Effects of hydrogen on the electronic properties of synthetic melanin

Abstract

In this work the influence of thermal treatments on the structural and electronic properties of synthetic melanins was investigated. To that end, thermal gravimetry, infrared spectroscopy, electron spin resonance, and dc conductivity were employed. Our results indicate that the water corresponds to 10%–20% of the mass of synthetic melanin and also show that the latter is rich in mobile interlayer hydrogens or protons. Extended heating above 60°C decreases the conductivity by three orders of magnitude and increases the spin density by a factor of 3. This enhanced spin concentration and decreased conductivity are stable at room temperature, but can be reestablished by rehydration. Evidence is given that these properties are related to the presence of mobile interlayer protons. Prolonged heating is responsible for proton trapping by carboxylic groups forming COOH moieties, leading to the changes observed in electronic properties.

Eumelanin thin films: solution-processing, growth, and charge transport properties

“…14, 15 Our measurements, at short and long timescales support the hypothesis of mixed ionic-electronic conduction and give new insights into the contribution of these two types of charge carriers at different hydration states.…”

“…Furthermore, Au or Ag were typically used as electrode materials in melanin thin lm devices. 13,15,21,22,25 However the use of these metals should be avoided, since they can electrochemically react with eumelanin, as we recently demonstrated.…”

“…11,12 There is now growing evidence that protons play an important role in the charge carrier transport in eumelanin. [13][14][15] The potential for mixed ionic-electronic conduction combined with anti-oxidant, metal chelation properties, and the intrinsic biocompatibility of eumelanin make it an interesting candidate for organic bioelectronic applications.…”

“…Studies of electrical properties of eumelanin thin lms reported so far have been carried out in vacuum and ambient air (i.e. low lm hydration) 13,15,20,[22][23][24] or are limited to the determination of the lm conductivity at 100% relative humidity (RH). 21,25 Studies on (partly) hydrated eumelanin lms were commonly performed at high voltages ($10 V), 22,24,25 which might result in undesired currents due to water electrolysis, for example.…”

On the origin of electrical conductivity in the bio-electronic material melanin

“…7,[11][12][13][14][15][16] A full model of charge transport in melanin has remained elusive for a number of reasons including (1) its electrical properties are very sensitive to hydration 9,[16][17][18][19][20] and (2) there are difficulties associated with forming good ohmic contacts for accurate and reproducible measurements (due to surface roughness). These issues have led to observations of apparent n and/or p-type transport dependent upon experimental conditions, 19,21 anomalous Arrhenius behaviour, 9,13,16,18,22 dominant capacitative effects reminiscent of ionic materials, 17,18,20 and a general lack of agreement as to how electrically conductive melanin actually is. 7 In this letter, we report the first systematic and controlled study of the conductivity of melanin as a function of hydration (both in the dark and under illumination) with different electrical contact geometries.…”