Multi-functional MnO₂-doped Fe₃O₄ nanoparticles as an artificial enzyme for the colorimetric detection of bacteria

The authors describe a rapid enzyme-free colorimetric assay for food- and water-borne pathogens by using the multi-functional MnO₂-doped magnetic ferrite nanoparticles (MCDs-MnO₂ NPs). These nanoparticles not only can recognize, absorb, and separate the analyte from the matrix efficiently but also can directly catalyze the oxidation of TMB into a blue colored product without H₂O₂. In the presence of target, the nanoparticles preferentially bind to the target surface and the interaction between nanoparticles and TMB can be blocked, providing turn-off sensing of bacteria. By combining the superior capture efficiency of magnetic beads with the high catalytic activity of the enzyme mimic, the turn-off colorimetric assay can detect the gram-positive bacterium and the gram-negative bacterium by the naked eye at a low detection limit (10² cfu mL^-1) with a wide line arrange from 10 to 10⁶ cfu mL^-1. With the advantage of simplicity, sensitivity, and specificity, this proposed approach showed promise in rapid instrumental and on-site visual detection of bacteria for determination of water contamination. Graphical abstract.