Abstract
Recent studies showed that silk and human hair fibers develop thermoelectric properties at optimal water, temperature and light conditions. The nature of charge carriers and the role of water in mediating charge conduction in these fibers is an unexplored issue. By studying four different classes of natural fibers, viz., silk cocoon, human hair, jute and corn silk, we uncover their common electrical transport properties and its dependence on water concentration and temperature. All these fibers uniformly exhibit nonlinear, hysteretic current - voltage characteristics, which scale with water concentration. The optimal electrical conductivity shows thermally activated hopping transport mechanism. Scanning tunneling microscope (STM) and dielectric measurements of silk cocoon fibers showed the electronic density of states and dielectric properties of the hydrated medium enhances with water concentration. Electron paramagnetic resonance (EPR) study reveals that the charge carriers in these membranes are electronic in nature. Our results are explained through the mechanism of hopping of a Polaron, which is an electron surrounded by positive charge fluctuations created by water molecules. The mechanism unravels the peculiar role water plays in mediating electrical activity in these membranes and also opens the possibility for exploring such charge transport mechanism in other biological membranes.
Highlights
Recent studies show that silk and human hair fibers have novel thermoelectric properties[1,2]
Electron paramagnetic resonance (EPR) measurements unravel the electronic nature of the charge carriers in these membranes and the presence of water weakens the coupling of the charges with their complex chemical environment
A reason we study materials with chemically distinct macromolecules, like, silk cocoon membranes, human hair, corn silk and jute, is to check if the charge transport properties differ with the chemical composition of these materials
Summary
Recent studies show that silk and human hair fibers have novel thermoelectric properties[1,2]. In biological membranes charge carriers can be either electrons or protons[3] and charge conduction occurs in an aqueous medium. Studies suggest that the transport of protons across biological macromolecules[4,7,11] like proteins occur via the formation of an intermediate ionic complex of water molecules Our scanning tunneling microscope (STM) measurements show strong water induced enhancement in the electronic density of states and the development of a local internal electric field within the membrane. We show these highly resistive membranes develop significant dielectric polarizability with a large dielectric constant ~169 in the presence of water. We model the charge transport in these hydrated dielectric membranes occurs via the model of hopping of a Polaron complex
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