Congo red chromogenic paper strip as a rapid screening tool for organophosphate pesticide residues in fresh vegetables

Organophosphate pesticides are widely applied in vegetable production due to their broad-spectrum effectiveness and low cost. However, their residues, especially in leafy vegetables and tomatoes consumed raw pose significant health risks, including neurodevelopmental and metabolic effects. Conventional analytical methods, while accurate, are time-consuming, expensive, and impractical for rapid field-level monitoring. This study aimed to develop and validate a rapid, cost-effective Congo red-based paper strip assay for on-site rapid detection of organophosphate residues in fresh vegetables. The assay is designed to detect sulfur-containing organophosphate pesticides, including chlorpyrifos, dimethoate, ethion, triazophos, quinalphos, and profenofos, by producing a blue spot on a red background. It demonstrates a detection limit of 0.01 mg/kg for rapid screening and effectively identifies pesticides with phosphorothioate functional groups at concentrations as low as 1 mg/L. Although the color develops quickly, it fades rapidly at low concentrations below (< 0.1 mg/L), necessitating immediate observation for accurate detection. In contrast, at higher concentrations (> 0.5 mg/L), the color remains stable, persisting for 3–4 days under light and up to 10 days in the dark. Validation against GC–MS/MS confirmed substantial to almost perfect agreement (Cohen’s Kappa, κ = 0.71–1.00), demonstrating the assay’s reliability for qualitative screening of pesticides residue in market vegetables. The method is rapid, user-friendly, and suitable for large-scale monitoring in resource-limited settings, providing an effective tool for improving food safety and reducing consumer exposure to pesticide residues.