Elsevier

Polymer

Volume 46, Issue 25, 28 November 2005, Pages 11505-11509
Polymer

Electrochemical synthesis of melanin free-standing films

https://doi.org/10.1016/j.polymer.2005.10.068Get rights and content

Abstract

Free-standing melanin films were successfully synthesised electrochemically from dopa. The optimum synthetic conditions such as pH, concentration and current were determined, and it was found that free-standing films could only be formed when ITO glass electrodes were used. The films were analysed by solid state NMR and XPS which showed the presence of indolic moieties characteristic of melanin-type macromolecules. The film showed higher conductivity than chemically synthesised melanin previously reported in literature and also exhibited photoconductivity.

Introduction

Melanin is a class of biofunctional macromolecule which is the predominant pigment in the surface structure of vertebrates [1], [2]. It is most well known for its photoprotective role in humans, a property related to its strong absorbance in the UV and its antioxidant behaviour. There is also a suggestion that melanin may have some function in the nervous system due to its presence in the substantia nigra of the human brain [3], [4], [5], [6].

Eumelanin and phaeomelanin are the two predominant forms in humans. Eumelanin is the more common black–brown pigment commonly found in the skin, and is formed naturally by the oxidation of tyrosine through the Raper–Mason scheme [1] (Fig. 1).

Synthetically, melanin is usually made by the autooxidation of dopa in alkaline aqueous solution. However, due to its insolubility in most common solvents, the resultant melanin powder cannot be easily processed with conventional polymer processing methods. As far as technological materials are concerned, melanin has excellent chemical and photostability, and being a natural material it represents the ultimate in potential biocompatibility. Despite this, melanin-based devices are not widely reported, and so the main applications studied concerning melanin has been their use as UV filters [7]. This is mainly due to the processing difficulty outlined above, and hence for melanin-based devices to be fabricated a better synthetic route is required. An alternative method for melanin synthesis that has been reported recently is electrochemical synthesis, which has been widely used in the fabrication of conducting polymer thin films.

The first published work on electrochemically synthesised melanin was in an abstract by Zielinski [8]. However, this was not followed by a full publication until Horak and Weeks [9] published a paper showing melanin synthesis electrochemically through the oxidation of 5,6-dihydroxyindole (DHI). Since then, other researchers have synthesised melanin thin films from dopa [10], [11] or DHI [12], [13], [14]. However, these studies are concerned with melanin thin films as electrode coatings or modifiers, and none have so far synthesised melanin free-standing films, which is the next step towards the fabrication of melanin-based devices.

In this paper, we describe the fabrication of melanin free-standing films by means of electrochemical oxidation from basic aqueous solutions of dopa and the subsequent characterisation of these films.

Section snippets

Experimental

All chemicals were obtained from Sigma-Aldrich and used as received. Electrochemical synthesis was done using a Thandar TS3021S 30V/2A power supply connected to a Parameters 8040 multimeter in a two-electrode set-up, in which the potential was referenced against the counter electrode. The distance between the two electrodes was 3 cm.

The solution used was dl-dopa or l-dopa (Sigma-Aldrich) in 0.05 M borax (Sodium Tetraborate, Sigma-Aldrich) buffer (pH 9). The anode in the two-electrode set-up was

Results and discussion

Galvanostatic electropolymerisation of dl-dopa over a period of 6 days yielded thick, highly hydrated melanin films. The hydrated films were not uniform through their thickness, with the side attached to the electrode forming a film with reasonable mechanical stability while the side exposed to the electrolyte had the consistency of a thick paste. When dried, the melanin films reduced considerably in mass (the mass of dried film was about one-tenth the mass of the hydrated film) and became very

Conclusion

We have demonstrated the electrochemical synthesis of a free-standing melanin film. The optimum synthetic conditions were determined, and it was also found that films thick enough to be free standing can only form when ITO glass electrodes were used, with metallic electrodes resulting in a thin, dense film which does not grow thicker upon further oxidation. The solid state NMR and XPS analysis of the film showed distinct difference to its precursor dopa and confirmed the presence of the indolic

Acknowledgements

The author would like to thank the group in the Inorganic Materials Research Program (Queensland University of Technology), Mr Loc Duong (analytical electron Microscopy Facility, Queensland University of Technology), Eric Tavenner (Soft Solid State Physics, University of Queensland), Dr Barry Wood (Surface Analysis Facility, University of Queensland), and Dr Andrew Whittaker (Centre for Magnetic Resonance, University of Queensland) and the ARC for funding.

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