Comparative study of titania nanoparticles and nanotubes as antibacterial agents

Solid State Sciences 13 (2011) 1797e1803

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Comparative study of titania nanoparticles and nanotubes as antibacterial agents Zhihong Jing a, b, *, Daojun Guo a, Weihua Wang a, Shufang Zhang a, Wei Qi a, Baoping Ling a a b

College of Chemistry and Chemical Engineering, Qufu Normal University, West Jingxuan Road No. 57, Qufu, Shandong 273165, PR China Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, North Shanda Road No. 27, Jinan, Shandong 250100, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 April 2011 Received in revised form 30 June 2011 Accepted 13 July 2011 Available online 23 July 2011

Anatase titania nanoparticles with a high surface area (about 587.7 m2/g) were synthesized by solegel method using isobutyl alcohol as solvent, and then anatase titania nanotubes with needlelike shape, which had diameters of about 5 nm and wall thickness of about 1 nm, could be obtained by microwave process using the above titania nanoparticles as precursors. Both titania nanoparticles and nanotubes were characterized through X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, Fourier transform infrared spectroscopy, ultraviolet-visible spectroscopy, photoluminescence spectroscopy and nitrogen adsorptionedesorption isotherm technique. The antibacterial activities of both titania nanoparticles and nanotubes against Escherichia coli (E. coli) were developed by quantification and qualitative ways, e.g. microcalorimetric method and disk diffusion method. At the same time, their antibacterial activities against E. coli were also investigated in dark and under UV irradiation. As a result, both the titania nanoparticles and nanotubes had good antibacterial activities against E. coli due to their low inhibitory concentration and large diameter of antibacterial circle. In addition, the titania nanoparticles displayed higher antibacterial activities than those of the titania nanotubes under UV irradiation, though they presented similar antibacterial activities in dark. The differences in antibacterial activities between titania nanoparticles and nanotubes might be attributed to the changes of their microstructure in our works. Ó 2011 Elsevier Masson SAS. All rights reserved.

Keywords: Titania nanoparticles/nanotubes Solegel method Microwave process Microcalorimetric method Disk diffusion method Antibacterial activities

1. Introduction The study of nanoscale materials has been paid much more attention because of their novel properties compared to bulk materials. Among them, one-dimensional nanotube materials are an important category of nanostructured materials and have been widely fabricated through various approaches, such as carbon, NB, NbS2, MoS2, WS2, SiO2, Al2O3 and TiO2 [1e10]. Titania is one of the wide band gap semiconductors (3.2 and 3.0 eV for anatase and rutile, respectively), and widely utilized in photocatalyst and supports [11,12], sensors [13], solar cells [14] and antibacterial agent [15e18]. The titania nanotubes have also been synthesized by means of various processes, since they have large surface area and high photocatalytic activity [8e10,19,20]. Much work has been reported by applying TiO2 or metal/carbon doped TiO2 nanoparticles/films in antibacterials [12,17,18,21e23].

* Corresponding author. College of Chemistry and Chemical Engineering, Qufu Normal University, West Jingxuan Road No. 57, Qufu, Shandong 273165, PR China. Tel.: þ86 537 4455221; fax: þ86 537 4456305. E-mail address: [email protected] (Z. Jing). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2011.07.010

However, TiO2 nanotubes used as antibacterials are seldom studied simultaneously. Recently, microwave synthesis has been successfully applied to the synthesis of nanocrystals due to its several advantages over conventional method, such as short reaction time, small particle size, narrow size distribution, and high purity [20,24e26]. Therefore, we attempt to develop a microwave route to synthesize titania nanotubes using titania nanoparticles as precursor, and contrast titania nanoparticles with titania nanotubes in antibacterial characteristics. In this paper, anatase titania nanoparticles with high surface area were prepared by solegel method using isobutyl alcohol as a solvent, and follow by microwave treating above titania nanoparticles, needlelike titania nanotubes could be obtained. The titania nanoparticles and nanotubes were characterized through XRD, TEM, HRTEM, FTIR, UVevis, PL, nitrogen adsorptionedesorption isotherm technique, respectively. The antibacterial activities of the anatase titania nanoparticles and nanotubes against Escherichia coli were developed by microcalorimetric method and disk diffusion method. At the same time, their antibacterial activities against E. coli were also performed in dark and under UV irradiation. The results indicate that the titania nanoparticles display better antibacterial

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activities than those of the titania nanotubes under UV irradiation, but they present similar antibacterial activities against E. coli in dark. The reasonable reason and explanation are discussed in detail. 2. Experimental section 2.1. Preparation of titania nanoparticles and nanotubes All chemical were purchased from Shanghai Chemical Co. 2.1.1. Sol-gel process A quantity of 3 mL tetrabutyl titanate and 15 mL isobutyl alcohol were mixed and stirred at room temperature. Then, the 2 M HNO3 solution was added to control the pH value at about 3. The mixture of 15 mL deionized water and 15 mL isobutyl alcohol was added dropwise to the above sol under magnetic stirring within 2 h to form a gel. After aging for 2 h, the product was washed with distilled water, dried at 80  C for 24 h, and then calcined at 400  C for 2 h. The titania nanoparticles were obtained. 2.1.2. Microwave process A quantity of 20 mg above titania nanoparticles and 50 mL 10 mol/L NaOH aqueous solution were mixed in a Teflon flask and then kept in the microwave system under stirring. The reaction conditions were in the power of 500 W and at the temperature of w110  C for 30 min. Afterwards the resultant precipitate was centrifuged, washed with 0.1 mol/L HCl aqueous solution and deionized water, and dried at 80  C for 2 h. Finally, the titania nanotubes were obtained. 2.2. Characterization The X-ray powder diffraction (XRD) patterns were provided using a Bruker D8 advance X-ray diffractionmeter equipped with graphite monochromatized Cu Ka radiation (l ¼ 1.5418 Å). The transmission electron microscopy (TEM) images and highresolution transmission electron microscopy (HRTEM) images were recorded on a JEOL 2100 transmission electron microscope with an acceleration voltage of 200 kV. The Fourier transform infrared spectra (FTIR) were collected using a NEXUS-470 infrared spectrometer. The specific surface area was estimated by the BET equation based on the nitrogen adsorption isotherm (77 K) using a NAVA 2000e Surface Area & Pore Size Analyzer (Quantachrome instruments, USA). Pore size distribution was determined by the BJH method using the desorption branch of adsorptionedesorption isotherms. All absorbance measurements were carried out on a CARY-300 ultravioletevisible spectrophotometer with 1.0 cm quartz cells. Photoluminescence experiments were carried out on an F-4600 fluorescence spectrophotometer with a Xe lamp at room temperature. Colloid solutions in ethanol were prepared ultrasonically for the photoluminescence measurements. 2.3. Measurement of antibacterial activities 2.3.1. The procedure of antibacterial test Before the antibacterial experimentation, all glass wares and samples were sterilized by autoclaving at 120  C for 15 min. The bacteria E. coli were cultured on a nutrient agar plate at 37  C for 24 h. Then, the cultured bacteria were added in 10 mL of saline solution to reach the concentration of bacteria of 106 colony forming units per milliliter (cfu/mL). For the antibacterial test, 10 mL of LB agar medium were poured into a sterilized petri dish (90 mm diameter), then 100 mL of the slurry (containing 1 mg/mL TiO2 nanoparticles or nanotubes) and 1 mL of the saline solution with E. coli at the concentration of 106 cfu/mL was in turn spread

onto the surface of each LB agar medium, and then inoculated in the self-regulating thermostat for 24 h at 37  C to count the surviving bacterial colonies by using an optical microscope. The total number of the cells forming unit was determined by area-based estimation. The solutions containing the different samples were kept in the dark or under UV light. UV irradiation was performed using a 20 W UV lamp with wavelengths at 254 nm (YGZM-T5 Shanghai Yuanguang Illumination Ltd., China). The procedure of dish diffusion method has been reported in our previous literature [27]. Namely, 10 mL of LB agar medium were poured into a sterilized petri dish, and two Oxford cups (6 mm inner diameter and 10 mm height) were put in the middle of the petri dish and then buried under the surface of the LB agar medium. One milliliter of the saline solution with E. coli at the concentration of 106 cfu/mL was spread onto the surface of each LB agar medium. Fifty microlitre of the slurry mentioned above were put into each Oxford cup and then inoculated in the self-regulating thermostat for 24 h at 37  C. A vernier caliper was adopted to measure the diameter of the antibacterial circle. The reported data were the average value of three separate similar runs. 2.3.2. The procedure of microcalorimetric experiment A 3114/3236 Thermal Activity Monitor (TAM) Air Isothermal Calorimeter, manufactured by Thermometric AB Company of Sweden, was used to measure the metabolic poweretime curves of E. coli strain growth. The performance of this instrument and the details of its construction have been described previously [28]. 3. Results and discussion 3.1. Characterization of nanomaterials The XRD patterns of titania nanoparticles and nanotubes are shown in Fig. 1(a) and (b). From Fig. 1(a), the peaks at 2q ¼ 25.3, 37.8, 48.1, 55.0 and 62.7 are observed, which can be indexed to the anatase TiO2 structure (JCPDS card No. 21-1272). The mean crystallite size of the TiO2 nanoparticles is estimated to be around 8 nm. As we known, anatase phase of TiO2 is the favorable structure with better photocatalytic functional properties than its rutile phase. From Fig. 1(b), the XRD analyses show that the phase of TiO2 nanotubes is also anatase, suggesting that the anatase structure of the TiO2 can be maintained after nanoparticles change to nanotubes by microwave process. However, the peak intensities of the

(101)

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2Theta (degree) Fig. 1. XRD patterns of (a) the TiO2 nanoparticles, and (b) the TiO2 nanotubes.

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Fig. 2. TEM images of (a) the TiO2 nanoparticles, and (b) the TiO2 nanotubes.

TiO2 nanotubes are weaker compared to those of the TiO2 nanoparticles. The mean crystallite size of the TiO2 nanotubes is estimated to be around 10 nm. From Fig. 2(a), TEM image of the TiO2 nanoparticles shows the dispersed homogeneous particles with diameters of around 9 nm, which is agreement with the result of the XRD. As shown as the HRTEM image (seen Fig. 3(a)), the clear fringes separated by 0.351 and 0.189 nm correspond to (101) and (200) lattice spacing of a tetragonal TiO2 crystal respectively. From Fig. 2(b), the TEM image of the TiO2 nanotubes reveals a needlelike shape with a diameter of about 5 nm and a wall thickness of about 1 nm. Fig. 3(b) is the HRTEM image of the TiO2 nanotubes, and the clear fringes separated by 0.167 nm corresponding to the (211) plane of a tetragonal TiO2 have been observed. It indicates that the obtained nanotubes are well crystalline. In our experiments, the anatase TiO2 nanoparticles can absolutely become anatase TiO2 nanotubes with microwave irradiation, which is consistent with the previous reports of Wu [20] and Kasuga [29]. Fig. 4 shows the FTIR transmission spectra of the TiO2 nanoparticles and nanotubes. The peaks at around 3400 and 1630 cm1 originate from the adsorbed water and a large amount of hydroxyl groups arising on the surface of the products [24]. Generally, the greater the number of surface hydroxyl groups, the faster the photocatalytic reaction [30,31]. The large peak observed at 1630 cm1 for the TiO2 nanoparticles and the TiO2 nanotubes indicates a large number of surface hydroxyl groups and hence more photocatalytic activity. TieOeTi vibration bands appear in the range of 400e600 cm1 as a result of condensation reaction [32]. However, the surface chemical state of the TiO2 nanoparticles

and the TiO2 nanotubes is readily distinguishable. For example, there are only three peaks in the TiO2 nanotubes spectrum (around 1061, 1388 and 2960 cm1). These peaks can be mainly attributed to edge-shared [TiO6] octahedral layer after the wafers of the TiO2 nanoparticles formed the nanosheets and then rolled up into nanotubes [24]. It suggests that the different microstructure of the two samples could influence their antibacterial properties. The representative nitrogen adsorption and desorption isotherm and the corresponding Barrett-Joyner-Halenda (BJH) pore size distribution plots (inset) of the TiO2 nanoparticles and nanotubes are shown in Fig. 5(a) and (b), respectively. The N2 isotherm of the TiO2 nanoparticles corresponds to a type IV isotherm in the Brunauer classification [33]. Characteristic feature of the type IV isotherm plot is its hysteresis loop, which is associated with the filling and emptying of mesopores by capillary condensation. According to the IUPAC classifications, the loop observed is ascribed to type H3 loop, indicating the presence of mesopores (pores 2e50 nm in diameter) in materials. As calculated by the BJH method from the desorption branch of the nitrogen isotherm, the pore size distribution presents the material containing small mesopores with an average pore size centered about 4.3 and 17.5 nm, respectively. The pore volumes are equal to about 0.39 cm3/g and the specific surface area of the TiO2 nanoparticles is calculated to be 587.7 m2/g. For the TiO2 nanotubes, to a certain extent, isotherm plot corresponds to type I isotherm (Fig. 5(b)). The features of the type I isotherm plots are their gentle curve and especial hysteresis loop. As seen form Fig. 5(b), the hysteresis loop does not close at relative

Fig. 3. HRTEM images of (a) the TiO2 nanoparticles, and (b) the TiO2 nanotubes.

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a

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Wavenumbers (cm ) Fig. 4. FTIR spectra of (a) the TiO2 nanoparticles, and (b) the TiO2 nanotubes.

Absorption (arb. unit)

low P/P0, but continues to the minimum of P/P0, which may be attributed to the existence of micropores by low-pressure hysteresis effect. The hysteresis loop observed is ascribed to type H4 loop, indicating the presence of micropores (pores 2 nm in diameter) in

materials. As calculated by the BJH method, the pore size distribution shows a narrow microporous distribution with an average pore diameter at 2.1 nm. This result is in agreement with that of the isotherm plot, suggesting the presence of micropores in the TiO2 nanotubes. Moreover, the pore volumes are equal to about 0.11 cm3/g and the specific surface area of the TiO2 nanotubes is calculated to be 228.2 m2/g. As a consequence, isotherm plots and BJH adsorption pore distributions strongly depend on the pore size and the pore shape in the samples. For the TiO2 nanoparticles, it is of type H3 loop, the reason can be explained that the aggregated small TiO2 nanoparticles generate a mesoporous network, which results in the inkbottle shaped pores. However, for the TiO2 nanotubes, it is of type H4 loop, indicating the presence of some taper-capillary shaped pores. Here, the taper-capillary shaped pores can be mainly due to the existence of needlelike TiO2 nanotubes, as observed from TEM. The UVevis measurement of the TiO2 nanoparticles and TiO2 nanotubes all exhibits an absorption peak centered at 372 nm before and after UV irradiation, as shown in Fig. 6. A remarkable blue shift in the absorption edge is observed for the TiO2 nanoparticles and the TiO2 nanotubes in comparison to bulk anatase (387 nm), demonstrating the quantum confinement effect. Exceptionally, a broad absorption peak centered at around 606 nm is observed for TiO2 nanotubes before UV irradiation, but it disappears after UV irradiation for 2 h. It might be assigned to the changes of surface chemical bands and the microstructure of the TiO2 nanotubes exposed under UV light. The photocatalytic activities of TiO2-based material are well known. When TiO2 is exposed to UV light, the photoinduced electronehole pairs are generated. Photoluminescence emission analysis has been often used to investigate the transfer behavior of the photogenerated electronehole pairs, and also to understand the separation and recombination of the pairs as the primary processes in the field of photocatalysis. Fig. 7 shows the PL emission spectra of the TiO2 nanoparticles and TiO2 nanotubes excited at a wavelength of 285 nm. The spectra of both samples all present two emission peaks at wavelengths of 420 and 518 nm. The emission band at 420 nm (2.95 eV) is due to free exciton emission of TiO2, while the emission band at 518 nm (2.39 eV) corresponds to the surface state of Ti4þeOH [34,35]. It can be found that the emission spectrum of the TiO2 nanoparticles exhibits the same energies for the peaks

a b c d 200

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Wavelength ( nm) Fig. 5. Typical N2 isotherms and BJH pore size distribution plots (inset) of two samples: (a) the TiO2 nanoparticles, and (b) the TiO2 nanotubes.

Fig. 6. UVevis absorption spectra of (a) the TiO2 nanotubes, (b) the TiO2 nanoparticles under UV irradiation for 2 h, (c) the TiO2 nanoparticles, and (d) the TiO2 nanotubes under UV irradiation for 2 h.

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Fig. 7. Photoluminescence spectra of the TiO2 nanoparticles and the TiO2 nanotubes.

3.2. Investigation of the antibacterial activities by microcalorimetric method For inhibitory conditions, the model of bacterial growth followed the Logistic equation [28,34]. By integrating and arranging the Logistic equation, we obtained

(1)

where m was the growth rate constant, a was the integral constant, Pm and Pt was the maximum heat production rate and the heat production rate at time t, respectively. Using the experimental data of Pm, Pt and t obtained from the poweretime curves, the growth rate constant m could be calculated from linear regression analysis. Subsequently, the relationship equation between the growth rate constant m and the inhibitory concentration C could be

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Pt (mw)

related to the TiO2 nanotubes, but the PL intensity of the peaks is lower than that of the TiO2 nanotubes. This indicates that the TiO2 nanoparticles is better sensitive to UV emission compared with the TiO2 nanotubes related to the PL intensity of the peaks, due to some changes in the electronic structure of the TiO2 nanomaterials. In fact, the delay in the recombination rate of the photogenerated electronehole pairs led to the reduction of the PL peak intensity [31]. Thus, it suggests that the TiO2 nanoparticles can effectively suppress the recombination rate of the photogenerated electronehole pairs under UV irradiation, thereby promoting the photocatalytic activities compared with the TiO2 nanotubes in our experiments. It is concluded that the absolute anatase phase TiO2 nanoparticles can become anatase phase TiO2 nanotubes by microwave treating. XRD figures clearly demonstrate the higher crystallinity of the TiO2 nanoparticles compared to the TiO2 nanotubes. In addition, the TiO2 nanoparticles present not only larger specific surface area but also bigger pore volume compared with the needlelike TiO2 nanotubes by BET analysis. In particular, the pore shape changes from ink-bottle to taper-capillary, and the pore size decreases from mesopores to micropores. The UVevis and PL measurements of the TiO2 nanoparticles and the TiO2 nanotubes reveal some changes in the electronic structure of them. Therefore, we guess that the microstructure changes may have a noticeable influence on their antibacterial properties.

  Pt ¼ Pm = 1 þ aemt

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t (min) Fig. 8. The metabolic power-time curves of E. coli of the TiO2 nanoparticles (I) and the TiO2 nanotubes (II) at different concentrations: (a) 0 mg/mL; (b) 0.01242 mg/mL; (c) 0.02470 mg/mL; (d) 0.04878 mg/mL; (e) 0.05996 mg/mL; (f) 0.07228 mg/mL; (g) 0.09524 mg/mL; (h) 0.1176 mg/mL.

established from linear regression analysis. Consequently, the minimum inhibitory concentration (MIC) was obtained when m ¼ 0. The lower the value of the MIC was, the better the antibacterial activity of the sample was. For E. coli strain, the metabolic poweretime curves of both titania nanoparticles and nanotubes under different concentration conditions were determined using a 3114/3236 Thermal Activity Monitor, and shown in Fig. 8. Here, we chose the metabolic poweretime curves of E. coli of the TiO2 nanotubes as an example to illustrate how to deal with the data. From Fig. 8 (II), the growth rate constant m could be obtained according to Eq. (1) and shown in Table 1, and then the relationship equation was established from linear regression analysis with the best relative coefficient (r), e.g. m ¼ 0.11036e0.2613C. Thus, the MIC value of the TiO2 nanotubes was obtained, CMIC ¼ 0.4220 mg/mL when m ¼ 0. For comparison, the fitted equations (m  c) and MIC of E. coli of the TiO2 nanoparticles were obtained by means of the same treatment process. The fitted equations (m  c) of the TiO2 nanoparticles was represented as m ¼ 0.1194e0.5418C r ¼ 0.9988, and the MIC value of the TiO2 nanoparticles was 0.2204 mg/mL.

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Table 1 Values of m of the TiO2 nanotubes at different concentrations. C (mg ml1) m (min1) r

0 0.01242 0.1112 0.1063 0.9982 0.9986 Fitted equation (m  c)

0.02470 0.04878 0.1044 0.09673 0.9986 0.9992 m ¼ 0.1103e0.2613C

3.3. Investigation of the antibacterial activities by disk diffusion method The antibacterial activities of the TiO2 nanoparticles and nanotubes in the dark or under UV light are investigated by counting the surviving bacterial colonies by using an optical microscope. The survival ratio can be obtained by the ratio of the number survived to the total ones. The quantitative results confirm that the survival ratio of E. coli for the TiO2 nanoparticles was 70.3% in the dark and decreased to 2.8% under UV irradiation for 2 h; while the survival ratio of E. coli for the TiO2 nanotubes was 69.6% in the dark and decreased to 6.8% under UV irradiation for 2 h. No significant changes on the survival ratio of two samples could be seen in the dark, but evident difference was presented under UV irradiation for 2 h. We find that both TiO2 nanoparticles and TiO2 nanotubes have less antibacterial activities in the dark than that under UV light. It is convincible that UV irradiation is an intensifier for nano-TiO2 photocatalysis as previously reported [12,17]. By the way, the control test has been investigated and no considerable change in the antibacterial activity was observed. It is generally that if the diameter of antibacterial circle of one sample is larger than 7 mm, which means that the sample has better antibacterial activity. The antibacterial circle photos of the TiO2 nanoparticles and nanotubes are shown in Fig. 9(a) and (b). The antibacterial circle diameters of two samples for E. coli were measured, which was 26.0 and 23.1 mm for the TiO2 nanoparticles and nanotubes, respectively. It reveals that both TiO2 nanoparticles and nanotubes exhibit better antibacterial activities against E. coli. Furthermore, the TiO2 nanoparticles present higher antibacterial activity than that of the TiO2 nanotubes in the same conditions. This result is in accordance with that of the microcalorimetric method. The reason why the TiO2 nanoparticles exhibit better antibacterial activity than the nanotubes may be summarized as follows. It is well known that the surface chemical states of the TiO2 products, such as a large number of surface-adsorbed water and hydroxyl groups, would improve their photocatalytic performances. But the intensities of these peaks in the FTIR spectra of two samples are similar in our experiment, implying that the existence of a large number of hydroxyl groups is not primary element making

0.05996 0.09446 0.9987 CMIC ¼ 0.4220

0.07228 0.09141 0.9986

0.09524 0.08564 0.9943 r ¼ 0.9984

0.1176 0.07987 0.9985

difference between the TiO2 nanoparticles and nanotubes. However, some exceptive peaks (around 1060, 1388 and 2960 cm1) only in the nanotube spectrum maybe explain the difference in the antibacterial activity. When the structure transformed from nanoparticles to nanotubes, these peaks appear, suggesting the interfaces between the nanomaterials and bacteria vary evidently. Bacteria are known to attach to nanomaterial particles by means of strong van der Waals forces between the bacterial and the nanomaterial surface. Therefore, the TiO2 nanoparticles with more active sites behave much more strongly reactions on bacteria surface, which in turn lead to better antibacterial activity compared to nanotubes. Recently, several studies suggest that peptide or lipid nanotubes can penetrate through cell membranes because of their cylindrical shape and high aspect ratio and lead to cell death [21,35,36]. Owing to similarities in geometry, we postulate that the TiO2 nanotubes were also able to penetrate cell membranes and damage to the E. coli bacteria by physical damage to the outer membrane of the cells, causing the release of intracellular content. It seems that nanotubes can easily interact with the bacteria through direct contact with sharp edges and hence represent better antibacterial activities than nanoparticles. However, in our experiment, the results are inconsistent with previous reports. Because of limited experiment conditions, we can not take out the direct evidence now, but further research and confirmation will be achieved in contiguous study. On the other hand, the TiO2 nanoparticles have high crystallinity and small particle size by XRD figures, which result in higher specific surface area and offer more active sites. As calculated from BET plots, for the TiO2 nanoparticles, the specific surface area is 587.7 m2/g; for the TiO2 nanotubes, the specific surface area is 228.2 m2/g. The specific surface area of the TiO2 nanoparticles is 2.5 times as high as the TiO2 nanotubes. These conditions are of great importance in photocatalyst. Generally, the higher the specific surface area is, the stronger the antibacterial activity is. It is also considered that the TiO2 nanoparticles with small particle size offer more chance to contact bacteria compared to the TiO2 nanotubes, consequently the lower concentration necessary can inhibit bacterial growth. The UVevis and PL measurements have been applied to explore the structural and optical properties. In photocatalytic applications,

Fig. 9. The antibacterial circle photos of E. coli of the samples: (a) the TiO2 nanoparticles, and (b) the TiO2 nanotubes.

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an increased band gap enhances the photocatalytic efficiency. It can be explained that larger band gap resulted from the quantum size effect for the TiO2 nanoparticles and the TiO2 nanotubes, compared with bulk anatase, is a key factor for improving their photocatalytic properties [36]. The decrease of the PL peak intensity for the TiO2 nanoparticles suggests the delay in the recombination rate of the photogenerated electronehole pairs, which indicates the TiO2 nanoparticles would behave better antibacterial activities than the TiO2 nanotubes. Furthermore, the pore size distribution and pore structure are crucial factors, too. The TiO2 nanoparticles are of typical mesoporous materials and the pore size distribution shows a maximum centered about 4.3 nm and a bigger mesopore at about 17.5 nm. At the same time, its hysteresis loop reveals the presence of some inkbottle shaped pores. It is said that most industrial catalysts with mesoporous features and the ink-bottle shaped pores always present good catalytic/photocatalytic performances. Our experiment results set just a good example with the previous works. While the TiO2 nanotubes favor the absorption in narrow pores, their characteristic is close to the micropores. The pore size distribution only shows a maximum centered about 2.1 nm and its hysteresis loop suggests the presence of some taper-capillary shaped pores, which are similar with the observation from TEM. Thus, the significant difference in pore size distribution and pore shape will be a sufficient cause to influence the antibacterial properties. We believe that research on the feature of the pore structure is beneficial for effectively selecting photocatalyst in industry. In summary, the changes of microstructure result in the difference of antibacterial properties for the TiO2 nanoparticles and TiO2 nanotubes. It is expected that both TiO2 nanoparticles and nanotubes can be applied widely in clinical and environmental field not only as antibacterial agents but also as photocatalyst. 4. Conclusion In this study, anatase titania nanoparticles with high surface area were synthesized by solegel method using isobutyl alcohol as a solvent, and then anatase titania nanotubes with needlelike shape, which had diameters of w5 nm and wall thickness of w 1 nm, could be obtained by microwave process. Both titania nanoparticles and nanotubes present good antibacterial activities against E. coli by microcalorimetric method and disk diffusion method. Compared with the titania nanotubes, the titania nanoparticles display higher antibacterial activities under UV irradiation, though they present similar antibacterial activities in dark. The differences in antibacterial activities mainly depend on the changes of the microstructure in our works, such as small average particle size and high surface area, surface chemical states, large band gap, pore size distribution and pore structure. It was proven that excellent antibacterial activities of both titania nanoparticles and nanotubes rendered them potentially used as antibacterial agents in clinical and environmental application.

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Acknowledgement The work was financially supported by Shandong Natural Science Foundation (ZR2010BM010) and the Foundation of Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education (201002). Support from the Foundation for Outstanding Young Scientist in Shandong Province (BS2009HZ014, BS2010NJ008) is also acknowledged. References [1] T. Akasaka, F. Watari, Acta Biomater. 5 (2009) 607. [2] G. Ciofani, V. Raffa, A. Menciassi, A. Cuschieri, Nano Today 4 (2009) 8. [3] G. Seifert, H. Terrones, M. Terrones, T. Frauenheim, Solid State Commun. 115 (2000) 635. [4] Y. Tian, Y. He, Y.F. Zhu, Mater. Chem. Phys. 87 (2004) 87. [5] Y.Q. Zhu, W.K. Hsu, S. Firth, M. Terrones, R.J.H. Clark, H.W. Kroto, D.R.M. Walton, Chem. Phys. Lett. 342 (2001) 15. [6] C.H. Rüscher, I. Bannat, A. Feldhoff, L.R. Ren, M. Wark, Microporous Mesoporous Mater. 99 (2007) 30. [7] C.L. Li, Y.W. Jiang, S.G. Yang, R.K. Zheng, W.R. Yang, Z.W. Liu, S.P. Ringer, Mater. Chem. Phys. 120 (2010) 443. [8] A. Kodama, S. Bauer, A. Komatsu, H. Asoh, S. Ono, P. Schmuki, Acta Biomater. 5 (2009) 2322. [9] S.C. Roy, M. Paulose, C.A. Grimes, Biomaterials 28 (2007) 4667. [10] L.R. Hou, C.Z. Yuan, Y. Peng, J. Hazard. Mater. 139 (2007) 310. [11] J.R. Xiao, T.Y. Peng, R. Li, Z.H. Peng, C.H. Yan, J. Solid State Chem. 179 (2006) 1161. [12] G. Williams, B. Seger, P.V. Kamat, ACS Nano 2 (2008) 1487. [13] D. Morris, R.G. Egdell, J. Mater. Chem. 11 (2001) 3207. [14] M. Thelakkat, C. Schmitz, H.W. Schmidt, Adv. Mater. 14 (2002) 577. [15] W. Su, S.S. Wei, S.Q. Hu, J.X. Tang, J. Hazard. Mater. 172 (2009) 716. [16] K. Hashimoto, H. Irie, A. Fujishim, Jpn. J. Appl. Phys. 44 (2005) 8269. [17] O. Akhavan, E. Ghaderi, J. Phys. Chem. C 113 (2009) 20214. [18] O. Akhavan, J. Colloid Interface Sci. 336 (2009) 117. [19] Y.C. Zhu, H.L. Li, Y. Koltypin, Y.R. Hacohen, A. Gedanken, Chem. Commun. 24 (2001) 2616. [20] X. Wu, Q.Z. Jiang, Z.F. Ma, M. Fu, W.F. Shangguan, Solid State Commun. 136 (2005) 513. [21] O. Akhavan, E. Ghaderi, Surf. Coat. Technol. 204 (2010) 3676. [22] Y. Liu, X.L. Wang, F. Yang, X.R. Yang, Microporous Mesoporous Mater. 114 (2008) 431. [23] O. Akhavan, R. Azimirad, S. Safa, M.M. Larijani, J. Mater. Chem. 20 (2010) 7386. [24] X. Wu, Q.Z. Jiang, Z.F. Ma, W.F. Shangguan, Solid State Commun. 143 (2007) 343. [25] Y. He, H.T. Lu, L.M. Sai, Y.Y. Su, M. Hu, C.H. Fan, W. Huang, L.H. Wang, Adv. Mater. 20 (2008) 3416. [26] A. Qurashi, N. Tabet, M. Faiz, T. Yamzaki, Nanoscale Res. Lett. 4 (2009) 948. [27] Z.H. Jing, C.C. Wang, G.L. Wang, W.J. Li, D.M. Lu, J. Sol-Gel Sci. Technol. 56 (2010) 121. [28] I. Wadsö, Thermochim. Acta 269e270 (1995) 337. [29] T. Kasuga, M. Hiramatst, A. Hoson, T. Sekino, K. Niihara, Langmuir 14 (1998) 3160. [30] N. Machado, V.S. Santana, Catal. Today 107 (2005) 595. [31] N. Brundabana, K.M. Parida, S.G. Chinnakonda, J. Phys. Chem. C 114 (2010) 19473. [32] Z.J. Li, B. Hou, Y. Xu, D. Wu, Y.H. Sun, W. Hu, F. Deng, J. Solid State Chem. 178 (2005) 1395. [33] S.J. Gregg, S.W.K. Sing, Adsorption, Surface Area and Porosity, second ed. Academic Press, London, UK, 1982. [34] W.J. Kong, Y.L. Zhao, L.M. Shan, X.H. Xiao, W.Y. Guo, J. Chromatogr. B 871 (2008) 109. [35] D.V. Bavykin, S.N. Gordeev, A.V. Moskalenko, A.A. Lapkin, F.C. Walsh, J. Phys. Chem. B 109 (2005) 8565. [36] J. Joo, S.G. Kwon, T. Yu, M. Cho, J. Lee, J. Yoon, T. Hyeon, J. Phys. Chem. B 109 (2005) 15297.