Ferrocene-functionalized polydopamine as a novel redox matrix for H₂O₂ oxidation

We describe a simple approach for the synthesis of a ferrocene-derivative of dopamine by reacting ferrocene dicarboxylic acid with the amine functional groups present in dopamine via amide linkage. Polymerization of this derivative was effected at room temperature under mild alkaline conditions (pH ∼ 8.5). The formation of covalent amide linkage between polydopamine (polyDA) and ferrocene (Fc) units was confirmed by FTIR and NMR spectroscopy. Electrochemical characterization of the poly(DA-Fc)-modified glassy carbon electrodes shows a reversible response at E_pa = 0.40 V and E_pc = 0.28 V pertaining to the ferrocene/ferrocenium redox, in addition to polyDA redox peaks at E_pa = 0.63 V and E_pc = 0.57 V. Furthermore, the poly(DA-Fc) film displays excellent electrocatalytic oxidation of H₂O₂ in phosphate buffer solutions (pH ∼ 7.4). A Fenton-type reaction mechanism is proposed to explain the electrocatalytic oxidation, in which oxidation of ferrocene occurs along with the hydroxyl groups in the forward anodic scan, yielding a polymer film with Fc⁺ and hydroxyl radicals and also a polymer radical, upon reaction with H₂O₂ gives out oxyhydroxy radicals and ferrocene-polydopamine polymers. During the reverse scan, charge neutralization among the radicals occurs taking the polymer back to its original state. A linear relationship between the anodic current and [H₂O₂] allows quantitative determination of the latter with a detection limit and sensitivity of 5 μM and 0.7 μA μM^-1, respectively. Also, the poly(DA-Fc) film shows no interference from other small organic molecules such as glucose and ascorbic acid.