It has been demonstrated [31] that the presence of the iodide ions, I−, in the sensing system will increase the LOD value of the OPs, i.e. make it less favorable. This occurs because the iodide ions can induce the aggregation of AgNPs. Thus, two preprocessing steps were introduced: (1) 10.2 mg (0.06 mmol) AgNO3 was added to 5 mL 10 mmol L−1 ATChI aqueous solution to remove theI−, and (2) after the removal of the AgI precipitate by centrifugation,1.17 mg (0.02 mmol) NaCl was added to the solution to remove any excess Ag+; any AgCl precipitate was removed by further centrifu-gation. The resulting ATCh solution (∼10 mmol L−1) was decanted into a glass reagent bottle, and this solution was stored at 5◦Cfor future work. A concentration series of solutions was preparedfrom the bottled AChE solution, and the activity of these solutionswas determined by the improved Ellman method [38]. Thus, a setof AChE solutions with different activities (50, 100, 200, 300, and 400 mU mL−1) was prepared and stored at 5◦C. The dipterex solution (0.5 mg mL−1) was prepared by dissolving a suitable amountof it in water and this was stored at 5◦C; a concentration series was prepared from this stock solution as required.