Multiphonon assisted upconversion thermal enhancement for optical temperature sensing and high penetration depth

Thermal quenching that the upconversion (UC) luminescence intensities decrease with increasing temperature limits the application of UC luminescence materials in the field of optical temperature sensing. Herein, we report that Tm³⁺/Yb³⁺ doped Gd₂O₃ phosphors achieve thermal enhancement of UC luminescence with the multiphonon assisted process. Significantly, a possible mechanism of Yb³⁺ ions in thermal enhancement and multiphonon assisted UC luminescence process is proposed. Based on the luminescence intensity ratio technique of non-thermally coupled energy levels, research shows that thermal enhancement can effectively improve the optical temperature sensing absolute sensitivity. Owing to the near-infrared excitation and strong near-infrared emission, the UC luminescence of the Gd₂O₃: Tm³⁺/Yb³⁺ phosphors can penetrate 12 mm pork tissue and achieve UC thermal enhancement in 287-314 K after penetrating 6 mm pork tissue, which shows its potential in vivo application. The results not only provide a pathway to realize the thermal enhancement of UC luminescence and the improvement of the temperature sensing sensitivity, but also promote the understanding and utilization of the UC luminescence thermal enhancement.