Upconverting Lanthanide Fluoride Core@Shell Nanorods for Luminescent Thermometry in the First and Second Biological Windows: β-NaYF:Yb- Er@SiO Temperature Sensor.

Upconverting core@shell type β-NaYF:Yb-Er@SiO 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 = 975 nm), i.e., upconversion luminescence of Er and down-shifting emission of Yb, 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 and Yb observed in the vis-near-infrared (NIR) range monotonously change with temperature, i.e., the thermalized Er levels (H → I/S → I) and the nonthermally coupled Yb/Er levels (F → F/I → I or F → I). 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 525/545 nm (max S = 1.31% K) and Yb/Er 1010/810 nm (1.64% K) or 1010/660 nm (0.96% K). 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/Er bands, vs. the typically used Er 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.