Upconversion luminescence and temperature sensing properties of NaGd(WO(4))(2):Yb(3+)/Er(3+)@SiO(2) 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 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.