Ultrasensitive sandwich-type electrochemical immunosensor based on a novel signal amplification strategy using highly loaded palladium nanoparticles/carbon decorated magnetic microspheres as signal labels

An ultrasensitive sandwich-type electrochemical immunosensor for quantitative detection of alpha fetoprotein (AFP) was proposed based on a novel signal amplification strategy in this work. Carbon decorated Fe 3 O 4 magnetic microspheres (Fe 3 O 4 @C) with large specific surface area and good adsorption property were used as labels to anchor palladium nanoparticles (Pd NPs) and the secondary antibodies (Ab 2 ). Pd NPs were loaded on Fe 3 O 4 @C to obtain Fe 3 O 4 @C@Pd with core–shell structure by electrostatic attraction , which were further used to immobilize Ab 2 due to the bonding of Pd-NH 2 . A signal amplification strategy was the noble metal nanoparticles , such as Pd NPs, exhibiting high electrocatalytic activities toward hydrogen peroxide (H 2 O 2 ) reduction. This signal amplification was novel not only because of the great capacity, but also the ease of magnetic separation from the sample solution based on their magnetic property. Moreover, carboxyl-functionalized multi-walled carbon nanotubes (MWCNTs-COOH) were used for the immobilization of primary antibodies (Ab 1 ). Therefore, high sensitivity could be realized by the designed immunosensor based on this novel signal amplification strategy. Under optimal conditions, the immunosensor exhibited a wide linear range of 0.5   pg/mL to 10   ng/mL toward AFP with a detection limit of 0.16   pg/mL ( S / N =3). Moreover, it revealed good selectivity, acceptable reproducibility and stability, indicating a potential application in clinical monitoring of tumor biomarkers.