Environ. Sci. Technol. 2009, 43, 148–151
Photocatalytic Inactivation of E. coli with a Mesoporous TiO2 Coated Film Using the Film Adhesion Method DONG SUK KIM AND SEUNG-YEOP KWAK* Department of Materials Science and Engineering, Seoul National University, 599 Gwanangno, Gwanak-ku, Seoul 151-744, Korea
Received April 14, 2008. Revised manuscript received August 11, 2008. Accepted September 17, 2008.
The photocatalytic inactivation of Escherichia coli with the film adhesion method by using Degussa P25 TiO2 and mesoporous TiO2 coated on glass was investigated. Monodisperse sphericalmesoporousTiO2 withamorphologysizeofapproximately 800 nm was synthesized via the sol-gel approach and coated onto glass substrates without cracking by using the doctor blade method with various amounts of polyethylene oxide (PEO) and polyethylene glycol (PEG). Photocatalytic disinfection was tested by varying the UV-A light intensity, cell concentration, and UV-A irradiation time. The photocatalytic inactivation achieved with mesoporous TiO2 was found to be higher than that with thecommercialP25TiO2.Byvaryingthesurfaceareaandcrystallite size of mesoporous TiO2 through control of the calcination temperature, we found that the efficacy of photocatalytic disinfection with the film adhesion method is strongly dependent on the surface area and crystallite size: the larger the surface area and the smaller the crystallites, the higher the efficacy of photocatalytic inactivation.
Introduction Titanium dioxide (TiO2) is known as a semiconductor with interesting applications in gas sensors, dye-sensitized solar cells, and photochromic devices (1-5). In particular, TiO2 has diverse uses, such as the decomposition of air and water contaminants, as well as deodorization, self-cleaning, antifogging, and antibacterial actions under UV-A irradiation (6, 7). These functions of TiO2 are generally based on the action of active oxygen species such as OH · , hydrogen peroxide, and superoxide radicals formed on the surface of the photocatalyst (8-12). Since the photocatalytic inactivation of Escherichia coli in the presence of TiO2/Pt was first reported by Matsunaga et al., research into the use of TiO2 in photocatalytic disinfection under UV-A irradiation for the detoxification and inactivation of micro-organisms has been carried out throughout the past 20 years (13). To increase the efficacy of photocatalytic disinfection, various new materials such as TiO2 nanorods, water-soluble TiO2 nanosized particles, and metal-doped TiO2 have been tested and compared with Degussa P25 TiO2 (P25 TiO2) (14, 15). However, most of this research has been performed on photocatalytic inactivation of water and wastewater by TiO2 photocatalyst (16-22). * Corresponding author e-mail:
[email protected]. 148
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The range of practical applications of photocatalysts, such as TiO2 coated materials, plastics, papers, and clothes that are produced by either coating or mixing with TiO2, has recently rapidly expanded. Testing of the photocatalytic inactivation provided by these materials is still required. The aim of this study was to investigate the efficacy of photocatalytic disinfection with P25 TiO2 and mesoporous TiO2 coated materials by using the film adhesion method which is the test to evaluate antibacterial performance of photocatalytic flat surface materials with covering polymer film. Mesoporous TiO2 has been widely used in the photocatalytic degradation of organic compounds, especially environmental pollutants, because of its confined porous structure and high surface area to volume ratio, and it is known to have a high photocatalytic activity, because of its large surface area and high crystallinity (23, 24). To successfully use the film adhesion method, a flat and well dispersed TiO2 coating surface without cracking is necessary. Therefore, monodisperse and spherical mesoporous TiO2 was synthesized and coated onto glass with modified doctor blade method which is the coating process for the removal of excess substances from a moving surface being coated, and the performances of photocatalytic disinfection with the film adhesion method were tested under various conditions. We found that the photocatalytic inactivation achieved with mesoporous TiO2 is better than that of P25 TiO2-coated samples, which is thought to be because of the large surface area and high crystallinity of mesoporous TiO2. By varying the surface area and crystallite size of the mesoporous TiO2 through control of the calcination temperature, we found that the efficacy of photocatalytic disinfection is strongly dependent on the surface area and crystallite size.
Experimental Section Preparation of Mesoporous TiO2. The P25 TiO2 was purchased from a Degussa Korea Ltd. Spherical and monodisperse mesoporous TiO2 was synthesized with the sol-gel method by using a triblock copolymer surfactant and titanium isopropoxide mixed with 2,4-pentanedione in aqueous solution. 14.6 g (1 mmol) of poly(ethylene oxide)-blockpoly(propylene oxide)-block-poly(ethylene oxide) (Aldrich, average MW ) 14 600) was dissolved was dissolved in 100 mL of distilled water at 40 °C. After the surfactant had dissolved with magnetic stirrer for 6 h, 1.5 g (15.3 mmol) of sulfuric acid was added. Titanium (IV) isopropoxide (7.84 g, 27.6 mmol) was mixed with 2,4-pentanedione (2.76 g, 27.6 mmol) in a separate beaker and dropped slowly into the surfactant solution with vigorous stirring. The reaction was then carried out at 55 °C for 10 h without stirring. The resulting materials were treated hydrothermally at 90 °C for 10 h without stirring. The resulting powders were collected by filtration with Whatman GF/C glass-fiber filters with pore size of 0.4 µm and thoroughly washed with water and alcohol. To eliminate the residual surfactant, the powders were calcined at 400 °C (the rate of temperature increase was 1 °C min-1) in air. Preparation of TiO2 Coated Samples. The P25 TiO2 materials were coated onto glass substrates by using the doctor blade method. 1.2 g of P25 TiO2 was added to 420 µL of 2,4-pentanedione (0.1 M aqueous solution) in a mortar, and ground for 10 min to produce a homogeneous mixture. The mixture was dispersed by grinding in 500 µL of distilled water and then a further 500 µL of water. 0.24 g of polyethylene oxide (Aldrich, average MW ) 100 000) was added to the mixture, which was mixed to homogeneity. Next, 0.24 g of polyethylene glycol (Aldrich, average MW ) 10 000) was 10.1021/es801029h CCC: $40.75
2009 American Chemical Society
Published on Web 11/19/2008
FIGURE 1. Schematic diagram of the experimental setup for photocatalytic disinfection with the film adhesion method using TiO2 coated materials. added to the mixture, which was ground for 30 min to reach homogeneity. The viscous suspension was spread onto a glass plate (5 × 5 cm) with two tracks of one layer of adhesive tape (any adhesive tape such as Scotch tape can be used) as a spacer to obtain a unique film thickness and dried for 30 min under air and 30 min at 90 °C. After the tape was removed, the glass plate was dried using an oven at 90 °C for 1 h, and then calcined at 400 °C for 3 h under ambient conditions. The mesoporous TiO2 was also coated onto glass substrates with the same procedure. Cell Culture and Photocatalytic Disinfection Test. Further details about the cell culture conditions are provided in the Supporting Information (SI). Before conducting the photocatalytic disinfection tests, each TiO2 coated glass was irradiated with UV-A light for 3 h in order to sterilize the surface. The cell density was adjusted to the required final concentration (∼ 106 cells mL-1). An aliquot (0.5 mL) was dripped onto each mesoporous TiO2 coated glass and a polyethylene film (5 × 5 cm) placed on top of the dripped suspension [film material is not specified, but one with good UV-A light and oxygen transparentness, adherence shall be used, such as polyethylene, polypropylene, etc. (thickness of film is 30∼60 µm)]. UV-A irradiation was carried out using black light blue (BLB) lamps (20 W, with a light intensity of approximately 1 mW cm-2). After irradiation, the samples were washed with 4.5 mL of 0.2% Tween 20 solution, and 1 mL of washing solution was diluted 10-fold serially with phosphate buffer solution. Finally, 0.1 mL sample of the diluted solution is spread in order to count the number of colonies. The number of colony forming units (CFUs) is determined after 24 h incubation at 37 °C. To conduct the control tests, an uncoated sample was tested within the present experimental time scale. The measurements were carried out three times, and their average values with statistical deviation were used in the data analysis. Schematic diagrams of the experimental setup and the photocatalytic disinfection test are shown in Figure 1 and S1.
Results and Discussion Analysis of the TiO2 Coated Samples. To successfully conduct photocatalytic disinfection test by using TiO2 coated materials, highly dispersed samples with flat surfaces and no cracking are required. After several tests, we found that the use of PEO and PEG with 10 000 MW and 100 000 MW respectively results in better coatings. SI Figure S2 shows the variation in the condition of the coating of mesoporous TiO2 on glass with increases in the total amount of PEG and PEO.
With increases in the weight % of PEG and PEO from 10 wt. % to 40 wt. %, the number of cracks in the films gradually decreases. The optimal conditions for reducing cracking on the surface and obtaining highly dispersed TiO2 are achieved when the total concentration of PEG and PEO is above 40 wt. %. Figure 2 show SEM images of P25 TiO2 and mesoporous TiO2 coated on glass without cracks after sintering. Figure 2a shows the SEM image of P25 TiO2 coated on glass. Images with additional magnification show that the small nanoparticles are aggregated randomly, as in Figure 2b. For comparison of the condition of the coatings, the surface morphologies of mesoporous TiO2 were also investigated. Except for the P25 TiO2 samples, the monodisperse (approximately 900 nm) spherical mesoporous TiO2 were found to be well dispersed on the glass plates and to be free of cracking, as shown in Figure 2c. The monodisperse mesoporous TiO2 pack efficiently and are well-dispersed. To investigate the surface features and mesoporous structure of the particles in more detail, a further analysis of a magnified FE-SEM image was carried out. Figure 2d shows a magnified image of one mesoporous TiO2 particle, in which pores between nanosized TiO2 are present. The pore structure consists of a wormhole-like pore array, and the wall of the mesoporous framework consists of aggregated TiO2 nanoparticles with a size of approximately 10 nm. The disordered channels are packed randomly and form a three-dimensional mesoporous framework. Photocatalytic Disinfection with P25 TiO2 Coated Samples. To evaluate the photocatalytic disinfection achieved with the TiO2 coated samples, the effects of varying the light intensity, the concentration of E. coli, and the irradiation time were investigated for P25 TiO2 coated samples. The effects of varying the light intensity on the photocatalytic inactivation of E. coli are shown in SI Figure S3. The rate of cell inactivation increases with increases in the incident light intensity from ∼0.48 to ∼1.0 mW cm-2. These results indicate that a higher light intensity produces a higher concentration of active oxygen species such as OH · and results in improved photocatalytic disinfection of E. coli. The control uncoated sample also exhibits photoinduced disinfection that increases with increases in the UV-A light intensity. However, the efficacy of photoinduced disinfection for the control is lower than photocatalytic disinfection for the P25 TiO2 coated sample. Varying the initial E. coli concentration also influences the photocatalytic disinfection. SI Figure S4 shows the results of experiments conducted with a UV-A irradiation time of 3 h and various initial concentrations: ∼106, ∼107, and ∼108 cells mL-1. The efficacy of photocatalytic disinfection slowly decreases with increases in the E. coli concentration. Furthermore, increasing the UV-A exposure time improves the photocatalytic inactivation of E. coli, as shown in SI Figure S5. Photocatalytic Disinfection with Mesoporous TiO2 Coated Samples. The photocatalytic disinfection tests with the mesoporous TiO2 coated samples were conducted with a light intensity of 1.0 mW cm-2 and an E. coli concentration of ∼106 cells mL-1. Figure 3 shows the results of the tests for mesoporous TiO2 and P25 TiO2 coated samples. In the case of uncoated glass under UV-A irradiation, slow linear destruction occurs. In the case of the P25 TiO2 coated sample, the efficacy of E. coli inactivation reached 47% in 90 min. However, in the case of the mesoporous TiO2 coated sample, the E. coli bacteria cells were inactivated completely (approximately 99.99%) within 60 min of UV-A irradiation. In plots of ln(N0/N) versus irradiation time, straight lines were found for all the materials, indicating that the photocatalytic inactivation is a first order kinetics. The inactivation rate constant was 0.118 min-1, which is ca.11 times faster than that of P25 (0.010 min-1). The higher photocatalytic disinfection efficacy of mesoporous TiO2 can be explained VOL. 43, NO. 1, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. SEM images of TiO2 coated with 40 wt. % of PEG and PEO and calcined at 400 °C. (a) a P25 TiO2 coated sample, (b) a magnified image of the P25 TiO2 coated sample, (c) a monodisperse spherical mesoporous TiO2 coated on glass, (d) a magnified image of the coated mesoporous TiO2.
FIGURE 3. The fraction of surviving E. coli cells as a function of the UV-A irradiation time. The cells were treated with a mesoporous TiO2 coated sample (9), a P25 TiO2 coated sample (b), and an uncoated sample (2). in terms of its larger surface area and smaller crystallites. A larger surface area is likely to result in better photocatalytic disinfection, because it provides more active sites (25). The mesoporous TiO2 material coated on glass has a larger surface area (approximately 214 m2g-1) than the P25 TiO2 (approximately 50 m2g-1). Therefore, mesoporous TiO2 is likely to exhibit better photocatalytic inactivation than P25 TiO2. Another factor that influences photocatalytic activity is crystallite size. It is commonly accepted that smaller crystallites have a more powerful redox ability because smaller crystallites induce a larger band gap due to the quantum size effect, which arises due to a dramatic reduction in the number of free electrons (26). Since the mesoporous TiO2 was formed by the aggregation of nanocrystalline anatase that has a smaller crystallite size (approximately 10 nm) than P25 TiO2 (approximately 25 nm), its photocatalytic activity is expected to be better. The band gap of mesoporous TiO2 (3.3 eV) was also higher than that of P25 TiO2 (3.1 eV). In the repeated experimental test with the same coated glass plates, the 150
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efficacy of photocatalytic disinfection was reduced by ca. 20% comparing with that of first experiment test. With a view to further investigating the effects of surface area and crystallite size on photocatalytic disinfection, mesoporous TiO2 coated samples with various surface areas and crystallite sizes were synthesized, and film adhesion tests were conducted. The surface area and crystallite size of mesoporous TiO2 were varied through control of the calcination temperature. The physical properties of mesoporous TiO2 calcined at various temperatures are shown in SI Figure S6 and listed in SI Table S1. The surface area of mesoporous TiO2 decreases with increases in the calcination temperature: 213.9 m2g-1 (400 °C), 173.6 m2g-1 (500 °C), and 118.6 m2g-1 (600 °C). However, the crystallite size of mesoporous TiO2 increases with increases in the calcination temperature: 9.8 nm (400 °C), 12.3 nm (500 °C), and 20.5 nm (600 °C). Figure 4 shows the effects of varying the calcination temperature of the mesoporous TiO2 on the fraction of surviving E. coli cells. When mesoporous TiO2 calcined at 400 °C is used, the surviving fraction decreases sharply. However, the surviving fraction gradually increases with increases in the calcination temperature. In the case of the sample calcined at 500 °C, the surviving fraction is slightly higher. For a calcination temperature of 600 °C, the surviving fraction is significantly increased, up to 50% for 30 min UV-A irradiation. These results indicate that the efficiency of photocatalytic disinfection is strongly dependent on the surface area and the crystallite size: the larger the surface area and the smaller the crystallites, the better the photocatalytic inactivation. This study has shown that the photocatalytic disinfection test can be successfully conducted with TiO2 coated materials on the surface with film adhesion by measuring the enumeration of bacteria under UV-A irradiation. The photocatalytic inactivation achieved with mesoporous TiO2 was better than that of P25 TiO2 coated samples, which was thought to be because of the large surface area and high crystallinity of mesoporous TiO2. This method can be used
FIGURE 4. The fraction of surviving E. coli cells as a function of UV-A irradiation time. The cells were treated with mesoporous TiO2 coated samples: control (1), a sample calcined at 400 °C (9), a sample calcined at 500 °C (b), and a sample calcined at 600 °C (2). with different kinds of materials such as ceramics, plastics, and paper, and materials are produced by either coating or mixing of a TiO2 photocatalyst. Since it is very effective for determining the photocatalytic inactivation of microorganisms or bacteria, this approach can be of broad interest to environmental research, especially antibacterial activity.
Acknowledgments We thank the Korea Science and Engineering Foundation (KOSEF) for sponsoring this research through the SRC/ERC Program of MOST/KOSEF (R11-2005-065) and Dr. KyungBok Lee at KBSI for the expert technical assistance. The authors of this paper would like to thank the Korea Science and Engineering Foundation (KOSEF) for sponsoring this research through the SRC/ERC Program of MOST/KOSEF (R11-2005-065).
Supporting Information Available Experimental procedure for the photocatalytic disinfection tests, FE-SEM images of the coatings, BET analysis of surface area, XRD analysis of crystallite size, and HR-TEM image. This material is available free of charge via the Internet at http://pubs.acs.org.
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