Multifunctional Tm3+/Yb3+:Na5Y (MoO4)4 nanocrystals for simultaneous luminescent nanothermometry, photothermal therapy and deep bioimaging in the NIR-I window

Abstract

In this study, Tm³⁺/Yb³⁺ co-doped Na₅Y(MoO₄)₄ (NYMO) nanoparticles were successfully synthesized via the sol–gel method, and their structural, morphological, upconversion (UC) luminescence, and photothermal properties were comprehensively investigated. Upon 975 nm near-infrared (NIR) laser excitation, the NYMO nanoparticles exhibited two prominent UC emission bands at 477 nm and 796 nm, corresponding to the ¹G₄ → ³F₆ and ³H₄ → ³H₆ transitions of Tm³⁺ ions, respectively. The fluorescence intensity ratio (FIR) between these emissions enabled precise optical temperature sensing; at 298 K, the highest relative sensitivity is 2.05% K⁻¹, and at 388 K, the highest absolute sensitivity is 0.53% K⁻¹. The FIR value increased with excitation power, indicating a rise in internal temperature due to photothermal effects. This temperature increase is attributed to efficient NIR absorption and nonradiative relaxation processes, enhanced by electron–phonon interactions at elevated temperatures. Furthermore, the nanoparticles demonstrated significant NIR emission penetration into biological tissue up to a depth of 4 mm, confirming their suitability for deep-tissue imaging. Operating within the optically transparent biological window, the Tm³⁺/Yb³⁺-doped NYMO nanoparticles exhibit multifunctional capabilities, combining optical thermometry, photothermal conversion, and in vivo bioimaging. These results highlight NYMO nanoparticles as promising candidates for luminescent nanothermometers and photothermal agents in biomedical applications.