A novel membrane charge sensor: Sensitive detection of surface charge at polymer/lipid composite films on indium tin oxide electrodes

We report the design of a novel membrane charge sensor by the deposition of highly insulating polymer/lipid composite films on indium tin oxide (ITO) semiconductor electrodes. The lipid monolayers were deposited on soft Langmuir-Blodgett (LB) multilayers of cellulose derivatives ('hairy-rod' polymers) by continuous exchange of solvent. The optical transparency of ITO enables the parallel characterization of the polymer-supported lipid monolayers by electrochemical impedance spectroscopy and fluorescence microscopy. The polymer/lipid composite system yielded an electric resistance of 2.5 × 10⁶ Ω cm² and a lateral diffusion constant for the lipids of 0.1 μm²/s. Such highly insulating and fluid composite films on ITO semiconductor electrodes can be utilized as membrane charge sensors to detect changes in surface charge by treating this electrolyte/(organic) insulator/semiconductor (EIS) system as an analogue of the metal/oxide/semiconductor (MOS) system. For this purpose, we incorporated 10 mol % of lipids with a chelator headgroup (nitrilotriacetic acid, NTA) to switch the membrane charge. A difference in surface charge density of ΔQ = 2.2 × 10^-6 C/cm² changed the flat band potential of the EIS system by nearly 50%. This result suggests that the sensitivity limit for our setup is sufficient to detect the binding of charged proteins to a membrane surface.