PEG-nanotube liquid crystals as templates for construction of surfactant-free gold nanorods

Article abstract of DOI:10.1039/c8cc02013b

Lyotropic liquid crystals, in which nanotubes coated with polyethylene glycol were aligned side-by-side in aqueous dispersions, acted as templates for the construction of surfactant-free gold nanorods with controllable diameters, functionalizable surfaces

Full text of DOI:10.1039/c8cc02013b

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
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PEG-nanotube liquid crystals as templates for
construction of surfactant-free gold nanorods
Cite this: DOI: 10.1039/x0xx00000x
Naohiro Kametaa* and Hidenobu Shiroishib
Received 00th March 2018,
Accepted 00th March 2018
DOI: 10.1039/x0xx00000x
www.rsc.org/
As previously reported,7 selfꢀassembly of glycolipidꢀbased
amphiphiles with a hydrophilic ꢀglucose headgroup at one end and
Lyotropic liquid crystals, in which nanotubes coated with
polyethylene glycol were aligned side-by-side in aqueous
dispersions, acted as templates for the construction of
surfactant-free gold nanorods with controllable diameters,
functionalizable surfaces, and tunable optical properties.
D
a different hydrophilic headgroup at the other end produces
molecular monolayer nanotubes. In this study, we synthesized
amphiphiles 1(n) and PEG derivatives 2(n), where n = 2, 6, and 10
(Fig. 1a), and carried out binary selfꢀassembly of 1(n) and 2(n) with
identical n values by refluxing stoichiometric mixtures of 1(n) and
2(n) in pure water and then gradually cooling the reaction mixtures
to room temperature (see ESI for experimental details).
Transmission electron microscopy (TEM) revealed that this process
Gold nanorods (GNRs) are attractive materials for basic research and
technological applications because of the unique optical properties
that arise from their morphological anisotropy. For example, the
localized surface plasmon resonance (LSPR) and photothermal
properties exhibited by GNRs are widely utilized for sensing and
imaging, drug delivery, photothermal therapy, catalysis, and solar
cells.1 GNRs are generally produced by chemical reduction of gold
ions in surfactant solutions. The surfactants are indispensable for
achieving oneꢀdimensional growth of the crystal nucleus; in the
absence of surfactants, morphologically isotropic spherical and
plateꢀlike structures are obtained.2 However, GNRs produced in
surfactants are covered with dense layers of surfactant molecules,
which often are highly cytotoxic and which prevent chemical
modification of the GNRs.3
Various alternatives to surfactants have been investigated as
templates for the construction of metal/inorganic nanostructures,
including GNRs. Previously reported alternatives include
nanoporous materials such as polycarbonate and alumina
membranes,4 as well as organic nanotubes5 formed by selfꢀassembly
of amphiphilic molecules.6 However, such template methods present
some problems. For example, the GNRs obtained by template
methods tend to have structural defects resulting from the low
efficiencies with which the starting agents are loaded into the
nanochannels of the templates; and the isolated yields of GNRs are
low because the templates are difficult to remove and separate.
Herein we describe the use of lyotropic liquid crystals, consisting
of organic nanotubes coated with hydrated polyethylene glycol
(PEG) and aligned sideꢀbyꢀside, as templates to construct surfactantꢀ
free GNRs. This template method enabled us not only to tune the
diameters and LSPR properties of the GNRs but also to densely
functionalize the GNR surface with thiol–crown ether compounds.
The thiolꢀfunctionalized GNRs exhibited selective ion recognition
and chiral molecular recognition.
Fig. 1 (a) Schematic representation of PEG nanotubes formed by binary selfꢀ
assembly of 1(n) and 2(n) and subsequent thermal dehydration and
rehydration of the exterior PEG chains. (b, c) Photographs of NTꢀLCs
composed of 1(2) and 2(2) obtained without (b) and with (c) a cross polarizer.
(d) SEM and (e) TEM images of sideꢀbyꢀside aligned nanotubes in dryꢀstate
NTꢀLCs.
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DOI: 10.1039/C8CC02013B
gave only nanotubes, whereas selfꢀassembly of singleꢀcomponent scattering (Fig. 1b). Birefringence was also observed under a cross
dispersions of 2(2), 2(6), and 2(10) formed micelles, nanofibers, and polarizer (Fig. 1c), indicating formation of lyotropic liquid
sheets, respectively (Fig. S1, ESI). The differential scanning crystals.7,8 Scanning electron microscopy (SEM) and TEM of the
calorimetry profile of each of the three types of nanotubes showed liquid crystals in the dry state showed sideꢀbyꢀside alignment of the
an endothermic peak corresponding to an isotropic melting point that nanotubes (Fig. 1d,e). In contrast, the nanotubes were found to be
was different from the melting points of the corresponding oriented randomly in the clear aqueous dispersions obtained at
assemblies of 1(n) alone or 2(n) alone (Fig. S2, ESI). These results concentrations of <0.2 wt%, and networked structures of nanotubes
confirmed that the nanotubes were composed of 1(n) and 2(n).
were observed in the hydrogels that were obtained at concentrations
Powder Xꢀray diffraction and infrared spectroscopy revealed that of >5 wt%. The nanotube liquid crystals (hereafter abbreviated as
the nanotubes consisted of a monolayer membrane in which 1(n) and NTꢀLCs) are assignable to a nematic type.
2(n) were packed parallel to each other, as indicated by the presence
The NTꢀLCs were stable at room temperature, but they broke
of intermolecular hydrogen bonding between the amide moieties, as apart at elevated temperatures; and, as a result, the nanotubes formed
well as by hydrophobic and van der Waals interactions between the aggregates and precipitated from the aqueous solutions. This
oligomethylene chains (Fig. S3, ESI). The thicknesses of the phenomenon is ascribable to dehydration of the PEG chains on the
membrane walls of the nanotubes were estimated from TEM images
outer surface of the nanotubes, as indicated by changes in the C−C
to be approximately 5−7 nm (Fig. S1, ESI), a range that is bonds from the gauche conformation at low temperatures to the anti
comparable to the extended molecular lengths (L) of 2(n): 4.82, 5.78, conformation at high temperatures.9 Lowering the temperature
and 6.74 nm for n = 2, 6, and 10, respectively. The inner diameters resulted in rehydration of the PEG chains. The dehydration and
(i.d.) of the nanotubes were uniform and increased with increasing L: rehydration temperatures (T–H2O and T+H2O) of the PEG chains were
i.d. = 18, 23, and 31 nm for n = 2, 6, and 10, respectively. For
nanotubes with monolayer membranes composed of asymmetric
amphiphiles, the i.d. can sometimes be calculated using the
following equation: i.d. = 2asL/(al − as), where as and al are the
crossꢀsectional areas of the small and large headgroups,
respectively.7 That is, i.d. is proportional to L if as and al are constant.
Elongation of the oligomethylene chain theoretically increases the
i.d. by 6 nm for every eight methylene units. Although the
experimentally observed increment in the i.d. values in this study
was more irregular than predicted by the aboveꢀmentioned equation,
the trend we observed was in accordance with the equation. The
parallel molecular packing of 1(n) and 2(n) induced curvature of the
55−60 and 40−45 °C, respectively, as determined by means of
turbidity measurements (Fig. S6, ESI). The
appearance/disappearance cycle for the NTꢀLCs—that is, the
dispersion/aggregation cycle of the nanotubes that occurred as a
result of thermal dehydration/rehydration of the PEG chains—could
be repeated several times. This unique behavior was useful for
separation of these nanotube templates from GNRs prepared as
described in the next paragraph.
The NTꢀLCs were used as templates for the production of
surfactantꢀfree GNRs (see ESI for experimental details). First, we
added 3.5ꢀnm gold seed particles10 to the NTꢀLCs (Fig. 2a). TEM
clearly showed oneꢀdimensional alignment of the seed particles on
the surfaces of the sideꢀbyꢀside aligned nanotubes (Fig. 3a). The seed
particles did not become encapsulated in the nanotube channels
under these conditions, because encapsulation would have required
capillary action or some specific interaction between the seed
particles and the nanotube channels.6b We expected that the seed
particles would be trapped in grooves between the nanotubes
comprising the NTꢀLCs (Fig. 2b), and in fact seed particles were
observed only in the grooves and not around isolated nanotubes or at
the ends of the aligned nanotubes (Fig. 3a). Second, we treated the
monolayer membrane, owing to the size difference between the Dꢀ
glucose headgroup (or the PEG moiety) and the hydroxyl headgroup,
the former two being larger than the latter. The large headgroups and
the small headgroups must be located on the outer and inner surfaces
of the nanotubes, respectively (Fig. 1a).
The nanotubes prepared as described above had lengths of
several micrometers. Sonication of the nanotubes shortened their
lengths and enhanced their dispersibility in water. The length
distributions of the shortened nanotubes were estimated from TEM
images, and the peaks of the distributions were around 400−600 nm NTꢀLCs with a growth solution containing HAuCl4 and ascorbic
(Fig. S4, ESI). The visual appearance of aqueous dispersions of the acid as a weak reducing agent, and the result was the formation of
nanotubes was remarkably influenced by nanotube concentration GNRs with uniform diameters (Figs. 2c,d and 3b). Growth solutions
(Fig. S5, ESI). When the concentration was <0.2 wt%, the usually include surfactants such as cetyltrimetylammonium bromide
dispersions were clear, whereas when the concentration was in the
0.2−5 wt% range, the dispersions were bluish, owing to Rayleigh
Fig. 3 (a) TEM image of oneꢀdimensional alignments of gold seed particles
trapped in the grooves between sideꢀbyꢀside aligned nanotubes in a NTꢀLC
template composed of 1(2) and 2(2). (b) TEM image of GNRs grown from the
gold seed particles on the NTꢀLC template.
Fig. 2 Production of GNRs using NTꢀLC templates.
2 | J. Name., 2012, 00, 1-3
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(CTAB), which are required to obtain GNRs with anisotropic heating of the entire system at temperatures above T–H2O (=
morphology and controllable diameters. However, in the present
system, the monolayer membrane walls of the nanotubes assumed
the role played by CTAB micelles or bilayer membranes. We found
that when nanotube hydrogels consisting of networked structures of
randomly oriented nanotubes were used in this procedure in place of
the NTꢀLCs, we obtained gold nanoparticles instead of GNRs (Fig.
S7, ESI); and nanoparticles also formed in the absence of the
nanotube hydrogels and in the absence of CTAB (data not shown).
These results suggest that the grooves formed by the sideꢀbyꢀside
aligned nanotubes in the NTꢀLCs were necessary not only for
entrapment of the seed particles but also for growth of the entrapped
particles into GNRs.
When thiolꢀcrown ether compounds were present in the system,
thermal dehydration of the PEG chains on the outer surface of the
nanotubes induced nanotube aggregation and spontaneous release of
thiolꢀfunctionalized GNRs (Fig. 2e–g). We found that the
photothermal effect induced by nearꢀinfrared irradiation (NdꢀYAG
laser, 1064 nm, 0.7 W) was more effective than general direct
55−60 °C). The use of nearꢀinfrared irradiation led to complete
recovery of the GNRs, whereas direct heating left almost all of the
GNRs in the NTꢀLC templates (Fig. S8, ESI). The temperature
during nearꢀinfrared irradiation was estimated to be 42 °C, which
was lower than T–H2O. We attributed the initiation of PEG chain
dehydration to localized heating via a photothermal effect of the
GNRs, which were in close contact with the nanotubes, because we
confirmed that irradiation of the NTꢀLCs under the same conditions
but in the absence of the GNRs failed to induce NTꢀLC destruction
and nanotube aggregation.
The diameters and lengths of the isolated thiolꢀfunctionalized
GNRs were estimated from SEM and TEM images (Fig. 4a; Fig. S9,
ESI) and were plotted against the outer diameters (i.d. + membrane
thickness) of the nanotubes in the NTꢀLC templates (Fig. 4b,c).
GNR diameter was found to be proportional to nanotube outer
diameter, a result that supports our conclusion that GNR growth
occurred only in the grooves between the aligned nanotubes in the
NTꢀLC templates (Fig. 3d). The GNR diameters were reasonably
consistent with the size of the grooves, which increased with
increasing nanotube outer diameter (Fig. 4e). In contrast, GNR
length was independent of nanotube outer diameter (Fig. 4c) and
was also unrelated to nanotube length (400−600 nm, Fig. S4, ESI).
Absorption spectroscopy revealed that the GNRs exhibited LSPR
bands in the visible and nearꢀinfrared regions (Fig. 4d). The
absorption intensity in the visible region and the maximum
absorption wavelength in the nearꢀinfrared region reflected the
aspect ratio (= length/diameter) of the GNRs. Our results were
consistent with the generally observed trend that higher aspect ratios
result in a less intense absorbance in the visible region and a longer
maximum absorption wavelength.11 The present template method
enabled us to produce GNRs with tunable optical features.
After removal of the NTꢀLC templates by nearꢀinfrared
irradiation, the GNRs functionalized with thiolꢀ12ꢀcrownꢀ4 ether
compound 3 or thiolꢀ15ꢀcrownꢀ5 ether compound 4 (hereafter
abbreviated as 12crown4ꢀGNRs and 15crown5ꢀGNRs) were
Fig. 5 (a) Absorption spectra of 12crown4ꢀGNRs ([3] = 7.2 × 10−4 M) in water
in the absence and presence of 1.4 × 10−6 M Li+ (pink), Na+ (blue), and K+
(yellow). Inset: Plot of absorbance at 970 nm versus initial ion concentration.
The 12crown4ꢀGNRs were prepared by using a NTꢀLC template composed of
1(10) and 2(10). The diameters and lengths of the 12crown4ꢀGNRs are
shown in blue in Fig. 4b,c. (b, c) Plots of absorbance at 970 nm of 15crown5ꢀ
GNRs ([4] = 6.5 × 10−4 M) versus initial concentration of Dꢀ and Lꢀalaninols.
The 15crown5ꢀGNRs in panel (b) were prepared by using a NTꢀLC template
composed of 1(10) with a Dꢀglucose headgroup and 2(10), and the GNRs in
panel (c) were prepared by using a template composed of an enantiomer
(with an Lꢀglucose headgroup) of 1(10) and 2(10).
Fig.
4 (a) SEM image of thiolꢀfunctionalized GNRs (12crown4ꢀGNRs)
separated from a NTꢀLC template composed of 1(2) and 2(2). (b, c) Plots of
12crown4ꢀGNR diameters (b) and lengths (c) against the outer diameters of
the nanotubes in the NTꢀLC templates. Solid symbols indicate experimental
values estimated from SEM and TEM images, and open symbols indicate
values calculated by using the equation in panel (e). (d) Absorption spectra of
aqueous dispersions of 12crown4ꢀGNRs; the colors correspond to the colors
in panels (b) and (c). (e) Schematic representation of the relationship
between GNR diameter and nanotube diameter.
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VJieowuArrtnicalelONnlainme e
DOI: 10.1039/C8CC02013B
investigated for molecular recognition of alkaline metal ions and
chiral amines, respectively. Upon addition of Li ions to an aqueous
dispersion of 12crown4ꢀGNRs led to a decrease in the intensities of
the LSPR bands but no shift in the peak wavelengths (Fig. 5a). The
lack of peak shifts indicates that the 12crown4ꢀGNRs did not
undergo aggregation. The decrease in peak intensity is attributable to
a 1:1 host–guest interaction between the Li ions and the 12ꢀcrownꢀ4
ether moieties on the surface of the 12crown4ꢀGNRs. In contrast, the
LSPR bands showed no response to the addition of Na or K ions,
despite the fact that the formation of sandwichꢀtype 2:1 host–guest
complexes between 12ꢀcrownꢀ4 ether and Na and K ions is known to
occur. GNRs functionalized with 3 by means of a ligandꢀexchange
reaction of CTABꢀcoated GNRs are reported to form aggregates
with Na and K ions by means of a sandwichꢀtype 2:1 host–guest
interaction.12 The Liꢀion selectivity observed for the present
12crown4ꢀGNRs was attributed to the direct functionalization of the
surfactantꢀfree GNRs with 3. The 12ꢀcrownꢀ4 ether moieties densely
functionalized on the 12crown4ꢀGNRs prohibited sandwichꢀtype 2:1
host–guest interactions with Na and K ions (Fig. S10, ESI).
Eꢀmail: nꢀkameta@aist.go.jp; Fax: +81ꢀ29ꢀ861ꢀ4545; Tel: +81ꢀ29ꢀ861ꢀ
4478
b
National Institute of Technology, Tokyo College, Kunugida 1220ꢀ2,
Hachiouji, Tokyo, 193ꢀ0997, Japan.
† Electronic Supplementary Information (ESI) available: Synthesis of
1(n), 2(n), 3 and 4; selfꢀassembly procedure; TEM, molecular packing
analysis, and length distributions of the nanotubes; pictures of nanotube
hydrogels; determination of T–H2O and T+H2O; production procedure of
gold seed particles and GNRs; TEM images of GNRs and gold
nanoparticles; release profiles of GNRs from NTꢀLCs; schematic images
of molecular recognition; circular dichroism spectra of GNRs. See
DOI: 10.1039/c000000x/
1
2
3
(a) N. D. Burrows, W. Lin, J. G. Hinman, J. M. Dennison, A. M.
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Dꢀ or Lꢀalaninol to an aqueous dispersion of
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,
15crown5ꢀGNRs led to decreases in the intensities of the LSPR
bands but no shifts in the peak wavelengths (Fig. 5b). The decrease
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Dꢀalaninol than for Lꢀalaninol, indicating that the
15crown5ꢀGNRs exhibited chiral recognition ability. In contrast,
15crown5ꢀGNRs, which are prepared from a NTꢀLC template
(a) J. G. Hinman, A. J. Stork, J. A. Varnell, A. A. Gewirth, C. J.
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containing an enantiomer (with a
Lꢀglucose headgroup) of 1(n),
showed higher affinity for ꢀalaninol (Fig. 5c). To obtain chiral
L
information of the GNRs, we conjugated pyrene acting as a VIS
absorption marker to the two 15crown5ꢀGNRs. Both GNRs each
other have positive/negative symmetrical Cotton bands at the
absorption wavelength of the pyrene moieties anchored to the most
outer surface through the alkyl chain and crown ether, even though
the pyreneꢀ15crown5 itself is achiral (Fig. S11, ESI). This indicates
that the supramolecular chirality of the NTꢀLC templates is
conferred and transcribed to the surface of the GNRs. The induction
of chirality in polymers encapsulated in nanotube channels and
nanoparticles adsorbed on the nanotube outer surface with
supramolecular chirality has previously been reported.7, 13 However,
the present study is the first to indicate that grooves shaped by the
outer surfaces of sideꢀbyꢀside aligned nanotubes can act as chirality
inducers and, furthermore, that the induced chirality of 15crown5ꢀ
GNRs generated in the grooves was retained after separation of the
GNRs from the NTꢀLC templates.
In conclusion, we constructed surfactantꢀfree GNRs with
controllable diameters and tunable optical properties by using NTꢀ
LCs as templates. GNRs densely functionalized with thiol–crown
ethers exhibited highly selective ion recognition and chiral
recognition abilities. The present method is superior not only to
previously reported template methods using nanochannels of
nanoporous materials and nanotubes but also to conventional ligandꢀ
exchange methods involving GNRs precoated with surfactants, in
terms of the dimensional control, isolation, and functionalization
efficiency. Precise design of NTꢀLC templates can be expected to
facilitate the use of GNRs for various applications, including as
sensors, drugs, and catalysts.
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Graphical Table of Contents
PEG-nanotube liquid crystals as templates for construction of
surfactant-free gold nanorods
Naohiro Kameta,* and Hidenobu Shiroishi
Nanogrooves shaped by side-by-side alignment of PEG nanotubes in water precisely
assisted growth of gold seeds to nanorods without surfactants.