Nd³⁺ doped Gd₃Sc₂Al₃O₁₂ nanoparticles: towards efficient nanoprobes for temperature sensing

Development of contactless temperature-probing nanoplatforms based on thermosensitive near-infrared (NIR) light-emitting nanoparticles opens up new horizons for biomedical theranostics at a deep tissue level. Here, we report on the crystallinity and relative thermal sensitivity of NIR emitting Nd³⁺ doped Gd₃Sc₂Al₃O₁₂ (GSAG:Nd³⁺) nanoparticles synthesized by a solvothermal method. The obtained nanoparticles are well-crystallized, with sizes less than 100 nm, and can be dispersed in water without any additional functionalization. Upon excitation at 806 nm, the nanoparticles exhibit emission in the first and second biological optical transparency windows. The temperature sensing properties were evaluated from the luminescence intensity ratio of the thermally coupled emission lines corresponding to the R₁, R₂→Z₅ transitions between the Stark sublevels of the ⁴F_3/2 and ⁴I_9/2 electronic states of Nd³⁺ in the physiological temperature range of 20-50 °C. GSAG:Nd³⁺ nanoparticles exhibit a maximal relative thermal sensitivity of 0.20% °C^-1, higher than that of YAG:Nd³⁺ nanoparticles used as a control, due to the difference in the crystal field of the host matrices. A higher synthesis temperature in the range of 300-400 °C was also provided to improve the crystallinity of the GSAG:Nd³⁺ nanoparticles which results in a higher relative thermal sensitivity. Our results demonstrate the potential of GSAG:Nd³⁺ nanoparticles as luminescence nanothermometers and emphasize the interest of the GSAG matrix itself, which with the presence of Gd, could lead to multimodal diagnostic applications in nanothermometry and magnetic resonance imaging (MRI).