Upconversion-mediated Boltzmann thermometry in double-layered Bi2SiO5:Yb3+,Tm3+@SiO2hollow nanoparticles

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 (Sr > 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 3F2,3 → 3H6 (∼700 nm) transitions. Despite the impossibility to be in thermal equilibrium due to the large energy gap between 1G4 and 3F2,3 excited states, their relative populations are demonstrated to follow the Boltzmann distribution, reflecting, through the upconversion processes, the thermalization between the 3F2,3 and 3H4 excited states. Consequently, the system features high thermal sensitivity (Sr = 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.