Self-assembly characteristics of gold nanoparticles in the presence of cysteine

Gold nanoparticles in aqueous dispersions were prepared by two methods, using sodium citrate as reduction agent, and their interaction with l-cysteine was investigated. UV–vis, dynamic light scattering (DLS) and zeta potential measurements were used for physical and chemical characterization of mixtures with different gold:cysteine ratios, coupled with transmission electron microscopy (TEM) and atomic force microscopy (AFM) observations. Even a diluted (0.001 M) cysteine solution leads to a rapid assembly of gold nanoparticles and a broad absorption band at longer wavelength (characteristic for the aggregation of gold nanoparticles) develops and surpasses rapidly the peak of non-aggregated gold particles. TEM images for gold nanoparticles mixed with a 0.001 M cysteine solution show randomly arranged aggregates of gold nanoparticles on TEM carbon coated copper grids. AFM images indicate a rather ordered surface of self-assembled monolayers deposited on silanized glass. From thermodynamic considerations on the protolytic equilibria in aqueous cysteine solutions, the species present at different pH values were ascertained. A possible model for cysteine binding to gold nanoparticles and for the formation of gold particle assemblies is also suggested.