Development of red-luminescent hybrids as contrast agents for cell imaging: A correlation among surface, luminescence, and biological properties

TTA (2-thenoyltrifluoroacetone), DBM (dibenzoylmethane), and EPHEN (5,6-Epoxy-5,6-dihydro- [1,10] phenanthroline) have generated europium complexes with excellent red emission properties. However, for bio applications, these complexes have been grafted on the surface of silica nanoparticles (SNP), by different approaches, due to their low water solubility. Here, we report how three approaches influence the surface, luminescent, and biological properties of 50 and 170 nm-SNP functionalized with carboxylic acid or phenanthroline ligands, useful to anchor TTA and DBM-Eu-complexes. TTA-hybrids exhibited better photophysical properties over the DBM ones, although intermolecular interactions between TTA and silica surface groups are likely to influence their properties. Aiming at such biological application, approach 3 resulted in the luminescent PHEN-hybrids with the highest values of intrinsic (φEuEu) and absolute quantum yields (φEuL), color purity of 100%, and high sensibilization efficiency (η). However, depending on the approach used, different surface charges were observed for the hybrids that directly impacted their cytotoxicity in Huh 7.5 cells. The larger the size of the nanoparticle or the more positive its surface charge is, the lower the toxicity of the hybrid. Even considering the presence of the protein corona effect, estimated in DMEM medium, the initial surface charge, observed in the buffer solution, plays an important role in the nanoparticle-cell interaction. Confocal microscopy images revealed that the hybrids were internalized by cells regardless of their size and surface charge, and their luminescence was also detectable, making them promising candidates for cell imaging applications as luminescent contrast agents.