Local symmetric distortion boosted photon up-conversion and thermometric sensitivity in lanthanum oxide nanospheres

Abstract

It is essential to simultaneously boost the luminescence intensity and thermometric sensitivity of up-converted optical thermometers towards potential biomedical sensing applications. Herein, the effects of local site symmetry on the up-conversion (UC) emission and thermal sensing ability in trigonal-phased La₂O₃:Er³⁺/Yb³⁺ nanospheres were qualitatively explored using cubic-phased Lu₂O₃ and Y₂O₃ with a similar shape and phonon energy as contrasts. Under 980 nm light excitation, much stronger UC emissions were detected in La₂O₃ samples than that in cubic Lu₂O₃ and Y₂O₃ samples, and the possible mechanisms were elaborately proposed using Eu³⁺ as a luminescent probe. Thermo-responsive emission intensity from ²H_11/2/⁴S_3/2 levels was monitored to evaluate the absolute sensitivity of three samples, which strongly depends on the dopant-induced local site symmetric distortions according to the Judd–Ofelt theory. The potentiality of La₂O₃:Er³⁺/Yb³⁺ for sub-tissue thermometry was also validated by ex vivo experiments. Results open a promising avenue for realizing highly sensitive thermometry with a large signal-to-noise ratio in sub-tissues via finely tailoring the local site symmetry.