Engineering efficient upconverting nanothermometers using Eu3+ ions

Upconversion nanothermometry combines the possibility of optically sensing temperatures in very small areas, such as microfluidic channels or on microelectronic chips, with a simple detection setup in the visible spectral range and reduced heat transfer after near-infrared (NIR) excitation. We propose a ratiometric strategy based on Eu3+ ion luminescence activated through upconversion processes. Yb3+ ions act as a sensitizer in the NIR region (980 nm), and energy is transferred to Tm3+ ions that in turn excite Eu3+ ions whose luminescence is shown to be thermally sensitive. Tridoped SrF2:Yb3+,Tm3+,Eu3+ nanoparticles (average size of 17 nm) show a relative thermal sensitivity of 1.1%