In this paper, we present a smartphone-readable barcode assay for the qualitative detection of methyl
parathion residues, a toxic organophosphorus pesticide that is popularly used in agriculture worldwide.
The detection principle is based on the irreversible inhibition of the enzymatic activity of acetylcholinesterase
(AchE) by methyl parathion; AchE catalytically hydrolyzes acetylthiocholine iodine to thiocholine
that in turn dissociates dithiobis-nitrobenzoate to produce a yellow product (deprotonated thio-nitrobenzoate).
The yellow intensity of the product was confirmed to be inversely dependent on the concentration
of the pesticide. We have designed a barcode-formatted assay chip by using a PDMS
(polydimethylsiloxane) channel plate (as the reaction reservoir), situated under a printed partial barcode,
to complete the whole barcode such that it can be directly read by a barcode scanning app installed on a
smartphone. The app is able to qualitatively present the result of the pesticide test; the absence or a low
concentration of methyl parathion results in the barcode reading as “−”, identifying the test as negative for
pesticides. Upon obtaining a positive result (the app reads a “+” character), the captured image can be
further analyzed to quantitate the methyl parathion concentration in the sample. Besides the portability
and simplicity, this mobile-app based colorimetric barcode assay compares favorably with the standard
spectrophotometric method.