Spectrophotometric Determination of Melamine in Milk Based on Agglomeration of Gold Nanoparticles

A red-colored solution of gold nanoparticles (AuNPs) with its particle diameter of about 12 nm was prepared by the reaction of 0.1 g·L^-1 HAuCl₄ solution (100 mL) and 10 g·L^-1 trisodium citrate solution (2 mL). The AuNPs solution shows its characteristic absorption peak at the wavelength of 518 nm. When melamine was added to the AuNPs solution, agglomeration of AuNPs happened due to induction by melamine and the original red color of the AuNPs solution was turned to blue color, showing a new absorption band near 680 nm. Detection range of melamine was 0.1-28.0 μmol·L^-1, with lower limit (3σ) of determination 0.1 μmol·L^-1. Based on these facts, a spectrophotometric method for determination of melamine in milk was proposed. Test for recovery was made by standard addition method; values of recovery were in the range of 99.0%-108%.