Upconversion luminescence and temperature sensing properties of NaGd(WO₄)₂:Yb³⁺/Er³⁺@SiO₂ core-shell nanoparticles

Optical thermometry based on the fluorescence intensity ratio (FIR) of two thermally coupled levels in lanthanide ions has potential application in non-contact optical temperature sensing techniques. In this work, a shell of SiO₂ with tunable thickness was uniformly coated on NaGd(WO₄)₂:Yb³⁺/Er³⁺ core upconversion nanoparticles (UCNPs). The effects of the silica shell on UC luminescence and thermal sensing properties of core-shell NaGd(WO₄)₂:Yb³⁺/Er³⁺@SiO₂ UCNPs were investigated. Under 980 nm laser excitation, the temperature-dependent UC emission spectra of obtained samples were measured. The FIR was analyzed based on the thermally coupled ²H_11/2 and ⁴S_3/2 levels of Er³⁺ in the biological temperature range of 300-350 K, in which the Boltzmann distribution is applied. The emission from the upper ²H_11/2 state within Er³⁺ was enhanced as temperature increased due to the thermal effect. Absolute sensitivities (S _A) and relative sensitivities (S _R) of the core and core-shell UCNPs were calculated. It was found that after SiO₂ coating, the maximum S _A was enhanced by ∼2-fold (1.03% K^-1 at 350 K). Especially, S _A was as high as 2.14% K^-1 at 350 K by analyzing the FIR of the non-thermally coupled ²H_11/2 and ⁴F_9/2 levels.