Immunosensors and DNA Sensors Based on Impedance Spectroscopy

Abstract

Impedance spectroscopy is a rapidly developing electrochemical technique for the characterization of biomaterial-functionalized electrodes and biocatalytic transformations at electrode surfaces, and specifically for the transduction of biosensing events at electrodes. The immobilization of biomaterials, e.g., antigen/antibodies or DNA on electrode surfaces alters the capacitance and interfacial electron transfer resistance of the electrodes. The impedance features of the modified electrodes can be translated into the equivalent electronic circuits consisting of capacitances and resistances. The kinetics and mechanisms of the electrochemical processes occurring at modified electrode surfaces could be derived from the analysis of the equivalent circuit elements. For example, electron transfer resistances can be found upon analysis of Faradaic impedance spectra in the form of Nyquist plots. The electron transfer rate constants can be calculated from the measured electron transfer resistances. Different immunosensors that use impedance measurements for the transduction of antigen-antibody complex formation on electronic transducers were developed. These include: (i) in-plane impedance measurements between electrodes separated by a nonconductive gap modified with antigen or antibody molecules, (ii) the amplified detection of antigen-antibody complex formation using biocatalyzed precipitation of an insoluble product as an amplification route and Faradaic impedance spectroscopy as readout signal, or (iii) the amplified detection of antigen-antibody complex by the biocatalytic dissolution of a polymer film associated with the electrode and Faradaic impedance spectroscopy as transduction method. Similarly, DNA biosensors using impedance measurements as readout signal were developed. The assembly of nucleic acid primers and their hybridization with the complementary DNA were characterized by Faradaic impedance spectroscopy using [Fe(CN)6]3−/4− as a redox probe. Amplified detection of the analyte DNA using Faradaic impedance spectroscopy was accomplished by the coupling of functionalized liposomes or by the association of biocatalytic conjugates to the sensing interface, providing biocatalyzed precipitation of an insoluble product on the electrodes. The amplified detections of viral DNA and single-base mismatches in DNA were accomplished by similar methods. The theoretical background of the different methods and their practical applications in analytical procedures were outlined in the paper.