Optical nanothermometry under multiphonon assisted anti-Stokes excitation operating in the biological windows

In this work, we report a potential nanothermometer with high thermal sensitivity based on sodium niobate (NaNbO3) nanocrystals doped and co-doped with Tm3+ and Tm3+/Yb3+, respectively, synthesized by the Pechini (sol-gel) method. Under anti-Stokes excitation (1064 nm) it was investigated the fluorescence intensity ratio (FIR = I-700nm/I-660nm) of the 700 nm (F-3(2,3) -> H-3(6)) and 660 nm ((1)G(4) -> F-3(4)) emissions, which are within the first biological window, as a function of temperature in the 300-493 K range. Relative thermal sensitivities constant with the temperature were obtained, with values of 1.60%K-1 and 1.86%K-1 for NaNbO3:1%Tm3+ and NaNbO3:1%Tm3+/Yb3+ samples, respectively, which is the highest value in the literature for Tm3+/Yb3+ co-doped system under 1064 nm excitation. The importance of the anti-Stokes excitation relies on the fact that emissions (up and downconversion) in general increase with the temperature, making possible obtaining fluorescence and thermal images with better resolutions. Additionally, as both excitation and emission are within the biological windows, these nanoparticles might be useful for biological applications. (C) 2019 Elsevier B.V. All rights reserved.