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<sup>–1</sup> cm<sup>–1</sup> / nm<sup>3</sup>)
with 400 dyes packed into 16-nm particles, corresponding to an absorption
coefficient of 4 × 10<sup>7</sup> M<sup>–1 </sup>cm<sup>–1</sup>. UV
excitation of the cyanostar component leads to highest brightness (>6000 M<sup>–1</sup> cm<sup>–1</sup> /
nm<sup>3</sup>) by energy transfer to rhodamine emitters. Coated nanoparticles
stain cells and are thus promising for bioimaging.