Electrostriction techniques for preparation of thin lipid films on different solid supports

Abstract

Two types of electrostriction techniques that apply constant (DC) or alternate (AC) voltage are used in the work to prepare thin lipid films on three different types of electrodes – glassy carbon (GC), gold and SiO/SiO2. The films are obtained by a self-thinning of lipid organic solution placed between the electrode and an electrolyte solution. This process leads to relatively thick films with thicknesses in the range 100-1000 Å depending on the solid support. Further thinning is achieved by electrostriction that generates additional external pressure P∼ULF2/h2, where ULF is the voltage drop across the lipid film and h is the film thickness. ULF depends on the applied external voltage UEXT and the voltage drops across the other elements in the overall circuit that represents the electrochemical cell consisting of the film-supporting electrode, the lipid film, the electrolyte solution and the reference electrode. On the other hand, the voltage drop across each element depends on the type of the applied voltage - constant (DC) or alternate (AC). In the case of glassy carbon and gold electrodes, the most insulating element is the lipid film and the voltage drop ULF is close to the applied UEXT, which permits to obtain easily thin films by DC-electrostriction. In the case of Si/SiO2 wafers, the insulating SiO2 layer represents an almost ideal capacitor in series with the other elements in the circuit. Applying DC-electrostriction leads to a predominant voltage drop across the SiO2 layer and small voltage drop ULF. To obtain sufficiently high ULF that would lead to the thinning of SiO/SiO2-supported lipid layer, an AC-electrostriction is used. At higher frequencies of AC-voltage, the impedance of the SiO2 layer becomes small and the voltage drops across the SiO2 layer and across the lipid film become comparable. Thus, according to the type of the support either DC-electrostriction or AC-electrostriction could be used to obtain very thin lipid films.