Ultra-Bright Fluorescent Organic Nanoparticles Based on Small-Molecule Ionic Isolation Lattices

Ultra-bright fluorescent nanoparticles hold great promise for demanding bioimaging applications. Recently, extremely bright molecular crystals of cationic fluorophores were obtained by hierarchical co-assembly with cyanostar anion-receptor complexes of associated counterions. These small-molecule ionic isolation lattices (SMILES) ensure spatial and electronic isolation to prohibit dye aggregation quenching. We report a simple, one-step supramolecular approach to formulate SMILES materials into nanoparticles. Rhodamine-based SMILES nanoparticles stabilized by glycol amphiphiles show high fluorescence quantum yield (30%) and brightness per volume (5000 M^–1 cm^–1 / nm³) with 400 dyes packed into 16-nm particles, corresponding to an absorption coefficient of 4 × 10⁷ M^–1 cm^–1. UV excitation of the cyanostar component leads to highest brightness (>6000 M^–1 cm^–1 / nm³) by energy transfer to rhodamine emitters. Coated nanoparticles stain cells and are thus promising for bioimaging.