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A core-shell structure with a NaYF:Sm/Yb core for photothermal conversion nanocalorifier and a NaYF:Er/Yb shell as temperature probe for potential applications in photothermal therapy (PTT) were synthesized by a thermal decomposition technique of rare-earth oleate complexes. The optical temperature reading-out property for the NaYF:Sm/Yb@NaYF:Er/Yb core-shell structure was systematically investigated and it was found that in comparison with pure NaYF:Er/Yb particles, the temperature sensing performance of the NaYF:Er/Yb shell did not become worse due to the presence of NaYF:Sm/Yb core. Furthermore, the photothermal conversion behavior for core-shell nanoparticles was successfully examined by dint of temperature sensing of the NaYF:Er/Yb shell, and it was found that an excitation-power-density-dependent temperature increase of up to several tens degrees can be achieved. All the experimental results suggested that the core-shell structure may be an excellent nanocalorifier candidate for advanced temperature-controllable PTT.

It is difficult to calculate the Judd-Ofelt (J-O) parameters for trivalent rare earth (RE)-doped powders due to the unavailable absorption spectrum that is necessarily used in the conventional J-O calculation procedure. In this study, a universal method starting from the diffuse-reflection spectrum for calculating the J-O parameters of RE3+-doped powdered samples was proposed. In this proposed method, by taking the Kubelka-Munk function into account, the absorption cross-section spectrum was derived from the diffuse-reflection spectrum in the RE3+-doped powdered sample using the connection between the absorption cross section and the radiative transition rate of RE3+. Then, the J-O parameters might be calculated from the absorption cross-section spectrum via the traditional J-O calculation technique. The NaYF4:Er3+/Yb3+ and NaYF4:Er3+ phosphors were prepared via an auto-combustion-assisted fluoridation technique, and the J-O calculation was carried out for the obtained samples. The obtained J-O parameters were compared with those reported in the literature and also verified by comparing the calculated radiative transition lifetimes with the experimental values. Finally, it was deduced that the proposed J-O calculation route was practicable.

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