Engineering proton conductivity in melanin using metal doping

Abstract

Long range electrical conduction in biomaterials is an increasingly active area of research, which includes systems such as the conductive pili, proteins, biomacromolecules, biocompatible conductive polymers and their derivatives. One...

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

“…For Fe(III) this equated to around 8.0% by mass of melanin. Similar values can be obtained for Cu(II) [88] and Zn(II) [113]. Again, whether one has a natural or synthetic melanin is an important consideration.…”

“…Proton conductivity has also been demonstrated [108], with protons apparently being the dominating charge carrier when hydrated [109], though suggestions of the presence of proton conductivity has a long history [77,80]. Finally, this model has been utilized, successfully, in creating copper doped melanin systems to enhance proton conductivity [88]. Again, it should be mentioned, that there are differences between natural and synthetic samples of melanin.…”

“…Initial work suggested a standard redox reaction between Cu 2+ ions [86] and the radical that modulates the signal of both, but later it was suggested that no chemical reaction occurs but is instead due to a physical, magnetic interaction [87], which is unusual. However, as with melanin research, this has been brought into question recently [88], leaving open questions for researchers to answer, not just philosophically, but with real material implications. Finally, as pertaining to melanin solutions, the extent of free radical production at high pH is much less than what would be expected for a standard ortho-quinone system.…”

Natural biopolymers as proton conductors in bioelectronics

“…In Figure 11C, by adjusting the humidity of the environment (0 mbar water vapor pressure-dry, 8 mbar-low hydration, 18 mbar-high hydration) the authors further verified that Recently, the Meredith group chelated the transition metal ion Cu (II) into melanin to enhance and control melanin's proton conductivity and the performance as a transducing material in OECTs. [162] The authors proposed that the generation of semiquinone radicals from the comproportionation reaction increases the reduction of Cu(II) along with the formation of quinone reactants, which in turn feedback into the comproportionation reaction to generate more free protons. Thus, the free proton concentration and proton conductivity of melanin are adjustable with controlled Cu upon hydration.…”

“…[38] The study of proton conducting bioelectronic devices is relatively more recent. [39][40][41][42][43][44][45] In this review, we discuss three types of natural biopolymers and their applications in bioelectronic devices as proton conductors (Figure 1). We discuss polysaccharides (chitosan and glycosaminoglycans [GAGs]), peptides and proteins, and the pigment melanin.…”