Silica-Coated Mn-Doped ZnS Nanocrystals for Cancer Theranostics

Doped nanocrystals such as manganese-doped zinc sulfide (ZnS:Mn) are useful nanomedicine probes for cancer cell labeling and anticancer drug delivery. However, the synthesis and retention of fluorescence of these nanocrystals is highly indispensable for efficient cell theranostics. Herein, we report a modified synthesis of highly fluorescent hydrophobic ZnS:Mn nanocrystals with the advent of dual ligands. Our results demonstrate that the alkylamine ligand with the carbon chain length of C₁₈ promotes the diffusion of Mn from the surface into the interior of ZnS nanocrystals. Optical measurements show that the quantum yield of Mn (QY_Mn) can reach as high as 80% in the presence of a dual ligand combination of oleylamine–octadecylamine because of the increased probability of ⁴T₁ → ⁶A₁ emission, originating from the energy transfer of ligated nanocrystals. These doped nanocrystals after ligand exchange of organic ligands with glutathione exhibited a high retention of quantum yield (QY: ∼50–60%), and further coating with silica showed the QY of ∼35–40%. Finally, we show the application of these doped nanocrystals for cancer theranostics such as HeLa cell labeling and anti-cancer drug delivery.