pH-controlled UV−Vis sensing strategy for indirect, rapid detection of paraoxon based on molecular form conversion

In this paper, an indirect strategy based on the pH-dependent molecular form conversion was established for rapid, selective detection of the total paraoxon (methyl paraoxon (MP) and ethyl paraoxon (EP)) by using UV−Vis spectroscopy. Under pH 13, the paraoxon (λmax 275 nm) can be quickly hydrolyzed to 4-nitrophenol (4-NP) in 5 min at 50 ℃, with hydrolysis efficiencies of 96.63% (MP) and 96.27% (EP), then the phenolic hydroxyl of 4-NP (λmax 315 nm) rapidly converted to phenoxy anion, accompanied by a red shift from 315 nm to 400 nm and an enhanced peak intensity from 0.070 to 0.157. Based on the molar ratio of 4-NP and paraoxon (1:1), an indirect, sensitive sensing assay for detection of the total paraoxon was achieved based on the quantitative analysis of 4-NP at 400 nm. A low detection limit of 1 ppb for the 4-NP was obtained by UV−Vis (linear range 1–1000ppb, R2 = 0.999), and as calculated the detection limits were 1.77 ppb and 1.98 ppb for MP and EP, respectively. As expected, the proposed UV−Vis method possessed excellent selectivity to the paraoxon over other commonly used pesticides. Furthermore, the assay exhibited good performance in real water and vegetable eluent samples, with acceptable recoveries in the range of 88.33–108.42% for MP, 91.67–108.33% for EP and 91.91–100.12% for the total paraoxon. This successful UV−Vis sensing platform, without the participation of biological recognition elements (protein, nucleic acid, antibody), offers a new alternative for rapid, selective paraoxon detection in practical application and holds a great prospective for the assessment of food safety and environmental risks.