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Synthesis and characterization of ZnO and Ag nanoparticle-loaded TiO2 nanotube composite layers intended for antibacterial coatings
TSF-32894; No of Pages 6 Thin Solid Films xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Synthesis and characterization of ZnO and Ag nanoparticle-loaded TiO2 nanotube composite layers intended for antibacterial coatings A. Roguska a,⁎, M. Pisarek a, M. Andrzejczuk b, M. Lewandowska b a b
Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland Faculty of Material Science and Engineering, Warsaw University and Technology, Wołoska 141, 02-507 Warsaw, Poland
a r t i c l e
i n f o
Available online xxxx Keywords: Titania nanotubes Ag nanoparticles ZnO nanostructures Magnetron sputtering Electrodeposition
a b s t r a c t In this work we have fabricated nanoporous oxide layers on Ti with the addition of Ag or ZnO nanoparticles in order to obtain functional coatings ensuring both biocompatibility and antibacterial properties. The morphological and chemical features of the composite coatings fabricated were studied with the aid of high-resolution scanning electron microscopy and surface analytical techniques such as Auger electron spectroscopy and X-ray photoelectron spectroscopy. Our results have shown that Ag and ZnO nanoparticles can be incorporated in a simple and economic manner, suitable for the fabrication of a bactericidal material. The amount of the nanoparticles is variable and depends on the deposition process conditions. The nanoparticles are distributed homogeneously at the top surface of the coating, however they exhibit in depth distribution gradient. This may be promising for maintaining a steady antibacterial effect. © 2013 Elsevier B.V. All rights reserved.
1. Introduction TiO2 nanotubes (NT) offer encouraging implications for the development and optimization of biomedical-related treatments, with precise control over desired cell growth and functionality [1–5]. It was shown that different tube diameters influence mesenchymal stem cell fate by having different effects on cytoskeletal stresses which in turn modulate differentiation into different types of cells [1,2]. The interior of TiO2 nanotubes can be filled with some bioactive species or antibacterial agents, e.g. antibiotics, thereby providing an implant material suitable for local antibiotic therapy (at site of an implantation) [3–5]. The precise control of the nanotube length and diameter (e.g. by the end voltage of the anodic polarization) enables to load different amounts of drug and control the release rate. Extensive effort has been made recently to develop antimicrobial materials containing various organic antibiotics and inorganic substances. Among these fine functional materials nanosized inorganic structures have attracted increased attention in medical applications due to their unique properties and adaptability to biological functionalization [6,7]. Titanium dioxide (TiO2) is now one of the most promising candidates for an antibacterial coating, mostly because of its photocatalytic bactericidal activity and long-term chemical stability [8]. The bactericidal activity of TiO2 is directly related to the ultraviolet light absorption and the formation of various reactive species such as superoxide and hydroxyl radicals [9,10]. Thus, TiO2 itself and TiO2 deposited
⁎ Corresponding author. Tel.: +48 22 343 2078. E-mail address: [email protected] (A. Roguska).
materials have a potential to kill bacteria and also simultaneously degrade the toxic compounds exhausted from the bacteria [11]. However, the TiO2 has inherent drawbacks as an efficient biocidal agent due to the large bandgap energy and fast recombination rate of photogenerated electron–hole pairs [12]. In addition, the fact that the photocatalytic decomposition processes on TiO2 can usually be induced only by ultraviolet light hinders its practical application [13]. In order to improve the photocatalytic performance of TiO2 various approaches have been developed, e.g. it was reported that loading of silver nanoparticles may enhance the overall photocatalytic efficiency of TiO2 [14–16]. Noble metal doping can extend the range of light absorption to the visible spectrum due to the decrease in the bandgap energy of TiO2. Furthermore, the difference in the conduction band edges between TiO2 and the coupled metal (Pt, Pd or Ag) offers pathways to the photogenerated electrons and holes, causing increment in a recombination time. Silver is well described in literature and – also as a modifying agent deposited on medical devices – exhibits good antibacterial properties with possible applications in treatment of burns, severe chronic osteomyelitis and infections of urinary tract and central venous catheters [17,18]. Recent literature data evidenced that silver nanoparticles are more active and reactive than the bulk metallic counterpart, first of all because of their larger specific surface area [19]. Furthermore, it was reported that silver particles with a preferential diameter of about 1–10 nm had direct interactions with bacteria, while larger did not [20]. In the view of this, a system combining both titania and silver nanoparticles is thought to expand the antibacterial function of TiO2 to a wider variety of working conditions. Recently, antibacterial activities associated with ZnO nanoparticles has been also investigated [12,13,21,22]. For example, it was shown
0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.11.057
Please cite this article as: A. Roguska, et al., Synthesis and characterization of ZnO and Ag nanoparticle-loaded TiO2 nanotube composite layers intended for antibacterial coatings, Thin Solid Films (2013), http://dx.doi.org/10.1016/j.tsf.2013.11.057
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A. Roguska et al. / Thin Solid Films xxx (2013) xxx–xxx
The titanium oxide layers were fabricated by the electrochemical anodization of Ti samples (Ti foil, 0.25 mm-thick, 99.5% purity) in an optimized electrolyte: NH4F (0.86 wt.%) + deionized (DI) water (47.14 wt.%) + glycerol (52 wt.%) under a constant voltage of 30 V. After anodization, the samples were rinsed with DI water and dried in air. Subsequently, thermal annealing was performed at 450 °C for 3 h to transform the TiO2 nanotube structure from amorphous (after anodic oxidation) to crystalline (anatase) and to obtain mechanically stable nanotubes well integrated with Ti substrate.
Silver (2.5 and 10 nm thick layer) was deposited using the sputter deposition technique (magnetron sputtering) in a vacuum chamber (Leica EM MED020) with a base pressure of about 2 × 10−5 Pa. The thickness values are nominal values, as measured on the surface of a quartz crystal micro balance. Due to the highly developed specific surface area of the titanium oxide substrates and a very low amount of a metal deposited, silver does not form a continuous layer but an inhomogeneous film with islands (nanoparticles). ZnO was electrodeposited from 0.05 M Zn(NO3)2 aqueous solution at a potential of −1.1 V vs SCE at 60 °C. The duration of the deposition was 3 or 5 min. After the deposition the samples were rinsed with DI water and annealed at 400 °C for 1 h. For the morphological characterization of the anodized, annealed and nanoparticle-loaded samples, examinations were carried out with a scanning electron microscope (SEM, Hitachi S-5500) at the accelerating voltages of 5 kV. Chemical composition of the oxide layers before and after silver or zinc oxide deposition processes was analyzed using the Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS), Microlab 350 (Thermo Electron), with a lateral resolution of about 20 nm for AES and several mm for XPS. The chemical state of surface species was identified using the high-energy resolution spherical sector analyzer of the Auger and XPS spectrometer (the maximum energy resolution is 0.83 eV for XPS method). The appropriate standards for AES and XPS reference spectra were also used. XPS spectra were excited using AlKα (hν = 1486.6 eV) radiation as a source. Survey spectra and high resolution spectra were recorded using 150 and 40 eV pass energy. A linear or Shirley background subtraction was made to
Fig. 1. SEM images of the Ag/TiO2 NT composite layer: (a) top and (b) cross section view.
Fig. 2. SEM images of the ZnO/TiO2 NT composite layer: (a) top and (b) cross section view.
that the electrospun ZnO/TiO2 composite nanofibers exhibited superior antibacterial activity without light irritation [12]. In other study it was revealed that coupled semiconductor photocatalyst of TiO2/ZnO enhanced photodegradation efficiency of TiO2 by increasing the recombination time of photogenerated electron-holes pairs [13]. In addition, silver modification is found to be effective for the fabrication of p-type ZnO, as the naturally occurring ZnO displays n-type conductivity due to its native defects such as zinc interstitials and oxygen vacancies [23,24]. In this work, we present a simple fabrication method of nanotubular oxide layers on Ti loaded with Ag and/or ZnO nanoparticles. As such composite coatings are expected to provide both biocompatibility and antibacterial properties, they are discussed here in terms of possible disinfection-related applications.
2. Methods
Please cite this article as: A. Roguska, et al., Synthesis and characterization of ZnO and Ag nanoparticle-loaded TiO2 nanotube composite layers intended for antibacterial coatings, Thin Solid Films (2013), http://dx.doi.org/10.1016/j.tsf.2013.11.057
A. Roguska et al. / Thin Solid Films xxx (2013) xxx–xxx
obtain XPS signal intensity. The peaks were fitted using an asymmetric Gaussian/Lorentzian mixed function. The measured binding energies were corrected referring to energy of C1s at 285 eV. An Avantage based data system software (Version 5.41) was used for data processing. 3. Results and discussion The optimized anodization conditions applied resulted in the formation of TiO2 nanotubes (“hollow cylinders”) arranged perpendicular to the substrate. SEM examinations (data not shown) revealed that the nanotubes are separated from each other, open at the top and closed from the bottom. The average diameter of the tubes obtained under anodizing potential of 30 V is about 120 nm. The as-grown TiO2 porous anodic layers exhibit poor adhesion to the Ti substrate. Good adhesion and mechanical stability of TiO2 nanotubes are crucial for their suitability as an antibacterial coating in biomedical applications. In order to improve these parameters, the samples were annealed in air at 450 °C for 3 h. Annealing of TiO2 nanotubes at 450 °C leads to their crystallization to anatase phase. More details are given in [5,25]. Fig. 1a and b shows typical SEM images of TiO2 nanotubular layers covered with 10 nm of Ag. Detailed inspection of Fig. 1a reveals that silver forms a thin solid coating on the top edges of the nanotubes, which is composed of Ag nanoparticles. Some single spherical Ag particles can be found on the inner and outer side walls of the tubes (Fig. 1b). The diameter of these particles is below 50 nm. A careful analysis of the above SEM data suggests that the deposited Ag nanoparticles exhibit in depth distribution gradient. The highest amount of Ag is located at the nanotube top edges and decreases along the side wall of the nanotube to reach the lowest value at the bottom of the tube. TiO2 nanotubes loaded with ZnO exhibit quite different morphology. Fig. 2 shows SEM images of ZnO deposited nanotube layers on Ti. Electrodeposition of ZnO for 3 min leads to the formation of elongated and pointed particles, similar in shape to a rice bean (Fig. 2a). The length
3
of these particles is below 100 nm. They are located at the nanotube top edges as well as on the nanotube inner and outer side walls. Some smaller spherical particles are present along the whole nanotube (Fig. 2b). Increasing the electrodeposition time leads to the formation of ZnO “nanoflowers” and complete coverage of the TiO2 nanotube surface. In case of ZnO electrodeposition the in depth deposition gradient is much lower than that observed for Ag loaded composite layers. TiO2 nanotubes are loaded with ZnO nanoparticles on the whole length. This may be due to the nature of the deposition processes. Ag was deposited on the titania nanotube layer by the magnetron sputtering technique using a device, where a Ag sputtering target is located directly above the samples (in a parallel configuration), so only the top surface may be covered by the Ag deposit. During the ZnO electrodeposition process the samples are immersed in the electrolyte which penetrates the nanotubes due to the capillary action and, as a result, a higher number of nucleation sites are available. Both Ag and ZnO nanoparticles are reported to exhibit an antibacterial activity [21]. In addition, ZnO was found to be good match for Ag in order to obtain enhanced and synergistic antibacterial activity for both Gram positive and Gram negative bacteria [23]. Bearing the above in mind we made an attempt to fabricate a composite coating containing both ZnO and Ag nanoparticles. Fig. 3 shows the ZnO electrodeposited on the titania nanotube layer previously loaded with 5 nm Ag layer. At this amount deposited silver forms single spherical nanoparticles with diameter below 25 nm. The presence of single Ag nanoparticles on the titania nanotubular layer resulted in the higher number of nucleation sites for ZnO (when compared to pure TiO2 nanotubes). This in turn has influenced the size and shape of the electrodeposited ZnO particles. They are mostly spherical with diameter below 50 nm, which is two times less than for the only ZnO loaded TiO2 nanotubes. This is in agreement with literature data [24], where the crystallite size of silver doped zinc oxide nanoparticles was seen to decrease with increased silver
Fig. 3. SEM images of the ZnOAg/TiO2 NT composite layer: (a) top and (c) cross section view; (b) and (d) corresponding BSE images. The white arrows correspond to Ag nanoparticles.
Please cite this article as: A. Roguska, et al., Synthesis and characterization of ZnO and Ag nanoparticle-loaded TiO2 nanotube composite layers intended for antibacterial coatings, Thin Solid Films (2013), http://dx.doi.org/10.1016/j.tsf.2013.11.057
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content. Fig. 3b and d shows the distribution of Ag and ZnO nanoparticles on the top surface and the side walls of TiO2 nanotubes, respectively (the backscattered electron images, BSE). The lighter areas are attributed to the Ag deposit (white arrows). Both Ag and ZnO nanoparticles are distributed homogeneously at the top surface of the coating. It seems also that the nucleation of the ZnO took place in the areas where the Ag nanoparticles are present. The in depth distribution gradient is more evident here than for ZnO loaded TiO2 nanotubes (without predeposited Ag), compare Figs. 2b and 3c. The AES investigations of Ag sputter deposited TiO2 nanotubes confirmed the presence of Ag, Ti and O in the as-obtained composite layer. Fig. 4a shows Auger survey spectra measured locally at Ag/TiO2 NT surface. Fig. 4c shows typical AES spectrum of silver measured at a resolution of 0.2 eV on Ag loaded TiO2 nanotube surface, together with the appropriate silver reference spectrum. There is no distinct shift for the Ag MNN signal at the composite layer surface as compared to that for the Ag reference. This suggests that the Ag appears in the composite layer as metal silver. The AES investigations of ZnO electrodeposited TiO2 nanotubes confirmed the presence of Zn, O and Ti in the as-obtained composite layer (Fig. 4b). The contamination with nitrogen (possible contamination from the electrolyte) was not observed. Fig. 4d shows typical AES spectrum of zinc measured at a resolution of 0.2 eV on ZnO loaded TiO2 nanotube surface, together with the appropriate zinc reference
spectrum. There is a distinct shift to lower energies of LMM Auger electron emission of zinc for the ZnO/TiO2NT composite layer. Similar shift was observed for Zn LMM spectrum measured for ZnO/Ag/TiO2NT. The observed shift is due to the fact that Zn (metal) is a good conductor element while ZnO, as a semiconductor, has lower amount of electron density, consequently there is a change in Auger electron energy. The chemical states of the elements in the Ag and/or ZnO loaded TiO2 nanotube composite sample layer were analyzed by X-ray photoelectron spectroscopy (XPS). The survey spectra (Fig. 5a) clearly indicate three major sets of signals from Ti2p, O1s and Ag3d for Ag/TiO2 NT sample, Ti2p, O1s and Zn2p for ZnO/TiO2 NT sample and Ti2p, O1s, Ag3d and Zn2p for ZnO/Ag/TiO2 NT sample. No trace of any impurity is observed, except for a small amount of adventitious carbon (C1s). A high-resolution XPS spectrum confined to the Ag window (Fig. 5b) gave the binding energies of Ag3d doublet peaks located at 368.4 eV (Ag3d5/2) and 374.4 eV (Ag3d3/2). According to the literature [26] the Ag3d5/2 binding energy for Ag is approximately 368.3 eV. This confirms that the Ag particles are located on the Ag/TiO2 NT surface as metallic silver. A slight broadening of the main Ag peak at higher energies site may suggest that the XPS signals of Ag3d are slightly modified by the interaction of the Ag nanoparticles with the TiO2 substrate. Fig. 5c and d shows the XPS spectra of the binding energy of Zn2p3/2 and O1s for the ZnO nanoparticles electrodeposited on the TiO2 nanotubes. The Zn2p3/2 peak was fitted by two Lorentzian–Gaussian peaks
Fig. 4. Survey AES spectra taken locally at (a) Ag/TiO2 NT surface, (b) ZnO/TiO2 NT surface; (c) high resolution Ag MNN spectrum for Ag/TiO2 NT surface; (d) high resolution Zn LMM spectrum for ZnO/TiO2 NT surface.
Please cite this article as: A. Roguska, et al., Synthesis and characterization of ZnO and Ag nanoparticle-loaded TiO2 nanotube composite layers intended for antibacterial coatings, Thin Solid Films (2013), http://dx.doi.org/10.1016/j.tsf.2013.11.057
A. Roguska et al. / Thin Solid Films xxx (2013) xxx–xxx
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Fig. 5. (a) Survey XPS spectra for Ag/TiO2 NT, ZnO/TiO2 NT and ZnO/Ag/TiO2 NT samples; (b) Ag3d XPS high resolution spectrum for Ag/TiO2 NT sample; (c) Zn2p3/2 and (d) O1s XPS high resolution spectra for ZnO/TiO2 NT sample.
centered at 1021.6 and 1022.8 eV (Fig. 5c). The dominant peak at 1021.6 eV is associated with the Zn2+ ions in ZnO. The weak peak at 1022.8 eV is attributed to the zinc hydroxide species or Zn2+ in oxygen-deficient regions [27]. The O1s peak in Fig. 5d was fitted by two Lorentzian–Gaussian peaks. The low energy peak at 530.4 eV corresponds to the O2− ions surrounded by the Zn2+ ions in the stoichiometric ZnO. The second binding energy of 532.1 eV is typically assigned to the O1s in the zinc hydroxide species or in the oxygen-deficient regions of ZnO [27]. The above presented results confirmed that Ag or/and ZnO loaded TiO2 nanotube composite coatings can be easily formed by simple and cost effective methods. An evident advantage of nanotube layer is that it offers a large specific surface area. An empty volume of the tubes can be filled with antibacterial agent in the form of silver or ZnO nanoparticles. The beneficial effect of Ag/TiO2 NT composite layers may be due to (i) the close contact of bacteria with Ag nanoparticles present in the TiO2 oxide layer and/or (ii) the release of Ag ions, detachment of Ag particles or (iii) release of free radicals from the porous TiO2 oxide layer. ZnO/TiO2 nanotube composite coatings may show significant antibacterial activity due to (i) increased number of hydroxyl radicals (when exposed to UV irritation) or (ii) bactericidal effect of ZnO itself (in the dark). In addition, synergistic antibacterial activity, for both Gram positive and Gram negative bacteria, may be expected for a ZnO/Ag/TiO2 NT composite layers. This is, however, a subject of another study aimed at characterization of the cytotoxicity/biocompatibility of the composite layers developed here.
4. Conclusions A serious problem common to all biomaterials and medical devices, the risk of infection, may be alleviated by developing an antibacterial coating containing Ag or ZnO nanoparticles. Our results have shown that these nanoparticles can be incorporated into TiO2 nanotubular layer in a simple and economic manner, suitable for fabrication of bactericidal materials. The macroscopic distribution of both Ag and ZnO nanoparticles at the top surface of the coating is uniform, however the nanoparticles exhibit the in depth distribution gradient. This may be promising to maintain a steady antibacterial effect. The highly ordered Ag and ZnO loaded metal oxide nanotube arrays may offer unique surface features of biomedical-related treatments assuring both biocompatibility and antibacterial properties. Acknowledgments This work was financially supported by the National Science Center (DEC 2011/03/N/ST5/04388). A.R. gratefully acknowledges financial support received from the Foundation for Polish Science (START Program). References [1] K.S. Brammer, S. Oh, C.J. Cobb, L.M. Bjursten, H. van der Heyde, S. Jin, Acta Biomater. 5 (2009) 3215. [2] K.S. Brammer, S. Oh, Ch.J. Frandsen, S. Jin, JOM 62 (2010) 50.
Please cite this article as: A. Roguska, et al., Synthesis and characterization of ZnO and Ag nanoparticle-loaded TiO2 nanotube composite layers intended for antibacterial coatings, Thin Solid Films (2013), http://dx.doi.org/10.1016/j.tsf.2013.11.057
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[3] C.K. Popat, M. Eltgroth, J.T. La Tempa, A.C. Grimes, A.T. Desai, Biomaterials 28 (2007) 4880. [4] N.K. Shrestha, J.M. Macak, F. Schmidt-Stein, R. Hahn, C.T. Mierke, B. Fabry, P. Schmuki, Angew. Chem. Int. Ed. 48 (2008) 969. [5] A. Roguska, M. Pisarek, M. Andrzejczuk, M. Dolata, M. Lewandowska, M. Janik-Czachor, Mater. Sci. Eng. C 31 (2011) 906. [6] D.R. Monteiro, L.F. Gorup, A.S. Takamiya, A.C. Ruvollo-Filho, E.R. de Camargo, D.B. Barbosa, J. Antimicrob. Agents 34 (2009) 103. [7] A.J. Huh, Y.J. Kwon, J. Control. Release 156 (2011) 128. [8] H.A. Foster, I.B. Ditta, S. Varghese, A. Steele, Appl. Microbiol. Biotechnol. 90 (2011) 1847. [9] K. Sunada, Y. Kikuchi, K. Hashimoto, A. Fujishima, Environ. Sci. Technol. 32 (1998) 726. [10] P-Ch. Maness, S. Smolinski, D.M. Blake, Z. Huang, E.J. Wolfrum, W.A. Jacoby, Appl. Environ. Microbiol. 65 (1999) 4094. [11] A. Belcarz, J. Bieniaś, B. Surowska, G. Ginalska, Thin Solid Films 519 (2010) 797. [12] S.H. Hwang, J. Song, Y. Jung, O.Y. Kweon, H. Song, J. Jang, Chem. Commun. 47 (2011) 9164. [13] A. Stoyanova, H. Hitkova, A. Bachvarova-Nedecheva, R. Iordanova, N. Ivanova, M. Sredkova, J. Chem. Technol. Metall. 48 (2013) 154. [14] L. Ren, Catal. Commun. 10 (2009) 654.
[15] B. Yu, K.M. Leung, Q. Guo, W.M. Laum, J. Yang, Nanotechnology 22 (2011) 115603. [16] P.A. Gross, S.N. Pronkin, T. Cottineau, N. Keller, E.R. Savinova, Catal. Today 189 (2012) 93. [17] M. Bosetti, A. Masse, E. Tobin, M. Cannas, Biomaterials 23 (2002) 887. [18] B.S. Atiyeh, M. Costagliola, S.N. Hayek, S.A. Dibo, Burns 33 (2007) 139. [19] M. Rai, A. Yadav, A. Gade, Biotechnol. Adv. 27 (2009) 76. [20] J.R. Morones, J.L. Elechiguerra, A. Camacho, K. Holt, J.B. Kouri, J.T. Ramírez, M.J. Yacamen, Nanotechnology 16 (2005) 2346. [21] Y.-Y. Chang, Ch.-H. Lai, J.-T. Hsu, Ch.-H. Tang, W.-Ch. Liao, H.-L. Huang, Clin. Oral Invest. 16 (2012) 95. [22] M. Premanathan, K. Karthikeyan, K. Jeyasubramanian, G. Manivannan, Nanomedicine: NBM 7 (2001) 184. [23] A.H. Shah, E. Manikandan, M. Basheer Ahmed, V. Ganesan, J. Nanomed. Nanotechol. 4 (2013) 1000168. [24] N. Sharma, J. Kumar, S. Thakur, S. Sharma, V. Shrivastava, Drug Invent. Today 5 (2013) 50. [25] M. Pisarek, A. Roguska, A. Kudelski, M. Andrzejczuk, M. Janik-Czachor, K.J. Kurzydłowski, Mater. Chem. Phys. 139 (2013) 55. [26] Y. Lai, H. Zhuang, K. Xie, D. Gong, Y. Tang, L. Sun, C. Lin, Z. Chen, New J. Chem. 34 (2010) 1335. [27] Q. Tian, J. Li, Q. Xie, Q. Wang, Mater. Chem. Phys. 132 (2012) 652.
Please cite this article as: A. Roguska, et al., Synthesis and characterization of ZnO and Ag nanoparticle-loaded TiO2 nanotube composite layers intended for antibacterial coatings, Thin Solid Films (2013), http://dx.doi.org/10.1016/j.tsf.2013.11.057
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Synthesis and characterization of ZnO and Ag nanoparticle-loaded TiO2 nanotube composite layers intended for antibacterial coatings A. Roguska a,⁎, M. Pisarek a, M. Andrzejczuk b, M. Lewandowska b a b
Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland Faculty of Material Science and Engineering, Warsaw University and Technology, Wołoska 141, 02-507 Warsaw, Poland
a r t i c l e
i n f o
Available online xxxx Keywords: Titania nanotubes Ag nanoparticles ZnO nanostructures Magnetron sputtering Electrodeposition
a b s t r a c t In this work we have fabricated nanoporous oxide layers on Ti with the addition of Ag or ZnO nanoparticles in order to obtain functional coatings ensuring both biocompatibility and antibacterial properties. The morphological and chemical features of the composite coatings fabricated were studied with the aid of high-resolution scanning electron microscopy and surface analytical techniques such as Auger electron spectroscopy and X-ray photoelectron spectroscopy. Our results have shown that Ag and ZnO nanoparticles can be incorporated in a simple and economic manner, suitable for the fabrication of a bactericidal material. The amount of the nanoparticles is variable and depends on the deposition process conditions. The nanoparticles are distributed homogeneously at the top surface of the coating, however they exhibit in depth distribution gradient. This may be promising for maintaining a steady antibacterial effect. © 2013 Elsevier B.V. All rights reserved.
1. Introduction TiO2 nanotubes (NT) offer encouraging implications for the development and optimization of biomedical-related treatments, with precise control over desired cell growth and functionality [1–5]. It was shown that different tube diameters influence mesenchymal stem cell fate by having different effects on cytoskeletal stresses which in turn modulate differentiation into different types of cells [1,2]. The interior of TiO2 nanotubes can be filled with some bioactive species or antibacterial agents, e.g. antibiotics, thereby providing an implant material suitable for local antibiotic therapy (at site of an implantation) [3–5]. The precise control of the nanotube length and diameter (e.g. by the end voltage of the anodic polarization) enables to load different amounts of drug and control the release rate. Extensive effort has been made recently to develop antimicrobial materials containing various organic antibiotics and inorganic substances. Among these fine functional materials nanosized inorganic structures have attracted increased attention in medical applications due to their unique properties and adaptability to biological functionalization [6,7]. Titanium dioxide (TiO2) is now one of the most promising candidates for an antibacterial coating, mostly because of its photocatalytic bactericidal activity and long-term chemical stability [8]. The bactericidal activity of TiO2 is directly related to the ultraviolet light absorption and the formation of various reactive species such as superoxide and hydroxyl radicals [9,10]. Thus, TiO2 itself and TiO2 deposited
⁎ Corresponding author. Tel.: +48 22 343 2078. E-mail address: [email protected] (A. Roguska).
materials have a potential to kill bacteria and also simultaneously degrade the toxic compounds exhausted from the bacteria [11]. However, the TiO2 has inherent drawbacks as an efficient biocidal agent due to the large bandgap energy and fast recombination rate of photogenerated electron–hole pairs [12]. In addition, the fact that the photocatalytic decomposition processes on TiO2 can usually be induced only by ultraviolet light hinders its practical application [13]. In order to improve the photocatalytic performance of TiO2 various approaches have been developed, e.g. it was reported that loading of silver nanoparticles may enhance the overall photocatalytic efficiency of TiO2 [14–16]. Noble metal doping can extend the range of light absorption to the visible spectrum due to the decrease in the bandgap energy of TiO2. Furthermore, the difference in the conduction band edges between TiO2 and the coupled metal (Pt, Pd or Ag) offers pathways to the photogenerated electrons and holes, causing increment in a recombination time. Silver is well described in literature and – also as a modifying agent deposited on medical devices – exhibits good antibacterial properties with possible applications in treatment of burns, severe chronic osteomyelitis and infections of urinary tract and central venous catheters [17,18]. Recent literature data evidenced that silver nanoparticles are more active and reactive than the bulk metallic counterpart, first of all because of their larger specific surface area [19]. Furthermore, it was reported that silver particles with a preferential diameter of about 1–10 nm had direct interactions with bacteria, while larger did not [20]. In the view of this, a system combining both titania and silver nanoparticles is thought to expand the antibacterial function of TiO2 to a wider variety of working conditions. Recently, antibacterial activities associated with ZnO nanoparticles has been also investigated [12,13,21,22]. For example, it was shown
0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.11.057
Please cite this article as: A. Roguska, et al., Synthesis and characterization of ZnO and Ag nanoparticle-loaded TiO2 nanotube composite layers intended for antibacterial coatings, Thin Solid Films (2013), http://dx.doi.org/10.1016/j.tsf.2013.11.057
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A. Roguska et al. / Thin Solid Films xxx (2013) xxx–xxx
The titanium oxide layers were fabricated by the electrochemical anodization of Ti samples (Ti foil, 0.25 mm-thick, 99.5% purity) in an optimized electrolyte: NH4F (0.86 wt.%) + deionized (DI) water (47.14 wt.%) + glycerol (52 wt.%) under a constant voltage of 30 V. After anodization, the samples were rinsed with DI water and dried in air. Subsequently, thermal annealing was performed at 450 °C for 3 h to transform the TiO2 nanotube structure from amorphous (after anodic oxidation) to crystalline (anatase) and to obtain mechanically stable nanotubes well integrated with Ti substrate.
Silver (2.5 and 10 nm thick layer) was deposited using the sputter deposition technique (magnetron sputtering) in a vacuum chamber (Leica EM MED020) with a base pressure of about 2 × 10−5 Pa. The thickness values are nominal values, as measured on the surface of a quartz crystal micro balance. Due to the highly developed specific surface area of the titanium oxide substrates and a very low amount of a metal deposited, silver does not form a continuous layer but an inhomogeneous film with islands (nanoparticles). ZnO was electrodeposited from 0.05 M Zn(NO3)2 aqueous solution at a potential of −1.1 V vs SCE at 60 °C. The duration of the deposition was 3 or 5 min. After the deposition the samples were rinsed with DI water and annealed at 400 °C for 1 h. For the morphological characterization of the anodized, annealed and nanoparticle-loaded samples, examinations were carried out with a scanning electron microscope (SEM, Hitachi S-5500) at the accelerating voltages of 5 kV. Chemical composition of the oxide layers before and after silver or zinc oxide deposition processes was analyzed using the Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS), Microlab 350 (Thermo Electron), with a lateral resolution of about 20 nm for AES and several mm for XPS. The chemical state of surface species was identified using the high-energy resolution spherical sector analyzer of the Auger and XPS spectrometer (the maximum energy resolution is 0.83 eV for XPS method). The appropriate standards for AES and XPS reference spectra were also used. XPS spectra were excited using AlKα (hν = 1486.6 eV) radiation as a source. Survey spectra and high resolution spectra were recorded using 150 and 40 eV pass energy. A linear or Shirley background subtraction was made to
Fig. 1. SEM images of the Ag/TiO2 NT composite layer: (a) top and (b) cross section view.
Fig. 2. SEM images of the ZnO/TiO2 NT composite layer: (a) top and (b) cross section view.
that the electrospun ZnO/TiO2 composite nanofibers exhibited superior antibacterial activity without light irritation [12]. In other study it was revealed that coupled semiconductor photocatalyst of TiO2/ZnO enhanced photodegradation efficiency of TiO2 by increasing the recombination time of photogenerated electron-holes pairs [13]. In addition, silver modification is found to be effective for the fabrication of p-type ZnO, as the naturally occurring ZnO displays n-type conductivity due to its native defects such as zinc interstitials and oxygen vacancies [23,24]. In this work, we present a simple fabrication method of nanotubular oxide layers on Ti loaded with Ag and/or ZnO nanoparticles. As such composite coatings are expected to provide both biocompatibility and antibacterial properties, they are discussed here in terms of possible disinfection-related applications.
2. Methods
Please cite this article as: A. Roguska, et al., Synthesis and characterization of ZnO and Ag nanoparticle-loaded TiO2 nanotube composite layers intended for antibacterial coatings, Thin Solid Films (2013), http://dx.doi.org/10.1016/j.tsf.2013.11.057
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obtain XPS signal intensity. The peaks were fitted using an asymmetric Gaussian/Lorentzian mixed function. The measured binding energies were corrected referring to energy of C1s at 285 eV. An Avantage based data system software (Version 5.41) was used for data processing. 3. Results and discussion The optimized anodization conditions applied resulted in the formation of TiO2 nanotubes (“hollow cylinders”) arranged perpendicular to the substrate. SEM examinations (data not shown) revealed that the nanotubes are separated from each other, open at the top and closed from the bottom. The average diameter of the tubes obtained under anodizing potential of 30 V is about 120 nm. The as-grown TiO2 porous anodic layers exhibit poor adhesion to the Ti substrate. Good adhesion and mechanical stability of TiO2 nanotubes are crucial for their suitability as an antibacterial coating in biomedical applications. In order to improve these parameters, the samples were annealed in air at 450 °C for 3 h. Annealing of TiO2 nanotubes at 450 °C leads to their crystallization to anatase phase. More details are given in [5,25]. Fig. 1a and b shows typical SEM images of TiO2 nanotubular layers covered with 10 nm of Ag. Detailed inspection of Fig. 1a reveals that silver forms a thin solid coating on the top edges of the nanotubes, which is composed of Ag nanoparticles. Some single spherical Ag particles can be found on the inner and outer side walls of the tubes (Fig. 1b). The diameter of these particles is below 50 nm. A careful analysis of the above SEM data suggests that the deposited Ag nanoparticles exhibit in depth distribution gradient. The highest amount of Ag is located at the nanotube top edges and decreases along the side wall of the nanotube to reach the lowest value at the bottom of the tube. TiO2 nanotubes loaded with ZnO exhibit quite different morphology. Fig. 2 shows SEM images of ZnO deposited nanotube layers on Ti. Electrodeposition of ZnO for 3 min leads to the formation of elongated and pointed particles, similar in shape to a rice bean (Fig. 2a). The length
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of these particles is below 100 nm. They are located at the nanotube top edges as well as on the nanotube inner and outer side walls. Some smaller spherical particles are present along the whole nanotube (Fig. 2b). Increasing the electrodeposition time leads to the formation of ZnO “nanoflowers” and complete coverage of the TiO2 nanotube surface. In case of ZnO electrodeposition the in depth deposition gradient is much lower than that observed for Ag loaded composite layers. TiO2 nanotubes are loaded with ZnO nanoparticles on the whole length. This may be due to the nature of the deposition processes. Ag was deposited on the titania nanotube layer by the magnetron sputtering technique using a device, where a Ag sputtering target is located directly above the samples (in a parallel configuration), so only the top surface may be covered by the Ag deposit. During the ZnO electrodeposition process the samples are immersed in the electrolyte which penetrates the nanotubes due to the capillary action and, as a result, a higher number of nucleation sites are available. Both Ag and ZnO nanoparticles are reported to exhibit an antibacterial activity [21]. In addition, ZnO was found to be good match for Ag in order to obtain enhanced and synergistic antibacterial activity for both Gram positive and Gram negative bacteria [23]. Bearing the above in mind we made an attempt to fabricate a composite coating containing both ZnO and Ag nanoparticles. Fig. 3 shows the ZnO electrodeposited on the titania nanotube layer previously loaded with 5 nm Ag layer. At this amount deposited silver forms single spherical nanoparticles with diameter below 25 nm. The presence of single Ag nanoparticles on the titania nanotubular layer resulted in the higher number of nucleation sites for ZnO (when compared to pure TiO2 nanotubes). This in turn has influenced the size and shape of the electrodeposited ZnO particles. They are mostly spherical with diameter below 50 nm, which is two times less than for the only ZnO loaded TiO2 nanotubes. This is in agreement with literature data [24], where the crystallite size of silver doped zinc oxide nanoparticles was seen to decrease with increased silver
Fig. 3. SEM images of the ZnOAg/TiO2 NT composite layer: (a) top and (c) cross section view; (b) and (d) corresponding BSE images. The white arrows correspond to Ag nanoparticles.
Please cite this article as: A. Roguska, et al., Synthesis and characterization of ZnO and Ag nanoparticle-loaded TiO2 nanotube composite layers intended for antibacterial coatings, Thin Solid Films (2013), http://dx.doi.org/10.1016/j.tsf.2013.11.057
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content. Fig. 3b and d shows the distribution of Ag and ZnO nanoparticles on the top surface and the side walls of TiO2 nanotubes, respectively (the backscattered electron images, BSE). The lighter areas are attributed to the Ag deposit (white arrows). Both Ag and ZnO nanoparticles are distributed homogeneously at the top surface of the coating. It seems also that the nucleation of the ZnO took place in the areas where the Ag nanoparticles are present. The in depth distribution gradient is more evident here than for ZnO loaded TiO2 nanotubes (without predeposited Ag), compare Figs. 2b and 3c. The AES investigations of Ag sputter deposited TiO2 nanotubes confirmed the presence of Ag, Ti and O in the as-obtained composite layer. Fig. 4a shows Auger survey spectra measured locally at Ag/TiO2 NT surface. Fig. 4c shows typical AES spectrum of silver measured at a resolution of 0.2 eV on Ag loaded TiO2 nanotube surface, together with the appropriate silver reference spectrum. There is no distinct shift for the Ag MNN signal at the composite layer surface as compared to that for the Ag reference. This suggests that the Ag appears in the composite layer as metal silver. The AES investigations of ZnO electrodeposited TiO2 nanotubes confirmed the presence of Zn, O and Ti in the as-obtained composite layer (Fig. 4b). The contamination with nitrogen (possible contamination from the electrolyte) was not observed. Fig. 4d shows typical AES spectrum of zinc measured at a resolution of 0.2 eV on ZnO loaded TiO2 nanotube surface, together with the appropriate zinc reference
spectrum. There is a distinct shift to lower energies of LMM Auger electron emission of zinc for the ZnO/TiO2NT composite layer. Similar shift was observed for Zn LMM spectrum measured for ZnO/Ag/TiO2NT. The observed shift is due to the fact that Zn (metal) is a good conductor element while ZnO, as a semiconductor, has lower amount of electron density, consequently there is a change in Auger electron energy. The chemical states of the elements in the Ag and/or ZnO loaded TiO2 nanotube composite sample layer were analyzed by X-ray photoelectron spectroscopy (XPS). The survey spectra (Fig. 5a) clearly indicate three major sets of signals from Ti2p, O1s and Ag3d for Ag/TiO2 NT sample, Ti2p, O1s and Zn2p for ZnO/TiO2 NT sample and Ti2p, O1s, Ag3d and Zn2p for ZnO/Ag/TiO2 NT sample. No trace of any impurity is observed, except for a small amount of adventitious carbon (C1s). A high-resolution XPS spectrum confined to the Ag window (Fig. 5b) gave the binding energies of Ag3d doublet peaks located at 368.4 eV (Ag3d5/2) and 374.4 eV (Ag3d3/2). According to the literature [26] the Ag3d5/2 binding energy for Ag is approximately 368.3 eV. This confirms that the Ag particles are located on the Ag/TiO2 NT surface as metallic silver. A slight broadening of the main Ag peak at higher energies site may suggest that the XPS signals of Ag3d are slightly modified by the interaction of the Ag nanoparticles with the TiO2 substrate. Fig. 5c and d shows the XPS spectra of the binding energy of Zn2p3/2 and O1s for the ZnO nanoparticles electrodeposited on the TiO2 nanotubes. The Zn2p3/2 peak was fitted by two Lorentzian–Gaussian peaks
Fig. 4. Survey AES spectra taken locally at (a) Ag/TiO2 NT surface, (b) ZnO/TiO2 NT surface; (c) high resolution Ag MNN spectrum for Ag/TiO2 NT surface; (d) high resolution Zn LMM spectrum for ZnO/TiO2 NT surface.
Please cite this article as: A. Roguska, et al., Synthesis and characterization of ZnO and Ag nanoparticle-loaded TiO2 nanotube composite layers intended for antibacterial coatings, Thin Solid Films (2013), http://dx.doi.org/10.1016/j.tsf.2013.11.057
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Fig. 5. (a) Survey XPS spectra for Ag/TiO2 NT, ZnO/TiO2 NT and ZnO/Ag/TiO2 NT samples; (b) Ag3d XPS high resolution spectrum for Ag/TiO2 NT sample; (c) Zn2p3/2 and (d) O1s XPS high resolution spectra for ZnO/TiO2 NT sample.
centered at 1021.6 and 1022.8 eV (Fig. 5c). The dominant peak at 1021.6 eV is associated with the Zn2+ ions in ZnO. The weak peak at 1022.8 eV is attributed to the zinc hydroxide species or Zn2+ in oxygen-deficient regions [27]. The O1s peak in Fig. 5d was fitted by two Lorentzian–Gaussian peaks. The low energy peak at 530.4 eV corresponds to the O2− ions surrounded by the Zn2+ ions in the stoichiometric ZnO. The second binding energy of 532.1 eV is typically assigned to the O1s in the zinc hydroxide species or in the oxygen-deficient regions of ZnO [27]. The above presented results confirmed that Ag or/and ZnO loaded TiO2 nanotube composite coatings can be easily formed by simple and cost effective methods. An evident advantage of nanotube layer is that it offers a large specific surface area. An empty volume of the tubes can be filled with antibacterial agent in the form of silver or ZnO nanoparticles. The beneficial effect of Ag/TiO2 NT composite layers may be due to (i) the close contact of bacteria with Ag nanoparticles present in the TiO2 oxide layer and/or (ii) the release of Ag ions, detachment of Ag particles or (iii) release of free radicals from the porous TiO2 oxide layer. ZnO/TiO2 nanotube composite coatings may show significant antibacterial activity due to (i) increased number of hydroxyl radicals (when exposed to UV irritation) or (ii) bactericidal effect of ZnO itself (in the dark). In addition, synergistic antibacterial activity, for both Gram positive and Gram negative bacteria, may be expected for a ZnO/Ag/TiO2 NT composite layers. This is, however, a subject of another study aimed at characterization of the cytotoxicity/biocompatibility of the composite layers developed here.
4. Conclusions A serious problem common to all biomaterials and medical devices, the risk of infection, may be alleviated by developing an antibacterial coating containing Ag or ZnO nanoparticles. Our results have shown that these nanoparticles can be incorporated into TiO2 nanotubular layer in a simple and economic manner, suitable for fabrication of bactericidal materials. The macroscopic distribution of both Ag and ZnO nanoparticles at the top surface of the coating is uniform, however the nanoparticles exhibit the in depth distribution gradient. This may be promising to maintain a steady antibacterial effect. The highly ordered Ag and ZnO loaded metal oxide nanotube arrays may offer unique surface features of biomedical-related treatments assuring both biocompatibility and antibacterial properties. Acknowledgments This work was financially supported by the National Science Center (DEC 2011/03/N/ST5/04388). A.R. gratefully acknowledges financial support received from the Foundation for Polish Science (START Program). References [1] K.S. Brammer, S. Oh, C.J. Cobb, L.M. Bjursten, H. van der Heyde, S. Jin, Acta Biomater. 5 (2009) 3215. [2] K.S. Brammer, S. Oh, Ch.J. Frandsen, S. Jin, JOM 62 (2010) 50.
Please cite this article as: A. Roguska, et al., Synthesis and characterization of ZnO and Ag nanoparticle-loaded TiO2 nanotube composite layers intended for antibacterial coatings, Thin Solid Films (2013), http://dx.doi.org/10.1016/j.tsf.2013.11.057
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Please cite this article as: A. Roguska, et al., Synthesis and characterization of ZnO and Ag nanoparticle-loaded TiO2 nanotube composite layers intended for antibacterial coatings, Thin Solid Films (2013), http://dx.doi.org/10.1016/j.tsf.2013.11.057