A novel capacitive immunosensor for transferrin detection based on ultrathin alumina sol–gel-derived films and gold nanoparticles

A novel capacitive immunosensor was successfully developed for the direct detection of transferrin based on an ultrathin γ-alumina sol–gel-derived film and gold nanoparticles. The thin film was formed and air-dried by dripping Al2O3 sol with a microliter syringe on a gold electrode modified with a self-assembled mercaptoacetic acid (MAA) monolayer. After defects in the film were blocked with a long-chain alkylthiol, gold nanoparticles were deposited on the surface of the modified film via a potential step from +1.1 to 0.0 V (versus Ag|AgCl|KCl) for 15 s in a 0.5 M H2SO4 solution containing 0.1 mM HAuCl4. Finally, the antibody was immobilized on the gold nanoparticles under the optimized experimental conditions. The capacitive sensor prepared by the present method can provide high sensitivity because of the ultrathin inorganic film with high permittivity and good biocompatibility of gold nanoparticles. Compared with a capacitive immunoassay based on antibody-embedded ultrathin γ-alumina sol–gel-derived films, the novel immunosensor presented a lower detection limit of 0.05 ng/ml and a wider linear response range of 1–75 ng/ml for transferrin detection. The prepared procedure of the novel immunosensor also provided a new approach to fabrication of capacitive immunosensors.