On the origin of electrical conductivity in the bio-electronic material melanin

The skin pigment melanin is one of a few bio-macromolecules that display electrical and photo-conductivity in the solid-state. A model for melanin charge transport based on amorphous semiconductivity has been widely accepted for 40 years. In this letter, we show that a central pillar in support of this hypothesis, namely experimental agreement with a hydrated dielectric model, is an artefact related to measurement geometry and non-equilibrium behaviour. Our results cast significant doubt on the validity of the amorphous semiconductor model and are a reminder of the difficulties of electrical measurements on low conductivity, disordered organic materials. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3688491

Decoupling Ionic and Electronic Currents in Melanin

Melanin, the human skin pigment, has emerged as a model material for bioelectronic interfaces due to its biocompatibility, ability to be processed into electronic‐device‐grade thin films, and transducing charge transport properties. These charge transport properties have been suggested to be of a mixed protonic/electronic nature, regulated by a redox reaction that can be manipulated by changing the material's hydration state. However, to date, there are no detailed reports which clarify, quantify, or disentangle the protonic and electronic contributions to long‐range current conduction in melanin. Described herein, is a systematic hydration controlled electrical study on synthetic melanin thin films utilizing impedance/dielectric spectroscopy, which rationally investigates the protonic and electronic contributions. Through modeling and inspecting the frequency dependent behavior, it is shown that the hydration dependent charge transport is due to proton currents. Results show a real dielectric constant for hydrated melanin of order ≈1 × 103. Surprisingly, this very high value is maintained over a wide frequency range of ≈20–104 Hz. The electronic component appears to have little influence on melanin's hydration dependent conductivity: thus the material should be considered a protonic conductor, and not as previously suggested, a mixed protonic/electronic hybrid

Biomimetic Peptide Nanowires Designed for Conductivity

The filamentous peptide-based nanowires produced by some dissimilatory metal-reducing bacteria, such as Geobacter sulfurreducens, display excellent natural conductivity. Their mechanism of conduction is assumed to be a combination of delocalized electrons through closely aligned aromatic amino acids and hopping/charge transfer. The proteins that form these microbial nanowires are structured from a coiled-coil, for which the design rules have been reported in the literature. Furthermore, at least one biomimetic system using related synthetic peptides has shown that the incorporation of aromatic residues can be used to enhance conductivity of peptide fibers. Herein, the de novo design of peptide sequences is used to enhance the conductivity of peptide gels, as inspired by microbial nanowires. A critical factor hampering investigations in both microbiology and materials development is inconsistent reporting of biomaterial conductivity measurements, with consistent methodologies needed for such investigations. We have reported a method herein to analyze non-Ohmic behavior using existing parameters, which is a statistically insightful approach for detecting small changes in biologically based samples. Aromatic residues were found to contribute to peptide gel conductivity, with the importance of the peptide confirmation and fibril assembly demonstrated both experimentally and computationally. This is a small step (in combination with parallel research under way by other researchers) toward developing effective peptide-based conducting nanowires, opening the door to the use of electronics in water and physiological environments for bioelectronic and bioenergy applications

Redox chemistry in the pigment eumelanin as a function of temperature using broadband dielectric spectroscopy

Conductive biomolecular systems are investigated for their promise of new technologies. Onebiomolecular material that has garnered interest for device applications is eumelanin. Its unusualproperties have led to its incorporation in a wide set of platforms including transistor devices andbatteries. Much of eumelanin's conductive properties are due to a solid state redox comproportionationreaction. However, most of the work that has been done to demonstrate the role of the redox chemistryin eumelanin has been via control of eumelanin's hydration content with scant attention given totemperature dependent behavior. Here we demonstrate for the first time consistency between hydrationand temperature effects for the comproportionation conductivity model utilizing dielectric spectroscopy,heat capacity measurements, frequency scaling phenomena and recognizing that activation energies inthe range of ~0.5 eV correspond to proton dissociation events. Our results demonstrate thatbiomolecular conductivity models should account for temperature and hydration effects coherently.NOTE: Abstract is from RSC Advances, 2019, by Motovilov et al, under the creative commons licence CC BY-NC 3.0. No changes made to the abstract. Not used for commercial purposes

Hydration-controlled X-band EPR spectroscopy: a tool for unravelling the complexities of the solid-state free radical in eumelanin

Melanin, the human skin pigment, is found everywhere in nature. Recently it has gained significant attention for its potential bioelectronic properties. However, there remain significant obstacles in realizing its electronic potential, in particular, the identity of the solid-state free radical in eumelanin, which has been implicated in charge transport. We have therefore undertaken a hydration-controlled continuous-wave electron paramagnetic resonance study on solid-state eumelanin. Herein we show that the EPR signal from solid-state eumelanin arises predominantly from a carbon-centered radical but with an additional semiquinone free radical component. Furthermore, the spin densities of both of these radicals can be manipulated using water and pH. In the case of the semiquinone radical, the comproportionation reaction governs the pH- and hydration-dependent behavior. In contrast, the mechanism underlying the carbon-centered radical’s pH- and hydration-dependent behavior is not clear; consequently, we have proposed a new destacking model in which the intermolecular structure of melanin is disordered due to π–π destacking, brought about by the addition of water or increased pH, which increases the proportion of semiquinone radicals via the comproportionation reaction

Gaseous adsorption in melanins: Hydrophilic biomacromolecules with high electrical conductivities

The melanins are an important class of multifunctional biomacromolecules that possess a number of intriguing physical and chemical properties including electrical and photoconductivity. Unusually for a conducting organic material, eumelanin is hydrophilic and its electrical properties are strongly dependent on its hydration state. We have therefore measured adsorption isotherms for two polar adsorbates, water and ethanol, in the pressed powder pellets of synthetic eumelanin typically used in electrical studies. We show that a simple kinetic monolayer Langmuir model describes the adsorption and find that there are strong adsorbate−eumelanin interactions in both cases. These isotherms allow the proper scaling of electrical conductivity data and in doing so make progress toward a better understanding of eumelanin electrical properties, which is a critical prerequisite to the design of new eumelanin-like bioelectronic materials

Microbial nanowires – Electron transport and the role of synthetic analogues

Electron transfer is central to cellular life, from photosynthesis to respiration. In the case of anaerobic respiration, some microbes have extracellular appendages that can be utilised to transport electrons over great distances. Two model organisms heavily studied in this arena are Shewanella oneidensis and Geobacter sulfurreducens. There is some debate over how, in particular, the Geobacter sulfurreducens nanowires (formed from pilin nanofilaments) are capable of achieving the impressive feats of natural conductivity that they display. In this article, we outline the mechanisms of electron transfer through delocalised electron transport, quantum tunnelling, and hopping as they pertain to biomaterials. These are described along with existing examples of the different types of conductivity observed in natural systems such as DNA and proteins in order to provide context for understanding the complexities involved in studying the electron transport properties of these unique nanowires. We then introduce some synthetic analogues, made using peptides, which may assist in resolving this debate. Microbial nanowires and the synthetic analogues thereof are of particular interest, not just for biogeochemistry, but also for the exciting potential bioelectronic and clinical applications as covered in the final section of the review.Some microbes have extracellular appendages that transport electrons over vast distances in order to respire, such as the dissimilatory metal-reducing bacteria Geobacter sulfurreducens. There is significant debate over how G. sulfurreducens nanowires are capable of achieving the impressive feats of natural conductivity that they display: This mechanism is a fundamental scientific challenge, with important environmental and technological implications. Through outlining the techniques and outcomes of investigations into the mechanisms of such protein-based nanofibrils, we provide a platform for the general study of the electronic properties of biomaterials. The implications are broad-reaching, with fundamental investigations into electron transfer processes in natural and biomimetic materials underway. From these studies, applications in the medical, energy, and IT industries can be developed utilising bioelectronics

Heavy Water as a Probe of the Free Radical Nature and Electrical Conductivity of Melanin

Melanins are pigmentary macromolecules found in many locations throughout nature including plants and vertebrate animals. It was recently proposed that the predominant brown-black pigment eumelanin is a mixed ionic–electronic conductor which has led to renewed interest in its basic properties as a model bioelectronic material. This exotic hybrid electrical behavior is strongly dependent upon hydration and is closely related to the free radical content of melanin which is believed to be a mixed population of two species: the semiquinone (SQ) and a carbon-centered radical (CCR). The predominant charge carrier is the proton that is released during the formation of the SQ radical and controlled by a comproportionation equilibrium reaction. In this paper we present a combined solid-state electron paramagnetic resonance (EPR), adsorption, and hydrated conductivity study using D₂O as a probe. We make specific predictions as to how the heavy isotope effect, in contrast to H₂O, should perturb the comproportionation equilibrium and the related outcome as far as the electrical conductivity is concerned. Our EPR results confirm the proposed two-spin mechanism and clearly demonstrate the power of combining macroscopic measurements with observations from mesoscopic probes for the study of bioelectronic materials