Cobalt Single-Atom Nanozyme-Enabled Multimodal Lateral Flow Immunoassay for On-Site Ultrasensitive Detection of Tetracycline Residues in Agri-Food Products

Tetracycline (TC) residues in agricultural products pose serious food safety risks, necessitating sensitive and field-deployable detection tools. Conventional lateral flow immunoassays (LFIAs) based on gold nanoparticles (AuNPs) are widely used but suffer from limited sensitivity and high cost. To overcome these challenges, we developed a cobalt single-atom nanozyme (CoSAN) using a template-pyrolysis strategy. The resulting Co–N₅ coordination structure exhibited excellent peroxidase-like activity (K_m: 0.622 mM for TMB, 0.294 mM for H₂O₂), generating ^•OH, ^•O^2–, and ¹O₂ to enhance both colorimetric and chemiluminescent signals. A multimodal LFIA platform was constructed by integrating CoSAN, enabling colorimetric screening, catalytic amplification, and chemiluminescence quantification. Detection limits reached 0.091, 0.062, and 0.056 ng/mL, representing 43-, 63-, and 70-fold sensitivity improvements over those of AuNP-LFIAs. Application to real milk and honey samples showed no TC levels above the regulatory limits. This study presents a robust, sensitive, and cost-effective approach for on-site food contaminant screening.