Luminescent nanothermometry using short-wavelength infrared light

Highlights: • Analysis of SWIR emissions of lanthanide ions for luminescence thermometry. • Tm³⁺ and Ho³⁺ more suitable than Er³⁺ for this purpose. • Thermal sensitivities achieved comparable to those in the visible and higher than those in the NIR biological windows. • Potentiality of SWIR emissions for luminescence thermometry and imaging in ex vivo experiments. • Especial importance for biological tissues containing melanin, not considered yet in luminescence thermometry. We analyzed the potentiality of the short-wavelength infrared (SWIR) emissions of different lanthanide ions (Er³⁺, Tm³⁺ and Ho³⁺) embedded in different hosts for luminescence thermometry. The 1.55 μm emission band generated by Er³⁺ has different Stark sub-levels that can be used in temperature sensing purposes. However, the thermal sensitivity that can be achieved with this emission is relatively low, ranging from 0.06 to 0.15% K⁻¹. In the case of Tm³⁺, the emissions arising from the ³F₄ and ³H₄ electronically coupled energy levels are useful for luminescence thermometry, with a linear evolution for the intensity ratio in the biological range as the temperature increases, which simplifies the calibration procedure for luminescent thermometers based on this parameter. When co-doped with Ho³⁺, an efficient energy transfer between the Tm³⁺ and Ho³⁺ ions is generated, that results in a new emission line centered at 1.96 μm that can be also used for luminescence thermometry purposes, with an enhanced thermal sensitivity when pumped at 808 nm. The thermal sensitivities achieved with these doping ions are higher than those obtained with Er³⁺ and are comparable to those reported previously for some lanthanide-doped materials operating in the visible, and in the I- and II-BWs. We demonstrated the potentiality of these emissions in the SWIR region for luminescence thermometry and imaging in ex-vivo experiments by monitoring the increase of temperature induced in chicken breast meat, with an experimental thermal resolution of ∼0.5 K, below the theoretical value of 0.8 K predicted for particles operating in this spectral region, and a penetration depth of at least 0.5 cm.