Single-Nanoparticle Luminescence Nanothermometers with Enhanced Sensitivity in Physiological Temperature Range

In this paper, we presented the unique optical properties of monodispersed and uniform luminescence nanothermometers (LNTs), composed of core (NaYF₄:Yb³⁺,Er³⁺) and core-shell (NaYF₄:Yb³⁺,Er³⁺@NaYF₄) upconverting nanoparticles (UCNPs). We observed a significant influence of the NaYF₄ shell on the reduction of the luminescence energy loss, which appeared as a steeper temperature vs. luminescence intensity ratio (I _G2/I _G1) curve. Moreover, the addition of nonionic IGEPAL CO-520 surfactant and a small amount of deionized water led to the additional improvement of the nanothermometer's sensitivity, especially in the physiological temperature range between 35 and 40 °C. Such behavior was explained by the physical and chemical interactions of the surfactant molecules with the particle surface, which led to a significant reduction in luminescent energy loss. The developed method of lanthanide-based LNT synthesis and characterization is suitable for the preparation of in vitro nanothermometers and could be applied, for example, in microelectronics or environmental monitoring.