)quinolone, acac is acetylacetone and dpm is dipropionylmethane) have been investigated for their LD-specific imaging properties in detail. Complex Ir1 was demonstrated to selectively accumulate in cellular LDs with high specificity, good biocompatibility and excellent photostability. Moreover, complex Ir1 can be utilized to observe LDs by two-photon phosphorescence lifetime imaging microscopy (PLIM), which is a great benefit for phosphorescent bioprobes. Further studies reveal that probe Ir1 can be utilized to analyze LDs accumulation in living cells and visualize in vivo lipid metabolism in zebrafish. Overall, complexes Ir1 and Ir2 are promising alternative phosphorescent probes for LD analysis in LD-related scientific research. Our work also provides useful clues for the construction of highly specific phosphorescent LD probes through rational structural modification of phosphorescent metal complexes.