Modulating Peroxidase-like Activity of Fe3O4@Pt@poly-LDOPA and Its Application as Multifunctional Magnetic Probes Towards SARS-CoV-2 Detection

We report a Fe3O4@Pt@poly-LDOPA nanozyme that displays enhanced peroxidase (POD)-like activity. Polymerisation of levodopa onto the surface of Fe3O4@Pt yields a carboxyl-rich poly-LDOPA shell that is available for bioconjugation with antibodies and other types of receptors. Physicochemical characterisation confirmed the integrity of the Fe3O4 core, successful Pt modification, and formation of the polymer coating under acidic and basic conditions. Steady-state kinetic analysis using the Michaelis–Menten model revealed robust catalytic performance toward both substrates: for H2O2, Vmax = 4.0 × 10−8 M·s−1 and Km = 25.13 mM; for TMB, Vmax = 6.07 × 10−8 M·s−1 and Km = 0.229 mM, indicative of high turnover and strong apparent affinity for the chromogenic substrate. A nanozyme-linked immunosorbent assay for the SARS-CoV-2 nucleocapsid was developed. The anti-nucleocapsid antibodies were immobilised onto Fe3O4@Pt@poly-LDOPA via EDC/NHS. In buffer, the calibration range (1.0–100 ng·mL−1) afforded an LOD of 6.95 ng·mL−1. In 10% human serum, reduced background and improved nanozyme dispersion yielded a linear low-concentration response (0.1–10 ng·mL−1), with an LOD of 0.0036 ng·mL−1. These results establish Fe3O4@Pt@poly-LDOPA as a promising inorganic–organic nanozyme platform that combines catalytic effectiveness, magnetic manipulability, and facile bioconjugation for immunosensing of various disease-related biomarkers.