Fabrication of Polyaniline Coated Plasma Modified Polypropylene Filter for Antibioaerosol Application

The present study describes the fabrication of polyaniline (PANI) coated high power plasma treated polypropylene (PP) filter for antibioaerosol application. PANI was synthesized by emulsion polymerization method. PP filter was modified by plasma treatment followed by coating of PANI was performed by dip coating method and successfully characterized using SEM and FT-IR analysis. Antibioaerosol property of PANI coated plasma modified PP filter was assessed against E. coli, B. subtillis and S. aureus bioaerosols. Results revealed that water absorption property of plasma modified PP filter was significantly increased (2488%) as compared to unmodified PP filter (404%). Stability study shows that PANI coating is highly stable; concentration of PANI is higher on modified PP filter than the unmodified counterpart. PANI coated modified PP filter showed higher antibacterial efficiency than unmodified filter. It revealed nearly 100% inhibition efficiency against E. coli and B. subtillis and lower for S. aureus, but completely inhibited at high PANI concentration. Bioaerosol filtration study showed that after 120 min standing both E. coli and B. subtillis were completely inhibited by PANI coated modified PP filter. It is concluded that PANI coated plasma modified PP filter is an efficient antibioaerosol agent for above mentioned bioaerosols under given reaction conditions, could be further applied in the indoor air quality control.


INTRODUCTION
Bioaerosol are the biological particles such as bacteria, virus and fungi suspended in the air, which are mainly present in schools, colleges, offices, hospitals, airports and other places (Lee et al., 2012;Chow et al., 2015;Kallawicha et al., 2015).The concentration of bioaerosol in the indoor environment is increasing with continuous rise in the population with urbanization and industrialization, which causes severe adverse effect on the indoor air quality (Douwes et al., 2003, Lee, 2011;Kang et al., 2015).Therefore, continuous exposure of humans to the polluted air (presence of bioaerosol) causes many infectious diseases like severe acute respiratory syndrome (SARS), H1N1 influenza, acute toxic effects, allergies, cancer etc. (Dawood et al., 2009;Lee et al., 2011).To control the bioaerosol and protect the people, various bioactive filters or fabrics with solid bioaerosol capture particles have been designed (Huang et al., 2012;Han et al., 2015;Lin et al., 2016).However, the developed bioactive filter possess an appropriate proportion which blocks the bioaerosol particles of a minimum of 98% and the air flow resistance is minimum of 200 Pa (Majchrzycka et al., 2012).Therefore, recently polymer fibers or filters have been used to hold the biocidal agents permanently and enhances the antibacterial efficiency.
Polypropylene (PP) has been effectively used to prepare bioactive filter due to its good thermoplastic and recycling properties.Moreover PP has been used for different sanitary applications like filters, diapers, surgical masks, medical devices etc. Subsequently it has been efficiently utilized as an antibioaerosol agent against S. aureus and E. coli (Fages et al., 2011;Majchrzycka et al., 2012).Further increasing its efficiency, it was modified with metal and metal oxides nanoparticles, made composite with another polymeric materials.Fages et al. (2011) reported antimicrobial properties of PP filled with surfactant coated Ag nanoparticles against S. aureus and E. coli bioaerosols.Subsequently, Delgado et al. (2011) prepared copper and copper oxide nanoparticles loaded PP matrix for E. coli inhibition.Recently, Demir et al. (2015) synthesized N-Halamine modified polypropylene nonwoven fabrics and studied their antimicrobial efficiency against bioaerosols of S. aureus and E. coli.Similarly, various studies have been reported on the antibacterial application of PP filter (Torlak and Nizamlioğlu, 2011;Oliani et al., 2015;Prorokova et al., 2015).However, the antibioaerosol efficiency of PP filter is sufficiently inefficient and commercialization of this process is not up to the mark; it may be due to the less hydrophilicity.To enhance the hydrophilicity, polymers surface has been subjected into plasma treatment, moreover the direct plasma has also been used to kill the microorganism (Gweon et al., 2009).Noeske et al. (2004) modified the polyethylene terephthalate (PET), polyamide 6 (PA6), polyvinylidenefluoride (PVDF), polyethylene (HD-PE), and polypropylene (PP) polymers surface by plasma jet treatment, revealed that hydrophilicity of polymers have been improved.Subsequently, Lai et al. (2006) reported that hydrophilicity of polycarbonate, PP and PET samples were improved by microwave-induced argon plasma treatment.Similarly Hegemann et al. (2003) and Cheng et al. (2006) improved the surface and adhesion property of the PP, PC, PET and PMMA polymers by plasma treatment.Therefore, we expect that plasma treatment of PP filter may enhance its hydrophilicity, which may have positive impact on the antibacterial efficiency.
Furthermore, the polymers are widely employed for antimicrobial applications, mainly in the fields of health care, pharmacy, food packaging and tissue implants (Kenawy et al., 2007;Bonilla and Garcia, 2012;Jain et al., 2014).Moreover due to their low cytotoxicity and biocompatibility, it has been potentially applied in biomedicine (Saikia et al., 2010).Among the polymers, polyaniline (PANI) is highly attractive due to its electroactivity, easy synthesis and doping-dedoping chemistry (Syed and Dinesan, 1991).It has been extensively used in the field of biosensors, neural probes, controlled drug delivery systems and tissue engineering materials (Wei and Ivaska, 2006;Banerjee et al., 2010;Gizdavic-Nikolaidis et al., 2010;Balint et al., 2014).Moreover, it possess potential antibacterial properties, especially PANI, PANI/metal nanoparticles, PANI/polyvinyl alcohol/Ag, PANI/aromatic nitro compounds etc., are effectively controlled the E. coli and S. aureus bacteria (Seshadri and Bhat 2005;Kucekova et al., 2013;Boomi et al., 2014;Moghaddam and Eslahi 2014;Dhivya et al., 2016).Furthermore, the literature on preparation of PANI/PP filter composite and their application towards antibacterial efficiency is very scarce.Therefore, in order to make use of these properties and also improve the antimicrobial efficiency of PP filter, the PP filter surface was modified using plasma treatment followed by coating of PANI was performed in this study.
Herein, the synthesis of PANI was carried out using emulsion polymerization method followed by coating of PANI on plasma modified PP filter was performed by dip coating method and characterized using SEM and FT-IR analysis.Subsequently, the antibioaerosol application of synthesized samples was investigated against E. coli, B. subtillis and S. aureus bacteria respectively.The results revealed that the PANI coated plasma modified PP filter had superior antibacterial efficiency than the uncoated counterparts under present reaction conditions.

Materials
Nonwoven polypropylene (PP) filter, aniline and dodecylbenzene sulfonic acid (DBSA) were purchased from Sigma Aldrich.Ammonium persulfate was purchased from J. T. Baker, Avantor performance materials, USA.Concentrated phosphoric acid (H 3 PO 4 ) was bought from King Ming chemical.Toluene was procured from Tedia Company USA.Soy protein casein agar decomposition (Tryptic Soy Agar (TSA)), Soy protein casein decomposition medium (Tryptic Soy Broth (TSB)) and Culture nutrient solution (Nutrient Broth (NB)) were purchased from Sharlau, USA.Double distilled water (DDW) was used for the preparation of experimental solutions.

Synthesis of Polyaniline
Polyaniline was synthesized by emulsion polymerization of aniline (Wo, 2006).Dodecyl benzene sulfonic acid (DBSA, 0.1 mole) was used as an emulsifier and dopant, dissolved in a 150 mL of DDW which was taken in the four-necked bottle.Then it was slowly stirred, placed in a water-bath and maintained at low temperature.Subsequently phosphoric acid (0.2, 0.4, 0.6 and 0.8 mole) and aniline monomer (0.1 mole) was slowly added into the mixture under stirring.The milky solution was formed and continuously stirred until the water bath meet homogenous temperature.Finally 150 mL of ammonium persulfate (APS, 0.125 M) solution as the initiator; was added dropwise into reaction mixture using peristaltic pump with the flow rate of 0.014 mL per second to form an emulsion polymerization.After complete addition of APS solution, the reaction was maintained at low temperature and used for coating.

Plasma Modification Process of Nonwoven Polypropylene Filter
The plasma modification of nonwoven polypropylene filter was performed as follows: Initially, the PP filter were cut into circular shape with the diameter of 1.5 cm and ultrasonicated for 10 min to clean the filter and then it was dried in an oven.Then the oven dried filter was subjected to plasma treatment which was schematically shown in Fig. 1 and their photographic images were shown in Figs.1(b)-1(c).Oxygen gas passed with the flow rate of 5 sccm O 2 , 0.05 torr pressure and 30 W for 15 seconds.Similar procedure was repeated with different power (10, 20, 40 and 50 W) and time (30 and 45 seconds).Among these 30 W and 15 seconds delivered better efficiency in modifying PP filter.

Coating of Polyaniline on Modified Nonwoven Polypropylene Filter
Different amount of PANI powder was mixed with toluene to make the different concentration of PANI solution.Then the different size of modified PP filter was immersed in the PANI solution, thereafter it was placed in a shaker for 1 h at 100 rpm and 37°C temperature, to prepare the different concentration of PANI coated PP filter.Subsequently, the PANI coated PP filter was dried in an oven at 75°C for 1 h and the amount of coating was determined by following formula.(1)

Assessment of Bacterial Inhibition Efficiency of PANI Coated Nonwoven PP Filter
Initially, the bacteria inhibition efficiency of different percentage of PANI coated nonwoven PP filter was evaluated for different bacterial species.The filter was immersed in the dilute concentration of bacterial species and incubated at 37°C for 24 h.Subsequently, it was transferred into 30 mL sterile water and centrifuged (3600 rpm).Then the 100 µL of solution was wiped on the appropriate medium using glass rod and incubated for 24 h at 37°C.After 24 h, the number of colonies were calculated by measuring the colonyforming unit (CFU) to determine the bacterial inhibition efficiency by following formula.

Determination of Biological Aerosol Filtration Efficiency of PANI Coated Nonwoven PP Filter
Bioaerosol production experimental set-up and processes are shown in Fig. 2. First, the compressed air is divided into three streams.Whereas the two streams are mixing the wet air to adjust the relative humidity and third streams produces desired bioaerosol concentration to evaluate the biological aerosol filtration efficiency.

Characterization
Surface morphology of PP filter and PANI coated filter was observed using environmental scanning electron microscope analysis (ESEM).Fourier transform infrared spectroscopy (FT-IR) analysis of samples were performed using JASCO FT/IR 650 Fourier transform infrared spectrometer.Plasma treatment of polypropylene was performed using the highfrequency plasma reactor system (German Dressler high frequency technology company, Cesar).

Characterization of PANI Coated Polypropylene Filter
The surface morphology of nonwoven PP filter and PANI coted modified PP filter was characterized by environmental scanning electron microscopy analysis and the results are shown in Fig. 3.The results demonstrated that very thin film morphology of PP filter (Figs. 3(a) and 3(b)).Moreover, the PANI coating on PP filter was clearly observed from the Figs.3(c) and 3(d), it forms the thin layer on the surface of PP filter.
The PANI coating on PP filer was further determined by FT-IR analysis of unmodified and modified PP filter and PANI coated modified PP filter and the results are shown in Fig. 4. The results revealed that the significant reduction in the intensity of all the peaks of PP filter was observed (Fig. 4(c)) which obviously proved that PANI was successfully coated on the PP filter.

Effect of Plasma Treatment on Water Absorption
Fig. 5 shows the influence of plasma treatment on water absorption characteristics of nonwoven PP filter.The results revealed that water absorption of nonwoven PP filter was significantly enhanced after plasma treatment.For unmodified nonwoven PP filter, the water absorption is 404%, whereas it was 2488% for plasma modified PP filter.Similarly, after back plasma discharge treatment, the water absorption is increased to 2205.9%, however lower than the main plasma discharge.From these results it is observed that hydrophilic characteristics of nonwoven PP filter was significantly improved after plasma treatment.

Coating of PANI on Unmodified and Modified PP Filter
Different concentration of PANI solution like 25, 50 and 100 mg mL -1 were prepared for coating on unmodified and modified PP filter and the results were shown in Fig. 6.The results demonstrated that coverage of PANI on PP filter is increased with increase in the PANI concentration.The coating percentage of PANI on modified PP filter is 15% higher than the unmodified PP filter (Fig. 6).When increasing the PANI concentration from 25 mg mL -1 to 100 mg mL -1 , the coating percentage of PANI on modified PP filter was significantly increased from 39.0 to 121.9%.

Stability Study of PANI Coating
The stability of PANI coating on unmodified and modified PP filter was tested by immersing the filter in the distilled water (10 mL), sonicated (10 min), dried at 75°C for 2 h and measured the weight of the filter and desorption percentage was calculated.The same procedure repeated for three times.The results revealed that in the case of 25 mg mL -1 of PANI concentration, concentration of PANI coating was decreased from 39% to 32% with the loss of 7%; after second and third time sonication, it was decreased to 27.4 and 26.7% with loss of 4.6 and 0.7% respectively (Fig. 7(A)).Similarly, in the case of 50 and 100 mg mL -1 of PANI concentration, the coverage of PANI concentration was decreased from 65.6% and 121.9% to 46.8% and 101.7% (Figs.7(B) and 7(C)).From the results, it is observed that coating of PANI is highly stable and desorption rate is very slow and less; it can filter biological aerosol in the environment for longer time.Furthermore, after desorption, the coating percentage of PANI on modified PP filter is still higher than the unmodified counterpart (Fig. 7(D)).

Effect of PANI Concentration on Bacterial Inhibition Efficiency of PP Filter
The influence of PANI concentration on bacterial inhibition efficiency of PANI coated PP filter was evaluated by E. coli, B. subtilis and S. aureus.The results revealed that without PANI coating, PP filter showed poor inhibition efficiency to these bacteria, whereas after PANI coating PP filter exhibited significantly improved inhibition efficiency.When increasing the PANI coating concentration from 5.74% to 29.7%, the E. coli inhibition efficiency was enhanced from 82.2% to 100% (Fig. 8(a) and Table 2).This was clearly proved by bacterial colonies (CFU) measurement (Table 2).Results revealed that initially 698 CFU observed and it was decreased to 124 when the PANI concentration was 5.74% and further increasing the PANI coating concentration to 12.0% results in the no bacterial colonies were observed.Similarly for B. subtilis bacteria, when increasing the PANI coating concentration from 5.0% to 24.0%, result in the significant enhancement in the inhibition efficiency of PANI coated PP filter from 96.9% to 100.0% (Fig. 8(b) and Table 3).This was further confirmed by CFU determination, revealed that initially 327 CFU was observed and increasing the PANI concentration to 5.0, 13.5 and 24.0% leads to complete inhibition of B. subtilis bacteria and no bacterial colonies were obtained (Table 3).For S. aureus bacteria, increasing the PANI concentration from 5% to 8.7%, the inhibition efficiency of PP filter was decreased to 55.6% as compared to other two bacteria, however further increasing the PANI concentration to 24.7%, leads to 100% inhibition (Fig. 8(c)), which was confirmed by no bacterial colonies were observed (Table 4).This significant enhancement in the inhibition efficiency of PANI coated PP filter is may be due to that PANI is a non-diffused (eluting) bacteriostatic agent.From the results, we conclude that PANI coating on PP filter is highly indispensable for bacterial inhibition.

PANI Coated Nonwoven PP Filter for Biological Aerosol Filtration Efficiency via Antibacterial Test
The antibiological aerosol efficiency of PANI coated plasma modified PP filter was evaluated by E. coli and B. subtilis antibacterial test.The uncoated and PANI coated PP filter were used to filter both the bacteria for different timings (5, 15, 30 and 60min) and kept stationary for different standing time (30, 120 and 480 min).Afterwards the bacterial inhibition efficiency was evaluated by determining the amount of bacterial colonies and the results were shown in Figs.9(a) and 9(b).The results revealed that the uncoated modified filter showed 30% inhibition efficiency to E. coli bacteria after filtered for 30 min followed by 30 min standing (Fig. 9(a)), whereas in the case of B. subtilis, it was found that no inhibition efficiency (Fig. 9(b)), remains alive and increasing the growth which leads to secondary pollution.However, in the presence of PANI coated modified PP filter, the bacteria inhibition efficiency was increased with increase in the filtration and standing time.For E. coli bacteria, after 5 min filtration and 30 min standing, the filter showed 75% inhibition efficiency and increased with increase in the filtration time and 100% inhibition was observed after 60 min filtration.Subsequently increasing the standing time of different time interval filtered filter to 120 and 480 min, results in the 100% inhibition of E. coli bacteria was observed (Fig. 9(a)).Similarly, for B. subtilis bacteria, after 5 min filtration and 30 min standing, 100% inhibition was obtained, however increasing the filtration time to 60 min, result in the reduction in the inhibition efficiency was obtained.Moreover, enhancing the standing time (120 and 180 min) leads to the 100% inhibition of B. subtilis bacteria (Fig. 9(b)).From this results, it is observed that both the bacteria were significantly inhibited using PANI coated plasma modified PP filter.However the initial lower inhibition efficiency against B. subtilis bacteria is may be ascribed to the production of surviving forms during the course of reaction which is responsible for the high resistance of B.subtilis.The enhancement in bacterial inhibition efficiency of PANI coated PP filter is may be due to that molecular structure of PANI was changed after interaction with bacteria species.Moreover the acidic dopants on the molecular chains of the PANI might have reacted with bacterial species which resulted in their death.In addition, electrostatic adherence between the PANI and the bacteria carries the charges of different polarity, which resulted in the breaking of bacteria wall and their constituents were broken, which led to bacteria die.Similarly Prorokova et al. (2015) reported the antimicrobial property of PP yarn modified by metal nanoparticles (manganese, iron, gold, nickel and palladium) stabilized by polyethylene against E. coli and S. aureus bacteria.The results revealed that PP yarn modified iron metal nanoparticles showed highest inhibition efficiency against E. coli (64%) and S. aureus (97%) bacteria.Delgado et al. (2011) reported that copper and copper oxide nanoparticles embedded PP matrix showed 95% inhibition against E. coli bacteria.Oliani et al. (2015) reported that silver nanoparticle-poly(N-vinyl-2-pyrrolidone) (PVP) surfactant loaded PP film displayed 68 and 100% reduction of E. coli and S. aureus bacteria.Gutarowska et al. (2010) studied the antimicrobial efficiency of alkylammonium microbiocide substance loaded nonwoven PP filter against gram-positive and gram-negative bacteria.Results revealed that PP filter exhibited 99.9, 95 and 86% reduction of E. coli, S. aureus and B.subtilis respectively.In all of these studies antimicrobial efficiency of the PP filter was enhanced by addition of active substances or nanoparticles; however in this work PP filter was coated with PANI polymer resulted in enriched antimicrobial efficiency in shorter reaction time.Therefore from the above mentioned discussion, it is concluded that PANI coated plasma modified PP filter is an efficient antimicrobial agent against E. coli and S. aureus bacteria under present reaction conditions.

CONCLUSION
In summary, the high frequency oxygen plasma modified nonwoven PP filter was successfully prepared followed by coating of PANI on the filter surface was efficiently achieved.Water absorption property of PP filter was significantly improved after plasma treatment and made more hydrophilic property to PP filter.The PANI coating on PP filter is increased with increase in the PANI concentration, however plasma modified PP filter showed 15% higher coating efficiency than the unmodified counterpart.The stability test revealed that PANI coating is highly stable and desorption rate is very slow and less.PANI coated modified nonwoven PP filter exhibited excellent antibacterial efficiency towards E. coli and B. subtillis bacteria and less effective to S. aureus bacteria.Biological aerosol filtration efficiency results revealed that PANI coated unmodified PP filter showed that 30% inhibition to E. coli and not effective to B. subtillis bacteria after 30 min filtration and standing.PANI coated modified filter exhibited 75 and 66.7% inhibition to E. coli and B. subtillis bacteria after 30 min standing, further increase in the standing time leads to 100% inhibition was obtained.From the aforementioned results, conclusion was made that PANI coated plasma modified nonwoven PP filter is an efficient antibioaerosol agents for E. coli and B. subtilis inhibition under present reaction conditions and it could be useful for improving the indoor air quality.

Fig. 1 .
Fig. 1.(a) Schematic diagram and (b and c) photographic images of high power plasma treatment setup.

Fig. 5 .
Fig. 5. Effect of high-frequency plasma treatment on water absorption property of nonwoven PP filter.

Fig. 6 .
Fig. 6.Percentage coating of PANI on unmodified and modified PP filter.

Fig. 8 .
Fig. 8. Effect of PANI concentration on the antibacterial efficiency of PP filter for (a) E. coli, (b) B. subtillis and (c) S. aureus inhibition.

Fig. 9 .
Fig. 9. Effect of standing time on the inhibition efficiency of PANI coated PP filter for the inhibition of (a) E. coli and (b) B. subtilis bacteria at different filtration time.

Table 1 .
E. coli inhibition efficiency of nonwoven polypropylene filter.

Table 2 .
Antibacterial activity results of PANI coated nonwoven PP filter for E. coli inhibition.

Table 3 .
Antibacterial activity results of PANI coated nonwoven PP filter for B. subtillis inhibition.

Table 4 .
Antibacterial activity results of PANI coated nonwoven PP filter for S. aureus inhibition.