The detailed principle of ECA impedimetric biosensor is illus- trated in Fig. 1. Firstly, the aptamer functionalized with a thiol group at its 5′ end was covalently attached on the Au surface. The mod- ified electrode was then blocked with MCH to form a mixed SAM in order to effectively prevent the nonspecific adsorption of OMPs on the electrode surface and also to align the aptamers to facilitate their interaction with the ligand, the OMPs.
In the absence of OMPs, the added redox probe (ferro- cyanide/ferricyanide) molecules can exchange electrons with the modified electrode surface but a certain resistance is observed. In the presence of OMPs, the ECA folds around the proteins and forms an OMPs–ECA complex. The formation of this complex blocks the electron transfer and consequently the Ret increases.