Near infrared absorbing near infrared emitting highly-sensitive luminescent nanothermometer based on Nd3+ to Yb3+ energy transfer

Abstract

A new type of near infrared absorbing near infrared emitting (NANE) luminescent nanothermometer is presented, with a physical background that relies on efficient Nd³⁺ to Yb³⁺ energy transfer under 808 nm photo-excitation. The emission spectra of LiLa_0.9−xNd_0.1Yb_xP₄O₁₂ (x = 0.05, 0.1, 0.2, 0.3, 0.5) nanocrystals were measured in a wide 100–700 °C temperature range. The ratio between the Nd³⁺ (⁴F_3/2 → ⁴I_9/2) and Yb³⁺ (²F_5/2 → ²F_7/2) luminescence bands, and the thermometer sensitivity were found to be strongly dependent on the Yb³⁺ concentration. These phenomenological relations were discussed in terms of the competition between three phenomena, namely (a) Nd³⁺ → Yb³⁺ phonon assisted energy transfer, (b) Yb³⁺ → Nd³⁺ back energy transfer and (c) energy diffusion between Yb³⁺ ions. The highest sensitivity of the temperature measurement was found for x = 0.5 (LiLa_0.4Nd_0.1Yb_0.5P₄O₁₂), which was equal to 4 × 10⁻³ K⁻¹ at 330 K. In stark contrast to conventional approaches, the proposed phosphate host matrix allows for a high level of doping, and thus, owing to the negligible concentration quenching, the presented luminophores exhibit a high absorption cross section and bright emission. Moreover, such optical remote thermometers, whose excitation and emission wavelengths are weakly scattered or absorbed and fall into the optical transmission window of the skin, may therefore become a practical solution for biomedical applications, such as remote control of thermotherapy.