The imaginary axis (-Z") has a negative value as capacitance its contributing component, which always has a negative phase shift. A positive imaginary value would be due to inductance. The real axis (Z') shows the resistive contribution of the impedance. An equivalent circuit is constructed to account for the interactions at the electrode/electrolyte interface, such as the formation of the double layer or the occurrence of charge-transfer. The most commonly used circuit is the Randles circuit, which consists of solution resistance (R s ), charge-transfer resistance (R ct ), double layer capacitance (C dl ) and a Warburg element (W). The corresponding Nyquist is shown in Figure ; the semicircle data is produced by the kinetically controlled process and the diagonal line shows the effects of diffusion on the system. Alternatively, the Bode plot can be considered, for the frequency-dependency of the overall impedance or of the phase shift. By immobilising the bioreceptor to the electrode surface, the antibody-antigen binding event will create a change at the interfacial region and elicit an impedimetric response. Since EIS is sensitive to fluctuations on the electrode surface it facilitates label-free detection. To create an impedimetric immunosensor the bioreceptor must first be immobilised onto the electrode surface. To optimise the immobilisation of the bioreceptor to the electrode, the surface can first be modified.