Highly-sensitive Eu3+ ratiometric thermometers based on excited state absorption with predictable calibration

Abstract

Temperature measurements ranging from a few degrees to a few hundreds of Kelvin are of great interest in the fields of nanomedicine and nanotechnology. Here, we report a new ratiometric luminescent thermometer using thermally excited state absorption of the Eu³⁺ ion. The thermometer is based on the simple Eu³⁺ energy level structure and can operate between 180 and 323 K with a relative sensitivity ranging from 0.7 to 1.7% K⁻¹. The thermometric parameter is defined as the ratio between the emission intensities of the ⁵D₀ → ⁷F₄ transition when the ⁵D₀ emitting level is excited through the ⁷F₂ (physiological range) or ⁷F₁ (down to 180 K) level. Nano and microcrystals of Y₂O₃:Eu³⁺ were chosen as a proof of concept of the operational principles in which both excitation and detection are within the first biological transparent window. A novel and of paramount importance aspect is that the calibration factor can be calculated from the Eu³⁺ emission spectrum avoiding the need for new calibration procedures whenever the thermometer operates in different media.