Multispectral luminescence of core-shell rare-earth NaLuF4:Yb,Er@NaLuF4:Yb,Tm@NaLuF4 upconversion nanomaterials for ratiometric optical temperature sensing

This paper focuses on the study of core-shell rare earth NaLuF 4 :Yb,Er@NaLuF 4 :Yb,Tm@NaLuF 4 upconversion nanoparticles (UCNPs) with multispectral emission for applications in the field of ratiometric temperature sensors. This approach lays the foundation for exploring more luminous intensity ratios (LIRs) between different spectra for temperature sensing. Additionally, it facilitates the use of different LIRs for various temperatures, yielding higher sensitivity. Under 980 nm laser excitation, these UCNPs produce multispectral luminescence covering blue, green, red, and infrared luminescence intervals. Within the temperature range of 293–473 K, we measured the luminescence intensity of different emission peaks and observed that the 803 nm peak exhibits an anomalous thermal enhancement effect. The LIR between the luminescence intensities of different emission peaks is fitted using formulas to calculate the absolute sensitivity ( S a ), relative sensitivity ( S r ), uncertainty ( ΔT ), and repeatability ( R ) of the corresponding LIR. Comparisons revealed that LIR(I 803 /I 540 ) demonstrates greater S a compared to the other LIRs. Furthermore, the S r of LIR(I 803 /I 540 ) is higher in the elevated temperature range, whereas LIR(I 522 /I 540 ) shows greater values in the lower temperature range. By combining the advantages of these two LIRs at different temperatures, LIR(I 522 /I 540 ) is utilized at low temperatures and LIR(I 803 /I 540 ) at high temperatures, achieving a combined S r of 0.67–1.06 %K −1 and a combined ΔT of less than 0.045. The R of LIR(I 803 /I 540 ) is tested and found to be greater than 99 % averaged over five cycles. In summary, the NaLuF 4 :Yb,Er@NaLuF 4 :Yb,Tm@NaLuF 4 UCNPs proposed in this paper demonstrate potential applications in the field of temperature sensing.