Time-resolved fluorescence imaging with color-changing, “turn-on/turn-on” AIE nanoparticles

Summary

Two aggregation-induced emission (AIE) 1,8-naphthalimides (1 and 2) exhibiting "turn-on/turn-on" emission behavior are reported. They are emissive in good solvents of low/intermediate polarity (THF/hexane) but undergo drastic quenching in polar solvents (DMSO/MeOH) due to solvatochromic and energy gap law effects. Water also quenches the emission up to a critical volume (<50% water in THF), after which hydrophobicity drives them to aggregate into nanoparticles, restoring their emission. The mechanisms are revealed through spectroscopy and theory with distinct excited-state decay kinetics observed between the two turn-on/turn-on states. Self-assembly of 2 with the biocompatible poloxamer P188 generates luminescent particles that are taken up into MDA-MB 231 human breast cancer cells, at which point they disassemble, releasing 2, which then localizes in the lipid droplets. Time-resolved fluorescence lifetime imaging (FLIM) distinguishes extracellular 2-P188 particles emitting from the "aggregated on-state" and intracellular, free molecules of 2 emitting from the "disaggregated on-state" within the lipid droplets.