Single Dye Molecule Behavior in Fluorescent Core–Shell Silica Nanoparticles

Understanding the parameters that control the intermolecular interactions of chromophores encapsulated within nanoparticles is of fundamental importance to various fields of nanoscience. Employing single-molecule time and spectral domains, we studied a red-absorbing hemicyanine analogue (DY-630) covalently encapsulated in the core of ∼20 and ∼30 nm core–shell silica nanoparticles. We find that on average 4 and 7 dyes are encapsulated within these particles, respectively. Steady state and fluorescence correlation spectroscopy show unusually strong enhancements (up to 16 times) in the relative fluorescence efficiency of the nanoparticles as compared to the free dye in aqueous solution. This increase is explained in terms of restriction of the trans–cis isomerization process due to the more rigid local environment provided by the silica, and protection from the solute–solvent interaction, while preserving the spectral characteristics of the constituent dye. Single molecule measurements reveal that the majority of the nanoparticles behave as systems of independently emitting chromophores. Two subpopulations of molecules are identified and assigned to molecules embedded within and on the surface of the core, respectively. Fluorescence lifetime and polarization trajectories of single molecules provide evidence that under certain conditions intermolecular interactions between several encapsulated molecules, such as energy hopping and singlet–singlet annihilation, can occur within single nanoparticles. We find that the energy transfer processes are more efficient in the smaller nanoparticles (∼11%), probably due to the limited space provided by the core and the shorter distance between the trapped molecules (4 nm). In the bigger nanoparticles energy hopping is present only in 5% of the studied cases.