Fast Room-Temperature Synthesis of K₃ZrF₇:Yb³⁺/Ho³⁺ Nanocrystals for Highly Sensitive Nanothermometry

The emerge of thermoresponsive upconversion emissions opens up new possibilities for temperature sensing. However, it has remained a challenge to achieve anti-thermal quenching luminescence from fast-synthesized nanocrystals for thermometry. Here, we report a strategy to realize highly thermosensitive upconversion luminescence from K₃ZrF₇:Yb³⁺/Ho³⁺ nanocrystals that were rapidly synthesized in 1 min at room temperature, remarkably faster and simpler than the conventional synthetic methods. Interestingly, the upconverted emissions of Ho³⁺ show an efficient enhancement with an increasing temperature instead of thermal quenching, as shown in conventional phosphors. We demonstrated that the removal of the water molecules adsorbed at the surface plays a key role in enhancing Ho³⁺ upconversion by suppressing the non-radiative relaxation channels from its ⁵I₆ level in the thermal field. When we take advantage of the luminescence intensity ratio of non-thermally coupled green and red emissions, a maximum relative sensitivity of 2.53% K^-1 was achieved, which ranks in the high values for Ho³⁺-based thermometers. Our results provide a fast-synthesis and low-cost platform for highly sensitive non-contact nanothermometry toward cutting-edge applications.