Improvement of thermal stability of gold nanoparticles by synergistic interligand interactions

Article abstract of DOI:10.1246/cl.2012.708

Three types of novel gold nanoparticles containing a different condensed ring in their ligands were prepared. Their thermal stability was found to be much higher than that of typical gold nanoparticles surrounded by long-chain alkanethiols because of strong interligand interaction formed by the condensed rings (ππ stacking). A clear correlation was found between the thermal stability and the strength of π π stacking.

Full text of DOI:10.1246/cl.2012.708

doi:10.1246/cl.2012.708
708
Published on the web June 23, 2012
Improvement of Thermal Stability of Gold Nanoparticles
by Synergistic Interligand Interactions
Atsushi Okumura,1 Yoshitaka Tsuchido,1 Kou Yamada,2 Kenji Todori,2 and Shigeru Machida*1
1Tokyo National College of Technology, 1220-2 Kunugida-machi, Hachioji, Tokyo 193-0997
2Corporate Research and Development Center, Toshiba Corporation,
1 Komukai-Toshiba-cho, Saiwai-ku, Kawasaki, Kanagawa 212-8582
(Received March 21, 2012; CL-120242; E-mail: smachida@tokyo-ct.ac.jp)
Three types of novel gold nanoparticles containing a
prepared with these ligands, and their thermal stability was
different condensed ring in their ligands were prepared. Their
thermal stability was found to be much higher than that of
typical gold nanoparticles surrounded by long-chain alkanethiols
because of strong interligand interaction formed by the
condensed rings (³-³ stacking). A clear correlation was found
between the thermal stability and the strength of ³-³ stacking.
evaluated based on TG analysis.
The synthetic routes for ligands 5, 6, and 7 are shown in
Schemes 1, 2, and 3, respectively. 3-(Tritylthio)propanoic acid
was synthesized according to Polidori et al.’s method.7 Tetra-
hydrofuran (THF) was purified by distillation and stocked under
a nitrogen atmosphere. Other chemicals were used without
further purification.
1H NMR spectra were measured using a JEOL AL-400, and
infrared absorption was measured with a JASCO FT/IR-620.8
All of the gold nanoparticles Au-5, Au-6, and Au-7 were
prepared in accordance with the Brust et al. method.5 Perylene-
type (Au-6) and coronene-type (Au-7) gold nanoparticles were
found to disperse in chloroform, while pyrene-type (Au-5)
gold nanoparticles readily dispersed in chloroform as well as
toluene.
Recently, gold nanoparticles have been extensively inves-
tigated owing to their characteristic behavior compared to bulk
gold.1-4 Since gold nanoparticles have a strong affinity, they are
generally surrounded by organic molecules called ligands that
prevent the nanoparticles from aggregating. Brust et al. reported
that gold nanoparticles surrounded by long-chain alkanethiols 1
show high dispersibility in nonpolar organic solvents and do not
aggregate even in the solid condition.5 The thiol group anchors
the ligand molecule to the surface of the gold core, while the
long-chain alkyl groups interact intermolecularly with each other
by van der Waals forces and stabilize the gold core. However,
since the van der Waals interaction is weak, the gold nano-
particles surrounded by the typical long-chain alkanethiols need
to be improved in terms of their thermal resistance.
Therefore, with the aim of developing thermally resistant
gold nanoparticles, we focused on functional groups such as the
amide group and aromatic rings. In our previous study,6 ligands
containing an amide group and/or a tolyl group (Figure 1) were
synthesized, and the thermal stability of the corresponding gold
nanoparticles was evaluated by thermogravimetry (TG). It was
found that the gold nanoparticles with a tolyl group in their
ligands 3 and 4 did not aggregate up to about 300 °C. This result
suggests that introducing aromatic rings into ligands is effective
for enhancing the thermal stability.
O
O
O
O
HS
O
N
H
HS
11 O
N
H
11
Perylene-type 6
Pyrene-type 5
O
O
HS
O
N
H
11
Coronene-type 7
Figure 2. Chemical structures of novel ligands 5, 6, and 7.
(a)
H2N
11OH
Br
11OH
5a
O
(b)
S
N
11OH
H
Therefore, in this study, we synthesized three novel ligands
containing condensed rings (Figure 2), with the aim of further
improving the thermal stability. Gold nanoparticles were then
5b
O
O
(c)
(d)
S
N
H
11O
5c
O
n = 7; 2a
O
O
HS
HS
N
H
n = 11; 2b
n = 15; 2c
n = 17; 2d
n
11
HS
11O
N
H
Amide type 2
Long-chain alkanethiol 1
5
O
O
Scheme 1. Synthetic route for pyrene-type ligand 5: (a) NaN3,
CH3CN, reflux, overnight, then LiAlH4, THF, 0 °C, 1 h; (b) 3-
(tritylthio)propanoic acid, 1-ethyl-3-(3-dimethylaminopropyl)-
carbodiimide hydrochloride (EDC), 4-dimethylaminopyridine
(DMAP), CH2Cl2, rt, overnight; (c) 1-pyrenebutanoic acid,
EDC, DMAP, CH2Cl2, rt, overnight; (d) trifluoroacetic acid,
triethylsilane, rt. Total yield of the four steps: 20%.
HS
N
H
O
HS
O
O
11
11
Tolyl-Amide type 4
Tolyl-ester type 3
Figure 1. Chemical structures of long-chain alkanethiol 1 and
synthesized ligands 2, 3, and 4.
Chem. Lett. 2012, 41, 708-710
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

709
1
0.8
0.6
0.4
0.2
O
(a)
(b)
HO
O
6a
O
HO
6b
O
O
(c)
(d)
S
N
H
11 O
6c
11O
400
500
600
700
O
O
Wavelength / nm
HS
N
H
Figure 3. UV-vis absorption spectrum of gold nanoparticles
Au-5.
6
Scheme 2. Synthetic route for perylene-type ligand 6: (a)
succinic anhydride, AlCl3, CH2Cl2, rt, 13 h; (b) N2H4 (79%
hydrate), NaOH, diethylene glycol, reflux, 4 h; (c) compound 5b,
EDC, DMAP, CH2Cl2, rt, overnight; (d) trifluoroacetic acid,
triethylsilane, rt. Total yield of the four steps: 17%.
O
(a)
HO
O
7a
10 nm
O
(b)
HO
7b
Figure 4. TEM image of gold nanoparticles Au-5.
O
O
(c)
(d)
100
80
S
N
H
11O
7c
O
O
60
HS
11O
N
H
7
40
Onset temperature
of decomposition
Scheme 3. Synthetic route for coronene-type ligand 7: (a)
succinic anhydride, AlCl3, nitrobenzene, rt, 5 h; (b) N2H4 (79%
hydrate), NaOH, diethylene glycol, reflux, 4 h; (c) compound 5b,
EDC, DMAP, CH2Cl2, rt, overnight; (d) trifluoroacetic acid,
triethylsilane, rt. Total yield of the four steps: 15%.
20
0
100
200
Temperature / °C
500
300
400
Gold nanoparticles are known to have a broad absorption
range of 450-600 nm, which is attributed to the surface plasmon
resonance of the Au cores. Therefore, UV-vis absorption spectra
were measured using a SHIMADZU UV-3600 to confirm the
generation of the gold nanoparticles. Figure 3 shows the
absorption spectrum of the pyrene-type gold nanoparticles
Au-5, where the broad absorption attributed to the surface
plasmon resonance was observed in the 450-600 nm range.
Similar absorption was also confirmed in the spectra of the other
gold nanoparticles Au-6 and Au-7.
Particle sizes of the gold nanoparticles were estimated on
the basis of transmission electron microscopy (TEM) images
taken with JEOL JEM-2000EX. Figure 4 shows the TEM image
of Au-5; its average size was visually estimated to be ca. 2 nm.
The other gold nanoparticles were found to be about the same
size as Au-5. The calculated standard deviations of Au-5, Au-6,
and Au-7 were 0.3, 0.4, and 0.7 nm, respectively, and their size
distributions were similar in shape.
Figure 5. TG chart of gold nanoparticles Au-5.
In order to evaluate the thermal stability of the gold
nanoparticles, TG measurements were carried out using SII TG/
¹1
DTA 6300 at a heat rate of 20 °C min under a nitrogen
environment (150 mL min¹1).
Figure 5 shows the TG chart of Au-5, where the weight loss
was observed in one step, owing to desorption of the ligand
molecules from the gold surface. Since similar TG curves were
obtained for the other gold nanoparticles, the onset temperature
of decomposition was defined as shown in Figure 5. This
temperature was then used as an indicator of the thermal stability
of each gold nanoparticle (Figure 6).
The three novel gold nanoparticles Au-5, Au-6, and Au-7
containing condensed rings showed a remarkably high onset
temperature of decomposition in comparison with the others.
Among the three, the pyrene-type gold nanoparticle Au-5 had
Chem. Lett. 2012, 41, 708-710
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

710
O
440
390
340
290
240
O
417°C
397°C
S
O
N
H
390°C
O
O
O
S
O
N
H
Au
O
O
S
N
H
298°C
280°C
Amide bond
(Hydrogen
Bonding)
Alkyl chain
(van der
Aromatic ring
(π−π stacking)
310°C
277°C
Waals force)
249°C
279°C
253°C
Figure 7. Schematic illustration of interligand interactions of
novel gold nanoparticles Au-5, Au-6, and Au-7.
Au-1
Au-2b Au-2cAu-2d Au-3 Au-4 Au-5 Au-6 Au-7
Au-2a
n = 7; 2a
n = 11; 2b
n = 15; 2c
n = 17; 2d
O
bonding is formed near the gold surface and stabilizes the
orientation of the alkyl chain. This enables aromatic rings at the
end of the ligand to readily form ³-³ stacking. This synergistic
effect is considered to boost the contribution of ³-³ stacking to
the enhancement of thermal stability.
Au
S
S
Au
S
N
10
m
n
m
H
Au-1
Au-2
O
O
O
Au
S
O
N
H
11 O
Au
11 O
m
m
m
Au-3
Au-4
O
O
O
This paper belongs to “Innovative Nanophotonics Compo-
nents Development Project” which OITDA contracted with The
New Energy and Industrial Technology Development Organ-
ization (NEDO) (2006-2010).
S
N
H
O
Au
S
N
H
O
11
Au
11
m
Au-5
Au-6
O
O
S
N
O
11
Au
m
H
Au-7
References and Notes
1
2
3
M.-C. Daniel, D. Astruc, Chem. Rev. 2004, 104, 293.
G. Schmid, B. Corain, Eur. J. Inorg. Chem. 2003, 3081.
A. C. Templeton, D. E. Cliffel, R. W. Murray, J. Am. Chem. Soc.
1999, 121, 7081.
J. van Herrikhuyzen, R. A. J. Janssen, E. W. Meijer, S. C. J.
Meskers, A. P. H. J. Schenning, J. Am. Chem. Soc. 2006, 128,
686.
Figure 6. Onset temperature of decomposition of each gold
nanoparticle.
Table 1. Correlation between the number of ³ electrons in
each condensed ring and onset temperature of decomposition
4
Gold
Number of Onset temperature of Weight
5
6
7
8
M. Brust, M. Walker, D. Bethell, D. J. Schiffrin, R. Whyman,
J. Chem. Soc., Chem. Commun. 1994, 801.
Y. Tsuchido, K. Yamada, K. Todori, S. Machida, Trans. Mater.
Res. Soc. Jpn. 2010, 35, 271.
A. Polidori, N. Michel, A. S. Fabiano, B. Pucci, Chem. Phys.
Lipids 2005, 136, 23.
nanoparticle ³ electrons
decomposition/°C
loss/%
Au-5
Au-6
Au-7
16
20
24
390
397
417
59
56
57
Spectroscopic data of 5: 1H NMR (400 MHz, CDCl3): ¤ 8.31-7.84
(m, 9H, pyrene), 5.56 (br, 1H, NH), 4.08 (t, 2H, J = 6.6 Hz,
CH2OCO), 3.37 (t, 2H, J = 7.8 Hz, CH2pyrene), 3.19 (q, 2H,
J = 6.7 Hz, CH2NH), 2.78 (q, 2H, J = 7.3 Hz, CH2SH), 2.45 (m,
4H, CH2CONH and CH2COO), 2.18 (m, 2H, CH2), 1.61 (m, 3H,
SH and CH2), 1.44-1.20 (br, 16H, CH2); IR (KBr): 3304, 3038,
2920 (alkyl), 2850 (alkyl), 1726 (ester), 1638 (amide I), 1547
(amide II), 1467, 1263, 1177, 840, 720 cm¹1. Spectroscopic data
of 6: 1H NMR (400 MHz, CDCl3): ¤ 8.20-8.08 (m, 4H, perylene),
7.90 (d, 1H, J = 8.3 Hz, perylene), 7.67 (d, 1H, J = 5.4 Hz,
perylene), 7.65 (d, 1H, J = 5.1 Hz, perylene), 7.52-7.43 (m, 3H,
perylene), 7.32 (d, 1H, J = 7.8 Hz, perylene), 5.51 (br, 1H, NH),
4.08 (t, 2H, J = 6.8 Hz, CH2OCO), 3.21 (q, 2H, J = 6.7 Hz,
CH2NH), 3.05 (t, 2H, J = 7.8 Hz, CH2perylene), 2.79 (q, 2H,
J = 7.3 Hz, CH2SH), 2.43 (m, 4H, CH2CONH and CH2COO),
2.09 (m, 2H, CH2), 1.61 (m, 3H, CH2 and SH), 1.44-1.20 (br,
16H, CH2); IR (KBr): 3301, 3053, 2920 (alkyl), 2853 (alkyl),
1726 (ester), 1635 (amide I), 1547 (amide II), 1465, 1188, 812,
768 cm¹1. Spectroscopic data of 7: 1H NMR (400 MHz, CDCl3): ¤
8.47-8.26 (m, 11H, coronene), 5.31 (br, 1H, NH), 4.09 (t, 2H,
J = 7.1 Hz, CH2OCO), 3.35 (t, 2H, J = 7.8 Hz, CH2coronene),
2.98 (q, 2H, J = 6.8 Hz, CH2NH), 2.67 (q, 2H, J = 7.3 Hz,
CH2SH), 2.51 (t, 2H, J = 7.2 Hz, CH2CONH), 2.28 (m, 2H,
CH2), 2.20 (t, 2H, J = 6.5 Hz, CH2COO), 1.59 (m, 2H, CH2),
1.47 (t, 1H, J = 8.3 Hz, SH), 1.31-1.01 (br, 16H, CH2); IR (KBr):
the lowest thermal stability, followed by the perylene-type
nanoparticle Au-6; the coronene-type nanoparticle Au-7 had the
highest stability. They were also found to have comparable
amounts of ligands since their weight losses were of approxi-
mately the same value. Therefore, this result indicates that there
is a clear correlation between the thermal stability and the
number of ³ electrons (Table 1). Since the strength of the ³-³
stacking highly depends on the number of ³ electrons, there is a
high degree of relativity between the ³-³ stacking strength and
the thermal stability of the gold nanoparticles. Consequently,
³-³ stacking proved quite important for the development of
thermally resistant gold nanoparticles.
In conclusion, three types of novel ligand molecules
containing a different condensed ring were synthesized in order
to evaluate the correlation between the strength of ³-³ stacking
and the thermal stability of the corresponding gold nano-
particles. The onset temperature of decomposition was obtained
based on TG analysis and used as an indicator of thermal
stability. The temperature rose with an increase in ³ electrons.
Therefore, ³-³ stacking was revealed to play an important role
in the development of thermally resistant gold nanoparticles.
Figure 7 is a schematic illustration of the interligand
interactions of the gold nanoparticles. The interligand hydrogen
3287, 2919 (alkyl), 2849 (alkyl), 1728 (ester), 1633 (amide I),
¹1
1544 (amide II), 1465, 1176, 846, 544 cm
.
Chem. Lett. 2012, 41, 708-710
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/