Lanthanide-Doped Bi₂SiO₅@SiO₂ Core–Shell Upconverting Nanoparticles for Stable Ratiometric Optical Thermometry

The development of nanomaterials with high sensitivity to external stimuli such as temperature is critical to investigate the driving force of not only biological processes but also catalytic mechanisms in extreme environments. However, the instability of nano-objects at high temperatures and different environments is a serious drawback limiting often their real use. This is particularly severe in the case of bismuth-based compounds, making the development of highly stable bismuth-based nanosystems a challenge. Here, we report the synthesis of uniform crystalline lanthanide-doped Bi2SiO5 nanoparticles into a silica shell of a controlled thickness (Bi2SiO5:Ln@SiO2) for the design of a reliable ratiometric optical thermometer stable at high temperatures and extreme acid environments. The fine control of the SiO2 shell thickness is modeled based on a theoretical and experimental approach. The formation of the Bi2SiO5 single phase is triggered by the local reactivity between Bi2O3 and SiO2 in the Bi2O3@SiO2 system, leading to a double-layered Bi2SiO5@SiO2 hollow nanosystem. The potential of the Bi2SiO5:Ln@SiO2 nanosystem as a ratiometric nanothermometer is demonstrated for the upconverting Yb–Er couple. The performances were evaluated in the wide range of linearity of the Boltzmann law (280–800 K) showing suitable values of relative sensitivity, temperature uncertainty, and repeatability (R > 99%) not only for biological applications but also to probe the temperature in extreme environments. In fact, the strategy results in an acid-inert thermal probe up to pH < 1 overcoming the weakness of bismuth-based materials to acid environments with promising properties for in situ thermometry of catalytic reactions.