Mono-exponential Current Attenuation with Distance across 16 nm Thick Bacteriorhodopsin Multilayers

TL;DR

This study demonstrates a mono-exponential decay of electrical conductance with distance in bacteriorhodopsin multilayers up to 16 nm thick, revealing efficient long-range electron transport that challenges existing theories.

Contribution

It provides the first detailed measurement of exponential current attenuation in thick bacteriorhodopsin layers and suggests the need for new theoretical models for protein-based charge transport.

Findings

Conductance decays mono-exponentially with a small attenuation coefficient (~0.8 nm^{-1})

Effective energy barriers of about 100 meV are observed from temperature-dependent measurements

Transport is consistent with tunneling through protein-protein and protein-electrode interfaces

Abstract

The remarkable ability of natural proteins to conduct electricity in the dry state over long distances remains largely inexplicable despite intensive research. In some cases, a (weakly) exponential length-attenuation, as in off-resonant tunneling transport, extends to thicknesses even beyond 10 nm. This report deals with such charge transport characteristics observed in self-assembled multilayers of the protein bacteriorhodopsin (bR). About 7.5 nm to 15.5 nm thick bR layers were prepared on conductive titanium nitride (TiN) substrates using aminohexylphosphonic acid and poly-diallyl-dimethylammonium electrostatic linkers. Using conical EGaIn top contacts, an intriguing, mono-exponential conductance attenuation as a function of the bR layer thickness with a small attenuation coefficient $\beta \approx 0.8 \space {\rm nm}^{-1}$ is measured at zero bias. Variable-temperature measurements using evaporated Ti/Au top contacts yield effective energy barriers of about 100 meV from fitting the data to tunneling, hopping, and carrier cascade transport models. The observed temperature-dependence is assigned to the protein-electrode interfaces. The transport length and temperature dependence of the current densities are consistent with tunneling through the protein-protein and protein-electrode interfaces, respectively. Importantly, our results call for new theoretical approaches to find the microscopic mechanism behind the remarkably efficient, long-range electron transport within bR.