Chirality control for in situ preparation of gold nanoparticle superstructures directed by a coordinatable organogelator

Article abstract of DOI:10.1021/ja403722t

Imposing chirality into nanoscale superstructures is a major step forward toward systematic understanding and utilization of nanomaterials. In an attempt to achieve tunable chirality during in situ preparation of hybrid nanomaterials, we here report a novel unimolecular strategy of employing a coordinatable organogelator for the realization of chirality control in the formation of gold nanoparticle superstructures. The work takes advantage of thermally reversible sol-gel transition of the chiral dispersion as template, which causes different micelle properties that can influence the coordination ability between the organogelator and Au(III) ions. Followed by a reduction reaction, gold nanoparticle superstructures with P-helicity were prepared from the sol form of the template through a coordination-induced chiral inversion, whereas those with M-helicity were obtained from the gel form with chiral holding. Such superstructures are solvent-stable and the chirality difference between them could be observed in many solvent environments.

Full text of DOI:10.1021/ja403722t

Article
pubs.acs.org/JACS
Chirality Control for in Situ Preparation of Gold Nanoparticle
Superstructures Directed by a Coordinatable Organogelator
Liangliang Zhu,†,§ Xin Li,‡,§ Shaojue Wu,† Kim Truc Nguyen,† Hong Yan,† Hans Ågren,‡
and Yanli Zhao*,†,∥
†Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
21 Nanyang Link, Singapore 637371
‡Department of Theoretical Chemistry and Biology, School of Biotechnology, KTH Royal Institute of Technology, SE-10691
Stockholm, Sweden
∥School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798
S
* Supporting Information
ABSTRACT: Imposing chirality into nanoscale superstructures
is a major step forward toward systematic understanding and
utilization of nanomaterials. In an attempt to achieve tunable
chirality during in situ preparation of hybrid nanomaterials, we
here report a novel unimolecular strategy of employing a
coordinatable organogelator for the realization of chirality
control in the formation of gold nanoparticle superstructures.
The work takes advantage of thermally reversible sol−gel
transition of the chiral dispersion as template, which causes
different micelle properties that can influence the coordination
ability between the organogelator and Au(III) ions. Followed by a reduction reaction, gold nanoparticle superstructures with P-
helicity were prepared from the sol form of the template through a coordination-induced chiral inversion, whereas those with M-
helicity were obtained from the gel form with chiral holding. Such superstructures are solvent-stable and the chirality difference
between them could be observed in many solvent environments.
preparation process for realization of the hypotheses. The
strategy is inspired by the chirality regulation of self-assembled
organic structures via certain external stimuli,11 especially by
the coordination-induced structural transformation.12 Our idea
is to employ the coordination approach to aid our design and
meanwhile to make use of an orthogonal factor (i.e.,
temperature) for the modulation of ordered self-assembly
properties through selective control of the coordination effect.
Thus, a novel organogelator was designed and synthesized by
covalently incorporating lipoic acid, azobenzene, and choles-
terol into a unimolecular platform (compound SAC, see Figure
1). Cholesterol is a highly efficient initiator for the construction
of chiral low-molecular-weight organogels.13 Azobenzene is
well-known for its photochemical characteristics,14 redox
properties,15 and the coordination ability toward heavy metal
species like auric chloride.16 It can also serve as a chromophore
label for the chiroptical outputs in this work.13 The disulfide
unit has a very high affinity for the surfaces of many inorganic
materials.17 Overall, the preparation of hybrid gold nanoma-
terials was designed to take advantage of the Au(III)−azo
coordination as well as the gold−thiol binding.18 With these
functions in mind, we envision that a chiral sol−gel system may
INTRODUCTION
■
Chirality has become a very important topic in nanoscience
since a chiral nano-object generally presents distinct properties
from those of the individual subunits or building blocks.1 In
particular, chiral hybrid nanomaterials having structure-specific
stability and morphological diversity exhibit promising
potentials for applications in asymmetric catalysis, selective
separation, chiral sensing, and novel photonics.2 The past
decade has witnessed extensive progress in the nanoengineering
of chiral composite nanomaterials,3 where a typical approach
developed thus far to impart chirality into nanoarchitectures is
to employ organic chiral molecules, for example, specifically
designed peptides,4 DNAs,5 surfactants,6 sol−gel systems,7
etc.,8 as template ligands or intermediates to be incorporated
with inorganic counterparts. However, facile tuning of chirality
in these materials is still difficult, and conformational alterations
for most of the superstructures are not readily accessible. To
the best of our knowledge, intentional control of chirality has
largely relied on either the employment of different enantiomer
templates9 or the assistance of specific treatments.10
straightforward approach for the control of chiral expressions
in an in situ preparation process remains a challenge.
A
On this basis, we propose a unimolecular strategy of
harnessing controllable interactions between functional organic
chiral template and the inorganic counterpart during the
Received: April 15, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja403722t | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Figure 2. Solvent selectivity for micelle formation: CD spectra of SAC
dissolved in different solvents. These spectra were determined at a
concentration of 0.03 mM calculated from the azobenzene unit in a
cuvette with path length 10 mm at 298 K.
upon the formation of the helically self-assembled nanostruc-
tures in a counterclockwise fashion (M-helicity). The self-
assembly properties of SAC were examined by the absorption
variation upon the concentration change. The critical
aggregation concentration (cac) of SAC in n-butanol was
determined to be 10.5 μM (Figure S1a in Supporting
Information), and the aggregates formed exhibit a fiber-shaped
morphology as visualized by transmission electron microscopy
(TEM) (Figure S2 in Supporting Information). SAC is prone
to be monodispersed in other organic solvents since no obvious
CD signals were observed (Figure 2). The critical micelle
concentration (cmc) of this kind of organogelators is usually 1
or 2 orders of magnitude higher than the corresponding cac.21
In this way, the cmc of SAC was also determined as 420 μM
(Figure S1b in Supporting Information).
SAC can completely gelate n-butanol above a certain
concentration regarded as the critical gelation concentration
(cgc = ∼0.3 wt %; Figure S3 in Supporting Information) at
room temperature. To ensure a favorable condition for phase
transition, a sol−gel system with a moderate concentration of
SAC (∼0.5 wt %) was optimized for the following experiments.
The gel-to-sol transition temperature at this concentration is 52
°C. Similar to most other low-molecular-weight gelators,22 the
sol state (SAC-sol) can be reversibly changed into its
corresponding gel state (SAC-gel) by cooling it to room
temperature and returned to the sol state by heating (Figure 1).
In our experiments, the micelles self-assembled from SAC
always display a counterclockwise helicity as indicated by the
Cotton effects in Figure 2 and Figure S4 in Supporting
Information, which is attributed to the molecular structure itself
and the sol−gel preparation procedure.23 The helical self-
assembly was prone to be disordered or dissociated upon
increasing the temperature, and the CD signal intensity of SAC
was reduced accordingly (Figure S4 in Supporting Informa-
tion). Therefore, temperature can serve as an orthogonal factor
to aid in the preparations of helical self-assembly as well as the
chirality control.
Figure 1. Chemical structures and synthetic routes of SAC,
photographs of thermoreversible sol−gel transition of SAC in n-
butanol, and representation of chirality control in the in situ
preparations of SAC-based gold nanoparticle hybrid superstructures
via corresponding micelle template. The superstructure Au@SAC-1
with P-helicity is prepared from the sol state (SAC-sol) through a
chiral inversion, whereas the superstructure Au@SAC-2 with M-
helicity is prepared from the gel state (SAC-gel) with chiral holding.
Reagents and conditions: (i) cholesteryl chloroformate; (ii) triethyl-
amine; (iii) ( )-α-lipoic acid; (iv) 4-(dimethylamino)pyridine and
dicyclohexylcarbodiimide.
be formed based on SAC and that the micelles of SAC could be
purposely used for the in situ construction of hybrid gold
nanoarchitectures under different temperatures with the
synergy of coordination reactions.
The synthetic route for preparation of SAC is outlined in
Figure 1 (see Experimental Section for synthesis details). The
intermediate compounds SA (azobenzene coupled with lipoic
acid) and AC (azobenzene conjugated with cholesterol) were
used as reference molecules for control studies. It has been well
documented that superstructures involving gold nanoparticles
possess fascinating plasmonic properties and promising
supportive features.19 In this way, it is necessary to employ a
series of characterization techniques to investigate the in situ
preparation process as well as chirality control.
RESULTS AND DISCUSSION
■
Gelation and Sol−Gel Transition. The solvent selectivity
for the gelation of SAC was investigated first by circular
dichroism (CD) spectroscopy, since it is an effective method-
ology to probe nanoscale chiral characteristics.20 Cotton effects
of a strong positive CD signal at ∼300 nm and a negative one at
∼350 nm, corresponding to the electronic transition of the
azobenzene unit, were observed when SAC was dispersed in n-
butanol at a relatively low concentration (Figure 2). Since there
is no CD signal observed from the cholesterol unit itself in the
spectral region above 250 nm, this phenomenon indicates that
the macrochirality of the achiral azobenzene skeleton emerges
Coordination-Induced Chiral Inversion. To shed light
on the correlation between micelle formation and chirality
tuning, we turned to explore the interaction between the
micelle template and Au(III) ions. When HAuCl4 was
B
dx.doi.org/10.1021/ja403722t | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
respectively introduced into SAC-sol and SAC-gel, different
apparent behavior was observed. The color of SAC-sol
immediately turned to red upon the addition of 5 equiv of
HAuCl4, and the mixture no longer gelated even upon cooling
down to room temperature (vial 1 in Figure 3a). In contrast,
2Au(III)−SAC, in n-butanol with a relatively high association
constant (6139 255 M−2) (Figures S6 and S7 in Supporting
Information). Electrospray ionization mass spectrometry (ESI-
MS) provides additional evidence for formation of the bis-
coordinated complex (Figure 3c). Two peaks at m/z = 1313.42
and 1349.62 were observed in the mass spectrum of 2Au(III)−
SAC, corresponding to [M − Cl]+ and [M + H]+, respectively.
Titration experiments shown in Figure S8 of Supporting
Information clearly demonstrate the coordination-induced
chiral inversion process.
To gain further insight into the fine superstructures of the
aggregates formed from the organogelator and the coordinated
complex, we employed molecular dynamics (MD) simula-
tions25 to study the self-assembly behavior of compounds SAC
and 2Au(III)−SAC (see Supporting Information for simulation
details). A representative comparison between the M- and P-
helices is shown in Figure 4 by illustrating the stacking patterns
Figure 3. Coordination-induced chiral inversion of the micelle
template. (a) Photographs of SAC-sol (vial 1) and SAC-gel (vial 2)
after addition of 5 equiv of HAuCl4. (b) CD spectra of n-butanol
solutions of SAC-sol (curve 1) and SAC-gel (curve 2) after addition of
5 equiv of HAuCl4. (c) ESI-MS spectrum and chemical structure of
2Au(III)−SAC.
the color of SAC-gel slightly changed after the addition of 5
equiv of HAuCl4, and the gel form was preserved after blending
and re-gelation (vial 2 in Figure 3a). More interestingly, dilute
solutions of the mixtures clearly exhibit chiral differences (see
CD spectra in Figure 3b). The Cotton effect of SAC-gel after
addition of 5 equiv of HAuCl4 retained a positive−negative pair
of peaks from 280 to 400 nm (M-helicity),24 while that of SAC-
sol after addition of 5 equiv of HAuCl4 became a negative−
positive pair of peaks (P-helicity).24 On the basis of these
observations, we infer that a significant interaction between
SAC-sol and HAuCl4 is responsible for the chiral inversion. For
SAC-gel, the micelles may not get enough contact with HAuCl4
in the gel state, so that the original chirality was kept
unchanged.
New absorption bands at around 380 and 470 nm emerged
after addition of HAuCl4 into SAC-sol (Figure S5 in Supporting
Information). Employing this signal change, we conducted a
series of control experiments to investigate the interaction
between SAC-sol and HAuCl4. Protonation of SAC was
excluded since no absorption change was observed when
SAC and excessive trifluoroacetic acid (TFA) are mixed (Table
S1 in Supporting Information). The addition of TFA did not
influence the chemical structure of SAC under the current
experimental conditions. Redox reaction was also ruled out as
no cyclic voltammetric (CV) signal was found in the micelle
dispersions. As these interactions were excluded, it was inferred
that this chiral inversion was caused by coordination of the
azobenzene unit with auric chloride, that is, chelation of a
phenyl group and a nitrogen atom on the azobenzene unit with
[AuCl2] (see the proposed structure in Figure 3c).16 Such a
coordination site on the azobenzenyl moiety is reasonable since
the disulfide unit is prone to bind with Au0 and Au(I) rather
than Au(III). From this aspect, we further explored that SAC is
able to coordinate with Au(III) to form a 1:2 complex,
Figure 4. Representative (top) M-helical stacking pattern of SAC self-
assembly and (bottom) P-helical stacking pattern of 2Au(III)−SAC
self-assembly, extracted from trajectories of MD simulations.
of the aromatic units in the self-assemblies of SAC and
2Au(III)−SAC, respectively. In the packing mode of SAC, the
azobenzenyl unit stacks over the phenyl ring through weak
interactions, where the macroscopic helicity is most likely
guided by the chiral cholesterol units.23a Notably, the stacking
pattern and helicity of the 2Au(III)−SAC self-assembly are
altered in the presence of gold atoms (Figure 4). Inspection of
the intermolecular interaction energies within the self-
assemblies (Figure S13 and Table S2 in Supporting
Information) indicates that the electrostatic attraction26
among the 2Au(III)−SAC molecules is significantly enhanced
as compared with that in the self-assembly of SAC, due to the
presence of gold and chlorine atoms. For both SAC and
2Au(III)−SAC self-assemblies, the vertical distance between
two adjacent stacking units is around 3.6 Å, typical of π−π
stacking character, and the relatively short distance between the
gold and chlorine atoms (3.33 Å in Figure 4) reflects that the
electrostatic attraction takes effect in altering the packing mode
and optical rotation of the 2Au(III)−SAC self-assembly.
Chirality Control for in Situ Preparation of Gold
Nanoparticle Superstructures. The hybrid gold nanostruc-
tures Au@SAC-1 and Au@SAC-2 were prepared from SAC-sol
and SAC-gel, respectively, through the addition of HAuCl4
followed by a reduction reaction using tetrabutylammonium
borohydride (TBAB) (see Experimental Section for details).
Through TEM studies, their superstructure morphologies were
observed. Au@SAC-1 with an average cross-sectional diameter
of 40 nm tends to curl, whereas Au@SAC-2 with an average
C
dx.doi.org/10.1021/ja403722t | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
cross-sectional diameter of 200 nm is relatively widespread (see
TEM images in Figure 5 and microscopy images with relatively
their surface plasmon resonance (SPR) band to more than 570
nm (Figure 6a and Figure S14 in Supporting Information), as
compared to the ones observed from monodispersed gold
nanoparticles (ca. 520 nm).27 The shifting of the SPR band in
Au@SAC-1 shows a slight difference from that of Au@SAC-2,
which was also reflected by the corresponding colors of the
dispersions (inset in Figure 6a). Fourier transform infrared
spectroscopy (FT-IR) and energy-dispersive X-ray spectrome-
try (EDS) studies also confirm the formation of gold
nanoparticle hybrid superstructures (see the corresponding
spectra and discussions in Figures S15 and S16 in Supporting
Information).
When these nanostructures were further examined by CD
spectroscopy, an encouraging phenomenon was observed.
Dilute dispersions of the gold nanoparticle superstructures
exhibit similar chiral behavior as their corresponding
precursors, that is, mixtures of HAuCl4 and the SAC micelle
in sol and gel states, respectively (see CD spectra in Figure 6b).
The Cotton effect of Au@SAC-2 also reveals a positive−
negative pair of peaks at the electronic transition region of the
azobenzene unit, while that of Au@SAC-1 displays a negative−
positive pair of peaks. These results indicate that the difference
in chirality can be preserved with subsequent formation of gold
nanoparticle superstructures. Electron microscopy with rela-
tively large scale provides further insight into the chiral
character of these superstructures. It can be seen that Au@
SAC-1 normally adopts a clockwise helical stacking mode (P-
helix),24 whereas Au@SAC-2 exhibits an anticlockwise pattern
(M-helix)24 (Figure 6c−f). The pitch in these hierarchical
nanoarchitectures of Au@SAC-1 and Au@SAC-2 can also be
measured as 60 and 130 nm, respectively (Figure 6g,h). Such
pitch difference is in agreement with their cross-sectional
diameter difference shown in Figures 5 and 6. These interesting
results came from optimization of a series of parallel
experiments for preparation of the hybrid nanostructures
under different strategies (see Table S3 and Figures S4 and
S17 in Supporting Information for details). Although the
temperature and addition sequence of the reaction materials
can affect the chiral expression, it is clearly demonstrated that
chirality control for in situ preparation of the hybrid
superstructures can be realized by employing the sol and gel
states of SAC as template. In addition, chiroptical properties of
the hybrid superstructures in the plasmonic region (∼570 nm)
were also explored. The CD signals of Au@SAC-1 and Au@
SAC-2 around 570 nm are relatively weak as a result of low
chiral transfer efficiency in the systems as well as small oscillator
strength of the gold nanoparticles.28 Nevertheless, magnified
CD spectra feature the opposite plasmonic signals between
Au@SAC-1 and Au@SAC-2 (Figure S18 in Supporting
Information).
Figure 5. TEM images of (a) Au@SAC-1 and (b) Au@SAC-2,
prepared from the corresponding n-butanol dispersions.
large scale in Figure 6). In addition, it can be clearly seen from
the TEM images that these superstructures were formed
through the growth of small gold nanoparticles with diameter
less than 5 nm along the micelle surfaces via gold−thiol binding
(Figure 5). The distribution of gold nanoparticles in Au@SAC-
1 is more compact than that in Au@SAC-2. Formation of the
superstructures results in a significantly bathochromic shift of
Solvent Stability of the Superstructures. The chiral
appearance of the unimolecular template is largely solvent-
dependent, as exemplified in Figure 2. However, such a
limitation was well overcome by the formation of gold
nanoparticle superstructures. Beyond n-butanol, the chirality
difference between Au@SAC-1 and Au@SAC-2 can also be
observed in several other solvents with different polarities (e.g.,
toluene, acetone, and water; see the corresponding CD signals
in Figure 7). This finding suggests that the whole nano-
architectures of Au@SAC-1 and Au@SAC-2 remain intact and
the inorganic−organic hybrid entities are retained in these
media, owing to the immobilization effect generated from the
equilibrium between destabilization of the organic components
Figure 6. Formation of Au@SAC-1 and Au@SAC-2 and their chirality
difference. (a) Absorption spectra (SPR band) of n-butanol
dispersions of Au@SAC-1 (curve 1) and Au@SAC-2 (curve 2).
(Inset) Corresponding photographs of the dispersions. (b) CD spectra
of n-butanol dispersions of Au@SAC-1 (curve 1) and Au@SAC-2
(curve 2). (c, e) TEM images with relatively large scale for (c) Au@
SAC-1 and (e) Au@SAC-2, prepared from the corresponding n-
butanol dispersions. (d, f) SEM images with relatively large scale for
(d) Au@SAC-1 and (f) Au@SAC-2, prepared from the corresponding
n-butanol dispersions. Some clear helical twists of the nanostructures
in panels c−f are highlighted by red arrows. Scale bar = 100 nm. (g, h)
Model representations of (g) Au@SAC-1 and (h) Au@SAC-2.
D
dx.doi.org/10.1021/ja403722t | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
point system. The photo images were photographed by a Nikon
Coolpix S8000 digital camera.
Synthesis of Compounds A and SA. These compounds were
prepared according to our previous report.17b
Synthesis of Compound AC. A solution of cholesteryl
chloroformate (0.42 g, 0.93 mmol) in anhydrous acetone (10 mL)
was added dropwise into a mixed solution of compound A (0.2 g, 0.93
mmol) and triethylamine (0.1 mL) in anhydrous acetone (20 mL) at
room temperature. After the mixture was stirred at room temperature
for 16 h, the byproduct quaternary ammonium salt was removed by
filtration. The filtrate was concentrated under vacuum, and the residue
was washed with ethanol (20 mL) and petroleum ether (5 mL). Then
the crude product was purified through silica gel chromatography
(petroleum ether:ethyl acetate = 20:1) to afford yellow compound AC
(195 mg, 33.6%), mp 148−150 °C. 1H NMR (400 MHz, CDCl3, 298
K) δ = 7.93 (d, J = 8.8 Hz, 2H), 7.89 (d, J = 8.8 Hz, 2H), 7.35 (d, J =
8.8 Hz, 2H), 6.97 (d, J = 8.8 Hz, 2H), 5.44 (s, 1H), 4.62 (m, 1H), 2.48
(m, 1H), 2.04 (m, 1H), 1.59−0.71 (m, 41H). 13C NMR (100 MHz,
CDCl3, 298 K) δ = 149.80, 138.50, 125.01, 123.83, 123.75, 123.30,
121.63, 115.81, 83.03, 79.15, 77.22, 56.64, 56.13, 49.93, 42.31, 39.67,
39.52, 37.58, 36.76, 36.50, 36.18, 35.78, 31.89, 31.80, 28.21, 28.02,
27.37, 24.27, 23.83, 22.83, 22.57, 21.04, 19.22, 18.72, 11.86. HRMS
(ESI) m/z [M + H]+ calcd for C40H55N2O4, 627.4162; found,
627.4166.
Synthesis of Compound SAC. A solution of cholesteryl
chloroformate (1.26 g, 2.79 mmol) in anhydrous acetone (20 mL)
was added dropwise into a mixed solution of SA (585 mg, 1.45 mmol)
and triethylamine (0.1 mL) in anhydrous acetone (20 mL) at room
temperature. After the mixture was stirred at room temperature for 24
h, the byproduct quaternary ammonium salt was removed by filtration.
The filtrate was concentrated under vacuum, and the residue was
washed with ethanol (50 mL) and petroleum ether (15 mL). Then the
crude product was purified through silica gel chromatography
(petroleum ether:ethyl acetate = 40:1) to afford yellow compound
SAC (682 mg, 57.7%), mp 131−133 °C. 1H NMR (400 MHz, CDCl3,
298 K) δ = 7.97 (d, J = 8.8 Hz, 4H), 7.36 (d, J = 8.8 Hz, 2H), 7.25 (d,
J = 8.8 Hz, 2H), 5.45 (s, 1H), 4.62 (m, 1H), 3.65 (m, 1H), 3.20 (m,
2H), 2.64 (t, J = 7.2 Hz, 2H), 2.49 (m, 2H), 2.04 (m, 1H), 1.90 (m,
1H), 1.82 (m, 2H), 1.59−0.71 (m, 45H). 13C NMR (100 MHz,
CDCl3, 298 K) δ = 171.63, 153.03, 152.75, 152.54, 150.14, 139.07,
124.11, 123.31, 122.23, 121.71, 79.19, 77.23, 56.69, 56.30, 56.13,
49.99, 42.33, 40.26, 39.71, 39.52, 38.53, 37.93, 36.84, 36.57, 36.19,
35.79, 34.62, 34.18, 31.92, 31.85, 28.73, 28.23, 28.02, 27.65, 24.60,
24.29, 23.83, 22.83, 22.57, 21.06, 19.30, 18.72, 11.87. HRMS (ESI) m/
z [M + H]+ calcd for C48H67N2O5S2, 815.4491; found, 815.4503.
Gelation Test in Different Organic Solvents. SAC (10 mg) and
different solvents (0.5 mL) were placed in a capped vial, which was
heated until the solid was dissolved (SAC cannot be entirely dissolved
in some solvents even by heating). Then the solution was cooled
naturally to room temperature. In our study, we found that only n-
butanol can be effectively gelated by SAC. Once the gel was formed, it
can be reversibly transformed between the gel and sol states by heating
and cooling, respectively. For a batch sol−gel system with 0.5 wt %
SAC, the gel-to-sol transition temperature is 52 °C.
Figure 7. Stability of Au@SAC-1 and Au@SAC-2 in different solvents:
CD spectra of Au@SAC-1 in (1) toluene, (2) acetone, and (3) water
and of Au@SAC-2 in (4) toluene, (5) acetone, and (6) water.
and the Hamaker attraction of gold nanoparticles.29 Thus, the
solvent stability of the nanoarchitectures is strengthened
through a rational combination of the organic component
with the gold component. This property broadens the potential
application scope of chiral hybrid nanomaterials.
CONCLUSION
■
Chirality control in the in situ preparation of hybrid gold
nanoparticle superstructures has been realized via the
unimolecular strategy of using a coordinatable organogelator
as template. Upon addition of HAuCl4 into the sol form of the
template, coordination between the azobenzene moiety and
Au(III) leads to chirality inversion of the helical micelles. Upon
introduction of HAuCl4 into the gel form, the thickness of the
micelles that cannot get enough contact with HAuCl4 prevents
the coordination, and thus the original chirality is maintained.
After subsequent reduction by TBAB, the chirality difference
between the resulting gold nanoparticle superstructures can be
preserved and cannot be disrupted in several solvents. These
observations have been well supported by multiple exper-
imental characterization techniques as well as by theoretical
calculations. The strategy demonstrated herein could thus be
valuable for potential exploitation toward the development of
smart chiral nanomaterials.
EXPERIMENTAL SECTION
■
1
General. H NMR and 13C NMR spectra were measured on a
Bruker BBFO-400 spectrometer. Electronic spray ionization mass
spectra were recorded on a ThermoFinnigan LCQ quadrupole ion trap
mass spectrometer. High-resolution mass spectrometry (HRMS) was
performed on a Waters Q-tof Premier MS spectrometer. Absorption
spectra were recorded on a Shimadzu UV-3600 UV−vis−NIR
spectrophotometer, while the CD spectra were recorded on a Jasco
J-810 CD spectrophotometer. All the optical spectra discussed in this
work were determined at a concentration of 0.3 mM calculated from
the azobenzene unit in a cuvette with a path length of 1 mm. TEM
images were collected on a JEM-1400 (JEOL) operated at 100−120
kV. SEM images were obtained on a JSM 6340 scanning electron
microscope (0.5−30 kV) equipped with cold cathode field-emission
gun (FEG) as electron source. Energy-dispersive X-ray spectra (EDS)
were collected from JED-2300F (JEOL) equipped with SEM of field-
emission JSM-6700F (JEOL) operated at 15 kV. FT-IR spectra were
recorded on a Shimadzu IR Prestige-21 spectrophotometer. Melting
points were determined by use of an OptiMelt automated melting
Preparation of Au@SAC-1. SAC (4.0 mg) and n-butanol (0.8
mL) were placed in a capped vial, which was heated until the solid was
entirely dissolved. The mixture was cooled to 55 °C and remained as a
fluid sol under stirring. Then HAuCl4 (8 mg) in n-butanol (0.2 mL)
was added into the sol, immediately accompanied by an obvious color
change of the mixture from yellow to red. After Au(III) was reduced
by the addition of tetrabutylammonium borohydride (TBAB, 2.4 mg)
in n-butanol (0.2 mL), the mixture was kept stirring for another 20
min. The resulted hybrid gold nanostructure Au@SAC-1 was isolated
by centrifugation (1500 rpm, 15 min) and washed by n-butanol to
sufficiently remove free ligand and excessive reagents. Au@SAC-1 was
either dried under vacuum or redispersed in different solvents for
further investigations.
Preparation of Au@SAC-2. SAC (4.0 mg) and n-butanol (0.8
mL) were placed in a capped vial, which was heated until the solid was
entirely dissolved. The mixture was cooled to room temperature and
E
dx.doi.org/10.1021/ja403722t | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
became the gel state. The gel was minced under stirring for better
interactions with additional agents. Then HAuCl4 (8 mg) in n-butanol
(0.2 mL) was added to the minced gel, and only the surface color of
the micelles slightly turned red. TBAB (2.4 mg) in n-butanol (0.2 mL)
was added under manual stirring for the Au(III) reduction, and the
resulting mixture was kept stirring for another 20 min. The obtained
hybrid gold nanostructure Au@SAC-2 was isolated by centrifugation
(1500 rpm, 15 min) and washed by n-butanol to sufficiently remove
free ligand and excessive reagents. Au@SAC-2 was either dried under
vacuum or redispersed in different solvents for further investigations.
Computational Details. The geometries of compounds SAC and
2Au(III)−SAC were optimized by density functional theory
calculations employing the hybrid B3LYP functional30 and the 6-
31G* basis set31 as implemented in the Gaussian 09 program.32 For
gold atoms in 2Au(III)−SAC, the Los Alamos effective core potential
basis set (LANL2DZ)33 was used. At the optimized geometry, the
electrostatic potential (ESP) was calculated at the Hartree−Fock level
of theory according to the Merz−Singh−Kollman scheme.34 Atomic
partial charges for subsequent molecular dynamics (MD) simulations
were derived on the basis of the restrained electrostatic potential
(RESP).35 The general Amber force field (GAFF)36 was employed to
model the organic part of the molecules, and the Hess2FF scheme37
was used to derive force field parameters for bonded interaction
involving gold atoms. Here, the equilibrium bond lengths and bond
angles as well as the force constants for dihedral potentials were
further refined so that the molecular mechanical force field could well
reproduce the quantum chemically predicted geometries. The
Lennard-Jones parameters for gold atoms were adopted from
literature.38 MD simulations were carried out with the GROMACS
program package.25 For each compound, 10 molecules were solvated
in TIP3P water39 and simulated under constant-NpT conditions (T =
298 K, p = 1 atm) for 50 ns. For comparison, simulations of SAC
molecules were also performed in methanol for 65 ns. Snapshots were
extracted from each simulation trajectory with an equal interval of 500
ps, and the computed electronic circular dichroism (ECD) spectra
were averaged over snapshots of the last 10 ns. The semiempirical
ZIndo/S method40 and time-dependent density functional theory
(TDDFT) calculations with the PBE0 functional41 were employed to
compute the ECD spectra.
thanks C. Y. Ang, Z.-N. Liu, Z. Luo, X. Ma, X. Yao, M.-H. Li,
and L.-Y. Bai for helpful discussions. X.L. acknowledges a grant
from the Carl Tryggers Foundation and computational
resources provided by the Swedish National Infrastructure for
Computing under the project Multiphysics Modeling of
Molecular Materials, SNIC 022/09-25.
REFERENCES
■
(1) (a) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418−
2421. (b) Kinbara, K.; Aida, T. Chem. Rev. 2005, 105, 1377−1400.
(c) Lehn, J. M. Science 2002, 295, 2400−2403. (d) Aida, T.; Meijer, E.
W.; Stupp, S. I. Science 2012, 335, 813−817. (e) Kiyonaka, S.; Sada, K.;
Yoshimura, I.; Shinkai, S.; Kato, N.; Hamachi, I. Nat. Mater. 2004, 3,
58−64. (f) Valev, V. K.; Baumberg, J. J.; Sibilia, C.; Verbiest, T. Adv.
Mater. 2013, 25, 2517−2534.
(2) (a) Kondo, Y.; Takayanagi, K. Science 2000, 289, 606−608.
(b) Wu, Y.; Cheng, G.; Katsov, K.; Sides, S. W.; Wang, J.; Tang, J.;
Fredrickson, G. H.; Moskovits, M.; Stucky, G. D. Nat. Mater. 2004, 3,
816−822. (c) Shopsowitz, K. E.; Qi, H.; Hamad, W. Y.; MacLachlan,
M. J. Nature 2010, 468, 422−425. (d) Zhu, J.; Peng, H.; Marshall, A.
F.; Barnett, D. M.; Nix, W. D.; Cui, Y. Nat. Nanotechnol. 2008, 3, 477−
481. (e) Zhou, Y.; Yang, M.; Sun, K.; Tang, Z.; Kotov, N. A. J. Am.
Chem. Soc. 2010, 132, 6006−6013. (f) Thomas, J. M.; Raja, R. Acc.
Chem. Res. 2008, 41, 708−720. (g) Gabashvili, A.; Medina, D. D.;
Gedanken, A.; Mastai, Y. J. Phys. Chem. B 2007, 111, 11105−11110.
(h) Fireman-Shoresh, S.; Popov, I.; Avnir, D.; Marx, S. J. Am. Chem.
Soc. 2005, 127, 2650−2655. (i) Hodgkinson, I.; Wu, Q. H. Adv. Mater.
2001, 13, 889−897. (j) Kong, X. Y.; Wang, Z. L. Nano Lett. 2003, 3,
1625−1631.
(3) (a) Qiu, H.; Che, S. Chem. Soc. Rev. 2011, 40, 1259−1268.
(b) Yang, X.; Tang, H.; Cao, K.; Song, H.; Sheng, W.; Wu, Q. J. Mater.
Chem. 2011, 21, 6122−6135. (c) Wang, Y.; Xu, J.; Wang, Y.; Chen, H.
Chem. Soc. Rev. 2013, 42, 2930−2962.
(4) (a) Slocik, J. M.; Govorov, A. O.; Naik, R. R. Nano Lett. 2011, 11,
701−705. (b) Chen, C.-L.; Rosi, N. L. Angew. Chem., Int. Ed. 2010, 49,
1924−1942.
(5) (a) Li, Z.; Zhu, Z.; Liu, W.; Zhou, Y.; Han, B.; Gao, Y.; Tang, Z. J.
Am. Chem. Soc. 2012, 134, 3322−3325. (b) Shemer, G.; Krichevski,
O.; Markovich, G.; Molotsky, T.; Lubitz, I.; Kotlyar, A. B. J. Am. Chem.
Soc. 2006, 128, 11006−11007.
ASSOCIATED CONTENT
* Supporting Information
■
S
́
(6) (a) Delclos, T.; Aime, C.; Pouget, E.; Brizard, A.; Huc, I.; Delville,
Additional text, 18 figures, and three tables describing cac and
cmc measurements for SAC, observations for cgc, coordination
studies between SAC and Au(III), complexation constant K
between SAC and HAuCl4, supplementary computational
results and characterizations of superstructures, optimization
for in situ preparation strategy, and full citation for ref 32. This
material is available free of charge via the Internet at http://
pubs.acs.org.
M.-H.; Oda, R. Nano Lett. 2008, 8, 1929−1935. (b) Wang, R.-Y.;
Wang, H.; Wu, X.; Ji, Y.; Wang, P.; Qu, Y.; Chung, T.-S. Soft Matter
2011, 7, 8370−8375. (c) Yokoi, T.; Ogawa, K.; Lu, D.; Kondo, J. N.;
Kubota, Y.; Tatsumi, T. Chem. Mater. 2011, 23, 2014−2016. (d) Jin,
H.; Liu, Z.; Ohsuna, T.; Terasaki, O.; Inoue, Y.; Sakamoto, K.;
Nakanishi, T.; Ariga, K.; Che, S. Adv. Mater. 2006, 18, 593−596.
(7) (a) Jung, J. H.; Lee, S.-H.; Yoo, J. S.; Yoshida, K.; Shimizu, T.;
Shinkai, S. Chem.Eur. J. 2003, 9, 5307−5313. (b) Llusar, M.;
Sanchez, C. Chem. Mater. 2008, 20, 782−820.
AUTHOR INFORMATION
Corresponding Author
zhaoyanli@ntu.edu.sg
(8) To mention a few examples: (a) Chen, W.; Bian, A.; Agarwal, A.;
Liu, L.; Shen, H.; Wang, L.; Xu, C.; Kotov, N. A. Nano Lett. 2009, 9,
2153−2159. (b) Oh, H. S.; Liu, S.; Jee, H.; Baev, A.; Swihart, M. T.;
Prasad, P. N. J. Am. Chem. Soc. 2010, 132, 17346−17348. (c) Tseng,
W.-H.; Chen, C.-K.; Chiang, Y.-W.; Ho, R.-M.; Akasaka, S.; Hasegawa,
H. J. Am. Chem. Soc. 2009, 131, 1356−1357. (d) Karatchevtseva, I.;
Cassidy, D. J.; Man, M. W. C.; Mitchell, D. R. G.; Hanna, J. V.; Carcel,
C.; Bied, C.; Moreau, J. J. E.; Bartlett, J. R. Adv. Funct. Mater. 2007, 17,
3926−3932.
■
Author Contributions
§L.Z. and X.L. contributed equally.
Notes
The authors declare no competing financial interest.
(9) (a) Li, C.; Deng, K.; Tang, Z.; Jiang, L. J. Am. Chem. Soc. 2010,
132, 8202−8209. (b) George, J.; Thomas, K. G. J. Am. Chem. Soc.
2010, 132, 2502−2503.
ACKNOWLEDGMENTS
■
This work was financially supported by the Singapore National
Research Foundation Fellowship (NRF2009NRF-RF001-015),
the Singapore National Research Foundation CREATE
program−Singapore Peking University Research Centre for a
Sustainable Low-Carbon Future, and the Centre of Excellence
for Silicon Technologies (A*Star SERC 112 351 0003). L.Z.
(10) (a) Chen, Q.; Whitmer, J. K.; Jiang, S.; Bae, S. C.; Luijten, E.;
Granick, S. Science 2011, 331, 199−202. (b) Gautier, C.; Burgi, T. J.
̈
Am. Chem. Soc. 2008, 130, 7077−7084. (c) Wang, H.; Wu, J.-C.; Shen,
Y.; Li, G.; Zhang, Z.; Xing, G.; Guo, D.; Wang, D.; Dong, Z.; Wu, T. J.
Am. Chem. Soc. 2010, 132, 15875−15877.
F
dx.doi.org/10.1021/ja403722t | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
(11) (a) Huang, Z.; Kang, S.-K.; Banno, M.; Yamaguchi, T.; Lee, D.;
Seok, C.; Yashima, E.; Lee, M. Science 2012, 337, 1521−1526.
(b) Zhang, Y.; O’Callaghan, M. J.; Baumeister, U.; Tschierske, C.
Angew. Chem., Int. Ed. 2008, 47, 6892−6896. (c) Gopal, A.;
Hifsudheen, M.; Furumi, S.; Takeuchi, M.; Ajayaghosh, A. Angew.
Chem., Int. Ed. 2012, 51, 10505−10509. (d) Peterca, M.; Imam, M. R.;
Ahn, C.-H.; Balagurusamy, V. S. K.; Wilson, D. A.; Rosen, B. M.;
Percec, V. J. Am. Chem. Soc. 2011, 133, 2311−2328. (e) Lohr, A.;
(26) Mayoral, M. J.; Rest, C.; Stepanenko, V.; Schellheimer, J.;
Albuquerque, R. Q.; Fernan
2151.
́
dez, G. J. Am. Chem. Soc. 2013, 135, 2148−
(27) Jana, N. R.; Gearheart, L.; Murphy, C. J. Langmuir 2001, 17,
6782−6786.
(28) (a) Kuzyk, A.; Schreiber, R.; Fan, Z.; Pardatscher, G.; Roller, E.-
M.; Hogele, A.; Simmel, F. C.; Govorov, A. O.; Liedl, T. Nature 2012,
̈
483, 311−314. (b) Shen, X.; Song, C.; Wang, J.; Shi, D.; Wang, Z.; Liu,
N.; Ding, B. J. Am. Chem. Soc. 2012, 134, 146−149. (c) Zhu, Z.; Liu,
W.; Li, Z.; Han, B.; Zhou, Y.; Gao, Y.; Tang, Z. ACS Nano 2012, 6,
2326−2332.
Wurthner, F. Chem. Commun. 2008, 2227−2229. (f) Moriyama, M.;
̈
Mizoshita, N.; Yokota, T.; Kishimoto, K.; Kato, T. Adv. Mater. 2003,
15, 1335−1338.
(12) (a) Crassous, J. Chem. Commun. 2012, 48, 9684−9692.
(b) Ousaka, N.; Takeyama, Y.; Yashima, E. Chem. Sci. 2012, 3,
466−469. (c) Shin, S.; Lim, S.; Kim, Y.; Kim, T.; Choi, T.-L.; Lee, M. J.
Am. Chem. Soc. 2013, 135, 2156−2159.
(29) (a) Jin, Q.; Xu, J.-P.; Ji, J.; Shen, J.-C. Chem. Commun. 2008,
3058−3060. (b) Zhang, H.; Wang, D. Angew. Chem., Int. Ed. 2008, 47,
3984−3987.
(30) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652.
(b) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J.
Phys. Chem. 1994, 98, 11623−11627.
(13) (a) Zhao, Y. L.; Aprahamian, I.; Trabolsi, A.; Erina, N.; Stoddart,
J. F. J. Am. Chem. Soc. 2008, 130, 6348−6350. (b) Wang, S.; Shen, W.;
Feng, Y.; Tian, H. Chem. Commun. 2006, 1497−1499. (c) Klawonn,
(31) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56,
2257−2261.
T.; Gansauer, A.; Winkler, I.; Lauterbach, T.; Franke, D.; Nolte, R. J.
̈
(32) Frisch, M. J.; et al. Gaussian 09, Revision A.2; Gaussian, Inc.,
Wallingford, CT, 2009.
M.; Feiters, M. C.; Borner, H.; Hentschel, J.; Dotz, K. H. Chem.
̈
̈
Commun. 2007, 1894−1895.
(33) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270−283.
(34) (a) Singh, U. C.; Kollman, P. A. J. Comput. Chem. 1984, 5, 129−
145. (b) Besler, B. H.; Merz, K. M., Jr.; Kollman, P. A. J. Comput.
Chem. 1990, 11, 431−439.
(14) (a) Zhu, L.; Yan, H.; Wang, X.-J.; Zhao, Y. L. J. Org. Chem.
2012, 77, 10168−10175. (b) Zhu, L.; Yan, H.; Ang, C. Y.; Nguyen, K.
T.; Li, M.; Zhao, Y. L. Chem.Eur. J. 2012, 18, 13979−13983.
(c) Zhu, L.; Lu, M.; Qu, D.; Wang, Q.; Tian, H. Org. Biomol. Chem.
2011, 9, 4226−4233.
(15) (a) Willner, I.; Pardo-Yissar, V.; Katz, E.; Ranjit, K. T. J.
Electroanal. Chem. 2001, 497, 172−177. (b) Chiu, K. Y.; Tu, Y.-J.; Lee,
C.-J.; Yang, T.-F.; Lai, L.-L.; Chao, I.; Su, Y. O. Electrochim. Acta. 2012,
62, 51−62.
(16) (a) Ghedini, M.; Aiello, I.; Crispini, A.; Golemme, A.; Deda, M.
L.; Pucci, D. Coord. Chem. Rev. 2006, 250, 1373−1390. (b) Omae, I.
Coord. Chem. Rev. 2011, 255, 139−160. (c) Wang, H.; Bao, C.; Li, F.;
Kong, X.; Xu, J. Microchim. Acta 2010, 168, 99−105.
(35) Bayly, C. I.; Cieplak, P.; Cornell, W.; Kollman, P. A. J. Phys.
Chem. 1993, 97, 10269−10280.
(36) (a) Wang, J.; Wolf, R. M.; Caldwell, J. W.; Kollman, P. A.; Case,
D. A. J. Comput. Chem. 2004, 25, 1157−1174. (b) Duchstein, P.; Neiss,
C.; Gorling, A.; Zahn, D. J. Mol. Model. 2012, 18, 2479−2482.
̈
(37) (a) Seminario, J. M. Int. J. Quantum Chem. 1996, 60, 1271−
1277. (b) Nilsson, K.; Lecerof, D.; Sigfridsson, E.; Ryde, U. Acta
Crystallogr. D 2003, 59, 274−289.
(38) Schoen, P. A. E.; Walther, J. H.; Arcidiacono, S.; Poulikakos, D.;
Koumoutsakos, P. Nano Lett. 2006, 6, 1910−1917.
(17) (a) Sperling, R. A.; Parak, W. J. Philos. Trans. R. Soc. London, Ser.
A 2010, 368, 1333−1383. (b) Zhu, L.; Yan, H.; Nguyen, K. T.; Tian,
H.; Zhao, Y. L. Chem. Commun. 2012, 48, 4290−4292. (c) Zhu, L.;
Ang, C. Y.; Li, X.; Nguyen, K. T.; Tan, S. Y.; Ågren, H.; Zhao, Y. L.
Adv. Mater. 2012, 24, 4020−4024.
(39) Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D.; Impey, R.
W.; Klein, M. L. J. Chem. Phys. 1983, 79, 926−935.
(40) Zerner, M. C. In Reviews of Computational Chemistry; Lipkowitz,
K. B., Boyd, D. B., Eds.; VCH Publishing: New York, 1991; Vol. 2, pp
313−366.
(18) Gautier, C.; Burgi, T. ChemPhysChem 2009, 10, 483−492.
̈
(41) Adamo, C.; Barone, V. J. Chem. Phys. 1999, 110, 6158−6169.
(19) (a) Wang, L.; Xu, L.; Kuang, H.; Xu, C.; Kotov, N. A. Acc. Chem.
Res. 2012, 45, 1916−1926. (b) Fafarman, A. T.; Hong, S.-H.;
Caglayan, H.; Ye, X.; Diroll, B. T.; Paik, T.; Engheta, N.; Murray, C. B.;
Kagan, C. R. Nano Lett. 2013, 13, 350−357.
(20) (a) Berova, N.; Bari, L. D.; Pescitelli, G. Chem. Soc. Rev. 2007,
36, 914−931. (b) Canary, J. W. Chem. Soc. Rev. 2009, 38, 747−756.
(21) (a) Jain, N.; Trabelsi, S.; Guillot, S.; McLoughlin, D.; Langevin,
D.; Letellier, P.; Turmine, M. Langmuir 2004, 20, 8496−8503.
(b) Okuzaki, H.; Osada, Y. Macromolecules 1995, 28, 380−382.
(22) (a) Estroff, L. A.; Hamilton, A. D. Chem. Rev. 2004, 104, 1201−
́
1217. (b) Svobodova, H.; Noponen, V.; Kolehmainen, E.; Sievanen, E.
̈
RSC Adv. 2012, 2, 4985−5007. (c) Zhu, L.; Ma, X.; Ji, F.; Wang, Q.;
Tian, H. Chem.Eur. J. 2007, 13, 9216−9222. (d) Wang, X.-J.; Xing,
L.-B.; Cao, W.-N.; Li, X.-B.; Chen, B.; Tung, C.-H.; Wu, L.-Z.
Langmuir 2011, 27, 774−781. (e) de Loos, M.; Feringa, B. L.; van
Esch, J. H. Eur. J. Org. Chem. 2005, 70, 3615−3631. (f) Gao, P.; Zhan,
C. L.; Liu, M. H. Langmuir 2006, 22, 775−779.
(23) (a) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.;
Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994,
116, 6664−6676. (b) Cui, J.; Liu, A.; Guan, Y.; Zheng, J.; Shen, Z.;
Wan, X. Langmuir 2010, 26, 3615−3622.
(24) (a) Lu, X.; Guo, Z.; Sun, C.; Tian, H.; Zhu, W. J. Phys. Chem. B
́
2011, 115, 10871−10876. (b) Danila, I.; Riobe, F.; Piron, F.;
Puigmartí-Luis, J.; Wallis, J. D.; Linares, M.; Ågren, H.; Beljonne, D.;
Amabilino, D. B.; Avarvari, N. J. Am. Chem. Soc. 2011, 133, 8344−
8353. (c) Zhu, L.; Li, X.; Zhang, Q.; Ma, X.; Li, M.; Zhang, H.; Luo,
Z.; Ågren, H.; Zhao, Y. L. J. Am. Chem. Soc. 2013, 135, 5175−5182.
(25) Hess, B.; Kutzner, C.; van der Spoel, D.; Lindahl, E. J. Chem.
Theory Comput. 2008, 4, 435−447.
G
dx.doi.org/10.1021/ja403722t | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX