Tailoring the Plasmonic Response of Gold Nanobipyramids via Enzymatic Etching for a Highly Sensitive and Multicolor Competitive Immunoassay for the Detection of Aflatoxin B 1

Rapid on-site monitoring of aflatoxin B 1 (AFB 1 ) is critical for food and medicine safety. Herein, a multicolor plasmonic immunoassay was developed for AFB 1 detection by combining competitive enzyme-linked immunosorbent assays (ELISA) with enzyme-mediated etching of gold nanobipyramids (AuNBPs). Compared with spherical gold nanoparticles, AuNBPs have sharp tips that provide stronger field enhancement; compared with gold nanorods, the double-conical tips have higher curvature, making them more sensitive to morphological changes during etching. In the competitive system, AFB 1 concentration negatively regulates the amount of horseradish peroxidase (HRP) on the solid phase. HRP catalyzes TMB oxidation to trigger an oxidation–reduction etching pathway, which gradually reshapes AuNBPs from bipyramids to spheres, causing a distinct LSPR shift and multicolor change for naked-eye readout. The method shows a limit of detection of 0.67 ng/mL with a linear range of 1–20 ng/mL and performs well in real food samples. This strategy provides a simple, sensitive, and field-applicable tool for on-site AFB 1 screening.