Proteins as Electronic Materials: Electron Transport through Solid-State Protein Monolayer Junctions

Abstract

Electron transfer (ET) through proteins, a fundamental element of many biochemical reactions, is studied intensively in aqueous solutions. Over the past decade, attempts were made to integrate proteins into solid-state junctions in order to study their electronic conductance properties. Most such studies to date were conducted with one or very few molecules in the junction, using scanning probe techniques. Here we present the high-yield, reproducible preparation of large-area monolayer junctions, assembled on a Si platform, of proteins of three different families: azurin (Az), a blue-copper ET protein, bacteriorhodopsin (bR), a membrane protein-chromophore complex with a proton pumping function, and bovine serum albumin (BSA). We achieve highly reproducible electrical current measurements with these three types of monolayers using appropriate top electrodes. Notably, the current-voltage (I-V) measurements on such junctions show relatively minor differences between Az and bR, even though the latter lacks any known ET function. Electron Transport (ETp) across both Az and bR is much more efficient than across BSA, but even for the latter the measured currents are higher than those through a monolayer of organic, C18 alkyl chains that is about half as wide, therefore suggesting transport mechanism(s) different from the often considered coherent mechanism. Our results show that the employed proteins maintain their conformation under these conditions. The relatively efficient ETp through these proteins opens up possibilities for using such biomolecules as current-carrying elements in solid-state electronic devices.

Solid-state electron transport via cytochrome c depends on electronic coupling to electrodes and across the protein

“…1A). The order of magnitude of the measured currents (at RT) is similar (although severalfold lower at negative bias) to what we measured via azurin (Az), a Cu-containing electron-mediating protein with a molecular weight similar to that of CytC, which forms monolayers with similar density (27,29).…”

“…For measuring the ETp via adsorbed CytC, we functionalized the surface of high-doped Si with a carboxylate (see Experimental Procedures for further details). The successful formation of the CytC monolayers on the carboxylate-carrying surface was confirmed by ellipsometry (from which an optical thickness of 17-18 Å is deduced, comparable to that found for monolayers of other proteins of this size) (27)(28)(29), and by atomic force microscopy. The electronic conductance via the protein at room temperature (RT) was measured with an Hg drop as the top electrode, with an ∼400-μm diameter contact area, yielding rather symmetric current-voltage (I-V) curves (Fig.…”

“…These were bound covalently to one electrode via a thiolate-terminated linker, a procedure that we used earlier to form monolayers of Az (27,29). It has been argued that the electrostatically bound CytC monolayer has a preferred orientation, where the heme of the CytC is located proximal (<1 nm) to the carboxylate-terminated surface (15)(16)(17)(18).…”

“…1A). The intrinsic Cys residues of Az allow forming monolayers by covalent binding to the surface of an Au electrode (S-Au bond) (31) or via a thiolate-carrying linker (S-S bond) (27,29). In the produced orientation, the Az redox center, the Cu ion is very close (∼0.7-0.8 nm) to the top electrode.…”

Sensing of molecules using quantum dynamics

“…Eq. 10 can be used directly in the modeling of electron transport through solid-state protein junctions (27,28) or redox junctions (29)(30)(31)(32). The semiconductor's band structure should be used for quantitative descriptions of conduction through typical FET bridges.…”