<scp>CTAB</scp> ‐capped Au@Ag core–shell nanoparticles as a colorimetric platform for sensitive detection of ziram fungicide in aqueous and complex food matrices

Abstract Cetyltrimethylammonium bromide (CTAB)‐capped Au@Ag core–shell nanoparticles (NPs) were developed as a rapid, sensitive, and cost‐effective colorimetric platform for detecting the dithiocarbamate fungicide ziram. Hydrophobic Au@Ag NPs were successfully transferred into the aqueous phase through CTAB functionalization, which enhanced colloidal stability, introduced a positive surface charge, and preserved strong localized surface plasmon resonance (LSPR) characteristics. Transmission electron microscopy (TEM) and Dynamic light scattering (DLS) analysis confirmed near‐spherical, monodisperse NPs with an average hydrodynamic diameter of ~17 nm, while Fourier‐transform infrared (FTIR) and X‐ray diffraction (XRD) verified the CTAB coating and crystalline Au@Ag core–shell structure. Upon exposure to ziram, NPs aggregated via sulfur–metal coordination and electrostatic interactions, producing a visible color change from yellow to grayish‐brown and a pronounced ratiometric absorbance shift (A 640 /A 440 ). The sensor exhibited a linear response up to 30 μg/mL, a detection limit of 0.3 μg/mL, high selectivity over other pesticides and mercapto compounds, and excellent reproducibility. Recovery tests in milk and apple samples demonstrated satisfactory accuracy (94%–98%). This visually readable, portable sensing platform provides a versatile approach for rapid on‐site monitoring of ziram and related dithiocarbamate fungicides in aqueous and food matrices.