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

• Published in: ACS applied materials & interfaces Vol. 11; no. 14; pp. 13389 - 13396
• Main Authors: Runowski, Marcin, Stopikowska, Natalia, Szeremeta, Daria, Goderski, Szymon, Skwierczyńska, Małgorzata, Lis, Stefan
• Format: Journal Article
• Language: English
• Published: American Chemical Society 10.04.2019
• Subjects: energy transfer; optical thermometer; upconversion luminescence; luminescence intensity ratio (LIR); functional nanomaterials; rare earth ions
• ISSN: 1944-8244; 1944-8252
• DOI: 10.1021/acsami.9b00445

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 (λex = 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 (2H11/2 → 4I15/2/4S3/2 → 4I15/2) and the nonthermally coupled Yb3+/Er3+ levels (2F5/2 → 2F7/2/4I9/2 → 4I15/2 or 4F9/2 → 4I15/2). 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.