Eu3+-doped titanium oxide nanoparticles for optical thermometry in the first biological window

Lanthanide-based optical nanothermometers, operating in the physiological temperature range (288-323 K), with excitation and emission in the first biological transparent window have special interest for biological applications. In this context, trivalent europium doped titanium oxide (Eu3+:TiO2) nanoparticles were prepared via a sol-gel method and their spectroscopic properties were studied. In order to assess their potential for temperature sensing, excitation and luminescence spectroscopies were performed. We observed that the intensities of the excitation bands for the F-7(0)-> D-5(0) (576 nm) and F-7(2)-> D-5(0) (610 nm) transitions, monitoring the D-5(0)-> F-7(4) (700 nm) transition have a strong dependence on temperature. This dependence, which is explained in terms of a thermal coupling between the Eu3+:F-7(J) levels, was used for the construction of an optical nanothermometer. Relative sensitivity values between 1.78 and 1.41% K-1, when the temperature of the material increases from 288 to 323 K, were obtained. We show that the nanothermometer calibration can be obtained by a single luminescence room temperature measurement. Our results indicate the potential application of Eu3+:TiO2 nanoparticles for temperature sensing in the first biological window and physiological temperature range.