Rapid visual detection of quaternary ammonium surfactants using citrate-capped silver nanoparticles (Ag NPs) based on hydrophobic effect

• A rapid, sensitive and low-cost colorimetric method for detection of quaternary ammonium surfactants using citrate-capped Ag NPs was developed.
• The Ag nanoparticles' aggregation is induced by the hydrophobic interaction.
• The response of Ag NPs upon the addition of different cationic surfactants with different lengths of the alkyl chain is different.
• This method was successfully utilized to analyze the disinfectant residual sample.

In this work, a rapid, sensitive and low-cost colorimetric method for detection of quaternary ammonium surfactants using citrate-capped silver nanoparticles (Ag NPs) was developed. The quaternary ammonium surfactants induce the aggregation of Ag NPs through the hydrophobic effect, which is a novel aggregation mechanism of Ag NPs. The addition of cationic surfactant results in color change of Ag NPs solution from yellow to red and finally to colorless, which is due to the broadening of the surface plasmon band. The color change was monitored using a UV–vis spectrophotometer. The LOD of different cationic surfactants was in the range of 0.5–5 µM. More importantly, this detection method was successfully utilized to the disinfectant residual sample.