NIR luminescence lifetime nanothermometry based on phonon assisted Yb3+–Nd3+ energy transfer

Abstract

Luminescence thermometry in biomedical sciences is a highly desirable, but also highly challenging and demanding technology. Numerous artifacts have been found during steady-state spectroscopy temperature quantification, such as ratiometric spectroscopy. Oppositely, the luminescence lifetime is considered as the most reliable indicator of temperature thermometry because this luminescent feature is not susceptible to sample properties or luminescence reabsorption by the nanothermometers themselves. Unfortunately, this type of thermometer is much less studied and known. Here, the thermometric properties of Yb³⁺ ions in Nd_0.5RE_0.4Yb_0.1PO₄ luminescent temperature probes were evaluated, aiming to design and optimize luminescence lifetime based nanothermometers. Temperature dependence of the luminescence lifetimes is induced by thermally activated phonon assisted energy transfer from the ²F_5/2 state of Yb³⁺ ions to the ⁴F_3/2 state of Nd³⁺ ions, which in turn is responsible for the significant quenching of the Yb³⁺:²F_5/2 lifetime. It was also found that the thermal quenching and thus the relative sensitivity of the luminescent thermometer can be intentionally altered by the RE ions used (RE = Y, Lu, La, and Gd). The highest relative sensitivity was found to be S_R = 1.22% K⁻¹ at 355 K for Nd_0.5Y_0.4Yb_0.1PO₄ and it remains above 1% K⁻¹ up to 500 K. The high sensitivity and reliable thermometric performance of Nd_0.5La_0.4Yb_0.1PO₄ were confirmed by the high reproducibility of the temperature readout and the temperature uncertainty being as low as δT = 0.05 K at 383 K.