Tethered Bilayer Membranes Containing Ionic Reservoirs: The Interfacial Capacitance

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• 作者：Gowri Krishna ; Jurgen Schulte ; Bruce A. Cornell ; Ron Pace ; Lech Wieczorek ; and Peter D. Osman
• 刊名：Langmuir
• 出版年：2001
• 出版时间：August 7, 2001
• 年：2001
• 卷：17
• 期：16
• 页码：4858 - 4866
• 全文大小：220K
• 年卷期：v.17,no.16(August 7, 2001)
• ISSN：1520-5827

文摘

The use of polar linkers to tether lipid bilayer membranes to a gold substrate results in a hydrophiliclayer between the membrane and the gold surface. The tethering of lipid bilayer membranes to goldsubstrates using tetraethylene glycol chains results in a polar layer between the membrane and the goldsurface. This region may sequester ions and can act as a reservoir for ions transported across the tetheredlipid membrane. In the present article, we report on the electrical properties of this ionic reservoir. Inparticular, the Stern model of ionic distribution is used to describe the interfacial capacitance. The modelcombines a surface adsorption layer (Helmholtz model) and a dynamic diffuse layer of ions (Gouy-Chapmanmodel) to describe the interfacial capacitance. This model is used to interpret data from measurementsof the interfacial capacitance obtained over a range of ionic species and concentrations. Four analoguesof the sulfur-tetraethylene glycol tethers have been investigated. These studies show the effects of varyingthe structure of the linker group and of introducing a passivation layer adjacent to the gold. Studies werealso 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 wasalso studied. These measurements were carried out using ac impedance spectroscopy on bilayers assembledusing the method of Cornell et al.⁶ Most data are successfully modeled as a constant Helmholtz capacitancein series with a diffuse region capacitance that depends on ionic concentration. The dependence on ionicconcentration has been modeled by the Gouy-Chapman formalism. At low ionic concentrations (<20 mM),the model becomes inadequate. Deviation from the model also occurs at higher concentrations for moretightly packed membranes, in the absence of tethered spacer molecules. According to the model at verylow concentrations of electrolyte, the ionic Debye length intrudes into the hydrocarbon region of the bilayer,violating the Gouy-Chapman assumption of a uniform dielectric medium in the diffuse double layer. TheHelmholtz capacitance is insensitive to potential and ionic concentration. This is consistent with Helmholtzcapacitance being defined by a hard sphere distance of closest approach of the ions to the gold interfaceover the range of concentrations studied here. The model suggests that the application of a dc potentialalters 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 reservoir region varies only from 27 to 54, suggestingthe reservoir has properties more akin to a dense hydrated gel with restricted ionic mobility than to a bulkelectrolyte.