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Journal of Photochemistry and Photobiology A: Chemistry 283 (2014) 29–37

Contents lists available at ScienceDirect

Journal of Photochemistry and Photobiology A:Chemistry

jo ur nal homep age: www.elsev ier .com/ locate / jphotochem

hotocatalytic antibacterial activity of TiO2–SiO2 thin films: The effectf composition on cell adhesion and antibacterial activity

eril Erdural, Ufuk Bolukbasi, Gurkan Karakas ∗

iddle East Technical University, Chemical Engineering Department, Dumlupinar Blv., 06800 Ankara, Turkey

r t i c l e i n f o

rticle history:eceived 11 February 2014eceived in revised form 24 March 2014ccepted 29 March 2014vailable online 8 April 2014

eywords:

a b s t r a c t

Colloidal solutions of SiO2–TiO2 mixed oxides having different SiO2 contents were synthesized by sol–geltechnique and thin films were deposited over glass slides by using dip coating technique. The sampleswere characterized by XRD, SEM, AFM and methylene blue adsorption methods. The surface free energyof thin films was measured by sessile drop method by using diiodomethane, glycerol and water as probeliquids. The photocatalytic antibacterial activity of the samples was determined for Escherichia coli XL1-blue by using artificial solar irradiation. The surface characterization studies revealed the presence of

hotocatalysisntibacterial surfacesiO2

iO2

acterial adhesion. coli

small crystallites of anatase phase, enhanced dispersion of TiO2 particles and increasing surface freeenergy at elevated SiO2 content. Highest photocatalytic antibacterial activity was obtained over 92 wt%SiO2 containing sample and strong correlation was found between the bacterial adhesion and antibacte-rial activity. Surface-adhered E. coli cells having direct contact with TiO2 particles were inactivated at ahigher rate than suspended cells.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Microbial contamination and infection risks can be controlledy disinfection of surfaces by using chemical agents (biocides) or

rradiation sources. The semiconductor metal oxide photocatalystsre promising alternatives to conventional disinfection techniques1–8]. Photocatalytic oxidation and reduction reactions over metalxides have been extensively studied in literature and TiO2 is theost commonly used semiconductor metal oxide having sufficient

hemical stability, high photocatalytic efficiency, non-toxicity, andow cost [9,10]. Many factors affect the photocatalytic performancef semiconductor metal oxides such as, bulk defects, impurities,article size, specific surface area, crystal structure, porosity, sur-ace acidity and hydrophobicity [10–19].

The photocatalytic disinfection and inactivation kineticsave been studied for many model microorganisms such asscherichia coli, Lactobacillus acidophilus, Streptococcus sobrinus,almonella cholerasuis, Aspergilus niger, Saccharomyces cervisia and

he photocatalytic disinfection has been explained by several

echanisms [1–5,7–9]. Although, the precise mechanism is stillnder debate, the attack of generated reactive oxygen species (ROS)

∗ Corresponding author. Tel.: +90 312 2102630; fax: +90 312 2102600.E-mail address: gkarakas@metu.edu.tr (G. Karakas).

ttp://dx.doi.org/10.1016/j.jphotochem.2014.03.016010-6030/© 2014 Elsevier B.V. All rights reserved.

to the cell surface was suggested as the initiation step of microbialinactivation. The cell inactivation has been related to the inhibitionof cell respiration, decomposition of the lipopolysaccharide layer,decomposition of outer membrane and structural disarrangementof the cytoplasmic membrane due to lipid per oxidation. Most ofthe studies in the literature reported the viability of microorgan-isms suspended in aqueous phase with respect to the light exposuretime in the presence of semiconductor metal oxide particles. There-fore, photocatalytic disinfection of suspended microorganisms inaqueous phase is comprised of complex kinetics incorporating masstransfer, life time of oxidative free radicals, catalyst concentra-tion, light intensity and spectrum, particle size and accessibility,entry of nanosized photocatalyst particles into cell, cell defensemechanisms and toxic effects [19–22]. On the other hand, the pho-tocatalytic disinfection over thin film surface has different aspectssuch as presence of thin layer or lack of water over film surface,the lack of mixing and cell adhesion into semiconductor film sur-face. Thus, the bacterial adhesion and the colonization propensityare important surface properties for antibacterial photocatalyticcoatings. The bacterial adhesion over film surface is initiated bynon-specific, reversible physical adhesion step, which is followed

by the formation of irreversible molecular phase between the cellsurface structure and the material surface [23]. The first phase ofbacterial adhesion and retention is mostly dominated by surfacecharacteristics such as chemical structure, surface hydrophobicity,

30 B. Erdural et al. / Journal of Photochemistry and Ph

Nomenclature

Ce equilibrium concentration of remaining MB in thesolution after adsorption (ppm)

x/m quantity of MB adsorbed per unit glass substratearea (mg/cm2)

xm amount of MB adsorbed at monolayer per unit areaof thin film (mg)

� contact angle�sl solid/liquid interfacial free energy�sv solid/vapor interfacial free energy�lv liquid/vapor interfacial free energy�tot

surfacesurface free energy of solid

�LW Lifshitz–van der Waals force�− basic (electron-donor) component�+ acidic (electron acceptor) component�AB

surfaceacid–base component

�FAdh work of adhesion (mN/m)�sb solid-bacterium interfacial free energy (mN/m)�sl solid–liquid interfacial free energy (mN/m)�bl bacterium-liquid interfacial free energy (mN/m)

ssidats[hbtr[mrlbd

iaahpipattsaba

amsa

The relative surface area can be defined as the surface area of thinfilm sample over per unit area of glass substrate which is calcu-lated by assuming the surface area of one molecule of MB and thecoverage factor as 1.2 nm2 and 2, respectively [40].

urface charge, surface free energy, roughness, surface area, poretructure and size [24–29]. Therefore, the first phase of adhesions established by the physicochemical forces such as, Lifshitz–vaner Waals, Lewis acid–base, electrostatic and hydrophobic inter-ctions [24–27]. Considerable amount of work has been devotedo correlate the relationship between the bacterial adhesion andurface free energy of the bacteria and/or the material surface26–31]. In the literature it has been revealed that the cell surfaceydrophobicity plays and important role on bacterial adhesion overoth hydrophilic and hydrophobic material surfaces [27,29,30]. Onhe other hand, surface hydrophobicity of the substrate is alsoeported as important contributing factor for bacterial adhesion28,31]. The differences on the controlling factors of adhesion

ight be attributed to the steric interactions between the bacte-ia and material surface such as, length of carbohydrate chains ofipopolysaccharide (LPS) molecules on the surface of gram-negativeacteria or other extracellular polymeric substances (EPS) pro-uced by bacteria [28,29,31].

The bacterial adhesion and photocatalytic disinfection activ-ty of semiconductor metal oxide thin films might be tailored byltering textural and optical properties of their surfaces. Photocat-lytic TiO2–SiO2 composites offer low cost, high thermal stability,igh wear resistance, transparent and porous thin films that can beroduced by sol–gel technique [32–35]. The photocatalytic activ-

ty of TiO2–SiO2 mixed oxides is enhanced by larger surface area,orosity, dispersed TiO2 phase and the formation of peculiar cat-lytic active sites such as Ti O Si and Si O O bridging bonds athe TiO2–SiO2 interface [35]. The effect of SiO2 addition on the pho-ocatalytic activity and surface properties of TiO2 thin films weretudied extensively in the past [32–39]. However, the effect of SiO2ddition on the photocatalytic antibacterial activity of TiO2–SiO2inary mixture has not been reported to the best knowledge of theuthors.

In this study, the effect of SiO2 addition on the photocatalyticntibacterial activity of TiO2 was studied by using E. coli as theodel microorganism. The effect of surface composition on the film

tructure, surface free energy on the adhesion and the antibacterialctivity were investigated.

otobiology A: Chemistry 283 (2014) 29–37

2. Materials and methods

2.1. Sample preparation

TiO2–SiO2 binary mixtures were synthesized by using sol–gelmethod and thin films were deposited over soda-lime glass plates.The effect of SiO2 loading was examined by using various amountsof titanium and silica precursors during synthesis. Hydrolysis stepwas performed by mixing 5 ml of titanium isopropoxide (extrapure grade, Sigma–Aldrich) with 200 ml of distilled water, 1 mlof acetic acid (99% Sigma–Aldrich) and 1 ml of HNO3 (0.7%v) at80 ◦C for 30 min. Appropriate amount of SiO2 (Ludox SM-30) wasadded and the resulting mixture of TiO2–SiO2 mixture was agedfor additional 24 h at room temperature. Finally, 10 g of PEG-4000 (Sigma–Aldrich) was added to the TiO2–SiO2 mixture. Glassmicroscope slides (25 mm × 75 mm × 1 mm) which are pre-cleanedby rinsing with isopropyl alcohol were coated by the preparedcolloidal solution by using dip coating method with 4.5 cm/minwithdrawing speed and dried at 120 ◦C for 20 min. The desired filmthickness was accomplished by repeating the coating–drying cyclesfive times. Finally, the samples were calcined at 500 ◦C for 15 minunder air flow.

2.2. Surface characterization

The crystal structure of thin films were characterized by thegrazing incidence X-ray diffractometer (Rigaku Ultima-IV) withCu target and Ni filter (�Cu K � = 1.5405981 A) between 20 and 60Bragg angles. The band gap of the thin film samples were measuredby UV–vis spectrophotometer (Shimadzu, UV-1601) in wavelengthrange of 290–550 nm.

The surface roughness of the thin films was determined by usingnon-contact dynamic force mode (190 Hz) AFM (Nanosurf easyScan2) images. The surface and crossections of the thin film sampleswere examined by SEM imaging (Quanta 400F Field Emission). SEManalysis was also performed to observe the effect of photocatalyticinactivation process on cell morphology. For this purpose, the cellsobtained from the coated and uncoated surfaces were fixed anddehydrated in 4% glutaraldehyde and ethanol solutions. Sampleswere further dried by using supercritical CO2 (Polaron, CPD 7501)and finally coated with gold before SEM (Jeol JSM 6060, 10 kV)analysis.

N2 sorption (BET) technique is not applicable for prepared thinfilm samples having very small mass of TiO2–SiO2 layer per unitweight of glass plates. The methylene blue (MB) adsorption tech-nique was adopted for the determination of relative surface areaof the TiO2–SiO2 thin film samples. The adsorption of MB wasachieved by immersing thin film samples into 25 ml of 2 ppm MB(Merck) solution for 72 h at room temperature in dark. The amountof MB adsorbed was determined by measuring the absorbance ofremaining MB in solution at 663 nm by using UV–vis spectropho-tometer (Nicolet Evolution 100 UV–vis) with respect to time. TheMB adsorption equilibrium data was fitted to the following Lang-muir equation [40].

Ce

(x/m)= 1

k · xm+ Ce

xm(1)

where Ce is the equilibrium concentration of MB in the solution, andx/m is the quantity of MB adsorbed per unit glass substrate area, xm

is the adsorption capacity and k is the binding energy. The adsorp-tion capacity can be obtained by linear plot of Ce/(x/m) versus Ce.

and Photobiology A: Chemistry 283 (2014) 29–37 31

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2

tsbcwAbwPbpcFatp

cse3sttc1c

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3

od(st

B. Erdural et al. / Journal of Photochemistry

Surface free energy of thin films was studied by sessile dropethod at ambient temperature (20 ◦C) by using contact angle

nalyzer (KSV-CAM 100). Contact angle measurements were per-ormed by applying 5 �l droplets of diiodomethane, glycerol andater over samples. The measurements were repeated at least at

en different random spots over the sample surface and the aver-ge values were evaluated. The surface free energy of E. coli cellsere also measured by using lawns, which are obtained by vacuumltering of 20 ml of bacterial suspension (106–107 cells/ml) on cel-

ulose membrane filters (0.45 �m, Millipore) after drying at roomemperature for 1 h.

.3. Photocatalytic antibacterial activity

E. coli XL1-blue (Invitrogen Life Technologies, USA) was used ashe model bacterium in the study. LB agar and broth were used asolid and liquid media, respectively. Cultivations were performedy using shaker (170 rpm) and incubator at 35 ◦C. Stock culturesontaining glycerol were stored in a deep-freezer at −80 ◦C andorking cultures were maintained on slants and stored at 4 ◦C.fter cultivation of E. coli in liquid medium, cells were separatedy using a centrifuge (5000 × g) for 10 min and the harvested cellsere washed twice with sterilized phosphate buffer solution (0.2 M

BS) at pH 7.2. Cells were then re-suspended in PBS, and the num-er of cells was adjusted as 106–108 per ml liquid which is used inhotocatalytic cell inactivation experiments. The number of cells inell suspensions was determined by using viable cell count method.or this purpose, 100 �l sample was diluted serially between 10−1

nd 10−6 dilution in phosphate buffer solutions (pH 7.2) and pipet-ed onto Luria Bertani (LB) agar plates. The numbers of colonies onlates were counted after overnight incubation at 35 ◦C.

The antibacterial activity of the thin film samples for suspendedells was determined by exposing 200 �l of cell suspension overample surface to artificial solar radiation (Suntest Atlas CPS+quipped with Special window glass filter) with a power flux of00 W/m2 between 310 and 800 nm at 35 ◦C. The flux was mea-ured and controlled by built in radiometer of solar simulator. Eachhin film compositions were tested by at least 6 different parallelhin film samples, initially and the time course of number of viableells was determined by removing one plate out of test chamber.00 �l samples were collected from thin film surfaces and the timeourse of number of viable cells was determined;

Antimicrobial activity

= initial number of microorganisms − final number of microorgainitial number of microorganisms

Photocatalytic cell inactivation experiments were also per-ormed for surface-adhered cells. For this purpose, thin filmamples were immersed into 20 ml of E. coli cell suspension at7 ◦C in a rotary shaker (170 rpm) for 24 h in dark. Non-adheredacteria were removed by washing with PBS and the samples withurface-adhered bacteria were irradiated by using the same testrotocol applied for suspended cells. The number of viable cellsas determined after removal of adhered E. coli cells by sonication

or 25 min.

. Results and discussions

Fig. 1 shows the XRD patterns of SiO2–TiO2 thin films depositedver glass substrates. The characteristic peaks of anatase were

etected at 2� values around 25, 38 and 48◦, which correspond to1 0 1), (1 1 2) and (2 0 0) planes of anatase phase of TiO2. Pure TiO2ample (Fig. 1(a)) presented low intensity broad peaks indicatinghe presence of small crystallites of anatase.

s × 100 (2)

Fig. 1. XRD patterns of TiO2–SiO2 thin film samples; effect of SiO2 content: (a) 0 wt%,(b) 49 wt%, (c) 60 wt%, (d) 74 wt%, (e) 85 wt%, (f) 92 wt% and (g) 95 wt%.

The addition of 49 and 60 wt% SiO2 yields higher peak intensi-ties at 25◦ 2� values (Fig. 1(b) and (c)) as well as emerging peaksof (1 1 2) and (2 0 0) planes of anatase phase (2� = 37.8◦ and 48◦).The maximum peak intensity of (101) plane was observed for the60 wt% SiO2. Further addition of SiO2 resulted in the smaller peakof (1 0 1). Although addition of SiO2 causes dilution of TiO2 phase inthe mixture, the anatase peak at 25◦ is still apparent for 92 wt% SiO2

sample. The addition of SiO2 causes higher consistency and filmthickness. Therefore higher intensity diffraction peaks obtained forthe samples containing 49% and 60 wt% SiO2 can be explained byincreased film thickness. The amorphous structure of SiO2 is evi-dent with a wide peak around 2� of 23.4◦.

The band gap of the pure TiO2 sample was determined as 3.64 eVwhich is higher than the band gap of bulk crystalline anatase(3.2 eV) indicating the indirect transitions specific to amorphousstructure [24,41]. The addition of SiO2 increases the band gapslightly and maximum band gap was observed 3.71 eV for 92 wt%SiO2 sample which can be attributed to improved dispersion of TiO2particles in SiO2 matrix and resulting quantum size effect. Theseresults were in good agreement with literature for the samplesobtained by sol–gel method [42,43].

SEM images of samples coated with pure TiO2 and 92 wt%SiO2–TiO2 thin films are presented in Fig. 2(a) and (b), respectively.As seen from Fig. 2(a), pure TiO2 has glassy and smooth surfacewith cracks due to thermal stresses during drying and calcination.

32 B. Erdural et al. / Journal of Photochemistry and Photobiology A: Chemistry 283 (2014) 29–37

m sam

TasFbcp

Tt1t

abrnrSpt

Fig. 2. SEM images of TiO2–SiO2 thin fil

he addition of SiO2 introduces surface roughness and porosity to great extent. The SiO2 containing TiO2 thin films are comprised ofpherical particles with approximately 10 nm diameter as shown inig. 2(b). The growth of TiO2 particles in silica skeleton is controlledy diffusion. Hence the formation of larger TiO2 particles during thealcination due to agglomeration and sintering is hindered in theresence of SiO2 [44–46].

The cross section of the samples showed the presence of uniformiO2–SiO2 film structure (Fig. 3(a) and (b)). The film thickness ofhe samples were determined by SEM imaging between 30 nm and

�m within the range of 0–92 wt% SiO2, which is consistent withhe peak intensities of XRD patterns.

The surface roughness of the thin films and the relative surfacereas were measured by using AFM image analysis and methylenelue adsorption technique, respectively. The results given in Table 1eveal the impact of the SiO2 addition on the average surface rough-ess and on the relative surface area of the samples. The surface

oughness and relative surface area are improved by the addition ofiO2 to a great extent which can be attributed to the change of mor-hology of thin films. The surface area and porosity are importantextural properties on the kinetics of photocatalytic degradation

Fig. 3. Cross sectional view of TiO2–SiO2 thin fi

ples: (a) pure TiO2 and (b) 92 wt% SiO2.

of organic chemicals, which includes the pore diffusion, adsorp-tion into active sites and generation rate of free radicals. However,the photocatalytic inactivation of microorganisms takes place overthe exterior surface due to large size of microorganisms and associ-ated with the free radical generation in surface and pores. Thus, thephotocatalytic degradation efficiency of any thin film surface can-not be extrapolated to photocatalytic anti microbial activity [47]. Inthis study, both relative surface area and surface roughness of thesamples were determined. The relative surface area and surfaceroughness are strongly related with the surface free energy whichis an important factor for free radical generation rate, adsorptionand bacterial adhesion steps.

3.1. Photocatalytic antibacterial activity

The photocatalytic inactivation of E. coli cells over the TiO2–SiO2thin films was examined by using cell suspensions as described

in the experimental part. The efficacy of the photocatalytic dis-infection was tested by control experiments in dark/light and inthe presence/absence of photocatalytic thin films, respectively.No activity was observed over TiO2–SiO2 films in the dark and

lms: (a) 49 wt% SiO2 and (b) 92 wt% SiO2.

B. Erdural et al. / Journal of Photochemistry and Photobiology A: Chemistry 283 (2014) 29–37 33

Table 1Surface roughness values of thin films.

SiO2 (wt%) 0 49 60 74 85 92 95Roughness (RMS, nm) 2.0 ± 1.0 3.6 ± 0.2 4.1 ± 0.7 5.4 ± 1.0 6.6 ± 1.0 7.0 ± 1.0 9.4 ± 2.0Relative Surface Area (cm2/cm2) ND 4 ± 2 6 ± 3 22 ± 3 43 ± 5 83 ± 7 85 ± 4

B) ove

bi3tgs7ac(air9csob(c

rtc9idsab

tTeabTeobc

of TiO2–SiO2 composites, and effect of surface composition on theE. coli adhesion must be clarified.

Fig. 4. SEM images of E. coli cells (A) over the bare glass and (

are glass surface under irradiation. However, significant activ-ty was observed over TiO2–SiO2 thin film sample surfaces under00 W/m2 irradiation. The morphology of E. coli cells irradiated overhe TiO2–SiO2 thin film surfaces containing 92 wt% SiO2 and barelass were compared by SEM imaging. Before fixing the cells on theurfaces by 4% glutaraldehyde solution, 0.2 M sterilized PBS (pH.2) was added to E. coli suspension over the thin film surfacesnd kept at 4 ◦C overnight to clearly observe the morphologicalhanges of the cells and minimize the preparation defects. Fig. 4a) and (b) are representative images of E. coli cells over bare glassnd irradiated over 92 wt% SiO2–TiO2 for 60 min, respectively. SEMmages revealed the formation of morphological lesions such asumples, dents and irregular contours on the cells irradiated over2 wt% SiO2–TiO2 sample (Fig. 4(b)) which can be attributed to theell surface damage by reactive oxygen species and the oxidativetress environment created by photocatalytic conditions. On thether hand, no defect was observed for the cells irradiated overare glass (Fig. 4(a)) and TiO2–SiO2 coated glass samples in darknot shown). No antibacterial activity was observed for thin filmsontaining pure SiO2 as expected in both dark and light conditions.

The effect of SiO2 addition on the photocatalytic antibacte-ial activity of TiO2–SiO2 thin films at the end of 1 h exposureo 300 W/m2 is depicted in Fig. 5. The results showed a signifi-ant increase of antibacterial activity by the addition of SiO2 up to2 wt%. The maximum antibacterial activity was achieved as 50%

nactivation over 92 wt% SiO2–TiO2. Higher SiO2 loadings caused arastic decrease in the photocatalytic activity as a result of exces-ive dilution of TiO2 phase in SiO2, reduced accessibility of photonsnd limited transport of oxygen, water and reactive oxygen speciesetween TiO2 surface and bacterial suspension.

The enhancement of photocatalytic antibacterial activity withhe addition of SiO2 can be explained by improved dispersion ofiO2 in silica matrix and better accessibility of TiO2 particles. How-ver, the effect of composition is not limited to the dispersion andccessibility of titania in silica phase alone. The surface structure-acterial adhesion relationship should be also considered carefully.he attack of reactive oxygen species (ROS) to cell surface is an

ssential step for photocatalytic inactivation. ROS are generatedver the TiO2 surface and react readily with the cell wall of adheredacteria. On the other hand, the suspended bacteria are more sus-eptible to the attack of short-lived reactive oxygen species in

r 92 wt% SiO2–TiO2 thin film after 1 h irradiation (300 W/m2).

aqueous phase. Therefore, the inactivation rate of adhered bacteriais supposed to be higher than the diffusion controlled inactivationof suspended bacteria. The photo-generated electrons and holesconvert surface ad-species of oxygen and water (surface hydroxylOH−) into ROS such as hydroxyl radicals (•OH), superoxide ion(•O2

−) and hydrogen peroxide H2O2 [48,49]. The surface concen-tration of hydroxyl species is replenished easily in aqueous phasewhile the oxygen concentration is rather limited by solubility anddiffusion of gas phase oxygen. The hydroxyl (•OH) and super oxy-gen radicals (•O2

−) generated over the semiconductor surface candirectly interact with the cell surface of adhered bacteria. There-fore, the adhesion of the cells over the thin film surfaces whichfacilitates the interaction of reactive oxygen species with cell sur-face becomes an important issue for antibacterial activity of thinfilms. These results require detailed analysis of surface structure

Fig. 5. Effect of surface composition of TiO2–SiO2 thin films on the photocatalyticantibacterial activity at the end of 1 h under 300 W/m2 irradiation at 35 ◦C.

34 B. Erdural et al. / Journal of Photochemistry and Photobiology A: Chemistry 283 (2014) 29–37

Fs

3

ruiwc

ppganidtdssrafOgpriiSdfigEdk

(

Ht(

ig. 6. Effect of surface composition of TiO2–SiO2 thin films on the bacterial adhe-ion.

.2. Bacterial adhesion and surface thermodynamics of thin films

In order to clarify the effect of surface composition or TiO2–SiO2atio of thin films on cell adhesion, the number of adhered cells pernit substrate area was determined experimentally. As it is shown

n Fig. 6, bacterial adhesion to TiO2–SiO2 thin film surfaces increasesith SiO2 loading and reaches its maximum over the same surface

omposition where the maximum antibacterial activity observed.These results show that antibacterial activity is directly pro-

ortional to the bacterial adhesion. It is well known that, the firsthase of bacterial adhesion is the interaction of cells with a surfaceoverned by van der Waals, electrostatic, and hydrophobic inter-ctions [30]. Bacterial adhesion can be explained by specific andon-specific interactions between cell and surface. Non-specific

nteractions have been elucidated by DLVO theory (Derjaguin, Lan-au, Verwey, Overbeek) and thermodynamic theory [50–52]. DLVOheory describes short range electrostatic interactions and vaner Waals forces between the electrical double layer of cell andurface [50]. On the other hand, thermodynamic approach con-iders the equilibrium between various forms of attractive andepulsive interactions in terms of interfacial free energies of inter-cting surfaces [27,51,52]. The thermodynamic theory is applicableor closed systems, where no energy is introduced from outside.n the other hand, the energy required for cell adhesion can beenerated by physiological mechanisms such as the synthesis ofroteins e.g. fimbriae. Although, DLVO and thermodynamic theo-ies are well founded for microbial adhesion, extended DLVO theorys more successful in predicting the bacterial adhesion by includ-ng hydrophobic attractive and hydrophilic repulsive forces [53].urface free energy of the thin film samples and E. coli cells wereetermined and the effect of surface composition of TiO2–SiO2 thinlms on the surface free energy and cell adhesion were investi-ated. The surface free energy of surfaces is defined by Young’squation (Eq. (3)) and the surface free energy components can beetermined by measuring the contact angle of probe liquids withnown surface tension values.

1 + cos �)�totl = 2

(√�LW

s �LWl

+√

�+s �−

l+

√�−

s �+l

)(3)

ere, � is the contact angle, � tot is the total surface energy, �LW ishe Lifshitz–van der Waals (dispersive), �− is the electron donorbasic) and �+ is the electron acceptor (acidic) components of solid

Fig. 7. The electron donor component of TiO2–SiO2 thin films and E. coli adhesion.

(s) and liquid (l) phase free energies. The acidic (�+s ) and basic (�−

s )components of surfaces were obtained by using water, glycerol anddiiodomethane as probe liquids. The contact angle measurementresults and calculated surface free energy components of TiO2–SiO2thin film samples and E. coli cells are presented in Table 2.

Both TiO2–SiO2 coated samples and E. coli cells have hydrophilicsurface structures and water contact angle decreases signifi-cantly with increasing amount SiO2. The superhydrophilicity wasobserved over 95 wt% SiO2–TiO2. Although, the contribution of dis-persive (�LW) component (ranging from 44 to 50 mN/m) in surfacefree energy is more significant than the polar (acid–base) (�AB)component (ranging from 1.3 to 16.6), SiO2 does not have a sig-nificant effect on the dispersive (�LW) component of surface freeenergy. The acid–base component (�AB) is generally established bycarboxyl ( COOH), hydroxyl (OH−) and hydroperoxyl (HO2

•) func-tional groups over the surface [54–56]. Therefore the increase inpolar component (�AB) with SiO2 amount can be attributed to sur-face hydroxyl OH− groups associated with silica surface [57]. Thetotal surface free energy (�TOT) of the samples increases with SiO2loading in parallel to surface roughness and relative surface area ofthe samples.

The repulsive forces between the particles are caused by elec-tron donor components over the solid surfaces [58]. The isoelectricpoint of E. coli has been reported in literature between 2 and 4. Theacidic cell wall functional groups of E. coli are deprotonated abovethis pH level and cells become negatively charged. Therefore, therepulsive forces between the thin film surface and E. coli cells aredominant under the experimental conditions employed (pH = 7.2)[59]. The relationship between the electron donor component ofthe surface free energy of samples and cell adhesion is depicted inFig. 7.

As the concentration of electron donor component of thin filmsincreases, the number of adhered E. coli cells decreases indicat-ing the presence of repulsive forces between the thin film surfaceand E. coli cells which is in good agreement with the literature[58]. As seen from Table 2, the highest electron donor compo-nent (�−) was determined for pure SiO2. Therefore, reduction ofnumber of bacterial adhesion at that point can be explained by theincrease of repulsive forces between the surface and free cells. The

AB

contribution of surface free energy by polar component (� ) isdominated by electron donor component (�−) for both thin filmsamples and E. coli surface rather than the electron acceptor compo-nent (� +). Thus, the E. coli cells and thin film surfaces are negatively

B. Erdural et al. / Journal of Photochemistry and Photobiology A: Chemistry 283 (2014) 29–37 35

Table 2Contact angles and surface free energy components of the thin film surfaces and bacterial surfaces at 25 ◦C (W, water; G, glycerol; D, diiodomethane).

Surfaces Contact angle � (degrees) Surface energy components (mN/m)

SiO2 (wt%) �w �D �G �LW �+ �− �AB �TOT

0 26.0 ± 0.7 46.2 ± 1.5 31.0 ± 4.2 44.0 <0.1 56.3 1.3 45.349 24.0 ± 2.0 39.6 ± 9.0 16.5 ± 3.3 49.0 0.1 50.1 3.6 52.460 27.0 ± 3.7 40.1 ± 7.0 20.5 ± 7.0 48.0 0.1 48.0 4.6 52.274 13.0 ± 9.0 33.7 ± 12.0 19.3 ± 8.3 48.0 0.2 54.2 7.1 55.185 11.5 ± 2.0 33.0 ± 6.0 20.0 ± 2.0 48.0 0.3 53.9 7.8 55.792 9.2 ± 3.0 23.0 ± 5.0 13.2 ± 2.5 49.5 0.8 48.0 12.4 62.095 5.4 ± 3.0 18.4 ± 6.0 12.2 ± 4.5 49.1 1.0 47.0 16.6 63.0

0

0

cEtof

ralpbaeE

�

badlTul4imr(ts1trbaifSr

TTa

experiment, the survival ratio of E. coli cells in suspension overthe 92 wt% SiO2–TiO2 sample surface was determined. Free E. colicells in suspension are separated from adhered cells by rinsing

100 8.2 ± 2.0 36.6 ± 3.0 11.0 ± 2.

E. coli XL1 15.0 ± 1.0 49.0 ± 5.0 32.7 ± 3.

harged [58] under the experimental conditions. Therefore, the. coli adhesion is more affected by hydrophobic interactions thanhe electrostatic interactions. In addition, the hydrophilic characterf SiO2–TiO2 surfaces can be ascribed by the domination of surfaceree energy by the electron donor component [31,60–62].

In order to understand the bacterial adhesion mechanism andelate the surface free energy of the thin film samples to the E. colidhesion, the solid–liquid (�SL), solid-bacteria (�SB) and bacteria-iquid (�BL) interfacial energy values were also measured by usingrobe liquids. The total interaction energy or work of adhesion cane defined as the energy required per unit area to separate bacteriand surface initially in contact to infinity. Therefore, the adhesion isnergetically favorable if �FAdh < 0. The work of adhesion between. coli and TiO2–SiO2 samples can be determined by Eq. (4) [63].

FAdh = 2

⎛⎜⎜⎝

√�LW

s �LWL +

√�+

s �−L +

√�−

s �+L

+√

�LWB �LW

L +√

�+B �−

L +√

�−B �+

L

−√

�LWs �LW

B −√

�+s �−

B −√

�−s �+

B − �L

⎞⎟⎟⎠ (4)

The measured solid–liquid (�SL), solid-bacteria (�SB) andacteria-liquid (�BL) interfacial energy values and the work ofdhesion of the thin film samples are also shown in Table 3. Theifference between the contact angle of water and suspending

iquid medium (0.2 M PHS, pH = 7.2) was also taken into account.he solid-suspending liquid medium and bacteria-suspending liq-id medium interfacial energies and surface tension of suspending

iquid medium were determined as �TotPHS = 65 mN/m (�LW

PHS =4.30 mN/m, �+

PHS = 4.64 mN/m, and �−PHS = 23.05 mN/m). The

nterfacial energy between the bacteria and suspending liquidedium (�BL) was determined as −3.8 mN/m. Table 3 shows the

esults of solid-suspending liquid medium (�SL), solid-bacteria�SB) interfacial energy values and calculated work of adhesion ofhe thin film surfaces. As seen from Table 3, the work of adhe-ion of TiO2–SiO2 thin film surfaces varies within the range of5.7–10.0 mN/m2 which reveals the adhesion of E. coli on TiO2–SiO2hin films are not energetically favorable (�FAdh > 0) and the energyequired for adhesion is generated by cells. The interfacial energyetween the thin film surfaces and suspending liquid media (�SL)nd work of adhesion (�FAdh) decreases significantly with the

ncrease of SiO2 weight fraction over SiO2–TiO2 thin films. There-ore, the driving force for adhesion of any organic substance toiO2–TiO2 thin films increases with SiO2 loading which is directlyelated with the change in solid–liquid interfacial free energy (�SL).

able 3he solid–liquid (�SL), solid-bacteria (�SB) interfacial energy values and the work ofdhesion of the thin film surfaces.

SiO2 (wt%) 0 49 60 74 85 92 95 100�SL (mN/m) −11.1 −8.5 −7.7 −8.5 −8.2 −5.2 −4.5 −11.0�SB (mN/m) 0.3 2.0 2.00 1.2 1.2 1.7 1.7 0.9�FAdh (mN/m) 15.3 14.3 13.5 13.5 13.2 10.8 10.0 15.7

50.0 <0.1 58.2 2.8 53.0

35.0 1.7 55.1 19.5 54.1

The solid–liquid interfacial free energy (�SL) of pure SiO2 and pureTiO2 was found higher (−11 mN/m) than all SiO2–TiO2 contain-ing samples indicating the interaction between the SiO2 and TiO2phases.

The strong correlation between the work of adhesion of E. coli toTiO2–SiO2 thin film surfaces (�FAdh) and E. coli adhesion is shownin Fig. 8.

Bacterial adhesion is favored by decreasing work of adhe-sion thermodynamically. Therefore, the number of adhered cellsincreases with increasing SiO2 content and SiO2–TiO2 thin filmsbecome thermodynamically more favorable to bacterial adhesionwith increasing SiO2 loading. Then, direct contact between TiO2 andE. coli cells increases. Similarly, the relationship between the workof adhesion of E. coli cells over TiO2–SiO2 thin film surfaces (�FAdh)and antibacterial activity of thin films are depicted in Fig. 9. Stronglinear correlation was observed between the work of adhesionand antibacterial activity. Thus the increase in the photocatalyticantibacterial activity of TiO2–SiO2 thin films with the increase ofSiO2 loading can be explained by the favored bacterial adhesionwhich enhances direct contact of bacteria with TiO2 particles andsurface ROS. The consistency of the proposed mechanism was alsotested by control experiments.

The photocatalytic antibacterial activity tests were performedfor both adhered and suspending E. coli cells separately. In the first

Fig. 8. The effect of surface composition on interfacial free energy and E. coli adhe-sion.

36 B. Erdural et al. / Journal of Photochemistry and Ph

Fig. 9. The effect of work of adhesion of TiO2–SiO2 samples on antibacterial activity.

Fig. 10. Antibacterial activity over 92 wt% SiO2–TiO2 sample; (�) suspended E. colicells and (�); adhered E. coli cells.

tsockvtpS(

ioroabt

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[

[

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[16] K. Sunada, T. Watanabe, K. Hashimoto, Studies on photokilling of bacteria onTiO2 thin film, J. Photochem. Photobiol. A 156 (2003) 227–233.

[17] Y. Lan, C. Hu, X. Hu, J. Qu, Efficient destruction of pathogenic bacteria withAgBr/TiO2 under visible light irradiation, Appl. Catal. B: Environ. 73 (2007)354–360.

he samples in 3 ml PBS. The adhered bacteria were removed fromurface by 25 min of sonication in 3 ml of PBS and the numberf adhered cells was determined [64]. The effect of sonication onell viability was checked by blank experiments by sonicating thenown number of E. coli cells in PBS and no significant effect oniability was observed. Both experiments were performed underhe same temperature and irradiation rate (35 ◦C, 300 W/m2). Thehotocatalytic inactivation of suspended E. coli cells over 92 wt%iO2–TiO2 surface (�), demonstrates two different kinetic regimesFig. 10).

Slow inactivation rate was observed during the first 15 min ofrradiation which can be named as shoulder followed by the periodf higher inactivation rate. On the other hand, constant inactivationate without any initial lag was observed for adhered E. coli cells (�)ver the same sample surface. However the activity slows downfter 20 min. The time lag observed for suspended E. coli cells mighte explained by the low concentration of ROS in aqueous phase andhe presence of cells with minor damage which are still viable.

otobiology A: Chemistry 283 (2014) 29–37

4. Conclusions

The effect of composition of TiO2–SiO2 thin films on the surfacethermodynamic properties and the photocatalytic antibacterialactivity against E. coli were investigated. The addition of SiO2enhanced the dispersion of TiO2, surface area, roughness and pho-tocatalytic antibacterial activity of TiO2–SiO2 thin films and thehighest antibacterial activity was observed over 92 wt% SiO2–TiO2surface. Activity loss was observed above 92 wt% SiO2 loadings dueto the decrease of relative concentration of TiO2 particles in silicamatrix and loss of accessibility of photons and free radicals. Thenumber of surface OH− groups and hydrophilic character of sur-faces increases with SiO2 content of thin films. Strong relation wasobserved between the antibacterial activity and bacterial adhesion.The adhesion of E. coli over SiO2–TiO2 thin film surfaces becamethermodynamically more favorable by increasing SiO2 content andthe direct contact between TiO2 and E. coli cells can be accom-plished by adhesion. Higher antibacterial activity was observed foradhered E. coli cells than the suspended cells in aqueous phasewhich can be explained by the short half life of reactive oxygenspecies and slow diffusion in aqueous phase.

Acknowledgements

This work was financially supported by Turkish Scientific andResearch Council (TUBITAK) Grant 106M168. We acknowledge Dr.Demet Cetin for SEM analysis.

References

[1] T. Matsunaga, T. Tomoda, N. Nakajima, H. Wake, Photoelectrochemical steril-ization of microbial cells by semiconductor powders, FEMS Microbiol. Lett. 29(1985) 211–216.

[2] Z. Huang, P.C. Maness, D.M. Blake, E.J. Wolfrum, S.L. Smolinski, W.A. Jacoby,Bacterial mode of titanium dioxide photocatalysis, J. Photochem. Photobiol. A130 (2000) 163–170.

[3] K. Page, M. Wilson, I.P. Parkin, Antimicrobial surfaces and their potential inreducing the role of the inanimate environment in the incidence of hospital-acquired infections, J. Mater. Chem. 19 (2009) 3819–3831.

[4] A. Erkan, U. Bakir, G. Karakas, Photocatalytic microbial inactivation over Pddoped SnO2 and TiO2 thin films, J. Photochem. Photobiol. A 184 (2006) 313–332.

[5] J.M. Robertson, P.K. Robertson, L.A. Lawton, A comparison of the effectiveness ofTiO2 photocatalysis and UVA photolysis for the destruction of three pathogenicmicro-organisms, J. Photochem. Photobiol. A 175 (2005) 51–56.

[6] A.G. Rincon, C. Pulgarin, Bactericidal action of illuminated TiO2 on pure E. coliand natural bacterial consortia: post-irradiation events in the dark and assess-ment of the effective disinfection time, Appl. Catal. B: Environ. 49 (2004)99–112.

[7] L. Caballero, K.A. Whitehead, N.S. Allen, J. Verran, Inactivation of E. coli onimmobilized TiO2 using fluorescent light, J. Photochem. Photobiol. A 202 (2009)92–98.

[8] P. Amezaga-Madrid, R. Silveyra-Morales, L. Cordoba-Fierro, G.V. Nevarez-Moorillon, M. Miki-Yoshida, E. Orrantia-Borunda, F.J. Solis, TEM. evidence ofultrastructural alteration on Pseudomonas aeruginosa by photocatalytic TiO2

thin films, J. Photochem. Photobiol. B 70 (2003) 45–50.[9] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemannt, Environmental appli-

cations of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96.10] A. Fujishima, T.N. Rao, D.A. Tryk, Titanium dioxide photocatalysis, J. Photochem.

Photobiol. C 1 (2000) 1–21.11] K. Tanaka, M.F.V. Capule, T. Hisanaga, Effect of crystallinity of TiO2 on its pho-

tocatalytic action, Chem. Phys. Lett. 187 (1991) 73–76.12] A.A.C. Magalhaes, D.L. Nunes, P.A.R. Dutenhefner, E.M.B de Sousa, Catalytic

activity of porous TiO2 obtained by sol–gel process in the degradation of phenol,J. Non-Cryst. Solids 348 (2004) 185–189.

13] R. van Grieken, J. Aguado, M.J. López-Munoz, J. Marugán, Synthesis of size-controlled silica-supported TiO2 photocatalysts, J. Photochem. Photobiol. A 148(2002) 315–322.

14] K. Nakata, A. Fujishima, TiO2 photocatalysis: design and applications, J. Pho-tochem. Photobiol. C 13 (2012) 169–189.

15] C.J. Ireland, P. Klostermann, W.E. Rice, M.R. Clark, Inactivation of Escherichia coliby titanium dioxide photocatalytic oxidation, Appl. Environ. Microb. 59 (1993)1668–1670.

and Ph

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

B. Erdural et al. / Journal of Photochemistry

18] T. Saito, T. Iwase, J. Horie, T. Morioka, Mode of photocatalytic bacterial actionof powdered semiconductor TiO2 on mutans streptococci, J. Photochem. Pho-tobiol. B 14 (1992) 369–379.

19] L.Y. Ong, A. Razatos, G. Georgiou, M.M. Sharma, Adhesion forces between E. colibacteria and biomaterial surfaces, Langmuir 15 (1999) 2719–2725.

20] T.B. Dasari, P. Kavitha, H. Hwang, Determination of the mechanism of pho-toinduced toxicity of selected metal oxide nanoparticles (ZnO, CuO, Co3O4 andTiO2) to E-coli bacteria, J. Environ. Sci.: China 25 (2013) 882–888.

21] J. Marugán, R. Grieken, C. Sordo, C. Cruz, Kinetics of the photocatalytic disinfec-tion of Escherichia coli suspensions, Appl. Catal. B: Environ. 82 (2008) 27–36.

22] A.K. Benabbou, Z. Derriche, C. Felix, P. Lejeune, C. Guillard, Photocatalyticinactivation of Escherischia coli effect of concentration of TiO2 and microorgan-ism, nature and intensity of UV irradiation, Appl. Catal. B: Environ. 76 (2007)257–263.

23] J.A. Lichter, K.J.V. Vliet, M.F. Rubner, Design of antibacterial surfaces andinterfaces: polyelectrolyte multilayers as a multifunctional platform, Macro-molecules 42 (2009) 8573–8586.

24] A.L. Linsebigler, G. Lu, J.T. Yates, Photocatalysis on TiO2 surfaces: principles,mechanisms, and selected results, Chem. Rev. 95 (1995) 735–758.

25] K. Banaszek, W. Szymanski, P. Pietrzyk, Adhesion of E. coli bacteria cells toprosthodontic alloys surfaces modified by TiO2 sol–gel coatings, Adv. Mater.Sci. Eng. (2013) 1–6.

26] Y.H. An, R.J. Friedman, Concise review of mechanisms of bacterial adhesion tobiomaterial surfaces, J. Biomed. Mater. Res. 43 (1997) 338–348.

27] D.R. Absolom, F.V. Lamberti, Z. Policova, W. Zingg, C.J. Van Oss, A.W. Neumann,Surface thermodynamics of bacterial adhesion, Appl. Environ. Microbiol. 46(1983) 90–97.

28] Y. Liu, S.F. Yang, Y. Li, H. Xu, L. Qin, J.H. Tay, The influence of cell and substratumsurface hydrophobicities on microbial attachment, J. Biotechnol. 110 (2004)251–256.

29] B. Li, B.E. Logan, Bacterial adhesion to glass and metal-oxide surfaces, ColloidsSurf. B 36 (2004) 81–90.

30] N. Satou, J. Satou, H. Shintani, K. Okuda, Adherence of streptococci to surface-modified glass, J. Gen. Microbiol. 134 (1988) 1299–1305.

31] H.C. van der Mei, R. Bos, H.J. Busscher, A reference guide to microbial cell surfacehydrophobicity based on contact angles, Colloids Surf. B 11 (1998) 213–221.

32] C. Anderson, J.A. Bard, Improved photocatalytic activity and characterizationof mixed TiO2/SiO2 and TiO2/Al2O3 materials, J. Phys. Chem. B 101 (1997)2611–2616.

33] N.R. Viswananth, S. Ramasamy, Study of nanocrystallites in TiO2–SiO2 compos-ites, Colloids Surf. A 133 (1998) 49–56.

34] K.S. Guan, Y.S. Yin, Effect of rare earth addition on super-hydrophilic propertyof TiO2/SiO2 composite film, Mater. Chem. Phys. 92 (2005) 10–15.

35] G. Xingtao, R.S. Bare, G.L.J. Fierro, A.M. Banares, E.I. Wachs, Preparation and in-situ spectroscopic characterization of molecularly dispersed titanium oxide onsilica, J. Phys. Chem. 102 (1998) 5653–5666.

36] M. Cho, H. Chung, W. Choi, J. Yoon, Linear correlation between inactivation ofE. coli and OH radical concentration in TiO2 photocatalytic disinfection, WaterRes. 38 (2004) 1069–1077.

37] Y.K. Jung, B.S. Park, Anatase-phase titania: preparation by embedding silicaand photocatalytic activity for the decomposition of trichloroethylene, J. Pho-tochem. Photobiol. A 127 (1999) 117–122.

38] K. Guan, B. Lu, Y. Yin, Enhanced effect and mechanism of SiO2 addition in super-hydrophillic property of TiO2 films, Surf. Coat. Technol. 173 (2003) 219–223.

39] T. Yuranova, R. Mosteo, J. Bandara, D. Laub, J. Kiwi, Self-cleaning cotton textiles

surfaces modified by photoactive SiO2/TiO2 coating 244 (2006) 160–167.

40] M.L. Fetterolf, H.V. Patel, J.M. Jennings, Absorption of methylene blue and acidblue 40 on titania aqueous solution, J. Chem. Eng. Data 48 (2003) 831–835.

41] H. Tang, H. Berger, P.E. Schmid, F. Levy, G. Burri, Photoluminescence in TiO2

anatase single crystals, Solid State Commun. 87 (9) (1993) 847–850.

[

otobiology A: Chemistry 283 (2014) 29–37 37

42] M. Gartner, V. Dremov, P. Müller, H. Kisch, Bandgap widening of titania throughsemiconductor support intearctions, ChemPhysChem 6 (2005) 714–718.

43] H.Y. Kim, Y.G. Shul, H. Han, Photocatalytic properties of silica supported TiO2,Top. Catal. 35 (2005) 287–293.

44] J. Yu, X. Zhao, The effect of SiO2 addition on the grain size and photocatalyticactivity of TiO2 thin films, J. Sol-Gel Sci. Technol. 24 (2002) 95–103.

45] J. Zhai, L. Zhang, X. Yao, Effects of composition and temperature on gel-formedTiO2/SiO2 films, J. Non-Cryst. Solids 260 (1999) 160–163.

46] H. Sankur, W. Gunning, Crystallization and diffusion in composite TiO2–SiO2

thin films, J. Appl. Phys. 66 (1989) 4747–4751.47] D. Gumy, A.G. Rincon, R. Hajdu, C. Pulgarin, Solar photocatalysis for detoxifi-

cation and disinfection of water: different types of suspended and fixed TiO2

catalysts study, Sol. Energy 80 (2006) 1376–2138.48] Y. Kikuchi, K. Sunada, T. Iyoda, K. Hashimoto, A. Fujishima, Photocatalytic bac-

tericidal effect of TiO2 thin films: dynamic view of the active oxygen speciesresponsible for the effect, J. Photochem. Photobiol. A 106 (1997) 51–56.

49] O. Carp, L.C. Huisman, A. Reler, Photoinduced reactivity of titanium dioxide,Prog. Solid State Chem. 32 (2004) 33–177.

50] P.R. Rutter, B. Vincent, The Adhesion of Microorganisms to Surfaces: Physico-Chemical Aspects, Microbial Adhesion to Surfaces, Ellis Horwood, Chiester,1980, pp. 79–93.

51] R. Bos, H.C. van der Mei, H.J. Busscher, Physico-chemistry of initial microbialadhesive interactions – its mechanisms and methods for study, FEMS Microbiol.Rev. 23 (1999) 179–230.

52] H.J. Busscher, A.H. Weerkamp, H.C. Van der Mei, A.W.J. Van Pelt, H.P. De Jong, J.Arends, Measurement of the surface free energy of bacterial cell surfaces andits relevance for adhesion, Appl. Environ. Microbiol. 48 (1984) 980–983.

53] C.J. Van Oss, R.J. Good, M.K. Chaudhury, The Role of Van der Waals forces andhydrogen bonds in hydrophobic interactions between biopolymers and lowenergy surfaces, J. Colloid Interface Sci. 111 (1986) 378–390.

54] R. Roots, S. Okada, Estimation of life times and diffusion distances of radicalsinvolved in X-ray-induced DNA strand breaks of killing of mammalian cells,Radiat. Res. 64 (1975) 306–320.

55] C.J. Van Oss, K.M. Chaudhury, J.R. Good, Interfacial Lifshitz–van der Waals andpolar interactions in macroscopic systems, Chem. Rev. 88 (1988) 927–941.

56] T.J. Kinnari, J. Esteban, N. Zamora, R. Fernandez1, C. López-Santos, F. Yubero, D.Mariscal, J.A. Puertolas, E. Gomez-Barrena, Effect of surface roughness and ster-ilization on bacterial adherence to ultra-high molecular weight polyethylene,Clin. Microbiol. Infect. 16 (2010) 1036–1041.

57] G. Karakas, P. Yetisemeyen, Room temperature photocatalytic oxidation of car-bon monoxide over Pd/TiO2–SiO2 catalysts, Top. Catal. 56 (2013) 18–20.

58] M. Hermansson, The DLVO theory in microbial adhesion, Colloid Surf. A 14(1999) 105–119.

59] K. Konhauser, Introduction to Geomicrobiology, 1st ed., Wiley-Blackwell, UK,2007.

60] F. Marciano, D.A. Lima-Oliveira, N.S. Da-Silva, A.V. Diniz, E.J. Corat, V.J. Trava-Airoldi, Antibacterial activity of DLC films containing TiO2 nanoparticles, J.Colloid Interface Sci. 340 (2009) 87–92.

61] W. Su, S. Wang, L. Wu, X. Wang, P. Liu, X. Fu, Surface anti-bacterial adhesioncharacteristics of TiO2-coated PMMA, J. Struct. Chem. 28 (2009) 1497–1502.

62] A.A. Mafu, C. Plumety, L. Deschenes, J. Goulet, Adhesion of pathogenic bacteriato food contact surfaces influence of pH of culture, Int. J. Microbiol. 1 (2011)1–10.

63] R.P. Schneider, Conditioning film-induced modification of substratumphysicochemistry-analysis by contact angles, J. Colloid Interface Sci. 182 (1996)

204–213.

64] T.J. Kinnari, J. Esteban, N. Zamora, R. Fernandez, C. Loıpez-Santos, F. Yubero, D.Mariscal, J.A. Puertolas, E. Gomez-Barrena, Effect of surface roughness and ster-ilization on bacterial adherence to ultra-high molecular weight polyethylene,Clin. Microbiol. Infect. 16 (2009) 1036–1041.