Adsorption phenomena of ofloxacin at a hanging mercury drop electrode in Britton Robinson buffer solution with pH of 8.36 were studied by means of square-wave voltammetry

Gulaboski, Rubin

Jordanoski, Blagoja

UGD Academic Repository

GHTMDD- 366Received:June 29,2000Accepted:November9,2000BulletinoftheChemistsandTechnologistsofMacedonia,Vo!.19,No.2,pp.177-181(2000)ISSN0350- 0136UDC:543.552Shortcommunication"SQUARE-WAVE VOLTAMMETRY OF OFLOXACINRubin Gulaboski,Blagoja JordanoskiInstituteof Chemistry,FacultyofNaturalSciencesandMathematics,The "Sv.Kiril & Metodij"University,MK-1000Skopje,RepublicofMacedoniaAdsorptionphenomenaof ofloxacinathangingmercurydropelectrode(HMDE) in Britton-RobinsonbuffersolutionwithpH of 8.36werestudiedbymeansof square-waveoltammetry(SWV).TheSWV responseofoflox-acinis sensitivetopH,typeof thesupportingelectrolyte,theofloxacinconcentration,theaccuI11lllationtimeandpo-tential,aswellastheexcitingsignalparameterssuchasfrequency,amplitudeandpotentialincrement.Theoptimiza-tionofexperimentalconditionsforquantitatived terminationfofloxacinwasachieved.A square-waveoltammet-ricmethodforquantitatived terminationfofloxacinisdeveloped.Thedetectionlimitof4'10-8molllofloxacinwasfound.Keywords:ofloxacin;adsorption;surfaceredoxreaction;square-waveoltammetryINTRODUCTIONThedemandfor detectionof tracelevelsforsubstancesof environmental,c inicalandforensicimportanceis growingtremendously.Electro-chemicaltechniquessuchassquare-waveoltam-metry(SWV), differentialpulse voltammetry(DPP)andlinearscanvoltammetry(LSV) havebeena powerfultool for analyticalpurposes.Amongthem,theSWV is themostadvancedtech-niquebothforelucidationof electrodemechanismsaswell as for analyticalapplications,especiallywhenadsorptionof reactantor/andproductoccur[1-5].Theuniqueadvantagesof square-waveolt-ammetrysuchasfastscanrate,largefrequency,largeamplitude,shorttimerequiredfor measure-mentsaswellasthespecificapproachin thecur-rentsamplingprocedurerankthistechniqueasoneofthemostadvancedamongthefamilyof pulsevoltammetrictechniques[1-7].Thecompoundstudiedin thispaper,knowninthepharmacyasofloxacin(see.Fig.1)is asyn-theticfluorinatedquinolonederivativehavingac-tivityagainstbothgramnegativeandgrampositivebacteriathroughinhibitionof theirDNA gyrase[8].It is widelyusedinthetreatmentofrespiratoryandurinarytractinfections[9]. Severalpolaro-graphicandvoltammetrictechniquesforitsquanti-tativedeterminationhavebeenutilized[10-12].Nooneofthepublishedpapershasbeenconcernedtotheadsorptionpropertiesof ofloxacinatamer-curyelectrode.Also, theeffectof themethanolconcentrationaddedin thepolarographiccellhasnotbeendiscussed.TherearealsoreportsthatH+ionshavebeenincludedin theelectrochemicalre-actionof ofloxacin,howeveritsnumberis notyetdeterminedprecisely[12].Thispaperdealswiththeadsorptionbehaviorof ofloxacinataHMDE inordertoelucidatesomespecificpropertiesof its electrodereaction.Theobjectiveofthisworkis alsotodevelopastrippingsquare-waveoltammetricmethodforquantitativedeterminationfofloxacin.EXPERIMENTALAll measurementswereperformedwith amultimodepolarographicanalyzerPrincetonAp-pliedResearchModel384BconnectedwithastaticmercurydropelectrodePAR 303A.An Ag/AgCI(saturatedKCI) wasthereferenceandPtwirewasthecounterelectrode.Thestocksolutionof oflox-178 R. Gulaboski, B. lordanoskiacin(purchasedfromMERCK) waspreparedbydissolvinganappropriateamountin 98 % metha-nol.All chemicalsusedsuchas methanol,boricacid,aceticacid,phosphoricacidandNaOH(allobtainedfromSIGMA) wereof analyticalgradeand wereusedas received.0.1 molll Britton-Robinsonbuffersolutions(mixtureof 0.1molllboricacid,0.1 molll aceticacidand0.1 molllphosphoricacid,adjustedtodifferentpHvaluesby0.1mol/1NaOH)wereusedassupportingelectro-Ilytes.All solutionswerepreparedwithultrapurewater.Solutionsweredegassedwith high-purityInitrogenfor8 minpriortothemeasurementsa d Iforanadditional20sbeforeeachscan.A nitrogenIatmospherein thecellwasmaintainedthroughoutIexperiments.Each scan was done on a separatemercurydropatroomtemperature.RESULTS AND DISCUSSIONOfloxacin(seeFig. 1)hasbeena subjectofseveralelectrochemicalinvestigations.However,itselectrochemicalbehaviorata hangingmercurydropelectrode(HMDE)isnotyetfullyclear.Belalatal. [12]haverecentlystudiedofloxacinelectro-chemicalbehaviorby meansof differentialpulsepolarography(DPP).Theyfoundthatofloxacin'selectrochemicalactivityis duetothereductionofthecarbonylC=Ogroupandit proceedsaccordingtothefollowingScheme:R-C=O+2H++2e--+RCH-OHeOOHrH3C'VFig. 1.StructureofofloxacinObviously,thereductionofofloxacinis anir-reversibleredoxprocessfollowedby protonation.Rizketat.[12]pointedoutthatthenumberof ex-changedprotonsin theredoxreactionof ofloxacinis 1.13andtheyapproximateit for2. In thesamepaper,adsorptionphenomenaof ofloxacinmole-culesat a HMD electrodein Britton-RobinsonbuffermediumwithpH of 8.36arejustnoted,butnotinvestigatedin details.Keepingin mindthefactthattheadsorptionofelectroactivecompoundsatamercuryelectrodecouldbeusefulfordevelop-ingof sensitivestrippingvoltammetricmethodforquantitativedetermination,adsorptionphenomenaof ofloxacinataHMDE deserveseriousattention.Aiming to inspectelectrochemicalbehaviorofofloxacinataHMD electrodeaswellastodevelopasensitiveSW voltammetricmethodforitsquanti-tativedetermination,supplementaryvoltammetricinvestigationswere carried out by means ofsquare-wavevoltammetry.The SW voltammetricresponseof ofloxacinis sensitiveto pH, accumulationtime,accumula-tion potential,as well as the excitingsignalpa-rametersuchasfrequencyamplitudeandpotentialincrement.All thesedependencieswere investi-gatedfurther.The pH of supportingelectrolytesolutionsin-fluencesboththepeakpotentialandpeakcurrentsof the SW voltammetricresponsesof ofloxacin.ChangingthepH from4.0 to 11.0causestheSWpeakpotentialof ofloxacinto shift towardsmorenegativevalues.For instance,where pH of thesupportingelectrolytesolutionwas changedfrom4.0 to 11.0,thepeak-potentialEpwasshiftedfrom-1.436 V to -1.780 V, respectively.The linearde-pendencebetweenthe peak potentialand pH ofsupportingelectrolytesolutionsexist,representedby thefollowing equation:EpIV =-0.057V/pH-1.180withR2=0.986.The slopeof thelinearline,whichreads-57 mV/pHimplicatesthatoneH+ionis involvedin theelementaryactof theelectro-chemicalreaction.Thisresultdiffersfromthatpre-sentedbyBelaletal. [12].Aimingto selecthebestbuffersolutionforanalyticalpurposes,severalcalibrationcurvesofofloxacinsolutionswereconstructedin Britton-RobinsonbuffersolutionswithdifferentpHvalues(seeFig.2).Theslopeof thelinearlineaswellasthecorrelationcoefficientof thelinearregressionlineareonesof themajorcriteriatoselectanopti-mal electrolytesolutionfor analyticalpurposes.Regardingtheconstructedcalibrationcurvesde-pictedin theFig. 2, oneshouldobservethatthegreatestslopeof theIpvs.pH aswellasthebestlinearityin Britton-RobinsonbuffersolutionwithpHof8.36exist.Thus,alladditionalvoltammetricmeasurementsofofloxacinwereperformedinBrit-Bull.Chem.Technol.Macedonia.19,2,177-181(2000)Square-wavevoltammetryof ofloxacin 179ton-RobinsonbuffersolutionwithpH of 8.36.IntheFig.3 is presenteda square-wavevoltammo-gramof ofloxacinsolutionrecordedin Britton-RobinsonbufferwithpH of 8.36.A welldefined43.5~//"' ///////"'///////.; ----0.500 10 2015C (ofloxacin x 105(mol r')reductionpeakof ofloxacinwithpeakpotentialofEp=-1.56 V andhalf-peakwidth/)'Ep/2of 80mVis observed.///"'~ pH='.17. pH=6.14pH=7.04"' pH=8.36- - - L;,,,, (pH=8.36)U"" (pH=7.04)-L;,,,, (pH=6.14)-U"" (pH='.I7)25 30 4535 40Fig. 2. The calibrationplotsof ofloxacinconstructedin Britton-RobinsonbufferswithdifferentpH values.The experimentalconditionsare:frequencyf= 120Hz, amplitudeEsw =20 mY, scanincrementdE =4 mY-1.70 -1.80-'.50 -'.60PotentialIVFig.3.Square-wavevoltammogramof5x 10-5molllofloxacinrecordedinBritton-RobinsonbuffersolutionwithpHof8.36.All otherconditionsarethesameasinthecaptionof theFig.2AdditionalinformationabouttheSW volt-ammetricbehaviorof ofloxacinin 0.1mol/lBrit-ton-RobinsonbuffersolutionwithpHof 8.36havebeencollectedbyvaryingthefrequencyofthesig-nal.Thepeakpotentialis almostunaffectedbytheSWfrequency.For instanceit shiftsfor only 12fJIac. xeM.TeXHOJI.MaKe)\OHlfja,19,2,177-181 (2000)mV in negativedirectionchangingthe SW fre-quencyfrom30 to 120Hz, respectively.However,the SW peakcurrentis severelysensitiveto theSW frequency.A linear relationshipbetweenthepeakcurrentsandtheSW frequencywasobservedin 0.1molll Britton-RobinsonbuffersolutionwithpH of 8.36.The linear line is representedby thefollowing equation: 1p/~A =0.0069 ~A/Hz +0.008.ThelinearitybetweenthepeakcurrentandtheSW frequencyis characteristicpropertiesfortheredoxprocessescontrolledbyadsorption.Thispropertyappearsto be on of themostimportantdiscrepanciesto the diffusioncontrolledredoxprocessesin whichthepeakcurrentincreasesinproportiontothesquarerootofthefrequency.Intendingto inspectheadsorptionphenom-enaof ofloxacinmoleculesat a HMD electrode,additionalexperimentswerecarriedout.RecordingtheSW voltammogramsof ofloxacinafterpro-longedaccumulationtime,significantincreaseoftheSW peakheightswasobserved.In thesametime,the peakpotentialremainedtotallyunaf-fectedby theaccumulationtime.ThefunctionofSW peakcurrentsof theaccumulationtimeis de-pictedin theFig.4 A. Obviously,therelationship2.5<-'i.: 2....1.5CurrentI AAI140012001000800600400200I, , , I-1.40180 R. Gulaboski, B. JordanoskihasfollowedtheLangmuireisotherm,reachingalimitingpeakcurrentvalueataccumulationtimeof150s.Thelinearpartof thecurvedepictedin theFig.4A suggestshatalinearrelationshipbetweenthesurfaceandbulk concentrationof ofloxacinexistinthatregion.1,31,27': 1) A1,1C'''oo''J'A1600 Q.,1400 h ~1200JV~~ it1000 . \~BOO600400200~.1.40 .1.51J.1.60 .1.70 .1.00P,'oo,;,IIV300 400100 200taeelsFig. 4.The dependenceof theSW peakcurrentof ofloxacinon theaccumulationtime:withoutmethanol(curveA) andwith 10%v/vmethanol(curveB). Eacc.=-1.35V, pH=8.36,C (ofloxacin) =5 x 10-6mol/I.All otherconditionsarethesameasin theFig. 2. In theinsetareshownSWvoltammogramsof ofloxacinrecordedataccumulationtimeof 0 s (curveon thebottom),5, 10and15s (towardstop),respectivelyThe accumulationpotentialEacc.alsoaffectsthe SW voltammetricresponseof ofloxacin.Aparabolicdependencyof theSW peakcurrentsofofloxacinon theaccumulationpotentialwasob-served,withamaximalpeakcurrentlocatedatEacc.=-1.35V.Theeffectof themethanoladdedinthebuffersolutionwasalsostudied.As it is shownintheFig.4 B, 1 ml methanoladdedin the supportingelectrolytesolutioncausedasignificantdecreaseofthepeakheightsof ofloxacin.ThisisforexpectingknowingthefactthatmethanolisadsorbeditselfataHMD electrodeandit showsconcurrentadsorp-tionto theofloxacinmolecules.Generallyspeak-ing,oneshouldexpectappreciableinterference'soriginatedfromconcurrentadsorptionif somesur-faceactivecompoundsarepresentedinthesampleandthesupportingelectrolyte.All theseresultsindicatethattheredoxreac-tionofofloxacininBritton-RobinsonbuffersolutionwithpH of 8.36occurspredominantlyasa surfaceredoxprocessin whichtheofloxacinmoleculesaresignificantlyconfinedtotheelectrodesurface.It shouldbepointedoutthattheSW voltam-metricresponseof ofloxacinis particularlysensi-tiveto thesignalamplitude.If theamplitudewasincreasedto 80mY, a significantenhancementoftheSW peakheightswasobserved.However,anamplitudegraterthan50mV significantlydisturbstheSW peakof ofloxacin.The maximumratio1p//j,Ep/2,whichappearstobemaincriterionforse-lectingparticularamplitudeforanalyticalapplica-tion,atamplitudeof30mV wasobserved.Finally, the followingset of experimentalconditionshasbeenevaluatedasoptimalforquan-titativedeterminationof ofloxacin,utilizingad-sorptivestrippingsquare-wavevoltammetry:0.1mol/lBritton-RobinsonbuffersolutionwithpH of8.36,frequencyof 120Hz, amplitudeof 30 mVandscanincrementof4mV.Undertheseoptimalconditions,a linearde-pendencebetweenpeakcurrentsandconcentrationof ofloxacinexistsovera wideconcentrationrange.The calibrationplot in 10-7molll ofloxacincon-centrationregionwasconstructedwithaccumula-tiontimeof 90s (seeFig.5).Thelinearlineis as-sociated with following equation: Ip/flA =0.0335flA l/flmol +0.505,with R2of 0.998.In thehigherconcentrationregion, accumulationtime of 10 swasappliedandthecorrespondingequationof thelinear regression line reads Ip/flA =0.0524 flAlIflmol+0.602.0,95]0,9 ~0,850,8~0,75~ 0,70,650,60,550,50./.....2 4 6 8 10 12C (of1oxacin)x 107/molrIFig. 5. Calibrationplotof ofloxacinrecordedin Britton-Robinsonbuffersolutionwith pH of 8.36.The experimentalswere:accumulationtimetacc.=90 s,Eacc.=-1.35 V, Esw=30mY. Otherconditionswerethesameasin theFig. 2Bull.Chem.Technol.Macedonia,19,2,177-181(2000)0,9.....0,80,70,60,50Square-wavevoltammetryof ofloxacin 181Under accumulationtime of 90 s, detectionlimit,estimatedas threetimesto noise,of 4.10-8mol/lwasfound.It is thelowestvaluefor thede-tectionlimit from all publishedfor ofloxacin.Therelativestandardeviation(RSD) of theresultsvariedfrom0.2to 7.1% (eightmeasurementsa5.10-7and5.10-6molll).Thereproducibilityof theresultsrangesfrom89.49to102.65%.CONCLUSIONOfloxacinis electrochemicallyactivecom-poundthatundergoesirreversiblereductionataHMDelectrode.Thevoltammetricresponseof of-loxacinis sensitivetopHofthesupportingelectro-lytesolutions,aswellastheinstrumentalparame-tersuchasSW frequency,SW amplitudeandpo-tentialincrement.Thispaperis mainlyconcernedtoelucidateadsorptionphenomenaof ofloxacininBritton-RobinsonbuffersolutionwithpH of 8.36.Wehavedemonstratedthatin thispHmediumtheredoxreactionof ofloxacinappearsas a surfaceredoxprocessin whichtheofloxacinmoleculesareconfinedto themercuryelectrodesurface.Thesepropertiesenableonetodevelopa sensitivestrip-pingsquare-waveoltammetricmethodforquanti-tativedeterminationf ofloxacin.An optimizationof theexperimentalparametersforquantitatived -terminationof ofloxacinwasalsoachieved.A de-tectionlimitof 4 x 10-8molllof ofloxacinwases-timated.REFERENCES[1]1. 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EI-Enany,Talanta,46,83(1998).Pe311MeKBAL(PATHO-EPAHOBA BOJITAMETPIIJA HA OFLOXACINPy6HHrYJIa60CKH,EJIarojaJopp;aHocKHfblCiUuruyiU3axeMuja,IJpupooHo-MaiUeMaiUu'lKUcjJaKYJliUeiU,YHuBep3uiUeiU"CB. KupUJl U MeiUoouj",MK-1OOOCKouje,Peuy6JlUKaMaKeooHujaKJIYQIlII 36opOBII: ofloxacin; aTcopIIIJ;Hja; nOBpIIIHHCKa pe)),OKc-peaKu,Hja;KBa)),paTHo-6paHOBa BOJITaMeTpHjaBo OBOj TPY))" co npHMCHa Ha KBa)),paTHo-6paHOBaBOJITaMeTpHja, 6ea CTY)),HpaHHaTcopnu,HOHHTe OC06HHHHa ofloxacin-OT Ha BHce'IKa )KHBHHa KanKa, BO EpHTOH-P06HHCOHOBnycpepCKH paCTBOp co pH =8.36. KBa)),paT-Ho-6paHoBHoT BOJITaMeTpHcKH o)),rOBOpHa ofloxacin-oT C<PYHKu,Hja0)),pH Ha OCHOBHHTeeJIeKTpOJIHTli 0)),BpeMeTO11nOTeHu,lijaJIOT Ha aKYMYJIau,lijaTa, 0)),KOHu,eHTpau,lijaTaHa ofloxacin-OT H 0)), napaMeTpHTe Ha eKcu,HTau,HOHliOTCIIrHaJI KaKBli IIITO ce cppeKBeHu,HjaTa, aMnJIHTy)),aTa lifJ1ac.XeM.TeXHOJI.MaKe)10Hllja,19,2, 177-181 (2000)nOTeHu,lijaJIHHOT liHKpeMeHT. I1pliToa 6eIIIe licnHTaHO liBJIHjaHHeTo Ha KOHu,CHTpau,lijaTa Ha MeTaHOJI, )),o)),a)),eHBOOCHOBHliOT eJIeKTpOJIliT, BP3 BOJITaMeTpHCKliOT o)),rOBOpHa ofloxacin. EeIIIe H3BpIIIeHa onTHMH3au,Hja Ha eKcncpli-MeHTaJIHliTe YCJIOBli co u,eJI )),ace pa3pa60TH KBa)),paTHo-6paHoB BOJITaMeTplicKli MeTo)), 3a KBaHTliTaTHBHO onpe-)),eJIYBaIbe Ha ofloxacin. I1pli OnTliMaJIHHTe YCJIOBH, npec-MeTaHaTa rpaHHu,a Ha )),eTeKu,Hja3a ofloxacin H3HecYBaIIIe4 '10-8 mol/I.

Square-wave voltammetry of Ofloxacin

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