A temperature sensor based on the enhanced upconversion luminescence of Li+ doped NaLuF4:Yb3+,Tm3+/Er3+ nano/microcrystals

Abstract

In this paper, fluorescent and optical temperature sensing bi-functional Li⁺-doping NaLuF₄:Ln (Ln = Yb³⁺, Tm³⁺/Er³⁺) nanocrystals were synthesized via a simple hydrothermal method using oleic acid as a capping ligand. The crystal phase, size, upconversion (UC) properties, and optical temperature sensing characteristics of the crystals can be easily modified by Li⁺ doping. The results reveal that additional Li⁺ can promote the transformation from the hexagonal phase to the cubic phase and reduce the size of the nanocrystals. In addition, NaLuF₄:Ln (Ln = Yb³⁺, Tm³⁺, Li⁺) nanocrystals present efficient near infrared (NIR) emission, which is beneficial for in vivo biomedical applications due to the increased penetration depth and low radiation damage of NIR light in bio-tissues. More importantly, under 980 nm excitation, the temperature dependent UCL from the ²H_11/2 and ⁴S_3/2 levels of Er³⁺ ions in NaLuF₄:Yb³⁺,Er³⁺,Li⁺ microcrystals was investigated systematically. The fluorescence intensity ratios (FIR) of the pairs of thermally coupled levels were studied as a function of temperature in the range of 298–523 K. The maximum sensor sensitivities were found to be about 0.0039 K⁻¹ (523 K) by exploiting the UC emissions from the ²H_11/2 and ⁴S_3/2 levels. This suggests that the Li⁺-doped upconversion luminescence (UCL) materials are promising prototypes for application as multi-mode probes for use in bio-separation and optical thermometers.