Conducting polyaniline-graphene oxide fibrous nanocomposites: Preparation, characterization and simultaneous electrochemical detection of ascorbic acid, dopamine and uric acid

Article abstract of DOI:10.1039/c3ra42322k

Polyaniline/graphene oxide (PANI-GO) fibrous nanocomposites have been prepared and the electrochemical catalytic activity towards the electro-oxidation of ascorbic acid (AA), Dopamine (DA) and Uric acid (UA) has been investigated. The nanocomposites were synthesized via an in situ chemical polymerization method. The morphology, composition, thermal and electrochemical properties of the resulting nanocomposites were characterized by scanning electron microscopy, X-ray diffraction, Raman spectroscopy, FT-IR spectroscopy, thermo gravimetric analysis and cyclic voltammetry. The catalytic behavior of PANI-GO nanocomposite modified glassy carbon electrode (GCE) towards AA, DA and UA has been investigated by cyclic voltammetry (CV), differential pulse voltammetry (DPV). The PANI-GO/GCE showed excellent catalytic activity towards electrochemical oxidation of AA, DA and UA compared to the bare GCE. The electrochemical oxidation signal of AA, DA and UA are well separated into three distinct peaks with peak potential separation of 343 mV, 145 mV and 488 mV between AA-DA, DA-UA and AA-UA respectively in CV studies and the corresponding peak potential separation in DPV mode are 320 mV, 230 mV and 550 mV. Under the optimized DPV experimental conditions, the peak current of AA, DA and UA give linear response over the range of 25-200 μM (R² = 0.9955), 2-18 μM (R² = 0.9932) and 2-18 μM (R² = 0.9902) with detection limit of 20 μM, 0.5 μM and 0.2 μM at S/N = 3, respectively. The attractive features of PANI-GO provide potential applications in the simultaneous detection of AA, DA and UA. The excellent electrocatalytic behavior of PANI-GO may lead to new applications in electrochemical analysis. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra42322k

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Conducting polyaniline-graphene oxide fibrous
nanocomposites: preparation, characterization and
simultaneous electrochemical detection of ascorbic acid,
dopamine and uric acid
Cite this: DOI: 10.1039/c3ra42322k
P. Manivel,a M. Dhakshnamoorthy,b A. Balamurugan,a N. Ponpandian,a
D. Mangalaraja and C. Viswanathan*a
Polyaniline/graphene oxide (PANI-GO) fibrous nanocomposites have been prepared and the electro-
chemical catalytic activity towards the electro-oxidation of ascorbic acid (AA), Dopamine (DA) and Uric acid
(UA) has been investigated. The nanocomposites were synthesized via an in situ chemical polymerization
method. The morphology, composition, thermal and electrochemical properties of the resulting
nanocomposites were characterized by scanning electron microscopy, X-ray diffraction, Raman spectro-
scopy, FT-IR spectroscopy, thermo gravimetric analysis and cyclic voltammetry. The catalytic behavior of
PANI-GO nanocomposite modified glassy carbon electrode (GCE) towards AA, DA and UA has been
investigated by cyclic voltammetry (CV), differential pulse voltammetry (DPV). The PANI-GO/GCE showed
excellent catalytic activity towards electrochemical oxidation of AA, DA and UA compared to the bare GCE.
The electrochemical oxidation signal of AA, DA and UA are well separated into three distinct peaks with
peak potential separation of 343 mV, 145 mV and 488 mV between AA-DA, DA-UA and AA-UA
respectively in CV studies and the corresponding peak potential separation in DPV mode are 320 mV, 230
mV and 550 mV. Under the optimized DPV experimental conditions, the peak current of AA, DA and UA
give linear response over the range of 25–200 mM (R2 = 0.9955), 2–18 mM (R2 = 0.9932) and 2–18 mM (R2 =
0.9902) with detection limit of 20 mM, 0.5 mM and 0.2 mM at S/N = 3, respectively. The attractive features of
PANI-GO provide potential applications in the simultaneous detection of AA, DA and UA. The excellent
electrocatalytic behavior of PANI-GO may lead to new applications in electrochemical analysis.
Received 10th May 2013,
Accepted 29th May 2013
DOI: 10.1039/c3ra42322k
www.rsc.org/advances
diseases such as cancer, diabetes mellitus, hepatic disorders,
schizophrenia, Parkinson, hyperuricaemia, pneumonia and
Introduction
gout.2,6–9
Ascorbic acid, dopamine and uric acid are compounds of great
biological and chemical interest and play a potential role in
the metabolic system of human body.1,2 Ascorbic acid (AA) is
commonly used in large scale as an antioxidant in food,
animal feed, pharmaceutical formulation and cosmetic appli-
cations.3 Dopamine (DA) is a neurotransmitter and plays a very
important role in the function of central nervous, renal,
hormonal and cardiovascular systems.4 Uric acid (UA) is the
major final product of urine catabolism in human body. In a
healthy human, the normal level of UA in urine is in milli
molar range whereas in serum it is in micro molar range.5
Detection and quantification of AA, DA and UA are important
in diagnosis, monitoring, prevention and treatment of certain
However, electrochemical determination of these species
based on anodic oxidation has a disadvantage of overlapping
oxidation peaks that is obtained at solid electrodes. At present,
various modified electrodes, such as poly(acid chrome blue K)
modified glassy carbon electrode,10 single-walled carbon
nanohorn modified glassy carbon electrode,11 chitosan-gra-
phene modified glassy carbon electrode,2 hollow nitrogen-
doped carbon microsphere modified glassy carbon electrode,12
iron ion doped natrolite zeolite-multiwall carbon nanotube
modified glassy carbon electrode13 and graphene oxide-
polyaniline composite modified electrode,14 etc. have been
attempted to solve the problems encountered in simultaneous
determination of AA, DA and UA. However, development of
new materials for simultaneous electrochemical determina-
tion of various compounds is still one of the most attractive
subjects which researchers strive to accomplish.
aDepartment of Nanoscience and Technology, Bharathiar University, Coimbatore-641
046, Tamil Nadu, India. E-mail: viswanathan@buc.edu.in; Fax: +91 422 2422387;
Tel: +91 422 2428422
bPolymer Nanotechnology centre, B.S.Abdur Rahman University, Chennai-600 048,
Tamil Nadu, India
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Conducting polymers are in use as an alternative to metal
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However, to best of our knowledge, there are no reports on
highly selective, sensitive and simultaneous detection of AA,
DA and UA based on PANI-GO and PANI-RGO nanocomposite
modified electrodes in their ternary mixture.
In this work, Polyaniline/graphene oxide (PANI-GO) fibrous
nanocomposites were synthesized via an in situ chemical
oxidation polymerization method. The PANI-GO fibrous
nanocomposite electrocatalysts were drop coated on the glassy
carbon electrode (GCE) to detect AA, DA and UA simulta-
neously with high selectivity and sensitivity.
oxide materials for potential applications. Among the con-
ducting polymers polyaniline (PANI) has become one of the
technologically important conducting polymers, because of its
relatively low cost, simple synthesis, good optical and electrical
properties with excellent environmental stability.15–17 It also
has electrochemical properties and finds potential applica-
tions for electronic, optical devices,18–22 such as light emitting
diodes, sensors, electronic circuit boards, and bioelectrochem-
ical sensing.17,22 However, PANI was reported to be less
sensitive to bio molecules. This issue could be alleviated by
introducing suitable functional groups into polymer backbone
or conjugating PANI with others materials which are sensitive
to neurotransmitter DA,14,23 such as metal nanoparticles and
carbon nanomaterials, etc. Feng et al. have synthesized PANI/
Au composite hollow sphere and studied the bioelectrocata-
lytic activity in the oxidation of DA.24 Ali et al. have reported
the preparation of self-doped PANI/Carbon nanotube compo-
sites for the sensitive and selective detection of DA.25
In recent years, numerous reports have been published on
graphene oxide and graphene. Graphene, a flat monolayer of
hexagonally arrayed sp2-bonded carbon atoms tightly packed
into a two-dimensional (2D) honeycomb lattice, has been the
major focus of recent research due to its excellent electrical,
mechanical, thermal and optical properties,26,27 leading to
their potential applications in many different areas such as
nanoelectronics, super capacitor, gas sensors and chemical
sensors28–31 and also many graphene based electrode materi-
als and nanocomposites have been developed for sensing
biomolecules such as glucose,32 nicotinamide adenine dinu-
cleotide,33 dopamine and mixture of AA, DA and UA.5,31
Researchers particularly endeavor to prepare functionalized
graphene layer with enhanced solubility, dispersibility,
mechanical and electrochemical properties. Ping et al. have
reported selective, sensitive and simultaneous detection of AA,
DA and UA using high-performance screen-printed electrode.3
The potential separation in this analysis indicates the superior
selectivity of this system. Wu et al. have reported an
electrochemical method on selective detection of DA by
porphyrin-functionalized graphene.34 The simultaneous elec-
trochemical detection of mixture of bio-molecules (AA, DA and
UA) using chitosan-graphene modified electrode system was
reported by Han et al.2
Experimental
Materials
Natural graphite flakes (10 meshes) were purchased from Alfa
Aesar. Aniline was purchased from Sd-fine chemical and was
distilled under reduced pressure and kept below 0 uC before
use. Ascorbic acid, dopamine and uric acid were obtained
from Sigma-Aldrich. Sodium nitrate, sulfuric acid, hydrochlo-
ric acid, potassium permanganate, ammonium peroxydisul-
fate, Sodium hydroxide, potassium dihydrogen phosphate and
sodium phosphate were obtained from Himedia chemicals
and hydrogen peroxide obtained from Sd-fine chemicals. All
the analytical-reagent grade chemicals were used without
further purification. Phosphate buffer solution (PBS 0.05 M,
pH 7) was prepared with potassium dihydrogen phosphate and
sodium phosphate. All the aqueous solutions were prepared
with double-distilled water.
Synthesis of graphene oxide (GO)
Graphene oxide was synthesized from natural graphite flakes
by a modified Hummers method.38 Graphite (1 g) and NaNO3
(1 g) were mixed with 75 mL H2SO4 (95%) in a 250 mL flask.
The mixture was stirred for 30 min on an ice bath. While
maintaining vigorous stirring, potassium permanganate (3 g)
was added to the suspension. The rate of addition was
carefully controlled to keep the reaction temperature well
below 20 uC. The ice bath was then removed, and the mixture
was stirred at room temperature for 7 h. Additional potassium
permanganate (3 g) was added in one portion, and stirring was
continued for 12 h at room temperature. As the reaction
progressed, the mixture gradually became the consistency of
the mixture increased as the reaction progressed and the color
turned light brown. At the end, 100 mL of deionized water was
slowly added to the viscous mixture with vigorous agitation.
The reaction temperature rapidly increased to 98 uC while the
system color changed from brown to yellow and effervescence
was observed. The dilute suspension was stirred for 12 h.
Finally 20 mL of 30% H2O2 was added to the mixture. For
purification, the mixture was washed by rinsing and centrifu-
ging with 5% HCl and then washed with deionized (DI) water
for several times. After drying under vacuum, the brown color
graphene oxide (GO) was obtained.
Furthermore, graphene oxide (GO) and reduced grphene
oxide (RGO) have also been intercalated or used to make
composites with PANI through in situ chemical polymerization
and in situ anodic electrochemical polymerization to prepare
PANI-GO and PANI-RGO nanocomposites for potential appli-
cation in energy storage.35–38 However, the electrocatalytic
activities of both graphene oxide and graphene with polyani-
line nanocomposites have very rarely been analyzed. Very
recently, Bai et al.39 reported the electrocatalytic activity of
functionalized graphene sheets incorporating sulfonated PANI
electroactive composite towards AA and Bao et al.14 reported
graphene oxide-templated polyaniline microsheet towards
simultaneous electrochemical analysis of AA, DA and UA.
Synthesis of polyaniline nanofibers (PANI)
Aniline was first distilled under vacuum to remove the
oxidation impurities. The purified aniline (0.2 M) was
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dissolved in 100 mL of 1 M HCl aqueous solution. While
maintaining vigorous stirring at room temperature, the
ammonium peroxydisulfate (0.25 M) in 50 mL of 1 M HCl
aqueous solution was slowly added drop wise into the aniline
solution. Polymerization was observed after complete addition
when the characteristic green color of polyaniline emeraldine
salt appeared. The mixture was allowed to stir at room
temperature overnight. The precipitated polyaniline was
collected by filtration and repetitively washed with double
distilled water, ethanol and acetone successively until the
filtrate became colorless and dried in vacuum oven at 60 uC
overnight.
Synthesis of polyaniline-graphene oxide nanocomposites
(PANI-GO)
Graphene oxide and polyaniline fiber nanocomposites were
prepared by in situ polymerization of aniline in a suspension
of graphene oxide in acidic solution. The weight ratio of
aniline to graphene oxide was varied as 100 : 1, 100 : 2,
100 : 3, 100 : 4 and 100 : 5 and the resulting composites were
named as PANI-GO1, PANI-GO2, PANI-GO3, PANI-GO4 and
PANI-GO5, respectively. Typically, the graphene oxide was
dispersed in 100 mL of 1 M HCl aqueous solution by bath
sonicating for 1 h. Then 0.2 M aniline was dissolved in the
dispersed GO containing acidic aqueous solution. While
maintaining vigorous stirring at room temperature, ammo-
nium peroxydisulfate solution, with a mole ratio to aniline of
1 : 2, in 1 M HCl was added drop wise into the resulting
mixture. After about 5 min of addition, the aniline started to
polymerize while the color of mixture changed into green. The
mixture was stirred at room temperature overnight. The
composites were collected by filtration and repetitively washed
with double distilled water, ethanol and acetone successively
until the filtrate became colorless and dried in vacuum oven at
60 uC overnight.
Fig. 1 FTIR spectra of (a) GO, (b) PANI and (c) PANI-GO nanocomposite.
distilled water to form a homogenous dispersion. Then 5 mL of
the above solution was drop cast on the surface of a freshly
polished GCE and dried at ambient atmosphere for 5 h.
Results and discussions
Structure, morphology and properties of PANI-GO
nanocomposites
Characterization
FTIR spectroscopy was used to characterize the in situ
chemical polymerization of aniline on the GO sheets. Fig. 1
displays the typical spectra of oxidized GO, PANI, and PANI-GO
nanocomposites. Fig. 1a, shows the presence of the oxygen-
containing functional groups on GO. The bands at 1050, 1242,
1384, 1630 and 1732 cm21 corresponds to the C–O–C
stretching vibrations, C–OH stretching peak, O–H deformation
of the C–OH group, CLC stretching mode and CLO stretching
vibrations of the –COOH group, respectively. The broad and
intense peak corresponding to the stretching vibrations of O–
H at 3409 cm21 indicates that the GO samples contain large
quantity of adsorbed water molecules. These features suggest
that graphene oxide is heavily oxidized and consists mainly –
OH and other oxygen containing functional groups. As shown
in Fig. 1b, the strong absorption peaks has been assigned to
A QUANTA 250 field emission scanning electron microscope
(FE-SEM) was used to determine the morphology of the
composites. The FT-IR spectra of the samples were recorded
on a Thermonicolet is10-ATR mode FT-IR spectrometer by
using KBr pellets to confirm the presence of functional groups.
Raman spectra were recorded at ambient temperature on a
Horiba Jobin-LabRam-HR UV spectrometer with Argon laser at
an excitation wavelength of 514 nm. The electrochemical
measurements including cyclic voltammetry were performed
with an electrochemical work station CHI 1120A (CH instru-
ments, USA). A conventional three-electrode cell was used,
including Ag/AgCl (saturated KCL) electrode as the reference
electrode, a platinum wire as the counter electrode and glassy
carbon electrode (GCE, 3 mm in diameter) as the working
electrode.
the stretching of quinonoid ring (CLN) appeared at 1567 cm21
,
benzenoid rings (C–C) appeared at 1483 cm21, The peak at
1387 cm21 corresponds to C–N stretching in the neighborhood
of a quinonoid ring. The band at 1294 cm21 is assigned to the
C–N stretch of a secondary aromatic amine. The aromatic C–H
in-plane bending modes appeared at 1126 cm21 and aromatic
CH out-of-plane bending appeared at 811 cm21. As shown in
Preparation of PANI-GO modified electrode
The GCE surface was carefully polished to a mirror like finish
in alumina slurries with 0.5 and 0.05 mm (mesh), successively.
Afterwards, the electrode was washed thoroughly with excess
amount of water and dried in ambient atmosphere. A certain
amount of PANI-GO nanocomposite was ultrasonicated in
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Fig. 1c, the FTIR spectrum of PANI-GO is almost the same as
PANI (Fig. 1b). The absorption peaks of the amide group were
observed at 1656 cm21 for PANI-GO. This showed amidation
between the carboxylic acids group of GO and the amino group
of aniline and also, the absorption peak of CLO groups has red
shifted to 1743 cm21 from 1732 cm21 of composites. These
results indicate that the carboxylic groups on the edge region
of GO have partially reacted with the amino group of the
aniline monomer. The structure of the interface between GO
and PANI features a combination of electrostatic interaction,
hydrogen bonds and p–p interactions.35 The characteristic
peaks of PANI at 1567 and 1483 cm21 manifests the presence
of PANI in the PANI-GO composite. In addition, matching with
Chen et al. reports, we also observed the reappearance of a
band at 2926 cm21 for the CH2 groups (Fig. 1c) implying a
restoration of the carbon basal planes on account of reduction
by hydrogen.40
Fig. 2 shows, the Raman spectra for the developed PANI-GO
nanocomposite as well as for pure PANI fiber and GO layer. In
the Raman spectrum of GO layer (Fig. 2a), the two intense
bands at 1357 cm21 and 1595 cm21 have been assigned to D
band and G band respectively. The G band assigned to the first
order scattering of the E2g phonon of GO represent the in-
plane bond stretching vibration of sp2- bonded carbon atoms
in a 2D hexagonal lattice. The D band is associated with the
breathing mode of K-point phonons of A1g symmetry with
vibration of carbon atoms with dangling bonds in plane
terminations of disordered graphite.41,42 In the Raman
spectrum of pure PANI nanofiber (Fig. 2b), the characteristic
bands appear at 1585 and 1347 cm21, which are assigned to
the CLC stretching vibration of the quinonoid ring and C–H
bending vibration of benzene ring in the backbone of PANI,
respectively. The band at 1046 cm21 is attributed to
symmetrical C–H in-plane bending vibrations.43 The peaks
Fig. 3 SEM images of (a) PANI, (b) GO and (c, d) PANI-GO nanocomposite.
observed at 647 cm21 can be attributed to bipolaronic benzene
ring deformations, while the peak related to the polaronic C–
N–C torsion was obtained at 418 cm21 ref. 35,44. In the Raman
spectrum of PANI-GO nanocomposite (Fig. 2c), the D band
becomes broad during the in situ polymerization process,
implying in-plane bending vibration of C–H molecules over-
lapped in the D band of GO layer. In addition, the composites
presented a shift of D and G bands towards higher wavelength
when GO was introduced into the PANI. This is probably due
to the doping of carboxyl group of graphene oxide to PANI
backbone and p–p interaction between PANI and GO sheets.35
To visually learn the dimensional self-construction of the
prepared nanocomposite, the surface morphologies and shape
of the PANI, GO and PANI-GO are shown in Fig. 3. The pure
PANI shows short agglomerated fibers with some granular
particles in the order of y150 to 200 nm (Fig. 3a). The GO
sheets was composed of a layered structure are stacks wrinkled
(Fig. 3b), while the PANI-GO composites were composed of
smooth surface fibers. Since the uniform PANI nanofibers
have been grown on the surface of the GO nanosheets in the
order lower than y100 nm. In analyzing the morphology of
the PANI-GO nanocomposite (Fig. 3b), it is important to take
into consideration the electron transport properties of both
GO and aniline monomer in order to determine the molecular
binding mechanism.44 The strong affinity between the
negatively charged carboxyl group and the positively charged
amine nitrogen groups makes the PANI fibers firmly attach
with the GO nanosheets.42 The aniline monomer is first
adsorbed onto the surface of the GO nanosheets during the
formation of PANI nanofiber on GO nanosheets due to
electrostatic attraction. Oxidation of aniline by oxidizing agent
gives a nanostructured product shown in Fig. 3. This result
was reported earlier by Sadek et al.45 Hence, the GO
nanosheets can be considered as a support material providing
a large number of active nucleation sites for PANI growth.46
TGA and DTG studies were carried out to study the thermal
stability of PANI, GO and PANI-GO nanocomposites shown in
Fig. 2 Raman spectra of (a) GO, (b) PANI and (c) PANI-GO nanocomposite.
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Fig. 4 TGA curve of (a) GO, (b) PANI and (c) PANI-GO nanocomposite. Inset: DTG
Fig. 5 Cyclic voltammogram curves of bare GCE, PANI and PANI-GO nanocom-
curves of GO, PANI and PANI-GO nanocomposite.
posite.
Fig. 4. As shown in Fig. 4a, GO first starts to lose weight upon
heating from 50 uC to 150 uC owing to the evaporation of water
present between the layers in GO; a second major weight loss
occurs at about 300 uC which is due to the decomposition of
oxygen containing functional groups (such as C–OH, C–O–C,
COOH) producing CO2, CO and H2O which are removed from
the GO nanosheets. For the pure PANI (Fig. 4b), the gradual
weight loss in the temperature range from 70 uC to 200 uC
shows 20% mass loss which is due to dehydration and
deprotonation of the PANI.47 However, a rapid change in
major weight loss of 40% takes place in the range 230 uC to 650
uC, which corresponds to the degradation and decomposition
of PANI at different polymerization stage. In PANI/GO
nanocomposite (Fig. 4c), the slow weight loss near 70 uC to
260 uC can be attributed to the loss of co-intercalated water
molecules and the release of co-intercalated HCl. After 260 uC,
major weight loss occurs due to decomposition of the PANI in
the nanocomposite. Finally the weight loss of 50% takes place
in the range, at about 650 uC for GO-based PANI nanocompo-
site. In general, the thermal stabilities of the PANI-GO
nanocomposite are greater than that of the GO presumably
because of the elimination of the OH groups and the
incorporation of PANI chain molecules.40 Also, the percen-
tages of major weight loss of the molecules corresponding to
the temperature are shown in inset Fig. 4.
(PANI and PANI-GO) were controlled to be the same on each
modified electrode.
Each CV curve is nearly rectangular in shape, indicating
good charge propagation within the electrode. A couple of
oxidation and reduction peaks are observed from the CV curve
of PANI, corresponding to transition of leucoemeraldine/
polaronic emeraldine form and Faradic transformation of
emeraldine/pernigraniline.38,49
It is clear from Fig. 5 that the CV curve current densities of
the PANI nanofiber modified GC electrode is higher than those
of the PANI-GO nanocomposite modified GC electrode and
bare GC electrode. The GO containing nanocomposite (PANI-
GO) shows typically a couple of well defined oxidation and
reduction peaks when compared to pure PANI nanofiber
(Fig. 5). This, suggests that the more oxygen containing
functional groups of GO nanosheets in the composite
contribute a little to the current density, and the total pairs
of oxidation and reduction peaks are well defined by the
Faradic process (cathodic and anodic) than the conducting
PANI nanofiber. This is mainly due to the PANI nanofiber
grown on the surface of GO nanosheets improve the charge
transfer between the active materials and GC electrode.
Therefore, the electrochemical catalytic behavior of PANI was
further improved by intercalation with GO sheets.
The PANI-GO nanocomposite modified GC electrode was
also observed with enhanced electrochemical stability. Fig. 6A
reveals that in the CV curve of PANI modified electrode,
current decreases significantly with increase in the number of
cycles. The electrode current decreases by 0.8 mA at 100th
cycle and by 1.2 mA at 500th cycle from first cycle. On the other
hand the CV curve of PANI-GO nanocomposite modified GC
electrode was almost the same as the first cycle as shown in
Fig. 6B. Also, there is only a slight change in the CV curve of
PANI-GO nanocomposite modified GC electrode after 500th
cycle. Based on the electrochemical properties described
above, it can be concluded that the small amount (1–5%) of
Electrochemical characteristics of PANI/GO nanocomposite
modified GCE electrodes
Following previous report48 the working electrodes were
prepared by casting a Nafion-impregnated composite sample
onto a glassy carbon electrode. Fig. 5 compares the CV curve of
the bare GCE, PANI and PANI-GO nanocomposite modified GC
electrodes at a scan rate of 20 mV s21 within the potential
range from 20.2 to + 0.8 V vs. Ag/AgCl in the aqueous solution
of 1 M H2SO4 electrolyte. The weights of the active materials
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Fig. 7 Cyclic voltammogram curves of 1.0 mM AA, 1.0 mM DA and 1.0 mM UA
Fig. 6 Cyclic voltammogram curves of (A) PANI and (B) PANI-GO nanocompo-
at bare GCE (A) and PANI-GO/GCE (B) in 0.05 M (pH 7.0) PBS. Scan rate: 50
site.
mVs21
.
GO introduced into the PANI can greatly improve the
electrochemical stability of the PANI nanofiber.
potential of AA is negatively shifted to 82 mV and oxidation
peak potential of DA and UA are a positively shifted to 425 mV
and 596 mV. The difference between AA and DA peak
potentials is above 300 mV, which is enough to characterize
AA and DA. The current densities observed at PANI-GO
nanocomposite modified GC electrode are more or less six
times higher than that of the bare GC electrode. These results
suggest that the electron transfers for the oxidation of AA, DA
and UA at the PANI-GO nanocomposite is much easier, due to
the increased surface area. Such an improvement in electron
transfer kinetics could be attributed to the doping of
carboxylic group of graphene oxide to PANI backbone and p–
p interaction between PANI and GO sheets which may offer
some favorable sites for transferring the electrons to biomo-
lecules and would accelerate the electron transfer between
electrode surface and electroactive molecules.50
Electrocatalytic oxidation characteristics of AA, DA and UA at
PANI-GO modified GCE
Fig. 7 illustrates the cyclic voltammetric responses of the bare
GC electrode and PANI-GO nanocomposite modified GC
electrode in 0.05 M PBS solution (pH 7.0) containing 1 mM
AA, 1 mM DA and 1 mM UA within the potential window of
20.2–0.8 V and at scan rate of 50 mV s21. As can be seen in
Fig. 7A, AA, and UA shows irreversible oxidation peaks at 514
and 299 mV, respectively. DA shows quasi-reversible electro-
chemical behavior with the anodic and cathodic peak potential
at 399 mV and 300 mV at the bare GC electrode. It seems that
the separation of the anodic peak potentials for these three
molecules is not enough to obtain good selectivity at the bare
GC electrode.
For the PANI-GO nanocomposite modified GC electrode as
shown in Fig. 7B, the corresponding anodic oxidation peak
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Scheme 1 Schematic representation of the simultaneous electrocatalytic
Fig. 8 DPV curves at bare GCE, GO/GCE, PANI/GCE and PANI-GO/GCE in 0.05 M
(pH 7.0) PBS containing ternary mixture of 1 mM AA, 1 mM DA and 1 mM UA.
DPV settings: step potential: 5 mV; amplitude: 60 mV; pulse width: 0.05 s;
sample width: 0.01 s and pulse period: 0.2 s.
oxidation of AA, DA and UA.
and UA, but rejection to positively charged DA due to the
charge characters of AA (pKa = 4.1), DA (pKa = 8.8), and UA (pKa
= 3.7).14 Both the CV and DPV measurements demonstrate that
PANI-GO nanocomposite possess good electrocatalytic activity
towards the oxidation of these three molecules.
Electrocatalytic oxidation characteristics of AA, DA and UA in a
ternary mixture
The electrocatalytic activity of PANI-GO nanocomposite mod-
ified GC electrode towards the oxidation of these three bio-
molecules in a mixture is also investigated by the sensitive
technique of differential pulse voltammetry (DPV). Fig. 8
shows the DPV peaks of ternary mixture of AA, DA and UA at
bare GCE, GO/GCE, PANI/GCE and PANI-GO/GCE. At the bare
GC electrode, the electro-oxidation of AA and DA molecules is
completely overlapped and presents a broad overlapped peak
(at 222 mV) so that simultaneous determination of these three
molecules is impossible. On the contrary, well defined DPV
oxidation peaks were observed at PANI-GO/GCE with peak
potential value of 230 mV, 290 mV and 520 mV for AA, DA and
UA, respectively. The separation of electro oxidation peak
potential for AA-DA, DA-UA and AA-UA are 320 mV, 230 mV
and 550 mV, respectively. The large separation of the peak
potentials reveals that PANI-GO nanocomposite with higher
electrocatalytic activity can be used to construct electrochemi-
cal sensor for selective determination of AA, DA and UA in the
presence of the other two molecules, or simultaneous
detection of them in their ternary mixture. The plausible
mechanisms (Scheme 1) behind the simultaneous detection of
AA, DA and UA with wide potential separation of PANI-GO
nanocomposite modified GC electrodes can be explained as
follows. The molecular structure of DA and AA are different
from each other. The improved oxidation could be due to the
p–p interaction between phenyl structure of DA and PANI-GO
nanocomposite and the hydrogen bonds formed between
hydroxyl or amine groups of DA and nitrogen atom within
nanocomposite fiber.51 Although the p–p interaction between
penta-heterocyclic system of AA and PANI-GO nanocomposite
is weak, the presence of cationic emeraldin salt provide a
significant electrostatic attraction to negatively charged AA
In the control experiment, only one distinct anodic peak of
AA is observed in GO/GC electrode at 69 mV and two small
intense anodic peaks observed at about 340 mV and 480 mV
indicating that the oxidation peaks of DA and UA cannot be
distinguished from each other in GO/GCE. It seems that it is
possible for PANI/GCE to differentiate individual analyte
species. However, compared to PANI-GO/GCE, the DA and
UA oxidation peak is much lower, and the DA-UA peak
separation is smaller. PANI-GO has electrochemical activity in
neutral solution, and its electrochemical activity has been
greatly enhanced than the PANI fibers. The possible reason for
this preferable electrocatalytic behavior may be attributed to
the fiber structure of nanocomposite, which provides larger
surface area thereby enhancing the sensing signal and
accelerating the electron transport which is due to the strong
p–p stacking between PANI and GO.39
Effects of the mass ratio of aniline/GO and pH on the
electrochemical oxidation behavior of AA, DA and UA
Fig. 9 shows the different mass ratio of aniline to GO during
the polymerization process on the DPV responses of AA, DA
and UA in their ternary mixture at the PANI-GO nanocompo-
site modified GC electrode. The different mass ratios of
aniline to GO chosen for analysis were 100 : 1, 100 : 2, 100 : 3,
100 : 4 and 100 : 5 and they were named as PANI-GO 1, PANI-
GO 2, PANI-GO 3, PANI-GO 4 and PANI-GO 5 respectively. The
anodic peak potentials for ternary mixture of these three
molecules are well defined in the five ratios, and the PANI-GO
3 nanocomposite modified GC electrode obtained a large peak
potential separations. The PANI-GO 3 modified GC electrode
can effectively resolve the merged voltammetric peak into
three well defined anodic oxidation peak at potential of about
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Fig. 9 DPV curves at PANI-GO/GCE in 0.05 M (pH 7.0) PBS containing ternary
mixture of 1.0 mM AA, 1.0 mM DA and 1.0 mM UA. Different ratios of aniline to
GO (mass ratio) during PANI-GO polymerization process. The ratios of aniline to
GO in 100 : 1, 100.2, 100 : 3, 100 : 4 and 100 : 5. DPV settings: step potential:
5 mV; amplitude: 60 mV; pulse width: 0.05 s; sample width: 0.01 s and pulse
period: 0.2 s.
230 mV, 290 mV and 520 mV for AA, DA and UA, respectively.
The separation of electro oxidation peak potential for AA-DA,
DA-UA and AA-UA are 320 mV, 230 mV and 550 mV,
respectively. These large anodic peak potential separations
indicates that 100 : 3 aniline-GO ratio is a very suitable ratio
for selective determination of AA, DA and UA in the presence
of the other two molecules, or simultaneous detection of them
in their ternary mixture.
The influence of solution pH on the peak current and peak
potential of the electrocatalytic anodic oxidation of AA, DA and
Fig. 11 DPVs at the PANI-GO/GCE in 0.05 M (pH 7.0) PBS containing10 mM DA,
10 mM UA and different concentrations of AA (from inner to outer): 25, 50, 75,
100, 125, 150, 175, 200, 225 mM (A), containing 100 mM AA, 10 mM UA and
different concentrations of DA (from inner to outer): 2, 4, 6, 8, 10, 12, 14, 16, 18
mM (B) and containing 100 mM AA,10 mM DA and different concentrations of UA
(from inner to outer): 2, 4, 6, 8, 10, 12, 14, 16, 18 mM (C). DPV settings: step
potential: 5 mV; amplitude: 60 mV; pulse width: 0.05 s; sample width: 0.01 s
and pulse period: 0.2 s. Inset: Plot of peak currents vs. sample concentrations.
Fig. 10 DPV curves at PANI-GO/GCE in 0.05 M (pH 5.0, 7.0 and 9.0) PBS
containing ternary mixture of 1.0 mM AA, 1.0 mM DA and 1.0 mM UA. DPV
settings: step potential: 5 mV; amplitude: 60 mV; pulse width: 0.05 s; sample
width: 0.01 s and pulse period: 0.2 s.
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Table 1 Comparison of the analytical performance of the different modified electrodes for the simultaneous determination of AA, DA and UA
Linear range (mM)
AA DA
Detection limit (mM)
Sensitivity (mA mM21
)
Potential separation (mV)
Electrode materials
Ref.
UA
AA
1.0
DA
UA
AA
DA
UA
AA-DA
DA-UA
AA-UA
Poly-ACBK
SWCNH
CG/GCE
50-1000 1–200 1–120
30–400 0.2–3.8 0.06–10 5.0
50–1200 1.0–24 2.0–45 50
0.5
0.06
1.0
0.5
0.02
2.0
0.002
0.020
0.012
0.027
—
0.04
0.92
0.147
3.5
0.375
0.93
—
1.3
2.0
0.007
5.6
0.45
0.59
—
0.7
2.2
193
211
165
136
270
250
320
166
152
90
212
150
190
230
359
363
255
248
430
440
550
10
11
2
12
13
14
HNCMS/GC
100–1000 5–70
3–30
0.91
1.11
50
20
0.02
1.05
0.5
0.04
0.03
1.0
MWCNT-FeNAZ/GCE 20–355
GO-PANI/GCE
65–260 6–25
150–850 2–14
2–16
2–18
PANI-GO/GCE
25–200
2–18
0.5
0.2
Our work
UA at the PANI-GO nanocomposite was also investigated by
DPV techniques in the pH ranging from 5 to 9 as shown in
Fig. 10. The anodic oxidation peaks of AA and DA were
overlapped, when the solution pH was below 6. It is clear that
with the increase in solution pH between 7 and 9, the anodic
oxidation peak potential of AA, DA and UA shifted negatively.
This indicates, protons are directly involved in the overall
electrochemical oxidation reactions, i.e., the oxidation reaction
occur via electron transfer step followed by protonation
process.52 Furthermore, it is evident that the large separation
of the peak potential for AA-DA and DA-UA are obtained at pH
7. Therefore, individual or simultaneous determination of AA,
DA and UA from their ternary mixture can be realized at the
PANI-GO nanocomposite modified GC electrode in a wide pH
range. Based on the peak potential separation and detection
sensitivity, the buffer solution of pH 7 was selected in our
measurements.
catalytic activity towards the oxidation of AA, DA and UA.
Furthermore, in AA, DA and UA ternary mixture, three well
defined peaks at about 230 mV, 290 mV and 520 mV were
obtained, which is ascribed to the p–p interaction and
electrostatic attraction between the target biomolecules and
PANI-GO nanocomposite. In addition, the electro oxidation of
AA, DA and UA on the PANI-GO nanocomposite modified
electrode shows rapid response of three well defined peaks
with larger peak potential separation. Therefore, an electro-
chemical sensor for individual or simultaneous determination
of these three biomolecules on PANI-GO/GCE can be achieved
with higher sensitivity and selectivity. Thus PANI-GO nano-
composite with unique structural and electrocatalytic proper-
ties is a promising candidate for the fabrication of sensitive
and selective biosensors.
Simultaneous determination of AA, DA and UA
References
Simultaneous determination of AA, DA and UA at the PANI-GO
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other two species remained constant. As shown in Fig. 11, the
electro oxidation peak current of AA, DA, or UA increases
linearly with the increase of the biomolecule concentration.
The corresponding linear range for AA, DA and UA determina-
tion are 25 to 200 mM (R2 = 0.9955), 2 to 18 mM (R2 = 0.9932)
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such a high sensitivity and large potential separation is rarely
reported. This PANI-GO nanocomposite with unique structural
and electrocatalytic properties is a promising candidate for the
fabrication of sensitive and selective biosensors.
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