Hybrid multifunctionalized mesostructured stellate silica nanoparticles loaded with β-diketonate Tb³⁺/Eu³⁺ complexes as efficient ratiometric emissive thermometers working in water

Despite the great effort made in recent years on lanthanide-based ratiometric luminescent nanothermometers able to provide temperature measurements in water, their design remains challenging. We report on the synthesis and properties of efficient ratiometric nanothermometers that are based on mesoporous stellate nanoparticles (MSN) of ca. 90 nm functionalized with an acetylacetonate (acac) derivative inside the pores and loaded with β-diketonate-Tb³⁺/Eu³⁺ complexes able to work in water, in PBS or in cells. Encapsulating a [(Tb/Eu)₉(acac)₁₆(μ₃-OH)₈(μ₄-O)(μ₄-OH)] complex (Tb/Eu ratio = 19/1 and 9/1) led to hybrid multifunctionalized nanoparticles exhibiting a Tb³⁺ and Eu³⁺ characteristic temperature-dependent luminescence with a high rate Tb³⁺-to-Eu³⁺ energy transfer. According to theoretical calculations, the modifications of photoluminescence properties and the increase in the pairwise Tb³⁺-to-Eu³⁺ energy transfer rate by about 10 times can be rationalized as a change of the coordination number of the Ln³⁺ sites of the complex from 7 to 8 accompanied by a symmetry evolution from C_s to C_4v and a slight shortening of intramolecular Ln³⁺-Ln³⁺ distances upon the effect of encapsulation. These nanothermometers operate in the 20-70 °C range with excellent photothermal stability, cyclability and repeatability (>95%), displaying a maximum relative thermal sensitivity of 1.4% °C^-1 (at 42.7 °C) in water. Furthermore, they can operate in cells with a thermal sensitivity of 8.6% °C^-1 (at 40 °C), keeping in mind that adjusting the calibration for each system is necessary to ensure accurate measurements.