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A novel apigenin modified glassy carbon sensor electrode for the determinationof copper ions in soil samples
_Ibrahim Ender M€ulazımo�glu*a and Ali Osman Solakbc
Received 6th June 2011, Accepted 11th August 2011
DOI: 10.1039/c1ay05328k
In this study, electrochemical modification of a glassy carbon (GC) electrode with apigenin was carried
out and the modified electrode was used for determination of copper(II) (Cu(II)) in soil samples. The GC
was modified through the electrochemical polymerization of apigenin (PolyApi/GC) on the electrode
surface in aqueous media. The electrode surface was modified with apigenin in phosphate buffer
solution (PBS), pH 7, from 0 mV to +1400 mV potential ranges, using 100 mV s�1 sweep rate and 30
cycles by cyclic voltammetry (CV). The surface characterizations of this sensor electrode were
performed by CV, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy
(SEM). Britton-Robinson (BR) buffer solution at pH 5 was used for determination of Cu(II) by
differential pulse voltammetry (DPV). The detection limit was obtained as lower as 1.0 � 10�11 M. By
using this calibration curve, the amount of Cu(II) was determined as 7.34� 10�7 M in soil samples. The
results showed that pH, incubation time and interferences of some cations and anions were significant.
1. Introduction
Trace metals commonly exist as environmental pollutants.
Copper is a heavy metal extensively examined in environmental,
industrial and biological applications. Copper is vital and toxic
for many biological systems,1,2 so its determination in various
samples is very important. Copper is one of the essential trace
elements in the body. Very low and high intakes of this element
can cause adverse health effects.3 Copper has a very complex role
in many body functions such as normal function of the central
nervous system, connective tissue development, hemoglobin
synthesis, and oxidative phosphorylation.4 However, excessive
copper intake could result in deposition of the metal in liver cells
and thus can cause hemolytic crisis, jaundice, and neurological
disturbances.5
Different analytical techniques have been proposed for Cu(II)
determination such as, flame atomic absorption spectrometry
(FAAS),6,7 inductively coupled plasma mass spectrometry (ICP-
MS),8 electrothermal atomization atomic absorption spectro-
metry (ET-AAS),9 inductively coupled plasma optical emission
spectrometry (ICP-OES),10 graphite furnace atomic absorption
spectrometry (GF-AAS),11 inductively coupled plasma-atomic
emission spectrometry (ICP-AES)12 and anodic stripping
aDepartment of Chemistry, Ahmet Kelesxo�glu Education Faculty, SelcukUniversity, Konya, Turkey. E-mail: [email protected]; Fax:+90 332 3238225; Tel: +90 332 3238220bDepartment of Chemistry, Faculty of Science, Ankara University, Ankara,Turkey. E-mail: [email protected]; Fax: +90 312 2232395;Tel: +90 332 2126720cDepartment of Chemical Engineering, Faculty of Engineering, Kyrgyz-Turk Manas University, Bishkek, Kyrgyzstan
2534 | Anal. Methods, 2011, 3, 2534–2539
voltammetry.13 These techniques have been applied in various
samples, for example water, soil, food, mineral, biological
samples etc.14–20
Among these methods, much importance has been attached to
the electroanalytical techniques due to their simplicity, simulta-
neous determination, low-cost, accurateness, sensitivity and high
stability. Some researchers have shown that chemically modified
electrodes can be successfully applied to the analysis of heavy
metal ions.21–25 In particular, trace levels of heavy metal ions
could be pre-concentrated at the electrode surface by electro-
static attraction or complexation with the chemical modifier,
hence improving sensitivity and selectivity.
Electrode modification, which has an important part in elec-
trochemical studies, has been extensively used for the last decade.
As a matter of fact, these modified electrodes have come into
prominence in the determination of organic and inorganic
species, especially in that of trace amounts in natural samples.
Electrode modification with complexing polymer films is an
attractive approach as it yields large amounts of ligand at the
electrode surface and hence allows large amounts of metal ions to
be accumulated. Among the different processes that can be
carried out for electrode surface modification, electro-polymeri-
zation of heteroaromatic monomers provides a straightforward
and efficient route to coat polymer films onto electrode surfaces.
Therefore, conducting polymer films have received considerable
attention for generating modified electrodes with analytical
utility, especially for trace metals detection. In order to increase
the selectivity of the polymer film for one or another metal ion,
the imprinted polymer strategies could be followed by conduc-
ting the electro-polymerization in the presence of the metal ion of
interest.
This journal is ª The Royal Society of Chemistry 2011
Fig. 1 Chemical structures of flavonoids: (a) general structure, (b)
apigenin.
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Two different approaches for the modification of glassy
carbon electrodes using a mercury film and mercury-Nafion have
been compared byMerkoci et al.32 They have used the mixture of
mercury(II) chloride solution with a Nafion solution diluted in
ethanol to coat the polished glassy carbon surface. Poly(2-
amino-4-thiazoleacetic acid)/multiwalled carbon nanotubes
modified glassy carbon electrodes obtained by electro-
polymerization of 2-amino-4-thiazoleacetic acid, have been used
for the voltammetric determination of copper ions by Zhao
et al.21 They have evaluated the voltammetric response of copper
ions at poly(2-amino-4-thiazoleacetic acid)/multiwalled carbon
nanotubes modified glassy carbon electrodes by differential pulse
stripping voltammetry. Square-wave anodic-stripping voltam-
metry (SWASV) has been set up and optimized for simultaneous
determination of cadmium, lead, and copper in siliceous spicules
of marine sponges, directly in the hydrofluoric acid solution
(�0.55 mol L�1 HF, pH �1.9) by Truzzi et al.22 They have used
a thin mercury-film electrode (TMFE) plated on to an HF-
resistant epoxy-impregnated graphite rotating-disc support. The
complexing properties of poly(3-(pyrrole-1-yl)propylmalonic
acid) and poly(N,N0-ethylenebis[N-[(3-(pyrrole-1-yl)propyl)car-
bamoyl) methyl]-glycine coated electrodes towards Cu(II), Pb(II),
Hg(II) and Cd(II) cations using the open circuit chemical pre-
concentration anodic stripping technique have been studied by
Pereira et al.23 The electrochemical behavior of a series of metal
ions (Ag(I), Hg(II), Cu(II), Pb(II), Cd(II)) and the ternary Cu(II)–
Pb(II)–Cd(II) system in the solutions of water-soluble complexing
polymers poly(ethylenimine) (PEI), poly(1-vinyl-2-pyrrolidone)
(PVP), their thiourea-containing derivatives poly(ethyleneimine)
methylthiourea (PEI-TU) and poly(1-vinil-2-pyrrolidone)
methylthiourea (PVP-TU) has been investigated using cyclic and
anodic stripping voltammetry (ASV) at different carbon elec-
trodes by Osipova et al.25 A novel method of generating a rapidly
renewable and reproducible polymer coated electrode surface has
been proposed by Khoo and Guo.24 This involves in situ electro-
polymerization at a monomer modified carbon paste electrode.
They have used a carbon paste electrode bulk modified with 2-
methyl-8-hydroxyquinoline to demonstrate this approach. The
polymer modified carbon paste electrode obtained by electro-
polymerization was found to be useful for trace determination of
Cu(II), involving pre-concentration and anodic stripping proce-
dures. Polyviologen has been formed on glassy carbon electrodes
using potentiostatic electro-polymerization in pH 4.2 Britton-
Robinson buffer solution by Hsu et al.38 They have employed the
polyviologen-modified glassy carbon electrode (PVGCE) to
determine Cu(II) in chloride-rich solutions in order to demon-
strate the electroanalytical application of polyviologen. For the
synthesis of complexing polymer film modified electrodes, the
oxidative electro-polymerization of (3-pyrrol-1-ylpropyl)malonic
acid monomer has been performed by Heitzmann et al.39 They
have applied this electrode to the electroanalysis of Cu(II), Pb(II),
Cd(II) and Hg(II) ions by pre-concentration upon complexation,
followed by anodic stripping analysis.
Modification of carbon surfaces is an important objective in
electrochemistry and material science. In electrochemistry,
carbon electrodes are widely used because of their low back-
ground current, low cost, wide potential window, speed, low
equipment, chemical inertness and minimum sample pretreat-
ment required prior to analysis.26,27 Electrochemical methods are
This journal is ª The Royal Society of Chemistry 2011
based on the direct oxidation or reduction of a substrate on an
electrode surface. Electrode reactions are very suitable for
analytical applications due to their requirements of high poten-
tial. Moreover, these surfaces can be modified by a reductive
substrate for analytical applications. Recently, the application of
inorganic modified electrodes has increased.28–31
Flavonoids are the best example of polyphenols. The flavonoid
term refers to a class of aromatic, oxygen-containing heterocyclic
pigments widely distributed among higher plants as secondary
metabolites. Flavonoids constitute one of the most characteristic
classes of compounds containing hydroxyl groups attached to
ring structures.26 Many flavonoids are easily recognized as flower
pigments in most angiosperm families. However, their occur-
rence is not restricted to flowers but includes all parts of the
plants. They constitute most of the yellow, red and blue colors in
flowers and fruits.27 Flavonoids are broken down into categories
of isoflavones, anthocyanidins, flavans, flavonols, flavones, and
flavanones.32 The molecule structure of apigenin, a derivative of
flavonoids, is given in Fig. 1.
The main purpose of this study was to demonstrate an elec-
trochemically modified PolyApi/GC electrode in aqueous media
by CV, characterize the PolyApi/GC electrode by CV and EIS,
propose the structure of the complex formed between the Poly-
Api/GC electrode with Cu(II), investigate the interference effects
and apply the PolyApi/GC sensor electrode for Cu(II) determi-
nation at trace levels in soil samples for the first time.
2. Experimental
2.1. Chemicals, electrodes and apparatus
Apigenin and other chemicals were of analytical-reagent grade
supplied from Sigma-Aldrich. Ultra pure quality water with
a resistance of 18.3 MU cm (Millipore Milli-Q purification
system, Millipore Corp. Bedford, MA, USA) was used in
preparations of aqueous solutions, cleaning of the glassware and
polishing the electrodes. Apigenin solution used in modification
was prepared in 1 mM concentration in 10 mL acetonitrile
(MeCN) + 40 mL PBS, pH 7, mixture. The PBS was prepared by
mixing 0.05 mM Na2HPO4 and 0.05 mM KH2PO4 and then
adjusting the pH by addition of NaOH or HCl. CuSO4$5H2O
solutions were prepared at different concentrations (ranging
from 1.0� 10�11 M to 1.0� 10�6 M) in BR buffer solution, pH 5,
which was prepared from H3PO4 + CH3COOH + H3BO3
according to preparation conditions in the literatures33,34 and
then adjusting the pH by addition of 0.2 M or 1 M NaOH. A
traditional three-electrode cell system was used in all electro-
chemical and spectroelectrochemical experiments. In our experi-
ments, a GAMRY Reference PCI4/750 series Potentiostat/
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Galvanostat/ZRA from GAMRY Instruments (PA, USA) elec-
trochemical analyzer with BAS (Bioanalytical Systems, West
Lafayette, IN, USA) Model MF-2012 and Tokai GC-20 GC
electrodes were used. Ag/Ag+ (10 mMAgNO3) (BASModel MF-
2042) for non-aqueous media and a Ag/AgCl/3 M KCl (BAS
Model MF-2063) for aqueous media were used as reference
electrodes. Pt wire (BASModel MW-1032) was used as a counter
electrode. A Jenway 3010 pH meter was used for the measure-
ment of pH values. The CV technique was applied with PHE 200
software, EIS was applied with EIS 300 software and DPV was
applied with PV 220 software. The morphology of apigenin film
on the GC electrode surface was investigated by using SEM Carl
Zeiss LS10 Series SEM, Missouri, USA.
2.2. Preparation and modification of the working electrodes
The GC electrodes were prepared for the experiments by poli-
shing to gain a mirror-like appearance, first with fine wet emery
papers (grain size 4000) and then with 1.0 mm and 0.3 mm
alumina slurry on micro cloth pads (Buehler, USA). After the
initial polishing, the GC electrodes were resurfaced with 0.05 mm
alumina slurry. First, in the following order, the GC electrodes
were sonicated both in water, and in 1 : 1 (v/v) isopropyl alcohol
(IPA) and MeCN (IPA + MeCN) mixture for 10 min.34–37
The electrochemical modification of the GC electrode was
performed with 1 mM apigenin in 10 mL MeCN + 40 mL PBS,
pH 7, mixture from 0 mV to +1400 mV potential range, using
100 mV s�1 sweep rate and 30 cycles.
3. Result and discussion
3.1. Modification and characterization of apigenin on the GC
surface
In this study, the stability of the GC electrode surfaces modified
with apigenin in aqueous medium was investigated and the
PolyApi/GC electrode obtained by polymerizing with multi-
cycles (30 cycle) after the modification in aqueous medium was
used as the sensor electrode for the determination of Cu(II) ions.
The cyclic voltammogram of the apigenin modified GC electrode
surface is shown in Fig. 2.
Fig. 2 Cyclic voltammogram of 1 mM PolyApi/GC in 10 mL MeCN +
40 mL PBS mixture, pH 7, vs. Ag/AgCl/(3 M KCl), 1st (a) and 30th (b)
cycles. Sweep rate is 100 mV s�1.
2536 | Anal. Methods, 2011, 3, 2534–2539
Surface characterizations after the modification process were
carried out by CV and EIS. In the characterizations with CV,
1 mM ferrocene solution in 0.1 M tetrabutylammonium tetra-
fluoroborate (TBATFB) was carried out in the potential range
from �200 mV to +500 mV in Fig. 3A and 1 mM Fe(CN)63� in
BR buffer solution, pH 2.0, was performed the potential range
from +600 mV to 0.0 mV in Fig. 3B at a sweep rate of 100 mV s�1.
The surface voltammograms of the modified electrode were
compared with surface voltammograms of the bare GC elec-
trode. The electrode surface was negatively charged after the
modification process. Thus, negatively charged ferrocyanide ions
are repelled by the negatively charged electrode surface. Conse-
quently, no electron transfer occurs.
Impedance measurements were carried out in 1 mMFe(CN)63�
and Fe(CN)64� mixture (in 0.1 M KCl) in the range from 100.000
Hz to 0.05 Hz frequency and the Nyquist plots were recorded.
The Nyquist plot of the modified electrode was compared with
the EIS data of the bare GC electrode. The Nyquist plots of the
EIS investigations are shown in Fig. 4.
In addition to CV and EIS measurements, SEM was applied
for characterization of the bare GC and PolyApi/GC layers
grafted on the GC electrode surface. The SEM images are pre-
sented in Fig. 5. The bare GC electrode surface is shown in
Fig. 5A. The granular structure of the PolyApi/GC electrode
(Fig. 5B) formed a larger surface area suitable for more efficient
binding of apigenin molecules.
3.2. Detection of Cu(II) on modified PolyApi/GC electrode by
DPV
The complex of Cu(II) ions in BR buffer solution at pH 5 with
apigenin which was oxidized on the modified GC electrode
surface was studied by the DPV technique (Fig. 6). Prior to the
complex formation, the apigenin modified GC electrode surface
was characterized by the CV and EIS techniques.
In DPV experiments, the potential range was from �400 mV
to 0.0 mV, the potential sweep rate was 50 mV s�1, the pulse
amplitude was 25 mV, the pulse period was 0.05 s and the sample
period was 1.0 s. For the optimum conditions, the pH of Cu(II)
solution and modified PolyApi/GC electrode incubation time
were investigated. For this aim, 1.0 � 10�6 M Cu(II) solutions
were prepared in BR buffer solution at 2–12 pH range. The
modified electrodes were incubated in these Cu(II) solutions and
then Cu(II) ions on PolyApi/GC electrode surface were
Fig. 3 Cyclic voltammograms of the bare GC and PolyApi/GC. A)
1 mM ferrocene redox probe solution vs. Ag/Ag+ (10 mM) in MeCN +
0.1 M TBATFB, a) bare GC and b) PolyApi/GC. B) 1 mM Fe(CN)63�
redox probe solution vs. Ag/AgCl/ (3 M KCl) in BR buffer solution, pH
2.0, a) bare GC and b) PolyApi/GC. Sweep rate was 100 mV s�1.
This journal is ª The Royal Society of Chemistry 2011
Fig. 4 Nyquist plots of 1 mM of Fe(CN)63�/Fe(CN)6
4� in 0.1 M of KCl
of bare GC (a), and PolyApi/GC electrode (b). Frequency range is from
100.000 Hz to 0.05 Hz, the modulation amplitude is 10 mV. Inset:
Equivalent circuit applied for calculations.
Fig. 5 SEM images of (A) bare GC, (B) PolyApi/GC electrode surfaces.
Fig. 6 Differential pulse voltammograms of different concentrations of
CuSO4$5H2O for a) bare GC electrode surface and b) 1 � 10�11; c) 1 �10�10, d) 1 � 10�9, e) 1 � 10�8, f) 1 � 10�7, g) 1 � 10�6 on the PolyApi/GC
electrode surface. The measurements were performed in BR buffer
solution, pH 5.0, vs. Ag/AgCl/(3 M KCl). Sweep rate was 50 mV s�1.
Fig. 7 A) Oxidation of apigenin on the GC electrode surface and B) the
proposed structure of the complex formed between the PolyApi/GC
electrode with Cu(II) ion.
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determined using DPV. Similar to the literature,40 the optimum
pH value was determined as 5 for the determination of Cu(II)
ions. Cu(II) can’t be detected due to the precipitation of Cu(II) as
hydroxide at higher pH value. The optimum incubation time was
determined by incubating the PolyApi/GC electrode in Cu(II)
solutions in BR buffer solution, at pH 5.0, for different time
periods (30, 60, 90, 120, 150, 180 min). As the incubation time
increased, the DPV signals also increased up to 120 min
This journal is ª The Royal Society of Chemistry 2011
incubation time. Above this time, the steady state was achieved.
The optimum conditions for the most Cu(II) complexation with
apigenin on the GC electrode surface are as follows: BR buffer
solution, pH 5.0, incubation time 120 min. The Cu(II)-apigenin
complex was investigated using the DPV technique. Electro-
chemical oxidation of apigenin on the GC electrode surface and
the DPVs of the complex41 formed between the apigenin with Cu
(II) ion are shown in Fig. 7A and 7B.
3.3. Calibration curve and calculations
A series of CuSO4$5H2O from 1 � 10�11 M to 1 � 10�6 M was
prepared for the calibration curve for the determination of Cu(II)
ions in soil samples at optimum conditions. First, the modifica-
tion of the GC electrode surface with apigenin was done by CV,
and then the surface voltammograms of PolyApi/GC electrodes
by the DPV technique following the incubation of these elec-
trodes in the prepared solution for 120 min. A calibration curve
of Cu(II) concentration versus peak current obtained from the
voltammograms was drawn.
The calibration graph is linear in the range from 1 � 10�11 M
to 1 � 10�6 M Cu(II) ions under the optimum conditions of the
general procedure. According to the following equation for Cu
(II) determination: Ip¼ 0.527C� 2,599, Ip is the peak current and
C is the Cu(II) concentration. The correlation coefficient (R2) was
0.998. Cu(II) ions were determined under the optimum conditions
in soil samples.
3.4. Interference effects
The interferences of some ions on the determination of Cu(II)
were investigated. The PolyApi/GC electrode was incubated in
a mixture of ions (cation ions: Cd2+, Ni2+, Co2+ and Zn2+, anion
ions: NO3�, SO4
2�, CO32� and Cl�, (1.0� 10�6 M each one)). The
voltammogram of the PolyApi/GC electrode was taken using the
differential pulse technique after 120 min duration. The DPVs
after incubation in solution of Cu(II) ions with the interference
ions were compared. The tolerance limit is defined as the ion
concentration causing a relative error smaller than �5% related
to the determination of Cu(II) ions. The ions normally present in
water do not interfere under the experimental conditions used.
This modified electrode can be used successfully for the deter-
mination of Cu(II) ions in the presence of different interferents.
3.5. Determination of Cu(II) ions in soil samples
The proposed method was successfully used for the determina-
tion of Cu(II) ions in soil samples in Meram region in Konya,
Anal. Methods, 2011, 3, 2534–2539 | 2537
Fig. 8 Differential pulse voltammogram of Cu(II) ions in soil sample on
the a) bare GC and b) PolyApi/GC electrode surfaces. The measurement
was performed in BR buffer, pH 5.0, potential is referred vs. Ag/AgCl/
(3 M KCl).
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Turkey without any pretreatment. In this aim, 10 g of the soil
sample taken from the area where the soil was not covered by
decayed leaves and other organic substances, was kept in 50 mL
BR buffer solution, pH 5, for 24 h. Then the mixture was filtered.
10 mL portion of this filtrated solution was used for the Cu(II)
ions analysis.
The apigenin modified GC electrode was incubated for two
hours in the prepared mixture. The voltammogram of the incu-
bated PolyApi/GC electrode was taken by differential pulse
technique in BR buffer solution at pH 5 (Fig. 8). After the
binding of the Cu(II) ions to the apigenin modified GC surface,
the peak current was measured and then, the obtained peak was
used to find the Cu(II) concentration in the soil sample from the
calibration curve by interpolating the peak current obtained
from the voltammogram. The concentration of Cu(II) ions in the
real sample is found to be 0.734 mM.
4. Conclusions
The voltammetric technique, an electrochemical technique, is
advantageous compared to the others because this technique is
inexpensive and reliable. Besides, all colored and turbid solutions
can be easily analyzed using the voltammetric technique. For this
reason, we tried to develop a specific sensor electrode for the
determination of Cu(II) ions in aqueous media by modifying the
GC surface using apigenin in aqueous-acidic media. Although
Cu(II) ions have been determined for years using various tech-
niques, there are few studies for the determination of Cu(II) ions
with a sensor electrode using an electrochemical technique.
Although similar studies have been done using related molecules
by other researchers, our study has the advantage that the newly
developed sensor electrode has been applied to natural samples.
In some studies natural samples have been used for Cu(II)
determination. However, the low detection limit of our study is
the other advantage of this study. This applied method to soil
samples can be easily applied to water, food and air samples
similarly. This study was successfully applied to soil samples for
Cu(II) ions determination. Lastly, by using this developed sensor
electrode one can easily quantitatively determine Cu(II) at very
low concentrations. The main advantages of this proposed
2538 | Anal. Methods, 2011, 3, 2534–2539
method are that the electrochemical modification of apigenin on
the GC electrode in aqueous media at neutral pH is reported, the
PolyApi/GC electrode is developed for determination of Cu(II)
ions for the first time, the polyphenol structure has a significant
role in the formation of complexes with Cu(II) ions, the proposed
method is simple, sensitive and quick, the determination of Cu(II)
ions is carried out in soil samples without any pretreatment, it is
cheap with no need of using expensive reagents or equipment,
and it has a low detection limit.
Acknowledgements
This study was financially supported by the Research Founda-
tion of Selcuk University, Konya-TURKEY (BAP-09401118).
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