Electrochemical immunoassay for amyloid-beta 1–42 peptide in biological fluids interfacing with a gold nanoparticle modified carbon surface

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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 (Aβ) in both serum and plasma. Both bioreceptors forming the assay are highly selective antibodies for Aβ, namely antiAβ (12F4) and (1E11) which possess different binding sites for the Aβ 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 antiAβ (12F4) to MPA using a heterobifunctional cross linker. Surface sandwich complexes of antiAβ (12F4)/Aβ/antiAβ (1E11)-ALP were then formed via sequential adsorption with the latter antiAβ (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 Aβ concentration. Differential pulse voltammetry was applied to establish a lowest detectable concentration of 100 fM of Aβ 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 Aβ and the results were validated using a commercially available ELISA test.