Publication date: 8th January 2019
The human skin pigment melanin is emerging as a promising bioelectronic material due its unique electrical properties such as broadband optical absorbance, electron paramagnetic free-radical signal, bistable electrical switching with water-dependent conductivity and potential protonic conduction [1]. Among all the available melanin derivatives, sulfonated species have interesting properties such higher thermal stability, biocompatibility, proton transport properties and solubility in dimethylsulfoxide, N,N-dimethylformamide and N-methyl-2-pyrrolidone which enables the production of homogeneous thin films with good adhesion even on hydrophobic surfaces [2]. Given that melanin’s stable free radicals could be a charge carrier or, at least, a spin label for charge transport [3], the evaluation of the paramagnetic properties could shed light on the sulfonated-melanin charge transport behavior and thus help to achieve its full bioelectronics potential. In such a way, in this work we performed hydration-controlled electron paramagnetic resonance (EPR) measurements on different sulfonated-melanins. At low and high microwave power, our results show that these derivatives present at least two different paramagnetic species with no significant difference upon hydration; behavior that was found to be similar to non-functionalized melanin. However, simulation analysis of the EPR spectra indicates that the sulfonated groups attached to the melanin structure may be responsible for an additional paramagnetic species in the system due to the presence of a new resonance line. Based on these results, we speculate that these derivatives could form a non-functionalized-like nanoparticle with the sulfonate groups located only on the surface, which in turn may indicate a similar charge transport behavior.
We thank São Paulo Research Foundation (FAPESP, Grant: 18/02411-1), CAPES, and CNPq for financial support.
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