Design, synthesis, and characterization of mesogenic amine-capped nematic gold nanoparticles with surface-enhanced plasmonic resonances

Article abstract of DOI:10.1021/ja300492d

The use of the liquid-crystalline state to control the assembly of large (>5 nm) gold nanoparticles (NPs) is of considerable interest because of the promise of novel metamaterial properties of such systems. Here we report on a new approach for the prepara

Full text of DOI:10.1021/ja300492d

Communication
pubs.acs.org/JACS
Design, Synthesis, and Characterization of Mesogenic Amine-Capped
Nematic Gold Nanoparticles with Surface-Enhanced Plasmonic
Resonances
Chih H. Yu, Christopher P. J. Schubert, Chris Welch, Bai J. Tang, M.-Gabriela Tamba,
and Georg H. Mehl*
Department of Chemistry, University of Hull, Cottingham Road, Hull HU6 7RX, United Kingdom
S
* Supporting Information
thin films, increase significantly. Moreover, because of the small
ABSTRACT: The use of the liquid-crystalline state to
control the assembly of large (>5 nm) gold nanoparticles
(NPs) is of considerable interest because of the promise of
novel metamaterial properties of such systems. Here we
report on a new approach for the preparation of large
nematic gold NPs using a bifunctional capping agent that
enables control over the particle size and serves as a
linkage for subsequent functionalization with mesogenic
groups. Properties of the NPs were characterized by
HRTEM, NMR, DSC, TGA, UV/vis, OPM, and XRD
studies. The results confirmed the formation of a stable
nematic mesophase above 37.5 °C for NPs in the 6−11
nm size range.
size of the particles (typically less than 3 nm), they may exhibit
various morphological structures other than the typically most
stable face-centered-cubic (fcc) structure that would be
expected under thermodynamic control.17
Thus, it is an important challenge to prepare Au NPs with
sizes larger than 6 nm, where plasmonic effects play a strong
role, and to design an organic corona that promotes self-
assembly and allows for the modulation of the viscosities of the
target materials.
To address these issues, a novel two-step approach was
pursued (Figure 1). In the first step, NPs capped with organic
old nanoparticles (NPs) functionalized with mesogenic
G
groups1−5 have recently attracted considerable attention
as potential candidates for metamaterials with tuned optic-
electric behavior, enhanced plasma-splitting properties, and
electrically controlled light-scattering properties.6−10 The
physical and optical properties of the NPs are strongly affected
by the size and shape of the particles and by the chemical
structure of the attached organic ligands. Moreover, the design
of the organic corona affects the self-assembly, and thus,
chemical control of the composition of the particles is critical.
This is particularly important because small gold particles in the
1−2 nm size range are known to exhibit quantum size effects11
and only larger gold particles (>6 nm) show strong surface
plasmatic properties critical for metamaterial behavior.12,13 For
relatively small particles in the 1.4−3 nm range that are capped
with a corona of thiol-bonded hydrocarbon groups terminated
with mesogenic groups, formation of a liquid crystal (LC)
phase has been reported, and 2D and 3D self-assembly of the
Au NPs has been detected.1,7,8c In most cases, mesomorphic
behavior has been introduced in polymer-like exchange
reactions with molecules promoting mesomorphic phase
behavior after the synthesis of the NPs, following the classical
Brust−Schiffrin method or using Murray’s exchange li-
gands.3,4,8,14−16
Figure 1. (top) Structures of dec-9-enylamine-functionalized organic-
coated Au NPs (1), the mesogen (2), and mesogen-coated Au NPs
(3). (bottom) NMR spectra of (a) dec-9-enylamine-functionalized Au
NPs, (b) the mesogen, and (c) nematic Au NPs.
chains bearing reactive end groups were prepared. In the
second step, reactions of these end groups to introduce groups
that lower the viscosity and induce mesomorphic behavior were
carried out. As a capping agent, amine functionalities were used,
as it was found that the classical approach for NPs (<6 nm)
using thiol groups as capping agents did not yield acceptable
results. In initial experiments, the thiol groups were found to
However, with increasing size of the particles and the
concomitant increase in the size of the ensuing organic corona,
it was found that the control of mesomorphic properties
becomes more difficult and that the viscosities of the materials,
which determine to some extent their technological potential in
Received: February 2, 2012
Published: March 5, 2012
© 2012 American Chemical Society
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Journal of the American Chemical Society
Communication
interfere with reactions of the organic corona involving
transition-metal catalysts. Since the successful synthesis of
larger particles using primary amines as ligands has been
reported previously, this route was pursued.18,19 To modulate
the melting behavior, a tetramethyldisiloxane unit, a group
known to lower thermal transition temperatures, was
introduced into the hydrocarbon chain. Laterally connected
mesogens were used to induce LC behavior in the Au NPs.20
For the synthesis of larger NPs (>6 nm), a water/oil
microemulsion technique was developed. This method was
adapted from earlier work on the synthesis of iron oxide
particles.21,22 To obtain Au NPs in the 6−11 nm size range, it
was found that the optimal molar ratio of water to
tetraoctylammonium bromide (TOAB, used as a surfactant)
was 18.4 [for details, see the Supporting Information (SI)].25
Dec-9-enylamine was prepared and used as a digestive-ripening
capping agent bearing a terminal alkene group for post-NP-
synthesis functionalization.23
as the solvent. The chemical structure of the organic corona
surrounding the NP 3 was confirmed by 1H NMR spectroscopy
(Figure 1c). The signals relating to the alkene and silane groups
were no longer present in the spectrum [peaks at δ 5.7−5.8
(−CH), δ 4.85−5.0 (−CH2), and δ 4.65−4.68 (−SiH)]. The
attachment of the amine function to the gold surface was
evidenced by the absence of the signals of the methylene group
connected to the amine function at δ 2.65−2.85 (−CH2−
NH2). TLC and GPC experiments confirmed the high purity of
the material.
High-resolution transmission electron microscopy
(HRTEM) (Figure 2a,c; also see the SI) confirmed that the
1
The H NMR spectrum of the NPs (Figure 1a) confirmed
that all of the impurities (e.g., TOAB, NaBH4, and dodecyl-
amine) were removed by washing using ethanol or acetone. In
addition, the alkene group in the functionalized Au NPs can be
1
clearly observed from the H NMR signals at δ 5.70−5.80 (m,
1
1H, CH) and 4.85−5.00 (m, 2H, CH2). Moreover, the H
NMR spectrum of the NPs (Figure 1a) confirmed the chemical
composition of the material and the attachment of the amine
group to the gold surface, as evidenced by the absence of the
signal for the protons adjacent to the amine group. The
accuracy of these measurements was tested in additional
experiments by adding dec-9-enylamine (5% w/w) to the
purified capped NPs, and here a signal for the protons in the
methylene group adjacent to the amine functionality could be
detected. This shows that in the purified NPs no free amine
groups were present, and this result was confirmed by thin-layer
chromatography (TLC) and gel-permeation chromatography
(GPC) experiments (see the SI).
Figure 2. (a) TEM image of nematic Au NPs. (b) UV/vis spectra of
nematic Au NPs with SPRs at ∼520 nm before (blue) and after (red)
size exclusion. (c) Histogram depicting the size distribution of the
particles. (d) OPM texture of nematic Au NPs at 60 °C (×100).
As a surface functionalization reaction method, the hydro-
silylation of terminal olefin groups mediated by Karstedt’s
catalyst was selected. In successful tests with pentamethyldisi-
loxane, complete functionalization of the organic surface was
observed, as evidenced by the disappearance of the signals for
the terminal olefin groups at δ 5.7−5.8 (−CH) and 4.85−5.0
(−CH2) and the appearance of signals at δ 0.3 (−CH2) and
0.15 from the five methyl groups of the pentamethyldisiloxane
group. Subsequently the organic mesogenic group 2 bearing a
reactive silane group (Figure 1) was selected as a suitable LC
moiety. This particular group was selected on the basis of its
structural similarity with related mesomorphic Au NP systems
previously reported.8 The use of a siloxane functionality
permitted attachment to the organic surface of the NPs while
decreasing the viscosity and melting point of the system.20
The rodlike mesogen 2 contains four aromatic rings with
eight methylene groups as flexible chains and melts at 97.7 °C
into a nematic phase before clearing to an isotropic liquid at
115.7 °C. This nematic-to-isotropic transition is associated with
an enthalpy of 0.59 J g−1, a somewhat low value for a low-
molar-mass calamitic nematic system that is attributed to the
presence of the long lateral chain bearing a siloxane group.
Following the hydrosilylation reaction using Karstedt’s catalyst
in dry toluene at room temperature, removal of the excess LC
group monomer was carried out, initially by intensive washing
and subsequently by size-exclusion chromatography (SEC)
with Biobeads SX-1 as the stationary phase and tetrahydrofuran
size of the Au NPs ranged from 6 to 18 nm (average size 9.98
2.2 nm) (see the SI). As shown in the histogram of the size
distributions of the NPs (Figure 2c), the majority of the NPs
have sizes of 6−11 nm, although larger particles are found as
well, shifting the average size to a value close to 10 nm. The
lattice-fringe spacings of the Au NPs were confirmed as dhkl
=
0.235 nm {111} and 0.204 nm {200} with 55° crossover
(Figure 2a inset). The image shows the {111} planes of an fcc
structure of the Au NP, and this is the most stable facet. It is
likely that the geometry exhibits truncated icosahedral or
decahedral NPs.8,24 The size and structure of the material
before coating with mesogen was also confirmed by powder X-
ray diffraction (XRD).25
The Au NPs show intensive surface plasmon resonances
(SPRs), as confirmed by UV/vis spectrometry, which showed a
maximum at ∼520 nm with only a single peak, a feature typical
for spherical Au NPs (Figure 2b).27 The spectra in Figure 2b
show the UV/vis adsorption before (blue line) and after (red
line) purification by SEC. The two spectra exhibit similar
positions of the SPR, indicating, according to Mie theory,26 that
the purification process removed the low-molar-mass impurities
without fundamentally altering the structure of the NPs.
The LC properties of 3 were investigated by optical
polarization microscopy (OPM) and differential scanning
calorimetry (DSC), and the results are shown in Table 1.
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Communication
Table 1. Transition Temperatures (Ttrans) and Enthalpies
(ΔHtrans) Determined by DSC
compound transition Ttrans/°C transition Ttrans/°C (ΔHtrans/J g−1
ASSOCIATED CONTENT
* Supporting Information
Synthesis procedures for 1−3 and TEM, DSC, GPC, NMR,
and XRD characterizations. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
a
S
)
2
3
Cr → N
Cr → N
97.7
37.5
N → Iso
N → Iso
115.7 (0.59)
94.5 (2.76)
a
The scan rate was 10 °C min−1 in the heating cycle. Abbreviations:
AUTHOR INFORMATION
Corresponding Author
g.h.mehl@hull.ac.uk
■
Cr, crystalline phase; N, nematic LC phase; Iso, isotropic phase.
DSC experiments revealed a melting point of 37.5 °C in freshly
prepared samples. The material cleared from an LC phase to an
isotropic phase at 94.5 °C, a transition associated with an
enthalpy change of 2.76 J g−1. Relative to the monomeric
material (mesogen 2 in Figure 1), the isotropization temper-
ature was reduced by 21.2 °C; however, as the melting point
was reduced by 60.3 °C, the LC range increased by ∼39.1 °C. A
decrease in the transition temperature has been observed
previously in Au NPs functionalized with laterally connected
LC groups as well as in other LC NPs and can be attributed to
a destabilization of the LC phase due to the presence of the
central nondeformable NP core, which reduces the orienta-
tional mobility of the mesogens, differentiating the NPs from
LC dendrimers.7a,8,10 The lowering of the melting point is
attributed to the increased spacer length relative to earlier
results but is mainly due to the inclusion of the disiloxane group
in the middle of the spacer unit separating the NP and the
mesogens. OPM studies confirmed the nematic structure of the
mesophase; a typical OPM micrograph showing the formation
of a coarse-grained gray schlieren-type texture is presented in
Figure 2d. In comparison with other reported materials,
compound 3 is characterized by the rapid formation of a
defect texture, indicative of a relatively low viscosity, and
shearing of the sample by moving the microscopy coverslip is
easily possible. Preliminary XRD experiments on nonoriented
samples in the nematic phase were carried out. It should be
noted that aligning the samples by shearing or the use of
capillary force was not possible. Weak reflections at 2Θ = 3.32°,
17.62° (broad), and 18.89° (broad), equivalent to d = 2.66,
0.507, and 0.470 nm, respectively, were detected (see the SI).
The wide-angle reflections can be associated with the liquidlike
ordering of the hydrocarbon and siloxane chains and the
mesogenic groups. The reflection at d = 2.66 nm can be
associated with the longitudinal dimension of the aromatic core
of the mesogens and is close to the d value of 2.80 nm detected
for 2 (see the SI). Though an increase in the diffracted
scattering intensities at smaller angles that could not be
attributed to background scattering was detected, no indication
of the formation of long-range positional ordering of the NPs
was found. This increase of the scattering intensity is likely to
be associated with local short-range ordering of the particles
assumed to be distributed in the nematic organic matrix.28
In summary, we have designed and prepared mesogenic-
group-coated Au NPs in which the organic ligands are linked to
the NPs via amine groups. A novel two-step approach was used.
First, Au NPs were prepared by a digestive ripening method,
using a microemulsion of appropriate size for plasmonic
properties, and then capped with end-group-functionalized
ligands. The mesomorphic group was introduced in the second
step by a reaction of the organic surface. The use of a long
spacer that includes a siloxane group helped to lower the
viscosity. The resulting materials are thermally and chemically
stable and show nematic phase behavior close to room
temperature.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This research was supported by the EU under Grant 228455.
■
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