All near-infrared multiparametric luminescence thermometry using Er³⁺, Yb³⁺-doped YAG nanoparticles

This paper presents four new temperature readout approaches to luminescence nanothermometry in spectral regions of biological transparency demonstrated on Yb³⁺/Er³⁺-doped yttrium aluminum garnet nanoparticles. Under the 10 638 cm^-1 excitation, down-shifting near infrared emissions (>10 000 cm^-1) are identified as those originating from Yb³⁺ ions' ²F_5/2 → ²F_7/2 (∼9709 cm^-1) and Er³⁺ ions' ⁴I_13/2 → ⁴I_15/2 (∼6494 cm^-1) electronic transitions and used for 4 conceptually different luminescence thermometry approaches. Observed variations in luminescence parameters with temperature offered an exceptional base for studying multiparametric temperature readouts. These include the temperature-dependence of: (i) intensity ratio between emissions from Stark components of Er^{3+ 4}I_13/2 level; (ii) intensity ratio between emissions of Yb³⁺ (²F_5/2 → ²F_7/2 transition) and Er³⁺ (⁴I_13/2 → ⁴I_15/2 transition); (iii) band shift and bandwidth and (iv) lifetime of the Yb³⁺ emission (²F_5/2 → ²F_7/2 transition) with maximal sensitivities of 1% K^-1, 0.8% K^-1, 0.09 cm^-1 K^-1, 0.46% K^-1 and 0.86% K^-1, respectively. The multimodal temperature readout provided by this material enables its application in different luminescence thermometry setups as well as improved the reliability of the temperature sensing by the cross-validation between measurements.