An ultrasensitive enzyme-free electrochemical immunosensor for CA125 using Au@Pd core–shell nanoparticles as labels and platforms for signal amplification

A facile and sensitive enzyme-free sandwich-type immunosensor for the detection of carbohydrate antigen 125 (CA125) was designed based on Au@Pd core–shell nanoparticles. Au@Pd nanoparticles were used both as electrode-modifying materials to immobilize the primary antibody (Ab1) of CA125 and as labels of secondary antibody (Ab2). The covalent bond between palladium nanoparticles and available amine groups of Ab1 enables the anchoring of larger amounts of Ab1 while the high stability and bioactivity of Ab1 can be reserved. Au@Pd nanoparticles show high electrocatalytic activity toward hydrogen peroxide (H2O2) reduction. The encapsulated Au nanoparticles can also accelerate the electron transfer. A high-performance electrochemical immunosensor was constructed based on the signal amplification of Au@Pd nanoparticles. Under optimal conditions, the immunosensor exhibited a wide linear response to CA125 ranging from 0.002 to 20 U mL−1 with a low detection limit of 0.001 U mL−1. The designed immunosensor displayed an excellent analytical performance with high sensitivity and stability. This strategy was successfully proved to be a simple method, which could be easily extended to other protein analysis.