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A visible light exitable Chrome Sensor Actuator Paper 2018.pdf

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A probe, quinoline-2-carboxylic acid (4-oxo-4H-chromen-3-ylmethylene)-hydrazide, (HL), acts as selective and specific fluorogenic sensor to Al+3 in the visible light (435 nm) excitation in presence of biologically available large number of cations
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  SensorsandActuatorsB257(2018)545–552 ContentslistsavailableatScienceDirect Sensors   and   Actuators   B:   Chemical  journalhomepage:www.elsevier.com/locate/snb Research   Paper A   visible   light   excitable   chromone   appended   hydrazide   chemosensorfor   sequential   sensing   of    Al +3 and   F − in   aqueous   medium   and   in   Verocells Rakesh   Purkait a ,   Chiranjit   Patra a ,   Ananya   Das   Mahapatra b ,   Debprasad   Chattopadhyay b ,Chittaranjan   Sinha a , ∗ a DepartmentofChemistry,JadavpurUniversity,Kolkata700032,India b ICMRVirusUnit,InfectiousDiseases&BeliaghataGeneralHospital,GB-4,57,S.C.BannerjeeRoad,Beliaghata,Kolkata − 700010,India a   r   t   i   c   l   e   i   n   f   o  Articlehistory: Received13May   2017Receivedinrevisedform27October2017Accepted28October2017Availableonline7November2017 Keywords: Chromone-hydrazideAl +3 sensorQuenchingwithF − CellimagingDFTcomputation a   b   s   t   r   a   c   t A   probe,   quinoline-2-carboxylic   acid   (4-oxo-4H-chromen-3-ylmethylene)-hydrazide,   (HL),   acts   as   selec-tive   and   specific   fluorogenic   sensor   to   Al +3 in   the   visible   light   (435   nm)   excitation   in   presence   of biologically   available   large   number   of    cations   and   emission   appears   at   (  em )   520nm.   The   limit   of detection   (LOD)   for   Al +3 is7.6   nM   in   aqueous   medium   which   is   less   than   10 − 3 times   of    WHO   recom-mendeddata   (7.41   mM).   The    Job’s   plot   and   mass   spectral   data   support   1:1   composition   of    the   complex[Al(HL)(OH)H 2 O](NO 3 ) 2 .   The   strongly   emissive   complex   turns   off    upon   addition   of    F − and   isdetected   atthe   level   of    (LOD)   7.4   nM.   Thus,   F − ,   aharmful   water   pollutant,   could   be   identified   at   much   lower   levelof    WHO   recommended   toxic   limit   (3.68    M).   The   absorption   and   emission   spectral   features   of    HL    andits   Al +3 -complex   have   been   explained   byDFT   computation   of    optimized   geometries   and   calculation   of molecular   functions.   The   devised   receptor   is   non-toxic   and   hasbeen   used   indetecting   Al +3 in   the   intra-cellular   region   of    African   green   monkey   kidney   cells(Vero   cells)   and   exhibits   an   INHIBIT   logic   gate   withAl +3 and   F − aschemical   inputs   bymonitoring   the   emission   mode   at   520   nm.©2017   Elsevier   B.V.   All   rights   reserved. 1.Introduction Aluminium,anon-essentialelementandsignificantlyavail-ableontheearth’scrust(8.3%oftotalmass),isusefulinthemanufacturingofhouseholdutensils,waterpurificationinstru-ments,electricalwirings,andisenteringintohumanbodythroughfoodsandbeverages[1].ItcausesAl-relatedbonedisease(ARBD), encephalopathy,myopathyandvariousneurodegenerativedis-easessuchas,Alzheimer’sdisease,Parkinsonismdementiaetc.inhumanbodyandcanalsodamageplantroots[2–5].WorldHealth Organization(WHO)hasassignedAl +3 asprimefoodpollutantswithlimitingconcentration200  g/litre(7.41mM)   andhasrecom-mendedthattolerableweeklydietaryhumanintakeis7.0mg/kgbodyweight[6–8].ThusdetectionofAl +3 inwaterisofurgentimportantformonitoringhumanhealth.Explorationofselectiveandsensitivechemosensorfordetectionofionsinsolutionhasbeenofconsiderableattentionwithbiologicalandenvironmen- ∗ Correspondingauthor. E-mailaddresses: debprasadc@gmail.com(D.Chattopadhyay),crsjuchem@gmail.com(C.Sinha). talinterest[9–14].Schiffbaseligandshavegainedrecentinterest asfluorescentsensorsformetalionsincludingAl +3 duetotheirrelativelyeasyonestepsynthesis[15–24].However,sensingof  aluminuminaqueousmediumhavebeenrarelyreported[25,26],mainlyduetosparingorinsolubilityoforganicprobeinaqueousmedia.Thereforeitisachallengingtasktodaytodevelopnewsen-sorsforselectivelydetectingAl +3 inaqueoussolutions.Besides,emissiveAl +3 -complexesareservingasanionsensor,especiallytoF − [27–30] . Amongtheentirerangeofbiologicallyusefulanions,F − possessessignificantpotentialinthepreventionofdentalcariesandtreatmentforosteoporosis[31,32].Conversely presenceofexcessoffluorideinthehumanbodymaybedumpedasfluorapatiteinthebonesandteethleadingtoosteoporosis,osteosclerosis,dentalfluorosisandskeletalfluorosis[33–35].Thus, itisquiteimportanttodevelopanefficientF − sensor.ThedesignofdualsensorwhichiscapabletodetectsequentiallyAl +3 andF − byfluorescenceONOFFsignalingresponseishighlyeffec-tive.Recently,coumarinbasedmolecularswitchforthesequentialdetectionofAl +3 followedbyitsuseforF − sensingisreported[27,28].Besides,rhodaminebased[29]andquinaldineappended [30]chemosensorsarealsoavailableforthedetectionofAl +3 and https://doi.org/10.1016/j.snb.2017.10.1680925-4005/©2017ElsevierB.V.Allrightsreserved.  546 R.Purkaitetal./SensorsandActuatorsB257(2018)545–552 F − [29].However,problemappearsaboutthesolubilityofprobesinaqueousmedium.Chromonederivativeshavebeenextensivelyappliedasanti-tumoralagents,cardiocerebrovasculardrugsandfluorophoresduetotheirexcellentpharmacologicalandspectroscopicprop-erties[34–37].However,thereportsaboutfluorescentsensors basedonchromonetodetectmetalionsarerelativelyfew[38,39].Inthepresentwork,wehavesynthesizedahydrazidebasedchromonederivative,Quinoline-2-carboxylicacid(4-oxo-4H-chromen-3-ylmethylene)-hydrazide,whichshowsselectiveandsensitivedetectionofAl +3 inaqueousmediuminpresenceof othercommonlyavailablemetalions.HL-Al +3 complex(1:1com-plex)actsasanagentforthedetectionofF − inpresenceofotheranions.TheDFTcomputationofoptimizedgeometryofHLandthecomplexhasbeenusedtoexplaintheelectronicspectralproper-ties.Thepracticalapplicabilityoftheligand(HL)hasbeentestedbyaddingtoAfricangreenmonkeykidneycells(Verocells,ATCC,Manassas,VA,USA)forthedeterminationofexogenousAl +3 ionsbyfluorescencecellimagingprocesses. 2.Experimentalsection  2.1.Materialsandmethods Quinaldicacidand4-oxo-4 H- chromene-3-carbaldehydewerepurchasedfromSigma-Aldrichandquinoline-2-carbohydrazidewassynthesizedfollowingthepublishedprocedure[40].Allother organicchemicalsandinorganicsaltswereobtainedfromMerckandusedwithoutfurtherpurification.Aqueoussolutionswerepre-paredusingMilli-Qwater(Millipore).ElementalanalyseswereperformedusingaPerkin-Elmer2400Series-IICHNanalyzer,PerkinElmer,USAelementalanalyzer.ThespectrawererecordedonPerkinElmerinstruments:Lambda25spectrophotometerforUV–visspectra;modelLS55forfluorescencespectraandFT-IR spectra(KBrdisk,4000–400cm − 1 )frommodelLX-1FTIRspec-trophotometer.NMR    spectrawereobtainedonaBruker(AC)300MHz   FT-NMRspectrometerusingTMS   asaninternalstan-dard.ESImassspectrawererecordedfromaWaterHRMSmodelXEVO-G2QTOF#YCA351spectrometer.Allofthemeasurementswereconductedatroomtemperature.CellimagingstudieswereexecutedusingZeissAxiovert40CFLfluorescencemicroscope.Thefluorescencequantumyieldwasdeterminedusingfluores-ceinasreferencewithaknownquantumyield,  R  =0.79in0.1MNaOH[41].Theexperimentalsampleandreferencewereexcited atsamewavelength,maintainingalmostsameabsorbanceandflu-orescence.Areaofthefluorescencespectraweremeasuredusingthesoftwareavailableintheinstrumentandthequantumyieldwascalculatedbyfollowingtheformula  s   ⁄    R  =   A S   ⁄    A R  ×  (  Abs ) R   ⁄   (  Abs ) S  ×   S 2   ⁄    R 2  where,  s and  R  arethefluorescencequantumyieldofthesamplesandreference;A s  andA R   aretherespectiveareasunderemissionspectraofthesampleandreferencerespectively.(Abs) R  ,(Abs) s aretheabsorbanceofsampleandreferenceattheexcitationwavelengthand  s2 ,  R 2 aretherefractiveindexofthesolventusedforthesampleandthereference.  2.2.Synthesisofprobe,HL Quinoline-2-carbohydrazide(0.187g,1.0mmol)was   addedtodrymethanolsolution(15ml)   of4-oxo-4 H- chromene-3-carbaldehyde(0.174g,1.0mmol)atroomtemperatureandstirredfor5h.ApaleyellowprecipitateappearedandfilteredoffandwashedseveraltimeswithcoldMeOHanddriedindesiccator(fusedCaCl 2 ).Yield:0.3g,87.5%.M.P.:  200 ◦ C.( SchemeS1 )   .ThemassspectralpeakofHLappearsat( m /  z  )344.102(M + +H)   and366.162(M + +Na)( Fig.S1 )   (Mwt.343.336)Microanalyticaldata:C 20 H 13 N 3 O 3  calcd(found):C,69.96(69.85);H,3.82(3.76);N,12.24(12.40)%.  1 HNMR    (300MHz,DMSO- d 6 ):12.43(s,1H,NH),8.88(s,1H,imine-H),8.87(s,1H),8.59(d,1H,9Hz),8.22-8.09(m,   5H),7.93-7.83(m,   2H),7.75(t,2H,7.5Hz),7.55(t,1H,7.5Hz)( Fig.S2 ); 13 CNMR    (300MHz,DMSO- d 6 ):175.55,161.27,156.28,155.21,150.16,146.47,142.47,138.48,135.18,131.14,129.75,129.42,128.85,128.64,126.59,125.71,123.87,119.56,119.22,119.00( Fig.S3 );Mass m /  z    344.102(M + +H)and m /  z  366.16(M + +Na)(ESI,Fig.S3)IR:3234cm − 1 (HydrazideNH),1675cm − 1 (chromoneringCO),1633cm − 1 (amideCO)1523cm − 1 (azomethine,CN),( Fig.S4 ). Synthesisofcomplex[Al(HL)(OH)H   2 O](NO  3 )  2 ToMeOH(10ml)   solutionofHL(0.34g,1mmol),aqueoussolu-tionofAl(NO 3 ) 3 · 9H 2 O(0.37g,1mmol)wasaddedandstirredfor3hthenthesolutionwasstandforslowevaporation.Awhitecrystallineproductobtained,filteredoffandprocessedforfur-therinvestigation.MicroanalyticalData:Calcd.(%)C,59.26;H,3.98;N10.37;  1 HNMR    (300MHz,DMSO- d 6 )12.44(s,1H),9.23(s,1H),8.87(s,1H,8.87),8.60(d,1H,18Hz),8.22-8.08(m,4H),7.90(m,   2H),7.74(t,2H,7.5Hz),7.55(t,1H,7.5Hz)( Fig.S5 ).TheMS( m /  z  )peakappearsat202.544whichsupportstheforma-tionof[HLAl(OH)H 2 O] +2 andpeakat418.17supportsformationof[(HL)Al(OH)(OCH 3 )] + ( Fig.S6 )   IR:3407cm − 1 (–OH),3234cm − 1 (HydrazideNH),1627cm − 1 (chromoneringCO),1558cm − 1 (amideCO)1468cm − 1 (azomethine,CN),1380cm − 1 (NO 3 − )( Fig.S7 ).  2.4.GeneralmethodforUV–visandfluorescencestudies HL(1.72mg,   0.001mmol)wasdissolvedinmethanol(5ml)and100  lofHLsolutiondilutedusing2mlwatercontainingHEPESbuffer(pH7.2)tomakethesolutionwithtotalvolume2.1ml.   Al(NO 3 ) 3 · 9H 2 O(3.7mg,   0.001mmol)was   dissolvedinwater(10ml).TheAl +3 solution(100  l)weretransferredtoHLsolutionpreparedabove.Thisprocedureforsamplesolutionpreparationalsomaintainedforothercations.Aftermixingthoseforafewsecondsemissionspectrawererecordedatroomtemperature.F − solutionwas   addedtothesamesolutionwhereAl +3 wereaddedgraduallytoHLandthefluorescencespectrawererecorded.Forfluorescencestudyexcitationwavelengthusedwas438nm(excita-tionslit=10.0andemissionslit=10.0).ForUV–vismeasurements,2ml   MeOH-Waterbuffer(7/3v/v,HEPESbufferpH7.2)wereused.  2.5.Theoreticalcomputation HLandHL-Al +3 wereoptimizedtogeneratethestructuresbyDFT/B3LYPmethodusingGaussian09software[42,43].6–31+G* basissetwasusedforC,H,N,OandLanL2DZbasissetwasusedaseffectivepotential(ECP)setforAl.Toensuretheoptimizedgeome-triesrepresentthelocalminima,vibrationalfrequencycalculationswereperformed,andtheseonlyyieldedpositiveEigenvalues.TheoreticalUV–visspectrawerecalculatedbytime-dependentDFT/B3LYPmethodinWaterusingconductor-likepolarizablecon-tinuummodel(CPCM)[44–46].GAUSSSUMwasusedtocalculate thefractionalcontributionsofvariousgroupstoeachmolecularorbital[47,48].  2.6.Immunofluorescence(IFA)examination ToprepareAfricangreenmonkeykidneycells(Verocells,ATCC,Manassas,VA,USA)Dulbecco’sModifiedEagleMedium(DMEM)wasusedin5–10%fetalbovineserum(FBS).AmonolayerofVerocells(1.0 × 10 6 cells/ml)was   grownonto6-wellplatesat5%CO 2  for  R.Purkaitetal./SensorsandActuatorsB257(2018)545–552 547 24h.Thecellswerefixedinparaformaldehyde(4%)solutionandblockedwith1%bovineserumalbumin(BSA)in0.1%PBS(phos-phatebufferedsaline)-Triton-X100solution.Thesewerewashedandpermeabilizedwith0.1%Triton-X100inPBS.Thepermeabi-lizedVerocellmonolayerwastreatedwithHL(100mg/ml)for1hatroomtemperatureandwashedtwicewithPBS(pH7.2)toelim-inatecellrubbish.Then,Al +3 [Al(NO 3 ) 3 · 9H 2 O](100mg/ml)wasaddeddropwiseviamicro-syringefor10min,andthecellswerewashedtwicewithPBS.Then,thecellswereobservedunderanepifluorescencemicroscope[49].  2.7.Cellcytotoxicityassay CytotoxicityofHLonVerocellswereperformedfollowingtheMTT   assayaspertheprotocoldescribedearlier[50,51].Thecells seededin96wellplatesweretreatedwithdifferentconcentrationsofHLandincubatedfor24hin5%CO 2  at37 ◦ C.FortheMTT   assay,thiazolylbluetetrazoliumbromidesolution(100ml;   1mg/   ml)   inincompletemediumwasaddedandthismixtureincubatedfor4h.Afterthat,100ml   ofdimethylsulphoxide(DMSO)was   addedandtheplateswererotatedfor5min.Opticaldensitywas   recordedat550nmwithDMSOastheblank.PercentageofcellviabilitywasplottedagainstdifferentconcentrationsofHLandthecellstreatedwithoutanycompoundservedascontrol. 3.Resultanddiscussion  3.1.Al +3 sensoractivity:UV-visspectroscopicstudies TheUV–visspectrumofHL( Fig.S8 )   inMeOH-water(v/v,7/3)buffersolution(HEPESbuffer,pH7.2)showsasharpintensebandcentredat316nmalongwithvibrationalcounterpartsat295and335nm( Fig.S8 )   thosemay   assignto  ®  *transitionsandaweakpeakat425nmcorrespondington →  *transition.ToHL (inMeOH-water(v/v,7/3))anaqueoussolutionofdifferentmetalions(Na + ,K + ,Ca +2 ,Mg +2 ,Ba +2 ,Hg +2 ,Ni +2 ,Co +2 ,Pb +2 ,Pd +2 ,Mn +2 ,Cd +2 ,Cu +2 ,Fe +3 ,Fe +2 andZn +2 )hasbeenaddedandtheabsorp-tionspectrashownewbandscentredat435nmforZn +2 ,Cd +2 ,Hg +2 andAl +3 .ThemixtureofHLandAl +3 (1:2molarratio)showshighestabsorbanceat435nmwithtwovibrationsat413and457nm(435 ± 22nm)   ( Fig.S9 )   whichmay   beassignedtoredshift-ing( ∼ 119nm)   of   →  *transitionsofprobe.TheJob’splotsupportsthe1:1molarcompositionofthecomplex( Fig.S10 ).Naked-eyecolorchangeoftheHLsolutionuponadditionofAl +3 ionisverystriking(Fig.1).TheinteractionofHLwithAl +3 hasbeenexaminedbyspectrophotometrictitrationofHLwithincrementaladditionofconcentrationAl +3 inHEPESbuffer(10mM,   pH7.2)at25 ◦ Cinthesamesolvent,andhasshownabsorptionenhancementwithbroughthumpcontainingthreesharppeaksat413,435and457nmfollowedbythedecreaseinintensityat316nm,   withtheisosbesticpointat352(Fig.1).Thechangeofabsorbanceislinearuntilthe molarratio[Al +3 ]:[HL]reaches1:1,andnolongerchangeswithincreasein[Al +3 ].ItsuggeststhatthestoichiometrybetweenHL andAl +3 is1:1andtheassociationconstant(Ka)is3.32 × 10 4 M − 1 ( Fig.S11 ).  3.2.FluorescenceOFF–ONsensingforAl +3 andF  − Visiblelightexcitationoftheprobe(HL)at435nmshowsweakemissionat630nm(quantumyield,  HL  ,0.0009)anduponadditionofAl +3 theemissionbandisblueshiftedto520nm.   ThefluorescencespectrumofHLwithothercations(Na + ,K + ,Ca +2 ,Mg +2 ,Mn +2 ,Fe +2 ,Al +3 ,Co +2 ,Ni +2 ,Pd +2 ,Cd +2 ,Hg +2 ,Cu +2 ,Ba +2 ,Pb +2 andZn +2 )inaque-ousmedium(pH,7.2)hasbeenstudiedandtheturn-onemissionisobservedonlyinpresenceofAl +3 (Fig.2).Ongradualadditionof  Al +3 tothesolutionofHLthefluorescenceintensityincreasesand Fig.1. ChangeinabsorptionspectrumofHL(50  M)   upongradualadditionofAl +3 ions(5  Meach)in7:3v/vMeOH:H 2 O(pH=7.2). Fig.2. ChangeinabsorptionspectrumofHL(50  M)   upongradualadditionof differentmetalions(100  Meach)inwater(pH=7.2). becomessaturatedwhenreachedat1:1molarratio;thecomplex-ationalsoenhancesthequantumyield(0.049,54foldcomparedtoHL).TheemissionintensityofthemixturedoesnotchangeonexcessadditionofAl +3 .SuchanenhancementinfluorescenceintensityisveryselectiveforAl +3 (Fig.3)inaqueousmedium.The incrementinfluorescenceintensityforHL+Al +3 may   arisefromtheeliminationofphotoinducedelectrontransfer(PET)infreeHLandchelationenhancementeffect(CHEF)throughtheco-ordinationof chromonecarbonyl-O,azomethine-Nandhydrazide-Otometalion(Scheme1).HLdoesnothaveanypossibilitytoinducequenching byESIPTorICTprocess.Fluorimetrictitrationdeterminesthebind-ingconstant[K d ,6.8 × 10 4 M − 1 ]( Fig.S12 ).Thelimitofdetection(LOD)ofAl +3 hasbeencalculated7.6nMfollowingthe3  method( Fig.S13 ).ThefluorescenceenhancementofHL-Al +3 complexhasbeenexaminedinpresenceofothermetalions( Fig.S14 ).EffectofpHvariationonfluorescenceintensityofHL    andHL-Al +3 complexhasbeenstudied;ithasobservedthatthereisnosignificantfluorescenceemissionofHLatthepHrange2–12andinpresenceofAl +3 theligandemitsinthepHrangebetween4.0–8.5( Fig.S15 )andhighemissionoccursinthepHrangebetween4–7.5.Inacidicmediumligandmay   beprotonatedandthebasicmedium  548 R.Purkaitetal./SensorsandActuatorsB257(2018)545–552 Fig.3. ChangeinemissionspectrumofHL(50  M)   upongradualadditionofAl +3 ions(5  Meach)inwater(pH=7.2)(arrowindicatestheenhancementofemissionuponincreasingadditionofAl +3 ). Scheme1. Schematicrepresentationofsensing. Fig.4. DecayprofileofHLand[HL-Al +3 ]complex. (pH>7)may   precipitateAl(OH) 3  thatmay   inhibitthecomplexa-tion.ThusHLisusefulfordetectionofAl +3 inbiologicalpHoffood,beverages,environmentalsampleseveninpresenceofothermetalionsinaqueousmedium.Lifetimedatawereobtaineduponexcitationat450nm,   andthefluorescencedecaycurvewasdeconvolutedwithrespecttothelampprofile.Theobservedflorescencedecayfitsnicelywiththemono-exponentialdecayprofileforthecomplexes(Fig.4),which Fig.5. ChangeinabsorptionspectrumofFig.6Changeinemissionspectrumof [HL-Al +3 ]upongradualadditionofdifferentanionsin7:3v/vMeOH:H 2 O(pH=7.2). Fig.6. Changeinemissionspectrumof[HL-Al +3 ]upongradualadditionofdifferentanionsinwater(pH=7.2). issupportedbygoodness-offit(  2 )dataintheregressionanaly-ses.Radiativeandnon-radiativerateconstants(k r  andk nr ;  TableS1 )   arecalculated,anddatashowtheusualhigherk nr  thank r  val-ues.Theaveragelifetimevalueof[HL-Al +3 ](0.98ns)islongerthanthatofHL(0.61ns).We   examinedthereversibilityeffectoftheHL-Al +3 com-plexwithdifferentanionssuchasS 2 O 3 − 2 ,S − 2 ,SCN − ,I − ,OAc − ,ClO 4 − ,PO 4 − 3 ,H 2 PO 4 − ,SO 4 − 2 ,HSO 4 − ,Cl − ,F − ,NO 3 − ,Br − ,NO 2 − ,N 3 − .Amongtheaforesaidanions,onlyF − hasinducedaconspicuouschangeintheabsorptive(Fig.5)andemissive(Fig.6) behaviourofthecomplex.TheUV–visabsorptionresponseof[HL-Al +3 ]complextowardsaddedF − showsthatthebandat438nmcharacteristictotheHL-Al +3 complexisdecreased(Fig.7)while thebandat316nmisrecoveredgradually,whichischaracteristicofthefreeligand,HL( Fig.S8 ).Thefluorescenceintensityof[HL-Al +3 ]at520nmgraduallydiminishesuponadditionofF − dueto  R.Purkaitetal./SensorsandActuatorsB257(2018)545–552 549 Fig.7. ChangeinabsorptionspectrumofHL-Al +3 complex(50  M)   upongradualadditionofF − ions(2.5  Meach)in7:3v/vMeOH:H 2 O(pH=7.2). Fig.8.  1 HNMR    spectrumofHLanHL-Al +3 complexinDMSO- d 6. thesubstitutionofHLfromtheemissivecomplex.Al +3 hasaffin-itytobindF − comparedtothatwiththereceptorHL.Asaresult,the[HL-Al +3 ]complexactsasadetectingassembleforF − (Fig.8).TocheckthespecificityofHL-Al +3 complextowardsthedetec-tionofF − ,changeinemissionintensityofthecomplexisstudiedbyadditionofseveralotheranionssuchasS 2 O 3 − 2 ,S − 2 ,SCN − ,I − ,OAc − ,ClO 4 − ,PO 4 − 3 ,H 2 PO 4 − ,SO 4 − 2 ,HSO 4 − ,Cl − ,NO 3 − ,Br − ,NO 2 − ,N 3 − andnosignificantchangeinemissionintensityisobserved.Thisclearlyillustratestheremarkableselectivityofthe[HL-Al +3 ]towardsF − comparedtoseveralotheranions(Fig.6).Thelimitof  detectionofF − bythetransducer[HL-Al +3 ]is7.4nM( Fig.S16 )thatislessthan1/500timesofWHO   recommendedtoxiclimit,3.68  M[52].TherearesomereportsavailableinliteraturerelatedtoF − sen-sorsuchasB.Sui et.al reportedLODofF − 1.9  M[53].Y.Zhou et.al suggestedtwoF − sensorswithLODof6.7  Mand9  M[54].R.Hu et.al accountedLOD<4  MforF − [55].R.ArabahmadiandothersdemonstratefluorophoreofF − atLODof1.02  M[56].Thepresent resultshowslowestLODofF − ,7.4nMsofarreportedinliterature.  3.3.Stoichiometricstudyby  1 HNMR   spectroscopictechnique ToelucidatethenatureofthebindingofHLtoAl +3 the  1 HNMR spectrumofHLhasbeeninvestigatedinabsenceandinpresenceof Al +3 inDMSO- d 6  (Fig.8).InpresenceofAl +3 the  1 HNMRspectrumsignalsforhydrazide-NH(-CONHN=)at12.43,HCN(azome-thine)at8.88ppmwereshifteddownfieldtowards12.54ppm(  d,0.11)and9.23ppm(   ,   0.35ppm)respectively.Shiftingofaro-maticprotonsofchromoneandno-shiftingofquinolinearealsoobserved.SotheNMR    observationsuggeststhatHLmay   chelateAl +3 throughbindingwithcarboxylate-O,imine-Nandcarbonyl- Fig.9. FrontiermolecularorbitalofHLand[HLAl(OH)H 2 O]complex. Oofchromonewhilequinoline-Nremainsuncoordinatedbutnonchelatethroughquinonenitrogen(Scheme1).Thestoichiometryof[HL-Al +3 ]complexhasbeenfurtherestab-lishedbymeansofmassspectroscopicdata.Job’splotforthereactionbetweenHLandAl +3 inmethanol-water(7:3,v/v)usingabsorptionspectroscopyalsosupportstheformationof1:1com-plex( Fig.S10 ).ESI–MSdataofHLand[HL-Al(OH)H 2 O] +2 givesapeakat m /  z    202.54and[HL-Al(OH)(OCH 3 )] + at m /  z    418.174 { [Al(HL)(OH) 2 (H 2 O) 2 ] + +Na } at m /  z    463.168( Fig.S6 )   respectivelywhichcorrespondstoformationthe1:1complex.  3.4.Densityfunctionaltheorycalculation GeometryoptimizationofHLand[(HL)Al(OH)H 2 O] +2 hasbeenperformedusingDFTcalculationwithB3LYPmethod.Basedonmassspectraldatacompositionofthecomplexionhasbeenassignedas[Al(HL)(OH)(H 2 O)] +2 .AccordingtoDFToptimizedstructureAlispenta-coordinatedanddistortedsquarepyramidingeometry(Scheme1).HLactsasneutralO,N,OchelatortoAl +3 ;thecalculatedAl–N(imine),Al–O(amidecarbonyl)andAl–O(chromonecarbonyl)distancesare2.12,1.87and1.85Årespectivelyandhavebeencomparablewithsimilarstructure[57,58].Duetocomplex formationtheCObonddistancehasbeenelongatedby0.02Å − 0.03Å(1.25Å(amidecarbonyl)and1.25Å(chromonecarbonyl)to1.27Å(amidecarbonyl)and1.28Å(chromonecarbonyl))(  TableS2 ).TheresultshowsthatelectrondensityinHOMOofHLisdis-tributedonthequinoline(42%)andchromone(58%)andLUMOisdistributedmainlyonquinoline(90%)moiety.ForHOMOof [(HL)Al(OH)H 2 O] +2 ,the  electrondensityisallocatedmainlyonquinolinebuttheLUMOisspreadoverquinolineandchromonemoiety(  TablesS3andS4;Fig.S17andS18 ).TheHOMO–LUMOgapinHL(3.79eV)hasbeendecreasedin[(HL)Al(OH)H 2 O] +2 (2.18eV)whichsupportstheredshiftofabsorptionbandfrom316nmto435nminUV–visspectra(Fig.9).Thecalculatedtransitionof  HLappearsasadmixtureofHOMO → LUMO+1,HOMO–1 → LUMO( Fig.S19 )whichcorrespondstocalculated317.5nm(f,0.0765)andtheexperimentaltransitionwavelengthis316nm.   Othercalculatedtransitionsappeararound317.5nm.   TheTD-DFTcomputedtran-sitionin[Al(HL)(OH)H 2 O)] +2 iscentredat447.17nm(f,0.1394)whichisassignedtomixtureofHOMO → LUMO/LUMO+1( Fig.S20 ).AbsorptionenergiesofHLanditsAl +3 complexalongwiththeiroscillatorstrengtharegivenin  TableS5 whoareobservedindeed.
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