Investigation of the NaBH4-Induced Aggregation of Au Nanoparticles

Colloidal Au nanoparticles (AuNPs) with a diameter of 17 rim were prepared by the reduction of HAuCl4 with citrate trisodium. The addition of NaBH4 to the prepared citrate-stabilized AuNP solutions not only induced a blue shift in the surface plasmon resonance peak (lambda(max)) because of the increased number of electrons in the NPs injected by NaBH4 but also affected the stability of citrate adsorbed on AuNPs. The zeta potential of AuNPs after the addition of 6 mM NaBH4 decreased (67%) but was restored (88%) after the discharge of the injected electrons. The effect of NaBH4 treatment on the stability of citrate ions on AuNPs was investigated by X-ray photoelectron spectroscopy (XPS). The XPS data showed that citrate ions partially desorbed from the surfaces of AuNPs (67%) after NaBH4 treatment but readsorbed onto the AuNPs (80%) after the discharge of the NPs; this result agrees well with the zeta potential data. The partial removal of citrate ions from AuNPs results in an anisotropic charge distribution around the AuNPs. By increasing the amount of NaBH4 and the electrolyte concentration of the solution, non-close-packed aggregates of AuNPs can be formed, from monomers to small aggregates containing a few AuNPs and 3D network aggregates.