Immobilization of Cyclic N-Halamine on Polystyrene-Functionalized

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Immobilization of Cyclic N-Halamine on Polystyrene-Functionalized Silica Nanoparticles: Synthesis, Characterization, and Biocidal Activity Alideertu Dong, Qing Zhang, Tao Wang, Weiwei Wang, Fengqi Liu, and Ge Gao* College of Chemistry and MacDiarmid Laboratory, Jilin UniVersity, Changchun 130021, People’s Republic of China ReceiVed: May 4, 2010; ReVised Manuscript ReceiVed: July 15, 2010

Antimicrobial composites with a well-defined core-shell nanostructure were prepared through immobilization of N-halamine on polystyrene-functionalized silica nanoparticles. Evidence for immobilization of N-halamine onto polystyrene-modified silica has been inferred from different techniques like transmission electron microscopy (TEM), dynamic light scattering (DLS), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), zeta potential analyses, and Fourier transform infrared (FTIR). Experimental results showed that structures and morphologies of the as-prepared hybrid nanoparticles could be well controlled. Resultant nanosized particles displayed 2-8 times higher biocidal activity against S. aureus and E. coil than the bulk counterparts, and tests indicated that these powerful and stable nanosized antimicrobials had higher biocidal efficacy against S. aureus than E. coli. The biocidal behavior makes these composite nanoparticles an ideal candidate for various important applications such as in disinfection of hygienic areas, water purification, and food packaging. 1. Introduction Free halogen,1-4 ozone,5-8 and chlorine dioxide9-12 were used as disinfectants in the treatment of water and wastewater at one time. However, all of these compounds have some limitations such as short-term stability in aqueous solution and reactivity with organic impurities in water to form undesired byproducts.13-15 As a result, there is continuous effort to develop new insoluble polymeric disinfectants. For example, polymeric quaternary ammonium salts,16-19 phosphonium salts,20 and N-halamines21-25 have been investigated for a wide range of applications such as disinfection of hygienic areas, water purification, and food packaging.26-30 In particular, N-halamines containing one or more nitrogen-halogen covalent bond are of great importance due to their inherent advantages such as long-term stability, high durability, and regenerability. Considerable research efforts have been devoted to synthesize various N-halamine-based antibacterial materials.21-25 In general, the antibacterial performance of N-halamine-based materials strongly depends on materials’ surface area and the contact time of microorganisms with materials.31-33 Recently, modern research on materials science has focused on the fabrication of spheric structures with dimensions ranging from nanometers up to micrometers. Interest in these kinds of materials has been driven from many fields because of their exceptional properties (chemical, optical, electrical, etc.), depending on the particle’s size, composition, and structure.34-37 Among these, nano-SiO2 particles are most widely chosen as supports as SiO2 nanoparticles were nearly nontoxic, biocompatible, chemically inert, optical transparent, optimistically water dispersible, and surface functions as the silanol groups on the silica surface offer versatile possibilities for covalently functionalizing the silica-coated particles. Accordingly, silica nanoparticles have been used in many fields such as ceramics, chromatography, and catalysis, etc.38-42 Besides, fine silica powder (submicrometer-sized particles) has been used for the * To whom correspondence should be addressed. Phone: +86-43182276325. Fax: +86-431-88499187. E-mail: [email protected].

wide-ranged applications including electronic substrates, thermal and electrical insulators, humidity sensors, and optoelectronic devices.43-46 In this work, the performance of antibacterials has been improved by using silica nanoparticles as supports to enhance the activated surface. Since antibacterials adsorbed physically on the support surface can leach out easily, immobilization by means of chemical bonding is an effective approach. However, SiO2 particles with Si-OH groups on surfaces are difficult to firmly attach to N-halamine chemically.47-49 Therefore, it is difficult for the antimicrobial properties of the composites to reach a desirable level. To overcome this defect, polystyrene (PS) has been coated on the surface of SiO2 particles, and after chloromethylation these grafted PS could be used to interlink tightly silica and N-halamine precursor 5,5-dimethylhydantoin (DMH) so as to form a SiO2-PS-DMH composite. The asprepared materials were characterized and analyzed by TEM, DLS, TGA, DSC, FTIR, and zeta potential. The antibacterial performance against Escherichia coli (E. coli) as a typical Gramnegative bacterium and Staphylococcus aureus (S. aureus) as a Gram-positive bacterium was successfully performed eventually. 2. Experimental Section 2.1. Materials. Tetraethoxysilane (TEOS) and styrene were obtained from Tianjin Guangfu Fine Chemical Research Institute. 3-(Methacryloxy)propyl trimethoxysilane (MPS) and azobisisobutyronitrile (AIBN) were available from Shanghai Chemical Reagent Plant. 1,4-Bis(chloromethyoxy)butane (BCMB) and 5,5-dimethylhydantoin (DMH) were purchased from Westingarea Co., Ltd. Tin chloride pentahydrate, potassium hydroxide, and sodium hypochlorite were provided by Sinopharm Chemical Reagent Co., Ltd. The other reagents were analytical grade and used without any purification. 2.2. Characterizations. The structures and morphologies of hybrid nanoparticles were characterized with a Hitachi H-8100 transmission electron microscope. The zeta-potential and particle size distribution were measured by a ZetaPlus Zeta Potential Analyzer (ZZPA). TGA was performed using a Perkin-Elmer

10.1021/jp104083h  2010 American Chemical Society Published on Web 09/20/2010

Immobilization of Cyclic N-Halamine

Figure 1. Schematic illustration and structural model of N-halamine immobilization onto polystyrene-functionalized silica nanoparticles.

thermogravimetric analyzer. DSC was achieved with a Shimadzu DSC-60 instrument. FTIR spectra were recorded on a Thermo Nicolet (Woburn, MA) Avatar 370 FTIR spectrometer. 2.3. Preparation of SiO2-PS-CDMH Nanoparticles. The general procedure to immobilize N-halamine onto PS-decorated silica nanoparticles consists of four steps: fabrication of MPS-SiO2, coating with PS, DMH immobilization, and chlorination. An outline of the synthesis is shown in Figure 1. 2.3.1. Preparation of MPS-Modified Silica Nanoparticles. About 25 mL of tetraethyl orthosilicate (TEOS) was added to a mixture of 40 mL of ethanol, 50 mL of deionized water, and 30 mL of ammonia (25 wt %). The mixture was stirred vigorously at room temperature for 24 h. About 2 mL of 3-(methacryloyloxy)propyl trimethoxysilane (MPS) was then introduced dropwise into the silica sol over a reaction period of 24 h to introduce the carbon-carbon double bonds onto the surface of silica template cores. The MPS-modified silica nanoparticles (MPS-SiO2) were purified by several cycles of centrifugation and redispersion in a 1:1 (V:V) mixture of ethanol and water. 2.3.2. Preparation of Silica-Polystyrene Nanoparticles. The composite nanoparticles silica-polystyrene (SiO2-PS) were prepared by precipitation polymerization. Typically, 0.5 g of particle MPS-SiO2 and 3 mL of styrene were added into 20 mL of toluene, and polymerization initiated by azoisobutyric dinitrile was performed under a nitrogen atmosphere at 80 °C for 7 h to obtain SiO2-PS nanoparticles. The resultant nanoparticles were centrifuged and washed several times to remove the impurities and dried in a vacuum.

J. Phys. Chem. C, Vol. 114, No. 41, 2010 17299 2.3.3. Anchoring of 5,5-Dimethylhydantoin onto SilicaPolystyrene Nanoparticles. Immobilization of DMH on SiO2PS nanoparticles was accomplished via two-step processes including chloromethylation of SiO2-PS and DMH immobilization. First, chloromethylation of the polystyrene chain of the nanoparticles SiO2-PS was described as follows. A 0.5 g amount of SiO2-PS was dispersed into 20 mL of dichloromethane, and 3.0 g of chloromethylation reagent 1,4-bis(chloromethyoxy)butane and 1.0 mL of catalyst SnCl4 (0.2 mol/L) were injected slowly into the mixture, which was allowed to be reacted at room temperature for 24 h. The mixture was treated with diluted hydrochloric acid solution to destroy catalyst in the reaction end. After being filtrated out, the crude products chloromethyled SiO2-PS (SiO2-CMPS) were washed with dioxane and distilled water successively. In the second step, 0.1 g of DMH was dissolved in 20 mL of ethanol in the presence of 0.1 g of potassium hydroxide. This mixture was heated at 78 °C for 30 min, after an appropriate amount of SiO2-CMPS and 10 mL of methanol were added into the mixtures. The reaction was continued for 12 h at 60 °C. The products SiO2-PS-DMH were collected by centrifugation and washed by repeating redispersion in deionized water and pure ethanol, respectively. 2.3.4. Chlorination of SiO2-PS-DMH Nanoparticles. Chlorination of SiO2-PS-DMH nanoparticles was carried out as follows. About 0.2 g of SiO2-PS-DMH was dispersed into 20 mL of sodium hypochlorite solution, and chlorination was performed by vigorously stirring for 12 h at room temperature. In the end, products SiO2-PS-CDMH were collected by centrifugation and washed by repeating redispersion in deionized water and pure ethanol. 2.4. Antibacterial Assessment. S. aureus (S. aureus 29213 and S. aureus 25923) and E. coli (E. coli 15597) were used as model microorganisms to determine the antimicrobial properties of the samples. The minimum inhibition concentration (MIC) of SiO2-PS-CDMH nanoparticles was determined by a similar agar plate method. The sample concentration varied from 32 to 64, to 128, to 256, to 512, and to 1024 µg/mL. The culture of each bacterium was diluted by sterile distilled water to ca. 100 CFU/mL, and the inoculated plates were incubated at 37 °C for a contact time of 12 h. 3. Results and Discussions 3.1. Preparation of SiO2-PS-DMH Nanoparticals. Figure 2a displays the TEM photograph of MPS-modified silica, which indicates that quasi-monodisperse silica nanospheres have spherical shapes and smooth surfaces without any coagulation. From Figure 2b, one can see that the silica nanoparticles coated

Figure 2. TEM micrographs of MPS-modified silica (a), SiO2-PS (b), and SiO2-PS-DMH (c) nanoparticles.

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Figure 3. Particle size distributions of MPS-modified silica (a), SiO2-PS (b), and SiO2-PS-DMH (c) nanoparticles.

Figure 4. FTIR spectra of MPS-modified silica (a), SiO2-PS (b), and SiO2-PS-DMH (c) nanoparticles.

with polystyrene are perfectly spherical with smooth particle surfaces and represent clear core-shell structures. The picture of SiO2-PS-DMH nanoparticles shown in Figure 2c suggests that the resultant nanoparticles with a core-shell structure have spherical shapes and relatively rough surfaces, which distinctly expresses that DMH is immobilized onto the SiO2-PS nanoparticles surface. Furthermore, the aggregation of a few SiO2-PS-DMH nanoparticles is attributed to cross-linking during chloromethylation of the polystyrene chain.50,51 The mean particle sizes of MPS-SiO2, SiO2-PS, and SiO2-PS-DMH nanoparticles estimated by TEM are 201.6, 242.4, and 247.8 nm, and the corresponding standard deviations are 11.26, 14.80, and 22.14 nm, respectively. The distributions of particles sizes characterized by dynamic light scattering are unimodal and narrow. Figure 3 shows that the particle sizes of the as-prepared nanoparticles are in the range of 180-280, 230-370, and 200-440 nm, and the most probable size is around 216.8, 273.6, and 285.5 nm for MPS-silica, SiO2-PS, and SiO2-PS-DMH nanoparticles, respectively. As expected, the sizes of nanoparticles determined with DLS are larger than those obtained from the TEM micrograph. The reason is that the specimens of the particles for TEM are at the dry state but the particles in the specimens for ZZPA are more or less swelled, and the shrinking of the particles for TEM caused by electron beam damage is also an explanation for the size differences measured by TEM and DLS.52,53 FTIR spectra of MPS-modified silica, SiO2-PS, and SiO2-PSDMH nanoparticles are shown in Figure 4. The peaks around 800, 950, and 1100 cm-1 are attributed to a symmetric stretching vibration of Si-O-Si, stretching vibration of Si-O-H, and antisymmetric stretching vibration of Si-O-Si, which can be seen in all three curves. The peaks at 2982, 2931, and 1722 cm-1, reflected at the spectrum of MPS-modified silica particles, are assigned to the stretching of C-H and CdO bonds, respectively.54 The peak at 1402 cm-1 is ascribed to a symmetric bending vibration of the C-H band, and the absorption bands 1448 and 1639 cm-1 are, respectively, ascribed to the unhydrolyzed SiOCH3 and the residual water.54 In the case of SiO2-PS nanoparticles, the characteristic absorption bands at

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Figure 5. TGA curves of SiO2-PS nanoparticles (a) and SiO2-PS-DMH nanoparticles (b).

1628, 1495, and 1452 cm-1 for CdC stretching and 754 cm-1 for C-H bending of the benzene ring are clearly observed. The peak at 700 cm-1 is attributed to a corrugation vibration of the benzene ring, which is well matched with those reported elsewhere.55,56 In the spectrum of SiO2-PS-DMH nanoparticles, besides characteristic absorption bands of SiO2-PS nanoparticles, the peak of the N-H stretching bands is observed at 3190 cm-1. The absorption peak of the N-H bending vibration and C-N stretching vibration appear at around 1545 cm-1, and the peak at 1674 cm-1 is caused by the CdO stretching vibrations of the imide and amide groups.21 Thermogravimetric analysis results are shown in Figure 5. From TGA results, the thermodynamic information on the composites and the content of each layer can be obtained. Curves a and b correspond to SiO2-PS and SiO2-PS-DMH nanoparticles, respectively. The weight loss below 200 °C is attributed to evaporation of water and residual organic solvent.57 In curve a, SiO2-PS nanoparticles begin decomposing at 320 °C, which corresponds to the decomposition temperature of polystyrene.58 When the temperature reaches about 600 °C, the PS shell completely vanishes and the residuals with a content of around 80 wt % are SiO2 particles. From curve b, it can be confirmed that there are three different substances in SiO2-PS-DMH nanoparticles,57 and it is also verified that DMH was immobilized onto the SiO2-PS nanoparticles surface. Nanoparticles SiO2-PS-DMH begin decomposing at 220 °C, which is ascribed to decomposition of DMH, and when the temperature is up to 320 °C, the decomposition rate increases, which is caused by degradation of the PS layer, which completely disappears at about 600 °C and about 75 wt % SiO2 residues are left behind. To further elucidate the immobilization of DMH onto silica templates, zeta-potential analyses of the nanoparticles were performed at pH 7. The zeta potential of silica nanoparticles shows a negative value (-43.70 mV) due to the negatively charged -OH groups on the surface.59 In the case of silica-polystyrene core-shell nanoparticles, the zeta-potential value is -27.66 mV, indicating that negatively charged sites on the silica surface were protected by the coated polystyrene shell.60 The zeta-potential value of SiO2-PS-DMH is down to -37.06 mV, which suggests that DMH was entrapped at the interface of the polystyrene-functionalized silica. The DMH/SiO2-CMPS mass ratio plays an important role in the morphologies of SiO2-PS-DMH nanoparticles. The morphologies of SiO2-PS-DMH nanoparticles are controlled in a straightforward fashion through tuning the mass ratio of DMH to SiO2-CMPS when all other parameters remain constant. As shown in Figure 6, on increasing the DMH/ SiO2-CMPS ratio from 1/4 to 1/2, to 1/1, to 2/1, and then to 5/1, the surfaces of the core-shell nanoparticles become more and more rough. When the DMH/SiO2-CMPS ratio is below

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Figure 6. TEM micrographs of SiO2-PS-DMH nanoparticles with different mass ratio of DMH to SiO2-CMPS: (a) 1/4, (b) 1/2, (c) 1/1, (d) 2/1, and (e) 5/1.

Figure 7. FTIR spectra of SiO2-PS-DMH nanoparticles before (a) and after chlorination (b).

Figure 8. DSC curves of SiO2-PS-DMH nanoparticles before (a) and after chlorination (b).

1, DMH is not enough to cover the whole particle surfaces, resulting in the resultant SiO2-PS-DMH nanoparticles with legible spherical shapes and little coarse surfaces. When the DMH/SiO2-CMPS ratio is above 2, DMH is excessive and particle surfaces are fully surrounded by DMH, leading to the shapes of SiO2-PS-DMH nanoparticles no longer being defined as spherical and the surfaces are completely rough and irregular. In other words, the roughness of particle surfaces increases as the content of DMH in SiO2-PS-DMH nanoparticles increases. 3.2. Chlorination of SiO2-PS-DMH Nanoarticles. Upon treatment with sodium hypochlorite solution, the amide groups of the hydantoin are readily transformed into N-halamine structures. Such a transformation can be observed with Fouriertransform infrared spectroscopy. The IR spectra of SiO2-PSDMH nanoparticles before and after bleach treatment are shown in Figure 7. In the spectrum of SiO2-PS-CDMH nanoparticles (Figure 7b), the N-H stretching vibration present before chlorination around 3190 cm-1 disappears (Figure 7a). Moreover, the transformation of N-H bonds to N-Cl groups is associated with breakage of N-H · · · OdC hydrogen bonding in SiO2-PS-DMH nanoparticles, and this results in the shifts

of the CdO band from 1672 cm-1 in SiO2-PS-DMH nanoparticles to 1630 cm-1 in SiO2-PS-CDMH nanoparticles. The thermal properties of the nanoparticles were examined with differential scanning calorimetry (DSC). Figure 8 shows the DSC curves of the chlorinated and unchlorinated SiO2-PS-DMH nanoparticles. The chlorinated nanoparticles reveal an exothermic peak at 121 °C, which is attributed to decomposition of the N-Cl bond. The same peak is not observed in the DSC curve of the unchlorinated sample. These results further confirm the N-H f N-Cl transformation upon hypochlorite bleach treatment. 3.3. Antimicrobial Functions. The minimum inhibitory concentration (MIC) is considered to be the lowest concentration that completely inhibits against on agar plates comparing, disregarding a single colony or a faint haze caused by the inoculum.61 The MIC values of SiO2-PS-CDMH nanoparticles against S. aureus 29213, S. aureus 25923, and E. coli are shown in Table 1. As a control, bulk powder PS-CDMH without colloidal silica templates was prepared, and its biocidal efficiency was estimated as well. From Table 1, we can see that the SiO2-PS-CDMH nanoparticles show powerful antibacterial activity against both Gram-negative and Gram-positive bacterium whereas the bulk counterparts show moderate activity. The

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TABLE 1: MIC Values of Bulk Powder PS-CDMH and SiO2-PS-CDMH Nanoparticles Against S. aureus 29213, S. aureus 25923, and E. coil at a Contact Time of 12 h and after Two Month Storage MIC (µg/mL) sample

time

S. aureus 29213

S. aureus 25923

E. coil

PS-CDMH PS-CDMH SiO2-PS-CDMH SiO2-PS-CDMH

12 h 2 month 12 h 2 month

1024 1024 128 128

512 1024 64 64

1024 1024 512 512

MIC values of 1024, 512, and 1024 µg/mL against S. aureus 29213, S. aureus 25923, and E coil are observed for bulk powder PS-CDMH at a contact time of 12 h. However, the MIC values are merely 128, 64, and 512 µg/mL for SiO2-PS-CDMH nanoparticles. As mentioned previously, the nanosized particles SiO2-PS-CDMH show 2-8 times higher biocidal activity than the bulk counterparts. In brief, SiO2-PS-CDMH nanoparticles intrinsically have more active sites to contact bacteria and thus provide enhanced antibacterial activity. Long-term stability is an important feature of N-halamine structural biocides.62 For both of these two samples, after storage for two month in an environment of 25 °C and 65% RH, the antibacterial properties are almost unchanged (as Table 1). Except for the increase of the MIC value (to 1024 µg/mL) of bulk powder against S. aureus 25923, no significant reduction in biocidal activity is noticed, indicating the N-halamine structural antimicrobial possessing proper stability in the dry state. The results indicate that the SiO2-PS-CDMH nanoparticles have higher biocidal efficacy against S. aureus than E. coli. This phenomenon may be attributed to the different cell structures of Gram-positive and Gram-negative bacteria. The lipid bilayer cell wall of Gram-positive bacteria is mostly covered by a porous peptidoglycan layer, which does not exclude most antibacterial agents. The cell wall of Gram-negative bacteria also contains ca. 20% peptidoglycan. However, Gram-negative bacteria are surrounded by two membranes, and the outer membrane acts as an efficient permeability barrier because it includes lipopolysaccharides and porins.23,63-66 For this reason, it is considered that Gram-positive bacteria would be more vulnerable than Gram-negative bacteria against N-halamine structural antibacterial agents. 4. Conclusions We presented an efficient method for the design and preparation of antimicrobial hybrid nanoparticles with PS-coated silica as a support and cyclic N-halamine immobilized on the surface. To synthesize these antibacterial materials, PS was coated with the silica nanoparticles and then DMH was anchored onto the PS chain. The morphologies and structures of SiO2-PS-DMH nanoparticles could be controlled by adjusting the mass ratio of DMH to SiO2-CMPS. After treatment with a bleach solution, resultant materials SiO2-PS-CDMH displayed vigorous and everlasting antimicrobial activities against S. aureus and E. coil, which was 2-8 times higher than that of bulk counterparts PS-CDMH. Due to the difference between cell structures of S. aureus and E. coil, SiO2-PS-CDMH nanoparticles had higher biocidal efficacy against S. aureus than E. coli. We believe that composite materials SiO2-PS-CDMH could find applications in medical devices, hygienic materials, and the foodprocessing industry. Acknowledgment. We thank the National Natural Science Foundation of China for financial support of this research (50673033).

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