The ion selectivity of monensin incorporated phospholipid/alkanethiol bilayers

Monensin was incorporated into phospholipid/alkanethiol bilayers on the gold electrode surface by a new, paint-freeze method to deposit a lipid monolayer on the self-assembled monolayers (SAMs) of alkanethiol. The advantages of this assembly system with a suitable function for investigating the ion selective transfer across the mimetic biomembrane are based on the characteristics of SAMs of alkanethiols and monensin. On the one hand, the SAMs of alkanethiols bring out their efficiency of packing and coverage of the metal substrate and relatively long-term stability; on the other hand, monensin improves the ion selectivity noticeably. The selectivity coefficients K-Na+,K-K+, K-Na+,K-Rb+ and K-Na+,K-Ag+ are 6 x 10(-2), 7.2 x 10(-3) and 30 respectively. However, the selectivity coefficient K-Na+,K-Li+ could not be obtained by a potentiometric method due to the specific interaction between Li+ and phospholipid and the lower degree of complexion between Li+ and monensin. The potential response of this bilayer system to monovalent ions is fairly good. For example, the slope of the response to Na+ is close to 60 mV per decade and its linearity range is from 10(-1) to 10(-5) M with a detection limit of 2 x 10(-6) M, The bilayer is stable for at least two months without changing its properties. This monensin incorporated lipid/alkanethiol bilayer is a good mimetic biomembrane system, which provides great promise for investigating the ion transfer mechanism across the biomembrane and developing a practical biosensor.

Self-assembled monolayers of thiols on gold electrodes for bioelectrochemistry and biosensors

Monolayers of biological compounds including redox proteins and enzymes, and phospholipids have been immobilized on a gold electrode surface through self-assembling. These proteins and enzymes, such as cytochrome c, cytochrome c oxidase and horseradish peroxidase (HRP), immobilized covalently to the self-assembled monolayers (SAMs) of 3-mercaptopropionic acid on a gold electrode, communicate directly electrons with the electrode surface without mediators and keep their physiological activities. The electron transfer of HRP with the gold electrode can also be mediated by the alkanethiol SAMs with electroactive group viologens on the gold electrode surface. All these direct electrochemistries of proteins and enzymes might offer an opportunity to build a third generation of biosensors without mediators for analytes, such as H2O2, glucose and cholesterol. Monensin and valinomycin have been incorporated into the bilayers on the gold electrode consisting of the SAMs of alkanethiol and a lipid monolayer, which have high selectivity for monovalent ions, and the resulting Na+ or K+ sensor has a wide linear range and high stability. These self-assembly systems provide a good mimetic model for studying the physiological function of a membrane and its associated enzyme. (C) 1997 Elsevier Science S.A.