Controllable self-assemblies of micro/nano-tubes and vesicles from arylamides and their applications as templates to fabricate Pt micro/nano-tubes and hollow Pt nanospheres

Article abstract of DOI:10.1039/b917576h

A novel class of nonamphiphilic aromatic amides (T1-T3) have been designed and synthesized from naphthalene-2,7-diamine or 2-amino-naphthalene and a 5-hydroxy-isophthalic acid segment, which is revealed to selectively assemble into vesicular or tubular ar

Full text of DOI:10.1039/b917576h

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Controllable self-assemblies of micro/nano-tubes and vesicles from arylamides
and their applications as templates to fabricate Pt micro/nano-tubes and
hollow Pt nanospheres†
*
Yun-Xiang Xu, Gui-Tao Wang, Xin Zhao, Xi-Kui Jiang and Zhan-Ting Li
*
Received 26th August 2009, Accepted 7th December 2009
First published as an Advance Article on the web 18th January 2010
DOI: 10.1039/b917576h
A novel class of nonamphiphilic aromatic amides (T1–T3) have been designed and synthesized from
naphthalene-2,7-diamine or 2-amino-naphthalene and a 5-hydroxy-isophthalic acid segment, which is
revealed to selectively assemble into vesicular or tubular architectures, depending on the solvents and
concentrations. T1 and T2 form vesicles in methanol, but can be converted into micro/nano-tubes when
water is added and further gelate the binary solvent when the concentration is high enough. In contrast,
T3 self-assembles into fine tubular structures in methanol, but can be transformed into vesicles upon
being diluted or adding chloroform. The morphology transition has been investigated by SEM, AFM,
TEM and fluorescent microscopy, which also reveal that microtubes of large size are formed through
the fusion of vesicles of small size, which is driven by the cooperative hydrogen bonding and aromatic
stacking interactions, as evidenced by the X-ray investigation on an analogue of T3. The novel organic
micro/nano-tubes and vesicles are further used to template the fabrication of Pt micro/nano-tubes or
hollow Pt spheres by in situ reduction of the absorbed K2PtCl4 with ascorbic acid, as confirmed by
SEM, TEM and EDX analyses.
concentration,25–27 and polarity of the medium28,29 have also been
Introduction
explored to offer efficient control over the self-assembling
process. This approach is attractive because different architec-
tures may be assembled from the identical building blocks.
Herein we report a novel class of aromatic amide-based non-
amphiphiles that can self-assemble into vesicular and tubular
structures by simply changing the solvents and/or concent-
rations. In addition, the new assembled architectures have been
further utilized to template the formation of fine Pt micro/nano-
tubes and hollow Pt spheres, the preparation of which was
mainly limited to an inorganic template approach.30–36
The self-assembly of low-molecular-weight organic molecules
has drawn considerable attention in the past decade for its
powerful applications in fabricating nano- and microscale
objects.1,2 In such a process, small molecules are drawn together
by noncovalent interactions, such as hydrogen bonding,
aromatic stacking and van der Waals interactions, resulting in
the formation of discrete ordered complex structures. As basic
building blocks for this ‘‘bottom-up’’ approach, organic
compounds have advantages of structural variety, ready modi-
fications and multifunctionality. Based on this strategy, a variety
of well-defined nanoscale structures, including vesicles,3–7 nano-
tubes,8–12 and nanofibers,13–15 have been constructed.
Results and discussion
Owing to the intrinsic features of noncovalent interactions
such as reversibility and sensitive response to external stimuli and
weak binding strength, one crucial challenge in organic molec-
ular self-assembly is the selective formation of specific assembled
targets in solution or on surfaces. To reach this goal, rational
design and modification of the self-assembling building blocks at
the molecular level have been the most widely adopted
approach.16–20 In addition to structural modification, changing
the environmental conditions such as pH,21,22 temperature,23,24
Design and synthesis
We have previously reported a series of amphiphilic aromatic
amide oligomers that are capable of self-assembling into vesic-
ular structures in methanol.37 Since vesicles have been mainly
constructed from amphiphilic molecules, such as surfactants and
lipids, and copolymers,38–42 and the fabrication of vesicular
structures from nonamphiphilic compounds are quite rare,43–45
we therefore further designed and synthesized a new class of non-
amphiphilic aromatic amides, i.e., T1–T3, to explore their
capacity of forming vesicular structures. The synthesis of T1–T3
is straightforward. For the preparation of T1, diamine 1 was first
reacted with di-tert-butyl dicarbonate to afford 2 in 34% yield,
which was then reacted with diacyl chloride 3 to give T1 in 65%
yield, while T3 was prepared from the coupling reaction of diacid
4 with 2-amino-naphthalene. For the synthesis of T2, compound
5 was first treated with iso-butanol to afford 6 in 71% yield. The
diester was further hydrolyzed with lithium hydroxide to give 7
State Key Lab of Bioorganic and Natural Products Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences, 345
Lingling Lu, Shanghai 200032, China. E-mail: xzhao@mail.sioc.ac.cn;
ztli@mail.sioc.ac.cn; Fax: +0086-21-64166128; Tel: +0086-21-54925122
† Electronic supplementary information (ESI) available: Experimental
details, synthesis and characterization, fluorescenct micrographs, AFM
image, additional SEM and TEM images, and XRD result. CCDC
reference number 715335. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/b917576h
1246 | Soft Matter, 2010, 6, 1246–1252
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Fig. 1 SEM images of (a) T1 (5 mM), (b) T2 (5 mM), (c) T3 (2 mM), and
(d) T3 (0.2 mM) on silicon surface. The samples were obtained by
evaporation of the methanol solutions.
Scheme 1 The synthetic routes for compounds T1–T3.
quantitatively. The diacid was then coupled with 2 to yield T2 in
63% yield (Scheme 1).
Fig. 2 TEM images of (a) T1 and (b) T2. The samples were obtained
from their methanol solution (0.01 mM).
AFM images showed that the spherical aggregates of T1 had
a large ratio of diameter over height (ca. 13.2, Fig. S2 of ESI†),
which might be attributed to hollowness of the spheres or the
high local force applied by the AFM tip.49 The hollow feature of
the spheres was further evidenced by TEM (Fig. 2), which
showed a contrast between the inner part and the periphery.50
The wall thickness was estimated to be ca. 2.5 and 2.6 nm for T1
and T2, respectively, which is close to the length of their extended
backbone (ca. 2.7 nm). Therefore, the wall of the vesicles might
be generated by a monolayered stacking of the molecules with
the Boc groups being located outside.
Self-Assembly properties in methanol
The self-assembly of T1–T3 were first investigated in methanol.
Morphologies of the aggregates of T1–T3 were characterized by
SEM, which revealed that T1 and T2 self-assembled into
spherical structures. The sizes of the spheres formed by them
were comparable, with an average of 2 mm in diameter.
Compared to those of T2, the spheres formed from T1 showed
a little tendency of fusing (Fig. 1a and 1b). Under similar
conditions, T3, which has no NHBoc groups at the ends of the
backbone, gave rise to fine tubular microstructures when the
concentration was 1 to 5 mM (Fig. 1c and Fig. S1 of ESI†). The
hollow nature of the tubes was evidenced by their open-ended
feature. Upon being diluted to 0.5 or 0.2 mM, tubular structures
could not be observed. Instead, spheres much smaller than those
of T1 and T2 were generated exclusively (Fig. 1d and Fig. S1 of
ESI†). Similar conversions were reported previously for peptide-
based nanotubes and attributed to the collapse of compact
stacking structures of the tubes into less compact stacking
vesicular structures.46–48 No similar tubular structures were
observed for T1 and T2. Moreover, their spherical morphology
almost did not change even when the concentration was
decreased to 0.01 mM (vide infra, Fig. 2).
Effect of the solvent polarity on the assembling selectivity
Experiments in the methanol–water binary solvent were carried
out to investigate the effect of the solvent polarity on the
assembling behavior of T1–T3. It was found that in this more
polar media T1 and T2 self-assembled into tubular structures, as
evidenced by SEM, TEM and fluorescent microscopy images
(Fig. 3 and Fig. S3 of ESI†). Moreover, they could further gelate
the solvent at higher concentrations. The minimum gelation
concentrations were 1.09 and 1.77 wt%, respectively, for the
methanol–water system (5 : 1, v/v). SEM images showed that the
gels of both compounds were generated by hollow tubes of
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Fig. 3 (a) SEM image of T1, (b) SEM image of T2, (c) SEM image of T3
and (d) TEM image of T1 (80 kV). The samples (5 mM) were obtained
from methanol–water (5 : 1).
several micrometres in width and dozens of micrometres in
length (Fig. S4 of ESI†). Attempts to measure the wall thickness
of the hollow tubes by high resolution TEM was not successful
because the tubular structures could not survive the condition
used (200 kV). Different from T1 and T2, T3 exclusively
formed tubular microstructures, independent of the methanol–
water ratio.
Fig. 4 SEM images of (a) T1 (5 mM) from methanol–chloroform (9 : 1),
(b) T1 (5 mM) from chloroform, (c) T2 (5 mM) from methanol–chloro-
form (9 : 1), (d) T3 (2 mM) from methanol–chloroform (9 : 1), (e) T3
(5 mM) from methanol–chloroform (3 : 1) and (f) T3 (5 mM) from
chloroform.
The assembling behavior in the methanol–chloroform binary
solvent was also investigated (Fig. 4 and Fig. S5 of ESI†).
When the content of chloroform was #10%, T1 (5 mM) gave
rise to vesicles exclusively (Fig. 4a). When the content was
increased to 25%, SEM images did not show any vesicular
structures. In pure chloroform, T1 formed belt-like aggregates.
Their salient feature was illustrated by the twisted belt in the
middle of the SEM image shown in Fig. 4b. T2 showed
a similar trend. However, in pure chloroform no well-defined
aggregates were generated. Adding chloroform also strongly
induced T3 (2 mM) to form vesicular structures. Thus, in the
presence of 10% of chloroform, T3 could form vesicles selec-
tively (Fig. 4d). The vesicles retained their structure even when
the content of chloroform was increased to 50%. Increasing the
concentration facilitated the formation of the tubular struc-
tures. For example, the solution of 5 mM still selectively
formed microtubes even when 25% of chloroform was added.
Further increasing the content of chloroform gradually
destroyed these well-ordered tubular structures. Interestingly,
microtubes could be generated again in pure chloroform
(Fig. 4f). It should be noted that the cross sections of the
resulting tubes are not exclusive. For example, while tubes with
a hexagon-like section were observed in methanol–water (5 : 1)
as shown in Fig. 3c, microtubes with a rectangular cross section
were also obtained (Fig. S6a of Supporting Information) from
the same sample. In addition, microtubes with hexagon-like
section and rectangular section were observed for a sample
prepared from methanol–chloroform (3 : 1) (Figures S 6b–c of
ESI†), besides the squared section illustrated in Fig. 4e.
However, the reason for the formation of such cross sections is
yet unclear. In nonpolar decalin, T1 and T2 also formed
spherical structures, while T3 produced tubular aggregates
(Fig. S7 of ESI†). These results were similar to those in polar
methanol, suggesting that T1–T3 might adopt similar packing
modes in the two solvents.
Self-assembling mechanism
The assembling process of T1–T3 should be driven by coopera-
tive intermolecular hydrogen bonding and aromatic stacking
interactions.37 The IR spectra of their dried samples of the tubes,
obtained after the solvent was removed, exhibited the N–H
stretching vibrations around 3290 cmꢀ1, indicating that the
amide proton was engaged in intermolecular H-bonding. The
benzyl or iso-butyl units should also contribute significantly,
serving as ‘‘glue’’ or an ‘‘anchor’’ to crosslink the stacking layers,
because compound T4 (Fig. 5), which lacks such units, did not
form any ordered microstructures under the above conditions.
Attempt to grow single crystals of T1–T3 suitable for the X-ray
crystallography was not successful. However, the crystal struc-
ture of T5‡ (Fig. 5), an analog of T3, revealed that the neigh-
boring molecules were packed with each other in oppositely
alternate manner stabilized by two pairs of C]O/H–N
hydrogen bonds, even though the appended benzyl group was
‡ Crystal data for T5: C27H22N2O3, M ¼ 422.47, Monoclinic, space
ꢀ
ꢀ
ꢀ
group C2/c, a ¼ 18.473(2) A, b ¼ 14.611(2) A, c ¼ 8.4322(12) A, a ¼
3
ꢀ
90.00, b ¼ 103.972(4), g ¼ 90.00, V ¼ 2208.6(5) A , T ¼ 293(2) K, Z ¼
4, Dc¼ 1.271 g cmꢀ3, m ¼ 0.083 mmꢀ1, R1 ¼ 0.0822, wR2 ¼ 0.2481(I >
2s(I)), R1
¼
0.1027, wR2
¼
0.2460 (all data). Reflections
collected/unique: 5722/2050 (Rint ¼ 0.0848). GOF ¼ 1.073.
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Fig. 6 SEM images of T1 (2 mM) from methanol–water (20 : 1): (a)
freshly prepared, (b) incubated for 17 days. Fluorescence microscopy
images of T1 (3 mM) from methanol of (c) freshly prepared and (d)
incubated for 50 days. The scale bar for fluorescent images is 10 mm.
and 6d, freshly prepared sample from methanol gave rise to
discrete spherical structures, which was consistent with the SEM
observation. The spherical structures gradually connected each
other to form necklace-like structures after 50 days (4 ꢁC), which
could further transform into tubes after another 42 days at room
temperature. This result also indicated that the tubular structures
were more stable than the vesicular structures and water accel-
erated the vesicle-tube transition by enhancing the stacking
interaction. The powder XRD profiles of vesicles and tubes of T1
exhibited almost identical patterns (Fig. S10 of ESI†), suggesting
that both structures had a similar stacking pattern.53
Fig. 5 The packing structure of T5 in the crystal, highlighting the
intermolecular hydrogen bonding and alternate stacking of the mole-
cules. The appended benzyl unit is disordered due to two different
orientation.
disordered in the crystal structure.51 T5 could also self-assemble
into tubular structures in methanol (Fig. S8 of ESI†). The
powder XRD patterns of the nanotubes of T5 were very close to
the theoretical ones calculated from the crystal structure, indi-
cating that the molecules in the tubes of T5 adopted a similar
packing mode as that in the crystal structure (Fig. S9 of ESI†).
Therefore, we propose that the assembly of T1–T3 was also
driven by the two similar interactions. Moreover, their stacking
interaction should be stronger than that of T5 due to the exis-
tence of the two larger naphthalene units. Furthermore,
compared to T3, T1 and T2 bear two NHBoc groups at their
ends of the backbones, which can form two additional inter-
molecular hydrogen bonds. On the other hand, the steric
hindrance imposed by the bulky Boc groups might push the
monomers to align in a slightly twist arrangement, which should
has influence on the self-assembling process and might improve
the assembling selectivity by inhibiting the intermolecular head-
to-tail contact.52
The conversion between vesicles and tubes should be a dynamic
process.47 As mentioned above, both IR and X-ray crystallog-
raphy studies suggested that the self-assembling process was
dictated by intermolecular hydrogen bonding and aromatic
stacking interactions between the monomers. It is generally
accepted that aromatic stacking is more favorable in higher polar
solvents but hydrogen bonding becomes stronger in less polar
solvent. Thus, the origin of the transition between vesicles and
tubes might be ascribed to different dominating noncovalent
interactions and a subtle balance of these interactions between the
molecules in the solvents of different polarity. Aromatic stacking
should be enhanced in the methanol–water binary solvent, which
is proposed to be responsible for the formation of the tubular
structures. For compound T3, aromatic stacking is more decisive
than those of T1 and T2 because the latter two bear two more
hydrogen-bonding sites. As a result, T3 could self-assemble into
tubular structures in methanol and even in chloroform (5 mM)
while T1 and T2 could not under the same conditions.
A systematic SEM investigation of T1 revealed that the
tubular structures were generated through the time-dependent
fusion of the smaller vesicles. For example, the sample of its
freshly prepared methanol–water (20 : 1) solution (2 mM)
showed that most vesicles fused into sheet-like structures,
although there were some complete vesicles that could be
observed (Fig. 6a). After the solution was incubated for 17 days
at room temperature, most of these sheet-like structures evalu-
ated into microtubes, on which some unconverted vesicles could
also be found (Fig. 6b). This fusion process was also supported
by the fluorescence microscopy study in methanol. However, in
this case, the fusion took even longer time. As shown by Fig. 6c
Templates for Pt nanostructure
It has been established that the N and O atoms of amides have
a high affinity towards Pt(II).54 The O– and N-rich micro/nano-
tubes and vesicles from T1–T3 are therefore potentially useful in
templating the fabrication of Pt micro/nano-tubes and spheres, if
Pt(II) ions are anchored on their surface and further reduced to Pt
(0) in situ.55
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T1 and T3 were chosen to test this assumption, because the
assembling objects from these two compounds were generally
more stable. We started with the tubular template first as follows:
to a 5 mM solution of T1 in methanol–water (5 : 1), which had
been confirmed to assemble into tubes, an aqueous solution of
K2PtCl4 (20 mM, 0.1 mL) was added. The mixture was stirred
slowly for 5 min and then an aqueous solution of ascorbic acid
(0.15 M, 0.1 mL) introduced. The mixture was gently shaken and
the resulting solution incubated at room temperature for 10 h.
The solution changed from light yellow (that of K2PtCl4) to
black, indicating that the salt was reduced to metal Pt (0). The
resulted materials were collected by filtration and then re-
dispersed in hot methanol, followed by centrifugal purification to
remove the organic material. This procedure was repeated three
times. The resulting material was investigated by SEM and TEM.
The results strongly supported that Pt nanotubes were con-
structed. As shown in Fig. 7, a low resolution TEM image shows
a tube of ca. 5 micrometres long with a diameter around 500 nm
(Fig. 7a), whose hollow nature was clearly evidenced by its naked
end through the SEM and the high resolution TEM images
(Fig. 7b–d). The formation of the Pt (0) metal was further
confirmed by the energy dispersive X-ray (EDX) experiment,
which exhibited the strong peaks of Pt (Fig. 7e). Furthermore,
HR-TEM images also illustrated that the as-formed Pt nano-
tubes were built by Pt nanoparticles, which could be observed
directly (Fig. 8d) and further confirmed to be nanocrystallites of
Pt by the selected area electron diffraction (SAED) experiment
(Fig. 8d, inset).34 They should be formed through the reduction
of Pt(II) ions on the surfaces of the organic nanotubes and then
further aggregated to produce the Pt tubes.
As expected, when the spherical templates were used (a
0.1 mM solution of T1 in methanol was employed here) and the
above procedure was followed, Pt nanospheres could also be
generated. Again, SEM and TEM images revealed that congre-
gated Pt nanospheres were produced (Fig. 8a and 8b). This as-
prepared product was also investigated by HRTEM image
(Fig. 8c and 8d). The dark edge and pale centre in the HRTEM
image of the spheres indicated that the spherical structures were
also hollow, and the magnified part clearly showed Pt nano-
particles again by which the Pt sphere were constructed, while the
Fig. 7 (a) TEM, (b) SEM, (c) high-resolution TEM images, (d) partial
magnification of image c and SAED pattern (inset), and (e) EDX diagram
of Pt nanotubes, obtained on T1 templates. The carbon and copper peaks
in the EDX diagram were generated from the copper support grid.
Fig. 8 (a) SEM, (b) TEM (c) high-resolution TEM images (d) partial
magnification of image c and (e) EDX diagram of the as-prepared Pt
spheres with T1 as template.
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14 Z.-Y. Xiao, X. Zhao, X.-K. Jiang and Z.-T. Li, Org. Biomol. Chem.,
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EDX diagram supported that these hollow nanospheres con-
sisted of Pt metal (Fig. 8e).
The templating effect of T3 was also investigated. Both Pt
nanotubes and nanospheres were observed when the above
procedures were carried out with this compound. Because the
introduction of aqueous K2PtCl4 partially destroyed the well-
ordered tubular or vesicular structures of T3 in methanol, the
resulting Pt nanotubes and nanospheres were always accompa-
nied by Pt agglomeration (Fig. S11 of ESI†).
19 Y. Wang, H. Fu, A. Peng, Y. Zhao, J. Ma, Y. Ma and J. Yao, Chem.
Commun., 2007, 1623–1625.
20 B.-S. Kim, D.-J. Hong, J. Bae and M. Lee, J. Am. Chem. Soc., 2005,
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Conclusions
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24 L. Gao, L. Shi, W. Zhang, Y. An, Z. Liu, G. Li and Q. Meng,
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26 Y. Chen, B. Zhu, F. Zhang, Y. Han and Z. Bo, Angew. Chem., Int.
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27 S. Y. Kim, K. E. Lee, S. S. Han and B. Jeong, J. Phys. Chem. B, 2008,
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28 B. Guan, M. Jiang, X. Yang, Q. Liang and Y. Chen, Soft Matter,
2008, 4, 1393–1395.
In this study, we describe a novel class of aromatic amides that
are capable of self-assembling into vesicles or micro/nano-tubes.
The shape and size of the aggregates can be tuned by changing
the polarity of the solvents and the concentration. The vesicle-to-
tube transformation represents a new promising strategy for the
construction of well-ordered supramolecular systems of different
shape and size from single molecular building block. By taking
advantage of simple surface chemistry, we also demonstrate that
these organic micro/nano-tubes and vesicles can be used as
degradable supporting materials to fabricate Pt tubes and hollow
Pt spheres. This strategy provides an alternative method to
prepare well-defined metal nanostructures in desired morpho-
logies, which may be applicable to the fabrication of other
transition metal nanostructures.
29 C. Ott, R. Hoogenboom, S. Hoeppener, D. Wouters, J.-F. Gohy and
U. S. Schubert, Soft Matter, 2009, 5, 84–91.
30 Y. Ding, A. Mathur, M. Chen and J. Erlebacher, Angew. Chem., Int.
Ed., 2005, 44, 4002–4006.
31 Z. Chen, M. Waje, W. Li and Y. Yan, Angew. Chem., Int. Ed., 2007,
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32 C. Mu, Y. Yu, R. Wang, K. Wu, D. Xu and G. Guo, Adv. Mater.,
2004, 16, 1550–1553.
33 A. Takai, Y. Yamauchi and K. Kuroda, Chem. Commun., 2008,
4171–4173.
Acknowledgements
This research is supported by the National Science Foundation
of China (Nos. 20621062, 20672137, 20732007, 20872167), the
National Basic Research Program (2007CB808000) and the
Science and Technology Commission of Shanghai Municipality
(09XD1405300).
34 B. Mayers, X. Jiang, D. Sunderland, B. Cattle and Y. Xia, J. Am.
Chem. Soc., 2003, 125, 13364–13365.
35 S. Guo, S. Dong and E. Wang, Chem.–Eur. J., 2008, 14, 4689–4695.
36 For examples fabricating Pt nanotubes from organic templates, see(a)
L. Yu, I. A. Banerjee and H. Matsui, J. Mater. Chem., 2004, 14, 739–
743; (b) T. Kijima, T. Yoshimura, M. Uota, T. Ikeda, D. Fujikawa,
S. Mouri and S. Uoyama, Angew. Chem., Int. Ed., 2004, 43, 228–232.
37 Y.-X. Xu, G.-T. Wang, X. Zhao, X.-K. Jiang and Z.-T. Li, Langmuir,
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