Upconversion-mediated Boltzmann thermometry in double-layered Bi2SiO5:Yb3+,Tm3+@SiO2 hollow nanoparticles

Abstract

Ratiometric optical thermometry is one of the most promising techniques for contactless temperature sensing. However, despite the efforts devoted in the last decades, the development of nanothermometers characterized by high reliability along with suitable sensitivity (S_r > 1% K⁻¹) and thermal resolution (δT < 0.5 K) in the physiological temperature range is still a critical challenge in the biological field. Here, we propose uniform Yb,Tm co-doped crystalline Bi₂SiO₅@SiO₂ hollow upconverting nanoparticles as red-NIR emitting nanophosphors for ratiometric optical thermometry. The synthetic procedure leads to double-layered Bi₂SiO₅:Yb,Tm@SiO₂ hollow nanoparticles. The thermometric performances are investigated in a wide temperature range (80–800 K) demonstrating the reliability of the thermometer based on the emission ratio between the ¹G₄ → ³F₄ (∼650 nm) and ³F_2,3 → ³H₆ (∼700 nm) transitions. Despite the impossibility to be in thermal equilibrium due to the large energy gap between ¹G₄ and ³F_2,3 excited states, their relative populations are demonstrated to follow the Boltzmann distribution, reflecting, through the upconversion processes, the thermalization between the ³F_2,3 and ³H₄ excited states. Consequently, the system features high thermal sensitivity (S_r = 1.95% K⁻¹ at 300 K) and excellent thermal resolution (0.28 K at 300 K) for a highly reliable system following the Boltzmann-distribution. In addition, the superior performances of the investigated system in comparison with other NIR-to-NIR thermometers such as Nd³⁺-based ones and the biocompatibility of the NPs prove its potential in the physiological temperature range.