Aniline pentamer-based electroactive polyimide prepared from oxidation coupling polymerization for electrochemical sensing application

Article abstract of DOI:10.1016/j.polymer.2012.07.064

This article details electrochemical investigations of carbon paste electrode (CPE) modified with electroactive polyimide (EPI) as ascorbic acid sensor (vitamin C, AA). EPI, with aniline-pentamer-based in the main chain, was synthesized from oligoaniline and p-phenylenediamine by oxidative coupling polymerization. The well-defined molecular structure of the oligoaniline and EPI was confirmed by LC-Mass, ¹H NMR and FTIR spectroscopy. The in-situ chemical oxidation of the reduced form of soluble, electroactive poly(amic acid) (EPAA) in N-methyl-2-pyrrolidone was monitored by UV-Visible absorption spectra. Moreover, the electroactivity of the EPAA and EPI were evaluated by performing electrochemical cyclic voltammetry studies. A linear relationship between the concentration of AA added and the change of peak current obtained, as shown by the linear calibration curve of the amperometric response of the CPE modified with EPI sensor to the concentration of AA (R² = 0.996, n = 15). The limit of quantitation measured was 32.2 μM and the limit of detection for the EPI-CPE was estimated 9.6 μM at signal/noise (S/N) of 3.

Full text of DOI:10.1016/j.polymer.2012.07.064

Polymer 53 (2012) 4373e4379
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Aniline pentamer-based electroactive polyimide prepared from oxidation
coupling polymerization for electrochemical sensing application
Tsao-Cheng Huang, Shih-Ting Lin, Lu-Chen Yeh, Chun-An Chen, Hsiu-Ying Huang, Zheng-Yong Nian,
*
Hsin-Han Chen, Jui-Ming Yeh
Department of Chemistry, Center for Nanotechnology, Chung Yuan Christian University, Chung Li 32023, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
This article details electrochemical investigations of carbon paste electrode (CPE) modified with elec-
troactive polyimide (EPI) as ascorbic acid sensor (vitamin C, AA). EPI, with aniline-pentamer-based in the
main chain, was synthesized from oligoaniline and p-phenylenediamine by oxidative coupling poly-
merization. The well-defined molecular structure of the oligoaniline and EPI was confirmed by LC-Mass,
1H NMR and FTIR spectroscopy. The in-situ chemical oxidation of the reduced form of soluble, electro-
active poly(amic acid) (EPAA) in N-methyl-2-pyrrolidone was monitored by UVeVisible absorption
spectra. Moreover, the electroactivity of the EPAA and EPI were evaluated by performing electrochemical
cyclic voltammetry studies. A linear relationship between the concentration of AA added and the change
of peak current obtained, as shown by the linear calibration curve of the amperometric response of the
CPE modified with EPI sensor to the concentration of AA (R2 ¼ 0.996, n ¼ 15). The limit of quantitation
Received 7 June 2012
Received in revised form
25 July 2012
Accepted 28 July 2012
Available online 3 August 2012
Keywords:
Electroactive polymer
Electrochemical sensing
Ascorbic acid
measured was 32.2
(S/N) of 3.
mM and the limit of detection for the EPIeCPE was estimated 9.6 mM at signal/noise
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
polymerization reported by Zhang et al. [7e9]. The fabricated
copolymers not only contain well-defined conjugated segments,
but also provide an opportunity to present a better understanding
about the structureeproperty relationships and the conducting
mechanism of conjugated polymers; knowledge which is limited
by the complexity of their molecular structure and their poor
solubility in organic solvents. Zagorska and co-workers prepared
new electroactive polymers combining the electrochromic prop-
erties of polythiophenes and oligoanilines [10,11]. Both the oli-
goaniline side chains and the polythiophene main chains could be
doped in various ways. Wei et al. and Chen et al. incorporated
aniline oligomers into polylactide and obtained copolymers that
showed good electroactive, biodegradability, and processing
properties [12,13]. Wang et al. also prepared electroactive polymers
containing aniline oligomer in the main chain and side chain, which
exhibited electrochemical properties, electrochromic behavior and
fluorescent sensing properties [14e18]. In our group, electroactive
polymers have been well studied as functional membranes [19],
electrochromic materials [20,21] and anticorrosion coating mate-
rials [22e25]. To the best of our knowledge, the practical applica-
tions of electroactive polymers used as electrochemical sensor have
not been reported. In this study, we synthesized electroactive
polyimide (EPI) through simple oxidative coupling polymerization
and used it to modify the carbon paste electrode (CPE) as
Since the first discovery of electroactive polyacetylene in 1977
[1,2], a considerable amount of attention has been paid on con-
ducting polymers which exhibited interesting chemical and phys-
ical properties due to their special
p-conjugated electron system
[3]. Polyaniline (PANI), polypyrrole, polythiophene, poly(3,4-
ethylenedioxythiophene) related polymers were all interesting
materials for both fundamental research and technological appli-
cations. Among these types of polymers, PANI has been considered
one of the most promising electrode materials due to its facile
synthesis, environmental stability, unique electronic properties,
and simple acid-base doping/dedoping chemistry [4]. However, the
poor solubility of PANI in common organic solvents has limited its
practical application in many fields [5]. Recently, electroactive
polymers incorporated with aniline oligomers have attracted
research attention because of their superior properties such as
good solubility, mechanical strength and the ability to form films
[6]. By far the most well-known and most frequently used reaction
for the synthesis of electroactive polymers is the oxidative coupling
* Corresponding author. Tel.: þ886 3 2653340; fax: þ886 3 2653399.
E-mail address: juiming@cycu.edu.tw (J.-M. Yeh).
0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2012.07.064

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T.-C. Huang et al. / Polymer 53 (2012) 4373e4379
electrochemical sensor. The electroactive property enable EPI to
have a potential to apply in the advanced sensors for detecting
minimal amounts of chemical species such as the ascorbic acid
(vitamin C, AA). AA is known for its anti-oxidant properties in foods
and drinks. It is also important in several human metabolic
processes involving oxidation and reduction. Therefore, the
detection of AA is of great importance in pharmaceutical, clinical
and food industry. Several techniques have been reported for the
determination of AA, including spectroscopic, chromatographic,
enzymatic and electroanalytical methods [26e29]. Among them,
electrochemical methods are considered one of the best potential
approaches because of their high sensitivity and simplicity. Here,
we used AA as a model to test the practical applications of the
EPIeCPE sensor. A linear relationship between the concentration of
AA added and the change of peak current obtained, as shown by the
linear calibration curve of the amperometric response of the sensor
to the concentration of AA (R2 ¼ 0.996, n ¼ 15).
d
¼ 7.63e7.60 (2H, due to H7, H9),
d
¼ 7.29e7.23 (t, 4H, due to H2,
¼ 6.87 (t, 1H, due to H1).
H6),
d
¼ 7.17e7.12 (t, 4H, due to H3, H5),
d
2.2.2. Synthesis of electroactive poly(amic acid) (EPAA)
EPAA was prepared by simultaneously dissolving 1.28
g
(2 mmol) of oligoaniline and 0.216
g (2 mmol) of 1,4-
phenylenediamine into 25 mL of a stirring solution that con-
tained 20 mL of NMP, 2.5 mL of distilled water, and 2.5 mL of
concentrated hydrochloric acid. Subsequently, a solution contain-
ing 0.913 g of APS and 5 mL of 1.0 M aqueous HCl was added
dropwise while stirring at room temperature. A black product was
then precipitated by pouring the obtained solution into 300 mL of
distilled water with continuous stirring for 12 h. The mixture was
filtered and washed with distilled water and acetone several times.
The as-obtained product was then dried under dynamic vacuum at
40 ꢀC for 24 h. The typical yield of as-prepared EPAA powder was ca.
88%. The EPAA characterization is given as follows (Figs. 1 and 2(a)):
FTIR (KBr, cmꢁ1): 3260 (m, yNH), 1711 (m,
quinoid rings), 1506 (vs, yC of benzenoid rings), 1309 (s, yCeN),
yC]O), 1587 (s, yC] of
C
2. Experimental
]
C
1150 (s, N ¼ Q ¼ N, where Q represents the quinoid rings), 972
2.1. Materials and instrumentation
(m, dCH), 825 (m, dCH).
N-Phenyl-p-phenylenediamine (98%, Aldrich), 4,40-oxy-
diphthalic anhydride (ODAD, 97%, Aldrich), 1,4-phenylenediamine
(99%, Aldrich), ammonium persulfate (APS, 98%, Merck), dichloro-
methane (99.5%, Merck), N-methyl-2-pyrrolidone (NMP, 99%,
Merck), hydrochloric acid (37%, Riedel-deHaën), ammonium
hydroxide (30%, Riedel-deHaën), and acetone (99%, Acros) were
used as received without further purification. All of the chemicals
were of reagent grade unless otherwise stated. Mass spectra were
obtained on a Bruker Daltonics IT mass spectrometer model Esquire
2000 (Leipzig, Germany) with an Agilent ESI source (model G1607-
6001). The chemical structure of the oligoaniline and the electro-
active poly(amic acid) (EPAA) were determined by 1H NMR spec-
troscopy on a Bruker 300 spectrometer, using deuterated dimethyl
sulfoxide (DMSO) as the solvent. Fourier transform infrared spectra
were collected using an FTIR spectrometer (JASCO FT/IR-4100) at
room temperature. UVeVisible absorption spectra were collected
using a UVeVisible spectrometer (JASCO V-650). Electroactive
experiments were performed on VoltaLab 40 (PGZ 301) analytical
voltameter using a conventional three-electrode system.
2.2.3. Preparation of EPI
The as-prepared EPAA was dried under vacuum at 260 ꢀC for
10 h to complete the conversion of EPAA into the EPI materials, as
shown in Scheme 1.
2.3. Electrochemical cyclic voltammetry (CV) studies of EPAA and
EPI coatings
To study the redox behavior of EPAA and EPI, two representative
samples were examined systematically by CV in 100 mL of 1.0 M
H2SO4 in the ꢁ0.2e1.0 V range at a scan rate of 50 mV sꢁ1. The as-
prepared EPAA and EPI film-coated electrode were prepared by
casting as-prepared polymer solution in DMAc on top of the
working electrode and then drying it with used hotplate in air.
Indium tin oxide (ITO) glass substrate was acted as working elec-
trode, platinum wire and saturated calomel electrode (SCE) were
used as counter and reference electrode, respectively. All electro-
chemical cyclic voltammetric measurements were performed in
a double-wall jacketed cell.
2.2. Synthesis of EPI
2.4. Preparation of electrodes
2.2.1. Synthesis of an imidic monomer of oligoaniline
The unmodified CPE was prepared by mixing 30 mg of paraffin
oil with 150 mg graphite thoroughly in a mortar to form a homo-
geneous carbon paste. A portion of the carbon paste was filled
firmly into one end of a Teflon tube (about 3.0 mm i.d.), and
a copper wire was inserted through the opposite end to establish an
electrical contact. Appropriate packing was achieved by pressing
the surface against a bond paper until a smooth surface was ob-
tained. The modified electrode was fabricated using the same
procedure except the introduction of EPI into the unmodified
carbon paste. The total mass of graphite and EPI was 30 mg.
A total of 0.26 g (2 mmol) of ODAD was dissolved in 10 mL of NMP
and then introduced dropwise over 30 min into a stirred solution of
1.11 g (6 mmol) of N-phenyl-p-phenylenediamine dissolved in
10 mL of NMP. The solution was magnetically stirred for 3 h and
subsequently poured into 100 mL of distilled water to precipitate
the electroactive oligoaniline. The as-prepared oligoaniline was
then filtered, washed with an excess of distilled water and
dichloromethane several times, and finally dried under dynamic
vacuum at room temperature for 24 h, Mass spectrum (m/e):
calculated for C32H26N4 ¼ 678.6. Found 677.2. Subsequently, the
obtained fine oligoaniline powder was heated under dynamic
vacuum at 260 ꢀC for 5 h to complete the chemical imidization
process of the oligoaniline. The imidic form of oligoaniline was
obtained in a yield of ca. 90%,. The detailed characterizations for
imidic monomer of oligoaniline were listed as follows: FTIR (KBr,
3. Results and discussion
3.1. Polymer synthesis and characterization
The synthetic routes for the preparation of oligoaniline, EPAA,
and EPI are shown in Scheme 1. Fig. 2 shows the FTIR spectra of the
obtained EPAA and EPI materials. For example, in both of the FTIR
spectra, the characteristic peak found at 3330 cmꢁ1 was attributed
to the NeH stretching modes. Moreover, characteristic peaks at
1598 cmꢁ1 and 1510 cmꢁ1 were designated as the stretching modes
cmꢁ1): 3380 (s, yNH), 1774 (m, yC
(vs, yC O symmetric stretching), 1592 (s,
1521 (vs, C]C of benzenoid rings),1323 (s, yCꢁN),1102 (m, dCH), 890
(m, dCH), 740 (m, imide ring deformation). 1H NMR (d6-DMSO):
¼ 8.40 (s, 1H, due to H4), ¼ 8.05e8.03 (d, 1H, due to H8),
] asymmetric stretching), 1708
O
]
yC]C of benzenoid rings),
y
d
d

T.-C. Huang et al. / Polymer 53 (2012) 4373e4379
4375
Fig. 1. (a) LC-mass of oligoaniline and (b) 1H NMR of imidic monomer of oligoaniline.
of N ¼ Q ¼ N and NeBeN, respectively. (Q represents the quinoid
ring, and B represents the benzene ring structure.) The character-
istic peak at 1309 cmꢁ1 was assigned to C]N stretching. The
absorption band found at 823 cmꢁ1 corresponded to the out-of-
plane CꢁH deformation mode. In the case of EPAA (Fig. 2 (a)), the
After thermal imidization of AP-based EPAA, vibrational modes
were found at 1773 cmꢁ1 and 1713 cmꢁ1, which may be associated
with asymmetric and symmetric carbonyl stretching. Moreover, the
characteristic band found at 750 cmꢁ1 was designated as the
deformation of the imide groups, as shown in Fig. 2(b). These
changes in the characteristic peaks of EPAA and EPI indicate that
EPAA was almost completely converted into the corresponding EPI
through the thermal imidization process at 260 ꢀC.
characteristic peaks for C]O in COOH were found at 1711 cmꢁ1
.
3.2. Chemical oxidation of EPAA
The obtained EPAA (0.4 g) was dispersed into a solution of 4 mL
hydrazine hydrate in 40 mL 1.0 M ammonium hydroxide and stirred
for 10 h. The reaction mixture was then filtered, washed with
distilled water several times, and dried under dynamic vacuum at
40 ꢀC for 24 h. Finally, the EPAA was reduced to the leucoemer-
aldine oxidation state (0.35 g, 88%). The leucoemeraldine base (LB)
form of the EPAA was dissolved in an NMP solution. Subsequently,
trace amounts of the oxidant, (NH4)2S2O8, were introduced into the
EPAA solution gradually for in-situ monitoring of the sequential
oxidation process of the conjugated aniline pentamer components
by UVeVisible absorption spectroscopy, as shown in Fig. 3. It should
be noted that upon oxidation, the color of the EPAA solution varied
from dark blue to purple. Initially, only one absorption band was
Fig. 2. FTIR spectra of (a) EPAA and (b) EPI.

4376
T.-C. Huang et al. / Polymer 53 (2012) 4373e4379
Scheme 1. Synthesis route of the oligoaniline, EPAA and EPI.
visible at 319 nm, which was associated with the
p
to p* transition
the continuous oxidation of EPAA in the LB oxidation state, the
EPAA first converted to the emeraldine oxidation state with each
aniline pentamer segment containing only one quinoid ring, which
showed a second absorption at a longer wavelength. Subsequently,
it was oxidized to the second emeraldine oxidation state with each
aniline pentamer segment containing two quinoid rings, which
corresponded with the increase in intensity of the second absorp-
tion. After the second absorption peak reached maximum intensity,
it exhibited a blue shift. This blue shift is indicative of the conver-
sion past the emeraldine base state to the pernigraniline oxidation
state. The chemical oxidation process of EPAA is similar to that of
oligoaniline [33].
of the conjugated ring system [30]. Upon the addition of trace
amounts of the oxidant, slow oxidation of the aniline pentamer
segments in EPAA was observed. The original absorption band
detected by UVeVisible spectroscopy underwent a blue shift from
319 to 301 nm, accompanied with a decrease in intensity. At the
same time, a new absorption peak appeared at 595 nm, which was
designated to the exciton-type transition between the HOMO
orbital of the benzoid ring and the LUMO orbital of the quinoid ring
[31,32]. After its intensity reached a maximum, lmax of a second
absorption began to undergo a blue shift from 595 to 580 nm. One
possible interpretation of this result is presented as follows. During
Fig. 3. UVeVis spectra monitoring the chemical oxidation of the EPAA in the leucoemeraldine oxidation state.

T.-C. Huang et al. / Polymer 53 (2012) 4373e4379
4377
Fig. 4. CV measurements of (a) EPAA and (b) EPI measured in aqueous H2SO4 (1.0 M) at a scan rate of 50 mV sꢁ1
.
3.3. Electroactivity of EPAA and EPI coatings
food field to have rapid and sensitive methods for its routine and
reliable determination. Here, we used it as a model to test the prac-
tical applications of the EPI modified CPE. A feature of the electrolysis
of AA is that only an oxidation peak appears in its cyclic voltammo-
gram and no reduction peak is observed consistent with [35]. The
oxidation of AA to dehydroascorbic acid involves the transfer of two
electrons. The mechanism of AA oxidation process has been studied
in previous literature [36,37], as shown in Scheme 3.
Electrochemical cyclic voltammetry (CV) studies have been
widely used to characterize the redox properties of electroactive
polymers. In this article, the polymers were characterized by CV
using a typical three-electrode electrochemical cell. As shown in
Fig. 4, the CV curve of EPAA shows a three-pair redox peak (323,
534, and 690 mV), which is different from the typical two-pair
redox peak (350 and 800 mV) observed for PANI [34]. Four
different oxidation states of EPAA can be found: LB, emeraldine
base I (one quinoid ring in oligoaniline segment, EBI), emeraldine
base II (two quinoid rings in oligoaniline segment, EBII), and per-
nigraniline base (PNB), as shown in Scheme 2. In CV studies on AP-
based EPI, a two-pair redox peak appeared with oxidation peaks at
329 mV and 520 mV, which was different from that of EPAA likely
due to the differences in molecular structure between the two
systems. During the thermal imidization of oligoaniline, the
secondary amines on both sides of the oligoaniline segment are
converted from carboxylic acids to imines, which would not allow
for quinoid ring formation.
Fig. 5 shows the cyclic voltammograms of the EPIeCPE electrode
response at different AA concentrations in a pH 7.0 PBS solution and
at 50 mV/s. The detection was attributed to the oxidation of AA and
the reduction reaction of EPI. The AA concentrations in the buffer
ranged from 0.0 to 8.0 mM. The current (oxidation peak) increased
with increasing the concentration of AA, which occurred at 0.4 V
versus Ag/AgCl, corresponding to the catalytic properties of AA at
the modified electrode. The inset shows the peak currents plotted
versus amount of AA added. A linear relationship was observed for
concentration of AA beyond the 0.0e8.0 mM range used (R ¼ 0.992,
n ¼ 5).
Fig. 6 shows the amperometric response of the EPIeCPE with
continuous addition of AA in the buffer solution. At an applied
potential of 0.4 V, the successive addition of AA in 0.1 M PBS (30 ml)
to the EPIeCPE electrode increased the current successively. Upon
addition of an aliquot of AA to the stirring solution, the oxidation
current of sensor increased to reach a stable value. Each step
3.4. Electrochemical response of EPI modified CPE toward AA
Ascorbic acid (AA) is a powerful antioxidant that is naturally
present in many foods, especially fruits and vegetables, and plays an
important role in the prevention of infectious diseases. Because of its
biological and technological importance, it is of great interest in the
current observed represents an increase by 75 mM of AA (20
ml).
There was a linear relationship between the concentration of AA
Scheme 2. Molecular structures of (a) EPAA and (b) EPI at various oxidation states.

4378
T.-C. Huang et al. / Polymer 53 (2012) 4373e4379
Scheme 3. The oxidation and hydrolysis of AA by EPI.
added and the change of peak current obtained, as evidenced by the
linear calibration curve of the amperometric response of the sensor
to the concentration of AA in the inset (R2 ¼ 0.996, n ¼ 15). The
measuring limit of quantitation and detection for the EPIeCPE AA
sensor were 32.2
of 3, respectively.
mM at S/N of 10 and approximately 9.6 mM at S/N
4. Conclusions
In this work, we present an investigation of carbon paste elec-
trode modified with EPI as ascorbic acid sensor. EPI, with
aniline-pentamer-based in the main chain, was synthesized from
oligoaniline and p-phenylenediamine by oxidative coupling poly-
merization. The well-defined molecular structure of the oligoani-
line and EPI was confirmed by LC-Mass, 1H NMR, and FTIR
spectroscopy. The CV study shows that EPI exhibited redox prop-
erties, this properties give it an opportunity to apply to detect the
chemical materials. There was a linear relationship between the
concentration of AA added and the change of peak current ob-
tained, as evidenced by the linear calibration curve of the amper-
ometric response of the sensor to the concentration of AA
(R2 ¼ 0.996, n ¼ 15). The measuring limit of quantitation and
Fig. 5. Cyclic voltammograms of EPI-modified electrodes in PBS (pH
different AA concentrations.
¼
7) with
detection for the EPIeCPE AA sensor were 32.2
mM at S/N of 10 and
approximately 9.6 M at S/N of 3, respectively. According to the
m
results, the CPE modified with EPI electrode may have potential for
using as an AA sensor in the future.
Acknowledgments
This research was supported by the National Science Council of
the Republic of China under grant numbers NSC 98-2113-M-033-
001-MY3 and NSC 101-2811-M-033-010.
Appendix A. Supplementary data
Supplementary data related to this article can be found online at
http://dx.doi.org/10.1016/j.polymer.2012.07.064.
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Fig. 6. Amperometric current responses of EPIeCPE for successive addition of AA into
a 30 mL PBS stirred constantly. Inset shows calibration curve between the current and
the concentration of AA.
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