A Hierarchical Au@PDA-PdCu Nanozyme for Signal-Amplified Immunochromatographic Detection of Carbofuran in Leafy Vegetables

Carbofuran residues threaten food safety and public health, yet conventional instrumental methods require expensive equipment and complicated pretreatment, while traditional immunochromatographic assays still lack sufficient sensitivity. Herein, a hierarchical nanozyme composed of an Au core, a polydopamine (PDA) interlayer, and a PdCu shell (Au@PDA-PdCu) was developed as a signal probe for a competitive immunochromatographic assay (ICA) for rapid carbofuran detection. The Au core served as a stable nanoplatform, the PDA interlayer enabled antibody immobilization, and the PdCu shell provided strong peroxidase-like activity for post-assay signal amplification. Three probe formats, namely liquid, sprayed, and lyophilized probes, were constructed and optimized. After 3,3′-diaminobenzidine (DAB) enhancement, the detection limit of the liquid-format strip decreased from 0.055 to 0.016 ng/mL, while the working ranges of the sprayed-format and lyophilized-format strips expanded to 0–100 ng/mL. In Shanghai bok choy, Chinese chive, and spinach, all three formats exhibited good concentration-dependent responses. The visual limits of detection were 5 ng/mL for the liquid- and sprayed-format strips and 1 ng/mL for the lyophilized-format strip. Recoveries ranged from 70% to 119%, with coefficients of variation of 3.7–25.7%. Good specificity was observed, with cross-reactivity below 0.1% for most tested compounds except 3-hydroxycarbofuran. The ICA results agreed well with those obtained by LC–MS/MS. Overall, this method provides a rapid and sensitive proof-of-concept platform with potential for on-site carbofuran screening in leafy vegetables.