Scheme 1. Schematic Diagram of the Quantitative Immunoassay Based on Glucose Oxidase (GOx)-Catalyzed Growth of Gold Nanoparticles (AuNPs, 5 nm in Diameter).

Prostate-specific antigen (PSA) is first immobilized by the capture antibody (Ab1) and then recognized by the detection antibody (Ab2) conjugated with GOx on the surfaces of magnetic beads (MBs). The immobilized GOx catalyzes the oxidation of β-d-glucose to generate H₂O₂, which induces the growth of the 5 nm AuNPs in the presence of AuCl₄^–. With the enlargement of AuNPs, the solution turns red from colorless.