Ultrabright Fluorescent Organic Nanoparticles Based on Small-Molecule Ionic Isolation Lattices**

Ultrabright fluorescent nanoparticles (NPs) hold great promise for demanding bioimaging applications. Recently, extremely bright molecular crystals of cationic fluorophores were obtained by hierarchical coassembly with cyanostar anion-receptor complexes. These small-molecule ionic isolation lattices (SMILES) ensure spatial and electronic isolation to prohibit aggregation quenching of dyes. We report a simple, one-step supramolecular approach to formulate SMILES materials into NPs. Rhodamine-based SMILES NPs stabilized by glycol amphiphiles show high fluorescence quantum yield (30 %) and brightness per volume (5000 M-1 cm(-1)/nm(3)) with 400 dye molecules packed into 16-nm particles, corresponding to a particle absorption coefficient of 4x10(7) M-1 cm(-1). UV excitation of the cyanostar component leads to higher brightness (>6000 M-1 cm(-1)/ nm(3)) by energy transfer to rhodamine emitters. Coated NPs stain cells and are thus promising for bioimaging.