Simultaneous determination of dopamine and uric acid in the presence of high ascorbic acid concentration using cetyltrimethylammonium bromide-polyaniline/activated charcoal composite

Article abstract of DOI:10.1039/c6ra18740d

We describe a simple, low-cost and mass producible composite made up of cetyltrimethylammonium bromide (CTAB) functionalized polyaniline (PANI) and activated charcoal (CTAB-PANI/AC) for simultaneous determination of dopamine (DA) and uric acid (UA). The composite formation was verified through scanning electron microscopy, electrochemical impedance spectroscopy and electrochemical methods. The CTAB-PANI/AC composite was used to modify a glassy carbon electrode (GCE) and the resulting modified electrode displayed excellent electrocatalytic activity to DA and UA and successfully separates their overlapped voltammetric peaks. The composite completely inhibits the AA signal and does not produce any voltammetric signal for AA up to 2 mM. The DA and UA can be selectively detectable up to detection limits of 0.06 (±0.006) μM and 0.20 (±0.008) μM, respectively. The effects of kinetics, analyte concentration and pH of the supporting electrolyte were investigated and optimized. The modified electrode has appreciable stability, repeatability and reproducibility. Besides, the practical feasibility of the sensor is demonstrated in biological samples, which delivered satisfactory recovery results.

Full text of DOI:10.1039/c6ra18740d

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Simultaneous determination of dopamine and uric acid in presence of high
concentration of ascorbic acid using cetyltrimethylammonium bromide–
polyaniline/activated charcoal composite
Mani Govindasamy1, ShenꢀMing Chen1*, Veerappan Mani1,2*, Anandaraj Sathiyan3, Johnson
Princy Merlin3 Fahad M.A. AlꢀHemaid4, M. Ajmal Ali4
,
1Department of Chemical Engineering and Biotechnology, National Taipei University of
Technology, Taipei, Taiwan (ROC)
2Graduate Institute of Biomedical and Biochemical Engineering, National Taipei University of
Technology, Taipei, Taiwan (ROC)
3Department of Chemistry, Bishop Heber College (Autonomous), Tiruchirappalliꢀ620 017, Tamil
Nadu, India
4Department of Botany and Microbiology, College of Science, King Saud University, Riyadh
11451, Saudi Arabia
Corresponding author: V. Mani, Email: veera.678@gmail.com Tel.: +886 2271ꢀ2171 2525; Fax:
+886ꢀ02ꢀ2731ꢀ7117. S.ꢀM. Chen, Email: smchen78@ms15.hinet.net Tel: +886 2270 17147,
Fax: +886 2270 25238
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Abstract
We described a simple, lowꢀcost and mass producible composite made up of
cetyltrimethylammonium bromide (CTAB) functionalized polyaniline (PANI) and activated
charcoal (CTAB–PANI/AC) for simultaneous determination of dopamine (DA) and uric acid
(UA). The composite formation was verified through scanning electron microscopy,
electrochemical impedance spectroscopy and electrochemical methods. The CTAB–PANI/AC
composite was used to modify glassy carbon electrode (GCE) and the resulting modified
electrode displayed excellent electrocatalytic activity to DA and UA and successfully separates
their overlapped voltammetric peaks. The composite completely inhibits the AA signal and does
not produce any voltammetric signal for AA upto 2 mM. The DA and UA can be selectively
detectable upto detection limits of 0.06 (± 0.006) µM and 0.20 (± 0.008) µM, respectively. The
effect of kinetics, analyte concentrations and pH of the supporting electrolyte pH investigated
and optimized. The modified electrode has appreciable stability, repeatability and
reproducibility. Besides, practical feasibility of the sensor is demonstrated in biological samples
which delivered satisfactory recovery results.
Keywords: Activated carbon materials, analytical science, conducting polymers,
electrocatalysis, electrochemical sensors, bioassays, neurotransmitters
1. Introduction
Dopamine (DA) is one of the key neurotransmitter in mammalian central nervous
systems which facilities communication between brain and neurons.1ꢀ3 Abnormal concentration
of DA is directly related to motor functions of the central nervous system which can cause
several neurological disorders.4, 5 In addition, DA is administered externally as a medication to
DA deficient patients, but its excess dosage causes neurological side effects.6 Therefore,
sensitive determination of DA is important in clinical analysis.7ꢀ9 Besides, uric acid (UA) is the
primary end product of purine metabolism and its abnormal levels are symptoms of several
diseases such as gout, hyperuricemia and LeschꢀNyan disease.10, 11 It is wellꢀknown that DA, UA
and ascorbic acid (AA) are usually coexisted in the extra cellular fluids; however, they are
indicators for different diseases.12, 13 AA is a significant vitamin required for human diet and
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usually its concentration in extra cellular fluids is much higher (about 1000 times) than that of
DA and UA. Hence, the detections of DA and UA always encountered strong interference from
AA.14, 15 In addition, the electrode surface is susceptible to get damaged by the adsorption of
oxidized products of AA which leads to surface fouling and film stability problems.16 The other
problems in the determinations of DA and UA are overlapped voltammetric responses, poor
selectivity and low reproducibility.17 The bare electrodes are not suitable for either selective
and/or simultaneous detections of DA and UA and hence rationally designed chemical modifiers
are required.18 The ideal modifiers should separate the voltammetric peaks of DA and UA with
wide peakꢀtoꢀpeak separation, completely eliminate the AA interference, high sensitivity in
presence of AA, and selectivity over coexisted electroactive species in biological fluids. So far,
several modified electrodes based on nanomaterials 7, 17, conducting polymers 1, 19, metal
7, 20
21, 22
nanoparticles
and metal oxides
were developed; however not all of them are ideal
modifiers.23 Xu et al., have reported Pt nanoparticles supported reduced graphene oxide for the
simultaneous detection of DA and UA which eliminates AA interference upto 1 mM.14 Liu et al.,
described poly(acrylic acid)ꢀmultiwalled carbon nanotubes composite for the detections of DA
and UA which hinders AA signal upto 0.3 mM.24 Alipour et al., described pretreated pencil
graphite electrode for the detections of DA and UA which is selective upto 0.5 mM AA.17 In
spite of the success of developed electrode materials, new materials are still in demand to
completely tackle the problems due to the biological significance of DA and UA.
Polyaniline (PANI) is the most popularly used conducting polymer in electrochemical
sensors attributed to its excellent electrocatalytic ability, conductivity, easy synthesis, high
environmental stability and thermal stability.25 The composites of PANI with carbon materials
such as, activated carbon, graphene and carbon nanotubes have shown improved electronic and
mechanical properties compared with pure PANI, making the composite more suitable for
electronic/electrochemical applications.26 Among other carbon materials, activated
carbon/charcoal (AC) is the cheapest, most abundant and it possesses high surface area and good
porosity27. In recent times, the composite prepared from AC and PANI is attracted significant
attention in supercapacitors.27ꢀ29 Nevertheless, the electrochemical sensor applicability of
AC/PANI composite is not investigated in the literature.
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Herein, we have prepared AC/PANI composite via simple softꢀtemplate synthetical
method using CTAB as template and explored its electrochemical sensing applicability towards
simultaneous determinations of DA and UA. Previous studies proved that CTAB is a good softꢀ
template for controlled polymerization of aniline and the resulting polymer is ordered in different
aspects, such as morphology, particle size, and conductivity30, while we have used potassium
peroxodisulfate (PDS) as a radical initiator in PANI formation. The positively charged PANI
surface is easy to bind with negatively charged AC surface via electrostatic interaction which
resulting in the formation of stable composite. The morphology, elemental composition,
electrodeꢀelectrolyte interface and electrochemical properties are revealing that the CTAB–
PANI/AC composite is highly suitable for electrochemical sensing applications. The
electrochemical studies proved that the composite is highly useful for simultaneous
determination of DA and UA in presence of high concentrations of AA (upto 2 mM) (scheme 1).
Scheme 1 Schematic representation for the preparation of CTAB–PANI/AC/GCE and its
application towards simultaneous determinations of DA and UA
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2. Experimental
2.1 Chemicals and Apparatus
Aniline, CTAB, AC and PDS were purchased from Merck. All the other reagents were
purchased from SigmaꢀAldrich and used as received. All the reagents used were of analytical
grade and used without any further purification. Double distilled water was used for all the
experiments. 0.1 M phosphate buffer (pH 7.0) was used as supporting electrolyte which was
prepared from sodium dihydrogen phosphate and disodium hydrogen phosphate. Rat brain
sample and human serum were acquired from Chang Gung University, Taiwan and the
experimental protocols were approved by the institutional Animal Ethic Committee. The real
sample analysis performed in rat brain and human serum samples were performed in compliance
with the laws and institutional guidelines of Chang Gung University, Taiwan. An informed
consent was obtained for human serum collection with human subject.
Electrochemical studies were performed in a conventional three electrode cell using
modified glassy carbon electrode (GCE) (Bioanalytical Systems, Inc., USA) as a working
electrode (area 0.071 cm2), saturated Ag|AgCl (saturated KCl) as a reference electrode and Pt
wire as a counter electrode. All the electrochemical measurements were carried out using CHI
1205a electrochemical work station (CH Instruments, Inc., U.S.A) at ambient temperature. Prior
to each electrochemical experiment, the electrolyte solutions were deoxygenated with preꢀ
purified nitrogen for 15 min unless otherwise specified. Surface morphological studies were
carried out using Hitachi Sꢀ3000 H scanning electron microscope (SEM) and transmission
electron microscope (TEM) (Hꢀ7600, Hitachi, Japan). Raman spectra were acquired using
MicroꢀRaman spectrometer (RENISHAW in Via system, U.K) by a 514.4 nm He/Ne laser. FTIR
spectra were carried out using a PerkinꢀElmer IR spectrometer. The Xꢀray photoelectron
spectroscopy was studied by using XPS, PerkinElmer PHIꢀ5702. EIM6ex Zahner (Kronach,
Germany) was used for electrochemical impedance spectroscopy (EIS) studies.
2.2 Preparation of CTAB–PANI/AC/GCE
First, 5 mM CTAB solution was prepared in 0.5 M H2SO4. Next, 40 mM aniline solution
was added to the CTAB solution and the mixture was stirred for 30 min using magnetic stirrer. 1
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g of AC was added to the solution and continued stirring for additional 30 min, while the
temperature was maintained below 0°C. Afterwards, a preꢀcooled solution of 50 mM PDS was
added in dropwise to aniline solution and stirred for 30 min. A precipitate was formed which was
filtered and washed several times with water and acetone, respectively. The purified CTAB–
PANI/AC composite was dried and redispersed in ethanol (1 mg mLꢀ1). Next, GCE surface was
polished with 0.05 µm alumina slurry using a Buehler polishing kit, then washed with water and
dried. 5 µl dispersion of CTAB–PANI/AC was dropped at the preꢀcleaned GCE and dried at
ambient conditions.
2.3 Sampling procedure for real sample analysis
Dopamine hydrochloride injection was obtained from local medical hospital and directly
used for the analysis. Realꢀtime analysis is directly carried out by injecting aliquots of dopamine
hydrochloride injection sample into phosphate buffer (pH 7). In all the realꢀsample studies, the
total volume of the electrochemical cell is kept at 1 mL. Rat brain sample and human serum were
acquired from Chang Gung University, Taiwan and the experimental protocols were approved by
the institutional Animal Ethic Committee. About 1 mL of rat brain or human serum was added to
20 mL of buffer. To this solution, known concentrations of DA are spiked and analyzed using
CTAB–PANI/AC film modified electrode. Human urine sample is collected from a healthy
human. About 1 mL of human urine is diluted with 50 mL of phosphate buffer and the resulting
solution is DA free. To this solution, known concentrations of DA are spiked and analyzed using
our method.31
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Fig. 1 SEM images of CTAB–PANI (a) and CTAB–PANI/AC composite (b). TEM images of
CTAB–PANI (c) and CTAB–PANI/AC composite
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Fig. 2 a) XPS survey spectrum of CTAB–PANI/AC composite. b) C 1s, c) O 1s, d) N 1s,
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Fig.3 (a) Raman spectra of CTAB–PANI (a’) and CTAB–PANI/AC (b’). (b) FTꢀIR spectra of
CTAB–PANI (a’) and CTAB–PANI/AC (b’)
3. Results and Discussions
3.1 Characterization of CTAB–PANI/AC
The surface morphology of the CTAB–PANI and CTAB–PANI/AC composites were
studied using SEM and TEM. The SEM of CTAB–PANI (Fig. 1a) exhibited porous like
morphology along with the presence of PANI fibers and this kind of morphology is also
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observed in the TEM (Fig. 1c). The SEM image of CTAB–PANI/AC is featured with highly
porous morphology which portrays abundant pores, cavities and randomly distributed flakes
(Fig. 1b). TEM image of the composite also displays the PANI covered porous carbon like
morphology which is consistent with SEM results (Fig. 1d). XPS is used to measure the
elemental composition of the asꢀprepared composite (Fig. 2a). The survey XPS spectrum
indicates that the composite consists of oxygen (O 1s, 532 eV), nitrogen (400 eV) and carbon
(C 1s 285 eV). The presence of nitrogen is clearly indicating the successful formation of
PANI/AC composite, because the nitrogen content is originated from polyaniline. The
enlarged XPS spectra for C1s (Fig. 3b), O1s (Fig. 3c) and N1s (Fig. 3d) are clearly show the
typical bands and revealed the successful formation of composite. The Raman spectrum of
CTABꢀPANI (curve a’, Fig. 4a) displays two sharp bands at ≈ 1261 cm−1 and ≈ 1539 cm−1
corresponding to the characteristic D and G bands. Besides, the D band to G band intensity ratio
(ID/IG) is considerably increased in CTAB–PANI/AC composite (curve b’, Fig. 4a) and the
bands are slightly red shifted compared to CTAB–PANI indicating the possible interaction
between CTAB–PANI and AC. FTꢀIR spectroscopy is used to investigate the functionality of the
asꢀprepared materials. The FTIR spectrum of CTAB–PANI (curve a’, Fig. 4b) displays FTIR
absorption peaks, ν(N–H) = 3430, ν(aromatic CꢀH bond)= 2891, ν(quinoid)= 1563,
ν(benzenoid)= 1490, ν(CꢀN stretching and bending vibrations)= 1299 and 1238, ν(CꢀN double
bond)= 1115 and ν(CꢀH out of plane bending)= 801 cmꢀ1 which are characteristic stretching
vibrations of PANI and this result is consistent with previous reports.32, 33 These vibration modes
are observed in CTAB–PANI/AC composite, but slightly red shifted which could be due to the
composite formation (curve b’, Fig. 4b).
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Fig. 4 EIS curves of bare GCE (a), CTAB–PANI (b) and CTAB–PANI/AC composite (c). Inset:
Randles equivalent circuit model. Here, Rs is electrolyte resistance, Rct is charge transfer
resistance, Cdl is double layer capacitance and Zw is Warburg impedance.
Fig. 4 displays the EIS obtained at bare GCE (a), CTAB–PANI (b) and CTAB–PANI/AC
(c) in 0.1 M KCl containing 5 mM Fe(CN)63–/4–. Randles equivalent circuit model was used to fit
the experimental data (inset to Fig. 4). The semicircles indicates the parallel combination of
electron transfer resistance (Rct) and double layer capacitance (Cdl) at electrode surface resulting
from electrode impedance. The charge transfer resistance values were obtained by fitting the
Nyquist plot results with Randles equivalent circuit model. The Rct values of bare GCE, CTAB–
PANI/GCE and CTAB–PANI/AC/GCE are 460 (± 2.18) Ω, 275 (± 3.05) Ω and 25 (± 1.41) Ω
respectively. The Rct value obtained at CTAB–PANI/AC/GCE is 11 and 18 times smaller than
that of CTAB–PANI/GCE and unmodified GCE, respectively. Thus, CTAB–PANI/AC has
lower electrode resistance over control electrodes which are due to the high conductivity and
large surface area of the composite.
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Fig. 5 CVs obtained at bare GCE (a), CTAB–PANI/GCE (b), and CTAB–PANI/AC/GCE (c) in
phosphate buffer (pH 7.0) containing 1 mM DA (a) or 0.5 mM UA (b). Scan rate = 50 mV s–1.
(d) CVs obtained at bare GCE (b), CTAB–PANI/GCE (c) and CTAB–PANI/AC/GCE (d) in
phosphate buffer (pH 7.0) containing mixture of 2 mM DA+2 mM UA. CV of CTAB–
PANI/AC/GCE (curve a) in the absence of DA and UA
3.2 Electrocatalysis of DA and UA
Fig.5 shows the cyclic voltammograms (CVs) obtained at unmodified GCE (a), CTAB–
PANI/GCE (b), and CTAB–PANI/AC (c) towards 1 mM DA and 0.5 mM UA. The scan rate is
50 mV s−1. Electrochemical parameters for the electrocatalysis of DA and UA such as, anodic
(Epa) and cathodic peak potentials (Epc), anodic (ipa) and cathodic peak currents (ipc), and peak
potential separation (ꢁEp) at these modified electrodes are given in Table S1. The unmodified
GCE displays poor electrocatalytic ability to oxidize DA and UA. Although, CTAB–PANI/GCE
displays good electrocatalytic ability, the oxidation potentials of DA and UA are closely
associated and the oxidation peaks are weak. Comparatively, the CTAB–PANI/AC has shown
excellent electrocatalytic ability to DA and UA which is evident from the observation of two
sharp and highly enhanced redox peaks. Compared with control electrodes, the CTAB–PANI/AC
composite has shown significantly improved electrocatalytic performance due to its large
electrochemically accessible surface area. Fig. 5(d) displays the CVs of bare GCE (b), CTAB–
PANI/GCE (c) and CTAB–PANI/AC/GCE (d) films modified GCEs in phosphate buffer (pH
7.0) containing mixture of 2 mM DA and 2 mM UA. As shown in figure, bare GCE has shown
unresolved and weak oxidation peak at 0.30. Although, the CTAB–PANI have showed two
oxidation peaks at 0.315 V and 0.40 V, the peakꢀtoꢀpeak separation is not sufficient.
Interestingly, CTAB–PANI/AC modified electrode is featured with two strong and wellꢀdefined
anodic peaks at 0.218 V and 0.335 V, respectively. These peaks are separated by wide peakꢀtoꢀ
peak separation gap of about 117 mV, which is fairly enough to perform simultaneous
determinations. Hence, CTAB–PANI/AC composite is a qualified hybrid material to perform
simultaneous detections of DA and UA.
In order to investigate stability of the films, 100 consecutive cyclic voltammograms of
bare GCE, CTAB–PANI/GCE, and CTAB–PANI/AC/GCE were studied in phosphate buffer
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containing mixture of DA and UA. After 100 consecutive cycles (100th cycle), the bare GCE,
CTAB–PANI/GCE, and CTAB–PANI/AC/GCE have been retained 86.54, 90.43 and 95.90% of
their initial catalytic response currents (1st cycle). For UA detection, 85.12, 91.80 and 94.65% of
the initial response currents were retained at bare GCE, CTAB–PANI/GCE, and CTAB–
PANI/AC/GCE, respectively. Apparently, the CTAB–PANI/AC/GCE is retained large percent of
catalytic response currents over control electrodes which indicates its good stability during
electrocatalysis. Next, the influences of different scan rates (ν) to the electrocatalysis reactions of
DA (Fig. S1a) and UA (Fig. S1b) are investigated. The oxidation peak currents of DA and UA
are linearly increases as the scan rate increases from 20 to 200 mV s–1 which revealing surface
confined oxidation process (insets to Fig. S1).
Fig. 6 (a) pH dependence of oxidation peak potentials of DA (2 mM) and UA (2 mM). (b) pH
dependence of oxidation peak currents of DA (2 mM) and UA (2 mM).
The effects of electrolyte solution pH on the oxidation peak currents and peak potentials
of DA and UA are examined. As shown in fig. 6a, the anodic peak potentials of DA and UA are
negatively shifted with the increase in the pH from 1 to 11. The slopes of the plot between pH
values and peak potentials exhibit good linearity (Fig. 6b). The slopes are – 55.3 and –56.4 (UA)
pH/mV for DA and UA, respectively which indicating that equal numbers of protons and
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electrons are involved in the oxidation process. The oxidation peak currents of DA and UA are
predominant at low pH, while it decreases steadily when the pH changes from 1 to 11. In order to
develop a sensor for biological sample, we have chosen pH 7 as the optimal working pH for our
studies.
Fig. 7a presents the voltammograms obtained at CTAB–PANI/AC/GCE in presence of
different concentrations of DA (Fig. 7a) and UA (Fig. 7b). As shown in figure, the modified
electrode exhibited sharp oxidation peaks for each concentration of DA and UA. The oxidation
peak currents are linearly increased as the concentrations of DA and UA increases. For DA, the
linear range is 50–500 µM and sensitivity is 0.0517 (± 0.007) µA µM–1 (inset to Fig. 7a). For
UA, the linear range is 50–500 µM and sensitivity is 0.0169 (0.009) µA µM–1 (inset to Fig. 7b).
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Fig. 7a CVs obtained at CTAB–PANI/AC/GCE in phosphate buffer (pH 7.0) containing DA
(a=50, b=100, c=150, d= 200, e=250, f=300 and g=350 µM). b) CVs obtained at CTAB–
PANI/AC/GCE in phosphate buffer containing UA (a= 50, b=100, c=150, d= 200, e=250, f=300
g=350, h=400, i=450 and j=500 µM)
3.3 Simultaneous determination of DA and UA
The main objective of the present work is to develop an electrode having higher antiꢀ
interference ability to AA. Therefore, the catalytic response of CTAB–PANI/AC/GCE towards
AA is investigated through cyclic voltammetry (CV) (Fig. S2a) and differential pulse
voltammetry (DPV) (Fig. S2b). The modified electrode doesn’t produce any measurable signal
for AA upto 2 mM in both CV and DPV analyses. Notably, the CTAB–PANI/AC composite
prepared without PDS has shown selectivity upto 1 mM AA (figure not shown), while the
composite prepared using PDS has shown improved selectivity upto 2 mM AA. The plausible
reasons are, (1) AC surface is negatively charged which is unfavorable for negatively charged
AA molecules and (2) the presence of PDS makes the electrode surface negatively charged
which hinders the negatively charged AA; as a result the electrode achieved improved
selectivity. Next, the electrolyte solution containing different concentrations of DA and AA in
presence of fixed concentration of AA is investigated. Fig. 8a presents the CV curves obtained at
CTAB–PANI/AC/GCE in phosphate buffer containing 2 mM AA and different concentrations of
DA and UA. As presented in figure, the Ipa corresponding to DA and UA are linearly increased
as the concentrations of DA and UA increased. The plots between peak currents and respective
concentrations of DA (Fig. 8b) and UA (Fig. 8c) have shown good linearity with slopes of 2.233
and 2.883 µA/mM, respectively. Remarkably, the coexistence of AA in the mixture didn’t
impose any interference to the detections of DA and UA. Therefore, the CTAB–PANI/AC
composite has high level of significance in the selective determinations of DA and UA. In
presence of 2 mM AA coꢀexisted solution, the modified electrode has showed linear range of 50–
500 µM for DA and UA.
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Fig. 8 (a) CVs obtained at CTAB–PANI/AC/GCE in phosphate buffer (pH 7.0) containing
different concentrations of DA and UA mixture (a=50, b=100, c=150, d=200, e= 250, f=300,
g=350, h=400, i=450, j=500 µM). b) Ipa/µA vs. [DA]/µM. c) Ipa/µA vs. [UA]/µM. d) DPVs of
CTAB–PANI/AC/GCE in phosphate solutions (pH 7.0) containing 1.0 mM AA and mixtures of
DA (0.3, 0.5, 1.5, 2.0, 4.0, 6.0, 12.0, 15.0, 17.0, 20.0 µM) and UA (1, 3, 5, 7, 9, 11, 13, 15, 17,
20 µM). e) Ipa/µA vs. [DA]/µM. f) Ipa/µA vs. [UA]/µM. The optimized parameters for DPV
measurements are amplitude = 0.05 V, sampling width = 0.0167 s and pulse period = 0.5 s
In order to improve sensitivity, DPV experiments were performed. Fig. 8d displays the
DPV curves obtained at CTAB–PANI/AC/GCE in phosphate buffer containing 2 mM AA and
mixture of DA and UA. The Ipa of the DA and UA are linearly increases as the concentrations of
DA and UA increases. The plots between concentrations of DA (Fig. 8e) and UA (Fig. 8f) with
respective peak currents showed good linearity. The coexisted AA doesn’t produce any
interference which is consistent with the CV results. For DA and UA, the regression equations
were obtained as, [Ip]/µA = 1.213 [DA] (µA/µM) + 0.304; R²= 0.997 and [Ip]/µA = 0.471 [DA]
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(µA/µM) + 1.159; R²= 0.990, respectively. For the DA determination, the linear range is 0.3–20
µM, the sensitivity is 17.08 (± 0.09) µAµM–1 cm–2 and the detection limit (LOD) is 0.06 (±
0.006) µM. For the UA determination, the linear range is 1–20 µM, the sensitivity is 6.63 (±
0.05) µAµM–1 cm–2 and the LOD is 0.20 (± 0.008) µM. The LOD was calculated using the
formula, LOD= 3 sb/S (where, sb= standard deviation of blank signal and S= sensitivity).34 The
linear range and LOD obtained for DA and UA detections at our modified electrode are
comparable with previously reported modified electrodes (Table 1).
Fig. 9 (a) DPVs obtained at CTAB–PANI/AC/GCE in phosphate solutions (pH 7.0) towards
different concentrations of DA: 1, 3, 10, 15, 20, 25, 30 and 35 µM containing fixed amount of (1
mM) of AA and UA. b) DPVs obtained at CTAB–PANI/AC/GCE in phosphate solutions (pH
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7.0) towards different concentrations of UA: 3, 7, 10, 20, 25, 30, 35 and 40 µM containing fixed
concentrations (1 mM) of AA and DA
Table 1 Comparison of the analytical performance of CTAB–PANI/AC composite with
previously reported modifiers for the determinations of DA and UA
Electrodes materials
Dopamine
Uric acid
Ref
Linear range/µM aLOD/µM Linear range/µM LOD/µM
35
23
10
36
37
16
14
38
17
poly(l–leucine)/DNA
MoS2/bRGO
0.1–100
5–545
0.04
0.05
24
0.5–100
25–2745
20–336
2–200
0.2
0.46
21
PtAu hybrid film
24–384
0.5–160
1–24
Pd NPs/carbon nanofibers
Chitosan‐graphene
Nꢀdoped graphene
0.2
0.7
1
2–45
2
0.5–170
10–170
0.25–20
0.15–15
0.3–20
0.25
0.25
0.08
0.033
0.06
0.1–20
10–130
0.3–13
0.3–150
1–20
0.045
0.45
0.12
0.12
0.20
Pt NPs/RGO
Pt NPs/polydopamine/cCNTs
pretreated pencil graphite
CTAB–PANI/PANI
This work
aLOD= Limit of detection; bRGO= Reduced graphene oxide; cCNTs=Carbon nanotubes
Next, selectivity of the electrode to detect DA in presence of UA and vice versa was
investigated. First, DPVs were performed using the modified electrode towards different
concentrations of DA containing fixed concentration of UA (1 mM) (Fig. 9a). As shown in
figure, the electrode shows linear enhancement in peak currents for each addition of DA. The
presence of UA in the supporting electrolyte wasn’t produced any interference to DA oxidation.
Similarly, the electrode selectively detects UA in presence of 1 mM DA and the presence of DA
has no influence towards voltammetric peak of UA (Fig. 9b).
3.4 Stability, repeatability and reproducibility
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In order to determine storage stability of the modified electrode, its electrocatalytic
response towards 1 µM DA and 1 µM UA were monitored every day. The modified electrode
was stored in phosphate buffer (pH 7) at 4°C when not in use. During 15 days of its storage
period, 92.16% and 91.05% of the initial oxidation peak currents were retained for DA and UA,
respectively. Next, repeatability of the electrode was evaluated by performing five repeatitive
measurements using independently prepared modified electrodes in phosphate buffer containing
mixture of DA and UA. The electrode has shown satisfactory repeatability with RSD of 4.12%
and 3.87% for DA and UA detections, respectively. Similarly, reproducibility of the electrode
was evaluated for five independent measurments performed using five different modified
electrodes in phosphate buffer containing mixture of DA and UA. The electrode delivers good
reproducibility for the determinations of DA and UA with RSD of 3.68 and 3.80%, respectively.
Table 2 Determination of DA and UA in real samples using CTAB–PANI/AC/GCE
Real Samples
DA
UA
Added/µM Found/µM Recovery/% *RSD/% Added/µM Found/µM Recovery/% *RSD/%
Human serum
Urine sample
Rat brain
10
20
10
20
10
20
10
20
10.45
19.34
9.86
104.5
96.7
98.6
96.2
96.1
95.9
97.4
96.85
3.51
3.95
3.82
3.40
4.22
4.58
3.61
4.08
5
10
5
4.78
9.62
4.82
9.81
–
95.6
96.2
96.4
98.1
–
2.54
3.95
4.20
3.18
–
19.24
9.61
10
–
19.18
9.74
–
–
–
–
Dopamine
Injection
–
–
–
–
19.37
–
–
–
–
* Related standard deviation (RSD) of 3 independent experiments
3.5 Real sample analysis
The realꢀtime application of the modified electrode was demonstrated in rat brain,
dopamine injection, human serum and urine samples. The sampling procedure is given in section
2. 3. Known concentrations of DA and UA (mixtures) were spiked into these solutions and DPV
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experiments were carried out. The dopamine injection sample was directly used without any
dilution. For each real sample, the CTAB–PANI/AC/GCE presents quick and sensitive DPV
signals which were consistent with the results obtained in lab samples analyses. The added,
found and recovery values are calculated and given as Table 2. It can be seen from the table that
the modified electrode able to detect DA and UA with satisfactory range of recoveries (95.6–
104.5%). Thus, the CTAB–PANI/AC composite has good practicality and it can be used for the
realꢀtime determinations of DA and UA.
4. Conclusions
In summary, a sensitive and highly selective electrochemical DA and UA detection
platform was developed using CTAB–PANI/AC composite. The successful formation of the
composite was confirmed by SEM, EIS and electrochemical methods. The CTAB–PANI/AC
composite film modified electrode exhibited excellent electrocatalytic ability to determine DA
and UA. The electrode is highly selective to detect DA and UA even in presence of high
concentrations of AA (upto 2 mM). The DPV based sensing platform was developed which
shown excellent analytical parameters for both DA and UA with detection limits of 0.06 (±
0.006) µM and 0.20 (± 0.008) µM. Also, the modified electrode has satisfactory stability,
repeatability and reproducibility. The composite has promising practical applicability in rat
brain, dopamine injection, and human serum and urine samples.
Acknowledgement
The authors extend their appreciation to the International Scientific Partnership Program
ISPP at King Saud University for funding this research work through ISPP#6. This work was
supported by the Ministry of Science and Technology (MOST), Taiwan (ROC).
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Table of content
Simultaneous determination of dopamine and uric acid have been described using CTAB–PANI/
activated charcoal composite with detection limits of 0.06 µM and 0.20 µM, respectively. The
composite does not produced any signal for ascorbic acid. Practicality is demonstrated in rat
brain, dopamine injection, and human serum and urine samples