Fereiro Lab - Bioelectronics

Aim: To develop single molecules and ensemble-based bio-electronic devices for the investigation and control of charge transport phenomena in and across biomolecules. These devices are operated at ambient conditions, in a vacuum, and in liquids. The gained knowledge provides the basis for designing novel electronic devices.

What is bioelectronics? "Bioelectronics is aimed at the direct coupling of biomolecular function units of high molecular weight and extremely complicated molecular structure with electronic or optical transducer devices". Wolfgang Gopel

Two key points are introduced here: the "direct coupling" and the "high molecular weight". The first indicates an intimate integration between biomolecular principles and microelectronics. The second implies large biomolecules such as proteins or complex biological systems such as cells or bacteria. That means we need to integrate biomolecular functions with extremely complicated biological structures.

Background: Integrating (bio)molecules into electronic junctions is a challenging research area, which combines nano-science and -technology, biophysics, and bio-electrochemistry. Apart from the principal aim of my research, which is to learn about electron transport through (bio)molecules, a potentially practical future goal may be the use of proteins as building blocks to develop multi-functional electronic devices for applications, such as biomedical ones, biofuel cells, and biosensing devices. As part of the effort to achieve my goal, I want to understand how the mechanism of electron transfer (ET) in the natural liquid surroundings of a protein compares to that of electron transport (ETp) in the solid-state, i.e., in dry proteins (dry in the sense that they keep only the structural water).

The last two decades have seen a great rise in research focused on the electron transport properties of various biological molecules. Behind this interest are two main goals: A) to deepen our understanding of natural processes involving charge transport, and B) to explore the potential of biomolecules to be incorporated into bio-electronic devices. Among other classes of biomolecules, proteins enjoy particular attention due to their natural diversity in structure and function, including those related to charge transport. Natural redox proteins are known to facilitate electron transport over long distances (nanometer scale), making cellular processes like photosynthesis and respiration possible. Although much effort has been put into investigating which mechanisms allow proteins to support electron flow – that is, in terms of efficiency comparable to that of organic semiconductors – many aspects remain unresolved.

The current view on electron transport through proteins envisions two major mechanisms: coherent tunneling and incoherent vibrationally-assisted hopping. They can be distinguished experimentally by the difference in distance dependence and temperature dependence. The tunneling efficiency depends on the width of the tunneling barrier, L (kET ~ exp(-L)) and at room temperature is not expected to contribute significantly at distances longer than ~2.5 nm for saturated and 3.5-4 nm for conjugated molecules; hopping is less sensitive to the distance to be traversed by the electrons (kET ~ 1/L) and supports electron transport over longer distances. In this scenario, a protein presents an extreme case. For proteins, studying distance dependence of electron transport rate to determine the mechanism requires a set of complicated experiments and is anyway inherently limited by the dimensions of the protein. Therefore, temperature-resolved current measurements are more suitable for this task. Most proteins measured so far conduct electrons by coherent tunneling and display high-efficiency temperature-independent conduction over the whole range of measured temperatures, and some other proteins, show very low activation energies (~1-10 meV) and can be considered essentially temperature-independent. This finding is surprising given the distances of electron transport through these proteins (~ 10 nm). The mechanism(s) and structural basis of this high-efficiency tunneling are unknown and are at the focus of the proposed research. In my research proposal, I wish to understand the fundamental basis for the long-range temperature-independent transport behavior, the importance of coherence in charge transport, and what leads to its destruction.

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