Fluorescent organic nanoparticles with enhanced fluorescence by self-aggregation and their application to cellular imaging.

We report a simple strategy to prepare organic biocompatible fluorescent nanoparticles with enhanced fluorescence. A significant fluorescence enhancement was realized by designing a fluorescent small molecule, 4,4'-(2,7-bis[4-{1,2,2-triphenylvinyl}phenyl]-9H-fluorene-9,9-diyl)bis(N,N,N-trimethylbutan-1-aminum)bromide (TPEFN), with aggregation-induced emission enhancement (AIEE) effect. Nanoparticles of TPEFN can be formed through molecular self-aggregation by gradually increasing the water fraction in TPEFN mixed solution (methanol/water). Fluorescence enhancement by about 120-fold was observed after nanoparticle formation. By addition of the biomolecule adenosine triphosphate (ATP), larger nanoparticles of TPEFN are formed and further fluorescence enhancement can be achieved, yielding a total fluorescence enhancement of 420-fold compared with the TPEFN molecular solution. Both of these nanoparticles show very good biocompatibility. Ultrabright spots present in the confocal laser scanning microscopy image again proved the formation of nanoparticles. Positively charged side chains of TPEFN endow these nanoparticles cationic surfaces. The size of the prepared TPEFN nanoparticles and their cationic surface allow them to be rapidly internalized into cells. Cell viability assays prove that the TPEFN nanoparticles have high biocompatibility. These organic fluorescent nanoparticles show great promise for applications in cellular imaging or biotechnology.