Non-Invasive Deep-Tissue Temperature Monitoring via High-Performance Optical Nanothermometer

Accurate, non-invasive temperature monitoring is vital for biomedical diagnostics and therapies, yet conventional thermometry often suffers from its invasiveness and limited tissue penetration. In this work, we present Er³⁺ and Tm³⁺ co-doped TiO₂ nanofibers as high-performance optical nanothermometers operating within near-infrared (NIR) biological windows. The materials are synthesized via a hydrothermal route and structurally confirmed by XRD, SEM coupled with EDS, and TEM analyses, showing successful incorporation of lanthanide ions without compromising TiO₂ morphology. Under 532 nm excitation, the probes exhibit dual emission bands at approximately 797 nm (Tm³⁺: ³H₄ → ³H₆) and around 1000 nm (Er³⁺: ⁴I_11/2 → ⁴I_15/2), enabling fluorescence intensity ratio (FIR)-based thermometry. Remarkably, the system demonstrates anti-thermal-quenching behavior, with emission intensity increasing with temperature due to the synergistic effects of TiO₂ host structure and energy transfer between dopants. The optimized sensor achieves an exceptional relative sensitivity of 3.59 % K^-1 at room temperature and a temperature resolution less than 1 K over the 298-398 K temperature range. Validation in intralipid tissue phantoms confirms reliable signal detection up to 17.35 mm depth, highlighting suitability for deep-tissue applications. These findings establish TiO₂ nanofibers co-doped with Er³⁺ and Tm³⁺ ions as ultrasensitive and stable optical probes, with strong potential for real-time, non-invasive thermal monitoring in biological and medical environments.