An in-situ surface electrochemistry approach toward whole-cell studies: Charge transfer between Geobacter sulfurreducens and electrified metal/electrolyte interfaces through linker molecules

Electrochemical reactivity and structure properties of electrogenic bacteria, Geobacter sulfurreducens (Gs) were studied to explore the heterogeneous electron transfer at the bacteria/electrode interface using electrochemical and in-situ spectroscopic techniques. The redox behavior of Gs adsorbed on a gold electrode, which is modified with a ω-functionalized self-assembled monolayer (SAM) of alkanethiols, depends strongly on the terminal group. The latter interacts directly with outermost cytochromes embedded into the outer membrane of the Gs cells. The redox potential of bacterial cells bound electrostatically to a carboxyl-terminated SAM is close to that observed for bacteria attached to a bare gold electrode, revealing a high electronic coupling at the cell/SAM interface. The redox potentials of bacterial cells adsorbed on amino- and pyridyl-terminated SAMs are significantly different suggesting that the outermost cytochromes changes their conformation upon adsorption on these SAMs. No redox activity of Gs was found with CH3-, N(CH3)3+- and OH-terminated SAMs. Complementary in-situ spectroscopic studies on bacteria/SAMs/Au electrode assemblies were carried out to monitor structure changes of the bacterial cells upon polarization. Spectro-electrochemical techniques revealed the electrochemical turnover of the oxidized and reduced states of outer membrane cytochromes (OMCs) in Gs, providing evidence that the OMCs are responsible for the direct electron transfer to metal electrodes, such as gold or silver, during the electricity production. Furthermore, we observed spectroscopic signatures of the native structure of the OMCs and no conformational change during the oxidation/reduction process of the microorganisms. These findings indicate that the carboxyl-anchoring group provides biocompatible conditions for the outermost cytochromes of the Gs, which facilitate the heterogeneous electron transfer at the microorganism/electrode interface.

Charge transport through perylene bisimide molecular junctions: An electrochemical approach

Single molecular junction conductances of a family of five symmetric and two unsymmetric perylene tetracarboxylic bisimides (PBI) with variable bay-area substituents were studied employing a scanning tunneling microscope (STM)-based break junction technique. The stretching experiments provide clear evidence for the formation of single molecular junctions and π–π stacked dimers. Electrolyte gating demonstrates a distinct gating effect in symmetric molecular junctions, which strongly depends on molecular structure and properties of the solvent/electrolyte. Weak π–π-coupling in the unsymmetric dimers prevents rectification.

Charge Transport in Photoswitchable Dimethyldihydropyrene-Type Single-Molecule Junctions

The conductance properties of a photoswitchable dimethyldihydropyrene (DHP) derivative have been investigated for the first time in single-molecule junctions using the mechanically controllable break junction technique. We demonstrate that the reversible structure changes induced by isomerization of a single bispyridine-substituted DHP molecule are correlated with a large drop of the conductance value. We found a very high ON/OFF ratio (>104) and an excellent reversibility of conductance switching.