Monitoring of parathion methyl using a colorimetric gold nanoparticle-based acetylcholinesterase assay

A colorimetric gold nanoparticles (AuNPs)-based acetylcholinesterase (AChE) assay was designed for the first time to measure the concentration of parathion-methyl (PM) in lake water samples. In this assay, the analyte PM inhibited the hydrolysis of acetylthiocholine (ATCh) by AChE, preventing the formation of thiocholine (TCh) that would otherwise react with the AuNPs catalyst and deactivate the catalyst. Therefore, in the presence of PM, the AuNPs catalyzed the oxidation of the 3,3',5,5'-tetramethylbenzidine (TMB) colorimetric indicator to oxTMB, inducing a visual color change from colorless to blue. However, in the absence of PM, AChE hydrolyzed ATCh to TCh, which then reacted with the AuNPs, preventing the oxidation of TMB to oxTMB and rendering the solution colorless. Therefore, the change in the color of the analyte solution indicated the presence of PM, and the absorbance of the resulting solution was measured by UV-Vis spectroscopy to calculate the concentration of PM after generation of a calibration curve. This method was then employed using the smartphone app Color Picker, which converted the color information from the photos of the solution into digital red (R), green (G), and blue (B) values. The ratio of green (G) to blue (B) (G/B) was then plotted against the corresponding concentration to calculate the standard curve, whose regression equation was expressed by y = -0.012x + 1.02 (ng/mL), and the coefficient of determination (R²) was 0.97. In addition, this method was also used to determine the amount of PM in real lake water samples with recovery of 90.2-133.3%.