Ratiometric luminescent thermometry for the third bioimaging window by Er³⁺ doped garnet with large Stark splitting

Based on the ⁴I_13/2 → ⁴I_15/2 emission (1450–1700 nm) of Er³⁺ in garnet, which is matching well with third bioimaging window (NIR-III), we propose a ratiometric near-infrared (NIR) thermometer (YAGG:Ce-Er) in NIR-III by utilizing thermal coupling between Stark levels of ⁴I_13/2, where energy levels were split into seven sublevels (I–VII). The energy gaps between Stark sublevel VII and I (ΔE_VII-I) and V and I (ΔE_V-I) of the ⁴I_13/2 level, calculated from the temperature dependence of intensity ratios of the two selected thermal couples, corresponded well with the theoretical values. This agreement supports the analysis that the system can be utilized as a Boltzmann thermometry. Moreover, the relative sensitivity (S_r) within the temperature range commonly encountered in organisms (25–60 °C) reached a value of 0.63 ± 0.03% K⁻¹ for the VII/I ratio and 0.57 ± 0.07% K⁻¹ for the V/I ratio. These high relative sensitivity values indicate the strong potential for practical application in various biological fields.