Colorimetric detection of melamine during the formation of gold nanoparticles

A sensitive and simple colorimetric method for determination of melamine was reported based on the seedless production of gold nanoparticles (Au-NPs). Au-NPs were synthesized by using pyrocatechol-3,5-disodiumsulfonate (PD) as reducer without adding nanoparticles seeds and stabilizing agent. PD can form intramolecular hydrogen-bonding in solution by adjacent sulfurnate and α-phenolic hydroxyl group, resulting in its weak reducing capacity and thus the synthesis of Au-NPs was slow. While in the presence of melamine, PD reacted with melamine through hydrogen-bonding. Therefore, the intramolecular hydrogen-bonding of PD was interrupted by melamine, and the α-phenolic hydroxyl group was free to reduce Au(3+), hence, the synthesis of Au-NPs was accelerated. Especially, the presence of melamine led to a shift in the surface plasmon bond and a color change of Au-NPs from green to yellow. Results showed that the absorbance ratio (A(436)/A(600)) was linear with the logarithm of melamine concentration in the range of 4.8×10(-9) to 1.6×10(-6) M with a correlation coefficient of 0.9949. The detection limit (3σ) obtained by UV-vis spectrum was 6.4×10(-10) M (i.e., 0.08 ppb). The proposed method was applied successfully to the determination of melamine in pretreated liquid milk products, and the recoveries were from 93% to 107%.