Multifunctional NaYF4 : Yb3+,Er3+@Ag core/shell nanocomposites: integration of upconversion imaging and photothermal therapy

Abstract

Here we present the synthesis of core–shell structured hexagonal-phase NaYF₄ : Yb³⁺,Er³⁺@Ag nanoparticles (NPs) and their unique bio-functional properties. The structure and morphology of the NPs are confirmed by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR) measurements. They displayed both strong ²H_11/2/⁴S_3/2–⁴I_15/2 (green) and ⁴F_9/2–⁴I_15/2 (red) upconversion luminescence (UCL) and photothermal transfer properties under the excitation of a continuous 980 nm laser diode. Furthermore, the UCL intensity ratio of ²H_11/2–⁴I_15/2 to ⁴S_3/2–⁴I_15/2 acted as a temperature sensor due to the low energy gap between ²H_11/2 and ⁴S_3/2 (∼740 cm⁻¹). After coating with the silver shell, the cytotoxicity of the composite NPs was reduced largely in contrast to the pure NaYF₄ : Yb³⁺,Er³⁺ NPs. HepG2 cells from human hepatic cancer and BCap-37 cells from human breast cancer incubated with the composite NPs in vitro were found to undergo photothermally induced death on exposure to 980 nm NIR light, and the optimum mortality approaches 95% with a power density of 1.5 W cm⁻² which is much lower than that reported for Au nanoshells and Au nanorods. Overall, this class of core/shell NPs is expected to be an attractive therapeutic agent for tumor ablation with bioimaging and thermal detection in real time.