Decoupling Ionic and Electronic Currents in Melanin

Abstract

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 × 10 3 . Surprisingly, this very high value is maintained over a wide frequency range of ≈20-10 4 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.

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

“…We also note that the temperature dependent features in the permittivities (including similar values and frequency features) show the same qualitative behavior as those for melanin thin lms 34 (see also Fig. S1 †) that are successively hydrated.…”

“…The aforementioned mixed ionic/ electronic viewpoint should actually be further narrowed to viewing melanin as a proton conductor exclusively based upon recent impedance work in which thin lms of melanin were subject to hydration control. 34 Based upon this hydration dependent behaviour, the origin of the conductivity in melanin is ascribed to a comproportionation reaction (Fig. 1, pathways IV and V) where adding water yields hydronium and semiquinone radicals for conduction.…”

Electronic and protonic transport in bio-sourced materials: a new perspective on semiconductivity

“…protons and for conduction in the biopigment). 37,38 From these considerations, the question whether in biosourced materials the band theory of transport holds for ions and for the special case of protons, naturally arises. In the quest to distinguish between semiconducting and nonsemiconducting bio-sourced materials, accounting for the main type of charge carrier may not be the right approach.…”

A Skin‐Inspired, Interactive, and Flexible Optoelectronic Device with Hydrated Melanin Nanoparticles in a Protein Hydrogel–Elastomer Hybrid

“…These functions are possible because of melanin's broadband light absorption and mixed electronic–ionic conductivity. [ 4–6 ] The pigment is composed of oligomeric and polymeric species such as indolic monomers 5,6‐dihydroxyindole (DHI), 5,6‐dihydroxyindole‐2‐carboxylic acid, and their various redox forms, including quinone (Q), hydroquinone (H 2 Q), and semiquinone (SQ). [ 7 ] Broadband light absorption originates from the existence of multiple bandgaps in the range of 2.0–3.7 eV that these distinct chemical species make.…”