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

Abstract

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. Keywords: Gold nanoparticles (AuNPs), infant formula, melamine, pH stability, Surface plasmon resonance (SPR), TEM, UV-vis -1 ) in the visible portion of the electromagnetic spectrum (13). This unique SPR depends on size, shape, the refractive indices of the surrounding media and the distance between particles (13). The solutions of smaller gold nanoparticles appear wine red in their dispersed state and larger particles or in their aggregate forms appear purple or blue. The observed color changes are associated with the shift in the surface resonance band to longer wavelengths in the aggregated state and can easily be observed visually with the naked eye. We take advantage of these analyte-induced color changes of gold nanoparticles for visual detection of melamine in infant formula without the need for an expensive and complex instrumentation. In this report, we studied the effect of pH on the stability of gold nanoparticles and their application for the detection of melamine in infant formula. As stated above, the method of detection depends on color change of gold nanoparticles from wine red to blue when mixed with melamine. This method involves simple treatment of the infant formula with trichloroacetic acid to remove proteins which otherwise may interfere with the detection of melamine. Furthermore, the method is fast and highly sensitive.