Upconverting Lanthanide Fluoride Core@Shell Nanorods for Luminescent Thermometry in the First and Second Biological Windows: β-NaYF4:Yb3+– Er3+@SiO2 Temperature Sensor

Upconverting core@shell type β-NaYF 4:Yb 3+ –Er 3+ @SiO 2 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 Er 3+ and down-shifting emission of Yb 3+, 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 Er 3+ and Yb 3+ observed in the vis–near-infrared (NIR) range monotonously change with temperature, i.e., the thermalized Er 3+ levels ( 2 H 11/2 → 4 I 15/2 / 4 S 3/2 → 4 I 15/2 ) and the nonthermally coupled Yb 3+ /Er 3+ levels ( 2 F 5/2 → 2 F 7/2 / 4 I 9/2 → 4 I 15/2 or 4 F 9/2 → 4 I 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., Er 3+ 525/545 nm (max S r = 1.31% K –1 ) and Yb 3+ /Er 3+ 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 Yb 3+ /Er 3+ bands, vs. the typically used Er 3+ 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.