Towards the fabrication of label-free amperometric immunosensors using SWNTs

A label-free immunosensor based on the modulation of the electrochemistry of a surface bound redox species, to detect the presence of antibodies, is demonstrated. In this proof of concept study the model epitope was biotin and the model antibody was anti-biotin IgG. Glassy carbon (GC) electrode surfaces were first modified with 4-nitrophenyl groups by electrochemical reductive adsorption of the corresponding aryl diazonium salt. Subsequently, the nitro group was reductively converted into an amine, giving 4-aminophenyl groups. Oxidatively shortened single walled carbon nanotubes (SWNTs) were then covalently attached to the electrode via self-assembly; a procedure that has previously been shown to give SWNTs aligned normal to the surface. 1,1-Di(aminomethyl)ferrocene was attached to the carboxylic acid terminated SWNTs followed by attachment of biotin to the remaining free amine of the ferrocene derivative. Binding of anti-biotin IgG to the surface bound epitope resulted in attenuation of the ferrocene electrochemistry. This label-free immunosensor was successfully able to detect anti-biotin between 30 and 450 ng mL⁻¹.