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Int. J. Electrochem. Sci., 11 (2016) 1213 - 1227
International Journal of
ELECTROCHEMICAL
SCIENCE
www.electrochemsci.org
Influence of Oxidation Stage and Exfoliation Extent of Carbon-
Based Materials on Electrochemical Detection of As(III)
Monika Kremplova1,2, Lukas Richtera1,2, Pavel Kopel1,2, Renata Kensova1,2, Iva Blazkova1,2,
Vedran Milosavljevic1, David Hynek1,2, Vojtech Adam1,2, Rene Kizek1, 2*
1 Department of Chemistry and Biochemistry, Faculty of Agronomy, Mendel University in Brno,
Zemedelska 1, CZ-613 00 Brno, Czech Republic, European Union
2 Central European Institute of Technology, Brno University of Technology, Technicka 3058/10, CZ-
616 00 Brno, Czech Republic, European Union
*E-mail: kizek@sci.muni.cz
Received: 5 May 2015 / Accepted: 23 May 2015 / Published: 1 January 2016
This study deals with the electrochemical detection of As(III) and especially with its interaction with
carbon-based materials such as graphene oxide, graphite oxide and partially reduced graphene oxide in
connection with the adsorption of As(III) to their surface. Using differential pulse voltammetry and
atomic absorption spectrometry, it was found that the As(III) reaches the best adsorption efficiency on
the carbon-based materials surfaces at very acidic pH after 1 hour interaction. We decided to use this
promising ability of graphene oxide to design a new method for As(III) detection using a modified
glassy carbon electrode (GCE). In order to enable detection of As(III), the surface of glassy carbon
electrode was firstly modified by gold nanoparticles (AuNPs). After this step the working electrode
was modified for with graphene oxide to enhance the electrochemical signal of As(III). Compared to
the working electrode modified only by gold nanoparticles, the electrochemical signal of As(III) on the
GCE/AuNPs/GO increased by approx. 50%. Interaction time of As(III) on graphene oxide (located on
the surface of the working electrode) demonstrated the process of As(III) adsorption to the surface,
which is manifested by increasing electrochemical signal of As(III) in individual time intervals.
Keywords: arsenic, graphene oxide, reduced graphene oxide, gold nanoparticles, glassy carbon
electrode, differential pulse voltammetry
1. INTRODUCTION
Arsenic in the form of non-volatile inorganic compounds occurs naturally in the Earth's crust
[1]. At low concentrations, it may be present in the aquatic environment and in atmosphere mostly due
to weathering and erosion of rocks and minerals, volcanic activity and biological processes [2,3].
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Higher concentrations of arsenic in the environment result from anthropogenic activities. Its
occurrence is mainly associated with the combustion of fossil fuels and metallurgical processes [1-3].
Appreciable concentrations of arsenic are released during burning of wood treated by chemicals
containing arsenic compounds, or with pesticide application [4,5]. In surface water and groundwater
arsenic exists in both organic and inorganic forms in the oxidation states of As(III) - arsenite and
As(V) – arsenate [6]. The most common organic forms include monomethylarsonic acid and
dimethylarsinic acid [1,7] which arise biosynthetically, mostly as a metabolic product of
microorganisms [8-10]. Inorganic forms of arsenic occur in natural waters more often and exhibit
higher toxicity than organic forms.
In many countries of the world, higher levels of arsenic exceeding the permitted limits were
recorded in industrial and drinking water, vegetables, cereals, fish, meat and milk [11-15]. Arsenic
toxicity is the cause of a number of diseases and tissue damage, such as atherosclerosis, hypertension,
skin lesions, mucous membrane disorders and nervous system damage [16-19]. Excessive exposure to
arsenic can lead to gastrointestinal and cardiovascular problems, and in the worst cases can have
genotoxic, mutagenic and carcinogenic effects [20-22].
With regards to environmental importance of arsenic, new ways of detection offering fast,
inexpensive and routine detection of this element are of high importance, as well as the optimization of
existing processes for detection of arsenic to increase the sensitivity and reproducibility. Nowadays
there is a number of methods usable for detection of arsenic, e.g. atomic fluorescence spectrometry
[23-25], hydride generation-atomic absorption spectrometry (HG-AAS) [26,27], inductively coupled
plasma mass spectrometry [28-31], electrospray mass spectrometry [32]. These methods are high-cost,
require expensive laboratory equipment, frequently a complicated sample preparation is required, and
last but not least, they tend to be time consuming. A preferred alternative to these techniques may be
electrochemical methods with the advantage of easy handling, high sensitivity of determination and the
relatively low cost with the low price of individual determination. It is possible to determine both
As(III) and As(V) forms of arsenic electrochemically, however it requires chemical pretreatment of
As(V) in the sample to As(III) form [33]. For the determination of arsenic in aqueous solutions several
electrochemical methods like cyclic voltammetry (CV), differential pulse voltammetry, linear sweep
voltammetry, square wave voltammetry and various types of working electrodes (mercury, glassy
carbon, gold, etc.) can be used [34-37]. The advantage of solid-state electrodes is a possibility of
modifications using different kinds of nanomaterial [38-40], which allows increasing the sensitivity of
detection.
The aim of this study is the monitoring of the trivalent arsenic, especially its adsorption on the
surface of the carbon-based materials such as graphene oxide, graphite oxide and partially reduced
graphene oxide. The method used for As(III) determination after the interaction with carbon based
materials was differential pulse voltammetry on hanging mercury drop electrode. Selected carbon-
based material with best adsorption properties was further used for second modification of glassy
carbon electrode. For determination of As(III) on glassy carbon electrode, this electrode was firstly
modified by gold nanoparticles and next the second modification step was based just on the carbon
material to increase the sensitivity of the electrochemical determination of As(III) using the ability of
its adsorption onto the sorbent surface.
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2. EXPERIMENTAL PART
2.1 Chemicals and material
All chemicals used in this study were purchased from Sigma Aldrich (St. Louis, MA, USA) in
ACS purity unless noted otherwise. Pipetting was performed by pipettes from Eppendorf (Hamburg,
Germany). Stock solutions were prepared with ACS water. High purity deionized water (Milli-Q
Millipore 18.2 MΩ/cm, Bedford, MA, USA) was used for rinsing, washing, and buffer preparation, pH
values were measured using an inoLab Level 3 (Wissenschaftlich-Technische Werkstatten GmbH,
Weilheim, Germany).
2.2. Preparation of graphite oxide
The preparation of graphite oxide was done according to the standard method of Hummers
[41]. The graphite (2 g) was added to 46 mL of concentrated sulfuric acid and mixed by stirring and
cooled with ice, followed by an addition of 1 g NaNO3 and 6 g of KMnO4. The mixture was left for 24
h at laboratory temperature, in order to thicken it. All the black graphene oxide was stirred in 300 mL
of ACS water.
2.3. Preparation of graphene oxide
Graphite oxide for graphene oxide (GO) synthesis was prepared by chemical oxidation of 5.0 g
graphite flakes in a mixture of concentrated H2SO4 (670 mL) and 30 g KMnO4 according to the
simplified Hummer's method [42]. The reaction mixture was stirred vigorously. The oxidation process
manifests outwardly by the gradual color change from dark purplish green to dark brown. After 4 days,
the oxidation of graphite was terminated by slow addition of H2O2 solution (250 mL) and the color of
the mixture turned to bright yellow, indicating high oxidation level of graphite. Formed graphite oxide
was washed 3 times with 1 M of HCl and repeatedly several times washed with deionized water (total
volume used - 12 L) until constant pH value (3–4) was achieved using a simple decantation until it was
possible and using centrifugation at the last steps. During the washing process with deionized water
exfoliation of graphite oxide led to the thickening of solution and formation of stable colloid of
graphene oxide.
2.4. Preparation of partially reduced graphene oxide
The synthesis of partially reduced GO was performed according to the Gao at al. [43]
procedure but simplified. Solution of graphene oxide was diluted with deionized water to
concentration 1 g/L (total volume 140 cm3) and pH of solution was adjusted by 5% sodium carbonate
solution to value 9.5. Sodium borohydride (1.15 g, reagent grade, 98.5%, Sigma-Aldrich) was slowly
added into the solution under magnetic stirring and then heated to 80°C. Reaction mixture was kept at
this temperature and under stirring for 3 h (till the formation of gaseous products can be observed).
Int. J. Electrochem. Sci., Vol. 11, 2016
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The reduction process was manifested by turning brown color of solution to black. Reduced product
was washed with deionized water to remove impurities and separated by centrifugation several times
until constant pH value (6–7) was achieved.
2.5. Preparation of samples for interaction with As(III) in different pH values
250 µL (5 mg/mL) of carbon material (graphite oxide, graphene oxide, partially reduced
graphene oxide) was mixed with 375 µL of As(III) standard solution (100 μM) and followed by
addition of 375 µL ACS water, hydrochloric acid or sodium hydroxide depending on required pH
value. Interaction of As(III) compound and carbon materials was carried out for 1 hour under shaking
(BIOSAN, Multi RS-60) at room temperature. After the interaction, the sample was centrifuged
(BIOSAN, FVL 2400N, Combi-Spin) for 30 min. The supernatant was carefully removed using a
syringe with a needle and filtered through a membrane filter (0.45 μm). The concentration of As(III) in
supernatant was detected by differential pulse voltammetry and compared with applied As(III)
concentration to observe the rate of adsorption.
2.6. Preparation of samples for time-interaction with As(III)
250 µL (5 mg/mL) of carbon materials (graphite oxide, graphene oxide, partially reduced
graphene oxide) was mixed with 375 µL of As(III) standard solution (100 μM) followed by addition of
375 µL of concentrated hydrochloric acid. Interaction of arsenic and carbon material was carried out in
five different time intervals (0, 30, 60, 90 and 120 minutes). Shaking, centrifugation, filtration and
As(III) determination was carried out under the same conditions as reported in section 2.5. (see above).
2.7. Preparation of gold nanoparticles
19.7 mg HAuCl4·3H2O was dissolved in 50 mL of Milli-Q water to form yellow solution (1
mM) followed by addition 1.25 mL of sodium citrate solution (0.1 M) was added. Reaction mixture
was stirred for 1 h at ambient temperature until the color of the solution turned purple [44].
2.8. Preparation of modified glassy carbon electrode
For modification of glassy carbon electrode, gold nanoparticles and graphene oxide were used.
At the first step, the surface of GCE was thoroughly polished using Alumina powder 0.05 µm (CH
Instruments, USA). On the clean GCE surface drop of AuNPs solution was applied and the fluid was
slowly evaporated at 30°C to form thin AuNPs layer. The second step of the modification included
covering the AuNPs layer with graphene oxide. Therefore, drop of graphene oxide suspension was
applied onto dried AuNPs layer and after subsequent evaporation of fluid at 30°C the modified GCE
was ready to use.
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2.9. Electrochemical determination of Arsenic using hanging mercury drop electrode
Determination of As(III) by differential pulse voltammetry were performed with 797 VA
Computrace (Metrohm, Switzerland), using a standard cell with three electrodes. A hanging mercury
drop electrode with a drop area of 0.4 mm2 was the working electrode. An Ag/AgCl/3M KCl electrode
was the reference and platinum electrode was auxiliary. For data processing 797 VA Computrace
software by Metrohm CH was employed. The analyzed samples were deoxygenated prior to
measurements by purging with argon (99.999%). 0.75 M hydrochloric acid with addition of CuSO4
(1g/L) as a supporting electrolyte was used. The supporting electrolyte was exchanged after each
analysis. The parameters of the measurements were as follows: initial potential of -0.1 V, end
potential - 1.2 V, deoxygenating with argon 120 s, deposition 0 s, pulse amplitude 0.05 V, pulse time
0.04 s, voltage step 0.003 V, voltage step time 0.4 s, sweep rate 0.0076 V/s, volume of injected
sample: 20 µL, volume of injected CuSO4: 110 µL, volume of measuring cell 2 mL (20 μL of sample +
110 μL of CuSO4 + 1870 μL acetate buffer).
2.10. The microscopy of complexes in ambient light
The inverted system microscope Olympus UIS2 series (Tokyo, Japan) was used for the
imaging of the carbon materials. Samples were pipetted on a microscope slide and covered by a cover
slip. The objective CPlanFLN 10× (N.A. 0.3, W.D. 9.5 mm, F.N. 22) was used for magnification 100×
and objective LUCPlanFLN 40× (N.A. 0.6, W.D. 2.7 – 4 mm, F.N. 22) was used for magnification
400×. The images were captured by Camera Olympus DP73 and processed by Stream Basic 1.7
Software, the resolution of the images was 1600 × 1200 pixels, ISO 200.
2.11. Scanning electron microscopy (SEM)
Structure of carbon materials was characterized by SEM. For documentation of the structure, a
MIRA3 LMU (Tescan, Brno, Czech Republic) was used. The SEM was fitted with In-Beam SE
detector. An accelerating voltage of 15 kV and beam currents about 1 nA gave satisfactory results.
2.10. Electrochemical determination of As(III) using modified glassy carbon electrode
Determination of As(III) by linear sweep voltammetry was performed by CH Instruments
Electrochemical Workstation (CH Instruments, USA) using a system of 3 electrodes. A modified
glassy carbon electrode was used as the working electrode; an Ag/AgCl/3M KCl was used as the
reference electrode, and a platinum wire as an auxiliary one. The analyzed samples were deoxygenated
prior to measurements using 99.999% argon for 120 s and 0.75 M hydrochloric acid was used as
supporting electrolyte. The parameters of the measurement were chosen as follows: initial potential of
-0.4 V, end potential 0.5 V, sample interval 0.001 V, preconditioning potential -0.4 V, deposition time
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60 s, scan rate 0.1 V/s. Volume of injected sample: 20 µL, volume of electrochemical cell was 2 mL
(20 µL of sample + 1980 µL of electrolyte).
2.11. Atomic absorption spectrometry (AAS)
Total arsenic was determined on 280Z Agilent Technologies atomic absorption spectrometer
(Agilent, USA) with electrothermal atomization. Arsenic ultrasensitive hollow cathode lamp (Agilent)
was used as the radiation source (lamp current 10 mA). The spectrometer was operated at 193.7 nm
resonance line with spectral bandwidth of 0.5 nm. The sample volume of 20 µL was injected into the
graphite tube. The flow of argon inert gas was 300 mL/min. Zeeman background correction was used
with field strength of 0.8 Tesla. Arsenic was determined in the presence of palladium chemical
modifier.
3. RESULTS AND DISCUSSION
Materials based on graphene oxide and reduced graphene oxide exhibit convenient sorption
abilities towards heavy metals, which can be used e.g. for decontamination of surface water and
wastewater polluted by these metals. Promising sorption capacity has been also reported to other
substances and elements of metallic or non-metallic nature [45-47]. Mentioned adsorption ability is
also one of the key steps in the determination of electrochemically active compounds. In
electrochemical assays, several physico-chemical processes, e.g. diffusion, oxidation-reduction
processes, the preconcentration of an analyte on electrode surface by preliminary electrolysis, and
finally adsorption are used [48,49]. Carbon-based materials (graphite oxide, graphene oxide, partially
reduced graphene oxide) were used for interacting with arsenic and for subsequent study of the
electrochemical behavior of this transition metal in their presence, due to their sorption abilities and
surface characteristics. It was expected, if the material exhibits a sorption ability towards specific
substance, it could be also a promising material for modification of the working electrode designed for
the detection of given analyte. With respect to these facts, three carbon-based materials were studied
and the material with the best qualities was then selected for modification of the glassy carbon
electrode.
3.1. Electrochemical determination of As(III) using hanging mercury drop electrode
For detection of arsenic in aqueous solutions and for the verification of sorption properties of
carbon-based materials, an electrochemical method differential pulse voltammetry as one of the most
sensitive techniques for the determination of metal ions, metalloids and other electrochemically active
substances have been used [50,51]. As a standard, solution of trivalent arsenic AsCl3 was used. The
addition of chloride hydrazine as the reducing agent is necessary in order to avoid spontaneous
oxidation of trivalent arsenic to a higher oxidation state. For the electrochemical determination of
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As(III) on the mercury working electrode there is strictly necessary to add Cu(II) ions, which leads to
the formation of the copper amalgam on the surface of the mercury electrode, and subsequently to
form an intermetallic compounds Cu3As2. At highly acidic pH used in cathodic stripping voltammetry,
this compound passes into AsH3, which provides an electrochemical signal [36].
Figure 1. Optimization of the parameters of the method for As(III) determination: (A) dependence of
relative peak height on the addition of CuSO4 (1g/L), red points indicates a potential values of
As(III) peak; (B) influence of deposition potential (0, -0.1, -0.2, -0.3, -0.4, -0.5, -0.6, -0.7, -0.8
and -0.9 V) on relative peak height of As(III); (C) dependence of relative peak height of As(III)
on deposition time (0, 60, 120, 180, 240 and 300 s); (D) calibration curves in concentration
range 0.78 – 12.5 µg/mL and 12.5 – 100 µg/mL containing inserted voltammograms (each line
for one measured concentration in concentration range 0.78 – 100 µg/mL, measured by
differential pulse voltammetry). 0.75 M hydrochloric acid with CuSO4 addition (1g/L) was
used as a supporting electrolyte. Parameters of the method were as follows: initial potential -0.1
V, end potential - 1.2 V, deoxygenating with argon 120 s, deposition 0 s, pulse amplitude 0.05
V, pulse time 0.04 s, voltage step 0.003 V, voltage step time 0.4 s, sweep rate 0.0076 V/s.
For obtaining a higher sensitivity of the method, an individual parameters were optimized. The
addition of copper ions was the first optimized parameter because Cu(II) ions influence the intensity
and the quality of the obtained electrochemical signal of As(III). The volume of added CuSO4
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(concentration of Cu(II) 1g/L) to the supporting electrolyte was gradually increased in the range 60 -
130 mL. From Figure 1A, it is clear that with increasing concentration of Cu(II) ions in the electrolyte
the electrochemical signal of As(III) increased and the peak potential shifted to more negative values.
The highest signal was obtained by the volume of 130 mL, but at this point an unstable potential of the
characteristic peak of As(III) was recorded. At applied volume of 110 mL, stable electrochemical
response was recorded and for this reason, this volume of CuSO4 was used for all other
electrochemical determination of As(III) using a mercury electrode.
Further, the deposition potential was tested (Figure 1B) in the values 0; -0.1; -0.2; -0.3; -0.4; -
0.5; -0.6; -0.7; -0.8 and -0.9 V. In this case, there was no dependence between the electrochemical
signal of As(III) and decreasing values of the applied potential. The best electrochemical response was
observed at deposition potential -0.1 V that was also selected as an initial potential during this method.
Deposition time associated with deposition potential was the next optimized parameter. Figure 1C
shows an increasing electrochemical signal of As(III) depending on the increasing deposition time.
The peak potential was shifted to more negative values. The increase in the peak height of As(III)
(compared zero) and the maximum applied deposition time did not exceed 25%, therefore considering
the length of the individual assay the deposition time of 0 s was used in all subsequent measurements.
After optimization steps, the calibration curve of As(III) in concentration range 0.78 –
100 µg/mL was determined. Due to the relatively wide concentration range, two areas of the
calibration curve with a linear course were selected. Between 12.5 – 100 µg/mL the concentration
dependence has a linear course with regression equation y = 4.4798x + 36.168 (n = 4) and coefficient
of determination R2 = 0.9978. A calibration curve in the lower concentration range (0.78 – 12.5
µg/mL) with linear course, regression equation y = 6.9344x – 3.5162 and coefficient of determination
R2 = 0.9987 is shown in the upper inset of the Figure 1D. . Characteristic peak of As(III) was detected
in potential of -0.68 V. Characteristic voltammograms are shown in the lower inset of Figure 1D.
3.2. Characterization of carbon-based materials using microscopy in ambient light and SEM analysis
In this study, the carbon-based materials such as graphite oxide, graphene oxide and partially
reduced graphene oxide were used because of their sorption properties and high surface area. For
verification of the carbon materials structure, the microscopy in an ambient light and SEM analysis
were used. Using microscopy in the ambient light, the Figure 2Aa (magnification 100×) shows
particles with size of several micrometers. Darker particles represent unexfoliated material (graphite
oxide) which does not meet the parameters of nanoparticles. At higher magnification (Figure 2Ab,
magnification 400×) disparate thickness of individual particles is more evident, while larger lightproof
particles probably contain a higher proportion of non-oxidized graphite. In the case of graphene oxide
in Figure 2Ba particle sizes were much smaller than in the previous case, the fundamental difference is
greater homogeneity of the sample and only a minor presence of particles with high thickness. The
graphene oxide nanoparticles are very thin due to almost perfect exfoliation, their thickness not
exceeding units or tens of nanometers, and therefore their edges are typically less recognizable for
microscopy in ambient light (Figure 2Bb).
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Figure 2Ca represents the structure of the partially reduced graphene oxide. For synthesis of
this carbon-based material graphene oxide (Figure 2Ba, 2Bb) as starting compound was used. During
reduction process the agglomeration of GO individual nanoparticles occur due to the loss of polar
functional groups on GO surface, leading to reduced solubility in aqueous medium. Nevertheless, in
comparison with Figures 2Aa and 2Ab, partially reduced graphene oxide has a much finer structure.
These observations are fully consistent with the results of SEM analysis.
Figure 2. Micrographs of carbon materials using microscopy in ambient light: (A) graphite oxide; (B)
graphene oxide; (C) partially reduced graphene oxide. Parameters were as follows: Device:
Microscopy; Zoom: 100× (“a” labelled micrographs) or 400× (“b” labelled micrographs);
Ambient light; ISO 200; Resolution: 1600 × 1200.
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For a more detailed description of the structure of carbon materials, SEM analysis was used.
Figure 3 shows representative SEM micrographs that express the morphology of individual carbon
materials obtained by drying dispersions on clean silicon wafers.
Figure 3. SEM micrographs of (A) graphite oxide, (B) graphene oxide, (C) partially reduced GO with
their representative morphology obtained by drying of appropriate dispersions on clean silicon
wafers. Magnification (a) 1000×, (b) 30 000×, standard detection mode SEI (secondary electron
imaging).
Using 1000× magnification in Figure 3Aa was noticeable bumpy surface of a material caused
by the uneven distribution of the size and thickness of the individual particles of graphite oxide.
Nevertheless it can be seen that this material contains partially exfoliated proportion, which shows
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only locally transparent veil structure at 30 000× (Figure 3Ab). Figure 3Ba (1000× magnification)
shows the characteristic structure formed by evaporating the solvent from a solution of graphene oxide.
Formed continuous layer tends to be transparent in the case of the residue from the solution with a low
concentration of solids and the substrate (silicon wafers) can be observed under this layer. Detailed
photo (Figure 3Bb) also shows the transparent veil undulating morphology characteristic for graphene
oxide [52]. The structure of the partially reduced graphene oxide (Figure 3C) is quite similar to the
starting material (Figure 3B), but it is clearly visible that the partially reduced graphene oxide have
more pronounced gossamer texture (Figure 3Ca) even at magnification 30 000× (Figure 3Cb). These
structural changes are consistent with the chemical changes that occur during the process of reduction
of graphene oxide.
3.3. Interaction of carbon-based materials with As(III) depending on pH
To improve the efficiency of the detection method, in general it is required to modify the
carbon material, which forms the working electrode. Characterization of the behavior of these
materials in the presence of analyzed samples is a crucial step. For this reason, the interaction of
carbon-based materials with As(III) in the pH range 0-7 was carried out. The acidic medium was
chosen according to the electrochemical determination of As(III). The ability of carbon-based
materials to adsorb As(III) ions on a carbon surface was investigated (Figure 4).
A standard solution of As(III) was added to individual aliquots of graphene oxide, graphite
oxide and the partially reduced graphene oxide, the mixtures were shaken for one hour. After this time
the concentrations of As(III) was determined by difference pulse voltammetry and atomic absorption
spectrometry in the filtered supernatant. The graph in Figure 4A shows the strongest adsorption onto
the carbon surface at the lowest pH (the value was approximated to zero for all three carbon materials).
The available literature shows that the adsorption of As(III) to the sorption materials is strongly
influenced by the pH of the environment caused by dissociation H3AsO3 gradually to H2AsO3-,
HAsO32- and finally in a strongly basic region to AsO33-. Generally, at alkaline pH the adsorption of
As(III) occurs with a higher efficiency than at the acidic range, but the character of the sorbent and
dissociation groups respectively the charge on its surface also plays the important role [53]. However,
due to the electrochemical determination of As(III) using the glassy carbon electrode, which takes
place at very low pH, the adsorption studies of the behavior of As(III) to the surface of the carbon
material was carried out in strongly acidic to neutral environment.
For the graphene oxide, the adsorption efficiency was determined as 24.6%, for graphite oxide
- 18.1% and for partially reduced graphene oxide - 13.4% after the interaction time of one hour.
Adsorption efficiency of As(III) to the surface of carbon-based materials was not higher in comparison
with the values obtained at the most acidic pH.
For maximum efficiency of adsorption, substantially acidic pH created by the addition of
concentrated HCl to the sample, was used in following experiment. The interaction between carbon-
based materials and As(III) in different reaction times was investigated in order to ascertain optimal
interaction time when the surface adsorption is utmost. For this experiment, 5 different interaction
Int. J. Electrochem. Sci., Vol. 11, 2016
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times (0; 30; 60; 90 and 120 minutes) were used. From Figure 4Bb can be seen that in the time range
of 60-90 minutes, the efficiency of adsorption reached the maximum which is comparable to others
carbon based materials [54]. Graphene oxide with the adsorption efficiency of 30.8% after 1 hour
interaction seemed to be the most effective and this trend was confirmed by atomic absorption
spectrometry (Figure 4Ba). This material was used for modification of glassy carbon electrode used in
the electrochemical detection of As(III) in aqueous solutions.
Figure 4. (A) relative adsorption of As(III) depending on pH, red line is for graphene oxide, blue line
for graphite oxide, grey line for partially reduced graphene oxide, measured by differential
pulse voltammetry with 0.75 M HCl as a supporting electrolyte; (B) relative adsorption of
As(III) depending on time interaction (0; 30; 60; 90 and 120 minutes) determined by (a) atomic
absorption spectrometry and (b) differential pulse voltammetry, red line is for graphene oxide,
blue line for graphite oxide, grey line for partially reduced graphene oxide; (C)
voltammograms of As(III) signal determined using bare GCE (violet line), GCE modified by
AuNPs (green line) and GCE modified by AuNPs and GO (red line), measured by linear sweep
voltammetry in 0.75 M HCl as a supporting electrolyte, parameters of the method were as
follows: initial potential of -0.4 V, end potential 0.5 V, sample interval 0.001 V,
preconditioning potential -0.4 V, deposition time 60 s, scan rate 0.1 V/s; (D) voltammograms
of As(III) signal depending on time interaction with GO on the GCE surface, (a) 0 minutes,
black line, (b) 15 minutes, green line, (c) 30 minutes, orange line, (d) 45 minutes, blue line, (e)
60 minutes, red line.
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3.4. The behaviour of As(III) on glassy carbon electrode modified with gold nanoparticles and
graphene oxide
For As(III) detection on glassy carbon electrode, it is necessary to modify electrode surface by
some substance with higher affinity to this transition metal, e.g. AuNPs because using bare GCE led to
the no electrochemical signal for As(III) [55,56]. Using the results obtained from analysis of the
interaction between arsenic and carbon-based materials (determined on hanging mercury drop
electrode), the glassy carbon electrode was modified using gold nanoparticles followed by graphene
oxide, which was selected as the material with the highest efficiency of As(III) adsorption on its
surface. At each change made to the surface of the glassy carbon electrode, this electrode was involved
in the electrochemical cell as a working electrode, and the electrochemical response of As(III) in
standard solution was detected. Figure 4C shows that the glassy carbon electrode without any
modification makes no electrochemical signal of As(III), Sakira et al shows the same trend on solid
carbon paste electrode [57]. Modification of gold nanoparticles (10-15 nm) was carried out by
applying AuNPs solution and evaporation of the liquid at 30°C. After this adjustment an
electrochemical signal of As(III) was recorded. In the next step, dual modification of the surface was
made using AuNPs and graphene oxide. After this modification, the electrochemical signal of arsenic
increased by more than 50% compared to the initial signal of As(III) on gold nanoparticles. This effect
could be seen also in Liu et al. article, where authors used electrodeposition as alternative possibility of
glassy carbon electrode modification [58].
To verify the process of As(III) adsorption glassy carbon electrode modified by AuNPs and GO
was immersed into the electrolyte (0.75 M HCl) containing As(III) ions at concentration of 1 µg/mL.
At times 0; 15; 30; 45; 60 minutes an electrochemical signal of As(III) in the mixture was measured
(Figure 4D). The initial signal of As(III) measured at time 0 was increased with increasing interaction
time. Electrochemical signal reached a maximum in 60 minutes, after that in the case of longer
interaction times (data for 75 and 90 minutes not presented) peak height of As(III) did not increase. It
is possible to say that after 60 minutes of interaction, the surface of graphene oxide was saturated by
As(III) ions.
4. CONCLUSIONS
Due to its toxicity and carcinogenicity, arsenic is considered one of the major environmental
threats of this millennium. With regard to the environmental importance of arsenic, it still makes sense
to seek new ways of detection and it is also important to design methods that allow rapid, inexpensive,
routine detection of this element. In this study, an alternative method for detection of As(III) using
GC/AuNPs/GO electrode has been proposed. This method of double modification resulted in a two-
fold increase of the electrochemical signal of As(III) in comparison with a simple modification of GCE
using gold nanoparticles only.
Int. J. Electrochem. Sci., Vol. 11, 2016
1226
ACKNOWLEDGEMENTS
Financial support from CEITEC CZ.1.05/1.1.00/02.0068 is greatly acknowledged.
CONFLICT OF INTEREST
The authors have declared no conflict of interest.
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