Doubly‐Encased DAST Nanoparticles with Bright Orange Fluorescence in Aqueous Solutions for Bioimaging

Abstract A new type of ternary nanoparticles is reported 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 4‐N,N‐dimethylamino‐4′‐N'‐methyl‐stilbazolium tosylate (DAST), which is first encapsulated in a β‐cyclodextrin (β‐CD) shell and then further by silica nanoparticles (SNPs) in the outer shell. In aqueous solutions, the fluorescence emission of the DAST@β‐CD@SNPs is 38.6 times stronger than that of DAST or 15.6 times over the single‐shelled DAST@β‐CD. Such a giant enhancement is attributed to two aspects: one is that the chemical anchor of the DAST molecule within the inner cavity of β‐CD restricts the physical rotation of the dimethylamine group in DAST, inhibiting the twisted intramolecular charge transfer of DAST; another is that further encapsulation of DAST@β‐CD in rigid SNPs effectively isolates DAST from external molecules, reducing its non‐radiative transitions. Particularly, the DAST@β‐CD@SNPs also exhibit excellent biocompatibility and emission stability, and in tests, bright orange fluorescence in HeLa cells is observed. These findings not only show great prospects of the deployment of such nanoparticles in bioimaging but also point to a new nanotechnology for enhancing the fluorescence and overcoming the leakage and pollution of organics in the solutions.