Near-infrared optical nanothermometry via upconversion of Ho-sensitized nanoparticles.

Recently, materials revealing the upconversion (UC) phenomenon, which is a conversion of low-energy photons to higher-energy ones, have attracted considerable attention in luminescence thermometry due to the possibility of precise and remote optical thermal sensing. The most widely studied type of luminescent thermometry uses a ratiometric approach based on changes in the UC luminescence intensity, mainly of lanthanide ions' thermally coupled energy levels. In this work, NaYF:Ho@NaYF and NaYF:Ho, Er@NaYF nanoparticles (NPs) were synthesized by the controlled reaction in oleic acid and octadecene at 573 K. The obtained nanoparticles had hexagonal structures, oval shapes, and average sizes of 22.5 ± 2.2 nm and 22.2 ± 2.0 nm, respectively. The spectroscopic properties of the products were investigated by measurements of the UC emission under 1151 nm laser excitation in the temperature range between 295 to 378 K. The sample doped with Ho and Er ions showed unique behavior of enhancing emission intensity with the temperature. The relative sensitivity determined for the NPs containing Ho and Er ions, reached the maximum value of 1.80%/K at 378 K. Here, we prove that the NaYF:Ho, Er@NaYF system presents unique and excellent optical temperature sensing properties based on the luminescence intensity ratios of the near-infrared bands of both Ho and Er ions.