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 beta-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 (lambda(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 (H-2(11/2) -> I-4(15/2)/S-4(3/2) -> I-4(15/2)) and the nonthermally coupled Yb3+/Er3+ levels (F-2(5/2) -> F-2(7/2)/I-4(9/2) -> I-4(15/2) or F-4(9/2) -> I-4(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.