Observation of high efficient photothermal conversion of sub-10 nm Au nanoparticles coated on upconversion nanoparticles

Abstract Gold nanoparticles have been confirmed to be very suited for applications in cancer photothermal therapy. However, nanoscale photothermal conversion mechanisms of Au nanoparticles are still not completely unveiled. Here, we employ upconversion nanoparticles (UCNPs) as nanoscale light sources, of which green emission centered at 540 nm can be efficiently absorbed by Au nanoparticles with an average diameter of 4 nm. Gold nanoparitcles (~4 nm) are coated on upconversion nanoparticles (UCNPs) with an average diameter of 90 nm, while the gap among these coated Au nanoparticles is smaller than 20 nm which is obviously shorter than the half wavelength (270 nm) of green emission of UCNP core nanoparticles. As a result, green emission of UCNPs could not penetrate the coating of Au nanoparticles without being absorbed. Based on UCNPs@Au nanosystems, high efficient photothermal conversion of Au nanoparticles can be directly observed, while a photothermal conversion efficiency of 41.2% is determined. Importantly, fluorescent resonance energy transfer (FRET) mechanisms from UCNPs to Au nanoparticles is confirmed to be the predominant route for this nanoscale photothermal system (UCNPs@Au), instead of direct photon absorption.