Effect of hydrophobic and hydrophilic surfaces on the morphological and physical properties of supported lipid membranes

The characteristics of supported lipid bilayers (SLBs) have long been studied as SLBs are commonly accepted as basic model systems of cell membranes. Atomic force microscopy (AFM) has proved to be effective in characterising physical and thermodynamic properties of SLBs, owing to its high resolution and the ability of environmental control. Through study of these properties, information such as lipid phase transitions can be investigated. Lipid phase transitions, from solid-ordered to liquid-disordered states, are well known to vary depending upon temperature, pH and ionic strength etc., and play a substantial role into events such as membrane protein mobility. Although previously studied on hydrophilic substrates, knowledge on the impact of hydrophobic substrates, such as van der Waals (vdW) crystals, on lipid phase transition is still lacking. This is particularly important for developing new bio- and electrochemical sensing technologies, where conductive vdW materials are used as sensing electrodes. It is expected that the thermodynamic properties of SLBs affect their conductive and dielectric polarisation and in turn molecular charge distribution and dielectric properties are known to strongly influence hydration layers occurring at the interface of lipids. Here, we present the study of adsorption of lipid membranes on various substrates, where morphological and physical (i.e. thermodynamic, electrical, and interfacial) properties of the membrane were investigated using advanced AFM modes as a function of temperature. We find that the transition temperature of the lipid monolayers greatly differs depending upon the chosen substrate. We then probe interfacial properties of these electrically charged interfaces. Finally, we demonstrate that substrates with different chemical-physical properties used as a lipids support have a substantial effect on the physical properties of lipid monolayers.