Decoration of Ag nanoparticles on the apatite nanosheet-coated silica nanofibers with enhanced anti-bacterial property and photo-catalytic activity

Materials Letters 230 (2018) 236–240

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Decoration of Ag nanoparticles on the apatite nanosheet-coated silica nanofibers with enhanced anti-bacterial property and photo-catalytic activity Aili Wei, Yunhong Yao, Tianlong Wang, Lulu Shen, Lan Jia, Song Chen ⇑ College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China

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Article history: Received 5 April 2018 Received in revised form 10 July 2018 Accepted 28 July 2018 Available online 30 July 2018 Keywords: Bioceramics Nanocomposites Colloidal processing

a b s t r a c t In this study, Ag-decorated apatite nanosheet-coated silica nanofibers (Ag-Ap-Si NFs) with enhanced anti-bacterial and photo-catalytic properties were synthesized through incubation of apatite nanosheet-coated silica nanofibers (Ap-Si NFs) in the suspension of chitosan-stabilized Ag nanoparticles (NPs). SEM observations showed that Ag-Ap-Si NFs had the diameter of 840–2340 nm and possessed a hierarchical structure. TEM observations showed the surface of Ag-Ap-Si NFs was constructed by the composites of apatite nanosheets with the thickness around 10 nm and Ag NPs with the size of 5–20 nm. It was found that Ag-Ap-Si NFs showed enhanced anti-bacterial property to inhibit the growth of both Escherichia coli and Staphylococcus aureus and superior photo-catalytic activity for reduction of 4-nitrophenol in the presence of NaBH4. The present Ag-Ap-Si NFs would be applicable as multifunctional materials with both photo-catalytic and anti-bacterial properties for wastewater treatment at weak acidic and basic pH. Ó 2018 Elsevier B.V. All rights reserved.

1. Introduction Wastewater treatment has become one of the promising methods to reduce environment pollution [1]. Removal of the harmful organic compounds [2] and microbial contaminants [3] is one of the promising strategies for water purification. Ag nanoparticles (NPs) show excellent photo-catalytic activity for degradation of organic compounds and strong anti-bacterial property against bacteria [4]. Therefore, Ag NP-based materials have been extensively applied for wastewater treatment. However, the agglomeration of Ag NPs strongly limited their practical applications. It has been demonstrated that the immobilization of Ag NPs on the material substrates would be advantageous for reducing their agglomeration and improving their stability. Several types of materials have been utilized for immobilization of Ag NPs, including polymers, carbon materials, and inorganic materials. Among them, the inorganic materials such as SiO2 [5] and hydroxyapatite (HAp) [6] are much more interesting as they present more stable size and morphology, easy synthesis route, and low-cost. Apatite crystals are the main inorganic phase in the hard tissues such as bone and teeth and show excellent biocompatibility and osteo⇑ Corresponding author. E-mail address: [email protected] (S. Chen). https://doi.org/10.1016/j.matlet.2018.07.129 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.

conductivity. Very recently, we found that apatite nanosheets possess the hierarchical microstructure and a large specific surface area [7,8]. They well supported the immobilization of Au NPs and the resultant apatite nanosheet/Au composites showed excellent photo-catalytic activity for reduction of 4-nitrophenol [7,8]. Compared with Au NPs, Ag NPs not only showed excellent photocatalytic activity, but also showed strong anti-bacterial property. Therefore, synthesis of apatite nanosheet/Ag NP composites would be much more interesting. In this study, novel and multi-functional Ag-decorated apatite nanosheet-coated silica nanofibers (Ag-Ap-Si NFs) were synthesized through decoration of Ag NPs on apatite nanosheet-coated silica nanofibers (Ap-Si NFs) and their microstructure, antibacterial and photo-catalytic properties were evaluated. 2. Experimental 2.1. Synthesis of Ap-Si NFs Ap-Si NFs were synthesized according to the method described in the previous study [8]. Briefly, silica nanofibers were synthesized through electrospinning of a sol-gel mixture of gelatin, acetic acid, tetraethoxysilane (TEOS), water, and calcium chloride followed by sintering process and then soaked in the Kokubo’s

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SBF (pH = 7.4) for 24 h to in situ induce the deposition of apatite nanosheets (Fig. 1a). Ap-Si NFs were then collected from SBF by centrifugation at 5000 rpm for 5 min, washed with water three times, and finally dried in a freezing-drier. 2.2. Synthesis of chitosan-stabilized Ag NPs Ag NPs were synthesized from AgNO3 with chitosan as stabilizing agent via a modified hydrothermal method [9]. Briefly, 1% (w/v) chitosan solution was prepared by dissolving appropriate amount of chitosan powders in 1% (v/v, pH = 5.0) HAc solution. Subsequently, 9 mL of chitosan solution was mixed with 1 mL of 1% (w/v) AgNO3 solution and the mixture was transferred to a 50 mL-Teflon-lined autoclave and then sealed and maintained at 120 °C for 4 h. A yellow solution was obtained, which indicated that chitosan-stabilized Ag NPs were produced. The resultant chitosan-stabilized Ag NPs were collected by centrifugation at 10,000 rpm for 10 min and then suspended in the water for the following experiments.

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2.3. Synthesis of Ag-Ap-Si NFs Ag-Ap-Si NFs were synthesized through decoration of Ap-Si NFs with chitosan-stabilized Ag NPs (Fig. 1a). Briefly, 5 mg of Ap-Si NFs were added to 0.2 mL of chitosan-stabilized Ag NP suspension held in a 5 mL-eppendorf tube. The mixture was placed in a shaking bath for 12 h for decoration of Ag NPs. After 12 h, the mixture was centrifuged at 10,000 rpm for 5 min and washed with water and finally freeze-dried. 2.4. Anti-bacterial property Both Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) are used to examine the anti-bacterial property of the as-synthesized samples. E. coli and S. aureus was separately grown in the Mueller-Hinton broth solution at 37 °C overnight. Ap-Si NFs (1 mg) and Ag-Ap-Si NFs (1 mg) were separately added to 1 mL of bacterial suspension at the concentration of 107 CFU/mL and incubated at 37 °C for 24 h. To quantitatively analyze the

Fig. 1. (a) Schematic illustration of synthesis route for Ap-Si NFs and Ag-Ap-Si NFs and possible formation mechanism for (b) Ap-Si NFs and (c) Ag-Ap-Si NFs.

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growth of bacteria, the optical density (OD) of bacteria incubated with those samples was measured with a micro-plate reader at the wavelength of 595 nm. To qualitatively analyze the growth of bacteria, the suspension of both bacteria incubated with different types of samples was also imaged with a digital camera. 2.5. Photo-catalytic activity To examine the photo-catalytic activity, 4-nitrophenol was used as model chemical. 0.4 mL of 4-nitrophenol (0.25 mmol/L) and 0.4 mL of NaBH4 (0.1 mol/L) were mixed in a 4 mL-quartz cuvette to obtain the mixture of 4-nitrophenol and NaBH4 at room temperature. Ap-Si NFs (0.5 mg) and Ag-Ap-Si NFs (0.5 mg) were separately added to the mixture of 4-nitrophenol and NaBH4. The quartz cuvette was immediately subjected to UV–vis measurements. The

absorption of the suspension was examined at the intervals of 2 min until the dye was completely reduced. 3. Results and discussion SEM image (Fig. 2a) showed that Ap-Si NFs had the fibrous structure with the diameter ranging from 840 nm to 2340 nm. Their surface was constructed by numerous apatite nanosheets with the thickness around 10 nm. After decoration of Ag NPs, AgAp-Si NFs still maintained the fibrous structure of Ap-Si NFs (Fig. 2b). EDS spectrum (Fig. 2c) clearly revealed that Ag-Ap-Si NFs contained Ca, O, P, and Ag elements. Further, the element mapping images (Fig. 2d) clearly showed that P, O, Ca, and Ag elements were well combined in Ag-Ap-Si NFs. TEM observations further gave the structural evolution from Ap-Si NFs to Ag-Ap-Si NFs.

Fig. 2. SEM images of (a) Ap-Si NFs and (b) Ag-Ap-Si NFs, (c) EDS spectrum and (d) element mapping images of Ag-Ap-Si NFs, and TEM images of (e, f) Ap-Si NFs and (g, h) AgAp-Si NFs. (Black arrows: apatite nanosheets; White arrows: Ag NPs).

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Fig. 3. (a) Quantitative result and (b) photographs of both E. coli and S. aureus after exposed to Ap-Si NFs and Ag-Ap-Si NFs as well as Ag NPs (Control: only bacteria), (c) UV spectra of the mixture of 4-nitrophenol and NaHB4 and (b) the plots of C/Co and ln(C/Co) verse reaction time for the reduction of 4-nitrophenol by NaHB4 in the presence of Ap-Si NFs and Ag-Ap-Si NFs. Co and C was the absorption peak at 400 nm initially and at different reaction time, respectively.

Ap-Si NFs presented the fibrous structure and their surface was very rough (Fig. 2e) and covered with numerous apatite nanosheets (Fig. 2f, black arrows). Ag-Ap-Si NFs well maintained the fibrous structure of Ap-Si NFs. However, a significant difference in surface morphology of the apatite nanosheets was clearly observed. Numerous smaller Ag NPs (Fig. 2g, black dots) were well immobilized on the surface of the apatite nanosheets (Fig. 2h, white arrows), further confirming the formation of Ag-Ap-Si NFs. The deposition of apatite nanosheets might be attributed to their Si-OH groups and the hydrated silicate networks both of which provided the active sites for nucleation and growth of apatite nanosheets (Fig. 1b) when soaked in the Kokubo’s SBF [10,11]. Moreover, Ca ions released from silicate materials would highly promote the deposition of apatite nanosheets as they increased super-saturation degree of SBF [12]. Chitosan is a natural polysaccharide and contains plenty of amino groups [13]. It is normally positively charged due to the rapid protonation of these amino groups. Apatite is normally negatively charged due to the presence of PAOAP bonds [14,15]. Therefore, the immobilization of chitosan-stabilized Ag NPs on Ap-Si NFs might be attributed to the electric interaction between them (Fig. 1c). Fig. 3a quantitatively shows the optical density of bacterial suspension incubated with Ap-Si NFs and Ag-Ap-Si NFs after 24 h. A significant difference in growth behaviour of E. coli and S. aureus was observed. Ap-Si NFs well supported the growth of bacteria, while Ag-Ap-Si NFs and Ag NPs highly inhibited the growth of bacteria and showed anti-bacterial property against bacteria. Fig. 3b qualitatively shows the anti-bacterial property of both types of samples. Ap-Si NFs showed weak antibacterial property and well supported the growth of the bacteria colonies. However, Ag-Ap-Si NFs and Ag NPs showed strong anti-bacterial property to inhibit the growth of bacteria.

Fig. 3c shows UV spectra of the mixture of 4-nitrophenol and NaBH4 after incubated with Ap-Si NFs and Ag-Ap-Si NFs. The band around 400 nm was normally assigned to 4-nitrophnolate ions [16,17]. No significant decrease in intensity of the peak around 400 nm was observed for Ap-Si NFs, while a significant decrease in intensity of peak around 400 nm and a new peak around 295 nm assigned to 4-aminophenol were observed for Ag-Ap-Si NFs. Fig. 3d quantitatively shows the dependence of C/Co and ln(C/Co) versus reaction time for the reduction of 4-nitrophenol. By calculating the slope of the fitting line, the kinetic constant k is 0.002 min 1 for Ap-Si NFs and 0.229 min 1 for Ag-Ap-Si NFs. In contrast, Ag-Ap-Si NFs presented much larger photo-catalytic activity. 4. Conclusions In summary, novel and multi-functional Ag-decorated apatite nanosheet-coated silica nanofibers were synthesized and presented enhanced anti-bacterial activity against E. coli and S. aureus and superior photo-catalytic performance for reduction of 4nitrophenol. Acknowledgement This work was conducted with the financial support from the Shanxi Scholarship Council of China (Grant No. 2016-024). References [1] P. Goh, A. Ismail, Desalination 434 (2018) 60–80. [2] J. Luek, M. Gonsior, Water Res. 123 (2017) 536–548. [3] P. Robertson, J. Robertson, D. Bahnemann, J. Hazard. Mater. 211–212 (2012) 161–171.

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[4] T. Ali, A. Ahmed, U. Alam, I. Uddin, P. Tripathi, M. Muneer, Mater. Chem. Phys. 212 (2018) 325–335. [5] X. Wang, H. Fan, P. Ren, H. Yu, J. Li, Mater. Res. Bull. 47 (2012) 1734–1739. [6] X. Hong, X. Wu, Q. Zhang, M. Xiao, G. Yang, M. Qiu, G. Han, Appl. Surf. Sci. 258 (2012) 4801–4805. [7] T. Wang, Y. Yao, A. Wei, L. Jia, S. Chen, Mater. Lett. 220 (2018) 129–132. [8] S. Chen, T. Wang, Y. Yao, A. Wei, Mater. Design 147 (2018) 106–113. [9] L. Biao, S. Tan, Y. Wang, X. Guo, Y. Fu, F. Xu, Y. Zu, Z. Liu, Mat. Sci. Eng. C-Mater. 76 (2017) 73–80. [10] S. Chen, A. Osaka, S. Hayakawa, K. Tsuru, E. Fujii, K. Kawabata, J. Sol-Gel Sci. Technol. 48 (2008) 322–335.

[11] S. Chen, A. Osaka, N. Hanagata, J. Mater. Chem. 21 (2011) 4332–4338. [12] K. Tsuru, Y. Aburatani, T. Yabuta, S. Hayakawa, J. Sol-Gel Sci. Technol. 21 (2001) 89–96. [13] S. Chen, H. Zhang, X. Shi, H. Wu, N. Hanagata, Lab Chip 14 (2014) 1842–1849. [14] A. Szczes´, L. Hołysz, E. Chibowski, Adv. Colloid Interfac. 249 (2017) 321–330. [15] M. Šupová, Ceram. Int. 41 (2015) 9203–9231. [16] G. Mele, E. Garcìa-Lòpez, L. Palmisano, G. Dyrda, R. Słota, J. Phys. Chem. C 111 (2007) 6581–6588. [17] L. Yang, S. Luo, Y. Li, Y. Xiao, Q. Kang, Q. Cai, Environ. Sci. Technol. 44 (2010) 7641–7646.