Effect of pH on the Stability of Gold Nanoparticles and Their Application for Melamine Detection in Infant Formula

A simple and sensitive colorimetric gold nanoparticle probe for detection of melamine in infant formula was developed by the reduction of Au(III) salt with sodium citrate. This method is rather simple that does not involve any surface modification of the nanoparticles or multistep sample treatment. This technique is based on the fact that the optical properties of gold nanoparticles depend on distance between particles. Gold nanoparticles are aggregated in a neutral media in the presence of melamine; this causes an easily measurable change in the absorption spectrum of the particles, which can be monitored with the naked eye or UV-vis spectrophotometer. Here, the color of gold nanoparticles changed from wine red to blue in the presence of melamine and no color change was observed before melamine was introduced into the milk sample. The observed color change is the result of the coupling of the surface plasmon resonance (SPR) between particles in close proximity. This method is also sensitive and a limit of detection of 0.90 ppm of melamine was obtained with the UV-vis spectrophotometer in aqueous solution and 0.46 ppm of melamine was detected visually with the naked eye in a solution of infant formula.