Silica-based nanohybrids containing europium complexes covalently grafted: structural, luminescent, and cell labeling investigation

Designing luminescent nanohybrids for bioimaging proposes has been explored by different approaches in the literature. In this context, here silica luminescent nanohybrids containing Eu³⁺-complexes were synthesized in three different approaches to determine the better methodology to obtain the most efficient emissive final hybrid and its applicability in cell imaging by using the Eu³⁺ luminescent probe properties. For this, the synthesized dense Stöber silica nanoparticles, SiO₂, had their surface functionalized with APTES, in which its amine group reacted with salicylaldehyde to form a Schiff base ligand (SB), yielding the SiO₂-SB system. Then, Eu³⁺ ion was coordinated to the SB, followed by the displacement of coordinated water molecules by dibenzoylmethane (dbm), resulting in the SiO₂-[Eu1] hybrid. SiO₂-[Eu2] hybrid, in turn, was obtained from tris-[Eu(dbm)₃] complexes coordinated to the imine groups grafted on the SiO₂-SB surface. For the third hybrid, SiO₂-[Eu3], a new Eu³⁺-Schiff base complex displaying a triethoxysilyl group was grafted onto the SiO₂ surface. The three luminescent hybrids are spheroidal shaped with 100 nm-size and they are red emitters with long lifetime (0.34–0.61 ms) and high photostability when exposed to continuous 340 nm UV radiation. Quantum efficiency (\(Q_{{\rm{Eu}}}^{{\rm{Eu}}}\)) as well as the number of coordinated water molecules (q_H2O) to the Eu³⁺ was estimated using the LUMPAC software package and Horrocks equation, respectively. Although the three strategies exhibited suitable photophysical results, SiO₂-[Eu1] was classified as the best hybrid considering its higher \(Q_{{\rm{Eu}}}^{{\rm{Eu}}}\) and color purity values, and it was evaluated as non-toxic according to its bio-viability in CHO-k1 cells in different doses. Exploratory cell imaging tests using such hybrid indicated cell marking near the nucleus with the internalization of nanoparticles in the cell confirmed by Eu³⁺ (⁵D₀ → ⁷F_J) narrow emission bands. Therefore, SiO₂-[Eu1] hybrid manifested suitable shape and size, optical, and biocompatibility features that make it promising to be applied as a luminescent stain for cell imaging.