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.