Development of an immunoassay based on impedance measurements utilizing an antibody–nanosilver probe, silver enhancement, and electro-microchip

This study reports a new, simple and sensitive immunoassay that uses the impedance variation caused by silver nanoparticles (AgNPs) coated with an antibody. In this method, an electro-microchip is used to detect the immuno-reaction signal of antibody–antigen recognition, AgNPs as a label of antibody and a catalyst for silver precipitation, and the silver enhancement reaction is used to magnify the detection signal. Our model format is based on the sandwich immunoassay (three-layer format). The AgNPs were introduced into the electro-microchip by the specific binding of the antibodies and then coupled with silver enhancement solution to reduce silver ions to silver metal. The silver precipitation constructs a “bridge” between two electrodes of the electro-microchip allowing the electrons to pass. There was a significant difference in impedance between the experimental sample and the negative control after 10 min of reaction time. The proposed method requires less time and fewer steps than enzyme-linked immunosorbent assay (ELISA). It also has a high detection sensitivity (10 μg/mL of 1st antibody (IgG) immobilized on slides and 0.1 ng/mL of antigen (protein A)). Therefore, a new immunoassay is constructed using an electro-microchip, antibody–AgNPs conjugates, and a silver enhancement reaction.