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graphene sheets nanocomposites under visible light irradiation
Materials Letters 93 (2013) 349–352
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
High antibacterial activity of ultrafine TiO2/graphene sheets nanocomposites under visible light irradiation Baocheng Cao a,n, Shuai Cao a, Pengyu Dong b, Jing Gao a, Jing Wang a a b
School of Stomatology, Lanzhou University, Lanzhou 730000, PR China Department of Materials Science, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, PR China
a r t i c l e i n f o
abstract
Article history: Received 18 October 2012 Accepted 29 November 2012 Available online 7 December 2012
The composites of ultrafine TiO2 nanoparticles and graphene sheets (GSs) were prepared via a direct redox reaction. Investigation of morphological features of the composite indicated that nano-sized TiO2 particles were distributed on the surfaces of GSs. The as-prepared TiO2/GSs photocatalysts possessed extended light absorption range, which could be excitated by visible light. These visible-light active photocatalysts were utilized for degradation of Escherichia coli bacteria, and their antibacterial activity against E. coli was much higher than that of pure TiO2 nanoparticles. Overall, a material with high antibacterial activity under ambient visible light illumination was developed, which could be widely used for indoor air disinfection. & 2012 Elsevier B.V. All rights reserved.
Keywords: TiO2 Graphene Antibacterial effect Photocatalysis Visible light
1. Introduction Bacterial attacks are frequent and widespread in people’s lives. Pathogenic bacteria spread mainly through water and food; for example, Escherichia coli could cause food poisoning, inflammation and meningitis through water contamination [1]. Antibiotics are commonly used to combat bacterial infection, but there are increasing concerns about the development of antibiotic resistance caused by their abuse [2]. To kill the pathogenic bacteria effectively, TiO2-based photocatalysts have been widely investigated as a disinfection agent in the past few decades [3,4]. In particular, TiO2/graphene composites with high photocatalytic activity have been developed in recent years [5–8]. However, these researches always focus on the photocatalytic degradation of organic pollutants. The antibacterial property of TiO2/graphene composite photocatalyst has not gained much attention. It is worth mentioning that Akhavan et al. prepared the TiO2–graphene films and investigated the antibacterial property in an aqueous solution under solar light irradiation [9]. Nevertheless, to the best of our knowledge, there is no research on the antibacterial activity of TiO2/graphene composite photocatalyst under visible light irradiation. The visible-light-induced antibacterial activity offers the potential for use as a disinfectant in public areas, specifically those indoor environments without adequate air circulation and solar irradiation, such as public toilets, schools, hospitals, stations, and hotels [10].s In addition, the surfaces of
n
Corresponding author. Tel.: þ86 931 891 2772; fax: þ 86 931 891 3554. E-mail address: [email protected] (B. Cao).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.11.136
objects such as door handles and push buttons are constantly contacted by people, which need a constant disinfection process to limit pathogen spread [11]. Since these objects are exposed to artificial light sources of emitting visible light, visible-lightinduced photocatalysts are essential for disinfection. Therefore, in this letter, the composite of ultrafine TiO2 and graphene sheets (GSs) was synthesized, and visible-light-induced antibacterial activity of the TiO2/graphene system was firstly investigated to meet the demands of indoor disinfection.
2. Experimental Synthesis: Graphene oxide (GO) was prepared from natural graphite by using a modified Hummers’ method [12]. In the preparation of TiO2/GSs, 2 mL of titanium trichloride (TiCl3, 15 wt%) was added to 50 mL of HCl solution (1 mL of 37 wt% HCl). The obtained solution was mixed with GO dispersion (50 mL) of various concentrations (0.1, 0.3, 0.5 mg/mL). The resulting mixture was sonicated for 5 min and stirred at 80 1C for 3 h. The precipitate was harvested by centrifugation, and washed with deionized water for several times. Finally, the obtained product was dried at 100 1C overnight before characterization. The nominal GSs contents are 1.4, 4.2, and 7 wt% for the TiO2/GSs nanocomposites synthesized using GO dispersions with various concentrations as the precursor. For comparison purpose, pure TiO2 nanocrystals were synthesized under the same condition without adding GO. Characterization: Phase identification was carried out by Rigaku D/Max-2400 X-ray diffractometer (XRD) with Cu Ka
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B. Cao et al. / Materials Letters 93 (2013) 349–352
radiation. X-ray photoelectron spectroscopy (XPS) was conducted on a PHI-5702 spectrometer. Transmission electron microscopy (TEM), selected-area electron diffraction pattern (SAED), and energy dispersive X-ray spectra (EDX) were collected on a FEI Tecnai G2 F30 S-TWIN electron microscope. Diffuse reflectance ultraviolet–visible (UV–vis) absorption spectra were measured using a Perkin Elmer 950 spectrometer, while BaSO4 was used as a reference. Antibacterial assay: E. coli was chosen as the standard bacterium and colony counting method was used in our antibacterial experiments. In a typical experiment, a Petri dish containing approximate 12 mL LB culture medium was seeded with fresh E. coli bacterium suspension at a concentration of 106 cfu/mL. Then, 0.1 g of the as-prepared sample was added and evenly dispersed in the Petri dish, which was used as the experimental group. Correspondingly, the Petri dish without the sample was used as the control group. Subsequently, the Petri dish was incubated at 37 1C for 12 h under indoor natural light irradiation (light wavelength: 400–700 nm, light intensity: 1.2 W/cm2). After irradiation, the number of colonies was counted. The experiment was repeated 3 times to obtain mean and standard deviation.
3. Results and discussion XRD and XPS analysis: Fig. 1a shows the XRD patterns of asprepared pure TiO2 and TiO2/GSs composite samples. The broad diffraction peaks of each sample can be indexed to the phase of anatase TiO2 (JCPDS no. 04-0477). For TiO2/GSs composite samples, no diffraction peaks of graphene can be seen, indicating the absence of layer-stacking regularity or the relatively low content of graphene in the composite. [13] The grain sizes are calculated from the broad anatase (101) plane peak using Scherrer’s equation, which are 5.3, 6.1, 7.2, and 5.2 nm for pure TiO2, TiO2/ 1.4 wt% GSs, TiO2/4.2 wt% GSs, and TiO2/7 wt% GSs, respectively. From Fig. 1b, it is clear that the intensity of the peak assigned to C–O bond markedly decreased in the composite sample compared with that of GO, indicating GO was effectively reduced to graphene by the reactive cations Ti3 þ [8]. In addition, the band located at 283.8 eV in the composite sample can be assigned to the Ti–C bond [9]. This result indicates that close interfacial contacts between TiO2 and GSs are obtained, which could be beneficial for the effective charge separation and transfer. The formation process of Ti–C bond can be explained as follows: the surface of GO is covered with abundant oxygen-containing groups, such as epoxy, hydroxyl, and carboxylic acid [14], which shows the negatively charged character in aqueous solution and is favorable for interacting with Ti3 þ [8]. During the synthesis
process, reactive Ti3 þ cations with a strong reduction ability gradually reduced the oxygen-containing groups of GO. Simultaneously, the oxidation of Ti3 þ ions resulted in the formation of TiO2 nanoparticles on the GSs. Thus, the chemical Ti–C bond was formed during the synthesis process. TEM analysis: From TEM image of pure TiO2 (Fig. 2a), agglomerations of ultrafine nanoparticles are observed. This is because ultrafine nanoparticles have relatively large surface areas, thus they tend to agglomerate to minimize the total surface energy. The particle sizes of pure TiO2 are in the range of 3–6 nm, which is consistent with the calculated value (5.3 nm) obtained from XRD pattern. Fig. 2b displays the TEM image of TiO2/4.2 wt% GSs. It can be seen that GSs are covered with spherical anatase TiO2 nanoparticles, and the TiO2 nanoparticle sizes are in the range of 6–12 nm, which is in agreement with the grain size value of 7.2 nm estimated by the Scherrer equation. The concentric ring of SAED pattern (Fig. 2c) further confirms the formation of anatase TiO2 phase. Compared with the EDX pattern of pure TiO2 (Fig. S1 in Electronic Supplementary Data), the intensity of C peak in composite sample (Fig. 2d) is enhanced significantly. This result indicates the intense C peak should be attributed to the existence of GSs in composite sample instead of organic compound/film on the TEM grid, further confirming the formation of TiO2/GSs composite. Optical absorption property and antibacterial activity: From Fig. 3a, it is obvious that the composite samples show red-shifts of absorption edges and increased absorption intensities compared to pure TiO2. The red-shifts of absorption edges should be attributed to the chemical Ti–C bonds between nanocrystalline TiO2 and GSs [5], as shown in the XPS results (Fig. 1 b). Moreover, this result clearly indicates that the composite samples could utilize visible light for photocatalytic antibacterial application. The visible-light-induced photocatalytic antibacterial activity of as-prepared samples is shown in Fig. 3b. The cell viability of E. coli on bare Petri dish (control) after 12 h illumination is set as 100%. Pure TiO2 nanoparticles exhibit lower bactericidal activity compared to that of TiO2/GSs nanocomposite samples since it cannot absorb visible light resulting from its larger band gap. The TiO2/ 4.2 wt% GSs sample shows the lowest E. coli viability at 9.5%, which means the best antibacterial activity. The excellent antibacterial activity of TiO2/GSs nanocomposites under ambient visible light illumination could be attributed to the extended light absorption range (as shown in Fig. 3a) and efficient separation of photogenerated electron–hole pairs since graphene can be used as an electron acceptor and transporter [5]. This suggests that more reactive species such as O2 and OH involved in the antibacterial activity. In addition, the antibacterial activity decreases when the graphene content exceeds 4.2 wt% in the
Fig. 1. (a) XRD patterns of the prepared samples and (b) C 1s XPS spectra of GO and TiO2/1.4 wt% GSs.
B. Cao et al. / Materials Letters 93 (2013) 349–352
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Fig. 2. TEM images of pure TiO2 (a) and TiO2/4.2 wt% GSs (b); SAED (c) and EDX and (d) of TiO2/4.2 wt% GSs.
Fig. 3. (a) UV–vis absorption spectra of as-prepared samples and (b) viability of E. coli treated on different samples after 12 h of ambient visible light illumination. The insets are the corresponding photographs of bactericidal effects.
composite. This result can be explained as follows: a large number of graphene sheets exist in the sample of TiO2/7% GSs, then the active TiO2 might be covered by other graphene sheets [7], which could lead to a shield of the active sites of TiO2 during the antibacterial process. Thus, the sample of TiO2/7% GSs possesses lower activity than TiO2/4.2% GSs. In addition, the viability of E. coli for TiO2/4.2% GSs without light irradiation is nearly 100% (the corresponding photograph of bactericidal effect is shown in Fig. S2 in Supplementary Data), indicating there is no antibacterial property for composite samples without light irradiation.
4. Conclusion Chemically bonded ultrafine TiO2/GSs nanocomposites have been successfully produced. These composites possessed extended photoresponding range and enhanced absorption intensity in the visible light region. With optimization of the content of GSs in the composite, the TiO2/4.2 wt% GSs sample exhibits the best photocatalytic antibacterial activity under ambient visible light illumination, which could find promising application in the field of indoor air disinfection.
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B. Cao et al. / Materials Letters 93 (2013) 349–352
Acknowledgments This work is financially supported by the Science and Technology Planning Project of Lanzhou in Gansu Province (Grant no. 2009-1-70).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2012.11.136.
References [1] Kang Q, Lu QZ, Liu SH, Yang LX, Wen LF, Luo SL, et al. 2010;31:3317–26.
Biomaterials
[2] He Z, Xu Q, Yang Tan TT. Nanoscale 2011;3:4977–83. [3] Benabbou A, Derriche Z, Felix C, Lejeune P, Guillard C. Appl Catal B 2007;76:257–63. ¨ [4] Sayılkan F, Asilturk M, Kiraz N, Burunkaya E, Arpac- E, Sayılkan H. J Hazard Mater 2009;162:1309–16. [5] Zhang H, Lv X, Li Y, Wang Y, Li J. ACS Nano 2009;4:380–6. [6] Wang Y, Shi R, Lin J, Zhu Y. Appl Catal B 2010;100:179–83. [7] Dong P, Wang Y, Guo L, Liu B, Xin S, Zhang J, et al. Nanoscale 2012 4641–9. [8] Zhang J, Xiong Z, Zhao XS. J Mater Chem 2011;21:3634–40. [9] Akhavan O, Ghaderi E. J Phys Chem C 2009;113:20214–20. [10] Wong MS, Chu WC, Sun DS, Huang HS, Chen JH, Tsai PJ, et al. Appl Environ Microbiol 2006;72:6111–6. [11] Booth TF, Kournikakis B, Bastien N, Ho J, Kobasa D, Stadnyk L, et al. J Infect Dis 2005;191:1472–7. [12] Marcano DC, Kosynkin DV, Berlin JM, Sinitskii A, Sun Z, Slesarev A, et al. ACS Nano 2010;4:4806–14. [13] Jiang G, Lin Z, Chen C, Zhu L, Chang Q, Wang N, et al. Carbon 2011;49:2693–701. [14] Gao W, Alemany LB, Ci L, Ajayan PM. NatChem 2009;1:403–8.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
High antibacterial activity of ultrafine TiO2/graphene sheets nanocomposites under visible light irradiation Baocheng Cao a,n, Shuai Cao a, Pengyu Dong b, Jing Gao a, Jing Wang a a b
School of Stomatology, Lanzhou University, Lanzhou 730000, PR China Department of Materials Science, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, PR China
a r t i c l e i n f o
abstract
Article history: Received 18 October 2012 Accepted 29 November 2012 Available online 7 December 2012
The composites of ultrafine TiO2 nanoparticles and graphene sheets (GSs) were prepared via a direct redox reaction. Investigation of morphological features of the composite indicated that nano-sized TiO2 particles were distributed on the surfaces of GSs. The as-prepared TiO2/GSs photocatalysts possessed extended light absorption range, which could be excitated by visible light. These visible-light active photocatalysts were utilized for degradation of Escherichia coli bacteria, and their antibacterial activity against E. coli was much higher than that of pure TiO2 nanoparticles. Overall, a material with high antibacterial activity under ambient visible light illumination was developed, which could be widely used for indoor air disinfection. & 2012 Elsevier B.V. All rights reserved.
Keywords: TiO2 Graphene Antibacterial effect Photocatalysis Visible light
1. Introduction Bacterial attacks are frequent and widespread in people’s lives. Pathogenic bacteria spread mainly through water and food; for example, Escherichia coli could cause food poisoning, inflammation and meningitis through water contamination [1]. Antibiotics are commonly used to combat bacterial infection, but there are increasing concerns about the development of antibiotic resistance caused by their abuse [2]. To kill the pathogenic bacteria effectively, TiO2-based photocatalysts have been widely investigated as a disinfection agent in the past few decades [3,4]. In particular, TiO2/graphene composites with high photocatalytic activity have been developed in recent years [5–8]. However, these researches always focus on the photocatalytic degradation of organic pollutants. The antibacterial property of TiO2/graphene composite photocatalyst has not gained much attention. It is worth mentioning that Akhavan et al. prepared the TiO2–graphene films and investigated the antibacterial property in an aqueous solution under solar light irradiation [9]. Nevertheless, to the best of our knowledge, there is no research on the antibacterial activity of TiO2/graphene composite photocatalyst under visible light irradiation. The visible-light-induced antibacterial activity offers the potential for use as a disinfectant in public areas, specifically those indoor environments without adequate air circulation and solar irradiation, such as public toilets, schools, hospitals, stations, and hotels [10].s In addition, the surfaces of
n
Corresponding author. Tel.: þ86 931 891 2772; fax: þ 86 931 891 3554. E-mail address: [email protected] (B. Cao).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.11.136
objects such as door handles and push buttons are constantly contacted by people, which need a constant disinfection process to limit pathogen spread [11]. Since these objects are exposed to artificial light sources of emitting visible light, visible-lightinduced photocatalysts are essential for disinfection. Therefore, in this letter, the composite of ultrafine TiO2 and graphene sheets (GSs) was synthesized, and visible-light-induced antibacterial activity of the TiO2/graphene system was firstly investigated to meet the demands of indoor disinfection.
2. Experimental Synthesis: Graphene oxide (GO) was prepared from natural graphite by using a modified Hummers’ method [12]. In the preparation of TiO2/GSs, 2 mL of titanium trichloride (TiCl3, 15 wt%) was added to 50 mL of HCl solution (1 mL of 37 wt% HCl). The obtained solution was mixed with GO dispersion (50 mL) of various concentrations (0.1, 0.3, 0.5 mg/mL). The resulting mixture was sonicated for 5 min and stirred at 80 1C for 3 h. The precipitate was harvested by centrifugation, and washed with deionized water for several times. Finally, the obtained product was dried at 100 1C overnight before characterization. The nominal GSs contents are 1.4, 4.2, and 7 wt% for the TiO2/GSs nanocomposites synthesized using GO dispersions with various concentrations as the precursor. For comparison purpose, pure TiO2 nanocrystals were synthesized under the same condition without adding GO. Characterization: Phase identification was carried out by Rigaku D/Max-2400 X-ray diffractometer (XRD) with Cu Ka
350
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radiation. X-ray photoelectron spectroscopy (XPS) was conducted on a PHI-5702 spectrometer. Transmission electron microscopy (TEM), selected-area electron diffraction pattern (SAED), and energy dispersive X-ray spectra (EDX) were collected on a FEI Tecnai G2 F30 S-TWIN electron microscope. Diffuse reflectance ultraviolet–visible (UV–vis) absorption spectra were measured using a Perkin Elmer 950 spectrometer, while BaSO4 was used as a reference. Antibacterial assay: E. coli was chosen as the standard bacterium and colony counting method was used in our antibacterial experiments. In a typical experiment, a Petri dish containing approximate 12 mL LB culture medium was seeded with fresh E. coli bacterium suspension at a concentration of 106 cfu/mL. Then, 0.1 g of the as-prepared sample was added and evenly dispersed in the Petri dish, which was used as the experimental group. Correspondingly, the Petri dish without the sample was used as the control group. Subsequently, the Petri dish was incubated at 37 1C for 12 h under indoor natural light irradiation (light wavelength: 400–700 nm, light intensity: 1.2 W/cm2). After irradiation, the number of colonies was counted. The experiment was repeated 3 times to obtain mean and standard deviation.
3. Results and discussion XRD and XPS analysis: Fig. 1a shows the XRD patterns of asprepared pure TiO2 and TiO2/GSs composite samples. The broad diffraction peaks of each sample can be indexed to the phase of anatase TiO2 (JCPDS no. 04-0477). For TiO2/GSs composite samples, no diffraction peaks of graphene can be seen, indicating the absence of layer-stacking regularity or the relatively low content of graphene in the composite. [13] The grain sizes are calculated from the broad anatase (101) plane peak using Scherrer’s equation, which are 5.3, 6.1, 7.2, and 5.2 nm for pure TiO2, TiO2/ 1.4 wt% GSs, TiO2/4.2 wt% GSs, and TiO2/7 wt% GSs, respectively. From Fig. 1b, it is clear that the intensity of the peak assigned to C–O bond markedly decreased in the composite sample compared with that of GO, indicating GO was effectively reduced to graphene by the reactive cations Ti3 þ [8]. In addition, the band located at 283.8 eV in the composite sample can be assigned to the Ti–C bond [9]. This result indicates that close interfacial contacts between TiO2 and GSs are obtained, which could be beneficial for the effective charge separation and transfer. The formation process of Ti–C bond can be explained as follows: the surface of GO is covered with abundant oxygen-containing groups, such as epoxy, hydroxyl, and carboxylic acid [14], which shows the negatively charged character in aqueous solution and is favorable for interacting with Ti3 þ [8]. During the synthesis
process, reactive Ti3 þ cations with a strong reduction ability gradually reduced the oxygen-containing groups of GO. Simultaneously, the oxidation of Ti3 þ ions resulted in the formation of TiO2 nanoparticles on the GSs. Thus, the chemical Ti–C bond was formed during the synthesis process. TEM analysis: From TEM image of pure TiO2 (Fig. 2a), agglomerations of ultrafine nanoparticles are observed. This is because ultrafine nanoparticles have relatively large surface areas, thus they tend to agglomerate to minimize the total surface energy. The particle sizes of pure TiO2 are in the range of 3–6 nm, which is consistent with the calculated value (5.3 nm) obtained from XRD pattern. Fig. 2b displays the TEM image of TiO2/4.2 wt% GSs. It can be seen that GSs are covered with spherical anatase TiO2 nanoparticles, and the TiO2 nanoparticle sizes are in the range of 6–12 nm, which is in agreement with the grain size value of 7.2 nm estimated by the Scherrer equation. The concentric ring of SAED pattern (Fig. 2c) further confirms the formation of anatase TiO2 phase. Compared with the EDX pattern of pure TiO2 (Fig. S1 in Electronic Supplementary Data), the intensity of C peak in composite sample (Fig. 2d) is enhanced significantly. This result indicates the intense C peak should be attributed to the existence of GSs in composite sample instead of organic compound/film on the TEM grid, further confirming the formation of TiO2/GSs composite. Optical absorption property and antibacterial activity: From Fig. 3a, it is obvious that the composite samples show red-shifts of absorption edges and increased absorption intensities compared to pure TiO2. The red-shifts of absorption edges should be attributed to the chemical Ti–C bonds between nanocrystalline TiO2 and GSs [5], as shown in the XPS results (Fig. 1 b). Moreover, this result clearly indicates that the composite samples could utilize visible light for photocatalytic antibacterial application. The visible-light-induced photocatalytic antibacterial activity of as-prepared samples is shown in Fig. 3b. The cell viability of E. coli on bare Petri dish (control) after 12 h illumination is set as 100%. Pure TiO2 nanoparticles exhibit lower bactericidal activity compared to that of TiO2/GSs nanocomposite samples since it cannot absorb visible light resulting from its larger band gap. The TiO2/ 4.2 wt% GSs sample shows the lowest E. coli viability at 9.5%, which means the best antibacterial activity. The excellent antibacterial activity of TiO2/GSs nanocomposites under ambient visible light illumination could be attributed to the extended light absorption range (as shown in Fig. 3a) and efficient separation of photogenerated electron–hole pairs since graphene can be used as an electron acceptor and transporter [5]. This suggests that more reactive species such as O2 and OH involved in the antibacterial activity. In addition, the antibacterial activity decreases when the graphene content exceeds 4.2 wt% in the
Fig. 1. (a) XRD patterns of the prepared samples and (b) C 1s XPS spectra of GO and TiO2/1.4 wt% GSs.
B. Cao et al. / Materials Letters 93 (2013) 349–352
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Fig. 2. TEM images of pure TiO2 (a) and TiO2/4.2 wt% GSs (b); SAED (c) and EDX and (d) of TiO2/4.2 wt% GSs.
Fig. 3. (a) UV–vis absorption spectra of as-prepared samples and (b) viability of E. coli treated on different samples after 12 h of ambient visible light illumination. The insets are the corresponding photographs of bactericidal effects.
composite. This result can be explained as follows: a large number of graphene sheets exist in the sample of TiO2/7% GSs, then the active TiO2 might be covered by other graphene sheets [7], which could lead to a shield of the active sites of TiO2 during the antibacterial process. Thus, the sample of TiO2/7% GSs possesses lower activity than TiO2/4.2% GSs. In addition, the viability of E. coli for TiO2/4.2% GSs without light irradiation is nearly 100% (the corresponding photograph of bactericidal effect is shown in Fig. S2 in Supplementary Data), indicating there is no antibacterial property for composite samples without light irradiation.
4. Conclusion Chemically bonded ultrafine TiO2/GSs nanocomposites have been successfully produced. These composites possessed extended photoresponding range and enhanced absorption intensity in the visible light region. With optimization of the content of GSs in the composite, the TiO2/4.2 wt% GSs sample exhibits the best photocatalytic antibacterial activity under ambient visible light illumination, which could find promising application in the field of indoor air disinfection.
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B. Cao et al. / Materials Letters 93 (2013) 349–352
Acknowledgments This work is financially supported by the Science and Technology Planning Project of Lanzhou in Gansu Province (Grant no. 2009-1-70).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2012.11.136.
References [1] Kang Q, Lu QZ, Liu SH, Yang LX, Wen LF, Luo SL, et al. 2010;31:3317–26.
Biomaterials
[2] He Z, Xu Q, Yang Tan TT. Nanoscale 2011;3:4977–83. [3] Benabbou A, Derriche Z, Felix C, Lejeune P, Guillard C. Appl Catal B 2007;76:257–63. ¨ [4] Sayılkan F, Asilturk M, Kiraz N, Burunkaya E, Arpac- E, Sayılkan H. J Hazard Mater 2009;162:1309–16. [5] Zhang H, Lv X, Li Y, Wang Y, Li J. ACS Nano 2009;4:380–6. [6] Wang Y, Shi R, Lin J, Zhu Y. Appl Catal B 2010;100:179–83. [7] Dong P, Wang Y, Guo L, Liu B, Xin S, Zhang J, et al. Nanoscale 2012 4641–9. [8] Zhang J, Xiong Z, Zhao XS. J Mater Chem 2011;21:3634–40. [9] Akhavan O, Ghaderi E. J Phys Chem C 2009;113:20214–20. [10] Wong MS, Chu WC, Sun DS, Huang HS, Chen JH, Tsai PJ, et al. Appl Environ Microbiol 2006;72:6111–6. [11] Booth TF, Kournikakis B, Bastien N, Ho J, Kobasa D, Stadnyk L, et al. J Infect Dis 2005;191:1472–7. [12] Marcano DC, Kosynkin DV, Berlin JM, Sinitskii A, Sun Z, Slesarev A, et al. ACS Nano 2010;4:4806–14. [13] Jiang G, Lin Z, Chen C, Zhu L, Chang Q, Wang N, et al. Carbon 2011;49:2693–701. [14] Gao W, Alemany LB, Ci L, Ajayan PM. NatChem 2009;1:403–8.