Gold nanoparticles stabilized by thioether dendrimers

Article abstract of DOI:10.1002/chem.201101837

Ligand-stabilized gold nanoparticles (Au NPs) are promising materials for nanotechnology with applications in electronics, catalysis, and sensors. These applications depend on the ability to synthesize stable and monodisperse NPs. Herein, the design and s

Full text of DOI:10.1002/chem.201101837

FULL PAPER
DOI: 10.1002/chem.201101837
Gold Nanoparticles Stabilized by Thioether Dendrimers
Jens Peter Hermes,[a] Fabian Sander,[a] Torsten Peterle,[a, b] Raphael Urbani,[c]
Thomas Pfohl,[c] Damien Thompson,[d] and Marcel Mayor*[a, e]
Abstract: Ligand-stabilized gold nano-
particles (Au NPs) are promising mate-
rials for nanotechnology with applica-
tions in electronics, catalysis, and sen-
sors. These applications depend on the
ability to synthesize stable and mono-
disperse NPs. Herein, the design and
synthesis of two series of dendritic
ies. A comparison between the two li-
gands shows how both size control and
the stability of the NPs are influenced
by the nature of the ligand–NP wrap-
ping interaction. The meta-xylene-
bridged ligands provided NPs with a
narrow size distribution centered
around a diameter of 1.2 nm, whereas
the NPs formed with ethylene-bridged
dendrimers lack long-term stability
with NP aggregation detected by UV/
Vis spectroscopy and transmission elec-
tron microscopy. The bulkier tert-butyl-
functionalized meta-xylene bridges
form larger ligand shells that inhibit
further growth of the NPs and thus
provide a simple route to stable and
monodisperse Au NPs that may find
use as functional components in nano-
electronic devices.
thio
stabilize Au NPs is presented. The
dendrimers have 1,3,5-trisubstituted
ACHTUNGTRENNUNGether ligands and their ability to
Keywords: dendrimers · gold · li-
ACHTUNGTRENNUNG
gand design
thioethers
·
nanoparticles
·
benzene branching units bridged by
either meta-xylene or ethylene moiet-
Introduction
approach widely used since the pioneering work of Brust
et al.[28] In addition to free thiols, the less reactive thioethers
have also been used to ligate NP surfaces.[29–34] The thioeth-
er–gold coordination is much weaker than the covalent thio-
late–gold interaction.[35] Therefore multidentate thioether li-
gands may be used to form self-assembled, multivalent—
bound, stable and monodisperse ligand-wrapped NPs with a
distinct low-integer number of ligands wrapping and effec-
tively ensnaring each NP.[31–33] The first application of multi-
dentate macromolecular ligands for the stabilization of Au
NPs was the use of thioether polymers.[36–39] The use of thio-
ether dendrimers as stabilizing ligands has also been report-
ed.[40–43] The advantage of dendrimers over polymers is the
control over their monodispersity. The molecular structures
of reported dendritic ligands vary from stiff arylic sul-
fides[40,41] to partially flexible benzylic/arylic sulfides[42] and
highly flexible benzylic thioether dendrimers.[43] Superior
stability and monodispersity has been reported for the
latter. Unfortunately, one cannot unambiguously relate
these findings to thioether properties as the presence of ad-
ditional ether moieties may have played a role, with recent
work showing that ether moieties present in poly(ethylene
glycol) (PEG) dendrimers are also able to stabilize Au
NPs.[44] Other known stabilizing units for the formation of
dendrimer-encapsulated metal NPs are poly(amidoamine)
(PAMAM)[45–47] and poly(propyleneimine) (PPI) struc-
tures.[48,49] Thioether dendrimers used for applications other
than the stabilization of NPs have also been reported.[50–53]
The goal of this work was to develop dendritic thioether
structures that are able to stabilize Au NPs with monodis-
perse size through the formation of NP–ligand complexes
that allow a low-integer number of ligands to cover each NP
The research field of gold nanoparticles (Au NPs) has been
steadily advancing in the past decade. The chemical stability
and size-dependent properties of Au NPs make them attrac-
tive materials for use in nanotechnology.[1–4] The scope of
future applications is broad,[1] ranging from advanced elec-
tronic[5–9] and photonic[10,11] devices to ultrasensitive chemi-
cal[12–16] and biological sensors.[17] In addition, Au NPs have
current and potential applications in biological labeling,[18–22]
medical diagnostics,[23] and catalysis.[24,25] Aqueous Au NPs
are often formed as citrate-stabilized NPs and then function-
alized by using peptides[26,27] or DNA.[20,21] However, within
this study we focused on nonpolar organic solvents in which
mainly alkanethiols have been used to stabilize Au NPs, an
[a] J. P. Hermes, F. Sander, Dr. T. Peterle, Prof. Dr. M. Mayor
Department of Chemistry, University of Basel
St. Johanns-Ring 19, 4056 Basel (Switzerland)
E-mail: marcel.mayor@unibas.ch
[b] Dr. T. Peterle
Evonik Degussa GmbH
Untere Kanalstraße 3, 79618 Rheinfelden (Germany)
[c] R. Urbani, Prof. Dr. T. Pfohl
Department of Chemistry, University of Basel
Klingelbergstrasse 80, 4056 Basel (Switzerland)
[d] Dr. D. Thompson
Theory Modelling and Design Centre, Tyndall National Institute
Lee Maltings, University College Cork, Cork (Ireland)
[e] Prof. Dr. M. Mayor
Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT)
P.O. Box 3640, 76021 Karlsruhe (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101837.
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while also providing the long-term stability that is a prereq-
uisite for technological applications. Dendrimers are ideal
candidate ligands with their branched, flexible architecture
potentially allowing for extensive NP surface coverage and
therefore providing monodisperse NPs that do not aggregate
over time. The two dendritic ligands synthesized in this
work are both based on benzylic thioethers, the combination
of flexibility and weak individual thioether anchoring
groups providing a multivalent ligand for the assembly of
NPs complexed by a low number of ligands. Different gener-
ations and structural motifs of the dendritic ligands were
synthesized to determine their NP-stabilizing abilities and
their influence on the size distributions of the NPs obtained.
proach. The dendrons were synthesized by starting from the
terminal groups and working back towards the central unit.
The dendritic ligands are branched with a 1,3,5-trisubstitut-
ed benzene. The use of benzylic thioethers should give flexi-
ble molecular structures that allow all three sulfide groups
to be orientated towards the NP surface. Note that a similar
building block has already been reported to stabilize Au55.[54]
The dendrimers differ by the bridging unit that separates
the branching units from each other. The nomenclature of
the ligands emphasizes the different bridging units, which
are a focus of this work. The bridges were introduced into
the ligand design to 1) provide more separated thioether an-
choring points and 2) to increase the amount of free space
in the center of the dendrimers. This reduced branching den-
sity should improve the ability of the dendrimers to adapt to
the convex NP surface by forming a concave pocket. We
thus hypothesized considerably improved wrapping features
for such dendrimers with “diluted” branching units. The first
series (mX ligands, Scheme 1) use a tert-butyl-functionalized
meta-xylene to interconnect two sulfur atoms, the same moi-
eties used for the previously studied linear ligands.[31–33] The
second series (Et ligands, Scheme 2) uses ethylene bridges
for the interconnection of two neighboring sulfur atoms.
Two generations of dendrimers were synthesized for each
dendrimer series to investigate the correlation between den-
drimer generation and stabilizing or size-steering features.
1
The NPs were investigated by UV/Vis and H NMR spec-
troscopy, thermogravimetric analysis (TGA), small-angle X-
ray scattering (SAXS), and both standard transmission elec-
tron microscopy (TEM) and high-resolution scanning trans-
mission electron microscopy (HRSTEM).
Results and Discussion
Concept and strategy: We have recently shown that linear,
unbranched thioether ligands with a certain threshold length
are able to stabilize Au NPs and provide a narrow size dis-
tribution of NPs with a diameter of around 1.1 nm that do
not aggregate over time.[31–33] These linear thioethers are oli-
gomeric structures constructed from a meta-xylene-bridged
thioether motif. In this work we designed and synthesized
two series of dendritic thioether ligands (Scheme 1 and
Scheme 2) and investigated their potential for stabilizing Au
NPs. The dendrimers were synthesized by a convergent ap-
Ligand synthesis: The synthesis of the mX dendrimers is
shown in Scheme 1. The basic building blocks 1 and 4 were
synthesized by using literature protocols.[55,56] The branching
unit 2 was obtained after substitution of the bromides of
starting material 1 with lithium chloride in dimethylforma-
Scheme 1. Synthesis of a,a’-meta-xylene-bridged dendrons and dendrimers of various generations. Reagents and conditions: a) LiCl, DMF, 08C, 30 min,
RT, 2 h, 90%; b) BnSH, NaH, THF, RT, 1 h, 44%; c) 6, NaH, THF, RT, 1 h, 99%; d) TrtSH, NaH, THF, RT, 2 h, 49%; e) 1. KSAc, THF, RT, 1 h;
2. MeOH, K2CO3, RT, 1 h, 80%; f) 2, NaH, THF, RT, 2 h, 49%; g) TFA, Et3SiH, CH2Cl2, RT, 15 min; h) 7, NaH, THF, RT, 1 h, 90%; i) 4, NaH, THF,
RT, 1 h.
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Gold Nanoparticles Stabilized by Thioether Dendrimers
FULL PAPER
mide (DMF). The bromides were substituted because chlor-
ides are more stable in the presence of protected thiols. The
dendron terminal unit 3 was synthesized by statistical nucle-
ophilic substitution with benzyl mercaptan (BnSH) and
sodium hydride (NaH) as base in tetrahydrofuran (THF).
The G0 dendron [mX-G0.STrt] was formed from the termi-
nal unit 3 with the monothiol 6. Compound 6 was prepared
from the monofunctionalized bromide 5 by a mild one-pot
procedure for the conversion of benzylic bromides into
thiols.[57] After deprotection of the trityl group with tri-
Ligand-stabilized nanoparticles: Au NPs were prepared in
the presence of the dendritic thioether ligands mX-Gn and
Et-Gn to investigate the ability of these ligands to stabilize
NPs by preventing aggregation. The NPs were prepared in a
two-phase water/dichloromethane system closely following
the procedure developed by Brust et al.[28] (see the Experi-
mental Section for the synthetic protocol). The goldACHTUNGTRENNUNG(III)
precursor, tetrachloroauric acid, dissolved in water was
transferred to the organic phase by tetra-n-octylammonium
bromide (TOAB). To keep the ratio between the goldACHTUNGTRENNUNG(III)
fluoroacetic acid (TFA), the
[mX-G0.SH] dendron can be
precursor and thioether moieties comparable to earlier stud-
ies,[31–33] the amount of added ligand was normalized to the
number of thioether groups. The starting point for investi-
gating the ability of a ligand to stabilize Au NPs was in all
cases equal numbers of ligand sulfur atoms and gold atoms
extended by branching unit 7 to the next generation den-
dron. Precursor 7 was assembled from the bridging unit 6
with an excess of the dendritic branching unit 2. The respec-
tive ACHTUNGTRENNUNG[mX-Gn.SH] dendrons were used to form the final den-
drimers mX-G1 and mX-G2 with central unit 4. In view of
the statistical nature of some monofunctionalizations, all the
reactions gave good-to-excellent yields.
in the precursor. Thus, an eight-fold excess of the goldACHTUNGTRENNUNG(III)
precursor was used for mX-G1 and a twenty-fold excess was
used for mX-G2. Although for these mX ligands the ratios
were maintained, the concentration of the ligand was raised
Scheme 2 depicts the synthesis of the Et dendrons. The
first dendron [Et-G0.STrt] was synthesized starting from
thiirane (ethylene sulfide). The bridging unit 8 was synthe-
sized by ring-opening of thiirane with an excess of trityl
thiol (TrtSH) in the presence of triethylamine (TEA) as
base. As for the mX ligands, subsequent nucleophilic substi-
tution and deprotection reactions of the trityl groups led to
the terminal thiols as powerful nucleophiles. All the reac-
tions gave good-to-excellent yields considering the statistical
nature of the monofunctionalization reactions.
in the case of the Et ligands. The reduction of goldACHTUNGTRENNUNG(III) in
the presence of the thioether ligands was carried out by
quickly adding an aqueous solution of sodium borohydride
to the two-phase system. After aqueous workup, the organic
phases were dried over MgSO4 and filtered.
In the case of the mX ligands, the change in color to dark
brown indicated the formation of the NPs Au-mX-G1 and
Au-mX-G2. Precipitation of gold was not observed, which
indicates an efficient stabilization of the Au NPs formed. In
analogy to linear oligomers,[31–33]
coating by mX ligands provided
the NPs with enough stability
to allow removal of TOAB by
applying
a precipitation and
centrifugation protocol[32] and
of the excess ligand by size ex-
clusion chromatography (SEC).
Analysis by 1H NMR spectros-
copy (see Figure S1 in the Sup-
porting Information, SI) corro-
borated the total removal of
TOAB. The spectra also
showed the presence of surface-
bound dendrimer ligands, cor-
roborating
their
stabilizing
nature as a coating of NPs. As
far as the gold atoms were con-
cerned, the synthetic procedure
and removal of TOAB led to
the formation of NPs in a yield
of around 95%. However, ap-
proximately 10–20% of the
NPs were lost during SEC be-
cause some late SEC vials still
showed the presence of excess
ligand and were therefore dis-
carded to obtain only ligand-
Scheme 2. Synthesis of ethylene-bridged dendrons and dendrimers of various generations. Reagents and condi-
tions: a) TrtSH, TEA, DMF, RT, 92%; b) 2, K2CO3, THF, reflux, 46%; c) NaSMe, DMF, RT, 88%;
d) 1. Et3SiH, TFA, CH2Cl2, RT; 2. 9, NaH, THF, RT; e) 1. Et3SiH, TFA, CH2Cl2, RT; 2. 4, NaH, THF, RT, 1 h. stabilized NPs. This loss is due
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M. Mayor et al.
to the overlap in the retention times of ligand-stabilized
NPs and the free ligand.
around 2 nm from the very beginning (Figure 1A, black
line). A color change from reddish brown to dark red was
observed upon storing the isolated and dried organic phase
under ambient conditions in CH2Cl2 in the presence of
excess ligand for several weeks. As shown in Figure 1A
(gray line), a prominent plasmon resonance band was ob-
served after 4 weeks, which indicates an increase in NP size
upon storage. Interestingly, in spite of this aggregation of in-
itially formed NPs to give larger NPs, the precipitation of
aggregated NPs was not observed.
The weak plasmon resonance band in the UV/Vis spectra
of both the mX-G1- and mX-G2-stabilized NPs (Figure 1B)
point to NP sizes of around and below 1.6 nm.[58,59] The two
samples show similar absorption spectra. The minor differ-
ences between 300 and 400 nm may be ascribed to the pres-
ence of different amounts of excess ligand. Interestingly,
these UV/Vis spectra remained unchanged when the solu-
tions were retested after 6 months, which indicates the ex-
cellent long-term stability of mX-ligand-stabilized NPs even
on exposure to air and light. However, higher temperatures
than room temperature were avoided as a slight growth of
NPs has previously been reported at temperatures of around
408C.[31]
The formation of NPs in the presence of Et-G2 led to im-
mediate and complete precipitation of aggregated NPs after
addition of the reducing agent. The 1:1 ratio of gold equiva-
lents to sulfur atoms in the ligand design used initially was
then adjusted to a ratio of 1:2. A quick and complete precip-
itation of aggregated NPs was still observed. The same 1:2
ratio was used during the formation of NPs in the presence
of the fourth generation ligand Et-G4. In this case, upon ad-
dition of the reducing agent, the organic phase turned a red-
dish brown color pointing to the formation of stable NPs;
the precipitation of NPs was not observed for Et-G4.
To analyze the ligand-stabilized NPs UV/Vis spectra were
recorded (Figure 1). In the case of the stable and redissolva-
ble mX-ligand-stabilized NPs new solutions were prepared
from dried NPs in CH2Cl2, whereas in the case of Et-G4-sta-
bilized NPs, the organic layer was investigated directly by
UV/Vis spectroscopy. The organic layer of Au-Et-G4
showed a weak plasmon resonance band, which indicates a
NP distribution comprising a few NPs with diameters of
HRSTEM analysis of the Et-ligand-stabilized NPs and
TEM analysis of the mX-ligand-stabilized NPs were per-
formed to determine the diameters (sizes) of the NPs
formed. Micrographs were taken of CH2Cl2 solutions of NPs
deposited on carbon-coated copper grids (Figure 2). Large
differences between the Et-G4- (Figure 2A) and mX-ligand-
stabilized NPs (Figure 2B and C) are readily visible to the
naked eye. A solution of CH2Cl2, aged for 4 weeks, was de-
posited on the carbon grid (Figure 2A) and the diameters of
about 500 Au-Et-G4 NPs were measured. As expected on
the basis of the UV/Vis investigation, rather large NPs with
diameters of up to 15 nm were observed. Analysis of the ob-
served size distribution (Figure 3A) revealed a large disper-
sity of 1–15 nm. The broad distribution of Au-Et-G4 NPs
has a mean value of 6.2 nm with a standard deviation of
Æ2.4 nm. Although the NP growth of Au-Et-G4 is interest-
ing, we did not investigate it further because nanoelectronic
device components require NPs with a distinct number of li-
gands for further coupling to organic–inorganic superstruc-
tures.[32,33] In contrast to these large NPs stabilized by the
Et-G4 dendrimer, very different NP diameters were ob-
served for the mX-Gn-stabilized NPs. In this case the re-
corded TEM micrographs were analyzed by an automated
procedure using imageJ[60] (see the SI for a detailed descrip-
tion). The size distributions for both NPs are displayed in
Figure 3B and C. Interestingly, within the precision of the
measurement, similar NP sizes of 1.1Æ0.3 nm and 1.2Æ
0.4 nm were determined for Au-mX-G1 and Au-mX-G2, re-
spectively.
Figure 1. UV/Vis absorption spectra in CH2Cl2 of ligand-stabilized Au
NPs. A) Spectra of Au-Et-G4 NPs directly after NP formation (black)
and after 4 weeks (gray) in CH2Cl2. The arising plasmon resonance band
indicates the aggregation of NPs. B) Spectra of Au-mX-G1 (black) and
Au-mX-G2 (gray). The spectra are normalized to match at 520 nm. The
weak plasmon resonance peaks indicate NPs with diameters of around
and below 1.6 nm.[59]
The diameters of the NPs were also analyzed by SAXS,
performed by dissolving the Au NPs in benzene. The 2D
scattering signal was integrated to obtain intensity profiles,
which are shown as log–log representations in Figure 4. The
plots of Au-mX-G1 and Au-mX-G2 are similar, indicating
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Gold Nanoparticles Stabilized by Thioether Dendrimers
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Figure 3. Size distributions of ligand-stabilized Au NPs: A) Au-Et-G4
after storage in solution (500 NPs were measured manually), B) Au-mX-
G1, and C) Au-mX-G2 (5000 NPs were measured automatically for B
and C).
spheres. The intensity plots were fitted with Nanofit soft-
ware version 1.2 from Bruker, using a least-squares method
for polydisperse, spherical particles. The analysis revealed
both samples to have diameters of around 1.6 nm by assum-
ing a Gaussian distribution of the NP diameters of s=
0.4 nm.
Figure 2. A) Representative HRSTEM image of Au-Et-G4 after being
dissolved in CH2Cl2 for 4 weeks. Representative TEM images of B) Au-
mX-G1 and C) Au-mX-G2 NPs, respectively.
similar NP sizes, as expected on the basis of TEM investiga-
tions. The shapes of the plots suggest form factors for
The diameters of the NPs measured by small-angle X-ray
scattering (SAXS) differ from the values found in TEM in-
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M. Mayor et al.
should be able to coat a considerably larger surface area
than mX-G1.The ratio of organic ligand to gold should give
a closer insight into the assembly of the NP and ligand shell.
The excess ligand was first removed by SEC. Small amounts
of dried NPs were then studied by thermogravimetric analy-
sis (TGA). The sample was heated up to 9008C to remove
all organic components. The results for Au-mX-G1 and Au-
mX-G2 are shown in Figure 5. The weight loss for both sam-
Figure 5. Thermogravimetric analyses of Au-mX-G1 (black) and Au-mX-
G2 (gray).
ples follows the same trend. Decomposition starts at around
2008C and reaches a plateau between 600 and 7008C. The
weight loss is attributed to the decomposition and removal
of the organic shell from the NP surface and the plateau is
interpreted as the end of this process, when all the organic
coating has been removed. Comparable weight losses of 26
and 23% were measured for Au-mX-G1 and Au-mX-G2,
respectively.
Figure 4. SAXS intensity plots as log–log representations and best fits for
A) Au-mX-G1 and B) Au-mX-G2.
vestigations (diameters 1.1 and 1.2 nm). This deviation has
only recently been reported[33] and may be due to a slight
growth of the Au NPs triggered by the X-ray irradiation;
similar thermal expansion has previously been reported.[31]
In addition, the organic ligand shell might add to the scatter-
ing signal leading to larger radii. Although the SAXS meas-
urements corroborate the similarity of the sizes of both NPs,
the deviation from the diameters measured by TEM is not
yet understood and is the topic of further investigations.
Our previous studies relied on diameters measured by TEM
assuming ligand coating, which were corroborated by the
chemical behavior of these NPs.[32,33] We thus currently
prefer to refer to the diameters obtained by TEM over
those measured by SAXS to allow comparison between the
results obtained.
Despite the considerable increase in the number of sulfide
groups from eight for the dendritic ligand mX-G1 to twenty
for mX-G2, similar NP sizes were stabilized, as found by
TEM and SAXS analyses. This indicates that increasing the
dendrimer generation from G1 to G2 has no significant in-
fluence on the size of the NP obtained. It rather seems that
NPs grow until they reach a size that allows their enwrap-
ping by the dendritic ligand. However, with more than twice
the number of sulfide groups, the dendritic ligand mX-G2
Knowing the size of the NPs from the TEM investigations
allows calculation of their mass and thus the average mass
of ligand coating per NP can be estimated from the weight
percentage obtained by TGA. First, the mass of gold per
ligand is derived from the rule of proportion from the mass
of the ligand and the mass percentage of both the gold and
ligand [Eq. (1) in the SI]. This value is divided by the molec-
ular mass of gold to obtain the number of gold atoms per
ligand [Eq. (2) in the SI]. For the Au-mX-G1 NPs a ratio of
19 gold atoms per mX-G1 ligand was obtained. By using the
density of bulk gold (1Au) the number of gold atoms per NP
was estimated to be 41 for NPs with an average diameter of
1.1 nm (from TEM). The calculated 19 gold atoms per octa-
dentate mX-G1 ligand indicate a ratio of two mX-G1 li-
gands per gold NP. A similar pairwise coating of the NP sur-
face has been observed for linear octadentate ligands.[32,33]
For the Au-mX-G2 NPs, the ratio of gold atoms per mX-
G2 ligand was determined to be 54. The number of gold
atoms per 1.2 nm NP was calculated to be 53 atoms on aver-
age, which indicates that a single mX-G2 ligand can stabilize
the entire 1.2 nm NP. Note that Au55 clusters are known to
have a diameter of 1.4 nm.[8] The calculation with 1Au seems
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Gold Nanoparticles Stabilized by Thioether Dendrimers
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to overestimate the number of gold atoms in the NP. As the
sizes of the NPs determined by TEM are smaller than those
determined by SAXS studies, this overestimation to some
extent compensates the deviation in size. In view of the ex-
tended structure of the mX-G2 ligand with more than twice
the number of phenyl subunits and sulfide groups compared
with the first generation analogue mX-G1, this ability to
enwrap the entire surface of a NP of comparable size is not
surprising. This specific ratio of one ligand stabilizing one
NP is very rare. To our knowledge this has only been ach-
ieved by the use of a single polymer chain[61] or by radical-
chain polymerization on the NP surface.[62]
the branched ligand structure to the NP surface and the con-
siderable dimension of the ligand only allows for a single
ligand per NP in the case of Au-mX-G2. As the thioether–
gold bond is weak we can expect that the NP–ligand assem-
bly does not contain any “staples”, which have been found
in the crystal structures of several thiol-stabilized Au
NPs.[63,64] The discrete and integer number of ligands per NP
may even allow use of the supramolecular notations Au41ꢀ-
N
Au-mX-G2, respectively. However, this notation is mislead-
ing as it suggests that the number of gold atoms forming the
NPs is not controlled. In view of the NP size distributions
displayed in Figure 3B and C, a more appropriate descrip-
A molecular dynamics model of a Au55 cluster coated
with mX-G2 is depicted in Figure 6. The greed of the sulfide
groups for noble metal surfaces guarantees the adhesion of
tion would be Au41Æ8
spectively. To avoid confusion we prefer the old notations
ꢀ(mX-G1)2 and Au53Æ10ꢀmX-G2, re-
N
Au-mX-G1 and Au-mX-G2, respectively.
The two dendrimer structures Et-G4 and mX-G2 display
large differences in the long-term stability of the coated Au
NPs. Although NPs stabilized by Et-G4 quickly aggregate to
form larger NPs, mX-G2-stabilized NPs display excellent
long-term stability, which makes them very interesting
ligand structures for obtaining monofunctionalized NPs, for
example, for use as TEM labels. As a working hypothesis
we attribute this unequal long-term stability of ligand-stabi-
lized NPs to the different bridging units. The bulky tert-
butyl-functionalized meta-xylene bridges create
a large
ligand shell around the Au NP surface preventing further
aggregation. This steric protection of the NP surface pro-
vides not only a certain size control during the growth of
the NPs, but also long-term stability for the NPs Au-mX-G1
and Au-mX-G2. The loss of long-term stability in the case
of Au-Et-G4 is attributed to the reduced bulkiness of the
ethylene bridges in this dendritic structure. It seems that this
motif is not able to provide a strong protective shell and
thus NPs get close enough to aggregate. In addition, both
series of dendritic ligands differ in their terminal groups:
The mX-Gn series has terminal benzyl sulfides whereas the
terminal groups of the Et-Gn series are methyl sulfides.
However, the considerable differences in long-term stability
probably arise from the dendritic skeleton and not from the
terminal groups. This assumption is supported by a model in
which the terminal benzene rings (orange) do not coordi-
nate to the gold surface.
Conclusion
Figure 6. Quantum mechanical calculations of thioether–gold bond
strengths were combined with a classical molecular dynamics model to
calculate one-dendrimer (shown) and alternative two-dendrimer com-
plexes with the Au NP, modeled to a first approximation by a 55-atom
cluster (diameter 1.4 nm)[8] in dichloromethane. The details of the calcu-
lations will be presented elsewhere.[65] The calculations indicate that the
alternative two-dendrimer state is less stable for these sized NPs. The low
likelihood of replacement of the fully bound single dendrimer by two
partially bound dendrimers was determined by using an energy function
summed over beneficial wrapping interactions (individual thioether–gold
bond strengths plus van der Waals dendrimer–gold contacts) and wrap-
ping penalties (loss in dendrimer conformational freedom plus dendrimer
and gold desolvation).
Two dendrimer motifs have been synthesized, both based on
thioethers mounted on a 1,3,5-trimethylbenzene scaffold as
branching units but with different spacers to “expand” the
dendrimer structure. The spacer units reduce the density of
the branching units and should therefore allow the dendritic
ligand to adapt to the convex curvature of NPs. As spacer
units, a,b-ethynyl bridges and a,a’-meta-xylene structures
comprising a bulky tert-butyl group have been considered.
Although from the ethynyl-bridged dendrimer the second
and fourth generation ligands Et-G2 and Et-G4 were syn-
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M. Mayor et al.
discarded. Some loss also occurred during the filtration in advance of
SEC, performed to protect the column.
thesized, the first and second generation ligands mX-G1 and
mX-G2 were prepared in the case of the meta-xylene
spacers. The ability of these dendrimers to control the
growth and to stabilize particular sizes of Au NPs was inves-
tigated by using them as reagents during the biphasic reduc-
tion of chloroauric acid. With the two ethynyl-bridged den-
drimers only the larger Et-G4 displayed some limited NP
stabilizing features. Et-G4 was neither able to control the
size of NPs during their formation nor to stabilize the
formed NPs in solution over time. In contrast, both meta-
xylene-bridged dendrimers were able to stabilize small Au
NPs with average diameters of between 1.1 and 1.2 nm
(from TEM) in very good yields and with excellent long-
term stability. The limited surface area of these small NPs
allows all the thioethers of only two dendritic ligands mX-
G1 to coordinate to a NP. In the case of the further expand-
ed dendrimer mX-G2, the spatial limitation only allows a
single ligand to coordinate its 20 thioethers to the NP sur-
face. Thermogravimetric analysis corroborated the expected
1:2 and 1:1 ratios between the NP and dendritic ligands mX-
G1 and mX-G2, respectively. The considerable increase in
both the control over NP size and NP stability has been at-
tributed to the bulkiness of the dendritic coating with a tert-
butyl-functionalized meta-xylene linker, which prevents ag-
gregation by sterically separating the metal cores of the
NPs.
Acknowledgements
We gratefully acknowledge financial support by the EU through the pro-
ject FUNMOL (number 213382 of the call FP7-NMP-2007-SMALL-1),
the Gebert Rꢁf Foundation, the Swiss National Science Foundation, and
the National Research Project (No. 62 Smart Materials).
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Received: June 16, 2011
Published online: October 26, 2011
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