UV-Curable Aliphatic Silicone Acrylate Organic–Inorganic Hybrid Coatings with Antibacterial Activity

13 Pages·2017·15.11 MB·English

molecules
Article
UV-Curable Aliphatic Silicone Acrylate
Organic–Inorganic Hybrid Coatings with
Antibacterial Activity
VirginijaJankauskaite˙ 1,*,AlgirdasLazauskas2,EgidijusGriškonis3,Aiste˙ Lisauskaite˙ 1
andKristinaŽukiene˙ 1
1 FacultyofMechanicalEngineeringandDesign,KaunasUniversityofTechnology,Studentu˛St.56,
LT-51424Kaunas,Lithuania;[email protected](A.L.);[email protected](K.Ž.)
2 InstituteofMaterialsScience,KaunasUniversityofTechnology,BaršauskoSt.59,LT-51423Kaunas,
Lithuania;[email protected]
3 FacultyofChemicalTechnology,KaunasUniversityofTechnology,Radvile˙nu˛St.19,LT-50254Kaunas,
Lithuania;[email protected]
* Correspondence:[email protected];Tel.:[phone]
Received:31March2017;Accepted:7June2017;Published:9June2017
Abstract: The most effective means to protect against bacterial invasion and to reduce the risk
of healthcare-associated infections are antibacterial components synthesis. In this study, a novel
process for the synthesis of organic–inorganic hybrid coatings containing silver nanoparticles is
presented. Silver nanoparticles and polymer formation proceeds simultaneously through the
in situ photoreduction of silver salt to silver nanoparticles and UV-crosslinking of bifunctional
aliphatic silicone acrylate. The nanocomposite films with 0.5–1.43 wt % of silver nanoparticles
concentration were obtained and investigated. The formation of silver nanoparticles in polymer
matrixwasconfirmedviaUV-visiblespectroscopy,X-raydiffraction(XRD),Fouriertransforminfrared
spectroscopy,scanningelectronspectroscopy,andenergydispersivespectroscopy. Ourinvestigations
clearlyshowtheformationofsilvernanoparticlesinsiliconeacrylatenetwork. Directphotoreduction
ofsilversaltbyUV-radiationintheorganicmediaproducedsilvernanoparticlesexhibitingcubic
crystalstructure. Thesizeofnanoparticleswasdeterminedtobenear20±5nm. Theantibacterial
activitiesofcoatingsweredeterminedusingthediscdiffusionanddirectcontactmethods.UV-curable
siliconeacrylatehybridcoatingsexhibitedantibacterialactivityagainstharmfulbacteriastrains.
Keywords: siliconeacrylate;silvernanoparticles;UV-curable;antibacterial
1. Introduction
Hybridmaterialscontainingorganic–inorganicconstituentshavegainedincreasedinterestdue
to their exceptional multifunctional properties. Different combinations allow the achievement of
exceptional electrical, catalytic, antibacterial, radiation-resistant, optical, thermal, and mechanical
properties. The potential applications of these materials can cover multiple fields, ranging from
engineeringtomedical[1–3]. Hybridorganic–inorganicmaterialscanbedefinedasnanocomposites
obtainedbyintimatemixingoforganicandinorganiccomponents.Thecommonrouteforthesynthesis
of these nanocomposites corresponds to the use of bridged and polyfunctional precursors, very
convenientsol–gelchemistry,andhydrothermalsynthesis[4]. Moresophisticatedtechniquesinclude
theuseofself-assembly,integrativesynthesisroutes,ortemplatedgrowthbyorganicsurfactants[5].
Organic–inorganicpolymerhybridsarebasedonthecombinationsofpolymermatrixeswith
metals,ceramics,orboth,andcanbepreparedbyavarietyofmethodsfordifferentapplications[6,7].
The most widely used organic matrices for the incorporation of inorganic nanoparticles include
Molecules2017,22,964;doi:10.3390/molecules22060964 www.mdpi.com/journal/molecules
Molecules2017,22,964 2of13
poly(methyl methacrylate), poly(vinyl chloride), polycarbonate, poly(ethylene terephthalate), and
polystyrene[8–12].
Silicone-modified acrylates form a new class of hybrid surface-modifying materials. These
substances are not merely blends of silicone and acrylates. Instead, they are hybrids obtained by
thechemicalincorporationofsiliconechainsintobasicacrylatestructure[13]. Silicone-acrylatesare
obtainedviatheradicalpolymerizationofacrylatesormethacrylateswithsiliconemacromers(i.e.,long
chains of polydimethylsiloxane) [14]. A single terminal reactive group (e.g., acrylic, methacrylic,
orvinylfunctionalgroups)isattheendofthechain. Hereby,insuchacomposition,siliconesoflow
surfacetensionarecombinedwithpolyacrylateshavingrelativelyhighsurfacetension. Thestrong
surface orientation of the silicone chains creates a very strong repulsion effect against water and
contaminants,suchasoilanddirt,andbacterialadhesion.
Biofilm-producingpathogensarefrequentlyassociatedwithhealthcare-associatedinfectionsthat
oftenneedsurgicalinterventionandthewidespreaduseofbroad-spectrumantibiotics[15]. Therisk
of such infections can be reduced by the modification of medical device materials with additives
possessingantimicrobialactivitywiththeabilitytodecreasebacterialadhesion.
Antibacterialcomponentssynthesisisthemosteffectivemeansofprotectingagainstcolonization
by bacterial pathogens and dangerous infections. Silver nanoparticles are a well-known potent
antimicrobial agent. A number of different synthesis methods of silver nanoparticles have been
developed,includingchemical,photochemical,andthermal[16–19]. Specifically,thephoto-induced
synthesisstrategyisveryattractive,asitisaversatileandconvenientprocesswithdistinguishing
advantages, such as space-selective fabrication [20]. Direct photoreduction is a very important
techniquefornanoparticlesynthesis,wherethedirectexcitationofametalsourceisemployedfor
theformationofmetalnanoparticles. Additionally,thereisnoneedforreducingagents. Moreover,
itcanbeemployedinvariousmedia,includingpolymercoatingsandthinfilms. Silvernanoparticles
withtheirattractivephysicochemicalproperties,size,andshapevarietypushtheintensiveresearch
with great scientific interest into a wide range of applications, including biological, chemical, and
physicalsectors[21].Theyareknowntoexerttoxicitythroughmechanismsdifferentfromconventional
antibiotics[22,23];theseincludedisruptionofcellmorphology,inactivationofvitalcellularenzymes
and proteins, DNA condensation [24], inhibition of ribosome interaction, accumulation at lethal
concentrationincells[25],proteindenaturation[26],modulationofcellularsignaling,generationof
reactiveoxygenspecies, oxidativestress[27], lossofDNAreplication, anddepletionofadenosine
triphosphate[28]. Thiscanbeespeciallyusefultoensureasterileenvironmentinmedicalapplications
(e.g.,surgicalsettings)bypreventingbacteriafromgrowingonoradheringtothesurface[29,30].
In the present study, a simple and fast hybrid organic–inorganic polymer synthesis method
is investigated. UV-cured aliphatic silicone acrylate coatings containing silver nanoparticles were
formedviaphoto-polymerizationtechnique. Thesecoatingswerecharacterizedemployingmultiple
analyticaltechniques.
2. ResultsandDiscussion
2.1. StructureAnalysis
In order to confirm the presence of silver nanoparticles in the fabricated UV-cured silicone
acrylatecompositestructure,UV-visiblespectroscopywasusedinthefirstinstance. Figure1shows
the UV-visible light spectrum of silicone acrylate-silver nanocomposite, presented as a Gaussian
functionwhichhasacharacteristicabsorptionpeakataround416nmduetothesurface-plasmon
resonancebandofthesilvernanoparticles[31,32]. Inmostcases,theabsorptionpeakisobservedin
therangeof410–460nmduetosurfaceplasmonresonanceinsilvernanoparticles. Itisknownthat
theabsorptionbandwidthoftheplasmonbanddependsheavilyonthesizeandinteractionwiththe
surroundingmedium[33]. Thebandcenteredat416nmintheUV-visiblelightabsorbancespectrum
Molecules 2017, 22, 964 3 of 12
Molecules2017,22,964 3of13
surrounding medium [33]. The band centered at 416 nm in the UV-visible light absorbance spectrum
(3(33377––884400 nnmm) )ininddiciacatetes sththee ppreresesennccee oof fsislivlveer rnnaannooppaartritcilceles sinin ththee ppoolylymmeer rmmaatrtirxix, ,yyieieldldiningg aa uunniqiquuee
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acarcyrylaltaet ecocommppoosistiet ecocoaatitningg. .
The structural peculiarities of synthesized silver nanoparticles were determined using X-ray
The structural peculiarities of synthesized silver nanoparticles were determined using X-ray
diffraction (XRD). Figure 2 shows an XRD pattern of a UV-cured aliphatic silicone acrylate
diffraction (XRD). Figure 2 shows an XRD pattern of a UV-cured aliphatic silicone acrylate
nanocomposite coating which contains silver nanoparticles. After the reduction process, silver
nanocomposite coating which contains silver nanoparticles. After the reduction process, silver
nanoparticles crystallized into cubic crystal structure (as per JCPDS file no. 04-016-5006) with space
nanoparticlescrystallizedintocubiccrystalstructure(asperJCPDSfileno. 04-016-5006)withspace
group Fm-3m (group number = 225), and the (2 0 0), (2 2 0), (3 1 1), (4 0 0), and (4 2 0) crystallographic
groupFm-3m(groupnumber=225),andthe(200),(220),(311),(400),and(420)crystallographic
plane orientations were observable. The diffraction pattern also reveals two broad components in the
planeorientationswereobservable. Thediffractionpatternalsorevealstwobroadcomponentsinthe
range of 2θ = 8.0–28.0°, which are attributed to the amorphous structure of aliphatic silicone acrylate.
rangeof2θ=8.0–28.0◦,whichareattributedtotheamorphousstructureofaliphaticsiliconeacrylate.
The chemical structure of UV-cured silicone acrylate nanocomposite was identified via FTIR
The chemical structure of UV-cured silicone acrylate nanocomposite was identified via FTIR
spectroscopy (Figure 3). The sharp peak at 1720 cm−1 is attributed to the stretching vibration of C=O
spectroscopy(Figure3). Thesharppeakat1720cm−1isattributedtothestretchingvibrationofC=O
groups, and a peak at 1258 cm−1 is related to C–O stretching vibration of the acrylic moiety [35].
groups, and a peak at 1258 cm−1 is related to C–O stretching vibration of the acrylic moiety [35].
An absorption peak at 2960 cm−1 corresponds to C–H symmetric and asymmetric stretching of
Anabsorptionpeakat2960cm−1correspondstoC–Hsymmetricandasymmetricstretchingofmethyl
methyl and methylene groups. The presence of this group is also confirmed by peaks at 1450 cm−1
and methylene groups. The presence of this group is also confirmed by peaks at 1450 cm−1 and
and 1387 cm−1, which are associated with a bending vibration of C–H groups.
1387cm−1,whichareassociatedwithabendingvibrationofC–Hgroups.
The silicone part could be characterized by Si–CH3 symmetric deformation and stretching
The silicone part could be characterized by Si–CH symmetric deformation and stretching
3
vibration at 1260 and 788 cm−1, respectively. The bands in the 1070–1015 cm−1 region are attributed to
vibrationat1260and788cm−1,respectively. Thebandsinthe1070–1015cm−1regionareattributed
Si–O–Si stretching vibration [36–39]. There are no noticeable shifts in the peak positions when the
toSi–O–Sistretchingvibration[36–39]. Therearenonoticeableshiftsinthepeakpositionswhenthe
silicone acrylate polymer without nanoparticles and polymer with embedded silver nanoparticles
siliconeacrylatepolymerwithoutnanoparticlesandpolymerwithembeddedsilvernanoparticles
are compared, except for a broader and stronger O–H absorption band at 3350 cm−1. It can be
are compared, except for a broader and stronger O–H absorption band at 3350 cm−1. It can be
assumed that the OH group is engaged in the formation of hydrogen bonds or complexation with
assumedthattheOHgroupisengagedintheformationofhydrogenbondsorcomplexationwith
metal nanoparticles.
metalnanoparticles.
Molecules2017,22,964 4of13
Molecules 2017, 22, 964 4 of 12
Molecules 2017, 22, 964 4 of 12
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wFiigtuhrcer y2s. tXalRloDg rpaapttheircnp olaf nseilvoerrie nnatantoiopnarstiocflessi lvcoenrtpahinaisnegi nUdVic-cauterded. silicone acrylate composite coating
with crystallographic plane orientations of silver phase indicated.
with crystallographic plane orientations of silver phase indicated.
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Figure 3. Fourier transform infrared (FTIR) spectra of UV-curable aliphatic silicone acrylate coating
(FFaii)gg wuurrieeth 33o..u FFtoo auunrrdiiee r(rb tt)rr aawnnisstfhfoo r1rmm.0 iwinnftfr r%aarr eoedfd (s F(ilFTvTIeRIrR )n)s apsnpeoceptcrtaarraot ifoclUfe UsV. -Vcu-cruarbalbelael iaplhipahtiactisci lisciolincoenaec rayclraytelactoe actoinagtin(ag)
w(ai)t whoituhtoauntd a(nbd) (wbi)t hw1it.h0 1w.0t %wto %fs oilfv seirlvnearn noapnaortpicalretsic.les.
The surface morphologies of the silicone acrylate–silver nanocomposites were investigated by
The surface morphologies of the silicone acrylate–silver nanocomposites were investigated by
scannTihneg seulercfatrcoenm moircprhosocloogpiye s(SoEfMth) easnidli coenneergaycr ydliastpee–rssiilvvee rXn–arnayo csopmecptorosistceospwy e(rEeDiSn)v easntiaglyasteisd.
scanning electron microscopy (SEM) and energy dispersive X–ray spectroscopy (EDS) analysis.
Tbhyes cparnenseinngtedel eScEtMro nmmicircorgorsacpohpsy ((FSiEguMr)e a4n) dcleeanrelryg syhdowisp tehres ifvoermXa–triaoyn sopf escitlvroesrc noapnyo(pEaDrtSic)leasn ainly tshies.
The presented SEM micrographs (Figure 4) clearly show the formation of silver nanoparticles in the
aTlhipehparteics esniltiecdonSeE Macrmyliactreo gnreatpwhosr(kF. igSiulvreer4 )ncalneaorplayrstihcolews athree fworemll adtiisopneorsfesdil viner tnhaen pooplayrmtieclre msiantrtihxe.
aliphatic silicone acrylate network. Silver nanoparticles are well dispersed in the polymer matrix.
Haloipwheavteicr, stihliec oinncereaacsrey loaft esinlveetrw noarnko.pSairlvtieclrens acnoonpceanrttricalteiosna rceauwseesl ltdheis pacecrusemduilnatitohne opfo nlyamnoepramrtaictlreisx,.
However, the increase of silver nanoparticles concentration causes the accumulation of nanoparticles,
wHhoiwchev fearc,itlihteieisn tchreea fsoermofastiilovne ronf acnluosptaerrtsi c(lFeisgucorne c4ebn)t.r Tathioe nEcDaSu ssepsetchtreuamcc oufm thuela stiiolincoonfen aacnroyplaatret iaclneds,
which facilities the formation of clusters (Figure 4b). The EDS spectrum of the silicone acrylate and
swilhviecrh nfaanciolictoiemspthoesitfeo rimn aFtiigounreo f5 calulssot ecrosn(fFirimguerde t4hbe) .foTrhmeaEtDioSn sopf escitlvruemr noafntohpeasritliiccloesn eina tchrye lsaitleicaonnde
silver nanocomposite in Figure 5 also confirmed the formation of silver nanoparticles in the silicone
asiclrvyelratnea mnoactorimx.p ositeinFigure5alsoconfirmedtheformationofsilvernanoparticlesinthesilicone
acrylate matrix.
acrylCatheamngaetrsi xo.f surface free energy due to the incorporation of silver nanoparticles into the aliphatic
Changes of surface free energy due to the incorporation of silver nanoparticles into the aliphatic
silicone acrylate coating were determined using contact angle (CA) measurements. The 5 μL water
silicone acrylate coating were determined using contact angle (CA) measurements. The 5 μL water
droplet measurements were employed (Figure 6). The UV-cured aliphatic silicone acrylate was
droplet measurements were employed (Figure 6). The UV-cured aliphatic silicone acrylate was
found to be hydrophobic (CA = 94° ± 1°). The formation of silver nanoparticles in aliphatic silicone
found to be hydrophobic (CA = 94° ± 1°). The formation of silver nanoparticles in aliphatic silicone
Molecules 2017, 22, 964 5 of 12
Molecules 2017, 22, 964 5 of 12
acrylate during the UV-curing process reduces the contact angle, because metallic silver particles
acrylate during the UV-curing process reduces the contact angle, because metallic silver particles
have a large surface energy. Importantly, CA values do not depend on the nanoparticles
have a large surface energy. Importantly, CA values do not depend on the nanoparticles
concentration—an increase of the content of silver nanoparticles from 0.5 to 1.43 wt % resulted in a
concentration—an increase of the content of silver nanoparticles from 0.5 to 1.43 wt % resulted in a
CA value of 84 ± 2°. It is possible that with the increase of metallic silver concentration the
CA value of 84 ± 2°. It is possible that with the increase of metallic silver concentration the
nanoparticles tend to accumulate, thereby creating clusters. The smaller the silver particles, the
nanoparticles tend to accumulate, thereby creating clusters. The smaller the silver particles, the
higher are their surface energy and surface activity. According to [40], polymer composites exhibit
higher are their surface energy and surface activity. According to [40], polymer composites exhibit
hMioglehceulre sc2o0n1t7a,2c2t ,a9n64gle when the concentration of silver nanoparticles is increased due to the forma5toifo1n3
higher contact angle when the concentration of silver nanoparticles is increased due to the formation
of silver clusters of large dimensions.
of silver clusters of large dimensions.

(a) (b)
(a) (b)
Figure 4. SEM micrographs of silver-containing UV-cured aliphatic silicone acrylate coating: (a)
Figure4.SEMmicrographsofsilver-containingUV-curedaliphaticsiliconeacrylatecoating:(a)0.5wt%
Figure 4. SEM micrographs of silver-containing UV-cured aliphatic silicone acrylate coating: (a)
0.5 wt % silver nanoparticles; (b) 1.43 wt % silver nanoparticles.
silvernanoparticles;(b)1.43wt%silvernanoparticles.
0.5 wt % silver nanoparticles; (b) 1.43 wt % silver nanoparticles.

Element wt % at % Errorin%
Element wt % at % Errorin%
Carbon K 41.53 61.16 6.79
CEarlebmone Kn t 41w.5t3 % 61a.1t6% Er6r.o7r9 in%
Oxygen K 10.81 11.95 3.47
Oxygen K 10.81 11.95 3.47
CSailrvbeor nL K 64.817.5 3 16.11.31 6 06.1.79 9
Silver L 6.87 1.13 0.19
OSixliycgonen KK 318.04.88 1 2141.2.49 5 13.6.43 7
Silicon K 38.48 24.24 1.63
SilverL 6.87 1.13 0.19
Figure 5. EDS spectrum of silver containing (1 wt %) UV-cured aliphatic silicone acrylate coating.
SiliconK 38.48 24.24 1.63
Figure 5. EDS spectrum of silver containing (1 wt %) UV-cured aliphatic silicone acrylate coating.
Figure5.EDSspectrumofsilvercontaining(1wt%)UV-curedaliphaticsiliconeacrylatecoating.
Changesofsurfacefreeenergyduetotheincorporationofsilvernanoparticlesintothealiphatic

siliconeacrylatecoatingweredeterminedusingcontactangle(CA)measurements. The5µLwater
dropletmeasurementswereemployed(Figure6). TheUV-curedaliphaticsiliconeacrylatewasfound
tobehydrophobic(CA=94◦ ±1◦). Theformationofsilvernanoparticlesinaliphaticsiliconeacrylate
duringtheUV-curingprocessreducesthecontactangle,becausemetallicsilverparticleshavealarge
surfaceenergy. Importantly,CAvaluesdonotdependonthenanoparticlesconcentration—anincrease
Molecules2017,22,964 6of13
ofthecontentofsilvernanoparticlesfrom0.5to1.43wt%resultedinaCAvalueof84 ± 2◦. Itis
possiblethatwiththeincreaseofmetallicsilverconcentrationthenanoparticlestendtoaccumulate,
therebycreatingclusters.Thesmallerthesilverparticles,thehigheraretheirsurfaceenergyandsurface
activity. Accordingto[40],polymercompositesexhibithighercontactanglewhentheconcentrationof
silvernanoparticlesisincreasedduetotheformationofsilverclustersoflargedimensions.
Molecules 2017, 22, 964 6 of 12

(a) (b)
(c)
Figure 6. Five microliter water droplet on UV-cured aliphatic silicone acrylate coating (a) without
Figure6. FivemicroliterwaterdropletonUV-curedaliphaticsiliconeacrylatecoating(a)without
nanoparticles and containing (b) 0.5 wt % and (c) 1.43 wt % of silver nanoparticles.
nanoparticlesandcontaining(b)0.5wt%and(c)1.43wt%ofsilvernanoparticles.
2.2. Antibacterial Activity
2.2. AntibacterialActivity
The antibacterial activity of the hybrid silver nanoparticles containing UV-cured aliphatic silicone
TheantibacterialactivityofthehybridsilvernanoparticlescontainingUV-curedaliphaticsilicone
acrylate coatings obtained via in situ formation method was investigated by different methods. At first,
acrylate coatings obtained via in situ formation method was investigated by different methods.
qualitative assessment of the antibacterial activity was used by measuring the diameter of the
At first, qualitative assessment of the antibacterial activity was used by measuring the diameter
growth inhibition zone. Disks of hybrid silicone acrylate films were placed on the agar plates
ofthegrowthinhibitionzone. Disksofhybridsiliconeacrylatefilmswereplacedontheagarplates
inoculated with the Gram-negative bacteria Escherichia coli ATCC 25922 and the Gram-positive
inoculatedwiththeGram-negativebacteriaEscherichiacoliATCC25922andtheGram-positivebacteria
bacteria Staphylococcus aureus ATCC 25923. Figure 7 illustrates the antibacterial activity of the silicone
StaphylococcusaureusATCC25923. Figure7illustratestheantibacterialactivityofthesiliconeacrylate
acrylate organic–inorganic hybrid coatings containing different amounts of silver nanoparticles (from
organic–inorganichybridcoatingscontainingdifferentamountsofsilvernanoparticles(from0.5to
0.5 to 1.43 wt %). The antibacterial effect increased gradually as the silver nanoparticles concentration
1.43 wt %). The antibacterial effect increased gradually as the silver nanoparticles concentration
increases. The zone of inhibition for E. coli increased by 20–30%, while for S. aureus this increase was
increases. ThezoneofinhibitionforE.coliincreasedby20–30%,whileforS.aureusthisincreasewas
noticeably higher, at 60–90% (Table 1). Importantly, silver nanoparticles synthesized during silicone
noticeablyhigher,at60–90%(Table1). Importantly,silvernanoparticlessynthesizedduringsilicone
acrylate polymerization process acted as an effective antibacterial agent.
acrylatepolymerizationprocessactedasaneffectiveantibacterialagent.
Table1. MeaninhibitionzoneofsilvernanoparticlescontainingUV-curedsiliconeacrylatecoating
againstdifferentpathogenspresentedasanintervalrange.
InhibitionZone*
SilverNanoparticles
Concentration(wt%) E.coli S.aureus
0 0 0
0.5 1.14–1.27 1.61–1.66
1.0 1.15–1.34 1.82–1.94
1.43 E. col1i .26–1.34 1.83–2.01
Notes:*Threereplicateswerecarriedoutforeachtesting.

S. aureus
Figure 7. Antibacterial activity against E. coli and S. aureus of UV-cured silicone acrylate composite
samples with different concentration of silver nanoparticles: (a,d) 0.5 wt %; (b,e) 1 wt %;
(c,f) 1.43 wt %.
Molecules 2017, 22, 964 6 of 12

(a) (b)
(c)
Figure 6. Five microliter water droplet on UV-cured aliphatic silicone acrylate coating (a) without
nanoparticles and containing (b) 0.5 wt % and (c) 1.43 wt % of silver nanoparticles.
2.2. Antibacterial Activity
The antibacterial activity of the hybrid silver nanoparticles containing UV-cured aliphatic silicone
acrylate coatings obtained via in situ formation method was investigated by different methods. At first,
qualitative assessment of the antibacterial activity was used by measuring the diameter of the
growth inhibition zone. Disks of hybrid silicone acrylate films were placed on the agar plates
inoculated with the Gram-negative bacteria Escherichia coli ATCC 25922 and the Gram-positive
bacteria Staphylococcus aureus ATCC 25923. Figure 7 illustrates the antibacterial activity of the silicone
acrylate organic–inorganic hybrid coatings containing different amounts of silver nanoparticles (from
0.5 to 1.43 wt %). The antibacterial effect increased gradually as the silver nanoparticles concentration
increases. The zone of inhibition for E. coli increased by 20–30%, while for S. aureus this increase was
nMootleiccuelaesb2ly01 7h,i2g2h,9e6r4, at 60–90% (Table 1). Importantly, silver nanoparticles synthesized during sili7coofn1e3
acrylate polymerization process acted as an effective antibacterial agent.

E. coli
Molecules 2017, 22, 964 7 of 12
Table 1. Mean inhibition zone of silver nanoparticles containing UV-cured silicone acrylate coating
against different pathogens presented as an interval range.
InhibitionZone *
Silver Nanoparticles Concentration (wt %)
E. coli S. aureus
0 0 0

0.5 1.14–1.27 1.61–1.66
S. aureus
1.0 1.15–1.34 1.82–1.94
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Notes: * Three replicates were carried out for each testing.
(c,f) 1.43 wt %.

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pfoerrfqourmanetdifi cfaotri oqnuoafntthifeicbaaticotenr iocifd tahlee fbfeacctt.eAriccioduanl teofffetchte. cAo locnouienstr eovfe tahleed ctohlaotntihees UreVv-ecaulreedd athliapth tahtiec
UsiVlic-counreedac arylilpahteatoicr gsailnicico–nien oarcgryanlaitcec oormgapnoisci–teincooragtainngic sciogmnipfiocasnittel ycoinathiinbgit seidgnthifeicgarnotwlyt hinohfibSi.teadur tehues ,
garsoswhothw nofi nSF. iaguurreeu8s,. Tash eshbaocwtenr iaind eFcirgeuarsee d8.t oTzheer obiancttherrieae oduetcroefafsoeudr rteop zeteirtoio nins atfhtererein ocuubt aotifo fnofuorr
r2e4phet.itions after incubation for 24 h.

FFiigguurree 88.. BBacatcetreirciicdiadla lefeffefcetc toof fUUVV-c-cuureredd ssiliilcicoonnee aaccrryylalatete ccooaatitningg ccoonnttaainininingg 11.4.433%% ooff ssiillvveerr
nnaannooppaarrttiicclleess oonn SS.. aauurreeuuss aafftteerr ddiiffffeerreenntt ccoonnttaacctt ttiimmee..
The antibacterial effect could be the result of the dissociation of silver nanoparticles into Ag+
ions and their accumulation on the coating surface. Silver ions accumulate on the bacterial cell
surface, which interacts with the microbial membrane to cause structural change, permeability, and
finally bacterial cell death [31]. The influence on bacteria viability depends extremely on the size,
shape, and concentration of nanoparticles [32,33]. In [34], it is reported that silver nanoparticles
accumulation on the E. coli cell membrane makes gaps in the entirety of the bilayer, which
predisposes it to the increased penetrability and finally bacterial cell death [31].
The model of the silicone acrylate formation with simultaneous conversion of silver perchlorate
to silver nanoparticles and possible bacterial inactivation mechanism is presented in Figure 9.
Molecules2017,22,964 8of13
TheantibacterialeffectcouldbetheresultofthedissociationofsilvernanoparticlesintoAg+ions
andtheiraccumulationonthecoatingsurface. Silverionsaccumulateonthebacterialcellsurface,
whichinteractswiththemicrobialmembranetocausestructuralchange,permeability,andfinally
bacterialcelldeath[31]. Theinfluenceonbacteriaviabilitydependsextremelyonthesize,shape,and
concentrationofnanoparticles[32,33]. In[34],itisreportedthatsilvernanoparticlesaccumulationon
theE.colicellmembranemakesgapsintheentiretyofthebilayer,whichpredisposesittotheincreased
penetrabilityandfinallybacterialcelldeath[31].
Themodelofthesiliconeacrylateformationwithsimultaneousconversionofsilverperchlorate
tMoosleicluvleesr 2n01a7n, o22p, a96r4ti clesandpossiblebacterialinactivationmechanismispresentedinFigure9. 8 of 12

FFiigguurree 99.. SScchheemmee ooff tthhee ffoorrmmaattiioonn ooff ssiillvveerr nnaannooppaarrttiicclleess ccoonnttaaiinniinngg UUVV--ccuurreedd aalliipphhaattiicc ssiilliiccoonnee aaccrryyllaattee
ccooaattiinngg aanndd ppoossssiibbllee bbaacctteerriiaall iinnaacctitvivaatitoionn mmeecchhaannisimsm vviai ainintetrearcatciotino nwwithit hanatnibtiabcatectreiarli aclocaotiantgin sgusrufarcfea.c e.
According to the studies of other researchers, silver nanoparticles possess a strong antibacterial
Accordingtothestudiesofotherresearchers,silvernanoparticlespossessastrongantibacterial
and antiviral activity. Acting with microorganisms, they impact the growth of bacterial biofilms.
andantiviralactivity. Actingwithmicroorganisms,theyimpactthegrowthofbacterialbiofilms. Silver
Silver nanoparticles interact with bacterial surfaces, as well as with their particular structure [22–29].
nanoparticlesinteractwithbacterialsurfaces,aswellaswiththeirparticularstructure[22–29]. When
When the size of silver nanoparticles is larger than 10 nm, the predominant bacteria inactivation
thesizeofsilvernanoparticlesislargerthan10nm,thepredominantbacteriainactivationmechanism
mechanism is through silver ions [30]. Although nanoparticles’ antibacterial effects have been
isthroughsilverions[30]. Althoughnanoparticles’antibacterialeffectshavebeendescribedindetail,
described in detail, their mechanism of action still requires further elucidation both from chemical
theirmechanismofactionstillrequiresfurtherelucidationbothfromchemicalandbiologicalpoints
and biological points of view.
ofview.
3. Materials and Methods
3. MaterialsandMethods
33..11.. MMaatteerriiaallss
BBiiffuunnccttiioonnaall aalliipphhaattiicc ssiilliiccoonnee aaccrryyllaattee oolliiggoommeerr wwiitthh vviissccoossiittyy ooff 5500––7700 PPaass,, ssuuiittaabbllee ffoorr uussee iinn
UUVV aanndd eelleeccttrroonn bbeeaamm ccuurriinngg ccoommppoossiitteess ((CCNN99880000)),, wwaass ppuurrcchhaasseedd ffrroomm SSaarrttoommeerr ((AArrkkeemmaa GGrroouupp,,
CCoolloommbbeess CCeeddeexx,, FFrraannccee)).. IIttss ppoollyymmeerriizzaattiioonn wwaass ccaarrrriieedd oouutt uussiinngg mmiixxttuurree bbiiss((22,,66--ddiimmeetthhooxxyybbeennzzooyyll))--
22,,44,,44--ttrriimmeetthhyyll ppeennttyylplphhoosspphhinineeooxxidideea nandd2 -2h-yhdyrdorxoyx-y2--2m-methetyhl-y1l--p1-hpehneynl-yplr-poproapna-1n--o1n-oenine ainr aat iroatoifo1 o:3f
(1I:r3g a(Icrugraec1u7r0e0 1)7a0s0p) haost opihnoittioatinorit,isautoprp, liseudpbpylieBdA bSFy (BSAouStFh fi(Seoldu,tMhfiIe,lUdS, AM).I,S UilvSeAr)p. eSriclhvleorr apteercAhgloCrlaOte
4
(AAggC>lO540 (.A5%g )>a 5n0d.5a%cr)y alnicda accidrywlice raecpidu wrcehraes epdurfrcohmaseSdig fmroamA Sldigrmicha (ASltd. Lriochu i(sS,tM. LOo,uUisS, AM)O. , USA).
3.2. Nanocomposite Preparation
Silver perchlorate salt (0.05–0.15 g) was first dissolved in 1.00 mL of acrylic acid, and then
0.1–0.3 g of photoinitiator Irgacure 17,000 was added (in ratio AgClO4:photoinitiator = 1:2). The mixture
was continuously stirred at ambient temperature until a homogeneous solution was obtained. Silver
nanoparticles precursor, solvent, and photoinitiator mixture were mixed with 4.00 g of bifunctional
aliphatic silicone acrylate oligomer CN9800 for 10 min at ambient temperature until a homogeneous
suspension was formed and then kept under vacuum for 10 min at ambient temperature to remove
air bubbles. After that, the obtained mixture was poured onto a glass plate. The polymerization and
silver salt photoreduction to silver nanoparticles initiated by Irgacure 1700 was carried out with a
medium pressure mercury lamp (1 kW, Hibridas Photosensitive Paste UV Exposure Unit MA-4).
After irradiation of the composition for 120 s, silicone acrylate coatings having a thickness of 1 mm
without or with silver nanoparticles of 0.5–1.43 wt % concentration were formed. Higher silver
Molecules2017,22,964 9of13
3.2. NanocompositePreparation
Silverperchloratesalt(0.05–0.15g)wasfirstdissolvedin1.00mLofacrylicacid,andthen0.1–0.3g
ofphotoinitiatorIrgacure17,000wasadded(inratioAgClO :photoinitiator=1:2). Themixturewas
4
continuously stirred at ambient temperature until a homogeneous solution was obtained. Silver
nanoparticlesprecursor,solvent,andphotoinitiatormixtureweremixedwith4.00gofbifunctional
aliphaticsiliconeacrylateoligomerCN9800for10minatambienttemperatureuntilahomogeneous
suspensionwasformedandthenkeptundervacuumfor10minatambienttemperaturetoremove
airbubbles. Afterthat,theobtainedmixturewaspouredontoaglassplate. Thepolymerizationand
silversaltphotoreductiontosilvernanoparticlesinitiatedbyIrgacure1700wascarriedoutwitha
mediumpressuremercurylamp(1kW,HibridasPhotosensitivePasteUVExposureUnitMA-4). After
irradiationofthecompositionfor120s,siliconeacrylatecoatingshavingathicknessof1mmwithout
Molecules 2017, 22, 964 9 of 12
orwithsilvernanoparticlesof0.5–1.43wt%concentrationwereformed. Highersilvernanoparticles
concentrationinpolymermatrixviainsituphoto-reductionmethodbecomesproblematic. Visual
nanoparticles concentration in polymer matrix via in situ photo-reduction method becomes problematic.
obsVeirsvuaatl ioobnseorfvtahteiorne souf ltthaen rtemsualttaenrita mlsastheroiwalss sthhoawttsh tehayte tlhloew yieslhlowfilimshs fwilmitsh owuitthsoiluvte srilnvaern onpanaortpiaclretisclheas ve
aghoaovde oap gtiocoadl troapntiscpala rteranncsyp;amreenacny;w mhielaen,wthhoislee, wthiothsee wmibthe dedmebdedsidlveder snilavnero pnaarntoicplaerstifculersn ifsuhrnaisbhr oaw n
colborroawtinon co(Floirgautiroen1 (0F)i.gure 10).

(a) (b)
FigFuigreur1e0 1.0. AAlliipphhaattiicc ssiililcicoonnee acarcyrlyaltae tceoacotiantgi nfgilmfisl:m (as): w(ait)howuitt hsiolvuetr snilavneorpanratincolepsa; r(tbi)c lwesi;th( bsi)lvweri th
silvnearnnopanarotpicalertsi.c les.
3.3. Nanocomposite Characterization
3.3. NanocompositeCharacterization
An optical spectrometer Avantes that is composed of a deuterium halogen light source
AnopticalspectrometerAvantesthatiscomposedofadeuteriumhalogenlightsource(AvaLight
(AvaLight DHc, Avantes, Apeldoorn, The Netherlands) and spectrometer (Avaspec-2048, Avantes,
DHc,Avantes,Apeldoorn,TheNetherlands)andspectrometer(Avaspec-2048,Avantes,Apeldoorn,
Apeldoorn, The Netherlands) was used to record UV-visible light absorbance spectra. The analysis
TheNetherlands)wasusedtorecordUV-visiblelightabsorbancespectra. Theanalysiswasperformed
was performed in the wavelength range of 337–840 nm.
inthewavelengthrangeof337–840nm.
The structural peculiarities of the synthesized silver nanoparticles was determined using X-ray
ThestructuralpeculiaritiesofthesynthesizedsilvernanoparticleswasdeterminedusingX-ray
diffractometer D8 Discover (Bruker AXS GmbH, Billerica, MA, USA) with parallel beam focusing
diffractometer D8 Discover (Bruker AXS GmbH, Billerica, MA, USA) with parallel beam focusing
geometry of Cu Kα1 (λ = 0.154 nm) radiation. Processing of the resultant diffractograms was
geometryofCuKα (λ=0.154nm)radiation.Processingoftheresultantdiffractogramswasperformed
performed with D1IFFRAC.EVA software (version 3.0, Billerica, MA, USA). Phase identification and
withDIFFRAC.EVAsoftware(version3.0,Billerica,MA,USA).Phaseidentificationandinterpretation
interpretation involved matching the diffraction pattern of free-standing silver nanoparticles
invcoolnvteadinimnga tcshiliicnognteh eacdryiflfartaec tciooantipnagt tetor npoafttferrenes- sotaf nsdininggles-pilhvaesre nraenfoerpeanrcteic lmesatceorniatlasi. nRinegfesreilniccoe ne
acrpyaltatteerncso aftrionmg ttohep aJtotienrtn sCoofmsminitgtleee- pohna sPeorwefdeerre nDceiffmraacttieornia lSst.anRdeaferdresn–Icneteprantatteironnsalf rComenttrhee fJoori nt
CoDmimffriattcetieoonn DPaotaw (dJCerPDDSif–fIrCacDtiDo)n dSattaanbdaaserd ws–eIrne tuesrenda.t ionalCentreforDiffractionData(JCPDS–ICDD)
databasTehwe eFroeuursieerd .transform infrared (FTIR) spectra were obtained by using Spectrum GX FTIR
speTchtreomFoeuterri e(rPetrraknins-fEolrmmeri,n Wfraarlethdam(F,T MIRA), spUeScAtr)a ewqueirpepeodb tawinitehd ab hyouriszionngtaSl paetctetnruumateGd XtoFtaTl IR
spercetfrleocmtieotne r((HPeArTkiRn)- Ealmcceers,sWorayl.t hTahme, MHAA,TURS AF)TeIRqu ispppeecdtraw iothf ashamorpizleosn tawleartete nruecaoterddetdo taaltr erfloeocmtio n
(HtAeTmRp)eraactcuerses oinr yt.heT wheavHenAuTmRbFeTr IrRansgpee ocft r4a00o0f–s6a5m0 pcmle−s1 wwietrhe ar erecsoorlduetdiona torfo 1o cmmt−1e. mEapcehr astpuercetriunmt he
wawveans uamvebreargerdan fgroemof 1460 0sc0a–n6s5 0atc ma s−c1anw irtahtea orfe s0o.2l uctmio∙ns−1o. fC1ocllmec−te1d. Esapcehctsrap ewcterruem prwocaesssaevde rwagitehd tfhreo m
16Sspcaecntsruamt a® vs5c.a0n.1r saotfetwoafr0e. 2frcomm ·Ps−er1k.inC-oElllmecetre (dWsapltehcatrma, wMeAre, UpSrAoc)e. ssed with the Spectrum® v5.0.1
The CA measurements were performed at room temperature (23 °C) using sessile drop method.
softwarefromPerkin-Elmer(Waltham,MA,USA).
One droplet of deionized water (~5 μL) was deposited on the sample surface. Optical images of the
droplet were obtained, and contact angle was measured using a method based on B-spline snakes
(active contours).
SEM analysis of coatings structure was performed with a microscope Quanta 200 FEG (FEI,
Eindhoven, The Netherlands) operating in a low vacuum at 20.0 kV using an LDF detector. The
chemical analysis of nanocomposites was performed by EDS technique with a Bruker XFlash 4030
detector (Berlin, Germany) (accelerating voltage 10 kV, distance between the bottom of the objective
lens and the object 10 mm).
3.4. Antibacterial Studies
The antibacterial activities of the silver nanoparticles containing UV-cured aliphatic silicone
acrylate organic–inorganic composite coatings were tested using standard strains of E. coli and
Molecules2017,22,964 10of13
TheCAmeasurementswereperformedatroomtemperature(23◦C)usingsessiledropmethod.
Onedropletofdeionizedwater(~5µL)wasdepositedonthesamplesurface. Opticalimagesofthe
dropletwereobtained,andcontactanglewasmeasuredusingamethodbasedonB-splinesnakes
(activecontours).
SEM analysis of coatings structure was performed with a microscope Quanta 200 FEG (FEI,
Eindhoven, The Netherlands) operating in a low vacuum at 20.0 kV using an LDF detector.
ThechemicalanalysisofnanocompositeswasperformedbyEDStechniquewithaBrukerXFlash4030
detector(Berlin,Germany)(acceleratingvoltage10kV,distancebetweenthebottomoftheobjective
lensandtheobject10mm).
3.4. AntibacterialStudies
The antibacterial activities of the silver nanoparticles containing UV-cured aliphatic silicone
acrylateorganic–inorganiccompositecoatingsweretestedusingstandardstrainsofE.coliandS.aureus.
Twomethods—theagardiskdiffusiontestandthedirectcontacttest—werechosenforevaluationof
antibacterialactivity.
Forthediskdiffusionprocedure,E.coliandS.aureuswereculturedinTryptoneSoyaBroth(TSB)
solution.Followingthat,about1mLofthebacteriawaspipettedfromtheovernightphaseintoanother
flaskwith30mLoffreshly-preparedTSB.Bacteriaweregrownat37◦Cintheincubatorshakerfor
another5htoobtainbacteriawithhigheractivityandmoreviabilitythaninothergrowthphases.
Commercially-availableMueller-Hintonagarplateswereusedinantibacterialtesting. Thestationary
phasebacteriumwasdilutedintoaconcentrationofabout103CFU/mLwithNaClsolution(0.85%).
Dilutedsuspensions(0.1mL)ofthepreviouslymentionedmicroorganismsweretransferredandspread
ontothesolidsurfaceoftheplate. Intheinhibitionexperiment,thesilvernanoparticlescontaining
siliconeacrylatediscswereplacedonMueller-HintonagarplatesinoculatedwithE.coliandS.aureus
andincubatedat37◦Cfor24h. Later,thezoneofinhibitionwasnotedandtabulated. Threereplicates
werecarriedoutforeachtesting.
Toquantifytheantimicrobialactivityofthehybridcomposite,amodifieddirectcontacttestwas
used [41]. Overnight cultures were diluted into a concentration at 105 CFU/mL with 0.85% NaCl
suspension. Thebacterialsuspension(0.1mL)wasdepositedonglassslidesandcoveredwiththin
film(thicknessca. 0.4mm; dimension25mm × 25mm)ofUV-curedsiliconeacrylatetofacilitate
directcontactofthebacteriawiththesilvernanoparticlesontheacrylatesiliconecompositesurface.
Two-layer assembly with the liquid film containing bacteria were placed in petri plate containing
1mLofphosphate-bufferedsaline(PBS)tomaintainadampenvironmentandincubatedatroom
temperaturefor3,6,and24h. Afterthat,9mLofPBSwasaddedintoeachpetriplatetodetachthe
assemblyofglassslideandpolymerfilmbysoftshaking. Lastly,thebacterialsuspensionsweregrown
innutrientagarmediumtocountcoloniesandcalculateCFU/mL.Lowerthan500bacterialcounts
intheculturemediumwereattributedtotheantibacterialactivityoftheinvestigatedhybridcoating.
Fourrepetitionswerecarriedoutforeachtesting.
4. Conclusions
Thealiphaticsiliconeacrylatecompositescontainingsilvernanoparticlescanbeeasilysynthesized
viain-situphotopolymerizationtechnique. Theformationofapolymericmatrixandtheconversion
of silver salt into silver nanoparticles proceeds simultaneously. Silicone acrylate composites with
0.5–1.43wt%ofsilvernanoparticleswereobtained. SiliconeacrylatepolymerOHgroupisengagedin
theformationofhydrogenbondsorcomplexationwithsilvernanoparticles. Thesynthesizedsilver
nanoparticleshavingasizeof20±5nmarehomogeneouslydispersedinsiliconeacrylatehybrid
coating. DirectphotoreductionofsilversaltbyUV-radiationintheorganicmediaproducedsilver
nanoparticlesexhibitingcubiccrystalstructurewithspacegroupFm-3m.Silvernanoparticlesincreased
thesurfaceenergyofthesiliconeacrylatehybridcoating. Aliphaticsiliconeacrylatehybridcoatings
exhibitedantibacterialactivity,andcanbeusedtoprotectagainstbacterialinvasion. Thepredominant