Europium Complex-Loaded Albumin Nanoparticles as Probes for Time-Resolved Luminescent Immunoassay.

We report the first analytical application of albumin nanoparticles loaded with luminescent europium complexes for immunoassay development. These nanoparticles, synthesized via a desolvation method, exhibited a uniform spherical morphology with a hydrodynamic diameter of 263 nm and strong, long-lived luminescence at 615 nm (λex = 360 nm). Surface functionalization with streptavidin enabled specific binding to biotinylated proteins. The nanoparticles were applied as labels in a sandwich time-resolved solid-phase immunoassay for human IgG detection in black 96-well plates. Unlike commercial DELFIA assays, the method eliminates the need for signal enhancement steps, as the nanoparticles intrinsically contain high concentrations of europium complexes. Optimization studies revealed that the sharp emission peaks of europium can compromise assay reproducibility; however, employing surface scanning and increasing measurement replicates per well partially mitigated this effect. Time-resolved detection reduced background by two orders of magnitude and increased signal intensity nearly tenfold in IgG-positive samples. The assay demonstrated minimal cross-reactivity with IgA and IgM (~2%) and enabled IgG detection at serum dilutions up to 1:100,000. Comparative analysis showed strong concordance with commercial immunoassays and no concentration-dependent bias. The primary limitation observed was suboptimal intra-assay reproducibility (CV > 20% in four of six tested sera).