Water-soluble gold nanoparticles (3.1±0.6 nm) with polymerization ability and affinity to streptavidin are prepared by reducing HAuCl4 in the presence of the capping ligands 2-mercaptoethanesulfonic acid, biotin–cysteamine, and p-aminothiophenol. This colloid, HAuCl4, is used to modify gold electrodes by forming a 3D electropolymerized network of bis-aniline-cross-linked nanoparticles on the metal surface. The modified electrode is used as a scaffold for the assembly of an amperometric immunosensor system to detect human fibrinogen. The immunosensor shows excellent analytical characteristics, with a dynamic range of detection between 0.018 and 2.208 μg mL−1, a detection limit of 4 ng mL−1, and a half-maximal inhibitory concentration (IC50) of 177 ng mL−1. The immunosensor is markedly stable, retaining full analytical capacity after 45 days of storage at 4 °C.