Ultrabright fluorescein-labeled antibodies near silver metallic surfaces

Fluorescein-labeled antibodies are widely used in clinical assays and fluorescence microscopy. The fluorescent signal per labeled antibody is limited by fluorescein self-quenching, which occurs when the antibody is heavily labeled with multiple fluoresceins. We examined immunoglobulin G (IgG) when labeled with 0.7 to about 30 fluoresceins per antibody molecule. The extent of self-quenching was decreased, and the signal increased, when the labeled antibody was in close proximity to metallic silver particles. Time-resolved measurements showed that the intensity increase was due in part to a silver-induced increase in the radiative decay rate. These results suggest the use of labeled antibodies conjugated to silver particles as ultrabright probes for imaging or analytical applications.