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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: iemulazimoglu@gmail.com; Fax:+90 332 3238225; Tel: +90 332 3238220bDepartment of Chemistry, Faculty of Science, Ankara University, Ankara,Turkey. E-mail: osolak@science.ankara.edu.tr; 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/

Anal. Methods, 2011, 3, 2534–2539 | 2535

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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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