Temperature sensing with Er3+ doped Y2O3 nanoparticles operating within the 1st and 2nd biological window: The influence of particle size on the relative sensitivity of thermally decoupled levels

The luminescent properties of three nanocrystalline samples of Y ₂O ₃:Er ³⁺ produced via PVA-assisted sol-gel route, with mean particle diameter of 21, 44 and 86 nm, were investigated aiming their application as optical temperature sensors within the first and second biological windows. Luminescence spectra were analyzed using three different wavelengths (532 nm, 660 nm and 800 nm) as excitation sources. The three samples exhibited up to four emission bands centered at 660 nm, 800 nm, 860 nm and 1000 nm, which were related to the ⁴F _9/2 → ⁴I _15/2, ⁴I _9/2 → ⁴I _15/2, ⁴S _3/2 → ⁴I _13/2 and ⁴I _11/2 → ⁴I _15/2 transitions of Er ³⁺ , respectively. Three luminescence intensity ratio (LIR) schemes associated with thermally decoupled levels were proposed and their dependence on ambient temperature was investigated, two for excitation at 660 nm and one for excitation at 800 nm. In all the three cases, excitation and emission wavelengths were within the first or second biological window. Relative sensitivities of up to 2.02 ± 0.06% K ⁻¹ were obtained, with good repeatability and reproducibility and with temperature uncertainties below 1 K. The influence of particle size on luminescence intensity and sensor sensitivity was also presented and discussed, considering the potential applications. The results showed that particle size can affect the sensitivity of each LIR differently depending on the phonon-assisted processes involved.