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Colloids and Surfaces B: Biointerfaces 74 (2009) 75–83

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Colloids and Surfaces B: Biointerfaces

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iofouling studies on nanoparticle-based metal oxide coatingsn glass coupons exposed to marine environment

. Dineshram a, R. Subasri b, K.R.C. Somaraju b, K. Jayaraj a, L. Vedaprakash a, Krupa Ratnam a, S.V. Joshi b,

. Venkatesan a,∗

National Institute of Ocean Technology, Velachery-Tambaram main road, Pallikaranai, Chennai 600100, IndiaInternational Advanced Research Centre for Powder Metallurgy and New Materials, Balapur, Hyderabad 500005, India

r t i c l e i n f o

rticle history:eceived 29 August 2008eceived in revised form 24 June 2009ccepted 29 June 2009

a b s t r a c t

Titania, niobia and silica coatings, derived from their respective nanoparticle dispersions or sols andfabricated on soda lime glass substrates were subjected to field testing in marine environment for anti-macrofouling applications for marine optical instruments. Settlement and enumeration of macrofoulingorganisms like barnacles, hydroides and oysters on these nanoparticle-based metal oxide coatings sub-

vailable online 7 July 2009

eywords:etal oxide coatingsarine optical instruments

iofouling

jected to different heat treatments up to 400 ◦C were periodically monitored for a period of 15 days. Thedifferences observed in the antifouling behaviour between the coated and uncoated substrates are dis-cussed based on the solar ultraviolet light induced photocatalytic activities as well as hydrophilicities ofthe coatings in case of titania and niobia coatings and the inherent hydrophilicity in the case of silicacoating. The effect of heat treatment on the photocatalytic activity of the coatings is also discussed.

hotocatalysisydrophilicity

. Introduction

Marine fouling is the major problem for the application of mate-ials in marine environment. Studies on the environmental changesn the coastal ocean on timescales from minutes to decades can bechieved by the use of moorings deployed at multiple depths. Thesean-made objects placed in the marine environment encounter

ouling by marine organisms and biofilm formation on surfaces.rogress in coastal ocean research will be enhanced with optical

nstrumentation that provides valuable tools for understanding thehysical, chemical, biological and geological process in the coastalcean. Most of the in situ water quality monitoring instrumentsnd sensors made of glass need proper maintenance to ensure con-inued accuracy. The greatest disadvantage of these platforms isiofouling of the sensors. These moored buoy instruments are sus-eptible to fouling in the form of microbial and algal films thatan cover optical windows and degrade data quality [1]. The settle-ent of larvae of sessile invertebrates on the optical windows and

heir subsequent growth is also a common problem [2]. Organisms

uch as barnacles, algae and bacteria that foul surfaces in aquaticnvironments are determined by interfacial interactions within theutermost, few nanometers of the surface and involve a wide rangef bonding mechanisms. Furthermore, fouling organisms especially

∗ Corresponding author. Tel.: +91 44 66783424; fax: +91 44 66783430.E-mail address: rvenkatin@yahoo.com (R. Venkatesan).

927-7765/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfb.2009.06.028

© 2009 Elsevier B.V. All rights reserved.

invertebrates and algae are highly selective in their preferences forcertain surfaces, the signal that exerted by the surface might initiatecharacteristic behavior which either promotes or inhibits attach-ment [3]. There are continuous efforts to investigate on methods tokeep the optical windows clean during their extended deploymentunder marine environments.

Fouling in one form or other can occur in most locations, dur-ing all seasons, and the use of antifouling methods is essential toreduce the consequences for offshore installations. One method ofcombating biofouling is by using toxic antifouling coatings contain-ing metal compounds such as copper and tri-n-butyltin (TBT) or bythe use of biocides like silver or surfactants [4,5]. However, thesecoatings are not recommended for use by virtue of their poten-tial damage to marine organisms causing environmental pollutionproblem. Even though organotin-based coatings are most efficientfor long-life, due to the increased risks in the application of thesetoxic coatings, International Maritime Organization aims to com-pletely remove tin-based paints from all ships around the world by2008 in an effort to limit their use.

Since 2003 ultraviolet (UV) radiation treatment has also beenidentified as one of the promising areas of new biofouling con-trol methods [6]. In a recent study, Patil et al. [7] reported UV-C

(254 nm) irradiation as a potential tool for control of biofouling inmarine optical instruments. The advantages of using UV-C radiationas an antifouling tool are they are less harmful when comparedto biocides and also there is no physical contact with the opticalwindows when employing the irradiation method, which reduces

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urface damage due to abrasion. However, the disadvantage of thisechnique is that it consumes lot of energy for the UV irradiationnd is consequently cost intensive.

TiO2 coatings are good choices for investigation, due to theirhotocatalytic properties, upon UV light irradiation. Li and Logan8] studied the impact of UV light on bacterial adhesion on metalxide coated glass substrates and concluded that UV irradiation ofiO2 coated glass surfaces can be an effective method for reducingacterial adhesion. An extended investigation recently conductedy Ma et al. in this direction reports on the importance of surfaceeterogeneity of engineered metal oxide (TiO2) coated glass sur-

aces in deciding the bacterial and colloid adhesion [9]. The coatingsere fabricated by vapor deposition. Nb2O5 is a semiconductor like

iO2 with a band gap of 3.4 eV and is also capable of exhibitinghotocatalysis on UV light exposure. Though there have been inves-igations on new methods of synthesis of Nb2O5 powders like solvohermal synthesis and their photocatalytic activities in aqueoususpensions [10], Nb2O5 coatings have not so far been investigatedor their antifouling properties.

Silica-based organic–inorganic hybrid coatings with antifoulingroperties have already been investigated for applications on opti-al instruments by Meinema et al. [11]. These authors fabricatedoatings using functionally substituted (organo) silicon oxides RnSiOR′)4−n, where R was an alkyl, fluorine substituted alkyl or phenylroup; R′ was a methyl or ethyl group and n was either 0 or 1. Theyound a correlation between surface energy and marine antifoulingroperties and concluded that biological fouling could be reducedut cannot be completely prevented using these coatings. How-ver, these coatings had an easy-to-clean property compared ton uncoated glass, which holds promise. Another set of interestingnvestigations by Tang et al. [12] reported on silica-based hybriderogel films as novel coatings for antifouling and fouling release.n this case, it was found that coatings of xerogel surfaces with low

ettability presented reduced zoospore settlement and increasedemoval of zoospores.

Based on the above, it can be seen that efforts are still ongoingo develop fouling resistant coatings that are stronger, durable andafer. In this aspect, the use of nanoparticle-based metal oxide coat-ngs represents a promising approach for development of non-toxicontrol technologies for macrofouling organisms by engineeringhe surface with either low surface energy coatings that minimizeiological adhesion strength, allowing “fouling release” with mod-st brushing/water spray pressures or through coatings that canrevent biofouling through their photocatalytic activity. There is aeed to develop economically viable and environmentally friendlyethods, not involving obnoxious paint coatings, for biofouling

ontrol. These non-toxic coatings are not expected to prevent staticeposits of fouling layers, but rather function through a commonechanism by which most biofouling can be easily removed on the

asis of controlled surface chemistry [13].Development of new techniques like a metal oxide coating

eems to be an effective method for combating fouling on sensorarts [14]. Due to the increasing emphasis and importance of usef non-toxic materials as antifoulants, the main objective of thetudy is to monitor the antifouling properties of metal oxide (sil-ca, titania and niobia) nanoparticle coated glass coupons derivedsing commercial nanoparticle dispersions/sols, in a natural marinenvironment and to investigate the fouling organisms attachedo these coated coupons submerged within the eutrophic zonehere photosynthesis takes place. This paper reports the results

erived from the field testing of nanoparticle metal oxide coating

ased antifoulants deposited on glass substrate in marine envi-onment for potential use in optical devices. These coatings havehe potential to be used for marine applications, since they arenvironment friendly with low cost and good compatibility ineawater.

s B: Biointerfaces 74 (2009) 75–83

2. Materials and methods

2.1. Antifouling coatings

The coating materials were commercially available, water-basedtitania and niobia nanoparticle dispersions with a titania averagecrystallite size of 8.6 nm and niobia crystallite size of 145 nm. Theaverage crystallite sizes were determined from the X-ray diffraction(XRD) patterns of powders obtained from the as-dried dispersions.XRD patterns were collected by using a Brukers D8 Advance X-raydiffractometer over a 2 theta range of 10–80◦ and the crystal-lite sizes evaluated by using the Debye-Scherrer formula. In thecase of the coating material for fabrication of a silica coating, acommercial silica sol was used. Glass substrates of dimensions(70 mm × 70 mm × 5 mm) were carefully cleaned using 1% NaOHsolution followed by rinsing thoroughly in running distilled waterand finally by high pure low conductivity water. Traces of waterwere removed by blowing filtered compressed air onto the sub-strate.

2.2. Coating deposition

The nanoparticle-based metal oxide coatings were depositedon the glass substrates by a spin coating technique usinga spin coater (Laurell Technologies Corporation, USA, modelWS–400B–6NPP–LITE). 3 ml of the sol or the dispersion was dis-pensed on the substrate by means of a graduated syringe for spincoating. Silica coatings were prepared from a silica sol. Titaniaand niobia coatings were prepared from nanoparticle dispersions.The coating program used for the spin coating was 100 rpm-15 s;1000 rpm-30 s; 250 rpm-15 s. After few minutes of air drying, thecoatings were heat treated (HT) to temperatures of 300 and 400 ◦Cat the rate of 1 K/min and a soaking time of 1 h at the set tempera-ture.

2.3. Coating characterization

The thickness of the coatings was determined using a surfaceprofilometer (Mahr Germany Perthometer). XRD analysis for theTiO2 and Nb2O5 powders obtained by drying the dispersion andheating to different temperatures was carried out to study the crys-tallinity and variation of crystallite size with temperature. No suchattempt was made in the case of silica sol since the coatings areexpected to be amorphous at least up to 400 ◦C. Scanning electronmicroscope analysis was carried out using a Hitachi SE/3400 N fieldemission scanning electron microscope.

2.4. Contact angle measurement

Water contact angles on glass and metal oxide surfaces weremeasured using a Krüss DSA 100 drop shape analyzer. A drop ofliquid from a graduated syringe was dispensed on the surface andthe contact angle formed by the liquid drop on the surface wasevaluated by image analysis software using the captured image ofthe water drop. For each sample, measurements were carried outat least on 15 locations and their average was reported as the watercontact angle.

2.5. Adhesion testing for coatings

Adhesion of the coatings to the substrate was measured by a

tape test using a cross hatch cutter, according to ASTM 3359-02.The test is carried out by applying and removing pressure sensitivetape over cuts made in the film. A 25 mm wide semi transparentpressure sensitive tape manufactured by Permacel, New Jersey wasused along with a cross hatch cutter at a cutting edge angle between

R. Dineshram et al. / Colloids and Surfaces B: Biointerfaces 74 (2009) 75–83 77

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Table 1Thickness and roughness of the coatings investigated in the present study.

Sample Coating thickness [�m] Roughness [�m]

Bare glass substrate – 0.00083Glass|silica – HT 400 ◦C 0.1 0.003

◦

Fa

Fig. 1. Test rig with frames containing nanoparticle-based metal oxide coupons.

5 and 30◦. Initially, approximately 2 cm long cuts in the form ofrids with 2 mm spacings are made on the film. The tape was placedver the grid to ensure good contact with the film and then the tapermly rubbed. The tape was then removed within 90 s and the gridrea inspected for removal of the coating from the substrate. Thedhesion testing was carried out at least on 3 different locations onach sample and a minimum of 2 samples with the same coatingarameters were used for each testing.

.6. Seawater exposure

The nanoparticle-based metal oxide coated coupons wereeployed at Ennore, Chennai, east coast of India at the location3◦15′25.03′′N and 80◦20′29.32′′E, to the natural marine environ-ent, suspended from a floating raft at 0.3 m depth below the

urface of mean sea level (Fig. 1).

.7. Biofilm characterization

Physicochemical data corresponding to pH, turbidity, dissolvedxygen, temperature and salinity were collected using a water qual-

ty monitoring system (Hydrolab, Quanta instruments). Total viableacterial count was enumerated by swabbing 1 cm surface of thexposed coating and the control sides (without coating) to a glassest tube containing filtered, autoclaved seawater. All samples wereeld in a cooler at 4 ◦C and returned directly to the laboratory for

ig. 2. Profilometer results on a bare soda lime glass substrate for assessing the (a) rought 300 ◦C (1 h).

Glass|titania – HT 300 C 0.06 ± 0.01 0.003 ± 0.001Glass|titania – HT 400 ◦C 0.05 ± 0.01 0.005 ± 0.001Glass|niobia – HT 300 ◦C 0.061 ± 0.009 0.004 ± 0.001Glass|niobia – HT 400 ◦C 0.053 ± 0.01 0.004 ± 0.001

analysis. The samples were further serially diluted 100-fold usingsterile seawater and 0.1 ml aliquots from each tube were plated onZobel marine agar, incubated at 28 ± 1 ◦C which is close to the seaenvironment conditions and resulting colonies counted after 48 hof incubation [15]. All results were compared relative to controlwithout metal oxide coatings. Zobel marine agar plates were usedas a control.

2.8. Assessment of macrofoulants

The submerged coatings were monitored continuously for 4 daysafter deployment to observe initial settlement of macrofoulers andsamples were retrieved at periodic intervals after day 4, 9 and15 for macrofouling and larval observation. The collected couponswere transferred to the laboratory in separate containers filledwith seawater. Enumeration of the numerical abundance of foulingorganisms were made manually by counting the number of indi-viduals after identifying the organisms with the help of trinocularstereo zoom microscope (NIKON SMZ 1500). The percentage of foul-ing resistance was rated according to the portion of coating that wascovered by a particular type of fouling organism.

3. Results and discussion

3.1. Coating characterization

The thickness and roughness of the coatings investigated inthe present study as measured by the profilometer is presented

in Table 1. Typical profiles of a bare soda lime glass substrate toevaluate its roughness and that of a titania coating heat treated at300 ◦C are shown in Fig. 2. It can be seen that due to slight increasein shrinkage of the films at higher temperatures, the 400 ◦C HT coat-ings exhibit slightly lower thickness than the 300 ◦C HT ones. The

ness of bare substrate and (b) thickness of a titania coating on glass substrates HT

78 R. Dineshram et al. / Colloids and Surfaces B: Biointerfaces 74 (2009) 75–83

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ig. 3. Scanning electron micrographs of metal oxide coatings on glass substrate (aoating on glass HT at 300 ◦C HT (d) niobia coating on glass HT at 400 ◦C and (e) sili

oatings as observed under the SEM were found to be crack-free.he SEM images of the coatings are shown in Fig. 3. In the case ofilica, the coatings were featureless due to the amorphous naturef the coating material (as shown in Fig. 3(e). Fig. 4 (a) and (b) showhe results of the elemental composition analysis of the surface ofhe coated metal oxides, titania and niobia, respectively, on glassubstrates. The presence of Ti and Nb in the coatings along withhe elements present in the glass substrate is confirmed from theDS spectra. The same was not attempted for the silica coatingsue to the possibility of interference from the presence of Si in theubstrate.

.2. Adhesion testing

All the coatings investigated under the present study, when

bserved under a magnifying glass after the tape test, showed thathey belonged to rank 5B, which means that, the adhesion of theoatings to the substrate was very good and that there was noemoval of coating onto the tape, after it was removed from therid area.

ia coating on glass HT at 300 ◦C (b) titania coating on glass HT at 400 ◦C (c) niobiating on glass HT at 400 ◦C.

3.3. Biofilm characterization and assessment of macrofoulants

The hydrobiological parameters were found to be at ambi-ent levels during the course of the study period. On an average,attachment of colonies were lower on Nb2O5 followed by that onSiO2 and finally on TiO2 (Fig. 5). The total viable counts on TiO2HT 300 ◦C were 7 × 104 and 5 × 104 CFU cm−2on TiO2 HT 400 ◦C.The corresponding values on Nb2O5 HT 300 ◦C were 1 × 104 and6 × 103 CFUcm−2on Nb2O5 HT 400 ◦C and those on SiO2 HT 400 ◦Cwere 2 × 104 CFUcm−2, respectively. In this study, all the coatingswere subjected to static immersion conditions and therefore differ-ences in the microbial counts reflected differences in the coatings.An increase in colony counts was observed in bacterial density oncontrol surfaces compared to the nanoparticle-based metal oxidecoatings.

Salinity (34.3 ± 0.2 pss) and temperature (28 ± 1.2 ◦C) valueswere normal to favor the larval settlement. The major speciesidentified on metal oxide coatings were barnacles such as Balanusamphitrite while minor foulants include hydrozoans (Hydroideselegans) and oysters (Crassostrea madrasensis). After 1 day there

R. Dineshram et al. / Colloids and Surfaces B: Biointerfaces 74 (2009) 75–83 79

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ig. 4. Results from energy-dispersive X-ray spectroscopy (EDS) analysis of the (a)itania coating and (b) niobia coating on glass substrates.

as no attachment of macrofoulers on metal oxide coatings,mmersed under static conditions in sea, but deposition of sus-ended particles of silt were initially observed. Settlement ofarnacles increased rapidly on the nanocoatings and on to thencoated surface (control) after 4 days of exposure. Cyprid stagef barnacle was found starting from day 4 and their numbers

ncreased till the completion of the experiment (Fig. 6a). H. elegansettlement was visible after 9 days onwards and their numbersncreased gradually on all the coated coupons till the completion ofhe experiment (15 days) (Fig. 6b). Oyster settlement was observedrom the day 2 of deployment and was found to be least on SiO2oated coupons (Fig. 6c). Accumulation of slime was seen both onhe control and coated sides and it was found to be higher on theoated side than on the control side.

Barnacle settlement varied between coatings with the maxi-um on titania HT 400 ◦C (2 cm−2) followed by niobia HT 400 ◦C

1 cm−2) and their number was minimum on titania HT 300 ◦C−2

0.15 cm ). Hydrozoans followed a pattern different from that of

arnacles and its maximum was on (0.35 cm−2) niobia HT 400 ◦C,0.25 cm−2) on SiO2 HT 400 ◦C and (0.20 cm−2) on TiO2 400 ◦C,espectively.

ig. 5. Total viable bacterial count (in colony forming units) on metal oxide coatings1 cm−2) relative to the control (without metal oxide coatings) after static immersionor 15 days. Each value is the mean of duplicate counts (n = 2), and the error bar ishe standard deviation.

Fig. 6. (a) Barnacle (B. amphitrite) count (b) Hydroides (H. elegans) count and (c)Oyster (C. madrasensis) count after static immersion for 4, 9 and 15 days of differentmetal oxide coatings on glass substrate (front side) relative to their control, withoutmetal oxide coatings (back side).

The highest fouling resistance for the oyster (C. madrasensis) wasobserved on SiO2 HT 400 ◦C coatings. All the coatings exhibited rel-atively low levels of polychaete and oyster attachments comparedto barnacle settlement. The other fouling groups like seaweeds andbryozoans were uncommon on the coatings and were not consid-ered in this experiment.

Screening against macrofouling organisms has gained signifi-cance apart from the inhibitory screening studies on microfilmconstituents like bacteria and diatoms [16]. As seen from the abovedata, settlement of the macrofoulants is considerably less on SiO2coatings. The effectiveness of antifouling resistance of the heat

treated nano coatings varied between the macrofouling organisms(Fig. 7). Coatings heat treated at 400 ◦C accumulated more numberof barnacles compared to 300 ◦C heat treated TiO2 coating. Hencein this present study, there is no consistent pattern of inhibition ofmacrofoulants on all the metal oxide coatings. In general barnacles

80 R. Dineshram et al. / Colloids and Surfaces B: Biointerfaces 74 (2009) 75–83

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before and after UV illuminations (only for titania and niobia coat-ings), by exposing the coated glass coupons to bright sunlight for5 h. Table 2 presents the results of the contact angle measurementsfor the coatings, after heat treatment. It can be seen that the silica

Table 2Water contact angles of coatings measured before and after UV illumination.

Sample Contact angle of coatingbefore UV illumination[◦]

Contact angle of coatingafter UV illuminationa

[◦]

Bare glass substrate 52 ± 2Glass|silica – HT 400 ◦C 24 ± 3 –

Fig. 7. Percentage variation of settlement of (a) Barnacle (B. amphitrite), (b) Hyd

re more abundant than the other two macrofoulants. Fig. 8 showshe photomicrographs of metal oxide coated glass coupons docu-

ented on day 15.

.4. Possible reasons for the metal oxide nanoparticle coatings toxhibit antifouling behaviour

The following section will discuss on possible reasons forhe metal oxide coatings to exhibit antifouling behaviour, wheneployed 30 cm below the sea level. The difference observedetween the antifouling behaviour of the coated and uncoated glassurfaces could be due to the solar ultraviolet induced photocatalyticctivities in addition to some degree of induced hydrophilicity, inase of titania and niobia coatings. If we consider the spectral dis-ribution of solar energy at sea level to be approximately 3, 44 and3% in the UV, visible and IR regions, respectively, the solar UV-radiation with a wavelength range of 315–400 nm has a greater

enetration depth in water than UV-B (280–315 nm) or visible light.ence, the coatings of TiO2 and Nb2O5 with band gaps of 3.2 and.4 eV, respectively, should be expected to get sufficient UV-A lighto exhibit photocatalysis even when deployed 0.3 m below the seaevel.

The role of photocatalytic coatings on glass for combating bio-ouling on exposure to UV light has two implications; one is to killhe microorganisms through the photocatalytic activity induced byV radiation, which can be understood from the reactions givenelow:

− +

iO2 + h� → TiO2(e + h )

2O + h+ → •OH− + H+

H− + h+ → •OH

s (H. elegans), and (c) Oyster (C. madrasensis) on different metal oxide coatings.

O2(ads) + e− → O2•−

Thus in the presence of aqueous environments, TiO2 exposed toUV, leads to the generation of both •OH and O2

•− radicals that arehighly reactive and microcidal. This could be one factor responsiblefor the antifouling nature of the photocatalytic metal oxide coatingon the glass substrate.

Another mechanism that operates is the UV light inducedhydrophilicity. It has been reported [8] that a 1 h UV (254 or340 nm with intensity 2800 or 4600 �W cm−2) exposure reducesthe contact angle from 59◦ to less than 5◦, changes the surfacefrom hydrophobic to hydrophilic. So, the photocatalytic metal oxidecoated surfaces can be expected to become hydrophilic on exposureto UV light that could reduce the biofouling in addition to decreas-ing adhesion by photocatalysis. In order to verify this, water contactangle measurements were carried out on coated glass surfaces

Glass|titania – HT 300 ◦C 40 ± 3 20 ± 2Glass|titania – HT 400 ◦C 33 ± 2 18 ± 2Glass|niobia – HT 300 ◦C 63 ± 5 50 ± 5Glass|niobia – HT 400 ◦C 67 ± 2 53 ± 2

a Samples exposed to sunlight for 5 h.

R. Dineshram et al. / Colloids and Surfaces B: Biointerfaces 74 (2009) 75–83 81

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ig. 8. Photographs of coated glass coupons before and after experiment exposed innd silica coatings.

oating has the least contact angle after heat treatment, even with-ut any UV light exposure. The silica coating is more hydrophilichen compared to titania or niobia coatings as expected, because of

he Si–OH bonds formed on the surface. This inherent hydrophilic-

ty of a silica coating could make it easy for the marine biofilm to be

ashed away by water and hence, be favourable for it to be resistantgainst oyster settlement.

In case of titania coating, the contact angles immediately mea-ured after 5 h of sunlight illumination have decreased to some

a for 15 days show less macrofouler recruitment observed on titania than on niobia

extent, indicating a photoinduced hydrophilicity. However, thecoatings have not become super hydrophilic (contact angles ∼0◦)as reported in [8]. This situation is possible to be achieved withan external source of UV light. In case of niobia coatings, though

there is some indication of a slight decrease in contact anglesbefore and after UV illumination, a substantial decrease of con-tact angles is possible only with an external UV source irradiation.This might be due to a higher band gap in case of niobia whencompared to titania. However, it is also possible that a continuous

82 R. Dineshram et al. / Colloids and Surfaces B: Biointerfaces 74 (2009) 75–83

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ig. 9. XRD patterns of (a) as-dried and 400 ◦C HT TiO2 powders (indexed with anatispersions.

xposure to UV from solar radiation will induce a slight hydrophilic-ty.

.5. Effect of heat treatment temperature on photocatalyticctivity of metal oxide coating

The results from the present studies show that the barnaclepecies counts on the TiO2 and Nb2O5 coatings heat treated at00 ◦C are effective after 15 days when compared to other coat-

ngs. The reason for this observation could be explained as givenelow. Generally, photocatalytic activity is expected to increaseith smaller particle size (higher surface energy) and increased

rystallinity [17]. In the present investigation, TiO2 and Nb2O5anoparticle dispersions were used for fabrication of coatings andre nanocrystalline in nature. The XRD patterns of TiO2 and Nb2O5owders heated to different temperatures are shown in Fig. 9 (a)nd (b). The crystallite size in the case of TiO2 after a 400 ◦C heatreatment increased from 8.6 up to 11.5 nm, as determined from theRD pattern considering that a major fraction of it is the anatasehase. With increase in heating temperature, the crystallinity is

ncreased with increasing particle size thereby reducing the photo-atalytic activity. This could be a reason for the 300 ◦C heat treatedoating to be better than a 400 ◦C HT coating. The same has beenbserved and reported by Ge et al. [18], who found that a 300 ◦Calcined TiO2 coating shows higher photocatalytic activity than a00 ◦C calcined coating. However, there could be one more reasonor the photocatalytic activity to reduce with heat treatment tem-erature beyond 300 ◦C. The substrates used here are soda limelass substrates with 13–15 wt% Na2O as one of the components.he diffusion of Na+ into the TiO2 layer takes place at temperaturesf 400 ◦C forming sodium titanate, which is reported to be detri-ental to the photocatalytic activity of TiO2 [17]. The Na+ diffusion

ould be negligible at 300 ◦C heat treatment, and hence, a higherhotocatalytic activity could be expected in these samples. Theame justification holds good for Nb2O5 photocatalytic coatings.owever, not many studies have been carried out in the literature

o study the effect of Na+ diffusion on the photocatalytic activity ofiobia coatings on glass substrates.

It is evident from the present studies that UV light induced

ydrophilicity as well as photocatalytic activity is more effectivehan just engineering the surface to be hydrophilic. Hence, the tita-ia and niobia photocatalytic coatings present higher antifoulingfficiency by inhibiting barnacle settlement than a plain silica coat-ng.

rm-A) and (b) as-dried and 400 ◦C HT Nb2O5 powders obtained from the respective

Nanoparticles-coated glass coupons do not show substantialbenefit in comparison with the uncoated sides in inhibiting thecomplete larval settlement. Similarly, a previous study has demon-strated that the coated surface of the technological objects createsa type of barrier that may lessen the toxic action of the chem-ical substances present in antifouling coatings [19]. The titania,niobia and silica coatings derived from the respective nanoparti-cle dispersions have been studied for the first time in a real-timefield investigation in a south Indian marine environment. Thereis some evidence from field studies of similar coatings in sea-water environments that upon conclusion of active exudation,these coatings lose their advantage. As noted earlier, the fieldand lab results of the epoxy based coatings immersed demon-strate that they are not good fouling release materials [13]. Thenanoparticle-derived coatings investigated in the present studyyielded better results when compared to epoxy based coatings.Among the three nanocoatings used in this study, TiO2 holds mostpromise for future use as the results shown are positive. These pos-itive results could be useful in replacing organic based coatingswhich are not recommended due to their toxicity. It is possible toobtain coatings with good photocatalytic activity by a low tem-perature heat treatment, which is a very important point whensuch coatings have to be commercially implemented on the opticalwindows of marine instruments. New approach should be basedon the analysis and inhibition of the most sensitive colonizationprocess that impedes further development of fouling on hard sub-strates.

4. Conclusions

Viable bacterial counts on metal oxide coatings were low whencompared to control. The present results show colonization ofbiofilms on the nanocoatings and clearly indicate biofilm forma-tion which in turn allows macrofouling organisms to deposit easily.It is possible that the adsorption of organic matter onto the coatings,leads to the formation of a barrier between the fouling organismsand the coatings. This may be responsible for the observed decreasein the antifouling resistance of the metal oxide coatings (9 daysonwards), which show settlement and growth of macrofoulants.

Niobia coatings along with titania and silica have been investi-gated for the first time in biofouling applications. The differentresults presented allow the authors to conclude that TiO2 presentsmore promise in minimizing biofouling on optical surfaces. Furtherstudies are in progress to study the mechanical properties of such

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cknowledgements

The authors are grateful to Dr G. Sundararajan (Director, ARCI)nd Dr S. Kathiroli, (Director, NIOT) for providing constant encour-gement throughout the course of this investigation. The authorsre thankful to Dr Tadashi Shinohara, National Institute for Materi-ls Science, Tsukuba, Japan for the nanoparticle dispersions and to. Venkatesh and Dr G. V. N. Rao for the SEM and the XRD analysis.hanks are due to Ministry of Earth Sciences for extending facilities.

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