Expanding NIR-IIc Nanothermometry: Architectural Control of Tm3+-doped NaGdF4 Core/Shell Nanoparticles

Near-infrared (NIR) nanothermometers are promising for biomedical applications due to reduced optical scattering and absorption of NIR light that matches the biological transparency windows when compared to UV or visible light. Yet, the exploration of nanothermometers that operate in the NIR-IIc (1700–1880 nm) and NIR-III (2080–2340 nm) spectral regions remains scarce. To address this gap, we propose a series of Tm³⁺-, Er³⁺-, and Yb³⁺ doped NaGdF₄ core/shell/shell nanoparticles dispersed in toluene for double-band ratiometric nanothermometry operating in the NIR-IIc region. The influence of the doping concentration of activator and sensitizer ions Tm³⁺ and Yb³⁺ on the Er³⁺ and Tm³⁺ emissions has been systematically investigated. The maximal S_r value based on the Tm³⁺³F₄ → ³H₆ (1850 nm) and Er³⁺⁴I_13/2 → ⁴I_15/2 (1550 nm) radiative transitions reached values as high as 2.3 % °C^-1 at 50 °C. This is significantly higher than previously reported S_r values, particularly for ratiometric NIR nanothermometers. To further explore the even longer wavelengths, Tm³⁺ and Ho³⁺ co-doped NaGdF₄ core/shell nanoparticles were designed, exhibiting emissions at 1850 nm (Tm³⁺) and 2000 nm (Ho³⁺) reaching the NIR-III window and a maximum S_r value of 0.58 % °C^-1 (T = 20 °C). These findings contribute to the establishment of design principles and a library of sought-after novel NIR optical thermal nanosensors.