Synthesis of Stable Silver Nanoparticles with Antimicrobial Activities in Room-temperature Ionic Liquids

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1 CHEM. RES. CHINESE UNIVERSITIES 2009, 25(4), Synthesis of Stable Silver Nanoparticles with Antimicrobial Activities in Room-temperature Ionic Liquids AN Jing 1,2, WANG De-song 2 and YUAN Xiao-yan 1* 1. School of Materials Science and Engineering, Tianjin Key Laboratory of Function and Composite Materials, Tianjin University, Tianjin , P. R. China; 2. College of Sciences, Hebei University of Science and Technology, Shijiazhuang , P. R. China Abstract Stable silver nanoparticles was successfully synthesized by chemical reduction of silver nitrate in an ionic liquid, 1-n-butyl-3-methylimidazolium tetrafluoroborate([bmim] BF 4 ) at room temperature. Results of UV-Vis diffuse reflectance spectroscopy show as-prepared Ag nanoparticles exhibit a typical emission peak at nm. By varying the reaction temperature and the precursor concentration, the size and the shape of the silver nanoparticles could be easily controlled under mild conditions. Analyses of transmission electron micrographs, X-ray diffraction pattern and X-ray photoelectron spectrum further reveal that the silver nanoparticles were coated incompletely by [BMIM] BF 4. Microbial experiments indicate that as-prepared silver nanoparticles show a wide spectrum of antimicrobial activities and have better antimicrobial activities to Pseudomonas aeruginosa than silver nitrate with the same concentration of silver. Keywords Silver nanoparticle; Ionic liquid; Antimicrobial activity Article ID (2009) Introduction Silver particles have been investigated extensively in recent years because of their potential applications in catalysis, antimicrobial as well as surface-enhanced Raman scattering detection [1 3]. Their application depends greatly on their physical properties, such as apparent density, surface area, and morphology that is strongly related to the preparation method and synthesis medium. In recent years, the synthesis of silver nanoparticles in microemulsion by chemical reduction that could prevent the nanoparticles from agglomeration was reported [4 6]. In spite of Ag nanoparticles with different shapes and sizes successfully obtained in various aqueous solution systems [7 9], most of the reported synthetic methods rely heavily on the use of organic solvents and toxic reducing agents(e.g. N,N-dimethylformamide and sodium borohydride). All these chemicals are highly reactive and can induce potential environmental and biological pollutions [10 12]. Therefore, it is desirable to develop more efficient and environmentally friendly medium for the synthesis of Ag nanoparticles and their composites. Room-temperature ionic liquids(rtils) [13] have *Corresponding author. yuanxy@tju.edu.cn Received August 4, 2008; accepted October 16, attracted intensive interests only in recent years as a replacement for classical molecular solvents in fundamental researches and application including separation, catalysis, organic synthesis, and so on [14 16]. Compared with other solvents, ionic liquids have proven to be a class of less harmful, highly productive, and readily adoptable ones [17 19]. Kumar et al. [20] have recently reported the assembly of conducting organic-metallic composite submicrometer hexagonal rods based on electrostatic complexation between an RTIL and tetrachloroaurate anions. Among various RTILs, 1-n-butyl-3-methylimidazolium tetrafluoroborate([bmim] BF 4 ) has received special attention due to its application as solvent in the synthesis of nanoparticles. [BMIM] BF 4 is hydrophilic and its viscosity is much higher than that of water at room temperature [21,22]. So [BMIM] BF 4 is very useful for the preparation of new materials with specific characteristics, especially the transition elements and semiconductor compounds. For instance, Scheeren et al. [23] have reported that the controlled decomposition of the organometallic Pt(0) precursor Pt 2 (dba) 3 in ion liquid [BMIM] BF 4 yielded stable and catalytically active Pt(0) nanoparticles with an average diameter of 2.3 nm.

2 422 CHEM. RES. CHINESE UNIVERSITIES Vol.25 In this work, the authors reported a simple method to synthesize silver nanoparticles in ion liquid [BMIM] BF 4 with sodium citrate as a reducing agent. Effects of the reduction temperature and the precursor concentration on the as-prepared silver nanoparticle size were investigated, the silver nanoparticles were characterized and their antimicrobial activities were also investigated. 2 Experimental 2.1 Materials N-Methylimidazole and 1-bromobutane(butyl bromide) were supplied by Beijing Fuxing Chemical Reagent Factory. They were used after decompression distillation to remove the impurities and dried with anhydrous sodium sulfate. Sodium fluoroborate, silver nitrate, sodium citrate and other chemicals with analytical grade were used as received. Escherichia coli (ATCC 44752), Bacillus subtilis(atcc 63501), Staphylococcus aureus(atcc 26003), and Pseudomonas aeruginosa(atcc 10110) were purchased from Beijing Center for Disease Prevention and Control, China. 2.2 Synthesis of Ag Nanoparticles in [BMIM] BF 4 [BMIM] BF 4 was synthesized according to the reference [24] by substitution reaction and colloidal silver nanoparticles were synthesized in it. Two solutions of silver nitrate with a concentration of 0.03 mol/l and sodium citrate with a concentration of mol/l in [BMIM] BF 4 were, respectively, prepared. The sodium citrate solution was then added dropwise into the AgNO 3 solution under a vigorous stirring at a given temperature in the range of C. The process was conducted for 0.5 h and the solution was continuously stirred for another 3 h. After adjusting the ph value at 8.0, the brown colloid contained silver nanoparticles could be formed. The products were separated by centrifugation, washed with absolute ethanol for several times, then vacuum-dried at 60 C for 48 h for further characterization. 2.3 Characterization The morphology of silver nanoparticles was viewed under a Transmission electron microscope (TEM, Tecnai G2 F20) equipped with a selected area electron diffraction(saed) detector and an energy dispersive X-ray(EDX) detector. The X-ray diffraction (XRD, Rigaku D/MAX-2500) pattern was performed in a range of 2θ= A Varian Cary 100 Scan UV-Visible system(uv-vis) equipped with an integrating sphere attachment was used to obtain the reflectance spectra of Ag nanoparticles over a range of nm. X-ray photoelectron spectroscopic (XPS) measurement was conducted with a PHI 5000C ESCA spectrometer(perkin-elmer, USA). 2.4 Microbial Experiments Approximately 10 5 colonies of four kinds of bacteria, i.e., Escherichia coli, Bacillus subtilis, Staphylococcus aureus and Pseudomonas aeruginosa were cultured on nutrient agar plates supplemented with 5 ml of a silver nanoparticle-suspension solution or an aqueous AgNO 3 solution, which contained g/ml Ag nanoparticles and g/ml Ag +, respectively. Control experiments were performed with distilled water only. The plates were incubated at 37 C for 24 h. Each of the samples was done in six parallel experiments and the average number of colonies was counted. The antibacterial rates were obtained from the the calculation via following equation: Antibacterial rate(%)=(n 0 N 1 )/N 0 100% where N 0 and N 1 are referred to the numbers of bacterium colonies in the control culture plates and the experiment culture plates, respectively. 3 Results and Discussion 3.1 Synthesis of Silver Nanoparticles Actually, the high performance of nanomaterials was achieved by controlling their phase structure in the nano-sized range. The size and microstructure of nanoparticles are firmly dependent on the temperature, the concentration of precursor and the properties of synthesis systems. Fig.1(A) shows that the absorption peak at approximate nm is attributed to silver nanoparticles as the temperature increased from 25 C to 40 C. At 30 C the resulting silver nanoparticles had a smaller average diameter and were in more monodisperse. At a lower preparation temperature, chemical reduction could not finish completely at a short time so that a smaller amount of nuclei formed in the system, and then the Ag nanocrystals grew bigger when the reduction reaction was completed. The maximum absorption gave rise to a red shift from to nm with the temperature rise from 30

3 No.4 AN Jing et al. 423 to 40 C, indicating an increase of the particle size. The phenomenon could be caused by the high rate nuclei and then the formation of more silver nanocrystals at the higher temperature. could absorb [BMIM] BF 4 molecules to prevent the particles from aggregating. But, the collisions between silver particles became more frequently with further increasing the AgNO 3 concentration in RTIL and the protection of [BMIM] BF 4 for nanoparticles was thus cancelled out, resulting in silver particles aggregating. In conclusion,the absorption peak of the particles at nm suggests that the as-prepared Ag particles were nanosized and stable without large aggregates under the reaction conditions[25 40 C, c(agno 3 )= mol/l]. 3.2 Particle Size and Morphology Fig.1 UV-Vis diffuse reflectance spectra of silver nanoparticles at different temperatures(a) and concentrations of AgNO 3 (B) (A) c(agno 3 )=0.033 mol/l; t/ C: a. 25; b. 30; c. 35; d. 40. (B) t=30 C; c/(mol L 1 ): a ; b ; c ; d The final wavelength for the absorption maximum was dependent on the initial silver ion concentration. Fig.1(B) shows the absorption peak, at approximate nm, of silver nanoparticles [25,26]. The UV-Vis absorption intensity increased with increasing AgNO 3 concentration, which reflects the formation of more Ag nanoparticles. However, the absorption maximum was a little different when the AgNO 3 concentration was varied from to mol/l. The maximum absorption peak existed at a lower wavelength when AgNO 3 concentration was mol/l. It is because the more AgNO 3 molecules the more silver nuclei, then the more Ag nanoparticles with smaller sizes formed. The maximum absorption gave rise to a red shift from to nm when the AgNO 3 concentration was increased from to mol/l, meaning the particle size increased [27,28] and large silver aggregates formed. It is believed that the formation rate of nuclei increased significantly with increasing the AgNO 3 concentration. There were large amounts of small nanoparticles with high special surface area produced in a short time. These particles The TEM micrograph with SAED pattern of as-prepared Ag nanoparticles is shown in Fig.2. The particles obtained in RTIL exhibited a regular spherical shape with smooth surfaces and the size was distributed in a range of about nm. Structure and cubic crystallinity of as-prepared particles could be proved by the inset SAED pattern, but the lattice fringes of the Ag crystal structure could not be observed clearly by the TEM image. These observations could lead to the conclusion that the silver nanoparticles were encapsulated in the RTIL beads in the course of the crystal growth. The size distribution of Ag nanoparticles is narrow and the average diameter of the silver nanoparticles is approximately 12.4 nm. They are consistent with the deduction from the analysis of XRD patterns as shown below. Fig.2 TEM image and SAED pattern(inset) of as-prepared Ag nanoparticles Result of EDX spectrum shows that in addition to H element, the composite is composed of C, O, N, B, F and Ag elements. The contents of elements are listed in Table 1. The contents of N and F elements confirm that Ag particles were coated by [BMIM] BF 4 partially. Cu atoms should be the copper grids in the performance of TEM.

4 424 CHEM. RES. CHINESE UNIVERSITIES Vol.25 Table 1 EDX elemental composition of Ag nanoparticles synthesized in [BMIM] BF 4 Element C O N B F Ag Cu Content(%) not participate in the reaction fully. 3.3 XRD Analysis The XRD pattern of the as-prepared Ag nanoparticles(fig.3) shows that they held a cubic crystal structure. The major strong characteristic peaks of Ag particles are at 2θ=38.16, 44.28, 64.38, and 81.54, which were corresponding to crystal faces of (111), (200), (220), (311) and (222) of Ag [29]. All the reflection peaks could be indexed to face-centered cubic(fcc) silver and the results indicate that Ag nanoparticles prepared in RTIL did not change the crystalline structure of neat Ag nanoparticles [30]. According to the full width at half-maximum of the diffraction peaks, the average size of the particles could be estimated from the Scherrer equation to be about 15.3 nm[reaction conditions: t=30 C, c(agno 3 )= mol/l, ph=8.0]. The results are consistent with the TEM images. Fig.3 XRD pattern of Ag nanoparticles coated by RTIL molecules 3.4 XPS Analysis In order to gain more surface information of the as-prepared Ag nanoparticles with RTIL coating, XPS technique was employed to detect the product. Survey and Ag 3d spectra shown in Fig.4 exhibit that there were Ag, F, N, O and C(from the support) elements on the Ag nanoparticles. The F and N signals could indicate the nanoparticles contained residues of RTIL. Furthermore, the two clear peaks of Ag in Fig.4(B) are attributed to Ag 3d 5/2(367.5 ev) and Ag 3d 3/2(373.5 ev) orbits, which represent Ag 0 and Ag + on the surface of the nanoparticles, respectively. The result indicates that the silver nanocrystals were coated by RTIL incompletely and a few AgNO 3 dissolved in RTIL did Fig.4 XPS spectra of as-prepared Ag nanoparticles (A) Survey spectrum; (B) Ag 3d. 3.5 Antimicrobial Activities To study the bacteriostatic effects of Ag nanoparticles on the bacteria of Escherichia coli, Bacillus subtilis, Staphylococcus aureus and Pseudomonas aeruginosa, the bacteriostatic tests were carried out. The antimicrobial activities of Ag nanoparticles suspension and aqueous AgNO 3 solution are shown in Fig.5. It shows that the antibacterial rates of Ag nanoparticles are similar high to that of Ag + at the same concentration of silver nitrate and the nanoparticles had a wide spectrum of antimicrobial activities against all the tested bacteria although the concentration of Ag particles is only g/ml in the suspension. It is also noticed that Ag nanoparticle has better antimicrobial activities on Pseudomonas aeruginosa than AgNO 3 aqueous solution with the same Fig.5 Relationship of antibacterial rate of as-prepared Ag nanoparticles and AgNO 3 with different kinds of microorganisms

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