Thioglycerol-capped Mn-doped ZnS quantum dot bioconjugates as efficient two-photon fluorescent nano-probes for bioimaging.

Water-dispersible 1-thioglycerol (TG)-capped Mn-doped ZnS quantum dots were prepared in aqueous solution using the nucleation-doping strategy. Using 4% Mn relative to Zn and a Zn(OAc)2/Na2S ratio of 0.9, Mn:ZnS nanocrystals with an average diameter of 3.9 ± 0.5 nm, with pure Mn2+-related photoluminescence (PL) at 585 nm, and with a PL quantum yield of 13.2% were obtained. Transmission electron microscopy, X-ray powder diffraction, electron spin resonance, X-ray photoelectron spectroscopy, UV-visible spectroscopy and spectrofluorometry have been used to characterize the crystal structure, the doping status, and the optical properties of the doped-dots. Folic acid (FA) was linked to TG-capped Mn:ZnS nanocrystals to produce Mn:ZnS@TG-FA nanobioconjugates that were used for targeted in vitro delivery to a human cancer cell line. Folate receptor mediated cellular uptake of FA-functionalized dots is proven via confocal and two-photon imaging.