Download Regioregular polythiophene/gold nanoparticle hybrid materials

C O M M U N I C AT I O N
Department of Chemistry, Carnegie Mellon University, 4400 Fifth Ave., Pittsburgh,
PA 15213, USA
Journal of
Lei Zhai and Richard D. McCullough*
Materials
Chemistry
www.rsc.org/materials
Regioregular polythiophene/gold nanoparticle hybrid materials
Received 13th May 2003, Accepted 25th November 2003
First published as an Advance Article on the web 8th December 2003
DOI: 10.1039/b305407a
We report a one-pot synthesis of regioregular polythiophene stabilized gold nanoparticles with narrow size
distributions that can form fibril-like structures due to
the self-assembly of regioregular polythiophene.
Gold nanoparticles have attracted intense interest in materials
and physical chemistry since the development of simple
solution-phase methods for their synthesis and stabilization.1
The unique physical properties of gold nanoparticles have
created a wide variety of potential applications including
biological markers,2 DNA sensors,3 molecular recognition
systems4 and nanoscale electronics.5 Encapsulation of inorganic nano-particles in core-shell structures with conducting
polymers has been attempted with a variety of potentially
conductive organic systems.6 These materials may differ from
the pure polymer in that some of the physical and chemical
properties, such as the conductivity,7 are enhanced. The
stabilization of nanoparticles with polymers has been investigated by a number of groups. Walker et al. have synthesized
gold nanoparticles by using poly(methylphenylphosphazene)
(PMPP) where gold nanoparticles were stabilized by the
nitrogen lone pair electrons on the polymer backbone.8 Zhou
and coworkers have reported the preparation of novel
p-conjugated polydithiafulvene (PDF) protected PbS colloidal
nanoparticles.9 The authors believe that the PbS nanoparticles
are stabilized by weak interactions between the sulfur moieties
of PDF and the surface unsaturatedly coordinated Pb21 ions
present on the surface of the nanoparticles.9 Gold nanoparticles
inside well-defined polystyrene–oligothiophene–polystyrene
triblock copolymer (PS–OT–PS) micelles10 were fabricated
through the reduction of gold salt in an oligothiophene
core by the electrons transferred from the oligothiophene.
Advincula et al. have used a polyelectrolyte complex (PEC)
with water-soluble terthiophene derivatives to reduce HAuCl4
to gold nanoparticles.11 Since regioregular polythiophene are
amongst the most widely studied conjugated polymers, we
sought to prepare and characterize polythiophene/gold nanoparticle hybrid materials.
The synthesis of regioregular poly(3-hexylthiophene) stabilized gold nanoparticles was accomplished using a room temperature, two-phase, one-pot reaction involving the reduction
of tetrachloroauric acid by sodium borohydride in the presence
of regioregular poly(3-hexylthiophene)s (Scheme 1).{ Although
Advincula and coworkers reported that tetrachloroauric acid
oxidized the water-soluble terthiophene derivatives,11 we found
that using a large amount of tetraoctylammonium bromide
prevented the oxidation of poly(3-hexylthiophene)s. Work by
Torsi et al. has shown that conjugated polymers can stabilize
inorganic nanoparticles by coordinating or forming complexes
with the inorganic nanoparticles.12 Furthermore, work by Ng
et al. has demonstrated that sulfur atoms of polythiophenes can
interact with metal ions.13 Hence, we surmise that the gold
nanoparticles are stabilized by the interaction of the sulfur
atoms of poly(3-hexylthiophene)s with the gold surface.
We have used various amounts of polythiophenes (25, 20, 15
and 10 mg) in preparing the gold nanoparticles. We have found
that nanoparticle size distribution was large when 25, 20 and
10 mg of the polythiophene was used. However, a narrow size
distribution of nanoparticles was obtained by using 15 mg of
regioregular poly(3-hexylthiophene). This surprising result is
reproducible. While it has been found that as the concentration
of oligothiophenes increases, nanoparticle size also increases,11
we have found no study that shows that the size distribution of
gold nanoparticles can be controlled by tuning the concentration of polythiophenes. At high concentration, e.g. 25 mg, we
believe that the aggregation of polythiophenes results in simple
nanoparticle fusion or growth, leading to larger gold nanoparticles; while at low concentration, there may not be sufficient polymer to stabilize the nanoparticles.
UV-Vis behavior of polythiophene-stabilized nanoparticles
has been investigated by comparing the UV-Vis absorptions
of regioregular polythiophene, polythiophene-gold stabilized
nanoparticles, and dodecanethiol-stabilized nanoparticles
(synthesized according to the simple and efficient procedure
reported by Brust et al.)1a in toluene at room temperature. The
main absorption band observed at 450 nm in the UV-Vis
spectrum of polythiophene-stabilized nanoparticles (Fig. 1b) is
attributed to the absorption of free polythiophenes in the
solution. This is consistent with the UV-Vis spectrum of
pure polythiophene solution which shows a strong absorbance
around 450 nm due to the p–p* transition of the conjugated
polymer chain (Fig. 1a).14 In their studies of preparing gold
nanoparticles from terthiophene, Youk et al. have reported
gold surface plasmon peaks at around 530 nm.11 Our spectra
are quite similar to the oligothiophene gold nanoparticle
spectra.11 Two factors contribute to the broad absorption from
500 to 600 nm in the UV-Vis spectrum of polythiophenestabilized nanoparticles: one is the long-range order of the
polythiophene caused by self-assembly of polymers on nanoparticles,15 and the other is gold surface plasmon peak of gold
nanoparticles which can also be observed around 530 nm in the
spectrum of dodecanethiol-stabilized nanoparticles (Fig. 1c).
Scheme 1 Synthesis of
polythiophene.
This journal is ß The Royal Society of Chemistry 2004
gold
nanoparticles
with
regioregular
J. Mater. Chem., 2004, 14, 141–143
141
Fig. 3 Tapping mode AFM images of nanoparticles stabilized by
polythiophene film solution casting on silicon wafer. Left and right are
sample height and phase images, respectively. The size of images is 3 6
3 mm2. The z-range of height and phase is 50 nm and 50u, respectively.
Fig. 1 UV-Vis absorption of (a) pure poly(3-hexylthiophene), (b) poly(3-hexylthiophene)-stabilized gold nanoparticles, and (c) dodecanethiolstabilized gold nanoparticles.
The formation of gold nanoparticles was also confirmed by
transmission electron microscopy (TEM). The TEM images of
typical poly(3-hexylthiophene)-Au nanoparticles produced
using 15 mg of polythiophenes show a narrow size distribution.
The gold nanoparticles are uniformly distributed at low
concentration of nanoparticles (Fig. 2). On the other hand,
nanoparticles formed a hexagonal packed structure at high
concentration (Fig. 2, inset). The particle size for this sample
was found to be around 10 nm according to TEM.
We have found that ultrathin films of polythiophenestabilized gold nanoparticles, prepared by casting from toluene
followed by free evaporation of the solvent, form fibril-like
structures (Fig. 3). The film was cast from a 0.03 mg mL21
solution in toluene and then was imaged using the light-tapping
force technique in order to study the topology of the film.16
In contrast, there was no indication of any order in the films
prepared by spin-coating from toluene. The high rate of
solidification associated with spin-coating usually suppresses
the aggregation of polythiophenes. The conductivity of the
polythiophene-stabilized nanoparticle film was measured by
four-point probe. The conductivity of neutral film was found to
be 6.4 6 1024 S cm21 while the conductivity of iodine doped
film was found to be 0.2 S cm21.
In conclusion, we present the preparation of regioregular
polythiophene stabilized gold nanoparticles with a narrow
size distribution using a one-pot reduction reaction at room
temperature. The gold nanoparticles can form a nanowire
structure due to the self-assembly of regioregular polythiophene. These gold nanoparticles may find applications
in single-electron tunneling studies as well as field effect
transistors.
Acknowledgements
We are very grateful to the NSF (CHE0107178) for support of
our work. We also acknowledge Professor Tomasz Kowalewski
and Dr Shijun Jia for help with AFM.
Notes and references
{ All the chemicals were purchased from Aldrich and used as received.
Tetraoctylammonium bromide (0.27 g, 0.5 mmol) was added to 8 mL of
toluene in a 25 mL round bottom flask and stirred until dissolved.
Then, an aqueous solution of HAuCl4 (3 mL, 0.1 mmol) was added and
the two phase mixture was stirred vigorously until all the tetrachloroaurate was transferred into the organic layer. This was determined by
observing a color change from yellow to colorless. Regioregular poly(3-hexylthiophene)14 (15 mg, 0.09 mmol) (M n ~ 16350 by GPC) was
dissolved in 3 mL of toluene and added to the mixture. The mixture was
stirred for 3 h. A freshly prepared aqueous solution of sodium
borohydride (2.5 mL, 10 mmol) was added slowly to the mixture with
vigorous stirring. The dark purple polymer gold nanoparticles were
formed and the mixture was stirred overnight before quenching in
methanol.
1
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Fig. 2 TEM image of poly(3-hexylthiophene)-stabilized gold nanoparticles. (Inset) a hexagonal packing from concentrated poly(3-hexylthiophene)-stabilized gold nanoparticles.
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J. Mater. Chem., 2004, 14, 141–143
8
(a) M. Brust, M. Walker, D. Bethell, D. J. Schiffrin and
R Whyman, J. Chem. Soc., Chem. Commun., 1994, 801;
(b) M. Brust, J. Fink, D. Bethell, D. J. Schiffrin and C. Kiely,
J. Chem. Soc., Chem. Commun., 1995, 1655; (c) M. Green and
P O’Brien, Chem. Commun., 2000, 183.
J. W. Slot and H. J. Geuze, J. Cell. Biol., 1981, 90, 533.
(a) R. Elghanian, J. J. Storhoff, R. C. Mucic, R. L. Letsinger and
C. A. Mirkin, Science, 1997, 277, 1078; (b) J. J. Storhoff,
R. Elghanian, R. C. Mucic, C. A. Mirkin and R. L. Letsinger,
J. Am. Chem. Soc., 1998, 120, 1.
(a) J. Liu, R. Xu and A. E. Kaifer, Langmuir, 1998, 14, 7337;
(b) A. Labande and D. Astruc, Chem. Commun., 2000, 1007.
(a) R. P. Andres, J. D. Bielefeld, J. I. Henderson, D. B. Janes,
V. R. Kolagunta, C. P. Kubiak, W. J. Mahoney and R. G. Osifchin,
Science, 1996, 273, 1690; (b) T. Sato, H. Ahmed, D. Brown and
B. F. G. Johnson, J. Appl. Phys., 1997, 82, 696.
(a) S. Maeda and S. P. Armes, Chem. Mater., 1995, 7, 171;
(b) T. Hori, N. Kuramoto, H. Tagaya, M. Karasu, J. I. Kadokawa
and K. Chiba, J. Mater. Res., 1999, 14, 5; (c) Z. Qi and
P. G. Pickup, Chem. Commun., 1998, 2299; (d) C. T. Hable and
M. S. Wrighton, Langmuir., 1991, 7, 1305; (e) G. Wang, H. Chen,
H. Zhang, Y. Shen, C. Yuan, Z. C. Lu, G. Wang and W. Yang,
Phys. Lett. A, 1998, 237, 165.
(a) A. Bhattacharya, K. M. Ganguly, A. De and S. Sarkar, Mater.
Res. Bull., 1996, 31, 527; (b) R. Gangopadhyay and A. De, Eur.
Polym. J., 1999, 35, 1985.
C. H. Walker, J. V. St. John and P. Wisian-Neilson, J. Am. Chem.
Soc., 2001, 123, 3846.
9 Y. Zhou, H. Itoh, T. Uemura, K. Naka and Y. Chujo, Langmuir,
2002, 18, 5287.
10 M. A. Hempenius, B. M. W. Langeveld-Voss, J. A. E. H. van Haare,
R. A. J. Janssen, S. S. Sheiko, J. P. Spatz, M. Möller and
E. W. Meijer, J. Am. Chem. Soc., 1998, 120, 2798.
11 J. H. Youk, J. Locklin, C. Xia, M. Park and R. Advincula,
Langmuir, 2001, 17(15), 4681.
12 N. Cioffi, L. Torsi, i. Losito, C. D. Franco, I. D. Bari,
L. Chiavarone, G. Scamaracio, V. Tsakova, L. Sabbatini and
P. G. Zammbonin, J. Mater. Chem., 2001, 11, 1434.
13 S. C. Ng, X. C. Zhou, Z. K. Chen, P. Miao, H. S. O. Chan,
S. F. Y. Li and P. Fu, Langmuir, 1998, 14, 1748.
14 R. D. McCullough, R. D. Lowe, M. Jayaraman and
D. L. Anderson, J. Org. Chem., 1993, 58, 904.
15 R. D. McCullough, S. P. Tristram-Nagle, R. D. Lowe and
M. Jayaraman, J. Am. Chem. Soc., 1993, 115, 4910.
16 J. Liu, E. Sheina, T. Kowalewski and R. D. McCullough,
Angew. Chem., Int. Ed., 2002, 41, 329.
J. Mater. Chem., 2004, 14, 141–143
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