Ultra-Bright Orange Fluorescence from Doubly-Encased DAST Nanoparticles for Bioimaging

We report on a new type of ternary organic nanoparticles that are double-shelled, biocompatible and brightly fluoresce by an order of magnitude over the bulk counterpart as well as the single-shell predecessor. In the core of the nanoparticles is 4-N,N-dimethylamino-4'-N'-methyl-stilbazolium tosylate (DAST), which is encapsulated in a β-cyclodextrin (β-CD) shell and then again by silica nanoparticles (SNPs) in the outer shell. Results indicate that in the aqueous solutions, the fluorescence intensity emitting from the DAST@β-CD@SNPs is more than 38 times stronger than that measured from pristine DAST and ~15 times over the single-shelled DAST@β-CD. Such giant enhancement of fluorescence intensity can be attributed to two mechanisms: one is that the chemical anchor of DAST molecule within the inner cavity of β-CD restricts the physical rotation of dimethylamine group in DAST, thus inhibiting the twisted intramolecular charge transfer of DAST; another is that the second encapsulation of the DAST@β-CD supermolecules in SNPs evokes the aggregation-induced emission of DAST. Furthermore, the double-encapsulated core-shell-shell ternary nanostructures also exhibit excellent biocompatibility, and in tests stable and bright orange fluorescence in Hela cells have been observed. These findings not only suggest great prospects in their deployment in bioimaging, but also point to a new route for significantly enhancing the fluorescence of ionic organic materials.