Twofold increase in the sensitivity of Er³⁺/Yb³⁺ Boltzmann thermometer

Luminescence thermometry is the most versatile remote temperature sensing technique and can be employed from living cells to large surfaces and from cryogenic temperatures to the melting points of metals. Ongoing research aims to optimize the sensitivity of the ratio between the emission intensity from two coupled excited states. However, this approach is inherently limited to temperature-dependent processes involving only the excited states. Here, we develop a novel measurement technique, called luminescence intensity ratio squared (LIR²) for the Yb³⁺/Er³⁺ pair, that combines the temperature sensitivity of ground- and excited-state populations. We use Y₃Al₅O₁₂:Er³⁺,Yb³⁺ nanoparticles as a promising model system with both visible and infrared emissions. To apply our method, we record two luminescence spectra at different excitation wavelengths and determine the LIR² using one emission in each of the two spectra. The LIR² testing with Y₃Al₅O₁₂ nanoparticles showed a sensitivity increase of 70% in the visible region and an impressive 230% increase in the NIR region compared to the conventional LIR method. This enhances the measurement precision by a factor of 1.5-2.5. The LIR² based on the visible upconversion emission is particularly useful for measurements of high temperatures, while the LIR² based on the downshifted ∼1.5 μm emission may revolutionize temperature measurements of biological samples in the range of physiological temperatures.