Synthesis and characterization of gold-nanoparticle-cored dendrimers stabilized by metal-carbon bonds

Article abstract of DOI:10.1002/asia.200900388

Synthesis and characterization of gold-nanoparticle-cored dendrimers (NCDs), in which the dendrons are attached to the gold core through gold-carbon bonds, are described. Synthesis of these materials involved the simultaneous reduction of HAuCl ₄ and a Frechet-type dendron with a diazonium group at the focal point, all in an organic solvent such as toluene. These materials possess a nanometer-sized gold core surrounded by a shell of polyaryl ether dendrons, which are connected radially to the core. The NCDs were characterized by TEM, thermogravimetric analysis (TGA), and IR, UV, and NMR spectroscopic techniques. Average particle diameter of the NCDs ranged from 4.7 to 5.5 nm for the different generations. All NCDs exhibit the characteristic plasmon absorption of gold nanoparticles at 520 nm. Average numbers of dendrons per NCD in AuG_n were calculated using results from TGA and TEM studies. Multiple layering of the dendrons is proposed as a possible reason for the high dendron/NCD value.

Full text of DOI:10.1002/asia.200900388

DOI: 10.1002/asia.200900388
Synthesis and Characterization of Gold-Nanoparticle-Cored Dendrimers
Stabilized by Metal–Carbon Bonds
Venugopal K. Ratheesh Kumar and Karical R. Gopidas*[a]
Abstract: Synthesis and characteriza-
tion of gold-nanoparticle-cored den-
drimers (NCDs), in which the dendrons
are attached to the gold core through
gold–carbon bonds, are described. Syn-
thesis of these materials involved the
simultaneous reduction of HAuCl4 and
a Frꢀchet-type dendron with a diazoni-
um group at the focal point, all in an
organic solvent such as toluene. These
gold core surrounded by a shell of
polyaryl ether dendrons, which are con-
nected radially to the core. The NCDs
were characterized by TEM, thermog-
ravimetric analysis (TGA), and IR,
UV, and NMR spectroscopic tech-
niques. Average particle diameter of
the NCDs ranged from 4.7 to 5.5 nm
for the different generations. All NCDs
exhibit the characteristic plasmon ab-
sorption of gold nanoparticles at
520 nm. Average numbers of dendrons
per NCD in AuGn were calculated
using results from TGA and TEM
studies. Multiple layering of the den-
drons is proposed as a possible reason
for the high dendron/NCD value.
Keywords: dendrimers · diazonium
salts · monolayers · nanoparticles ·
self-assembly
materials possess
a
nanometer-sized
Introduction
been made in the design and synthesis of NCDs that contain
different metals and also dendrons of different structures.
NCDs with gold,[1a,b,e] platinum,[1h] and palladium[1c,e,g] cores
and with polyaryl ether- or polyamidoamine-type dendrons
were synthesized and thoroughly characterized. Most of
these systems make use of the Brust reaction, which was de-
signed originally for the preparation of monolayer-protected
clusters (MPC) of gold atoms.[6] The Brust synthesis of Au-
MPCs involved the partitioning of hydrogen tetrachloroau-
rate (HAuCl4) into an organic solvent such as toluene using
a phase-transfer agent, followed by reduction using sodium
borohydride (NaBH4) in the presence of an alkanethiol cap-
ping agent. The alkanethiols get attached to the growing Au
nanoparticle, thereby stabilizing it from aggregation and
also arresting its growth, thus leading to the formation of
the MPC.[7] Synthetic methodologies for NCDs were recent-
ly reviewed and were classified into three categories.[8] The
first involved self-assembly of a suitably capped dendron on
the growing nanoparticle and this is achieved by replacing
the alkane thiol in the Brust synthesis by the capped den-
dron.[1a–c] NCDs prepared by this method would have large
void spaces on the metal surface and a large fraction of the
metal-core surface is unpassivated. The second method in-
volves replacement of some of the alkane thiolate protecting
groups of a MPC by suitably capped dendrons through
place-exchange reactions.[9] A third strategy involved synthe-
sis of MPCs with functional groups on their periphery fol-
lowed by reaction of these functional groups with suitably
Nanoparticle-cored dendrimers (NCDs) are inorganic–or-
ganic core–shell materials that possess nanometer-sized
metal clusters at the core surrounded by a shell of dendrons
of different generations (G), which are attached radially to
the core.[1] These core–shell hybrid materials are significant-
ly different from dendrimer-encapsulated nanoparticles
(DENs) in that the metal nanoparticles in NCDs are stabi-
lized through specific bonding interactions with the den-
drons, whereas in DENs, encapsulated nanoparticles are sta-
bilized through nonspecific interactions of the coordinating
atoms of the dendrimers with the metal. The synthesis of
DENs has been pioneered by the groups of Crooks,[2] Toma-
lia,[3] and Esumi,[4] and their applications, especially in the
field of catalysis, have been well documented in the litera-
ture.[5] During the last few years, significant progress has
[a] V. K. R. Kumar, Dr. K. R. Gopidas
Photosciences and Photonics Section, Chemical Sciences and Tech-
nology Division
National Institute for Interdisciplinary Science and Technology
Council of Scientific and Industrial Research (CSIR)
Trivandrum 695 019 (India)
Fax : (+91) 471-2490186
E-mail: gopidaskr@rediffmail.com
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.200900388.
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FULL PAPERS
functionalized dendrons.[10,11] For example, upon reaction
with hydroxyl-functionalized Frꢀchet-type dendrons, MPCs
with carboxylic acid end groups gave NCDs of different gen-
erations.[10] NCDs prepared by the latter methods will have
most of the metal surface sites passivated by alkane thio-
lates, and the metal cluster will not have any catalytic sites.
Most of the approaches for NCD synthesis involved self-
assembly of ligands with thiol or disulfide groups on the
nanoparticle surface. For example, Fox and co-workers have
synthesized Au–NCDs of different generations using thiol-
functionalized Frꢀchet-type dendrons.[1a] Self-assembly of
dendrons with CH2OH and R4N+ groups were also reporte-
d.[1e,f] An alternate approach was to employ dendrons with
metal-coordinating groups at the focal point of the dendron.
Thus, polyamidoamine (PAMAM) or Frꢀchet-type dendrons
capped with pyridine,[12] 4-pyridone,[13] or diphenylphosphi-
ne[1g] groups at the focal point were also used in NCD syn-
thesis. Since the NCDs enclose large void spaces close to the
nanoparticle surface, they can function as “reaction vessels”
for metal-catalyzed reactions.[1c,f,g] NCDs thus serve as cata-
lysts in several reactions. For example, Fox and co-workers
have reported that a Pd nanoparticle-cored Frꢀchet dendri-
mer could catalyze Heck and Suzuki reactions.[1c] Wu et al.
have shown that the Pd-nanoparticle-cored phosphine den-
drimer is a good catalyst for Suzuki–Miyaura and hydroge-
nation reactions.[1g] Yang and co-workers have shown that
Pt-NCDs are capable of catalyzing the hydrogenation reac-
tion of nitrobenzenes.[1f]
gands on the metal surface. This report is an attempt in that
direction.
In this paper, we report the synthesis and characterization
of Au–NCDs of generations 1–4 (abbreviated as AuGn, in
which n stands for the dendron generation number), where-
in the dendrons are linked to the gold surface through
ꢀ
metal–carbon bonds. The Au C bond is reported to be
stronger (48 kcalmolꢀ1)[16] than the Au S bond in SAMs and
ꢀ
hence these NCDs are expected to be more stable than thio-
late-capped NCDs. Our synthetic approach is based on the
propensity of the aryl radical generated by the one-electron
reduction of diazonium salts to form covalent bonds with
metal/solid surfaces.[17f,m,n] When a diazonium salt is reduced
electrochemically in an aprotic solvent, aryl radicals are
formed that exhibit a tendency to get chemically attached to
the cathode surface.[17f,m] This reaction has been exploited
for the modification of solid surfaces with mono- or multi-
layers of organic materials. Surfaces modified in this way in-
clude carbon (carbon fibers,[17b] carbon black,[17c] carbon
nanotubes,[17g] highly ordered pyrolytic graphite (HOP-
G),[17a,c,d] diamond[17j]), semiconductors (Si,[17d,h] GaAs[17k]),
and metals (Au,[17p,q] Pt,[17i] Fe,[17o] Zn,[17i] Cu,[17i,l] Pd,[17k] and
so forth). Recently, Mirkhalaf et al. have synthesized Au
and Pt MPCs with metal–carbon bonds by using diazonium
salts with alkyl chains.[18] Under the reaction conditions, the
diazonium salts decompose to generate aryl radicals that get
attached to metal nanoparticles, thereby leading to the for-
mation of MPCs. Subsequently, synthesis of Pd, Ru, and Ti
nanoparticles stabilized by metal–carbon bonds were also
reported.[16,19] In this paper we show that reduction of
HAuCl4, phase-transferred into toluene in the presence of
diazonium salt-capped Frꢀchet-type dendrons (G1–G4), re-
sults in the formation of Au–NCDs that have carbon–gold
covalent bonds. Idealized structures of the NCDs are shown
in Scheme 1.
ꢀ
ꢀ
Although the Au S bond in diatomic Au S is very strong
(100 kcalmolꢀ1),[14a] the Au S bond in R-S-Au systems such
ꢀ
as alkanethiolate self-assembled monolayers (SAM) of gold
and Au-NCDs are much weaker (ꢁ40 kcalmolꢀ1).[14b] In the
case of alkanethiolate SAMs on gold, it has been shown that
prolonged exposure to air leads to oxidation of the gold–thi-
olate bond, thus leading to formation of sulfinates and sulfo-
nates.[15f] Annealing of the SAMs at 1008C or above for long
periods also lead to oxidation of thiolate species to sulfona-
tes.[15d] Irradiation by UV light leads to photo-oxidation of
alkanethiolates, which provides yet another pathway for the
formation of sulfonates.[15b,e] The sulfinates and sulfonates
thus formed have lower affinity to the gold surface and ex-
hibit a tendency to desorb from the metal surface. It is also
reported that oxidized SAMs can undergo rapid exchange
reactions with thiols present in solution.[15a,c,e] The gold–thio-
late bond in NCDs is also expected to behave similarly,
thereby casting doubt on the longtime stability of metal–thi-
Results and Discussion
Synthesis
The various steps involved in the synthesis of AuGn are
shown in Scheme 2. Frꢀchet-type dendritic bromides (G1Br–
G4Br) were prepared using reported procedures.[18,20] The
bromides were treated with p-nitrophenol to get the nitro-
dendrons, which were reduced to get the amino derivatives.
The amino dendrons were diazotized to get the diazoden-
drons (see the Supporting Information for details of synthe-
sis and characterization). Characterization data for the vari-
ous diazodendrons are given in the Experimental Section.
ꢀ
olate NCDs. In addition, the C S bonds in NCDs are also
not very strong if the carbon is benzylic type (ꢁ53 kcalmo-
lꢀ1)[14a] and hence NCDs with a metal-S-dendron type of
ꢀ
structure may undergo cleavage at the C S bond under cer-
tain conditions. For example, Fox and co-workers have re-
ported that Pd–NCDs are unstable under hydrogenation re-
action conditions, most probably owing to hydrogenolysis of
Synthesis of AuGn
Gold NCDs with carbon–metal bonds AuG1–AuG4 were
prepared by reducing HAuCl4 phase-transferred into tolu-
ene in the presence of the diazodendrons. The diazonium
salt (0.25 mmol) solution (in 20.0 mL toluene/CH2Cl2, 9:1)
was stirred for 1.5 h with aqueous HAuCl4 (0.125 mmol in
ꢀ
the C S bond, which leads to precipitation of the Pd
metal.[1c] It was felt that these problems could be eliminated
if we could develop methodologies for SAM, MPC, or NCD
synthesis that avoid chemisorption of thiolate-capped li-
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Gold-Nanoparticle-Cored Dendrimers
Scheme 2. Synthesis of NCDs (the wedge represents Frꢀchet-type den-
drons of different generations).
The G1 and G2 diazodendrons were dark red powders,
freely soluble in toluene and THF. The G3 and G4 salts were
dark red glassy solids, slightly soluble in toluene but freely
soluble in CH2Cl2. The diazodendrons were stable for about
a week at room temperature. These were characterized by
Scheme 1. Idealized structures of AuG3 and AuG4.
1
FTIR and H and 13C NMR spectroscopy. For example, IR
spectra of the diazodendrons exhibited the characteristic di-
azonium peak at 2245 cmꢀ1.[16,18,19] 1H NMR spectra of these
salts exhibited proton signals at d=8.2 and 7.0 ppm (2H
each), which is attributable to protons of the phenyl group
attached to the diazo moiety. The 13C NMR spectra exhibit-
ed a peak at d=168 ppm for the phenyl carbon to which the
diazo group is attached. The contribution owing to the diaz-
ophenyl group to the total mass of the diazodendron de-
creases with dendron generation and as a result the intensi-
5 mL water). Tetraoctylammonium bromide (TOAB;
0.18 mmol) was added and stirring continued for 1 h. For G1
and G2 diazonium salts, the reduction was carried out using
aqueous NaBH4 (1.9 mmol in 15 mL water) under ice-cold
conditions, whereas for G3 and G4 diazonium salts, the re-
duction was carried out by methanolic NaBH4 (1.9 mmol in
15 mL methanol) at ꢀ308C. For G1 and G2, the reaction
mixture was stirred for 24 h, and for G3 and G4, stirring was
carried out for 48 h. After the reaction time, the mixture
was extracted twice with 0.5m H2SO4, 0.5m Na2CO3, and fi-
nally with water. The organic layer was dried and the solid
was suspended in ethanol (50 mL) and sonicated for 30 min.
It was allowed to settle and the ethanol was decanted. Soni-
cation with ethanol and decantation were continued until
the ethanol layer was colorless. The ethanol was decanted
and the product dried in air and then under vacuum for two
days.
1
ties of the FTIR and H and 13C NMR spectroscopic signals
arising from the diazophenyl group decrease with an in-
crease in dendron generation.
AuIII is reduced to give Au atoms, which undergo nuclea-
tion and growth to form small clusters. Upon receiving an
electron from the reducing agent, the dendritic diazonium
salt decomposes into nitrogen and dendron with a phenyl
radical[17] at the focal point. At some stage of the Au cluster
growth, the dendron radicals get attached to the gold sur-
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K. R. Gopidas et al.
FULL PAPERS
face, thereby arresting their growth and leading to the for-
mation of the Au–NCDs. AuGn NCDs thus prepared were
black powders, stable in the solid form or in solution, freely
soluble in CH2Cl2, chloroform, THF, and toluene and insolu-
ble in methanol, ethanol, and ether. For example, a solution
of AuG2 in toluene was observed for nearly one year and no
decomposition or precipitation was observed. Dilute solu-
tions of all the NCDs exhibited a deep wine-red color. We
observed that the NCDs incorporate a small percentage of
TOAB (used as the phase-transfer agent). NCDs can be
subjected to chromatography over silica gel without under-
going decomposition, and this procedure also could not
remove the TOAB. Soxhlet extraction using ethanol also
could not remove the residual TOAB.
1H and 13C NMR Spectra
1
In Figure 2, we compare the H NMR spectra of G1 and G3
diazodendrons and the corresponding NCDs. Signals at d=
8.2 and 7.0 ppm (shown by arrows) characterize the protons
The NCDs synthesized were analyzed by IR, UV, and
NMR spectroscopy. Information about the size of the Au
core was obtained using transmission electron microscopy
(TEM). Information about the organic contents in the NCD
samples is obtained by thermogravimetric analysis (TGA).
Figure 2. 1H NMR spectra of a) G1 and b) G3 diazodendrons and corre-
sponding NCDs in CDCl3.
FTIR Spectra
In the case of NCDs reported previously, FTIR spectra of
the NCDs and precursor disulfide dendrons were nearly
identical.[1a,b] In the present case, IR spectra of the precursor
diazodendrons were different from those of NCDs, and
hence the IR spectra could be used to check the completion
of the reaction. In Figure 1, we compare the FTIR spectra
of two diazodendrons and the corresponding NCDs. FTIR
spectra of the diazodendrons clearly show the vibrational
stretching at 2245 cmꢀ1 (indicated by arrows).[16,18,19]
of the diazophenyl group (in G3 diazodendron the peak at
d=7.0 ppm is very small and merged with the bigger peak
1
at d=7.3 ppm). In the H NMR spectra of AuGn, the diazo-
phenyl proton signals have vanished. In AuG1 and AuG2,
the 1H NMR spectroscopic signals exhibited considerable
broadening. For higher generation NCDs, line broadening
was found to be negligible. Broadening of the NMR spectro-
scopic peaks arising from fast spin–spin relaxation of atoms
close to a metal core or size-dependent spin–spin relaxation
clearly supports the metal core–organic shell-type structure
of AuGn.[16,18,19,21] The signals seen at d<3 ppm in the
1H NMR spectra of AuGn are assigned to protons of TOAB,
which is present as an impurity (vide supra).
Figure 3 is a comparison of the 13C NMR spectra of two
diazodendrons and corresponding NCDs. In the case of the
diazodendrons, signals at d=168, 135, and 118 ppm are
Figure 1. FTIR spectra of a) G1 and b) G3 diazodendrons and correspond-
ing NCDs.
The diazo group decomposes during the reaction, and
hence this peak is absent in the IR spectra of NCDs. This
also shows the absence of any diazo precursor in the NCDs.
IR spectra of the diazodendrons and NCDs were featured
ꢀ1
ꢀ
by methylene C H stretching modes at n˜s =2870 cm , n˜as =
ꢀ1
ꢀ
2927 cm , and aromatic C H vibration modes at n˜s =
3030 cmꢀ1 and n˜as =3061 cmꢀ1. Both systems exhibited bend-
ꢀ1
ꢀ
ing modes of aromatic C H at 698, 746 cm and C=C
breathing modes at 830, 910, 1155, and 1496 cmꢀ1
.
Figure 3. 13C NMR spectra of a) G1 and b) G3 diazodendrons and corre-
sponding NCDs in CDCl3.
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Gold-Nanoparticle-Cored Dendrimers
identified as belonging to the diazo-bearing phenyl ring
(shown by arrows in Figure 3).
For the higher-generation dendrons, the diazophenyl
group constitutes only a small fraction of the total mass, and
hence the signals that correspond to this group are very
small. In the 13C NMR spectra of AuGn the above peaks are
not seen. Similar observations were made previously for
MPCs, and it was suggested that the signals arising from the
carbon atoms on the phenyl ring linked directly to the metal
have broadened so much that they are indistinguishable
from the background.[16,18,19] Absence of these peaks also
suggests complete removal of the diazo group during the re-
duction reaction. It can be concluded that the 1H and
13C NMR spectra provide strong evidence that the dendron
is directly linked to the metal core through the phenyl ring
at its focal point.
Figure 5. TEM images for AuGn.
Absorption Spectroscopy
UV/Vis spectroscopy is very useful in characterizing metal
nanoparticles. The absorption spectra of AuG2 and AuG3 in
CH2Cl2 are shown in Figure 4. Absorption spectra of all
Figure 4. Absorption spectra of a) AuG2 and b) AuG3 in CH2Cl2.
these materials are characterized by a broad band around
520 nm, which is termed the surface plasmon resonance
(SPR) band. According to Mie theory, the plasmon band of
spherical particles can be attributed to the dipole oscilla-
tions of free electrons of the conduction band.[24] The SPR
band maximum and width are dependent on the metal and
also on the particle size, shape, medium dielectric constant,
and temperature. In the present case, the presence of an
SPR band centered around 520 nm indicates the formation
of nanoparticles that have diameters in the 3–6 nm range.[24c]
The intensity of the SPR band did not show any regular de-
pendence on generation. Also, the band maxima and inten-
sity did not exhibit any solvent dependence.
Figure 6. Core-size histograms for AuGn (F=frequency).
MPCs, the core–core separation corresponds to the length
of the alkyl chain.[21c] In the present case, occurrences of rel-
atively identical core–core separations were rare in the
TEM images, and hence thickness of the dendritic shell
could not be estimated.
We have stated previously that the AuGn contained small
amounts of TOAB as an impurity. There are reports that
suggested that phase-transfer catalysts such as TOAB are
capable of stabilizing metal nanoparticles.[25] Hence, it may
be argued that the nanoparticles are stabilized by TOAB
and not by the metal–carbon bonds in the present case. This
argument can be rejected on two counts. The amount of
TEM Studies
1
TOAB present is very small, as is evident from the H NMR
TEM images of different generation NCDs are shown in
Figure 5. The images were taken by drop-casting the solu-
tion of NCD in CH2Cl2. Corresponding core-size histograms
are given in Figure 6. The TEM images show dispersed
metal particles with nonuniform size of about 4–6 nm. There
is no regular change in the size of the particle with an in-
crease in the dendron generation. TEM shows only the
image of the gold core. In the case of several alkanethiolate
spectra of AuGn (Figures 2 and 3), and at these low concen-
trations TOAB will not be able to protect the metal nano-
clusters from aggregation. We also obtained the energy-dis-
persive X-ray (EDX) spectrum of AuGn, which allows us to
determine the elemental composition of the nanoclusters. If
TOAB were present in considerable quantity close to the
metal surface, the EDX spectrum would show peaks that
correspond to N and Br. Figure 7 shows the EDX spectrum
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Figure 8. Thermograms of a) AuG2 and b) AuG3.
Figure 7. EDX spectrum of AuG3 (inset shows the expanded low-energy
region)
moiety; these aspects are projected as major evidence for
the formation of metal–carbon bonds in AuGn. It may be
argued that the dendron radical produced can undergo radi-
cal–radical coupling, thereby leading to dimeric products, or
addition to other dendrons, thereby leading to the formation
of polymeric products, and the Au nanoparticles formed are
stabilized by encapsulation within these structures. We have
carried out control experiments, as shown in Scheme 3, to
rule out this possibility. The diazodendrons were first re-
duced using NaBH4 in the absence of HAuCl4. After stirring
the reaction mixture overnight, the products were isolated
(see the Supporting Information for synthesis and character-
ization details). In the case of G1 and G2 diazodendrons
dimers (Gn)2 that arise out of radical–radical (Scheme 3)
coupling were identified as major products using 1H and
13C NMR spectroscopy and FAB mass spectra. In the case
of higher-generation dendrons, products could not be identi-
fied clearly.
of AuG3, which did not show any peaks that correspond to
these atoms. The N peak at 0.392 keV may not be observa-
ble in the present case because of the very strong carbon
signal, which could obscure any small peaks in this region.[25]
The signals owing to Br, if present, should be observed at
1.48, 11.36, and 11.90 keV.[25a] Since no peaks are observed
in these regions, we believe that TOAB is not present on
the surface of the Au cluster. We propose that the small
amount of TOAB present in the NCDs is not present on the
metal surface and is entrapped in cavities in the dendritic
shell.
We have carried out an additional experiment to show
that Au nanoparticles in the present case are not stabilized
by TOAB. Gittins and Caruso have shown that gold nano-
particles, if protected by TOAB in an organic solvent such
as toluene, can be phase-transferred into aqueous solutions
using a 0.1m aqueous solution of 4-dimethylaminopyridine
(DMAP).[26] Solutions of AuGn in toluene were stirred with
aqueous solutions of DMAP for several hours, and we did
not observe partitioning of gold into the aqueous phase,
which confirms that the gold nanoparticles are present as
covalently linked to dendrons and not as TOAB-protected
particles.
Products of the above reaction, along with TOAB, were
then used to phase-transfer HAuCl4 into toluene. The mix-
ture was then reduced by NaBH4 to obtain the gold nano-
particles, which we designate here as Au@(Gn)2. The nano-
particles thus obtained exhibited absorption spectra and
color similar to AuGn, but they exhibited several differen-
ces.
In Figure 9, the 1H and 13C NMR spectra of (G1)2 are
compared with those of Au@(G1)2 (isolated (G1)2 was used
for synthesis of Au@(G1)2). NMR spectra of (G1)2 exhibited
sharp peaks, and the peaks owing to the biphenyl group (1H
and 13C) are identified by arrows. NMR spectra of
Au@(G1)2 also exhibited these sharp peaks in addition to
the peaks owing to TOAB. Since all the NMR spectroscopic
peaks in (G1)2 are also present in Au@(G1)2, we can safely
suggest that the Au nanoparticle in this case is encapsulated
Thermal Studies
Progressive heating of NCDs led to the volatilization of the
organic shell, thereby leaving a residue of gold. TGA is
therefore a very useful technique to estimate the organic
content of organic–inorganic hybrid materials. Figure 8
shows the thermograms of AuG2 and AuG3. For AuGn, the
volatilization started around 2508C and occurred in a single
step. The high decomposition
temperatures suggested that the
thermal stabilities of these ma-
terials are similar to C16-coated
MPCs.[21]
Control Experiments
IR spectra of AuGn did not
show the peak owing to the
diazo group, and NMR spectra
of AuGn did not exhibit peaks
owing to the phenyl group that
Scheme 3. Mechanism of formation of dendritic dimers (Gn)2 and synthesis of Au nanoparticles in the presence
was attached to the diazo of dendritic dimers Au@(Gn)2.
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Gold-Nanoparticle-Cored Dendrimers
Table 1. Optical and structural parameters of AuGn.
AuGn lmax [nm] Particle size [nm] Au [%] No. of dendrons/NCD
AuG1 518
AuG2 520
AuG3 523
AuG4 522
5.5
4.7
5.4
4.9
45
48
54
48
3161
845
475
229
NAu ¼ dðp=6ÞD3
ð1Þ
in which d is the density of gold (59 atomsnmꢀ3). Since
the atomic weight of Au is 197, the weight of the gold core
is given by Equation (2):
Figure 9. 1H and13C NMR spectra of (G1)2 and Au@(G1)2 in CDCl3.
WAu ¼ NAu ꢂ 197
ð2Þ
in the dendrimer and not chemically attached to it. These
NMR spectra are in sharp contrast to the NMR spectra of
AuGn shown in Figures 2 and 3, in which we see considera-
The value thus obtained can be equated to the percentage
weight of Au obtained from TGA, to obtain the total weight
of the NCD. Weight owing to the organic core in the NCD
can be calculated, and since we know the molecular weight
of the dendron, we can calculate the average number of
dendrons per NCD for each AuGn. The values obtained are
shown in Table 1 and also plotted in Figure 10. The number
of dendrons/NCD obtained here is much larger than report-
ed previously for thiolate-based NCDs.[1a,b]
1
ble broadening of the H NMR spectroscopic signals and ab-
sence of 13C signals owing to the phenyl group at the focal
point, which is covalently linked to the gold cluster.
The Au@(Gn)2 thus prepared were not very stable. Solu-
tions of these materials in toluene or CH2Cl2, when kept for
long periods of time, became colorless with precipitation of
the metal. Precipitation occurred within a few minutes in
the case of Au@(G1)2, but other systems were stable for one
to two days. We also observed that the gold nanoparticles in
Au@(Gn)2 could be extracted into the aqueous phase using
DMAP. These control experiments clearly show that the
dendrons are attached covalently to the Au clusters in
AuGn, whereas in Au@(Gn)2, the Au clusters are physically
entrapped in the dendritic structure.
Figure 10. Plot of the average number of dendrons (n) against NCD gen-
eration.
Stability of AuGn
There are several studies dealing with the labile nature of
metal–sulfur bonds in MPCs.[15b–f] Scott et al. have recently
studied the decomposition of dithiolate-protected MPCs by
oxygen using 1H NMR spectroscopy. They observed that
bubbling oxygen for 2 h resulted in the development of
1H NMR spectroscopic signals that corresponded to free li-
gands.[27] This was attributed to oxidation of the dithiolate
ligand and its subsequent removal from the metal surface.
The AuGn we report here are not oxidized under similar
We attribute the large dendron/NCD ratio to multiple
layering of dendrons on the NCD surface. It has been
shown that multiple organic layers can form on metal surfa-
ces during electrografting of aryl diazonium salts.[17m] The
aryl radical formed in the decomposition of the diazoden-
dron is expected to be highly reactive, and these can attack
the first grafted dendron, as shown in Scheme 4. In
Scheme 4, we have shown the addition of two dendrons to a
1
conditions. H NMR spectroscopy of AuGn did not exhibit
any change, even after bubbling oxygen for 4 h.
Structure of AuGn
Table 1 summarizes the TEM, TGA, and absorption spectral
data for AuGn. Information from the TGA and TEM can be
combined to obtain useful information about the actual
structure of AuGn. From the TEM size distributions, one
can obtain the average core diameter (D) of the nanoparti-
cles. The average number of gold atoms in a cluster can
then be calculated using Equation (1):[1a,28]
Scheme 4. Schematic of possible dendron layering in AuGn.
Chem. Asian J. 2010, 5, 887 – 896
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
893

K. R. Gopidas et al.
FULL PAPERS
dendron in the first layer, but the actual number depends on
the availability of dendron radicals and steric factors.
While the self-assembly of thiols on Au surfaces can yield
only a monolayer of ligands, grafting of aryl groups on surfa-
ces through the diazonium route generally results in multi-
ple layering owing to secondary reactions of the radicals
with terminal aryl groups in the first layer. The mechanism
responsible for the multiple layering has been established by
the groups of Podvorica and McDermott.[22] The reaction in-
volves the addition of the aryl radical onto a phenyl group
of the first layer to get a cyclohexadienyl-type adduct, which
is rearomatized by electron exchange with another diazoni-
um salt. We expect the same mechanism to operate here as
well.
As we proceed from one generation to the next higher
generation, the molecular mass and the dendron size nearly
doubles. Because of this size increase, the number of den-
drons that can be accommodated on the metal core decrease
drastically as we go from AuG1 to AuG4. Thus the higher-
generation AuGn are forced to assume sterically induced
stoichiometries (SIS).[23] In the case of NCDs in general, SIS
would lead to a reduction in the number of dendrons at-
tached onto the gold cluster and a corresponding increase in
the volume of the void space available on the metal surface.
For the AuGn reported here, all the dendrons in the NCD
are not connected to the metal core. As a result, the void
space available on the metal surface cannot be correlated di-
rectly to the dendron number. A quantitative treatment of
this aspect requires a combined morphological study (TEM
and AFM) along with particle-size analysis, which will be
considered in the future.
Experimental Section
Materials
3,5-Dihydroxybenzoic acid, benzyl bromide, carbon tetrabromide, triphe-
nylphosphine, [18]crown-6, tetraoctylammonium bromide (TOAB),
K2CO3, HAuCl4, LiAlH4, NaBH4, HBF4, and tert-butyl nitrite were pur-
chased from Aldrich and used as received. Solvents such as acetone, tolu-
ene, and dichloromethane were obtained from Merck and used as re-
ceived. Dry tetrahydrofuran (THF) used for the synthesis was freshly dis-
tilled from sodium benzophenone ketyl. For the UV/Vis spectra, spectro-
scopic solvents from Merck were used.
Methods
Melting points were determined using a Mel-Temp II melting point appa-
ratus and are uncorrected. Proton NMR spectroscopic data were ob-
tained using a 300 MHz Bruker Avance DPX spectrometer. 13C NMR
spectra were recorded using a 500 MHz Bruker Avance DPX spectrome-
ter. FTIR spectra were recorded using a Shimadzu IR Prestige 21 spec-
trometer. High-resolution mass spectra were obtained by using a JOEL
JMS600 mass spectrometer. Absorption spectra were obtained using a
Shimadzu 3101PC UV/Vis/NIR scanning spectrophotometer. TGA ex-
periments were performed using a Perkin–Elmer Pyris Diamond TG/
DTA analyzer. TEM images of the particles were obtained using a
100 kV FEI-Tecnai 30G2S-Twin transmission electron microscope
equipped with an EDAX energy-dispersive X-ray analysis system. Sam-
ples of TEM were prepared by drop-casting one drop of an approximate-
ly 1 mgmLꢀ1 solution of AuGn in CH2Cl2 onto standard carbon-coated
Formvar films on copper grids (300 mesh) and drying in air for 30 min.
Spectroscopic Data
G1 Diazodendron: 1H NMR (300 MHz, CDCl3, 258C, TMS): d=4.92–
5.09 (m, 6H; CH2), 6.51–6.69 (m, 3H; Ar), 7.26–7.4 (m, 12H; Ar),
8.33 ppm (s, 2H; Ar); 13C NMR (125 MHz, CDCl3, 258C, TMS): d=70.0,
101.68, 102.31, 106.62, 118.08, 127.47, 127.96, 128.51, 135.73, 136.36,
136.52, 160.13, 168.49 ppm; IR (KBr): n˜ =3084.18, 3062.21, 3030.87,
2885.23, 2237.43, 1595.13, 1577.77, 1493.26, 1450.47, 1382.26, 1338.60,
1282.66, 1217.08, 1149.57, 1083.99, 1062.78, 1026.13, 910.40, 839.03,
804.32, 742.39, 698.23, 648.08, 607.58, 518.85, 487.99, 460.99, 408.91 cmꢀ1
.
G2 Diazodendron: 1H NMR (300 MHz, CDCl3, 258C, TMS): d=4.76–
4.93 (m, 14H; CH2), 6.23–6.58 (m, 9H; Ar), 7.14–7.3 (m, 22H; Ar),
8.2 ppm (s, 2H; Ar); 13C NMR (75 MHz, CDCl3, 258C, TMS): d=69.94,
70.04, 101.52, 102.42, 106.37, 118.06, 127.45, 127.88, 128.14, 128.46, 135.68,
136.38, 136.69, 139.05, 160.03, 168.48 ppm; IR (KBr): n˜ =3030.15,
2931.80, 2873.94, 2245.14, 1595.13, 1494.83, 1450.47, 1375.25, 1340.53,
1290.28, 1215.15, 1157.29, 1056.29, 910.40, 835.18, 738.74, 698.23, 632.65,
Conclusion
We have shown that simultaneous reduction of diazo-func-
tionalized Frꢀchet-type dendron and HAuCl4, taken in an
organic medium, leads to the formation of Au nanoparticle-
cored dendrimers. Au–NCDs of generations 1–4 were thus
prepared and characterized. Our studies showed that the
Au–NCDs consisted of a gold core of approximately 5 nm in
diameter to which the dendrons of generations 1–4 are con-
nected radially by means of gold–carbon bonds. These mate-
rials were almost indefinitely stable, thus indicating the sta-
bilization of the gold cluster through covalent bonds and not
by encapsulation in dendron cavities. The NCDs exhibited a
wine-red color and were characterized by the plasmon ab-
sorption band of gold clusters around 520 nm. IR and 1H
and 13C NMR spectra suggested that the dendrons are
linked to the Au core through the phenyl ring at the focal
point of the dendron. By using the information from TGA
and TEM studies, we calculated the average number of den-
drons attached to the metal core. The results suggested that
the number of dendrons/NCD decreased with an increase in
the dendron generation. The large dendron/NCD value
could arise from multiple layering of dendrons.
522.71, 459.06 cmꢀ1
.
G3 Diazodendron: 1H NMR (300 MHz, CDCl3, TMS): d=4.84–4.99 (m,
30H; CH2), 6.5–6.66 (m, 21H; Ar), 7.27–7.37 (m, 42H; Ar), 8.21 ppm (s,
2H; Ar); 13C NMR (125 MHz, CDCl3, TMS): d=70.01, 101.53, 105.77,
106.39, 114.87, 118.07, 126.96, 127.50, 127.94, 128.20, 128.52, 129.34,
135.50, 136.73, 139.21, 140.83, 157.92, 160.09, 168.50 ppm; IR (KBr): n˜ =
3062.96, 3030.17, 2995.01, 2910.58, 2872.01, 2243.21, 1593.23, 1496.76,
1444.68, 1371.39, 1342.46, 1321.26, 1294.24, 1215.15, 1157.29, 1055.06,
908.47, 833.25, 736.81, 696.30, 522.71 cmꢀ1
.
G4 Diazodendron: 1H NMR (300 MHz, CDCl3, 258C, TMS): d=4.86–
4.96 (m, 62H; CH2), 6.50–6.61 (m, 45H; Ar), 7.23–7.32 (m, 82H; Ar),
8.08 ppm (br, 2H; Ar); 13C NMR (125 MHz, CDCl3, 258C, TMS): d=
67.31, 69.62, 69.34, 100.55, 107.29, 107.67, 124.60, 126.11, 126.50, 126.93,
127.19, 127.51, 127.96, 128.43, 128.71, 133.44, 135.72, 138.18, 139.93,
159.09 ppm; IR (KBr): n˜ =3086.11, 3030.17, 2872.01, 2235.50, 1662.64,
1595.13, 1537.27, 1496.76, 1450.47, 1373.32, 1321.24, 1296.16, 115.36,
1053.13, 910.40, 835.18, 746.45, 698.23, 632.65, 520.78, 484.13, 460.99 cmꢀ1
.
AuG1: 1H NMR (500 MHz, CDCl3, 258C, TMS): d=4.997 (s, 6H), 6.37–
6.57 (m, 3H), 7.25–7.35 ppm (m, 10H); 13C NMR (125 MHz, CDCl3,
258C, TMS): d=70.09, 101.56, 106.13, 127.53, 127.99, 128.56, 136.71,
160.15 ppm; IR (KBr): n˜ =3084.18, 3061.03, 3030.17, 2872.01, 1597.06,
894
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ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2010, 5, 887 – 896

Gold-Nanoparticle-Cored Dendrimers
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AuG3: 1H NMR (300 MHz, CDCl3, 258C, TMS): d=4.92–4.97 (m, 30H),
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Acknowledgements
The authors thank the Council of Scientific and Industrial Research and
the Department of Science and Technology, Government of India (DST
grant no. SR/S5/OC-15/2003) for financial support. V.K.R.K. thanks the
University Grants Commission (UGC), Government of India, for a re-
search fellowship. This is contribution number NIIST-PPG-292.
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Received: August 24, 2009
Revised: October 20, 2009
Published online: February 22, 2010
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