Color-coded detection of malathion based on enzyme inhibition with dark-field optical microscopy

Malathion is widely applied in agriculture as one of organophosphorus pesticides (OPs) and easily causes a number of health and environmental problems. Traditional methods for analyzing malathion residues have failed to meet the current requirements of detection. To address this challenge, we designed a colorimetric-based single particle detection (SPD) method to quantify malathion using MnO2-coated gold nanoparticles (GNP@MnO2 NPs) as the probe. In the existence of alkaline phosphatase (ALP), p-aminophenyl phosphate (p-APP) can be hydrolyzed to p-aminophenol (p-AP), which employs as a reductant to induce MnO2 to Mn2+ and make the gold core exposure. With the dark field optical microscopy (DFM), the color change of the probe can be readily observed at the single particle level. However, malathion is able to inhibit the activity of ALP and makes the color of the probe remain. Thus, the content of malathion can be accurately quantified by monitoring the changes in color and scattering intensity of the probe. The linear range of this assay is 0.001-0.1 ng/mL and the limit of detection (LOD) is as low as 0.82 pg/mL. In addition, degradation of malathion can be achieved due to the interaction between malathion and ALP. Therefore, this strategy offers new insights on the design of an ultrasensitive assay for OPs detection and degradation in the future.