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

Abstract

We demonstrate on synthetic eumelanin that biomolecular conductivity models should account for temperature and hydration effects coherently.

Melanin, the What, the Why and the How: An Introductory Review for Materials Scientists Interested in Flexible and Versatile Polymers

“…The photoconductivity behavior was also shown to be hydration dependent [106], and hence, trap states may not be the preferred explanation but that water desorption due to localized heating causes a drop in available charges. Furthermore, the temperature dependence can also be explained by the same equilibria, since equilibrium chemistry also exhibits Arrhenius, semiconducting like behavior [99]. This model has been bolstered by kinetic isotope work utilizing D 2 O hydration [89] and demonstration of photo induced semiquinone production alongside photoconductivity [92].…”

“…As such, melanin has been investigated in semiconductor related devices. One thing that is interesting about all the observations above, is that melanin's conductivity in the condensed matter state can be significantly changed depending on its water content (see Figure 10) [77,80,99,[106][107][108]. Indeed, the switching behavior of melanin could not be induced in the dry state [107], though recent work suggests that it can (i.e., Figure 9, left) [105].…”

“…Melanin has been viewed as an example of an amorphous semiconductor since the 1970s due to a couple of observations: its broad band optical absorbance profile indicating localized states in the gap [93], the presence of the aforementioned free radical signal suggesting electrons at the Fermi level available for conduction, semiconductor like behavior of the conductivity as a function of temperature (e.g., [80,[94][95][96][97][98][99]), drop in current after photocurrent measurement indicating trapped states as well as temperature dependent photocurrent behavior [100,101] and finally the ability of melanin to demonstrate bi-stable switching behavior after an application of a high enough voltage (Figure 9) [102][103][104][105] (a key characteristic of an amorphous semiconductor [7]). As such, melanin has been investigated in semiconductor related devices.…”

Melanins as Sustainable Resources for Advanced Biotechnological Applications

“…[ 399,400 ] Studies of reduction/oxidation processes and electron transfer using cyclic voltammetry are particularly useful for melanins, with fundamental studies on melanins formed chemically from single monomers (e.g., l ‐DOPA, [ 401 ] DHI, [ 402 ] 3,4‐dihydroxyphenylacetic acid, [ 403 ] HGA, [ 404,405 ] DHN [ 231 ] ), combinations of DHI and DHICA, [ 406–408 ] and natural melanins from bacteria (e.g., Shewanella oneidensis MR‐1, [ 157 ] Pseudomonas aeruginosa [ 409 ] ), plants (including fungi: basidial fungi, [ 410 ] Cryptococcus neoformans [ 411 ] and Nigella sativa [ 412 ] ), cuttlefish ( Sepia officinalis [ 407 ] ), and human hair‐derived pheomelanins. [ 413 ] Electrochemical impedance spectroscopy and dielectric spectroscopy enabled the rational investigation of the protonic and electronic contributions, suggesting melanins are protonic conductors, [ 414–416 ] which is important because the electrical properties of melanins [ 417–422 ] underpin their potential technical and medical applications, [ 423–425 ] and it is noteworthy that the potential for melanins in electronics has seen an explosion of interest (see Figure [ 426 ] ).…”

Evaluating the Conductive Properties of Melanin-Producing Fungus, Curvularia lunata, after Copper Doping

“…When the two melanin molecules undergo redox, free radicals and protons are generated, allowing for the transfer of charge (Figure 3) (Meredith and Sarna 2006;Mostert et al 2016). Because the transfer of charge commonly occurs between water molecules and melanin molecules, the resulting conductivity of melanin is primarily influenced by hydration state (Jastrzebska et al 1996;Meredith and Sarna 2006); but temperature, and structure also play a crucial role (Meredith and Sarna 2006;Motovilov et al 2019) and together potentially allow the conductance to be controllable. Early research into the conductivity of melanin established a conductive range between 10 −13 and 10 −5 S/cm * (McGinness et al 1974;Jastrzebska et al 1996).…”