Fig. 1: Schematic diagram of the temperature detection mechanism of the nanocomposite.

Lanthanide ions doped nanocomposite as bidirectional ratiometric nanothermometer (NaErF₄:Yb@NaYF₄:Yb, abbreviated as LIBRA) with core-shell structure is developed for temperature imaging in vivo. Under 808 nm excitation, LIBRA generates 1550 nm emission caused by ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ions and 980 nm emission derived from ²F_5/2 → ²F_7/2 transition of Yb³⁺ ions and ⁴I_11/2 → ⁴I_15/2 transition of Er³⁺ ions. Upon temperature elevation, the quenching effects of H₂O stretching vibration transitions on the ⁴I_11/2 state of Er³⁺ and ²F_5/2 state of Yb³⁺ are promoted that induce the decrement of 980 nm emission and increment of 1550 nm emission of LIBRA. This process is utilized for temperature detection in aqueous environment. With the doping of Yb³⁺ in the active shell layer, energy migration among lanthanide ions is achieved to improve 1550 nm emission intensity and enhance thermal sensitivity.