Assembly of Gold Nanoparticles Mediated by Amino Acids

Gold nanoparticles capped with citrate anions were synthesized and characterized by UV-Vis spectroscopy and transmission electron microscopy. This aqueous colloidal gold solution is stable in time, indicating a good electrostatic stabili-zation of gold nanoparticles through the adsorbed monolayer of citrate anions on nanoparticle surface. The interaction between the citrate capped gold nanoparticles in aqueous solution and various amino acids was investigated monitoring their UV-Vis spectra. Some of the tested amino acids, like glycine, isoleucine and asparagine, showed little interaction with gold nanoparticles. However, other amino acids, such as lysine, arginine, glutamic acid, histidine, cysteine and methionine, presented an absorp-tion decrease of the colloidal gold solution at 528 nm. At high amino acid/gold ion mo-lar ratios a new, large absorption band appears at 600–700 nm, which increases in time, while the colour of the colloidal gold solution changes from red to blue, as a conse-quence of the assembly of gold nanoparticles. The nanostructure of amino acids capped gold nanoparticles assembly is analyzed by TEM and atomic force microscopy (AFM). The analysis of these images evidenced the assembly of gold nanoparticles mediated by amino acids, where some gold nanoparticles appear to be overlapped or slightly sin-tered and others are not in direct physical contact.