Abstracts

Red light-emitting contrast agents have been developed for applications in bioimaging, biosensing, and thermometry, based on europium complexes, however, their low water solubility and photostability issues limit their application, which can be overcome by their covalent anchoring on silica nanoparticles (SNP). Yet, the bioapplication depends on several factors, with the surface charge being a key factor in the toxicity and internalization of nanoparticles by cells. Herein, we evaluated the modulation of the surface charge on SNP by anchoring Eu-complexes of different structures, combining the ligands pamba (4-(Aminomethyl)benzoic acid) and tta (2-thenoyltrifluoroacetone), a carboxylic acid and a b-diketone, respectively, and their impact on the toxicity of the hybrids towards Huh 7.5 cells. For this, 50 nm-SNPs decorated with carboxylic acid (COOH) were produced, then, Eu³⁺ ions were coordinated to the carboxylate groups, and their coordination sphere was completed using pampa or tta, separately or combined, resulting in three samples, NPS‑[Eu(COO^-)(pamba)_x(H₂O)_y] (hybrid A), NPS‑[Eu(COO^-)(tta)_x(H₂O)_y] (hybrid B), and NPS‑[Eu(COO^-)(pamba)_x(tta)_z(H₂O)_y] (hybrid C). The ligands sensitized the europium ion, which then emits in the red region with high color purity. Hybrids B and C showed the best, and similar, photophysical properties such as higher excited state lifetimes and intrinsic quantum yields, indicating that tta works as a better sensitizer for Eu³⁺ ions than pamba. The surface charge of the SNPs changed according to the charge of the complexes, with zeta potential values of +18, ‑13, and ‑39 mV observed for hybrids A, C, and B, respectively. The presence of the NH₃⁺ group in pamba induces positive charge values in hybrid A while tta, which contains a negative charge, induces the opposite effect in hybrid B. The combination of the two ligands produced an intermediate charge in hybrid C. The change in charge directly influenced the toxicity of the hybrids analyzed through the MTT assay. The more negative the surface charge, the more toxic the hybrid was. IC₈₀ values of 3.13, 25, and 50 µg/mL were obtained for hybrids B, C, and A, respectively. Although europium complexes are promising in bioimaging assays, the application of their hybrids must be carefully investigated, as the surface charge plays an important role in the toxicity of the materials.