A new electrochemical sensor based on a nitrogen-doped graphene/CuCo2O4 nanocomposite for simultaneous determination of dopamine, melatonin and tryptophan

Article abstract of DOI:10.1039/c5ra12020a

A new nanocomposite based on nitrogen-doped graphene nanosheets/CuCo₂O₄ nanoparticles was prepared and used as an electrode material for simultaneous determination of dopamine, melatonin and tryptophan. For this purpose, CuCo₂O₄ nanoparticles were supported on porous nitrogen-doped graphene nanosheets by a simple method. The nanocomposite was characterized by transmission electron microscopy and X-ray diffraction spectroscopy. Incorporation of the prepared nanocomposite in a carbon paste electrode (CPE) increased the oxidation peak currents and reduced the overpotential of dopamine, melatonin and tryptophan. The new developed sensor was then employed for the simultaneous determination of target analytes with linear ranges of 0.010-3.0 μM, using differential pulse voltammetry. Detection limits of 0.0033, 0.0049 and 0.0041 μM were achieved for dopamine, melatonin and tryptophan, respectively. The selectivity of the method was studied and the results showed that the fabricated sensor is free from interference of organic compounds especially uric acid and ascorbic acid. Finally, the proposed electrochemical sensor was employed to determine analytes in urine, serum and pharmaceutical samples.

Full text of DOI:10.1039/c5ra12020a

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A new electrochemical sensor based on a nitrogen-
doped graphene/CuCo2O4 nanocomposite for
simultaneous determination of dopamine,
melatonin and tryptophan
Cite this: RSC Adv., 2015, 5, 65560
a
F. Tadayon and Z. Sepehrib
*
A new nanocomposite based on nitrogen-doped graphene nanosheets/CuCo2O4 nanoparticles was
prepared and used as an electrode material for simultaneous determination of dopamine, melatonin and
tryptophan. For this purpose, CuCo2O4 nanoparticles were supported on porous nitrogen-doped
graphene nanosheets by a simple method. The nanocomposite was characterized by transmission
electron microscopy and X-ray diffraction spectroscopy. Incorporation of the prepared nanocomposite
in a carbon paste electrode (CPE) increased the oxidation peak currents and reduced the overpotential
of dopamine, melatonin and tryptophan. The new developed sensor was then employed for the
simultaneous determination of target analytes with linear ranges of 0.010–3.0 mM, using differential
pulse voltammetry. Detection limits of 0.0033, 0.0049 and 0.0041 mM were achieved for dopamine,
melatonin and tryptophan, respectively. The selectivity of the method was studied and the results
showed that the fabricated sensor is free from interference of organic compounds especially uric acid
and ascorbic acid. Finally, the proposed electrochemical sensor was employed to determine analytes in
urine, serum and pharmaceutical samples.
Received 22nd June 2015
Accepted 27th July 2015
DOI: 10.1039/c5ra12020a
www.rsc.org/advances
hydroxyl (–OH) and carboxylate (–COOH) groups in the struc-
ture. These active functional groups enable the G structure to
Introduction
interact with metal NPs. These unique properties make G very
useful for supporting metal NPs, and the obtained metal-NPs/G
nanocomposites exhibit the synergistic effects and good sensi-
tivity in their electrochemical detection behavior.11,12 The cata-
lytic activity of the metal-NPs upon the electron transfer process
paves the way for the production of metal-NPs/G composite
based electrochemical sensors.12,13 However, G sheets tend to
irreversible agglomeration and re-stacking because of their
strong van der Waals interactions and p–p stacking,14 which
causes some loss of performance for metal-NPs/G nano-
composites in the detection process. Furthermore, low reduc-
tion degree (especially for chemically reduced GO) and no-
doping properties will also inuence their efficiency.15,16 Thus,
the prevention of aggregation and improvement of reduction
degree and doping properties are greatly important for reduced
graphene oxide (R-GO) in the preparation processes of metal-
NPs/G nanocomposites. Nitrogen-doped graphene (N-G) can
further improve the reactivity and electrocatalytic performance
of G by forming of a delocalized conjugated system with the sp2-
hybridized carbon frameworks ascribed to the lone electron
pairs of nitrogen atoms. Moreover, N-G provides abundant
binding sites for non-covalent functionalization as well as the
enhanced biocompatibility and sensitivity in biosensing appli-
cations.14–17 Also, the N-doping and high reduction degree
Over the past decades, to meet the growing demands for
ultrasensitive detection, researchers have developed many
techniques to enhance the response of electrochemical sensors
by modifying them with different functional materials. With the
continuous development of nanotechnology, great attention
has been paid to different nanomaterials, such as carbon-based
nanomaterials, metal nanoparticles (NPs), quantum dots,
magnetic NPs and polymeric NPs.1–4 Because of their biological
compatibility, high surface area, chemical stability, excellent
catalytic activity and conductivity, the introduction of nano-
materials has really improved the analytical performance of the
sensing strategies to obtain amplied detection signals and a
stabilized sensing interface with good selectivity.1–5
As a two-dimensional single-atom-thick carbon network,
graphene (G) has been a research focus due to its unique
structure and physiochemical properties.6 G has exhibited
specic characteristics including extremely electric conduc-
tivity, large specic surface area, unusual electronic structure,
and upstanding thermal conductivity.6–10 The production of G
by the reduction of G oxide (GO) in chemical way produce
aDepartment of Chemistry, Islamic Azad University, North Tehran Branch, Tehran,
Iran. E-mail: f_tadayon@iau-tnb.ac.ir
bDepartment of Internal Medicine, Zabol University of Medical Sciences, Zabol, Iran
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properties of N-G as supporter can enhance their own electrical oil and graphite powder were obtained from Merck Company
conductivities and improve their own crystal structure, the and used as received. Dp, Me and Tp were prepared from Sigma-
obtained metal NPs/N-G composites should possess excellent Aldrich. Deionized distilled water (DDW) was used to prepare all
properties for sensing.14–17
the solutions. The commercial pharmaceuticals available from
Among the G based nanocomposite, there are seldom a local pharmacy were subjected to the analysis. Britton–Rob-
reports about G-spinel cobaltites composite. Complex oxides inson (B–R) solutions were prepared in DDW and were tested as
(containing two or more types of cations) with spinel structure the supporting electrolytes.
are of intense interests in material research because of their
Transmission electron microscopic (TEM) images were
remarkable optical, electrical, magnetic, catalytic properties taken using Zeiss EM902A (Germany). X-ray powder diffraction
and widespread applications in science and engineering. (XRD, 38066 Riva, d/G.Via M. Misone, 11/D (TN) Italy) was
Among these, spinel cobaltites (MCo2O4, M ¼ Cu, Mn, Ni, Zn, employed to analyze the chemical components of the compos-
Mg, etc.) have recently drawn considerable attention by virtue of ites. Electrochemical experiments were carried out at room
their superior physicochemical properties and tremendous temperature using a Behpajoh potentiostat/galvanostat system
potential for many technological applications, ranging from (model BHP-2065). The electrochemical cell was assembled
catalysts and sensors to electrode materials and electrochromic with a conventional three electrode system: a saturated calomel
devices.18–20 The combination of N-G with CuCo2O4 NPs can electrode (SCE) as a reference electrode (Azar electrode) and a
result in the synergetic effects of two components and exhibit platinum disk as an auxiliary electrode. Different working
the enhanced performance. The N-G/CuCo2O4 with rough electrodes including CPE and modied CPEs were used. The pH
interface can provide sufficient spaces for the some redox measurements were carried out using a Metrohm pH meter
reactions with fast electron transfer rate.
(model 713) with a combined pH glass electrode.
Dopamine (Dp), melatonin (Me) and L-tryptophan (Tp) are
important biomolecules that coexist in the extracellular uid of
the central nervous system and serum. These compounds play
implicit roles in neurochemistry and biomedicine. Dp is one of
Preparation of nanocomposite
Preparation of GO. Natural graphite powder was used to
the most prominent catecholamines, functioning as a neuro- synthesize GO according to a modied Hummers method.27
transmitter in the central nervous system. Me, an indoleamine Briey, a mixture of 1 g of graphite and 23 mL of 98% H2SO4 was
synthesized from an essential amino acid (Tp), is a hormone stirred at room temperature over 24 h, and then 100 mg of
synthesized by the pineal parenchymal cells from serotonin by NaNO3 was added to the mixture and stirred for another 30 min.
N-acetylation and O-methylation and secreted by them into the Subsequently, the mixture was stirred below 5 ꢀC in an ice bath,
blood and the cerebrospinal uid. Dp deciency and variations followed by the slow addition of 3 g of KMnO4 to the graphite
in Me levels have been linked to Parkinson's disease. Also, powder solution. Aer vigorous stirring at 35–40 ꢀC for another
abnormal metabolism of neurotransmitters, particularly DA 30 min, 46 mL of DDW was added to the above mixture. Finally,
and ML, is frequently observed in individuals with phenylke- 140 mL of deionized water and 10 mL of 30% H2O2 were
tonuria and is believed to be involved.21–23
introduced into the mixture to complete the oxidation reaction.
Tp is an essential amino acid for humans and a precursor for The GO solution was washed and ltered with HCl. Before
serotonin, Me and niacin. A signicant correlation has been complete drying, GO was dispersed in 800 mL of deionized
reported between plasma Tp concentrations and depressive water. The as-synthesized GO was ultrasonically dispersed in
illness.24,25 Recently, effects of Tp and tyrosine supplementation anhydrous ethanol for further use.
were studied to optimize treatment in phenylketonuria with Dp
Preparation of CuCo2O4 NPs supported on N-RGO. The
and Me as biomarkers.26 Therefore, development of a sensitive N-RGO/CuCo2O4 nanocomposite was prepared by a simple
and selective method for the determination of Dp, Me and Tp is solvothermal method. In a typical synthesis, 1.5 mL of a 0.2 M
desirable.
Co(OAc)2 aqueous solution and 0.75 mL of a 0.2 M Cu(OAc)2
To the best of our knowledge, no study has been published so aqueous solution were introduced in to 20 mL of a 0.5 mg mLꢁ1
far reporting the simultaneous determination of Dp, Me and Tp GO ethanolic suspension, followed by the addition of 1 mL of
by using any kind of modied electrodes. These species always NH3$H2O at room temperature. The reaction was kept at 80 ꢀC
coexist in biological uids, and it is very important to develop in a water bath with stirring for 24 h. Aer that, the reaction
sensitive and selective sensors for their simultaneous determi- mixture was transferred to a 50 mL teon-lined autoclave and
ꢀ
nation of in analytical applications and diagnostic researches. In placed in an oven at 160 C for 3 h. The resulting product was
this work, a simple electrochemical method based on the collected by centrifugation and washed with ethanol and water.
differential pulse voltammetry (DPV) for the direct determina- For control experimentation, N-RGO was made through the
tion of Dp, Me and Tp at a N-G/CuCo2O4/CPE was proposed.
same procedure as N-RGO/CuCo2O4 except for adding Co and
Cu salts. Free CuCo2O4 NPs were made through the same
procedure except for adding GO.28
Preparation of the electrodes. The conventional CPE was
prepared by hand-mixing of graphite powder and paraffin oil
Experimental
Reagents and apparatus
All chemicals and reagents used in this work were of analytical (with a 30/70 paraffin oil/graphite ratio (w/w)) in a mortar and
grade and used as received without further purication. Paraffin pestle and a portion of the composite mixture was packed rmly
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with a piston-driven CPE holder. The G/CPE was prepared by
mixing the components of unmodied mixture with G (12%,
w/w). The CuCo2O4/CPE and N-RGO/CuCo2O4/CPE were
prepared by mixing the unmodied mixture with 12% w/w
CuCo2O4 and N-RGO/CuCo2O4, respectively.
Results and discussion
Characterization of materials
The size, morphology and structure of the resulting materials
were investigated by TEM. The morphology of GO, consisting of
thin stacked akes and having a well dened few-layer structure
at the edge, can be clearly seen in Fig. 1a. Typical TEM micro-
graph of CuCo2O4 (Fig. 1b) shows the formation of uniform and
monodispersed CuCo2O4 NPs with mean diameter and length
about 15–25 nm. As shown in Fig. 1c, CuCo2O4 NPs are grown
on a 3D ame like N-RGO and G sheets are completely covered
with NPs.
Fig. 2 Nyquist plots for CPE (a) Cu2Co2O4/CPE (b) G/CPE (c) N-RGO/
CuCo2O4 (d) in the presence of 1.0 mM [Fe(CN)6]4ꢁ/3ꢁ in B–R buffer
solution (pH 7.0) with 0.1 M KCl.
The phase structure of synthesized samples was determined
by XRD. The XRD patterns of N-RGO, CuCo2O4 NPs and N-RGO/
CuCo2O4 are shown in Fig. 1d. The synthesized GO displayed a
typical characteristic (002) peak at 2q ¼ 10ꢀ.The crystal structure
of CuCo2O4 is in a cubic structure and the tted lattice
parameter of 8.131(5) are in good agreement with the standard
implying that CuCo2O4 and G were excellent electric conducting
materials and accelerated the electron transfer.
˚
JCPDS card no. 78-2177 (a ¼ 8.133 A). For N-RGO/CuCo2O4 the
Electrochemical behavior of analytes
diffraction peak arising from GO nearly disappeared at 2q ¼ 10ꢀ,
suggestive of less agglomeration of the G sheets in the nano-
composite. All of the other diffraction peaks can be indexed to
the structure of CuCo2O4 in nanocomposite.
CVs of Dp, Me and Tp at CPE, G/CPE, CuCo2O4/CPE and N-RGO/
CuCo2O4/CPE in buffer solutions (pH ¼ 3.0) are shown in Fig. 3.
At CPE, the peaks of Dp, Me and Tp are rather broad and weak,
indicating a slow process of electron transfer (curve b in
Fig. 3A–C). For G/CPE and CuCo2O4/CPE, peaks with dened
shape and with the large peak currents were observed (curve c
Electrochemical impedance spectroscopy (EIS)
Fig. 2 exhibits the Nyquist plots of the CPE, G/CPE, Cu2Co2O4/ and d). The large oxidation peak current and shi in peak
CPE and N-RGO/CuCo2O4/CPE. The CPE (curve a) revealed a potential to less positive potentials for these electrodes as
very large semicircle domain, implying a very high electron compared to the CPE indicates the electrocatalytic effect of
transfer resistance (Ret) of the redox probe. Aer CPE was modied surfaces. Moreover, at N-RGO/CuCo2O4/CPE, oxida-
modied with Cu2Co2O4 (curve b) or G (curve c), the Ret was a tion and reduction peaks of Dp are located at 457 mV and
small semicircle domain, showing that the G/CPE and 151 mV showing a smaller peak-to-peak separation than that at
Cu2Co2O4/CPE promoted conductivity. When the electrode was CPE. The oxidation peak current is 5.5 times higher than that
conjugated with N-RGO/CuCo2O4/CPE (curve d), the Ret again CPE suggesting the excellent electrocatalytic activity of N-RGO/
decreased. This was attributed to that the coating of N-RGO CuCo2O4 toward Dp. The similar electrocatalytic activity for Me
surface with CuCo2O4 nanoparticles increased the ability of is also found, which occurs with an increased peak current
the redox probe to electron transfer. It can be seen that the (5.1 times) and positive shi compared with bare CPE, indi-
electron transfer resistance decreased in the order of: bare cating good electron transfer promotion ability of N-RGO/
CPE < Cu2Co2O4/CPE < G/CPE < N-RGO/CuCo2O4/CPE, which CuCo2O4/CPE, also the oxidation peak current of Tp is 9.8 mA,
Fig. 1 Comparison of TEM images of (A) GO, (B) CuCo2O4, (C) CuCo2O4/N-RGO and (D) XRD patterns.
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Fig. 3 CVs for (A) 2 mM Dp, (B) 2 mM Me, and (C) 2 mM Tp in B–R buffer solution (pH 3.0) on the surface of various electrodes; curve (a) CPE (b)
G/CPE, (c) CuCo2O4/CPE and (d) CuCo2O4/N-RGO/CPE in presence of analytes. Insets show CuCo2O4/N-rGO/CPE in absence of analytes.
supporting electrolyte. Of these, B–R buffer solution gave the
best response.
The effect of pH on the response of Dp, Me and Tp evaluated
using DPV employing N-RGO/CuCo2O4/CPE. The effect of pH on
peak potential and peak current of Dp, Me and Tp was inves-
tigated over the pH range of 2–9 employing B–R buffer solu-
tions. As shown in Fig. 5b, for Dp, with increasing pH, the
current increases and reaches a maximum value at pH ¼ 5.0 and
decreases with increasing in pH values. The peak currents of Me
and Tp initially go up with increasing pH (¼3), and then
decreases with increasing pH (>3). In addition, Fig. 5c shows
that the peak potential of Dp, Me and Tp is also pH dependent
Fig. 4 DPVs of 2 mM Dp, 2 mM Me and 2 mM Tp in B–R buffer solution
and the potentials shied negatively when the pH of the solu-
tions increases due to the participation of protons in the elec-
trode reaction. The potentials of Dp, Me and Tp followed the
linear regression equations with pH: Epa (V) ¼ ꢁ0.0602 pH +
0.708 (R ¼ 0.990), Epa (V) ¼ ꢁ0.0264 pH + 0.748 (R ¼ 0.984) and
Epa (V) ¼ ꢁ0.0520 pH + 1.130 (R ¼ 0.992), respectively. For Dp
and Tp the slopes is closed to the theoretical value of 59 mV per
(pH 3.0) on the surface of various electrodes; curve (a) CuCo2O4/N-
rGO/CPE in absence of analytes, (b) CPE (c) G/CPE, (d) CuCo2O4/CPE
and (e) CuCo2O4/N-RGO/CPE in presence of analytes.
which is about 5.8 times higher than that at CPE. Insets of Fig. 3
show CVs for N-RGO/CuCo2O4/CPE in absence of analytes.
Fig. 4 shows the DPV responses at various electrodes in a
ternary mixture solution of Dp, Me and Tp with 2.0 mM in B–R
buffer solution (pH 3.0). At the CPE only a broad and overlapped
oxidation peak is obtained and the potentials of Me and Tp are
indistinguishable so that simultaneous determination of these
biological molecules is impossible (Fig. 4b). However, at the
N-RGO/CuCo2O4/CPE (Fig. 4e), three sharp and well dened
oxidation peaks with larger peak separation potential and
higher peak currents corresponding to the oxidation of Dp, Me
and Tp were appeared. The three oxidation peaks for Dp, Me
and Tp are well resolved at 440, 690 and 840 mV, respectively,
with the peak separation potential is 250 mV (Dp–Me) and
150 mV (Me–Tp). All the voltammetric responses can demon-
strate that the N-RGO/CuCo2O4/CPE possess excellent electro-
catalytic activities towards the oxidation of Dp, Me and Tp,
which can be attributed to its unique structural features and
excellent electrochemical properties.29
ꢀ
pH at 25 C expected from the Nernst equation, indicates that
the oxidation process is proton dependent and the electron
transfer is accompanied by the transfer of an equal number of
protons. Two electrons and two protons are involved for the
oxidation of Dp and Tp. In the case of Me, the slope of 0.0264 V
pH obtained on N-RGO/CuCo2O4/CPE was almost similar to
that shown in previous electrochemical investigations and it is
suggested that the number of electrons transferred in the
oxidation of Me is double that of protons.30,31 The reaction
mechanism for the oxidation of Dp, Me and Tp is as given in
Scheme 1.
Effect of scan rate on the electrochemical oxidation of Dp, Me
and Tp
To investigate the reaction kinetics, the effect of scan rate on the
peak currents and peak potentials of Dp, Me an Tp was studied
(Fig. 6). Insets of Fig. 6 show that the peak currents changed in a
linear relationship with the square root of scan rates in ranges
Effect of pH and supporting electrolyte
A series of supporting electrolytes were tested (B–R, phosphate of 10–400, 10–300, and 10–250 mV sꢁ1 for Dp, Me and Tp,
and acetate buffer solutions). Both the peak height and the peak respectively, and by R-squared values of above 0.99 for all ana-
shape were taken into consideration when choosing the lytes. These plots indicate that the electrochemical reactions for
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Fig. 5 (a) 3D plots of influence of buffer pH on peak currents and potentials of analyte species, (b) plots of Ip vs. pH and (c) plots of Ep vs. pH from
differential pulse voltammograms of 2.5 mM Dp, Me and Tp in different buffer solution.
Individual and simultaneous voltammetric determination of
Dp, Me and Tp
The above ndings enabled us to develop a simple and suitable
method for Dp, Me and Tp determination using N-RGO/
CuCo2O4/CPE. As Fig. 7A–C shown, when detecting the indi-
vidual, the concentration of one substance was changed with
the others remained constant. Keeping the concentration of Me
and Tp both at 2.0 mM, the peak current increased proportion-
ally with adding concentrations of Dp from 0.010 to 3.0 mM. A
linear regression equation between the oxidation peak current
with the concentration of Dp was obtained Ip ¼ 14.98C + 0.4382
(Ip in mA, C in mM) with R2 ¼ 0.9997. The detection limit for Dp
was calculated to be 0.0032 mM. Similarly, to keep the concen-
Scheme 1 The oxidation mechanism of Dp, Me and Tp.
tration of Dp and Tp both at 2.0 mM, the concentration of Me
was changed within the range of 0.010 to 3.0 mM. The linear
regression equation of Me was got: Ip ¼ 10.00C + 0.507 with R2 ¼
these molecules are diffusion controlled. Also for more inves-
tigations, log peak currents versus log scan rates were plotted for
analytes. For these plots when the slope is 0.5, the electro-
chemical reaction is a diffusion controlled process, and when
0.9993. The detection limit for Me was calculated to be 0.0049
mM. In the same way, the concentration of Dp and Me remained
were kept at 2.0 mM and the concentration range of Tp was from
0.01 to 3.0 mM, the linear regression equation of Tp was: Ip ¼
11.94C + 0.492 with R2 ¼ 0.9907 and the detection limit for Tp
equals to 1, the electrochemical reaction occurs via an
adsorption-controlled process.32 From insets of Fig. 6 the plots
was 0.0041 mM.
of log anodic peak currents versus log scan rates have slopes of
The DPVs for different concentrations of Dp, Me and Tp were
0.485, 0.565 and 0.514 for Dp, Me and TP, respectively. These
values show that the electrochemical reactions for these
biomolecules are governed by diffusion control and the surface
of N-RGO/CuCo2O4/CPE was not fouled by them.
illustrated in Fig. 8. The resulting calibration plots are linear
over the range from 0.01 to 3.0 mM for Dp, Me and Tp. The
calibration curves and correlation coefficients are
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Fig. 6 CVs for CuCo2O4/N-rGO/CPEin B–R Buffer (pH 3.0) containing 2 mM of (A) Dp with scan rates ranging from a to h as 10–400 mV sꢁ1
,
respectively (B) Me with scan rates ranging from a to g 10–300 mV sꢁ1 and (C) Tp with scan rates ranging from a to f as 10–250 mV sꢁ1. Insets
show the linear relationship of the peak current versus square root of the scan rate (y1/2) and log peak currents versus log scan rate.
Dp:Ip (mA) ¼ 14.71CDp (mM) + 0.521 R2 ¼ 0.999
The limits of detection were 0.0033 mM for Dp, 0.0049 mM for
Me and 0.0041 mM for Tp based on 3sb/m, where sb is the
standard deviation of the mean value for 5 independent vol-
tammetric response of the blank solution.
Me:Ip (mA) ¼ 9.95CMe (mM) + 0.509 R2 ¼ 0.997
Tp:Ip (mA) ¼ 11.97CTp (mM) + 0.516 R2 ¼ 0.998
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Fig. 8 DPVs for different concentrations of Dp, Me and Tp in pH 3 B–R
buffer solution. Concentrations of Dp, Me and Tp from (a) to (n): 0.01,
0.05, 0.1, 0.2, 0.3, 0.4, 0.7, 1, 1.5, 2, 2.3, 2.5, 2.7 and 3 mM.
compounds were tested by analyzing a standard solution of
1.5 mM Dp, 1.0 mM Me and 2.0 mM Tp. The tolerance limit is
dened as the maximum concentration of inuence substances
which causes a ꢂ5% relative error. It is found that species like
glucose, lactose and sucrose, uric acid, ascorbic acid (till 100
fold excess), Na+, K+, Mg2+, Al3+, CO32ꢁ, NO3ꢁ, ClO4ꢁ, SCNꢁ(till
1000 fold excess) did not interfere in the analysis of Dp, Me and
Tp. The interference caused by same concentration of tyrosine
was serious on tryptophan signal. Because tyrosine has similar
electroactive group as tryptophan, its oxidation potential is
close to tryptophan and thus affect the oxidation peak current of
tryptophan. Hence determination of Dp, Me and Tp was not
Table 1 Results for Dp, Me and Tp determination (mM) in various real
samples obtained by the proposed method under the optimum
conditions
Samples
Urine
Analyte Added (mM) Found (mM) Recovery (%)
Dp
Me
Tp
1.00
2.00
1.00
2.00
1.00
2.00
1.50
2.50
1.50
2.50
1.50
2.50
0.000
0.500
1.20
0.500
1.20
0.500
1.20
0.500
1.20
0.000
0.500
1.20
0.500
1.20
0.970
2.05
1.01
2.06
0.980
2.04
1.46
2.45
1.45
2.57
1.53
2.48
0.810
1.29
2.03
0.520
1.24
0.480
1.18
0.510
1.22
0.540
1.05
1.73
0.510
1.19
97.0
102
101
103
98.0
102
97.3
Human serum Dp
98.0
96.6
102
102
99.2
Me
Tp
Fig. 7 (A) DPVs for Dp at the CuCo2O4/N-rGO/CPE in the presence of
2.0 mM Me and Tp in B–R buffer solution (pH ¼ 3.0). Dp concentrations
(from a to n): 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.7, 1, 1.5, 2, 2.3, 2.5, 2.7 and
3 mM. (B) DPVs for Me at the CuCo2O4/N-rGO/CPE in the presence of
2 mM Dp and. Tp concentrations (from a to n): 0.01, 0.05, 0.1, 0.2, 0.3,
0.4, 0.7, 1, 1.5, 2, 2.3, 2.5, 2.7 and 3 mM (C) DPVs for Tp at the CuCo2O4/
N-rGO/CPE in the presence of 2.0 mM Dp and Me in B–R buffer
solution (pH ¼ 3.0). Tp concentrations (from a to n): 0.01, 0.05, 0.1, 0.2,
0.3, 0.4, 0.7, 1, 1.5, 2, 2.3, 2.5, 2.7 and 3 mM.
Dp injection
Dp
—
97.5
102
104
103
96.0
98.3
102
Me
Tp
Me tablet
Dp
Me
102
—
Interferences, stability and reproducibility studies
102
Several compounds and ions from common co-existing
substances were selected to evaluate the anti-interference
ability of N-RGO/CuCo2O4/CPE. The effect of foreign
98.1
102
99.1
Tp
65566 | RSC Adv., 2015, 5, 65560–65568
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Table 2 Comparison of some characteristics of the different modified electrodes for the determination of Dp, Me and Tp
Linear range (mM)
ML DA
0.02–5.8 0.02–5.8
Detection limit (mM)
Simultaneously with
other analytes
Electrode
Method
SWV
TP
ML
DA
TP
Ref.
6
G–Fe3O4/CPE
—
—
0.0084 0.0065
—
—
—
ZnO nanorods modied SWV
carbon paste electrode
Boron-doped diamond CV
electrode
N.R.
0.3–10.0 and
N.R.
10.3
0.056
Methionine and caffeine
33
10.0 to 100.0
345–688
—
—
—
—
—
34
MWNT/GCE
Boron-doped diamond SWV
electrode
CV
0.08–10
0.5–4
—
—
—
—
0.02
0.11
—
—
—
—
—
—
35
27
MWCNTs–CHNPs/CILE DPV
0.01–50
100–4600
—
—
—
—
—
1–400
3.0–34.0 and
84.0–464
10–800
0.4–140
0.05–4.25
0.9–1000
0.004
100
—
—
—
0.3
0.08
—
—
0.3
0.7
L-Dopa
Catechin
—
36
37
38
MnHCFPEDOT/GCE
TiO2–GR/4-ABAS/GCE
GNPs/PImox
CV
DPV
1–400
5.0–268.0
—
—
Ascorbic acid and uric acid 39
AgNPs/rGO
GS–PTCA
LSV
DPV
—
—
—
—
10–800
0.4–370
—
—
—
—
—
5.4
0.13
—
7.5
Ascorbic acid, uric acid
Ascorbic acid, uric acid
—
40
41
42
0.06
0.008
0.7
MWCNT/GO
NiCo2O4/Nano-ZSM-5/
GCE
Amperometry
DPV
0.6–900
0.5
Ascorbic acid and uric acid 43
MIP/G
N-RGO/CuCo2O4
SWV
DPV
0.05–100
0.01–3.0 0.01–3.0
—
—
0.01–3.0
0.006
—
—
—
—
44
0.0049 0.0033 0.0041
This work
considerably affected by common interfering species, which literatures. Some gures of merit related to previous reports and
shows that the method is more selective towards the drugs. The the present study are shown in Table 2.
stability of the electrode was also tested. The peak current only
decreased less than 5% aer the electrode was stored at room
temperature for 22 days. In addition, in order to evaluate the
Conclusions
reproducibility of the modied electrode, a series of ve N-RGO/
CuCo2O4/CPEs were prepared for the detection of 2.0 mM of
analytes. The RSD (n ¼ 5) of peak currents for Dp, Me and Tp
were calculated as 3.3%, 3.5% and 3.1%, respectively, which
suggests that the reproducibility of the proposed electrode was
good. The above results indicate that the as-obtained N-RGO/
CuCo2O4/CPE presents well anti-interference ability, stability
and reproducibility.
In this work, a novel sensor based on N-RGO/CuCo2O4 nano-
composite as modier in CPE, has fabricated that provides an
extremely sensitive and selective method for the simultaneous
determination of Dp, Me and Tp. Both CV and DPV studies
reveal that the potential separations between them were large
enough to allow their simultaneous detection. Moreover,
appreciable reproducibility and lower detection limits were
attained for the three biomolecules at N-RGO/CuCo2O4/CPE.
Hence, it was employed for the analysis of Dp, Me and Tp in
human urine, serum and pharmaceutical samples. Recovery
values ranging from 97–104% were obtained for real sample
analysis. The cost-effectiveness and easy preparative method are
added advantages of this sensor. All these factors amalgamate
to make this sensor a potential candidate for the detection of
these species in routine analysis.
Determination of Dp, Me and Tp in pharmaceutical, urine and
serum samples
To conrm the practical usefulness of this proposed method for
simultaneous determination of Dp, Me and Tp in pharmaceu-
tical samples and biological uids such as human urine, serum,
Dp injection and Me tablet were selected as real samples for
analysis using the standard addition technique. For sample
pretreatments, both urine and serum samples were centrifuged
at 5000 rpm for 3 min and the supernatants were collected,
nally the prepared samples were diluted 50 times with B–R
buffer solution (pH 3.0).
Acknowledgements
The authors gratefully acknowledge the support of this work by
the North Tehran Branch, Islamic Azad University.
The results are summarized clearly in Table 1, with the
recoveries of the spiked samples ranged from 96.0% to 104.0%.
It was evident that the N-RGO/CuCo2O4/CPE could be success-
fully applied for the simultaneous determination of Dp, Me and
Tp in real samples. Moreover, the proposed method has good
gures of merits in comparison with other reported methods in
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