X-ray photoelectron spectroscopy characterization of gold nanoparticles functionalized with amine-terminated alkanethiols.

Gold nanoparticles (AuNPs) functionalized with a short chain amine-terminated alkanethiol (HS-(CH(2))(2)NH(2) or C2 NH(2)-thiol) are prepared via a direct synthesis method and then ligand-exchanged with a long chain amine-terminated alkanethiol (HS-(CH(2))(11)NH(2) or C11 NH(2)-thiol). Transmission electron microscopy analysis showed the AuNPs were relatively spherical with a median diameter of 24.2 ± 4.3 nm. X-ray photoelectron spectroscopy was used to determine surface chemistry of the functionalized and purified AuNPs. The ligand-exchange process was monitored within the time range from 30 min to 61 days. By the fourth day of exchange all the C2 NH(2)-thiol molecules had been replaced by C11 NH(2)-thiol molecules. C11 NH(2)-thiol molecules continued to be incorporated into the C11 NH(2) self-assembled monolayer between days 4 and 14 of ligand-exchange. As the length of the exchange time increased, the functionalized AuNPs became more stable against aggregation. The samples were purified by a centrifugation and resuspension method. The C2 NH(2) covered AuNPs aggregated immediately when purification was attempted. The C11 NH(2) covered AuNPs could be purified with minimal or no aggregation. Small amounts of unbound thiol (∼15%) and oxidized sulfur (∼20%) species were detected on the ligand-exchanged AuNPs. Some of the unbound thiol and all of the oxidized sulfur could be removed by treating the functionalized AuNPs with HCl.