Nyquist plots are consist of two portions. The linear portion at low frequencies is associated with electrochemical behavior limited by diffusion. The semicircle portion at high frequencies is associated with the electrochemical process subject to Rucaparib transfer, where the diameter corresponds to the resistance. Simply, resistance change could be judged by observing the diameter change of semicircle portion. Thus, A.C. impedance is a suitable method for monitoring the changes in the surface features during the assembly process . In this study, A.C. impedance method was used to characterize fabrication process of the proposed sandwich-type electrochemical immunosensor. Fig. 4 shows the Nyquist plots of A.C. impedance spectroscopy in the process of modifying electrode, which were recorded from 1 to 105 Hz at 0.18 V in a solution containing 0.1 M KCl and 2.5 mM Fe(CN)63−/Fe(CN)64−. It could be observed that the bare GCE exhibited a very small resistance (curve a). After electrodeposited with Au NPs, a smaller resistance (curve b) was observed attributing to the good conductivity of Au NPs. Gradually increasing resistance indicated the successful modification of the non-conductive bioactive substances when Ab1 (curve c), BSA (curve d) and TPA (curve e) were modified layer by layer on the GCE. When Au@MGN-Ab2 labels (curve f) were modified on the electrode, an obvious decrease of resistance could be observed, suggesting that Au@MGN with good electron transfer capability were modified successfully by the interaction between antibodies and antigens.