Gold Nanoparticle Included Graphene Oxide Modified

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(AuNPs) were prepared following a method described ... 0.1 M sodium acetate–acetic acid buffer solution .... 0.33 to 1.98 for intra-day and from 0.40 to 2.02 for.

ISSN 1061-9348, Journal of Analytical Chemistry, 2016, Vol. 71, No. 7, pp. 685–695. © Pleiades Publishing, Ltd., 2016.

ARTICLES

Gold Nanoparticle Included Graphene Oxide Modified Electrode: Picomole Detection of Metal Ions in Seawater by Stripping Voltammetry1 İlknur Üstündağa, *, Aslı Erkalb, Tamer Koralayc, Yusuf Kağan Kadıoğlud, and Seungwon Jeone aDumlupınar

University, Faculty of Arts and Science, Department of Physics 43100 Kütahya, Turkey b Dumlupınar University, Faculty of Arts and Science, Department of Chemistry 43100 Kütahya, Turkey c Pamukkale University, Faculty of Engineering, Department of Geology Denizli, Turkey d Ankara University, Faculty of Engineering, Geological Engineering 06100 Tandoğan, Ankara, Turkey eChonnam National University, Department of Chemistry and Institute of Basic Science Gwangju 500-757, South Korea *e-mail: [email protected] Received August 4, 2015; in final form, December 28, 2015

Abstract—We obtained a gold nanomaterial/graphene oxide-modified glassy carbon electrode and characterized it using transmission electron microscope, scanning electron microscope, cyclic voltammetry (CV), and X-ray photoelectron spectroscopy techniques. A response of the electrode using square wave anodic stripping voltammetry for Pb2+, Cu2+, and Hg2+ was found linear in the range from 1 × 10–7 to 1 × 10–11 M. The detection limits of Pb2+, Cu2+ and Hg2+ were 0.14, 0.5 and 1.2 pM, respectively. The method was applied to the simultaneous determination of Pb2+, Cu2+ and Hg2+ in seawater samples from a coastal region of Anatolia, and the results corresponded well with the values obtained by inductively coupled plasma-optical emission spectroscopy. Keywords: seawater sample, voltammetry, gold nanomaterial, graphene oxide, heavy metal determination, modified electrode DOI: 10.1134/S1061934816070108

Determining the chemical composition of seawater is highly important as it is largely related to geological and geochemical processes. Heavy metals and metalloids have great importance in terms of human health and environmental pollution. High concentrations of these metals (such as Cd, Cu, Cr, Pb, Hg, etc.) in water have been found to cause damage to gills, liver, kidneys, cardiovascular and nervous systems of marine organisms [1–3]. Many of heavy metals are toxic and cause cancer [2]. Determination of the concentrations of these heavy metals have great significance for live organisms and humans, as well as for monitoring environmental pollution. In recent years, multiple analytical approaches have been suggested for the heavy metals assay. Commonly, the analyses of environmental samples from soil, rock, water, and sediments are performed using 1 The article is published in the original.

spectroscopic techniques, including atomic absorption spectrometry [4], inductively coupled plasmaoptic emission spectrometry (ICP-OES)—mass spectrometry [5, 6] and X-ray fluorescence spectroscopy [7]. In recent years, electrochemical techniques such as voltammetry [8, 9] and impedimetry [10] have been used widely for the electrochemically sensitized ion analysis of environmental and food samples. Nanomaterial-modified electrodes are highly sensitive in sensor applications. The modified surface is composed of conductive nanofilms such as nanoparticles [11], carbon nanotubes [12], nanoribbons, graphene, graphene oxide [13], etc. The structure of the glassy carbon (GC) electrode is very useful for sensor studies [14], and GC is inherently compatible with carbonbased nanomaterials [15]. Carbon materials have become extremely important today and are used in many fields of electronics. This adventure began with the discovery of fullerenes

685

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and continued with carbon nanotubes and graphene. Graphene and its derivatives are experiencing a golden age. The graphene family is a single carbon layer of the graphite structure with a hexagonally arranged polycyclic aromatic hydrocarbon of quasi-infinite size [16– 21]. Graphene oxide (GO) comprises several functional groups, such as >C=O, –COOH and –OH with a 2D structure and sp2 hybridization. The graphene family provides a significant advantage to science in a variety of technological areas such as electronic systems [22–24], sensor technology [25], electromechanics [26], solar cells [27, 28], memory devices [29], hydrogen storage [30, 31], supercapacitors [32], fieldeffect transistors, and biomedical applications [33]. Graphene and its derivative-based electrodes have been used widely for heavy-metal determination in water samples. Liang et al. [34] studied a new modified electrode on ITO surface with organic moleculefunctionalized multi-walled carbon nanotubes for copper(II) determination using differential pulse anodic stripping voltammetry (DPASV) in the presence of other heavy metal ions in river water. The detection limit of the method was reported to be 2.5 nM. Zhao and colleagues [35] prepared a Hg2+modified GO in a Nafion-attached GC for the determination of lead(II) with square wave anodic stripping voltammetry (SWASV) in tap, pool, river, and lake water. The detection limits (LODs) were 0.13 ng/L for Pb2+ with the linear calibration curves ranging from 5 to 70 ng/L. A sensor was investigated [36] for use in the determination of merucry(II) in river water on a monodispersed gold nanoparticles-decorated graphene electrode surface by SWASV with a LOD of 6 ppt. Wang et al. [37] studied a novel sensor for use in the determination of cadmium(II) on a tin film/ poly(p-aminobenzenesulfonic acid)/electrochemically reduced graphene composite-modified electrode via SWASV in real-water samples. The linear response was over the range from 1.0 to 70 ppb with a LOD of 0.05 ppb. Some researchers determined heavy metals such as Pb2+, Cu2+, Cd2+, etc. in seawater samples. Locatelli et al. [38] determined heavy metals in seawater of Salerno Gulf by DPASV with LODs of 0.15 μg/g for Cu2+ and 0.08 μg/g for Pb2+. Süren and coworkers [39] developed a method for heavy metals determination of Dardanelles seawater by DPASV with hanging mercury drop electrode. The LOD for Pb2+ was found as 0.9 μg/L. In this study, we developed a gold nanomaterial/graphene oxide composite-modified glassy carbon electrode (AuNM-GO-GC). The aim was to develop a novel electrode for the determination of Pb2+, Cu2+, and Hg2+ in seawater using the SWASV technique. The samples were gathered from several

coastal regions of the Aegean and Mediterranean Seas (Turkey) in August 2014. EXPERIMENTAL Reagents. We obtained chemicals and solvents from chemical companies such as Merck, SigmaAldrich, Fluka, and Riedel. The ultra pure water (UPW) used to prepare aqueous solutions (18.2 MΩ·cm) was purified with the Human Power 1+ purification system (S. Korea). All experiments were applied under a refined argon gas (99.999%) atmosphere at room temperature (25 ± 1°C). A triple-electrode system was used for all electrochemical experiments. The reference electrode was Ag/AgCl/KCl (sat.) in aqueous media. Platinum was used as a counter electrode. Glassy carbon was acquired from BAS (Bioanalytical Systems, MF-2012, USA) and used as a working electrode. Glassy carbon electrode surfaces were polished using 100 and 50 nm Al2O3 suspensions (Baikowski Int. Corp., USA) on cleaning cloths (Buehler, Lake Bluff, IL, USA), washed two times by UPW, and all electrodes were sonicated (Ultrasonic Cleaner, SK1200H, China) in a mixture of 1 : 1 (v/v) isopropyl alcohol–acetonitrile. Synthesis of the nanomaterials. Gold nanoparticles (AuNPs) were prepared following a method described by Frens and Turkevich [40–43]. Briefly, 25 mL of 38.8 mM sodium citrate solution were added to 250 mL of boiling 1.0 mM HAuCl4 solution. The mixture was stirred vigorously, and the color of the solution changed to red. After 15 min the solution was allowed to cool to room temperature. The AuNPs were centrifuged (Hermle Z36HK, Germany, 10000 g, 10 min) and washed with UPW and ethanol two times to remove organic pollutants. AuNM was prepared by amidization reaction of solutions A and B. Solution A included AuNPs selfordered by 3-mercaptopropanoic acid (MPA) (1 mmol MPA was added to 10 mL AuNPs in pH 7 phosphate buffer solution, PBS), and solution B contained AuNPs self-assembled by 4-aminothiophenol (ATP) (1 mmol ATP was added to 10 mL of AuNPs ethanolic solution). The carboxylate group of MPA in solution A was activated with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide for 12 h. The activated AuNPs in solution A were separated by centrifuging for 10 min, washed with pH 7 PBS solution, added to solution B and stirred with a magnetic stirrer for 12 h. The AuNM was centrifuged for 10 min and washed with UPW three times to remove unreacted compounds. The AuNM solution was diluted with MeCN to 25 mL.

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GOLD NANOPARTICLE INCLUDED GRAPHENE OXIDE MODIFIED ELECTRODE

GO nanoparticles were prepared with electrochemical exfoliation of graphite. Pristine graphite and platinum foil in 2.4 g H2SO4 and 11 mL 30% KOH in 100 mL deionized water were used as the anode and cathode, respectively. Oxidation was performed under the constant direct current (DC) potential of +20 V. The GO was centrifuged (10 000 g, 10 min), washed with UPW and ethanol

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and dried at 100°C for 10 min. Next, 1.0 g GO was added to 25 mL of AuNM in MeCN solution. The suspension was sonicated with an ultrasonic cleaner for 30 min. The obtained nanostructure was denoted as AuNM-GO. Structure of the nanomaterials is provided in Scheme. The nanofilm attached to glassy carbon electrode surface was denoted as AuNM-GO-GC.

HO O HOOC

COOH OH O

O

O

O

S

HOOC S S

O

O

O

NH O

O

O

HO

NH

O

HOOC

NH S

O

H N

S

O

S

S

S

S

S S

N H

O

S

HO

HO

S

O

O

HOOC

O

COOH OH

HN HN

O

O

O

O

O

O

HN S

HOOC

S

O

O

O S

O

HO O

HOOC

O

HO

Schematic diagram of the AuNM and GO. Characterization of the nanomaterials. X-ray photoelectron spectroscopy (XPS) analysis was carried out using a PHI 5000 Versa Probe (Φ ULVAC-PHI. Inc., Japan/US) X-ray photoelectron spectrometer equipped with monochrome AlKα radiation (1486.6 eV) as an X-ray anode operated with 50 W at 7–10 Pa. Transmission electron microscope (TEM) and scanning electron microscope (SEM) images were recorded with a TEM (JEOL, Tokyo, Japan) and SEM (ZeissEvo, Germany). Voltammetric measurements were performed using an Ivium CompactStat (US) electroanalyzer system. Determination of simultaneous Pb2+, Cu2+, and Hg in aqueous media with SWASV. Volumes of 2, 4, 6, and 8 μL of AuNM-GO were dripped on the glassy 2+

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carbon electrode and dried under an infrared lamp. The linear response was investigated for Pb2+, Cu2+, and Hg2+ in the range from 1 × 10–11 to 1 × 10–7 M in 0.1 M sodium acetate–acetic acid buffer solution under optimal conditions such as pH, incubation temperature, deposition time, and suspension volume. Before recording the stripping currents, metal ions complexed on the AuNM-GO-GC surface were reduced at –1.1 V for 10 s in a measuring blank solution (0.1 M sodium acetate–acetic acid, pH 5.25). The detection limits of all ions were determined via the developed method. The seawater samples were collected from different coastal regions of the Aegean Sea and Mediterranean Sea near Turkey. They were collected and stored in polypropylene bottles at +4°C in

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AuNM-GO-GC GC 20 µA

−400

−200

0 200 Potential, mV

400

600

Fig. 1. Cyclic voltammograms of K3Fe(CN)6 on AuNM-GO-GC and bare GC vs. Ag/AgCl/KClsat in 0.1 M KCl; scan rate is 200 mV/s.

a refrigerator. Three replicate determinations using the standard addition method were done.

current of K3Fe(CN)6 on nanoparticle-included graphene surface increased via tunneling effect of the conductive nanomaterials [44].

RESULTS AND DISCUSSIONS

XPS high-resolution core spectra of C1s, N1s, S2p, and Au4f, are shown in Fig. 2. The C1s (Fig. 2a) binding spectra for AuNC-GO-GC were fitted as four peaks, which were C=O, C–N, C–S, and C–C at 287.5, 287.0, 286.3, and 284.5 eV, respectively [45]. The N1s (Fig. 2b) binding energy of the nanomaterials on GC was fitted at 406.3 eV (NH–C=O) [46]. The S2p (Fig. 2c) binding energy of the surface was fitted at 167.4 and 162.6 eV. The Au4f signal is shown in Fig. 2d in which the Au4f7/2 and Au4f5/2 peak signals appear at 88.1 and 84.5 eV, respectively [47].

Characterization of the nanomaterial. Bare and modified electrode surfaces were carried out with a 1 mM K3Fe(CN)6 redox probe in 0.1 M KCl aqueous solution using cyclic voltammetry. The voltammograms are shown in Fig. 1. The electron transfer rate of the redox probe significantly increased on AuNMGO-GC surfaces according to bare GC. The value of cathodic peak current of the redox probe on AuNMGO-GC and bare GC was determined as 28.9 and 20.8 μA, respectively. The electron transfer rate with the nanomaterial electrode was accelerated. The peak

Table 1. Analytical characteristics of the proposed method (n = 6) Analytical characteristic Regression equation

Pb

Cu

Hg

ip(μA) =

ip(μA) =

ip(μA) =

−0.269(–logPb2+) Standard error of the slope R2 Linearity range, M LOD, pM

+ 4.206

−0.220(–logCu2+)

+ 3.510

−0.158(–logHg2+) + 2.558

–0.053

–0.22

–0.014

0.988

0.997

0.998

1 × 10–11 to 1 × 10–7

1 × 10–11 to 1 × 10–7

1 × 10–11 to 1 × 10–7

0.14

0.5

1.2

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(a)

689

(b) N1s

3000 Cps

402.0

C–S 284.5

C−S 286.3

200 Cps

C−N 287.0 C=O 287.5

294

292

290

288 286 284 282 Binding energy, eV

280

278

406

404 402 400 Binding energy, eV

(c) 12

S 2p

(d) 32

S 2p

52

Au 4f

167.4

150 Cps

398

52

162.6

Au 4f

150 Cps

84.5

88.1

175

170 165 160 Binding energy, eV

155

92

90

88 86 84 Binding energy, eV

82

80

Fig. 2. XPS high resolution core spectra of C1s, N1s, S2p, and Au4f of the AuNM-GO-GC. Positions (a)–(d) are discussed in the text. @Key: 1. a; 2. b; 3. c; 4. d; 5. Binding energy, eV; 6.

TEM images of the GO and AuNPs covalently attached AuNM are given in Fig. 3. The diameter of the nanomaterial was determined as nearly 50 nm. TEM images of AuNM-GO are given in Fig. 4a. In Fig. 4b, SEM images of the AuNM-GO composite material are shown. Determination of Pb2+, Cu2+, and Hg2+ on the AuNM-GO-GC. The optimization parameters were detected using SWASV for Pb2+, Cu2+ and Hg2+ in a 0.1 M acetate buffer solution. The optimal deposition time, pH, temperature of incubation, and volume of suspension were determined to be 30 min (Fig. 5a), pH 5.25 (Fig. 5b), 28°C (Fig. 5c) and 4 μL (Fig. 5d), respectively. The SWASV voltammograms on the modified carbon electrode were obtained for various concentraJOURNAL OF ANALYTICAL CHEMISTRY

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tions of Pb2+, Cu2+ and Hg2+ ranging from 1.0 × 10–11 to 1.0 × 10–7 M in 0.1 M acetate buffer solution under the optimized conditions (Fig. 6a). Analytical characteristics of the proposed method are given in Table 1. Three different concentrations of 1.0, 3.0, and 5.0 nM Pb2+, Cu2+ and Hg2+ in the linear range were determined in five independent series on the same day for intra-day precision and on seven consecutive days for inter-day precision from five measurements of every series. The results as data percentages are given in Table 2. The precision (RSD, %) values varied from 0.33 to 1.98 for intra-day and from 0.40 to 2.02 for inter-day precision. The accuracy of the method was calculated as the percent of relative error. Both of the results obtained for intra- and inter-day accuracy were ≤2.3%.

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(a)

10 nm (b)

AuNP

AuNM 50 nm

Fig. 3. TEM imaging of the GO (a) and AuNM (b).

(a)

100 nm (b)

2 µm Fig. 4. TEM (a) and SEM (b) images of the AuNM-GO. JOURNAL OF ANALYTICAL CHEMISTRY

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GOLD NANOPARTICLE INCLUDED GRAPHENE OXIDE MODIFIED ELECTRODE

(a)

2.5

2.0 Current, µA

Current, µA

(b)

2.5

2.0

691

1.5 1.0 0.5

1.5 1.0 0.5 0

0

10

20 30 40 50 Deposition time, min

60

3.0

(с)

2.5

4.5 pH

5.0

5.5

6.0

2.0 Current, µA

Current, µA

4.0

(d)

2.4

2.0

3.5

1.5 1.0

1.6 1.2 0.8

0.5

0.4

0 18 20 22 24 26 28 30 32 34 36 Temperature, °C

2

4 6 8 Suspension volume, µL

10

Fig. 5. Optimization conditions of the metals on GC-GO-MnO2 electrode: deposition time (a), pH (b), temperature (c) and volume of suspension (d). (j), Pb, (d), Cu, (m), Hg.

Table 2. Intra- and inter-day precision and accuracy Added (Pb2+, Cu2+, Hg2+), nM 1 3 5 1.02 ± 0.02 3.01 ± 0.02 4.96 ± 0.03 1.01 ± 0.02 2.96 ± 0.03 5.03 ± 0.02 0.98 ± 0.01 3.07 ± 0.01 5.01 ± 0.02

Method characteristics Intra-day

Found, nM

Precision %

Accuracy %

Inter-day

Found, nM

Precision %

Accuracy %

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Pb2+ Cu2+ Hg2+ Pb2+ Cu2+ Hg2+ Pb2+ Cu2+ Hg2+ Pb2+ Cu2+ Hg2+ Pb2+ Cu2+ Hg2+ Pb2+ Cu2+ Hg2+ Vol. 71

1.96 1.98 1.02 2.00 1.00 –2.00 1.00 ± 0.02 1.01 ± 0.01 0.99 ± 0.02 2.00 0.99 2.02 0 1.00 –1.00 No. 7

2016

0.66 1.01 0.33 0.33 –1.33 2.33 3.02 ± 0.03 3.04 ± 0.02 3.01 ± 0.02 0.99 0.66 0.66 0.67 1.33 0.33

0.60 0.40 0.40 –0.80 0.60 0.20 5.06 ± 0.03 4.97 ± 0.02 5.02 ± 0.03 0.59 0.40 0.60 1.2 –0.6 0.4

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(a) Pb2+

Cu2+

1 µA Hg2+

Blank −800 −600 −400 −200 0 200 400 600 800 1000 Potential, mV (b)

2.4 2.2 Current, µA

2.0 1.8 1.6 1.4 1.2 1.0 0.8 7

8

9

10

11 −log c

Fig. 6. Calibration voltammograms (a) and their calibration curves (b) on the modified surface. (j), Pb, (d), Cu, (m), Hg.

The modified electrode was used to assay Pb2+, Cu2+ and Hg2+ in the seawater samples taken from the coastal region of Anatolia in Turkey. The values of concentrations in seawater samples were checked with ICP-OES. Chemical composition of the samples is given in Table 3. The concentrations of Pb2+, Cu2+ and Hg2+ determined by the proposed method range from 2.89 to 6.55 nM, 8.83 to 22.62 nM and 0.51 to 1.27 nM, respectively. The analysis results are very similar to concentrations in stream or potable waters [49–52]. The high concentrations of Pb, Cu and Hg may be related to human-caused contaminants originating from industrialization of coastal regions. The results reveal that the modified surface provides an analytically useful response for the metals in the seawater samples in terms of sensitivity, reproducibility,

and accuracy. The recovery values were up to 99.2% for Pb2+, 99.7% for Cu2+, and 98.4% for Hg2+. The elements can be found in more than one chemical form in waters. Concentrations of single forms are so low that their direct measurements are currently difficult or at least unreliable. Electrochemical techniques are commonly used by numerous researchers in order to determine metal concentrations in water. This study has presented a successful electrochemical method for the determination of trace Pb2+, Cu2+, and Hg2+ in seawater. The AuNP selfordered nanomaterial included graphene oxide nanostructure modified at the glassy carbon surface. The nanomaterial was characterized using cyclic voltammetry, TEM, SEM, and XPS techniques. The electrode was used for the determination of heavy metals,

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Added, nM

– 1 3

– 1 3

– 1 3

– 1 3

– 1 3

– 1 3

– 1 3

– 1 3

– 1 3

– 1 3

Çanakkale/Assos

Balıkesir/Ayvalık

İzmir/Çeşme

JOURNAL OF ANALYTICAL CHEMISTRY

Aydın/Kuşadası

Vol. 71

Muğla/Marmaris

No. 7

Antalya/Belek

2016

Mersin/Anamur

Adana/Karataş

Hatay/Samandağ

– 1 3

– 1 3

– 1 3

– 1 3

– 1 3

– 1 3

– 1 3

– 1 3

– 1 3

– 1 3

– 1 3

– 1 3

– 1 3

– 1 3

– 1 3

– 1 3

– 1 3

– 1 3

– 1 3

– 1 3

Pb2+ Cu2+ Hg2+

Edirne/Enez

Sample

4.16 ± 0.27 5.13 ± 0.33 7.10 ± 0.29

5.78 ± 0.32 6.81 ± 0.33 8.79 ± 0.29

4.63 ± 0.15 5.59 ± 0.13 7.67 ± 0.21

2.89 ± 0.11 3.88 ± 0.09 5.92 ± 0.13

3.27 ± 0.09 4.26 ± 0.08 6.33 ± 0.09

4.27 ± 0.18 5.23 ± 0.09 7.26 ± 0.14

6.55 ± 0.19 7.52 ± 0.17 9.56 ± 0.13

3.08 ± 0.14 4.04 ± 0.16 6.12 ± 0.23

4.61 ± 0.24 5.59 ± 0.22 7.64 ± 0.35

5.03 ± 0.41 6.01 ± 0.37 7.98 ± 0.23

Pb2+

14.33 ± 0.21 15.28 ± 0.26 17.34 ± 0.22

12.76 ± 0.14 13.83 ± 0.17 15.72 ± 0.21

11.68 ± 0.12 12.73 ± 0.26 14.66 ± 0.17

9.92 ± 0.13 10.93 ± 0.18 12.95 ± 0.27

10.27 ± 0.12 11.30 ± 0.09 13.31 ± 0.10

14.41 ± 0.37 15.44 ± 0.12 17.46 ± 0.26

22.62 ± 0.35 23.60 ± 0.27 25.58 ± 0.41

8.83 ± 0.21 9.80 ± 0.21 11.86 ± 0.26

12.77 ± 0.12 13.74 ± 0.11 15.80 ± 0.16

11.51 ± 0.12 12.54 ± 0.14 14.48 ± 0.17

Cu2+

0.96 ± 0.08 2.02 ± 0.08 3.93 ± 0.11

0.77 ± 0.06 1.82 ± 0.09 3.73 ± 0.12

0.67 ± 0.04 1.69 ± 0.09 3.71 ± 0.12

0.53 ± 0.05 1.56 ± 0.08 3.54 ± 0.11

0.51 ± 0.05 1.52 ± 0.08 3.55 ± 0.08

1.03 ± 0.09 2.08 ± 0.13 3.99 ± 0.12

1.27 ± 0.09 2.31 ± 0.11 4.27 ± 0.13

0.96 ± 0.05 2.01 ± 0.08 3.92 ± 0.12

0.72 ± 0.05 1.73 ± 0.09 3.66 ± 0.11

0.65 ± 0.04 1.63 ± 0.05 3.66 ± 0.04

Hg2+

Found by the suggested method

3.98 ± 0.17 5.02 ± 0.46 6.90 ± 0.18

5.93 ± 0.44 7.09 ± 0.25 8.98 ± 0.33

4.55 ± 0.18 5.58 ± 0.16 7.52 ± 0.11

9.56 ± 0.19 10.58 ± 0.27 12.61 ± 0.22

3.37 ± 0.17 4.33 ± 0.09 6.37 ± 0.11

4.39 ± 0.11 5.43 ± 0.17 7.41 ± 0.23

6.83 ± 0.33 7.80 ± 0.17 9.86 ± 0.19

3.23 ± 0.22 4.27 ± 0.16 6.21 ± 0.18

4.56 ± 0.13 5.60 ± 0.17 7.57 ± 0.20

5.11 ± 0.53 6.16 ± 0.48 8.05 ± 0.36

Pb2+

14.27 ± 0.24 15.33 ± 0.30 17.26 ± 0.32

11.62 ± 0.27 12.58 ± 0.26 14.78 ± 0.41

11.71 ± 0.13 12.75 ± 0.11 14.78 ± 0.15

10.21 ± 0.40 11.18 ± 0.17 13.26 ± 0.20

10.32 ± 0.14 11.36 ± 0.18 13.35 ± 0.09

13.86 ± 0.28 14.90 ± 0.17 16.88 ± 0.23

23.16 ± 0.56 24.13 ± 0.33 26.18 ± 0.18

8.86 ± 0.20 9.91 ± 0.32 11.95 ± 0.42

12.91 ± 0.26 13.86 ± 0.18 16.03 ± 0.21

11.63 ± 0.21 12.71 ± 0.13 14.57 ± 0.21

Cu2+

Hg2+

1.11 ± 0.13 2.17 ± 0.18 4.14 ± 0.21

0.68 ± 0.09 1.65 ± 0.06 3.72 ± 0.09

0.71 ± 0.06 1.76 ± 0.12 13.68 ± 0.14

0.55 ± 0.04 1.59 ± 0.07 3.56 ± 0.09

0.67 ± 0.05 1.64 ± 0.08 3.66 ± 0.09

1.12 ± 0.09 2.15 ± 0.11 4.11 ± 0.08

1.23 ± 0.11 2.27 ± 0.09 4.20 ± 0.11

1.04 ± 0.09 2.07 ± 0.07 4.01 ± 0.11

0.68 ± 0.11 1.71 ± 0.14 3.59 ± 0.22

0.61 ± 0.05 1.60 ± 0.06 3.57 ± 0.08

Found by ICP-OES

Table 3. The results (nM) of simultaneous determination of Pb2+, Cu2+, and Hg2+ in seawater (n = 6)

– 99.4 99.2

– 100.4 100.1

– 99.3 100.5

– 99.7 100.5

– 99.8 101.0

– 99.2 99.9

– 99.6 100.1

– 99.0 100.7

– 99.6 100.4

– 99.7 99.4

Pb2+

– 99.7 100.1

– 100.5 99.7

– 100.4 99.9

– 100.1 100.2

– 100.3 100.3

– 100.2 100.3

– 99.9 99.8

– 99.7 100.3

– 99.8 100.2

– 100.2 99.8

Cu2+

– 103.1 99.2

– 102.8 98.9

– 101.2 101.1

– 102.0 100.3

– 100.7 101.1

– 102.5 99.0

– 101.8 100.0

– 102.6 99.0

– 100.6 98.4

– 98.8 100.3

Hg2+

Recovery with the suggested method, % GOLD NANOPARTICLE INCLUDED GRAPHENE OXIDE MODIFIED ELECTRODE 693

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