Electrochemical sensing of membrane potential and enzyme function using gallium arsenide electrodes functionalized with supported membranes

We deposit phospholipid monolayers on highly doped p-GaAs electrodes that are precoated with methylmercaptobiphenyl monolayers and operate such a biofunctional electrolyte-insulator-semiconductor (EIS) setup as an analogue of a metal-oxide-semiconductor setup. Electrochemical impedance spectra measured over a wide frequency range demonstrate that the presence of a lipid monolayer remarkably slows down the diffusion of ions so that the membrane-functionalized GaAs can be subjected to electrochemical investigations for more than 3 days with no sign of degradation. The biofunctional EIS setup enables us to translate changes in the surface charge density Q and bias potentials U_bias into the change in the interface capacitance Cp. Since C_p is governed by the capacitance of semiconductor space charge region C_SC, the linear relationships obtained for 1/Cp2 vs Q and 1/C_p² vs U_bias suggests that C_p can be used to detect the surface charges with a high sensitivity (1 charge per 18 nm²). Furthermore, the kinetics of phospholipids degradation by phospholipase A₂ can also be monitored by a significant decrease in diffusion coefficients through the membrane by a factor of 104. Thus, the operation of GaAs membrane composites established here allows for electrochemical sensing of surface potential and barrier capability of biological membranes in a quantitative manner.