Upconversion nanoparticle-decorated gold nanoshells for near-infrared induced heating and thermometry

The present work involves the design of a multifunctional system based on gold nanoshells (AuNSs) decorated with lanthanide-based upconversion nanoparticles (UCNPs) intended as an optical heater and a temperature probe at the nanoscale. The synthesis of NaGdF4 UCNPs doped with ions Yb3+:Er3+ was performed via thermal decomposition of lanthanide fluoride precursors at high temperatures (>300 degrees C) in the presence of a coordinating ligand (oleic acid). UCNPs were synthesized at three different temperatures (310, 315 and 320 degrees C) and characterized in terms of morphological, structural and emission properties. In view of the intended biological applications, the surface of hydrophobic oleate-capped UCNPs was modified using a silica coating to achieve sufficient water dispersibility, through a modified Stober process using a reverse micro-emulsion method. Monodisperse NaGdF4:Yb3+:Er3+ upconversion nanocrystals (similar to 25 nm dia.) were obtained in cubic (at 310, 315 degrees C) and hexagonal (at 320 degrees C) phases. The UCNPs in the hexagonal phase were shown to be more suitable as temperature sensors, due to their lower red-to-green emission ratios and higher thermal sensitivities. The emission spectrum of NaGdF4:Yb3+:Er3+ (oleate-or silica-coated) UCNPs was recorded at different temperatures in the vicinity of the physiological range (20-70 degrees C) and presented suitable properties for application as temperature sensors, such as excellent linearity (r(2) > 0.99) and sensitivity (>3 x 10(-3) K-1). The heating capacity of AuNSs@UCNPs was verified by monitoring the Er3+ emission, showing their potential for application as a hyperthermia agent controlled using a nanothermometer function.