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Antibacterial Acrylamide Hydrogels Containing Silver Nanoparticles by Simultaneous Photoinduced Free Radical Polymerization and Electron Transfer Processes Mustafa Uygun, Muhammet U. Kahveci, Dilek Odaci, Suna Timur,* Yusuf Yagci*
Ag-nanoparticle-containing hydrogels were successfully prepared by in situ reduction of silver nitrate in the presence of citrate molecules as stabilizing agent during photoinduced copolymerization of AAm and BAAm. Swelling-deswelling behavior and thermal properties of the synthesized hydrogels were investigated. The interior morphology of the gels exhibit continuity, which is a common feature for hydrogel networks. Antimicrobial activities of the hydrogels were also investigated against pathogenic E. coli O157:H7, S. aureus, and non-pathogenic E. coli K12, which are model microorganisms for testing bactericidal properties. The hydrogels containing well-dispersed Ag NPs showed significant antibacterial activity.
Introduction Polymeric materials containing metallic nanoparticles (NPs) have attracted tremendous scientific and technological interest in recent years due to their unique and versatile properties.[1–5] Research effort has been focused on
M. Uygun, M. U. Kahveci, Y. Yagci Department of Chemistry, Faculty of Science and Letters, Istanbul Technical University, Maslak TR-34469, Istanbul, Turkey Fax: þ90 212 285 6386; E-mail:
[email protected] D. Odaci, S. Timur Department of Biochemistry, Faculty of Science, Ege University, Bornova 35100, Izmir, Turkey E-mail:
[email protected] Macromol. Chem. Phys. 2009, 210, 1867–1875 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
the design and development of such materials containing noble metal NPs dispersed in a polymeric matrix, with potential applications in the field of optical,[6] electrical,[7] chemical,[8,9] biological,[10–12] and medical[13,14] devices as well as in data storage.[15] In particular, polymers with silver NPs (Ag NPs) have been identified as materials commonly used in biological and medical applications.[10,16] In fact, the use of silver metal as a material to overcome infections, dates back as early as ancient times.[17] Important applications include employing Ag NPs as an antibacterial agent in dressing materials, medical devices, and equipments such as surgical gloves and covers coated with the antimicrobial polymers.[12,14] It is wellknown that biological activity of silver, especially the antibacterial property, is size-dependent.[12] Thus, silver
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clusters should be small enough to pass through any bacterial cell membrane. Consequently, various strategies have been employed to prepare nanocomposites containing Ag NPs with controlled size. One successful approach that has been employed is the reductive processes in homogeneous matrices. However, this requires separated processes leading to low homogeneity and more effort.[8,9,18] Mainly, two different methods have been employed for preparation of polymer networks containing Ag NPs, which involve two distinct steps; conventional polymerization for formation of supportive and stabilizing matrix, and reduction of silver salts for preparation of Ag NPs.[8,18] In the first method, hydrogel is firstly synthesized, then formerly prepared Ag NPs are incorporated into the polymeric material. This is called breath-in and breath-out technique.[18] In the second approach, previously formed cross-linked polymer is swollen with a solution of silver salts and reducing agent, and then reduction takes place within the polymer.[8] As stated previously, these techniques involve at least two distinct processes emerging some complications and difficulties. Although nanocomposites containing metal NPs, including hydrogels with Ag NPs, have elegant features, the homogeneous dispersion of metal NPs is not easy using a simple process because of their high surface free energy, which may cause agglomeration. Therefore, preparation of such nanocomposites with desired properties (i.e., Ag NPs with convenient size) is an important issue. In our laboratory, we have focused on the development of new methods to produce such materials in a comparatively simple way by combining the two distinct steps. One approach that has been successfully employed is the photoinduced processes involving simultaneous reduction of metal salts to NPs and polymerization of di-functional monomers to cross-linked polymeric matrix.[19–22] In this approach, photolysis of a benzoin-type photoinitiator, generates free radicals which can induce the polymerization and the reduction of metal cations concomitantly (Scheme 1). In the current study, the described photoinduced process was extended to in situ synthesis of acrylamide (AAm)/
N,N’-methylenebisacrylamide (BAAm) hydrogels containing Ag NPs. Measurements of water uptake and water retention were carried out to understand hydrogel behaviors of the gels. Thermal stability and interior porous structures of the gels were analyzed. NP size was revealed by atomic force microscopy (AFM) grain analysis. Antibacterial activities of the hydrogels were also investigated against Gram-negative, pathogenic and non-pathogenic Escherichia coli (E. coli) and Gram-positive (pathogenic), Staphylococcus aureus (S. aureus) which are the model microorganisms for testing bactericidal properties.
Experimental Part Materials Acrylamide (AAm) (Fluka), BAAm (Merck), trisodium citrate dihydrate (Riedel-de Haen), 1-[4-(2-hydroxyethoxy)phenyl]-2hydroxy-2-methyl-1-propane-1-one (Irgacure 2959) (Ciba), silver nitrate (Sigma-Aldrich), and citric acid monohydrate (Merck) were used as received. Nutrient Broth, Agar, Trypton, and yeast extract were purchased from Sigma (St. Louis, MO, USA) and Oxoid (Hampshire, England), respectively.
Synthesis of AAm/BAAm Hydrogels Containing Silver Nanoparticles The hydrogels containing Ag NPs were prepared through free radical aqueous photopolymerization of AAm/BAAm by using a water-soluble type I photoinitiator, Irgacure 2959, and reduction of silver nitrate occurred in the meantime. For this purpose, nitrogen-flushed samples listed in Tables 1 and 2 were prepared in glass tubes with a diameter of 3.2 mm. The tubes were sealed off and irradiated for 15 min at room temperature (22 8C) in a Rayonet merry-go-round photoreactor in which the samples were surrounded by a circle of 16 lamps emitting light nominally at 350 nm. The light intensity at the location of the samples was measured by a Delta Ohm model HD9021 radiometer (3.0 mW cm2). After synthesis of AAm/BAAm hydrogel containing Ag NPs, the tubes were broken gently and the gels were transferred into deionized water immediately. Each sample was stored in an excess of water at room temperature for at Table 1. Formulations for free radical photopolymerization of AAm/BAAm (9.876 and 0.124 wt.-%, respectively) and reduction of AgNO3 induced by Irgacure 2959.
Gel
Scheme 1. In situ synthesis of polymer network containing metal NPs by light-induced processes.
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AgNO3
Irgacure 2959
Citrate
wt.-%
wt.-%
wt.-%
Gel-1
0.12
0.032
0.182
Gel-2
0.12
0.047
0.182
Gel-3
0.12
0.063
0.182
Gel-4
0.24
0.063
0.364
Gel-5
0.36
0.063
0.546
DOI: 10.1002/macp.200900296
Antibacterial Acrylamide Hydrogels Containing Silver Nanoparticles by Simultaneous . . .
Table 2. Formulations for free radical photopolymerization of AAm/BAAm (9.876 and 0.124 wt.-%, respectively) and reduction of AgNO3 induced by Irgacure 2959b) (0.032 wt.-%) at different pH values.
Gel
AgNO3
Citric acid
Citrate
pH of solution
wt.-%
wt.-%
wt.-%
Gel-6
0.12
0.135
0
2.9
Gel-7
0.12
0.101
0.045
3.5
Gel-8
0.12
0.068
0.091
4.5
Gel-9
0.12
0.034
0.136
5.5
Gel-1
0.12
0
0.182
7.1
Gel-10
0
0
0
6.2
regular time intervals by removal of the samples from acetone, wiping excess acetone on the surface of the gels, and weighing the gels. The water loss was recorded periodically during the deswelling process. Investigation of change of water retention (WR) [Equation (3)] with time is a common way to understand deswelling kinetics of hydrogels. WR ¼ 100
Characterization
Swelling and Deswelling Kinetics Swelling and deswelling properties of the hydrogels were analyzed as described in literature.[23] Swelling behaviors of hydrogels at different temperature (20–60 8C) were investigated by the means of swelling ratios (SR) which can be defined as follows: Ws Wd
(1)
where Ws and Wd are the weight of water in the swollen gel and weight of dry gel, respectively. The SR measurements were carried out gravimetrically using samples wiped with moisturized filter paper to remove excess water after incubation of samples in distilled water for 24 h at each temperature. Swelling kinetics of the gels were analyzed by measuring water uptake (WU) by the gels dried completely under vacuum. The dried gels were placed in deionized water at 20 8C, and the gel samples were weighed periodically after removing of excess water on their surfaces. The swelling kinetics of the gels was analyzed gravimetrically by using WU which can be defined as follows: WU ¼ 100
Wt Wd Ws
(2)
where Wd and Ws are the same as defined above, and Wt equals to total weight of the gel at a certain time interval. The deswelling kinetics of the gels were analyzed gravimetrically in acetone at 20 8C. Fully swollen gel samples were immersed in acetone. The water loss from the gels was measured at Macromol. Chem. Phys. 2009, 210, 1867–1875 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
(3)
where parameters are the same as defined above.
least 24 h to keep them at their equilibrium state and to remove unreacted monomer and initiator molecules from the gels. Washing solutions were treated with aqueous sodium chloride (1 mL, 0.86 mol L1) and no precipitation was noted indicating the complete reduction of the salt. Monomer conversions were determined gravimetrically after the hydrogels were washed and dried (average conversion >94%).
SR ¼
Wt Wd Ws
Scanning electron microscopy (SEM) images were taken with a Jeol (JSM-639OLV SEM) operating at an acceleration voltage of 15 kV. SEM specimens were prepared by coating freeze-dried gels with Pt. The morphological images of Ag NPs were determined by AFM (Q-Scope 250 Scanning Probe Microscope Ambios Tech., UK). Samples for AFM measurement were obtained from the solutions in which the hydrogels were stored. When the hydrogels containing Ag NPs were placed in water for a long period, the water became colored probably due to escape of Ag NPs from the gels to the water. Investigation of these water samples by AFM can give information about the size of Ag NPs. For this reason, AFM specimens were prepared by placing a drop of the water sample containing Ag NPs on a glass slide; and after slow solvent evaporation, thin film Ag NPs were obtained. Thermogravimetric analysis (TGA) was performed on a Perkin-Elmer Diamond TA/TGA with a temperature scan from 40 to 900 8C with a heating rate of 10 8C min1 under nitrogen flow (200 mL min1).
Antibacterial Activities of the Hydrogels Three bacterial strains, namely E. coli K-12 and E. coli O157: H7 (from DSMZ; German Collection of Microorganisms and Cell Cultures, Braunschweig, Germany) and S. aureus 6538P (from Microbiology Department Culture Collection of Ege University, Faculty of Science) were subjected to this analysis. S. aureus was cultured on nutrient broth and E. coli K-12 and E. coli O157: H7 were inoculated on Luria-Bertani medium (containing (g L1): Tryptone, 10.0; Yeast extract, 5.0; NaCl, 10.0; agar, 15). All cultures were subcultured monthly and subsequently stored at 4 8C. The strains were incubated for 18 h at 37 8C on a rotary shaker in 250 mL flasks filled with 50 mL of mediums.[24] The effect of hydrogels on Gramnegative (pathogenic and non-pathogenic) and Gram-positive, pathogenic bacteria was investigated according to the agar diffusion method.[25] Initially, each strain was cultured on the medium at 37 8C for 18 h. After that, the cultured organisms were added to 10 mL of saline solution (0.9% NaCl) to reach approximately the 105 colony forming units per milliliter (CFU mL1). Each gel sample (15 mg) was placed in the middle of sterilized Petri dishes. Then, 1 mL of saline solution containing bacteria and agar solution were added onto the surface of each material. After incubation for 24 h at 37 8C, hydrogels having antimicrobial activity inhibited the growth of the microorganisms and a clear,
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distinct zone of inhibition was visualized surrounding the sample. The antimicrobial activity of each material was determined by measuring the diameter of inhibition zones in millimeter. Plates without hydrogel were used as controls. The counts on three plates corresponding to a particular sample were averaged.
Results and Discussion
Figure 1. Photographs of the freeze-dried hydrogels with (Gel-9) or without (Gel-10) Ag NPs.
Synthesis of Acrylamide/N,N’-Methylene Bisacrylamide Hydrogels Containing Silver Nanoparticles As reported in previous publications, Ag NPs were successfully formed by in situ reduction of silver nitrate during polymerization induced by photochemically[19,20] and thermally[26] generated free radicals. Using the same strategy, free radical photoinduced copolymerization of AAm and N,N’-methylene bisacrylamide (BAAm) along with formation of Ag NPs[27–29] were achieved by photolysis of a benzoin-type photoinitiator, Irgacure 2959, which generates free radicals in aqueous media with high quantum efficiency (Scheme 2).[30,31] The assembly of Ag NPs on nylon 6 nanofibers through interfacial hydrogen bonding interactions was demonstrated by Hinestroza and coworkers.[32] In our work, similar hydrogen bonding was achieved between citrate molecules attached to Ag NPs and amide moieties of the gel. Thus, the added citrate (or citric
acid), stabilized and assembled Ag NPs on the cross-linked polyacrylamide gel. At the end of 15 min of irradiation, the colorless aqueous solutions of formulations containing silver nitrate, turned to fully brown-yellow solid. However, although successful cross-linking was readily achieved, the irradiation of the solutions in the absence of silver nitrate under identical experimental conditions resulted in no color change. The difference between colors of freeze-dried gels formed in the presence and absence of silver nitrate shown in Figure 1 supports the existence of Ag NPs with a size distribution ranging between 10 and 50 nm.[19] In order to understand the size of the Ag NPs incorporated in the gels more precisely, AFM grain analysis was performed for the NPs obtained from storage solution of Gel-5. The NPs visualized by AFM (Figure 2) had an average size of 37 nm. Furthermore, the AFM micrographs also revealed the homogeneous size distribution of the NPs. Swelling-Deswelling Kinetics and Characterization of the Hydrogels
Scheme 2. Synthesis of AAm/BAAm hydrogel containing Ag NPs by photoinduced free radical aqueous polymerization and reduction of silver cations simultaneously.
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Figure 3 displays SR values of the hydrogels plotted against temperature. Chemical composition of hydrogels significantly affects their SR. Principally, hydrogels consisting of more hydrophilic moieties swell more than hydrogels with lower polar composition. As expected, a variation in SR values of the hydrogels with and without Ag NPs was found at certain temperatures. For example, the hydrogels containing Ag NPs (Gel-1 and 6–9) swelled up to 15–18 times with respect to their original weight at 20 8C, whereas pure hydrogel (Gel-10) swelled just 11 times which is consistent with literature (Figure 3).[33] This is because Gel-10 does not contain citrate (or citric acid) which provides more hydrophilicity to the hydrogels and interacts relatively more with water. Moreover, although at different rates, both radicals
DOI: 10.1002/macp.200900296
Antibacterial Acrylamide Hydrogels Containing Silver Nanoparticles by Simultaneous . . .
Figure 2. AFM micrographs of Ag NPs obtained from the solution in which hydrogel containing Ag NPs (Gel-5) was stored.
generated from photolysis of the initiator participate[34] in the initiation of polymerization and lead to the formation of highly cross-linked network. Figure 3 also shows the effect of temperature on SRs of all hydrogels. Temperature affected all gels in same fashion where SR increases with increasing temperature.[33] The effect of pH of formulations on SRs was also investigated and presented in Figure 3. As a result of the reason stated above, the hydrogel obtained from more basic
Figure 3. SRs of the hydrogels at various temperatures. Macromol. Chem. Phys. 2009, 210, 1867–1875 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 4. Effect of the initial silver nitrate concentration on swelling (a) and deswelling (b) kinetics of the hydrogels.
formulations draws more water inside the gel than the others prepared from more acidic formulations. The effect of the silver nitrate concentration in polymerization formulations on swelling and deswelling kinetics of corresponding hydrogels in acetone was summarized in Figure 4. Both swelling and deswelling behaviors of the hydrogels were independent from amount of Ag NPs within the gels. However, the pure hydrogel showed a little variation with fast response both in swelling and deswelling. This probably resulted from the fact that existence of Ag NPs stabilized with citrate molecules, modified the ionic character of the gels. Increase in amount of free radicals generated from any initiator in formation of cross-linked structures can increase cross-linking density and decrease diameters of pores of resulted gels leading to slower solvent mobility. Therefore, amount of the photoinitiator generating free radicals affected slightly the swelling and deswelling kinetics of the hydrogels containing Ag NPs as a result of change in porous structure of the gels. The gels produced from the formulations containing higher concentration of
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Figure 5. Effect of the initial photoinitiator concentration on swelling (a) and deswelling (b) kinetics of the hydrogels.
Figure 6. Effect of pH of the formulations on swelling (a) and deswelling (b) kinetics of the hydrogels.
Irgacure 2959, showed slightly slower swelling and deswelling behavior (Figure 5). However, all hydrogels containing Ag NPs had almost same final SR ranging from 15.6 to 17.1, which means that the photoinitiator amount affects rate of swelling or deswelling but not swelling level. Slightly faster response was observed in swelling and deswelling behavior of the hydrogels with more ionic character (the hydrogels containing more citrate) compared to those with less ionic nature (the hydrogels containing more citric acid) (Figure 6). In other words, increase in pH enhances responses of the gels since citrate molecules with sodium ions rather than hydrogen ions have more tendencies to water molecules compared to citric acid molecules. Enhancement of thermal stability of hydrogels due to incorporation of Ag NP was reflected in the thermograms of Gel-3, Gel-4, and Gel-5 (Figure 7). The char yield of Ag NP containing hydrogels was 14% when heated up to 750 8C, whereas that for hydrogel without Ag NPs was only 5%. SEM images of the hydrogels with [Figure 8(a), (c), and (e)] and without [Figure 8(b), (d), and (f)] Ag NPs showed
Figure 7. Effect of the amount of Ag NPs on thermal stabilities of the hydrogels. Gels contain 0.12 (Gel-4), 0.24 (Gel-4), and 0.36 (Gel-5) wt.-% Ag NPs. Gel-10 does not contain Ag NPs.
Macromol. Chem. Phys. 2009, 210, 1867–1875 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Antibacterial Acrylamide Hydrogels Containing Silver Nanoparticles by Simultaneous . . .
same properties. The structures of both hydrogels had similar interior porous morphology consisting of honeycomb-like pores with the diameters ranging from 15–40 mm. As a consequence, incorporation of Ag NPs did not affect the morphology and pore size of the hydrogel. Antibacterial Activities of The Hydrogels Silver nanoparticles (Ag NPs) are being considered as a nontoxic environmentally friendly antibacterial material. Recent advances are aimed to discover promising paths and nanopots to prepare Ag NPs within the polymeric gel network architectures that brought a concept for newer composite/hybrid materials in biomedical applications.[35] Nutrient agar (for S. aureus) and LB agar plates (for E. coli) incorporating hydrogels in different compositions were inoculated with 105 CFU mL1 from different bacterial
strains. Antibacterial effect of gels containing Ag NPs was obvious as shown in Table 3. As can be seen from the size of inhibition zones, similar antibacterial activity was observed for both types of E. coli strain. However, this effect was found to be more pronounced against Gramnegative bacteria than Gram-positive organism as already studied in early works in which it was reported that the interaction between NPs and cell wall of bacteria could be facilitated by the relative abundance of negative charges on gram-negative bacteria so that Ag NPs might easily penetrate into the cell and strongly associate with the cellular components that caused more effective inhibition of bacterial growth.[36] It was noticed that higher inhibition zone sizes against all strains were observed with Gel-5 [Figure 9(a)] which had highest NP content in contrast to Gel-10 (without NP) [Figure 9(b)] which did not exhibit any antibacterial activity. Hence, it can be also stated that this would be an evidence of the dose-dependent effect.
Figure 8. SEM micrographs of hydrogels with [Gel-5 (a, c, and e)] or without [Gel-10 (b, d, and e)] Ag NPs. Macromol. Chem. Phys. 2009, 210, 1867–1875 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Table 3. Anti-bacterial activities of gels containing Ag NP against pathogenic E. coli O157:H7, S. aureus, and non-pathogenic E. coli K-12 (cell density: 105 CFU mL1).
Gel
Zone diameter mm S. aureus E. coli O157:H7, E. coli K-12, pathogenic, non-pathogenic, pathogenic, Gram-negative Gram-negative Gram-positive
Gel-1
16
17
12
Gel-2
16
17
11
Gel-3
18
17
10
Gel-4
17
16
7
Gel-5
19
19
14
Gel-6
16
16
9
Gel-7
16
17
11
Gel-8
17
17
12
Gel-9
14
16
10
Gel-10
–
–
–
Figure 9. Anti-bacterial activity of the hydrogels with (a) or without (b) Ag NPs against pathogenic E. coli O157:H7, S. aureus, and non-pathogenic E. coli K-12 (Cell density: 105 CFU mL1).
Conclusion A two-component photochemical system based on the free radical polymerization and redox processes has been designed for the in situ preparation of hydrogels with Ag NPs. UV light is used to excite the photoinitiator which then undergoes homolytic scission to generate two radicals. While either or both radicals initiate the cross-linking
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copolymerization of AAm and BAAm, the electron transfer reaction between the electron donor radical and silver nitrate, results in the formation of Ag NPs. In this communication, the various experimental and compositional parameters that affect the hydrogel behavior, were investigated, and swelling and deswelling properties were evaluated. The hydrogels containing well-dispersed Ag NPs showed significant antibacterial activity. This property together with swelling behavior may be of great significance for extensive use in water-based applications. Received: June 18, 2009; Revised: July 17, 2009; Published online: September 22, 2009; DOI: 10.1002/macp.200900296 Keywords: antibacterial properties; hydrogels; nanoparticles; photopolymerization; radical polymerization
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