Multifunctional nanoheater based on NaGdF₄:Yb³⁺, Er³⁺ upconversion nanoparticles.

Upconversion nanoparticles (UCNPs) provide an ideal platform for achieving multifunction, such as multimodal imaging, sensing, therapy, etc., mainly by combining with other nanomaterials to construct complicated heterogeneous nanostructures. Multifunctional integration on a simple single-phase structure still is an open question and poses a big challenge. Here we show that small-sized NaGdF(4):Yb(3), Er(3+) UCNPs (~7.5 nm) can simultaneously possess upconversion luminescence (UCL), temperature sensing, paramagnetic and photothermal conversion properties, endowing them great potential for photothermal treatments with real-time imaging and temperature monitoring. Effects of Yb(3+) concentrations, nanoparticle sizes and core/shell structures on the light-to-heat conversion capability of UCNPs were also investigated, and the results were discussed on the basis of the variation in absorption rates and non-radiative relaxation probabilities of UCNPs. There is a competition between UCL and light-to-heat conversion processes. Higher UCL efficiency and enhanced photothermal conversion properties can be realized on UCNPs with the active-core/active-shell structure due to enhanced absorption rates.