Electrochemical behavior of morphine at ZnO/CNT nanocomposite room temperature ionic liquid modified carbon paste electrode and its determination in real samples

Article abstract of DOI:10.1016/j.molliq.2013.02.002

In this paper we report synthesis and application of ZnO/CNT nanocomposite and 1-methyl-3-butylimidazolium bromide as high sensitive sensors for voltammetric determination of morphine using carbon paste electrode. The ZnO/CNT nanocomposite was characterized with different methods such as TEM, SEM and XRD. The electrochemical oxidation of morphine on the new ZnO/CNTs ionic liquid carbon paste electrode (ZnO/CNTs/IL/CPE) was carefully studied. The oxidation peak potential of morphine on the ZnO/CNTs/IL/CPE appeared at 520 mV, which was about 75 mV decrease of the overpotential compared to that obtained on the traditional carbon paste electrode (CPE) and the oxidation peak current was increased for about 5.5 times. The electrochemical parameter of morphine on the ZnO/CNTs/IL/CPE was calculated with the charge transfer coefficient (α). Based on the relationship of the oxidation peak current and the concentration of morphine a sensitive analytical method was established with cyclic voltammetry. The linear range for morphine determination was in the range from 0.1 to 700 μmol L^{- 1} and the detection limit was calculated as 0.06 μmol L^{- 1} (3σ). Finally, the proposed method was also examined as a selective, simple and precise electrochemical sensor for the determination of morphine in real samples such as urine and ampoule.

Full text of DOI:10.1016/j.molliq.2013.02.002

Journal of Molecular Liquids 181 (2013) 8–13
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Electrochemical behavior of morphine at ZnO/CNT nanocomposite room temperature
ionic liquid modified carbon paste electrode and its determination in real samples
a
c
Elahe Afsharmanesh a, Hassan Karimi-Maleh b, , Ali Pahlavan , Javad Vahedi
⁎
a
Department of Physics, Science and Research Branch, Islamic Azad University, Mazandaran, Iran
Department of Chemistry, Science and Research Branch, Islamic Azad University, Mazandaran, Iran
b
c
Department of Physics, Sari Branch, Islamic Azad University, Sari, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper we report synthesis and application of ZnO/CNT nanocomposite and 1-methyl-3-butylimidazolium
bromide as high sensitive sensors for voltammetric determination of morphine using carbon paste electrode. The
ZnO/CNT nanocomposite was characterized with different methods such as TEM, SEM and XRD. The electro-
chemical oxidation of morphine on the new ZnO/CNTs ionic liquid carbon paste electrode (ZnO/CNTs/IL/CPE)
was carefully studied. The oxidation peak potential of morphine on the ZnO/CNTs/IL/CPE appeared at 520 mV,
which was about 75 mV decrease of the overpotential compared to that obtained on the traditional carbon
paste electrode (CPE) and the oxidation peak current was increased for about 5.5 times. The electrochemical
parameter of morphine on the ZnO/CNTs/IL/CPE was calculated with the charge transfer coefficient (α). Based
on the relationship of the oxidation peak current and the concentration of morphine a sensitive analytical
method was established with cyclic voltammetry. The linear range for morphine determination was in the
range from 0.1 to 700 μmol L−1 and the detection limit was calculated as 0.06 μmol L−1 (3σ). Finally, the pro-
posed method was also examined as a selective, simple and precise electrochemical sensor for the determination
of morphine in real samples such as urine and ampoule.
Received 25 January 2013
Accepted 2 February 2013
Available online 22 February 2013
Keywords:
Morphine
Ionic liquid
ZnO/CNT nanocomposite
Sensor
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
entirely composed of organic cations and various inorganic anions
that exist in the liquid state around room temperature. Because
Carbon paste based electrode (CPE) is a special kind of heteroge-
neous carbon electrode consisting of a mixture prepared from carbon
powder and a suitable water-immiscible or non-conducting binder
[1–3]. CPEs are widely applicable in both electrochemical studies
and electroanalysis thanks to their advantages such as very low back-
ground current, facility to prepare, large potential window, low cost,
simple surface renewal processes and easiness of miniaturization
[4,5].
As all we know, carbon nanotubes (CNTs) have received a tremen-
dous amount of attention due to their unique and highly desirable
electrical, thermal, and mechanical properties and have been widely
used in analytical chemistry. This tendency is well manifested by sev-
eral recently published excellent reviews [6–9]. The combination of
CNTs with other nanomaterials is expected to be useful for catalysis,
or making sensors. On the other hand, ZnO nanoparticles have advan-
tages such as narrow size distribution, efficient surface modification,
and desirable biocompatibility.
RTILs have many specific physical and chemical properties such as
wide electrochemical window, high ionic conductivity and good solu-
bility [10–12], they have been recognized as a useful non-aqueous
media for various electrochemical processes [13–16].
Morphine is the most abundant alkaloid found in opium, the dried
sap derived from shallowly slicing the unripe seedpods of the Papaver
somniferum poppy [17]. Morphine is used to treat moderate to severe
pain. Short-acting formulations are taken as needed for pain. To date,
many analytical methods have been developed to determine of
morphine concentrations, including high performance liquid chroma-
tography [18–20], gas chromatography–mass spectroscopy [21,22],
fluorimetry [23,24], chemiluminescence [25–27], surface plasma res-
onance (SPR) [28], and electrochemical methods [29–36].
We recently reported ionic liquid/carbon nanotube paste electrode
as a novel electrochemical sensor for rapid and sensitive voltammetric
determination of morphine [31,32]. In continuing efforts to improve
this kind of sensors in morphine analysis, in this study we describe
synthesis and application of novel ZnO/CNT nanocomposite as a novel
nanosensor and 1-methyl-3-butylimidazolium bromide as suitable
binder in carbon paste matrix for voltammetric determination of mor-
phine. Compared with previous reports for determination of morphine
using ionic liquid/carbon nanotube paste electrodes [31,32]; this
modified electrode has the best dynamic range, limit of detection and
Recently room temperature ionic liquids (RTILs) are beginning to
be used as a new kind of modifier to make an ionic liquid modified
carbon paste electrode. RTILs are generally regarded as a compound
⁎
Corresponding author. Tel.: +98 911 2540112 (mobile); fax: +98 151 2277733.
E-mail address: h.karimi.maleh@gmail.com (H. Karimi-Maleh).
0167-7322/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molliq.2013.02.002

E. Afsharmanesh et al. / Journal of Molecular Liquids 181 (2013) 8–13
9
sensitivity for morphine analysis. We also evaluate the analytical per-
formance of the modified electrode for voltammetric determination of
morphine in real samples such as patient urine and ampoule.
and mixed well for 40 min until a uniformly wetted paste was
obtained. The paste was then packed into a glass tube. Electrical con-
tact was made by pushing a copper wire down the glass tube into the
back of the mixture. When necessary, a new surface was obtained by
pushing an excess of the paste out of the tube and polishing it on a
weighing paper. ZnO/CNTs/ILCPE was prepared by mixing of 0.3 g of
1-methyl-3-butylimidazolium bromide, 0.7 g of the liquid paraffin,
0.20 g of ZnO/CNTs, and 0.80 g of graphite powder. Then the mixture
was mixed well for 40 min until a uniformly wetted paste was
obtained. A portion of the paste was filled firmly into one glass tube
as described above to prepare ZnO/CNTs/ILCPE.
2. Experimental
2.1. Chemicals
All chemicals used were of analytical reagent grade purchased
from Merck (Darmstadt, Germany) unless otherwise stated. Doubly
distilled water was used throughout.
A 1.0×10−2 mol L−1 morphine solution was prepared daily by dis-
solving 0.19 g morphine sulfate in water and the solution was diluted to
25 mL with water in a 25-mL volumetric flask. The solution was kept in
a refrigerator at 4 °C in dark. More dilute solutions were prepared daily
by serial dilution of the stock solution with water.
2.5. Preparation of real samples
Urine samples were stored in refrigerator immediately after col-
lection (from the Sari Health Centre). Ten milliliters of the sample
was centrifuged for 30 min at 2000 rpm. The supernatant was filtered
out using a 0.45 μm filter and then diluted 5-times with the PBS
(pH 8.0). The solution was transferred into the voltammetric cell to
be analyzed without any further pretreatment. The standard addition
method was used for the determination of morphine in real samples.
0.10 mL of the solution injection solution (APP Pharmaceuticals,
LLC Schaumburg 0.5 mg mL−1) plus 10 mL of 0.1 mol L−1 buffer
(pH 8.0) were used for the analysis.
Phosphate buffer (sodium dihydrogen phosphate and disodium
monohydrogen phosphate plus sodium hydroxide, 0.1 mol L−1) solu-
tions (PBS) with different pH values were used.
High viscosity paraffin (d=0.88 kg L−1) from Merck was used as
the pasting liquid for the preparation of the carbon paste electrodes.
2.2. Apparatus
Cyclic voltammetry, chronoamperometry, and square wave vol-
tammetry were performed in an analytical system, μ-Autolab with
(μ3AUT 71226) PGSTAT (Eco Chemie, the Netherlands). The system
was run on a PC using NOVA software. A conventional three-
electrode cell assembly consisting of a platinum wire as an auxiliary
electrode and an Ag/AgCl/KClsat electrode as a reference electrode
was used. The working electrode was either an unmodified carbon
paste electrode (CPE), ZnO/CNTs/CPE, IL/CPE or a ZnO/CNTs/ILCPE.
X-ray powder diffraction studies were carried out using a STOE diffrac-
tometer with Cu-Ka radiation (k=1.54 Å). Samples for transmission
electron microscopy (TEM) analysis were prepared by evaporating a
hexane solution of dispersed particles on amorphous carbon coated
copper grids.
3. Results and discussion
3.1. Nanostructures characterization
ZnO/CNT nanopowders were analyzed by XRD analyses. The XRD
pattern of ZnO/CNT nanopowders, in the 2θ range of 10–80°, is shown
in Fig. 1. It clearly proves the presence of ZnO nanoparticle, with a diffrac-
tion peak at about 26° from CNTs. An average diameter of as-synthesized
ZnO nanoparticle was calculated from the broadness peak of (2θ=36)
ꢀ
ꢁ
Kλ
by using Scherrer equation D ¼
, and it is about 22.0 nm.
The morphology of the as-groβwconsθnanostructures was characterized
by SEM and TEM techniques. Typical SEM micrograph of the ZnO/
CNTs is shown in Fig. 2a. Results show the presence of ZnO nanostruc-
ture grown on carbon nanotubes. The elemental composition of ZnO/
CNT nanocomposites was confirmed by the EDX measurement as
shown in Fig. 2b. The EDX measurement shows that the composition
of ZnO/CNTs alloy was 23 at.% of Zn, 62% C and 15.0 at.% O, indicating
a ratio of 1:2.7 to ZnO/CNTs. Fig. 3 presents a typical TEM image of
ZnO/CNT nanocomposite. Results show the core of particles supported
on carbon nanotubes. Since the corresponding XRD pattern presented
in Fig. 1 detected only CNTs and ZnO, it was believed that the core
and nanotubes of particles should be ZnO and carbon nanotubes,
respectively.
1-Methyl-3-butylimidazolium bromide (MBIDZBr) was synthe-
sized according to the previous paper [16].
2.3. Synthesis of ZnO/CNTs
The commercial multi-walled carbon nanotubes with tube diame-
ters of about 20–50 nm were used. The preparation of ZnO/CNT cata-
lysts includes three steps. First, the chemical pretreatment of carbon
nanotubes is required. A definite amount of carbon nanotubes was in-
troduced into 40 cm3 of nitric acid and sulfuric acid (3:1 in volume)
solution, then 10 cm3 of ethanol was dropped into the solution slowly,
and the solution was agitated in a shaker at 70 °C and 150 rpm for 24 h.
In the second step, certain amounts of purified CNTs (6 g) were dis-
persed into distilled water solution of NaOH (0.5 M; 100 ml) by
ultrasonication for 15 min. The third step is the supporting of zinc
oxide on carbon nanotubes by a direct deposition process. 7.4 g ZnO
(NO3)2·2H2O was dissolved in 100 cm3 distilled water. In the constant
magnetic stirring, the solution of ZnO (NO3)2·2H2O was added drop
wise to the solution of CNTs at 50 °C through a dropping funnel. The
rate of addition of the salt solution was kept approximately at 20 ml/h.
After completion of the precipitation procedure, the mixture was stirred
at room temperature for 12 h, washed and filtered continually in dis-
tilled water (pH 7.5), and dried at 120 °C. The solid samples were then
calcined at 200 °C for 1 h.
2.4. Preparation of the modified electrode
ZnO/CNTs/CPE was prepared by hand-mixing of 0.80 g of graphite
powder and 0.20 g ZnO/CNTs plus paraffin at a ratio of 70/30 (w/w)
Fig. 1. XRD patterns of as-synthesized ZnO/CNT nanocomposite.

10
E. Afsharmanesh et al. / Journal of Molecular Liquids 181 (2013) 8–13
Fig. 2. (a) SEM image of ZnO/CNTs. (b) EDX pattern of as-synthesized ZnO/CNT composites.
The direct electrochemistry of morphine on the modified electrode
3.2. Electrochemical investigation
was investigated by linear sweep voltammetry. Fig. 5 showed the
typical linear voltammogram responses of 500 μmol L−1 morphine
at pH 8.0 at the surface of different electrodes with a scan rate of
50 mV s−1. ZnO/CNTs/ILCPE exhibited significant oxidation peak cur-
rent around 520 mV with the peak current of 62.3 μA (Fig. 5, curve a).
In contrast, low redox activity peak was observed at ZnO/CNTs/CPE
(Fig. 5, curve c) and at unmodified CPE (Fig. 5 curve d) over the
same potential range. The morphine oxidation peaks potential at
ZnO/CNTs/CPE and at CPE were observed around 580 and 595 mV
vs. the Ag/AgCl/KClsat reference electrode with the oxidation peak
currents of 21.0 and 11.5 μA, respectively. In addition, at the surface
of bare IL/CPE, the oxidation peak appeared at 540 mV with a peak
current of 35.0 μA (Fig. 5, curve b), which indicated that the presence
of ILs in CPE could enhance the peak currents and decrease the oxida-
tion potential (decreasing the overpotential). A substantial negative
shift of the currents starting from oxidation potential for morphine
and dramatic increase of current of morphine indicated the catalytic
ability of ZnO/CNTs/ILCPE to morphine oxidation. The results indicat-
ed that the presence of ZnO/CNTs on ZnO/CNTs/ILCPE surface had
great improvement with the electrochemical response, which was
partly due to excellent characteristics of ZnO/CNT nanocomposite
such as good electrical conductivity, high chemical stability, and
high surface area. The suitable electronic properties of ZnO/CNTs
together with the ionic liquid gave the ability to promote charge trans-
fer reactions, good anti-fouling properties, especially when mixed with
a higher conductive compound such as ILs when used as an electrode.
Morphine can be oxidized at positive potential depending on
the electrode type and solution pH [37]. Scheme 1 shows the
electrooxidation reaction of morphine. According to Scheme 1, we
anticipated that the oxidation of morphine would be pH dependent.
In order to ascertain this, the voltammetric response of morphine at
a surface of ZnO/CNTs/ILCPE was obtained in solutions with varying
pH. Result shows that the peak potential of the redox couple was
pH dependent with a slope of −69.0 mV/pH unit at 25 °C which
was equal to the anticipated Nernstian value for a one-electron,
one-proton electrochemical reaction. It can be seen that the maxi-
mum value of the peak current appeared at pH 8.0 (Fig. 4), so this
value was selected throughout the experiments.
Fig. 5 (inset) shows the current density derived from the cyclic
voltammogram responses of 500 μmol L−1 morphine (pH 8.0) at
the surface of different electrodes with a scan rate of 50 mV s−1
.
The results show that the presence of ZnO/CNT nanocomposite and
IL together causes the increase in the active surface of the electrode.
Fig. 3. TEM image of ZnO nanoparticles deposited on sidewalls of MWCNTs.
Scheme 1. The mechanism for electrooxidation of morphine at a surface of electrode.

E. Afsharmanesh et al. / Journal of Molecular Liquids 181 (2013) 8–13
11
Fig. 4. Current-pH curve for electrooxidation of 150.0 μmol L−1 morphine at ZnO/CNTs/
ILCPE with a scan rate of 50 mV s−1. Inset: influence of pH on linear sweep voltammograms
of morphine at a surface of the modified electrode, (pH 5, 6, 7, 8, and 9, respectively).
Fig. 6. Plot of Ipa versus ν1/2 for the oxidation of morphine at ZnO/CNTs/ILCPE. Inset
shows linear sweep voltammograms of morphine at ZnO/CNTs/ILCPE at different
scan rates of a) 6, b) 10, c) 18, d) 35, and d) 50 mV s−1 in 0.1 M phosphate buffer,
pH 8.0.
The effect of scan rate (υ) on the oxidation current of morphine
was also examined (Fig. 6 inst). The results showed that the peak cur-
rent increased linearly with increasing the square root of scan rate
that ranged from 6 to 50 mV s−1 according to regression equation of:
The value of m=0.098 is calculated from Eq. (4). Therefore, the
electron transfer coefficient (α) is approximately 0.73 for the irre-
versible electrode process. In addition, the value of α was calculated
for the oxidation of morphine at pH 8.0 for both the ZnO/CNTs/
ILCPE and unmodified CPE using the following equation [39]:
ꢀ
ꢁ
Ip ¼ 5:8124ν1=2−9:5450 r2 ¼ 0:9926; I in cA; ν in mVs–1
:
ð1Þ
ꢀ
ꢁ
αnα ¼ 0:048= EP–EP=2
ð5Þ
The result shows that the electrode process is controlled under the
diffusion step. On the other hand, the peak potential shifts in negative
direction when the scan rate increases, meaning that the electrochem-
ical reaction is quasi-reversible. At higher scan rate, the dependence of
the peak potential (Epa) and ln(ν) showed a linear relationship with a
regression equation of:
where EP/2 is the potential corresponding to iP/2. The values for αnα
were found to be 0.73 and 0.59 at the surface of both ZnO/CNTs/ILCPE
and unmodified CPE, respectively. Those values show that the over-
potential of morphine oxidation is reduced at the surface of ZnO/
CNTs/ILCPE, and also that the rate of electron transfer process is greatly
enhanced. This phenomenon is, thus, confirmed by the larger Ipa values
recorded during linear sweep voltammetry at ZnO/CNTs/ILCPE.
ꢀ
ꢁ
Ep ¼ 0:0493lnðνÞ þ 0:3661 r2 ¼ 0:9919; Ep in V; ν in Vs–1
:
ð2Þ
The chronoamperometry as well as the other electrochemical
methods was employed for the investigation of electro-oxidation of
morphine at ZnO/CNTs/ILCPE. Chronoamperometric measurements
of morphine at ZnO/CNTs/ILCPE were done (not shown) for various
concentrations of morphine. For an electroactive material (morphine
in this case) with a diffusion coefficient of D, the current for the
According to the following equation [38]:
h
ꢀ
ꢁ
i
Epa ¼ E0= þ m 0:78 þ ln D1=2ks–1 –0:5lnm þ ðm=2ÞlnðνÞ
ð3Þ
ð4Þ
with
m ¼ RT=½ð1–αÞnαF:
Fig. 7. The plots of the electrocatalytic peak current as a function of morphine concentration.
Inset shows the SWVs of ZnO/CNTs/ILCPE in 0.1 mol L−1 phosphate buffer solution
(pH 8.0) containing different concentrations of morphine. From inner to outer correspond
to 0.1, 1.0, 10.0, 30.0, 50.0, 100.0, 150.0, 250.0, 350.0, 450.0, 550.0, 600.0 and 700.0 μmol L−1
of morphine.
Fig. 5. Linear sweep voltammograms of a) ZnO/CNTs/ILCPE, b) IL/CPE, c) ZnO/CNTs/CPE and
d) CPE in the presence of 500 μmol L−1 morphine at a pH 8.0, respectively. Inset: the
current density derived from linear sweep voltammogram responses of 500 μmol L−1
morphine at pH 8.0 at the surface of different electrodes with a scan rate of 50 mV s−1
.

12
E. Afsharmanesh et al. / Journal of Molecular Liquids 181 (2013) 8–13
Table 1
Determination of morphine in drug and urine samples.
Sample
Added (μmol L−1
)
Expected (μmol L−1
)
Founded (μmol L−1
)
Published method (μmol L−1) [37]
Fex
Ftab
tex
ttab(95%)
Injection solution
–
10.0
30.0
50.0
–
10.3 0.5
29.6 0.7
50.8 0.9
bLimit of detection
5.5 0.6
15.5 0.5
4.7 0.3
10.5 0.6
10.5 0.7
30.5 0.6
51.0 1.1
–
4.5
–
19
–
2.2
–
3.8
–
20.0
20.0
–
–
–
–
–
Urine
–
–
–
–
5.0
10.0
–
5.0
15.0
–
5.6 0.8
15.7 0.8
5.1 0.6
10.6 0.8
8.5
–
19
–
2.8
–
3.8
–
⁎
Urine
7.5
–
19
–
2.3
–
3.8
–
5.3
10.0
Shows the standard deviation.
⁎
Sampling was made after 2.5 h from a man who is sick and used morphine.
electrochemical reaction (at a mass transport limited rate) is described
by the Cottrell equation [40]:
data analysis presents the value of lower limit detection of morphine
to be 0.06 μmol L−1
.
I ¼ nFAD1=2 Cbπ−1=2 t−1=2
:
ð6Þ
3.6. Determinations of morphine in real samples
In order to demonstrate the ability of the modified electrode to
the determination of morphine in real samples, determinations of
morphine in pharmaceutical and in urine samples were examined.
The results are given in Table 1. These results demonstrated the ability
of ZnO/CNTs/ILCPE for voltammetric determination of morphine with
high selectivity and good reproducibility. Also, the results from the
statistical calculation indicate good agreement between the mean
values (t-test) and the precisions (F-test) of the data obtained by the
proposed and the published method [37].
Under diffusion control, a plot of I versus t−1/2 will be linear, and
from the slope the value of D can be obtained. The mean value of the
D was found to be 5.55×10−6 cm2/s.
3.3. Stability and reproducibility
The repeatability and stability of ZnO/CNTs/ILCPE were investigated
by linear sweep voltammetric measurements of 50.0 μmol L−1 mor-
phine. The relative standard deviation (RSD%) for seven successive
assays was 0.92%. When using five different electrodes, the RSD% for
five measurements was 1.5%. When the electrode is stored in the labo-
ratory, the modified electrode retains 97% of its initial response after a
week and 94% after 40 days. These results indicate that ZnO/CNTs/
ILCPE has a good stability and reproducibility, and could be used for
morphine analysis.
4. Conclusion
In this paper we report synthesis of ZnO/CNT nanocomposite as
a high sensitive sensor for voltammetric determination of morphine
using carbon paste electrode. The synthesized nanocomposite was char-
acterized with different methods such as SEM, TEM, XRD, and EDX. ZnO/
CNTs/ILCPE combines the features of ZnO/CNT nanocomposite and room
temperature ionic liquid to play a good voltammetric sensor. Hence it
shows high sensitivity and reproducibility in sensing of morphine.
Compared with traditional CPE, a decrease of overpotential of oxidation
of morphine was about 75 mV with 5.5-fold increment in the oxidation
peak current when using ZnO/CNTs/ILCPE. Under the optimum condi-
tions, the oxidation peak current was proportional to the morphine con-
centration in the range of 0.1 to 700 μmol L−1 with the detection limit of
0.06 μmol L−1. The proposed method was successfully applied to the
morphine detection in real samples such as drug and urine.
3.4. Interference studies
The influence of various substances as potentially interfering
compounds for the determination of morphine was studied under
the optimum conditions with 5.0 μmol L−1 morphine at pH 8.0. The
potential interfering substances were chosen from the group of sub-
stances commonly found with morphine in pharmaceuticals and/or
in biological fluids. The tolerance limit was defined as the maximum
concentration of the interfering substance that caused an error of
less than 5% for the determination of morphine. After the experi-
ments, we found that neither 1000-fold of glucose, sucrose, lactose
and fructose, nor 800-fold of K+, Li+, Ca2+, Mg2+, Al3+, NH4+
,
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