Binder–Block Copolymer Micelle Interactions in ... - ACS Publications

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Binder−Block Copolymer Micelle Interactions in Bactericidal Filter Paper Nura Mansur-Azzam,† Su Gyeong Woo,† Adi Eisenberg,‡ and Theo G. M. van de Ven*,† †

Pulp and Paper Research Center, Department of Chemistry, McGill University, Montreal, Quebec H3A 2A7, Canada Department of Chemistry, McGill University, Montreal, Quebec H3A 2K6, Canada

‡

S Supporting Information *

ABSTRACT: We previously produced a bactericidal filter paper loaded with PAA47-b-PS214 block copolymer micelles containing the biocide triclosan (TCN), using cationic polyacryamide (cPAM) as a binder. However, we encountered a very slow filtration, resulting in long bacteria deactivation times. Slow drainage occurred only when the filter paper was left to dry. It appears that the filter paper with cPAM and micelles develops hydrophobic properties responsible for this very slow filtration. Three approaches were taken to accelerate the very slow drainage all based on modification of binder−micelle interactions: (i) keeping the micelles wet, (ii) modification of the corona, and (iii) replacing cPAM with smaller and more highly charged cationic poly(isopropanol dimethylammonium) chloride (PIDMAC). In all cases, the drainage time of bactericidal filter paper became close to that of untreated filter paper, without decreasing its efficiency. Moreover, replacing cPAM with PIDMAC led to a much more efficient bactericidal filter paper that reduced bacteria viability by more than 6 orders of magnitude. In addition to resolving the hydrophobic drainage hurdle, the three solutions also offer a better understanding of the interaction between cPAM and micelles in the filter paper.

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INTRODUCTION The reduction of living pathogenic bacteria in water is important because of the obvious public health implications.1,2 It is especially relevant in situations where drinking water must be obtained from untreated sources during recreational activities, military operations, or major catastrophes and in lower income and developing countries that commonly lack safe drinking water. We recently developed a new antibacterial filter paper containing block copolymer micelles filled with a biocide, which are glued to the fibers in the filter by a polyelectrolyte.3 This filter paper complements other commercially available products, such as drinking straws,4 lifesaver bottle5,6 and antimicrobial paper,7−11 or disinfecting pills.12 The present method offers the simplicity of passing the water through antibacterial filter paper by gravity to achieve safe drinking water while simultaneously removing particulates. It is simple, usable anywhere without any special equipment, easy to transport, easy to deploy, inexpensive, safely disposable, and most importantly, extremely efficient in bacteria reduction while producing water with safe levels of the biocide. The first filter paper that we produced contained cationic polyacrylamide (cPAM) as the cationic polyelectrolyte (CPE) used to keep the micelles attached to the fibers. The micelles were made of poly(acrylic acid)-block-poly styrene (PAA-b-PS) filled with triclosan (TCN), the antibacterial agent.7,13 Although these filters are effective in killing bacteria passing through the filter, a major problem that we encountered was © 2013 American Chemical Society

extremely slow drainage times, ascribed to the hydrophobic properties of a cPAM/micelle complex formed during or after drying. Electrostatic attraction between polyanions and polycations results in the formation of polyelectrolyte complexes (PECs).14−16 In solution, three types of PECs can form simultaneously: (i) soluble, (ii) colloidal, and (iii) coacervate complex. The formation of colloidal and coacervate complexes from cPAM and the polyanion sulfonated Kraft lignin (SKL) was investigated by Vanerek and van de Ven,17 who found that the molecular weight of cPAM was a key factor in coacervate complex formation. The preferential formation of either the colloidal complex for low molar mass cPAM or the coacervate complex for high molar mass cPAM was attributed to a competition between clustering of cPAM molecules, prevalent for long macromolecules, and polymer reconformation, dominant for shorter chains. From the results in this paper, it appears that complexes can also be formed when the polyelectrolytes are adsorbed on a surface and that these complexes undergo configurational changes upon drying. In this paper, we discuss a variety of ways drainage times can be reduced by modifying the hydrophobic/hydrophilic properties of the complex. Interestingly, replacing cPAM with poly(isopropanol dimethylammonium) chloride (PIDMAC) led to Received: May 1, 2013 Revised: June 26, 2013 Published: July 1, 2013 9783

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Table 1. Structure and Properties of Block Copolymers Used in This Study

a

Determined by gel permeation chromatography (GPC). For PAA-b-PS, calculated from the corresponding PtBA-b-PS before hydrolysis.18 bMixing ratios of 4:1 and 8:1.

a much more efficient bactericidal filter paper, reducing bacteria viability by more than 6 orders of magnitude. In addition to the solutions to the slow drainage, the approaches also offer a better understanding of the interaction between polyelectrolytes and block copolymer micelles.

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loading procedure and evaluation of the TCN concentration into the micelles using ultraviolet (UV) spectroscopy against the TCN calibration curve are described in detail in the Supporting Information in the publication by Vyhnalkova et al.19 Preparation of Bactericidal Filter Paper and Other Filter Paper Samples. The desired filter paper samples were prepared from the commercially available Whatman cellulose filter paper grade 2. The average weight of each filter paper type was obtained from weight measurements of 10 filter paper samples. The volume of water absorbance of the filter paper was evaluated by allowing DIW to be completely absorbed into the filter paper, followed by weight measurements. The 110 mm diameter circle filter paper was found to have an average weight of 1 g, and 2 mL of water was completely absorbed into the fibers. Bactericidal filter paper was loaded first with the cationic binder (cPAM or PIDMAC) and left to air dry for overnight, followed by the addition of TCN-loaded micelles. The filter was air-dried again, as described later. To homogenously load the desired amount of the cationic binder into 1 g of the filter paper fibers, a solution of 1 mg of cPAM or 12 mg of PIDMAC per absorbed volume of DIW (2 mL) for each gram of fibers was prepared. These amounts correspond approximately to full coverage of the binder on the fibers in the filter.3 The filter paper was soaked for 1−2 s in this solution to absorb and then left overnight to dry. At this stage, the initially negatively charged fibers had become positive by adsorption of the positively charged binder, allowing us to bind the negatively charged micelles loaded with the hydrophobic biocide, TCN. The volume of TCN-loaded micelle solution was the same as the amount of water absorbed per each gram of filter paper (2 mL). The filter paper was left to dry again overnight to obtain the desired dry bactericidal filter paper or only for 30 min to obtain the wet bactericidal filter paper. Additional filter paper samples as needed were prepared to be compared to the bactericidal filter paper. In each filter paper sample, one of the components (cationic binder or TCN-loaded micelles) was eliminated and replaced by DIW in the appropriate stage

MATERIALS AND METHODS

Reagents and Materials. Whatman filter paper grade 2 or 2 V, 110 mm diameter circle, typical thickness of 190 and 8 μm pore size (1002-110) was purchased from Whatman. Gram-negative Escherichia coli ATCC 11229 were kindly donated by Prof. Mansel Griffiths (Canadian Research Institute for Food Safety, University of Guelph, Guelph, Ontario, Canada). cPAM with degree of substitution (DS) of 20% (PERCOL 292, Mw of 3 MDa) was purchased from Allied Colloids, Inc. (Canada). The commercial PIDMAC (Alcofix 158 by Ciba) was donated by Cascades, Inc. Structure, properties, and other information of PIDMAC and cPAM are summarized in Table 1S of the Supporting Information. The block copolymer PAA47-b-PS214 and the mixed PEO/PAA corona block copolymers PAA47-b-PS356/ PEO114-b-PS350 in ratios of 1:4 and 1:8, respectively, were prepared by Dr. Tony Azzam (Department of Chemistry, McGill University, Montreal, Quebec, Canada) using atom-transfer radical polymerization.18 Structure and properties are shown in Table 1. For culture preparation and testing, we purchased Mueller Hinton broth (MHB) Powder (Fluka), Difco Mueller Hinton Agar (Fisher Scientific). Deionized water (DIW) was used to prepare all solutions. The following chemicals were purchased and used without further purification: HPLC-grade TCN (Irgasan) purchased from Fluka BioChemika and HPLC-grade 1,4-dioxane purchased from SigmaAldrich. Micelle Preparation and TCN Loading into Micelles. Micelle preparation and characterization using dynamic light scattering (DLS) and transmission electron microscopy (TEM) as well as the TCN 9784

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Figure 1. E. coli deactivation by various bactericidal filter papers (indicated in the inset) treated with cPAM as the cationic binder. (A) Growth of bacteria after passing through the filter paper samples. (B) Plate counts in contaminated water after filtration through the bactericidal filter paper compared to blank filter paper (control). of preparation. In blank filter paper, both components were excluded and replaced by DIW. Moreover, some filter paper samples were loaded with empty micelles. Two or three filter papers of each sample were prepared (for duplicates or triplicates). All of the various designed filter paper samples were tested for bactericidal efficiency as described in the Bactericidal Efficiency of Filter Paper section. Preparation of Bacteria Suspensions. E. coli (ATCC 11229) was prepared for efficiency assays of bactericidal filter papers. An inoculum from frozen glycerol stock (kept at −80 °C) was grown in growth medium (85 mL) in a water bath shaker incubator (150 rpm) at 37 °C overnight. Then, a sample from the overnight culture was diluted to the desired concentration and plated onto a Mueller Hinton Agar plate. To prepare an E. coli suspension for the efficiency tests of bactericidal filter paper, a single colony from this plate (kept at 4 °C for up to 1 month) was inoculated into 85 mL of MHB and incubated overnight at 37 °C with shaking at 150 rpm. A 300 μL aliquot of the overnight culture was inoculated into 85 mL of fresh MHB and grown for 2 h to the linear phase. The optical density (OD) of the bacteria measured at 600 nm was around 0.6. This bacteria suspension was further diluted with sterile DIW to reach an OD of 0.1 ± 0.02. At this stage, the bacteria concentration was around 108 colony forming units (CFU)/mL. Bactericidal Efficiency of Filter Paper. The bactericidal efficiency of the designed filter paper samples (cf. the Preparation of Bactericidal Filter Paper and Other Filter Paper Samples section above) was tested by their ability to deactivate E. coli. The modified filter paper samples were placed on glass funnels (previously sterilized) that were set on a stand, specially designed for this test. Sterilized glass beakers or sterile conic polystyrene tubes (BD Falcon) were set under the funnels to collect the filtrate. A total of 50 mL of freshly prepared bacteria suspension (preparation described in the Preparation of Bacteria Suspensions section above) was passed through the specified filter paper samples by gravity filtration. Filtration lasted about 5−7 min. A filtrate that had passed through the blank (untreated) filter paper and a sample of unfiltered bacteria were the two controls in each experiment. Between stages, all filtrates as well as the unfiltered bacteria were kept on ice or at 4 °C to prevent further growth of E. coli. The OD of the filtrates and the unfiltered bacteria was immediately taken. Bacteria viability was estimated by the following two methods. Plating. Filtrate samples and the unfiltered bacteria suspensions were diluted with sterilized phosphate-buffered saline (PBS). Dilutions ranged from 0 to 10−6. A total of 100 μL of the diluted and/or undiluted filtrate samples were spread on the Difco MH agar plates and incubated overnight at 37 °C. Live bacteria counts were performed and considered significant when above 30 colonies.20,21 To determine the number of colony forming units per milliliter, the colony numbers found on each plate were first multiplied by 10 (for the 100 μL samples) and then by appropriate dilution. Results were the average of

duplicates of each filtrate sample, and each experiment was repeated 3 times to confirm results. Only one sample of unfiltered bacteria suspension was available. Bacteria Growth. Growth experiments were performed to monitor bacteria deactivation. Even if bacteria pass the filter, they might not be fully deactivated or killed and would grow normally when the optimal conditions are provided. In this experiment, the filtrates from the various filter paper samples and the unfiltered bacteria were 10 times diluted in fresh MHB in a sterile 15 mL polystyrene conic tube (BD Falcon). OD measurements were performed (time 0). The caps were released to allow in air, and all samples were placed in a 37 °C water bath incubator with shaking (150 rpm). The growth was monitored by UV/vis at 600 nm every 1 h (or as indicated) up to 5 h. When applicable, samples were left to grow overnight, and then final OD measurements were taken. Results were presented as relative absorbance (each absorbance at a given time point was divided by OD at time 0) and were the average of duplicates of each filtrate sample. Only one sample of unfiltered bacteria suspension was available. Each experiment was repeated 3 times to confirm results. Contact Angle Evaluation of the Water Drop on PAA-b-PS Micelle Film. A solution of unloaded micelles (5 mL, prepared as described in the Micelle Preparation and TCN Loading into Micelles section above) was added to a small glass Petri dish that contained a microscope cover (thin glass square). The micelles were dried in an oven (50 °C) for 5 h to form the film. A contact angle (θ) of a 3 μL DIW drop on micelle film was measured by a contact angle system (OCA) every 30 s. Triplicates of this sample were prepared and measured.

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RESULTS AND DISCUSSION Hydrophobic Bactericidal Filter Paper. Filter paper impregnated with cPAM and TCN-loaded micelles was able to deactivate bacteria growth and reduce E. coli viability (panels A and B of Figure 1). The growth of bacteria that were filtrated through the bactericidal filter paper was compared to that of bacteria that had passed through filter paper treated with cPAM, cPAM and empty micelles, and TCN-loaded micelles only. In comparison to the blank (untreated) filter paper control, growth deactivation was observed with the bactericidal filter paper (treated with cPAM and TCN-loaded micelles) and filter paper treated with TCN-loaded micelles only. Most likely, without the cationic binder, the micelles were released into the filtrate during the filtration process. Bacteria passing through filter paper samples treated with cPAM only or with empty micelles only were able to maintain normal growth (Figure 1A). As a result of filtration through the bactericidal filter paper, around 90% of the bacteria was lysed, as appears from Figure

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hydrophobic (non-wetting). Although the initial value of the contact angle is less than 90°, the initial value of 84° is close to the non-wetting angle. This angle decreased and reached a plateau of around 67° in 9 min (no contact angles on cPAM/ micelle were performed, because cPAM flocculated the micelles and no good films could be formed). It appears that the presence of cPAM in the paper with micelles increased its hydrophobicity (Figure 2). Upon drying, a complex formation is initiated or modified by the electrostatic attraction between the anionic corona and the high-molecular-weight polymeric cationic binder, cPAM, which could entangle with the PAA chains to maximize contact with micelles (as illustrated schematically in Figure 4). The interaction of cPAM with

1B. Similar results were shown in a previous study in our lab, using the same combination of cPAM and TCN-loaded micelles.3,13 The efficiency can be improved by passing the contaminated water through two filter papers, in which case the efficiency is 99%.22 Despite the promising bactericidal efficiency of the filter paper, we faced a very slow drainage hurdle. As appears in Figure 2, an extremely slow drainage occurred when

Figure 2. Drainage rates of 50 mL of contaminated water through the filter paper samples tested in Figure 1A. The blank is untreated filter paper. When cPAM and micelles (loaded or empty) are combined, the drainage becomes very slow.

cPAM and micelles, either empty or loaded with TCN, were impregnated into the filter paper, which increased the drainage time from minutes to hours. To develop an innovative product requires reasonably short filtration times of the contaminated water. Thus, a solution to this drainage problem is essential for further progress. An interesting observation showed that the bactericidal filter paper adopted hydrophobic properties after drying. During filtration, the bactericidal filter paper does not become wet instantly like the other filter paper samples that do not have the combination of cPAM/micelles. Although the PAA47-b-PS214 micelles are hydrophilic in water, when dried, the micelles are initially almost hydrophobic, with a water contact angle of about 84° (Figure 3). Surfaces with contact angles >90° are

Figure 4. Schematic illustration of cPAM forming a complex with the PAA corona of PAA47-b-PS214 micelles. cPAM rearranges on the surface and entangles with the PAA chains to maximize contact with the micelle corona.

PAA is reminiscent of a coacervation complex, with much reduced charge, thus making the system less hydrophilic (more hydrophobic). It is likely that, as result of this complex formation, PAA is not exposed and not readily accessible to the water during the filtration; therefore, it cannot recover its hydrophilicity quickly when exposed to water during the filtration. As part of the complex, the cationic portion of cPAM is not accessible as well. As a result, the hydrophilic character of the filter paper is lost. The reason that this happens upon drying may be that the affinity of cPAM for fibers is diminished during drying, because of a corresponding increase in ionic strength, which screens the electrostatic attraction between fibers and cPAM (neutral PAM does not adsorb on fibers). Three solutions were proposed to resolve the hydrophobic drainage problem, all on the basis of the modification of the interactions between the binder and the PAA corona, as described in the next sections below. First Solution: Keeping Micelles Wet. Micelles are hydrophilic in water. However, when dried, the PAA corona of the micelles collapses. As described in the Hydrophobic Bactericidal Filter Paper section, the initial contact angle (84°) of the dried corona showed that, upon drying, the micelles become close to being hydrophobic. When in contact with water, the micellar corona becomes wet again and its hydrophilicity increases. It is likely that the complex between the micellar corona and cPAM slowly rearranges upon wetting but too slow to regain its hydrophilicity in a reasonable amount of time. To test this idea, we prepared damp filter paper, in which micelles were added to filter paper treated with a cPAM

Figure 3. Contact angle of the water drop (3 μL) on the PAA47-bPS214 micelle film. The contact angle was measured every 30 s. The micelle film was formed on the glass surface, and the contact angle was determined, as illustrated in the inset. 9786

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solution and left to dry for 30 min only, during which time the micelles remained wet, thus maintaining the hydrophilic properties of PAA. As shown in Figure 5, as a result of using

Figure 6. Mixed corona micelles as a second solution to the drainage problem. Drainage rates of 50 mL of contaminated water through the filter paper samples. Micelles were loaded with TCN. Dry micelles on filter paper are compared to blank (untreated control) filter paper and to dry filter paper samples treated with cPAM and mixed corona micelles, as indicated in the inset.

Figure 5. Wet micelles as a first solution to the drainage problem. Drainage rates of 50 mL of contaminated water through the filter paper samples. Micelles were loaded with TCN. Wet micelles on filter paper are compared to dry micelles on filter paper and to untreated blank (untreated control) filter paper.

electrostatic attraction remains sufficient to keep the micelles attached to the filter paper. This modification maintains the bactericidal ability of the filter paper: UV/vis experiments (Figure 7) show that deactivation obtained for filter paper

wet filter paper, the drainage time was decreased from over 2 h to minutes. Wet micelles did not affect the ability of the filter paper to deactivate bacteria growth. Both dry and damp filter paper with empty or loaded wet micelles had the same drainage time that was only a few minutes longer than the blank (untreated) filter paper. As expected, only loaded micelles deactivated bacteria growth. A full series of experiments was performed with filter paper treated with cPAM and empty micelles, cPAM only, and loaded micelles only; both dry and wet forms were tested (data not shown). Keeping the micelles damp did not change any of the filter paper characteristics. Using wet micelles in filter paper pretreated with cPAM had provided us with a solution to the drainage hurdle without affecting the bactericidal activity of the filter paper. However, production of wet bactericidal filter paper is more costly, because the filters will require sealing. Therefore, our solution for fast drainage should preferentially involve dry filter paper. Second Solution: Modifying the Micellar Corona. It was suggested in the previous section that, when dried in the presence of cPAM, the PAA corona will not easily recover its hyrophilicity during the filtration process. It seems that, upon drying the micelles, strong interactions take place between cPAM and the PAA corona, leading to the loss of the hydrophilic properties of both cPAM and the PAA corona. Accordingly, the second solution proposes to decrease the interaction between cPAM and micelles by lowering the negatively charged PAA content in the corona. Thus, micelles with a mixed PEO/PAA corona were prepared, in which part of the hydrophilic poly(acrylic acid) (PAA) in the corona was replaced with neutral polyethylene oxide (PEO) (Table 1). As described in Table 1, although the polystyrene (PS) chain is longer (∼350 units in PEO-b-PS compared to ∼200 units in PAA-b-PS), the change in the micellar size is minor. Two mixed corona micelles were prepared, in which the ratio of PEO/PAA is 4:1 and 8:1. As seen in Figure 6, fast drainage was achieved as result of a lower PAA content in the corona. Both tested compositions of mixed corona micelles (PEO/PAA = 4:1 and 8:1) showed similar drainage times. The replacement of part of PAA with PEO in the corona reduces the strong interaction of PAA with cPAM, but the

Figure 7. Growth deactivation of bacteria suspension (50 mL) filtered through filter paper samples treated with cPAM and mixed corona micelles loaded with TCN and compared to filter paper samples treated with cPAM and PAA corona micelles loaded with TCN and with blank filter paper.

samples treated with cPAM and PS/PAA corona micelles was very similar to that for filter paper treated with cPAM and mixed corona micelles. As for the empty PS/PAA corona micelles, empty mixed corona micelles could not deactivate E. coli growth either. Third Solution: Modifying the Cationic Binder. In the previous sections, we successfully showed that the formation of hydrophobic filter paper was due to strong interactions between the high-molecular-weight cationic binder (cPAM) and the PAA in the corona of the micelles. Upon designing coronas with a lower PAA portion and, thus, weaker interactions with cPAM, we successfully maintained the hydrophilic properties of the filter paper. Are the electrostatic interactions the only force that drives the loss of hydrophobicity in the filter paper? In this part of the study, we replaced the cPAM with low-molecular-weight and highly cationic PIDMAC (see Table 1S of the Supporting 9787

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compares relative absorbance, and Figure 9B compares the number of live bacteria after filtration for filter papers with PIDMAC, in the presence and absence of loaded micelles. PIDMAC shows a clear bactericidal effect, and together with the TCN, both act in synergy to provide better deactivation of bacteria.3 As indicated in Figure 9B, the concentration of live bacteria is reduced by ∼7 orders of magnitude when bacteria are filtered through filter paper treated with PIDMAC and micelles loaded with TCN.

Information). This modification actually allowed for a stronger electrostatic interaction, yet it resolved the drainage problem (Figure 8).

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CONCLUDING REMARKS We have shown that bactericidal filter paper can be prepared with extremely high bacteria deactivation efficiencies and fast drainage rates. The type of polyelectrolyte used to bind the micelles to the fibers in the filter paper is crucial for its performance. High-molecular-weight cPAM is able to rearrange on the surface of the fibers and associate with PAA in the micelle corona. This leads to hydrophobic micelles after drying, which take a long time to restore their hydrophilicity upon rewetting. PIDMAC likely also forms a complex with PAA, but the micelles remain hydrophilic, probably because of the hydrophilic hydroxyl groups in its structure. Reducing the amount of PAA in the micelle corona also results in fast drainage, without affecting the deactivation efficiency. Adding PIDMAC, which itself is an antibacterial agent, together with TCN-loaded micelles to filter paper results in very effective bactericidal filters, because of synergistic effects of PIDMAC and TCN.

Figure 8. PIDMAC as a third solution to the drainage problem. Drainage rates of 50 mL of contaminated water through the filter paper samples. Micelles were loaded with TCN. The blank (untreated) filter paper sample was used as a control.

The difference between cPAM and PIDMAC may be due to the fact that, besides the cationic charge groups, which neutralize upon complexation, cPAM does not have any hydrophilic groups, whereas PIDMAC contains one hydrophilic hydroxyl group per repeat unit. PIDMAC alone, in the absence of TCN-loaded micelles, shows very significant antibacterial properties.3,23 Figure 9A

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ASSOCIATED CONTENT

S Supporting Information *

Structure and properties of cationic binders used in this study (Table 1S). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 514-398-6177. Fax: 514-398-8254. E-mail: theo. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Strategic Research Network on Bioactive Papers (Sentinel) of the Natural Sciences and Engineering Research Council of Canada (NSERC) and their industrial partners for financial support. The authors are also grateful to Dr. T. Azzam for the synthesis of the block copolymers, Prof. M. Griffiths for providing E. coli, and Dr. R. Gaudreault (Cascades, Inc.) for providing PIDMAC.

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ABBREVIATIONS cPAM, cationic polyacryleamide; PIDMAC, poly(isopropanol dimethylammonium) chloride; TCN, triclosan; PAA-b-PS, poly(acrylic acid)-block-poly styrene; CPE, cationic polyelectrolyte; PEC, polyelectrolyte complex

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Figure 9. (A) E. coli growth deactivation by bactericidal filter paper. (B) Plate counts (CFU/mL) in contaminated water after filtration through the filter paper treated with PIDMAC and TCN-loaded micelles compared to filter paper treated with PIDMAC only and blank filter paper. TFTC stands for too few to count (statistically reliably).

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