Polyvinylpyrrolidone-modified iron single-atom nanozymes: preparation and application to highly sensitive immunomagnetic bead assay for tetracycline detection

A one-pot method was adopted to prepare polyvinylpyrrolidone (PVP)-modified iron single-atom nanozymes (FeSANs) as signal amplification probes, establishing a rapid and sensitive immunomagnetic bead assay (IMBA) for tetracycline (TC) detection. After optimization, Fe10-ZIF-8@PVP demonstrated the highest oxidase-like and peroxidase-like activities in comparison with Fe5-ZIF-8@PVP, Fe20-ZIF-8@PVP, and Fe10-ZIF-8. The Michaelis-Menten constants (Km) for the substrates 3,3’,5,5’-tetramethylbenzidine (TMB) and hydrogen peroxide (H₂O₂) were measured as 0.31 mM and 0.67 mM, respectively. The Fe10-ZIF-8@PVP-assisted IMBAs demonstrated the highest sensitivity, achieving a limit of detection (LOD) as low as 0.196 ng/mL, The sensitivity was enhanced by 48-fold compared to conventional ELISA. Furthermore, the Fe10-ZIF-8@PVP-based IMBA was successfully applied to analyze honey, milk, water and soil samples, yielding recoveries of 97.3% to 109.3%, 92.5% to 105.6%, 89.1% to 108.5% and 93.5% to 107.8%, respectively. Practical analysis of environmental samples revealed TC residues not exceeding regulatory limits, with concentrations above the detection limit found only in three of 32 samples analzed (i.e. 0.43 ng/ml, 1.24 ng/ml, and 0.36 ng/ml in one of the honey, milk and water samples, respectively). This method offers a highly sensitive, rapid, and cost-effective alternative to conventional ELISA for antibiotic detection in food safety.