Advanced optical thermometry for in vitro applications: Leveraging Nd3+;Er3+ co-doped TiO2 nanorods for high-sensitivity, non-invasive temperature sensing

Luminescence thermometry, which leverages fluorescence-emitting nanomaterials, is crucial for a wide range of applications. The approach detailed in this study marks a significant synthesis of a high-performance radiometric nanothermometer. TiO 2 nanorods were synthesized at first via a sonication process combined with hydrothermal treatment. For optical temperature sensing, particularly within the first biological optical window, TiO 2 nanorods were co-doped with Er 3+ and Nd 3+ ions. The incorporation of rare earth ions into the host matrix was confirmed by XRD, while SEM showed a well-defined nanorod morphology further confirmed by TEM. The optical characterization under 532 nm laser excitation revealed distinct emission peaks corresponding to Er 3+ and Nd 3+ transitions in the 600-1100 nm spectral range, with a strong intensity variation as the temperature increased from 298 K to 398 K. The fluorescence intensity ratio (FIR) of the emission peaks at 810 nm and 900 nm was used to develop a ratiometric temperature sensor, showing a maximum relative sensitivity of 2.81% K -1 at room temperature with a temperature resolution of 0.39 K. Additionally, the luminescent material's performance was evaluated with an intralipid 5% solution as an optical tissue phantom, demonstrating its potential for in vitro applications. These findings highlight the potential of TiO 2 :Er 3+ ;Nd 3+ nanorods as a promising material for non-invasive, high-sensitivity optical temperature sensing in biological and medical contexts.