Visible light photoinactivation of bacteria by tungsten oxide nanostructures formed on a tungsten foil

Applied Surface Science 338 (2015) 55–60

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Visible light photoinactivation of bacteria by tungsten oxide nanostructures formed on a tungsten foil Fariba Ghasempour a , Rouhollah Azimirad b , Abbas Amini c , Omid Akhavan b,d,∗ a

Plasma Physics Research Centre, Science and Research Branch, Islamic Azad University, P.O. Box 147789-3855, Tehran, Iran Department of Physics, Sharif University of Technology, P.O. Box 11155-9161, Tehran, Iran School of Computing, Engineering and Mathematics, University of Western Sydney, Kingswood, NSW 2751, Australia d Institute for Nanoscience and Nanotechnology, Sharif University of Technology, P.O. Box 14588-89694, Tehran, Iran b c

a r t i c l e

i n f o

Article history: Received 18 October 2014 Received in revised form 6 January 2015 Accepted 28 January 2015 Available online 1 March 2015 Keywords: Tungsten oxide Nanostructures Antibacterial materials E. coli Photocatalysts

a b s t r a c t Antibacterial activity of tungsten oxide nanorods/microrods were studied against Escherichia coli bacteria under visible light irradiation and in dark. A two-step annealing process at temperatures up to 390 ◦ C and 400–800 ◦ C was applied to synthesize the tungsten oxide nanorods/microrods on tungsten foils using KOH as a catalyst. Annealing the foils at 400 ◦ C in the presence of catalyst resulted in formation of tungsten oxide nanorods (with diameters of 50–90 nm and crystalline phase of WO3 ) on surface of tungsten foils. By increasing the annealing temperature up to 800 ◦ C, tungsten oxide microrods with K2 W6 O19 crystalline phase were formed on the foils. The WO3 nanorods showed a strong antibacterial property under visible light irradiation, corresponding to >92% bacterial inactivation within 24 h irradiation at room temperature, while the K2 W6 O19 microrods formed at 800 ◦ C could inactivate only ∼45% of the bacteria at the same conditions. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Various metal-oxide materials have recently been developed for photocatalytic activities [1–5]. For instance, the semiconductor titanium dioxide (TiO2 ) has shown a good chemical stability and high reactivity under UV (ultraviolet) light irradiation [6]. TiO2 has a wide band gap of 3.2 eV and absorbs light with wavelengths ≤387 nm, and thus like many other metal-oxides, it can only absorb a small fraction of the UV solar light [7]. This undesirable property necessitates the development of new visible light photocatalysts to extend the absorption wavelength range into the visible light region. As such, among the transition metal oxide groups, the tungsten oxide family has shown to be an appropriate semiconductor with many applications in sensors [8] and photocatalytic devices [9]. From this group, tungsten trioxide (WO3 ) with different nanostructures is considered as an active catalytic material with a small band gap in the range of 2.4–2.8 eV. This alloy can be used as a visible-light-driven photocatalysis due to its strong absorption of the solar spectrum as well as its stable

∗ Corresponding author at: Institute for Nanoscience and Nanotechnology, Sharif University of Technology, P.O. Box 14588-89694, Tehran, Iran. Tel.: +98 21 66164566; fax: +98 21 66022711. E-mail address: [email protected] (O. Akhavan). http://dx.doi.org/10.1016/j.apsusc.2015.01.217 0169-4332/© 2015 Elsevier B.V. All rights reserved.

physico-chemical properties [9–11]. In addition to these findings, a recent study by the authors has newly found that tungsten oxide possesses an interesting biophotocatalytic property [10]. With this feature and the non-hazardousness of tungsten oxide [11], the compound can have a great potential for applications in nano-bio-technology with a strong photocatalytic tracking characteristic. In order to investigate the properties of tungsten oxide at an extremely small scale, many synthetic methodologies have been utilized to grow 1D and 2D nano-scaled structures of tungsten oxides [12]. For instance, for growing 1D nanostructure of tungsten oxides, vapor–liquid–solid (VLS) and vapor–solid (VS) methods have been proposed [13,14]. Most of these growth methods need high processing temperatures; nevertheless, with a proper catalyst, tungsten oxide nanostructures can be synthesized at a much lower temperature [9]. In this paper, we first proposed a simple method to synthesize tungsten oxide using potassium hydroxide as a catalyst on a tungsten foil substrate through a two-step heating process at a low temperature. Then, we reported the antibacterial property of the 1D synthesized tungsten oxide nanorods (TON) and tungsten oxide microrods (TOM). While considering the annealing temperature effect, the antibacterial activity of the prepared nanorods was evaluated by examining the photoinactivation of Escherichia coli (E. coli) bacteria in an aqueous solution.

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2. Experimental Tungsten foils with 99.95% purity and dimensions of 1 cm × 1 cm × 1 mm were cleaned with ethanol and acetone before the experiments. Then, 0.1 mL (milli-liter) of 0.7 M (5 wt%) KOH (potassium hydroxide) solution was dropped on the foil. When the solvent vaporized, tiny KOH seeds precipitated on the surface of the foil. The foil was put in a horizontal quartz boat that was placed in the uniform temperature zone of a conventional high-temperature furnace. Under the atmospheric pressure, the temperature of the furnace was raised from the room temperature up to 390 ◦ C at a ramping rate of 30 ◦ C min−1 (step 1). This temperature was maintained for 30 min, and then raised to 400, 500, 600, 700 and 800 ◦ C at a similar ramping rate (step 2). After 2 h, the furnace was gradually cooled down to room temperature. The cooled samples were rinsed with de-ionized water gently and then dried at 50 ◦ C in air for 5 min. The surface morphology of the samples was examined using field emission-scanning electron microscopy (FE-SEM, Hitachi S4160 at 30 kV). Before FE-SEM, the surfaces of the samples were coated by a gold thin film using the desktop sputtering (Nanostructured Coating Co.). X-ray diffraction (XRD) patterns of the samples were obtained using a Stoe Stadimp system equipped with a Cu-K␣ radiation source with a step size of 0.05◦ . The surface chemical composition of the films was investigated using X-ray photoelectron spectroscopy (XPS, Specs-EA 10 Plus). A concentric hemispherical analyzer was used to analyze the binding energy of the surface photoelectrons excited by an Al-K␣ X-ray source at the energy of 1486.6 eV. All binding energy values were determined by calibration of a fixed core level line of C(1s) at 285.0 eV as a reference point. Antibacterial activity of the films was investigated against E. coli bacteria (ATCC 25922, USA) by using a method called drop-test. The E. coli was selected as a model for the Gram-negative bacteria, because it is one of the most common bacteria causing many serious infections such as bacteremia, urinary tract infection and food poisoning [15]. Before any microbiological experiment, the glassware and samples were sterilized by autoclaving at temperature of 120 ◦ C and pressure of 15 lbs for a period of 15 min, as also mentioned in literatures (see, e.g., [16]). The bacteria were cultured on a nutrient agar plate at 37 ◦ C for 24 h. The cultured bacteria were

Fig. 1. SEM images of the tungsten oxide films prepared at 700 ◦ C (in step 2) (a) in the presence and (b) in the absence of KOH as a catalyst.

added to 10 mL of saline solution to obtain the bacterial concentration of ∼108 colony forming units (CFU)/mL. Then, a portion of the saline solution containing the bacteria was diluted to ∼106 CFU/mL. To carry out the antibacterial drop-test, each film was put in a sterilized Petri dish, and 100 ␮L of the diluted saline solution containing the bacteria were spread on surface of each film. The films were exposed to irradiation of a 110 mW/cm2 Hg lamp (using a cut-off filter to remove the irradiation wavelengths below 400 nm) at room

Fig. 2. SEM images of the tungsten oxide films after annealing at (a) 400, (b) 500, (c) 600, (d) 700 and (e, f) 800 ◦ C, inset figure (f) shows long nanostructures synthesized at 800 ◦ C.

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temperature. After 24 h irradiation, the bacteria were washed from the surface of each film using 5 mL of phosphate buffer solution in the sterilized Petri dish. Then, 100 ␮L of each bacterial suspension was spread on a nutrient agar plate and incubated at 37 ◦ C for 24 h. The survived bacterial colonies were monitored and counted by using an optical microscope. The final data points were obtained by averaging on the results of three separate runs.

3. Results and discussion Fig. 1 shows SEM images of the tungsten substrate heated at 700 ◦ C using the KOH catalyst and without using KOH. It is seen that, in the presence of KOH, the surface of the substrate was thoroughly covered by synthesized tungsten oxide structures (Fig. 1a). In contrast, the morphology does not show such features in the absence of KOH (Fig. 1b). This demonstrates the important role of the KOH catalyst in the formation of tungsten oxide structures. Fig. 2(a–f) are SEM images of the overall morphological state of synthesized TON/TOM after the second step of the heating stage with temperatures of 400, 500, 600, 700 and 800 ◦ C. TON annealed at 400 ◦ C showed the growth of nano-features with widths of 50–90 nm and lengths from sub-micrometer to few micrometers (Fig. 2a). By increasing the annealing temperature to 500 ◦ C, the width and the length of the nanostructures gradually increased to ∼200 nm and ∼3 ␮m, respectively (Fig. 2b). At higher temperatures, the tungsten substrate was entirely covered by the rod-like TOM with an increased average width of ∼300 nm and ∼1 ␮m for synthesized temperatures of 600 ◦ C (Fig. 2c) and 800 ◦ C (Fig. 2e–f), respectively. In fact, at the 700 ◦ C (Fig. 2d) and 800 ◦ C annealing temperatures, suitable sites were developed for growing TOM and the previously sintered TON continually converted to high density TOM with bigger diameters. In addition, by increasing the temperature, the length of the rod-like features increased and reached to ∼200 ␮m (inset Fig. 2f). The XRD results in Fig. 3(1) show the characterization of phase formation and crystalline structure of as-prepared (unheated) tungsten plate and samples with different second step heating temperatures. The XRD pattern of the as-prepared sample in Fig. 3(1)a indicates the sole existence of the tungsten peaks. This confirms the absence of the crystalline phase of potassium–tungsten oxide during step 1 of the fabrication process. For the annealing temperature of 400 ◦ C, a new peak in Fig. 3(1)b appeared which was assigned to the monoclinic WO3 phase formation with (−1 1 2) crystalline orientation [JCPDS Card No: 087-237]. The low intensity of the WO3 phase can be attributed to the formation of the low-density nanostructures on the surface of the tungsten foil at this temperature (Fig. 2a). At higher temperatures (500–800 ◦ C), the WO3 peak disappeared and was replaced by new peaks of orthorhombic W3 O8 [JCPDS Card No: 081-2265] and/or orthorhombic K2 W6 O19 [JCPDS Card No: 031-1115] (Fig. 3(1)c–f). From the high domain and width of the K2 W6 O19 peaks and their shifts to higher degrees, it is concluded that the potassium atoms diffused into the structure of the tungsten oxide nanostructures and formed orthorhombic K2 W6 O19 at the preferred orientation of (0 0 2) creating the stress in the lattice crystal. This phenomenon was initiated from the 600 ◦ C condition (Fig. 3(1)d). By using the Scherrer equation, the average crystalline size of the nanostructures for the K2 W6 O19 (0 0 2) peak was estimated as 35 ± 10 nm for the 600 ◦ C condition. By increasing the temperature to 700 and 800 ◦ C (Fig. 3(1)e and f), the K2 W6 O19 (0 0 2) peak became more intense while the peaks of tungsten and tungsten oxide phases weakened. This is attributed to the long potassium–tungsten oxide nanostructures covering the whole surface of the samples (inset Fig. 2(f)). By using the Scherrer equation for the samples heated up to 800 ◦ C, the average crystalline size of the (0 0 2) peak was increased subsequently to 45 ± 10 nm.

Fig. 3. (1): XRD pattern of the potassium–tungsten oxide films prepared at different temperatures: (a) as-prepared, (b) 400, (c) 500, (d) 600, (e) 700, and (f) 800 ◦ C; (2): XPS spectra of the tungsten oxide samples: (a) as prepared, (b) 400, (c) 600 and (d) 800 ◦ C.

For the surface chemical composition, Fig. 3(2) represents the XPS survey scans of the as-prepared sample (a) and the samples heated at 400 ◦ C (b), 600 ◦ C (c) and 800 ◦ C (d). Fig. 3(2)a shows only the tungsten peak with a trace of adsorbed carbon and oxygen and no other impurities on the surface of the tungsten foil. After the heating process (Fig. 3(2)b–d), beside the tungsten peaks (W(4f) and W(4p)), the peaks relating to potassium (K(2s) and K(2p)) appeared on the surface of the samples and the oxygen peak became more intense. The substantial shifts of the W(4f) peaks to higher binding energies indicated the transformation of the W metallic state of the surface into W5+ and/or W6+ states (inset of Fig. 3(2)). In order to investigate this transformation, Fig. 4(1)a–c show the deconvolution of the W(4f) core level of the XPS spectra in the range of 41–31 eV after the heating process. It is well known that the binding energy of the W(4f7/2 ) core levels for W0 , W5+ and W6+ chemical states are ∼31.4, 34.1 and 35.7 eV, respectively [9]. Corresponding to each W(4f7/2 ) peak, there is one W(4f5/2 ) peak with a 2.15 eV spin-orbit separation energy and 0.75 area ratio. In the current study, the two chemical states of W6+ and W5+ respectively corresponded to binding energies of 35.64 and 34.92 eV for all heat-treated samples. The full-width at half-maximum (FWHM) of all deconvoluted peaks for the samples annealed at 400, 600 and 800 ◦ C were 1.77, 1.17 and 1.67 eV, respectively. By increasing

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Fig. 4. (1) Deconvoluted W (4f); (2) deconvoluted O (1s) core level XPS spectra of the tungsten oxide samples for: (a) 400, (b) 600 and (c) 800 ◦ C.

the temperature, the binding energies of the W)4f(peaks shifted to higher levels, (Fig. 4(1)a–c), and more W6+ chemical states were consequently formed on the surface of the samples (see Table 1). This fact can be attributed to the formation of the K2 W6 O19 phase which was also confirmed by XRD results in Fig. 3(1)d–f. With the oxygen vacancies within the surface of TON synthesized at 400 ◦ C, the XRD and XPS results (Figs. Fig. 33(1)b, Fig. 44(1)a) indicated the existence of tungsten in the form of WO3 in the bulk and the oxidation number of 5 on the surface. By increasing the annealing temperature up to 800 ◦ C, a gradual improvement of the oxygen vacancy was observed with the shift of oxidation number of tungsten from 5 to 6 (Table 1 and inset Fig. 3(2)). As the annealing temperature increased, the penetrated potassium to the crystal lattice of TOM contributed to the increase of the oxidation number and the transformation of structure from WO3 to K2 W6 O19 . Fig. 4(2)a–c depict the deconvolution of O(1s) core levels of the XPS spectra of the samples heated up to 400, 600 and 800 ◦ C in the range of 537–527 eV. The peak deconvolution was performed using three Gaussian components with FWHM of 2.08, 1.56 and 1.88 eV after a Shirley background subtraction. The first component located at the binding energy of ∼530.3 eV was attributed to the oxygen atoms of strong W O bonds of tungsten oxide (WO3 ) [17]. The second component located at ∼531.0 eV was assigned to the oxygen atoms in W–O bonds of WOx compositions (other than WO3 ) or O–H bond [18–20]. For 400 ◦ C, the second peak of O(1s) assigned to the O–H state and, by increasing the temperature, this binding energy was dedicated to oxygen atoms in the W–O bond with no obvious O–H binding energy. It should be noted that this binding energy for 600 and 800 ◦ C refers to W–O bonds in K2 W6 O19 structure of TOM. For 600 ◦ C, the W–O bond, the second peak of

O(1s) characterized up to 60.6% and, for 800 ◦ C, the percentage of this bond was increased to 95.8%. The third component at ∼532.9 eV was attributed to the oxygen in the H2 O content of the surface structure [19]. In short, similar chemical property of the surface and bulk of TOMs heated at temperature > 600 ◦ C was determined to be dissimilar with the ones of TONs heated at 400 ◦ C. When the annealing temperature increased, the oxygen of the surface sourced from converting O–H to the bonding energy between W and O was subsequently increased (Table 1 and Fig. 4(2)a–c). The antibacterial activity of the synthesized tungsten oxide films was investigated against E. coli bacteria under visible light irradiation and in dark. Fig. 5 shows the results of photoinactivation of bacteria by TON/TOM after 24 h visible light irradiation. While the 400 ◦ C-synthesized sample carried only 8% of the surviving bacteria, the samples synthesized at 500, 600, 700 and 800 ◦ C had 13%, 28%, 52% and 55% surviving E. coli bacteria, respectively. These results determined that by increasing the temperature from 400 to 800 ◦ C, the antibacterial activity of films decreased significantly. It should be noted that the probable cell adhesion could not affect the results relating to the antibacterial activity of the samples. Because, further micropores (which could effectively be involved in probable cell adhesion onto the film surface) were formed by increasing the annealing temperature (see Fig. 2), while the antibacterial activity decreased by increasing the temperature. Hence, the 400 ◦ C-synthesized TON exhibited not only the best antibacterial property in the dark, but also the highest antibacterial activity under visible light irradiation up to five times stronger than those of dark environment. Since the as-prepared films (the films prepared at step 1) were effectively tungsten foils (see Fig. 3) with a smooth surface, they were used as control samples with low

Table 1 Percentage of W5+ , W6+ , H2 O, W–O/O–H, and W O for samples synthesized at different annealing temperatures in step 2. 2nd step temperature (◦ C)

W+5 (%)

W+6 (%)

H2 O (%)

W–O (WOx )/O–H %

W O (WO3 ) %

400 600 800

81.6 30.5 3.4

18.4 69.5 96.6

58 1.7 0

10.7 60.6 95.8

31.3 37.7 4.2

F. Ghasempour et al. / Applied Surface Science 338 (2015) 55–60

Fig. 5. Percentage of surviving E. coli bacteria on surface of various tungsten oxide films (as-prepared films prepared at step 1 (as a control sample with effectively tungsten phase and a smooth surface) and the films prepared at various annealing temperatures of step 2) under visible light irradiation and in dark for 24 h.

antibacterial activity under light irradiation and in dark. It should be noted that, the tungsten foils (the starting materials) also showed the antibacterial activity the same as the as-prepared samples. The antibacterial activity of a catalyst depends on the density of the photogenerated electron-hole pairs on the surface and their recombination lifetimes [17]. Considering this, the mechanism of the photoinactivation of E. coli in the presence of potassium tungsten oxide can be explained by Eqs. (1–4) [20]. When WO3 was irradiated with the visible light, photogenerated electron-hole pairs were produced (Eq. (1), hv refers to the irradiation energy). The photogenerated electrons in the conduction band transferred to the surface of WO3 and were trapped by molecular oxygen (O2 ) to produce superoxide radical anion (O2 −• ) (Eq. 2). The photogenerated holes in the valence band transfer (WO3 (h+ )) were trapped by surface hydroxyl to form hydroxyl radicals OH• (Eq. 3). Depending on the amount of superoxide radical ion or the hydroxyl radical, that degraded the E. coli bacteria, a lower percentage of surviving bacteria is obtained (Eq. 4). 2WO3 + h → WO3 (e− ) + WO3 (h+ ) −

WO3 (e ) + O2 → WO3 + O2

−•

WO3 (h+ ) + OH → WO3 + OH• O2

−•

orOH•

+ E.coli → degradedbacteria

(1) (2)

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death of E. coli bacteria [22,23]. Thus, the lower antibacterial activity of the samples with high annealing temperatures (e.g., 800 ◦ C) can be attributed to the overall morphology of TOM with a bigger diameter (∼1 ␮m). It should be noted that a previous study by authors showed that TON structures with an oxidation number less than 6 (W17 O47 ) had a distinguished photocalytic property because the decrease in Methylene Blue concentration represented the number of dangling bonds on the surface as well as the surface area of sample [20]. In the current study, the major TON fabricated at 400 ◦ C had the oxidation number 5 (Fig. 4(1)a) that determined the obvious photocalytic activity of TON in the inactivation of E. coli bacteria compared to other fabricated compounds. Regarding the potassium effect, the KOH solution was useful as a catalyst only in the formation of WO3 (rather than K2 W6 O19 ) nanostructures at low temperatures. Also, K gradually penetrated to the bulk and lead to a structure and morphology changed from TON WO3 to TOM orthorhombic K2 W6 O19 with ∼30% potassium. The higher potassium penetration and the resultant incorporation of K in the crystalline structure of tungsten oxide could significantly decrease the antibacterial activity on account of the alternation of crystalline structure and morphology. 4. Conclusions Potassium (as a catalyst) allowed the successful synthesis of tungsten oxide nanostructures (TONs) on tungsten foils at much lower temperatures (400–800 ◦ C) than the ones reported in literatures (<1400 ◦ C). The nanorods (with diameters of 50–90 nm) formed on TON showed a strong antibacterial activity against E. coli bacteria under visible light irradiation and in dark. For example, the TON could inactivate >92% of the bacteria after 24 h visible light irradiation. The stronger antibacterial activity of the samples synthesized at 400 ◦ C was assigned to the generation of photo exited electron–hole pairs and presence of more water content as well as O–H bonds on surface of the TON films. Correspondingly, the decrease in antibacterial properties of TOM (K2 W6 O19 ) formed at annealing temperatures >600 ◦ C was attributed to the significant change in the chemical property, structure and morphology. Acknowledgements The authors would like to thank Dr. R. Ghasempour for her contribution and advice during this study. The financial support from the Iranian National Science Foundation is acknowledged. O. Akhavan would like to thank the Research Council of Sharif University of Technology for supporting the work.

(3) (4)

The amount of oxygen bonds in the water content and O–H state is crucial for having an antibacterial effect. Authors have shown that more O–H state and more percentage of oxygen in H2 O component create more hydrophilic environment which lead to utilizing a high antibacterial activity [21]. According to Table 1, there is 58% oxygen, from the H2 O component, for 400 ◦ C and 1.7% and 0% for 600 and 800 ◦ C, respectively and 10.7% oxygen, from the O–H state, for 400 ◦ C. Here, according to our XPS results (Fig. 4(2)a and Table 1), only 400 ◦ C-synthesized TON possessed a high percentage of oxygen in O–H and H2 O components, and therefore, it had an excellent antibacterial property. The 400 ◦ C-synthesized TON has needle shape morphology with a diameter of 50–90 nm (Fig. 2a). When temperatures increased, tungsten oxide nanorods (TON WO3 ) changed to microrods (TOM K2 W6 O19 ) with less specific surface area. The needle type morphology of TON can damage the bacteria and lead to a leakage of cell contents and subsequent

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