Figure 6: Schematic representation of an faradaic impedance measurement. a) A sensing surface compromising a receptor attached above the gold electrode. A redox probe (here [Fe(CN)6]3-/4-) has to approach the surface to exchange electrons and is faced with a barrier, quantified by Rct, generated by the receptive film, any bound target and the supporting chemistry. The electrochemical behavior of the system (solution with the redox probe and sensing electrode) can be described by the Randles equivalent circuit, as shown in (b). When targets are recruited from the solution (receptor-target interaction) the additional layer formed at the surface further increases the blocking effect and Rct (situation 2 in Figure 6d). Change in Rct can thus be used as a direct reporter of specific target presence. In Figure 6d, a hypothetical Nyquist plot for items a and b is shown. In the abscissa is shown the resolution of Rs and Rct from experimental data