Chirality-controlled syntheses of double-helical Au nanowires

Article abstract of DOI:10.1021/jacs.8b00910

The selective large-scale syntheses of noble metal nanocrystals with complex shapes using wet-chemical approaches remain exciting challenges. Here we report the chirality-controllable syntheses of double-helical Au nanowires (NWs) using chiral soft-templates composed of two organogelators with their own active functions; one organogelator serves to introduce helicity into the template and the other acts as a capping agent to control the Au shape. One-dimensional twisted-nanoribbon templates are prepared simply by mixing the two organogelators in water containing a small amount of toluene, followed by the addition of LiCl; template chirality is controlled through the selection of the handedness of the helicity-inducing organogelator. Double-helical Au NWs synthesized on these chiral templates have the same helical structure as the template because the Au NWs grow along both edges of the twisted nanoribbons with right- or left-handed helicities. Dispersions of the right- and left-handed double-helical Au NWs exhibit opposite CD signals.

Full text of DOI:10.1021/jacs.8b00910

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Chirality-Controlled Syntheses of Double-Helical Au Nanowires
Makoto Nakagawa, and Takeshi Kawai
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b00910 • Publication Date (Web): 03 Apr 2018
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Chirality-Controlled Syntheses of Double-Helical Au
Nanowires
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Makoto Nakagawa and Takeshi Kawai*
Department of Industrial Chemistry, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601,
Japan
Supporting Information Placeholder
straightforward for the fabrication of nano-objects and afford
ABSTRACT: The selective large-scale syntheses of noble
complicated chiral nanostructures with high precision, alt-
hough there is significant room for improvement in produc-
tivity. The other is a chemical-production process that uses
chiral nanostructures induced by chiral organic molecules.5–10
This process can be applied to wet chemical syntheses using
chiral templates such as the organic biomolecular scaffolds of
DNA,5 peptides,6 and self-assembled amphiphiles.7 This ap-
proach has produced helical arrays of achiral metal nanopar-
ticles (NPs) deposited on chiral templates that exhibit strong
chiroptical activities.7d For example, Rosi and colleagues6a
fabricated a helical array of spherical Au NPs using left- and
right-handed helical molecular aggregates composed of L-
and D-peptide conjugates as templates. Although wet chemi-
cal synthesis is advantageous for preparing metal nano-
objects on the large scale, there are very few reports of the
fabrication of chiral metallic nano-objects by wet chemical
methods10 because most of the reported chiral templates do
not introduce shape-controlled functionality to the metal
NPs themselves. Consequently, the nano-objects produced
using these approaches are essentially chiral arrays of achiral
metal NPs.5-8
metal nano-crystals with complex shapes using wet-chemical
approaches remain exciting challenges. Here we report the
chirality-controllable syntheses of double-helical Au nan-
owires (NWs) using chiral soft-templates composed of two
organogelators with their own active functions; one organo-
gelator serves to introduce helicity into the template and the
other acts as a capping agent to control the Au shape. One-
dimensional twisted-nanoribbon templates are prepared
simply by mixing the two organogelators in water containing
a small amount of toluene, followed by the addition of LiCl;
template chirality is controlled through the selection of the
handedness of the helicity-inducing organogelator. Double-
helical Au NWs synthesized on these chiral templates have
the same helical structure as the template because the Au
NWs grow along both edges of the twisted nanoribbons with
right- or left-handed helicities. Dispersions of the right- and
left-handed double-helical Au NWs exhibit opposite CD sig-
nals.
Molecular chirality is a major topic in chemistry and biology
because chirality is a key molecular-recognition concept.1 In
addition to organic chiral molecules, chiral metallic nano-
materials have recently attracted significant attention due to
their unique chiroptical properties, such as large optical ac-
tivities2 and negative refractions3 induced by plasmonic chi-
rality. The key to realizing the desired chiroptical properties
relies critically on the technique used to fabricate the chiral
metallic nano-object.
To fabricate chiral-shaped metal nanocrystals using chiral
templates through wet chemical synthesis, the templates are
required to possess two functional moieties; the first induces
chirality into the template and the second controls the
growth direction of the metal nanocrystals along the chiral
template. The introduction of both functions into a single
compound through molecular design and synthesis is chal-
lenging. Accordingly, we introduce a role-sharing strategy in
this report; a template of chiral Au nanocrystals was con-
structed using two compounds; each compound provides the
individual functions required for introducing chirality into
the template and producing shape-controlled Au nanocrys-
tals.
Imparting chirality to the metallic nano-object is the most
intuitive approach for introducing chiroptical properties.
Two types of processes introduce chiral shapes to metallic
nano-objects with controllable chirality. One is a physical-
production process; for instance a mechanical-control pro-
cess in which the metallic nano-object is fabricated using
deposition or etching equipment that installs a rotational
component mechanically.4 For example, Fischer and col-
leagues4a fabricated left- and right-handed Au nanohelices by
glancing-angle deposition onto a rotating substrate in clock-
wise and anticlockwise directions, and showed that these
materials exhibit opposite CD spectra with distinct positive
and negative Cotton effects. Physical processes are the most
We report a chirality-controlled synthesis of ultrathin (aver-
age diameter ~3.0 nm) double-helical Au nanowires (NWs)
using a chiral soft-template. The template was prepared us-
ing two kinds of organogelators and facilitates both soft-
templating and the anisotropic growth of Au nanocrystals.
Chiral D-12-hydroxystearic acid (D-HSA, Figure 1) was used
as the organogelator that introduces chirality into the struc-
ture of the soft-template because D-HSA self-assembles to
form ribbon-like nanostructures with a right-handed twist.11
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D-HSA is a well-known organogelator used in commercial
and the apparent homogeneous structures depicted in Fig-
ures 2a and b, indicate that the nanoribbons have specific
chemical compositions. The molar ratios of HSA to C18AA in
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products, and enantiopure D-HSA is easily obtained in large
quantities by the hydrogenation and hydrolysis of castor oil.12
Furthermore, L-HSA (Figure 1), with the opposite chirality to
D-HSA, forms left-handed twisted nanoribbons, and can be
synthesized through the chiral inversion of D-HSA (Scheme
S1).11 A long chain amidoamine (C18AA, Figure 1) was selected
as the other organogelator because C18AA affords a platform
for the preparation of Au NWs due to its significant propen-
sity to control the growth of metal crystals, which is ascribed
to the selective adsorption properties the terminal amine
groups of C18AA for particular crystal faces of gold.13 We
have already fabricated straight ultrathin Au NWs,13a dendrit-
ic Au NWs,13b and Pd NWs14 assisted by the capping and soft-
template functions of C18AA.
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the nanoribbons were determined to be 4:1 by H-NMR spec-
troscopy of the filtered nanoribbons, although the ratio of
the original preparation mixture was 1:6 (Figure S8). In other
words, most of the water-soluble C18AA molecules remain in
the bulk aqueous solution. Furthermore, twisted nanorib-
bons with the same structures and compositions were ob-
tained using molar ratios that ranged from 1:12 to 1:4 in the
preparation mixture (Figure S9).
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From the nanoribbon thickness of 12.3 nm and the 1:4 molar
ratio of C18AA to HSA, these nanoribbons are probably com-
posed of one C18AA bilayer and two HSA bilayers, since bi-
layer thicknesses of 4.6715 and 3.73 nm13a were reported for
HSA and C18AA, respectively. Figures 2c and S10 show the
proposed structures of the nanoribbons; the C18AA bilayer is
sandwiched between two HSA bilayers, in which ionic bonds
between the NH2 groups of the C18AA and the COOH groups
of the HSA are formed through acid/base interactions at the
interface. Furthermore, the carboxyl groups on the outer
sides of HSA probably bind lithium ions. The formation of
the lithium salt of HSA was confirmed by the existence of a
carboxylate band and the absence of a carboxylic acid band
in the FT-IR spectrum of the hydrogel (Figure S11). Layers of
lithium hydroxystearate on both outer sides of the nanorib-
bon may prevent the adsorption of additional C18AA layers.
This proposal is consistent with the observation that large
tabular crystals, instead of nanoribbons, are formed in the
absence of LiCl (Figure S12).
OH
OH
D-HSA
L-HSA
C18AA
O
OH
OH
NH2
O
O
NH
NH
N
O
NH2
Figure 1. Chemical structures of D-HSA, L-HSA and C18AA.
Chiral soft-templates composed of D- or L-HSA, and C18AA
were prepared as described in the Supporting Information
(SI). In brief, D- or L-HSA, and C18AA (molar ratio = 1:6)
were mixed in water with a small amount of toluene, fol-
lowed by the addition of aqueous LiCl solution. The addition
of LiCl is crucial and promotes the formation of semi-
transparent hydrogels within 30 min at room temperature. It
should be noted that a mixture of the water-soluble C18AA
organogelator and water-insoluble HSA formed a hydrogel,
whereas each organogelator separately does not form a hy-
drogel. Furthermore, the hydrogel exhibits a thermal reversi-
ble phase transition from gel to sol upon heating (Figure S5),
indicating that the hydrogel is composed of a thermodynam-
ically stable product.
Figures 2a and 2b show scanning electron microscopy (SEM)
images of dried hydrogels prepared from D- and L-HSA, re-
spectively; many twisted nanoribbons with homogenous
structures are clearly visible. Both entangled nanoribbons are
highly uniform with periodic sizes and shapes, and exhibit
168 ± 12 nm (D-HSA) and 171 ± 9 nm (L-HSA) helical pitches,
26.9 ± 3.1 nm (D-HSA) and 26.3 ± 3.0 nm (L-HSA) diameters,
and 12.1 nm ± 0.9 nm (D-HSA) and 12.1 ± 0.8 nm (L-HSA)
thicknesses, whereas they exhibit opposite helicities; the
nanoribbons produced by the D- and L-HSA systems have
right- and left-handed twist structures, respectively (Figures
2, S6 and S7). This result verifies that the handedness of the
self-assembled twisted nanoribbon is completely controllable
by the chirality of the HSA used as the chiral source.
Figure 2. SEM images of twisted nanoribbons prepared using
(a) D-HSA and (b) L-HSA. Scale: 500 nm. (c) Illustrating the
preparation of twisted nanoribbons as soft-templates with
controllable helicities.
As C18AA molecules that act as soft-templates for Au-
nanocrystal growth are exposed along both edges of each
nanoribbon (Figure 2c), double-helical Au NWs can be
grown along both edges. The Au NWs were synthesized by
adding a solution of a gold precursor, a reducing agent, a pH
adjusting agent and LiCl to the solution used to prepare the
HSA/C18AA soft-template described above (see SI for de-
tails). Even in the presence of the gold precursor and the
reducing agent, the mixture forms a hydrogel and produces
the same nanoribbon described above (Figures 3e, S6, and
The observation that the twisted nanoribbons are reversibly
produced following thermal sol-gel phase-transition cycles,
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S7). The color of the mixture gradually changed from yellow
respond to those of the twisted nanoribbons that served as
the templates. Furthermore, dispersions of the right- and
left-handed helical Au NWs exhibit opposite CD spectra;
these spectra were clearly different to those of the twisted
nanoribbons (Figure 4e). Weak CD signals in 300–350 nm
region are derived from soft-templates with right- and left-
handed twist. Thus, broad CD signals in 400–800 nm region
originated from helical Au NWs. Further, the profiles and the
signs of CD signals of right- and left-handed helical Au NWs
dispersions were similar to recently reported results4k that
obtained from helical stackings of anisotropic straight Au
nanowire layers. Accordingly, chirality was successfully tran-
scribed from the organic template to the Au nanocrystals by
wet chemical synthesis.
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to dark brown because of the formation of Au nanocrystals
(Figure 3a and b). Figure 3c shows a transmission electron
microscopy (TEM) image of the resultant Au-nanocrystalline
products. Multiple meandering Au NWs with diameters of
3.0 ± 0.4 nm (D-HSA) and 3.0 ± 0.5 nm (L-HSA) are clearly
visible, with few particulate byproducts (Figure S13). Interest-
ingly, many of the meandering Au NWs exhibit double-
helical structures with 170 ± 11 (D-HSA) nm and 168 ± 9 (L-
HSA) nm helical pitches, and 21.8 ± 2.3 nm (D-HSA) and 21.8
± 2.0 nm (L-HSA) widths (Figures S14 and S15). This observa-
tion clearly reveals that the morphology of the nanoribbon is
transferred to that of the Au NW because the double-helical
Au NWs have almost identical structures to those of the na-
noribbons used as the soft-templates (Figures 3d, e, and S16).
Note that there were sometimes two nanowires at one edge
of the nanoribbons (Figure 3c). This is probably due to the
fact that there are two boundaries between HSA and C18AA
in the nanoribbons, which can be used as the adsorption
sites of Au (Figure S10).
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Figure 4. (a, c) TEM images and (b, d) the corresponding
TEM-tomography images of Au NWs prepared from right-
and left-handed templates, respectively. Scale: 50 nm. (e) CD
spectra of dispersions of right- (blue solid line) and left-
handed (red solid line) double-helical Au NWs with twisted
nanoribbons and nanoribbons with right- (blue broken line)
and left-handed twist (red broken line).
Figure 3. Photographic images of (a) a hydrogel and (b) a
hydrogel containing double-helical Au NWs. (c, d) High- and
low-magnification TEM images of double-helical Au NWs
and (e) a twisted nanoribbon composed of C18AA and D-
HSA. Scale bars: (c) 500 nm and (d, e) 100 nm.
To investigate the mechanism for the formation of the dou-
ble-helical Au NWs, high-resolution TEM images of the re-
sultant Au NWs were acquired (Figure S18). A periodic fringe
of 0.24 nm corresponding to the Au (111) lattice spacing, and
multiple crystal grain boundaries were observed in the Au
To determine the helicities of the double-helical Au NWs,
TEM tomography was conducted by acquiring TEM images
of the Au NWs at various tilt angles. It is clear that Au NWs
prepared from the D- and L-HSA systems have right- and
left-handed helicities, respectively (Figure 4a–d), which cor-
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(5) (a) Zhou, C.; Duan, X.; Liu, N. Acc. Chem. Res. 2017, 50, 2906–
NWs. Interestingly, relatively large single-crystalline do-
mains were observed in the polycrystalline Au NWs. Accord-
ing to a previous report, the predominant growth in the (111)
crystal direction is ascribed to C18AA preferring to cap the
(100) and (110) crystal facets rather than the (111) facet.13 Fur-
thermore, the presence of crystal grain boundaries provides
valuable information regarding the Au-NW growth process;
i.e., Au NWs are probably produced by the gradual fusion of
nanocrystals on the twisted nanoribbon template formed
during the early stages of reduction. This growth-process
hypothesis is also supported by the presence of Au NPs and
short Au NWs on the edges of the templates during the early
reduction period (Figure S19).
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In summary, we introduced a wet chemical method for the
synthesis of chirality-controlled double-helical Au NWs us-
ing twisted nanoribbon templates of HSA, which serves as
the source of chirality, and C18AA, which acts as the Au cap-
ping agent. We demonstrated that the introduction of two
compounds that individually perform different functions into
the template is a highly effective method for the preparation
of chiral Au nano-objects using wet chemistry. We believe
that this bi-functional strategy can be applied to the synthe-
sis of other shape-controlled metallic nano-objects.
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ASSOCIATED CONTENT
Supporting Information
Experimental details and Supplementary Figures S1–S19. This
material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: kawai@ci.kagu.tus.ac.jp
Notes
The authors declare no competing financial interests.
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