Surface Charge Tunability and Size Dependent Luminescence Anisotropy of Aqueous Synthesized ZnS/Dendrimer Nanocomposites

Highly water-soluble, biocompatible, fluorescent, ZnS/Dendrimer nanocomposites have been synthesized, for the first time, with amino-, carboxyl- or hydroxyl-terminated Polyamidoamine dendrimer. Average size of ZnS nanoparticles within the dendrimer matrix was found to be in the range of 2.2-3.1 nm. ZnS nanoparticles with high monodispersity and photoluminescence efficiency was obtained by tuning various experimental parameters. An eight-fold increase in super-saturation of initial reactants caused a sharp focusing of size distribution by 38% and photoluminescence quantum efficiency also increased from 1.3% to 4.2%. ZnS/Dendrimer nanocomposites display constant positive polarized emission with the anisotropy value of 0.2, which makes it a promising candidate for biophysical probe. The anisotropy value was found to increase with increasing particle size of ZnS nanoparticles and also to depend on nature of surface end groups of the dendrimer molecule. Surface charge being an important biological parameter was tuned by varying pH of the reaction medium. Further, it is demonstrated that photoluminescence and surface charge of these nanocomposites are governed by the surface functionality of the dendrimer molecule.