Size-dependent flocculation behavior of colloidal Au nanoparticles modified with various biomolecules

Au nanoparticles with different sizes were prepared and modified with various biomolecules including amino acids (arginine, lysine and cysteine), glutathione (GSH), oligopeptides, and proteins (bovine serum albumin (BSA), human serum albumin (HSA) and mouse IgG). The flocculation behaviors of the modified Au nanoparticles were investigated by observing their colorimetric and morphological changes with UV–vis spectrophotometer and transmission electron microscope (TEM), respectively. Consequently, we found that the flocculation behavior of the modified Au nanoparticles was quite different depending on both the size of the Au nanoparticles and the modified biomolecules. When modified with an amino acid, small-sized Au nanoparticles flocculated more easily than large ones while the modification with oligopeptides resulted in the flocculations of all tested Au nanoparticles due to the large number of hydrogen bonding between adjacent particles. The small Au nanoparticles were very effectively stabilized by protein capping, while the stabilization effect was not so good with large ones. The possible explanations for these size-dependent flocculation behaviors were discussed. This study would widen the understanding for the interaction involved in the Au nanoparticles modified with biomolecules.