Simultaneous photoinduced electron transfer and photoinduced CuAAC processes for antibacterial thermosets

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ContentslistsavailableatScienceDirect

Progress

in

Organic

Coatings

jou rn a l h om ep a ge :w w w . e l s e v i e r . c o m / l o c a t e / p o r g c o a t

Simultaneous

photoinduced

electron

transfer

and

photoinduced

CuAAC

processes

for

antibacterial

thermosets

Elif

Oz

a

_,

_Tamer

_Uyar

b

_,

_Huseyin

_Esen

a,∗

_,

_Mehmet

_Atilla

_Tasdelen

a,∗

a_Department_of_Polymer_Engineering,_Faculty_of_Engineering,_Yalova_University,_77100,_Yalova,_Turkey

b_{UNAM-Institute}_of_Materials_Science_and_{Nanotechnology,}_Bilkent_University,_TR-06800_Ankara,_Turkey

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received22September2016

Receivedinrevisedform7December2016

Accepted14January2017

Availableonline5February2017

Keywords:

Antibacterialproperties

Copper-catalyzedazide-alkyne

cycloadditionclickchemistry

Nanocomposites Photopolymerization

Silvernanoparticles

Thermosets

a

b

s

t

r

a

c

t

AcombinationofsimultaneousphotoinducedelectrontransferandphotoinducedCuAACprocesses enablesthein-situpreparationofantibacterialthermosetscontainingsilvernanoparticles(AgNPs)in one-pot.Uponphotolysisofphotoinitator,thegeneratedradicalsnotonlyreduceCu(II)intoCu(I) acti-vatortocatalysttheCuAACclickreaction,butalsosimultaneouslygenerateAgNPsfromAgNO3through electrontransferreaction.Duetotheirreductionpotentialsdifference,thepolymermatrixisformed beforetheformationofAgNPs,assistingtoeliminatetheagglomerationofthem.Thethermoset struc-turesareconﬁrmedbyFT-IRandsolubilitytests,whereasthepresenceofAgNPsisprovenbytransmission electronmicroscopywithenergydispersiveX-raysystemanalyzer.Thesamplescontaining5and10% AgNPsexhibitedstronginhibitionzones,whereallkindsofbacteria(gram-positive(Staphylococcus Aureus)andgram-negative(EscherichiaColi))werekilledinthesurroundingoftheﬁlmsamples.

1. Introduction

Antibacterialmaterialshavebeenwidelyusedindailylifedue toitsimportantroleinhumanhealthandsafety[1].Basedontheir nature,theycanbedivided intotwo categories as organicand inorganicantibacterialmaterials[2].Whiletheorganic antibac-terial materials are considered less stable, particularly at high temperaturesand/orpressures,the inorganicmetals and metal oxideshavebeenwidelyusedduetotheirfinechemical durabil-ityandhighantibacterialactivity[3].Amongthem,thesilver(Ag) anditssaltshaveefficientlybeenemployedasantibacterialagent againstbacteria,fungi,andviruses[4].Sinceaparticlesizeisone ofthemostsignificantfactorsaffectingantibacterialefficacy,the Agnanoparticles(AgNPs)havedisplayedasuperiorantibacterial activitycomparedtothemicroparticles[3].Thesmaller nanopar-ticleshaveagreatersurfaceareatovolumeratiowheretheycan potentiallyreleasemoresilverionsandareabletodirectlyinteract withthemicroorganisms[5].Thereareseveralchemicalmethods toproduceAgNPssuchaschemicalreduction[6,7],photochemical

∗ Correspondingauthors.

E-mailaddresses:huseyin.esen@yalova.edu.tr(H.Esen),tasdelen@yalova.edu.tr

(M.A.Tasdelen).

reduction[8–17],sol-gelsynthesis[18,19],hydrothermal[20,21], electrodeposition [22,23]and polyol[24,25]methods. Recently, anewstrategybasedonasimultaneousphotoreductionofAg(I) andphotopolymerizationenablestofabricatepolymer nanocom-positescontainingAgNPsinone-pot[9–13].Thephotogenerated radicals can be utilized for the reduction of Ag(I)+ _cation _into

Ag(0) nanoparticles as wellas the initiation of multifunctional monomers[26–29].Thephotoreductionmechanismis basedon theelectron-transferfromreducingintermediates(freeradicals, solvatedelectrons,andanions,whichgeneratedbyaphotoactive compoundunderUVorvisiblelight)totheAg+_producing_a_very

rapidandefﬁcientformationofAgNPs[30].

The photoinduced copper(I)-catalyzed azide-alkyne cycload-dition (CuAAC) click reaction has been developed to combine advantages of photochemistry including spatial and temporal controls with the CuAAC reaction [31–34]. Furthermore, some drawbacksoftheCuAACprocesssuchassensitivityoftoxiccopper (I)againsttheair,whichleadstothedeactivationofthecatalyst andreducetheyield,andlackofpossibilitytoeffectthereaction withexternalstimulihavebeeneliminated[30,35].Thein-situ gen-erationof Cu(I)catalysthasbeenalsoachieved byusingeither suitablereductionagent,orotherchemicalorelectrochemicalways [36–38].Thephotoinduced-CuAACprocesshasbeenutilizedfor synthesisofvariouscomplexmacromolecularstructuresinvolving

http://dx.doi.org/10.1016/j.porgcoat.2017.01.011

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telechelicandstarpolymers,blockandgraftcopolymers,gelsand bioconjugation[33,35,39–50].Inthisstudy,anantibacterial ther-mosetcontainingAgNPsissuccessfullypreparedbycombination ofsimultaneousphotoinducedelectrontransferandphotoinduced CuAACprocessesinone-pot.Thephotochemicallygenerated rad-icalsnotonlyusedtogenerateAg0_{nanoparticles}_from_Ag+_cation,

butalsoreducetheCu(II)intoCu(I)catalyst,whichenableto cat-alyzetheCuAACofmultifunctionalazideandalkynecompounds. Thus,thedesiredantibacterialthermosetmaterialcanbesimply fabricatedatroomtemperature.

2. Materialandmethods

2.1. Materials

2,2-dimethoxy-2-phenylacetophenone(DMPA,99%,Aldrich), copper(II)bromide(CuBr2,≥98.0%,Aldrich),silvernitrate(AgNO3,

ACS grade, Merck), 1,1,1-tris[4-(2-propynyloxy)phenyl]-ethane (≥98%, TCI Chemicals), propargyl bromide (80wt.% in toluene, Aldrich),trimethylolpropanetriglycidylether(TTE,technicalgrade, Aldrich), sodium azide (NaN3, 99%, Merck), sodium hydroxide

(NaOH, ACS grade, Merck) and potassium carbonate (K2CO3,

≥99%, Sigma-Aldrich,) were used as received. N,N,N,N,N -Pentamethyl diethylenetriamine (PMDETA, ≥98%, Merck) was distilled over sodium hydroxide before use. Commercial grade solvents (methanol, chloroform, dimethylsulfoxide and N,N-dimethylformamide) were purchased from Merck and used as received.

1,1,1-Tris[4-(2-propynyloxy) phenyl]-ethane (Tris-alkyne) [51,52] and 3,3 -((2-((3-azido-2-hydroxypropoxy)methyl)-2-ethylpropane-1,3-diyl)bis(oxy))bis(1-azidopropan-2-ol) (TTE-N3)

[1,53]werepreparedaccordingtotheliteratureprocedures. 2.2. Methods

ThePerkin-ElmerFT-IRSpectrumOneBspectrometerwasused forFT-IRanalysis.TheAgilentNMRSystemVNMRS500 spectrom-eterwasusedatroomtemperatureinCDCl3withSi(CH3)4asan

internal standard for 1_H _NMR _analyses._The _{thermogravimetric}

analysiswasconductedbyPerkin-ElmerDiamondTA/TGAwitha heatingrateof10◦C/minundernitrogenﬂow(200mL/min). Trans-missionelectronmicroscopy(TEM)observationwasperformedon aFEI TecnaiTM_G2 _F30_electron_microscope_operating_at ₂₀₀_kV.

TheultrathinTEMspecimensaround100nmwerecutbya cryo-ultramicrotome(EMUC6+EMFC6,Leica)equippedwithadiamond

knife.BeforeTEManalyses,theobtainedspecimenswerelocated onholeycarbon-coatedgrid.Thesolubilitypropertieswere deter-minedusing ASTMD3132-84 (1996): standard test methodfor

solubilityrangeofresinsandpolymers.Thecoloranddissolution of thesampleswereobservedafterimmersing themin solvent (5%w/vinsolvent,acetone,chloroform,N,N-dimethylacetamide, dimethylsulfoxide,methanol,andtetrahydrofuran)atroom tem-peraturefor4days.

2.3. In-situpreparationofantibacterialthermosets

All formulations contain same amounts of TTE-N3 (506mg,

1.17mmol):Tris-alkyne (494mg, 1.17mmol): Cu(II)Br2 (5.2mg,

0.023mmol): PMDETA (30␮L, 0.14mmol): DMPA (36mg, 0.14mmol)=50:50:1:6:6, except AgNO3 with different

load-ings(1, 3, 5and 10%of the monomersby weight). They were dissolvedwithDMF(0.2mL) inaPyrex tubeandde-aeratedby bubbling nitrogen gas for 10min and the tube was irradiated for 30min by a merry-go-round type photoreactor equipped with12 Philips8W/06lampsemittinglight at_␭>350nmanda coolingsystem[54].Attheendofgiventime,theproductswashed withexcessmethanol andﬁltered.Theproduct wasthendried in avacuumoven atroomtemperatureovernight.Solubilityof thermosetswastestedbyadding20mgofeachsampleto10mL solventfor24hatroomtemperature.

2.4. Antibacterialtest

The effect of Ag NPs on Gram negative(pathogenic and non-pathogenic) and Gram-positive, pathogenic bacteria was investigatedaccordingtotheagardiffusionmethod[8].Initially, strainswereculturedonthemediumat37Cfor18h.Afterthat,the culturedorganismswereaddedto10mLofsalinesolution(0.9% NaCl) toreachapproximatelythe105 _colony_forming_units_per

milliliter(CFUmL−1).Eachthermosetsample(30mg)wasplaced inthemiddleofsterilizedPetridishes.Then,1mLofsalinesolution containingbacteriaandagarsolutionwereaddedontothesurface ofeachmaterial.Afterincubationfor24hat37◦C,aclearand dis-tinct zoneofinhibitionwasvisualizedsurroundingthesamples implyingtheantimicrobialactivityagainstthemicroorganisms.

3. Resultsanddiscussion

By usingmultifunctionalazide andalkynecompounds, ther-moset networks were simply formed via CuAAC click reaction under mild condition [55]. For instance, photoinduced CuAAC chemistrywasutilizedforthebuildingthermosetnetworkwitha highcross-linkeddensity[31,32,56–58].Becauseoftheirstructural characteristicsthathavestiffertriazolelinkages,thesepolymers exhibited higher glass transition temperaturesand mechanical propertiesthanthiol-enenetworksofsimilarcrosslinkingdensity

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Fig.1.Monitoringtheformationofthermosetcontainingsilvernanoparticlesby

FT-IRspectroscopy.

[59].Inourcase,Tris-alkyne andTTE-N3 compoundswere

syn-thesizedandcharacterizedaccordingtotheliteratureprocedures. Theirstructures,conﬁrmedbyFT-IRand1_H_NMR_techniques,_were

inaccordancewithrespecttotheliteraturedata.Thecharacteristic bandsbelongingtopropargylgroupofTris-alkyneweredetected at2120and3260cm−1inFT-IRspectrum,whereasthealkyneand methyleneprotonswerevisualizedwithproperintegrationsat2.5 and4.7ppmin1_H_NMR_spectrum._Similarly,_the_{disappearance}_of

characteristicepoxybandat905cm−1andappearanceof charac-teristicazideandhydroxylbandsat2090and3500cm−1wereclear evidenceoftheTTE-N3.Inaddition,aprotonnexttotheazidegroup

wasseenat3.8ppmin1_H_NMR_spectrum._After_successful

synthe-sisofmultifunctionalclickableTris-alkyneandTTE-N3,thermoset

networkswereobtainedfromﬁxedformulations(Tris-alkyne: TTE-N3: Cu(II)Br2: PMDETA: DMPA=50:50:1:6:6) containingvarious

amount of silver nitrate under UV irradiation (Scheme1).The crosslinkingreactionwereformedbyCuAACclickreactionbetween azideandalkynegroups,whichwascatalyzedbyphotochemically generatedCu(I)ions.ThereductionofCu(II)Br2saltintoCu(I)

acti-vatorwasefﬁcientlyaccomplishedbyradicalsofgeneratedfrom photolysisofDMPA.Ontheotherhand,thesephotogenerated

rad-Fig.2.TEMmicrographsofthermosetcontainingAgNPs(3wt%AgNO3)in(a)low(scalebar:200nm)and(b)high(scalebar:5nm)magniﬁcations.

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Fig.4.TGAthermogramsofthermosetcontainingsilvernanoparticleswithvarious

loadings1,3,5and10%ofmonomersbyweight.

icalscouldbealsoutilizedforthereductionofAg(I)+_cation_into

Ag(0)nanoparticlesviaphotoinducedelectrontransferreaction. Indeed,twodifferentreductionprocesseswereincompetencewith theradicals formedfromthephotolysisoftheDMPA.Sincethe reductionpotentialofCu(II)intoCu(I)(+0.153E◦(V))wassmaller thanAg(I)intoAg(0)(+0.799E◦ (V)),therefore,theCuAACclick reactionwascatalyzedbeforetheformationofAgNPs[60].Infact, thispreferenceenabledtopreparethecrosslinkedtemplate before-handandnotonlyeliminatetheagglomerationofAgNPs,butalso retaintheirnanoparticlesize.

Withthis template advantage, antibacterial thermosets con-taining AgNPswithdifferentloadings wereprepared underUV irradiation.ThecrosslinkingprocesswasmonitoredbyFT-IR spec-troscopyand thecharacteristic peaksof azide (2090cm−1)and alkyne(C C HandC C,3260and2120cm−1)werecompletely disappearedafterthephotoinducedCuAACclickreaction.In addi-tion,anewweakbandcorrespondingtoN Hbondoftriazolering wasseenaround3310cm−1 inFig.1.Accrodingtotheseresults, almostquantitativeyieldswereobservedfromtheclickreactions thatproceedatambientconditions.Theformationofthermoset polymers was also conﬁrmed by solubility test using several solvents acetone, chloroform, N,N-dimethylacetamide, dimethyl sulfoxide,methanol,andtetrahydrofuran.Allsampleswere insol-ubleandcolorofthesolutionswerealmosttransparent,implying theirhighlycrosslinkedstructures.

InordertoconﬁrmthesizeanddistributionoftheAgNPs, trans-missionelectronmicroscopy(TEM)withenergydispersiveX-ray system(EDX)analyzerwasperformedforthermosetsample con-tainingAgNPs(3wt%AgNO3).Thedarkspheresrepresentedthe

AgNPs,whereas thegrayareas werecorrespondedtothe ther-mosetmatrix.Itwasclearlyobservablethatthethermosetsample wasbuiltfromdenselypackedAgNPswithnon-uniformsize dis-tributionsbetween3 and80nm.Furthermore,noagglomerated AgNPswereobservedinbothlowandhighmagniﬁcations(Fig.2). TheinteractionbetweenAgNPsandthepolymermatrixenabled thediffusion-limitedgrowthof AgNPstopreventtheirpossible agglomerationduringtheprocess.

TheenergydispersiveX-ray(EDX)spectrometeranalysis con-ﬁrmed the presence of elemental silver signal of the silver nanoparticles,where peaksaround 3.5 22.3and 25.1keV were assignedtothebindingenergiesofAgL,AgKa1andAgKa2,

respec-tively(Fig.3b).Furthermore,theselectedareaEDXpatternofthe

Fig.5. Anti-bacterialactivityofthethermosetscontaining1,5and10%AgNPs,andcontrolexperimentagainstpathogenicStaphylococcusAureusandEscherichiaColi

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sampledisplayedconcentricrings,indicatingthattheseAgNPshad highlycrystallinestructures(Fig.3a).Inaddition,otherpeaks cor-respondingtoCuKaandCuKbintheEDXweredetectedatbinding

energiesof1.6,8.1and8.9keV.TheCupeakscouldberelatedto thecopperoftheCuAACcatalystorTEMholdinggrid,inwhichthe samplewascoated.Also,sharppeaksnear1.0keVcorresponding ofcarbonandoxygenatomsofthermosetswereclearlyobserved. Overall,theseresultsundoubtedlyconﬁrmedthepresenceofboth components(AgNPs and polymericmatrix) of the antibacterial thermosets.

Thermalbehavior of the samples was investigated by ther-mogravimetricanalysis witha heating rate of 10◦C/min under nitrogen atmosphere. All samples exhibited two-step degrada-tionprocessintherangeof150–320◦Cand320–500◦C.Theﬁrst decomposition was attributed to the bond cleavage of C O C anddehydrationoffreehydroxylgroupsinthenetwork.The sec-onddegradationcorrespondedtothedepolymerizationreaction involvingthe scissionof C C bonds, resultingto the complete degradation of the carbonaceous materials and char forming (Fig.4).Itwasalsonotedthatthecharyieldincreasedwiththe additionoftheAgNO3.Forexample,thecharcontentofsample

containing10%AgNO3 increasedapproximatelythreefold(30%)

comparedwithneatthermoset(13%).Theincreaseincharyield wasdirectlyrelatedtodeterminetheAgNPsformation.Thus,the yieldofAgNPsformationinthermosetwasincreasedinorderof AgNO3loadings.

The antibacterial activities of thermoset samples containing AgNPsformedby photoinducedelectrontransfer duringcuring processwereinvestigatedagainstgram-positive(Staphylococcus Aureus)andgram-negative(EscherichiaColi)bacteriaasshownin Fig.5.Theantibacterialactivitywasattributedtoitsneutralform, inwhichthepassagethroughthecellwallfollowedbystrong inter-actionswithcellularcomponents[8].Nutrientagarplateswere inoculatedwith105_CFU_mL−1_from_different_bacterial_strains._The

samplescontaining5and10%AgNPsexhibitedstronginhibition zones,whereallkindsofbacteriawerekilledinthesurrounding oftheﬁlmsamples.Theinhibitionzonefromgrampositive bacte-riawasslightlylargerthanthezonefromgramnegativebacteria, butallzoneswerecleararoundthecloseproximityofﬁlmsamples containingAgNPs.Whereas,thecontrolsamplewithoutAgNPsand samplecontaining1%AgNPsdidnotdisplayedanyzoneafter incu-bationfor24hat37◦C,indicatingthattherewasnoantibacterial activityagainsttothebacterialgrowth.

4. Conclusions

In conclusion, in-situ preparation of thermosets containing AgNPswasachievedbysimultaneousphotoinducedelectron trans-ferandCuAACprocessesusingmultifunctionalazideandalkyne moleculesinthepresence ofDMPAand AgNO3.The

photogen-erated radicals not only reduce Cu(II) into Cu(I) activator to catalysttheCuAACclickreaction,butalsoproduceAgNPsfrom AgNO3 through photoinducedelectron transfer. Totake

advan-tageofreductionpotentialsdifference,theCuAACclickreaction wascatalyzedbeforetheformationofAgNPs,enablingto elimi-natetheagglomerationofAgNPsinthepolymermatrix.Thehighly crosslinkedstructuresofthesampleswereprincipallyconﬁrmed byFT-IRandsolubilitytests,whichwerenotsoluble invarious organicsolvents.TheTEMwithEDXresultsundoubtedlyconﬁrmed thepresenceofAgNPswithnon-uniformsizedistributionsinthe polymer matrix. It was also shown that all products exhibited antibacterialactivitiesagainst togrampositive (Staphylococcus Aureus)andgramnegative(EscherichiaColi)bacterialcolonies.By applyingthisstrategy,limitationssuchashighcostand compli-catedexperimentalproceduresforthepreparationofantibacterial

materialareeliminatedanditsproductionhasbecomeeasierthan previousone.

Acknowledgement

Theauthors wouldlike tothank YalovaUniversity Research Fund(Projectno:2015/YL/054)forﬁnancialsupports.

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