Fabrication of Au-Ag nanocage@NaYF 4 @NaYF 4 :Yb,Er Core-Shell Hybrid and its Tunable Upconversion Enhancement.

Localized electric filed enhancement by surface plasmon resonance (SPR) of noble metal nanoparticles is an effective method to amplify the upconversion luminescence (UCL) strength of upconversion nanoparticles (UCNPs), whereas the highly effective UCL enhancement of UCNPs in colloids has not been realized until now. Here, we designed and fabricated the colloidal Au-Ag nanocage@NaYF4 @NaYF4 :Yb,Er core-shell hybrid with different intermediate thickness (NaYF4 ) and tunable SPR peaks from visible wavelength region to NIR region. After the optimization of the intermediate spacer thickness (~7.5 nm) of NaYF4 NPs and the SPR peak (~950 nm) of noble metal nanoparticles, an optimum enhancement as high as ~25 folds was obtained. Systematic investigation indicates that UCL enhancement mainly originates from the influence of the intermediate spacer and the coupling of Au-Ag nanocages with the excitation electromagnetic field of the UCNPs. Our findings may provide a new thinking on designing highly effective metal@UCNPs core-shell hybrid in colloids.