A novel hydrogen peroxide biosensor was fabricated that is based on horseradish peroxidase-Au nanoparticles immobilized on a viologen-modified glassy carbon electrode (GCE) by amino cation radical oxidation in basic solution. The immobilized BAPV acts as a mediator and a covalent linker between GCE and the Au nanoparticles. The biosensor exhibited fast response, good reproducibility, and long-term stability.