Polypeptide/Multiwalled carbon nanotube hybrid complexes stabilized through noncovalent bonding interactions

Article abstract of DOI:10.1002/pola.27000

In this study, we used click chemistry to synthesize new linear polypeptide-g-pyrene polymers from a mono-azido-functionalized pyrene derivative (N₃-Py) and several poly(γ-propargyl-l-glutamate) (PPLG) oligomers. Incorporating the pyrene units as side chains enhanced the α-helical conformations of these PPLG oligomers in the solid state, as determined using Fourier transform infrared (FTIR) spectroscopy; it also increased the temperature stability of the α-helical secondary structures of the grafted PPLG oligomers, relative to those of the pure PPLG species, as revealed through temperature-dependent FTIR spectroscopic analyses. In addition, the thermal properties of the PPLG-g-Py polypeptides (e.g., glass transition temperatures increased by ca. 100 °C) were superior to those of pure PPLG oligomers. Mixing the PPLG-g-Py oligomers with multiwalled carbon nanotubes (MWCNTs) in dimethylformamide led to the formation of highly dispersible PPLG-g-Py/MWCNT organic/inorganic hybrid complex materials. Fluorescence emission spectra revealed significant π-π stacking between the PPLG-g-Py oligomers and the MWCNTs in these complexes. 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 321-329 Click chemistry is used to synthesize new linear polypeptide-g-pyrene polymers from a mono-azido-functionalized pyrene derivative (N3-Py) and several poly(γ-propargyl-l-glutamate) (PPLG) oligomers. Incorporating the pyrene units as side chains enhances the α-helical conformations of these PPLG oligomers in the solid state and increases the temperature stability of the α-helical secondary structures of the grafted PPLG oligomers, relative to those of the pure PPLG species. Copyright

Full text of DOI:10.1002/pola.27000

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Polypeptide/Multiwalled Carbon Nanotube Hybrid Complexes Stabilized
Through Noncovalent Bonding Interactions
Yung-Chih Lin, Shiao-Wei Kuo
Department of Materials and Optoelectronic Science, Center for Nanoscience and Nanotechnology, National Sun Yat-Sen
University, Kaohsiung 80424, Taiwan
Correspondence to: S.-W. Kuo (E-mail: kuosw@faculty.nsysu.edu.tw)
Received 10 September 2013; accepted 23 October 2013; published online 18 November 2013
DOI: 10.1002/pola.27000
ABSTRACT: In this study, we used click chemistry to synthesize
new linear polypeptide-g-pyrene polymers from a mono-azido-
functionalized pyrene derivative (N3-Py) and several poly(c-pro-
pargyl-L-glutamate) (PPLG) oligomers. Incorporating the pyrene
units as side chains enhanced the a-helical conformations of
these PPLG oligomers in the solid state, as determined using
Fourier transform infrared (FTIR) spectroscopy; it also
increased the temperature stability of the a-helical secondary
structures of the grafted PPLG oligomers, relative to those of
the pure PPLG species, as revealed through temperature-
dependent FTIR spectroscopic analyses. In addition, the ther-
mal properties of the PPLG-g-Py polypeptides (e.g., glass tran-
sition temperatures increased by ca. 100 ꢀC) were superior to
those of pure PPLG oligomers. Mixing the PPLG-g-Py oligom-
ers with multiwalled carbon nanotubes (MWCNTs) in dimethyl-
formamide led to the formation of highly dispersible PPLG-g-
Py/MWCNT organic/inorganic hybrid complex materials. Fluo-
rescence emission spectra revealed significant p–p stacking
between the PPLG-g-Py oligomers and the MWCNTs in these
C
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complexes.
2013 Wiley Periodicals, Inc. J. Polym. Sci., Part
A: Polym. Chem. 2014, 52, 321–329
KEYWORDS: biomaterials; carbon tube; nanotechnology; poly-
peptides; thermal property
mation in the solid state, as determined using Fourier trans-
form infrared (FTIR) spectroscopy, solid-state nuclear
magnetic resonance (NMR) spectroscopy, and wide-angle X-
ray diffraction analyses. In addition to polypeptide/POSS
nanocomposites,20–24 the covalent surface functionalization
of carbon nanotubes (CNTs) with polypeptides has also
received much recent attention for its biological applica-
tions.25 For example, Yao et al.26 proposed a “graft-from”
approach to synthesize polypeptide-modified multiwalled
carbon nanotubes (MWCNTs); the MWCNTs were first
amino-functionalized and then used as initiators to form
PBLG-MWCNTs through ring-opening polymerization (ROP)
of the monomer c-benzyl-L-glutamate N-carboxyanhydride
(BLG-NCA). In addition, Tang and Zhang27 used amino-
functionalized single-walled carbon nanotubes (SWCNTs) as
initiators to synthesize grafted PBLG polymers through ROP
of the BLG-NCA. Tang et al.28 proposed another “graft-to”
method for the synthesis of polypeptide-modified SWCNTs
through addition reactions using azido-terminated PBLG
derivatives. Such covalent functionalization of CNTs, however,
often changes the hybridization of the carbon atoms from
sp2 to sp3, potentially deteriorating the optical, electronic,
and mechanical properties. Noncovalent approaches toward
the functionalization of CNTs, without destroying their
INTRODUCTION Polypeptides are interesting materials
because of their close relationship with proteins: they can
form hierarchically ordered structures containing a-helices,
which can be regarded as a rigid rods stabilized through
intramolecular hydrogen bonding interactions, and b-sheets,
stabilized by intermolecular interactions, as fundamental sec-
ondary motifs.1,2 Performing conformational studies of model
polypeptides is a necessary step toward mimicking the bio-
logical activity of more complex proteins.3,4
The study of polypeptides has often focused on block copoly-
mers featuring organic nonpolypeptide block segments for
tissue engineering and drug delivery applications5–15; these
block segments have included poly(ethylene oxide),16 poly(e-
caprolactone),17 poly(N-isopropyl acrylamide),18 and poly(2-
ethyl-2-oxazoline).19 In the previous studies, we combined
polyhedral oligomeric silsesquioxane (POSS) derivatives,
inorganic nanoparticles having well-defined macromolecular
architectures, with polypeptides to generate polymeric build-
ing blocks substituted at the chain end [POSS-b-poly(c-ben-
zyl-L-glutamate) (PBLG)]20–22 or on the side chains [poly(c-
propargyl-L-glutamate) (PPLG)-g-POSS].23,24 We found that
the incorporation of POSS units at the chain end or side
chains of a polypeptide moiety enhanced its a-helical confor-
Additional Supporting Information may be found in the online version of this article.
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SCHEME 1 Synthesis of PPLG-g-Py polypeptides. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
electronic configurations, can enhance their dispersibility
while maintaining their electronic and mechanical properties.
Tang and Zhang29 demonstrated that SWCNTs can be dis-
persed homogeneously through p–p stacking with a chain
end-modified pyrenyl-PBLG derivative, taking advantage of
the ability of pyrene groups to form strong p–p interactions
with CNT surfaces.
Bromomethylpyrene (Br-Py)
A solution of OH-Py (3.00 g, 12.9 mmol) and PBr3 (1.41 mL,
15.0 mmol) in dry tetrahydrofuran (THF, 10.0 mL) was
stirred at room temperature for 30 min under Ar. After fil-
tering, the residue was washed several times with Et2O. The
resulting product was filtered off and dried under vacuum at
room temperature (2.67 g, 70%).
If a pyrene moiety is located only at the chain end of a poly-
peptide, then the amount of pyrene in a sample would be
extremely low and less effective at stabilizing a CNT. There-
fore, in this study, we linked pyrene moieties to the side
chains of a polypeptide and investigated the effects of such
grafting on the self-assembly and secondary structures of the
polypeptide. First, we prepared PPLG oligomers with different
degrees of polymerization (DP) through ROP of c-propargyl-L-
glutamate N-carboxyanhydride (PLG-NCA), using butylamine
as the macroinitiator. Next, we synthesized PPLG-g-Py poly-
peptides through click reactions (Scheme 1) with an azido-
functionalized pyrene derivative (N3-pyrene). We character-
ized the secondary structures of PPLG and PPLG-g-Py using
FTIR spectroscopy and investigated the interactions and dis-
persibility of MWCNTs and PPLG-g-Py using fluorescence
spectroscopy and transmission electron microscopy (TEM).
1H NMR (CDCl3): d 5 8.35–7.97 (m, 9H), 5.21 (s, 2H). 13C
NMR (CDCl3): d 5 32.18, 122.73, 124.53, 124.77, 125.01,
125.53, 125.56, 126.19, 127.24, 127.60, 127.94, 128.14,
128.97, 130.46, 130.68, 131.12, 131.86 (Supporting Informa-
tion Figs. S2 and S3).
Azidomethylpyrene (N3-Py)
NaN3 (1.00 g, 15.4 mmol) was added to a solution of Br-Py
(1.50 g, 5.08 mmol) in DMF (20 mL) and then the mixture
was heated at 60 ꢀC for 8 h under Ar. The solution was
cooled to room temperature, diluted with Et2O (100 mL),
and washed with 1 M of NaHCO3 (2 3 50 mL) and water (2
3 50 mL). The organic phase was dried (MgSO4) and con-
centrated under vacuum to yield a yellowish solid (1.18 g,
90%).
1H NMR (CDCl3): d 5 8.25–7.95 (m, 9H), 5.01 (s, 2H). 13C
NMR (CDCl3): d 5 32.18, 122.61, 124.58, 124.98, 125.50,
125.57, 126.13, 127.26, 127.39, 127.82, 128.19, 128.32,
129.18, 130.66, 131.16, 131.71 (Supporting Information Figs.
S2 and S3).
EXPERIMENTAL
Materials
Phosphorus tribromide (PBr3) was purchased from Acros.
Propargyl alcohol and L-glutamic acid were purchased from
Tokyo Kasei Kogyo. Copper(I) bromide (CuBr) was purified
by washing with glacial AcOH overnight, followed by wash-
ing with absolute EtOH and Et2O, and then drying under vac-
uum. 1-Pyrenemethanol (OH-Py), N,N-dimethylformamide
(DMF), sodium azide (NaN3), and N,N,N0,N00,N00-pentamethyl-
diethylenetriamine (PMDETA, 99%) were purchased from
Aldrich. All solvents were distilled prior to use. PPLG was
prepared according to a previously reported procedure.23,30
PPLG, Through ROP of PLG-NCA
PLG-NCA was frozen under vacuum using liquid N2. DMF
was added via syringe and then the resulting solution was
warmed to 0 ꢀC and N2 gas was added to fill the reactor.
Butylamine, the initiator, was added and the mixture was
stirred for 48 h at 0 ꢀC. The solvent was evaporated under
vacuum and then the solid residue was dissolved in DMF
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FIGURE 1 1H NMR spectra of PPLG5, N3-Py, and PPLG5-g-Py polypeptides. [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
and precipitated with cold Et2O or MeOH. The polymer was
filtered off and dried under vacuum at room temperature.
residual proton resonance of the deuterated solvent acting
as the internal standard. Molecular weights and molecular
weight distributions were determined through gel permea-
tion chromatography using a Waters 510 high-performance
liquid chromatography system equipped with a 410 differen-
tial refractometer and three Ultrastyragel columns (100, 500,
and 103 Å) connected in series, with DMF as the eluent
(flow rate, 0.4 mL/min). Thermal analysis through differen-
tial scanning calorimetry (DSC) was performed using a TA-
Q20 instrument operated at a scan rate of 10 ꢀC/min over
the temperature range from 290 to 1200 ꢀC under a N2
atmosphere. FTIR spectra of the polymer films were
recorded using the conventional KBr disk method. The films
used in this study were sufficiently thin to obey the Beer–
Lambert law. FTIR spectra were recorded using a Bruker
Tensor 27 FTIR spectrophotometer; 32 scans were collected
at a spectral resolution of 1 cm21. As polymers containing
amide groups are hygroscopic, pure N2 gas was used to
purge the spectrometer’s optical box to maintain dry sample
films. Mass spectra were recorded using a BrukerDaltonic-
sAutoflex III MALDI-TOF mass spectrometer with the follow-
ing voltage parameters: ion source 1, 19.06 kV; ion source 2,
16.61 kV; lens, 8.78 kV; reflector 1, 21.08 kV; reflector 2,
9.73 kV. TEM images were recorded using a JEOL-2100
transmission electron microscope operated at an accelerating
voltage of 200 kV.
Poly(c-propargyl-L-glutamate)-g-Py
N3-Py (0.60 g, 2.33 mmol), PPLG (0.20 g, 1.198 mmol), and
CuBr (0.17 g, 1.180 mmol) were dissolved in DMF (30 mL)
in a flask equipped with a magnetic stirrer bar. After one
brief freeze/pump/thaw cycle, PMDETA (24.6 mL, 1.18
mmol) was added. The reaction mixture was then carefully
degassed through three freeze/pump/thaw cycles, placed in
ꢀ
an oil bath thermostated at 60 C, and stirred for 24 h. After
evaporating the solvent under reduced pressure, the residue
was dissolved in THF and passed through a neutral alumina
column to remove the copper catalyst. The concentrated THF
solvent was evaporated, and the product precipitated using
MeOH. The resulting solid was filtered off and dried under
vacuum at room temperature, yielding a yellowish powder.
PPLG-g-Py/MWCNT Complexes
Supramolecular complexes of PPLG-g-Py and MWCNTs were
prepared via solution blending as follows: A dispersion of 1
mg of MWCNTs in 5 mL of DMF was sonicated for 1 h and
then the dispersed solution was added to a solution contain-
ing 50 mg of PPLG-g-Py in 5 mL of DMF. The mixture was
sonicated for an additional 1 h, followed by centrifuge at
5000 rpm for 90 min. The residue was collected by filtration
and washed thoroughly with DMF (more than five times)
until the filtrate was no longer fluorescent when irradiated
with UV light (365 nm), indicating thatꢀall excess free poly-
mer had been removed and dried at 30 C under vacuum for
24 h.
RESULTS AND DISCUSSION
Synthesis of Pyrene-grafted Polypeptides (PPLG-g-Py)
1
Figure 1 shows the H NMR spectra of N3-Py, PPLG-g-Py, and
PPLG. The signal of the methylene group of N3-Py (5.01
ppm) shifted downfield significantly relative to the corre-
sponding signal of PPLG-g-Py (6.41 ppm), as did the signal
of the methylene units of the propargyl groups of PPLG
Characterization
1H and 13C NMR spectra were recorded at room temperature
using a Bruker AM 500 (500 MHz) spectrometer, with the
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FIGURE 2 13C NMR spectra of PPLG5, N3-Py, and PPLG5-g-Py polypeptides. [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
(4.80 ppm) after forming the corresponding methylene units
in PPLG-g-Py (5.18 ppm). In addition, the signals for the
pyrene moiety also appeared in the spectrum of PPLG-g-Py.
The sharp resonance at 7.90 ppm was owing to the protons
of the triazole structures that resulted from the click reac-
tions, confirming the successful synthesis of the PPLG-g-Py
polypeptides. Figure 2 shows the 13C NMR spectra of N3-Py,
PPLG-g-Py, and PPLG. The signals of the carbon atoms of the
ester and amide units of PPLG-g-Py appeared at 173.0 and
174.1 ppm, respectively; those of the pyrenyl aromatic ring
of N3-Py appeared at 124.0, 125.0, 126.3, 126.7, 128.3,
129.2, 129.4, 130.1, 130.8, and 133.3 ppm. The signal for the
methylene group of N3-Py (53.0 ppm) shifted downfield sig-
nificantly relative to the corresponding signal (56.1 ppm) for
the PPLG-g-Py polypeptides. The signals for the ester carbon
atom (C-j) and the amino acid a-carbon atoms (NHCOC)
appear at 57.4 and 54.3 ppm (a-helical conformation),
respectively. The signal of the alkyne carbon atoms of PPLG
(originally at 73.4 ppm) disappeared, but two new peaks
appeared—at 121.4 and 138.2 ppm—representing the car-
bon atoms of the triazole structures that resulted from the
click reactions, thereby confirming the synthesis of PPLG-g-
FIGURE 3 FTIR spectra of (a) N3-Py, (b) PPLG5-g-Py, and (c) PPLG5. [Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
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FIGURE 4 MALDI-TOF mass spectra of the oligopeptides (a) PPLG5 and (b) PPLG5-g-Py. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
Py. Figure 3 shows the complete disappearance of the char-
acteristic signals for the azido and acetylene groups in the
FTIR spectra of the product grafter polypeptides. The signal
at 2100 cm21, corresponding to the absorbance of the azido
group of N3-Py, and the signal at 2130 cm21, corresponding
to the acetylene groups of PPLG, were absent in the spectra
of the PPLG-g-Py oligomers. The absorption bands of the
pyrene units at 846, 1589, and 1602 cm21 and the amide I
groups of PPLG at 1655 cm21 were also evident in the spec-
tra of the PPLG-g-Py-grafted systems, indicating that it was
only the azido and acetylene functionalities that had partici-
pated in the click reactions. The well-defined structure was
confirmed in the corresponding MALDI-TOF MS spectra (Fig.
4), which revealed only one apparent distribution for both
PPLG and PPLG-g-Py. The mass differences between all adja-
cent pairs of peaks were m/z 167 for PPLG and m/z 424 for
PPLG-g-Py, as expected for the repeating units of PPLG and
Thermal Analyses of Pyrene-grafted Polypeptides
(PPLG-g-Py)
Figure 5 shows DSC thermograms of the PPLG and PPLG-g-
Py polypeptides. During the second heating run of each of
these systems, we observed only one glass transition temper-
ature (Tg, at ca. 20 ꢀC), which increased slightly upon
increasing the DP of the pure PPLG.23 After grafting the
pyrene moieties onto the PPLG oligomers, we observed a
dramatic change in the second heating run of the DSC analy-
ses. The Tgs of the PPLG-g-Py oligomers increased significantly
to approximately 117 ꢀC—that is, they were greater than those
of the PPLG oligomers by approximately 97 ꢀC. The values also
1
PPLG-g-Py, respectively. Taken together, the data from the H
and 13C NMR, FTIR, and MALDI-TOF MS spectra all con-
firmed the successful syntheses of the pyrene fluorophore-
containing polypeptides (PPLG-g-Py) and the results are
summarized in Table 1.
TABLE 1 Characterizations of PPLG-g-Py Used in this Study
PDIb
a
b
Polymers
Mn
Mn
PPLG5
910
3,090
6,890
2,960
11,870
1.07
1.13
1.07
1.16
PPLG30
5,080
2,190
12,790
PPLG5-Py
PPLG30-Py
a
Determined from 1H NMR spectra.
Determined from GPC analysis.
FIGURE 5 DSC traces (second heating runs) of the polypep-
tides (a) PPLG and (b) PPLG-g-Py.
b
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PBLG9, where the Py unit was located only on the terminus
of the polypeptide; because the amount of pyrene in this
polypeptide was extremely low, Py-b-PBLG9 possessed an a-
helical conformation of only 35.0%.29 Jeon et al.33 reported
that the mobility of the side chains of polyglutamates affects
their a-helical conformations; their experimental data
revealed that longer, flexible side chains induced weaker
hydrogen bonds between the ester C@O groups and the
amide linkages in the a-helical conformation. Thus, we sus-
pect that the rigid pyrene moieties enhanced the strength of
the hydrogen bonds between the ester C@O groups and the
amide linkages in the a-helical conformation, similar to the
behavior in grafted PPLG-g-POSS species.23
We also recorded temperature-dependent FTIR spectra to
obtain information relating to the conformations of the pep-
tide segments. Figure 7 shows the FTIR spectroscopic data
for PPLG5 and PPLG5-g-Py. The amide I and amide II bands
appeared at 1655 and 1545 cm21, respectively, in the a-
helical secondary structure; they shifted to 1626 and 1520
cm21, respectively, in the b-sheet conformation. Here, we
chose to examine the PPLG5 oligomer because it possessed
both a-helical and b-sheet conformations; the longer PPLG30
oligomer adopted the a-helical secondary structure almost
entirely, making it insensitive to changes in temperature. The
spectra of PPLG5-g-Py indicated that the a-helical secondary
structure was stabilized significantly upon attachment of the
rigid pyrene units, relative to the FTIR spectra of the PPLG
homopeptide. The fraction of a-helical secondary structures
of PPLG5 decreased significantly upon increasing the temper-
ature [Fig. 7(A)]; in contrast, the secondary structure of
PPLG-g-Py remained almost unchanged [Fig. 7(B)]. Thus, the
DP, the temperature, and the presence of pyrene moieties all
had a strong influence on the stability of the secondary
structure. Increasing the DP of all of the polypeptides and
attaching pyrene units both led to stabilization of the a-
helical secondary structures with respect to increasing
temperatures.
FIGURE 6 FTIR spectra of (A) the PPLG species (a) PPLG5 and
(b) PPLG30 and (B) the PPLG-g-Py species (c) PPLG5-g-Py and
(d) PPLG30-g-Py. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
increased slightly upon increasing the DP of the PPLG-g-Py-
grafted oligomers. The significant increases in the values of Tg
of the PPLG-g-Py species were owing to the effects of p–p
stacking and the rigid aromatic structures of the pyrene moi-
eties, which effectively restricted the chain motion of the PPLG
segments, a situation similar to that we had noted after the
modification of oligopeptides with POSS nanoparticles.23,24,31
Conformations of the Peptide Segments
We recorded FTIR spectra at room temperature to obtain
information about the conformations of the peptide seg-
ments of the PPLG and PPLG-g-Py oligomers (Fig. 6). Analy-
sis of these spectra using the second-derivative technique4
indicated that the amide I band at 1655 cm21 was character-
istic of a-helical secondary structures. For polypeptides pos-
sessing a b-sheet conformation, the amide I band would
have shifted to 1627 cm21; random coil or turn populations
would feature a band located at 1693 cm21. The signal of
the free C@O groups of the side chains of PPLG appeared at
1740 cm21 [Fig. 6(A)], with the signals at 1589 and 1602
cm21 corresponding to stretching of the aromatic rings of
the pyrene units of the side chains [Fig. 6(B)]. Next, we used
the deconvolution technique to obtain a series of Gaussian
distributions to quantify the fraction of each peak [inset, Fig.
6(A)]. The fraction of a-helical secondary structures
increased upon increasing the DP of the PPLG oligopeptides;
this result is similar to that reported by Papadopoulos
et al.32 for PBLG. At low DPs (<18), both secondary struc-
tures were present, but as the DP increased, the a-helical
secondary structure was favored. In addition, all of our
grafted PPLG-g-Py species obtained after performing the
click reactions possessed a-helical secondary structures, even
those having a relatively low DP (e.g., PPLG5-g-Py). This
behavior is different from that observed previously for Py-b-
FIGURE 7 FTIR spectra of the oligopeptides (a) PPLG5 and (b)
PPLG5-g-Py at various temperatures.
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FIGURE 9 Photograph of (a) pure PPLG-g-Py polypeptides, (b)
pristine MWCNTs, and (c) PPLG-g-Py/MWCNT hybrid com-
plexes in DMF (each 1 mg/mL).
pure PPLG-g-Py oligomers, and the PPLG-g-Py oligomers in
the presence of the MWCNTs. The pure PPLG-g-Py oligopep-
tides formed a clear solution, whereas the pure MWCNTs
precipitated completely; the addition of the PPLG-g-Py
oligomers into the suspension of the MWCNTs resulted in a
clear brown solution, indicating that soluble complexes were
formed as a result of noncovalent interactions (primarily p–p
stacking). Supporting Information Figure S5 shows TGA anal-
yses of pristine MWCNT, pure PPLG30-g-Py polypeptide, and
PPLG30-g-Py/MWCNT hybrid complexes. At 600 ꢀC, we
observed most of the pure PPLG30-g-Py polypeptide is about
20 wt % char yield, and the compounds of pristine MWCNT
and PPLG30-g-Py/MWCNT hybrid complexes were 95.9 and
62.6 wt % char yield, respectively. If a similar weight-loss
behavior of pure PPLG30-g-Py polypeptide and pristine
MWCNT is assumed to take place in the hybrid complexes,
then the pure PPLG30-g-Py polypeptide content in the hybrid
complexes can be roughly estimated by TGA analysis and to
be 33.2 wt %. We used TEM to further investigate the mor-
phologies of dispersions of the pristine MWCNTs and the
PPLG-g-Py/MWCNT hybrid complexes in EtOH (each at 1023
g/mL) after ultrasonic vibration. Figure 10(a) shows that the
FIGURE 8 PL spectra of PPLG-g-Py/MWCNT hybrid complexes.
PPLG-g-Py/MWCNT Complexes Stabilized Through
p–p Interactions
Figure 8 shows fluorescence spectra (UV spectra in Support-
ing Information Fig. S4) of the PPLG-g-Py oligomers and the
PPLG-g-Py/MWCNT complexes in solution after excitation at
343 nm. The spectra for PPLG-g-Py exhibited strong fluores-
cence signals for both monomeric (374 nm) and dimeric
(493 nm) pyrene units; the spectra of the PPLG-g-Py/
MWCNT complexes exhibited very weak excimer fluorescence
and quenched monomer fluorescence.34,35 These findings
suggest that most of the pyrene moieties of the PPLG poly-
mers interacted with the MWCNTs through p–p stacking,
with only a few of them available for excimer formation as a
result of steric hindrance; therefore, our proposed model
consists of coexisting PPLG-g-Py oligomers (Py/Py excimer)
and PPLG-g-Py/MWCNT nanohybrids. Scheme 2 shows possi-
ble mechanisms of formation and structures determined
from fluorescence spectroscopic analyses. Figure 9 shows
photographs of DMF solutions of the pure MWCNTs, the
SCHEME 2 Preparation of PPLG-g-Py/MWCNT hybrid complexes stabilized through p–p interactions. [Color figure can be viewed
in the online issue, which is available at wileyonlinelibrary.com.]
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FIGURE 10 Typical TEM images of (a) pristine MWCNTs and (b) PPLG-g-Py/MWCNT hybrid complexes. [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.com.]
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