Monofunctionalized Gold Nanoparticles Stabilized by a Single Dendrimer Form Dumbbell Structures upon Homocoupling

Article abstract of DOI:10.1021/ja306253t

The assembly of dumbbell structures as organic-inorganic hybrid materials is presented. Gold nanoparticles (NPs) with a mean diameter of 1.3 nm were synthesized in very good yields using a stabilizing dendrimer based on benzylic thioether subunits. The extended dendritic ligand covers the NP surface and contains a peripheral protected acetylene, providing coated and monofunctionalized NPs. These NPs themselves can be considered as large molecules, and thus, applying a wet-chemical deprotection/oxidative acetylene coupling protocol exclusively provides dimers of NPs interlinked by a diethynyl bridge. The concept not only enables access to novel organic/inorganic hybrid architectures but also promises new approaches in labeling technology.

Full text of DOI:10.1021/ja306253t

Communication
pubs.acs.org/JACS
Monofunctionalized Gold Nanoparticles Stabilized by a Single
Dendrimer Form Dumbbell Structures upon Homocoupling
Jens Peter Hermes,† Fabian Sander,† Ulrike Fluch,† Torsten Peterle,†,⊥ Damien Thompson,‡
Raphael Urbani,§ Thomas Pfohl,§ and Marcel Mayor*,†,∥
†Department of Chemistry, University of Basel, St. Johanns-Ring 19, CH-4056 Basel, Switzerland
‡Theory Modelling and Design Centre, Tyndall National Institute, University College Cork, Lee Maltings, Cork, Ireland
§Department of Chemistry, University of Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland
∥Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), P.O. Box 3640, D-76021 Karlsruhe, Germany
S
* Supporting Information
oligomers, NPs with narrow dispersity are obtained in excellent
ABSTRACT: The assembly of dumbbell structures as
organic−inorganic hybrid materials is presented. Gold
nanoparticles (NPs) with a mean diameter of 1.3 nm were
synthesized in very good yields using a stabilizing
dendrimer based on benzylic thioether subunits. The
extended dendritic ligand covers the NP surface and
contains a peripheral protected acetylene, providing coated
and monofunctionalized NPs. These NPs themselves can
be considered as large molecules, and thus, applying a wet-
chemical deprotection/oxidative acetylene coupling pro-
tocol exclusively provides dimers of NPs interlinked by a
diethynyl bridge. The concept not only enables access to
novel organic/inorganic hybrid architectures but also
promises new approaches in labeling technology.
yields. Furthermore, these NPs are decorated with only two
ligands, allowing the introduction of a controlled number of
peripheral functional groups dictating their chemical behavior.
Thus, these functionalized NPs can be considered as large
artificial molecules; as a first example, Au NPs containing two
peripheral ethynyl groups were interlinked to form oligomers by
a wet-chemical oxidative acetylene coupling protocol.15 By
expanding the benzylic thioether motif to dendritic structures, we
recently synthesized unfunctionalized dendrimers, and one
second-generation representative was able to stabilize exactly
one Au NP.16 The observed 1/1 ligand/Au NP ratio might make
available monofunctionalized NPs, which are much better suited
both for labeling and as building blocks for discrete organic−
inorganic hybrid architectures. The acetylene homocoupling of
functionalized NPs is a new approach for covalent assembly of
NP architectures in nonpolar organic media (the concept is
illustrated in Figure 1). Other approaches for the direct assembly
of NP dumbbells have been reported,10−12,17−20 and recent
review articles have presented the formation of various NP
architectures and their interparticle forces.21−24
The thioether dendrimer design presented here is based on a
structural motif that recently demonstrated its potential as a
coating and surface-passivating ligand during the synthesis of Au
NPs. Important design aspects are the dilution of the branching
points and the steric surface protection using bulky tert-butyl
groups in the “diluting” subunits. The reduced branching was
chosen to favor enwrapping by the dendritic ligand rather than a
scenario where the NPs grow inside a ball-shaped dendrimer.25,26
Our dendritic ligands were further functionalized by introducing
a central oligo(phenylene ethynylene) (OPE) rod comprising a
central pyridine unit and a triisopropylsilyl (TIPS)-protected
acetylene (Figure 1). The pyridine nitrogen not only provides an
additional coordination site for the Au NP but was recently also
shown to result in a perpendicular arrangement of the rod on the
NP surface.27,28 The masked acetylene is introduced as a
peripheral functionality, providing an integer number of wet-
chemically addressable functional groups in each obtained Au
NP. The thioether dendrimers G1 and G2 were synthesized by
SN2 reactions of already reported compounds (Scheme 1). The
old nanoparticles (Au NPs) are promising model
1,2
Gcompounds for nanoelectronic devices,
sensor applica-
tions3 and catalysis.4,5 Au NPs are also intensively used for
labeling, as represented in numerous review articles addressing
this topic.6−9 In many cases, Au NPs are functionalized with
DNA or peptides as recognition sites for labeling purposes.
These protocols are limited to aqueous or polar organic
conditions and are usually statistical reactions with low yields
of monofunctionalized Au NPs. To date, monofunctionalization
of Au NPs in apolar organic media has been achieved only by the
use of a solid support as a protecting group10,11 or by ligand
polymerization on the NP surface.12 In addition, single polymer
strands were reported to stabilize one NP on average.13 The
strategy presented here involves the design of an organic ligand
that (i) controls the particle formation by coating and passivating
the NP surface and (ii) allows the introduction of a controlled
number of functional groups at the NP periphery. Such NPs
exposing a restricted number of functional groups can be
considered as molecules with nanoscale dimensions and can be
addressed by wet-chemical protocols. While di- and mono-
functionalized NPs are promising building blocks of well-defined
organic−inorganic hybrid architectures, the latter ones are in
addition particularly appealing as potential labels.
It was recently shown that benzylic thioether oligomers have
promising stabilizing features as multidentate ligands during the
synthesis of Au NPs.14 In the presence of larger thioether
Received: June 27, 2012
Published: August 23, 2012
© 2012 American Chemical Society
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periphery of the NPs was manifested by a broad peak at ∼300
nm. Single drops of the solutions used for UV/vis spectroscopy
were transferred to carbon-coated copper grids (CCCGs) for
transmission electron microscopy (TEM) investigations. The
micrographs (Figure 2 and Figures S5 and S6) showed small Au
Figure 1. General concept of forming ligand-stabilized Au NPs and NP
dimers. (a) NP synthesis: HAuCl4, TOAB, NaBH4, H2O/CH2Cl2. (b)
Deprotection: TBAF, CH2Cl2. (c) Oxidative coupling: CuCl, TMEDA,
O2 (ambient air). For a space-filling representation, see Figure S4.
thiol dendrons16 [G0.SH] and [G1.SH] have substituted the
benzylic bromines of the OPE rod 1.27 The monofunctionalized
ligands were obtained in good to excellent yields.
Scheme 1. Synthesis of Monofunctionalized Thioether
Dendrimers G1 and G2 as Stabilizing Ligands for Au NPs
a
Figure 2. Representative dense TEM image of Au-G1 NPs. Inset: size
distributions for Au-G1 (black) and Au-G2 (red) NPs.
NPs with a narrow size distribution. An automatic investigation
method using ImageJ32 was applied to measure the size
(diameter) of the NPs formed (see the SI for the analysis
protocol). More than 5000 NPs from more than 10 dense TEM
images were analyzed. Similar size distributions were obtained
for Au-G1 and Au-G2, with mean diameters of 1.3 0.4 nm
(Figure 2 inset). These diameters are similar to those recently
measured for Au NPs stabilized by the parent unfunctionalized
dendrimers.16 For Au-G2, small-angle X-ray scattering (SAXS)
was employed as a second tool to determine the NP size. The
measured 2D scattering signal was integrated to obtain an
intensity plot (Figure S2) that suggested form factors of spheres.
The plot was fitted with a model for polydisperse, spherical NPs,
and a mean NP diameter of 1.3 nm with a standard deviation of
0.5 nm was obtained, corroborating the values obtained by TEM.
To analyze the coating ligand/Au NP ratio, thermogravimetric
analysis (TGA) of Au-G1 and Au-G2 was performed. To remove
the organic shell in the TGA experiments, the dry NPs were
heated to 950 °C. The weight loss curves were similar for the two
samples (Figure S3). The amounts of weight loss were 21% in the
case of Au-G1 and 26% for Au-G2. These values were used to
calculate the average number of gold atoms stabilized per
dendrimer (see the SI for details). We found that on average one
G1 dendrimer stabilizes 30 Au atoms, while G2 coats 50 atoms.
The dimensions of the Au-G1 and Au-G2 NPs determined by
TEM and SAXS suggest an average number of ∼55 gold atoms
per NP. In analogy to the parent ligand structure,16,28 a single G2
dendrimer or a pair of G1 dendrimers are required to stabilize
one Au NP.
a
(a) NaH, THF, rt; 85−95%.
The Au NP-stabilizing features of thioether dendrimers G1
and G2 were investigated using techniques that have been
successfully applied in earlier studies.15,16,27 The formation of
functionalized Au NPs was performed in a H2O/CH2Cl2 solvent
mixture following a protocol developed by Brust et al.29 The
dendritic ligands G1 and G2 dissolved in CH2Cl2 were added to
an aqueous solution of tetrachloroauric acid and the phase-
transfer agent tetraoctylammonium bromide (TOAB) in
CH2Cl2. Au NPs Au-G1 and Au-G2 were obtained after the
addition of sodium borohydride in water. After aqueous workup
and removal of TOAB with a precipitation/centrifugation
protocol, the excess ligand was removed by gel-permeation
chromatography (GPC). See the Supporting Information (SI)
for a more detailed protocol. The purified NPs were obtained in
yields exceeding 80% and were initially analyzed by UV/vis
spectroscopy. The UV/vis spectra obtained (Figure S1 in the SI)
showed the presence of Au NPs without a strong plasmon band
of gold at 520 nm, indicating that the NPs had diameters below 2
nm.30,31 The presence of the delocalized OPE structure in the
The discrete number of coating ligands also results in an
equally well-defined number of peripheral masked acetylene
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Communication
functions per Au NP, and thus, the extent of surface
functionalization per NP must be reflected in the connectivity
of the NP subunits in the hybrid architectures obtained upon
exposing them to acetylene coupling chemistry. While chains of
NPs would be expected for the bifunctional Au-G1, similar to
another example of bifunctional NPs,33 the monofunctionalized
analogue Au-G2 should form dimers exclusively. Samples of both
Au NPs (Au-G1 and Au-G2) were exposed to a Glaser−Hay34
wet-chemical oxidative acetylene coupling protocol. In brief,
tetrabutylammoniumfluoride (TBAF) was first added to liberate
the masked acetylene, and then tetramethylethylenediamine
(TMEDA) and CuCl were added under ambient atmosphere to
provide molecular oxygen. After 20 min, precipitation was
observed only for the reaction mixture containing Au-G1, while
the one containing Au-G2 remained as a dark solution. The
precipitation of bifunctional Au NPs due to the formation of too-
long oligomeric chains was previously found for comparable
systems.15,27 To keep the oligomers short and dissolved, a small
sample of the reaction mixture containing Au-G1 was worked up
after 10 min. After further dilution to avoid accidental proximities
of NPs that were not covalently bound, samples were deposited
on CCCGs and investigated by TEM. The micrographs showed
the presence of single NPs, dimers, and higher oligomeric
assemblies of NPs. These oligomers were predominantly found
as chains of NPs, and typical representatives of these Au-G1
superstructures are displayed in Figure 3. A more comprehensive
When the ligands bear an acetylene, chains are formed upon
oxidative acetylene coupling and precipitate if their lengths
exceed a certain threshold value.15,27
In contrast to these bifunctional NPs, the monofunctionalized
Au-G2 NPs were expected to form dimers only, and indeed, even
after prolonged reaction times or with excessive amounts of
coupling chemicals, precipitation was not observed. The
protected Au-G2 NP monomers were first deprotected with a
large excess of TBAF in CH2Cl2. After aqueous workup, the
solution was concentrated to dryness using a stream of nitrogen.
The homocoupling was then conducted in small amounts of
CH2Cl2 in an open reaction vessel for 3 h using excesses of CuCl
and TMEDA (see the SI for a detailed protocol). Samples of the
reaction mixture containing the NP hybrid architectures were
deposited as highly diluted solutions on CCCGs for TEM
investigations. Thus, only a few NPs were present in each TEM
image and showed sizes similar to the protected Au-G2
monomers (Figure 4 and Figures S9 and S10). In these
Figure 4. Representative TEM images of diluted solutions of Au-G2
dimers. The scale bars represent 10 nm.
micrographs of diluted samples, the yield of NP dimers was
analyzed. On 20 TEM images, ∼400 NPs were counted, and
almost half of them were dimers (47% yield); 46% of the NPs
found were still present as monomers, and only 7% NPs existed
in trimeric structures. The yield of dimers represents a 2−3-fold
increase compared with the yield of superstructures formed with
bifunctional NPs stabilized with linear thioether ligands (16−
20%).27 At first glance, the formation of trimeric structures from
monofunctionalized NPs was surprising, and only a more
detailed analysis of the interparticle spacing shed light on this
unexpected observation.
From the TEM images of the diluted samples, the distance
between the NPs in the dimers was analyzed, and the
interparticle distance distribution displayed in Figure 5 was
obtained. A bimodal distribution was found, with one maximum
matching the expected distance of the straight spacer at 2.5 nm.
This distance results from a perpendicular arrangement of the
rod on the gold surface27 triggered by the coordination of the
Figure 3. Representative TEM images of diluted solutions of Au-G1
superstructures. The scale bars represent 10 nm.
overview of the micrographs is displayed in Figures S7 and S8.
The chainlike arrangement of the NPs in these micrographs not
only corroborates the wet-chemical coupling chemistry as the
origin of the interlinked NPs but also supports the picture of
having two functional groups per Au-G1 NP on opposite sides.
Strong similarities between G1 and linear thioether ligands that
were investigated in earlier studies14,15,27,35 are obvious.
Dendrimer G1 and the linear ligands have similar sizes and
consist of eight thioether moieties. NPs stabilized by these two
ligands have comparable sizes with two coating ligands per NP.
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ASSOCIATED CONTENT
* Supporting Information
Descriptions of dendrimer synthesis, NP synthesis, NP coupling,
and NP analysis by UV/vis, TGA, TEM, and SAXS as well as
representative TEM images of NP monomers, dimers, and
oligomers. This material is available free of charge via the Internet
at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
marcel.mayor@unibas.ch
■
Present Address
⊥Evonik Industries AG, Untere Kanalstraße 3, D-79618
Rheinfelden, Germany
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support by the EU through
the project FUNMOL (no. 213382 of the call FP7-NMP-2007-
SMALL-1), the Swiss National Science Foundation, the Swiss
Nanoscience Institute, and National Research Project No. 62
“Smart Materials”.
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