ABSTRACT
Melanin, the human skin pigment, is an emerging bioelectronic
material due to its unique electrical properties as well as being read-
ily prepared in electronic grade thin films on the nanometer scale.
These electrical properties include bistable electrical switching,
broadband optical absorbance, Arrhenius-dependent conductivity,
and an electron paramagnetic free-radical signal. Furthermore,
melanin has other electrical properties, such as water-dependent
conductivity and potential protonic conduction. However, to use
melanin as a bioelectronic material, greater clarity is required on
its charge transport behavior. Here we show that the current charge
transport model for melanin, an amorphous semiconductor model,
cannot describe melanin’s hydration-dependent conductivity. We
go on to show with a hydration-dependent muon spin resonance
(μSR) experiment thatmelanin’s charge transport properties are de-
scribed by a comproportionation reaction, inwhichwater self-dopes
the system with extra charge carriers. This new understanding of
melanin’s charge transport properties opens up new avenues of
exploration.We specifically seemelanin as a candidate for nanoscale
devices, which can act as transducers of ionic signals to electronic
signals.
