Size-dependent flocculation of colloidal gold nanoparticles capped with biomolecules

Abstract

Gold nanoparticles with different sizes were prepared by adjusting the amount of sodium citrate added to a HAuCl4 solution as reducing agent. The surface of the gold particles were modified with various biomolecules with amine, carboxyl and/or thiol groups: amino acids (arginine, lysine, cysteine and glutathione); two oligopeptides with molecular weights of 953 and 1162 respectively; and proteins(bovine serum albumin (BSA) and human serum albumin (HSA)). The flocculation behavior of the modified gold nanoparticles was investigated by observing colorimetric changes and morphological changes with an UV-Visible spectrophotometer and a transmission electron microscope (TEM), respectively. Being capped with a small amino acid such as arginine, small gold nanoparticles flocculated more easily than large ones. On the contrary, large nanoparticles were observed to flocculate more easily when gold particles were capped with the proteins. The driving force for the flocculation of small biomolecules-capped gold particles was considered to be hydrogen bonding among the biomolecules, while the governing mechanism for the flocculation of protein-capped gold nanoparticles was considered to be van der Waals interaction among the gold particles