Improved thermometry via luminescence lifetime ratio of Er³⁺ and Tm³⁺ operated in the biological window

The luminescent intensity ratio of Lanthanide-doped nanocrystals has been widely used for non-contact thermometry but is still facing difficulties in biological applications due to spectrum distortion caused by tissues. Lifetime-based thermometry is the best alternative to other typical thermometry methods. However, its main drawback is the limited sensitivity. This paper introduces an innovative method known as luminescence lifetime ratio, which enhances luminescence thermometry by combining two emission bands with opposite decay behaviors with temperature. As a proof of concept, a core-shell structure separately containing Tm³⁺ and Er³⁺ was synthesized and exhibited two strong emission bands centered at 800 nm and 1530 nm, which are located in the NIR-I and NIR-III biological windows. The commonly used LIR thermometer, an Er/Yb co-doped NaYF₄ sample, was also synthesized for comparison. By using chicken tissues of varying thicknesses (1 mm and 3 mm), it was revealed that the deep-tissue penetration and accuracy in biological environments of luminescence lifetime ratio are evidently improved than the widely-used technique of luminescence intensity ratio, although the relative sensitivity of is not much better. In summary, the luminescence lifetime ratio technique enables novel and more accurate temperature sensing within the wavelength range that is suitable for biological applications.