Aggregation-Induced Orange Emission via Inhibition of Twisted Intramolecular Charge Transfer in Double-Shelled Nanoparticles

We report on a new type of ternary nanoparticles that are double-shelled, biocompatible, and brightly fluoresce by an order of magnitude over the pristine counterpart and single-shelled nano predecessor. In the core of the nanoparticles is organic 4-N,N-dimethylamino-4'-N'-methyl-stilbazolium tosylate (DAST), which is first encapsulated in an organic β-cyclodextrin (β-CD) shell and then further by inorganic silica nanoparticles (SNPs) in the outer shell. In aqueous solutions, the fluorescence emission of the as-formed DAST@β-CD@SNPs ternary nanoparticles is 38.6 times stronger than that of DAST or 15.6 times over the single-shelled DAST@β-CD. Such giant enhancement of fluorescence emission is attributed to two mechanisms: one is that chemical anchor of DAST molecule within the inner cavity of β-CD restricts the physical rotation of dimethylamine group in DAST, inhibiting the twisted intramolecular charge transfer of DAST; another is that further encapsulation of DAST@β-CD binary supermolecules in SNPs causes aggregation-induced emission of DAST. Particularly, the ternary double-encapsulated core-shell-shell nanostructures also exhibit excellent biocompatibility and emission stability, and in tests bright orange fluorescence in Hela cells are observed. These findings not only show great prospects of the deployment of such nanoparticles in bioimaging, but also point to a new nanotechnology for significantly enhancing the fluorescence and overcoming the leakage and pollution of organics in the solutions.