New Strategy for Improving the Sensitivity of Optical Nanothermometers Based on NaYF₄:Yb³⁺/Er³⁺/Nd³⁺/Gd³⁺ Nanorods

Abstract

For optical nanothermometers, it is difficult to improve the absolute sensitivity (SA) and relative sensitivity (SR) at the same time. In this paper, a new strategy is explored to improve the SA and SR of nanothermometers based on upconversion (UC) nanorods (NaYF4:Yb3+/Er3+/Nd3+/Gd3+). With the temperature increasing, the intensity of UC luminescence bands of Er3+ decreases, and that of the 4S3/2 → 4I15/2 (Er3+) drops the most. Meanwhile, the intensity of infrared UC luminescence bands of Nd3+ increases, and that of 4F3/2 → 4I9/2 (Nd3+) increases the most. A new luminescence intensity ratio (LIR) strategy is used, which is based on two luminescence intensities that show the strongest opposite change with the temperature. The SA and SR of different LIRs are compared. It is found that the LIR (4S3/2 → 4I15/2/4F3/2 → 4I9/2) can significantly increase SA and SR. The values of SA and SR in 303–573 K are in the range of 0.003–0.226 K–1 and 1.09–2.37 %K–1, respectively. This work provides new strategies for improving temperature sensitivity.