Tethered bilayer membranes containing ionic reservoirs: The interfacial capacitance

The use of polar linkers to tether lipid bilayer membranes to a gold substrate results in a hydrophilic layer between the membrane and the gold surface. The tethering of lipid bilayer membranes to gold substrates using tetraethylene glycol chains results in a polar layer between the membrane and the gold surface. This region may sequester ions and can act as a reservoir for ions transported across the tethered lipid membrane. In the present article, we report on the electrical properties of this ionic reservoir. In particular, the Stern model of ionic distribution is used to describe the interfacial capacitance. The model combines a surface adsorption layer (Helmholtz model) and a dynamic diffuse layer of ions (Gouy-Chapman model) to describe the interfacial capacitance. This model is used to interpret data from measurements of the interfacial capacitance obtained over a range of ionic species and concentrations. Four analogues of the sulfur-tetraethylene glycol tethers have been investigated. These studies show the effects of varying the structure of the linker group and of introducing a passivation layer adjacent to the gold. Studies were also made of the influence of spacer molecules included to vary the "in-plane" two-dimensional packing. The effect of applying a dc bias potential between an external reference electrode and the gold surface was also studied. These measurements were carried out using ac impedance spectroscopy on bilayers assembled using the method of Cornell et al. Most data are successfully modeled as a constant Helmholtz capacitance in series with a diffuse region capacitance that depends on ionic concentration. The dependence on ionic concentration has been modeled by the Gouy- Chapman formalism. At low ionic concentrations (