Investigating the Thermometric Performance of Inorganic Materials Doped with Nd3+ under Infrared LED Excitation: An Alternative for Deep Tissue Luminescent Thermometry

Abstract

Luminescent thermometers based on the luminescence intensity ratio between two thermally coupled levels have a strong appeal in the biomedical area due to the possibility of monitoring the temperature of deep tissues. In such procedures, it is necessary that the excitation and emission wavelengths are within the biological windows. Probes based on neodymium luminescence, with excitation and emission around 800 and 880 nm, are frequently proposed but have low relative sensitivity (0.2%.K−1) due to the small energy separation between the explored Stark sublevels. By changing the excitation wavelength to around 750 nm, it is possible to explore the thermal coupling between the 4F5/2 and 4F3/2 levels. However, lasers in this wavelength range are not common. An alternative is to use LEDs as an excitation source. As a proof of concept, we investigated the thermometric performance of three distinct Nd-doped luminescent probes under 730 nm LED excitation and 532 nm laser excitation: nanocrystalline Y2O3, LiBaPO4 microcrystals, and lithium-boron-aluminum (LBA) glass. The results indicated that the use of LEDs as an excitation source can be applied in nano-, micro- and macro-structured probes, as it does not compromise the thermometric performance of the systems, which exhibited relative sensitivities of approximately 2%.K−1.