Upconverting Lanthanide Fluoride Core@Shell Nanorods for Luminescent Thermometry in the First and Second Biological...

Upconverting core@shell type β-NaYF₄:Yb³⁺-Er³⁺@SiO₂ nanorods have been obtained by a two-step synthesis process, which encompasses hydrothermal and microemulsion routes. The synthesized nanomaterial forms stable aqueous colloids and exhibits a bright dual-center emission (λ_ex = 975 nm), i.e., upconversion luminescence of Er³⁺ and down-shifting emission of Yb³⁺, located in the first (I-BW) and the second (II-BW) biological windows of the spectral range, respectively. The intensity ratios of the emission bands of Er³⁺ and Yb³⁺ observed in the vis-near-infrared (NIR) range monotonously change with temperature, i.e., the thermalized Er³⁺ levels (²H_11/2 → ⁴I_15/2/⁴S_3/2 → ⁴I_15/2) and the nonthermally coupled Yb³⁺/Er³⁺ levels (²F_5/2 → ²F_7/2/⁴I_9/2 → ⁴I_15/2 or ⁴F_9/2 → ⁴I_15/2). Hence, their thermal evolutions have been correlated with temperature using the Boltzmann type distribution and second-order polynomial fits for temperature-sensing purposes, i.e., Er³⁺ 525/545 nm (max S_r = 1.31% K^-1) and Yb³⁺/Er³⁺ 1010/810 nm (1.64% K^-1) or 1010/660 nm (0.96% K^-1). Additionally, a fresh chicken breast was used as a tissue imitation in the performed ex vivo experiment, showing the advantage of the use of NIR Yb³⁺/Er³⁺ bands, vs. the typically used Er³⁺ 525/545 nm band ratio, i.e., better penetration of the luminescence signal through the tissue in the I-BW and II-BW. Such nanomaterials can be utilized as accurate and effective, broad-range vis-NIR optical, contactless sensors of temperature.