In this work, a unique zirconium dioxide solid superacid (ZrO₂/SO₄²⁻) was utilized for the fabrication of an IgG electrochemical immunosensor. The treated ZrO₂ formed a SO bond on the surface, which has the electron-attracting induction effect and can further enhance the quantity and stability of immobilized biomolecules. In addition, using a platinum nanoparticle–chitosan–reduced graphene oxide (PtNP–CHIT–rGO) nanocomposite as a substrate modification material not only improves the conductivity of the electrode, but also increases the effective surface area of the immunosensing platform. The proposed label-free immunosensor has a lower detection limit of 11.0 pg mL⁻¹ and a wide linear range of 0.01–10.0 ng mL⁻¹ and 10.0–4000.0 ng mL⁻¹. Importantly, the immunosensor exhibits good selectivity, excellent stability and reproducibility, and is expected to be applied to the detection of other biomolecules.