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. One biomolecular material that has garnered interest for device applications is eumelanin. Its unusual properties have led to its incorporation in a wide set of platforms including transistor devices and batteries. Much of eumelanin's conductive properties are due to a solid state redox comproportionation reaction. However, most of the work that has been done to demonstrate the role of the redox chemistry in eumelanin has been via control of eumelanin's hydration content with scant attention given to temperature dependent behavior. Here we demonstrate for the first time consistency between hydration and temperature effects for the comproportionation conductivity model utilizing dielectric spectroscopy, heat capacity measurements, frequency scaling phenomena and recognizing that activation energies in the range of ∼0.5 eV correspond to proton dissociation events. Our results demonstrate that biomolecular conductivity models should account for temperature and hydration effects coherently.

Incipient ferroelectricity of water molecules confined to nano-channels of beryl.

Water is characterized by large molecular electric dipole moments and strong interactions between molecules; however, hydrogen bonds screen the dipole-dipole coupling and suppress the ferroelectric order. The situation changes drastically when water is confined: in this case ordering of the molecular dipoles has been predicted, but never unambiguously detected experimentally. In the present study we place separate H2O molecules in the structural channels of a beryl single crystal so that they are located far enough to prevent hydrogen bonding, but close enough to keep the dipole-dipole interaction, resulting in incipient ferroelectricity in the water molecular subsystem. We observe a ferroelectric soft mode that causes Curie-Weiss behaviour of the static permittivity, which saturates below 10 K due to quantum fluctuations. The ferroelectricity of water molecules may play a key role in the functioning of biological systems and find applications in fuel and memory cells, light emitters and other nanoscale electronic devices.