Angewandte
Communications
Chemie
Gold Nanoparticles
Water-Soluble N-Heterocyclic Carbene-Protected Gold Nanoparticles:
Size-Controlled Synthesis, Stability, and Optical Properties
Kirsi Salorinne, Renee W. Y. Man, Chien-Hung Li, Masayasu Taki, Masakazu Nambo,* and
Abstract: NHC-AuI complexes were used to prepare stable,
water-soluble, NHC-protected gold nanoparticles. The water-
soluble, charged nature of the nanoparticles permitted analysis
by polyacrylamide gel electrophoresis (PAGE), which showed
that the nanoparticles were highly monodisperse, with tunable
core diameters between 2.0 and 3.3 nm depending on the
synthesis conditions. Temporal, thermal, and chemical stability
of the nanoparticles were determined to be high. Treatment
with thiols caused etching of the particles after 24 h; however
larger plasmonic particles showed greater resistance to thiol
treatment. These water-soluble, bio-compatible nanoparticles
are promising candidates for use in photoacoustic imaging,
with even the smallest nanoparticles giving reliable photo-
acoustic signals.
NHC-stabilized metal nanoparticles in catalytic reactions,
including the use of bidentate hybrid NHC-thioether ligands
for water-soluble Pd and Au nanoparticles.[4c,d] These systems
showed good pH stability; however, as they were destined for
catalytic rather than biological applications, stability tests in
high ionic strength media or to biologically relevant nucleo-
philes were not reported.
Considering the potential importance of NHCs for nano-
structures, the need to access water-soluble and biocompat-
ible structures cannot be overstated. In this regard, we set out
to prepare NHC-functionalized nanoparticles decorated with
carboxylate groups chosen for their water solubility, pH
tunability, and ease of derivatization.
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We began by preparing NHC Au Cl (1), (Figure 1),
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through reaction of Me2S Au Cl with the carboxylated
benzimidazolium triflate in the presence of K2CO3 at
608C.[5] Although effective, this method was often compli-
cated by the presence of [(NHC)2Au]+OTfÀ (2).[6] Consider-
ing the ready availability of 2, we explored its use as
G
old nanoparticles are one of the most often employed and
most recognizable nanostructures.[1] With a multitude of
catalytic, imaging, drug delivery, and theranostic applications,
these species have attracted considerable attention.[1b] Recent
interest in the use of N-heterocyclic carbenes (NHCs) as
alternatives to thiols for the functionalization of gold
surfaces[2] is providing new opportunities in this important
area of nanochemistry, and there are several examples of the
use of these ligands to stabilize gold nanoparticles.[3,4] How-
ever, for any applications in biological systems, solubility in
aqueous media and stability under biological conditions are
critical.
a nanoparticle precursor despite the presence of two strong
[7]
À
Au C bonds. As shown in Figure 1, both starting materials
gave rise to stable gold nanostructures.
Thus far, there are very few examples of water-dispersible
NHC-stabilized metal nanoparticles.[4] In 2015, Johnson
published a seminal paper describing Au nanoparticles
stabilized with polyethylene glycol (PEG)-functionalized
NHCs.[4a] The PEGylated NHC ligand provided high stability
in a range of pH and good stability under biologically relevant
electrolyte concentrations. With regard to non-noble metal
nanoparticles, Chaudret[4b] described the use of sulfonate-
terminated NHC ligands as stabilizing groups for Pt nano-
particles. Ravoo and Glorius have employed water-soluble
Figure 1. Synthesis of water-soluble NHC-Au nanoparticles by direct
reduction of molecular NHC-Au complexes 1 or 2, or a mixture of 2
and HAuCl4 (3). PAGE (Tris-HCl/glycine) analysis of the purified
products. Au102 =Au102(pMBA)44[9] as a reference, H+ =ripened NPs
under acidic conditions.
[*] Dr. K. Salorinne, Dr. R. W. Y. Man, Dr. M. Taki, Dr. M. Nambo,
Dr. C. M. Crudden
Institute of Transformative Bio-Molecules (WPI-ITbM)
Nagoya University
Furo, Chikusa, Nagoya, 464-8602 (Japan)
E-mail: cruddenc@chem.queensu.ca
Reduction of neutral complex 1 with NaBH4 took place
with an immediate color change to dark brown, indicating
nanoparticle formation. The carboxylate functionality on the
carbene ligand renders the nanoparticles water soluble when
deprotonated and makes the surface of the nanoparticles
Dr. C.-H. Li, Dr. C. M. Crudden
Department of Chemistry, Queen’s University
Chernoff Hall, Kingston, Ontario (Canada)
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charged, thus the purity and monodispersity of the NHC Au
nanoparticles could be readily analyzed by polyacrylamide
gel electrophoresis (PAGE),[8] which separates species in the
gel matrix based on size.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers!