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

•NaNbO3:Tm and NaNbO3:Tm/Yb as nanothermometers operating within biological windows.•Enhanced emission with temperature rising making possible high-resolution imaging.•Relative thermal sensitivity independent of the temperature in the 300–493 K range.•Nanocrystals as high-resolution multi-photon excited fluorescent nanothermometers.•Nanoprobe for high contrast and highly penetrating fluorescent nanothermometers.