Redox nanoreactor dendrimer boxes: In situ hybrid gold nanoparticles via terthiophene and carbazole peripheral dendrimer oxidation

Article abstract of DOI:10.1039/c0sm00796j

π-Conjugated dendrimer-protected gold nanoparticles in stable colloidal form have been successfully prepared via simultaneous reduction of AuCl ₃ with oxidative polymerization of terthiophene (PT) and carbazole (PC) peripheral functionalized polyamidoamine (PAMAM) dendrimers. The hybrid dendrimer-metal materials were characterized by UV-vis, fluorescence, and FT-IR spectroscopy. XPS and AFM analyses were also employed. The differences in gold nanoparticle (AuNP) formation between the two PAMAM derivatives were discussed. In case of PT dendrimer, AuNPs can be stabilized in situ to form PAMAM-PPT-AuNP hybrid materials. For the PC dendrimer, the redox reaction afforded carbazole polymerization without necessarily forming AuNPs.

Full text of DOI:10.1039/c0sm00796j

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Redox nanoreactor dendrimer boxes: in situ hybrid gold nanoparticles via
terthiophene and carbazole peripheral dendrimer oxidation†
a
a
b
a
Chatthai Kaewtong,ab Guoqian Jiang, Ramakrishna Ponnapati, Buncha Pulpoka and Rigoberto Advincula
*
Received 10th August 2010, Accepted 9th September 2010
DOI: 10.1039/c0sm00796j
material.12 Electrochemically polymerized carbazoles show inter-
esting properties via formation of p-conjugated species at the 3,6
linkages. The radical cations generated during polymerization are
responsible for coupling-propagation, whereas distinct redox states
lead to electrochromic effects.13
p-Conjugated dendrimer-protected gold nanoparticles in stable
colloidal form have been successfully prepared via simultaneous
reduction of AuCl3 with oxidative polymerization of terthiophene
(PT) and carbazole (PC) peripheral functionalized polyamidoamine
(PAMAM) dendrimers. The hybrid dendrimer–metal materials
were characterized by UV-vis, fluorescence, and FT-IR spectros-
copy. XPS and AFM analyses were also employed. The differences
in gold nanoparticle (AuNP) formation between the two PAMAM
derivatives were discussed. In case of PT dendrimer, AuNPs can be
stabilized in situ to form PAMAM–PPT–AuNP hybrid materials.
For the PC dendrimer, the redox reaction afforded carbazole
polymerization without necessarily forming AuNPs.
Herein we report the in situ synthesis of hybrid AuNPs inside the
redox dendrimer nanoreactor boxes based on PAMAM dendrimers
with terthiophene (PT) and carbazole (PC) groups on the periphery.
The formation of these hybrid DEMNs consisting of AuNPs pro-
tected with PAMAM (inner shelf) and p-conjugated species (outer
shelf) is described. Energy transfer properties between the nano-
particle-thiophene and carbazole peripheral groups are also reported.
Atomic Force Microscopy (AFM), FT-IR, and X-ray photoelectron
spectroscopy (XPS) were utilized to further differentiate AuNP
formation in PT and PC dendrimer derivatives. Unlike previously
reported DEMNs, no external reducing agent was required for
AuNP formation.
Dendrimer-encapsulated metal nanoparticles (DEMNs) are hybrid
nanomaterials synthesized by complexing metal ions within den-
drimers followed by reduction to yield zero valent metal nano-
particles. As widely reported by Crooks et al., dendrimers are
well-suited for hosting metal nanoparticles for a number of reasons
including monodispersity, stability, catalytic reactivity, and solu-
bility.1 Of special interest are polyamidoamine (PAMAM) den-
drimers which can bind a defined number of transition metal cations,
while creating an ideal environment for trapping guest species.2,3 They
are ideal templates for stabilizing metal oxide or metal nanoparticles.4
Furthermore, the nanoparticle size, composition, and structure can
be controlled by taking advantage of the dendrimer structure and
generation. In the past few decades, several methods for making gold
nanoparticles (AuNPs) by chemical reduction of precursors have
been developed. The most popular method used for a long time was
the citrate reduction of HAuCl4 in water.5 Recently, electroactive
groups such as pyrrole,6 aniline,7 thiophene,8a EDOT,8b and others
have been utilized together with the HAuCl4 precursor. In principle,
they can be oxidatively polymerized to form p-conjugated polymers
while simultaneously forming the reduced AuNPs.
The starting materials, ethyl 2-(2,5-di(thiophen-2-yl)thiophen-3-yl)
acetate14 and methyl-3,5-bis[4-(9H-carbazol-9-yl)butoxy]benzoate,15
were prepared according to literature. Hydrolyzation of both
compounds was performed using KOH in THF/MeOH, followed by
reaction with excess pentafluorophenol in the presence of dicyclo-
hexylcarbodiimide/dimethyl aminopyridine (DCC/DMAP) in THF.
The active ester perfluorophenyl-3,5-bis(4-(9H-carbazol-9-yl)
butoxy) benzoate (3) and perfluorophenyl-2-(2,5-di(thiophen-2-yl)
thiophen-3-yl)acetate (4) were obtained in 68% and 10% yield
Polymers of thiophene and carbazole have been of high interest
due to their electrochemical9 and electrochromic properties.10 For
example, electropolymerization of terthiophene leads to the forma-
tion of highly electrochromic and conducting polythiophenes.11 On
the other hand, polycarbazole is well-known as a hole transport
aDepartment of Chemistry and Department of Chemical Engineering,
University of Houston, Houston, Texas, USA. E-mail: radvincula@uh.
edu; Fax: +1 713 743 1755; Tel: +1 713 743 1755
bSupramolecular Chemistry Research Unit and Organic Synthesis Research
Unit, Department of Chemistry, Faculty of Science, Chulalongkorn
University, Bangkok, Thailand. E-mail: buncha.p@chula.ac.th; Fax: +66
2218 7598; Tel: +66 2218 7596
† Electronic supplementary information (ESI) available: Experimental
procedures, characterization, AFM images and spectroscopic data of
PC and PT before and after adding AuCl3. See DOI: 10.1039/c0sm00796j
Fig. 1 Electroactive groups (terthiophene, T and carbazole, C) of
peripheral modified PAMAM dendrimers used in this study.
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respectively. Amidation reactions of 3 and 4 with PAMAM were
accomplished in THF/DMF to obtain PC (85%) and PT (83%)
(Fig. 1). NMR and IR were used to confirm the structures as
described in the ESI†.
The attempt to prepare AuNPs encapsulated in PT and PC
dendrimers was initiated by adding aliquots of AuCl3 (1 ꢀ
10ꢁ3 M) in MeOH into a solution containing PT or PC (1 ꢀ
10ꢁ6 M) in toluene. These solutions were then stirred to incor-
porate AuCl3 into the dendrimer templates. The reduction to
zero valent AuNPs simultaneous with the oxidative chem-
ical polymerization to form polycarbazole (PPC) or poly-
terthiophene (PPT) in the periphery of the dendrimers is then
expected, i.e. PT / PPT and PC / PPC.
In a typical procedure, PT was reacted with 55 equiv. of the Au
precursor. Upon adding the AuCl3 solution into the PT solution, the
color of the solution changed immediately from pale yellow to blue.
After 30 min, a dark blue solution was observed. The blue color is
probably due to the initial electron transfer from the terthiophene to
Au3+ (metal to ligand charge transfer or MLCT) or blue color
generated by the plasmon band (440–490 nm). It has been reported
that polyelectrolyte complex (PEC) with water-soluble terthiophene
derivative also changed in color successively after addition of Au3+.8a
A similar procedure was also employed for the PC. From the initial
experiment, using 55 equiv. of gold precursor, no color change was
observed. Therefore, a larger ratio of AuCl3 to PC was employed.
When 110 equiv. of gold precursor were used, the color changed from
pale yellow to deep yellow and the color was found to be stable in the
media even after several weeks. The fact that a higher ratio of AuCl3
to PC was needed may be due to the higher oxidation potential
(monomer) (0.8–1.0 mV) for the carbazole compared to terthiophene
at about 0.22 V.17a The formation of DEMNs in PT in particular can
be tracked by monitoring the change in UV-visible absorbance at
various time intervals (Fig. 2a). The main absorption band at 345 nm
is attributed to the terthiophene unit. The observed peaks at 320 nm
and at 850 to 1000 nm corresponds to the p–p* and polaron–p, p–p,
and polaron–p* transitions, respectively. The peak shift from 345 nm
to 320 nm is due to presence of MLCT transition which was highest
at the initial mixing of the solutions. This peak gradually decreases
with time. After vigorous stirring for 30 min, the absorption band
of PPT and the surface plasmon band of AuNPs appeared at
410–550 nm. However, it may be difficult to totally distinguish the
AuNP surface plasmon band due to some overlap between the AuNP
and PPT peaks.18 Nevertheless, one interesting evidence is the coin-
cident decrease of the 320 nm region due to Au3+ to thiophene
MLCT, increase at the 500 nm for AuNP and PPT, and decrease in
polaron band with time (Fig. 2a). The latter indicates strong evidence
of Au0 formation since an electron charge transfer mechanism from
the polaron of the p-conjugated PPT species occurs only on the
surface of Au0 nanoparticles when present.16
Fig. 2 UV-vis absorption spectra of (a) PT at different time (0–30 min)
and (b) PC after adding AuCl3 30 min. Dendrimer PT and PC show their
characteristic absorption bands at 345, 333 and 346, respectively.
The steady state fluorescence emission spectra of solutions of PT,
PPT–AuNPs, PC and PPC–AuNPs (or Au3+) are shown in Fig. 3.
Without AuNPs, the terthiophene modified PAMAM dendrimer,
PT, exhibited a strong characteristic emission of terthiophene with
the emission maximum at 433 nm and one shoulder around 415 nm
upon excitation at 345 nm (in Fig. 3a) while the carbazole modified
PAMAM dendrimer, PC, showed a characteristic emission at 356
and 365 nm upon excitation at 333 nm (in Fig. 3b).17b The emission of
PPT was quenched dramatically in the PPT–AuNPs hybrid due to
a highly efficient energy transfer between PPT and AuNPs20 and
perhaps some self-quenching with formation of conjugated polymer
species.17b These were confirmed by the observation of a new peak at
545 nm with PPT. This is due to the AuNPs quenching for the PT.
For the PC, a decrease in emission was also observed and a new peak
at 510 nm was observed. However, this most likely represented an
Au3+-MLCT quenching effect for the PC dendrimer because of the
absence of the plasmon band from the UV-vis absorbance spectrum.
The preparation of the hybrid nanoparticles was also monitored by
FT-IR spectra (see ESI†). The characteristic peaks were observed at
1636 and 1690, corresponding to the two amide bands of PAMAM.
The ]C–H in-plane and out-of-plane vibrations of the substituted
terthiophene ring are assigned to 1240 and 825 cmꢁ1, respectively.
These two bands disappeared for the DEMNs which formed upon
the oxidation of terthiophene, PT by a–a0 coupling to form PPT with
Au3+ to Au0 conversion. The PAMAM characteristic peaks were also
observed for the PC and in addition, 1230, 1325, 1450, and 1533 cmꢁ1
The UV-vis spectrum for PC (1 ꢀ 10ꢁ6 M) to PPC conversion is
shown in Fig. 2b. The absorbances at 333 and 346 nm which are
characteristic of the carbazole group were observed.17 After mixing
the PC solution with 110 equiv. of AuCl3 for 30 min, a peak at
315 nm increased and a slight hypsochromic shift is observed.19 On
the other hand, the p–p* transition of carbazole was red shifted and
an absorption tail extending into the visible range centered at 385 nm
on the PPC is observed. This indicated the formation of higher
p-conjugated species. However, this spectrum seemingly lacked the
characteristic surface plasmon band at the 500 nm region.
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Fig. 4 (a) High-resolution XPS of the Au 4f7/2 and Au 4f5/2 doublet at
84.4 and 88.1 eV for the PPT–AuNPs and (b) AFM image (2 ꢀ 2 mM) of
PPT–AuNPs on mica substrate.
The particle morphology and distribution of the dendrimers
before and after treatment with AuCl3 were also studied by AFM
(Fig. 4b and ESI†). A droplet of a 10ꢁ6 M toluene solution was cast
on mica and the solution was allowed to evaporate with spin-coating.
Shown in Fig. 4b is the AFM image of an array of PPT–AuNPs on
a mica substrate. The image demonstrates the near uniform distri-
bution of the particles which tends to form aggregates if higher
concentrations were used. Some aggregation was also observed with
the PT alone (see ESI†). It is believed that the use of toluene as
a solvent is one reason for aggregation between the dendrimers on the
hydrophilic mica substrate.25 Detailed cross-sectional measurements
performed on a large number of isolated features revealed an average
height of 2.4 ꢂ 1.0 nm for the PT alone and after the formation of the
hybrid AuNP–PPT, a more uniform and twice higher average height
ꢃ5.2 ꢂ 0.2 nm of particles was observed (Fig. 4b).
Fig. 3 Steady-state fluorescent spectra of PT and PC 10ꢁ6 M before and
after mixing with AuCl3 in toluene. ET mechanism was proved by
quenching phenomena and new peaks of AuNPs.
peaks representing the carbazole aromatic (C–H), (C–N)ring and
carbazole (C]C)ring vibrations (see ESI†).17a We also observed the
disappearance of the ]C–H in-plane and out-of-plane vibrations of
PC at 1325 and 847 cmꢁ1 or wavenumber, respectively with
complexation. In general, both the loading of metal ions and the
presence of nanoparticles shifted the amide vibration to longer cmꢁ1.
The formation of the hybrid materials was also examined by XPS.
A solution of the AuNP–PPT dendrimer complex was solution cast
on planar Si wafer. The Au0 spectra based on Au 4f7/2 and Au 4f5/2
doublet appeared at 84.4 and 88.1 eV, respectively (Fig. 4a). These
features were in agreement with results reported for self-assembled
alkanedithiol,21 alkanethiols on AuNPs and/or bulk gold substrates22
as well as for bulk gold.23 Interestingly, the higher binding energy for
Au 4f5/2 also provides additional evidence for an Au0 and PPT
bipolaron charge transfer.16 However, it was hard to detect AuNPs
from the AuCl3 and PC procedure in toluene. This implies that the
carbazole-modified dendrimer PC was not able to fully convert
a majority of the Au3+ precursor to AuNPs. Although UV-vis and
fluorescence support the formation of a conjugated PPC, the MLCT
was primarily with Au3+ species. The fact that no distinct plasmon
band was observed with the UV-vis spectra also supports this fact.
However, the formation of Au0 clusters is still possible. Thus
compared to PC, the PT was a more effective oxidant to form AuNP
DEMNs while simultaneously forming the conjugated poly-
thiophene, PPT, in the outer shelf.24
Surprisingly, when we investigated the AFM of PC dendrimers
alone, we observed nano ring structures with different sizes together
with some individual dendrimers on mica substrate (see ESI†). These
structures are comparable to the results obtained by Nolte and co-
workers26 on the formation of ring liked structures of porphyrin
molecules when solutions of porphyrins were allowed to evaporate
fast. After the addition of AuCl3, much larger aggregates of the
dendrimer were observed together with some individual particles. The
ring structures also disappeared. However, the other supporting
results indicate a poor ability to mediate AuNP reduction, although
the PC to PPC oxidation (formation of more conjugated species) was
confirmed by spectroscopy measurements. In addition, the PC to
PPC conversion was also observed electrochemically by cyclic vol-
tammetry and spectroelectrochemical experiments (see ESI†). In the
future, it should be possible to test this concept with different
PAMAM generations.
In conclusion, we have successfully synthesized novel hybrid
materials in situ by using PAMAM dendrimers terthiophene (PT)
precursors as a reductant for simultaneous AuNP formation and
polythiophene (PPT) shell formation. While the carbazole dendrimer
(PC) afforded higher conjugated carbazole species, the interaction
was limited to mostly Au3+ MLCT interaction instead of AuNP
formation. UV-vis and strong fluorescence emission quenching after
treatment with AuCl3 and the appearance of new peaks were
attributed to an electron transfer mechanism from the PPT shell to
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Acknowledgements
The authors acknowledge funding from the NSF ARRA
CBET-0854979, DMR-10-06776, CHE-10-41300 and the
Robert A. Welch Foundation (E-1551). B.P. and C.K.
acknowledge supports from the Thailand Research Fund
(TRF) (RMU4880041) and the Royal Golden Jubilee PhD
Program of TRF (PHD/0137/2546). We also thank Pawilai
Chinwangso and La-ongnuan Srisombat for XPS.
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