Enzymatically biocatalytic precipitates amplified antibody–antigen interaction for super low level immunoassay: An investigation combined surface plasmon resonance with electrochemistry

We demonstrated a simple and efficient strategy, which based on the enzymatically biocatalytic precipitates amplified antibody–antigen interaction, for improving the response signals of surface plasmon resonance (SPR) immunosensing. The antibody–antigen-alkaline phosphatase (AP) labeled secondary antibody sandwich were successfully prepared and characterized by SPR, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The SPR signal amplification was accomplished through probing resonance angle shift and Faradaic electron impedance of [Fe(CN)6]3−/4− redox pair after the enzymatically biocatalytic products precipitating on the immunosensing electrode surface. As a result, the accumulation of the enzymatically biocatalytic precipitates leads to significantly resonance angle shift and increase of electron transfer impedance of [Fe(CN)6]3−/4− probe. The precipitates-enhanced sandwich SPR immunoassay for mouse immunoglobulin G (m-IgG) can easily detect solution protein concentrations in the linear range of 0.02–40 ng mL−1 and with a detection limit of 200 fg mL−1, which is more than four-orders and 10 times better compared with the values using streptavidin–biotinylated protein complex and biotinylated HRP biocatalyzation amplification methods. Moreover, this method is generally applicable to other sandwich immunoassays and also can be expanded to monitor other antibody–antigen interaction for immunosensing detection at low concentrations.