Solvatochromic dissociation of non-covalent fluorescent organic nanoparticles upon cell internalization.

Amorphous red-emitting materials involving solvatochromic small molecules have been processed by the reprecipitation method as non-doped nanospheres characterized by a remarkably low polydispersity. Their mean diameter could simply be tuned by the concentration of the organic solution giving rise to time-stable dispersion of 85-200 nm-sized nanoparticles. Time-resolved measurements performed on solid nanoparticles showed significant size-dependence effects of the emission lifetime and maxima evidencing populations with distinct molecular conformations. Nanoparticle internalization has proved successful in NIH-3T3 murine fibroblasts with normal toxicity effects after 48 h. Fluorescence confocal microscopy under one- and two-photon excitations revealed dual emission enabling localization of the organic material within the plasma membrane and the cytoplasm. Model experiments resorting to suspended artificial lipid bilayers allowed us to conclude on the dissolution of nanoparticles by the phospholipid membrane during the internalization process. They let us to assume that uptake of hydrophobic nanoparticles by living cells implies an endocytosis mechanism operating through the formation of plasmic vesicles.