Accepted Manuscript

Title: Development of a dimethyl disulfide electrochemicalsensor based on electrodeposited reduced grapheneoxide-chitosan modified glassy carbon electrode

Author: Somayeh Rajabzadeh Gholam Hossein RounaghiMohammad Hossein Arbab-Zavar Narges Ashraf

PII: S0013-4686(14)01048-2DOI: http://dx.doi.org/doi:10.1016/j.electacta.2014.05.064Reference: EA 22756

To appear in: Electrochimica Acta

Received date: 26-1-2014Revised date: 6-4-2014Accepted date: 12-5-2014

Please cite this article as: S. Rajabzadeh, G.H. Rounaghi, M.H. Arbab-Zavar, N. Ashraf,Development of a dimethyl disulfide electrochemical sensor based on electrodepositedreduced graphene oxide-chitosan modified glassy carbon electrode, ElectrochimicaActa (2014), http://dx.doi.org/10.1016/j.electacta.2014.05.064

This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.

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Development of a dimethyl disulfide electrochemical sensor based on electrodeposited

reduced graphene oxide-chitosan modified glassy carbon electrode

Somayeh Rajabzadeh, Gholam Hossein Rounaghi, Mohammad Hossein Arbab-Zavar*, Narges

Ashraf

Department of Chemistry, Faculty of Sciences, Ferdowsi University of Mashhad, Mashhad, Iran.

*Corresponding author,

Tel: +98 511 8797022

Fax: +98 511 8796416

E-mail address: arbab@um.ac.ir

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Abstract

This paper proposes a simple approach for sensing of dimethyl disulfide (DMDS) using reduced

graphene oxide (RGO) nanosheets and chitosan (CS) modified glassy carbon (GC) electrode.

Graphene oxide was synthesized chemically by employing a modified Hummer method and

characterized by UV-Vis, FT-IR and Raman spectroscopies and also X-ray diffraction (XRD).

The RGO nanosheets and CS layer were electrodeposited successively onto the surface of a

glassy carbon electrode in two separate steps. The morphology and electrochemical properties of

the CS/RGO film have been characterized by Transition Electron Microscopy (TEM) and cyclic

voltammetry (CV). The effects of relevant parameters such as the electrodeposition techniques,

the amounts of the graphene oxide (GO) and the chitosan, and also the pH of the solution have

been investigated and optimized. Analytical performance of the CS/ RGO/GC modified electrode

for determination of DMDS has been evaluated. Under optimized operating conditions, a

concentration linear range of 1- 500 mg L-1 with a detection limit of 0.9 mg L-1 of DMDS have

been obtained. Also, the RSD of the method was achieved to be 2.1%. Furthermore, the

voltammetric technique employed to determination of DMDS in various natural waters using the

CS/RGO/GC modified electrode.

Keywords: Reduced graphene oxide; Chitosan; Dimethyl disulfide; Modified electrode;

Voltammetry.

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1. Introduction

Dimethyl disulfide (DMDS) with an unpleasant smell is a member of substances known as the

volatile sulfur compounds (VSCs). Determination of total organic sulfur, a particular class of

sulfur compounds (thiols, sulfides, etc.), as well as individual components (speciation analysis) is

of important concern nowadays. The detection of DMDS is of particular importance in water

treatment process, food industries and also petroleum and chemical products [1]. DMDS can

enter the body either by inhalation of the air, ingestion of contaminated water, or by dermal

contact. The adverse effects include irritation of the lunges, skin and eyes [2, 3].

Determination of volatile sulfur compounds has several difficulties, including their broad range

of concentrations, their highly reactive nature, and complexity of matrices [4]. Today, the

popular method for quantitative analysis of sulfur compounds in environmental samples relies on

gas chromatography (GC) [5-7]. However, GC analysis of the sulfur compounds requires

efficient sample pretreatment and enrichment methods. But such procedures are not routinely

used because they are complicated, time consuming and tedious [8-10].

In contrast to these methods, electrochemical procedures are highly promising for obtaining high

sensitivity, a low detection limit and low cost. Recently, a few reports were presented on DMDS

determination by electrochemical approaches. Voltammetric techniques using modified electrode

for determination of DMDS in different matrices have been suggested but they are few. Hart et

al. used a screen printed carbon electrode modified with cobalt phthalocyanine as a gas sensor

for amperometric determination of DMDS [11, 12]. Also, Seraphim and Srtadiotto described a

methodology for determination of sulfur compounds including disulfides in gasoline using a

mercury film electrode [5].

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Graphene nanosheets are known as the thinnest and strongest material, which has a two-

dimensional honeycomb lattice and single atom thick nanosheet carbon atoms. It is a basic

building block for graphitic materials of other dimensionalities. This strictly two-dimensional

material exhibits remarkable electrical conductivity merit, a large theoretical specific surface

area, low cost and high yield approaches for its preparation [13-14]. Since every atom in a

graphene nanosheet is a surface atom, molecular interaction and thus electron transport through

graphene can be highly sensitive to any adsorbed molecules [15]. These unusual electronic

properties make it ideal for electrochemical applications [16]. However, graphene adsorbs on the

glassy carbon electrode surface via van der waals interactions and thus it suffers from low

stability.

The biopolymer chitosan is a natural polysaccharide with primary amino groups in the polymer

chain [17, 18]. It is commonly used to disperse nanomaterials and immobilize enzymes for

constructing biosensors due to its excellent capability for film formation, nontoxicity,

biocompatibility, mechanical strength, and good water permeability. Several investigations have

shown that the graphene-chitosan hybrid systems show enhanced electrochemical properties [19,

20].

To our knowledge, no report has been published to date on the application of graphene-chitosan

or sensing of DMDS in liquid samples. In this paper, an environmentally friendly and efficient

route for the reduction of graphene oxide (GO) and using chitosan (CS) as a stabilizer of reduced

graphene oxide (RGO) film is reported. Then, the application of CS/RGO film for DMDS

detection has been demonstrated.

2. Experimental

2.1. Chemicals and reagents

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All chemicals were of analytical reagent grade and used without further purification. Deionized

water was used throughout the entire work. Dimethyl disulfide (DMDS; 99% pure) was

purchased from Merck (Darmstadt, Germany). A stock solution of DMDS (600 mg L-1) has been

prepared by dissolving the proper amounts of DMDS in methanol. Working solutions were

prepared by diluting this stock standard solution with phosphate buffer solution (PBS, 0.1 M; pH

8.00) just before use. PBS was prepared by dissolving the proper amount of NaH2PO4.2H2O

(Merck, Darmstadt, Germany) in deionized water and adjusting the pH value to 8.00 by 0.1 M

NaOH Solution.

Graphite powder (� 50 µm) was purchased from Merck (Darmstadt, Germany). Chitosan (CS;

medium molecular weight) was obtained from Sigma-Aldrich (Iceland) and used as received. A

0.2 wt% CS solution was prepared by dissolving CS in acetic acid solution (1% V/V) at 40 °C.

The mixture was cooled to room temperature; its pH was adjusted to 6.00 using NaOH solution

(1 M), filtered and stored in the refrigerator (at 4 °C).

2.2. Apparatus

All of the electrochemical measurements were carried out using a µ-Autolab type III

electrochemical workstation with a conventional three electrode cell. A glassy carbon electrode

(GC, 2 mm in diameter, Azar electrode Co., Urmia, Iran) was used as working electrode. A

platinum wire and a saturated Ag/AgCl electrode (both from Azar electrode Co., Urmia, Iran)

were used as counter and reference electrodes, respectively.

The transmission electron microscope (TEM) image was obtained with a CM120, Philips

transmission electron microscope. X-ray diffraction (XRD) analysis was carried out on an X-ray

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diffractometer (D8 Advanced, Bruker). Raman spectrum was recorded using an Algema thermo

nicolet dispersive Raman spectrometer with a 532 nm laser excitation source.

Fourier transform infrared (FT-IR) spectrum was obtained by an AVATAR-370 Fourier

transform infrared spectroscopy system. Also, UV-Vis spectrum was recorded by a Unico 2800

UV/Vis spectrophotometer.

2.3. Synthesis of GO nanosheets

GO was prepared by employing a modified Hummer method [16]. Briefly, concentrated H2SO4

(23 mL) was added into a 250 mL flask filled with graphite (0.5 g) at 0 °C (ice bath), followed

by the addition of NaNO3 (0.5 g) and stirred for 10 min. Then, solid KMnO4 (3 g) was slowly

added (in 45 min) to the solution with stirring, while the mixture was kept below 5 °C in an ice

bath. The mixture was then stirred for another 2 h at 35 °C. Then, excess deionized water was

slowly added to the above mixture while the temperature was kept below 65 °C. Finally, a 30%

H2O2 aqueous solution (3 mL) was added to the deep brown mixture to reduce the residual

permanganate and manganese dioxide. The resulting suspension was filtered and then washed

with dilute aqueous HCl and deionized water to remove the acid. At last, the solid products were

dried for 24 h under vacuum at 60 °C to obtain graphene oxide as a brown powder.

2.4. Construction of modified electrodes

Prior to electrodeposition, the glassy carbon (GC) electrode was polished thoroughly by 0.5 µm

alumina slurry until a mirror-shiny surface was obtained. Then, it was ultrasonicated in a 1:1

mixture of ethanol and deionized water for 1 min, placed in 6 M HNO3 solution for 1 min, and

rinsed with deionized water. Thereafter, GC electrode was further cleaned by sweeping the

potential between −1.5 V and +1.5 V at 50 mVs−1 in 0.1 M PBS (pH 8.00) until stable cyclic

voltammograms were obtained.

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GO was electrodeposited using a pulsed chronoamperometry procedure. 0.006 g of GO was

suspended in 0.1 M PBS solution (pH 8.00), then GO was delaminated into GO nanosheets by

sonication for 20 min, forming a stable colloidal solution. The electrodeposition process was

performed by applying the potential pulses (-1.5 V, 5 s; 0 V, 5 s) for 80 cycles. Then, the RGO-

coated electrode was immersed into a 0.2% (W/V) chitosan solution and CS electrodeposition

process was performed at −2.5 V for 300 s. The modified electrodes were then rinsed with water

and air-dried.

2.5. Measurement procedure

In all of the voltammetric studies, 4.0 mL of the supporting electrolyte solution (with or without

the analyte), 0.1 mol L-1 PBS (pH 8.00), was transferred to the electrochemical cell and

deoxygenated with high-purity nitrogen (99.99%) for 5 min. A nitrogen environment was kept

over the solutions during measurements.

2.6 Preparation of real water samples

Water samples were simply filtered through Whatman® filter paper (No. 41) and stored in the

refrigerator at 4°C until analysis.

3. Results and discussion

3.1. Characterization of the synthesized GO and the deposited RGO film

The spectra of UV-Vis, FT-IR, Raman characterization and XRD pattern of GO have been

shown in Fig. 1. The UV-Vis spectrum of GO aqueous solution has been presented in Fig. 1(A).

Deionized water was used as a reference solution in taking the spectrum of GO solution. The

spectrum (A) has a distinct absorption peak at 233 nm which is attributed to the characteristic

absorption of GO [17]. The FT-IR spectra of graphite and GO powders have been shown in Fig.

1(B). Spectrum of GO shows the characteristic peaks of νO-H at 3384 cm-1, νC-O at 1051 cm-1, ν C-

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O-C at 1233 cm-1, ν C-OH at 1368 cm-1, and νC=O stretch in the carboxylic group at 1740 cm-1. These

bands are attributed to the oxygen-containing functional groups on GO, which confirm the

successful oxidation of graphite. The peak at 1613 cm-1 can be attributed to the skeletal

vibrations due to the presence of C=C bond [20, 21]. The XRD pattern of GO has been shown in

Fig. 1(C). Graphite has a strong diffraction peak at 2θ about 26.52° (002) which represents an

interlayer distance of 0.34 nm (not shown). However, this peak disappears in the GO and the

characteristic 2θ peak of GO appears at 11.00° which corresponds to a d-spacing of

approximately 0.78 nm. This is compatible with the interlayer spacing of GO nanosheets

reported in the literature, which is due to the existence of oxygen-rich groups on both sides of the

nanosheets and water molecules trapped between the nanosheets [17]. The Raman spectrum of

GO which is shown in Fig. 1(D) was recorded to obtain further information on GO

nanostructures. The GO has the characteristic peaks at 1587 cm-1 and 1348 cm-1 corresponding to

the G and the D bands of the GO structure, respectively. Generally, the G band is considered to

be arisen from the zone center E2g mode, which corresponds to the ordered sp2 bonded carbon;

while the D band is seemed to be resulted from sp3-hybridized carbon [22]. Fig. 1(D), with

strong D and G bands, suggests very small crystal size for GO nanosheets, which is ideal for its

electrochemical performance.

The morphology and structure of the electrodeposited RGO nanosheets were investigated using

TEM images (Fig. 2). As shown, RGO nanosheets exhibit Corrugation and scrolling structure.

This winkled nature of RGO was highly beneficial in maintaining a high effective surface area

on the electrode and facilitates the diffusion of the analyte. Also, TEM images show that the

RGO nanosheets are ultrathin transparent nanonanosheets. The black area is consisted of several

layers of RGO nanosheets which overlayed on the electrode surface [23].

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3.2. Electrochemical behavior of DMDS on the CS/RGO/GC modified electrode

The cyclic voltammograms of DMDS on the CS/RGO/GC and the bare GC electrodes are shown

in Fig. 3. Using the bare GC electrode, a broad oxidation wave is observed for DMDS with a

peak potential of about +1.2 V and a small oxidative current. The CS/RGO/GC modified

electrode exhibits a superior behavior with a small negative shift (~ 0.05 V) in peak potential of

DMDS and a large increase in the anodic peak current.

The electrochemical oxidation of DMDS at the bare GC electrode is irreversible (as shown in

Fig. 3), with only a single oxidation wave in the potential range employed. When the

concentration of DMDS in the solution increases, the anodic peak potential with CS/RGO/GC

modified electrode shifts about 29 mV toward more positive values which is much smaller

compared to 227 mV in the case of the bare GC electrode. This positive peak potential shift is

resulted from a slow kinetics in the electron transfer process at the electrode surface. However,

the smaller shift in peak potential with CS/RGO/GC modified electrode means that the kinetics

of the oxidation process is faster on the modified electrode compared to the bare electrode.

To further evaluate the oxidation process, the effect of scan rate on the anodic current was

investigated, as shown in Figure 4. The anodic current changes linearly with (scan rate)1/2,

indicating that the kinetics of the redox reaction is controlled by diffusion process.

The unique two-dimensional nanostructure of RGO enhances the electric conductivity of the

electrode, which could be accounted for the increased peak currents. CS has previously been

used as a stabilizer during the reduction of GO [24, 25]. This is because the RGO has hydroxyl

and carboxylic groups on its surface while the CS has –OH and –NH2 or O=C–NH2 groups on its

macromolecular chains. These functional groups can form hydrogen bonds or interact

electrostatically with each other and makes the RGO nanosheets more stable on the electrode

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surface. Thus, in this work a layer of CS is deposited on the RGO film as a separate layer to

stabilize it. However, the experimental results show that the presence of CS itself enhances the

sensitivity of the modified electrode towards DMDS. Fig. 5 shows the cyclic voltammograms

obtained by four different GC electrodes in 50 mg L-1 DMDS in pH 8.00 PBS (0.1 M) at 0.1 V

s−1. A single irreversible peak is observed for the bare GC electrode for DMDS. After

electrodeposition of the CS film onto the bare GC, a negative shift in the peak potential and an

enhancement in the peak current are observed which indicate the electrocatalytic activity of CS.

The response is also increased using a RGO modified GC electrode compared with that of a bare

GC electrode. Also, the synergistic effect of CS and RGO together has been shown and indicates

that the RGO/ CS film catalyzes the oxidation of DMDS.

3.3 Mechanistic aspects of electrochemical oxidation of DMDS

Depending on the reaction conditions, the oxidation of disulfides can lead to the formation of

thiolosulfinates, thiolosulfonates, disulfones, and sulfonic acids [26-29]. Previous studies on the

mechanistic aspects of the electrochemical oxidation of DMDS have been performed in

acetonitrile and dichloromethane solvents [3,30,31]. No report has been published on the

oxidation mechanism of DMDS in water. However, by comparison with electrochemical

oxidation of glutathione disulfide (GSSG) [32] some conclusions can be obtained. These results

suggest that an anodic oxygen transfer reaction to the sulfur atoms occurs during electrochemical

oxidation of DMDS. These oxygen transfer reactions involve water and are believed to occur

mainly by hydroxyl ions present in basic aqueous medium.

RSSR + 2OH- → RSOSR + H2O + 2e- (1)

RSOSR + 2OH- → RSOOSR + H2O + 2e- (2)

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RSOOSR + 2OH- → 2RSO2H + 2e- (3)

2RSO2H + 4OH- → 2RSO3H + 2H2O + 4e- (4)

In solvents of weak basicity (e.g. acetonitrile) the oxidation of DMDS occur in two steps. In the

first step, the compound gives an electron in its highest occupied molecular orbital (HOMO) to

the electrode to form a radical cation (CH3-S-S-CH3˙+); However if the solution contains a

nucleophile (e.g. OH-) or consists of a basic solvent (e.g. water), the radical cation formed at the

first oxidation step reacts to produce CH3-S-S˙OH-CH3. This product is easier to oxidize than the

CH3-S-S-CH3˙+, and again is oxidized to form CH3-S-S+OH-CH3, which chemically reacts with

OH- to form the electrically neutral final product. Thus, in aqueous solution, a two electron

oxidation occurs in the first step. The electrochemical oxidation of sulfur can be continued with

further movement to more positive potentials (than +1.2 V). However, on a CS/RGO/GC

electrode, the corresponding peak could not be observed due to the GC surface fouling and

increase in the background current because of the higher applied potentials [32]. However, to

exactly clarify the mechanism of DMDS oxidation, further studies such as coulometry and

macroscale electrolyses are required.

3.4. Optimization of the sensor performance

3.4.1. Electrodeposition of RGO

Despite that the GO can form well dispersed aqueous colloids; it is well known that the chemical

reduction of GO nanosheets in aqueous solutions results in their irreversible agglomerate [12].

Therefore, it is reasonable to suppose that when the GO nanosheets reduce in the vicinity of the

electrode surface, the resulted RGOs will directly attach to the electrode surface.

In order to find the most efficient experimental conditions for the electrodeposition of RGO,

several relevant factors such as the mode of RGO deposition, the amount of GO in the solution,

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and also the pH of the solution were investigated and optimized to obtain the highest peak

current for DMDS oxidation.

The method of deposition RGO is a crucial factor for the sensing applications. Usually, the RGO

film on the electrode surface is deposited by drop-casting of a RGO suspension obtained from

the chemical reduction of GO nanosheets [33]. Alternatively, RGO can be deposited

electrochemically by applying a potential more negative than -1.2 V. This electrochemical

deposition of RGO brings some advantages as it is simple, fast, reproducible and

environmentally friendly. Also, the resulted RGO film is clean, homogeneous and controllable

[14]. Therefore, deposition of RGO has been performed electrochemically in this study. Several

electrochemical techniques such as CV, chronoamperometry (CA) and chronopotentiometry

(CP) are available for electrodeposition of RGO on the GC surface. The sensitivity of the

proposed modified electrode showed to be greatly dependent upon the RGO elecrodeposition

technique. Pulsed CA performed by the application of 80 potential pulses between -1.5 V (for 5

s) and 0 V (for 5 s) showed to be the most efficient. The formation of RGO film at the surface of

GC electrode was confirmed by recording the TEM image (Fig. 2).

The effect of the amount of GO and the pH of solution were also investigated from which the

value of 6 mg and the pH 8.00 were obtained to be the most appropriate conditions for

electrodeposition of RGO.

3.4.2. Electrodeposition of chitosan

The RGO film at the GC surface is not very stable as a result of its poor attraction forces (van der

waals) with surface of GC electrode. On the other hand, CS biopolymer with many amino groups

along its macromolecular chains [14, 17] can interact strongly with RGO (which has hydroxyl

and carboxylic groups on its surface). Hence, CS can act as a protective layer to stabilize the

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RGO film. The CS biopolymer is positively charged in aqueous solutions with pH values lower

than 6.5. However, as the pH of the solution increases, the charged amino groups (NH3+)

becomes deprotonated and thus, insoluble. This issue opens a way for electrodeposition of CS at

the electrode surface. High pHs can be achieved at the cathode surface where the water

molecules reduce electrochemically and produce hydroxide ions. As a result, CS forms an

insoluble deposit.

To deposit the CS next to the RGO film, CA technique has been applied through the entire work.

The applied potential was varied in the range of -1.2 V to -3 V, and the maximum current

response obtained at -2.5 V. The influence of deposition time was studied at the different time

periods of 200, 300, 400 and 500 s and electrodeposition time of 300 s was proved to be

optimum value. Electrodeposition of chitosan was performed using various concentrations of

(0.01-0.4 %w/w) chitosan solutions at pH 6.00. Considering both the sensitivity and the stability

of electrode response showed that 0.2% (w/v) CS is the optimal concentration for construction

the sensor.

3.4.3. Conditions of the DMDS solution

The effect of pH of DMDS sample solution on the electrochemical response towards the

oxidation of DMDS was investigated from pH 3.00 to pH 11.00 as shown in Fig. 6A. Figure 6B

illustrates the influence of DMDS solution pH on the peak current and peak potential. The peak

current shows a maximum at the pH 8.00 while the peak potential was not influenced

significantly by pH of the solution. Indeed, the value of Ep shows a small shift toward more

negative values with the increasing the pH.

3.5. Analytical performance of the sensor

Analytical measurements have been performed by square-wave voltammetry technique. Under

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optimized operating conditions, the calibration curve was constructed in the concentration range

of 0.5 - 600 mg L-1 for DMDS (Fig. 7). Good linear correlation [I (µA) = 7.466 (±2.590) + 0.519

(±0.012) × c (mg L-1); R2= 0.9962] between the oxidation currents and the concentration of

DMDS was obtained in the range of 1 – 500 mg L-1. The detection limit (LOD) and

quantification limit (LOQ) of the method, calculated based on the 3sb/b criteria, was 0.9 mg L-1

and 3 mg L-1, respectively. The intra-day reproducibility of the method is reported as relative

standard deviation (RSD) and achieved by repetitive measurements (n=6) at 50 mg L-1 of DMDS

to be 2.1%. The inter-day reproducibility was measured to be 3.2% for 6 measurements.

3.6. Study of potential interferences

The interfering effect of some ions on the determination of DMDS with CS/RGO/GC modified

electrode was studied by addition the interfering ions to the supporting electrolyte containing 50

mg L-1 DMDS. The presence of 100 fold excess of Na+, K+, Mg2+, Ca2+, NH4+, Cl-, F-, NO3

-,

HCO3-, S2-, and S2O3

2- did not cause serious interference while the same concentration of SCN-,

SO32- and SO4

2- display noticeable interference which can be eliminated using an anion exchange

resin column prior to the voltammetric analysis.

3.7. Sensor applications to real sample analysis

In order to test the practical applications of the proposed sensor, the determination of DMDS in

sea water and river water samples were performed and the obtained recoveries are summarized in

Table 1. The recoveries in the range of 95.6% to 101.3% were obtained, indicating that the

sensor response is not affected considerably by sample matrix.

The performances of similar electrochemical methods in DMDS detection with different

modified electrodes are summarized in Table 2. Compared to others, the proposed electrode

shows a wider linear range and lower detection limit.

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4. Conclusion

Here, a CS/RGO/GC modified electrode has been proposed as a voltammetric sensor for

detection of DMDS in natural waters. The fabrication of the electrode is simple, fast and

environmentally friendly. The RGO at the electrode surface effectively facilitates the electron

transfer for DMDS oxidation and enhances of the current response. Under optimized conditions,

the proposed electrode exhibits a good performance in terms of sensitivity, detection limit,

response and linear calibration range.

Acknowledgments

Financial supports from the Ferdowsi University of Mashhad (Grant No. 3.20276) are gratefully

acknowledged.

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Fig.1. A) UV-Vis spectrum of GO, B) FT-IR spectra of: graphite (a) and GO (b). C) XRD

spectrum of GO and D) Raman spectrum of GO.

Figure(s)

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Fig. 2. Typical TEM images of the prepared RGO/ GC electrode (scale: 100 nm).

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Fig. 3. Cyclic voltammograms of the GC (bare GC) and CS /RGO /GC with or without DMDS in

0.1 M PBS solution, potential scan rate 0.1 V s−1

.

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Fig. 4. Linear variation of voltammetric currents upon the square rate of the scan rate (0.020- 0.4

V/ s) Ip= -22.0612+7.3104 v1/2

, R2=0.9934

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Fig. 5. Cyclic voltammograms of four different electrodes in 50 mg L-1

DMDS in 0.1 M PBS

solution, potential scan rate 0.1 V s−1

.

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Fig. 6. A) Square- wave voltammograms of CS/RGO/GC electrode in solution containing 92 mg

L-1

DMDS at different pHs. B) Influence of DMDS solution pH on the peak current (solid line)

and peak potential (dash line, Ep).

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Fig. 7. Square- wave voltammograms using chitosan- graphene- GC electrode in 0.1 M PBS (pH

8) in the presence of different DMDS concentrations: 1, 10, 25, 53, 106, 159, 212, 266 and 500

mg L-1

, potential scan rate: 0.1 V/s. Inset: calibration plot of anodic current recorded at 1.1

V/Ag/AgCl vs. DMDS concentrations.

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Table 1. Measurement of DMDS in natural waters. Results (mean ± standard deviation; based on three

replicate analyses) of determination of DMDS.

Sample Amount added (mg L-1) Amount found (mg L-1) Recovery (%)

Sea water 150 146±5 97.1

Sea water 200 203±6 101.3

River water 150 151±6 100.7

River water 200 191±8 95.6

Table 2. Comparison of some electrochemical methods for determination of DMDS.

Technique DMDS concentration/ mg L-1 Ref.

Square- wave

voltammetry

0.02-0.3 2

Hydrodynamic

voltammetry

6.2-100 10

Cyclic

voltammetry

200-4000 23

Amperommetry 5.3-26.5 11

Potentiometric

titration

2.83-212 34

Square- wave

voltammetry

1- 500 This study

Table(s)