Fast and Efficient Charge Transport across a Lipid Bilayer Is Electronically Mediated by C70 Fullerene Aggregates

Abstract

Fullerene anions, made by photoreduction in a lipid bilayer, produce the largest trans-membrane steady state photocurrents yet observed, ∼6.0 μA/cm2. Since these photocurrents are not light saturated, their maximum value is considerably larger. Dithionite was used as electron donor for its ability to reduce photoexcited fullerenes at the donor interface on a time scale faster than 15 ns. Both photovoltage and photocurrent increase 15-fold on adding the acceptor ferricyanide trans to the donor. There are two components to the transit time of negative charge across the bilayer, <100 ns and 6 μs, in the 100 mM dithionite|0.6 mM C70|5 mM ferricyanide system, where | stands for the water−bilayer interface. This is strong evidence that the conduction is electronic and not diffusive-ionic. The plot of the ratio of photovoltage for the dithionite|C70| system to that of the dithionite|C70|ferricyanide system versus concentration of C70 in the lipid-forming solution is highly nonlinear. This suggests that aggregates of the fullerene are responsible for the fast negative charge transport. The action spectrum of the photocurrent further supports the existence of photoative C70 aggregates in the lipid bilayer. These aggregates may form the conductive path for electrons across the lipid bilayer.