Chemical modification of TiO 2 nanotube arrays for label-free optical biosensing applications

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Monica

Terracciano

a

_,

_Vardan

_Galstyan

b,c

_,

_Ilaria

_Rea

a

_,

_Maurizio

_Casalino

a

_,

Luca

De

Stefano

a,∗

_,

_Giorgio

_Sbervegleri

b

a_Institute_for_{Microelectronics}_and_{Microsystems,}_National_Research_Council,_Via_P._Castellino_111,_80131,_Naples,_Italy b_Sensor_Lab,_CNR-INO_and_Department_of_Information_Engineering,_University_of_Brescia,_Via_Valotti_9,₂₅₁₃₃_Brescia,_Italy c_Department_of_Molecular_and_{Translational}_Medicine,_University_of_Brescia,_Viale_Europa_11,₂₅₁₂₃_Brescia,_Italy

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received21March2017

Receivedinrevisedform27April2017 Accepted3May2017

Availableonline4May2017 Keywords:

TiO2nanotubessurfacefunctionalization ProteinA

Interferometry Photoluminescence Bio-sensing

a

b

s

t

r

a

c

t

Inthisstudy,wehavefabricatedTiO2nanotubearraysbythepotentiostaticanodicoxidationofTifoilsin

ﬂuoride-containingelectrolyteandexploredthemasversatiledevicesforbiosensingapplications.TiO2

nanotubeshavebeenchemicallymodifiedinordertobindProteinAasaspecifictargetanalyteforthe opticalbiosensing.Theobtainedstructureshavebeencharacterizedbyscanningelectronmicroscopy, Fouriertransforminfraredspectroscopy,watercontactangle,fluorescencemicroscopy,spectroscopic reflectometryandphotoluminescence.InvestigationsshowthatthepreparedTiO2nanotubes,2.5␮m

longand75nmthick,canbeeasilyandefﬁcientlybio-modiﬁed,andtheobtainedstructuresarestrongly photoluminescent,thussuitableforthelabel-freebiosensingapplicationsintherangeof␮M,duetotheir peculiaropticalproperties.

1. Introduction

A growing demand of fast, efficient, low-cost and portable devices for real-time detection of specific analyte has meant biosensingoneofthemostrapidlyexpandingresearchfield[1]. Theterm“biosensor”referstoanintegratedreceptor-transducer deviceableofprovidingselectiveanalyticalinformationbyusing biologicalrecognitionelement(i.e.,bioprobe).Thetransducer con-vertsthechangescausedbythereactionbetweenthebioprobeand thetargetintoananalyticalsignal,dependingonthetechnology used[2].Overthelasttwodecades,severaltypesof electrochemi-cal,opticalorelectricalbiosensorshavebeendeveloped,resulting validlabel-freeanalyticaltoolsforclinicaldiagnostics,biomedicine, environmentalmonitoring,veterinaryandfoodqualitycontroland otherareasinwhichfastandreliableanalysisareneeded[3]. Tita-niumdioxide(TiO2)isawidelystudiednon-toxicsemiconductor

andhasbeeninvestigatedaspotentialtransducermaterialfor sens-ingapplicationduetoitsuniquephysicochemicalproperties[4–7]. IthasbeenobservedthatthepropertiesofTiO2canbeenhanced

by thenanoscale architectural features [8]: Lu and co-workers

∗ Correspondingauthor.

E-mailaddresses:luca.destefano@cnr.it,luca.destefano@na.imm.cnr.it (L.DeStefano).

demonstratedthattheanataseTiO2nanosheetsshowhigher

pho-tocatalyticactivityforthedegradationoforganicmoleculesthan anataseTiO2 crystals[9].Theseresultsencouragedthematerial

scienceresearchtofocusmainlyonnanostructuredTiO2.Recently

itwasdemonstratedthattheTiO2nanotubearraysdirectlygrown

onmetallictitaniumfoilbymeansofelectrochemicalanodization arevery attractivefunctionalmaterialsinthedesignof biosen-sors for biomedical applicationsowing theirhigh surface area, largerefractive index(n=2.5),highorientation and uniformity, aswellasthegoodbiocompatibility,great aqueousand chemi-calstability[10,11].Herein,wefabricatedTiO2 nanotubes(NTs)

bymeansofelectrochemicalanodizationandinvestigatedthe pre-paredsamplesastheplatformforlabel-freeopticalmonitoringof biomolecules.TheuseofTiO2 NTinbiosensordevelopmentasa

transducersurface,requiredthecreationofcouplingpointsforthe immobilizationofbiomolecules(theso-calledbioconjugation pro-cess),preservingthespeciﬁcfunctionalitiesofbiologicalreceptors throughagoodcontroloftheirorientationandorganizationonthe inorganicsurface[12].Tothisaim,thesurfaceofnanostructured titaniawaschemicallymodiﬁed inorder tocovalently bindthe proteinAbyusingthewell-knownsilaneandsilanolchemistries

[13,14].ProteinAimmobilizationwasmonitoredby theoptical methodsbasedonthespectroscopicreﬂectometryandthe steady-statephotoluminescence.OurresultsshowthattheTiO2NTscanbe

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236 M.Terraccianoetal./AppliedSurfaceScience419(2017)235–240

Fig.1.SEMimagesoftheobtainedTiO2NTs:(A)and(B)surfacemorphologiesofthenanotubearrayswiththedifferentresolutions,(C)and(D)cross-sectionalviewofthe nanotubeswiththedifferentresolutions.

Fig.2. SchematicrepresentationoftheTiO2NTsfunctionalizationwithProteinA.

appliedasversatileplatformforsensingofbiochemicalmolecules basedontwodifferentopticalmethods.

2. Experimental

2.1. FabricationandfunctionalizationofTiO2nanotubes

TiO2NTswerefabricatedbymeansofelectrochemical

anodiza-tionperformedintheelectrochemicalcellwiththetwo-electrode system.Ptfoilwasusedasacounterelectrode.Themetallicfoils wereanodizedinNH4FandH2Ocontainingglycerol.The

anodiza-tionwascarriedoutbypotentiostaticmodeatroomtemperature. Afteranodizationthepreparedsampleswerewashedindistilled wateranddriedatroomtemperature.Then,as-preparedNTswere crystallizedintheanatasestructureusingthethermaltreatment regimensreportedinourpreviousstudy[15,16].Thesurfaceof TiO2NTswasﬁrstactivatedbyPiranhasolution(H2O2:H2SO41:4)

for30min,inordertocreateOHgroups.Then,thesampleswere extensivelywashedinmilli-Qwatertoremoveanyadsorbedacid onthesurface.Thestructuresweresilanizedbyimmersionin5% 3-aminopropyltriethoxysilane(APT)solutioninanhydroustoluene for30minatroomtemperature[13].Thepreparedsampleswere

rinsedintoluene threetimesfor2mintoremovesilaneexcess. Afterwards,thesilanewascuredontheheaterat100◦Cfor10min. Fluoresceinisothiocyanate(FITC)wasusedtoattachaﬂuorescent labeltoProteinA(PrA,MW42kDa).ThelabelledProteinA(PrA*) andthenotlabelledproteinA(PrA)wereimmobilizedonthe sur-faceofTiO2NTarraysusingbis(sulfosuccinimidyl)suberate(BS3)

crosslinker[13].Theschemeofsamples’functionalizationprocess isreportedinFig.2.Theobtainedsampleswereincubatedwith 150␮lof1.6mMBS3_in_PBS_solution_(0.1_M;_pH₌_7.4)_at₄◦_C_for₄_h.

N-hydroxysulfosuccinimide(NHS)esterreacts(throughSN2)with primaryaminesofsilanizedsurfaceformingstableaminebonds andreleasingaNHSgroup.Then,thefunctionalizedsampleswere incubatedovernight(ON)at4◦Cwith150␮lof2mg/mlPrAinPBS (0.1M;pH=7.4)buffer.NHSesterreactedwithprimaryaminesin thesidechainoflysineresiduesofPrAformingstableaminebonds andreleasinganotherNHSgroup.Thechemicalsandthesolvents usedfortheexperimentswerepurchasedfromSigma-Aldrich.

2.2. Analysesofthesamples

The morphologiesof the obtainedNTs wereanalyzed using LEO 1525 scanning electron microscope (SEM) equipped with

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Fig.3. FTIRspectraofTiO2NTsaftereachstepoffunctionalization:aindicatesTi–O–Tibond,bCHxstretchingvibration,cthebendingmodeofthefreeNH2andtheC N stretching,eC Ostretchingvibration,dtherockingCHxvibration,respectively.

Fig.4.WatercontactanglemeasurementsperformedonTiO2NTsbeforeandaftereachstepoffunctionalization.

ﬁeldemissiongun.ThechemicalcompositionofTiO2NTsbefore

andaftersurfacemodiﬁcationwasinvestigatedbyFourier trans-forminfraredspectroscopy(FTIR)spectra.TheFouriertransform infrared spectra of all samples were obtained using a Nicolet Continu␮mXL(ThermoScientiﬁc)microscopeinthe wavenum-berregionof4000–650cm−1witharesolutionof2cm−1.Sessile

droptechniquewasusedforthewatercontactangle(WCA) mea-surementsontheFirstTenAngstromsFTA1000CClasscoupled withdropshape analysissoftware.TheWCAvaluesreportedin thisworkaretheaverageofatleastthreemeasurements.

ALeicaZ16APOﬂuorescencemicroscopeequippedwithaLeica cameraDFC320wasusedtoevaluatethebioconjugationofTiO2

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238 M.Terraccianoetal./AppliedSurfaceScience419(2017)235–240

Fig.5.Fluorescencecharacterizationofthenegativecontrol(A)andTiO2NTs-APT-BS3(B)aftertheincubationwiththesolutionscontainingProteinA*.Scalebar,100␮m.

Fig.6.Reﬂectivityspectra(A)andcorrespondingFouriertransforms(B)ofTiO2NTsbefore(blueline),afterAPT+BS3(redline)andafterPrA(blackline).(Forinterpretation ofthereferencestocolourinthisﬁgurelegend,thereaderisreferredtothewebversionofthisarticle.)

NTswithPrA*.ThefilterusedfortheacquisitionwasI3consisting ofa450–490nmband-passexcitationfilter,a510nmdichromatic mirroranda515nmsuppressionfilter.ReflectivityspectraofTiO2

NTswereacquiredbeforeandafterthefunctionalizationsbya sim-pleexperimentalsetup:awhitelightwassentonPSithroughaY opticalfiber(Avantes).Thesamefiberwasusedtoguidetheoutput signaltoanopticalspectrumanalyser(AndoAQ6315A).The spec-trawereacquiredatnormalincidenceovertherangefrom800to 1600nmwitha resolutionof5nm.Reflectivityspectrareported intheworkaretheaverageofthreemeasurements.Steady-state photoluminescence (PL) spectrabeforeand after TiO2 NTs

sur-facefunctionalizationwereexcitedbyacontinuouswaveHe–Cd laserat325nm(KIMMONLaserSystem).PLwascollectedat nor-malincidencetothesurfaceofsamplesthroughaﬁber,dispersed inaspectrometer(PrincetonInstruments,SpectraPro300i),and detectedusingaPeltiercooledchargecoupleddevice(CCD)camera (PIXIS100F).Alongpassﬁlterwithanominalcut-onwavelengthof 350nmwasusedtoremovethelaserlineatmonochromatorinlet.

3. Resultsanddiscussion

Fig.1showstheSEMimagesofthepreparedTiO2NTs.The

sur-face(Fig.1(A)and(B))andthecross-sectional(Fig.1(C)and(D)) morphologicalanalysesofthesamplesshowthatwereobtained well-alignedandhighlyorderedTiO2 NTswiththeinner

diam-eter of 75nm and the length of 2.5␮m. In order to fabricate

biosensorsbasedonpreparedTiO2 NTs,thematerialwasﬁrstly

hydroxylatedbyPiranhasolution(Fig.2,I)Thus, increasingthe reactivity of TiO2 surface by the introduction of OH groups

andmakingit abletograftalkylsilanemoleculesof APT(Fig.2, II)[13].Alkylsilanestrategy generatedself-assembled monolay-erswithwell-deﬁnedpacking.TheformationofcovalentTi–O–Si bondsbetweenhydroxylgroupinducedonTiO2 NTssurfaceand

hydrolyzedorganosilanemolecules,improvesthesurfacestability andintroducescouplingpoints(–NH2groups)forthe

immobiliza-tionofamino-terminatedbioprobes[17].ProteinA(PrA),derived fromStaphylococcusaureusbacteria,isapartofasmallcollection ofproteinsknowntospeciﬁcallybindtotheconstantdomainofa numberofantibodies.Thisproteinwasusedasspeciﬁcbio-receptor fortherealizationofTiO2NTsbiosensor.Moreover,sincePrAis

secretedbyanddisplayedonthecellmembraneofS.aureus,it isanimportantbiomarkerforthisbacterium.Therefore,itsrapid and specificdetection facilitatethepathogen identificationand initiationof proper treatment[18]. The protein wascovalently conjugatedtoamino-modifiedsurface(TiO2NTsA-APT)usingthe

crosslinkerBS3₍_Fig.₂_,_III_and_IV).

ThechemicalmodiﬁcationofTiO2 NTswasanalysed byFTIR

spectroscopy (Fig. 3). FTIRspectrum of bare TiO2 NTsshowed

atypicalpeak oftitaniaat 948cm−1 [19].Afterthesilanization process,theTiO2 NTs-APTdisplayedcharacteristicbandsofAPT

correspondingtotheCHxstretchingat2960and1720cm−1[20,21],

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akeyroleforthedevelopmentofbiosensorsaffectingsurface bio-functionalization,biomolecularinteractionandconsequenttarget recognition[24].ThevariationofsurfacewettabilityofTiO2 NTs

beforeandaftereachstepoffunctionalizationprocesseswas char-acterizedbymeasurementsofWCA,asshowninFig.4.TheTiO2

NTsarequasihydrophobic(Fig.4A),resultinginaWCAvalueof (86_±3)◦.APT,withitsamino-terminalalkylchaininducesaweak decreaseoftheWCAto(80±1)◦ (Fig.4B).BS3_,_due_to_the

intro-ductiononthesurfaceofhydrophilicN-hydroxysulfosuccinimide groups(NHS),decreasestheWCAdownto(41±6)◦,thusmaking thesamplesurfacemorehydrophilicthanbeforeandtherefore suit-ableforPrAbioconjugation(Fig.4C).ThebioconjugationofTiO2

NTsAdevicetoPrAwasconﬁrmedbyafurtherdecreaseofWCA valueto(35±2)◦duetothehydrophilicnatureoftheprotein.The biofunctionalizationofTiO2NTswiththePrAhasbeenmonitored

byspectroscopic reﬂectometry(Fig.5).To comparethe results, thesampleswerealsomonitoredbytheﬂuorescencemicroscopy (Fig.6).

Theopticalthickness(OT)oftheobtainedTiO2NTsdevicewas

calculatedfromthereﬂectivityspectrumbyFFT,whichdisplayed apeakwhosepositionalongthex-axiscorrespondedtotwotimes theoptical thickness(2OT) of thelayer [25]. Normalincidence reﬂectivityspectraofTiO2 NTsbeforeandafterAPT+BS3

modi-ﬁcation(TiO2NTsA-APT-BS3)andafterthefunctionalizationwith

thePrA(TiO2NTsA-APT-BS3-PrA)areshowninFig.5togetherwith

thecorrespondingFFTs(Fig.5B).Sincethephysicalthicknessdof theTiO2 NTslayerwasﬁxed,theFFTpeakshiftofabout10nm

afterAPT+BS3_and₅₀_nm_after_PrA_was_really_due_to_an_increase

oftheaveragerefractiveindex(n),i.e.morematterinthepores, whichwastheconﬁrmationofasuccessfulbio-functionalization

[17].In Fig.6 arereportedthe ﬂuorescenceimagesofnegative control(Fig.6A)andTiO2NTsA-APT-BS3afterincubationwiththe

PrA*(Fig.6B).TheTiO2 NTs-APT-BS3-PrA*showedahighgreen

ﬂuorescence,contrarytothedarknegativecontrol,conﬁrmingthe bio-conjugationofthebiologicalmoleculetosurfacedevice.

Steady-statePLspectraofTiO2NTsbeforeandaftereach

func-tionalizationsteparereportedinFig.7.TiO2NTsshowedaPLpeak

atabout390nmcorrespondingto3.2eVwhich iswellmatched withthebandgapenergyofanataseTiO2[26,27].Thebroadvisible

emissionisduetorecombinationofoppositelycharge-carriersat defectenergylevels.Inthevisibleregionofspectrumiswellevident thatthePLemissionincreasedaftereachfunctionalizationstepof TiO2 NTsAsurface.Thisfeatureisduetotheextrafreeelectrons

suppliedbythemolecularcomplexes(i.e.APT,BS3_,_PrA),_which

participatedtoexcitation-relaxationdynamicbetweenthesurface energylevels.Thisphenomenon wasless evidentincaseofthe peakat390nm,sincethislastwasduetotheradiativetransitions betweenconductionandvalencebandsthatwerenotaffectedby functionalizationtreatments.ItmustbementionedthatTiO2NT

arraysareexcellentphotocatalystforlightdegradationofmany substances[26],butinthepresentedcase,thechemical

passiva-Fig.7. PhotoluminescencespectrachangesofTiO2NTsaftereachstepof function-alization.

tionoftitaniananotubessurface,duetotheorganiclayersadded (APT±BS3_),_created_enough_distance_between_the_biological_active

molecule(PrA)andthephotocatalytictitaniasurfacesothatprotein degradationwasprevented.

4. Conclusions

Insummary,wefabricatedandinvestigatedthepropertiesof TiO2 NTs fortheir applicationin biosensor devices. In order to

developbiosensorbasedonTiO2NTarraystheobtaineddevices

were grafted byAPT alkylsilane compound, providing coupling points to immobilize the bioprobe, and then bioconjugated to PrAviaBS3 _molecule._The_device_{bio-functionalization}_was

con-firmedqualitativelyandquantitativelybyseveralcomplementary techniques,suchasFTIRspectroscopy,WCAmeasurements, flu-orescencemicroscopy,and label–freeopticalmethods basedon spectroscopicreflectometryfollowedbyFFTandPLanalysis.PrA wascovalentlygraftedonTiO2 NTssurfacein therange of␮M

andmonitoredasalargevariationinthePLintensity(morethan 1000a.u)withacorrespondingincreaseoftheopticalthicknessof thesample(about50nm).Inalabel-freeopticalbiosensing experi-ment,aprimaryantibodyofPrAshouldbeimmobilizedandusedas thebio-probe,andtheopticalresponseinbindingtoPrAshouldbe monitored.Manyphotoelectrochemical(PEC)sensorsusingTiO2

nanotubesarrayhavebeenreportedinliterature[29–31],someof themshowingveryhighsensitivity.Evenifverysensitive,PEC sen-sorsoftenrequireextradecorationofnanotubes(bypolymersor metals)inordertogettheseperformances,andtheymustusea lightsourceandelectricalequipmentforsignaldetection.Inour case,theexperimentalresults,evenifrepresentedonlya proof-of-conceptfortheapplicationofTiO2nanotubearraysastransducer

materials,were fabricatedand modiﬁed bystandard and well-establishedprocedures,easilyreproducibleinotherlaboratories. Moreover,theexperimentalsetuprequiredonlyalightsource (pas-siveforreﬂectionmeasurementsoractiveforphotoluminescent ones)andsimplesignalelaboration,makingtheproposedsystem veryattractiveforthedevelopmentoflabel–freeopticalbiosensors thatcouldbeusedforawiderangeofapplications,frombiomedical diagnosticandenvironmentalmonitoringtofoodqualitycontrol.

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