Mechanism Of Melamine Detection By Methanobactin-Mediated Synthesis Of Gold Nanoparticles

This paper mainly includes two parts. First, the synthesis conditions and mechanism of Mb-mediated synthesis of monodispersed gold nanoparticles were studied. Second, the application of this synthesis method of gold nanoparicles to detect melamine and its dection mechanism were explored detailedly.Preparation of gold nanoparticles with a narrow size distribution has enormous importance in nanotechnology. Methanobactin (Mb) is a copper-binding small peptide that appears to function as an agent for copper sequestration and uptake in methanotrophs. Mb can also bind and catalytically reduce Au(III) to Au (0). In this study, we demonstrate a facile Mb-mediated synthetic route to prepare monodispersed gold nanoparticles. Continuous reduction of Au(Ⅲ) by Mb can be achieved by using hydroquinone (HQ) as the reducing agent. Effect of molar ratios of Mb to Au(III), adding amount of HQ, pH value, reaction temperature, reaction time on Mb-mediated synthesis of monodispersed gold nanoparticles was studied. The optimal conditions of monodispersed gold nanoparticles synthesis were as follows:the molar ratio of Mb, Chloroauric acid and HQ was1:5:2, pH value was5.4, reaction temperature was60, reaction time was20min. The gold nanoparticles were characterized by UV-visible absorption spectroscopy, Fluorescence spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy(FT-IR) and transmission electron microscopy (TEM). The formation and the surface plasmon resonance (SPR) properties of the gold nanoparticles are highly dependent on the ratio of Au(III) to Mb in solution. XPS, Fluorescence spectra and FT-IR spectra suggest that Mb molecules catalytically reduce Au(III) to Au (0) with the concomitant production of gold nanoparticles, and then, Mb statically adsorbed onto the surface of gold nanoparticles to form an Mb-gold nanoparticles assembly. This avoids secondary nucleation. Fourier transform infrared spectroscopy showed Mb and benzoquinone as a protective agent present at the surface of gold nanoparticles to prevent aggregation. The formed gold nanoparticles have been demonstrated to be spherical, monodispersed and uniform by TEM images. Analysis of these particles shows an average size of14.9nm with a standard deviation of1.1nm. The gold nanoparticles are extremely stable and can resist aggregation, even after several months.The synthesis of gold nanoparticles and the detection of melamine were finished at the same time. The possible mechanism of Mb-mediated synthesis of gold nanoparticles to detect melamine was explored. UV-visible spectroscopy and Fluorescence spectroscopy results showed that melamine does not inhibit the combination of Mb with Au(Ⅲ), but inhibit the formation of gold nanoparticles. XPS photoelectron spectroscopy analysis showed that only Au (0) species exist when Mb react with chlorauric acid in the absence of melamine, however, Au (0) and Au (Ⅰ) present together in the presence of melamine. These results confirmed that the reduction process of Au(Ⅲ) by Mb was inhibited by melamin. Au (Ⅰ) present at the surface of gold clusters to prevent the generation of crystal nucleus, so that subsequent reaction can not to proceed to form gold nanoparticles. In addition, Melamin and HQ interact with each other forming a big compound, which decrease the reducing activity of HQ. Thus there is no color change in solution, so melamine was determined according to the color of the solution.