Opto-electronic properties of melanin: solid-state characterisation via EPR, conductivity, and copper chelation

Abstract

Melanin is a natural bio-macromolecule with hydration dependant ionic-electronic electrical conductivity, making it a promising material for bioelectronics. Previous temperature, pH and metal-ion studies using continuous wave electron paramagnetic resonance (CW EPR) support the proposed charge transport mechanism, which is a comproportionation reaction between the quinones, hydroquinones and semiquinone free radicals in melanin. The next step in developing melanin as a bioelectronic material is to enhance its conductivity. A promising method is to ‘dope’ melanin with copper ions. Copper ions have a high binding affinity for melanin, and helps regulate melanin synthesis and function in natural systems. However, the establishment and characterisation of the aforementioned conductivity mechanism remains a somewhat open question particularly in relation to any external doping. Therefore, the work described in this thesis firstly investigates and confirms the charge transport mechanism of neat melanin by using D2O as a probe in hydration controlled electrical and CW EPR measurements. Furthermore, simultaneous electrical and photoEPR data indicates that solid-state melanin contains a photoactive semiquinone radical linked to melanin photoconductivity. The proposed mechanism for the photoconductivity of melanin is via absorbed photon energy driving the comproportionation reaction to generate transient monoprotonated semiquinone radicals. This results in the observed simultaneous increase in melanin radical concentration, and electric current. The study then extends to copper doped melanin systems, by examining the enhanced melanin radical behaviour and conductivity by copper chelation. Characterisation using XPS and CW EPR shows Cu(II) ions and melanin free radicals undergo coupled redox exchange, which is enhanced under illumination with visible light. Furthermore, the addition of copper(II) to melanin influenced the melanin radical behaviour by shortening the melanin relaxation time T1. The shorter T1 of melanin is attributed to a strong interaction between the melanin radical and nearby copper(II) paramagnetic centres, although the interaction was too weak to be resolved in the CW EPR spectrum. Using a copper concentration of 5x10-4 mol/g [Cu(II)/melanin] effectively enhanced the response of the conductivity to hydration. The copper-melanin exhibited changes in conductivity of ~4 orders of magnitude as a function of hydration (from dry to fully saturated). This suggests that the comproportionation reaction, which determines charge transport in melanin, can be perturbed using copper chelation and makes the reaction, and hence the ability to generate charges, more sensitive.These results demonstrate Cu(II) is an effective dopant for improving melanin conductivity, and establishes solid-state CW EPR as a powerful tool for studying melanin free radical behaviour. This opens new opportunities for investigating melanin structureproperty-function relationships, as well as future bioelectronics applications.