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2012 82: 37 originally published online 2 August 2011Textile Research JournalSander De Vrieze, Nele Daels, Karel Lambert, Bjorge Decostere, Zeger Hens, Stijn Van Hulle and Karen De Clerck

Filtration performance of electrospun polyamide nanofibres loaded with bactericides  

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Orignal article

Filtration performance ofelectrospun polyamide nanofibresloaded with bactericides

Sander De Vrieze1, Nele Daels2, Karel Lambert1,Bjorge Decostere2, Zeger Hens1, Stijn Van Hulle2 andKaren De Clerck1

Abstract

Electrospinning is a process to generate nanofibrous nonwovens. With these nonwovens, many applications can be

targeted, such as water filtration. In this paper, polyamide nanofibrous membranes are evaluated for their pore size, a key

parameter in water filtration, and for their removal of microorganisms. To increase the removal efficiency to values

exceeding the state of the art, innovative functionalization of the nanofibres is studied. The nanofibrous membranes are

functionalized using a one step method. Different functionalization chemicals are investigated which are Ag nanoparticles

and bactericides. Ag functionalized nanofibres are used as a reference medium to compare with a novel bactericide based

functionalization system. It is seen that nanofibrous membranes functionalized with the bactericides exceed the normal

removal efficiencies obtained by microfiltration membranes. Furthermore, knowledge is built up on how these bacte-

ricides are inserted in the nanofibres themselves.

Keywords

Electrospinning, water filtration, microorganisms

Introduction

Electrospinning is an innovative process, capable ofproducing fibres with diameters typically one to twoorders of magnitude lower than extrusion and conven-tional solution-spun fibres. A variety of polymers canbe spun, each from a specific solution. In addition, theability to produce highly porous nanofibrous mem-branes with structural integrity is also an attractive fea-ture of electrospinning.1–3 Electrospun materials offeropportunities, for example, in medical applications,4

filtration5 and protective clothing.6

When using nanofibrous membranes in applications,large homogeneous samples are needed. Therefore areproducible method of nozzle electrospinning isneeded to supply these homogeneous samples. Thismethod is called steady state electrospinning.7

Electrospinning is in steady state when the amountof polymer that is transported through the needle perunit time equals the amount of polymer that is depos-ited as nanofibres on the collector per unit time. This

definition comprises two conditions. The first conditionis that in time all the polymer that is spun from thenozzle and collected at the target is converted intonanofibres, implying the absence of beads or drops inthe structure. The second condition for steady stateelectrospinning is a stable, time invariant, Taylorcone. Steady state electrospinning allows for the longterm stability needed for producing reproducible sam-ples of any desired size. As shown in previous work,7

obtaining steady state electrospinning conditions is pos-sible only if the right solvent mixture is used for theselected polymer, that is, the solvent mixture formic

1

Ghent University, Belgium.2

University College West Flanders, Belgium.

Corresponding author:

Karen De Clerck, Ghent University, Technologiepark 907, Gent 9000,

Belgium

Email: karen.declerck@UGent.be

Textile Research Journal

82(1) 37–44

! The Author(s) 2011

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DOI: 10.1177/0040517511416273

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acid/acetic acid for polyamide 6 and polyamide 6.6.Moreover specific process and ambient parameters areneeded to obtain reproducible samples. This conditionis a fundamental view in electrospinning that gives thepossibility to generate homogeneous nonwovens thatare applicable in demanding areas such as liquidfiltration.

Polyamide nanofibres made by electrospinninghave been extensively studied.8–10 Most of the researchfocuses on the influence of different parameters on theobtained materials or the use of polyamide nanofibresin air filtration. The use of nanofibres in the area ofliquid filtration is not fully explored yet, althoughsome initial performance assessment was alreadyreported in our previous work.11–13 In this initial work,the performance of unfunctionalized membranes wasexplored. Because of their specific pore size, electrospunnonwovens can be used in the class of microfiltration.14

Electrospun nonwovens are good candidates for liq-uid microfiltration because they have a high porositywhen compared with conventional substrates fabricatedusing other means. They result in a higher hydraulic per-meability than that of the conventional substrates.11 Assuch, they have a high clean water permeability (CWP).The CWP value of nanofibrous membranes is two orthree times higher than comparable microfiltrationmembranes. The unfunctionalized membranes need,however, a higher performance for certain aspects ofliquid filtration such as the removal of microorganisms.

Today, removal of microorganisms in water treat-ment is commonly done by dosing functional chemicalslike bactericides in the water itself. This is not good forthe ecology and the economy. As such, it would bebetter to work with functionalized membranes thathave the ability to work with a one step removal ofmicroorganisms. Nanofibres may be a good candidateto apply and dose the bactericides well. An advantageof the unique electrospinning method is that this pro-cess is able to fine tune the surface functionalities by theincorporation of the functional chemicals during pro-cessing. So for obtaining antimicrobial functionalitymostly Ag nanoparticles have been applied.10 Onlyrecent research describes the use of other functionalchemicals that are added to the electrospinning solutionto obtain functional nanofibrous membranes.15

Therefore in the present paper, steady state electro-spinning of polyamide with incorporated chemicals ispresented. After characterizing unfunctionalized nano-fibres, Ag nanoparticles are evaluated for their removalefficiency of microorganisms as these are the maindescribed functionalization agents in the literature. Asa further innovative system, nanofibres loaded withbactericides are compared with the Ag functionalizedfibres in respect of their removal efficiency of

microorganisms. Further knowledge is built up onhow these bactericides are inserted in the nanofibresthemselves.

Experimental

Materials

Polyamide 6 (PA 6, Mw 10000 g mol�1) was supplied bySigma-Aldrich and was used as received. Solventschosen for this research were 98wt% formic acid and99.8wt% acetic acid (both supplied by Sigma-Aldrich).Different biocides, thiocyanatomethylthiobenzothia-zole (TCMTB), dibromocyanoacetamide (DBNPA),Bronopol (BR), WSPC (unspecified proprietary quater-nary ammonium salt by Buckman) and chlorhexidine(CH), have been kindly provided by Buckman. Thesebiocides are 99.9% pure and acid stable. Ag nanopar-ticle solution in isopropanol (HAG78) has been kindlyprovided by Umicore and was used as obtained. Thissolution contains nanoparticles with a size rangebetween 5 and 100 nm.

The solutions for electrospinning were prepared bydissolving different wt% PA 6 in a 50:50 v% formicacid/acetic acid solvent mixture. Biocides or silvernanoparticles were added in the solutions at the desiredconcentrations. The solutions were gently stirred with amagnetic stirrer bar for at least 3 hours at roomtemperature.

Methods

Electrospinning. The electrospinning setup comprisesan infusion pump (KD Scientific Syringe Pump Series100) and a high voltage source (Glassman High VoltageSeries EH). A grounded aluminium foil collects thenanofibrous materials. The experiments are conductedat room temperature (20� 2�C) with room humidity(45� 3%). A BN-1838 Terumo mixing needle is usedto perform the experiments. The high voltage sourcecharges the needle directly (DC+). The flow rate ofthe solution is set on 2mL h�1. The tip to collectordistance is set at 6 cm. The applied voltage is changedto obtain steady state.

Scanning electron microscopy analysis. The morphol-ogy of the electrospun nanofibres was examined usinga scanning electron microscope (Jeol Quanta 200FFE-SEM) at an accelerating voltage of 20 kV. Prior toscanning electron microscopy (SEM) analysis, thesample was coated with gold using a sputter coater(Balzers Union SKD 030). The diameter of a certainset was calculated by taking an average of 50measurements.

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Transmission electron microscopy analysis. Thenanofibrous material is analyzed with a JEOL 2200FS, type 200 kV Cs-corrected electron microscope.

Bubble point test. The bubble point test is a methodfor the determination of the minimum and the maxi-mum diameter of the pores of a material. The bubblepoint test is performed on a CFP-1100-AEX. Thismethod uses a wet membrane that is pressurized (N2)at one side.16

Removal of colony forming units (CFU). To evaluatethe removal of microorganisms, water samples weretaken from waste water from a general hospital (107–108 CFU/100mL). These tests were performed in a flowthrough system11 in which the samples (100mL) werefiltered over a functionalized nanofibre membrane(0.0011m2) with a pressure filter (1–1.5 bar) in a dead-end filtration cell, grounded on a filter support. Thefiltration cell was previously autoclaved at 121�C for15min. Water samples were collected and diluted asneeded. Further the culturable microorganisms wereenumerated by inoculation in a nutrient agar culturemedium (www.oxoid.com) at 37�C for 48 h.17 Theremoval of microorganisms is expressed in a logarith-mic removal scale called ‘log removal’. For instance:when there is a log 3 removal, this means that thereare 1000 times less microorganisms after filtration.This corresponds to an absolute removal efficiencyof 99.9 %.

absolute removal efficiency

¼ 1�number of CFU before filtration

number of CFU after filtration

log removal

¼ �lognumber of CFU before filtration

number of CFU after filtration

� �

Results and discussion

Evaluation of the unfunctionalized nanofibrousstructures

One of the key specifications of a filter material is itspore size. This is the key parameter that determines theoptimum filtration class to which the envisaged mate-rial may belong.

Therefore the pore size of eight different electrospunPA 6 membranes is evaluated by the bubble point test.There are three different average diameter sizes: 80, 120and 160 nm. These sizes are selected because of theirbroad range. These sizes correspond with three differentelectrospinning solutions: 11, 13 and 16wt% PA 6

concentration in a 50:50 formic acid/acetic acid mix-ture. These optimal electrospinning parameters arebased on previous work.7 The results are summarizedin Table 1.

The results show that there is an influence of thethickness of the membrane. When the grammageincreases, the minimum and maximum pore sizedecreases. The thicker a nanofibrous membrane is, themore layers of nylon nanofibres are randomly put oneach other. The increase in layers results in a longertortuous path. This increase in tortuosity eventuallyresults in lower maximum and minimum pore sizes.As such a smaller range of pore sizes is obtained.

The average diameter of the nanofibres has an effecton the maximum pore size of the membrane. Whencomparing the same grammages, a smaller averagediameter results in a smaller maximal pore size. Thisis caused by the packing of the nanofibres: the smallerthe diameter of the nanofibres, the closer the packingcan be.

As to determine the optimal filtration class for pos-sible end application, the maximum pore size in Table 1is to be considered. Indeed, if a particle is bigger thanthe maximum pore size, it will be filtered by the mem-brane. It can be seen that the maximum pore size of allthe membranes is below 1 mm, but higher than 0.2 mm.These series of experiments imply that the polyamide 6nanofibrous membranes must be categorized in theclass of microfiltration.18

When evaluating the removal of microorganisms,unfunctionalized nanofibrous membranes (averagefibre diameter: 160 nm) were tested without any sup-portive layer. These polyamide nanofibrous membranescan filter these bacteria, expressed as CFU out of thewaste water. After evaluation, the unfunctionalizednanofibrous membranes exhibit a log removal of1.7� 0.3. This removal corresponds with an absoluteremoval efficiency of almost 98%. This filtration

Table 1. Bubble point test results for different PA 6 nanofibres

Diameter Grammage Min. pore size Max. pore size

nm (g m�2) nm nm

80 25 250 400

50 150 330

120 25 220 440

50 120 390

100 110 270

160 25 360 550

50 200 490

100 170 340

De Vrieze et al. 39

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result is based purely on mechanical filtration, wherethe CFU is regarded as a particle. This means thatthe efficiency of filtration is related to particle size.If the microorganism is bigger than the pore, it willbe filtered (Figure 1). Since the nonwoven nanofibrousnetwork tends to open when pressurized, the nanofi-brous filter pore size is not absolute. A way to solvethis problem is to add a supportive layer on the back ofthe nanofibrous membrane.12 Indeed, in Decostereet al. 12 the use of a supportive layer similarly increasedthe log removal of pathogens increases from 1.5� 0.3to 2.2� 0.3. The supportive layer helps to resist ruptureby water pressure. The log removal of CFU increases to2.7� 0.3 with the supportive layer. The absoluteremoval increases to 99.8%.

Previous work stresses that the nanofibrous mem-branes have an advantage for water filtration becauseof their high hydraulic permeability, resulting in highCWP values.11 However, the removal of microorgan-isms is very important in certain filtration streams.Normal log removal efficiency of commercial microfil-tration membranes are in the order of log 4.13 Thesecommercial membranes are typically made by phaseinversion. Nanofibrous membranes have the potentialto improve the removal efficiency of microorganisms.19

However, as seen above, unfunctionalized nanofibresdo not generate an improvement. Therefore, the routeto investigate is the functionalization of the nanofi-brous membranes. First Ag nanoparticles are used asa reference as this is the most applied functionalizationmethod in the literature.20–21 The major aim is, how-ever, to evaluate the performance of membranes func-tionalized with various biocides.

Incorporation of Ag nanoparticles in polyamide6 nanofibres

Ag nanoparticles have a biocidal effect against CFU.13

Therefore, they are investigated in this study. PA 6solutions of 16wt% in a 50:50 formic acid/acetic acidmixture were prepared. Silver nanoparticles wereinserted in the polymeric solution at 1% of the totalweight of the polymer.

The original dispersion of Ag nanoparticles is in iso-propanol. The combination of the isopropanol with theformic acid/acetic acid mixture has an influence on theprocess itself. To obtain steady state electrospinningwith these solutions, a change in applied voltage is nec-essary. Electrospinning unfunctionalized PA 6 solu-tions of 16wt% in a 50:50 v% of formic acid/aceticacid with the settings at a flow rate 2mL h�1 and aTip to Collector Distance (TCD) of 6 cm needs 20 kVto obtain steady state conditions.7 The voltage mustincrease to 26 kV to obtain steady state conditions forthe 1% Ag nanoparticles addition. The increase can becaused by the addition of the isopropanol to the polymersolution and the change of the dielectric properties of thesolution because of the insertion of theAg nanoparticles.

The nanofibrous material itself has only minorchanges (Figure 2). This observation is confirmed bydiameter measurements. The average diameter variesfrom 172� 19 to 180� 26 nm, which is statisticallyinsignificant (t-test, p¼ 0.03).

It is not possible to visually determine if there aresilver nanoparticles present in the nanofibres withSEM. Although an energy dispersive X-ray analysis(EDX) scan during SEM measurements indicates thatthe sample contains silver. Therefore, transmission elec-tron microscopy (TEM) analysis is used to study the1% Ag functionalized nanofibrous material (Figure 3).

The nanofibres (light grey) contain different blackspots. These black spots are, confirmed with EDX,silver particles. Figure 3b shows that the Ag nanopar-ticles have a wide range of diameters, ranging from5nm to over 50 nm. This is in line with the specificationof the original HAG78 solution. More interesting is,however, the very different distribution of nanoparticlesin Figure 3a and b. Screening several images showedthat there is no uniform distribution of nanoparticlesall over the nanofibrous sample. Indeed at some posi-tions a very low concentration of nanoparticles is found(Figure 3a) and at other locations a relatively high con-centration can be found (Figure 3b). This may beattributed to a local clustering in the solution beforespinning. Further research on the homogeneity ofthe distribution of the nanoparticles is therefore neces-sary. It is however also important to note that withina local region of high concentration (typical Figure 3b)the nanoparticles are homogeneously distributed.

Figure 1. Unfunctionalized PA 6 nanofibres filtering CFU.

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These TEM images make it clear that the nanoparticlesdo not group at the surface of the fibres. No agglom-eration of the nanoparticles was observed, contrary toother research, such as that reported by Tekmen et al.21

Electrospun Ag functionalized nanofibrous mem-branes generated a log 4 removal of CFU, which isalready significantly better than the unfunctionalizednanofibrous membranes. It is to be noted that a morehomogeneous distribution of the silver nanoparticles inthe nanofibres may even further improve the removalefficiency. This is however beyond the scope of the pre-sent paper. In this study, the current Ag functionalizednanofibrous membranes obtained, will be used as a ref-erence to evaluate the performance of a membranefunctionalized with bactericides.

Incorporation of bactericides in polyamide6 nanofibres

Today, a wide range of bactericides is used in watertreatment by direct addition to the water. This is,

however, not an acceptable procedure for all endusers. This paper will study the incorporation of thebactericides in a nanofibrous structure as an innovativealternative with an obvious ecofriendly dosing.

PA 6 solutions of 16wt% in a 50:50 formic acid/acetic acid mixture were prepared. The bactericidesare inserted in the spinning solution at concentrationsof 1, 3 and 5%. Electrospinning of all these differentsolutions was possible but again the applied voltageneeded to be altered to obtain steady state. Table 2compares the applied voltage needed for the differentsolutions. Again, the unfunctionalized PA 6 solutionneeds 20 kV to obtain steady state conditions.

The values that are described in Table 2 are in linewith the literature describing the effect of salts in electro-spinning.22 Adding different bactericides increases theapplied voltage needed. This increase is relatively lowfor the bactericides DBNPA, Bronopol and chlorhexi-dine, whereas for the bactericide TCMTB the increaseis very high, probably because of its chemical constitu-tion. WSCP has the lowest increase in applied voltage.

Figure 3. TEM images of PA 6 nanofibres with (a) and (b) 1% Ag nanoparticles.

Figure 2. SEM images of PA 6 nanofibres with (a) 0 and (b) 1% incorporated Ag nanoparticles.

De Vrieze et al. 41

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This effect is caused by the nature of WSCP, which is aquaternary ammonium salt. The charges in the WSCPdo not have a large negative effect on the electrospinningprocess.

The produced materials are analyzed with SEM tostudy the structure of the nonwoven, as well as to mea-sure the average fibre diameter.

All applied bactericides were found to allow electro-spinning of uniform nanofibrous material, except for theuse of TCMTB. The insertion of TCMTB does alter thenonwoven structure of the nanofibres (Figure 4).

Above a concentration of 3%, small nanofibres aresplit away from the normal nanofibrous structure

(Figure 4). This splitting is most noticeable in the 5%TCMTB mixture, resulting in a thick network. At thispoint, two nonwoven network structures exist: thenormal polyamide nanofibres and a secondary, eventhinner nanofibrous structure. The same phenomenonis seen in polymeric solutions which contain an abun-dance of chemical charges like chitosan.23 The splittingof the nanofibres in smaller nanofibres is attributed tothe increase in charges and fluctuations of the electricalfield close to the collector. This change in structure isonly observed in minor forms for the other bactericides.Because of their inhomogeneous structure, TCMTBfunctionalized nanofibres were not further evaluated.

Table 3 shows the average nanofibre diameters mea-sured by SEM. There is no significant change in theaverage fibre diameter values compared to the unfunc-tionalized value, which is 172� 19 nm. This is a positiveresult for the filtration properties of the membrane; dueto the unchanged values for the average fibre diameter,the pore size of the material will remain the same.

The log removal for all these membranes are mea-sured (Table 4). The confidence level is� log 0.3. Thestudy has been repeated for the 5% WSCP and

Figure 4. PA 6 nanofibres with (a) 1 (b) 3 and (c) 5 % TCMTB.

Table 2. Applied voltage (kV) needed for steady state for

different solutions (TCD 6 cm, 2 mL h�1)

(%) TCMTB DBNPA BR WSCP CH

1 25 22 22 21 22

3 29 24 24 22 24

5 - 25 25 22 25

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Bronopol samples. With an increase in % of the bacte-ricide in the nanofibres, the log removal also increases.WSCP and Bronopol show the highest removals thatcorrespond with absolute removals of 99.9997 and99.9998%. These values are extremely high for micro-filtration membranes, since common microfiltrationmembranes only obtain a log 2–log 4 removal.24,25

This high removal rate shows the high potential ofthe functionalized nanofibrous membranes for microfil-tration. The increase in removal of the CFU is attrib-uted to the incorporated functional chemicals. Thishypothesis will be further evaluated in the future.

Conclusions

Polyamide nanofibres are an exciting medium for waterfiltration which show high added value because of theirhigh CWP. Unfunctionalized nanofibrous membranesshow however a low log removal of microorganisms. Inthis study, the low removal efficiency is increased byfunctionalization. Different polyamide nanofibrousmembranes were made under steady state conditionswith the insertion of chemicals and Ag nanoparticles.It is seen that the insertion of the chemicals changes theprocess conditions. An increase in the applied voltage,which is the driving force in electrospinning, is neces-sary to keep the process stable. The diameter of thefunctionalized polyamide nanofibres does not changesignificantly. The average diameter is 160 nm, withlow standard deviation. We observe that the nanofibresfunctionalized with 5% Bronopol and WSCP exceed alog 5 removal of CFU. This is higher than the valuesfor common microfiltration membranes. With theseproperties, the nanofibrous membranes can be used inhigh demanding applications, such as the filtration ofprocess water.

Funding

This research received no specific grant from any funding

agency in the public, commercial, or not-for-profit sectors.

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3 146� 23 162� 25 167� 22 158� 21

5 155� 29 168� 18 176� 22 167� 24

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