Reversible dispersion and releasing of single-walled carbon nanotubes by a stimuli-responsive TTFV-phenylacetylene polymer

Article abstract of DOI:10.1039/c2cc17935k

A TTFV-phenylacetylene folding polymer was synthesized to exhibit the property of reversibly dispersing and releasing single-walled carbon nanotubes in organic solvents under the control of redox or pH stimuli. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc17935k

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COMMUNICATION
Reversible dispersion and releasing of single-walled carbon nanotubes by
a stimuli-responsive TTFV-phenylacetylene polymerw
Shuai Liang, Guang Chen, Jessica Peddle and Yuming Zhao*
Received 19th December 2011, Accepted 27th January 2012
DOI: 10.1039/c2cc17935k
A TTFV-phenylacetylene folding polymer was synthesized to exhibit
the property of reversibly dispersing and releasing single-walled
carbon nanotubes in organic solvents under the control of redox or
pH stimuli.
Single-walled carbon nanotubes (SWNTs) have attracted enormous
attention owing to their extraordinary electrical and mechanical
1
properties and wide-ranging applications. The strong bundling/
aggregation behaviour of pristine SWNTs, however, makes
direct processing and purification of SWNTs in the solution
phase very challenging. To overcome this barrier, various
functionalization methods for pristine SWNTs have been
2
developed. Non-covalent functionalization of SWNTs presents
an appealing approach not only to tame the solubility problem,
but more importantly to enable sorting out or enrichment of
3
specific-type SWNTs from as-prepared mixtures. Numerous
molecular and macromolecular systems have been studied to
2
effectively and/or selectively disperse SWNTs in solvents.
,3
Fig. 1 Bistable conformational changes of TTFV under (a) redox
and (b) acid/base conditions. (c) Reversible wrapping and unwrapping
of SWNTs by a TTFV polymer.
Nonetheless, many of these dispersants lack the reversibility
in terms of dispersing and releasing SWNTs. To address this
issue, a number of stimuli-responsive molecular systems have
been recently investigated. For instance, Zhang and co-workers
used a poly(ethylene glycol)-terminated malachite green derivative
8
attain conformational switching as shown in Fig. 1b. The
switching behaviours thus led us to the design of new types of
redox and pH responsive polymers to perform reversible
SWNT dispersion and release in a controlled manner as
illustrated in Fig. 1c. To achieve this goal, a TTFV-phenyl-
acetylene polymer 1 (see Fig. 2) was first synthesized. Both gel
(
PEG-MG) to reversibly control the dispersion and aggregation of
4
SWNTs by light. Moore and Zang et al. reported that
oligo(m-phenylene ethynylene)s could be wrapped around and
5
detached from SWNTs under the control of solvent polarity.
0
1
permeation chromatographic (GPC) and H NMR analyses
Zhang and co-workers used CO -responsive N,N-dimethyl-N -
2
indicated that the average chain length of polymer 1 is at the
8
stage of hexamer (n = 6). Molecular modelling using molec-
+
pyren-1-ylmethyl) acetimidamidinium (PyAH ) to manipulate
(
6
the dispersion and aggregation of SWNTs in water.
ular mechanics has revealed that the hexamer prefers a folded
To further expand the means of controlled SWNT dispersion
and releasing at the molecular level, novel stimuli-responsive
dispersants need to be developed. In our recent studies, phenyl-
substituted tetrathiafulvalene vinylogues (TTFV) have been
found to undergo cis-to-trans structural changes associated
7
with a two-electron transfer process (see Fig. 1a). Moreover,
the TTFV unit was found to be responsive to pH changes to
Department of Chemistry, Memorial University of Newfoundland,
St. John’s NL, Canada A1B 3X7. E-mail: yuming@mun.ca;
Fax: +1-709-864-3702; Tel: +1-709-864-8747
w Electronic supplementary information (ESI) available: Experimental
details and spectroscopic characterization. See DOI: 10.1039/
c2cc17935k
Fig. 2 Molecular structure of polymer 1 (left) and the space-filling
model of the hexamer (right).
3
100 Chem. Commun., 2012, 48, 3100–3102
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Fig. 3 (A) UV-Vis-NIR spectra of SWNTs dispersed in toluene
solution of polymer 1. (B) The RBM region of Raman spectra
Fig. 4 AFM image (tapping mode) of CoMoCAT SWNTs and
polymer 1 on a mica surface.
(lex = 534 nm) for SWNTs dispersed by polymer 1.
conformation in the neutral state with a hollow inner cavity
suitable for encapsulation of guests with diameters of 1–2 nm,
such as an individual strand of SWNTs. As such, it was predicted
that polymer 1 in the neutral state would spontaneously form
foldamers to wrap around SWNTs via favourable p–p inter-
actions, while upon oxidation or protonation the cis-TTFV
wherein the feature of polymer 1 wrapped around an individual
strand of SWNT can be clearly observed. AFM imaging of the
assemblies of HiPCO SWNTs and polymer 1, however, did not
reveal any discrete nanotube features despite numerous trials
under various conditions. Based on the Raman and AFM data,
it is rationalized that the interactions between folding polymer 1
and CoMoCAT tubes are predominately via the wrapping
mode as depicted in Fig. 4. For HiPCO SWNTs, it is likely
that the polymer, besides wrapping around small-diameter tubes,
also shows strong adhesion to the side-walls of large-diameter
tubes via p-stacking. Such an adhesion-type binding mode could
hence induce a significant degree of intertube stacking, making
individual SWNT features unobservable by AFM.
2
+
units in 1 were switched to trans-[TTFV]
to unfold the
polymer and dissociate it from the backbone of SWNTs.
With polymer 1 in hand, its ability to disperse SWNTs in
organic solvents was then examined. Two types of SWNTs,
HiPCO and CoMoCAT, were selected for study. They were first
subjected to ultrasonication for 30 min in the toluene solution of
1
to form black suspensions, which were subsequently filtered
through a tightly packed cotton plug to remove insoluble
components. The resulting black solution was analyzed by
UV-Vis-NIR spectroscopy. Fig. 3A gives the Vis-NIR absorp-
tion spectra of HiPCO and CoMoCAT SWNTs dispersed in
toluene, which clearly show distinctive absorption bands
characteristic of the electronic transitions between van Hove
singularities in the electronic density of states of SWNTs. In
the spectrum of CoMoCAT SWNT solution, a prominent
peak at 999 nm is observed, which is assigned to SWNTs with
a chiral index of (6,5). In addition, peaks at 576 and 668 nm
The polymer-induced HiPCO SWNT stacking is also evident
by the observation of gelation when the amounts of SWNTs
and polymer 1 were increased to certain levels. As shown in
Fig. 5C, a stable sol–gel was formed after ultrasonication of the
mixture of HiPCO SWNTs (5 mg) and polymer 1 (5 mg) in
toluene (1 mL) for 30 min. Scanning electron microscopic
(
SEM) imaging of the resulting sol–gel showed that it is made
of coarse interwoven micro-fibres (Fig. 5A). Dispersion of the
gel by toluene revealed individual strands of the micro-ribbons,
the widths of which are on the scale of tens of microns (Fig. 5B).
In contrast, CoMoCAT SWNTs did not give rise to any
gelation result when treated under the same conditions as
HiPCO tubes. These results substantiate the hypothesis that the
large-diameter tubes in HiPCO SWNTs interact with polymers
3a,e,9
can be assigned to (6,5) and (7,6) SWNTs respectively.
In the
spectrum of HiPCO SWNT solution, the peaks appearing in the
range of 930–1500 nm are attributed to inter-band transitions of
semiconducting SWNTs (S11), while the peaks from 500–930 nm
10
are due to metallic (M11) and semiconducting (S22) bands.
Evaporation of the SWNT–polymer solutions gave SWNT–
polymer supramolecular complexes, which were characterized by
Raman spectroscopy. Fig. 3B shows the Raman spectra in the
radial breathing mode (RBM) region. The frequencies of the
Raman scattering (oRBM) in this region cast light on the size
distributions of SWNTs dispersed by polymer 1 in toluene,
since they are inversely proportional to the diameters of
5
SWNTs (d
t
t
). Following the equation, oRBM = 223.5/d + 12.5,
ꢀ1
the Raman peaks in the region of 240–270 cm can be ascribed to
small-diameter SWNTs (d E 0.8–0.9 nm), while the peaks at
90–210 cm are assigned to relatively large-diameter tubes
E 1.2 nm). From Fig. 3B, it can be clearly seen that the
t
ꢀ
1
1
(d
t
CoMoCAT SWNTs dispersed by polymer 1 are mainly
composed of small-diameter tubes, while the dispersed HiPCO
3
SWNTs contain both small and large-diameter components.
c
Dilute SWNT–polymer suspensions were spin-cast on a
mica surface to be examined by atomic force microscopic
Fig. 5 (A) SEM image of gel of HiPCO SWNTs and polymer 1. (B) SEM
image of diluted gel of HiPCO SWNTs and polymer 1. (C) Photographic
image of sol–gel of HiPCO SWNTs and polymer 1. (D) Schematics of
polymer 1 induced stacking of HiPCO SWNTs.
(
AFM) imaging. Fig. 4 depicts the AFM image of the supra-
molecular assemblies of CoMoCAT SWNTs and polymer 1,
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3100–3102 3101

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the adhesion modes are at work. Hence the selectivity of polymer
1
for particular types of HiPCO SWNTs is poor.
In summary, the supramolecular interactions between
TTFV-phenylacetylene foldamer 1 and SWNTs have been
demonstrated to be tube diameter-dependent. The wrapping
mode facilitates the dispersion of individual small-diameter
tubes, giving stable and organic soluble SWNT–polymer
assemblies, while the adhesion mode works for large-diameter
tubes to form supramolecular networks and produce appeal-
ing SWNT/polymer sol–gels. Polymer 1 is responsive to
external stimuli (redox and acidity) to allow for controllable
dispersion and releasing of SWNTs in solution in a repetitive
way, which opens a new avenue for developing cost-effective
methods, such as continuous flow, to enrich or purify specific
types of SWNTs.
This work was supported by NSERC, CFI, and Memorial
University. Prof. J.-F. Morin at Laval University is acknowledged
for assistance in gel permeation chromatographic analysis.
Fig. 6 (A) Schematics of multi-cycle dispersion–releasing of SWNTs in
toluene solution of polymer 1. (B) UV-Vis-NIR absorption spectra moni-
toring multi-cycle dispersion of CoMoCAT SWNTs. (C) UV-Vis-NIR
absorption spectra monitoring multi-cycle dispersion of HiPCO SWNTs.
Notes and references
1
Carbon Nanotubes: Advanced Topics in the Synthesis, Structure,
Properties and Application, ed. A. Jorio, G. Dresselhaus and
M. S. Dresselhaus, Springer, Berlin, 2008.
12
The spectra were baseline-corrected using the Fityk software.
2
For recent reviews, see: (a) P. Singh, S. Campidelli, S. Giordani,
D. Bonifazi, A. Bianco and M. Prato, Chem. Soc. Rev., 2009, 38,
2214–2230; (b) L.-Y. Zhao and J. F. Stoddart, Acc. Chem. Res., 2009,
42, 1161–1171; (c) N. Karousis, N. Tagmatarchis and D. Tasis, Chem.
Rev., 2010, 110, 5366–5397; (d) B. I. Kharisov, O. V. Kharissova,
H. L. Gutierrez and U. O. Mendez, Ind. Eng. Chem. Res., 2009, 48,
via the surface adhesion mode to result in significant stacking,
whereas CoMoCAT SWNTs containing mainly small-diameter
tubes were dispersed by folding polymer 1 preferentially via
wrapping. It is worth noting that polymer 1 itself does not show
any gelation properties. Gelation of SWNTs with non-gelator
572–590; (e) D. A. Britz and A. N. Khlobystov, Chem. Soc. Rev., 2006,
35, 637–659.
1
1
dispersants has been rarely reported, while in our case the
interactions between polymer 1 and large-diameter tubes are
believed to be the key contributor to gelation.
3
For recent examples, see: (a) J. Feng, S. M. Alam, L. Y. Yan, C. M. Li,
Z. Judeh, Y. Chen, L.-J. Li, K. H. Lim and M. B. Chan-Park, J. Phys.
Chem. C, 2011, 115, 5199–5206; (b) F. A. Lemasson, T. Strunk,
P. Gerstel, F. Hennrich, S. Lebedkin, C. Barner-Kowollik,
W. Wenzel, M. M. Kappes and M. Mayor, J. Am. Chem. Soc., 2010,
To dissociate polymer 1 from SWNTs, sequential chemical
oxidation and reduction reactions were executed following the
concept described in Fig. 1. In our experiment, iodine was
chosen as the oxidant, since it can oxidize the TTFV unit of
133, 652–655; (c) Y. Maeda, M. Kanda, M. Hashimoto, T. Hasegawa,
S.-i. Kimura, Y. Lian, T. Wakahara, T. Akasaka, S. Kazaoui,
N. Minami, T. Okazaki, Y. Hayamizu, K. Hata, J. Lu and
S. Nagase, J. Am. Chem. Soc., 2006, 128, 12239–12242;
(d) R. Marquis, C. Greco, I. Sadokierska, S. Lebedkin,
M. M. Kappes, T. Michel, L. Alvarez, J. L. Sauvajol, S. Meunier
and C. Mioskowski, Nano Lett., 2008, 8, 1830–1835; (e) A. Nish,
J.-Y. Hwang, J. Doig and R. J. Nicholas, Nat. Nanotechnol., 2007, 2,
640–646; (f) M. Zheng, A. Jagota, M. S. Strano, A. P. Santos,
P. Barone, S. G. Chou, B. A. Diner, M. S. Dresselhaus,
R. S. Mclean, G. B. Onoa, G. G. Samsonidze, E. D. Semke,
M. Usrey and D. J. Walls, Science, 2003, 302, 1545–1548.
2
+ 7
polymer 1 into [TTFV]
. The addition of iodine to the
suspension of SWNTs and polymer 1 immediately led to
precipitation of SWNTs, which could be readily separated
by filtration. The resulting solution of oxidized polymer 1
could be recovered by treatment with excess aq. Na S O to
2
2
3
afford neutral polymer 1, which was reusable for dispersion of
SWNTs. These results validated the feasibility of reversible
dispersion and releasing of SWNTs. However, iodine oxidation
was found to cause significant decomposition of polymer 1,
giving a low efficiency of polymer recovery.
4 S. Chen, Y. Jiang, Z. Wang, X. Zhang, L. Dai and M. Smet,
Langmuir, 2008, 24, 9233–9236.
5
6
7
Z. Zhang, Y. Che, R. A. Smaldone, M. Xu, B. R. Bunes, J. S. Moore
and L. Zang, J. Am. Chem. Soc., 2010, 132, 14113–14117.
Y. Ding, S. Chen, H. Xu, Z. Wang, X. Zhang, T. H. Ngo and
M. Smet, Langmuir, 2010, 26, 16667–16671.
G. Chen, I. Mahmud, L. N. Dawe and Y. Zhao, Org. Lett., 2010,
12, 704–707.
See ESIw for details.
F. Hennrich, R. Krupke, S. Lebedkin, K. Arnold, R. Fischer, D. E.
Resasco and M. M. Kappes, J. Phys. Chem. B, 2005, 109, 10567–10573.
To avert this problem, an alternative pH approach (Fig. 1b)
was then attempted. Multi-cycle dispersion and releasing
experiments were undertaken through the procedures described
8
in Fig. 6A. In each cycle of dispersion, the resulting SWNT
8
9
suspension was subjected to UV-Vis-NIR analysis to monitor
the compositional changes in dispersed SWNTs. From Fig. 6B
and C, it can be seen that after three cycles of dispersion/
releasing, CoMoCAT SWNTs attained noticeable enrichment
of tubes with certain chirality indices. HiPCO SWNTs, on the
other hand, did not show any significant compositional variation.
Since polymer 1 prefers to wrap around CoMoCAT tubes, the
wrapping mode is believed to give rise to chirality-selectivity to
some extent. For HiPCO tubes, however, both the wrapping and
1
0 (a) T. Tanaka, H. Jin, Y. Miyata, S. Fujii, H. Suga, Y. Naitoh,
T. Minari, T. Miyadera, K. Tsukagoshi and H. Kataura, Nano
Lett., 2009, 9, 1497–1500; (b) X. Tu, S. Manohar, A. Jagota and
M. Zheng, Nature, 2009, 460, 250–253; (c) Y. Lian, Y. Maeda,
T. Wakahara, T. Akasaka, S. Kazaoui, N. Minami, N. Choi and
H. Tokumoto, J. Phys. Chem. B, 2003, 107, 12082–12087.
1 T. Ogoshi, Y. Takashima, H. Yamaguchi and A. Harada, J. Am.
Chem. Soc., 2007, 129, 4878–4879.
1
12 M. Wojdyr, J. Appl. Crystallogr., 2010, 43, 1126–1128.
3
102 Chem. Commun., 2012, 48, 3100–3102
This journal is c The Royal Society of Chemistry 2012