Multi-pathway mechanism of polydopamine film formation at vertically aligned diamondised boron-doped carbon nanowalls.
Boron-doped carbon nanowall (B:CNW) electrodes were used as a platform for studying the electropolymerisation of dopamine. Due to the unique properties of B:CNW, including the fast charge-transfer kinetics and high surface conductivity, a high degree of reversibility of redox reactions was achieved. Three separated redox peaks were observed on voltammograms and attributed to three fundamental reactions in the dopamine polymerisation mechanism: dopamine/dopamine quinone, leukodopaminechrome/dopaminechrome, and dihydroxyindole/indolequinone. The mechanism was also supported by the density functional theory calculations of the single point energy of the polydopamine structural units. Moreover, the electrochemical impedance spectroscopy experiments strongly suggest that the majority of th epolymerisation occurs only in the narrow range of potentials between +0.0 and +0.4 V vs. the silver chloride reference electrode. The concept of a resistor-like CPE element is introduced to facilitate the description of the electrochemical properties of B:CNW electrodes in a neutral electrolyte. Further, it is shown that electropolymerisation differs significantly as a function of pH. In acidic environments (pH 3–6) mostly the dopamine/dopamine quinone reaction can be seen, whereas in a more alkaline pH (8–10), the leukodopaminechrome/dopaminechrome reaction becomes dominant. The dihydroxyindole/indolequinone redox pair is seen only in a short pH interval between 7 and 8. Additionally, the kinetics of polymerisation is significantly boosted when the pH is more than 7. Together, these results provide insight into the complexity of the formation of polydopamine and can assist in better controlling the properties of electropolymerised dopamine films.