Conducting organic polymers find applications in bioelectronics and biomaterials. Eumelanin is one of a few examples that occur naturally and would consequently be more compatible for biotechnology applications.1 Eumelanin, a heterogenous biomacromolecule, appears to have hybrid ionic-electronic semiconductivity.2 Although it has been widely studied for many years, structure-property relationships of eumelanin in the solid state remain unclear. It is highly insoluble at neutral and acidic pH, making characterisation difficult. Electron Paramagnetic Resonance (EPR) spectroscopy has contributed to the majority of what has been established about eumelanin.1
In contrast to EPR studies of eumelanin in solution, which reveal the presence of a semiquinone radical whose concentration depends upon the comproportionation reaction, EPR studies of solid state samples reveal a resonance characteristic of a carbon centred radical.1,3,4 Recent hydration controlled EPR experiments of solid state eumelaninn from our laboratory suggests eumelanin has at least two populations of radicals centres, at least one governed by the comproportionation reaction.5
Herein we report the resolution of the multiple components of the eumelanin EPR signal, and determine which radical centre is responsible for electrical and photo conductivity in eumelanin. Power saturation measurements were taken as a function of pH and D2O hydration and compared with a previous H2O hydration controlled EPR study.5 The results demonstrate the presence of multiple radical centres as well as exchangeable protons. Utilising X-band resonators with different microwave magnetic field strengths has also allowed separation of the components.