Redox behavior of eumelanin derivates for bioelectronics

Eumelanins are an important class of the natural pigments with attractive physicochemical properties such as photoprotection and thermoregulation[1]. Unfortunately, eumelanins are poorly insoluble in several common solvents[2] making challenging the deposition of thin films for device applications. To improve their solubility, eumelanins derivatives have been synthetized using organic solvents such as DMSO (D-eumelanin) by incorporating sulfonate groups (-SO2CH3) in the phenolic hydroxyl positions of DHI (5,6-dihydroxyindole) and DHICA (5,6-dihydroxyindole-2-carboxylic acid) eumelanin building blocks [3][4][5]. To shed light onto the transport properties in D-eumelanin, in view of applications in bioelectronics, we performed a systematic study of the redox processes in D-eumelanin, synthetized at room temperature (RT) and at 100°C, by cyclic voltammetry (CV, using eumelanin on carbon paper as work electrode, platinum wire as the counter electrode, and aqueous ammonium acetate at pH 5.5 as the electrolyte, different cycle numbers and 5, 10, and 50 mV/s scan rates were conducted). Sigma eumelanin was used as our control material. D-eumelanin synthetized at RT and at 100 °C showed an intense oxidation peak 0.4 V with well-defined character and a reduction peak around 0.25 V. After five cycles, a capacitive behavior was dominant. An intense and irreversible oxidation peak, located at about 0.5 V was observed for Sigma eumelanin. The irreversible peak for the Sigma eumelanin has been interpreted as resulting from the covalent coupling of intermediate species forming at the positive electrode interface[6]. The peaks for the D-eumelanins are attributable to the hydroquinone/semiquinone/quinone redox species which exhibited an increased in the reversibility when the scan rate is increased. Raman spectroscopy revealed that there is no significant change in the molecular structure after the CVs except for the soufonate groups and hydroxyls. No significant difference was detected between D-eumelanin synthetized RT and 100ºC from the electrochemical and structural point of view. Based on the stability observed upon electrochemical tests, D-eumelanin has a good potential for applications in bioelectronics.

References: [1] P. Meredith, and T. Sarna. The physical and chemical properties of eumelanin. Pigment Cell Res., 2006, 19, 572-594 [2] G. Prota, Melanins and Melanogenesis, first ed., Academic Press, San Diego, USA, 1992 [3] S. N. Dezidério, C. A. Brunello, M. I. N. da Silva, M. A. Cotta, and C. F. O. Graeff. Thin films of synthetic melanin. Journal of Non-Cryst. Solid, 2004, 338-340, 634-638 [4] E. S. Bronze-Uhle, A. Batagin-Neto, P. H. P. Xavier, N. I. Fernandes, E. R. de Azevedo, and C. F. O. Graeff. Synthesis and characterization of melanin in DMSO. J. Molec. Struct., 2013, 1047, 102-108 [5] M. Piacenti-Silva, E. S. Bronze-Uhle. J. V. Paulin, and C. F. O. Graeff. Temperature-enhanced synthesis of DMSO-Melanin. J. Molec. Struct., 2014, 1056-1057, 135-140 [6] J. Wünsche, Y. Deng, P. Kumar, E. Di Mauro, E. Josberger, J. Sayago, A. Pezzella, F. Soavi, F. Cicoira, M. Rolandi, and C. Santato. Protonic and electronic transport in hydrated thin films of the pigment eumelanin. Chem. Mater., 2015, 27 (2), 436-442

Mots clés

• Biochemistry
• Cell Biology
• Analytical Chemistry
• Phytochemicals and Antioxidant Activities
• melanin and skin pigmentation
• Dye analysis and toxicity