A novel optical chemical sensor for the determination of nickel(II) based on fluorescence quenching of newly synthesized thiazolo-triazol derivative and application to real samples

ContentslistsavailableatSciVerseScienceDirect

Sensors

and

Actuators

B:

Chemical

j o u r n al hom 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 / s n b

A

novel

optical

chemical

sensor

for

the

determination

of

nickel(II)

based

on

fluorescence

quenching

of

newly

synthesized

thiazolo-triazol

derivative

and

application

to

real

samples

Nur

Aksuner

a,∗

_,

_Emur

_Henden

a

_,

_Ibrahim

_Yilmaz

b

_,

_Alaaddin

_Cukurovali

c a_{DepartmentofChemistry,FacultyofScience,UniversityofEge,35100Bornova, ˙Izmir,Turkey}

b_{DepartmentofChemistry,FacultyofScience,UniversityofKaramano˘gluMehmetBey,70200Karaman,Turkey} c_{DepartmentofChemistry,FacultyofArtsandSciences,UniversityofFırat,23169Elazı˘g,Turkey}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received16November2011

Receivedinrevisedform14February2012 Accepted20February2012

Available online 27 February 2012 Keywords: Thiazolo-triazol PVCmatrix Opticalsensor Fluorescencespectroscopy Nickel(II)

a

b

s

t

r

a

c

t

ThecharacterizationofanewopticalsensormembraneisdescribedforthedeterminationofNi(II)based ontheimmobilizationofthefluorescentthiazolo-triazolderivativeinPVCmatrix.Thisoptodehasawide linearrangeof1.0×10−9_–4.4×10−3_{MatpH6.0forNi(II)ionswiththedetectionlimitof8.5×10}−10_M.

TheresponseoftheoptodemembranetoNi(II)isfullyreversibleandrevealsaverygoodselectivity towardsNi(II)ionoverawidevarietyofothermetalionsinsolution.Themembraneshowedagood durabilityandshortresponsetimewithnoevidenceofreagentleaching.Theproposedopticalsensor givesgoodresultsforapplicationsindirectdeterminationofNi(II)inrealsamplesthataresatisfactorily comparablewithcorrespondingdatafromflameatomicabsorptionspectrometry.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Nickelisamoderatelytoxicelementcomparedtoother tran-sitionmetals.However,itisknownthatinhalationofnickeland itscompoundscanleadtoseriousproblems,includingrespiratory systemcancer[1,2].Moreover,nickelcancauseadisorderknown asnickel-eczema[3].Itsdeterminationisthusimportantinviewof toxicnatureandwidespreadpresenceinenvironment.The deter-minationoftracenickelinwaterand environmentalsamplesis difficultduetovariousfactors,particularlylowconcentrationand matrixeffects. Toovercometheseproblems,several preconcen-trationand separationtechniquesare neededbeforemeasuring [4–6].Manyofthesepretreatmenttechniquesare,however,time consuming or require complicated and expensiveinstruments. Therefore,developmentofaccurateandrapiddetectionmethod formonitoringthelevelofnickelinenvironmentalandbiological samplesisnecessaryandindispensable.

Chemicalopticalsensors(optode)offeradvantagessuchas sim-plepreparationprocedure,relativelyfastresponse,wideresponse range,reasonableselectivityandhighsensitivity[7–9].The immo-bilizationofvarioussensingreagentsofoptodemembraneshave been developed for many analytically relevant ions, especially

∗ Correspondingauthor.Tel.:+902323888264;fax:+902323888264. E-mailaddress:nur.erdem@ege.edu.tr(N.Aksuner).

heavymetalions.Immobilizationofdyesintoorontoasolid sup-port is a keyissue for theirapplicationin opticalsensing [10]. Thereagentisnormallyphysicallyentrappedbyadsorption, elec-trostaticallyattractedorchemicallybondedtothesolidsupport. Generally,sol–gelglasses[11,12]orpolymermatrices[13,14]are usedforthepreparationoftheoptodes.Poly(vinylchloride)(PVC) hasbeenusedforthepreparationofmembraneoptodesduetoits relativelylowcost,goodmechanicalpropertiesandamenabilityto plasticization[15].Recently,ourgrouphasbeeninvolvedinoptical sensorsforheavymetalionsembeddedinPVCfilms[16–18].

Uptonow,there areonlyafew reportsondeterminationof nickelbasedonchemicalopticalsensor.ANi(II)optodebasedon immobilizingof2-amino-1-cyclopentene-1-dithiocarboxylicacid totransparentacetylcellulosefilmwasdevelopedbyEnsafiand Bakhsi[19].Thedetectableconcentrationof nickel ina sample solution was in the range of 5.0×10−6–1.0×10−3M with the detectionlimitof5.2×10−7M(0.03␮g/ml)Ni(II).Shamsipuretal. [20]havedesignedanewfluorimetricbulkoptodemembranefor the determination of Ni2+ _{ions. The plasticized PVC-membrane}

incorporating2,5-thiophenylbis(5-tert-butyl-1,3-benzexazole),as a highly fluorescent chromoionophore, displays a calibration responseforNi2+_{ionswithalinearrangecoveringfrom1.0×10}−3

to1.0×10−8M.Anopticalsensorfornickelionbasedon immo-bilizationof2-(5-bromo-2-pyridylazo)-5-(diethylamino)phenolin NafionmembranewasofferedbyAminietal.[21].Hashemietal. [22]recentlyreportedaphotometricsenorbasedonthecovalently 0925-4005/$–seefrontmatter © 2012 Elsevier B.V. All rights reserved.

Table1

GeneralperformancecharacteristicsofsomeNi2+_optodes.

Reagent/supportmatrix Workingrange(M) Limitof detection(M) Response time Measured signal Reference 2-amino-1-cyclopentene-1-dithiocarboxylicacid/acetyl cellulosemembrane 5.0×10−6_–1.0×10−3 _5.2×10−7 _{10min Absorbance [19]} 2,5-thiophenylbis(5-tert-butyl-1,3-benzexazole)/PVCmembrane 1.0×10−8_–1.0×10−3 _8.0×10−9 _{<40s Fluorescence [20]} 2-(5-bromo-2-pyridylazo)-5-(diethylamino)phenol/Nafion

membrane

8.5×10−6_–3.4×10−4 _5.1×10−6 _{3min Absorbance [21]}

thionine/agarosemembrane 1.0×10−10–1.0×10−7 9.3×10−11 3min Absorbance [22]

2-amino-1-cyclopentene-dithiocarboxylicacid/PVCmembane 3.1× 10−8_{–8.0× 10}−3 _NRa _{3min Absorbance [23]}

2-

{6-(3-methyl-3-mesitylcyclobutyl)-thiazolo[3,2-b][1,2,4]triazol-2-yl}-phenol/PVC

membane

1.0×10−9_–4.4×10−3 _8.5×10−10 _{2min Fluorescence Thiswork}

a_{NR:notreported.}

immobilizedthionineinagarosemembrane.Thedetectionlimit ofthesensorforNi2+_was9.30×10−11_{M.Yarietal.[23]}

devel-opedanopticalsensorfordeterminationofnickel,whichwasbased ontheincorporationof2-amino-1-cyclopentene-dithiocarboxylic acidina plasticizedPVCmembrane.The sensordisplaysa cali-brationresponseforNi2+_{ionoverawideconcentrationrangeof}

3.1×10−8–8.0×10−3M.In Table1 therecentlypublished opti-calsensorsforNi(II)determinationwerecompared intermsof theirworkingranges,limitofdetections(LOD),sensingagentsand matrixmaterialswiththeofferedwork.

Herewepresentanewopticalthin-filmsensorbasedonthe fluorescentthiazolo-triazol derivativeentrapped inPVC matrix. Theproposedopticalsensorshowsasignificantfluorescence sig-nalchangeonexposuretoanaqueoussolutioncontainingNi(II) ion.Basedonthis,ahighlysensitive,selectiveandrapidmethod forthedetermination of nickelwas developed.The sensorwas appliedtodeterminetheconcentrationsofNi(II)inrealsamples, withsatisfactoryresults.

2. Experimental

2.1. Reagents

The polymer membrane components, polyvinylchloride (PVC) (high molecular weight) and the plasticizers, bis-(2-ethylhexyl) phtalate (DOP), bis(2-ethylhexyl)sebecate (DOS), bis-(2-ethylhexyl)adipate (DAO) and 2-nitrophenyl octyl ether (NPOE)wereobtainedfromFluka.Thelipophilicanionicadditive reagentpotassiumtetrakis-(4-chlorophenyl)borate(PTCPB)was supplied by Aldrich. Absolute ethanol (EtOH), tetrahydrofuran (THF)and dimethylformamide (DMF) were of analytical grade. Solventsforthespectroscopicstudieswereusedwithoutfurther purification.EDTAwasobtainedfromBDH.SheetsofMylar-type polyester(Dupont,Switzerland)wereusedassupport.Allsolutions werepreparedwithglass-distilledwater.

The pH values of the solutions were checked using a digi-talpH meter (WTW) calibrated with standard buffer solutions ofMerck.Buffercomponentsandmetalsaltswereof analytical grade (Merck and Fluka). All of the experiments were oper-ated at room temperature, 25±1◦C. Quinine sulphate (Sigma)

wasusedasreference(

Ф

st=0.54)forfluorescencequantumyield

calculations of the dye. Schematic structure of the employed dye molecule 2- {6-(3-methyl-3-mesitylcyclobutyl)-thiazolo[3,2-b][1,2,4]triazol-2-yl}-phenol(MMT)isshowninFig.1.

2.2. Instrumentation

UV–visabsorptionspectrawererecordedusingVarianCary100 bioUV–visiblespectrophotometer.Allfluorescencemeasurements werecarriedout ona ShimadzuRF-5301PCspectrofluorimeter withaXenonshortarclampasthelightsource.GBC904PBTatomic absorptionspectrophotometerwithanair-acetyleneflame(FAAS) wasalso usedfor nickel measurements.A CEM MARS 5 (CEM, Matthews,NC,USA)microwaveapparatusequippedwithPTFE ves-selswasusedformicrowavedigestion.Thefilmthicknessesofthe sensingslidesweremeasuredwithAmbiosTechnologyXP-1HGH Resolutionsurfaceprofiler.

2.3. Synthesisandthecharacterizationofthe2-

{6-(3-methyl-3-mesitylcyclobutyl)-thiazolo[3,2-b][1,2,4]triazol-2-yl}-phenol

(MMT)

Thecompoundwassynthesizedasin Fig.1bythefollowing procedure. To a stirred solution of 5-(2-hydroxy-phenyl)-2,4-dihydro-[1,2,4]triazole-3-thione (1.9323g, 10mmol) in 30mL ofethanol,2-chloro-1-(3-methyl-3-mesityl-cyclobutyl)-ethanone (2.6479g,10mmol)wasaddedinportions.Aftertheadditionof the_{␣-haloketone,}thetemperaturewaskeptat50–55◦Cfor2h. Aftercoolingtotheroomtemperature,thesolutionpHwasbrought about 6.8 with an aqueous solution of NH3 (5%). The

precipi-tatewasfilteredoff,washedwithaqueousNH3 solutionseveral

times and dried in air. Yellow crystals of the compound were obtainedbyslowevaporationofitsethanolsolution.Yield:93%, meltingpoint:152◦C.CharacteristicIRbands:3445cm−1(O H), 2951–2867cm−1 (aliphatics), 1624cm−1 (C N), 1586cm−1 (C N),754cm−1(C S C).Characteristic1_{HNMRshifts(CDCl}

3,

ı,ppm):1.73(s,3H, CH3),2.24(s,6H,o-CH3),2.26(s,3H,p-CH3),

2.69–2.74(m,2H, CH2 cyclobutane),2.86–2.92(m,2H, CH2

cyclobutane),3.88(quint,j=8.92Hz,1H, CH cyclobutane),6.56 (d,j=1.2Hz,1H,aromaticonthiazolering),6.80(s,2H,aromatics

onmesityl),6.94–6.98(m,1H,aromatic),7.04–7.06(m,1H, aro-matic),7.26–7.35(m,1H,aromatic),8.11(dd,j1=7.8Hz,j2=1.8Hz,

1H,aromatic),10.74(s,1H, OH,D2Oexchangeable).

Characteris-tic13_{CNMRshifts(CDCl}

3,ı,ppm):166.14,157.13,143.74,137.55,

135.33, 135.32, 131.60, 130.74, 130.72, 127.40,119.60, 117.50, 114.77,106.30,42.14,41.86,27.47,24.78,21.69,20.69.

2.4. Preparationofpolymerfilm

Themembranecocktailwaspreparedusingamixtureof120mg ofPVC,240mgofplasticizer(DOA),2.0mgofPTCPBand1.5mgof MMTdye.Themembranecomponentsweredissolvedin1.5mL driedTHFina glassvial.Thesolutionwasimmediatelyshaken vigorouslytoachievecompletehomogeneity.Theprepared mix-turescontained33%PVCand66%plasticizerbyweightwhichis inaccordancewithliterature[24,25].Theresultingcocktailswere spreadontoa125␮mpolyestersupport(MylarTMtype)byknife coatinglocatedinaTHF-saturateddesiccator.Thepolymersupport isopticallyfullytransparent,ionimpermeableandexhibitsgood adhesiontoPVC.Thefilmswerekeptinadesiccatorinthedark. Thiswaythephotostabilityofthemembranewasensuredandthe damagefromtheambientairofthelaboratorywasavoided.Each sensorfilmwascuttoasizeof13×50mm.Thefilmthicknessesof thesensingslidesweremeasuredwiththehighresolutionsurface profilerandfoundtobe4.78±0.024␮mforPVCmatrices(n=8).

Absorption and fluorescence emission spectra of PVC mem-braneswererecordedinquartzcellswhichwerefilledwithsample solution.Thepolymerfilmswereplacedindiagonalpositioninthe quartzcell.Theadvantageofthiskindofplacementwastoimprove thereproducibilityofthemeasurements.Alloftheexperiments wereoperated at roomtemperature,25±1◦C. Themembranes werenotconditionedbeforeuse.

2.5. Samplepreparation

Samplesolutionsoftealeaveandwildediblemushroom sam-ples were prepared by microwave digestion method. For the digestionofsamples,0.5gofeachsamplewasaccuratelyweighed andtransferredintotheTeflonvessels.Samplesweredigestedwith 3mlofHNO3and1mlofH2O2 inamicrowavedigestionsystem

anddilutedto10mlwithpurewater.Digestionprogramforthe microwavesystemwereappliedsequentiallyas3minfor180W, 5minfor360Wand3minfor180W.Certifiedreferencematerial andtheblankdigestionswerealsocarriedoutinthesameway.All thesolutionswerestoredintightlycappedpolythenebottles.

3. Resultsanddiscussion

3.1. Spectralcharacterizationstudies

InordertoperformthespectralcharacterizationoftheMMT dye,excitationandemissionspectrawererecordedinthesolvents ofdifferentpolaritiesandPVCfilm(Fig.2).Inalltheemployed solventsandPVCfilmtheStokes’shiftvalues, _ST (the

differ-encebetweenexcitationandemissionmaximum),calculatedfrom thespectraldatawerequitehighandwasfoundtospreadinthe wavelengthrangeof109–118nm(Table2).WhendopedinPVCthe

Fig.2.ExcitationandemissionspectraofMMTdyeindifferentsolventsandPVC. (a)THF(ex=380nm,em=493nm),(b)DMF(ex=379nm,em=495nm),(c)EtOH (ex=378nm,em=487nm),(d)PVC(ex=384nm,em=502nm).

Stokes’shiftsofMMTexhibitedanenhancementwithrespecttothe solutionphase.Therefore,whenimmobilized,theMMTdyecould beexcitedatlongerwavelengthswithrespecttothesolutionphase. Thisresultcanbeattributedtotherestrictedvibrationalrotational motionsinsolidstates.

3.2. Fluorescencequantumyieldcalculations

Fluorescencequantumyieldvalues(

Ф

F)oftheMMTcompound

were calculated employing the comparative William’s method whichinvolvestheuseofwell-characterizedstandardswithknown (

Ф

F)values[26].Forthispurpose,theUV–visabsorbtionand

emis-sionspectraofsixdifferentconcentrationsofreferencestandard (quininesulphatein0.1MH2SO4)andMMTwererecorded.The

integratedfluorescenceintensitieswereplottedversusabsorbance forthereferencestandardandthedye.Thegradientsoftheplots are proportional to the quantity of the quantum yield of the studiedmolecules.Theequationsoftheplotsarey=1,578,160x; R2_{=0.9988 for reference standard, y=36,341x; R}2_{=0.9954 for}

MMT dyein PVC, and y=17,845x; R2_{=0.9742 for MMT dyein}

EtOH.Thedataobtainedandquantumyield(

Ф

F)valuescalculated

accordingtoEq.(1)areshowninTable2. x=ST



_Grad x GradST

 

_n2 x n2 ST



(1) whereST andxdenotestandardand sample,respectively,Grad isthegradientfromtheplotandnistherefractiveindexofthe solventorpolymermatrixmaterial.Accordingtothedataobtained, theMMTdyeexhibitedhigherquantumyieldinplasticizedPVC comparedtothatobtainedinthesolventsused.

3.3. FluorescencequenchingofoptodebyNi2+

ToinvestigatetheopticalresponseofMMTembeddedPVCfilm towardNi2+_{,afluorescencedeterminationwascarriedoutinthe}

Table2

Theexcitation-emissionspectrarelatedcharacteristicsofMMTindilutedsolutionsofTHF,DMF,andEtOHandinsolidmatricesofPVC.

Matrix Excitationwavelengthex(nm) Emissionwavelengthem(nm) Stokes’shiftST(nm) Refractiveindexn QuantumyieldФF

THF 380 493 113 1.4070 0.025

DMF 379 495 116 1.4305 0.016

EtOH 378 487 109 1.3614 0.028

Fig.3.FluorescenceresponseoftheMMTdyedopedPVCfilmtoNi2+_ionsatpH6.0.

(a)Ni-freebuffer,(b)1.0× 10−9_{M,(c)5.0× 10}−9_M,(d)2.5×10−8_M,(e)1.3×10−7_M,

(f)6.5×10−7_M,(g)3.3×10−6_M,(h)1.7×10−5_M,(i)8.5×10−5_M,(k)4.3×10−4_M,

(m)2.2× 10−3_M,(n)4.4×10−3_{M(ex=384nm).}

Ni2+_{concentrationrangefrom1.0×10}−9_to4.4×10−3_M.A

signifi-cantdecreaseinfluorescenceintensityoftheoptodewasobserved

uponincreasingNi2+_{concentrationinthisrange(Fig.3).A}

cali-brationcurvewasobtainedfromtheplotoffluorescenceintensity withtheaddedNi2+_{concentration.Thecurveequationasshownin}

theinsetofFig.3wasy=−0.1023x+0.932,R2_{=0.9928.Thelimitof}

detectionbasedon3oftheblankwas8.5×10−10M.Thedetection limitobtainedforNi2+_{inthepresentstudywascomparedwiththe}

reportedmethodsisgiveninTable1.ItcanbeseenfromTable1 thatthelimitofdetectionobtainedinthepresentmethodisoneof thelowestforNi2+_.

Quenching can occur by different mechanisms. In dynamic quenching,chargetransferoccursand fluorescenceis quenched whenthequenhercollideswiththeexcitedfluorophore.Because thecollisionbetweenthequencherandfluorophoreaffectsonly theexcitedstateofthefluorophore,nochangesintheabsorption orexcitationspectrumareexpected.Onthecontrary,the forma-tionofground-statecomplexinstaticquenchingwillperturbthe absorptionspectraofthefluorophore[27].Thus,byexaminationof theabsorptionspectrum,staticanddynamicquenchingcanbe dis-tinguished.Fig.4showstheabsorptionspectraoftheMMTinthe absenceandpresenceofthequencher.Byconsideringthechanges intheabsorptionspectrumthequenchingtypeisassumedtobe static.

ThestoichiometryofNi2+_{–MMTcomplexwasdeterminedby}

meansofJob’smethod(Fig.5).ThefluorescencequenchingofMMT by Ni2+ _{wasattributed to the1:1 complex formation between}

Ni2+ _{and MMT and its association constant was calculated as}

2.24×106_M−1_.

3.4. Optimizationofmembranecomposition

Theresponsecharacteristicsofoptodessuchasdynamicrange andresponsetimedependonmembranecomposition[25]. Dif-ferentaspectsofthecompositionofmembranes-basedonMMT forNi2+_{ionswereoptimized,andtheresultsaresummarizedin}

Table3.Inallcases,themembraneswerepreparedaccordingto recommendedprocedures.

Fig.4. AbsorptionspectraofMMT(a)intheabsenceand(b)inthepresenceofthe quencher;Ni(II).

Forahomogeneousmembranephase,themembranesolvent (plasticizer) must be physically compatible with polymer. The natureoftheplasticizerisalsowellknowntoaffectthedynamic concentrationrangeandselectivitybehaviorofthesensing mem-braneandfacilitatethetransportoftargetions.Inordertostudy thenatureoftheplasticizer,severalsolventmediatorssuchasDOP, DOS,DAOandNPOEweretested.Duetoitslineardynamicrange towardNi(II)ions,whichisthelongest,anditssuperiorphysical properties,theDAOcontainingmembranewasselectedasthe opti-mumcompositionforpreparationofthemembranestobeusedin subsequentexperiments.

TheamountofPTCPBasanionicsitesinthemembraneisanother parameterthataffectstheoptoderesponse.Inthedesignofthe proposed optical sensor, the optode membrane working range becomeswiderandresponsetimeshorterastheamountofPTCPB intheoptodemembraneincreasesfrom1mgto2mg.Thus,2mg PTCPBwasselectedforfurtherstudies.

Theotherparameterofthemembranecomposition,whichhas tobeinvestigated,is theconcentrationoftheligand. Optimum responsewasfoundwhentheamountofMMTwas1.5mg.From thedatashowninTable3,themembranenumber8withoptimized PVC:DOA:MMP:PTCPB weight percentage ratio of 33:66:0.4:0.6 wasselectedforfurtherstudies.

Fig.5.Job’splotofMMTandNi(II)inwater(pH6.0).ThetotalconcentrationofMMT andNi(II)was1.0×10−6_M.

Table3

Optimizationofthemembranecomposition.

Composition Response

No. Plasticizer MMT(mg) PTCPB(mg) Responsetime(min) Workingconcentrationrange(M)

1 DOP 1 1 3 5.0×10−7–5.0×10−3 2 DOS 1 1 3 1.0×10−7_–1.0×10−4 3 DOA 1 1 3 5.0×10−8_–4.4×10−3 4 NPOE 1 1 4 1.0×10−6_–1.0×10−4 5 DOA 0.5 1 3 1.0×10−7_–1.0×10−4 6 DOA 1 1 3 5.0×10−8_–1.0×10−4 7 DOA 1.5 1 3 5.0×10−8–4.4×10−3 8a _{DOA 1.5 2 2 1.0×10}−9_–4.4×10−3

a_{Optimummembranecomposition.}

3.5. EffectofpH

Theresponsecharacteristicsoftheoptodesuchassensitivity, responserangeanddetectionlimitdependonpH.Theresponse curvedatawereobtainedbymeasuringthefluorescencevaluesfor 3.3×10−6MNi2+_{atdifferentpHvaluesandtheresultsareshown}

inFig.6.Fromthisfigure,weseethatthepHofthesolutionhasno considerableeffectontheresponseofthefilminpHrange5.0–8.0. Ontheotherhand,thedecreasedopticalresponseofthesensor atpH>8.0couldbeduetothehydroxideformationofnickelions aswellasapossibleslightswellingofthepolymericfilmunder alkalineconditionsofsolution.Therefore,apHof6.0adjustedbya 0.01MacetatebufferwasconsideredasoptimumandusedforNi2+

determinations.

3.6. Reversibility,reproducibilityandshort-termstability

Theregenerationoftheproposedmembranesensorwas stud-iedbyusingdifferentreagentsincludingHCl,HNO3andEDTAin

differentconcentrations.Theresultsindicatedthata0.1MEDTA solutioncanefficientlyremoveanyadsorbedNi2+_fromthe

mem-braneandreturnitsfluorescencetoitsinitialvalueinabout3min. Thereproducibilityoftheopticalmembranewasevaluatedby per-formingeightdeterminationswiththesamestandardsolutionof nickelionsusingasinglemembranesensor.Theresultsareshown in Fig.7. Ascanbe seenfrom this figure,thesystemis highly reversible.Therelativestandarddeviation(RSD)forthe determi-nationof3.3×10−6MNi(II)standardsolutionwas3.1%.

Theshort-termstabilityoftheoptodemembranewasdefined intermofthestabilityoffluorescenceoftheoptodemembrane. Tostudytheshort-termstabilityoftheoptodemembrane,its flu-orescenceintensityincontactwitha3.3×10−6MsolutionofNi2+

bufferedatpH6.0wasmeasuredoveraperiodof6h.Fromthe fluorescenceintensitiestakenevery30min(n=12),itwasfound thattheresponseisalmostconstantwithonlya1.6%increasein

Fig.6. TheeffectofpHonthesensorresponseinsolutionscontaining3.3×10−6_M Ni2+_.

Fig.7. Reproducibilityandreversibilityoftheresponseoftheoptodemembraneto 3.3×10−6_MNi2+_{andtotheregenerationsolution,0.1MEDTA.}

intensityafter6hmonitoring.Thisindicatedasatisfiedshort-term stability.

3.7. Selectivity

Obviously,theselectivityisoneofthemostimportant prop-ertiesof theresponse ofa sensor.Thisproperty representsthe preferenceofasensorresponsetotheprimaryionwithrespect tothepotentiallyinterferingions.Fortheevaluationofthe selec-tivityoftheproposedfilm,theresultingtoleratedrelativeerrorin thepresenceofaninterferingionwasdefinedas,Relativeerror (%)=[(F−F0)/F0]×100,inwhichFandF0denotethefluorescence

of the filmin the presence and absence of theinterfering ion, respectively.Theselectivityoftheoptodewastestedforthe deter-minationofNi2+_{inthepresenceofotherinterferingcationsnamely}

Ag(I),Cd(II),Co(II), Cr(III),Cu(II),Fe(III),Mg(II), Na(I),Pb(II)and Zn(II).Theconcentrationoftheinterferingionwas100timesas muchtheprimaryion(Ni2+_,3.3×10−6_{M).Theresultsof}

selectiv-itystudiesaresummarizedinFig.8.Ascanbeseeninthisfigure,

Table4

DeterminationofNi(II)intealeaveandwildediblemushroomsamplesofthree replicatemeasurementswiththeproposedsensorandFAAS.

Sample Ni2+_(␮gg−1_{) Relativeerror(%)}

Optode FAAS

Tealeave1 5.20±0.21 5.09±0.16 2.16 Tealeave2 4.22±0.12 4.17±0.15 1.19 Mushroom1 5.14±0.15 5.28±0.21 −2.65 Mushroom2 3.75±0.23 3.61±0.18 3.88

inthepresenceofalltheinterferingionsstudied,therelativeerror

islessthan5.0%,whichisrecognizedastolerable.

3.8. Analyticalapplication

Totestthepracticalapplicationofthepresentsensor,

applica-tionsfordirectdeterminationofNi(II)intealeaveandwildedible

mushroomsampleswerecarriedout.Threeparallelanalyseswere

doneforeachsample.Thesampleswerepreparedasdescribedin

Section2.5.Tocheckthevalidityoftheproposedmethod,the

con-centrationsofNi(II)inthesampleswerealsodeterminedbyflame atomicabsorptionspectrometry(FAAS).Therelativeerrorobtained withthesensorvariedintherange,1.19–3.88%comparedtothe resultsobtainedbyFAAS(Table4).

Inordertovalidatetheaccuracyofthedevelopedmethod, certi-fiedreferencematerial(NIST-SRM1547Peachleaves)wasanalyzed fornickel(II).Themeasuredvalue(0.67±0.06␮gg−1)wasingood agreementwiththecertifiedvalue(0.69±0.09␮gg−1).

4. Conclusion

Theproposedsensorisaprecise,lowcost,sensitiveandhighly selective metod for determination of Ni(II),based on the fluo-rescentthiazolo-triazol derivativeentrappedinPVCmatrix.The sensorproducedalinearresponseforNi(II)concentrationrangeof 1.0×10−9–4.4×10−3Mwiththedetectionlimitof8.5×10−10M. TheopticalsensorhasagoodselectivitytowardNi(II)versusother metalions.Thesensingmembranealsoexhibitedgood photosta-bilityandreproducibility.Theoptodewasfoundtobestableand reliablefor usein realsamples.Moreover,a comparison ofthe proposedoptodewiththepreviouslyreportedsensorsfor deter-minationofNi(II)(Table1)indicatesthattheproposedmethod, inadditiontofastandsimplicity,providesacomparabledetection limitwithmostoftheothermethods.

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Biographies

NurAksunerhasB.Sc.degreeinchemistry,M.Sc.andPh.D.degreein analyti-calchemistryfromEgeUniversity,Izmir,Turkey.Hercurrentresearchinterests includefluorescencespectroscopy,photo-characterizationofnewlysynthesized fluoroionophores,developingopticalchemicalsensorsformetalions.

EmurHendenhasB.Sc.degreeinchemistryfromEgeUniversity,Izmir,Turkey.He receivedhisM.Sc.degreeandPh.D.degreein1976inchemistryattheUniversityof Birmingham,UK.HeiscurrentlyaprofessorofanalyticalchemistryatEge Univer-sity.Hiscurrentresearchinterestsincludethedevelopmentofatomicspectrometric methodsandopticalsensors.

IbrahimYilmazhasB.Sc.degreeinchemistryfromInonuUniversity,M.Sc.andPh.D. degreeinchemistryfromFıratUniversity,Elazı˘gTurkey.Heworksasaprofessorin Karamano˘gluMehmetBeyUniversity.Hiscurrentresearchinterestsinclude synthe-sisofnewthiazoleandthiazoleringcontainingcompounds,andsubstitutedSchiff baseligands.

AlaaddinCukurovalihasB.Sc.degreeinchemistryfromAnkaraUniversity,M.Sc. andPh.D.degreeinchemistryfromFıratUniversity,Elazı˘gTurkey.Heiscurrently aprofessorofchemistryatFıratUniversity.Hiscurrentresearchinterestsinclude synthesisanddesignofnewheterocycles(thiazoleandthiazoleringcontaining compounds,cyclobutanederivatives,azomethineandhydrazonecompounds).