Lighting up Eu³⁺ luminescence through remote sensitization in silica nanoarchitectures

Hybrid organic-inorganic luminescent lanthanide-based nanomaterials are currently attracting great interest for a variety of applications from bioimaging/sensing to optics and photonics. Herein, we present a concept model system based on purely silica-based core-shell nanoparticles (NPs), where luminescent Eu³⁺ ions are confined to a thin silica layer and are efficiently remotely photosensitized through an antenna unit covalently grafted on the surface of the outer shell. The obtained core-shell NPs, synthesized through mild sol-gel methods, are of rare quality in terms of size distribution, homogeneity and smoothness of the coating shell, the absence of core-free silica, and dispersion of the dopant phase. Convenient indirect optical pumping through the remote photosensitizer allows a remarkable intensity enhancement of the Eu³⁺-based NP luminescence by 190-fold with respect to that achievable upon direct metal excitation, yielding the highest intrinsic (Φ_Eu = 49%) and overall (Φ = 19%) quantum yields and ligand-to-metal sensitization efficiency (η_sens ∼ 40%) reported so far for Eu³⁺-based remotely sensitized organic-inorganic nanosystems. These performances are achieved thanks to the suppression of unexpected nonradiative decay channels pertaining to the silica matrix as revealed by an in-depth analysis of the temporal dynamics of Eu³⁺ emission upon direct and indirect excitation. These results show that silica matrices are a suitable highly performing host alternative to commonly investigated nanocrystals such as fluorides for the development of lanthanide-based luminescent materials with the additional potentiality of high processing versatility through well-established sol-gel chemistry methods.