Silver nanocluster-silica composite antibacterial coatings for materials to be used in mobile telephones

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ContentslistsavailableatScienceDirect

Applied

Surface

Science

j o ur na l ho me pa g e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Silver

nanocluster-silica

composite

antibacterial

coatings

for

materials

to

be

used

in

mobile

telephones

Marta

Miola

a,∗

_,

_Sergio

_Perero

a

_,

_Sara

_Ferraris

a

_,

_Alﬁo

_Battiato

b

_,

_Chiara

_Manfredotti

b

_,

Ettore

Vittone

b

_,

_Davide

_Del

_Vento

c

_,

_Simona

_Vada

c

_,

_Giacomo

_Fucale

d

_,

_Monica

_Ferraris

a

a_Applied_Science_and_Technology_Department,_Politecnico_di_Torino,_Torino,_Italy b_Physics_Department,_NIS_excellence_centre_and_CNISM,_University_of_Turin,_Torino,_Italy c_Concept_Reply,_Turin,_Italy

d_Chemical,_Clinical_and_{Microbiological}_Analyses_Dept.,_CTO_Maria_Adelaide_Hospital,_Turin,_Italy

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received5February2014

Receivedinrevisedform20May2014 Accepted23May2014

Availableonline2June2014 Keywords: Coating Sputtering Silvernanocluster Antibacterial

a

b

s

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Antibacterialcoatingscontainingdifferentamountofmetallicsilvernanoclustersembeddedinasilica matrixhavebeendepositedbyco-sputteringtechniqueonseveraldifferentpolymersusedinmobile telephonescomponentssuchas:screens,covers,andmicrophonefelts.

Sputteringparametershavebeenvariedtoobtaindifferentcoatingthicknessandsilvercontent,in ordertomeetantibacterial,aestheticandfunctionalrequirementsforeachcomponent.Inparticular, anantibacterialfunctionalityhasbeenobtainedforscreens,coversandfeltswithoutaffectingtheir respectivetransparency,aestheticandacousticproperties.

Theoptimalparametersforeachparthavebeenusedtoobtainanantibacterialmobiletelephone suitableforpersonneloperatinginhospitalsorotherenvironmentswithpotentiallyhighbacterial con-tamination.

1. Introduction

Bacterialcontaminationisaproblemformedicaldevicesbut

alsoforawidenumberofeverydaylifeobjectsusedbyhundreds

ofpeople,especiallyinpublicplacesandhospitals.Forthese

rea-sons,theantibacterialfunctionalityismoreandmoreanimportant

addedvalue.

Variousantibacterial coatings have beeninvestigated in the

literature[1–11]andsomeofthemareavailableonthemarket

[12–15].Inparticularsilver-containingmaterialsarebeingwidely

usedsincesilverisknowntohaveapowerfulanti-bacterialactivity

[5,16,17]andtoinducelowbacterialresistancecomparedtothat

ofantibiotics.Thesefeaturesareofhighimportancetoﬁghtthe

worldwideplagueofbacterialincreasedresistancetoantibiotics,

inparticularinsidehospitals[18,19].

Severaltechniquesabletoprovideanantibacterialsurfaceon

differentmaterialshavebeendeveloped[1–11].

∗ Corresponding author.Present address:Presentafﬁliation:Department of HealthSciences,UniversitàdelPiemonteOrientale“A.Avogadro”,Novara,Italy. Tel.:+390110904717/+393471327373;fax:+390110904624.

E-mailaddress:[email protected](M.Miola).

The drawback of most techniques is that they do not give

thermalandmechanicalstablecoatingsorthattheyconsiderably

changetheappearanceofthecoatedmaterials.

Theprocessusedinthepresentworkallowsthesynthesisby

co-sputteringtechniqueofanantibacteriallayercharacterizedby

silvernanoclustersembeddedinasilicamatrix.Theco-sputtering

techniqueisextremelyversatile:thethicknessandsilver(orother

metals) content can be tailored to fulﬁl different antibacterial

andaestheticrequirements.Theselayerscombinethechemical,

mechanicalandthermalstabilityofsilicawiththebroadspectrum

antibacterialactivityofsilver.

Silvernanoclusters-silicacompositeantibacteriallayerson

sil-ica,soda-limeandsomepolymershavebeenfullycharacterized

anddiscussedin[20–25].Theyaresuitableformostofsubstrates,

inparticularforpolymers[25],whicharemostlyusedformobile

telephonecomponents.

Mobiletelephonesareinthetoptenofthemostusedobjectsby

billionsofpeopleeveryday.Themobiletelephoneismanipulatedin

differentsituations(atwork,athome,inpublicspaces,etc.),stored

invariousplaces(bags,pockets,desks,cars,etc.)anditisincontact

withourbody(ear,hand,mouth,hair,etc.)withstrongpotential

tocarryhighlevelsofbacteria.

Ascientiﬁcresearch[26]systematicallystudiedbacterial

con-taminationofpublictelephonesontheearandmouthpieces.The

http://dx.doi.org/10.1016/j.apsusc.2014.05.151

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Table1

Advantagesanddisadvantagesofexistingantibacterialtechnologyappliedtomobilephones.

Antibacterialtechnologyappliedto mobilephones

Antibacterialproductand(mobile phoneproducer)usingit

PROs CONs

Agdopedzeolitesorsilica microspheres

AgIonTM_(Motorola) _Low_cost,_industrial_products_available, suitabletocoatlargesurfaces

Possibleunwantedinteraction betweenmatrixandzeolites Ag-dopedpolymers:Agparticles

embeddedinpolymersorsynthesis ofpolymercontainingAgions

BiomasterTM_(Apple) _Low_cost,_industrial_products_available, suitabletocoatlargesurfacesandto largescaleproduction

Limitedmechanicalandthermal stabilityduetopolymersSilverions embeddedintoanadditivewhichis dispersedthroughouttheentire material,andisnotapplicableonother componentsofthemobilephonethan plasticcovers

Silverbasedionic-exchangeonglasses: Ag+_/Na+_or_Ag+_/K+_Ion-exchange_on glassescontainingNa+_or_K+

CorningTM _Low_cost,_industrial_products_available, suitabletocoatlargesurfacesandto largescaleproduction

SuitableonlyforglassescontainingK+ orNa+

NanoAg LGTM _Low_cost,_industrial_products_available,

suitabletocoatlargesurfacesandto largescaleproduction

Silverisnotembeddedinamatrix, highsilverrelease,lowdurabilityand mechanicalproperties

authorsrevealedtwelvedifferenttypologiesofbacteriaandtheir

densitywasfoundtobedangerouslyhighonthemouthcontact

zone of the telephone. Moreover a recent review reportedthe

bacterialcontaminationofmobilecommunicationdevices

demon-stratingthatabout25%ofthemarecontaminated[27].Ithasbeen

demonstratedthatbacteriacansurviveweeksonseveralsurfaces,

then come in contact withhuman skin [28,29]. Moreover,the

heatgeneratedbymobilephonescanincreasebacteria

concen-trationbyprovidingthema“comfortableenvironment”[29].The

bacteriacontaminationofmobiletelephonesisparticularly

dan-gerousinenvironmentswherethedevelopmentofinfectionscause

seriousconsequences,suchashospital,clinicsandsurgeryrooms

[29,30].Bacterialcontaminationofmobiletelephonesinthe

hospi-talenvironmentcanleadtothetransmissionofinfectiontoheath

compromisedpatients,butalsotothemedicalstaff,theirrelatives

andtoexternalpeople:infact,contaminatedtelephonesareoften

movedfromonehospitaldepartmenttoothersandalsoathome.

Thebacterialcontaminationofthesedeviceshasbeenwidely

documentedforhealthcareworkers[27–33]butalsoforpatients,

relativesandvisitors[34].Thebacterialtransferfromhandsto

tele-phonesurfaceandfromcontaminatedtelephonesurfacetoclean

handshavebeenreported[27],demonstratingthepossibilityof

infectiontransmissionthroughmobiledevices.

Despite of the numerous studies that reports these

poten-tialrisks,peopleawarenessofbacterialcontaminationofmobile

phonesisstillpoorormissing[27,30].

Antibacterialwipesare commercialized for telephone

clean-ing, but an extensive and systematic use of them is far from

beingreached,alsoinclinicalenvironment.Moreovermanymobile

phonesaresensitivetodetergentsordisinfectantsandproducers

discouragetheuseofthem;theuseofsiliconcoverstoprotectthe

telephoneandallowaneasycleaninghasbeensuggested[34],but

itdoesn’tworkonearandmicrophoneparts.

Some attempts to confer antibacterial properties to mobile

phoneshave beenreported:CorningTM _[35] _recently_advertised

aboutanantibacterialsilver-basedglassforsmartphonesandother

touchscreendevices.Anantibacterialtelephonehasbeen

commer-cializedinthepastbyMotorolaTM_:_it_is_based_on_Agion®_technology

[12],i.e.Agionsintroductioninzeolitecarriersforgradualrelease.

AlsoLGTM_{commercializes}_an_{antibacterial}_mobile_phone_[36]_:_in

thiscaseanon-speciﬁed“nano-silver”coatinghasbeenused.The

Biomaster®_technology_has_also_been_used_to_produce_a_cover_for

AppleTM _iPhone ₄TM _having _{antibacterial} _properties _EcoskinTM₎

[37].Thistechnologyisbasedonsilverionsembeddedintoan

addi-tivewhichisdispersedthroughouttheentirematerial,andisnot

applicableonothercomponentsofthemobilephonethanplastic

covers.

Compared to the existing solutions (Table 1), the coating

proposed here is more resistant from chemical, thermal and

mechanicalpointsofviewandcanbeappliedtovarioussubstrates

withoutparticular requirements (e.g.not only glass containing

exchangeableions,butalsoplasticsandfelts).Moreovermostofthe

claimed“antibacterial”deviceshaveactuallynon-stickingsurfaces,

insteadofatrueantibacterialproperty.

Co-sputteringtechniquehasbeenemployedinthisworkforthe

depositionofsilvernanoclusters-silicacompositelayersonseveral

differentpolymeric partsof mobile phones.Optimised

sputter-ingconditionshavebeenselectedforeachpart,obtainingseveral

antibacterialandperfectlyworkingmobilephones.

2. Materialsandmethods

Silvernanoclustersilicacompositecoatingsweredepositedon

selectedsubstratesbyRadioFrequency(RF)co-sputtering

(Micro-coatMS450).Silver(SigmaAldrich99.99%purity)andsilica(Franco

CorradiS.r.l.99.9%purity)targetswereused.Thesampleswere

pre-paredbyapplying200W(RF)toasixinchessilicatargetand1W

(PulsedDC)toaoneinchsilvertarget,byaco-depositionprocess.

Fourdifferentsetsofparameters(Table2)havebeenusedinorder

tovarysilverconcentrationandcoatingthickness.

TooptimiseAg/SiO2ratiointhecoatings,theplasmaonAg

cath-odewasswitchedonand offduringthewholedepositiontime,

(dutycycle).Thetotaldepositiontimeandtheplasmadutycycle

arereportedintheTable2.Asanexampleofdutycycle,12–1

cor-respondstoacycleduringwhichtheplasmaonAgwasswitched

on1secondonacycleof12s.

Thethicknessofthecoatingshasbeendeterminedbycontact

proﬁlometrymeasurements(KLA-TencorP15)afterdepositionon

partiallymaskedsamples,thenmeasuringthestepbetweenthe

coatedandtheuncoatedsurface.Detailsaboutthesecoatingsand

theircompletecharacterisationarein[20–25].

Threedifferentpartsofmobiletelephoneshavebeen

consid-eredfortheapplicationofanantibacteriallayer:protectivelenses

forscreen(screen,inthetext),covers(cover,inthetext),and

pro-tectivetissuefeltsforelectro–acoustictransducers(felt,inthetext).

Materialsusedforthesedifferentpartsarelistedbelow:

Protectivelensesfor screen:polycarbonate(SabicInnovative

Plastics,LexanTM_).

Covers:polycarbonateyellowcoverofSamsungTM_S3650_(cover

A)andblendpolycarbonate-ABS(Acrylonitrilebutadienestyrene)

blackcoverofNokiaTM₁₆₁₆_(cover_B).

Protective tissue felts for electro-acoustic transducers:

polyester monofilament fibre Saatifil AcoustexTM _B010 _(felt

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Table2

Sputteringparameters(totaldepositiontimeanddutycycle)onthesilvercathode,coatingthicknessmeasuredonscreens,coversandfelts,Ag/Siratio(rangeofobtained values)measuredbyEDS(*₎_and/or_XPS₍**_).

Sputteringparameters Totaldepositiontime Plasmadutycycle(s) Thickness(nm) Ag/Si(atomic%)

Condition1 8min 12–1 25–50 0.06–0.13(**₎ Condition2 2h 12–1 350–450 0.05–0.06(*₎ 0.07–0.13(**₎ Condition3 8min 6–2 25–50 0.22–0.30(*₎ Condition4 2h 6–2 350–450 0.36–0.63(*₎ 0.32–0.38(**₎

(felt3),SaatiﬁlAcoustexTM_B045_(felt_4),_Saatiﬁl_AcoustexTM_B090

(felt5)andSaatiﬁlAcoustexTM_B145_(felt_6)._All_felts_are_made_by

polyestermonoﬁlamentﬁbre,theonlydifferenceamongthemis

thetexture.

Aﬁrstevaluationoftheaestheticimpactofthecoatingonthe

mobiletelephonepartshasbeenperformedbyvisualobservation,

followedbyaquantitativestudyoftheproperties,consideringthe

mainrequirementsandstandardproceduresusedtovalidateeach

mobilephonepart.

2.1. Physical–chemicalcharacterization

Coated screens and felts have been characterised by X-ray

photoelectron spectroscopy (XPS) using a VSW TA10

non-monochromaticAlK␣(1486.6eV)X-raysourceequippedwitha

class100concentrichemisphericalanalyser.

Theroughnessofcoatedscreens,beforeandafterdeposition,

havebeeninvestigatedbynon-contactatomicforcemicroscopy

(AFM,ParkSystemsXE-100,dataanalysiswithXEI1.8.0software)

onseveralcoatedanduncoatedsamples.

2.2. Evaluationofcoatingadhesiontothesubstrate

Asemi-quantitativeanalysisofcoatingadhesionhasbeen

per-formed by cross-cut tape test, according to ASTM D3359 [38]

standard.Onlycoatedscreensandcovershavebeentested,because

feltsarenotsuitableforthiskindoftest.

2.3. Spectrophotometricanalysis

Inordertoverifytransparencyrequirementofcoatedscreens

and esthetical performance of the covers after the sputtering

process,analyseshavebeenperformedbya spectrophotometer

(XRiteTM _model_SP64)_and_the_color_variations_have_been

evalu-atedusingastandardCIELabcolorspace.Thespectrophotometer

lightsourceisatungstenlamp,andthespectralregionisbetween

400and700nm;astandardilluminantD65witha10◦observerhas

beenused.Thecolorvariation(E)isthedifferenceincolorofan

objectcomparedtoastandardcolor:itincludeshue,saturationand

brightnessusingthreechromaticcoordinates(a,b,L),asdeﬁnedin

theCIELabcolorspace.

2.4. Acousticmeasurements

ANokiaTM₁₆₁₆_telephone_has_been_used_for_these_experiments:

severalcoatedanduncoatedfeltshavebeenpositionedonthe

tele-phonemicrophoneandbothfrequencyresponseandsensitivityof

themicrophonesystemhavebeenmeasuredusingtheHeadAnd

TorsoSimulator(HATS)andtheartiﬁcialvoicedeﬁnedinITU-T

rec.P50[39].Inordertotestthehands-free/musicplayback

per-formance,thesameparameters havebeenevaluatedemploying

a telephoneloudspeaker (AAC14mm×20mm,8,0.5W)in a

200cm3_closed_box._The_chosen_input_test_signal_was_a_full_band

whitenoise.TheresponsehasbeencalculatedbymeansofFFT(Fast

FourierTransform)analysis(400rows,25.5kHzspan,64Hzfand

exponentialaverageon5000samples).Finally,thereceiving

per-formancehasbeenestimatedbyusinganearsimulatortype3.3

(HATS)andartiﬁcialvoiceandthemeasurewasrunwithseveral

felts,beforeandafterthecoating.

2.5. Antibacterialtests

Inordertoevaluatetheantimicrobialpropertiesofthedifferent

partsofamobilephone(screen,cover,felt),eachonewas

individ-uallycoatedbydifferentsputteringconditions(Table2),thenthe

inhibitionhalotestwasperformedinaccordancetoNational

Com-mitteeforClinicalLaboratoryStandards(NCCLSM2-A9[40])using

astandardStaphylococcusaureusstrain(ATCC29213),whichisa

leadingcauseofinfections[41].Thesampleswereplacedin

con-tactwithanagarplateuniformlycoveredwithastandardbacterial

brothandincubatedovernightat35◦C;subsequentlytheinhibition

halowasobservedandmeasured.

Coatedsamplesnotabletogiveaclearinhibitionzone were

subjectedtoa bacteriaadhesiontest(fromASTME2180andJIS

Z2801standard);thetestrequiresthepreparationofastandard

bacterialbrothcontainingapproximately5×105 _colonies

form-ingunits(CFU);thecoatedsampleswereplacedinamulti-well

plateandasmallamount(1ml)ofthebacterialbrothwasspread

overthesamplesurface,theplatewasclosedwithalaboratory

clingﬁlm,tomaintaintheappropriatehumidityandavoidbroth

evaporation,thentheplatewasincubatedovernightat35◦C.After

incubationthesampleswereremoved,gentlywashedin

physio-logicalsolution(0.9%w/vofNaClindistilledwater)andvortexed

alwaysinacleanphysiologicalsolutionfor1minat50Hztodetach

thebacteriaeventuallyadheredonthesamplesurface.The

vor-texedsolutionwasseriallydilutedandspreadonbloodagarplates

whichwereincubatedovernightat35◦CtoallowthegrowthofCFU.

AftertheincubationtimetheCFUgrownontheplatewere

manu-allycounted,theobtainednumberwasmultipliedbythedilution

factorinordertogetthetotalamountofCFUofthestarting

vor-texedsolutionandsothetotalamountofCFUadheredonsamples

surface.Allantibacterialtestswereperformedintriplicatealsoon

uncoatedsamplesforcomparisonpurposes.

3. Resultsanddiscussion

3.1. Screens

Screens must satisfy strict aesthetical and transmittance

requirements.Fig.1reportsthevisualappearanceofthescreen

aftercoatingwithdifferentsputteringconditions(Fig.1a–d).Itis

clearthatonlycondition1canmeettransparencyrequirements:a

verythincoating(25–50nm)wasmeasuredbyproﬁlometryanda

lowsilvernanoclustercontent(Ag/Siratiointherangeof0.06–0.13,

measuredbyXPSanddiscussedbelow)isresponsibleofthe

trans-parencyofthiscoating.

Spectrophotometric analysis conﬁrms this observation: only

sputtering condition1 presentsadifferenceincolorperception

closetostandard acceptability (E=1is themaximum

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Fig.1. Coatedscreen(sputteringconditionsasinTable2),before(a–d)andafter(e–h)tapetest.

phoneproducers).Theresultsofthespectrophotometricanalysis

aregraphicallyrepresentedinFig.2.

Thecoatingadhesiononthesubstrate(polycarbonate)isgood

foralltheconsideredsputteringconditions.Fig.1(e–h)showsthe

appearanceofthecoatedscreenafterthetapetest.Nodamagecan

benoticedbothinsideandoutsidethegrid.Ahighadhesionofthe

coating(0%damage,5Bclassiﬁcation,accordingtothestandard)

hasbeenmeasuredforalltheconsideredsamples.

EDSanalysesconﬁrmthepresenceofanincreasingAg/Siratio

for samples coated with sputtering condition from 1 to 4. As

expected,thesputteringconditionsinﬂuencethesilver

concentra-tionandthicknessofthecoating,asshowninTable2.Thesilver

concentrationismodulatedbyvaryingthedutycycleonthesilver

cathode:i.e.theplasmawasperiodicallyswitchedonandoff

dur-ingdepositiontime.Thecoatingthicknessisproportionaltothe

coatingtime.

Thepossibility to modulate theAg content and the coating

thickness is important for two main reasons: tomodulate the

antibacterialeffectwithoutoccurringintoxicityissues[42]andto

adjustthetransparencyoftheantibacteriallayer.Theadvantageof

thisapproach(optimizedsputteringconditionsforeachpartofthe

mobilephone)comparedtoothercommercialantibacterial

prod-ucts,isthattheantibacterialactivityandaestheticpropertiescanbe

modulatedaccordingtothedifferentrequirementsofthedifferent

mobilephoneparts.

Fig.3reportstheantibacterialtestsofthescreencoatedwith

differentsputteringconditions.Itcanbeobservedthatthesample

sputteredbyusingcondition1isnotabletogiveaninhibitionhalo;

nevertheless,nobacterialgrowthwasobservedunderthis

sam-ple.Sputteringconditions2and3allowaninhibitionzone≤1mm,

whilethesamplecoatedwithcondition4wasabletoproducean

inhibitionhaloofabout3–4mm(Fig.3a).Theinhibitionhalosshow

anirregularshape:probablythesilverconcentrationisnotuniform

andsomebacterialcoloniesbegintoproliferate.Asexpected,the

antibacterialeffectiscorrelatedtotheamountofsilverpresentin

thecoating,asdiscussedabove(Table2):theAg/Siratiois

simi-larforcoatingsdepositedwithsputteringcondition1and2,but

thedifferentthicknesscanexplainthedifferentbehaviorinterm

ofantibacterialactivity.

Althoughthesputteringconditions2,3and4producedalarger

inhibitionzone,thescreenscoatedwiththeseconditionshavean

unacceptabletransparencyloss.Aspreviouslydiscussed,the

trans-parencyisanessentialrequirementforscreens,sothecondition1

wasselectedtocoatscreensandafurtherantibacterialtest(the

countofCFU)wasperformedinordertoassessthecoatedscreen

abilitytoreducebacterialcontamination.

Fig.3breportstheresultsofCFUcountonthescreencoated

bycondition1:thesameprocedurewasusedforbothcoatedand

uncoated(control)samples,thusobtainingacomparisononthis

testsuitabilitytomeasuretheadheredbacteriainpresenceand

absenceofthecoating.ThenumberofCFUadheredontothecoated

screendecreased.ofabouttwoordersofmagnituderespecttothe

uncoatedone.Accordingtothistest,alsothesputteringcondition1

isabletoreducethebacterialcontaminationonthecoatedscreen.

Inordertobettercharacterizethecompositionofthecoating,

XPSanalyseshavebeendoneoncoatedscreen withsputtering

condition1;thesurveyspectrumand thechemicalcomposition

obtainedareshowninFig.4.Surfacechargeeffectsleadtoa7–8eV

shiftofrelevantpeaks[43],thusenergyscalecalibrationhasbeen

performedusingtheC1scoremainpeakasareference,locating

itat285eVandthenfollowingthestandardprocedurereportedin

literatureforinsulatingsamples[43].Theanalysesofthecoated

screensallowedidentifyingmainpeaksofAg(Ag3d),O(O1s),Si

(Si2p)(andC,C1spresentasasurfacecontaminant).

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Fig.3.(a)InhibitionhalotestoncoatedscreensputteredwithdifferentconditionsasinTable2;(b)countofadheredCFUonscreensputteredwithcondition1withimages ofagarplatefortheCFUevaluation.

Fig.4reportsalsotheobtainedconcentration(atomic%)ofthe

identiﬁedelements,togetherwithAg/SiandO/Siratio.Siwasused

asreferenceduetothehighandnotuniformpresenceofcarbon.

TheAg/Siconcentrationiscloseto0.1,forsputteringcondition1,

whileO/SirateisclosetothestoichiometricvalueforSiO2,the

matrixofthecompositecoatings.

Twotypicalatomic forcemicroscopymapsfor uncoatedand

coatedscreen(condition1)arereportedinFig.5asanexampleof

theobtainedresults.Ascanbeclearlyseenfrom3Dimages,the

antibacterialcoatingisnanostructuredandfollowsthe

topogra-phyofthepolymersurface.Theroughnessanalysisoftheobtained

mapsforboth uncoatedandcoatedscreenrevealedanegligible

increaseinallsurfaceroughnessparameters:therootmeansquare

roughness(RMS)ofthecoatedscreenisbelow5nm,whileforthe

uncoatedsampleiscloseto3nm.

3.2. Covers

Asdiscussedforscreens,thebestaestheticalresultwasgivenby

sputteringcondition1,butthisconditionshowedalower

antibac-terialprotectionthantheotherones,asevidencedbytheinhibition

halotest.On theotherside,sputtering condition3 gavehigher

antibacterialprotection,butthecoatedscreensshowedalow,not

acceptabletransmission.Inthecaseofcovers,sputteringcondition

3isabletoguaranteeahigherantibacterialprotection,suitablefor

mobilephoneswithayellowordarkercoverwheretheperceived

yellowisheffectofthecoatingcouldbenegligibleandwasthus

cho-senforthefollowinganalyses.Condition4,whichgavethelarger

inhibitionhalo,deﬁnitelygaveatoodarkcolortothesubstrate.

Visualinspectionand spectrophotometricanalysishavebeen

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Fig.4. XPSsurveyspectrumforcoatedscreen,sputteringcondition1,showingmain contributionsfromO,Ag,CandSiandsurfacestoichiometry.

(coverA)andtheblackcover ofaNokia 1616(coverB)coated

bysputteringcondition3.Fig.6showstheappearanceof cover

Aandcover Baftercoatingprocess(Fig.6a andc,respectively)

withthecorrespondentspectrophotometricresults(Fig.6bandd,

respectively).

Thevisualinspectionofcover Aand thespectrophotometric

analysis demonstrated a signiﬁcant color variation: the color

perceivedisdarkerand thereisaE≈10thatis tentimesthe

maximumacceptablevalue(_E=1),asspeciﬁedintheCIELabcolor

spacestandard.

Cover B showed a yellowish effect, conﬁrmed by

spectro-photometric results that showed a E≈4, but the perceived

estheticalresultlooksacceptable.

TapeadhesiontestoncoatedcoverAandBsputteredby

condi-tion3(Fig.7candd)indicatestheunsuitabilityofthecoatinginthe

caseofcoverA,asevidencedaftertaperemoval.Onthecontrary,

nodamagecanbeobservedonthecoatedcoverB(0%damage,5B

classiﬁcationaccordingtothestandard)aftertapetest:thereason

ofthisdifferenceinadhesionofthesamecoatingontwodifferent

substratesisunknown.Itmaybeduetoasurfacetreatmentdoneby

thesupplieronthepristinecovers,butneverofﬁciallyconﬁrmed.

InhibitionhalotestofcoversAandBsputteredbycondition3

isreportedinFig.8aandb:bothcoatedcoversareabletocreatea

smallinhibitionzone(thinnerthan1mm)buteffectivetolimitthe

bacteriaadhesionandproliferation.

Inorder toinvestigatetheability ofthesputtered coversto

reducetheadhesionofbacteria,thecountofCFUwasperformed

also in this case, but only for the black cover for the reasons

previouslydiscussed. Asitcanbeobserved (Fig.8c)thecoated

coverBdecreasedthebacterianumberby2–3ordersofmagnitude

ifcomparedtotheasreceivedcontrolcover.

3.3. Felts

Sinceacousticfeltsareblacktissueshiddeninsmallholesofthe

telephone,noaestheticalrequirementmustbesatisﬁed.Onthe

otherhand,theriskofbacterialcontaminationhereishigherthan

fortheothercomponents,sincetheseelementsarecontinuously

exposedtomouthandbreathandtheyareconﬁnedinsmallareas

difﬁculttoclean.

The applicability of the antibacterial layer to acoustic felts

dependsespeciallyontheeffectthatthecoatingcanhaveonthe

acousticperformanceofthematerial.

Differencesin theresponse signal have been grouped, with

respecttofrequency,consideringthedepositionconditionofeach

felt: Fig.9reports theworst casefor themicrophone response

(highersensitivitydifferencesaftercoating,coatingwithsputtering

condition2).Measurementsonthefeltscoatedbyother

deposi-tionconditionsresultedinanegligiblevariationofthemicrophone

response.Itcanbeobservedthatreadableeffectscanberevealed

onlyforhighacousticimpedanceonfelt5andfelt6,andfor

fre-quenciescloseto3kHz.Itcanbeconcludedthatthecoatingeffect

onthesendingperformanceisnegligible,becauseitoccursatthe

verytopofthetelephonefrequencybandwheretheresponseis

alreadyattenuatedbyinternalcircuitryﬁltering.Theeffectsonthe

receivingperformance(earpieceandloudspeaker)havebeen

eval-uatedbylookingatfrequencyresponseandharmonicdistortion.

Bothforfrequencyresponseandharmonicdistortionnosigniﬁcant

alterationhasbeendetected.

Theinhibitionhaloofallcoatedfeltswiththesputtering

con-ditions2,3and4wasevaluated(condition1wasavoideddueto

thelowthicknessandlowAg/Siratio,notsuitableforfelts).Felts

coatedwithcondition2didnotproduceauniforminhibitionhalo,

probablyduetothelowAg/Siratio;thesameunclearresultwas

obtainedforfeltscoatedwithcondition3,probablyduetothelow

thicknessofthecoating;condition4gaveareproducibleinhibition

zoneofabout3–4mm(Fig.10).Sinceforthiskindofapplication

theriskofbacterialcontaminationisveryhigh,thetransparency

isnotanissueandtheacousticperformancesaremaintained,the

condition4(Table2)wasselectedtocoatfelts.

Fig.11reportsasexampletheXPSsurveyspectrumofcoated

felt3,sputteredbycondition4.Resultsobtainedonsputteredfelts

1,2,4,5and6aresimilar;thecompletechemicalcompositionof

allthefeltsdepositedwiththecondition4isreportedinTable3.

Theenergyscalecalibrationhasbeenperformedusingthesame

(7)

Fig.6. YellowcoverofSamsungS3650(coverA)andblackcoverofNokia1616(coverB)aftersputteringcondition3(Table2)andthecorrespondentcolorvariation(E) measuredaftersputteringprocess.

procedureusedforscreens.Alsointhiscase,theanalysesofall

coatedfeltsallowedidentifyingthemainpeaksofAg(Ag3d),O

(O1s),Si(Si2p)andC(C1s,presentasasurfacecontaminant)and

tocalculatethesurfacecomposition(atomic%)oftheantibacterial

layer(Table3).

Fig.7.CoatedcoverAandcoverB(sputteringcondition3asinTable2),before(a andb)andafter(candd)tapetest.

Fig.8. (a)and(b)InhibitionhaloofcoverBcoverArespectively,coatedwith sput-teringcondition3(asinTable2);(c)countofadheredCFUoncoverB(controland coated)withsputteringcondition3(asinTable2)andimagesofagarplateforthe CFUevaluation.

Table3

SurfacestoichiometrycalculatedbyXPSonsputteredsilvernanoclustersilicacoated felts,sputteringcondition4.

Ag(at%) Si(at%) O(at%) C(at%) Ag/Si O/Si

Felt1 1.6 22.0 45.1 31.3 0.07 2.05 Felt2 7.7 19.9 42.6 29.8 0.38 2.14 Felt3 3.2 24.3 53.9 18.6 0.13 2.22 Felt4 2.6 27.2 51.7 18.5 0.10 1.91 Felt5 6.3 19.6 38.3 35.8 0.32 1.95 Felt6 6.1 17.0 42.2 34.7 0.36 2.48

(8)

Table4

Summaryoftheactivity,resultsoverviewandchoiceofthemostsuitablesputteringconditionforeachpartofthemobilephone.

Telephonepart Sputteringconditiontested Antibacterialactivity Opticalproperties/appearance Sputteringconditionselected

COVER 3 Discrete Semi-opaque 3

Felt 2 Discrete n.a. 4

3 Limited n.a.

4 Optimal n.a.

SCREEN 1 Limited Almosttransparent 1

2 Discrete Opaque

3 Discrete Semi-opaque

4 Optimal Dark

Fig.9.Sensitivitydifference(delta)insendingvoicesignal(dBV/Pa)fordifferent coatedfelts.

Fig.10. Inhibitionhaloofallcoatedfeltsusingsputteringcondition4(asinTable2).

Asfor coatedscreens,the O/Sirateis closeto 2(asfor the

expectedstoichiometryofSiO2).

SomediscrepanciesintheAgcontentcanbeattributedtothe

evidentirregulartextureoffelts.

Insummary(Table4),sputteringcondition1,with25–50nm

thicklayerandAg/Siratioof0.06–0.13wasselectedtocoatscreens

inordertohavebothantibacterialactivityandtransparency;

con-dition3with25–50nmthicklayerandAg/Siratioof0.22–0.30was

selectedtocoatcoversinordertoguaranteebothantibacterialand

aestheticissues;tocoatfelts,wheretransparencyandaesthetical

concernsarenotconsidered,condition4with350–450nmthick

layerandthehighestAg/Siratiogavethebestresults.

Finally,thedifferentcomponents(screen,cover andacoustic

felts)ofacommercialmobiletelephoneweresputteredwiththe

selectedconditions.The operationalperformance ofthemobile

phoneisnotinﬂuencedbyaddedantibacterialcoating:the

antibac-terialmobilephoneinFig.12isperfectlyworking.

Fig.11.XPSsurveyspectrumforcoatedfelt3,sputteringcondition4,showingmain contributionsfromO,Ag,CandSi.

Fig.12.Antibacterialmobiletelephone.

4. Conclusion

Silver nanoclusters-silica antibacterial layers have been

depositedondifferentpartsofmobiletelephones(screens,covers

andacousticfelts)byRFco-sputtering.

Depositionparametershavebeenoptimisedinordertotailor

thesilvercontentandthecoatingthicknesstomeetantibacterial

andaestheticrequirementsofeachpart.

Screens covered with condition 1 (duty cycle 12–1, 8min,

25–50nm)exhibita goodantibacterialaction maintainingtheir

transparency; antibacterial covers with acceptable aesthetic

appearance have been achieved with condition 3 (duty cycle

6–2,8min,25–50nm)andantibacterialfeltswithnosigniﬁcant

alterationoftheiracousticpropertieshave beenobtainedusing

(9)

Inconclusion,theco-sputteringprocesscanbeappliedto

differ-entmobiletelephonepartstobethenassembledinordertoobtain

aneffectiveandfunctioningantibacterialmobiletelephone.

Due to the reproducibility of the sputtering process, these

coatingsare suitable for mass production in the mobilephone

industryandforseveralotherpolymerbasedproducts.

Acknowledgments

This project was funded by Regione Piemonte, Italy, with

“NABLA—NanostructuredAntibacterialLayers”(2008-11).

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