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Development of an electrochemically reduced graphene oxide modified

disposable bismuth film electrode and its application for stripping

analysis of heavy metals in milk

Jianfeng Ping, Yixian Wang, Jian Wu , Yibin Ying

College of Biosystems Engineering and Food Science, Zhejiang University, Hangzhou 310058, PR China

a r t i c l e i n f o

Article history:

Received 14 June 2013Received in revised form 7 October 2013Accepted 5 November 2013Available online 17 November 2013

Keywords:

Electrochemically reduced graphene oxideAnodic stripping voltammetryScreen-printed electrodeHeavy metalsMilk

a b s t r a c t

A novel electrochemical sensing platform based on electrochemically reduced graphene oxide filmmodified screen-printed electrode was developed. This disposable electrode shows excellent conductivityand fast electron transfer kinetics. By in situ plating bismuth film, the developed electrode exhibitedwell-defined and separate stripping peaks for cadmium and lead. Several parameters, including electro-lytes environment and electrodeposition conditions, were carefully optimized to achieve best strippingperformance. The linear range for both metal ions at the disposable bismuth film electrode was from1.0 lg L1 to 60.0 lg L1. The detection limit was 0.5 lg L1 for cadmium ion and 0.8 lg L1 for leadion. Milk sample analysis demonstrates that the developed electrode could be effectively used to detectlow levels (lg L1) of cadmium ion and lead ion. Graphene based disposable bismuth film electrode is asensitive, stable, and reliable sensing platform for heavy metals determination.

2013 Elsevier Ltd. All rights reserved.

1. Introduction

Milk is an important food of animal origin and contains most ofessential nutrients for a healthy diet. It is the only food consumedby newborn babies, but in general, human beings continue to drinkmilk during the entire life cycle. However, current studies haverevealed that milk may contain varying amounts of different toxicheavy metals, such as Cd and Pb (Ataro, McCrindle, Botha,McCrindle, & Ndibewu, 2008). Especially in the developingcountries, numerous milk safety incidents associated with heavymetals contamination have been reported recently (Rahimi,2013; Xia, Chen, Liu, & Liu, 2011). Since the quick accumulationof heavy metals in human body and the toxicity towards thehuman organs, the determination of toxic heavy metals in milk is

becoming increasingly important.Current methods employed to detect toxic heavy metals, suchas atomic absorption spectrometry (AAS) (Bagheri, Afkhami,Saber-Tehrani, & Khoshsafar, 2012), inductively coupled plasmamass spectrometry (ICP-MS) (Ataro et al., 2008), and X-ray fluores-cence spectrometry (XFS) (Sogut, Bali, Baltas, & Apaydin, 2013), arealways labelled with expensive and time-consuming. Moreover,these detection techniques need to be operated in specializedlaboratory that makes them unsuitable for application in the field(Quintana et al., 2012). On the other hand, the complex emulsion

like matrices and low concentration levels of metal ions(commonly at lg kg1 level) in milk samples make the determina-tion a difficult task (Kazi et al., 2009). For these reasons, thedevelopment of sensitive, robust, and simple methods for quantify-ing trace metals is still highly desirable and challenging.

Electrochemical methods, particularly anodic stripping voltam-metry (ASV), have been proven to be a powerful tool for fast accessto metal ions information in complex samples due to their highsensitivity, easy operation, and low cost of analysis (Herzog & Beni,2013). Furthermore, analytical instruments for ASV are relativelyportable, compact and inexpensive compared with its spectro-scopic counterparts, providing considerable feasibility for on-sitemeasurements, e.g., biomedical, environmental and food monitor-ing (Ping, Wu et al., 2011;Wang, 2005). In most cases, mercury

film electrodes (MFEs) are preferred due to their excellent strip-ping characters. However, the toxicity of mercury limits its wideapplications, especially those involved in food contact. Recently,bismuth film electrodes (BiFEs) have been recognized as a promis-ing substitute for MFEs in view of their low toxicity and compara-ble analytical performance to MFEs (Economou, 2005). Bismuthcan normally be deposited on many electrode substrates, includingglassy carbon electrode, carbon paste electrode, screen-printedelectrode, boron doped diamond electrode, and some noble metalelectrodes (vancara, Prior, Hocevar, & Wang, 2010). Amongstthem, screen-printed electrodes (SPEs) have been attracted exten-sive attention in stripping analysis of heavy metals due to theinherent superiority of their manufacturing process, which is

0308-8146/$ - see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.foodchem.2013.11.026

Corresponding author. Tel./fax: +86 571 88982180.E-mail address:[email protected](J. Wu).

Food Chemistry 151 (2014) 6571

Contents lists available at ScienceDirect

Food Chemistry

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / f o o d c h e m
http://dx.doi.org/10.1016/j.foodchem.2013.11.026mailto:[email protected]://dx.doi.org/10.1016/j.foodchem.2013.11.026http://www.sciencedirect.com/science/journal/03088146http://www.elsevier.com/locate/foodchemhttp://www.elsevier.com/locate/foodchemhttp://www.sciencedirect.com/science/journal/03088146http://dx.doi.org/10.1016/j.foodchem.2013.11.026mailto:[email protected]://dx.doi.org/10.1016/j.foodchem.2013.11.026http://-/?-http://-/?-http://-/?-http://-/?-http://crossmark.crossref.org/dialog/?doi=10.1016/j.foodchem.2013.11.026&domain=pdfhttp://-/?-

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inexpensive, rapid, simple and capable of mass production (Met-ters, Kadara, & Banks, 2013; Serrano, Alberich, Daz-Cruz, Arino,& Esteban, 2013). There are several available methods for the prep-aration of BiF modified SPE (SPE/BiF), such as ex situ, in situ, andbulk approaches (Hwang, Han, Park, & Kang, 2008; Khairy, Kadara,Kampouris, & Banks, 2010; Serrano, Diaz-Cruz, Arino, & Esteban,2010). In the past years, these disposable sensors have been suc-cessfully used to determine heavy metals in various samples, suchas soil, vegetable, seawater, and blood (Metters, Kadara, & Banks,2011).

Graphene, an up-rising star of carbon-based nanomaterial, is asheet of sp2 bonded carbon atoms that are arranged into a honey-comb structure, drawing tremendous interest from both the exper-imental and theoretical scientific communities since its discoveryin 2004 (Novoselov et al., 2004). Like carbon nanotubes (CNTs),graphene provides an effective avenue for fabricating electrochem-ical sensing devices due to its large specific surface, high electronconductivity, and excellent biocompatibility (Liu, Dong, & Chen,2012). Graphene-based MFEs (Li, Guo, Zhai, & Wang, 2009b) orBiFEs (Li, Guo, Zhai, & Wang, 2009a;Sahoo et al., 2013) have beendeveloped for the determination of heavy metal ions with highsensitivity and good reproducibility. Until now, graphene filmmodified electrodes are mostly prepared by drop-casting graphenedispersion obtained from chemically reduced graphene oxide(CRGNO). However, such a preparation methodology suffers fromsome limitations. Firstly, the preparation of CRGNO always needsexcessive toxic reducing agents (Li, Mller, Gilje, Kaner, & Wallace,2008). Second, since the pure carbon materials are not soluble inmost kinds of solvents, a suitable dispersion method must be usedto obtain homogeneous carbon materials dispersion solution be-fore the drop-casting process, particularly in the preparation ofgraphene films (Chen, Tang, Wang, Liu, & Luo, 2011). Group func-tionalization or the use of surfactants could obtain well-dispersedgraphene solution (Georgakilas et al., 2012). However, the groupfunctionalization can adversely influence the electronic propertiesof graphene nanosheets. Whilst the residual surfactants in graph-

ene film may inhibit the stripping process in the determinationof cadmium ion (Brownson & Bank, 2011;Brownson, Kampouris,& Banks, 2012). Therefore, the search for green and rapid methodto build graphene film-based electrode are crucial to further ex-pand the application of graphene in detection of heavy metals.

Recently, a promising strategy in graphene synthesis based onelectrochemical method to produce electrochemically reducedgraphene oxide (ERGNO) has been introduced (Zhou et al., 2009).This method offers several advantages over other graphene prepa-ration methods, including green, efficient, inexpensive, and rapid(Ping, Wang, Fan, Wu, & Ying, 2011). Moreover, due to the insolu-bility of graphene in common solvents, the obtained ERGNO couldremain and further form a stable film on the electrode surface thatsignificantly reduces the fabrication time in graphene film-based

electrodes (Chen et al., 2011; Ping, Wang, Ying, & Wu, 2012). De-spite such remarkable advantages of the synthesis as well as theexcellent physicochemical properties of ERGNO film, its electro-chemical sensing applications in determination of heavy metalsyet have not been explored so far.

In this work, a novel disposable sensing platform based on ERG-NO film modified SPE (SPE/ERGNO) was present. The ERGNO filmwas prepared by on-step electrodeposition of the exfoliated graph-ene oxide (GNO) onto the SPE surface. For heavy metals sensing,the developed SPE/ERGNO was plated with BiF by in situapproach. Results shows that BiF modified SPE/ERGNO (SPE/ERG-NO/BiF) exhibited excellent stripping performance for simulta-neous analysis of cadmium ion (Cd2+) and lead ion (Pb2+) viasquare wave anodic stripping voltammetry (SWASV). Furthermore,

the developed disposable electrode was applied to determine Cd2+

and Pb2+ in milk sample extracts.

2. Experimental

2.1. Reagents

All chemicals were of analytical grade and used without anyfurther purification. Graphite powder (320 mesh, spectral pure)and N,N0-dimethylformamide (DMF) were purchased from Sinop-

ham Chemical Reagent Co., Ltd. (Shanghai, China). Graphite oxide(GO) was synthesized from graphite powder by using a modifiedHummers method (Hummers & Offeman, 1958; Kovtyukhovaet al., 1999). Exfoliation of GO to graphene oxide (GNO) wasachieved by ultrasonication of GO dispersion (0.5 wt%) using asupersonic cleaner (SK3300HP, 180W, Shanghai, China). Chemi-cally reduced graphene oxide (CRGNO) was synthesized by thechemical reduction of GNO with hydrazine (Li et al., 2008) andthen dispersed in DMF solvents. The commercially available silverink (Electrodag 427SS), conductive graphite ink (Electrodag 423SS),and insulating ink (Electrodag 452SS) were obtained from AchesonCo., Ltd. (USA). Standard solutions of Cd2+ and Pb2+ (1000 mg L1)were prepared and diluted as required. Except note, acetate buffersolution (NaAcHAc, 0.1 mol L1, pH 4.5) was used as the support-ing electrolyte. Millipore-Q (18.2 MX cm) water was used for allexperiments.

2.2. Apparatus

Tapping-mode atomic force microscopy (AFM) measurementswere made on the mica with a Dimension Icon AFM equipped witha ScanAsyst (Bruker AXS, Germany). Scanning electron microscopy(SEM) images were collected on a Zeiss Ultra-55 field emissionscanning electron microscope (Carl Zeiss Microscopy, Germany).Raman spectra were conducted using a micro-Raman spectrometer(Jobin Yvon LabRam HRUV, France, excited by 514 nm laser excita-tion). Electrochemical impedance spectroscopy (EIS) experimentswere carried out using a Solartron Analytical model 1260 Imped-

ance-Gain-Phase Analyzer in combination with a model 1287 Elec-trochemical Interface (Solartron Analytical, UK). Voltammetricmeasurements were carried out with using a PalmSens (PalmInstrument BV, Houten) that consists of a portable potentiostatinterfaced with a palmtop PC. The electrochemical cell was assem-bled with a conventional three-electrode system: a saturated Ag/AgCl reference electrode, a Pt wire auxiliary electrode, and the pre-pared disposable electrode. The electrochemical experiments werecarried out in a one-compartment electrochemical cell. All theexperiments were performed at room temperature.

2.3. Electrode preparation

The fabrication of SPE was performed on a semi-automaticscreen printer (Z-C3050A, Zheng Ting Screen Printing MaterialCo. Ltd., Shanghai, China). The size of screen mesh was about150 lm. Prior to the printing process, the ceramic substrate(5 mm thickness) was cleaned with ethanol and distilled water,and then dried at room temperature. The first layer on the ceramicsubstrate is made from silver ink which was used to act as the con-nections and reference electrode. The drying process of silver layerwas performed by putting the ceramic substrate in an oven for15 min at 93 C. Then, the conductive graphite ink was used toprint the counter electrode and working electrode. The evaporationof solvents was performed by heating the substrate in an oven for5 min at 121 C. Finally, an insulating ink was printed to cover theconnections and define the working electrode area. After irradiat-

ing with 254 nm ultraviolet, the home-made SPEs were obtained.The working area of these prepared SPEs was calculated as 0.08

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cm2 (4 mm 2 mm) with a connecting strip (8 mm 2 mm) andan electric contact end (6 mm 2 mm).

The preparation of SPE/ERGNO was performed by a one-stepelectrodeposition process, in which a working potential of0.8 V(vs. Ag/AgCl) was applied for 600 s in a nitrogen purged GNOdispersion solution (0.5 mg L1) with a magnetic stirrer. For com-parison, the CRGNO film modified SPE (SPE/CRGNO) was preparedby drop-casting 20

lL of CRGNO-DMF suspension (0.5 mg L1) on

the SPE surface and dried at room temperature in the air for 2 h.

2.4. Sample collection and treatment

Eight types of retail packaged milk were purchased from a localsupermarket (Hangzhou, China). The treatment (extraction pro-cess) of these milk samples was performed by ultrasonication-as-sisted acid digestion method (Quintana et al., 2012). In a typicalprocedure, 100 lL of hydrogen peroxide (30 wt%) was added into40 g of milk sample under stirring. Then the mixed solution wassonicated for 15 min. After that, 5 mL of hydrochloric acid(36.5 wt%) and 5 mL of acetic acid (50 wt%) were gradually addedinto the mixed solution and the solution was then sonicated for

10 min. The obtained solution was then subjected to centrifugationat 10,000gfor 20 min. The supernatant including the toxic metalions (Cd2+ and Pb2+) was collected and filtered through a mem-brane with a pore size of 0.22 lm. Before transferring to electro-chemical cells, the filtrate was adjusted to pH 4.5 by adding0.1 mol L1 NaOH solution under pH monitoring and diluted withdistilled water to a final volume of 80 mL.

2.5. Measurement procedure

The electrochemical analysis of Cd2+ and Pb2+ by these dispos-able SPEs was carried out using an in situ electroplating Bi pro-cedure in the presence of dissolved oxygen, in which the SPEswere immersed into 10 mL of metal ions standard solutions or

sample extracted solutions containing 800 lg L1

Bi(NO3)3. Themeasurements were performed in square wave anodic strippingvoltammetric (SWASV) mode, which contains an initial precondi-tion (or clean step), a time-controlled electrochemical deposition,an equilibration period, and a positive-applied potential squarewave stripping scan. The parameters of SWASV mode are as fol-lows: Econd= 0 V, tcond= 60 s; Edep= 1.2 V, tdep= 150 s; teq= 10 s;Ebegin= 1.2 V, Eend= 0.3 V, Estep= 5 mV, Eampl= 25 mV, f= 25 Hz.A magnetic stirrer was used to stir the test solutions during theelectrodeposition and precondition steps.

3. Results and discussion

3.1. Basic characteristics of SPE/ERGNO

The as-prepared ERGNO was first characterized by trapping-mode AFM. As shown inFig. 1A, the thickness of single ERGNOsheet is 0.8 nm, which is smaller than the thickness of GNO(1.2 nm, data not shown) and comparable to the reported appar-ent thickness of single-sheet CRGNO (Li et al., 2008), suggesting thesingle-sheet nature of ERGNO in this work. The morphology ofERGNO film on electrode surface was characterized by SEM(Fig. 1B), which presents uniform surface topography. The crystalstructures of GNO and ERGNO film were investigated by the Ramanspectra (Fig. 1C). Two prominent peaks at 1352 and 1594 cm1 arepresent on the spectra of GNO, corresponding to the well-docu-mented D and G bands, respectively. After electrochemical reduc-tion, these two peaks remain at the same position. However, the

intensity ratio (ID/IG) of the two peaks increases from 0.85 (GNO)to 1.12 (ERGNO), indicating the restored ordered crystal structure

of ERGNO and the smaller sp2 domains in ERGNO sheets (Zhouet al., 2009).

The electron transfer kinetics of a redox probe at SPE/ERGNOwas tested by impedance measurements shown in Fig. 1D. Thecharge transfer resistance (Rct) value for [Fe(CN)6]

3/4 redox probeis calculated by measuring the diameter of high-frequency semicir-cle in the Nyquist plots. By fitting the data (inset ofFig. 1D), the va-lue of

Rctfor SPE/ERGNO is 482Xwhich is much smaller than

that (1337X) for SPE, suggesting the presence of ERGNO filmmakes the electron transfer easier. The accelerated electron trans-fer kinetics could be attributed to the restoration of sp2-hybridizednetwork in ERGNO.

3.2. Stripping response of SPE/ERGNO

Fig. 2shows the square wave anodic stripping voltammogramsof Cd2+ and Pb2+ at BiF modified SPE (SPE/BiF), SPE/ERGNO, SPE/ERGNO/BiF, and BiF modified SPE/CRGNO (SPE/CRGNO/BiF). Ascan be seen in Fig. 2a, the stripping peaks for Cd2+ and Pb2+ atSPE/BiF were very low, probably due to the poor conductivity atthe electrode surface. In contrast, sharper and higher peaks fortarget metal ions were obtained at SPE/ERGNO/BiF (Fig. 2b). Thesignals of Cd2+ and Pb2+ at SPE/ERGNO/BiF were nearly five timeslarger than the ones obtained at SPE/BiF. This significant enhance-ment in stripping signals could be ascribed to the presence of ERG-NO nanosheets provides more effective active area for thenucleation of target metal ions with bismuth in short time duringthe electrodeposition procedure due to the their large surface-to-volume ratio (Li et al., 2009a). As the deposition time increases,and since the deposition is carried out under conditions of masstransfer control at potentials more negative than 0.95 V (here1.2 V), the deposit is expected to thicken in a non-uniform wayas it grows preferentially on top of already deposited nuclei (Bal-drianova, vancara, Vlcek, Economou, & Sotiropoulos, 2006). An-other potential explanation for the signal enhancement is theexcellent electron conductivity of ERGNO nanosheets that could

accelerate the growth process of nuclei.Compared with the response of SPE/ERGNO (Fig. 2b), the strip-

ping signals for the target metal ions at SPE/ERGNO/BiF increase sixtimes, suggesting the bismuth film could significantly improve theanalytical performance resulting from the fact that bismuth canform an alloy with Cd and Pb which makes Cd2+ and Pb2+ reducedmore easily. Furthermore, the stripping response of heavy metalions at SPE/ERGNO/BiF was also superior to that of SPE/CRGNO/BiF (Fig. 2d). The potential reason for this phenomenon is thenon-uniform and loose CRGNO film structure obtained from thedrop-coating method. Since graphene materials are insoluble incommon solvents, uniform and compact CRGNO film is hardly de-sired. This loose structure may induce large electron transport bar-rier and thus decrease the conductivity of CRGNO film and

reduction rate of Cd2+

and Pb2+

. While in the fabrication of ERGNOfilm, the raw material, GNO is soluble due to its abundant oxygen-containing functional groups. Upon electrochemical reduction, theresulted graphene sheets are insoluble, and thus directly attach tothe electrode surface and form an uniform film (seeFig. 1B).

3.3. Optimization of experimental parameters

Since the heavy metal ions have different electrochemicalbehaviours in different electrolytes, the effects of several commonelectrolytes, such as HCl, HNO3, HClO4, Na2SO4, acetate buffer solu-tion (NaAcHAc), and phosphate buffer solution (Na2HPO4NaH2-PO4) on stripping performance of Cd

2+ and Pb2+ were studied.Amongst these mentioned electrolytes, acetate buffer solution

(0.1 mol L1

) is the best choice due to the well-defined peaks withlargest peak current for Cd2+ and Pb2+ could be achieved in this

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electrolyte. The effect of pH (0.1 mol L1 acetate buffer solution) onthe determination of Cd2+ and Pb2+ was also investigated. As illus-trated in Fig. S1 (Supporting Information), the pH of buffer solution

has a profound influence on the formation of bismuth metal alloy.The best stripping response for Cd2+ and Pb2+ was obtained whenusing 0.1 mol L1 acetate buffer solution with a pH value of 4.5as the supporting electrolyte.

Fig. 3A shows the effect of deposition potential varied in therange from 0.8 V to 1.4 V on the stripping response of Cd2+

and Pb2+ at a deposition time of 150 s. It can be seen that whenthe selected deposition potential was 0.8 V, not enough impetuswas used to accelerate the reduction of Cd2+ on the electrode sur-face resulted in the low peak current. When shifting the potentialmore negative, the peak current of Cd2+ increased remarkably. Forthe stripping response of Pb2+, the peak current exhibited littlechange in the deposition potential range in 0.8 V and 1.2 V.The highest stripping peak current for both two metal ions could

be obtained when using 1.2 V as the deposition potential. Furthernegative shifting the potential, the stripping response began to

decrease due to other chemical species that may be reduced atthese potentials and interfere with the detection. Furthermore,since the hydrogen evolution frequently occurs in these potentialsthat partially suppresses the deposition of alloys, the stripping sig-nals became unstable (see the error bar inFig. 3A).

Fig. 3B shows the dependence of stripping response on thedeposition time in the range from 30 s to 300 s at a depositionpotential of1.2 V. As expected, the signals of both metal ions fol-lowed a similar and rapid increasing pattern when increasing the

deposition time from 30 s to 150 s. Further extending the deposi-tion time, the stripping response displayed a slight increase for

A

a b

a b

0

0.5

1.0

1.5

0 0.5 1.0 1.5 2.0

B

20 m

1000 1200 1400 1600 1800 2000

Intensity(a.u.)

Raman Shift (cm1

)

C

b

a

D G

0 0.5 1.0 1.5 2.0 2.5

Z (k)

-Z(k)

0

0.5

1.0

1.5

2.0

2.5

a

b

D

Rs

Rct

Cdl

ZW

Fig. 1. (A) Tapping-mode AFM image of ERGNO on freshly cleaved mica substrates. (B) SEM image of SPE/ERGNO. (C) Raman spectra of GNO (a) and ERGNO (b). (D)Impedance plots of SPE (a) and SPE/ERGNO (b) in 0.1 mol L1 KCl solution containing 5.0 103 mol L1 [Fe(CN)6]

3/4, inset: equivalent circuit.

-1.1 -0.9 -0.7 -0.5 -0.3 -0.1

Potential (V)

Current(A)

a

b

c

d

5 A

Cd2+

Pb2+

Fig. 2. Stripping voltammograms of SPE/BiF (a), SPE/ERGNO (b), SPE/ERGNO/BiF (c),and SPE/CRGNO/BiF (d) in acetate buffer solution containing 30 lg L1 Cd2+ andPb2+. For BiFEs, the test solution contains 800 lg L1 Bi(NO3)3. Deposition potential:1.2 V. Deposition time: 150 s.

0

2

4

6

8

10

12

0 60 120 180 240 300 360

Cd2+

Pb2+

B

PeakCurrent(A)

Deposition time (s)

0

1

2

3

4

5

6

7

8

9

-1.5-1.3-1.1-0.9-0.7

Deposition potential (V)

PeakCurrent(A)

A

Cd2+

Pb2+

Fig. 3. Influence of deposition potential (A) and deposition time (B) on the strippingpeak current of 30 lg L1 Cd2+ and Pb2+ at SPE/ERGNO/BiF. Supporting electrolyte:0.1 mol L1 acetate buffer solution (pH 4.5) containing 800 lg L1 Bi(NO3)3.

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both metals ions, probably due to the electrode surface saturation.Eventually, 150 s was chosen as an ideal compromise between sen-sitivity and efficiency.

3.4. Analytical performance

Calibration plots for the simultaneous determination of Cd2+

and Pb2+ at the developed SPE/ERGNO/BiF were obtained bySWASV under the optimal conditions described above. Fig. 4A de-picts the stripping response of SPE/ERGNO/BiF without any inter-ference while simultaneously increasing the concentrations ofboth metal ions from 0 lg L1 to 60 lg L1. Results show that thestripping peak current has a linear relationship with concentrationin the range of 160 lg L1. For Cd2+ (Fig. 4B), the linear regressionequation is calibrated asICd(current/lA) = 0.023 + 0.251CCd(con-centration/lg L1) (CCd: 160 lg L

1) with the correlation coeffi-cient of 0.9876 (R2) and sensitivity of 0.25 0.02 lA lg1 L. ForPb2+ (Fig. 4C), the linear regression equation is calibrated as IPb(current/lA)=0.018 + 0.212CPb (concentration/lg L

1) (CPb: 160 lg L1) with the correlation coefficient of 0.9892 (R2) and sensi-tivity of 0.21 0.01 lA lg1 L. Repetitive measurements of Cd2+

and Pb2+

with different concentrations (in the linear range) showedgood reproducibility with the relative standard deviations lessthan 6.1% and 5.9%, respectively (n= 10). Furthermore, the devel-oped SPE/ERGNO/BiF also possessed satisfactory detection limits(S/N = 3) of 0.5 lg L1 for Cd2+ and 0.8 lg L1 for Pb2+. Lower detec-tion limits for both target metal ions could be expected by prolong-ing the detection time. The comparison of the analyticalperformance of the developed electrode with other electrodes re-ported previously was summarized in Table S1 (Supporting Infor-mation). It can be seen that the SPE/ERGNO/BiF possessesimproved or comparable performance for the simultaneousdetermination of Cd2+ and Pb2+, nevertheless our electrodes areinexpensive and easy to be fabricated.

3.5. Interference study

The interferences from other metal ions were investigated,since other metals ions may be codeposited with Cd2+ and Pb2+,occupying the active sites on the electrode surface that are in-tended for target ions. For this reason, other metal ions togetherwith Cd2+ and Pb2+ (15 lg L1) in a 300-fold mass ratio of interfer-ent to analyte were used to study the possible interference at thedeveloped electrode. The peak current decrease was calculated inTable S2 (Supporting Information) and employed for the interfer-ence evaluation. Most of these metal ions have no significant influ-ence on the stripping response of Cd2+ and Pb2+. However, copperions (Cu2+) was found to reduce the response of target metal ions,probably due to the competition between electrodeposited bis-muth and copper at the active sites of the surface resulting from

0

2

4

6

8

10

12

14

0 20 40 60

Concentration (g L1)

Current(A)

IPb = - 0.018 + 0.212CPb

R2 = 0.9892

C

0

2

4

6

8

10

12

14

16

-1.1 -0.9 -0.7 -0.5 -0.3

Potential (V)

Current(A)

12 g L1

18 g L1

24 g L1

6 g L1

60 g L1

42 g L1

30 g L1

36 g L1

48 g L1

54 g L1

3 g L1

1 g L1

0 g L1

Cd2+

Pb2+

18

A

0

2

4

6

8

10

12

14

16

18

0 20 40 60


Current(A)

ICd = - 0.023 + 0.251CCd

R2 = 0.9876

B

Fig. 4. (A) Square wave anodic stripping voltammograms for different concentrations of Cd2+ and Pb2+ at the in situ plated SPE/ERGNO/BiF in 0.1 mol L1 acetate buffer

solution (pH 4.5) containing 800 lg L

1 Bi(NO3)3. From bottom to top, 0, 1, 3, 6, 12, 24, 30, 36, 42, 48, 54, and 60 lg L

1. (B) and (C) the corresponding calibration curves ofCd2+ and Pb2+, respectively.

0

2

4

6

8

10

12

14

16

-1.1 -0.9 -0.7 -0.5 -0.3 -0.1

Potential (V)

0

5

10

0 15 30 45


Current(A)

Cd2+

Pb2+

Cd2+

Pb2+Current(A)

Fig. 5. Typical stripping voltammograms for the determination of heavy metals inmilk extracted solution using standard addition method under optimized condi-tions. Dashed line represents the response of test solution, full lines represent theresponse of subsequent standard additions. The concentration of Cd2+ and Pb2+ foreach addition is 15 lg L1. Inset: the corresponding calibration curve.

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the close reduction potential of copper and bismuth. To solve thisproblem, we used the ferrocyanide to mask the copper ions. Re-

sults show that the stripping signals for Cd2+ and Pb2+ could berecovered by adding 0.1 103 mol L1 ferrocyanide.

Another common type of interference species in anodic strip-ping voltammetric determination of heavy metal ions is the surfac-tants. These substances could absorb on the electrode surface andcause deactivation of active sites. Three types of surfactants,including cetyltrimethylammonium (CTAB), sodium dodecylsul-fate (SDS) and Triton X-100, were selected to study the effect onthe stripping response of Cd2+ and Pb2+ at the developed electrode.As summarized in Table S3, these three surfactants exhibited pro-found influence on the analytical performance of target metal ions,especially for Cd2+. These results are in good agreement with thosereported byBrownson and Bank (2011), in which they found thepresence of surfactants may inhibit the transition of cadmium

metal to cadmium ions.

3.6. Sample analysis

To evaluate the feasibility of the developed disposable electrodefor routine analysis, the SPE/ERGNO/BiF was applied to detect Cd2+

and Pb2+ in milk sample extracts. The quantitative determinationof Cd2+ and Pb2+ levels in these extracted solutions at the SPE/ERG-NO/BiF was performed by standard addition method under opti-mized conditions. Fig. 5 illustrates the typical strippingvoltammograms of these analytical tests. As shown, the strippingresponse for three standard additions exhibits good linearity, indi-cating the availability of the disposable electrode in real sampleanalysis. The sensitivities for Cd2+ and Pb2+ were

0.15 0.04 lA lg1

L and 0.12 0.05 lA lg1

L, respectively. Thereduced sensitivity of the developed electrode in extracted solu-tions could be ascribed to the absorption of residual macromole-cules on electrode surface that occupy the active sites for metalions electrodeposition. The comparison of the developed methodwith other methods, such as AAS, XRF, and ICP-MS, was summa-rized in Table S4 (Supporting Information). As can been, electro-chemical method possesses comparable sensitivity and detectiontime, since those spectroscopic counterparts are expensive.

The analytical results of eight milk samples were summarizedin Table 1. To validate the accuracy of the test data, laboratorystandard method, inductively coupled plasma mass spectrometry(ICP-MS) measurements were performed. As can be seen inTable 1,the measured concentration values of Cd2+ and Pb2+ in these sam-

ples using the SPE/ERGNO/BiF are in good agreement with thoseobtained by the ICP-MS, indicating the good accuracy and reliabil-

ity of the developed electrode. The feasibility of the developedelectrode for the determination of heavy metal ions in milk sample

extracts was further examined by the recovery test. The goodrecovery values (seeTable 1) achieved for all the samples testeddemonstrate the suitability of the proposed method for a sensitiveand accurate determination of Cd2+ and Pb2+ at the appropriatelevels.

4. Conclusions

In this work, a disposable bismuth film electrode based on ERG-NO film was developed, and further used to simultaneous determi-nation of Cd2+ and Pb2+ by SWASV. With the excellent properties ofERGNO film and good stripping characters of bismuth film, thedeveloped electrode exhibited sharp and high peaks for target me-tal ions. The linear range of the bismuth film electrode was from

1.0 lg L

1 to 60.0 lg L

1 for both metal ions with a depositionpotential of 1.2 V and a deposition time of 150 s. The detectionlimits for the determination of Cd2+ and Pb2+ were 0.5 lg L1 and0.8 lg L1, respectively. In addition, the disposable electrode dem-onstrated high selectivity for the target metal ions determinationand was applied to quantitatively analyze Ca2+ and Pb2+ levels inmilk sample extracts with satisfactory results. This work providesa useful avenue for implementing ERGNO as a new generation ofelectrochemical transducer in heavy metal ions sensing, whichcould expand the scope of graphene based electrochemical sensingdevices and hold great promise for routine sensing applications.

Acknowledgements

We are grateful for the financial supports from the ZhejiangPostdoctoral Research Project (No. Bsh1201035) and China Post-doctoral Science Foundation (No. 2012M521177).

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.foodchem.2013.11.026.

References

Ataro, A., McCrindle, R. I., Botha, B. M., McCrindle, C. M. E., & Ndibewu, P. P. (2008).Quantification of trace elements in raw cows milk by inductively coupledplasma mass spectrometry (ICP-MS). Food Chemistry, 111, 243248.

Bagheri, H., Afkhami, A., Saber-Tehrani, M., & Khoshsafar, H. (2012). Preparation andcharacterization of magnetic nanocomposite of Schiff base/silica/magnetite as a

Table 1

Results for the simultaneous determination of Cd2+ and Pb2+ levels in milk sample extracts.

Sampleno.

SPE/ERGNO/BiFa ICP-MS Recovery test

Cd2+ Pb2+

Cd2+

(lg kg1)bPb2+

(lg kg1)bCd2+

(lg kg1)Pb2+

(lg kg1)Added(lg kg1)

Found(lg kg1)a

Recovery(%)

Added(lg kg1)

Found(lg kg1)a

Recovery(%)

1 1.35 0.14 3.29 0.43 1.43 0.11 3.42 0.38 5 5.73 0.67 90.2 5 7.24 0.96 87.32 1.12 0.21 9.86 0.78 1.16 0.14 9.47 0.52 5 5.47 0.54 89.4 5 13.55 1.73 91.23 - 1.84 0.21 0.53 0.06 2.01 0.27 5 4.33 0.36 86.6 5 6.20 0.64 90.64 3.42 0.64 7.63 0.64 3.19 0.36 7.98 0.43 5 7.21 0.59 85.6 5 10.68 1.18 84.65 - 2.12 0.28 - 2.28 0.21 10 8.73 1.23 87.3 10 10.50 1.38 86.66 - 5.49 0.62 0.61 0.03 5.68 0.39 10 9.03 1.34 90.3 10 13.66 1.61 88.27 - 1.97 0.27 - 2.08 0.16 10 8.49 1.62 84.9 10 10.27 1.29 85.88 1.23 0.24 11.72 1.23 1.33 0.21 12.29 0.82 10 10.19 1.93 90.7 10 19.27 2.24 88.7

All samples were analyzed using standard addition method ( n= 6).a Mean value standard deviation (n= 6).b The value was calculated according to the equation: Csample(lg kg

1) = Cextracted solution(lg L1) 80 mL / 40 g (Cextracted solutionis the measured value obtained by our

proposed method).

http://-/?-http://dx.doi.org/10.1016/j.foodchem.2013.11.026http://dx.doi.org/10.1016/j.foodchem.2013.11.026http://refhub.elsevier.com/S0308-8146(13)01649-X/h0005http://refhub.elsevier.com/S0308-8146(13)01649-X/h0005http://refhub.elsevier.com/S0308-8146(13)01649-X/h0005http://refhub.elsevier.com/S0308-8146(13)01649-X/h0005http://refhub.elsevier.com/S0308-8146(13)01649-X/h0005http://refhub.elsevier.com/S0308-8146(13)01649-X/h0010http://refhub.elsevier.com/S0308-8146(13)01649-X/h0010http://refhub.elsevier.com/S0308-8146(13)01649-X/h0010http://refhub.elsevier.com/S0308-8146(13)01649-X/h0010http://refhub.elsevier.com/S0308-8146(13)01649-X/h0005http://refhub.elsevier.com/S0308-8146(13)01649-X/h0005http://refhub.elsevier.com/S0308-8146(13)01649-X/h0005http://dx.doi.org/10.1016/j.foodchem.2013.11.026http://dx.doi.org/10.1016/j.foodchem.2013.11.026http://-/?-

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7/7

preconcentration phase for the trace determination of heavy metal ions inwater, food and biological samples using atomic absorption spectrometry.Talanta, 97, 8795.

Baldrianova, L., vancara, I., Vlcek, M., Economou, A., & Sotiropoulos, S. (2006). Effectof Bi(III) concentration on the stripping voltammetric response of in situbismuth-coated carbon paste and gold electrodes. Electrochimica Acta, 52,481490.

Brownson, D. A. C., & Banks, C. E. (2011). Graphene electrochemistry: Surfactantsinherent to graphene inhibit metal analysis. Electrochemistry Communications,13, 111113.

Brownson, D. A. C., Kampouris, D. K., & Banks, C. E. (2012). Grapheneelectrochemistry: Fundamental concepts through to prominent applications.Chemical Society Reviews, 41, 69446976.

Chen, L. Y., Tang, Y. H., Wang, K., Liu, C. B., & Luo, S. L. (2011). Directelectrodeposition of reduced graphene oxide on glassy carbon electrode andits electrochemical application. Electrochemistry Communications, 13, 133137.

Economou, A. (2005). Bismuth-film electrodes: Recent developments andpotentialities for electroanalysis. Trends in Analytical Chemistry, 24, 334340.

Georgakilas, V., Otyepka, M., Bourlinos, A. B., Chandra, V., Kim, N., Kemp, K. C., et al.(2012). Functionalization of graphene: Covalent and non-covalent approaches,derivatives and applications. Chemical Reviews, 112, 61566214.

Herzog, G., & Beni, V. (2013). Stripping voltammetry at micro-interface arrays: Areview.Analytica Chimica Acta, 769, 1021.

Hummers, W. S., & Offeman, R. E. (1958). Preparation of graphitic oxide. Journal ofthe American Chemical Society, 80, 1339.

Hwang, G., Han, W., Park, J., & Kang, S. (2008). An electrochemical sensor based onthe reduction of screen-printed bismuth oxide for the determination of tracelead and cadmium. Sensors and Actuators B: Chemical, 135, 309316.

Kazi, T. G., Jalbani, N., Baig, J. A., Kandhro, G. A., Afridi, H. I., Arain, M. B., et al. (2009).Assessment of toxic metals in raw and processed milk samples usingelectrothermal atomic absorption spectrophotometer. Food and ChemicalToxicology, 47, 21632169.

Khairy, M., Kadara, R. O., Kampouris, D. K., & Banks, C. E. (2010). In situ bismuth filmmodified screen printed electrodes for the bio-monitoring of cadmium in oral(saliva) fluid. Analytical Methods, 2, 645649.

Kovtyukhova, N. I., Ollivier, P. J., Martin, B. R., Mallouk, T. E., Chizhik, S. A., Buzaneva,E. V., et al. (1999). Layer-by-layer assembly of ultrathin composite films frommicron-sized graphite oxide sheets and polycations. Chemistry of Materials, 11,771778.

Li, D., Mller, M. B., Gilje, S., Kaner, R. B., & Wallace, G. G. (2008). Processableaqueous dispersions of graphene nanosheets. Nature Nanotechnology, 3,101105.

Li, J., Guo, S. J., Zhai, Y. M., & Wang, E. (2009a). High-sensitivity determination oflead and cadmium based on the Nafiongraphene composite film. AnalyticalChimica Acta, 649, 196201.

Li, J., Guo, S. J., Zhai, Y. M., & Wang, E. (2009b). Nafiongraphene nanocomposite filmas enhanced sensing platform for ultrasensitive determination of cadmium.

Electrochemistry Communications, 11, 10851088.Liu, Y. X., Dong, X. C., & Chen, P. (2012). Biological and chemical sensors based ongraphene materials. Chemical Society Reviews, 41, 22832307.

Metters, J. P., Kadara, R. O., & Banks, C. E. (2011). New directions in screen printedelectroanalytical sensors: An overview of recent developments. Analyst, 136,10671076.

Metters, J. P., Kadara, R. O., & Banks, C. E. (2013). Fabrication of co-planar screenprinted microband electrodes. Analyst, 138, 25162521.

Novoselov, K. S., Geim, A. K., Morozov, S. V., Jiang, D., Zhang, Y., Dubonos, S. V., et al.(2004). Electric field effect in atomically thin carbon films. Science, 306,666669.

Ping, J. F., Wang, Y. X., Fan, K., Wu, J., & Ying, Y. B. (2011). Direct electrochemicalreduction of graphene oxide on ionic liquid doped screen-printed electrode and

its electrochemical biosensing application. Bioelectronics and Bioelectronics, 28,204209.

Ping, J. F., Wang, Y. X., Ying, Y. B., & Wu, J. (2012). Application of electrochemicallyreduced graphene oxide on screen-printed ion-selective electrode. AnalyticalChemistry, 84, 34733479.

Ping, J. F., Wu, J., Ying, Y. B., Wang, M. H., Liu, G., & Zhang, M. (2011). Evaluation oftrace heavy metal levels in soil samples using an ionic liquid modified carbonpaste electrode. Journal of Agricultural and Food Chemistry, 59, 44184423.

Quintana, J. C., Arduini, F., Amine, A., Velzen, K. V., Palleschi, G., & Moscone, D.(2012). Part two: Analytical optimisation of a procedure for lead detection inmilk by means of bismuth-modified screen-printed electrodes. AnalyticaChimica Acta, 736, 9299.

Rahimi, E. (2013). Lead and cadmium concentrations in goat, cow, sheep, andbuffalo milks from different regions of Iran. Food Chemistry, 136, 389391.

vancara, I., Prior, C., Hocevar, S. B., & Wang, J. (2010). A decade with bismuth-basedelectrodes in electroanalysis. Electroanalysis, 22, 14051420.

Sahoo, P. K., Panigrahy, B., Sahoo, S., Satpati, A. K., Li, D., & Bahadur, D. (2013). In situsynthesis and properties of reduced graphene oxide/Bi nanocomposites: As anelectroactive material for analysis of heavy metals.Biosensors and Bioelectronics,43, 293296.

Serrano, N., Alberich, A., Daz-Cruz, J., Arino, C., & Esteban, M. (2013). Coatingmethods, modifiers and applications of bismuth screen-printed electrodes.Trends in Analytical Chemistry, 46, 1529.

Serrano, N., Diaz-Cruz, J. M., Arino, C., & Esteban, M. (2010). Ex situ depositedbismuth film on screen-printed carbon electrode: A disposable device forstripping voltammetry of heavy metal ions. Electroanalysis, 22, 14601467.

Sogut, O., Bali, T., Baltas, H., & Apaydin, G. (2013). Determination of trace elementsin ashes of milk samples by using xrf technique. Asian Journal of Chemistry, 25,43854388.

Wang, J. (2005). Stripping analysis at bismuth electrodes: A review. Electroanalysi,17, 13411346.

Xia, X. H., Chen, X., Liu, R. M., & Liu, H. (2011). Heavy metals in urban soils withvarious types of land use in Beijing, China. Journal of Hazardous Materials, 186,20432050.

Zhou, M., Wang, Y. L., Zhai, Y. M., Zhai, J. F., Ren, W., Wang, F. A., et al. (2009).Controlled synthesis of large-area and patterned electrochemically reducedgraphene oxide films. Chemistry A European Journal, 15, 61166120.

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