STM investigation of electron transport features in cytochrome c Langmuir–Blodgett films

The morphological and electron-transfer properties of cytochrome c monolayer Langmuir–Blodgett (LB) films deposited on a graphite electrode have been studied using scanning tunneling microscopy (STM) and tunneling spectroscopy techniques in a double tunnel junction configuration STM tip–cytochrome c monolayer–conducting substrate. Monolayer cytochrome c LB films were deposited successfully by horizontal lifting method via protein Langmuir monolayer formation using preliminary solubilization of the protein in water bubbles of reverse microemulsion of Aerosol OT in hexane. Electronic spectroscopy of a single cytochrome c molecule was carried out with sub-nanometer spatial resolution at the ambient conditions. The STM spectroscopic study, by recording tunneling current–bias voltage (I–V) curves, revealed nonsymmetric I–V curves with steps of variable widths and heights dependent on the STM tip position over the protein molecule. Complex-rich structure of the tunneling current–voltage spectra can be a result of the interplay between Coulomb charging effects and tunneling through discrete electronic levels of the cytochrome c molecule. It can also be determined by the tunnel junctions parameters, dependent on the state of cytochrome c molecules in the gap between the STM tip and the substrate, presence of absorbates and ‘electron paths’ effects in the protein molecule.