Improved Stability of “Naked” Gold Nanoparticles
Enabled by in Situ Coating with Mono and Multivalent Thiol PEG Ligands
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Abstract
Unprotected (“naked”)
gold nanoparticles with high
monodispersity (⟨<i>d</i>⟩, 5.5± 0.5
nm) were obtained in a facile and single-step microwave-assisted hydrolytic
decomposition of the molecular precursor [NMe<sub>4</sub>][Au(CF<sub>3</sub>)<sub>2</sub>]. Given their chloride-free surface chemistry,
the as-obtained gold nanoparticles were in situ functionalized with
mono-, di-, and trivalent thiolated PEG ligands in order to study
the influence of multivalent character of the ligands on the stability
of the colloidal solutions. For this purpose, a novel tridentate ligand
was synthesized and the previously reported syntheses of mono- and
divalent thiol ligands were improved. Owing to the pristine character
of the Au nanoparticles no ligand exchange was required, and the colloidal
and chemical stability of the mono- and multivalent functionalized
particles purely depended on the ligating ability of the thiolated
groups. In situ-functionalized Au nanoparticles showed a strikingly
(2 orders of magnitude higher) improved stability against small nucleophiles
such as sodium cyanide compared to gold nanoparticles coated with
citrate ligands and functionalized via a ligand-exchange reaction.
The monovalent thiol PEG ligand produced most stable colloids against
cyanide, which is explained by a strongly increased numerical ligand-density
on the surface. Gold colloids stabilized by di- and trivalent ligands
exhibited high stability in aqueous solutions with high NaCl concentrations
(2 M) in contrast to those functionalized with the monovalent PEG
ligand, which were only temporally stable in dilute NaCl solutions.
The beneficial effect of the multivalence of the ligands was further
demonstrated by the incorporation of an additional chelating ligand
(dithiothreitol) to the colloidal dispersions