Colorimetric sensing of chlorpyrifos through negative feedback inhibition of the catalytic activity of silver phosphate oxygenase nanozymes†
Intensive use of organophosphate chlorpyrifos pesticides in farming has become a serious issue due to their harmful effects on living beings. Most fruits, vegetables and soil contain chlorpyrifos, and it cannot be rinsed out completely by water washing. Therefore, a selective and sensitive detection of chlorpyrifos is significant. In the present study, the intriguing oxidase-mimicking activity of Ag3PO4 nanoparticles (NPs) is explored for the fast and selective detection of chlorpyrifos pesticides. Ag3PO4 NPs exhibit several advantages, such as great catalytic efficiency, high stability, monodispersity and reusability, over other expensive nanozymes via a facile one-step sensing. The size, shape, crystal planes and diffraction patterns of the Ag3PO4 NPs were observed via FESEM and HR-TEM. The surface properties and oxidation states were analyzed via XPS technique. Ag3PO4 NPs possess intrinsic excellent oxidase-mimicking properties against 3,3′,5,5′-tetramethylbezidyne (TMB). When chlorpyrifos and Ag3PO4 NP nanozymes come in proper orientation proximity, chlorpyrifos is oxidized. The oxidized chlorpyrifos produces sulfide ions and chlorpyrifos oxon. The produced sulfide ions in the reaction system interact with Ag3PO4 NPs and inhibit their catalytic activity by feedback inhibition. Indeed, neither any catalytic site is left to oxidize TMB nor any blue colour appears. Thus, this feedback inhibition phenomenon senses chlorpyrifos pesticides. The calculated limit of detection (LOD) for the standard chlorpyrifos is ∼9.97 ppm, and the efficacy of the Ag3PO4 NPs calculated in terms of the Km value was found to be 0.15 mM. A real sample analysis was carried out by the standard addition method with two soil samples collected from Pethapur and Chiloda villages.