On-site chip-based colorimetric quantitation of organophosphorus pesticides using an office scanner

Herein we report a scanner-readable plastic microchip for the colorimetric quantitation of organophosphorus (OP) pesticides in food products. The detection principle relies on the irreversible activity inhibition of acetylcholinesterase (AChE) by OP pesticides; AChE catalytically hydrolyzes acetylthiocholine (ATCh) to thiocholine, which in-turn reacts with dithiobis-nitrobenzoic acid (DTNB) to produce a yellow colored product (deprotonated 5-thio-2-nitrobenzoic acid, TNB). The yellow color diminishes upon increasing the concentration of the OP pesticides. In particular, we have fabricated a simple plastic microchip consisting of a transparent polydimethylsiloxane (PDMS) plate with embedded micro-channels (for the colorimetric reaction) attached to a polycarbonate (PC) base plate. A standard flatbed office scanner was used to image the colorimetric reaction at real time, and a commercial photo editing software to read the yellow intensity of the captured images. The detection limit for malathion (one of the most popularly used OP pesticides nowadays) was determined to be 100 ppb, which is below the food safety limits and comparable with conventional spectroscopic detection. The simplicity of the device fabrication and operation of this colorimetric protocol augments its potential for on-site food safety analysis in place of rather expensive and complex spectrometer-based methods.