Green Synthesis of Graphene Oxide Sheets Decorated by Silver Nanoprisms and Their

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Text of Green Synthesis of Graphene Oxide Sheets Decorated by Silver Nanoprisms and Their

Journal of Inorganic Biochemistry 105 (2011) 11811186

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Journal of Inorganic Biochemistryj o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j i n o r g b i o

Green synthesis of graphene oxide sheets decorated by silver nanoprisms and their anti-bacterial propertiesDanhui Zhang, Xiaoheng Liu , Xin Wang Key Laboratory of Education, Ministry for Soft Chemistry and Functional Materials, Nanjing University of Science and Technology, Nanjing 210094, China

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a b s t r a c tA widely soluble graphene oxide sheets decorated by silver nanoprisms were prepared through green synthesis at the room temperature using gelatin as reducing and stabilizing agent. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UVvisible spectroscopy and uorescence spectra. The results demonstrate that these silver-nanoprisms assembled on graphene oxide sheets are exible and can form stable suspensions in aqueous solutions. Furthermore, the formation mechanism of soluble graphene oxide sheets decorated by silver nanoprisms was successfully explained. The anti-bacterial properties of graphene oxide sheets decorated by silver nanoprisms were tested against Escherichia coli. This work provides a simple and green method for the synthesis of graphene oxide sheets decorated by silver nanoprisms in aqueous solution with promising antibacterial property. 2011 Elsevier Inc. All rights reserved.

Article history: Received 21 February 2011 Received in revised form 19 May 2011 Accepted 19 May 2011 Available online 27 May 2011 Keywords: Graphene oxide sheets Gelatin Silver nanoprisms Green synthesis

1. Introduction In recent years, the topic of green chemistry has been emphasized in academic circles since it could work out new route for chemical products with the process reducing or eliminating the use and generation of hazardous substances [1]. With the development of nanotechnology, the principles of green chemistry have been applied in synthesis and applications of nanomaterials. In the green synthetic strategy of nanoscale materials, two aspects including utilization of nontoxic chemicals and environmental friendly solvents have attracted considerable attention due to their advantage in reducing the environmental risk. In this sense, biocompatible nanomaterials have received considerable attention for the promising applications in bioimaging, biosensing, and developing of biomedicines [2,3]. Thus, commonly used methods in the preparation of biocompatible nanoparticles (NPs) should be evaluated again in terms of green chemistry viewpoints. As we know that the green-synthesis standard is to choose the environmental friendly solvents used for the synthesis, environmentally benign reducing agents, and nontoxic materials for the stabilization. The synthesis of silver nanoparticles using starch as stabilization and glucose as reducing agents reported by Wallen et al. [4] is an excellent example of green syntheses. Graphene, a single-atom-thick sheet of hexagonally arrayed sp2bonded carbon atoms, has attracted signicant attention from both experimental and theoretical elds recently [5]. Due to its unique electronic properties, graphene sheets provide potential applications in synthesizing nanocomposites [6] and fabricating eld-effect transistors Corresponding authors. Tel.: + 86 25 84315943; fax: + 86 25 84432747. E-mail addresses: xhliu@mail.njust.edu.cn (X. Liu), wxin@public1.ptt.js.cn (X. Wang). 0162-0134/$ see front matter 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jinorgbio.2011.05.014

[7], dye-sensitized solar cells [8], lithium ion batteries [9], electromechanical resonators [10], and electrochemical sensors [11]. However, just as for the other newly discovered allotropes of carbon (fullerenes and carbon nanotubes), material availability and process-ability will be the ratelimiting steps in the evaluation of putative applications of graphene. For graphene, that availability is encumbered by having to surmount the high cohesive van der Waals energy (5.9 kJ mol1) adhering graphitic sheets to one another [12]. Some methods including an epitaxial growth [13], chemical vapor deposition [14], the solvothermal reduction of graphene oxide [15], the electrochemical reduction of graphene oxide [16], and the chemical reduction of graphene oxide [17] have been used to prepare individual graphene sheets and to improve the properties of graphene. More recently, green synthesis of graphene has attracted considerable attention for their bio-application. For example, E.C. Salas et al. [18] showed that graphene oxide was reduced by the bacterial respiration and P. Laaksonen et al. [19] present a method for the exfoliation and functionalization of graphene sheets by an amphiphilic protein. In the past decade, many efforts have been paid in shaping the metal nanostructures, because the physical and chemical properties are highly dependent on their morphologies [20].The nanoprism, as one of the most important morphologies, is of unique optical properties [21] and has been prepared successfully by two strategies. The rst strategy includes a transformation procedure resulted from the Ostwald ripening process driven by thermal, photochemical or chemical treatment [22]. The other choice is to synthesize nanoprisms through a direct chemical reduction route [23]. In both strategies, the capping agents play an important role. Gelatin is the thermally and hydrolytically denatured product of collagen, which has been extensively applied as the immobilization matrix for the preparation of biosensors. It has a triple-helical structure

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and offers distinctive advantages such as good biocompatibility, nontoxicity, remarkable afnity to proteins, and excellent gel-forming ability [24]. In this study, we make a use of gelatin as a reducing and stabilizing agent to prepare graphene oxide sheets decorated by silver nanoprisms. The major advantage for gelatin as a stabilizing agent is that it can be used to tailor the nanocomposite properties and also to provide longterm stability of the nanoparticles by preventing particle agglomeration. This method did not introduce any environmental toxicity or biological hazards and thus was simple and green. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UVvis spectroscopy and uorescence were used to characterize the graphene oxide sheets decorated by silver nanoprisms. The results demonstrate that these silver-nanoprisms assembled on graphene oxide sheets are exible and can form stable suspensions in aqueous solutions. Moreover, the anti-bacterial activity of graphene oxide sheets decorated by silver nanoprisms was also displayed. 2. Experimental section 2.1. Reagents Silver nitrate (AgNO3) was obtained from Aldrich. Graphite was bought from Qingdao Zhongtian Company with a mean particle size of 44 mm. The gelatin was from Shanghai Chemical Reagent Co. (Shanghai, China). The other chemicals were all analytical grade, used as received without further purication, and the water was deionized. 2.2. Preparation of graphene oxide (GO) In this work, GO was synthesized from natural graphite powder by a modied Hummers method as originally presented by Kovtyukhova et al. [25]. The prepared GO has oxy-functional groups such as carboxyl (COOH), hydroxyl (OH), and epoxy groups on its surface. 2.3. Synthesis of graphene oxide sheets decorated by silver nanoprisms The typical procedure for the preparation of silver nanoprisms on graphene oxide sheets is shown below: (1) A certain amount of gelatin was completely dissolved H2O (30 mL) under magnetic stirring at about 60 C for about 30 min, and then cooled to room temperature. At this time, silver nitrate (0.03 M) was added in. After stirring for 12 h, the silver nanoprisms colloid was formed. (2) Graphite oxide powder (20 mg) was dispersed in water (30 mL) by sonication for 2 h to form a stable graphene oxide colloid. Finally, the colloid (1) mixed with colloid (2) still kept stirring at the room temperature over 12 h. The nal product was centrifuged (10,000 rpm for 15 min) and then vacuum-

dried at 60 C for overnight. Then the graphene oxide sheets decorated by silver nanoprisms were formed. 2.4. Antibacterial properties study First of all, liquid culture medium and solid culture medium were collocated for Escherichia coli. Briey, E. coli and 1 ppm, 5 ppm, and 10 ppm of GO and Ag/GO colloidal dispersions were added to 100 mL liquid culture medium, respectively in the Erlenmeyer ask shaking in thermostat shaker at rate of 180 rps. 0.1 mL so-made bacteriaGO/Ag mixture or bacteriaGO mixture was diluted with 0.9 mL. No GO/Ag or GO colloids were added to one Erlenmeyer ask containing 100 mL liquid culture medium, which was served as a control sample. Subsequently, the bacteria suspension was diluted 10 5 times. After the serial dilution had been carried out, 0.2 mL of each bacteriaGO/Ag mixture or bacteriaGO mixture was added to a Petri dish containing 10 mL warm agar medium. One additional plate was poured containing 10 mL of nutrient agar for control purposes. The plates were incubated for 24 h at 37 C and then analyzed for the number of bacterial colonies to determine the growth inhibition rates of GO/Ag or GO in accordance with the Eq. (1) R% = AB = A 100 1

where R = the growth inhibition rates, A = the number of bacterial colonies from control sample, and B = the number of bacterial colonies from GO or GO/Ag mixture. 2.5. Characterization UVvis spectra were recorded on a Shimadzu UV-2500 spectrophotometer in a 1 cm optical path quartz cuvette over a 200800 nm range at room temperature. X-ray diffr