J. Phys. Chem. B 2006, 110, 24923-24928
24923
Preparation and Characterization of the Antibacterial Cu Nanoparticle Formed on the Surface of SiO2 Nanoparticles Young Hwan Kim,† Don Keun Lee,‡ Hyun Gil Cha,† Chang Woo Kim,† Yong Cheol Kang,† and Young Soo Kang*,† Department of Chemistry, Pukyong National UniVersity, Busan 608-737, Korea, and Department of Chemistry and Chemical Biology, HarVard UniVersity, Cambridge, Massachusetts 02138 ReceiVed: August 31, 2006; In Final Form: October 10, 2006
Cu deposition on the surface of spherical SiO2 nanoparticles was studied to achieve the hybrid structure of Cu-SiO2 nanocomposite. SiO2 nanoparticles served as seeds for continuous Cu metal deposition. The chemical structure and morphology were studied with X-ray photoelectron spectroscopy (XPS), scanning electron microscope energy dispersive X-ray (SEM-EDX), and a transmission electron microscope (TEM). The antibacterial properties of the Cu-SiO2 nanocomposite were examined with disk diffusion assays. The homogeneously formed Cu nanoparticles on the surface of SiO2 nanoparticles without aggregation of Cu nanoparticles showed excellent antibacterial ability.
Introduction Novel metal nanoparticles, due to their conspicuous physicochemical properties, have received considerable attention in many fields.1-4 Extensive research in this area has been conducted for Ag and Au nanoscaled particles because of their attractive optical properties.5-6 To obtain high performance materials, it is extremely important that the particle’s size and morphology can be controlled. One significant approach is to synthesize the materials in the presence of a supporting material, such as alumina, titanium dioxide, polymer matrices, and mesoporous silica. These supporting materials can also act as novel hosts for the immobilization of nobel metal nanoparticles.7-10 These core-shell or hybrid structures have very recently been intensively studied, in particular since such structures exhibit peculiar properties which make them attractive for applications in optical and biological sensors, and in optoelectronics.11 In all coating methods, the monolayer covering is important, as the chemistry is specific to the shell. The monolayer of oxideshell materials is rather involved and requires multistep processes, and scale-up is difficult. To this purpose, oxide nanospheres of nearly equal size, offering great flexibility of composition, are well suited. The aim of producing hybrid structures requires one to achieve a high nucleation but low growth rate, and this results in the high number density of metal nanoparticles without the formation of aggregates. Up to now, Kawashita et al. and Park et al. reported that zeolites and carbon fiber have been developed as inorganic supports for antibacterial materials containing silver, respectively.12-13 The usefulness of Cu as an antibacterial agent has been known for a long time. It is an effective agent with low toxicity, which is especially important in the typical antibacterial treatment. The synthesis has been achieved via various routes, including radiation methods, microemulsion techniques, supercritical techniques, sonochemical reduction, laser ablation, chemical * Address correspondence to this author. Phone: +82-51-510-6379. Fax: +82-51-581-8147. E-mail:
[email protected]. † Pukyong National University. ‡ Harvard University.
vapor deposition, and so on.14-19 These synthetic methods are time-consuming and are carried out by using expensive instruments. Also Cu nanoparticles synthesized by using the above methods are easily aggregated and then cause deterioration of its chemical properties and consequently decreased antibacterial property. If Cu is deposited on supporting material, the releasing time of Cu can be delayed for a long time so that Cu-supported materials will be of great potential for antibacterial applications.20 At present, many antibacterial agents have been mainly based on organic materials, which are often not usable under conditions where chemical durability is required.21 However, Cu-supported inorganic materials can overcome this disadvantage. Few papers about the antibacterial properties of Cu nanoparticles have been published.22 Especially, metal (Cu or Ag)-supported silica materials, such as silica glass and silica thin films, are expected to be good candidates for antibacterial materials due to their fine chemical durability and high antibacterial activity. Unfortunately, only antibacterial Agcontaining silica glass is synthesized by some groups.23-27 Usually, the identification of the different oxidation states of copper can be easily carried out by XPS because of shifting binding energy of the Cu 2P3/2 photoelectron and Cu L3VV auger lines. Specific O1s binding energies and chemical states of the Cu-SiO2 nanocomposite have not been published yet. At present, Yao et al. and Goodman et al. have reported the XPS study of CuO nanorod synthesized by the solid-liquid phase arc discharge process and the correlation between the relative amounts of soft X-ray induced reduction from Cu2+ to Cu+, respectively.15-16 Also Pawlak et al. reported that the nonsingular O1s peak that occurred in the mixed perovskite crystals was due to the polarization of the oxygen valence electron density.28 In this study, we report the characterization of a welldispersed Cu-SiO2 nanocomposite. In particular, we report an XPS study of Cu-SiO2 nanocomposite including prepared SiO2 nanoparticles. Our purpose is to carry out the detailed comparative XPS study of pure SiO2 and Cu-SiO2 nanocomposite, and to see how the pure SiO2 nanoparticles differ from Cu-SiO2 nanocomposite. Also we report that the antibacterial properties
10.1021/jp0656779 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/16/2006
24924 J. Phys. Chem. B, Vol. 110, No. 49, 2006
Kim et al.
TABLE 1: Experimental Details and BET Values of the Cu-SiO2 Nanocomposite Series reactant materials
SiO2 (mmol)
H2O (mL)
CuCl2 (mmol)
Cu-SiO2-1 Cu-SiO2-2 Cu-SiO2-3 Cu-SiO2-4
25 25 25 25
100 100 100 100
11.16 44.63 11.16 11.16
NH3 (mmol)
pH
BET (m2/g)
11.74 23.48
9.80 9.78 10.94 11.32
30.45 31.44 441.82 194.05
of Cu nanoparticles formed on the surface of SiO2 nanoparticles show very excellent inhibitory effects to various microorganisms because of the ultrafine Cu nanoparticles homogeneously formed on the surface of the SiO2 nanoparticle without aggregation of the Cu nanoparticles. Experimental Section Cu Deposition on the Surface of the SiO2 Nanoparticle. SiO2 nanoparicles were synthesized according to the well-known Sto¨ber method by hydrolysis and condensation of tetraethoxysilane (TEOS, Aldrich Co., 98%, 0.5 mol) in a mixture of ethanol (1000 mL) and water (1 mol), using ammonia (1.5 mol) as a catalyst to initiate the reaction. The reaction was started with mixing and stirring of the solution, required for about 6 h, and dried at a temperature below 100 °C for 2 h.29 To deposit Cu on the surface of SiO2 nanoparticles, the specified amounts of copper chloride (CuCl2, Aldrich Co., 97%) as given in Table 1 were added to SiO2 nanoparticle slurry, which was prepared by 25 mmol of SiO2 nanoparticles dispersed in water. We synthesized the Cu-SiO2-1, Cu-SiO2-2, Cu-SiO2-3, and CuSiO2-4 by adding 11.16 mmol of CuCl2 in the absence of the catalyst, 44.63 mmol of CuCl2 in the absence of the catalyst, 11.16 mmol of CuCl2 in the presence of the catalyst (11.74 mmol of ammonia solution), and 11.16 mmol of CuCl2 in the presence of the catalyst (23.48 mmol of ammonia solution) into SiO2 nanoparticle slurry, respectively, at room temperature for 6 h under vigorous stirring. The products were filtered and purified by washing with ethanol, and then dried at room temperature for 2 h. Finally, Cu-SiO2 nanocomposite were obtained. Characterization. The size and morphology of the products were studied with a TEM (HITACHI H-7500). The N2 sorption isotherms and Brunaure-Emmitt-Teller (BET) surface area of the products were measured at 77 K, using a QUANTACHROME, Autosorb-1. All the samples were degassed at 623 K for 1 h under a vacuum before analysis. The elemental ratio of prepared nanocomposites was characterized by SEM-EDX (HITACHI S-2400). The chemical analysis on the elements was recorded on an XPS (MUTILLAB 2000). The samples were compressed into a pellet of 2 mm thickness and then mounted on a sample holder by utilizing double-sided adhesive tape for XPS analysis. The sample holder was then placed into a fast entry air load-lock chamber without exposure to air and evacuated under vacuum (
