Temperature sensing and bioimaging realized in NaErF4:40%Tm@NaYF4 with extremely intense red upconversion and suitable R/G emission ratio

The practical application of light thermometer based on the FIR (520/540) has been greatly limited due to the strong absorption of green light by biological tissues. The sensitivity of the temperature sensor located in a biological window (FIR-BW) also needs to be improved. Based on this, we designed and synthesized an upconversion nanoparticles (UCNPs) with suitable R/G ratio and red visual output to prebreaking the limitation of strong absorption of biological tissue, while improving the sensitivity of temperature sensor and in vivo fluorescence imaging. In this paper, NaErF4:40%Tm@NaYF4, a high efficient red UCNPs with R/G ratio (6.86), is first utilized as an optical thermometer, accomplished through the fluorescence intensity ratio (FIR) of thermally coupled stark sublevels of S-4(3/2)/F-4(9/2) (FIR(540/654)). In the studied temperature range, the maximum absolute and relative sensitivity (S-a and S-r) is 0.47% K-1 and 2.61% K-1 at 298 K, respectively, which is much higher than most previous reports about FIR-based temperature sensors. Additionally, we investigated the in vivo bioimaging behavior of NaErF4:40%Tm@NaYF4 in detail. The superior biocompatibility, the distribution to the main organs with the blood circulation system, and the bright red fluorescence signals were observed under 980 nm laser radiation. All the results imply that NaErF4:40%Tm@NaYF4 is a promising candidate for optical thermometry and in vivo bioimaging with high sensitivity.