Antimicrobial activity of transition metal acid MoO3 prevents microbial growth on material surfaces

Materials Science and Engineering C xxx (2011) xxx–xxx

MSC-03176; No of Pages 8

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Materials Science and Engineering C

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Antimicrobial activity of transition metal acid MoO3 prevents microbial growth onmaterial surfaces☆,☆☆

Cordt Zollfrank a,⁎, Kai Gutbrod a, Peter Wechsler b, Josef Peter Guggenbichler c

a University of Erlangen-Nuremberg, Department of Materials Science and Engineering 3-Glass and Ceramics, Martensstr. 5, D-91058 Erlangen, Germanyb LEONI Kabel GmbH, Stieberstraße 5, D-91154 Roth, Germanyc Laboratory for the Development of Healthcare Products, Leitweg 23, A-6345 Kössen, Austria

☆ The authors declare no competing financial interes☆☆ The authors are grateful to Dr. Carolin Körner (WNuremberg) for the kind gift of the titanium rods.⁎ Corresponding author. Tel.: +49 9131 85 27560; fa

E-mail address: [email protected]

0928-4931/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.msec.2011.09.010

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a b s t r a c t
a r t i c l e i n f o
Article history:Received 10 May 2011Received in revised form 22 August 2011Accepted 22 September 2011Available online xxxx

Keywords:Antimicrobial materialsTransition metal acidMolybdenum oxideAntibacterial coatingSol–gel technique

Serious infectious complications of patients in healthcare settings are often transmitted by materials anddevices colonised by microorganisms (nosocomial infections). Current strategies to generate material sur-faces with an antimicrobial activity suffer from the consumption of the antimicrobial agent and emergingmultidrug-resistant pathogens amongst others. Consequently,materials surfaces exhibiting a permanent antimi-crobial activitywithout the risk of generating resistantmicroorganisms are desirable. This publication reports onthe extraordinary efficient antimicrobial properties of transition metal acids such as molybdic acid (H2MoO4),which is based on molybdenum trioxide (MoO3). The modification of various materials (e.g. polymers, metals)with MoO3 particles or sol–gel derived coatings showed that the modified materials surfaces were practicallyfree of microorganisms six hours after contamination with infectious agents. The antimicrobial activity is basedon the formation of an acidic surface deteriorating cell growth and proliferation. The application of transitionmetal acids as antimicrobial surface agents is an innovative approach to prevent the dissemination ofmicroorgan-isms in healthcare units and public environments.

ts.TM, University of Erlangen-

x: +49 9131 85 28311.(C. Zollfrank).

rights reserved.

., Materials Science and Engineering C (2011)

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Health care associated infections (nosocomial infections: NI) are thefourth leading cause of disease and the most common complicationaffecting hospitalised patients in addition to a minimum of 175,000deaths every year in industrialised countries [1–2]. Reports from the USindicate that NI account for 2 million infections and 90,000 preventabledeaths per year [3]. Transmission of pathogens and resulting biofilmgrowth has also gained increasing importance in industrial applications(e.g. cooling towers of power plants), water treatment and sanitation,food packaging and public environments (e.g. public transportation).An increasing number of highly vulnerable patients together withemerging antibiotic-resistant microbes, especially methicillin-resistantStaphylococcus aureus (MRSA), vancomycin-resistant Enterococcusspecies (VRE) and Gram-negative microorganisms producing an extend-ed spectrum of beta-lactamases (ESBL) are encountered [4–7]. The situa-tion is aggravated by the fact that in the future there will be few newantibiotics under development to compensate for the increasing resis-tance, because of the high costs of research and clinical testing as wellas regulatory problems [8–11]. In view of increasing antibiotic resistance,

this trend may be considered dramatic and is seen in the USA, Japan andparticularly in Europe [12]. Unquestionably, barrier precautions likemeticulous hygienic measures, hand washing remain the main goalin the prevention of NI. However, it has been described that the surfacesof the inanimate environment such as instruments, cables, switches,accessories, doorknobs, bed gear, blankets and sanitary installationscan act as a reservoir for multi-resistant pathogens and in combinationwith invasive devices that bypass the body's natural lines of defenceprovide an easy route for infection [13,14]. Therefore, there is increasedevidence for the occurrence of NI connected to the transmission fromcontaminated surfaces [15–17], indicating that the concept of preven-tion cannot be limited to hand washing or antimicrobial biomaterials.Germ free surfaces close to patients in sensitive areas assume highestpriority. Therefore, effective strategies to reduce the number of NIby infection transmission through genuine bacteria free inanimate sur-faces will increase the state of health in society. Current approaches todecrease microbial contamination on inanimate surfaces are either pre-ventive or biocidal. The first category aims at preventing adhesion ofthe infectious agents on the surface through an anti-adhesive coating.These include poly(ethylene glycol) [18], diamond-like carbon [19],self-cleaning surfaces (Lotus effect) [20,21], and amphiphilic polymercoatings [22,23]. Since the infectious agents are not eliminated, theirpresence might be still a high risk for vulnerable patients. A morereliable approach is the use of biocidal coatings on materials surfaces[24]. Successfully applied technologies employ disinfectants such asTriclosan [25] or inorganic antimicrobials such as silver ions [26,27],

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copper ions [28] and photocatalytic agents (e.g. TiO2) [29,30]. Apartfor photocatalytically active materials, the techniques make use ofthe diffusion properties of the respective antimicrobial from the surfaceto the attachedmicrobes, hereby decreasing their viability rate. Existingantimicrobial modified surfaces suffer from a number of limitations,including the rapid release of the adsorbed antibiotic after implanta-tion [24]. This results in a time limited antimicrobial activity [31]. Anenhanced frequency of resistance of the infectious agents has to be en-countered by increasing exposure. This is eminently true for antibioticsand some disinfectants where cross-resistance with antibiotics hasbeendescribed.Microorganisms also developed resistance even to inor-ganic antimicrobials. For many existing antimicrobials e.g. copper andsilver ions cytotoxicity has been reported on mammalian cells [32],which limits their application in biomedical devices and healthcareenvironments. Last but not least, thesematerials often lack cost efficiency.It seems therefore essential to develop a new materials concept to copewith problems encountered with current technologies.

A cost-efficient method for the production of coatings with anantimicrobial effect might be the sol–gel technique. In a sol, smallparticles of a solid are dispersed in a liquid. Thereby, the precursorsare metallic or metalloid elements, which are surrounded by severalligands. By hydrolysis and a condensation step, a three-dimensionalnetwork is formed. Thereby, the viscosity is changed and the sol is trans-formed to a gel and finally to a solid, which normally is an oxide ceramic[33]. The sol–gel technique is an often usedmethod for the production ofthin layers of different materials for several applications, such as optic[34], electrochromic [35], photocatalytic [36] and antibacterial [37–39]use. Thereby, the antibacterial effect of standard systems strongly corre-lates to silver particles dispersed in the coating. Upon contact withwater, the silver will be dissolved by the formation of ions, whichare responsible for the reduction of the bacteria concentration. Thus,the antibacterial effect is time limited and strongly correlates to the silverconcentration.

Herein, the authors report on the development of an innovativeapproach for an antimicrobial coating using the antibacterial effectof transition metal acids. Molybdenum oxides (MoOx, 2bxb3) andtungsten oxides (WOx, 2bxb3) in contact to water are exceptionallyeffective agents against nosocomial pathogens such as Staphyloccocusaureus and Pseudomonas aeruginosa. Although molybdenum and tung-sten oxides are renowned for their gas sensing, [40] and electrochromiccharacteristics [35] in engineering applications, the anti microbial prop-erties of these materials have not been investigated so far. Recently, theantimicrobial properties of molybdenum and tungsten oxides wereshown for a number of substrates including polymers, metals, glassesand ceramics [41]. The investigated approach is a facile and cheap wayto produce long-lasting antibacterial surfaces. It can be applied to moststandard materials and thus can be a major goal for the reduction of NIin hospitals.

2. Materials and methods

2.1. Preparation of samples

In a typical experiment, the respective inorganic powders (Mo:3.8 μm;MoO2: 3.6 μm, andMoO3: 15.9 μm)were mixed with an acrylicresin powder (Transoptic, Buehler) attaining concentrations up to 50wt.-%. These high concentrations of the transitions metal acids wereselected at first to ensure a sufficient amount of the active inorganicparticles displayed on the surface of the samples. The powder mixtureswere pressed into small rods (2 cm in length, 1 cm in diameter) andsubsequently cured.

The PU tubes were manufactured by extrusion of a PU granulate.Various PU tubes containing different amounts of MoO3 filler particles(d50=0.8 μm) were fabricated. The MoO3 filled PU was prepared bymixing the PU granulate together with a given fraction of MoO3

Please cite this article as: C. Zollfrank, et al., Materials Science and Engin

particles (0.5, 2.0 and 5.0 wt.%). Unmodified and MoO3 filled PUtubes were manufactured on an existing industrial machine line.

Ti rods froman electronmelt process (Ti–6Al–4V) exhibiting a roughsurface with a diameter of 2 mmwere dip-coated in Mo containing sol,which was prepared by dissolution of metallic Mo in a mixture of aceticacid and hydrogen peroxide solution. The Mo oxide based gel coated Tirodswere air-dried and annealed at 300, 500 and 700 °C to assess theinfluence of the oxide phase formation during annealing of the Mooxide coating.

2.2. Investigated microorganisms

The antimicrobial activity of molybdenum oxide has been investi-gated with respect to S. aureus ATCC 25923 (MRSA) and P. aeruginosaATCC 10145.

2.3. Investigation of antimicrobial activity

Themicroorganisms are stored on slant agar at−25 °C single colonieshave been grown on Columbia agar with the addition of 5% defibrinatedsheep blood for 12 hours at 37 °C. The inoculum count was estimatedaccording to the Mc Farland standard [42]. In addition the concentrationwas evaluated by a photometric method: an OD of 0.8 at 475 nm revealsa concentration of 109 CFU/ml of Staphylococcus aureus. Final concentra-tions were determined with several 1:10 dilutions until colonies werecountable. A dispersion of 109 colony forming units (CFU)/ml in1/4 N saline was prepared by harvesting bacteria of an overnight cul-ture. The samples rods are incubated with a solution containing the109 CFU/ml for 4 h. This time is sufficient that bacteria are colonisingthe surface of the sample and start biofilm formation. After incubationthe samples were subsequently rolled onto an agar plate (OXOID TSBnutrient agar with the addition of 5% defibrinated sheep blood,OXOID) to document colonisation and then transferred into sterile1/4 N saline. This procedure was repeated every three hours for a totalof 12 hours. The agar plates were incubated at 37 °C for 24 h, Fig. 1a.At the same time negative controls were included in each experiment.By this method (“roll-on” culture method) not only the strength ofadherence, proliferation but also the bactericidal activity of microor-ganisms colonising a surface is assessed [27,43–45].

Additionally, the investigation of antimicrobial activity was docu-mented by a semi quantitative method [46]. A microplate lid wasequipped with test probes forming a ‘comb’. The teeth of the ‘comb’comprise the materials to be tested and fit precisely into the wells ofa commercial 96-well microplate. The teeth are colonised by turningthe lid upside down and filling it with a microbial dispersion. Afterremoval of the “comb”, the growth of released daughter cells intothe minimal medium was stimulated by addition of trypticase soybroth. Proliferation of microbes was followed online by optical densitymeasurements at 578 nm using a kinetic detection mode. Plates wereread with a microplate reader (Spectramax 250, Molecular Devices,Sunnyvale, California, USA). The processed data provided a time prolif-eration curve as testing result for each assay. If bacteriawere partially orcompletely inactivated by the sample surface they were able to seedonly few or even no cells resulting in lagging or absence of bacterialgrowth [47]. The time needed for a population to proliferate to an opti-cal density (OD) of 0.2 is defined as the onset time tOD. The differencetime tX of the onset time for the reference sample tOD,ref (no MoO3)and the onset time for the modified samples tOD,X were calculatedwith respect to the tOD,ref. An antimicrobial activity of the respectivesamples is defined for difference times larger than 6.0 correspondingto a threefold order of magnitude increase of onset of cell proliferation.

2.4. Cytotoxicity evaluation

Cytotoxicity was monitored using the 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay (MTT assay) [48]. This

eering C (2011), doi:10.1016/j.msec.2011.09.010


Fig. 1. a) Experimental protocol for the “roll-on” test to evaluate the antimicrobial activity of a materials surface. The sample rods were incubated with a bacteria dispersion androlled over a nutrient agar plate. The growth of transferred bacteria on the agar plates was evaluated. This procedure was repeated every three hours for 12 h. Tested agar platesfor S. aureus and P. aeruginosa for b) oxidised molybdenum, c) polymers modified with molybdenum metal, molybdenum dioxide MoO2 and molybdenum trioxide MoO3.

3C. Zollfrank et al. / Materials Science and Engineering C xxx (2011) xxx–xxx

assay measures the reducing potential of the cell using a colorimetricreaction and determines themetabolic activity of a cell. Viable cells willreduce the MTT reagent to a coloured formazan product, which can bemeasured by colorimetric techniques. This test system determines themetabolic activity of test cells and impairment by toxic substances.

For the measurement, a 24 h eluate of a 10 cm2 surface in 10 mlphysiologic saline was prepared. Dilutions 1:1, 1:2, 1:5 and 1:10 of theeluate with a dispersion of MRC5 mouse lung fibroblast cells wereused as control dispersions with 5×105 MRC5 cells in 1/4 N saline. Thenumber of test cells was adjusted to 5×105 cells in Dulbeccos modifiedEagleMedium. Simultaneously two control media were added: as nega-tive control for non-cytotoxicity 5×105 MRC5 mouse fibroblasts in1/4 N saline and as positive control for cytotoxicity MRC5 cells with0.1% copper sulphate. To 0.5 ml of these supernatants 10 μl of farmazanwas given. The colour change was measured in a photometer as opticaldensity (OD) at a wavelength of 475 nm after 4 h incubation at 37 °C.

3. Results

3.1. Investigation of antimicrobial activity

The “roll-on” culture method was applied as a screeningmethod forthe assessment of the antimicrobial activity of our modified samplerods, Fig. 1. The results indicate, that pure surface oxidised Mometal already showed antimicrobial activity, Fig. 1b. Very little colonieswere visible at 3 h, after 6, 9 and 12 h virtually nomicroorganismsweredetected. In contrast, negative controls showed unaffected growth ofbacteria throughout the entire experiment. To further elucidate, whichof the Mo phases are the active component, various Mo and MoOx

(x=2 or 3) loaded polymer samples were tested. Pure Mo did notshow any antimicrobial activity for P. aeruginosa. S. aureus showeda decrease in the number of detectable colonies after 9 h, Fig. 1c. In


contrast, MoO2 loaded polymer sample exhibited a noticeable anti-microbial activity. After 6 hmost of the infectious agentswere eradicatedfrom the incubated sample surface. The surface of the sample was free ofbacteria after 9 h. The antimicrobial activity drastically increased by theaddition of MoO3 particles to the polymer matrix. No infectious agentscould be transferred after 3 h in the roll-on test. This means, that the bac-teria vanished or were inactivated by the MoO3 phase displayed on thesurface of the polymer samples. This astonishing new effect of antimicro-bial activity for transition metal acids gives rise for a completely newmaterials surface design to prevent transmission of infectious agentsand thus to decrease the risk of NI.

3.2. Effect of MoO3 concentration

The potential of our materials concept using antimicrobial activeMoO3 particles was further evaluated on polyurethane (PU) hollowtubes, which are used as an isolation of electric cables. The outer sur-face of the PU tubes was subjected to the “roll-on” test to evaluate theantimicrobial activity, Fig. 2. Although, the entire polymer compositewas filled with inorganic particles, part of the MoO3 filler particleswere displayed on the surface of the PU tubes. A scanning electronmicroscopy (SEM) micrograph of an unfilled PU tube is shown inFig. 2a. The pure PU surface did not show any antimicrobial activity.Colonies of S. aureus and P. aeruginosawere transferred from the incu-bated tubes even after 12 h. The MoO3 particles of the modified PUtubes are clearly detectable in SEM investigation (back-scattered electronmode, BSE) as white spots, Fig. 2b–d. Whereas the particles were hardlyvisible at a filler content of 0.5 wt.%, the MoO3 agglomeration couldbe undoubtedly identified at filler concentrations of 2.0 and 5.0 wt.%.The inset in Fig. 2c shows a magnification of the agglomerate in rangeof a few microns consisting of submicron-sized MoO3 particles. Evalua-tion of the “roll-on” test distinctly showed a considerable antimicrobial



Fig. 2. SEMmicrographs (BSEmode) of the surfaces of extruded unmodified andMoO3 filled PU tubes and corresponding tested agar plates for S. aureus and P. aeruginosa: a) unmodifiedPU tube, b) 0.5 wt.% MoO3 (white circles highlight selected MoO3 particles), b) 2.0 wt.% MoO3 and c) 5.0 wt.% MoO3.


activity for MoO3 filler concentration of 0.5 wt.%, Fig. 2b. A substantialamount of bacteria was still viable after 6 h on the MoO3 modified PUtube, so that colonies of S. aureus and P. aeruginosa could be detectedon the agar plates. After 12 h colony density due to bacteria transferwas decreased. This means that the MoO3 exhibits an antimicrobialactivity, but themodified PU tubes were not free of bacteria. Increasingthe MoO3 fraction resulted in an increase of the microbial activity. Sig-nificant reduction of the bacteria on the PU surfaces was obtained for2.0 wt.% MoO3. Some transferred S. aureus were still present after 6 hwhereas P. aeruginosa was nearly absent. At 5.0 wt.% MoO3 filler con-centration, the modified PU rods were virtually free of bacteria after9 h. Both S. aureus and P. aeruginosa were hardly detectable after the


“roll-on” test on the agar plates. A SEM investigation of S. aureus onunmodified surface of the PU tubes showed, that the cells are intactand do not show any degradation effects, Fig. 3a. Contrary, S. aureus onMoO3 filled PU tubes (5.0 wt.%) exhibited severe deterioration after60 min of the cells, which resulted in cell death, and thus inactivation ofthe infectious agent, Fig. 3b. A similar SEM observation could be obtainedon E. coli. The bacteria are fully intact on unmodified PU tube surfaces,Fig. 3c, whereas degradation of the cells was clearly visible on 5.0 wt.%MoO3modifiedPU tubes after 10 min, Fig. 3d. As a result it could be dis-tinctly shown, that MoO3 modified PU tubes, even at low concentrationof MoO3 (0.5 wt.%) exhibit a distinct highly efficient antimicrobial activ-ity. Increasing thefiller concentration to 5.0 wt.% resulted in an enhanced



Fig. 3. a) Individual cell of S. aureus on an unmodified surface of a PU tube (60 min), b) deteriorated cell of S. aureus on PU tube containing 5.0 wt.-MoO3 60 min after incubation,c) intact E. coli. on an unmodified surface of a PU tube (10 min), d) deteriorated cells of E. coli. on a PU tube containing 5.0 wt.-MoO310 min after incubation.


antimicrobial activity of the modified PU tubes, which was sufficient toremove almost all bacteria after 9 h contact on the tube surface.

The semi-quantitative method for the investigation of antimicrobialactivity showed typical time-proliferation curves of microorganisms forthe microplate proliferation assay of our samples, Fig. 4a. The unmodi-fied samples were measured as a reference and the correspondingcurve shows uninhibited growth. The curves of the MoO3 modifiedsamples were shifted indicating a delayed proliferation. The calculateddifference time t0.5 from the diagram for the 0.5 wt.% MoO3 modifiedwas 13.5, and the calculated difference time for t2.0 and t5.0 were 17.5and 33.0, respectively. Since these values are well above the thresholdvalue of 6.0, the highly efficient antimicrobial activity of the MoO3

modified samples was independently confirmed.

3.3. Cytotoxicity measurements

The high measured OD of the negative references and our modifiedMoO3 samples indicates, that the MoO3 modified samples are non-cytotoxic, in contrast to the copper containing positive samples, wherethe low observed OD values points to the expected high cytotoxicity,Fig. 4b.

3.4. Temperature treatment

Further evidence of the antimicrobial activity of MoO3 and theactive phases was obtained on modified titanium (Ti) rods, Fig. 5. TheMo oxide based yellow gel coating is clearly visible on the as-prepareddip-coated Ti rods, Fig. 5c andd. According toX-ray diffraction, the nearlyamorphous gel coating consisted of a mixture of molybdates containing[Mo3O10]2− ions and hydrated MoO3, Fig. S1. After annealing the gel


layer was transformed into slightly blue crystalline MoO3 with broadpeaks consisting of a mixture of monoclinic and orthorhombic crystalstructures. The appearance of the coating did not change. The antimi-crobial properties were assessed using the “roll-on” test. The Mooxide gel coating on the Ti rods treated at 300 °C exhibited an excel-lent antimicrobial activity against P. aeruginosa. Transmission of thebacteria was no longer observed after 3 hours indicating a bacteriafree materials surface, Fig. 3e. The annealing treatment resulted infurther crystallisation of the gel product. The fraction of MoO3 ortho-rhombic phase was increased. However, a significant decrease of theantimicrobial activity was observed for samples treated at 500 °C.There was a considerable transmission of bacteria even after 12 h.A distinct antimicrobial activity could no longer be observed for thesamples treated at 700 °C. The samples were fully infectious after12 h. After annealing at 700 °C, the nearly white MoO3 was well crys-tallised and consisted almost exclusively of the orthorhombic crystalphase. For reason of comparison, a silver containing Ti-alloy, whichwas expected to show an enhanced antimicrobial activity was alsotested, Fig. 5f. The results indicated extensive transmission of bacteriaafter 12 h. As a result, Ti metal alloy, even if it contains silver, does notshow a noticeable antimicrobial activity. On the other hand, it couldbe definitely shown, that the sol–gel derived Mo oxide coating on Tialloy exhibits an extraordinary effective antimicrobial activity afterannealing at 300 °C. By further annealing and transformation into crys-talline Mo oxide phases, the antimicrobial activity is lost. The crystalstructure development with temperature indicates, that the mono-clinic phase might be correlated with the antimicrobial activity. Itcould be, however, shown for the first time, that sol–gel derived Mooxide coatings are efficient coatings to prevent growth of bacteria on in-animate surfaces.



Fig. 4. a) Evaluation of the antimicrobial activity using a semi quantitative microplateassay, [48] b) assessment of the cytotoxicity of the MoO3 modified samples.


4. Discussion

The antimicrobial principle of transition metal oxides (MoO3) canbe related to an acidic surface reaction (release of hydroxonium ions)according to the following reactions (1) and (2):

MoO3 þ H2O⇄H2MoO4 ð1Þ

H2MoO4 þ 2H2O⇄2H3Oþ þMoO2−

4 ð2Þ

First molybdic acid in the hydrated form of molybdenum trioxideis formed on the surface of the modified materials, reaction (1). Thesimplest solid form is the monohydrate MoO3×H2O (H2MoO4), butthe dihydrate MoO3×2H2O is also known. Hydroxonium ions (H3O+)will be released from H2MoO4 in the presence of water forming the cor-responding molybdates (MoO4

2−). In the equilibrium state, the molyb-dates will be retransformed into molybdic acid H2MoO4.

Acidic surfaces are well known to generally slow down bacterial andfungal growth and effectively kill microorganisms at pH values of 3.5–4.0 (e.g. staphylococci, streptococci, enterococci, Legionella pneumophila,Lactobacillus acidophilus spp., Candida spp., Aspergillus spp.). Many gram-negative microorganisms are killed at even higher pH values up to5.5 (e.g. E. coli, Pseudomonas aeruginosa, Clostridia, Campylobacter)


[49–52]. Thus, this mechanism is fairly non-specific (active againsta broad spectrum of gram-positive and gram-negative bacteria) and,in contrast to antibiotics, does not produce bacteria resistant to thismode of action. The acidic surface inhibits in many cases proliferationof the cells and the formation of biofilms leading to elimination of theinfectious agents within six to nine hours. The acid activity refers tothe diffusion of H3O+ ions through the cell membranes. This results ina distortion of the sensitive pH-equilibrium as well as the enzyme andtransport systems of the cell [53]. There is also a disruption of theDNA helix [54]. Furthermore, energy is required from the cell to adjustthe distorted pH-equilibrium resulting in further weakening of the cell[55]. Microorganisms growing in an acidic environment experiencealso an efficient blockage of adherence and biofilm formation. However,the direct contact of the microorganism with the acidic materials sur-face appears to be important.

As described above, it is necessary for the antibacterial effect that thematerial is in contact with water. Despite this, leaching and dissolutionof MoO3 is not relevant in our approach since we are dealing withinanimate materials surface that is not exposed to a permanent aqueousenvironment. The only contact with water might be through touchingthe surfaces (e.g. healthcare staff, patients) as well as general humidity.Under such “dry” static conditions leaching or dissolution of the MoO3

from the materials surface will only occur to a small amount, if any atall, and will only be of minor relevance. Additionally, it is well known,that MoO3 exhibits water-solubility only at high pH-values. The solubil-ity of MoO3 in water at neutral pH is 56.0±0.1 mg/l [56], and thusvery low. Interestingly, an antimicrobial activity of a MoO3 particledispersion in water could not be confirmed [57]. It was shown, thatMoO3 up to a concentration of 1 g/l had no effect on cell counts ofAcinetobacter sp. and several other bacteria. These results indicatethe low cytotoxicity and the importance of a direct contact betweenthe bacterium and the material surface.

5. Conclusions

We presented a novel materials concept to permanently preventgrowth of infectious agents (bacteria) on various materials surfaces,which were modified with transition metal oxides that can releasehydronium ions in contact with water. We confirmed, that MoO3

exhibits ahighly efficient antimicrobial activity towards severe infectiousagents such as S. aureus and P. aeruginosa. Polymers (PU) and metal (Ti)surfacesmodifiedwithMoO3 particles or sol–gel based coatingswere vir-tually free of bacteria within 6 h after incubation with an infectious solu-tion. We could show, that the MoO3 modified samples are not cytotoxic.The antimicrobial activity of transition metal acid MoO3 is related totheir surface acidity involving the intermediate formation of molybdicacid. Our developed materials concept is applicable to almost any mate-rials surface, because the antimicrobial active agents can be introducedusing conventional processing extrusion or in coatings. Materials withan antimicrobial coating are important to decrease the transfer andspreading of infectious agents wherever public interaction is involved.The modification of materials surfaces with antimicrobial MoO3 mightbe therefore important for materials used in public transportation andother frequented locations. The decrease of nosocomial pathogens suchas S. aureus and P. aeruginosa is extremely relevant in healthcare environ-ments, where transfer of infectious agents can cause severe infections ofalreadyweakened patients. Our developedMoO3 based antimicrobialmaterials concept is of general importance to decrease the risk ofhealth care associated infections, which is one of the most importantcauses of disease at present. Therefore, the modification of materialssurfaces with antimicrobial active transition metal acid is also ofgeneral interest to decrease transmission of infectious agents in publicenvironments.

Supplementary materials related to this article can be found onlineat doi:10.1016/j.msec.2011.09.010.



Fig. 5. SEM micrographs of the a and b) Ti rods and c and d) MoO3 gel coated Ti rods, e) antimicrobial activity of the MoO3 gel coated Ti rods after annealing at 300, 500 and 700 °C,f) antimicrobial evaluation of a reference titanium alloy sample.


Please cite this article as: C. Zollfrank, et al., Materials Science and Engineering C (2011), doi:10.1016/j.msec.2011.09.010



Acknowledgment

The authors gratefully acknowledge financial support from theGerman Research Foundation (DFG) under contract ZO113/13-1.

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