Synthesis and characterization of Yb3+:YAG nanocrystals: towards the development of an absolute luminescent thermometer operating in the second biological window.

Temperature range dimensions varying from a few to many Kelvin units are useful in the areas of nanomedicine and nanotechnology. Here, Yb3+-doped Y3Al5O12 nanoparticles were synthesized by six routes using the sol-gel-modified Pechini method, and their structural and morphological features were characterized using XRD and TEM. Moreover, we report an absolute luminescent thermometer which is based on the simple Yb3+ energy level structure and can operate between 298 and 333 K. Under excitation at 940 nm and 980 nm, the 5 at% Yb3+:YAG nanoparticles exhibited emission bands at 1030, 1048, 1000, and 1006 nm in the near-infrared region, corresponding to the 2F5/2 (0') → 2F7/2 (2), 2F5/2 (0') → 2F7/2 (3), 2F5/2 ( +phonon of 172 cm-1) → 2F7/2 (E1), and 2F5/2 ( + phonon of 228 cm-1) → 2F7/2 (E1) transitions of Yb3+, respectively, lying in the second biological window (II-BW). Emissions from both these excitations show appealing qualities for thermal sensing in this range of wavelengths. Two different thermometric parameters were defined as the ratios for both the excitations: first, the ratio between the emission intensities of the 2F5/2 (0') → 2F7/2 (2) and 2F5/2 (0') → 2F7/2 (3) transitions corresponding to the 1030 and 1048 nm peaks, respectively, and second, the ratio between the emission intensities of the 2F5/2 ( +phonon of 228 cm-1) → 2F7/2 (E1) and 2F5/2 ( +phonon of 172 cm-1) → 2F7/2 (E1) transitions corresponding to the 1000 and 1006 nm peaks, respectively. Another novel and outstanding perspective is that the calibration factor can be measured from the Yb3+ emission spectrum. Hence, these nanocrystals were selected as evidence of the functional principles for nanothermometry and imaging applications in the second biological window.