Visual quantification of melamine using citrate-capped gold nanoparticles

Due to its high nitrogen content and low cost, melamine, a widely used material in plastics industry, has been illegally added to food products to boost apparent protein content. We've previously developed a colorimetric method to detect melamine. This method utilizes the melamine-concentration dependent gradual color shift of gold nanoparticles (AuNPs) and an absorbance reader is usually required for reliable quantification of melamine. Herein, we present a novel strategy for reliable visual quantification of melamine without any equipment. First of all, we further explored the mechanism of the melamine-induced AuNP aggregation with various experimental techniques. Secondly, based on the findings from the mechanism studies, we've developed and demonstrated a critical coagulation concentration (CCC)-based salt titration as a reliable means to visually quantify the melamine. This has allowed us to accurately quantify melamine at a level as low as 30 ppb without any equipment.