1.319 μm excited thulium doped nanoparticles for subtissue thermal sensing with deep penetration and high contrast imaging

We report on the potential application of NIR-NIR two-photon excited fluorescence of Tm3+ doped sodium niobate (NaNbO3:Tm) nanoparticles with both emission (800 nm) and excitation (1.319 μm) operating respectively within first and second biological windows for high resolution and deeper penetration bio-images. Ex vivo tissue depth using 1.319 μm was achieved as high as twice in comparison with the already conventional 800 nm excitation. The spectral profile of Tm3+ upconversion luminescence was analyzed in detail in the biophysical temperature range. A twofold enhancement of the solely detected 800 nm upconversion emission was obtained in the 22-90 °C temperature interval. The population redistribution of the thermally coupled sub-levels was demonstrated useful for ratiometric nanothermometry. The outstanding penetration depth together with the fluorescence thermal sensitivity make the NaNbO3:Tm nanocrystal an excellent candidate as multifuncitional nanoprobe for high contrast and highly penetrating fluorescence imaging.