Multiwalled carbon nanotubes noncovalently functionalized by electro-active amphiphilic copolymer micelles for selective dopamine detection

Article abstract of DOI:10.1039/c4ra16923a

We have synthesized an electro-active amphiphilic copolymer with carbazole side chains via free radical polymerization using 7-(4-vinylbenzyloxy)-4-methyl coumarin and 9-(4-vinylbenzyl)-9H-carbazole as the monomers. The copolymer can self-assemble to form

Full text of DOI:10.1039/c4ra16923a

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Multiwalled carbon nanotubes noncovalently
functionalized by electro-active amphiphilic
copolymer micelles for selective dopamine
detection†
Cite this: RSC Adv., 2015, 5, 18233
*
Xiaoma Fei, Jing Luo, Ren Liu, Jingcheng Liu, Xiaoya Liu and Mingqing Chen
We have synthesized an electro-active amphiphilic copolymer with carbazole side chains via free radical
polymerization using 7-(4-vinylbenzyloxy)-4-methyl coumarin and 9-(4-vinylbenzyl)-9H-carbazole as
the monomers. The copolymer can self-assemble to form micelles (termed EACMs) in aqueous solution
and can adsorb onto the surfaces of MWCNTs via p–p interactions and thereby cause the efficient
dispersion of the MWCNTs in aqueous solution. The coumarin groups in the copolymer undergo
UV-induced photo-crosslinking, which further improves the stability of the suspension. Moreover, the
electro-active carbazole moieties in the EACMs can undergo electropolymerization to form
a
conducting network on the MWCNTs that significantly accelerates electron transfer. The EACM/
MWCNTs hybrid was applied to the amperometric sensing of dopamine (DA) as a model analyte. After
electropolymerization, the electrode exhibited good sensitivity and selectivity toward the determination
of dopamine with a 0.2 mM detection limit and a wide linear range. The method described here provides
a viable route to water-dispersible and stable carbon nanotubes while preserving their outstanding
electrical properties. We presume that the composite described here represents a valuable tool for the
construction of electrochemical sensors and electronics.
Received 23rd December 2014
Accepted 2nd February 2015
DOI: 10.1039/c4ra16923a
www.rsc.org/advances
functionalization of nanotube surfaces by using amphiphilic
copolymers has been recognized as one of the most useful
Introduction
Since the discovery of carbon nanotubes (CNTs) in 1991,1 CNTs
have been the subject of intense interest thanks to their
outstanding thermal, mechanical and electrical properties,
which lead to wide applications in molecular electronics,
sensors, eld-emission devices, biological systems, solar cells,
and high-performance composites.2–7 However, extended van
der Waals interactions between the side walls of CNTs lead to
their aggregation into insoluble and unprocessable bundles of
different lengths and diameters, which greatly limits their
practical utilities in these applications. To improve their solu-
bility and processability, various carbon nanotube surface
modication techniques have been developed, including cova-
lent and noncovalent functionalization.8–11 In recent years, the
approaches to improve the dispersion of CNTs in aqueous
media. Ideally, the hydrophobic segments of an amphiphilic
copolymer interact with the surface of the nanotube through
hydrophobic interactions, whereas the hydrophilic portions are
orientated toward the aqueous phase.12–15
Although homogeneous stable aqueous dispersions of CNTs
have been achieved using amphiphilic copolymers as disper-
sants, the electrical properties of the CNTs are sacriced due to
the presence of the insulating polymer dispersant, which is
detrimental for their applications in electronic or electro-
chemical devices because the insulating polymers act as inter-
facial resistors. In many cases, not only the dissolution of CNTs
but also the preservation of the optoelectronic properties of the
solubilized CNTs is desirable, leading to the development of
CNT hybrid materials with novel and enhanced functional
properties. Therefore, it is highly imperative to develop new
amphiphilic polymers that could efficiently disperse CNTs
without sacricing their outstanding electrical properties.
Carbazole is one of the electro-active units that could elec-
tropolymerize to form a large conjugated structure named pol-
ycarbazole (PCz). PCz shows good electrochemical and thermal
stabilities as well as photorefractive and photoconductive
characteristics.16–18 It has applications in cathode materials,
Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of
Chemical and Material Engineering, Jiangnan University, Wuxi, Jiangsu 214122,
China. E-mail: mq-chen@jiangnan.edu.cn; Fax: +86-510-85917763; Tel: +86-510-
85917763
† Electronic supplementary information (ESI) available: The 1H NMR spectra of
EAC; FT-IR spectra of EAC; TEM images of EACM; UV-Vis spectra of
EACM/MWCNTs; TGA curves of EACM/MWCNTs; increasing ratios of peak
currents for 100 mM DA using different EACM/MWCNTs modied GCE; and CV
of the EACM/MWCNTs modied GCE in 100 mM DA solution with the
electropolymerization of the carbazole moieties are included in the ESI. See
DOI: 10.1039/c4ra16923a
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Fig. 1 Scheme of the functionalization of MWCNTs by EACM and the fabrication of a GCE modified by EACM/MWCNTs for dopamine detection.
electrochromic devices and sensors. Several studies have been pendant groups of the EACM underwent electropolymerization
reported about the incorporation of CNTs into a polymer matrix by cyclic voltammetry (CV) to form a conducting network on the
containing carbazole through chemical and electrochemical surfaces of the MWCNTs. Electrochemical impedance spec-
methods with the goal of combining the unique properties of troscopy (EIS) and CV showed that the electropolymerization of
CNTs and electro-active carbazole.19,20 Considering the electro- EACM could signicantly enhance the electrical conductivity
activity of carbazole and the good electrical conductivity of and accelerate the electron transfer of a EACM/MWCNTs hybrid
polycarbazole, it has been questioned whether it is possible to lm. Application of the EACM/MWCNTs hybrid in electro-
achieve a balance of dispersibility and electrical conductivity for chemical sensing was demonstrated by using dopamine as a
CNTs by using an electro-active amphiphilic copolymer con- model analyte. We found that, compared to the non-
taining carbazole units as a dispersant. On one hand, the electropolymerized EACM/MWCNTs, which showed two over-
amphiphilic copolymer could disperse the CNTs efficiently in lapping peaks of DA and AA, the EACM/MWCNTs aer the
aqueous solution; on the other hand, the carbazole unit in the electropolymerization of the carbazole moieties showed sepa-
amphiphilic copolymer could electropolymerize to form a con- rate peaks of DA and AA with 10-fold higher peak currents in the
ducting network, which could enhance the conductivity and DPV measurements, indicating enhanced sensitivity and
accelerate the electron transfer of the CNT hybrid.
selectivity.
To demonstrate this hypothesis, in this paper, we synthe-
sized a novel electro-active amphiphilic copolymer (EAC) con-
taining carbazole pendants and used its self-assembled
micelles (EACM) to disperse MWCNTs in aqueous solution.
EACM not only served as an efficient MWCNTs dispersant in
aqueous solution but also as a precursor polymer to the
formation of a conducting network on the surface of the
MWCNTs, which could be formed with the electro-
polymerization of the carbazole moieties (Fig. 1). The electro-
active amphiphilic copolymer was synthesized through free
radical copolymerization, and then, it self-assembled into
micelles (EACM) in aqueous solution, which could adsorb onto
the surfaces of MWCNTs and efficiently disperse the MWCNTs
in aqueous solution, resulting in a homogeneous and stable
EACM/MWCNTs aqueous dispersion. The coumarin groups of
Experimental section
Materials
MWCNTs with a greater than 95% purity were obtained from
Timesnano. 2,2-Azobis (isobutyronitrile) (AIBN, Aladdin) was
recrystallized twice from ethanol before use. Carbazole
(Aladdin), 7-hydroxy-4-methylcoumarin (Acros Organics),
4-vinylbenzyl chloride (Acros Organics), dopamine (DA,
Aladdin), ascorbic acid (AA, Aladdin), maleic anhydride (Ma,
SCRC),ꢀDMF, THF, EtOH, KOH, K2CO3, and petroleum ether
(60–90 C, SCRC) were used without further purication.
Characterization
UV-Vis spectra were recorded on a TU-1901 spectrophotometer
(Beijing Purkinje, China). Fluorescence spectra were measured
on a Cary Eclipse spectrouorophotometer (Varian, USA). Gel
permeation chromatography (GPC) was performed at 35 ꢀC
the
copolymer
chain
undergo
crosslinking
under
UV-irradiation, which greatly improves the stability of the
obtained MWCNTs suspension. The obtained EACM/MWCNTs
dispersion was then casted on an electrode, and the carbazole
on
a
Waters-1515 instrument (Waters, USA) with N,N-
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dimethylformamide (DMF) as the eluent. The molecular weight
was estimated by comparison to a polystyrene (PSt) standard
curve. Differential scanning calorimetry (DSC) was carried out
with a Mettler Toledo DSC822e thermal analyzer. All of the
samples were heated at a 10 ꢀC minꢁ1 heating rate from 25 ꢀC to
200 ꢀC. The TGA was carried out on a Mettler Toledo TGA/
Preparation of the hybrid of MWCNTs noncovalently
functionalized by electro-active copolymer micelles (EACM/
MWCNTs)
For the preparation of the stable hybrid of MWCNTs non-
covalently functionalized by electro-active copolymer micelles
(EACM/MWCNTs), 2 mg of MWCNTs and 5 mg of EAC copol-
ymer were co-dissolved in 1 mL of DMF via sonication. Aer-
wards, the EAC chain was strongly attached to the surfaces of
the MWCNTs due to strong p–p interactions. Then, 0.1 mL of
deionized water was added dropwise to the solution with strong
stirring. During this process, the EAC copolymer self-assembled
into micelles on the surfaces of the MWCNTs and further
stabilized the MWCNTs. The solution was mildly sonicated and
water was added until the volume of DMF : H2O reached 1 : 9.
Then, UV irradiation was carried out with a UV-Vis spot curing
system to induce the photo-crosslinking of Vm. Aer this, there
were still some MWCNTs that could not be dispersed. There-
fore, the solution was ltered through cotton wool to remove the
insoluble MWCNTs, and the obtained ltrate was a photo-
crosslinked EACM functionalized MWCNTs dispersion.
Finally, the ltrate was dialyzed (molar mass cut off of 14 000)
against water to remove DMF and the trace of free copolymer.
Due to the strong p–p interactions and photo-crosslinking of
EACM around the MWCNTs, the EACM anchored on the
MWCNTs is very stable and will not be dialyzed out.
ꢁ1
ꢀ
ꢀ
1100SF instrument at a heating rate of 10 C min from 25
to 700 ꢀC under a nitrogen atmosphere. The SEM images of each
sample of EACM functionalized MWCNTs were taken with a
S-4800 eld emission scanning electron microscope (Hitachi,
Japan). TEM measurements were carried out on a JEOL JEM-
2100 microscope operating at 200 kV (JEOL, Japan). Raman
spectra were recorded using a Renishaw in Via Raman Micro-
scope (Renishaw, England) operating at 785 nm with a charge-
coupled device detector. The cyclic voltammetry (CV), electro-
chemical impedance spectroscopy (EIS) and differential pulse
voltammetry (DPV) were performed using a CHI660E electro-
chemical workstation (CH Instruments Inc., USA). All of the
measurements were conducted at room temperature in a stan-
dard three-electrode cell that consists of a glassy carbon elec-
trode as the working electrode, a platinum wire as the counter
electrode, and a saturated calomel electrode (SCE) as the
reference electrode. The EACM/MWCNTs modied electrodes
were prepared by drop-casting certain amounts of the corre-
sponding dispersion on glassy carbon electrodes (dried at
ꢀ
40 C).
Electrochemical measurements
The EACM/MWCNTs modied electrodes were prepared by
dropping certain volumes of the corresponding dispersion ꢀon
glassy carbon electrodes (GCE) and then drying them at 25 C.
All of the electrochemical measurements were performed using
a modied GCE as the working electrode, a platinum wire as the
counter electrode, a saturated calomel electrode (SCE) as the
reference electrode, and a CHI 660E electrochemical worksta-
tion (Shanghai, China). Electropolymerization was carried out
in a 0.1 M LiClO4 acetonitrile solution with increasing CV cycles
(0–1.5 V, 100 mV sꢁ1). Dopamine detection was carried out in a
0.1 M PBS buffer (pH ¼ 7.0).
Synthesis of the 7-(4-vinylbenzyloxyl)-4-methylcoumarin (Vm)
monomer
The Vm monomer was synthesized according to the literature.21
Synthesis of the 9-(4-vinylbenzyl)-9H-carbazole (VCz)
monomer
Carbazole (7.5 g, 45.0 mmol), potassium hydroxide (4.5 g,
80.0 mmol) and 120 mL of N,N-dimethylacetamide (DMF) were
added to a three-necked ask. Aer stirring for 15 min,
1-(chloromethyl)-4-vinylbenzene (6.8 g, 45.0 mmol) was added
slowly to the mixture, and the reaction was continued for 12 h at
room temperature. The mixture was ltered, and the lter cake
Results and discussion
was washed three times with deionized water to remove the Characterization of the EAC copolymer
KOH. Aerwards, the lter cake was washed with petroleum
Electro-active amphiphilic copolymer (EAC) was synthesized via
free radical polymerization. Different VCz/Vm feed ratios were
ether three times and then dried under vacuum at room
temperature for 24 h.
used to tune the contents of VCz and Vm in the polymer chain.
1
The structure of the synthetic copolymer was conrmed by H
NMR spectroscopy and FTIR. The results were demonstrated in
EIS. The basic characterization parameters of the EAC copol-
Synthesis of the EAC copolymer
Electro-active amphiphilic copolymer (EAC) was synthesized ymer such as the number average molecular weight (Mn), weight
through free radical copolymerization using Vm, VCz and Ma as average molecular weight (Mw), molecular weight distribution
monomers. Vm, VCz and Ma were dissolved in DMF at different index (Mw/Mn) and melting temperature (Tm) are given in
molar ratios in a round-bottom ask, and then, AIBN was Table 1.
ꢀ
added. The mixture was placed in an oil bath (65 C) for 24 h
The maleic anhydride units in the copolymer easily hydro-
under stirring. The resultant copolymers were puried by rep- lyze into carboxylic acid in water and thus serve as the hydro-
recipitation into EtOH/H2O (1 : 1 v/v) three times and then were philic segment. The Vm and VCz units function as the
dried under vacuum at room temperature for 24 h.
hydrophobic segment. Therefore, the obtained EAC copolymer
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Table 1 Basic characteristics of the EAC and the EACM/MWCNTs
Polymer content in
Sample
n(VCz) : n(Vm) : n(Ma)
Mn
Mw
Mw/Mn
Tm/ꢀC
EACM/MWCNTsa (wt%)
EAC0
EAC1
EAC2
EAC3
1 : 0 : 1
1 : 1 : 2
2 : 1 : 3
3 : 1 : 4
8655
9834
8209
12 703
9896
11 420
9120
1.14
1.16
1.11
1.21
168.9
164.5
168.3
169.1
49.6%
43.4%
41.0%
44.7%
15 390
a
Calculated by TGA.
is an amphiphilic macromolecule and thus can self-assemble homogeneous, and there was no optical behavior typically
into micelles in select solvents. The morphology of the self- caused by the aggregation of MWCNTs.
assembled copolymer micelles was observed by TEM. As
Our previous work has reported that the photo-crosslinking
shown in Fig. S3,† the self-assembled micelles were spherical, of the copolymer micelles around MWCNTs through the
and their diameters were in the range of 200 to 400 nm.
photo-dimerization of the Vm in the polymer chain could
encapsulate MWCNTs and further stabilize the noncovalent
functionalization.15 Here, the stabilities of photo-crosslinked
EACM/MWCNTs aqueous dispersions with different contents
of Vm were studied, and the results are given in Fig. 2b.
According to the Beer–Lambert law, the dissolved amount of
nanotubes is linearly proportional to the UV absorbance; thus, a
higher UV-Vis absorbance means a larger aqueous dispersibility
of the functionalized MWCNTs. For MWCNTs functionalized by
ECAM0 without Vm moieties, samples with or without photo-
crosslinking showed similar dispersibilities and stabilities.
The absorbance decreases to approximately 70% of its initial
value over a period of two weeks. For MWCNTs functionalized
by ECAM1 before photo-crosslinking, the absorbance also
decreased with time, and 72% of the initial value was preserved
aer two weeks. However, aer photo-crosslinking, the extent of
decrease was much less, and the absorbance remained at
almost 90% of its initial value aer two weeks. Such results
demonstrated that the dispersion stability of ECAM/MWCNTs
in water was greatly enhanced by the photo-crosslinking
process. The effect of the Vm content in the EAC copolymer
on the stability of the ECAM/MWCNTs dispersion was also
investigated. As shown in Fig. S5,† EAC1 exhibited best
dispersion stability for MWCNTs, which is possibly attributed to
the largest amount of Vm in EACM1, which thus leads to the
highest degree of photo-crosslinking. So EAC1 was chosen as
the original copolymer to investigate the subsequent charac-
terization of EACM/MWCNTs.
Characterization of the EACM/MWCNTs dispersion
Four types of EAC copolymers with different amounts of
carbazole and coumarin units were prepared by decreasing the
feed ratios of Vm as shown in Table 1, and subsequently, they
were assembled into micelles (EACM) along with simulta-
neously functionalizing MWCNTs. The obtained EACM func-
tionalized MWCNTs were named EACM/MWCNTs.
The dispersion efficiency of the MWCNTs in water with
EACM was rst investigated by visual observation. Pristine
MWCNTs and EACM/MWCNTs were sonicated in water for
0.5 h, and the corresponding photographs are shown in Fig. 2a.
It can be clearly observed that the pristine MWCNTs completely
settled at the bottom of vial within 0.5 h aer sonication due to
their high surface energy (van der Waals forces). In contrast, all
four of the EACM/MWCNTs samples exhibited good dis-
persibility in water and formed black-colored transparent
dispersions. No precipitation was observed aer one week of
standing.
UV-Vis spectroscopy was used to further investigate the dis-
persibilities and stabilities of the EACM/MWCNTs aqueous
dispersions. None of the EACM/MWCNTs samples exhibited
obvious absorption peaks, which was also observed by Rastogi
and co-workers.22 The absorption intensity increased with
increasing concentrations of the EACM/MWCNTs dispersion
(Fig. S4†), which obeyed Beer's law. These results indicated that
the aqueous dispersion of EACM functionalized MWCNTs was
Fig. 2 (a) Photographs of aqueous dispersions of (A) pristine MWCNTs, (B) EACM0/MWCNTs, (C) EACM1/MWCNTs, (D) EACM2/MWCNTs, and (E)
EACM3/MWCNTs; (b) the relationship between the stability of the EACM/MWCNTs aqueous solutions and time.
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The surface morphologies of pristine MWCNTs and EACM/ (Fig. 3d). The presence of interactions between the EAC micelles
MWCNTs hybrids were studied with SEM and TEM tech- and MWCNTs was investigated by uorescence spectroscopy.
niques. The SEM image (Fig. 3a) exhibits that the pristine The uorescence measurements of the EACM and EACM/
MWCNTs were entangled together with a distribution, whereas MWCNTs were carried out in aqueous solutions by using
the image of the EACM/MWCNTs clearly shows that the 295 nm as the excitation wavelength. As shown in Fig. 4, EACM
MWCNTs were tightly covered by a thick layer of copolymers, showed strong uorescence at 445 nm due to the presence of
supporting the functionalization of MWCNTs by EACM. The aromatic coumarin and carbazole groups in the polymer chain.
wrapping of EACM onto the surfaces of MWCNTs was also In contrast, the dispersion of the EACM/MWCNTs hybrid
visualized through TEM images. The image of the pristine showed very weak uorescence emission with the same EACM
MWCNTs (Fig. 3c) shows a smooth surface and agglomerated concentration. By comparing the uorescence intensities that
appearance. In contrast, the image of the EACM/MWCNTs were determined from the emission peak areas, 96% of the
(Fig. 3d) reveals well-separated individual nanotubes, and the uorescence emission was quenched in the EACM/MWCNTs
MWCNTs' surfaces were wrapped up by copolymer micelles. In with respect to that of EACM, suggesting intramolecular
addition, hemimicelles and globular micelles could be clearly energy and electron transfer between the excited singlet states
observed along the MWCNTs just like an untied pearl necklace of coumarin or carbazole moieties and the MWCNTs.23,24 This
strung together by MWCNTs. It should be noted that the signicant uorescence quenching strongly supports an effi-
diameters of the copolymer micelles along the surfaces of cient energy transfer from the aromatic moieties to the
MWCNTs were approximately 150 nm, as calculated by TEM. MWCNTs derived from the p–p stacking interactions via
Comparatively, the copolymer micelles without MWCNTs had vibrational coupling.
diameters of approximately 400 nm. Such a difference could be
Further evidence for existing intermolecular interactions
explained as follows: copolymer micelles were formed along the between the EACM and MWCNTs was provided by Raman
surfaces of the MWCNTs, and thus, their structures and diam- spectroscopic analysis. The results are shown in Fig. 5. As is
eters were inuenced by the MWCNTs. All of the results known, the graphite G band is related to the tangential mode
described above visually indicated the functionalization of vibration of the sp2 C atoms, whereas the D band is induced by
MWCNTs by EACM.
scattering from disordered sp3 C atoms.25 The ratio of the
The dispersing mechanism of EACM for MWCNTs in our D-band to G-band of the MWCNTs is a direct indication of the
study may be explained as follows: initially, the EAC copolymer degree of modication of the MWCNTs. In our case, the D
and MWCNTs were co-dissolved in good solvent (DMF) by
sonication. Due to the strong p–p interactions between the
MWCNTs and the aromatic segments along the EAC copolymer
chains, the EAC chains strongly attach to the surfaces of
MWCNTs. With the addition of water, the EAC copolymer self-
assembles into micelles along the surfaces of the MWCNTs,
which results in the stabilization of the MWCNTs in aqueous
solution through steric repulsion with a solvophilic hemisphere
Fig. 4 Fluorescence spectra of EACM and EACM/MWCNTs disper-
sions in aqueous solutions.
Fig. 3 SEM and TEM images of pristine MWCNTs (a and c), and EACM/
MWCNTs (b and d). Scale bar ¼ 200 nm.
Fig. 5 Raman spectra of pristine MWCNTs and EACM/MWCNTs.
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bands and G bands of the EACM/MWCNTs hybrids were
Electrochemical impedance spectroscopy (EIS) is a well-
observed at 1312 cmꢁ1 and 1608 cmꢁ1, respectively. The four known and powerful technique for determining the interfacial
EACM/MWCNTs hybrids showed almost the same Raman charge-transfer of lms and thus providing direct evidence of
spectra as pristine MWCNTs. This observation conrmed the electropolymerization. The impedance spectra of EACM/
noncovalent functionalization of EACM onto the MWCNTs, and MWCNTs before and aer electropolymerization were recor-
it also conrmed that the functionalization process did not lead ded and are shown in Fig. 7a. The EIS curves are composed of a
to any damage to the MWCNTs' structure. Whereas, the slight semicircle region at higher frequencies and a straight line at
decreases of the ID/IG values of the EACM/MWCNTs hybrids lower frequencies. The semicircle region corresponds to the
were probably because of the extra sp2 atom from the electron transfer limited process, and the diameter is equivalent
p-conjugation of the EACM. Overall, our data suggest to the electron transfer resistance, which normally reects the
adequately strong and stable p–p stacking interactions between conductivity and the electron transfer process. A GCE modied
the EACM and MWCNTs.
by EACM/MWCNTs before electropolymerization exhibited a
semicircle with a diameter of approximately 620 U at the lower
frequency region, which is higher than that of MWCNTs (560 U).
The larger diameter of the EACM/MWCNTs means a higher
resistance towards the electron transfer process at the electrode
surface, which should be attributed to the presence of the
insulating EAC on the MWCNTs. In contrast, aer electro-
polymerization, the modied GCE exhibited a much smaller
semicircle region with a diameter of approximately 95 U, which
indicated the formation of a conducting network through the
electropolymerization of carbazole moieties, thus enhancing
the conductivity and accelerating the electron transfer of the
EACM/MWCNTs. The enhanced electron transfer was also
conrmed by CV (Fig. 7b). It was obvious that the anodic and
cathodic peak currents of [Fe(CN)6]3ꢁ/4ꢁ on the GCE modied
by EACM/MWCNTs aer electropolymerization were much
larger than those of the electrode before electropolymerization.
In addition, the potential separation of the redox peaks also
became narrower aer electropolymerization, indicating faster
electronic transport. Both the EIS and CV measurements
demonstrate that the electropolymerization of the carbazole
enhanced the current response and accelerated the electron
transfer of the EACM/MWCNTs.
Electropolymerization study
Carbazole is one of the electro-active units that could electro-
polymerize to form a large conjugated structure named poly-
carbazole (PCz).26,27 With the introduction of carbazole moieties
into polymer chains and further electropolymerization, a
p-conjugated conductive network was formed and thus
enhanced the electrical properties of the EACM/MWCNTs. The
electropolymerization of the carbazole moieties was executed by
employing the CV technique, and the result is given in Fig. 6. An
obvious oxidation peak at approximately 1.1 V was observed in
the voltammogram, suggesting the formation of a carbazoly-
lium radical cation, which allowed the subsequent electro-
polymerization of the carbazole units via 3,6 connectivity.
DA detection
As is known, DA is one of the most important neurotransmitters
across all animals, including humans. Alterations in DA
contribute to the development of a number of severe mental
illnesses, such as Parkinson's disease, schizophrenia and
depression.28,29 Ascorbic acid (AA) has been one of the major
Fig. 6 CV for the electropolymerization of EACM/MWCNTs in 0.1 M
LiClO4/ACN at 100 mV sꢁ1
.
Fig. 7 (a) Nyquist diagrams in 5.0 mM [Fe(CN)6]3ꢁ/4ꢁ containing 0.1 M KCl recorded at a MWCNTs and EACM/MWCNTs modified GCE before and
after electropolymerization; (b) CV of the EACM/MWCNTs modified GCE before (red) and after (black) electropolymerization in 5.0 mM
[Fe(CN)6]3ꢁ/4ꢁ containing 0.1 M KCl.
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interfering species due to its abundance in serum samples and control experiments. The results are given in Fig. 9. Compared
its close Eox to that of DA.30 Therefore, the electrochemical to the bare GCE, the MWCNTs modied GCE showed a rela-
detection of DA in the presence of AA is of great importance. tively higher electrochemical response toward DA, which can be
The electrocatalytical ability of a GCE modied by EACM/ attributed to the good electrical conductivity of MWCNTs.
MWCNTs toward DA is investigated here. As shown in Fig. 8a, However, the peak of AA nearly overlaps with that of DA on the
compared to the EACM/MWCNTs modied GCE before carba- MWCNTs modied GCE, indicating a very poor selectivity
zole electropolymerization, which showed a very small peak toward DA and AA. The electrocatalytical ability of an EAC1
current even when the DA concentration was as high as modied GCE was also investigated. The EAC1 modied elec-
1000 mM, about a ten-fold higher current response was obtained trode showed a much smaller peak current of DA compared to
using the same electrode aer the electropolymerization of the the bare electrode, which is reasonable considering the insu-
carbazole moieties. Such a result suggested that the electro- lating property of EAC1. Aer electropolymerization, the peak
polymerization of carbazole could greatly enhance the electro- current of DA increased signicantly, indicating the formation
catalytical ability of the EACM/MWCNTs, which should be of a conductive network on the GCE. In addition, neither of the
attributed to the increased electrical conductivity and acceler- peak currents of DA on the MWCNTs (44 mA) or electro-
ated electron transfer across the EACM/MWCNTs hybrid. In polymerized EAC (18 mA) modied electrodes can reach as high
addition, before the carbazole moieties were electro- as that of the EACM/MWCNTs modied GCE aer electro-
polymerized, the peaks of DA and AA in the DPV measurement polymerization (67 mA), indicating that the MWCNTs and the
overlapped approximately 0.214 V. In contrast, aer electro- conducting EAC network cooperatively contribute to the great
polymerization, the current peak of AA in the DPV measurement response of EACM/MWCNTs toward DA.
shied to ꢁ0.02 V, and the peak of DA barely shied to 0.192 V,
The inuence of the carbazole content in different EACs on
suggesting separation of the peaks of DA and AA. This separa- the enhancement ratio of the current response of the corre-
tion made it possible for the EACM/MWCNTs modied GCE sponding EACM/MWCNTs modied GCE toward DA in DPV
aer electropolymerization to detect DA in the presence of AA, measurements was investigated, and the results are given in
suggesting an enhanced selectivity for the modied electrode.
Fig. S8.† It is clearly observed that among the four EACM/
To further realize the contribution of each component in the MWCNTs samples, ECAM3/MWCNTs showed the highest
EACM/MWCNTs to such a good sensitivity and selectivity for the enhancement ratio of the current response, which is in accor-
detection of DA, the response of DA on a MWCNTs modied dance with the highest content of carbazole in the ECAM3/
electrode and an EAC1 modied electrode were investigated as MWCNTs. Therefore, the subsequent detection performance
Fig. 8 (a) DPV of 1000 mM DA on a EACM/MWCNTs modified GCE after (red) and before (black) the electropolymerization of the carbazole
moieties; (b) DPV of 1000 mM DA in the presence of 1000 mM AA on a EACM/MWCNTs modified GCE before electropolymerization (black) and
DPV of 1000 mM DA (red) and AA (green) after electropolymerization.
Fig. 9 (a) DPV of 1000 mM DA on an EACM1 modified GCE before (green) and after (red) electropolymerization; (b) DPV of 1000 mM DA on a bare
GCE and on a MWCNTs modified GCE.
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Fig. 10 (a) DPV of different concentrations of DA ranging from 0.5 to 1000 mM on the EACM/MWCNTs modified GCE after electropolymerization
in PBS (pH ¼ 7.0); (b) calibration plots of the peak current versus the concentration of DA from DPV on the EACM/MWCNTs modified GCE after
electropolymerization.
Table 2 Comparison of the performance of our EACM/MWCNTs modified GCE sensor with some previously reported electrochemical sensors
for DA
Electrode
Detection method
Linear range (mM)
Detection limit (mM)
Reference
GNP/Ch/GCE
Ag/CNT-CPE
HCNTs/GCE
DPV
DPV
DPV
DPV
DPV
CV
0.2–80
0.8–64
2.5–105
1–199
1.9–771.9
1–40
1–200
10–196
0.1–18
0.12
0.3
0.8
Wang et al.31
Tashkhouriana et al.32
Cui et al.33
LDH/GC
0.3
Wang et al.34
IMWCNT-CPE
Poly(caffeic acid)/GCE
GNS/NiO/DNA-GC
SDS/CPE
C–Ni/GCE
EACM/MWCNTs/GCE
0.52
0.47
2.5
0.771
0.29
0.2
Nasirizadeh et al.35
Li et al.36
CP
Lv et al.37
38
´
DPV
DPASV
DPV
Colın-Orozco et al.
Yang et al.39
0.5–20, 200–1000
Present work
toward DA was investigated based on the ECAM3/MWCNTs
The stability of the EACM/MWCNTs modied GCE aer
modied GCE. Fig. 10 shows the DPV curves of various DA electropolymerization was examined by investigating its current
concentrations in PBS solutions on the ECAM3/MWCNTs response to 1000 mM DA over two weeks. The current response
modied GCE aer electropolymerization. The peak current decreased by less than 5.3% of its original response for the 1000
increased with increasing DA concentrations, and the calibra- mM DA solution, indicating the good stability of this electrode.
tion curve for DA showed the following two linear segments: the To check the reproducibility of our sensor, ve electrodes were
rst linear segment increases from 0.5 to 20 mM with a linear fabricated independently under the same conditions. A 1000
regression equation of I1 ¼ ꢁ0.5287CDA ꢁ 9.632 (R2 ¼ 0.989), mM DA solution was measured using the ve different elec-
and the second linear segment covers from 200 to 1000 mM trodes and a relative standard deviation (RSD) of 5.8% was
with a linear regression equation of I2 ¼ ꢁ0.02615CDA ꢁ 40.17 obtained, indicating good reproducibility. These results
(R2 ¼ 0.973). The detection limit was as low as 0.2 mM. In revealed that the EACM/MWCNTs modied GCE aer electro-
contrast, under the same conditions, the response of the non- polymerization has a high stability as well as a good reproduc-
electropolymerized control electrode showed very low peak ibility for the determination of DA, making it applicable for
currents for all of the DA concentrations within the test range, practical use.
and the detection limits were 100 mM, which is three orders of
magnitude higher than those aer electropolymerization. To
make a realistic comparison with previous procedures, the
Conclusion
characteristics of different electrochemical sensors for DA are
summarized in Table 2. It can be seen that the EACM/MWCNTs
In conclusion, a novel amphiphilic copolymer (EAC) with
carbazole side chains was synthesized. The obtained copolymer
modied GCE aer electropolymerization showed a low detec-
could adsorb onto the surface of MWCNTs via p–p interactions
tion limit and its linear range was much wider than those from
and self-assemble into micelles on the surfaces of the MWCNTs,
most of the other sensors.31–39 The comparison conrmed that
thereby causing the efficient dispersion of the MWCNTs in
our EACM/MWCNTs modied GCE was an appropriate plat-
form for the electrochemical sensing of DA. The above results
clearly demonstrated a signicantly enhanced sensitivity in DA
aqueous solution. The electroactive carbazole moieties could
electropolymerize with increasing CV cycles and could form a
conductive network on the surfaces of the MWCNTs, which
detection using EACM/MWCNTs modied GCE through
signicantly accelerated the electron transfer and enhanced the
carbazole electropolymerization.
conductivity of the EACM/MWCNTs. An electrochemical sensor
18240 | RSC Adv., 2015, 5, 18233–18241
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for detecting DA was fabricated using an EACM/MWCNTs 14 B. Panayiotis, K. Dimitrios, A. Apostolos and S. Georgios,
modied GCE. The results showed that the EACM/MWCNTs RSC Adv., 2014, 4, 2911–2934.
hybrid aer electropolymerization exhibited a good sensitivity 15 J. C. Liu, B. Q. Wang, X. M. Xiong, J. Luo and X. Y. Liu, Chem.
and selectivity with a ten-fold peak current enhancement. This Lett., 2012, 41, 50–52.
research paves the way for the fabrication of a carbon nanotube/ 16 Y. D. Zhang, T. Wada and H. Sasabe, J. Mater. Chem., 1998, 8,
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while preserving their outstanding electrical properties, which 17 J. F. Morin, M. Leclerc, D. Ades and A. Siove, Macromol. Rapid
`
would be valuable for the construction of electrochemical
sensors and electronics.
Commun., 2005, 26, 761–778.
18 T. M. Fulghum, P. Taranekar and R. C. Advncula,
Macromolecules, 2008, 41, 5681–5687.
¨
19 M. L. Mayo, D. Hogle, B. Yilmaz, M. E. Koseb and S. Kilina,
Author contributions
RSC Adv., 2013, 3, 20492–20502.
20 S. Lefrant, M. Baibarac and I. Balto, J. Mater. Chem., 2009, 19,
5690–5704.
The manuscript was written through contributions of all of the
authors. All of the authors have given approval to the nal
version of the manuscript.
21 J. Q. Jiang, Y. Feng, M. H. Wang, X. Y. Liu, S. W. Zhang and
M. Q. Chen, Acta Phys.-Chim. Sin., 2008, 24, 2089–2095.
22 R. Rastogi, R. Kaushal, S. K. Tripathi, A. L. Sharma, I. Kaur
and L. M. Bharadwaj, J. Colloid Interface Sci., 2008, 328,
421–428.
Conflict of interest
The authors declare no competing nancial interest.
¨
¨
23 H. Paloniemi, T. Aaritalo, T. Laiho, H. Like, N. Kocharova,
K. Haapakka, F. Terzi, R. Seeber and J. Lukkari, J. Phys.
Chem. B, 2005, 109, 8634–8642.
Acknowledgements
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25 M. S. Dresselhaus, G. Dresselhausb, R. Saitoc and A. Joriod,
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28 K. A. Conway, J. C. Rochet, R. M. Bieganski and
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We are grateful for the nancial support from the National
Natural Science Foundation of China (no. 51203063, 21174056),
the Fundamental Research Funds for the Central Universities
(JUSRP 51305A), and MOE & SAFEA for the 111 Project (B13025).
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