Abstract
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 SiO(2) with tunable thickness was uniformly coated on NaGd(WO(4))(2):Yb(3+)/Er(3+) core upconversion nanoparticles (UCNPs). The effects of the silica shell on UC luminescence and thermal sensing properties of core-shell NaGd(WO(4))(2):Yb(3+)/Er(3+)@SiO(2) 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 (2)H(11/2) and (4)S(3/2) levels of Er(3+) in the biological temperature range of 300-350 K, in which the Boltzmann distribution is applied. The emission from the upper (2)H(11/2) state within Er(3+) 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 SiO(2) 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 (2)H(11/2) and (4)F(9/2) levels.