A double-film screening card for rapid detection of organophosphate and carbamate pesticide residues by one step in vegetables and fruits

A double-film screening card, composed of an enzyme (acetylcholinesterase, AChE) and its substrate (indoxyl acetate) films, was developed for simple and rapid screening of organophosphate and carbamate pesticide residues. On the card, indoxyl acetate is decomposed by AChE to produce a blue-green compound. Compared with the speed of inhibition of AChE activity by pesticides, The color developing speed is relatively slow. When a sample solution with pesticides is dropped in the test window, this reaction of AChE is efficiently and rapidly inhibited to a degree depending on the level of pesticide. Any remaining active AChE would hydrolyze indoxyl acetate to show color. Qualitative and quantitative detection of pesticide residues could be achieved based on the color intensity. The selected suitable carrier films containing the enzyme and substrate were hydrophilic materials that were capable of adsorbing and releasing the full doze of enzyme or substrate. The optimum materials for containing AChE and indoxyl acetate on the card were glass fiber RB65 and polyester fiber VL78, respectively. The optimum quantities of AChE and indoxyl acetate per film in the test window were 15 μL of 3500 U/mL and 10 μL of 100 mM, respectively. The maximum color intensity was reached at room temperature in 15 min. The limits of detection were up to 0.05 μg/mL for phoxim, 0.1 μg/mL for acephate, 0.5 μg/mL for malathion, 0.5 μg/mL for omethoate, 0.04 μg/mL for carbofuran and 0.09 μg/mL for aldicarb. The card showed good reproducibility and high sensitivity when applied to real food samples.