Resumo:
Gold nanoparticles (AuNPs) are promising for electronic materials and biosensors applications. However, the synthesis methods of AuNPs involve toxic chemicals, posing environmental and biological risks. Alternatively, in the present study, green synthesis of gold nanoparticles was investigated using amino acids both as reducing agents for gold ions and as stabilizers of the metallic surfaces. The effect of reagent concentration (molar ratio of amino acid to gold ion precursor) and pH of the medium on nanoparticle synthesis was studied. The amino acids used in this study were aspartic acid, arginine, cysteine, histidine, tyrosine and tryptophan. The samples were characterized by UV-visible absorption spectroscopy, zeta potential, dynamic light scattering (DLS) and transmission electron microscopy (TEM). Based on UV-Vis spectroscopy analyses, aspartic acid, cysteine and tyrosine showed promising absorption spectra in terms of intensity at molar ratios ([amino acid]/[HAuCl4]) of 10, 0,25 and 0,5, respectively. Samples containing arginine and histidine exhibited lower ability to reduce gold ions, possibly due to their positive charge, among other factors. Tryptophan also showed poor results in terms of AuNP formation, which may be associated with its nonpolar and hydrophobic nature. TEM images and DLS measurements confirmed the formation of quasi-spherical gold nanoparticles with average diameters ranging from 10 to 50 nm. Zeta potential analysis revealed higher colloidal stability in samples containing aspartic acid, cysteine and tyrosine, with values above |30 mV|. In comparison, samples containing histidine and tryptophan had zeta potentials below |30 mV|, indicating lower stability. In an attempt to improve nanoparticle formation, pH adjustments were made to the synthesis with arginine, histidine, and tryptophan. However, these adjustments did not lead to significant improvements in terms of optical absorption, although more intense peaks were observed in the syntheses with tryptophan. The results of this study revealed that it is possible to obtain AuNPs under certain experimental conditions, based on principles of green chemistry, using amino acids as stabilizers and reducers of gold ions.
