Sr₂LuF₇:Yb³⁺–Ho³⁺–Er³⁺ Upconverting Nanoparticles as Luminescent Thermometers in the First, Second, and Third Biological Windows

The concept of optical temperature sensing, based mainly on the band intensity ratio, line shift, or luminescence lifetimes, is utilized for noninvasive and rapid detection of local temperature values in the inspected systems. For these purposes, lanthanide-doped nanoparticles (NPs) seem to be the most promising luminescence nanothermometers, because they can use temperature-dependent emission lifetimes, as well as band ratios of alike thermally coupled levels (TCLs) and non-TCLs. Such thermal effects are especially well pronounced in the case of upconverting NPs, whose optical response is strongly affected by temperature. Here, we use the multidoped, colloidal Sr2LuF7:Yb3+–Ho3+–Er3+ NPs, exhibiting bright yellow-green upconversion (λex = 975 nm) luminescence for multimode optical temperature sensing (≈20–56 °C) in a very broad spectral range (≈400–1700 nm), covering the visible, first, second, and third biological windows. Temperature sensing is realized based on the upconversion emission lifetimes, and numerous band intensity ratios of the rarely used and underestimated non-TCLs of Yb3+/Er3+ and Yb3+/Ho3+, which results in exceptionally high thermal sensitivity values, up to 5–9% K–1. Finally, the performed ex vivo experiments allow mimicking of biological conditions (tissue barrier) and optically monitoring of temperature changes of the aqueous system, using band intensity ratios and luminescence lifetimes in the NIR ranges.