Colloidal Stability of Gold Nanoparticles Coated with Multithiol-Poly(ethylene glycol) Ligands: Importance of Structural Constraints of the Sulfur Anchoring Groups

Gold nanoparticles (AuNPs) coated with a series of poly(ethylene glycol) (PEG) ligands appended with four different sulfur-based terminal anchoring groups (monothiol, flexible dithiol, constrained dithiol, disulfide) were prepared to explore how the structures of the sulfur-based anchoring groups affect the colloidal stability in aqueous media. The PEG-coated AuNPs were prepared by ligand exchange of citrate-stabilized AuNPs with each ligand. The colloidal stability of the AuNPs in different harsh environmental conditions was monitored visually and spectroscopically. The AuNPs coated with dithiol- or disulfide-PEG exhibited improved stability under high salt concentration and against ligand replacement competition with dithiothreitol compared with those coated with their monothiol counterpart. Importantly, the ligands with structurally constrained dithiol or disulfide showed better colloidal stability and higher sulfur coverage on the Au surface compared to the ligands with more flexible dithiol and monothiol. X-ray photoelectron spectroscopy also revealed that the disulfide-PEG ligand had the highest S coverage on Au surface on the Au surface among the ligands studied. This result was supported by energy minimization modeling studies: the structurally more constrained disulfide ligand has the shortest S S distance and could pack more densely on the Au surface. The experimental results indicate that the colloidal stability of the AuNPs is systematically enhanced in the following order: monothiol < flexible dithiol < constrained dithiol < disulfide. The present study indicates that the colloidal stability of thiolated ligand-functionalized AuNPs can be enhanced by (i) a multidentate chelating effect and (ii) use of the constrained and compact structure of the multidentate anchoring groups.