Photocatalytic antibacterial performance of n-doped TiO2 against Escherichia coli

JOURNAL OF SCIENCE OF HNUE Mathematical and Physical Sci., 2013, Vol. 58, No. 7, pp. 86-93 This paper is available online at PHOTOCATALYTIC ANTIBACTERIAL PERFORMANCE OF N-DOPED TiO2 AGAINST ESCHERICHIA COLI Do Minh Thanh, Nguyen Thi Khanh Hoa, Nguyen Cao Khang and Nguyen Van Minh Center for Nano Science and Technology, Hanoi National University of Education Abstract. Nitrogen-doped titanium dioxide (TiO2−xNx) material has been prepared using the hydrolysis method. The effect of nitrogen on the structural and optical properties of the catalyst was investigated. The antibacterial ability of synthesized N-doped TiO2 samples was evaluated using inactivated Escherichia coli (E. coli) as a model for Gram-negative bacteria under visible irradiation. Characterization results show that N-doped TiO2 samples have a broader absorption spectrum and a higher antibacterial efficiency against E. coli than pure samples. The DFT calculation suggests that nitrogen ion doping induces the formation of new states closed to the valence band leading to a narrowing of the band gap and a great improvement in photocatalytic activity in the visible light region of the doped material. Keywords: N-doped TiO2, E. coli, photocatalyst. 1. Introduction TiO2 is the best well-known photocatalyst among the metal oxides due to its excellent physical and chemical properties [1]. Titania in the anatase phase has been widely applied in environmental treatments [2-5]. Furthermore, TiO2 can be used as an antibacterial agent because of its strong oxidation activity and super hydrophilicity under appropriate irradiation. The photocatalytic biocidal effect of TiO2 was first reported by Matsunaga et al. in 1985 [6]. They observed that microbial cells can be killed when a homogeneous solution of TiO2-Pt catalyst and microbial cells was exposed to near-UV light. Afterwards, numerous studies related to the bactericidal effect of TiO2 have been Received September 10, 2013. Accepted October 28, 2013. Contact Nguyen Cao Khang, e-mail address: khangnc@hnue.edu.vn 86 Photocatalytic antibacterial performance of N-doped TiO2 against escherichia coli reported such as the successful killing of bacteria, viruses and cancer cells under UV illumination [7-9]. However, its application has been almost entirely limited to UV light due to its large band gap (3.2 eV for the anatase phase). In recent years, selectively doping of TiO2 has been an effective way to decrease the instintic band gap of the material, which promotes photocatalytic efficiency under visible light. Various non-metallic doped TiO2 materials exhibited a strong visible absorption and significant activity in the photocatalytic reactions [10-12]. Of the non-metallic elements, nitrogen is considered to be the most promising doping element because it has properties which are similar to oxygen [13]. We have previously reported doping of nitrogen into TiO2 anatase to achieve enhanced photocatalytic activities in the decomposition of methylene blue and phenol [14]. In the present work, we investigated the photocatalytic ability of this material through the inactivation of E. coli bacterium under visible irradiation. 2. Content 2.1. Experiments * Preparation of photocatalyst Titanium tetraisopropoxide (TTiP − Ti(OCH(CH3)2)4, 99%, Aldrich Co.) and urea ((NH2)2CO, 98%, China) were used as the titanium and nitrogen precursors, respectively. The N-doped TiO2 photocatalyst was prepared using the simple hydrolysis method, following the main steps. First, calculated quantities of TTiP and urea (corresponding to the weight ratio of urea/TTiP in the final solution equal to 1/1 and 4/1, respectively) were mixed and stirred into distilled water. The solution was then evaporated at 120 0C. Next, the dry residue was crushed and transferred to a horizontal muffle furnace and heated at 400 0C for 2 h to obtain the final powder sample. For comparison, TiO2 anatase was prepared by the same method without using urea. * Measurement of inactivated E. coli bacteria All of the photocatalysts and the equipment was disinfected prior to carrying out the experiment. An aliquot of homogeneous 0.9% saline solution containing photocatalyst particles was mixed with a prepared E. coli cell solution in a reactor vessel under magnetic stirring. The final photocatalyst and bacterial cell concentration was adjusted to 0.5 mg/ml and 107 cfu/ml, respectively. Illumination in the visible range was carried out using a 100 W filament lamp with a filter to remove all radiation with wavelength below 410 nm. At regular time intervals, 50 µl aliquots of the irradiated cell suspensions were withdrawn. After appropriate dilution, these suspensions were spread onto petri dishes containing an agar medium and incubated at 37 0C for 24 h to determine the number of viable cells in terms of CFU. 87 Do Minh Thanh, Nguyen Thi Khanh Hoa, Nguyen Cao Khang and Nguyen Van Minh 2.2. Results and discussion Figure 1 shows the XRD patterns of the samples. The diffraction peak lines at 2θ = 25.30, 37.80, 48.10, 53.90, 55.20, and 62.70 demonstrate that all three samples are of the anatase phase. No strange diffraction peak was observed in the phase containing nitrogen. This indicates that there was no phase change after nitrogen doping. Moreover, the average crystallite size determined from the strongest diffraction peak of the anatase (101) planes is listed in Table 1, this size estimated using the Debye-Scherrer equation. It is important to notice that the cell parameters of material were changed by the N presence and therefore we think that all the dopant ions entered the titania lattice. Figure 1. X-ray diffraction patterns of N-doped T iO2 samples with a urea/TT iP weight ratio of 4/1 (a), 1/1 (b) and a pure anatase sample (c). Table 1. Phase composition and crystal size of N-doped TiO2 material Sample Urea/TTiP Phase Average crystallite Lattice constant A˚ weight ratio composition size (nm) a c a 4/1 anatase 11 3.785 9.502 b 1/1 anatase 11 3.785 9.502 c (reference) 0/1 anatase 12 3.787 9.503 The particle morphology of the synthesized doping photocatalyst was observed using SEM (Figure 2). Analysis of the SEM image shows clearly that the doped samples contain irregular forms and aggregated particles with the average size of 15 nm. It is also clear that a porous structure with a large surface area that can promote catalytic efficiency was formed. 88 Photocatalytic antibacterial performance of N-doped TiO2 against escherichia coli Figure 3 shows the corresponding UV-vis diffusive reflectance spectra of prepared N-doped TiO2 and pure TiO2 nano anatase used as reference. It was clearly observed that the doped samples have an apparent adsorption in the visible region between 400 and 600 nm (the samples are bright yellow in color) whereas the absorbance threshold of a pure sample is below 400 nm. This suggests that the increased absorption of doped TiO2 could be due to the presence of nitrogen species in the samples. Figure 2. SEM image of N-doped T iO2 particles with a urea/TT iP weight ratio of 1/1. Figure 3. UV-vis diffusive reflectance spectra of N-doped T iO2 samples with a urea/TT iP weight ratio of 4/1 (a), 1/1 (b), and a pure anatase sample (c). In order to have a clearer view of the catalytic mechanism, a computational calculation was also performed to calculate the partial density of states (PDOS) of the N-doped anatase. The calculations in our work were carried out based on DFT using the Materials Studio package and the well-tested Castep code. Local density approximations (LDA) with PWC11 function were used to describe the exchange-correlation effects. A plane-wave cutoff energy of 380 eV was used with a Monkhorst-Pack scheme k-point grid sampling of 5 × 5 × 4 for the irreducible Brillouin zone. The primitive unit cell of TiO2 in the anatase structure and in the 2 × 2 × 1 supercell model was considered in this work. To archive realistic experimental dopant concentrations (3% -7%), the O atoms were displaced by one N atom, giving dopant concentrations of 3.125 %. 89 Do Minh Thanh, Nguyen Thi Khanh Hoa, Nguyen Cao Khang and Nguyen Van Minh The PDOS of the pure and N-doped anatase are plotted in Figure 4, with the Fermi energy being 0 eV on the energy axis. It shows that the band gap of the N-doped TiO2 (Eg = 2.2eV) was smaller than that of the pure anatase (Eg = 2.8eV). These gaps are very small compared to the experimental values due to the LDA supposition. The N 2p bands were located around the Fermi level and above the maximum valence band ofTiO2, which narrows the energy band gap of the material. The results of the analyses performed in the present research indicates that the nitrogen ions were incorporated into the structures of titania replacing oxygen ions or being located at interstitial sites. However, from the negligible difference in the analytic results of two doped samples, it is possible that the increase of the urea/TTiP weight ratio from 1/1 to 4/1 did not strongly affect the mentioned characteristic properties of the material. Figure 4. Density of states (DOS) of pure anatase (a) and a 3.125% N-doped T iO2 (b) sample. Figures 5 and 6 show the reduction of E. coli colony counts in agar after treatment. It is obvious that the visible light irradiation (control sample) has no bactericidal effect while the N-doped TiO2 (weight ratio of urea/TTiP being 1/1) has the bactericidal ability to kill 100% of the E. coli after 1 h of irradiation. The photocatalytic results are in good agreement with the visible light absorption of the prepared samples shown in Figure 4. Furthermore, this relationship confirms that the visible light bactericidal effect originated from the incorporation of nitrogen dopant atoms into the titania lattice. 90 Photocatalytic antibacterial performance of N-doped TiO2 against escherichia coli Figure 5. Images of E. coli colonies on agar plates before and after visible light irradiation. Figure 6. Number of E. coli colonies growing on MBA agar in petri plates under 1 h visible light irradiation. When N-doped TiO2 is illuminated with corresponding irradiation, the photo-induced electrons are transferred from the valence band (VB) into the conduction band (CB). The whole reaction process viewed on a schematic diagram at macroscopic scale is shown in Figure 7. It can be mentioned that visible light illumination produced holes in the occupied midgap level (N 2p) whereas UV irradiation produced holes in the valence band (O 2p). The photo-generated holes in the VB react with OH− to form a reactive hydroxyl radical (OH•) while the adsorbedO2 on the surface of the photocatalyst traps an excited electron in the CB which can trigger the formation of superoxide anion radicals (O•−2 ). It is well known these radical groups can decompose a cell wall and cell membrane causing E. coli cells to die [15]. Therefore, it thought that the doping of nitrogen atoms into the TiO2 lattice effectively decreased the band gap, leading to an efficient operation of the photocatalytic oxidation in antibacterial activities under visible light. 91 Do Minh Thanh, Nguyen Thi Khanh Hoa, Nguyen Cao Khang and Nguyen Van Minh Figure 7. A possible photocatalytic mechanism of N-doped T iO2 under visible irradiation. 3. Conclusion A high quality nano scale N-doped TiO2 catalyst has been successfully prepared using the simple hydrolysis method. 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