Boosting the sensitivity of Nd³⁺-based luminescent nanothermometers

Luminescence thermal sensing and deep-tissue imaging using nanomaterials operating within the first biological window (ca. 700–980 nm) are of great interest, prompted by the ever-growing demands in the fields of nanotechnology and nanomedicine. Here, we show that (Gd_1−xNd_x)₂O₃ (x = 0.009, 0.024 and 0.049) nanorods exhibit one of the highest thermal sensitivity and temperature uncertainty reported so far (1.75 ± 0.04% K⁻¹ and 0.14 ± 0.05 K, respectively) for a nanothermometer operating in the first transparent near infrared window at temperatures in the physiological range. This sensitivity value is achieved using a common R928 photomultiplier tube that allows defining the thermometric parameter as the integrated intensity ratio between the ⁴F_5/2 → ⁴I_9/2 and ⁴F_3/2 → ⁴I_9/2 transitions (with an energy difference between the barycentres of the two transitions >1000 cm⁻¹). Moreover, the measured sensitivity is one order of magnitude higher than the values reported so far for Nd³⁺-based nanothermometers enlarging, therefore, the potential of using Nd³⁺ ions in luminescence thermal sensing and deep-tissue imaging.