ChineseJournalofCatalysis34(2013)1333–1338 

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Article   

Electrocatalyticmeasurementofmethionineconcentrationwithacarbonnanotubepasteelectrodemodifiedwithbenzoylferrocene

HadiBEITOLLAHIa,*,AlirezaMOHADESIb,FarzanehGHORBANIb,HassanKARIMIMALEHc, MehdiBAGHAYERId,RahmanHOSSEINZADEHe

aEnvironmentDepartment,InstituteofScienceandHighTechnologyandEnvironmentalSciences,GraduateUniversityofAdvancedTechnology,Kerman,Iran

bDepartmentofChemistry,PayameNoorUniversity,19395‐4697,Tehran,IrancDepartmentofChemistry,GraduatedUniversityofAdvancedTechnology,Kerman,IrandDepartmentofChemistry,FacultyofScience,HakimSabzevariUniversity,POBox397,Sabzevar,IraneDepartmentofOrganicChemistry,FacultyofChemistry,UniversityofMazandaran,Babolsar,Iran

A R T I C L E I N F O  

A B S T R A C T

Articlehistory:Received7February2013Accepted15March2013Published20July2013

  Abenzoylferrocenemodifiedmulti‐wallcarbonnanotubepasteelectrodeforthemeasurementofmethionine(MET)concentrationisdescribed.METelectrochemicalresponsecharacteristicsofthemodifiedelectrodeinaphosphatebuffersolutionofpH7.0wereinvestigatedbycyclicvoltamme‐try, squarewavevoltammetry, andchronoamperometry.Underoptimizedconditions, the squarewavevoltammetricpeakcurrentofMETincreasedlinearlywithMETconcentrationintherangeof1.0×10−7to2.0×10−4mol/L.Thedetectionlimitwas58.0nmol/LMET.Thediffusioncoefficient(D=5.62×10−6cm2/s)andelectrontransfercoefficient(α=0.4)forMEToxidationwerealsodeter‐mined.ThesensorwassuccessfullyappliedforthemeasurementofMETconcentrationinhumanurine.

©2013,DalianInstituteofChemicalPhysics,ChineseAcademyofSciences.PublishedbyElsevierB.V.Allrightsreserved.

Keywords:MethionineElectrocatalysisCarbonnanotubeChemicallymodifiedelectrodeVoltammetryBenzoylferrocene

 

 

1. Introduction

Electrochemicalmethodsforthedetectionofinorganicandorganic compounds are selective and sensitive [1−4]. Chemi‐callymodifiedelectrodes(CMEs)areextensivelyresearchedforapplications in medical diagnostics, food analysis, and envi‐ronmentalmonitoring[5−9].AnimportantpropertyofCMEsistheir ability to catalyze the electrode process to significantlydecreasetheoverpotentialandgivemoreselectiveinteractionof the electron mediator with the target analyte. These elec‐trodes can enhance the selectivity in the electroanalyticalmethods.

Carbonpaste electrodes (CPEs) represent one of themostconvenient materials for the preparation of modified elec‐trodes.CPEsarewidelyappliedinbothelectrochemicalstudiesand electroanalysis for their advantages such as very lowbackgroundcurrent(comparedtosolidgraphiteornoblemetalelectrodes),easytoprepare, lowcost, largepotentialwindow,simple surface renewal process, and ease of miniaturization[10−13].

As a relatively novel nanomaterial, carbon nanotubes(CNTs) have received attention due to their small dimensionand structure sensitive properties. In electroanalytic and bio‐analyticapplications, thehighelectricalconductivityandelec‐

*Correspondingauthor.Tel:+98‐3426226613;Fax:+98‐3426226617;E‐mail:h.beitollahi@yahoo.comDOI:10.1016/S1872‐2067(12)60582‐8|http://www.sciencedirect.com/science/journal/18722067|Chin.J.Catal.,Vol.34,No.7,July2013

HadiBEITOLLAHIetal./ChineseJournalofCatalysis34(2013)1333–1338

trocatalyticactivityofCNTsallowthemtobeusedastheelec‐trodematerial tomediate electron transfer reactions [14,15].Several authors have reported the excellent electrocatalyticpropertiesofCNTsintheredoxreactionsofdifferentbiomole‐cules[16−20].

Protein and amino acid analysis continues to be an im‐portantareaofresearchinthefieldofchemicalandbiochemi‐calanalysis.Sulfur‐containingmoleculesarewidelypresentinplantsandlivingorganismsandtheyplayanimportantroleinliving systems [21]. Methionine (MET) is an essential sul‐fur‐containingproteinogenicaminoacidwithanimportantrolein biological methylation reactions. It is the main supply ofsulphurinthediet,anditpreventsdisordersinhair,skin,andnails. Moreover, it helps to reduce cholesterol levels by in‐creasinglecitinproductioninliver,anditisalsoanaturalche‐latingagentforheavymetals.METcanbepresentatdifferentconcentration levels from near 0.1% (m/v) in some drugs to10−5mol/Linbloodplasma[22].Homocysteineisaninterme‐diateproductofMETmetabolism[23].AdefectivemetabolismofMETwouldresultinhyperhomocysteinemia,whichhasbeenestablishedasanindependentriskfactorforvasculardiseasesin humans [24,25]. It has been reported that the improperconversionofMETleadstoatherosclerosis[26].METdeficien‐cyhasbeenreportedtocausetoxaemia,muscleparalysis,de‐pression, and impaired growth [27]. Hence, the detection ofMETisveryimportantfromaclinicalpointofview.

Severalmethodshavebeenproposedforthemeasurementof MET concentration including flow injection [28,29], chro‐matography [30,31], capillary electrophoresis [23], spectro‐photometry [32], chemiluminescence [33], atomic absorption[34], and colorimetric [35,36] and enzymatic methods [37].Thesemethodshaveseveraldisadvantages, suchashighcost,longanalysis time,and lowsensitivity,andthereforetheyareunsuitable for routine analysis. On the other hand, themeas‐urementofMETconcentrationbyelectrochemicalmethodshasseveral advantages, which include high sensitivity, selectivityand reproducibility. The measurement of MET concentrationwith a chemicallymodified electrode is an attractivemethod.Only few reports are available in the literature for themeas‐urement of MET concentration by electrochemical methods[38−43].

NostudyhasbeenreportedontheelectrocatalyticoxidationofMETwithabenzoylferrocene(BF)modifiedmulti‐wallcar‐bonnanotubepasteelectrode (BFCNPE).Thus, in thepresentwork,wedescribedthepreparationandsuitabilityofaBFCNPEas the electrode in the electrocatalysis and measurement ofMET concentration in an aqueous buffer solution. To demon‐strate the catalytic activity of the modified electrode in theelectrooxidationofMET,weexamined itsuse for thevoltam‐metricmeasurementofMETconcentrationininurinesamples.

2. Experimental

2.1. Apparatusandchemicals

The electrochemical measurements were performed withanAutolabpotentiostat/galvanostat(PGSTAT12,EcoChemie,

Netherlands).TheexperimentalconditionswerecontrolledbytheGeneralPurposeElectrochemicalSystem(GPES)software.Aconventional threeelectrodecellwasusedat(25±1)°C.AAg/AgCl/KCl (3.0mol/L) electrode, a platinumwire, and theBFCNPE were used as the reference, auxiliary, and workingelectrodes,respectively.AMetrohm710pHmeterwasusedforpHmeasurements.

All solutions were freshly prepared with doubly distilledwater.METandall other reagentswereanalytical gradepur‐chasedfromMerck(Darmstadt,Germany).Thebuffersolutionswereprepared fromorthophosphoricacidand its salts in thepHrangeof2.0to11.0.Multiwalledcarbonnanotubes(purity>95%)witho.d.between10and20nm, i.d.between5and10nm, and tube length from0.5 to200μmwereobtained fromNanostructured&AmorphousMaterials,Inc.

2.2. Synthesisofbenzoylferrocene

Amixtureof aluminumchloride (10.8 g, 80.66mmol)andbenzoylchloride(11.3g,80.66mmol) in100mlofdrymeth‐ylene chloride was added to a solution of ferrocene (15.0 g,80.66mmol)in100mlofdrymethylenechlorideoveraperiodof15min.Themixturewasstirredatroomtemperaturefor1hand then hydrolyzed with dilute hydrochloric acid (60 ml, 3mol/L).TheorganicphasewaswashedwithNaOHsolution(50ml,1mol/L),saturatedNaClsolution(50ml),andH2O(50ml),anddriedoveranhydroussodiumsulfate.Thesolidwhichre‐mainedafterevaporationofthesolventwasrecrystallizedfromtoluene and n‐hexane to give the pure product in 78% yield(redsolid);mp:106−108°C;1HNMR(400MHzCDCl3):δ4.17(s, 5 H), 4.58 (t, J = 2.5 Hz, 2 H), 4.90 (d, J = 8.4 Hz, 2 H),7.44−7.56(m,3H),7.90(t,J=2.5Hz,2H);13CNMR(100MHz,CDCl3): δ 70.22, 71.53, 72.51, 78.25, 128.07, 128.21, 131.45,139.87,199.02.

2.3. Preparationoftheelectrode

TheBFCNPEswerepreparedbyhandmixing0.01gofBFwith0.89ggraphitepowderand0.1gCNTswithamortarandpestle.Then,0.7mlofparaffinoilwasaddedtotheabovemix‐tureandmixedfor20minuntilauniformlywetpastewasob‐tained.Thepastewaspacked intotheendofaglass tube(ca.3.4mm i.d. and15 cm long).A copperwire inserted into thecarbonpasteprovidedtheelectricalcontact.Whennecessary,anew surface was obtained by pushing some paste out of thetubeandpolishingwithaweighingpaper.

For comparison, a BF modified CPE electrode (BF‐CPE)without CNTs, CNT paste electrode (CNPE) without BF, andunmodifiedCPEintheabsenceofbothBFandCNTswerealsopreparedinthesameway.

3. Resultsanddiscussion

3.1. ElectrochemicalpropertiesofBFCNPE

TheBFCNPEwaspreparedanditselectrochemicalproper‐ties were studied in a 0.1 mol/L phosphate buffer solution

HadiBEITOLLAHIetal./ChineseJournalofCatalysis34(2013)1333–1338

(PBS)of pH7.0using cyclic voltammetry (CV) (Fig. 1). Therewerewelldefinedandreproducibleanodicandcathodicpeaksfrom the benzoylferrocene/benzoylferricenium redox system,whichshowedquasireversiblebehaviorinanaqueousmedium[44].ThegenerationofareproduciblesurfacewasexaminedbyCVdataobtained intheoptimumsolutionofpH7.0 fromfiveseparatelypreparedBFCNPEs(Table1).

Inaddition,thelongtermstabilityoftheBFCNPEwastestedover a three week period. CVs recorded after the modifiedelectrodewas stored in theatmosphereat roomtemperatureshowedapeakpotentialforMEToxidationthatwasunchangedand thecurrent signals showed less than2.2%decreaserela‐tive to the initial response. The antifouling properties of themodified electrode toward MET and its oxidation productswere investigated by recording the cyclic voltammograms ofthemodifiedelectrodebeforeandafteruseinthepresenceofMET.CyclicvoltammogramswererecordedinthepresenceofMET after cycling the potential 20 times at a scan rate of 10mV/s. The peakpotentialswereunchanged, and the currentswere decreased by less than 2.3%. Therefore, the surface ofBFCNPE showed not only sensitivity increase but also de‐creasedfoulingbytheanalyteanditsoxidationproduct.

3.2. ElectrocatalyticoxidationofMETatBFCNPE

Figure 2 depicts the CV responses for the electrochemicaloxidationof100.0μmol/LMETatunmodifiedCPE(curve(3)),CNPE(curve(4)),benzoylferrocenemodifiedmulti‐wallcarbonnanotube paste electrode (BFCPE, curve (1)), and BFCNPE(curve(2)).WhiletheanodicpeakpotentialforMEToxidationat the CNPE and unmodified CPEwere 900 and 960mV, re‐spectively,thepotentialatBFCNPEandBFCPEwere~645mV.The peak potential for MET oxidation at the BFCNPE andBFCPEelectrodeswasshiftedby255and315mVtowardnega‐tive values compared to CNPE and unmodified CPE, respec‐tively.However, BFCNPE showed amuch higher anodic peak

current fortheoxidationofMETcomparedtoBFCPE, indicat‐ingthatthecombinationofCNTsandthemediator(BF)signif‐icantly improved the performance of the electrode for METoxidation.TheBFCNPEin0.1mol/LPBS(pH7.0)andwithoutMET in the solution exhibited awell behaved redox reaction(Fig. 1(1)). The addition of 100.0 μmol/LMET increased theanodicpeakcurrent(Fig.2(2)),indicatingastrongelectrocata‐lyticeffect[44].

The effect of scan rate on the electrocatalytic oxidation ofMETattheBFCNPEwasalsoinvestigatedbyCV(Fig.3(a)).Theoxidation peak potential shifted to more positive potentialswithincreasingscanrate,confirmingthekineticregimeintheelectrochemicalreaction.Also,aplotofpeakheight(Ip)versussquarerootofscanrate(ν1/2)waslinearintherangeof2−20mV/s,suggestingthatatsufficientoverpotentialtheprocessisdiffusion rather than surface controlled [44] (Fig. 3(b)). TheTafelslope(Fig.3(c))wasobtainedfromtheslopeofEpversuslogvusingEq.(1)[44]:

Ep=b/2logv+constant (1)TheTafelslopewas0.098V(Fig.3(c)),whichindicatesthat

aone‐electrontransferprocessistheratelimitingstepassum‐ingatransfercoefficient(α)of0.4.

3.3. Chronoamperometricmeasurements

ChronoamperometricmeasurementsofMETconcentrationat the BFCNPEwere carried out by setting theworking elec‐trodepotentialat0.7Vatthefirstpotentialstepandat0.5Vatthe secondpotential stepversusAg/AgCl/KCl (3.0mol/L) forthevariousconcentrationsofMETinPBS(pH7.0)(Fig.4).Foran electroactive material (MET in this case) with a diffusioncoefficient ofD, the current observed for the electrochemicalreactionunderamasstransportlimitedconditionisdescribedbytheCottrellequation[44].ThebestfitsofIvst−1/2wereob‐tainedforthedifferentconcentrationsofMET(Fig.4(b)).TheslopesofthestraightlineswerethenplottedversusMETcon‐centration(Fig.4(c)).FromtheresultingslopeandtheCottrellequation[44]:

I=nFAD1/2Cbπ−1/2t−1/2 (2)

300 400 500 600 700 800 900-3.0

-1.5

0.0

1.5

3.0

4.5

6.0I/A

E/mV

(1)

(2)

Fig.1.CVofBFCNPEin0.1mol/LPBS(pH7.0)(1)andatthesurfaceofabarecarbonpasteelectrodeatascanrate100mV/s(2).

Table1 Cyclicvoltammetricdataobtainedwith theBFCNPE in0.1mol/LPBS(pH7.0)at100mV/s.

Epa*/V Epc/V E1/2/V ΔEp/V Ipa/µA Ipc/µA0.645±1.3 0.540±1.4 0.592±1.3 0.105±1.4 5.2±1.7 –2.4±1.6

*VersusAg/AgCl/KCl(3.0mol/L)asreferenceelectrode.Allthe'±'valuesareRSD%(n=5).

200 400 600 800 1000 1200

0

3

6

9

12

15

I/A

E/mV

(1)

(2)

(4)(3)

Fig.2.CVs of BFCPE (1) and BFCNPE (2) in 0.1mol/L PBS (pH7.0)containing 100.0 μmol/L MET; CVs of CPE (3) and CNPE (4) in 0.1mol/LPBS(pH7.0)containing100.0μmol/LMET.Thescanratewas10mV/s.

HadiBEITOLLAHIetal./ChineseJournalofCatalysis34(2013)1333–1338

WhereD andCb are the diffusion coefficient (cm2/s) and thebulkconcentration(mol/cm3),respectively.ThemeanvalueofDwas5.62×10−6cm2/s.

3.4. Calibrationplotandlimitofdetection

The squarewavevoltammetrymethodwasused todeter‐mine theconcentrationofMET(Fig.5).Theplotofpeak cur‐rent versus MET concentration consisted of two linear seg‐mentswithslopesof0.634and0.049(μA·L)/μmolinthecon‐centration ranges of 0.1 to 10.0 μmol/L and 10.0 to 200.0μmol/L,respectively.Thedecreaseinsensitivity(slope)inthesecond linear segment was likely due to kinetic control. Thedetectionlimit(3σ)ofMETwas58.0nmol/L.Thesevaluesarecomparablewiththevaluesreportedbyotherresearchgroupsfor the electrocatalytic oxidation of MET at the surfaces ofchemicallymodifiedelectrodesbyothermediators(Table2).

3.5. Interferencestudy

The interferences of various foreign species on themeas‐urement of MET concentration were investigated. The toler‐ancelimitwastakenasthemaximumconcentrationofthefor‐eign substances, which caused a ± 5% relative error in themeasurement. L‐lysine, glucose, NADH, acetaminophen,L‐asparagine, glycine, folic acid, uric acid, tryptophan, andphenylalaninedidnotshowinterferenceinthemeasurementofMETbutL‐cysteine,cysteamine,captopril,andD‐penicillamineshowedinterferences.

3.6. MeasurementofMETconcentrationinurinesamples

In order to evaluate the analytic applicability of the pro‐posed electrode, itwas applied for themeasurement ofMETconcentrationininurinesamples.TheresultsaregiveninTable3. Satisfactory agreement of the experimental results wasfound for the MET concentration. The reproducibility of themethodwasdemonstratedbythesmallmeanrelativestandarddeviation(RSD).

400 500 600 700 800 900

0

4

8

12

15 mV/s

6 mV/s

4 mV/s

2 mV/s

8 mV/s

I/A

E/mV

(a)

20 mV/s

1 2 3 4 52

4

6

8

10

v1/2/(mV/s)1/2

I/A

(b)

y = 2.414x 0.798R2 = 0.998

-2.9 -2.5 -2.1 -1.70.66

0.70

0.74

y = 0.049x + 0.804R2 = 0.999

Ep/

V

logv (V/s)

(c)

Fig.3.(a)CVsofBFCNPEin0.1mol/LPBS(pH7.0)containing50.0μmol/LMETatvariousscanrates;(b)Variationofanodicpeakcurrentvsν1/2;(c)Anodicpeakpotentialvslogv.

0 5 10 15 20 25 30 35 40-50

0

50

100

150

I/A

t/s

1

7

(a)

70.0 1.5 3.0

20

40

60

80

100

t1/2/s1/2

I/A

(b)

1

6

0.0 0.3 0.6 0.9 1.215

20

25

30

y = 11.63x + 15.81R2 = 0.998S

lope

(A

s1/

2 )

[MET] (mmol/L)

(c)

Fig.4.(a)ChronoamperogramsobtainedatBFCNPEin0.1mol/LPBS(pH7.0)fordifferentconcentrationsofMET(thenumbers1–7correspondto0.0,0.1,0.2,0.4,0.6,0.8,and1.0mmol/LofMET);(b)PlotsofIvst–1/2obtainedfromchronoamperograms2–7;(c)PlotoftheslopeofthestraightlinesagainstMETconcentration.

HadiBEITOLLAHIetal./ChineseJournalofCatalysis34(2013)1333–1338

4. Conclusions

A benzoylferrocene modified carbon nanotube paste elec‐trode (BFCNPE) was fabricated. The modified electrodeshowedgoodelectrocatalyticactivityfortheoxidationofMETandgaveawidelinearrange(0.1to200.0μmol/L)withagooddetectionlimit(58.0nmol/L)forMETconcentration.Thissen‐sorwassuccessfullyappliedtodeterminetheMETconcentra‐tioninurinesamples.

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400 500 600 700 8000

10

20

30

40

50

60

70

80I/A

E/mV

1

6400 500 600 700 8000

25

50

I/A

E/mV

(a)

7

11

0 6 1222

26

30(b)

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I/A

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34

41(c)

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Fig.5. (a)SquarewavevoltammetrycurvesofBFCNPEin0.1mol/LPBS(pH7.0)containingdifferentconcentrationsofMET(thenumbers1–11correspondto0.1,0.5,1.0,2.0,5.0,10.0,30.0,70.0,100.0,150.0,and200.0μmol/LofMET);PlotsoftheelectrocatalyticpeakcurrentasafunctionofMETconcentrationintherangeof0.1to10.0μmol/L(b)10.0to200.0μmol/L(c).

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Electrode Modifier Method pH Dynamicrange(mol/L) Limitofdetection(mol/L) Ref.

Carbonpaste Colloidal‐goldcysteamine Voltammetry 7.0 1.0×10−6−1.0×10−4 5.9×10−7 [22]Glassycarbon Electropolymerizedfilmof

1,8,15,22‐tetraaminophthalocyanato‐copper(II)

Voltammetry 4.0 5.0×10−5−5.0×10−4

2.7×10−8 [38]

Glassycarbon Cobalthydroxidenanoparticles Amperometry 13.0 2.45×10−4−1.21×10−3 1.6×10−4 [45]Carbonnanotubepaste BF Voltammetry 7.0 1.0×10–7–2.0×10–4 5.8×10–8 thiswork

Table3 UseofBFCNPEfor themeasurementofMETconcentrationin inurinesamples(n=5).

SampleNo.

Originalcontent(μmol/L)

Added(μmol/L)

Found(μmol/L)

Recovery(%)

RSD(%)

1 — 15.0 14.9 99.3 2.32 — 30.0 31.1 103.7 3.43 — 45.0 45.6 101.3 2.44 — 60.0 58.3 97.2 1.9

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GraphicalAbstract

Chin.J.Catal.,2013,34:1333–1338 doi:10.1016/S1872‐2067(12)60582‐8

Electrocatalyticmeasurementofmethionineconcentrationwithacarbonnanotubepasteelectrodemodifiedwith benzoylferrocene

HadiBEITOLLAHI*,AlirezaMOHADESI,FarzanehGHORBANI,HassanKARIMIMALEH,MehdiBAGHAYERI,RahmanHOSSEINZADEHGraduateUniversityofAdvancedTechnology,Iran;PayameNoorUniversity,Iran;HakimSabzevariUniversity,Iran; UniversityofMazandaran,Iran

200 400 600 800 1000 1200

0

3

6

9

12

15

I/A

E/mV

Asensorwasfabricatedforthemeasurementofmethionineconcentrationthatreducedtheoxidationpotentialofmethionineascom‐paredtotheunmodifiedelectrode.