This paper describes the design and fabrication of a lipid film-modified electrical device and the relation between the chemical structure of the lipids and the impedance responses of the device in air. Eight different synthetic poly(ion-complexed) lipids including quaternary ammonium lipids, glutamic acid (Glu)-based lipids with a short or a long methylene spacer chain, and diethanolamine-based quaternary ammonium lipids with with a long or a short spacer chain were synthesized. Transparent multibilayer films with crystalline-to-liquid crystalline phase transition were formed, and impedance responses for interdigitated array electrodes coated with cast films of these lipids were examined. Complex plane plot analyses together with quartz crystal microbalance and FTIR experiments have revealed molecular mechanisms for the unique impedance responses which could be classified as the following three types. Type I is the phase transition-dependent impedance responses, where the impedance changes dramatically near the phase transition temperatures of the lipid bilayer films on the electrodes. The change is derived from the increase in the mobility of the ion-conducting carrier (protons) coupled with the phase transition. The lipid devices coated with cast films of quaternary ammonium lipids with no spacer or a short spacer belong to this type. Type II is a device coated with a cast film of a diethanolamine-based quaternary ammonium lipid with a long spacer. Tc-dependent impedance response similar to that in type I is observed, but the molecular mechanism of the response is different. Type III exhibits a phase transition-independent impedance response. Electrical devices using Glu-based primary ammonium lipids and a Glu-based quaternary ammonium lipid with a long spacer as electrode modifiers give this type of response. Conformational mobility of the hydrophilic head group moieties of these lipid bilayers is maintained rigid through hydrogen bonding even at temperatures higher than the phase transition which results in this response.
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