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 H-2(11/2) and S-4(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 H-2(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 similar to 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 H-2(11/2) and F-4(9/2) levels.