Sonochemical preparation of gold nanoparticles for sensitive colorimetric determination of nereistoxin insecticides in environmental samples.

A simple colorimetric method using gold nanoparticles (AuNPs) was developed as an efficient strategy for specific and sensitive detection of insecticides that are analogs of nereistoxin (NRT). The AuNPs were synthesized by a surfactant-free sonochemical reaction with ultrasonication at 430 kHz. A color change occurred in the presence of NRT because the AuNPs aggregated if they were coated with a small amount of thioctic acid (TA). At a pH of around 5, the TA adsorbed on the AuNPs was deprotonated, whereas NRT was protonated (NRT-H+). Adsorption of NRT-H+ onto the TA-coated AuNPs surface would decrease the surface charge of the AuNPs, and this resulted in aggregation. Because the aggregation of the TA-coated AuNPs could not be induced by amine compounds without thiol groups, this provided a surface-limited aggregation mechanism for specific sensing of NRT. The absorbance at 700 nm was dependent on the concentration of NRT, and the calibration curve was linear over the concentration range 85 nM (12 ng/mL) to 1000 nM (140 ng/mL). The applicability of the proposed method to detection of trace levels of NRT in environmental water samples was successfully demonstrated using a simple liquid-liquid reverse extraction technique.