Simple cubic packing of gold nanoparticles through rational design of their dendrimeric corona

Article abstract of DOI:10.1021/ja2095816

The first simple-cubic liquid crystal was obtained by coating monodisperse Au nanoparticles (NPs) with a thick corona of amino-substituted organic dendrons. This unusual structure was determined by grazing-incidence diffraction and electron density reconstruction and explained by analyzing the radial density profile of the corona. Another novel structure is proposed for the phase preceding the cubic one: a hexagonal superlattice composed of alternating dense and sparse strings of Au NPs.

Full text of DOI:10.1021/ja2095816

Communication
pubs.acs.org/JACS
Simple Cubic Packing of Gold Nanoparticles through Rational Design
of Their Dendrimeric Corona
Kiyoshi Kanie,*,† Masaki Matsubara,† Xiangbing Zeng,‡ Feng Liu,‡ Goran Ungar,*,‡,§ Hiroshi Nakamura,∥
and Atsushi Muramatsu†
†Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan
‡Department of Materials Science and Engineering, University of Sheffield, Sheffield S1 3JD, U.K.
§School of Chemical and Biological Engineering, Seoul National University, Seoul 151-744, Korea
∥Toyota Central R&D Laboratories, Inc., 41-1 Yokomichi, Nagakute, Nagakute-Cho, Aichi-Gun, Aichi 480-1192, Japan
S
* Supporting Information
convergent approach22 for precise control of the divergent shape
ABSTRACT: The first simple-cubic liquid crystal was
obtained by coating monodisperse Au nanoparticles (NPs)
with a thick corona of amino-substituted organic dendrons.
This unusual structure was determined by grazing-
incidence diffraction and electron density reconstruction
and explained by analyzing the radial density profile of the
corona. Another novel structure is proposed for the phase
preceding the cubic one: a hexagonal superlattice composed
of alternating dense and sparse strings of Au NPs.
of the organic ligands to give the appropriate RDP and produce
monodisperse gold NPs surrounded by a variety of double-shell
dendritic coronas. This approach has considerable potential as a
powerful alternative to the ligand-exchange method.11,12 The new
approach can be applied to various types of functional NPs, since
the organic layer on their surfaces is readily tunable.
abrication of nanoparticle (NP)-based periodic structures
F
has attracted considerable attention in materials science1−3
because new synergistic functions could be derived from such
structures. Top-down procedures could yield such structures,4
but bottom-up approaches relying on self-organization are
preferable. Considerable diversity of NP lattices has been
obtained by self-organization of two different NP types.5 Cubic
arrangements of NPs have also been achieved by modification
with self-complementary DNA6,7 or a hydrophilic surfactant.8
Moreover, an organic soft corona can promote periodic NP
structures, and various calamitic9−11 and dendritic12−14 meso-
gens have been grafted onto NPs. As a result, liquid-crystalline
(LC) phases with NPs in ordered arrays have been obtained.1,15
Among organic soft materials, LC dendrons are the most likely
to form spherical self-assembled aggregates. Such spherical
objects can spontaneously form cubic,16 tetragonal,17 or even
quasicrystalline18 3D structures. Modification by dendrons has
already been applied to introduce self-assembling ability into
functional materials such as light-harvesting antennae,19 full-
erenes,20 and vitamins.21
As dendrons, we synthesized the first-, second-, and third-
generation phenethyl ether-type dendrons G1, G2, and G3
with an amino group at the apex [for details, see the Supporting
Information (SI)]. G1−G3 themselves show a thermotropic
LC hexagonal columnar phase (Colh) upon heating (see
Figures S1−S6 in the SI). The lattice constants (a) of the Colh
phases at 40 °C are 5.4, 5.6, and 6.1 nm for G1, G2, and G3,
respectively. When they are heated, G2 and G3 form a liquid
quasi-crystal (LQC)18 phase via spontaneous formation of
spherical assemblies. The LQC of G2 is made up of spherical
dendron aggregates of 9.8 nm diameter assuming a hard-sphere
model (see the SI). Here we focused on encapsulating a hard
NP core into soft assemblies of G2 and G3 whose size is
compatible with that of the NP. The dendrons are attached as
the outer corona through amidation to the carboxylic groups at
the surface of the inner aliphatic corona encapsulating the NP.
Purpose-designed CO2H-modified monodisperse gold NPs A1,
A2, and A3 (see Figure 2b) were synthesized by the method of
Zheng et al.23 with some modifications using 12-dodecanethiol
(DT) and 16-mercaptohexadecanoic acid (MHA) (see the SI).
The initial DT/MHA molar ratio was fixed at 1/1, 3/2, and 4/1
for the preparation of A1, A2, and A3, respectively, to give dif-
As potential applications of hybrid NP-containing materials1
depend critically on the detailed spatial arrangement, it is desirable
to be able to design a wide range of lattices of NPs. One way to
achieve this is to introduce anisotropy by coating the NPs with
mesogenic ligands.11 Even while staying with cubic or similar high
symmetry, we may still wish to arrange the NPs on lattices other
than body-centered (BCC) or face-centered cubic (FCC).12 Thus,
we set out to place the NPs into the center of a soft dendritic
corona having a controllable radial density profile (RDP). It has
been shown that the RDP is the key determinant of the type of
packing for supramolecular dendrimers.17 Here we used the
Received: October 12, 2011
Published: December 19, 2011
© 2011 American Chemical Society
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ferent degrees of CO2H modification. As shown by the trans-
mission electron microscopy (TEM) images in Figure 1a−c,
weight fractions of G2 in G2/A1, G2/A2, and G2/A3, as
determined by subtracting the thiol component from the total
weight loss, were calculated as 28.2, 24.0, and 8.2%, respec-
tively. The numbers of G2 molecules per A1, A2, and A3 (NG2)
are also listed in Table 1, as are the ratios G2/A1, G2/A2, and
G2/A3 (see the SI). The fact that the NG2/NMHA ratios were
close to 1 means that the amidation reaction with G2 proceeded
almost stoichiometrically. In this regard, the modification amount
of dendron on A was precisely controlled by the present
convergent-like process.
Figure 1d−f shows TEM images of the dendronized NPs G2/
A1, G2/A2, and G2/A3 cast from toluene. In the case of G2/
A2, a well-ordered hexagonal monodomain with an interparticle
distance of 14 nm formed (Figure 1e). The ability to self-
organize seems to have been imparted by the dendritic corona.
In G2/A1, a hexagonal polydomain structure was observed
(Figure 1d), while only a random pattern was seen in G2/A3
(Figure 1f). In the case of G2/A3, there were NG2 = 80
dendrons per A3, which is 3.5 times lower than in G2/A2. Thus,
it would appear that dense grafting of G2 is required to achieve a
regular NP array. Furthermore, in the case of G2/A2, the gap
between the NPs was spontaneously and precisely controlled to
7 nm (surface-to-surface) simply by casting and drying the solution,
each NP being well isolated by the large interparticle distance. To
date, NPs have been modified by various types of organic
molecules, but the gap between the NPs is <3 nm in most cases.25
For the development of NP-based materials such as quantum dots
and magnetic memories,26 not only precise control of large-area
NP arrays but also perfect isolation of individual NPs by large gaps
is required. Thus, the present method involving self-assembly of
bulky dendrons, which gave a 2D hexagonal lattice of gold NPs
having an interparticle distance of 14 nm and gaps of 7 nm
between the surfaces of the NPs, may become a useful technique
for the fabrication of NP-based devices. The effect of dendron
generation on the 2D self-organization was also investigated using
G1/A2 and G3/A2. According to TEM, both systems formed
only random arrays. The average interparticle distances for G1/A2
and G3/A2 were 9.2 and 12.6 nm, respectively. Thus, modification
with G1 and G3 increased the distance between A2 particles
substantially but did not result in ordered arrays.
Next, we report on the 3D self-organization of the den-
dronized G/A NPs. Second-heating differential scanning calori-
metry (DSC) traces for G2/A showed endotherms at 0 °C
(G2/A1) and 9 °C (G2/A2) and a second broad endotherm at
∼220 °C. In the temperature interval bounded by these
transitions, the material is a viscous liquid; below it is rigid, and
above it is fluid and subject to degradation. On the basis of
experience with similar dendron systems, we attribute the lower
endotherm to melting of the alkyl chains and the higher one to
isotropization. No endotherms were seen for G2/A3. Temper-
ature-dependent small-angle X-ray scattering (SAXS) analysis
of G2/A2 gave evidence of ordered nanostructures (see below).
Other G/A systems produced only amorphous structures with
short-range NP order. Only two broad SAXS peaks at ∼10 and
5 nm were observed for G2/A1 between 50 and 200 °C
(Figure S16). G2/A3 showed even less evidence of order: only
scattering due to the NP form factor could be seen (Figure
S17). Thus, the grafting density of G2 on A plays a critical role
in the formation of ordered structures. The effect of dendron
generation on the LC structure was also investigated. Neither
G1/A2 nor G3/A2 showed order (see the SI). Thus, the
general ability to form ordered arrays in the bulk parallels that
on the surface (Figure 1d−f).
Figure 1. (a−f) TEM images of gold NPs: (a) A1; (b) A2; (c) A3; (d)
G2/A1; (e) G2/A2; (f) G2/A3. The scale bar in (a) is common for
(b−f). (g) SAXS pattern of G2/A2 at 150 °C. (h) GI-SAXS profile of
G2/A2 on a Si substrate at 170 °C.
the shapes of the gold cores were close to spherical with narrow
size distributions. The mean sizes and size distributions of the
core parts of A1, A2, and A3 were 5.9 0.6, 6.8 0.7, and
6.8 0.8 nm, respectively. Modification of A1−A3 to obtain
dendron-modified gold NPs G/A was carried out by amidation
reaction (see the SI). Complete removal of free G from
1
G/A was ascertained by H NMR and GPC measurements,
while the formation of amide bonding was observed
by IR spectra (see the SI). As a way of tailoring the corona,
G2/A1, G2/A2, and G2/A3 were prepared in this study.
Furthermore, to investigate the effect of dendron generation
on the ability of A2 to self-organize, G1/A2 and G3/A2 were
also synthesized using G1 and G3 dendrons. The methyl-to-
carboxylic ratio DT/MHA on the surface of A was determined
by 1H NMR analysis after removal of the thiols from A using I2
decomposition.24 The peak at 0.9 ppm was attributed to the
terminal methyl group of DT, and peaks around 1.2 ppm were
assigned to methylene groups of both DT and MHA. By
comparing the integral peak intensities, the DT/MHA
modification molar ratios on the surfaces of A1, A2, and A3
were determined as 10/8.0, 10/3.8, and 10/1.5, respectively
(see the SI). Thermogravimetric analysis (TGA) showed that
the weight losses of A1, A2, and A3 were 9.3, 8.8, and 8.0%,
respectively. These values were attributed to burning of DT and
MHA. The numbers of DT (NDT) and MHA (NMHA) on a gold
NP were calculated as described in the SI using the results of
1H NMR and TGA measurements (Table 1). NMHA could be
Table 1. Numbers and Ratios of DT, MHA, and G2
a
b
b
b
DT/MHA
NDT
NMHA
NG2
NG2/NMHA
A1
A2
A3
10/8.0
10/3.8
10/1.5
290
600
680
240
220
100
230
280
80
1.0
1.3
0.8
a
b
Molar ratio of DT to MHA on the surface of a gold NP A. Average
number of DT, MHA, or G2 molecules on the surface of a single A.
successfully controlled by adjusting the initial DT/MHA ratio.
The weight losses in TGA were 34.9, 30.7, and 15.5% for
G2/A1, G2/A2, and G2/A3, respectively (see the SI). These
losses were due to burning off of DT, MHA, and G2. Thus, the
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G2/A2 exhibited a series of moderately sharp SAXS peaks in a
wide range from room temperature to 200 °C (Figure S12),
indicatiing that several phases, some metastable, coexist.
Therefore the sample was annealed at increasing temperatures.
From room temperature to 130 °C, two relatively broad rings
remained dominant, with their corresponding spacings in a ratio
close to 31/2 (see Figure 4a). We will return to this phase below.
Upon further annealing at 150 °C, a series of sharp Bragg rings
developed, with their corresponding reciprocal spacings in the
ratio 1:21/2:31/2:41/2:51/2:61/2, as shown in Figure 1g (the two
broader rings from the previous phase are still visible). Although
the absence of the seventh Bragg ring suggests the simple cubic
phase (see Figure S12; the arrow shows where that peak would
have been if it had been observed), we cannot rely on this
evidence alone because the form factor approaches zero in this
region and all of the reflections become weak. Therefore, we
tried to obtain an oriented sample in order to establish this
unequivocally (i.e., to eliminate the possibility of BCC, which
would give the same ratio of spacings). Thus, grazing-incidence
SAXS (GI-SAXS) experiments were carried out on a thin film
of G2/A2 on a Si substrate. Good orientation was achieved
after annealing at 170 °C and subsequent cooling (Figure 1h).
The spatial arrangement of the reflections confirmed
unequivocally their indexing as (100), (110), (111), ... of a
simple-cubic (SC) lattice rather than (110), (200), ... of a BCC
lattice. For example, if the first reflection, at the scattering
vector q1, were indexed as (110)BCC rather than (100)SC, then
the distance between the layer lines in the pattern would
have been 21/2q1 rather than the observed q1. With no
To our knowledge, this is the first example of a self-
assembled SC array of spherical NPs, although close-packed
cube-shaped NPs with a thin corona have previously been seen
to pack on a SC lattice.27 It is also the first example of a SC
phase in a liquid crystal, either thermotropic28 or lyotropic.29
We are aware of only one example in block copolymers.30 The
absence of previous examples of SC packing is not surprising in
the case of NPs, as SC packing offers a very low packing
efficiency for spheres. In the case of dendrimers, the observed
prevalence of Pm3m, Pm3n (BCC), and P4 mnm phases has
been interpreted in terms of the RDP of the soft corona,
defined as RDP = A(r), where A is the cross-sectional area of
the dendron as a function of the distance r from its apex. A
given dendrimer tries to find a lattice for which the rate of space
filling by growing nonoverlapping spheres, dV/dr, most closely
matches its RDP. Here, V is the occupied volume and r the
sphere radius. A similar approach was also used to explain the
occurrence of different bicontinuous cubic LC phases.31 In
Figure 3, numerically calculated scaled dV/dr functions are
̅
̅
1
Figure 3. Rates of volume increase dV/dr as functions of radius r for
growth of nonoverlapping spheres arranged on common cubic lattices:
BCC = body-centered, FCC = face-centered, SC = simple-cubic. Hex
denotes the analogous curve for cylinders on 2D hexagonal lattice. The
curves are normalized to the same total volume and 50% of the
occupied volume. Inset: schematic structure of G2/A2.
systematic extinctions, space group Pm3m was selected.
̅
Diffraction intensities were measured (see Table S2) and
used in constructing the 3D electron density map presented in
Figure 2a (for details, including the volume fraction vs density
plotted against r for BCC, FCC, and SC as well as for the case
of growing cylinders on a hexagonal 2D lattice. The curves for
Pm3m and P4 mnm (not shown) are not too dissimilar from
̅
1
that for BCC. While spheres grow freely, dV/dr ∝ r2, but the
curves differ once the spheres clash. The SC curve differs
significantly from the others, as the drop in dV/dr starts
significantly earlier and continues for longer. This is because the
clash occurs at a relatively low occupied volume, whereas the
volume is not completely filled until r almost doubles. This
provides a clue to why G2/A2 chooses SC. As the DT/MHA
ratio is 10/3.8, only one in 3.6 chains of the inner corona
continues to the outer G2 corona. This would produce a sharp
drop in dV/dr for an actual G2/A2 sphere, as depicted sche-
matically in the Figure 3 inset and Figure 2b. Considerably
simplified, one could say that the touching spheres consist of
the gold core and the inner DT/MHA corona, while the den-
dron parts serve to fill the remaining volume, including inter-
stices (see the schematic in Figure 2c).
Figure 2. (a) 3D electron density map of a unit cell of the Pm3m SC
̅
phase reconstructed from the diffraction pattern in Figure 1g (blue,
lowest density; red, highest density). The isoelectron surface delimits
the spherical region of highest density (i.e., gold); organic matter fills
the continuum. (b) Schematic of part of the double corona of a G2/
A2 NP. (c) (110) section through the unit cell.
We now return to the phase that develops on heating and
annealing at 130 °C and is the precursor to the SC phase. Its
SAXS pattern consists of only two relatively broad rings with
spacings of 17.7 and 10.2 nm (i.e., in the ratio 31/2). It is
difficult to be certain about a structure on the basis of such
scarce data, and the one proposed here is rather speculative. We
assume a 2D hexagonal superlattice (HS), with the two reflec-
tions indexed as (10) and (11). One of the two possible elec-
tron density maps reconstructed from these two reflections is
shown in Figure 4c, and the alternative is shown in Figure S14a.
histogram, see the SI); this map shows that the spherical high-
density (i.e., gold) regions are located only at the corners of a
cubic unit cell having a = 12.5 nm. We simulated the diffraction
intensities using a model of solid spheres placed at the corners
of the cubic cell and obtained an excellent fit with experiment
for spheres of diameter 6.25 nm (Table S2), which is also in
good agreement with values from TEM (6.8 nm) and the
69.3% weight retention after the organic component was
burned off (the corresponding 9.5 vol % for Au indicates an Au NP
diameter of 7.0 nm).
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Journal of the American Chemical Society
Communication
Korea funded by the Ministry of Education, Science and
Technology (R31-10013). For help with the synchrotron
experiments we thank Prof. S. Collins and Dr. A. Bombardi
(beamline I16) and Drs. N. Terrill and J. Hiller (I22) at the
Diamond Light Source.
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AUTHOR INFORMATION
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Corresponding Author
kanie@tagen.tohoku.ac.jp; g.ungar@shef.ac.uk
ACKNOWLEDGMENTS
■
This work was financially supported by JSPS [Grants-in-Aid for
Scientific Research (A) 16685018, 19685017, and 22685019],
the METI & NEDO Super Hybrid Materials Development
Project, the European FP7 Project NANOGOLD (NMP3-SL-
2009-228455), and the WCU Program through the NRF of
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