As mentioned above, the colorimetric barcode assay for the
detection of organophosphorus pesticides (methyl parathion)
was based on the irreversible inhibition of the activity of acetylcholinesterase
(AChE, the enzyme) on the hydrolysis of acetylthiocholine
iodide (ATCh, the substrate)7–9 (Scheme S1,
ESI†). The intermediate (thiocholine) then reacts with DTNB(dithio-bis-nitrobenzoate, a chromogenic reagent) to generate
TNB (thionitrobenzoate), which has a strong absorbance peak
centered at 412 nm with a high extinction coefficient (14,150
M−1 cm−1) in dilute buffer solutions.22