Achieving thermochromic upconversion of Tm³⁺for high-sensitive nanoprobe and information encryption

Upconversion-based optical thermometry has many merits, such as quick temperature response, higher thermal sensitivity and robust reliability. However, the previous works were mainly focused on the thermally coupled emissions, limiting their thermal sensitivity due to the intrinsic constraint by small energy gaps (e.g., <2000 cm(-1)) according to Boltzmann distribution theory. Here, we report a design of NaYF4:Yb3+/Tm3+@NaYF4 core-shell nanostructure to achieve thermochromic upconversion and high thermal sensitivity by taking advantage of nonthermally coupled upconversion emissions of Tm3+. We show that the blue upconversion emission of Tm3+ at 475 nm ((1)G(4)-> H-3(6)) exhibits a regular decrease with elevating temperature, while its deep-red emission at 696 nm (F-3(2,3)-> H-3(6)) presents an increase. Such a feature leads to a rapid rise of luminescence intensity ratio (I-696 nm/I-475 nm) and the optimal relative sensitivity (S-r) reaches 2.79 % K- 1 (503 K). More interestingly, the sample gives rise to a gradual emission color change from blue to purplish-red with lifting temperature. Our results offer a new class of upconversion-based nonoprobe with high thermal sensitivity and observable emission color variation, showing a great potential for nanoprobe and information security.