The development of versatile platforms to construct novel and sensitive immunosensors is nowadays an intense research field. Nanomaterials and polymers are often combined to fabricate new platforms to immobilize capture antibodies. Here we evaluate for the first time the co-electropolymerization of dopamine (DA) and L-3,4-dihydroxyphenylalanine (L-DOPA) on carbon nano-onion (CNO) modified electrodes as versatile platform to develop electrochemical immunosensors. Mixtures of DA and L-DOPA at different molar rations were co-electropolymerized on CNO-modified glassy carbon electrodes to form a poly(L-DOPA/DA) film. Immobilization of aminoferrocene was used to estimate the number of accessible carboxylic acid groups on the surface (11.3 nmol/cm2), a value comparable to three-dimensional matrices. This platform was applied to the electrochemical detection of IgA antibodies using both a HRP-based sandwich type assay and label-free detection based on [Fe(CN)6]3-/4- signal blocking. The sandwich and the label-free assays showed a wide linear response with LOD of 19 and 48 ng/mL, respectively, allowing the detection of serum IgA deficiency. Most remarkably, the incorporation of CNO layer led to a significant improvement (three-orders of magnitude) of the analytical performance of these immunosensors due to a combination of high surface area and increased electron transfer rates provided by the CNO layer.