Colorimetric dipstick for assay of organophosphate pesticides and nerve agents represented by paraoxon, sarin and VX

A dipstick for fast assay of nerve agents and organophosphate pesticides was developed. Indicator pH papers were used as detectors. The principle of the assay is based on enzymatic hydrolysis of acetylcholine into acetic acid and choline by acetylcholinesterase. Acidification of the reaction medium due to accumulation of acetic acid was visible. The colour changed from dark red to yellow as the pH indicator recognized pH shift. Presence of an organophosphate pesticide or a nerve agent results in irreversible inhibition of acetylcholinesterase intercepted on the dipstick. The inhibition stops the enzymatic reaction. The inhibition appears as no change of the medium pH. Three compounds were assayed: paraoxon-ethyl as representative organophosphate pesticides and nerve agents sarin and VX. The achieved limit of detection was 5 × 10−8 M for paraoxon-ethyl and 5 × 10−9 M for sarin and VX. Dipsticks were found stable for at least one month. Suitability of these dipsticks for routine assay is discussed.