Strong Hydrogen Bonding of Gallic Acid during Synthesis of an Efficient AgNPs Colorimetric Sensor for Melamine Detection via Dis-synthesis Strategy

We developed a convenient, label-free, and nonaggregation-based silver nanoparticles probe for colorimetric detection of melamine. Ag nanoparticles (AgNPs) were synthesized using gallic acid, known as 3,4,5-trihydroxybenzoic acid, as a reducer and stabilizing agent simultaneously at room temperature. The formation of AgNPs was confirmed by various technical analyses, for example, UV-vis spectroscopy, Fourier transform infrared spectroscopy, energy dispersive spectroscopy, elemental mapping, powder X-ray diffraction, X-ray photon spectroscopy, dynamic light scattering, and high-resolution transmission electron microscopy. Since the gallic acid has three hydroxyl groups, it can interact with melamine via several hydrogen bonds which has been confirmed with theoretical study by density functional theory; thus, in the presence of melamine, the free gallic acid as a responsible agent for the synthesis of AgNPs, is decreased. Consequently, the formation of silver nanoparticles is disrupted because reducers are not enough for the reduction of Ag+ ion. Simultaneously, the color and spectral changes of the AgNPs depend on melamine concentration such that lower melamine concentration is equivalent to yellow (high absorbance) and higher concentration corresponds with colorless solution (low absorbance). In this way, the plasmon absorbance of AgNPs at 400 nm can be used for quantitative measurement of melamine. Our finding indicated that a sensitive linear calibration curve can be obtained between absorbance at 400 nm and the logarithm of melamine concentration in the range of 0.04-20 mu M with a linear coefficient of 0.992. The assay conveys a detection limit of 3.609 nM +/- 0.014 (3a) and shows high potential for melamine detection in a selectivity study.