Gold nanoshell coated NaYF4 nanoparticles for simultaneously enhanced upconversion fluorescence and darkfield imaging

Lanthanide-doped upconversion nanocrystals (UCN) converting low energy photons to high energy photons have emerged as an efficient and versatile bioimaging and therapeutic tool. However, the upconversion efficiency of the UCNs is low, which limits their applications. Plasmonic modulation makes it possible to enhance the luminescence of these nanocrystals. We hypothesize that the enhancement of the upconversion luminescence for all the emission peaks simultaneously could be achieved if the UCNs are coated with a gold nanoshell and the surface plasmon resonance (SPR) peak is tuned to the near-infrared (NIR) region and made resonant with the absorption of the UCNs, thereby substantially increasing the excitation flux via local field enhancement (LFE) effect. Furthermore, the nanoparticles could be used for darkfield imaging due to the light scattering caused by the gold nanoshell. Herein, we report a poly-(amino acid)-templated gold-shell encapsulation of the silica coated NaYF4:Yb,Er UCNs and show how a deft tuning of the SPR peak from visible to NIR region dramatically transforms the luminescence quenching into an enhancement effect and how the nanoparticles are used for combined upconversion fluorescence and darkfield light scattering imaging.