# Recent Lipid Membrane-Based Biosensing Platforms
## 2 The Electroanalytical Aspect of Lipid Membrane Biosensors
A two-electrode system can be used to measure the transmembrane potential, typically found to be ca. 70 mV at rest [10]. Many studies have shown that the transmembrane potential regulates voltage-sensitive phenomena for ion transporters [16], channels [17; 18], or gating-charge controlled receptors [19]. The dipolar potential, on the other hand, cannot be accessed directly. Voltage-sensitive fluorescent membranes have been used to probe hydration and polarity of bilayers [20], record potential changes [21] or the strength of the electric field [22]. The relation between the dipolar potential and the membrane permeability remains unclear as to which regulates what [23], but it has been shown that surface tension increases reduce the value of the dipolar potential due to changes in lipid packing and in lipid-bound water molecules [24], this yields a mechanical sensitivity for the bilayers. Following the Gouy-Chapman-Stern model, the bilayer can be seen as a plate capacitor, where capacitance measurements can be used to estimate membrane area, thickness, and surface potential. Typical values within the range of 0.5-0.8 \(\upmu\)F cm\({}^{-2}\) have been reported. Freely suspended bilayers can be represented by the simple circuit shown in Figure 4a, but circuitry modeling may be quite complicated to include more elements (electrodes, interfaces, etc.) in order to account for bias [25] or to represent complex platforms, such as tethered bilayers [11], unanchored (Figure 4b) [25] or anchored membranes (Figure 4c) [17] on electrode surfaces, metal-supported membranes [25], polymer supported bilayers [26], or high density biosensor arrays [27]. These approximations can lead to misestimating, unless interfacial molecular structure and hydration are taken into account [10]. In non-Faradaic transduction, the estimation of capacitance is well-suited for surface characterization. Faradaic responses (i.e., from redox proteins) rely on electron transfer across the bilayer, depending on solution pH and ion concentration, temperature, lipid charge, and lipid concentration. Monte Carlo simulations [28] and molecular dynamics algorithms [24] have been developed for electrochemical parameter calculations and implemented successfully in suspended bilayers. The supported formats present certain problems in the analysis of the membrane under varying conditions, as the pressure-area isotherms used cannot account for area and particle number variations at the same time [29]. Further, permeability models of freely suspended membranes have to be also considered for data interpretation. Impedance measurements have shown that long-range electrostatic attraction and short-range hydration repulsion dominate in unanchored membranes, whereas short-range van der Waals forces become significant at anchoring [25]. Additionally, in mixed bilayers with charged and uncharged lipids, the packing density of the lipid molecules may alter hydration repulsion and increase the number of pores in the membranes, thus reducing the value of its resistance [30]. Figure 4: Equivalent circuit models for (**a**) suspended bilayer (i.e., with no direct contact with any electrode); (**b**) supported unanchored bilayer (i.e., floating on the surface of an electrode); and (**c**) anchored bilayer (i.e., through a spacer). The serial resistance Reelectrolyte corresponds to the ohmic behavior of the electrolyte solution, whereas the capacitance Cmembrane in parallel with Rmembrane represents the bilayer system. In the case of anchored membranes, the spacer region contributes its own constant-phase element; Cs spacer indicates the electrochemical behavior of spacer and the water layer that separates the membrane from the surface of the electrode. Another inter-related critical parameter of membrane electrochemistry, especially as regards the modulation of permeability, is the phase shifting of the lipid sheets.