An Electrochemical immunosensor with a self-assembled monolayer and Protein A as a backbone structure

In developing an electrochemical immunosensor, an antibody is often immobilised on an electrode prior to the capture of its specific antigen. A primary consideration in such work is that the immobilised capture antibody is oriented with minimal steric hindrance to interact favourably with its target antigen. Equally important, it is desirable to immobilise the antibody without a significant change in its ability to bind its antigen. These features contribute to the level of sensitivity and dynamic range achievable by the immunosensor. In this work, we report the development of an electrochemical immunosensor consisting of a gold electrode coated with a thioctic acid self-assembled monolayer (SAM), to which the non-antigenic component of the bacterial protein, Protein A, is then covalently attached. Owing to the almost perpendicular alignment of SAM, the antigenic component of Protein A will likely be oriented in an upright direction, ready to bind the capture antibody. Following the binding of antigen to the capture antibody, a second, enzyme-conjugated, antibody is added to complete the molecular architecture of the immunosensor. By oxidising the product of an enzymatic reaction, the current obtained can be quantitatively related to the antigen in an analyte. In this paper, we will present an assessment on the performance of our electrochemical irnrnunosensor in terms of sensitivity and dynamic range in the determination of the hormone, human chorionic gonadotrophin, in standard solutions and in several serum samples. Comparisons will also be made to results obtained at a Sydney hospital.