Photocatalytic Antibacterial Capabilities of TiO2−Biocidal Polymer

Jun 17, 2010 - ... aureus (S. aureus) ATCC 6538 were purchased from Fisher Company. .... The degradation of organic dye (methylene blue) was carried o...
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Environ. Sci. Technol. 2010, 44, 5672–5676

Novel biocidal polymer-functionalized TiO2 nanoparticles were prepared by surface-initiated photopolymerization using titania as an initiator. Vinyl monomer mixtures of nontoxic secondary amine-containing biocidal 2-(tert-butylamino)ethyl methacrylate and antifouling ethylene glycol dimethacrylate were used for the antimicrobial polymer shell. It was shown that the synthesized TiO2/poly[2-(tert-butylamino)ethyl methacrylateco-ethylene glycol dimethacrylate] core/shell nanoparticles had enhanced photocatalytic antibacterial properties compared to the pristine TiO2 nanoparticles due to the combined antibacterial activities of light-driven anti-infective TiO2 core and biocidal polymer shell. In the dark condition, the TiO2/biocidal polymer nanoparticles exhibited high antimicrobial efficiency (95.7%) against gram-positive S. aureus. Furthermore, during UV irradiation, the TiO2/biocidal polymer showed improved inhibition of bacterial growth against gram-negative E. coli and gram-positive S. aureus in comparison to the pristine TiO2 nanoparticles.

organic medium (19) and combined merits of flexible polymers with functional TiO2 nanoparticles (23). Therefore, various attempts have been devoted to the synthesis of the polymer-TiO2 nanocomposites (22, 23). One example is a photopolymerization of vinyl monomer by the TiO2 nanoparticles as a photoinitiator and nanofiller (26). Typically, under UV irradiations, radicals at the surface of photoexcited TiO2 nanoparticles can initiate the polymerization, leading to filler-embedded polymer matrix. Though several TiO2 nanocomposites with antimicrobial capabilities have been reported, there are still barriers in their antibacterial application under dark conditions. In the TiO2 nanocomposites, biocidal efficiencies depend on their light absorbance under UV and visible light. Therefore, most TiO2 compounds exhibit no antimicrobial performances in the dark condition. For this reason, some approaches with metal-doped TiO2, i.e., silver, palladium, and copper coated titania compounds, have been reported to provide efficient antibacterial activities without light irradiation (27-30). However, a significant disadvantage of these metal-doped composites is that the released metal ions from the TiO2 surfaces may be an origin of an undesired environmental problem. Therefore, it is desirable to develop eco-friendly TiO2 nanocomposites with enhanced antibacterial properties with and without the light irradiation. This paper reports the synthesis of novel TiO2/biocidal polymer core/shell nanoparticles by photopolymerization to provide antimicrobial properties under both light and dark conditions. For this purpose, secondary amine containing monomer, 2-(tert-butylamino)ethyl methacrylate was used for the biocidal polycationic shell and ethylene glycol dimethacrylate as a comonomer was chosen for antifouling surface as well as cross-linking agent (4, 31-34). In the experimental procedure, monomers were adsorbed onto the surface of photoexcited TiO2 nanoparticles and the photopolymerization proceeded by encapsulating the TiO2 nanoparticles. Secondary amine-containing polymer afforded additional antibacterial properties to the photoactivated antimicrobial TiO2 nanoparticles. From this point of view, the TiO2/poly[2-(tert-butylamino)ethyl methacrylate-co-ethylene glycol dimethacrylate] nanoparticles displayed synergic antimicrobial properties with and without light irradiation.

Introduction

Materials and Methods

Photocatalytic Antibacterial Capabilities of TiO2-Biocidal Polymer Nanocomposites Synthesized by a Surface-Initiated Photopolymerization HYEYOUNG KONG, JOOYOUNG SONG, AND JYONGSIK JANG* WCU program of Chemical Convergence for Energy and Environment (C2E2), School of Chemical and Biological Engineering, College of Engineering, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-742, Korea

Received April 5, 2010. Revised manuscript received May 26, 2010. Accepted June 11, 2010.

In recent decades, there have been great demands for the preparation of antimicrobial agents to provide clean surfaces from the microbial infections related to many serious diseases (1-9). Especially, photocatalytic titanium dioxide (TiO2) is one of the most studied materials in the field of antibacterial applications due to its unique abilities such as photocatalytic bacterial-disrupting, nontoxicity, and self-cleaning properties (10-13). The photocatalytic disinfection performance of TiO2 can be explained by the following mechanism (14-17). First, the photoexcited TiO2 nanoparticles produce electron-hole pairs when exposed to UV light. Then, photogenerated holes and electrons in the TiO2 surfaces can react with adsorbed substances and consequently generate reactive oxygen radicals. Finally, these oxygen species attack and disrupt the bacterial cell wall, resulting in cell death. High-performance TiO2 nanocomposites can be prepared from polymer-based systems (18-25). Polymer-coated TiO2 nanocomposites have improved dispersion stability in * Corresponding author e-mail: [email protected]; phone: (+82) 2-880-7069; fax: (+82) 2-888-1604. 5672

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Materials. 1-Methyl-2-pyrrolidinone (99%) and 2-(tert-butylamino)ethyl methacrylate (97%) were purchased from Sigma Aldrich and used without further purification. Ethylene glycol dimethacrylate was purchased from Aldrich and used after inhibitor removal. TiO2 nanoparticles (anatase, needletype) were obtained from Miyoshi Kasei Co. (Saitama, Japan) and thermally pretreated at 500 °C for 2 h under N2 gas flow to remove adsorbed organic reagents on the TiO2 surface. For the bacterial test, Escherichia coli (E. coli) ATCC 8739 and Staphylococcus aureus (S. aureus) ATCC 6538 were purchased from Fisher Company. Characterization. Photographs of transmission electron microscopy (TEM) were obtained with a JEOL JEM-200CX. Acceleration voltage for TEM was 200 kV. In the sample preparation of TEM characterizations, the TiO2/polymer core/shell nanoparticles were dispersed in ethanol and cast onto a copper grid. Fourier transform infrared (FT-IR) spectra were recorded on a Bomem MB 100 spectrometer (Quebec, Canada) in the absorption modes at resolution of 4 cm-1 and 32 scans. Thermogravimetric analysis (TGA) was carried out in N2 flow using a TGA 2050 analyzer (TA Instruments). The weight loss of TiO2/polymer nanoparticle was measured from 10.1021/es1010779

 2010 American Chemical Society

Published on Web 06/17/2010

ambient temperature up to 700 °C, at the rate of 10 °C min-1. Field-emission scanning electron microscopy (FE-SEM) images were obtained using a JEOL 6700 at an acceleration voltage of 5 kV. The UV-vis spectra were taken at 25 °C with a Perkin-Elmer Lambda-20 spectrometer. Preparation of TiO2/Biocidal Polymer Nanoparticles. TiO2 nanoparticles (concentration of 0.5 mg/mL) were dispersed in 1-methyl-2-pyrrolidinone by ultrasonication for 10 min. Then, monomer mixture of 2-(tert-butylamino)ethyl methacrylate (65 mM) and ethylene glycol dimethacrylate (15 mM) was injected into the TiO2 solution with magnetic stirring. After inducing the monomer adsorption onto the TiO2 surface, the solution was set with a pyrex filter to obtain radiations with a wavelength greater than 290 nm (UVB) and purged with the Ar flow. Then, UV-irradiated polymerization was carried out for 4 h at room temperature by a mercury lamp (300 W), characterized by a main emission peak of 365 nm. The core/shell nanoparticles were precipitated by centrifugal precipitation after the photopolymerization. The fabricated TiO2/biocidal polymer core/shell nanoparticles were washed with ethanol to remove the residual monomers. For the characterization, the samples were dried under vacuum at 25 °C for 3 days. Photocatalytic Activity. The photocatalytic degradation of methylene blue (MB) was carried out at room temperature in a 50-mL vial containing 0.05 g of catalyst and 50 mL of 10 mg/L MB aqueous solution. After the vial was shaken for 30 min until reaching adsorption equilibrium, the photocatalytic reaction was performed by a mercury lamp (300 W), characterized by a main emission peak of 365 nm. With different UV-irradiation time, the solution was analyzed by UV-vis spectrophotometer at its characteristic wavelength (λ ) 665 nm) to determine the degradation yield (C/C0). For the comparison, the pristine TiO2 nanoparticles were also prepared and used as a photocatalyst for the MB degradation. Antimicrobial Tests. Microorganisms were cultivated in sterilized LB broth and then incubated overnight at 37 °C with a shaking incubator (microbial concentration was 105 CFU/mL). To investigate the antimicrobial performances of synthesized TiO2/biocidal polymer core/shell nanoparticles under UV light irradiation and dark condition, the samples were dispersed in methanol solvent and coated onto polystyrene Petri dishes. The coated Petri dishes were dried in a drying oven at 70 °C for the complete evaporation of methanol solvent. For a comparison, bulk poly(TBAM-coEGDMA) and pristine TiO2-coated and only methanol-treated Petri dishes were also prepared. The bulk polymer was synthesized via suspension polymerization at 80 °C for 24 h. Mixed monomer of TBAM and EGDMA (0.5 mL; 4:1 v/v) was suspended in aqueous solution and 0.03 g of AIBN was added to initiate the polymerization. Diluted microbial solution (104 CFU/mL) was sprayed onto the Petri dish and half of the dish was wrapped in black tape to protect the half from the light. After air-drying for 3 min, the Petri dish was set under the UV light of 312 nm (6 W) for 2 min. Similarly, the pristine TiO2, bulk polymer, and control surfaces were applied to the antimicrobial abilities with and without UV light. After drying or UV irradiated drying, the autoclaved growth bacterial medium (which was cooled to 40 °C) was added into the Petri dishes and solidified. The test dishes were incubated at 37 °C for 24 h and the bacterial colonies were inspected. For the observations of comparative antibacterial activities under UV irradiation, pristine TiO2 and synthesized TiO2/ polycation nanoparticles were palletized by a hydrolytic press. The pellets were inoculated with small amount of bacterial solution (105 CFU/mL) and set under UV light of 312 nm (6 W). After specific irradiation time, the pellets were incubated at 37 °C overnight without UV light. Then the pellets were gently washed with autoclaved distilled water. The number of surviving bacterial colonies was inspected with FE-SEM

FIGURE 1. TEM image of TiO2/poly[2-(tert-butylamino)ethyl methacrylate-co-ethylene glycol dimethacrylate] core/shell nanoparticles using photopolymerization. characterization. For the FE-SEM characterization, the pellets were fixed with 2.5 wt % of glutaraldehyde for 2 h and postfixed for an additional 1 h with 1% osmium tetroxide in distilled water. After fixation, the pellets were dehydrated with serial diluted ethanol (20 to 100%), air-dried, and coated with platinum using sputter coater for the FE-SEM observation.

Results and Discussion Photopolymerization at Surface of TiO2 Nanoparticles. To obtain novel antimicrobial TiO2/biocidal polymer core/shell nanoparticles, surface-initiated photopolymerization was employed using TiO2 nanoparticles as a photoinitiator and core material. Biocidal cationic monomer, 2-(tert-butylamino)ethyl methacrylate and antifouling ethylene glycol dimethacrylate were used for the antimicrobial polymer shell (31-34). In addition, ethylene glycol dimethacrylate also cross-linked the polymer shell, leading to stable nanocomposites (4). When exposed to UV light, TiO2 nanoparticles (band gap energy of 3.2 eV) absorb the light and produce electron-hole pairs at the excited metal oxide surfaces (14). In the photopolymerization using these electron-hole pairs as an initiator, the positive holes of the excited TiO2 nanoparticle can initiate polymerization by oxidizing the monomer and generating a radical (26, 35). In our experimental condition, vinyl monomer mixture of 2-(tert-butylamino)ethyl methacrylate and ethylene glycol dimethacrylate was adsorbed onto the surface of photoexcited TiO2 nanoparticles because ester group of monomers interacted with polar surface sites of the metal oxide (36). Concurrently, the hole initiation occurred via direct hole transfer from the excited TiO2 to the adsorbed monomers, resulting in the surface polymerization (26). During the polymerization, monomers diffused to the metal oxide surface and reacted with the adsorbed monomer radicals (36). As a result, TiO2/ poly[2-(tert-butylamino)ethyl methacrylate-co-ethylene glycol dimethacrylate] core/shell nanoparticles were obtained after the surface initiated photopolymerization. Figure 1 represents the TEM images of the synthesized TiO2/poly[2-(tert-butylamino)ethyl methacrylate-co-ethylene glycol dimethacrylate] (TiO2/poly(TBAM-co-EGDMA)) core/ shell nanoparticles. Inset TEM image indicates that the polymer shell is formed at the needle-shaped TiO2 surfaces, leading to the core/shell morphology. The polymer shell on the metal oxide surfaces is very thin layer of average 2 nm. TiO2/poly[2-(tert-butylamino)ethyl methacrylate-co-ethylene glycol dimethacrylate] core/shell nanostructures were further characterized by FT-IR spectroscopy and TGA VOL. 44, NO. 14, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Photocatalytic degradation of MB under UV light (365 nm) employing pristine TiO2 (solid line) and TiO2/polymer nanoparticles (dashed line) (C/C0 is relative concentration of MB in the tested solutions and initial solution, respectively).

FIGURE 2. (a) FT-IR spectra of pristine TiO2 (black line) and TiO2/poly[2-(tert-butylamino)ethyl methacrylate-co-ethylene glycol dimethacrylate] core/shell nanoparticles (red line) and (b) TGA graph of the TiO2/polymer nanoparticles. instrument as shown in Figure 2. In the FT-IR spectrum of pristine TiO2 (black line in Figure 2a), the peak at 1625 cm-1 is assigned to anatase TiO2 absorption peak (37). In contrast, new peaks at 3300 cm-1 of -NH stretching for amine, 1695 cm-1 of CdO stretching vibration, and 1445 cm-1 of -CH3 bending vibration of polymer appear in the spectrum of synthesized TiO2/poly(TBAM-co-EGDMA) nanoparticles (red line, Figure 2a). It is noteworthy that vibration band of Ti-O-C is also shown near 1275 cm-1 in the TiO2/polymer nanoparticles due to the metal oxide-polymer interfacial complex structures (38). The photopolymerization at the TiO2 surfaces was also characterized by thermogravimetric analysis (TGA) to obtain quantitative information of the polymer content (Figure 2b). Below 200 °C with a maximum at 120 °C, the amount of thermal degradation of TiO2/poly(TBAMco-EGDMA) was ca. 2.3%, which was mainly ascribed to the adsorbed moisture and organic substances. On the other hand, significant weight loss of TiO2/polymer between 200 and 700 °C (about 10%) was attributed to the thermal degradation of polymer shell. The weight loss with a maximum decomposition rate at 355 °C could be due to the loss of (tert-butylamino)ethyl side chains (39). Above 400 °C, the weight loss with a maximum decomposition rate at 484 °C could originate from the decomposition of cross-linked polymer backbone. Judging from these data, it is evident that the photopolymerization of vinyl monomers is successfully performed at the surfaces of TiO2 nanoparticles and the TiO2/poly(TBAM-co-EGDMA) nanoparticles are composed of ca. 10 wt % polymer. Photocatalytic Activity. The photoactivated surface properties of polymer-coated TiO2 nanoparticles should be considered for the light-activated disinfective TiO2 ability. The degradation of organic dye (methylene blue) was carried out by using the synthesized TiO2/polymer as a photocatalyst. Figure 3 shows that the UV light-irradiated degradation rate 5674

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FIGURE 4. Graph of % survival after treatment with pristine TiO2, bulk poly(TBAM-co-EGDMA), and TiO2/poly[2-(tert-butylamino)ethyl methacrylate-co-ethylene glycol dimethacrylate] nanoparticles in the presence and the absence of UV light irradiation at 312 nm for 2 min against S. aureus. of methylene blue by the TiO2/polymer catalyst is faster than that of the pristine TiO2 nanoparticles. The improved photocatalytic activity could be attributed to the increased stability and high dye-adsorption capability. The polymercoated TiO2 nanoparticles showed less aggregated structure because the metal oxide surface was protected by the polymer stabilizer (19). In addition, hydrophobic polymer shell provided improved dye adsorption ability due to the hydrophobic interaction between the organic dye and the polymer shell (40). As a result, the polymer-coated TiO2 nanoparticles had enhanced photocatalytic efficiency compared to the pristine TiO2 nanoparticles. This result demonstrates that the TiO2/polymer nanoparticles can generate reactive radicals at the photoexcited surfaces which can be correlated to the potency of antibacterial properties. Antibacterial Properties. It is anticipated that the TiO2/ biocidal polymer nanoparticles have improved antibacterial properties compared to the pristine TiO2 nanoparticles and the bulk poly(TBAM-co-EGDMA) with and without UV light because of the synergic antibacterial activities of nanosized secondary amine polymer and photodisinfective TiO2 nanoparticle. Figure 4 represents the inactivation of S. aureus on the TiO2 and the polymer-treated surfaces under UV irradiation and dark condition. The survival of bacterial growth can be calculated by % survival ) B/A × 100 (where A is the number of surviving microbial colonies in the control and B is the number of surviving microbial colonies in the tested sample). To rule out the biocidal effect of UV-ray itself, the

control was also UV-irradiated and its number of surviving bacterial colonies was used as a standard in the comparison for the UV irradiation condition. The pristine TiO2 nanoparticles inhibited bacterial growth by ca. 76% under the UV irradiations because TiO2 nanoparticles absorbed the UV light and generated toxic radicals to the bacterial cell wall. However, in the dark condition, the pristine metal-oxidetreated surface nearly did not affect the bacterial cell growth. On the bulk poly(TBAM-co-EGDMA) treated surfaces, about 67% bacterial inhibitions were observed in the UV irradiated and nonirradiated conditions owing to the biocidal secondary amine polymer. In contrast, the TiO2/poly(TBAM-co-EGDMA) core/shell nanoparticles had distinguishable antibacterial properties compared to the pristine TiO2 and the bulk polymer both in the UV light irradiation and the dark condition. The synthesized TiO2/biocidal polymer core/shell nanoparticles had high killing efficiency of 95.7% in the dark and 99.8% under UV light irradiation for 2 min. Notably, the TiO2/biocidal polymer core/shell nanoparticles displayed excellent antibacterial performance even in the dark condition because the secondary amine-containing antifouling polymer shell provided additional antimicrobial activity to the TiO2 nanoparticles. Furthermore, the nanosized particles provided enhanced antibacterial performances compared to the bulk polymer, which was attributed to the large surface area of nanoparticles. This result indicates that the TiO2/ poly(TBAM-co-EGDMA) core/shell nanoparticles have effective synergic antibacterial properties under dark condition as well as under UV irradiation. For the comparative antibacterial investigation on UV irradiation, the average numbers of surviving microbial colonies on the surfaces of pristine TiO2 and TiO2/polymer nanoparticles were inspected as a function of UV-irradiation time (min). Gram-negative E .coli and gram-positive S. aureus were selected as test bacteria and the % survival was obtained by the % survival ) D/C × 100 (where C is the number of surviving microbial colonies on each of the nonirradiated surfaces and D is that on each of the UV-irradiated surfaces). Figure 5a shows the E. coli growth on the tested surfaces with different UV irradiation time. On the 5-min irradiation, the pristine TiO2 surface was covered with 58.4% survival E. coli bacteria, whereas the TiO2/polymer surfaces had nearly half-reduced E. coli adhesion (24.6%) compared to the pristine TiO2 particles owing to their additional antibacterial performances of polymer shells. When the irradiation time increased up to 30 min, no microbial growth was detected on the TiO2/polymer core/shell surfaces. A similar result was observed in the antibacterial test against S. aureus. Under UV light irradiation for 5 min, the TiO2/polymer had about 3-times greater inhibition efficiency than the pristine particles. During the UV irradiation for 30 min, 99.9% S. aureus was inhibited on the surface of TiO2/polymer, whereas the pristine TiO2 still had 50% bacterial growth. For the comparative visual inspection of antibacterial properties, FESEM images are provided in Supporting Information. From these results, it is concluded that the TiO2/poly(TBAM-coEGDMA) core/shell nanoparticles have excellent antibacterial properties relative to the pristine TiO2 particles against both gram-negative and gram-positive bacteria due to the synergic biocidal performances. In summary, novel TiO2/biocidal polymer core/shell nanoparticles were synthesized by TiO2 nanoparticles as initiators for the photopolymerization of mixed vinyl monomers, 2-(tert-butylamino)ethyl methacrylate and ethylene glycol dimethacrylate. In the preparation, the monomers were adsorbed onto the TiO2 surfaces by the interaction between ester group of monomers and polar surface sites of metal oxide. Under the UV light irradiation, the excited TiO2 nanoparticles produced the electron-hole pairs and the positive holes concurrently initiated the surface polymeri-

FIGURE 5. Plot of % survival versus UV-irradiation time (min) on the surfaces of pristine TiO2 and TiO2/poly[2-(tert-butylamino)ethyl methacrylate-co-ethylene glycol dimethacrylate] core/shell nanoparticles against (a) E. coli and (b) S. aureus. zation via direct hole transfer to the adsorbed monomers. The obtained TiO2/poly(TBAM-co-EGDMA) core/shell nanoparticles had enhanced photocatalytic antibacterial properties compared to pristine TiO2 nanoparticles against both E. coli and S. aureus due to the synergic antibacterial performances of biocidal polymer shell and light-activated disinfective TiO2 core. Furthermore, even in the dark condition, the TiO2/biocidal polymer nanoparticles exhibited excellent antibacterial efficiency originated from the secondary aminecontaining antifouling polymer shell. The eco-friendly TiO2/ biocidal polymer nanoparticles can be applied to various antimicrobial applications ranging from the light activated system to the dark sterilization approach.

Supporting Information Available This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgments This research was supported by WCU (World Class University) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (R31-10013).

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