The Study of the Phase Transfer of Gold Nanoparticles by Surface Modification

Metal nanoparticles are of great interest for diverse applications due to their unique properties; however, for sensing and other targeted applications, it is necessary to modify the surface chemistry for their desired function. We have synthesized citrate-stabilized, spherical gold nanoparticles 6-8 nm in diameter and subsequently exchanged the surface-capping citrate with a model alkanethiol in a stirred, biphasic water/hexane medium. The change in surface chemistry switches the particle solubility preference from the aqueous to the organic environment. We have instigated the nanoparticle phase transfer with time resolved UV-Visible absorbance spectroscopy of the aqueous phase. We thereby monitor the absorbance intensity of a characteristic peak as it initially increases, then diminishes, illustrating the changes in nanoparticle surface chemistry and local solvent environment. The peak also shifts in wavelength from initially 506.1 nm to 610.1 nm throughout the phase transfer, which indicates the onset of the ligand exchange and a potential redistribution of particle size in the aqueous layer as the concentration begins to decrease. TEM images of the aqueous phase show the size distribution of the nanoparticles, and nanoparticles from the organic phase following ligand exchange.