Decoration of upconversion nanocrystals with metal sulfide quantum dots by a universal in-situ controlled growth strategy
Conjugating transition-metal sulfide quantum dots and upconversion nanocrystals (UCNCs) has aroused widespread concern due to enhanced physical and chemical properties in contrast to only the simple sum of them. However, the synthesis of such hybrid nanoparticles by a universal in-situ growth strategy has been scarcely reported so far. Herein, we developed a facile approach to functionalize NaYF4:Yb/Er with chitosan (NaYF4:Yb/Er@CS), which not only could improve the hydrophilicity of NaYF4:Yb/Er, but also can form stable chelates with transition-metal ions. Then, the ultrasmall metal sulfide (Mn+S, M = Ag, Cu, Cd) quantum dots (QDs) can be conjugated homogeneously on the surface of NaYF4:Yb/Er@CS. Taking Ag2S as an example, the growth behavior of Ag2S QDs on the surface of NaYF4:Yb/Er@CS was studied specifically. The influences of Ag:Y ratio, S:Ag ratio, pH value, reaction time and reaction temperature on the growth behavior of Ag2S on the surface of NaYF4:Yb/Er@CS were investigated systematically. Meanwhile, this innovative strategy is also suitable for the growth of ultrasmall QDs on various shapes, including plates, spheres and rods. The resultant NaYF4:Yb/Er@CS@Ag2S system possess both upconversion luminescence (UCL) properties of NaYF4:Yb/Er and good photothermal conversion effect of Ag2S, which is promising candidate for UCL imaging guided PTT of cancer.