However, these polymers do not always produce a completely conductive layer. Strategies to induce conductivity include the incorporation of dopants, co-polymers, or conductive nanostructures during the electropolymerisation process. Often several modification methods are combined for the immobilisation of the bioreceptor to the electrode surface. For instance, some groups have decorated polymers with NPs and then coated the NPs with thiols. The equivalent circuit for such sensors is simpler than the Randles' circuit as it does not require R ct or W. Instead it considers the R s and the capacitance in the circuit. The capacitance at the electrode/electrolyte interface will consist of a multitude of capacitors connected in series. Typically, these capacitors include the capacitance of the insulating layer; the capacitance of the bioreceptor (this includes the anchoring group, along with any trapped water molecules between the protein layer and diffuse layer); and the capacitance of the bulk solution. However, if the bioreceptor does not form a completely insulating layer, resistors will be connected in parallel with the capacitor to account for the contribution of pinholes. SAMs are a popular choice for modifying the electrode surface for the immobilisation of the bioreceptor. Unlike Faradaic sensors which often used mixed SAMs, long chain alkanethiols can be ideal for capacitive sensors, as they produce dense, well-organised layer which have insulative properties. However, a second SAM molecule may be added after the original SAM formation to cover empty regions. Capacitive sensors are attractive for POC devices as no redox reagent is needed however, pinholes in the insulating layer can decrease the sensitivity of the device. Interdigitated electrodes (IDE) are made from two individually addressable comb-like structures, whose fingers interlock with a set gap between neighbouring electrodes. IDE devices have been utilised as biosensor applications, including impedimetric biosensors. They provide a larger surface area than conventional electrodes, and have a great signal-to-noise ratio which enables increased sensor sensitivity. Additionally, the geometry of the electrodes can affect the sensitivity, especially the gap between electrodes. Hence Singh et al. investigated IDEs with a constant length, and similar widths and heights, but varying nanogaps (150 to 500nm). Overall, the sensitivity of the IDE increased as the pitch decreased. A simple two-electrode set up can be employed for EIS, which along with nanoscale gaps between the electrodes, results in a small sensor footprint and makes devices compatible with mass production, which is necessary for POC devices. As with conventional single electrodes or arrays of electrodes the surface can be modified to immobilise a bioreceptor to the surface and perform label-free detection. These can be adopted as either Faradaic or capacitive devices. Generally, the biolayer is formed on both electrodes, however, as mentioned in Section 4.1.1 thiols can be selectively desorbed from the electrode surface. Applying a suitable potential to the electrode will strip the SAM layer, 233 presenting the possibility to isolate one comb to perform the assay on, or modify each comb differently. Similarly, drastically accelerated the SAM formation by applying a negative potential to one comb, reducing the deposition time of 1-dodecanethiol to 1 min. The second comb was left at open circuit during deposition and remained relatively thiol free. Alternatively, rather than modifying the electrode surface in capacitive sensors, a biolayer can be formed between the electrode gaps. As previously described, the binding of the target to the bioreceptor will push water molecules and solvated ions away from the surface. Due to the low dielectric constant of the antibody compared with water there will be a change in the dielectric properties between the electrodes causing an alteration in the capacitance, also referred to as dielectrophoretic impedance measurement. 213