1. Is DNA a molecular conductor?
DNA is known as the bearer of genetic information of all living cells. The nucleic acid sequence holds the recipe to build all proteins needed to perform the ”daily work” of the cell. DNA is organized as a double-strand of complementary nucleic acids in a helix structure. DNA is also recognized as a potential electric conductor, see Figure, and electronic conductivity or long-range electron transfer believed to play a role in the cell’s control mechanisms (DNA repair). Conduction is via the bases.
This project investigates charge conduction mechanisms oligonucleotides (OGNs) as models for DNA using modified OGN duplexes and quadruplexes at solid surfaces. The technologies used are electrochemistry at single-crystal atomically planar electrode surfaces and in situ STM both in imaging and in scanning tunneling spectroscopy, directly in biological buffers at resolution right down to the single-molecule level. The results are supported by theoretical frames for single-molecule conduction also developed in the group. The results offer perspectives for future bioelectronics and bio-sensing for genetic disorders at ultra-high resolution, see Figure.
Left: Schematic view of a DNA (oligonucleotide) molecule immobilized on a single-crystal, atomically planar electrode surface via a thiol linker group.
Middle:Three-dimensional view of single oligonucleotide molecules on a single-crystal Au(111)-electrode surface recorded by electrochemical scanning tunnelling microscopy. The molecular scale features are single 13-base double-strand OGN molecules with a redox probe intercalated to monitor the single-molecule tunnelling current.
Right: Sensitivity of electrochemical current to mismatches in the base pairing. It is notable that these signals are of non-Faradaic, capacitive origin and reflect structural reorganization of the OGN adlayer.
2. Single-molecule and nanoscale molecular electron transport – molecular electronics
Electrical currents and electron transfer between molecules and biomolecules, or between (bio)molecules and enclosing electrodes are key notions in nano- and molecular-scale science in many of our projects. Rational approaches to this whole nanoworld of new phenomena require new techniques such as electrochemistry at single-crystal, atomically planar electrode surfaces, and the scanning probe microscopies, scanning tunneling microscopy and atomic force microscopy directly in aqueous chemical and biological media (in situ STM and AFM). New theoretical frames to understand the phenomena and offer guides to single-molecule electronic function are also paramount. Electrochemical single-molecule science and molecular electronics are pioneered by our group. Our projects in these exciting areas address for example electron transfer across molecular monolayers, electronic conductivity of single complex redox molecules and biomolecules, and mechanisms of single redox metalloenzyme molecules. We offer interesting projects in these novel areas of chemical nanoscale science that focus on single-molecule behavior in aqueous and ionic liquid media. The Figure shows some recent examples of single-molecule behavior for which detailed mapping of structure and function has been achieved.
Top left: An electron transfer protein (the four-helix bundle heme protein cytochrome b562) in action at a single-crystal, atomically planar Au(111) electrode surface. The corresponding single-molecule electrochemical signal, recorded by electrochemical scanning tunneling microscopy is also shown.
Top right: A large redox metalloenzyme (laccase, og broad biotechnological interest) with four Cu-atoms. The spots in both pictures are single protein molecules in direct electron transfer or enzyme action.
Bottom: A 1.5 nm Au-nanoparticle (Au145) can be successively charged by single-electron transfer. This raises the questions whether the nanoparticle is a molecule or a small chunk of metal, and what can be the physical origin of the highly efficient electroctrocatalysis of simple interfacial electrochemical and bioelectrochemical electron transfer processes.
3. Electronic properties of metallic nanoparticles
Metallic nanostructures of molecular and nanoscale size are good catalysts for electrochemical processes that involve major reorganization of the chemical bond system such as oxidation of methanol or formic acid eventually to carbon dioxide, but , they also catalyze much simpler interfacial electrochemical processes with electron transfer as the only step. Interfacial electron transfer catalyzed metallic nanoparticles and other nanostructures , however, involve complex molecules such transition metal complexes and even redox metalloproteins, for example heme proteins (cytochromes, Fe) or blue copper proteins (azurin, Cu) .
Our group is engaged in in theoretical research to understand the physical origin of the highly efficient electrocatalysis of molecular scale noble metal particles (Au, Pt, Pd, Ag). The electronic structure of the smallest particles (1-2.5 nm) give both intriguing single-electron charging reflrcted in voltammetric fine-structure of the cyclic voltammograms and in more favourable electronic overlap between the nanoparticles and the target redox moecules compared with the similar overlap at planar electrodes. Single-electron charging effects are congested for particles larger than abut 3 nm, while the electrocatalytic effects remain. The figure shows illustrations of some recent observations.