Electro-oxidation study of promethazine hydrochloride at the surface of modified gold electrode using molecular self assembly of a novel bis-thio Schiff base from ethanol media

Article abstract of DOI:10.1016/j.molliq.2015.12.094

In this study, a new Schiff base of bis-N-(2-mercaptophenyl) salicylaldimine (MPSI) has been immobilized on a bare gold electrode as a novel self assembled monolayer (SAM) sensor for determination of promethazine hydrochloride (PMZ). Cyclic voltammetry an

Full text of DOI:10.1016/j.molliq.2015.12.094

Journal of Molecular Liquids 216 (2016) 429–439
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Electro-oxidation study of promethazine hydrochloride at the surface of
modified gold electrode using molecular self assembly of a novel bis-thio
Schiff base from ethanol media
a,
a,b
a
a
Ebrahim Honarmand ⁎, Mohammad Hassan Motaghedifard , Mojtaba Hadi , Hossein Mostaanzadeh
a
Department of Chemistry, Faculty of Science, University of Qom, Qom 37161-46611, Islamic Republic of Iran
Young Researchers and Elites Club, Qom Branch, Islamic Azad University, Qom, Islamic Republic of Iran
b
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, a new Schiff base of bis-N-(2-mercaptophenyl) salicylaldimine (MPSI) has been immobilized on a
bare gold electrode as a novel self assembled monolayer (SAM) sensor for determination of promethazine hydro-
Received 14 October 2015
Received in revised form 11 December 2015
Accepted 24 December 2015
Available online xxxx
chloride (PMZ). Cyclic voltammetry and electrochemical impedance spectroscopy are used to study the proper-
3−/4−
ties of monolayer of Schiff's base on a gold electrode (MPSI-Au SAM electrode) using the [Fe(CN)
6
]
probe.
Also, the characterization of the MPSI-Au electrode was performed by scanning electron (SEM) and atomic force
microscopy (AFM). The electrochemical behavior of PMZ on the MPSI-Au SAM electrode was studied by several
voltammetry methods in phosphate buffer solution as supporting electrolyte. The modified electrode showed
good selectivity in the presence of interference of ascorbic and uric acid. A calibration curve was obtained for
Keywords:
Promethazine hydrochloride
Self-assembled monolayer
Electrocatalytic oxidation
Gold electrode
−
1
PMZ in a linear range of 1.0–240.0 μmol L . The detection limit for promethazine hydrochloride was found to
−
1
be 24.6 nmol L . The results indicated that the MPSI-Au SAM electrode could be employed for the determination
of promethazine hydrochloride in urine and plasma samples.
©
2015 Elsevier B.V. All rights reserved.
1
. Introduction
behavior and inhibition from fouling effects of the redox active mole-
cules [14,15].
The interest in developing sensing devices for use in environmental
Promethazine hydrochloride (PMZ) is a prominent compound in the
large group of phenothiazine derivatives widely used for its antihista-
minic, sedative, antipsychotic, analgesic and anticholinergic properties
[16,17]. However, adverse effects caused by PMZ in humans have also
attracted enormous attention, such as cardiac, reproductive alterations,
endocrinal, occasional hypotension and so on [18]. Therefore, its deter-
mination in commercial formulations is extremely important. Up to
now, many analytical techniques have been reported for the determina-
tion of PMZ, such as chemiluminescence [19], high performance liquid
chromatography [20,21], capillary zone electrophoresis [22], titrimetric
procedures [23], spectrophotometry allied to flow-injection analysis
[24–26] and spectrofluorimetry [27]. However, most of these methods
are complicated, time consuming and also require expensive instru-
mentation. Recently, electroanalytical techniques have been employed
to determine PMZ, since they are simple, cost little, and require relative-
ly short analysis times [28,29].
Electrochemical techniques have proved to be excellent alternatives
to determine pharmaceutical compounds, since they are simple, cost lit-
tle and require relatively short analysis times, without the need for de-
rivatizations or time-consuming extraction steps [30]. Moreover, these
techniques are less sensitive than other analytical techniques to the ef-
fects of excipient substances in commercial formulations. As for evalua-
tion of the electrochemical behavior and quantification of PMZ by
monitoring, clinical assays or process control is a rapidly growing field
of research. Electrochemical sensors satisfy many of the requirements
for such tasks, particularly owing to their simplicity of preparation,
high selectivity and sensitivity, and fast response. The utility of sensors
based on solid (bare) electrodes is often hampered by their low selectiv-
ity arising from high over potentials [1]. In particular, complexity of real
biological systems may result in overlapping voltammetric signals.
Moreover, the limited number of electrode materials makes only a re-
stricted number of analytes suitable for electrochemical detection
with high sensitivity and selectivity. Therefore, efforts have been
made to modify the electrode surfaces for the purpose of lowering the
over potential, improving the mass transfer velocity for effective enrich-
ment of the desired substance and/or restraining the effect of interfer-
ences [2–6].
Self-assembly procedure as a precise modification of the surface
structure in nanometer-scale [7] is recently employed in surface protec-
tion [8,9], fabrication of sensors [10,11] and biosensors [12,13]. It has
been revealed that by changing the charge state of electrodes, one can
control the electrochemical properties of the surface as insulation
⁎
Corresponding author.
E-mail address: e.honarmand@qom.ac.ir (E. Honarmand).
http://dx.doi.org/10.1016/j.molliq.2015.12.094
167-7322/© 2015 Elsevier B.V. All rights reserved.
0

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E. Honarmand et al. / Journal of Molecular Liquids 216 (2016) 429–439
Scheme 1. The FT-IR spectrum of synthesized MPSI Schiff base.
electroanalytical techniques, several electrodic surfaces such as highly
boron doped diamond electrode [31], glassy carbon electrode [32],
molecularly imprinted polymer carbon paste electrode [33] and gold
electrode [34] have already been used as working electrodes. Nano-
materials, owing the nanometer size and specific physicochemical
characteristics, will offer special opportunities for electrochemistry.
In the present work, an MPSI-Au SAM electrode as a new nanostruc-
ture was fabricated and applied for PMZ sensing. Scanning electron mi-
croscope (SEM), atomic force microscopy (AFM), cyclic voltammetry
(1.97 g, 16.10 mmol), dissolved in 10 mL of distilled methanol was
added drop by drop using a dropping funnel to the above solution.
The mixture was refluxed for 6 h and the yellow precipitate which ap-
peared was filtered off and recrystallized from chloroform/methanol
(94% yields). M.p: 169 °C. Anal. Calc. for MPSI: C = 68.40; H = 4.42;
N = 6.14. Found: C = 68.17; H = 4.33; N = 6.25%. Main IR
(KBr, cm ), νC_N (cm ): 1616. 1H NMR (400 MHz) chemical
shift (δ ppm), 14.80 (s, 2H, O–H), 9.08 (s, 2H, CH_N), 6.55–7.67
(m, 16H, H-aromatic). Also, to confirm synthesized MPSI, the FT-IR
study of MPSI was investigated which its spectrum is shown in
Scheme 1.
−
1
−1
(
CV) and electrochemical impedance spectroscopy (EIS) methods
were used for the characterization of monolayer of Schiff base at the sur-
face of gold electrode. Also, differential pulse voltammetry (DPV) was
used to measure of PMZ in real sample.
2.3. Preparation of MPSI-Au SAM electrode
2
. Experimental
Firstly, the gold electrode was immersed in Piranha solution (1:3, v/v;
3
2 2 2 4
0% H O and concentrated H SO for 3.0 min and rinsed thoroughly
2
.1. Chemicals and apparatus
with double-distilled water. Then, the gold electrode was carefully
polished with 0.5 μm and 0.05 μm Alumina powder on micro cloth
pads and rinsed with ethanol between each step (an ultrasonic bath is
required); then rinsed with water and sonicated in water/chloroform/
water each step 5 min, to remove physically adsorbed particles and con-
taminants. Then the cleanliness of the electrode surface was cleaned
electrochemically by cycling the electrode potential between 0.000
PMZ, AA, UA and other chemicals were of pure grade and purchased
2
from Merck. The gold electrode (Au, 99.99%, 0.0314 cm ) was purchased
from Azar Electrode Co., Urmia, I. R. Iran. The voltammetric and EIS mea-
surements were carried out using an Autolab Potentiostat/Galvanostat
PGSTAT30 (Eco Chemie, Utrecht, The Netherlands) controlled by Gener-
al Purpose Electrochemical Systems (GPES) and Frequency Response
Analyser (FRA) 4.9 software and adopted by a conventional three-
electrode system. The working electrode was MPSI-Au SAM electrode
and the auxiliary and reference electrodes were a Metrohm platinum
rod and Ag/AgCl/KCl (3.0 M) electrodes, respectively. All experiments
were done at room temperature (25 °C). A Metrohm 691 pH/ion
meter was also used for adjusting pH of buffers.
−
1
and +1.500 V vs. a reference electrode, with a 100 mV s scan rate
−
1
−1
in 0.5 mol L
2 4
H SO and 0.000 until −1.400 V with a 100 mV s
A scanning electron microscope (TESCAN, Czech Republic) and an
atomic force microscope (NT-MDT. noncontact) were used to study
of the surface of gold electrodes. Also, a Fourier transformation infra-
red (FT-IR) Magna 550 Nicolet spectrophotometer was used to char-
acterize of synthesized Schiff base using a KBr pellets in a range of
−
1
4
00–4000 cm
.
2
.2. Synthesis of bis-N-(2-mercaptophenyl) salicylaldimine (MPSI)
The aromatic Schiff base was prepared according to the literature
through a well-known procedure [35] as follows: Bis (2-aminophenyl)
disulfide (2.00 g, 8.05 mmol) was mixed with 50 mL distilled methanol
which was stirred using a magnetic stirrer. Then, salicylaldehyde
Scheme 2. The probable structure of immobilized MPSI at the surface of gold electrode.

E. Honarmand et al. / Journal of Molecular Liquids 216 (2016) 429–439
431
scan rate in 0.5 mol L− NaOH until reproducible voltammograms were
observed. A roughness factor of 5.28 was obtained for bare Au electrode
from the ratio of the real to geometric surface area [36,37]. The real sur-
face area was calculated by integration of a cathodic peak during the
1
3. Results and discussion
3.1. Characterization of MPSI-Au SAM electrode
−
2
redox of gold assuming 482 μC cm charge for reduction of one mono-
3.1.1. Scanning electron and atomic force microscopy
layer of AuO on Au (111) [37].
The response of an electrochemical sensor was related to the physi-
cal morphology of its surface. Several factors such as surface roughness,
porosity of films, and inclusion of defects should affect the current re-
sponse of the electrode. Surface morphological analysis of gold elec-
trodes without (A) and with (B) MPSI SAM in ethanolic solution after
6 h immersion was confirmed by scanning electron microscopy (SEM)
and was presented in Fig. 1. As can be observed in images, self assem-
bled monolayer of MPSI make a distinct appearance on the SEM image
−
1
The clean gold electrode was immersed in 1.0 mmol L solution
prepared of MPSI in ethanol for 6.0 h to form MPSI Au SAM modified
electrode. Finally, the MPSI-Au SAM electrode was removed, sonicated
in ethanol, washed thoroughly with ethanol and water to remove phys-
ically adsorbed MPSI, and used for electrochemical measurements. The
probable structure of immobilized MPSI at the surface of gold electrode
is shown in Scheme 2.
Fig. 1. SEM and 3-Dimension AFM images of the surface of bare Au electrode (A and C) and MPSI Au SAM electrode (B and D).

432
E. Honarmand et al. / Journal of Molecular Liquids 216 (2016) 429–439
−
1
Fig. 2. A) Cyclic voltammograms obtained for electrochemical oxidation/reduction of accessible Au atoms on the MPSI Au SAM and bare Au electrode in 0.5 mol L
H SO
2
4
. B) Cyclic
−
1
−1
voltammogram obtained for desorption of MPSI SAM from Au surface in 0.5 mol L
NaOH. C) Cyclic voltammograms obtained on MPSI Au SAM electrode in 0.1 mol L
and D) CVs of 2.4 × 10 mol L PMZ on bare Au (black line) and MPSI Au SAM electrode (red line) in 0.1 mol L PBS (pH 6.0). Scan rate: 25 mV s
KCl,
5
.0 m mol L− [Fe(CN)
1
6
]
-3/−4
−4
−1
−1
−1
.
(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
of gold while there is a smooth and unbroken surface on the SEM image
with bare gold, indicated that the modification of the gold electrode is
greatly done by the MPSI SAM.
at 835 mV. The results are summarized in Table 1. The isolation effect
can include the contribution of Au–S bond which impedes the surface
against charge-transfer process.
Atomic force microscopy is a powerful tool to measure topography
and properties of surfaces. The AFM technique was employed to illus-
trate the self-assembly process of MPSI Au SAM electrode and estimate
its average size. Fig. 1C and D shows the AFM 3D images of bare Au and
MPSI-Au SAM electrode by the non-contact mode. The images show
that the surface morphology of bare Au changes after the modification
of the surface with MPSI. It is clearly observed that the bare Au electrode
has a smooth and uniform surface which, after self-assembly of MPSI,
changes to rough and uneven sheets. These results confirm that the
MPSI is successfully immobilized on the surface of bare Au.
In NaOH media: The first CVs recorded in the potential range from 0.0
−
1
to −1.4 V in 0.5 mol L NaOH solution on a bare Au and the MPSI Au
SAM electrode (Fig. 2B). We attributed the large cathodic peak (the
maxima around −1077 mV) observed in the first potential cycle to
the reduction of adsorbed MPSI monolayer. A partial surface coverage
may be obtained for MPSI using desorption peak area (see Table 1).
−
3/−4
In [Fe(CN)
6
]
media: The recorded CVs of bare Au and MPSI Au
−
1
−1
SAM electrodes in 0.1 mol L KCl solution containing 5.0 mmol L
3
−/4−
3−/4−
[Fe(CN)
6
]
redox are shown in Fig. 2C. [Fe(CN)
6
]
produced
a couple of well-defined redox waves at bare Au with a peak-to-peak
−
1
separation (ΔE
p
) of 211 mV at 100 mV s . After the self assembly of
3
.1.2. Electrochemical characterization of MPSI SAM
Different electrochemical methods were used to verify formation of
MPSI on Au electrode, the shape of CV changed dramatically, a declined
3
−/4−
redox reaction of [Fe(CN)
6
]
was apparently observed because of
the MPSI Au SAM and evaluate the surface coverage of Au electrode in
this work.
the blocking effect of MPSI SAM on the electron and mass transfer of
6
[Fe(CN) ]
−/4−.
3
In H
trode in 0.5 mol L
pressed at the MPSI Au SAM in comparison with the bare-Au electrode
Fig. 2A). The electrode surface coverage (θ) can be used to estimate the
2
SO
4
media: First, the CV of the bare-Au and MPSI Au SAM elec-
Electrochemical impedance spectroscopy (EIS): The modification of Au
−
1
H SO
2 4
shows that the gold redox currents are sup-
electrodes was also evaluated by EIS technique. Fig. 3 shows the Nyquist
−
1
plots of the bare Au and MPSI Au SAM electrodes in 0.1 mol L
solution containing 5.0 mmol L [Fe(CN) ]
6
KCl
−
1
3−/4−
(
at a DC potential of
surface state of electrode. The surface coverage can be determined
through a decrease in the area (Q) associated with gold oxide reduction
+260 mV, an ac amplitude of 5.0 mV and a frequency range of
10 kHz–100 mHz. The Rct (semicircle diameter) for the bare gold
Table 1
Partial surface coverage obtained for MPSI Au SAM electrode by different methods.
Method
CV in H
Parameters
Equation
θ (%)
bare = 73.2 & Q modified = 24.5 μC cm−
2
% θ = 100 [1 − (Qmodified / Qbare)]
66.5
62.3
59.4
98.2
SO
2 4
Q
Q
modified = 7.6 μC cm−
2
% θ = [(3 × Qmodified in NaOH) / (Qbare in H2SO4 / 2)]
% θ = [1 − (ip modified / i°p bare)] × 100
% θ = [1 − (R°ct bare / Rct modified)] × 100
CV in NaOH
CV in probe Fe+
2/+3
2/+3
i
= 30.5 & i°
p
p
= 12.4 μA
EIS in probe Fe+
R°ct = 1056 & Rct = 58,400 Ω
θ: The electrode surface coverage.
Q: The charge density of surface.
i: The anode or cathode peak current of redox probe in cyclic voltammetry.
ct: The charge transfer resistant in electrochemical impedance spectroscopy.
R

E. Honarmand et al. / Journal of Molecular Liquids 216 (2016) 429–439
433
Fig. 3. Nyquist plots of bare Au and MPSI Au SAM electrode (curve a and b) in the presence of 5.0 mmol L−1 Fe(CN)3−/4−
and the frequency range was 10 kHz to 100 mHz. The inset shows the Nyquist plot of bare Au electrode in the same solution.
in 0.1 mol L−1 KCl solution. The EDC = +260.0 mV; Eac = 5.0 mV
6
electrode is 1.056 kΩ, and the assembly of MPSI layer on the elec-
3.2. Electrochemical behavior of PMZ on MPSI SAM
trode surface generates a barrier to the interfacial electron transfer
−
4
mol L−1 PMZ in
[
(Rct
)
modified = 58.4 kΩ]. Using these values, the surface coverage
The cyclic voltammograms of 2.4 × 10
−
1
value was estimated to be 98.2% (see Table. 1).
0.1 mol L PBS (pH 6.0) on the bare Au and Au MPSI SAM electrodes
are shown in Fig. 2D. It is verified that the presence of the MPSI SAM
at the electrode surface reduce the oxidation potential of PMZ, shifting
to more negative potential values by 76 mV. On the other hand, the ox-
idation peak current of PMZ on the MPSI Au SAM electrode was 1.6
times greater than that on the bare Au electrode. The lower over poten-
tial (−76 mV) and the current enhancement due to an increase in the
It can be seen that the semi-circular diameter enlarged dramatically
with modification, implying a high electron-transfer resistance, and
3
−/4−
very slow electron-transfer kinetics of [Fe(CN)
6
]
probe on Au elec-
trode due to insulation nature of the MPSI monolayer on the electrode
surface. These behaviors are also in agreement with those found by
cyclic voltammetry (Fig. 2A–C).
Fig. 4. A) CVs of 2.4 × 10− mol L−1 PMZ at various buffered pHs, performed with a scan rate of 25.0 mV s−1. The letters a–h correspond to pH values of 2, 3, 4, 5, 6, 7, 8 and 9, respectively.
4
B) Intensity, I (μA) vs. pH and C) Electrochemical potential, Epa (V) vs. pH.

434
E. Honarmand et al. / Journal of Molecular Liquids 216 (2016) 429–439
Fig. 5. DPVs of 2.4 × 10− mol L−1 PMZ on the MPSI Au SAM electrode to optimize of instrumental parameters. A) Interval times of 100, 300, 500, 750 and 1000 ms (corresponds to a–e). B)
4
Modulation amplitude in the range of 5.0–200.0 mV. C) Modulation time in the range of 10.0–100.0 ms and D) optimizing of scan rate in range of 10–35 mV s−
1
.
electron transfer rate could be attributed to a catalytic effect of the MPSI
SAM on the oxidation of PMZ [38]. The π–π interaction between the
phenyl structure of PMZ and phenyl structure of MPSI SAM on the sur-
face of Au electrode may accelerate the electron transfer of PMZ in com-
parison with the bare Au electrode [39].
at pH 6.0; after that, the peak currents decreased (Fig. 5). Therefore, ac-
cording to I (μA) vs. pH curve, the pH = 6.0 was selected for the further
studies (Fig. 5A). Also, the relationship between oxidation peak poten-
tial of PMZ and pH was obtained (Fig. 5B), which was according to the
Nernst equation (Eq. 1), Where n & m represents the number of elec-
trons and protons involved in reaction, a & b represent the coefficients
reagents in reaction equation:
3
.3. Optimization of instrumental parameters
h
i
ꢀ
a
b
Differential pulse voltammetry (DPV) method was used to improve
Ep ¼ E þ 0:0591 Vð1=nÞ log ðOxÞ =ðRÞ −0:0591 V ðm=nÞ pH
ð1Þ
the sensitivity of MPSI Au SAM electrode to measure of PMZ (Fig. 4A–D).
The optimum instrumental conditions were chosen from a study of
the variation of the peak current with interval time (A), modulation am-
plitude (B), modulation time (C) and scan rate (D). In the investigation
process, each variable was changed while the others were kept to be
constant. The variables of interest were studied over the ranges of
2
EpaðV vs:Ag=AgCl Þ ¼ 0:8774–0:0296 ½pHꢁ; R ¼ 0:9897:
It can be found that the peak potential shifted negatively with the pH
increase. The linear regression equation showed that the overall process
was proton dependent and the oxidation of PMZ involves two electrons
and one proton and finally, the resulting product will be subject to hy-
drolysis. The probable catalytic processes of PMZ may be expressed as
shown in Scheme 3 [40]:
The scan rate can influence the current responses of PMZ and the
corresponding electrochemical parameters could be deduced from the
relationship between the scan rates of potential sweep and current re-
sponses of PMZ oxidation. So, the influence of scan rate (ν) on the elec-
0
1
.1–1.0 s for interval time, 10–200 mV for modulation amplitude,
−
1
0–100 ms for modulation time and 10–35 mV s for the scan rate.
The results showed that the peak sensitivity was increased with increas-
ing the modulation amplitude and the scan rate, whereas increasing the
modulation time and interval time decreased the peak current. To ac-
quire voltammograms of a relatively high sensitivity and well-shaped
waves with a relatively narrow peak width, an interval time of
−
1
trochemical behavior of 150.0 μmol L of PMZ on the MPSI Au SAM
electrode was studied (Fig. 6). According to Eq. 2 for totally irreversible
diffusion controlled process [41]:
3
00 ms, modulation amplitude of 100 mV, modulation time of 10 ms
−
1
and scan rate of 25 mV s were chosen.
3
.4. The pH and scan rates effect
Ip ¼ 3:01 ꢂ 10 n ½ð1‐αÞ nαꢁ1=2A D1=2C ν1=2
5
:
ð2Þ
To investigate the oxidation mechanism of PMZ, the effects of vary-
ing pHs (3.0–9.0) and scan rates on its peak currents were studied.
Where D/(cm s−1) is the diffusion coefficient and C/(mol L−1) is the
concentration of reagent, in the scan rate range of 5.0–45.0 mV s , the
2
Using DPV method and in the presence of 2.4 × 10− mol L
4
−1
PMZ,
−1
with the pH increasing, the peak currents of PMZ reached the maximum
oxidation peak current increases linearly with the square root of scan

E. Honarmand et al. / Journal of Molecular Liquids 216 (2016) 429–439
435
Scheme 3. The probable oxidation process of PMZ at the MPSI Au SAM electrode.
rate, typical of a diffusion-controlled process of PMZ at the MPSI Au SAM
electrode (Fig. 6A). Therefore the linear regression equation for PMZ was
found to be:
A Tafel plot is a useful device for evaluating the kinetic parame-
ters. Inset B of Fig. 6 shows the Tafel plot (E vs. log I), drawn by
using the data derived from the rising part of the current–voltage
−
1
curve at a scan rate of 35 mV s . According to Eq. 3 [41], the number
of electrons involved in the rate-determining step (n ), charge
transfer coefficient (α) and the ionic exchange current (i ) can be es-
1
=2
2
α
Ipa ¼ 16:097 ν þ 0:2002; R ¼ 0:9947:
0
timated from the slope and the intercept, respectively, of the Tafel
plot (Eqs. 4–6). A Tafel slope of 0.1098 V per decade was obtained, in-
dicating that assuming a one-electron process involved in the rate-
determining step, a charge transfer coefficient (α) was 0.46. Also,
By considering (1-α) 0.54 (see below),
.15 × 10 cm s and A = 0.0314 cm , it is estimated that the total
number of electrons involved in the anodic oxidation of PMZ is n ~ 2.
n
α
=
D
=
−
5
2
−1
2
2
Fig. 6. CVs of 1.5 × 10 mol L−1 PMZ at the surface of MPSI Au SAM electrode in 0.1 mol L−1 PBS (pH 6.0) at various scan rates: 5, 15, 25, 35 and 45 mV s−1 from a to e. (A) Plot of intensity:
−
4
1
/2
I (μA) vs. ʋ . (B) Tafel plot, E
p
(V) vs. log (I/μA).

436
E. Honarmand et al. / Journal of Molecular Liquids 216 (2016) 429–439
−
1
Fig. 7. Chronoamperograms obtained at MPSI Au SAM electrode in 0.1 mol L PBS (pH 6.0) for different concentrations of PMZ. The letters a–f corresponds to 4.0, 34.0, 58.0, 88.0, 120.0
and 155.0 μmol L− PMZ. Insets: (A) Plots of I vs. t
1
−1/2
obtained from chronoamperograms a–f. (B) Plot of the slope of the straight lines against PMZ concentration.
was found to be 1.609 μA cm− from the intercept of
2
3.5. Chronoamperometry study
the value of i
the Tafel plot.
°
The catalytic oxidation of PMZ at the surface of MPSI Au SAM elec-
trode was studied by double potential step chronoamprometry
method. Double potential step chronoamprograms obtained by po-
larizing the potentials to 675.0 and 0.1 mV and are depicted in
Fig. 7. In chronoamperometric studies, we have determined the dif-
fusion coefficient of PMZ for a MPSI Au SAM electrode. For an electro
active material (PMZ in this case) with a diffusion coefficient of D,
the current observed for the electrochemical reaction at the mass
η ¼ ð2:3RT=n αFÞ logi
ꢀ
þ ½2:3RT=n ð1‐αÞ Fꢁ log i
ð3Þ
ð4Þ
ð5Þ
ð6Þ
α
α
η ¼ a þ b log i
a ¼ ð2:3RT=n αFÞ log i
ꢀ
α
b ¼ 2:3RT=n ð1‐αÞF
α
Fig. 8. DPVs of 1.0, 4.5, 6.3, 9.3, 14.0, 23.0, 50.0, 75.0, 120.0, 160.0 and 240.0 μmol L− PMZ on MPSI Au SAM electrode in 0.1 mol L−1 PBS (pH 6.0) and scan rate of 25 mV s−1. Insert shows
1
two linear relationships between the oxidation peak current and the concentration of PMZ.

E. Honarmand et al. / Journal of Molecular Liquids 216 (2016) 429–439
437
Table 2
Comparison of the efficiency of some electro analytical methods in the determination of PMZ.
Electrode
Method
Mediator
pH Potential shift (mV) LOD
LDR
Ref
(
μmol L−1) (μmol L−1)
GCE
CPE
Au
Amperometry
SWV (accumulation time of 10 min)
AS-LSV (accumulation time of 2 min)
BIA-MPA
SWV
Graphene oxide 5.0
MIP/NIP/VB-DVB 9.0
–
–
0.199
0.0032
0.01
0.2
5.9
0.026
0.004
0.065
0.0246
1.99–1030 [42]
0.007–0.4
0.05–10.0
15.0–78.0
[43]
[44]
[45]
MWCNT
BDD
DNA/MWCNT
−
4.0 +60
Poly crystalline silicon wafer
Matrix sol–gel silica (SiAlNb)
Boron-doped diamond electrode AS-SWV (accumulation potential of +0.78 V for 30 s)
GCE
CPE
Au
4.7
7.0
4.0
–
–
–
20.0–100.0 [46]
0.59–4.76 [47]
0.01–59. 8 [48]
DPV
DPV
DPV
ZnO/p-ABSA
Au-NPs
8.0 −20
6.0 −42
2.0–225.0
1.0–240.0
[49]
[This work]
MPSI Schiff base 6.0 −76
transport limited condition is described by the Cottrell equation
41]:
with the values reported [42–49] by other research groups for the elec-
trocatalytic oxidation of PMZ at the surfaces of chemically modified
electrodes by other mediators (Table 2).
[
I ¼ nFAD =2C t−1=2π−1=2
1
ð6Þ
To characterize the reproducibility of the MPSI Au SAM elec-
trode, repetitive measurements for five prepared electrodes were
where D (cm s−1) and C (mol cm−3) are the diffusion coefficient and
2
−5
−1
carried out in a solution containing of 7.5 × 10
mol L
PMZ,
the bulk concentration, respectively. Under diffusion limited transport
which the relative standard deviation (RSD) of voltammetric re-
sponses for 10 repeated detections on the MPSI Au SAM electrode
were 1.2%. The repeatability and stability of MPSI Au SAM electrode
were also investigated at the same molar concentration of PMZ.
The RSD in peak potential and current for ten successive assays
were 0.32% and 0.96%, respectively. When the electrode was stored
in PBS (pH = 6.0) at room temperature, the modified electrode
retained 96.1% of its initial response after a month. The results
showed that the modified electrode was not subjected to surfaces
fouled by the oxidation products and it has both good stability
and satisfactory reproducibility.
−
1/2
−1/2
(
mass transport), a plot of I vs. t
(s
) will be linear, and the
value of D can be extracted from the slope. Fig. 7A and B shows the fitted
experimental plots for different concentrations of PMZ. The slopes of
the resulting straight lines were plotted vs. the PMZ concentration,
−
5
2 −1
and the value of D was found to be 2.15 × 10 cm s
.
3
.6. Calibration plot and interference study
The differential pulse voltammetry method was used to determine
the concentration of PMZ and showed two linear relationship from
−
6
−5
1
.0 × 10
to 2.3 × 10
{ΔI
p
(μA) = (0.0976) CPMZ + (0.0484)
On the other hand, for evaluating selectivity of modified electrode,
the interference of UA and uric acid in oxidation of PMZ at the surface
of MPSI Au SAM electrode was studied. Results shown that UA and AA
had no oxidation peak at the surface of SAM electrode, but they had in-
2
−5
−4
(
R
= 0.9995)} and 2.3 × 10
to 2.4 × 10
{ΔI
PMZ + (1.2359) (R = 0.9979)} mol L PMZ (Fig. 8). The detection
/m (where S is the standard deviation of the
blank signal (n = 10) and m is the slope of the calibration curve)
p
(μA) = (0.0491)
2
−1
C
limits defined as 3S
b
b
−
1
terferences on current of PMZ oxidation from 0.5 mmol L of their con-
centrations and more.
−
1
were found to be 24.6 nmol L for PMZ. These values are comparable
Fig. 9. DPVs based on standard Addition method for recovery of PMZ from urine sample. A) The curve of I(μA) vs. μmol L−1 of PMZ in urine sample based on standard Addition method.
B) DPVs of MPSI Au SAM electrode in buffer (absence of PMZ) and plasma solutions.

438
E. Honarmand et al. / Journal of Molecular Liquids 216 (2016) 429–439
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[

E. Honarmand et al. / Journal of Molecular Liquids 216 (2016) 429–439
Ebrahim Honarmand is an assistant professor of Analytical
439
Mojtaba Hadi is assistant professor of Analytical Chemistry
at the University of Qom, Qom, Iran. He received a BC in Ap-
plied Chemistry from Karaj Branch, Islamic Azad University,
Karaj, Iran, in 2003, an MS in Analytical Chemistry from
K.N. Toosi University of Technology, Tehran, Iran, in 2005,
and a PhD in Analytical Chemistry from K.N. Toosi University
of Technology. His main research interests are fabrication and
preparation of voltammetric sensors, the nano-structured car-
bon films in electroanalytical sensing, advanced carbon mate-
rials, and nano-structured carbon-based electrochemical
sensors.
Chemistry (Electrochemistry) at the University of Qom,
Qom, Iran. He received a B.Sc. in Chemistry from the Univer-
sity of Kashan, Kashan, Iran, in 1999, and his M.Sc. in College
of Chemistry, Isfahan University of Technology, Isfahan, Iran,
in 2004, and his PhD in analytical chemistry from the Depart-
ment of Analytical Chemistry, Faculty of Chemistry, University
of Kashan, Kashan, Iran, in 2010. His main areas of interests
are: electrochemistry and drug sensors, nanoparticles modi-
fied electrodes, ion-selective electrode, electroanalytical
chemistry, self-assembled monolayers on gold and nanostruc-
tures, corrosion science and corrosion inhibitors, spectroscopy
with flow injection analysis and Uv–visible.
Hossein Mostaanzadeh is an assistant professor of chemis-
try at the University of Qom, Qom, Iran. He received a B.S.
in chemistry from the University of Arak, Arak, Iran, in
1997, and his M.S. in Inorganic chemistry from the Depart-
ment of Chemistry, Faculty of Science, University of Tarbiat
Modarres, Tehran, Iran, in 2003, and his Ph.D. in inorganic
chemistry from the Department of Chemistry, Faculty of Sci-
ence, University of Tarbiat Modarres, Tehran, Iran, in 2009.
His main areas of interest are: computational chemistry, syn-
thesis, characterization and structural analysis of lanthanides
complexes, metal organic framework and phosphoramidate
compounds.
Mohammad Hassan Motaghedifard completed his B.S. at
the Department of Chemistry, University of Kashan in
Applied Chemistry in 2006 and completed his M.S. at the
Department of Analytical Chemistry, University of Kashan
in 2009 and now he is a PhD student at University of Kashan
in Analytical Chemistry under supervision of Prof. M.
Behpour and invited assistant professor in department of
chemistry, University of Qom, Qom, Iran. His research inter-
ests include (a) electroanalytical chemistry, self-assembled
monolayers on gold and nanostructures, biosensors include
testing by electrochemical methods, mainly electrochemical
impedance spectroscopy, synthesis and application of nano-
particle, (b) phytochemistry and (c) application of chemo-
metric methods in electrochemistry.