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 B₁ (AFB₁) is critical for food and medicine safety. Herein, a multicolor plasmonic immunoassay was developed for AFB₁ 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, AFB₁ 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 AFB₁ screening.