Gold nanoparticle based colorimetric and Raman “turn-off” sensing of melamine in milk

Abstract

Here, we report the development of a sensitive colorimetric and Raman “turn-off” sensor based on gold nanoparticles (AuNPs), for the detection of melamine (MM) in real milk samples. The methodology relies on the effect of analyte-reducer interaction on the formation and stabilization of AuNPs. Gallic acid (GA), used as the reducing and capping agent imparts size and shape control to the AuNPs. In aqueous solution as well as milk extracts, the size control of AuNPs was found to be affected with increase in the concentration of analyte (MM). This resulted in the colour variation of the AuNPs solution from red to purple to blue and grey. The increasing size of AuNPs with higher MM concentration also resulted in Raman “turn-off” sensing due to the reduced effect of surface plasmon. From the least square fit of relative absorbance versus MM concentration, the limit of detection (LOD) was found to be ~1.15 ppb, which is lower than the WHO recommended limit of 1 ppm. This methodology for on-site visual colorimetric detection and Raman “turn-off” sensing involves ultra-filtration technique for the extraction of MM from milk matrix. The developed methodology is simple, rapid and cost effective. This study can thus, be further exploited for the development of low cost, robust colorimetric sensors for industrial use.