f. Applied Membrane Science & Technology, Vol. 10, December 2009, 21-33© Universiti Teknologi Malaysia

Effect of PVP Addition in the Preparationof Polyethersulfone (PES)-AgN0

3

Antibacterial Membrane

H. Basri', A. P. Ismail'" & M. Aziz-

1,2&3Advanced Membrane Technology Research Centre, Faculty of Chemical and Natural Resources Engineering,

Universiti Teknologi Malaysia, Skudai, 81300 [ohor Bahru, [ohor, Malaysia

'Department of Science and Mathematics, Faculty of Science, Art and Heritage,

Universiti 'Iun Hussein Onn Malaysia, Batu Pahat 86400, [ohor, Malaysia

"Chemistry Department, Faculty of Science, Universiti 'Ieknologi Malaysia,

Skudai, 81.300 Johor Bahru, [ohor; Malaysia

ABSTRACT

Association of antibacterial agent into polymer-based material is an interesting, recent extension in membrane

technologies research area. Asymmetric polyethersulfone-silver nitrate (PES-AgN03

) nanocomposite

membranes were synthesized by wet phase inversion with polyvinylpyrrolidone (PVP) as additive. Three different

molecular weights (MW) of PVP in the range of 10 K to 360 K were tested for their ability to stabilize and

disperse silver in the polymer solution. Results showed that PVP 360 K plays a significant role in altering silver

particle size with better dispersion compared to the membranes prepared without addition of PVP and with the

addition of lower MW of PVP. Results agreed with the leaching detected by ICP-MS where silver loss during

membrane fabrication for PES-Ag"P360 is only 0.4636 ppm (0.0093%). The antibacterial activity for the resultant

membranes was tested using inhibition zone method. Results indicated that PES"AgN03 and PES-AgN03~PVP

(loaded with only 0.5 wt% silver) were antibacterial to Escherichia coli and Staphylococcus aureus. The improvement

of antibacterial properties of the prepared membranes could be primarily attributed to the PVP addition as well

as the surface morphology and permeation properties.

Keywords: Polyethersulfone, polyvinylpyrrolidone, silver leaching, antibacterial

1.0 INTRODUCTION

Antibacterial compound is defined as a syntheticor natural compound that destroys bacteriaor suppresses their growth or their ability toreproduce [1], There are many antibacterialproducts commercially available for cleaningand hygienic purposes. In addition, invention

>:< Corresponding to: A F. Ismail (email: afauzi@utm.my)

on loading/incorporating antibacterial agentinto our daily needs such as clothing, soap,containers for food-packaging and waterfiltration system is seriously under taken. Watertreatment and effluents disinfection by usingmembrane technologies has been growinginterest topic recently due to the environmentalfriendly factor as compared to the commondisinfectant, chlorine. Therefore the combinationof membrane technologies and the relevancyof antibacterial agent incorporation such assilver is an area that should be seriously lookedinto.

22 H. Basri, A. F. Ismail & M. Aziz

Silver is a metal that has potential intechnological application such as catalysis [2],sensing [3], environmental [4] and biomaterialsdue to its distinctive properties such as goodconductivity, chemical stability, catalytic andantibacterial activity [4-5]. Therefore, extensiveresearch were done on loading silver into polymerin order to produce composites for antibacterialapplication such as clothing, membrane forwaste water treatment and food packaging. Thefact that silver can be dispersed well in variouspolymer matrix have attracted broad scientificinterest including optoelectronics, sensing andbiomaterials [3,6-7] applications. Previousresearch for polymer-silver loading has been doneon polyacrylonitrile (PAN) [8-9], polyurethane(PU) [10-11], polyimide (PI) [12], celluloseacetate (CA)[13-14] polysulfone (PSI) [15] andchitosan [16] embedded with silver for clothing,food-packaging and water treatment applications.Since polymer is known to be an excellent hostmaterial for metal, polyetbersulfone (PES)which is widely used for microfiltration (MF),ultrafiltration (UF) and gas separation membranes,bad been chosen in this study. Besides, PES isprincipally favorable due to its characteristicsof wide temperature limits, wide pH tolerances,fairly good chlorine resistance and good chemicalresistance to aliphatic hydrocarbons, alcohols andacids [17-19].

Poly(vinyl pyrrolidone) (PVP) is known asa water soluble polymer which had frequentlybeen studied to see the effects as additive inmembrane fabrication. It is also known as an agentfor suppressing macropore formation in phaseinversion membranes. In Rafizah and Ismail'swork, PVP with MW 10, 000 had been used asa sizing agent to promote the adhesion betweencarbon molecular sieve (CMS) with polysulfone(PSf) [20J. Other studies by Ismail and Hassandeduced that addition of 1 wt% of PVP (MW40,000) has increased membranes fluxes aswell as decreased salt rejection [21]. The effectsof adding PVPwith higher MW (40 K and 360 K)also resulted in significant morphology changefrom sponge-like morphology to finger-likemorphology in polyimide(PI)/NMP/watermembrane system [22].

Loading metal into polymer matrices has fewadvantages including to protect the metal, atthe same time trapped metals can be distributeevenly and finally made the fabrication easier. Inaddition, physical properties of metal can benefitthe performance of polymer embedded metal. Asuccessful preparation of metal embedded polymeris determined by the ability to produce particleswith uniform distribution and long stability, giventheir tendency to rapidly agglomerate in polymersolution. For polymer-silver, most of the releasingproducts are in the forms of sheet, sponge andfiber. Several methods have been establishedfor the preparation of various silver-polymercomposite including chemical reduction [23],photolysis [24] and in-situ fabrication [11,17,25J. The methods of preparation may lead to smalldifferences in physical and chemical properties ofthe end product.

To the best of our knowledge, this paperdescribes an easy synthetic route to load silvernitrate into PES by using PVP as dispersant.The PES-AgN0

3membranes were characterized

by various techniques to prove that silver issuccessfully loaded to PES which then exhibitantibacterial activity against Escherichia coli(Reali) and Staphylococcus aureus (S.aureus) inan inhibition zone test.

2,0 MATERIALS AND METHODS

2.1 Materials

PES was supplied by Solvay Advanced Material(USA) and N-Methyl-2-pyrrolidone (NMP) waspurchased from Merck. PVp, with average MW of10,000 (P10), 40,000 (P40) and 360,000 (P360)and silver nitrate, AgN0

3(analytical reagent

grade) were supplied by Pluka. For antibacterialtest, microbiology agar was purchased from Merck.Chemicals were used as received.

2.2 Preparation of PES-silver Membrane

PES, AgN03

and NMP was used as the basepolymer, as an additive as well as antibacterialagent and as the solvent respectively. PVP wasalso used as pore-former as well as dispersant

Effect of PVP Addition in the Preparation of Polyethersulfone-Agf-O, 23

which prevent particle aggregation and control theaverage particle size of silver in polymer solution[26~28]. To see the effects of PVP addition onesolution was prepared without PVP (as control).Detailed membrane composition is listed inTable 1. PES was dried in an oven at 120°Covernight before dope preparation. An in situapproach has been applied in preparing PES-AgN0

3

membrane [16-17] where PVP and AgN03

wereallowed to react in NMP with magnetic stirringfor an hour (Solution A). First, silver nitrate wasdissolved in NMP. Then, PVP was added slowlyand the agitation continued for an hour. In otherbeaker, after PES completely dissolved in NMP(solution B), A was added into B. For all solution,PES weight percent concentration is 15% withAgN0

3of 0.5%. The studied PVP/AgN0

3ratio is

constantly kept to -1 due to the work done by Leeet al. to discover that PVP/AgN03",1 is the bestto produce high dispersion and substantially smallparticle of silver [29]. The homogeneous solutionof PES/AgNO/PVP was agitated continuously forat least 24 hours. Control solution was prepared bydissolving silver nitrate in PES/NMP solution.

Flat sheet membrane was prepared accordingto the dry/wet phase inversion technique, asdescribed elsewhere [21, 30], The solution waspoured onto a clear, flat and smooth glass platethat was placed on the trolley. Stainless steelsupport casting knife was used to spread thesolution to a uniform thickness. The glass platewith the membrane filmwas then immersed into acoagulation bath containing tap water. During thisprocess, solvent exchange occurred and solidifiedthe thin film to a complete membrane structure. To

ensure all the solvent in the composite structureis removed, membrane was immersed in thecoagulation bath for one day. As the final stageof membrane fabrication, membrane was left air­dried for 24 hours.

2.3 Characterization of PES-silver Membranes

Thermogravimetric analysis (TGA) usingMettler Toledo thermogravimetry analyzer (TGATS0800GCIA) was performed to investigatethe compatibility of PVP with PES. Membraneswere prepared for thermal analysis as describedelsewhere [20]. Heating were carried out from50 to 750°C with a heating rate of 20uC/min.

Attenuated Total Reflection Fourier transformedinfrared (ATR-FTIR) was used to correlate thechanges in chemical bonding before and after silverand PVPwere added. The Ik-spectra were recordedon Thermo Nicolet 5700 ATRFTIR spectroscopy,which is supplied by Thermo Nicolet Corporation& Spectra Tech,USA. From the spectra, qualitativedifferences in the distribution of functional groupswere investigated.

Morphological studies were done by recordingSEM images using Ziess Supra 35 VP Field­Emission Scanning Electron Microscope,(FE­SEM). Silver content in membrane was evaluatedby energy dispersive spectroscope (EDS) whichis coupled with FE-SEM. For cross-section image,sampleswere dried and then fractured cryogenicallyin liquid nitrogen before mounting on samplestubs. The samples were then sputtered with athin layer of gold using a sputtering apparatus[21,31]. After gold sputtering, the samples were

Table 1 Composition of casting solutions

PES-AgN03 PES-AgN03-PIO PES-AgN03-P40 PES-AgN03-P360

PES 15.09 15.02 15.00 15.03

NMP 84.51 84.18 84.01 84.23

PVP-K15 0.52

PVP"K30 0.51

PVP-K90 0.51

AgN03 0.50 0.53 0.51 0.50

Coagulation bath content: tap waterMW P 10 '" 10,000, P 40 '" 40,000, P 360 = 360,000

24 H. Basri, A. F. Ismail & M. Aziz

examined using FE-SEM coupled with EDS withpotentials of 10.0 kV and magnifications rangingfrom 500 to 50,000x.

The membrane performance evaluation is doneby measuring the pure water permeation where thepermeation test was conducted by using a simplecross-flow permeation cell as described elsewhere[21]. Circular membrane discs were cut andmounted in a stainless steel cylindrical membranetest cell by a porous support and tightened by arubber O-ring. Effective permeation area of eachmembrane was about 13.2 cm-. Feed pressurewas controlled at 1-6 bar while the permeateside was opened to the atmosphere. Experimentswere carried out at ambient temperature(2JOC). Membrane characterization of pure waterpermeation (PWP) for the PES membrane wascalculated from the equation

PWP = QAxM

Where Q is volume of the permeate (L), A ismembrane surface area (m-) and ar is permeationtime (hour).

The antibacterial activity of resultant membraneswas tested by an inhibition zone method [19]. Inthis method, a nutrition agar culture medium(nutrient Agar, Merck) was melted and pouredin a Petri dish and solidified. Two kinds ofdifferent bacteria including Gram-positive bacteria(S.aureus) and Gram-negative bacteria (E.coli)were used. For measurement of the antibacterialactivity, membrane samples were punched to makedisks (diameter = 24 mm), and were put ontothe petri disk which have been inoculated withbacteria culture. The plates were visually examinedfor possible clear zones after incubation at37'C for 24 h. The presence of any clear zone thatformed around the membrane disk on the platemedium was recorded as an indication of inhibitionagainst the bacteria species. Suspension cultureswithout any membranes were used as a control.In this study, the four silver nitrate-loading PESmembranes were put on the agar surface and thenincubated at 37'C for 1 day.

During fabrication, the loss of silver wasassessed by measuring its concentration in thenon-solvent immersion bath, determined with a

Perkin Elmer Inductive Coupled Plasma MassSpectrometry (ICP-MS), model ELAN 6000 withargon as gas carrier. Reagents used for ICP-MSanalysis were ultrapure nitric acid and single­element plasma grade standards in 1-2 % nitricacid.

3.0 RESULTS AND DISCUSSION

By introducing PVP into PES-AgN03

, silver ions,Ag" is expected to coordinate with N in PVP or° in PVP or PES, Eq 1, 2 and 3 [27, 28J. Thecoordinate bond formed is responsible to theprotection from agglomeration and also reductionin silver leaching during fabrication. The detailedchemical reactions can be expressed in Eq (1-3)[27,28J.

TGA curves shown in Figure 1 show thethermal decomposition of PES-AgN0

3and PES­

AgN03-PVP

(PES-Ag-PI0/40/360) occurred ina single step starting at 450, 408, 395 and 350DC. The weight loss for PES-AgNO, membranesis in the order of PES-AgN0

3>PES-AgNO,­

P360>PES- AgN03-P40>PES-AgNO,-PI0

andthe loss in percent are (59.3%), (58.9%), (58.7%)and (57.3%) respectively. Results show PES­AgN0

3degrades latter than PES-AgN0

3-PVP

by mean, has better thermal stability relativelycompared to PES-AgNO,-PVP. When AgN03 wasadded into the PES-PVP matrix, they chemicallyinteract with the PVP molecules preferentially;as suggested in Eq. (1-3). However, the additionof only 0.5 wt% PVP of different molecularweight does not affect very much on the thermalstability, hence particularly PES-AgN0

3-P360

degrades at the lowest temperature, 350'C. Hereit is to be noted that the thermal degradationpattern of PES-AgN0

3and PES-AgNO,-PVP

(PES-AgN03-PI0/40/360) membranes are quitesimilar.

In order to investigate the interaction ofAgN03-PVP, PVP-PES and PES-AgN03 , ATR­FTIR spectra of the top surface on membranes aremeasured and showed in Figure 2. It was foundthat all the IR spectrum showed no apparentdifference in the range of 700-1600 crrr': onlyabsorption peak arising at 1666 cnr' which isattributed to amide carbonyl group from PVP [30,32]. Absorption peaks at 10lland 1106 crn' which

Effect of PVP Addition in the Preparation of Polyethersulfone-AglxO, 25

%0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 min

PES-Ag

00 PES~Ag-P10

90

80

70

60

50 PES-Ag-P360

4050 100 150 200 250 300 350 400 450 500 550 600 650 700 750 "C

Figure 1 Thermo gram of (a) PES-AgN03

and (b) PES-AgN03PIO, (c) PES-AgN03P40, (d) PES AgN03

P360 membranes

1700 1600 1300 1200 1100wavenumbers cm-1

1000 900

Figure 2 ATR-FTIR spectra of (a) PES-AgN03-PIO, (b) PES-AgN03-P40,

(c) PES-AgN03-P360,

(d) PES, (e) PES-AgN03

26 H. Basri, A. F. Ismail & M. Aziz

originally (PES- AgN03) correspond to stretchingmode of c-o bond has been weakened by theexistence of C-N at the close region (1074 and1019 cnr') similarly as studied by Wang et at. [27].The other reason might be due to coordination ofsilver to N, which strengthened the steric effectand prevented coordination of silver to ° [27]experienced that silver with size smaller than50 nm can be produced when silver is coordinatedwith N or else larger particle will be formed ifAg is coordinated to 0, as been discussed in the

. following subsection.Loading PVP as well as silver in PES might

induce specific changes in membrane morphologywhich in turn would affectthe porosity, macrovoidformation and also permeability of membranes.For example, it has been reported that by addingPVP into PES [33], PSf [34] and PVC [35],membrane produced will have the properties ofhigh porosity with well-interconnected pores andsurface. It is possibleto obtainsilverin PESmatrixbut the amount of particles is always reduced fromthe amount considered eluring dope preparationdue to silver leaching which occurred duringfabrication [11, 15-17]. Therefore, the additionof PVP is to overcome this problem in order toproduce membrane with better antibacterialproperty.

As shown in Figure 3, the membrane outersurfaces were recorded at 10,000 x magnificationswhile cross section images were analyzed at 25,000x magnifications. The EDS spectra confirmedthe presence of Ag component on membranesurface. It is clearly seen that silver distributiongets better after PVP addition. In addition, silvercontent analyzed by EDS techniques confirmedthat silver content in membranes get higher. Thepresence of PVP having molecular weight of

40,000 and 360,000 in membrane formulation(PES-AgN0

3-P40and PES-AgN0

3-P360)

(Figure 3) had also tremendously alter the silverparticle into smaller size, as listed in Table 2with reduction 63-74 % relatively compared toPES-AgNO,. The results agreed with Wang et.al., which PVP can protect Ag from growing andagglomerating [27]. However, by adding PVPwith molecular weight 10,000, as shown inFigure 3~ silver particle size is similar to the silverfrom the control (PES-AgNO,). This is due toPVP with smaller size (P10) may cover silversurface in general, reduce the protection fromagglomeration and therefore produced largediameter of silver particle [11]. Nevertheless,for PVP with higher MW(P40 and P360),protection from agglomeration occurred thatmade the particles obtained smaller. Studies byTaurozzi also noted that silver which formed insitu in PSf matrix resulted in smaller particlesize and more homogeneously distributed ascompared to the one prepared ex situ [18].Table 2 lists the silver particle size and the silvercontent of resultant membranes obtained fromFE-SEM images and EDS spectra respectively.

The cross sectional morphology of allmembranes shows finger-like structure, agreed toPSf/Ag obtained by Taurozzi et at. [18J in whichsilver were observed embedded within the polymerof the interpore walls and most nanoparticles werelocated along the pore surface. This result howeverdiffer from the one obtained by Chou et at. forcellulose acetate with silver loading hollow fiber[26J and Yu's work on PAN-based hollow fiberwith silver nitrate loading [11] which producedsponge-like structure. The different morphologyobtained might be ascribed to different polymerused.

Table 2 Analysis from FE-SEM micrographs

Membranes

PES-AgNO,

FES-AgNO,-P10

PES-AgNO,-P40

PES-AgNO,-P360

Content

PES-AgNO,

PES-AgNO,-PVP 105

PES-AgNO,-PVP ](30

PES-Ag-PVP ](90

Ag particle size (nm)

129.5

295.6

33.5

48.4

Skin layer thickness (nm)

51.4

34.6

54.7

30.9

Effect of PVP Addition in the Preparation of Polyethersulfone-Aglvtr,

Left (outer surface) Right (cross section)

Figure 3 FE - SEM surface and cross section images of membranes

27

28 H. Basri, A. F. Ismail & M. Aziz

0'

'-K) ~-\ )-+" Ag

0"

to ~-o----Ag t' \ j_S \ J ii •

11--0-.... Ag. (3)

[CH,-CH]

~.

C~O_ Ag (2)

+Ag'-~ J

Membrane permeability tests were conductedto make sure flat sheets are strong enough to avoidcollapse or bursting [11] for the pressure appliedwithin 1~6 bars. Furthermore, the hydraulicpermeability of pure water is an important indexfor the mass-transfer performance of PES-AgN03

antibacterial membrane. Figure 4 depicts the PWPresults of PES-AgN0

3membrane with and without

addition of PVP. As can be seen, membranepermeability increased linearly with increasingoperating pressure. It is because ultrafiltration(UP) membrane is one of the pressure-drivenmembrane processes. Results also revealed thatPES-AgN0

3exhibited the highest PWP followed

by PES-AgN03-P360.

Other than as dispersant,PVP is also known as a pore-forming agent;therefore, by adding PVP, membrane porositycould be affected, and thus explained for the lowPWP exhibited by PES-AgN0

3-P360.

Elemental silver has been believed to exhibitantibacterial effect against Staphylococcusaureus (5. aureusi and Escherichia coli (E. coli)[4,11,17,19,36]. A study done by Choi and co­workers revealed that silver nanoparticles andsilver chloride colloids strongly inhibited microbialgrowth depending on their size and bioavailability

(1)[CH,-CH]

I Ag_

(/N~o

+ Ag' ----.l

--"'"----~'"'"-- ..---,.--.--<O--+----+--<O---.~-----------..,,-""'_x ..... "" ..-"-..."

.- 400j--~----------~-----:I:a:::::!.a. 300~--

~

• PES-AgNO,-P1

Ii PES-AgNO,-P4

.. PES-AgNO,-P3

XPES-AgNO,

6

100l x ---ill

°--~J~-,---- 9! .__2 4 8

Figure 4 Pure water permeability (Lrrr-hr'] for PES-AgN03-PIO, PES-AgN03"P40, PES-AgN03"P360

and PES-AgN03

Effect of PVP Addition in the Preparation of Polyethersulfone-AgNO, 29

[28]; while 100% inhibition againstE. coli can bereached when concentration of silver ion (Ag')is as low as 4.2 !Lm. The principal antibacterialmechanism of silver was including inhibition ofprotein synthesis and the metabolic pathway inmicrobes as well as interference with nucleic acidand cell wall synthesis [37J. The antimicrobialactivity of silver is dependent on the silver cationAg-, which binds strongly to electron-donatinggroups in polymer matrix containing sulfur, oxygenor nitrogen. Hence the silver based antimicrobialpolymers have to release the Ag" to a pathogenicenvironment to be effective. Upon in contact withbacteria, silver will immediately inhibit the bacteriagrowth through the mechanism mentioned.

Previous studies have been done onincorporating silver, as an antibacterial agentinto polymers for the application of waste-watertreatment, clothing and food packaging. Forexample Ma and co-workers prepared chitosan, aknown biopolymer which also exhibit antibacterialproperty [38-39] blended with nylon-ti, chelatedwith silver ions, has resulted in an antibacterialbiopolymer potentially for food-packing material[19J. Yu [11], Son [10], Chou [17] and Taurozzi[18J who incorporated silver into polymer matrixfor waste water treatment reported silver leachingin the filtration but still exhibit antibacterialactivity against S. aureus and E. coli.

Using the disk diffusion method, thebactericidal effects of PES-AgNO" PES-AgNO,P10, PES-AgNO,-P40 and PES-AgNO,-P360 wereevaluated comparatively [19J. After 24 hoursincubation at 37°C, the membranes showedantibacterial effect on Gram positive S. aureus andGram-negative E. coli. The diameter of inhibitionzone for the PES-AgNO,P10, PES-AgNO,P40and PES-Ag-P360 is slightly larger than that forthe PES-AgNO, (see Figure 5), obviously seen in

S. aureus plates. PES-AgN03

membranes havebeen believed to function antimicrobially eitheras a release system for silver ions or as a contact­active material.

Even though silver has successfnlly beenincorporated in membrane matrices as provenby FE-SEM and EDS, silver-leaching duringfabrication still occurs. However by doping PVPinto the formulation, silver-loss has been reduced.Table 3 shows satisfactory resnits obtained fromthe analysis of silver content in the coagulationbath during fabrication by ICP-MS. Silver leachingis may be due to non-attached silver particles onthe membrane surface region which are moreaccessible to water (coagulation bath) [11].Other factors that may contribute to leaching ismembrane porosity as well as silver nanoparticlesize, by mean if the particle's size is larger thanits pore, it may easily be leached out during phaseinversion process.

4.0 CONCLUSION

The effects of PVP in the preparation of PES­silver membrane have been investigated in thiswork. PVP-K90 (MW 360 K) has shown greatcontribution in reducing silver-loss duringfabrication due to the attachment that bound thesilver ion and prevent it from leaching during phaseinversion. The use of PVP-K90 as additive has alsoaltered the silver nanoparticles size, to improve theupon-contact with bacteria during antibacterialactivity. By incorporating only 0.5 wt% of silvernitrate, the resultant membranes exhibited theantibacterial activity against S. aureus and E.coli. In summary, incorporation of silver intoPES membrane matrix appears to be an attractiveapproach in the design of antibacterial membranefor waste water treatment.

Table 3 Silver loss during fabrication

Membranes

PES-AgNO,

PES-AgNO,P10

PES-AgNO,P40

PES-AgNO,P360

Silverloss (pm)

17.3687

10.8623

1.6556

0.4636

Silverloss (%)

0.3474

0.2172

0.0331

0.0093

30 H. Basri, A. F. Ismail & M. Aziz

Reali S.Aureus

Figure 5 Photographs of the antibacterial test result of tbe membranes against E. Coli (left) and S.aureus (right)

Effect of PVP Addition in the Preparation of Polyethersullone-AgNO, 31

ACKNOWLEDGEMENTS [9] Li, Z., H. Huang, I. Shang, et.al. 2006.Facile Synthesis 01 Single-crystal and

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