Selective Inhibition toward Dual Enzyme-like Activities of Iridium Nanozymes for a Specific Colorimetric Assay of Malathion without Enzymes

A colorimetric assay based on an enzyme-inhibition strategy is promising for the on-site detection of pesticide residues. Due to the high cost and low stability of enzymes, nanozymes (nanomaterials with enzyme-like activities) are widely developed as substitutes of enzymes. However, the inhibition of pesticides toward enzymes and nanozymes generally lacks selectivity. It is of great significance and challenge to design a specific pesticide assay based on an activity-inhibition strategy. Here, we discovered that iridium nanoparticles possess both peroxidase-like and oxidase-like activities under the same conditions, and their catalytic mechanisms are different. The synergistic effect of dual enzyme-like activities enhanced the colorimetric signal. Interestingly, the dual enzyme-mimicking activities could be simultaneously inhibited, and the inhibition effect exhibited high selectivity toward malathion. Considering the popularity and the hazards of malathion, a malathion assay method based on activity inhibition was established without enzymes and a redundant process. The synergistic effect of the selective inhibition of dual enzyme-like activities enhanced the selectivity and sensitivity. The proposed assay strategy opens up an avenue for specific assay of various pesticides.