The lanthanide binuclear helicate [Ln(2)(L-C2)(3)] has been embedded into bare and NH2-functionalized silica nanoparticles (NPs) using water-in-oil microemulsion technique. TEM analysis reveals both [Ln(2)(L-C2)(3)]@SiO2 and [Ln(2)(L-C2)(3)]@SiO2/NH2 nanoparticles having a spherical morphology and being monodispersed with an average size of 55 +/- 5 and 90 +/- 10 nm, respectively. The energy of the ligand triplet state, similar to 21 800 cm(-1) ([Gd-2(L-C2)(3)]@NP), does not change upon incorporation into silica nanoparticles and is optimal for sensitizing Eu-III luminescence. As a consequence, [Eu-2(L-C2)(3)]@SiO2 and [Eu-2(L-C2)(3)]@SiO2/NH2 NPs display red emission due to characteristic D-5(0) > F-7(J) (J = 0-4) transitions with absolute quantum yield reaching 28% for the latter. NH2-functionalized NPs have then been conjugated with avidin (NP-avidin) or goat anti-mouse IgG antibody (NP-IgG) to test them as luminescent biomarkers. Time-resolved microscopy of immunocytochemical assays involving recognition of mucin-like proteins expressed on breast cancer MCF-7 cells by the 5D10 monoclonal antibody confirms that the NP-IgG bioprobe displays specific luminescent signal with signal-to-noise ratio approximate to 20% higher than the one obtained for the bioconjugate of molecular [Eu-2(L-C2(COOH))(3)] with IgG. In addition, immunoassays using a streptavidin-coated plate and the NP-IgG probe are able to detect 15 ng mL(-1) of the biotinylated 5D10 antibody with a signal-to-noise ratio of 100.