Proteins can conduct electrons efficiently over tens of nanometers when contact resistance is removed, revealing their potential as functional components in solid-state bioelectronic devices.
(Nanowerk Spotlight) Electricity in biology is usually associated with nerves firing, muscles contracting, or heart rhythms pulsing. But beneath those visible effects, there are electrons moving through molecular structures at nanometer scales. This movement is essential to life, yet its potential to carry charge in solid-state devices has remained largely unexplored.
Proteins, the folded molecular machines responsible for most biological functions, have attracted interest as possible components in bioelectronic circuits. Their complex structure, stability, and precise self-assembly make them appealing candidates for integration into miniaturized devices.
Still, a fundamental uncertainty remains. Can proteins support electronic charge transport over the kinds of distances relevant for solid-state technology? Most proteins are built from chains of amino acids with only sparse regions that can carry charge. They lack the extended conjugated structures that make organic semiconductors or metals efficient conductors.
Despite this, several studies have reported measurable current passing through dry protein films tens of nanometers thick. These observations do not fit well with standard models of charge transport. Tunneling is unlikely over such distances, and the lack of temperature dependence rules out activated hopping.
Before assuming that proteins exhibit a new kind of transport, however, another explanation must be considered. In many systems, the dominant resistance occurs not in the material itself but at the interface with the electrode. This contact resistance can distort or obscure the true electrical behavior of the molecule. In molecular electronics, it is well known that the quality of this interface can determine the outcome of an entire measurement. The same is likely true for proteins.
The researchers present an experimental framework that allows them to isolate the intrinsic charge transport properties of protein films by removing the influence of contact resistance. Their work not only provides clarity on the long-standing question of whether proteins can function as efficient conductors but also introduces a method for precisely measuring transport properties in molecular junctions.
Left: equivalent circuit of solid-state protein-based devices, irrespective of protein type and junction configurations; series resistance (RS), protein resistance (RP), and protein capacitance (CP). Specific color coding in figure on the left represents different circuit components: green—protein part, dark brown— whole circuit with terminal leads and external wire connection without protein, and red—protein/electrode interface regions that contribute to contact resistance
. In practice, the value that is measured as RP includes
. Right: schematic cross-section of the junction with the proteins (green) and the interface regions on both sides of the protein/electrode contact interfaces (light and darker red-brown). (Image: Reprinted from DOI:10.1002/adma.202507654, CC BY)
The study examines two proteins with distinct structures. Human serum albumin is a globular protein that lacks metal cofactors or conjugated systems. Bacteriorhodopsin is a membrane protein with an embedded light-sensitive group. These were chosen to test whether different types of proteins exhibit similar or distinct transport behavior once contact resistance is removed.
To measure electron transport, the researchers built three types of solid-state junctions. The first used silicon and gold electrodes. The second used a top contact made of a soft gallium-indium alloy. The third, referred to as the micropore device, used thermally evaporated gold and palladium electrodes applied through a defined opening in a thin insulating layer. Each configuration allowed electrons to pass through protein films of varying thickness.
In a typical two-terminal device, the total measured resistance includes contributions from three regions: the protein layer, the bottom contact, and the top contact. The challenge is to isolate the resistance of the protein layer itself. To do this, the team used both direct current and impedance spectroscopy measurements and plotted resistance as a function of film thickness. By extrapolating this relationship to zero thickness, they obtained a value known as the zero length resistance. This captures resistance from the contacts alone.
The researchers compared the zero length resistance to the series resistance of the electrodes, which they also measured independently. From this, they calculated a dimensionless contact resistance factor. This value reflects how much the electrode interfaces contribute to the total resistance of the system. A high value indicates that contact effects dominate. A value near zero indicates that the measurement captures mostly the intrinsic behavior of the protein.
The results varied sharply depending on the configuration. The silicon-gold and gallium-indium setups both showed large contact resistance factors. These configurations include oxide layers at the electrode surfaces, which appear to interact with the proteins in ways that block or distort charge injection. The micropore device, by contrast, showed contact resistance values close to zero. In this setup, the measured current reflects the behavior of the protein film itself.
Once contact resistance was eliminated, both proteins showed efficient electron transport across distances up to 60 nanometers. The resistance of the junction increased with thickness in a way that followed a clear exponential trend. The slope of this trend gave a decay constant, which describes how quickly the current drops off with distance. This value was about 0.7 per nanometer for bacteriorhodopsin and about 1.1 per nanometer for human serum albumin.
These are relatively low values. In systems made from short organic molecules or oligopeptides, decay constants are often in the range of 3 to 7 per nanometer. A lower decay constant means the material supports charge transport over longer distances with less resistance. The findings suggest that when measured cleanly, proteins can serve as effective conductors across scales relevant for solid-state devices.
It is important to note that the researchers do not claim to have identified the mechanism of transport. The distances involved rule out tunneling. The temperature independence rules out thermally activated hopping. Instead, the decay constant is treated as a measure of transport efficiency. The findings do not explain how electrons move through the proteins, only that they do so with surprising effectiveness under certain conditions.
The team extended their analysis to other molecular systems from the literature. They re-examined data from earlier studies on alkyl chains, peptides, and conjugated molecules. In each case, they applied the same method of separating contact resistance from the intrinsic properties of the junction. The reanalysis confirmed that the interface between the molecule and the electrode plays a critical role. It also showed that even molecules known to conduct well can appear insulating when the contacts are poorly matched.
The micropore device used in this study appears to solve several long-standing problems in molecular electronics. By providing a stable, clean metal interface and well-defined geometry, it enables measurements that are both reproducible and meaningful. It also avoids mechanical instabilities found in other methods, such as soft metal cones or manually applied contacts. This platform may serve as a new standard for evaluating molecular materials.
The results have broader implications for bioelectronics. Proteins are attractive candidates for electronic applications because of their biological compatibility, chemical diversity, and ability to self-assemble into ordered structures. The findings in this study suggest that with the right electrode design, proteins can be used in applications where long-range electron transport is required. This includes devices such as sensors, memory elements, or energy harvesters based on protein layers.
More broadly, the work shows that accurate measurement of molecular conductivity requires careful attention to the role of interfaces. This principle applies not just to proteins but to any system where soft or complex materials are integrated into solid-state devices. By establishing a method to separate contact effects from material properties, the researchers provide a tool that can be applied across a wide range of molecular and hybrid systems.
This work identifies contact resistance as the main barrier to measuring the true electronic behavior of proteins and offers a method to remove that interference. By separating interface effects from intrinsic transport, the study shows that proteins can move charge efficiently over long distances when measured under the right conditions. The micropore device provides a stable platform for such measurements and sets a standard for evaluating molecular conductivity in soft materials.
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