Comparison of YAG:Nd3+-Yb3+ nanothermometers synthesized by Pechini and solvothermal methods

Luminescence intensity ratio-based nanothermometry is a widely studied thermal sensing technique in the literature. Regarding biological purposes, it is essential to have thermal probes that are efficient in this type of environment. Thermal bioprobes demand highly crystallized nanocrystals, commonly smaller than 100 nm, with luminescence emissions in the near-infrared range that are not significantly absorbed by biological tissues. Several nanomaterials that have been studied for nanothermometry do not meet the requirements for this type of applications. Accordingly, researches are needed to develop suitable and reliable nanothermometers for thermal sensing. Therefore, our goal was to investigate the impact of Nd³⁺-Yb³⁺ co-doping on the thermometric performance of YAG matrix, a promising crystal because it presents a host structure favoring the insertion of lanthanide ions, which provide its luminescent features. In order to achieve this purpose, we first synthesized YAG:Nd³⁺-Yb³⁺ nanocrystals through a generic route - called modified Pechini method - to screen their thermal properties. Our results show that YAG:Nd³⁺-Yb³⁺ nanocrystals have the potential to work in vivo environments. The nanothermometers investigated here are excited in the first biological window (BW-I) at 805 nm with luminescence emissions within the BW-II, at 1030.5 and 1064 nm. By co-doping the YAG matrix with different Nd³⁺-Yb³⁺ concentrations, we studied the energy transfer process between the dopant ions and their impact on thermometry efficiency. By the efficient coupling of the Nd³⁺-Yb³⁺ pair, we improved the $S_r$ value by a factor of 3 of YAG compounds up to 0.6 %.K⁻¹. We then synthesized YAG:Nd³⁺-Yb³⁺ nanocrystals using a second type of synthesis, by solvothermal means, in order to obtain individual nanocrystals, well dispersed in aqueous solutions, and to adapt their morphology and size for biological purposes. Therefore, we compared the structural and luminescence properties and thermometry efficiencies of YAG:Nd³⁺-Yb³⁺ nanocrystals obtained through two distinct processes and showed that the nanothermometry properties are not affected by the synthesis method.