Journal of Electroanalytical Chemistry 714-715 (2014) 97–102

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Journal of Electroanalytical Chemistry

journal homepage: www.elsevier .com/locate / je lechem

Selective detection toward Cd2+ using Fe3O4/RGO nanoparticle modifiedglassy carbon electrode

1572-6657/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jelechem.2013.12.030

⇑ Corresponding author. Tel.: +86 551 65591142; fax: +86 551 65592420.E-mail address: syf@ahpu.edu.cn (Y.-F. Sun).

Yu-Feng Sun a,⇑, Wen-Kai Chen a,c, Wen-Juan Li b,c, Tian-Jia Jiang b,c, Jin-Huai Liu c, Zhong-Gang Liu b,c

a College of Mechanical and Automotive Engineering, Anhui Polytechnic University, Wuhu, Anhui 241000, PR Chinab Department of Chemistry, University of Science and Technology of China, Hefei 230026, PR Chinac Research Center for Biomimetic Functional Materials and Sensing Devices, Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei 230031, PR China

a r t i c l e i n f o

Article history:Received 15 October 2013Received in revised form 20 December 2013Accepted 23 December 2013Available online 3 January 2014

Keywords:Magnetite–reduced graphene oxideCd2+

SelectivityElectrochemical detection

a b s t r a c t

This work reports the detailed study on the magnetite–reduced graphene oxide (Fe3O4–RGO) modifiedglassy carbon electrode, which could be used for the selective electrochemical detection of Cd2+. TheFe3O4–RGO modified glassy carbon electrode (GCE) was characterized by cyclic voltammetry (CV) andelectrochemical impedance spectroscopy (EIS). Square wave anodic stripping voltammetry (SWASV)was applied for the detection of Cd2+. The limit of detection with the Fe3O4–RGO modified electrodetoward Cd2+ was calculated as 0.056 lM (3r method). The electrochemical parameters that exert influ-ence on deposition and stripping of metal ions, such as supporting electrolytes, pH values, depositionpotential and deposition time, were optimized. Besides, the interference and stability measurementswere also evaluated under the optimized parameters. More importantly, selective detection towardCd2+ was achieved. It is meaningful for electrochemical method to overcome the great challenge ofimproving the selectivity toward heavy metal ions.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Heavy metal ions poses adverse toxic risks to human health atvery low concentrations [1–4]. Over the past several decades, avariety of analytical methods have been used for heavy metal ionsdetection, such as solid phase spectrophotometry (SPS) [5], electro-thermal atomic absorption spectrometry (ET-AAS) [6], cold vaporgeneration-atomic absorption spectrometry (CVG-AAS) [7], induc-tively coupled plasma atomic emission spectrometric (ICP-AES) [8],flame atomic absorption spectrometry (FASS) [9], inductively cou-pled plasma mass spectrometric (ICP-MS) [10]. However, thesemethods require high operating cost and expensive instruments,which are not suitable for the detection of a large number of sam-ples. Therefore, it is extremely urgent to develop a simple andquick detection method towards heavy metal ions. The electro-chemical method, as an alternative to these spectroscopic tech-niques, has been accepted as an efficient method to detect heavymetal ions, which offers advantages in terms of high sensitivityand fast analysis speed, simple instrument, low cost, favorable sta-bility and easy to operate [11–14].

However, one of the great challenges is how to improve theselectivity of sensing heavy metal ions using electrochemicalmethods. To achieve the goal, much efforts have been devoted to

modifying the electrode with the functional groups or DNA bio-molecules. The good selectivity and sensitivity detection towardsheavy metal ions were obtained after modifying some functionalgroups on electrode surface or sensor [15–19]. For instance, Ali-zadeh et al. have reported the selective determination of Cd2+ usinga carbon paste electrode impregnated with Cd2+ imprinted poly-mer [20]. Complexing polymer film-coated electrodes realizedthe trace analysis of Cd2+ [21,22]. Huang et al. reported electro-chemical determination of Cd2+ using carbon-based nanomaterials[3,23–25]. Luo et al. have reported a multi-walled carbon nano-tubes-sodium dodecyl benzene sulfonate modified stannum filmelectrode to detect Zn2+ [26]. However, these methods suffer fromcomplex modification processes and synthesis of modifiers[16,18,27,28]. Besides, bismuth-plated carbon microelectrodes,bimetallic Hg–Bi/single-walled carbon nanotubes, bismuth-modi-fied multi-walled carbon nanotubes, etc., have also been reportedfor analysis of Cd2+ [29–31]. Therefore, it is important and neces-sary to develop novel and simple materials to improve the selec-tive of electrochemical detection method.

Magnetite–reduced graphene oxide (Fe3O4/RGO) has provokedincreasing research interest due to the large surface area and sta-bility of the reduced graphene oxide (RGO) and strong magneticof magnetite (Fe3O4), which has been applied in water treatment,lithium-ion batteries, drug delivery, magnetic-controlled switches,and microwave absorbing areas [32]. Bahadur et al. have reportedthe electrochemical detection of Cr3+ using Fe3O4–RGO modified

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electrodes [33]. Huang et Al. reported electrochemical detection ofAs(III) using Fe3O4–RTIL composite modified screen-printed carbonelectrode for the first time [34]. In this work, we modify a glass car-bon electrode (GCE) using Fe3O4–RGO composites for selectiveanalysis of Cd2+ by square wave anodic stripping voltammetry(SWASV), and the selective detection toward Cd2+ was achieved.The optimizing of experimental conditions, such as supportingelectrolytes, pH values, deposition potential, and deposition timewere investigated. Other heavy Metal ions, such as Pb2+, Hg2+,Cu2+ and Zn2+, were chosen as potential interfering ions for inves-tigating the electrochemical selective behavior of Fe3O4–RGO com-posites toward Cd2+. Furthermore, the stability of Fe3O4–RGOmodified electrode was also studied.

2. Experimental

2.1. Chemical reagents

Graphite (325 mesh) was purchased from Alfa Aesar, and allother reagents were obtained from Sinopharm Chemical ReagentCo., Ltd. (China). All reagents and solvents were analytical gradeand used as received without further purification. 0.1 M acetatebuffer solutions (HAc–NaAc) with different pH values were pre-pared by mixing stock solutions of 0.1 M HAc and 0.1 M NaAc.The deionized water (18.2 MX cm) used to prepare all solutionswas purified with the NANOpureDiamondTM UV water system.

2.2. Apparatus

All electrochemical experiments were recorded using a CHI660D computer-controlled potentiostat. Measurements were car-ried out in a conventional three-electrode cell using the modifiedor bare glassy carbon electrode (GCE, 3 mm in diameter) as a work-ing electrode, Ag/AgCl as a reference electrode and Pt wire as acounter electrode. The scanning electron microscopy (SEM) imageswere obtained by a field-emission scanning electron microscope(FESEM, Quanta 200 FEG, FEI Company, USA). X-ray diffraction(XRD) patterns were gained with a Philips X’Pert Pro X-ray diffrac-tometer with Cu Ka radiation (1.5418 Å).

2.3. Preparation of Fe3O4–RGO composites

Graphene oxide (GO) was prepared from natural graphite basedon Hummers’ method [35]. Fe3O4–RGO composites were preparedvia a hydrothermal method according to previous report [36].Briefly, 0.036 g GO was first dispersed in 40 mL deionized water.At the same time, 0.270 g FeCl3�6H2O and 0.528 g ascorbic acidwere added into the beaker, forming a homogenous solution byultrasonication. 10 mL hydrazine hydrate was added to the abovesolution under stirring. Then, the black solution was transferredinto a 50 mL Teflon-lined stainless steel autoclave and heated at180 �C for 8 h. After cooling to room temperature, solid precipitatewas collected by centrifugation and washed three times with ultra-pure water and alcohol, respectively. Finally, the Fe3O4–RGO com-posites were obtained by drying at 60 �C under vacuum for 12 h.

2.4. Fabrication of modified electrode

Prior to modification, bare GCE was polished with 1.0, 0.3 and0.05 lm of alumina power, respectively, and rinsed with ultrapurewater, followed by sonication in 1:1 HNO3, alcohol and ultrapurewater successively and dried under nitrogen. Then 5 mg Fe3O4–RGO composites were dissolved in 10 mL of the alcohol solutionand sonicated for 10 min to get a homogenous suspension. A4 lL aliquot of this suspension was then dropped onto the surface

of GCE. The electrode was then allowed to air-dry at room temper-ature. For comparison, Fe3O4 modified electrode was fabricatedusing the same process. Note that RGO as a modifier could adsorbmetal ions and produce a strong stripping current, but suffer fromdesorption, leading to that pure RGO modified electrode could notbe regenerated [24]. Thus, the electroanalysis on the pure RGOmodified electrode was not included in this work.

2.5. Electrochemical detection

Square wave anodic stripping voltammetry (SWASV) was usedfor the observation of electrochemical behavior under optimizedconditions. Cd, Pb, Cu, Hg, and Zn were deposited at the potentialof �1.0 V for 120 s by the reduction of Cd2+, Pb2+, Cu2+, Hg2+ andZn2+ in 0.1 M HAc–NaAc (pH 5.0). The anodic stripping (reoxidationof metal to metal ions) of electrodeposited metal was performed inthe potential range of �1.0 to �0.2 V at the following optimizedcondition: frequency, 15 Hz; amplitude, 25 mV; increment poten-tial, 4 mV; vs Ag/AgCl. A desorption potential of 0.1 V for 150 swas performed to remove the residual metals under stirring condi-tion. The same experiment conditions were applied in the interfer-ence, stability and selective studies. Cyclic voltammograms (CV)and electrochemical impedance spectra (EIS) were performed inmixing solution of 5 mM FeðCNÞ3�=4�

6 with 0.1 M KCl and the scan-ning rate was 100 mV s�1.

3. Result and discussion

3.1. Characterization of Fe3O4–RGO composites

The morphology and structure of the as-prepared Fe3O4–RGOcomposites were characterized by SEM and XRD, respectively.Fig. 1 shows the SEM images of RGO, Fe3O4 and Fe3O4–RGO. Asshown in Fig. 1a, the morphology of RGO was full of wrinkles.The wrinkles formed a curled morphology, which was representedthe intrinsical characterization of the RGO [37]. It can be found inFig. 1b that the size distribution of Fe3O4 was non-uniform due tothe agglomerates to large particles [36,37]. From the SEM imageshown in Fig. 1c, the obtained Fe3O4–RGO shows a layer-by-layerassembled structure. In addition, the presence of RGO can preventthe aggregation of Fe3O4 nanoparticles, which was consistent withthe previous reports [36–39].

Fig. 2 shows the XRD pattern of RGO, Fe3O4 and Fe3O4–RGOcomposites. Curve a shows a weak and broad diffraction peak at2h = 23.5� (002). The broad peak corresponded to the reflectionpeak of the RGO [36,40]. As seen in curves b and c, the peaks rep-resented the Bragg reflections of (220), (311), (440), (422), (511)and (440), which matched well with that of Fe3O4 nanoparticles(JCPDS Card No. 19-0629) [36,41,42]. However, for the Fe3O4–RGO composites, all peaks in the XRD pattern of Fe3O4 nanoparti-cles can also be observed. While no obvious peak diffraction ofRGO was observed, which indicated that the stacking of graphenesheets in the Fe3O4–RGO nanocomposite was disordered. The dis-tribution of RGO sheet can be confirmed by the SEM images [37].The result demonstrated that Fe3O4–RGO composites were suc-cessfully prepared by the hydrothermal method.

3.2. CV and EIS characterization of Fe3O4–RGO composites

The cyclic voltammetric responses of bare, Fe3O4 and Fe3O4–RGO composite modified GCE have been characterized using thesolution of 5 mM FeðCNÞ3�=4�

6 containing 0.1 M KCl (Fig. 3).Fig. 3a displays the CV responses of bare GCE, Fe3O4 and Fe3O4–RGO modified electrodes. As compared with the bare GCE, the ano-dic and cathodic peak currents were decreased at Fe3O4 modified

Fig. 1. SEM images of (a) RGO, (b) Fe3O4 nanoparticles and (c) Fe3O4–RGO composites.

Fig. 2. XRD pattern of the (a) RGO, (b) Fe3O4 nanoparticles and (c) Fe3O4–RGOcomposites.

Fig. 3. (a) Cyclic voltammograms (CV) and (b) electrochemical impedance spectra(EIS) on bare GCE (red line), Fe3O4 (black line) and Fe3O4–RGO (blue line) modifiedelectrode in the solution of 5 mM FeðCNÞ3�=4�

6 and 0.1 M KCl. In panel (a), the scanrate is 100 mV s�1. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)

Y.-F. Sun et al. / Journal of Electroanalytical Chemistry 714-715 (2014) 97–102 99

GCE. This was due to the poor conductivity of Fe3O4. On the con-trary, the well-defined peak and higher current on Fe3O4–RGOmodified electrode were obtained, which was attributed to theextraordinary electrical conductivity of RGO. This result indicatedthat the combination of Fe3O4 and RGO may provide the necessaryconduction pathways on the electrode surface and a better electro-chemical behavior, resulting in the promotion of electron transferprocess on the modified electrode surface.

EIS was employed to further characterize the interface proper-ties of the modified electrodes. Generally, EIS consists of two parts,a semicircle part and a linear part. From the semicircle at higherfrequencies, we can get the electron transfer resistance (Ret) andthe linear at the lower frequencies corresponds to the diffusionprocess [27,43,44]. As shown in Fig. 3b, the electron transfer resis-tance value was about 600 X corresponding to the Fe3O4 modifiedelectrode. After the electrode was modified with Fe3O4–RGO, thesemicircle domain with electron transfer resistance value de-creased and displayed an almost straight line, indicating the pro-motion of electron transfer process on the modified electrodesurface. These results were supported by the above CV data.

3.3. Electrochemical detection of Cd2+ with Fe3O4–RGO modifiedelectrode

The SWASV was applied in this work due to the better sensitiv-ity and lower background than the CV technology [23]. Fig. 4 pre-sents the SWASV analytical characteristic of bare, Fe3O4, andFe3O4–RGO modified GCE. When the accumulation process wasexecuted for 120 s at �1.0 V in a solution containing 0.6 lM Cd2+

in 0.1 M HAc–NaAc (pH 5.0), nearly no current peaks on bare (blackline) and Fe3O4 modified GCE (red line). The higher peak current forthe Cd2+ was obtained on Fe3O4–RGO modified GCE (blue line). Theincrease in peak current on Fe3O4–RGO modified GCE shows that

the Fe3O4–RGO composite was very suitable for the accumulationprocess of Cd2+ on the electrode surface (Cd2+ can be identified atpotentials of �0.83 V), thus improving the sensitivity.

Under the optimal experimental conditions, Cd2+ was deter-mined on Fe3O4–RGO modified electrode using SWASV. Fig. 5shows the SWASV response toward Cd2+ over the concentrationrange of 0–0.8 lM. The linearization equation was i/lA = �3.183 + 14.82 c/lM, with the correlation coefficients of0.985 (inset of Fig. 5). The limit of detection (LOD, calculated by3r method) was 0.056 lM. The sensitivity and detection limit ofthe present study, together with previously determined valuesfor Cd2+ electrochemical sensing in various other modified elec-trodes, are summarized in Table 1. It can be observed that the pref-erable sensitivity and lower detection limit can be obtained at theFe3O4–RGO modified electrode.

For comparison, bare GCE and Fe3O4 modified electrode werealso evaluated under the same condition. The sensitivities of bareGCE and Fe3O4 modified electrode toward Cd2+ were3.060 lA lM�1 and 1.283 lA lM�1, respectively. The calculated

Fig. 4. SWASV responses for 0.6 lM Cd2+ on bare GCE (black line), Fe3O4 (red line)and Fe3O4–RGO (blue line) modified electrode in 0.1 M HAc–NaAc (pH 5.0).Deposition potential, �1.0 V; deposition time, 120 s; amplitude, 25 mV; incrementpotential, 4 mV; frequency, 15 Hz; vs Ag/AgCl. (For interpretation of the referencesto color in this figure legend, the reader is referred to the web version of thisarticle.)

Fig. 5. Typical SWASV stripping signals of Cd2+ on a Fe3O4–RGO modified electrode.SWASV conditions are identical to those in Fig. 4. The dotted line refers to thebaseline. Inset is the calibration plot of SWASV peak currents against Cd2+

concentrations. Error bars correspond to standard errors measured from threeindependent measurements.

Fig. 6. Selectivity studies of Fe3O4–RGO modified electrode toward five heavy metalions (Zn2+, Cu2+, Pb2+, Hg2+ and Cd2+) in 0.1 M HAc–NaAc (pH 5.0).

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LODs (3r method) were 0.138 lM and 0.192 lM, respectively. Theresults further demonstrated that high sensitivity and low detec-tion limit toward Cd2+ can be obtained on the Fe3O4–RGO modifiedelectrode.

3.4. Selectivity and interference measurements

The selective response of Fe3O4–RGO modified GCE is betterinvestigated by observing the stripping signals of five heavy metal

Table 1Comparison of current sensitivity with previously reported values of different electrodes.

Electrodes

SnO2 Tube-in-Tube nanostructure modified glassy carbon electrodea-Cyclo-dextrin-modified carbon paste electrodeb-Cyclo-dextrin-modified carbon paste electrodeBi-film modified carbon paste electrodeBismuth film modified glass carbon electrodeA micro/nanoparticle bismuth film modified glass carbon electrodeN-BDMP modified carbon paste electrodeIonicliquid-functionalized ordered mesoporous silica SBA-15 modified carbon paste eGlass carbon electrodeFe3O4–RGO modified glass carbon electrode

LOD, limit of detection; N-BDMP, nitro benzoyl diphenylmethylenphosphorane.

ions (Hg2+, Cu2+, Zn2+, Pb2+ and Cd2+) under the optimal experimen-tal conditions are displayed in Fig. 6. It is obvious that the obtainedstripping current toward Cd2+ is around 3 times higher than thatPb2+, 15 times higher than that of Zn2+, Cu2+ and Hg2+. The resultsdemonstrated that Fe3O4–RGO modified electrode had a highselectivity toward Cd2+.

In order to investigate the interference of the other heavy metalions (Hg2+, Cu2+, Zn2+ and Pb2+) to the electrochemical detection ofCd2+, a series of interference measurements were studied. Fig. 7presents the SWASV responses of the Fe3O4–RGO modified elec-trode toward Cd2+ (0.8 lM) in 0.1 M HAc–NaAc (pH 5.0) with theexisting of various concentrations of Hg2+, Cu2+, Zn2+ and Pb2+,respectively. The interference results of Pb2+, Cu2+, Zn2+ and Hg2+

toward Cd2+ collected from SWASV responses were summarizedand showed in Fig. 8. According to Fig. 8, the peak currents ofCd2+ were decreased by 23%, 42% and 48% in the presence of2 lM Zn2+, Cu2+ and Pb2+, respectively. The possible reason wasthat unobvious interference of Pb2+, Cu2+, and Zn2+ on the adsorp-tion ability of Fe3O4–RGO toward Cd2+. On the contrary, the peakcurrent of Cd2+ became much higher after adding 2 lM Hg2+ intothe solution in which 0.8 lM Cd2+ was existed, which was sug-gested the formation of Cd–Hg intermetallic compounds on theelectrode [52,53]. It was known that the mercury film electrodehad been widely used to strengthen the sensitivity in the detectionof heavy metal ions. Thus, the formation of a Hg film and followedby the Cd–Hg intermetallic compound formation on the surface ofFe3O4–RGO modified electrode increased the peak current for Cd2+.

3.5. Stability measurement

Repetitive deposition by the reduction of Cd2+ anodic stripping(reoxidation of Cd0 to Cd2+) of electrodeposited Cd0 cycles werethen performed to characterize the reproducibility of the electrodeperformance. Fig. 9 shows the SWASV results of 0.8 lM Cd2+ onFe3O4–RGO modified electrode after continuous cycling for 16

Sensitivity (lA lM�1) LOD (lM) Ref.

12.88 0.1 [45]2.8 2.03 [46]0.42 2.51 [46]0.839 0.714 [47]0.011 0.223 [48]0.029 0.098 [48]0.1 0.059 [49]

lectrode 20.51 0.08 [50]0.74 0.086 [51]14.82 0.056 This work

Fig. 7. Interference studies of typical SWASV responses of 0.8 lM Cd2+ on Fe3O4–RGO modified electrode in HAc–NaAc (pH 5.0) in the presence of (a) Pb2+, (b) Cu2+, (c) Hg2+

and (d) Zn2+, respectively. The dotted line refers to the baseline. SWASV conditions are identical to those in Fig. 4.

Fig. 8. Interference studies of Fe3O4–RGO modified electrode at 0.8 lM Cd2+ in thepresence of 2.0 lM Zn2+, Cu2+, Pb2+ and Hg2+, respectively.

Fig. 9. Stability measurements of 0.8 lM Cd2+ on Fe3O4–RGO modified electrode in0.1 M HAc–NaAc (pH 5.0). Data were collected from every SWASV response at about�0.83 V shown in the inset. SWASV conditions are identical to those in Fig. 4. Thedotted line refers to the baseline.

Y.-F. Sun et al. / Journal of Electroanalytical Chemistry 714-715 (2014) 97–102 101

times. We found that the stripping current of the electrode wasnearly constant. The relative standard deviation (RSD) in the peakcurrents was 8.47%. Thus, it demonstrated that the Fe3O4–RGO

modified electrode showed favorable stability under the optimizedcondition, even when the cycling number was increased.

4. Conclusion

In summary, excellent electrochemical performance to selectivedetection toward Cd2+ has been obtained, based on SWASV onFe3O4–RGO modified electrode. In addition, the favorable sensitiv-ity (14.82 lA lM�1) and LOD (0.056 lM) for Cd2+ were achievedunder the optimized conditions. There were insignificant re-sponses to Hg2+, Cu2+, Zn2+ and Pb2+. This indicated that Fe3O4–RGO modified electrode showed a high selectivity toward Cd2+.Moreover, the Fe3O4–RGO modified electrode offered excellent sta-bility in the electrochemical determination of Cd2+. This findingprovided a potential material to improve the selectivity of electro-chemical detection of toxic metal ions.

Acknowledgements

This work was supported by the National Key Scientific andNanotechnology (2011CB933700) and National Natural ScienceFoundation of China (61174012).

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