Tethered bilayer membranes containing ionic reservoirs: The interfacial capacitance

The use of polar linkers to tether lipid bilayer membranes to a gold substr ate results in a hydrophilic layer between the membrane and the gold surfac e. The tethering of lipid bilayer membranes to gold substrates using tetrae thylene 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 fo r ions transported across the tethered lipid membrane. In the present artic le, we report on the electrical properties of this ionic reservoir. In part icular, the Stern model of ionic distribution is used to describe the inter facial capacitance. The model combines a surface adsorption layer (Helmholt z model) and a dynamic diffuse layer of ions (Gouy-Chapman model) to descri be the interfacial capacitance. This model is used to interpret data from m easurements of the interfacial capacitance obtained over a range of ionic s pecies and concentrations. Four analogues of the sulfur-tetraethylene glyco l tethers have been investigated. These studies show the effects of varying the structure of the linker group and of introducing a passivation layer a djacent to the gold. Studies were also made of the influence of spacer mole cules included to vary the "in-plane" two-dimensional packing. The effect o f applying a de bias potential between an external reference electrode and the gold surface was also studied, These measurements were carried out usin g ac impedance spectroscopy on bilayers assembled using the method of Corne ll et al.(6) Most data are successfully modeled as a constant Helmholtz cap acitance in series with a diffuse region capacitance that depends on ionic concentration. The dependence on ionic concentration has been modeled by th e Gouy-Chapman formalism. At low ionic concentrations (< 20 mM), the model becomes inadequate. Deviation from the model also occurs at higher concentr ations for more tightly packed membranes, in the absence of tethered spacer molecules. According to the model at very low concentrations of electrolyt e, the ionic Debye length intrudes into the hydrocarbon region of the bilay er, violating the Gouy-Chapman assumption of a uniform dielectric medium in the diffuse double layer. The Helmholtz capacitance is insensitive to pote ntial and ionic concentration. This is consistent with Helmholtz capacitanc e being defined by a hard sphere distance of closest approach of the ions t o the gold interface over the range of concentrations studied here. The mod el suggests that the application of a de potential alters the permittivity of the diffuse region as a result of water and ions being transported into the reservoir. However, the effective relative permittivity in the reservoi r region varies only from 27 to 54, suggesting the reservoir has properties more akin to a dense hydrated gel with restricted ionic mobility than to a bulk electrolyte.