Impedimetric immunoassays and immunosensors

In an EIS experiment, a low amplitude (5 to 10 mV peak-to-peak) sine wave potential signal is superimposed on a fixed DC potential applied to an electrochemical system. Based on Ohm s law, the impedance can be computed from the applied sinusoidal potential and the measured sinusoidal current. As the sinusoidal potential and current will 

Electrochemical Sensors, Biosensors and Their Biomedical Applications 

Very often, the electrode-solution interface can be represented by an equivalent circuit, as shown in Fig. 5.10, where Rs denotes the ohmic resistance of the electrolyte solution, Cdi, the double layer capacitance, the charge (or electron) transfer resistance that exists if a redox probe is present in the electrolyte solution, and Zw the Warburg impedance arising from the diffusion of redox probe ions from the bulk electrolyte to the electrode interface. Note that both R and Zw represent bulk properties and are not expected to be affected by an immunocomplex structure on an electrode surface. On the other hand, Qi and l ct depend on the dielectric and insulating properties of the electrode-electrolyte solution interface. For example, for an electrode surface immobilized with an immunocomplex, the double layer capacitance would consist of a constant capacitance of the bare electrode (Cbare) and a variable capacitance arising from the immunocomplex structure (Cimm n), expressed as in Eq. (4). 

As the immunocomplex structure is geueraUy electroinactive, its coverage on the electrode surface will decrease the double layer capacitance and retard the interfacial electron transfer kinetics of a redox probe present in the electrolyte solution. In this case, R i can be expressed as the sum of the electron transfer resistance of the bare electrode (/ bare) nnd that of the electrode immobilized with an immunocomplex 

There are several ways to present the Faradaic impedance data obtained at an electrode immobilized with an immunocomplex in the presence of a redox probe. For example, Zi is plotted vs Zrj as a function of decreasing frequency to obtain a 

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Currently, immunosensors provide a powerful tool for monitoring biospecific interactions in real time or for deriving information about the binding kinetics of an immunoreaction or the structure/function relationships of molecules. Amperometric, potentiometric, conductive, and impedimetric transducers have been applied in direct and indirect electrochemical immunoassays. 

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