Immobilization of double functionalized carbon nanotubes on glassy carbon electrodes for the electrochemical sensing of the biotin–avidin affinity

Multi-walled carbon nanotubes (MWCNTs) double functionalized with redox-active ferrocene and biotin (Fc–Biot-MWCNTs) were synthesized and used for the electrochemical detection of avidin. After dispersion in perfluorosulfonated polymer Nafion and immobilization on the electrode surfaces, the cyclic voltammetry response of the modified electrodes showed in aqueous medium a quasi-reversible one-electron system at 0.46 V vs. SCE, assigned to the bound ferrocene/ferrocenium redox couple. Upon the addition of avidin in the range 0.9–20 nM, a stepwise decrease of both anodic and cathodic peak currents ascribed to the ferrocene was observed. These electrochemical changes are specifically due to the formation of biotin–avidin complex and are explained by complexation-induced modifications in the environment of covalently bound ferrocene.