An electrochemical immunosensor involving the formation of a surface sandwich complex on a gold nanoparticle (NP) modified screen printed carbon electrode (SPCE) is demonstrated for the femtomolar detection of amyloid-beta 1-42 peptide (A beta) in both serum and plasma. Both bioreceptors forming the assay are highly selective antibodies for A beta, namely antiA beta (12F4) and (1E11) which possess different binding sites for the A beta peptide. In order to improve the sensing performance for complex biological fluidic matrix analysis, different mixed monolayers of thiol modified polyethylene glycol (PEG) and mercaptopropionic acid (MPA) were self-assembled onto the Au NP-SPCE followed by tethering antiAp (12F4) to MPA using a heterobifunctional cross linker. Surface sandwich complexes of antiAp (12F4)/A beta/antiA beta (1E11)-ALP were then formed via sequential adsorption with the latter antiAp (1E11) conjugated to alkaline phosphatase (ALP) enzyme. The reaction of surface immobilized ALP with the substrate, 4-amino phenyl phosphate (APP), generated voltammetric detection signals that linearly increased as a function of A beta concentration. Differential pulse voltammetry was applied to establish a lowest detectable concentration of 100 fM of A beta with a linear response range from 100 fM to 25 pM. Following optimization, the immunoassay platform was applied in diluted human serum and plasma samples to determine the native concentration of A beta and the results were validated using a commercially available ELISA test. (C) 2017 Elsevier B.V. All rights reserved.