Dual-functional Tm3+/Yb3+-doped LiCaLa(MoO4)3 phosphors: high-sensitivity thermal sensing and deep-tissue NIR bio-imaging

Abstract

Thermal sensing and optical bio-imaging are critical in materials science for applications ranging from biomedical diagnostics to industrial monitoring. Herein, we report the first-time synthesis of a novel Mo-based host matrix, LiCaLa(MoO₄)₃, co-doped with Tm³⁺/Yb³⁺, specifically designed for upconversion luminescence with dual optical functionalities. These phosphors were engineered to exhibit red and near-infrared (NIR) emissions for dual-functional optical sensing and imaging. The temperature-dependent NIR fluorescence (650–1000 nm) was systematically investigated to evaluate their potential as thermal sensors within the first biological window (650–950 nm), which is essential for in vivo applications due to minimal tissue absorption. Using the fluorescence intensity ratio (FIR) method under 975 nm excitation, the phosphor demonstrated exceptional thermal sensitivity in two distinct regimes: (i) a high relative sensitivity of 2.62% K⁻¹ at 298 K from thermally coupled energy levels, enabling precise low-temperature detection with sub-degree resolution, and (ii) a robust relative sensitivity of 1.4% K⁻¹ at elevated temperatures (up to 748 K) from non-thermally coupled levels, suitable for high-temperature industrial sensing. Unlike previously reported host lattices, our LiCaLa(MoO₄)₃ system integrates both temperature sensing and deep-tissue imaging capabilities into a single structure, offering a rare and efficient multifunctional solution. Furthermore, the material achieved tissue penetration depths of 4 mm, validating its dual utility for luminescence-based thermal sensing and deep-tissue bio-imaging. These findings not only highlight the unique properties of the LiCaLa(MoO₄)₃ matrix but also establish it as a pioneering multifunctional platform in the field of NIR-based nanodiagnostics.