Redox-active π-conjugated polymer nanotubes with viologen for encapsulation and release of fluorescent dye in the nanospace

Article abstract of DOI:10.1039/c1cc13853g

We report the encapsulation of negatively charged Au nanoparticles and anionic fluorescent (FL) dye in the inner cavity of redox-active cationic polymer nanotubes with viologen via electrostatic interaction and the release of the FL dye from the FL dye confined in the cavity of the polymer nanotubes by electrochemical and chemical reduction. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1cc13853g

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COMMUNICATION
Redox-active p-conjugated polymer nanotubes with viologen for
encapsulation and release of fluorescent dye in the nanospacew
Keisuke Sato, Tsukasa Nakahodo and Hisashi Fujihara*
Received 28th June 2011, Accepted 20th July 2011
DOI: 10.1039/c1cc13853g
We report the encapsulation of negatively charged Au nano-
particles and anionic fluorescent (FL) dye in the inner cavity of
redox-active cationic polymer nanotubes with viologen via electro-
static interaction and the release of the FL dye from the FL dye
confined in the cavity of the polymer nanotubes by electrochemical
Fig. 1 Fabrication of PT (polythiophene)-NTs and their composites:
and chemical reduction.
(a) bare alumina membrane (AM). (b) Modification of the nanopore
of AM with PT-NTs generated by template-based electropolymerization
of 1-VL2+ leads to 1-VL2+-PT-NT-AM as a polymer nanotube
membrane. The PT-NT-modified nanopore of AM: 1-VL2+-PT-NT-
AM. (c) SFꢀ-AuNPs or Pyra encapsulated in the PT-NT-modified
AM: SFꢀ-AuNP@1-VL2+-PT-NT-AM and Pyra@1-VL2+-PT-NT-AM.
(d) Release of Pyra from Pyra@1-VL2+-PT-NT-AM by reduction.
Nanotubes (NTs) and metal nanoparticles (NPs) represent a
rapidly emerging field of significant fundamental and practical
interest and are receiving worldwide attention.1–3 It is advanta-
geous or required to modify the nanotubes by physical or
chemical methods to optimize their use in many applications.
Recently, we reported new metal NP–polythiophene hybrid
NTs from the electropolymerization of thiophene-modified
metal NPs and the hybrid nanotube-catalyzed carbon–carbon
bond forming reaction in the nanocavity.4 Conducting poly-
thiophenes are indispensable materials for the development of
displays, electronic and energy storage devices, actuators,
sensors, and neural interfaces. The nanotubular structure of
polythiophene is one of the ideal structures that can enhance
the device performance by improving the charge-transport
rate as well as increasing the surface area. The template-
synthesized conducting polymer NTs can have relatively large
cavities and tunable interior chemical functionality.4 Espe-
cially, impregnation of the hollow NTs with guest materials
provides a potentially fascinating route to create functional
nanomaterials for a variety of applications. The development
of a generalized method for the controlled encapsulation/
release of a molecule in the conducting polymer NTs remains
an important and challenging issue.
The template-based electropolymerization of cationic
viologen-linked terthiophene (1-VL2+
) in a nanoporous
alumina membrane (AM) as a template gave the corresponding
polythiophene NT-modified alumina membranes, which were
converted into the redox-active cationic polythiophene NTs
(1-VL2+-PT-NTs) upon dissolution of the alumina membranes
by a sodium hydroxide solution (Chart 1 and Fig. 1).8 The
redox-active cationic polythiophene NT-modified alumina
membrane (denoted as 1-VL2+-PT-NT-AM) as a polymer
nanotube membrane was utilized to incorporate the anionic
fluorescent dye, pyranine (8-hydroxyl-1,3,6-pyrene-trisulfonate:
Pyra), in which the Pyra was expelled from the Pyra confined
within 1-VL2+-PT-NT-AM (denoted as Pyra@1-VL2+-PT-
NT-AM) by electrochemical and chemical reductions of the
viologen units in the cationic NTs (Fig. 1d and Scheme 1).9
There is currently considerable interest in using functional
nanotube membranes in molecular storage and release, and
sensing applications. This communication describes the
encapsulation of ionic metal NPs or Pyra into the cavity of
the redox-active p-nanotubes via electrostatic interaction, and
the preliminary realization of the electron-supply induced the
release of the Pyra from Pyra@1-VL2+-PT-NT-AM.
We have performed the synthesis of new polythiophene (PT)
NTs with redox-active viologen and the nanotube membranes,
and the encapsulation or release of metal NPs or a guest
molecule by the NTs (Chart 1 and Fig. 1). We have chosen
the cationic viologen (N,N0-disubstituted-4,40-bipyridinium
salt: VL2+) group to achieve the assembly in nanopores via
electrostatic attraction as a noncovalent approach.5–7
Department of Applied Chemistry, Kinki University, Kowakae,
Higashi-Osaka 577-8502, Japan. E-mail: h-fuji@apch.kindai.ac.jp;
Fax: +81-6-6727-2024; Tel: +81-6-6721-2332
w Electronic supplementary information (ESI) available: Synthesis of
1-VL2+ and SFꢀ-AuNPs, and the spectral and microscopic data. See
DOI: 10.1039/c1cc13853g
Chart 1 Structure of viologen-linked terthiophene (1-VL2+) and
protective ligand (SFꢀ) for gold NPs.
c
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Chem. Commun., 2011, 47, 10067–10069 10067

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Scheme 1 Proposed mechanism for the encapsulation and release of
Pyra in 1-VL2+-PT-NT-AM.
Fig. 3 Photographs of a solution of SFꢀ-AuNPs in (a) the presence
of bare AM, (b) the absence of bare AM, and (c) the presence of
1-VL2+-PT-NT-AM. (a-0, b-0, c-0) before and (a-1, b-1, c-1) after
overnight storage. (d) Each UV-vis spectrum (d-i, red line) corres-
ponds to the photograph of c-0 and (d-ii, blue line) corresponds to the
photograph of c-1.
The electropolymerization was performed by attaching the
alumina membrane (AM) (Whatman Anodisc; pore diameter
of about 200–250 nm, thickness of 60 mm) to a Pt electrode,
another Pt electrode was used as the counter electrode, and
Ag/0.1 M AgNO3 was used as the reference electrode. The
electrolysis solution contains 1-VL2+ (5 mM) in 0.1 M
Bu4NClO4–MeCN (1 mL). A voltage of +0.75 V (vs. Ag/
0.1 M AgNO3) was applied for 20 min. The 1-VL2+-PT-NT-
modified nanopore of AM (1-VL2+-PT-NT-AM) was formed.
After the electropolymerization, removal of the alumina
membrane of 1-VL2+-PT-NT-AM with 1 M NaOH led to
the release of 1-VL2+-PT-NTs. Scanning and transmission
electron microscopies (SEM and TEM), and energy-dispersive
X-ray (EDX) spectroscopy were used for the analysis of the
NTs. Fig. 2a shows an SEM image of the 1-VL2+-PT-NTs,
which were grown using the nanoporous alumina template;
most of them have outer diameters of about 200–230 nm. The
elemental composition of the 1-VL2+-PT-NTs was confirmed
by an EDX analysis, which reveals the presence of C, O, and S
(Fig. 2b). The redox behaviour of 1-VL2+-PT-NTs was studied
by cyclic voltammetry (vide infra).
The UV-vis spectra of SFꢀ-AuNPs in H2O showed a plasmon
resonance at 533 nm in the absence and the presence of the
bare alumina membrane (Fig. 3d–i). Interestingly, in contrast,
in the presence of 1-VL2+-PT-NT-AM (Fig. 3d–ii), the absorption
intensity decreased with the storage time, suggesting the con-
tinuing encapsulation of the Au NPs into the inner cavity of
the 1-VL2+-PT-NT-AM. The colour of the solution turned
from pink to colourless after overnight storage (Fig. 3c–0 and
c–1). The immersion of the 1-VL2+-PT-NT-AM into a solution
of SFꢀ-AuNPs was done, and any Au NPs remaining in solution
were removed by filtration and washed several times with water.
After removal of the alumina membrane template by the 1 M
NaOH solution, the Au NPs confined inside the NTs (denoted
as SFꢀ-AuNP@1-VL2+-PT-NTs) were obtained by filtration.
The composite, SFꢀ-AuNP@1-VL2+-PT-NTs, contains Au
and S, as evidenced by the EDX analysis (Fig. S1c, ESIw).
Thus, the electrostatic interaction should be effective for
fabricating a metal NP-PT-NT composite in the nanopore.
Importantly, this approach based on the electrostatic inter-
action provides an efficient method to attach other nano-
structures to functionalized polythiophene NTs and can be
used as an illustrative detection of functional groups on the
inner walls of polythiophene NTs.
In order to confirm the electrostatic interaction between an
anionic guest material and the inner surfaces of the cationic
NTs, 4-(10-mercapto-decyloxy)-benzenesulfonic acid sodium
salt (SFꢀ: Chart 1) and the SFꢀ-stabilized gold NPs (SFꢀ-
AuNPs) were prepared.8 We used the NT-modified alumina
membrane for the encapsulation of the Au NPs or Pyra into
the inner surfaces of the NTs because the outer surfaces of the
NTs are in contact with the pore wall of the alumina
membrane and thus masked.
The encapsulation of SFꢀ-AuNPs (4.6 ꢁ 0.5 nm, Fig. S1a and b:
We have succeeded in the encapsulation and release of an
ESIw) into 1-VL2+-PT-NTs was conducted as follows (Fig. 3).8
organic molecule, such as Pyra, using the redox-active 1-VL2+
-
PT-NTs via an electron transfer process.7,9 We postulate that
the positively charged inner surfaces of the 1-VL2+-PT-NTs
strongly accelerate the encapsulation of the negatively charged
Pyra via an electrostatic interaction. In contrast, such an
interaction between Pyra and the viologen neutral form gener-
ated by the two-electron reduction of the viologen dication is
unfavourable (Scheme 1); accordingly, the release of Pyra from
the Pyra confined within the 1-VL2+-PT-NTs will be confirmed
by the reduction (Fig. 1d). To prove this concept, the encapsu-
lation of Pyra into the 1-VL2+-PT-NTs was examined. The
UV-vis spectra of a Pyra solution in the absence or the presence
of a bare alumina membrane in water showed the characteristic
absorption due to the Pyra (Fig. 4a); while no absorption in
the UV-vis region was observed in the presence of 1-VL2+-PT-
NT-AM (Fig. 4b), indicating the formation of Pyra@1-VL2+
-
Fig. 2 (a) SEM image and (b) EDX spectrum of 1-VL2+-PT-NTs.
Scale: 100 nm. (c) Cyclic voltammogram of 1-VL2+-PT-NTs on a GC
PT-NT-AM by the encapsulation of Pyra into the NTs. On the
other hand, the cyclic voltammogram of the 1-VL2+-PT-NTs in
electrode in 0.1 M Bu4NClO4–MeCN; scan rate 100 mVꢀ1
.
c
10068 Chem. Commun., 2011, 47, 10067–10069
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In summary, we have shown that the nanopore functiona-
lization by the electropolymerization of terthiophene with a
positively charged pendant in the alumina membrane provides
new functional polymer NTs and their polymer NT membranes.
We have demonstrated the encapsulation and release of an
anionic Pyra fluorescent dye in the redox-active cationic polymer
NTs containing viologen via an electrostatic interaction. We
believe that this approach will have broad implications in the
reversible encapsulation/release of molecules using redox-active
polymer NTs. We expect this research to yield a broad range
of potential applications in catalysis, nanoelectrochemistry,
molecular separation, drug delivery, controlled-release devices,
and sensor developments.
Fig. 4 Structure of Pyra. UV-vis spectra and photographs of a solution
of Pyra (a) after overnight storage in the presence of bare AM and
(b) after overnight storage in the presence of 1-VL2+-PT-NT-AM in
water. The photograph of (a) shows a typical colour of Pyra in water.
This work was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas (No. 21108522 and 23108723,
‘‘pi-Space’’) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
Notes and references
1 (a) C. N. R. Rao and A. Govindaraj, Nanotubes and Nanowires,
RSC Publishing, 2005; (b) G. A. Ozin, A. C. Arsenault and
L. Cademartiri, Nanochemistry, RSC Publishing, 2009.
Fig. 5 Photographs of a solution of (a-0) before electrochemical
reduction of Pyra@1-VL2+-PT-NT-AM in water and (a-1) after
electrochemical reduction of Pyra@1-VL2+-PT-NT-AM in water.
The fluorescence of Pyra was monitored by irradiation at 365 nm
2 N. Karousis and N. Tagmatarchis, Chem. Rev., 2010, 110, 5366.
3 (a) A. N. Aleshin, Adv. Mater., 2006, 18, 17; (b) H. D. Tran, D. Li
and R. B. Kaner, Adv. Mater., 2009, 21, 1487; (c) D. T. Mitchell,
S. B. Lee, L. Trofin, N. Li, T. K. Nevanen, H. Soderlund and
C. R. Martin, J. Am. Chem. Soc., 2002, 124, 11864; (d) S. I. Cho,
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(f) C. Bae, H. Yoo, S. Kim, K. Lee, J. Kim, M. M. Sung and
H. Shin, Chem. Mater., 2008, 20, 756.
UV light. (b) Photographs for monitoring reaction of Pyra@1-VL2+
-
PT-NT-AM with dithionite: (b-0) before addition, (b-1) after 120 s,
and (b-2) after 180 s. The fluorescence of Pyra was monitored by
irradiation at 365 nm UV light. The photographs of b-1 and b-2
showed the fluorescence of the released Pyra.
4 (a) R. Umeda, H. Awaji, T. Nakahodo and H. Fujihara, J. Am.
Chem. Soc., 2008, 130, 3240; (b) H. Awaji, T. Nakahodo and
H. Fujihara, Chem. Commun., 2011, 47, 3547; (c) The unique
properties of conductive polymer NTs are as follows: (i) high
electrical conductivity, (ii) large specific surface area, (iii) short
path lengths for the transport of ions.
5 (a) S. W. Thomas, G. D. Joly and T. M. Swager, Chem. Rev., 2007,
107, 1339; (b) H. Jiang, P. Taranekar, J. R. Reynolds and
K. S. Schanze, Angew. Chem., Int. Ed., 2009, 48, 4300.
0.1 M Bu4NClO4–MeCN at a glassy carbon (GC) electrode
exhibited two reversible redox peaks corresponding to the VL2+
dication–VL+ radical cation–VL neutral system at E1/2
=
ꢀ0.69 and ꢀ1.13 V (vs. Ag/0.1 M AgNO3) (Fig. 2c). Thus, the
electronic state of the 1-VL2+-PT-NTs can be electrochemically
controlled. Consequently, at 1200 s, a potential of ꢀ1.25 V
(Ag/0.1 M AgNO3) was applied to the Pyra@1-VL2+-PT-
NT-AM. At this potential the viologen is present in the neutral
form by the reduction (Scheme 1), consequently, the Pyra
would be released from Pyra@1-VL2+-PT-NT-AM. Actually,
6 The cationic viologen and the anionic poly-sulfonate formed an
electrostatic complex.5a
.
7 Viologens (VLs) are well-known good electron acceptors that have
been found to quench the fluorescence of numerous dyes.11 The
charges facilitate two possible types of processes: electron transfer
and Coulombic attraction. Additionally, VL indicates that the one-
electron reduction of VL forms the radical cation and the two-electron
reduction forms the neutral species.
after the electrochemical reduction of the Pyra@1-VL2+
-
PT-NT-AM, the Pyra was released from the NT-alumina
membrane as evidenced from the UV-vis spectra and the
observed fluorescence due to the released Pyra (Fig. 5a-0 and
a-1). Moreover, the injection of sodium dithionite (Na2S2O4)
as a reductant into the Pyra@1-VL2+-PT-NT-AM induced
the release of the Pyra;10 i.e., the fluorescence of the released
Pyra from Pyra@1-VL2+-PT-NT-AM was observed when a
solution of sodium dithionite in water was injected into the
Pyra@1-VL2+-PT-NT-AM using a syringe pump (Fig. 5b-0–b-2).
It is noteworthy that the redox-active viologen-functionalized
polythiophene NT membrane is very unique from the standpoint
of electrochemical control of the reversible encapsulation/
release system. Further studies on this preliminary examination
for the encapsulation and release of Pyra in the redox-active
polythiophene NTs are now in progress.
8 See the ESIw.
9 Pyranine is an electron donor and a highly water-soluble anionic
fluorescent dye that has been most commonly used as a probe in
biochemical systems.12 Viologens act as fluorescence quenchers for
pyranine.13 The donor–acceptor charge-transfer complex of Pyra
and viologen is readily formed.13
.
10 The dicationic viologen can be reduced to the neutral species by the
dithionite ion: K. Tsukahara and R. G. Wilkins, J. Am. Chem.
Soc., 1985, 107, 2632.
11 P. M. S. Monk, The Viologens: Physicochemical Properties, Synthesis
and Applications of the Salts of 4,40-Bipyridine, Wiley, New York, 1998.
12 Y. Avnir and Y. Barenholz, Anal. Biochem., 2005, 347, 34.
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M. S. Baptista, Langmuir, 2000, 16, 5900; (b) S. Gamsey, A. Miller,
M. M. Olmstead, C. M. Beavers, L. C. Hirayama, S. Pradhan,
R. A. Wessling and B. Singaram, J. Am. Chem. Soc., 2007, 129, 1278.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 10067–10069 10069