Bilayer lipid membranes as electrochemical switches in reactions involving alteration of surface charge

Recent reports have indicated that some hydrolytic enzyme reactions at bilayer lipid membranes (BLMs) formed from a mixture of dipalmitoylphosphatidic acid (DPPA) and egg phosphatidyl choline (PC) can provide an electrochemical response in the form of a single transient of current. The time required for the appearance of a transient of constant magnitude has been observed to depend on substrate concentration, and the results suggest that BLMs may produce signals suitable for the development of chemically-selective switches. The mechanism of transient signal generation has not been established. Transient signals appear only when using BLMs prepared from mixtures of DPPA/PC, and are associated with pH changes at the surfaces of membranes. This work reports results of electrochemical studies of BLMs and fluorescence studies of monolayer lipid membranes, which are based on a continuous variation of pH in bulk solution to alter the surface charge density due to DPPA at the membrane surface. The origin of the transient current is due to a combination of double layer reorganization and lipid head group reorganization, and the magnitude and reproducibility of response can be adjusted by selection of DPPA/PC composition.