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% K^-1 at 25.0 °C, in the range of the best ones reported to date for Ln³⁺-based nanothermometers based on upconversion emission. The present nanoparticle design allows us to exploit upconversion of lanthanide ions that otherwise cannot be directly excited upon NIR excitation and that may provide operational wavelengths with a highly stable read out to fill the spectral gaps currently existing in upconversion-based nanothermometry.