Effect of the Spacer Structure on the Stability of
Gold Nanoparticles Functionalized with Monodentate Thiolated Poly(ethylene
glycol) Ligands
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Abstract
Poly(ethylene glycol)-
(PEG-) based ligands are well-established
for the stabilization of nanoparticles in aqueous solution and are
especially interesting for applications in medicine and biotechnology
because they are known to improve the pharmacokinetic properties of
nanomaterials. In this study, we prepared gold nanoparticles (AuNPs)
with ligand shells of different monodentate poly(ethylene glycol)–thiol
(PEG–SH) ligands. These ligands differed only in the segment
connecting the thiol group with the PEG moiety (<i>M</i><sub>w</sub> ≈ 2000 g/mol) through an ester bond, the spacer.
All ligands were synthesized by straightforward esterification. Specifically,
we used PEG ligands with a long (C<sub>10</sub>, PEGMUA) or short
(C<sub>2</sub>, PEGMPA) alkylene spacer or a phenylene (PEGMPAA) spacer.
The influence of the spacer on the stability of gold nanoparticle–PEG
conjugates (AuNP@PEG) was tested by cyanide etching experiments, electrolyte-induced
aggregation, and competitive ligand displacement with dithiothreitol
(DTT). In the presence of 100 mM cyanide, AuNPs stabilized with PEGMPA
or PEGMPAA were completely dissolved by oxidative etching within a
few minutes, whereas AuNPs stabilized with PEGMUA needed more than
20 h to be completely etched. By complementary experiments, we deduced
a simplified description for the etching process that takes into account
the role of excess ligand. In the presence of free ligand, significantly
fewer AuNPs are etched, suggesting a competition of etching and ligand
binding to AuNPs. We also compared the stabilizing effect of PEGMUA
with that of a bidentate PEG–thiol ligand (PEGLIP) and found
a reversed stability against cyanide etching and DTT displacement,
in agreement with previously reported observations. Our results clearly
demonstrate the strong impact of the spacer structure on conjugate
stability and provide valuable information for the rational design
of more complex AuNP@PEG conjugates, which are of much interest in
the context of biotechnology and medical applications