A sensitive impedance immunosensor based on functionalized gold nanoparticle-protein composite films for probing apolipoprotein A-I

A sensitive and label-free electrochemical impedance immunosensor via covalent coupling the antibody with functionalized gold nanoparticles (FAuNP) for probing apolipoprotein A-I was presented. The hybrid gold nanoparticles were prepared with a two-in-one strategy, i.e. via the stepwise employment of self-assembled monolayer (SAM) and sol-gel techniques, to improve the performance of such a label-free immunosensor, which was investigated by electrochemical impedance spectroscopy. It was found that this novel FAuNP immunosensor showed higher protein-loading capacity and better response properties (6-17 times) than that fabricated by normal SAM technique did. The remarkably improved properties of the immunosensor were ascribed to FAuNP with the larger surface-to-volume ratio, more free amino linkage groups, and the lower nonspecific protein adsorption. As a result, the thus-prepared antibody-modified immunosensor showed reproducible (R.S.D. = ±3.2%, n = 10) linear response to apolipoprotein A-I (Apo A-I) antigens in the range of 0.1-10 ng mL⁻¹. The detection limit of this immunosensor was 50 pg mL⁻¹ (corresponding to 1.8 pmol L⁻¹), which was two orders of magnitude lower than that of the traditional methods. These results exhibited the novel immunosensor had a high sensitivity, stability and selectivity for the determination of Apo A-I, especially in clinic microanalysis.