Synthesis and Characterization of Antibacterial Ag-SiO2 Nanocomposite

Young Hwan Kim,† Don Keun Lee,‡ Hyun Gil Cha,† Chang Woo Kim,† andYoung Soo Kang*,†

Department of Chemistry, Pukyong National UniVersity, Busan 608-737, Korea, and Department of Chemistryand Chemical Biology, HarVard UniVersity, Cambridge, Massachusetts 02138

ReceiVed: December 4, 2006; In Final Form: January 4, 2007

In order to increase antibacterial abilities and avoid aggregation of Ag nanoparticles, Ag-SiO2 nanocompositeswere studied to achieve hybrid structure. SiO2 nanoparticles synthesized by the Sto¨ber method served asseeds for immobilization of Ag. The chemical binding structure and morphology of Ag-SiO2 nanocompositesand SiO2 nanoparticles were investigated with X-ray photoelectron spectroscopy (XPS) and transmissionelectron microscopy (TEM). The antibacterial properties of Ag-SiO2 nanocomposites were examined withdisk diffusion assay and minimum inhibitory concentration (MIC). Results showed that Ag nanoparticles arehomogeneously formed on the surface of SiO2 nanoparticles without aggregation and showed excellentantibacterial abilities.

Introduction

The synthesis of nanosized particles is a growing researchfield in chemical science, in accordance with the extensivedevelopment of nanotechnology.1-3 The size-induced propertiesof nanoparticles enable the development of new applicationsor the addition of flexibility to existing systems in many areas,such as catalysis, optics, microelectronics, and so on.4-7 Mono-and bimetallic particles in the nanosize regime find extensiveapplications in catalysis, since with reduced size, increasedsurface area increases catalytic activity.8 Various routes areavailable for the synthesis of metal nanoparticles. These methodsare based on the reduction process of precursor metal ions thatincludes chemical reduction using different reducing agents,photoreduction, sonochemical, radiolytic methods, etc.9-15 Agnanoparticles have attracted considerable attention because oftheir catalytic, optical, and conducting properties.1,16,17 Theadvantages of inorganic antibacterial materials are superior tothose of organic antibacterial materials in durability, heatresistant, toxicity, selectivity, and so on. The usefulness of Agas an antibacterial agent has been known for a long time. It isan effective agent with low toxicity, which is especiallyimportant in topical antibacterial treatment.18 Its synthesis hasbeen achieved via various routes, including microemulsiontechnique, sonochemical reduction, photochemical technique,etc.11,19-21 These synthetic methods are time-consuming andrequire expensive instruments. Also, Ag nanoparticles synthe-sized by these methods are easily aggregated,which causesdeterioration of their chemical properties and decreases theirantibacterial properties. To improve these problems and toincrease antibacterial properties, we synthesized Ag nanopar-ticles formed on the surface of SiO2 nanoparticles. If Ag isformed on supporting materials, the release time of Ag can bedelayed for a long time so that Ag-supported materials will havegreat potential for antibacterial applications.22-27 At present,many antibacterial agents have been mainly based on organic

materials, which are often not usable under conditions wherechemical durability is required.28,29 However, Ag-supportedinorganic materials can overcome this disadvantage well. Upto now, zeolites, calcium phosphate, and carbon fiber have beendeveloped as inorganic supports for antibacterial Ag-containingmaterials.30,31 Especially, Ag-supported silica materials, suchas silica glass and silica thin films, are expected to be goodcandidates for antibacterial materials due to their fine chemicaldurability and high antibacterial activity.32-34 Core-shell orhybrid structures have been intensively studied very recently,in particular since such structures exhibit peculiar propertiesthat make them attractive for applications in optical andbiological sensors and in optoelectronics.35-37

Usually, identification of the different chemical states of ele-ments can be easily carried out by X-ray photoelectron spec-troscopy (XPS) because of shifting binding energy. Many groupshave been investigated the chemical state of Ag with XPS, butthey reported the position of binding energy or the ratio of Agcomponent in analytic systems.33,38,39 Specific O1s bindingenergies and chemical states of the Ag-SiO2 nanocompositehave not been published yet. Pawlak et al.40 reported that thenonsingular O1s peak that occurred in mixed perovskite crystalswas due to polarization of the oxygen valence electron density.

In this study, we report the chemical binding states of Ag-SiO2 nanocomposite and of prepared pure SiO2 nanoparticles.Our purpose is to report a detailed comparative XPS study ofpure SiO2 and Ag-SiO2 nanocomposite and to see how the pureSiO2 nanoparticle differs from Ag-SiO2 nanocomposite. Also,we report that the antibacterial properties of Ag nanoparticlesformed on the surface of SiO2 nanoparticles show very excellent

* Address correspondence to this author: tel+ 82 51 620 6379; fax+82 51 628 8147; e-mail yskang@pknu.ac.kr.

† Pukyong National University.‡ Harvard University.

TABLE 1: Experimental Details of Ag-SiO2 NanocompositeSeries

reactant

materialSiO2

(mmol)H2O(mL)

AgNO3

(mmol)NH3

(mmol) pH

Ag-SiO2-1 50 200 8.83 9.66Ag-SiO2-2 50 200 26.49 9.62Ag-SiO2-3 50 200 8.83 105.28 10.75Ag-SiO2-4 50 200 26.49 105.28 10.79

3629J. Phys. Chem. C2007,111,3629-3635

10.1021/jp068302w CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 02/09/2007

inhibitory effects on various microorganisms because of theultrafine Ag nanoparticles homogeneously formed withoutaggregation on the surface of the SiO2 nanoparticle.

Experimental Section

Synthetic Method for Ag Formation on the Surface ofSiO2 Nanoparticles. SiO2 nanoparticles were synthesizedaccording to the well-known Sto¨ber method by hydrolysis andcondensation of tetraethoxysilane (TEOS, Aldrich, 98%, 0.5mol) in a mixture of ethanol (1000 mL) and water (1 mol),with ammonia solution (1 mol, assay from 28% to 30%, JunseiCo.) as catalyst to initiate the reaction. The size of SiO2

nanoparticles was controlled by the molar ratio of TEOS, water,and ammonia solution.41 Ultra-high-purity water (18 MΩ,Millipore) was used throughout the whole experiment. Thereaction started with mixing and stirring of the components,required for 6 h, and drying at a temperature below 100 ˚C for2 h. To form Ag nanoparticles on the surface of SiO2

nanoparticles, the specified amounts of silver nitrate (AgNO3,Aldrich, 97%) as given in Table 1 were added into SiO2

nanoparticle slurry, which was prepared by dissolving 50 mmolof SiO2 nanoparticles in water. We prepared Ag-SiO2-1, Ag-SiO2-2, Ag-SiO2-3, and Ag-SiO2-4 by adding 8.83 mmol ofAgNO3 in the absence of catalyst, 2.65 mmol of AgNO3 in the

absence of catalyst, 8.83 mmol of AgNO3 in the presence ofcatalyst (105.28 mmol of ammonia solution), and 2.65 mmolof AgNO3 in the presence of catalyst (105.28 mmol of ammoniasolution), respectively, into SiO2 nanoparticle slurry at roomtemperature for 6 h under vigorous stirring. The obtainedproducts were filtered and purified by washing with ethanoland then dried at room temperature for 2 h.

Characterization. The size and morphology of the productswere studied by transmission electron microscopy (TEM;Hitachi H-7500). The elemental ratio of prepared nanocompos-ites was characterized by scanning electron microscopy-energy-dispersive X-ray spectrometry (SEM-EDX; Hitachi S-2400).

Figure 1. Tentative mechanism of Ag nanoparticles formed on thesurface of SiO2 nanoparticle, with and without catalyst, and schematicdiagram of chemical structure of Ag-SiO2 nanocomposite.

Figure 2. XP spectra of SiO2 and Ag-SiO2 nanocomposites: (a)Survey (1, Ag3d5/2; 2, Ag3d3/2; 3, Ag3p3/2; 4, Ag3p1/2; 5, O KL22L23; 6,O KL1L23; 7, Ag MV45V45). (b) O1s XP spectra of SiO2 and Ag-SiO2. The binding energy scale has been adjusted to C1s line at 285.1eV. (c) Electron density of O in Si-O-Ag, Si-O-Si, and Si-O-H.

3630 J. Phys. Chem. C, Vol. 111, No. 9, 2007 Kim et al.

Chemical analysis on the elements was recorded on an XPS(Multilab 2000). The samples were compressed into a pellet of2 mm thickness and then mounted on a sample holder byutilizing double-sided adhesive tape for XPS analysis. Thesample holder was then placed into a fast-entry air load-lockchamber without exposure to air and evacuated under vacuum(<10-6 Torr) overnight. Finally, the sample holder was trans-ferred to the analysis chamber for XPS study. The base pressureinside the analysis chamber was usually maintained at betterthan 10-10 Torr. Binding energy was referenced to the C1s lineat 285.1 eV from adventitious carbon.42 Curve deconvolutionof the obtained XP spectra was performed with the XPS PeakFitting Program (XPSPEAK41, Chemistry, CUHK).

Test of Antibacterial Properties. For antibacterial experi-mentation, Pseudomons aeruginosa(ATCC 17934, Gram-negative bacterium),Stphylococcus aureus(ATCC 25923,Gram-positive bacterium),Escherichia coli (ATCC 25922,Gram-negative bacterium),Enterobacter cloacae(ATCC 29249,Gram-negative bacterium),Candida albicans(ATTC 11282,yeast),Penicillium citrinum (ATCC 42504, fungus), andAs-pergillus niger(ATCC 64958, fungus) were selected as indica-tors. All disks and materials were sterilized in an autoclavebefore experiments. The antibacterial activities of Ag-SiO2

nanocomposites were measured by two methods: paper diskdiffusion assay and minimal inhibitory concentration (MIC).43-45

The disk diffusion assay was performed by placing a 8 mmdisk saturated with 5000µg/mL Ag-SiO2-1, Ag-SiO2-2, Ag-SiO2-3, or Ag-SiO2-4 nanocomposite aqueous slurry onto anagar plate seeded with various microorganisms. After 24 h ofincubation, the diameters of the inhibition zones were measured.MIC values were determined as the lowest concentration ofAgSiO2-3 nanocomposite where the absence of growth wasrecorded. At the end of the incubation period, the plates wereevaluated for the presence or absence of growth.

Figure 3. TEM images of (a) Ag-SiO2-1, (b) Ag-SiO2-2, (c) Ag-SiO2-3, and (d) Ag-SiO2-4 nanocomposites. All scale bars represent 97 nm.

Figure 4. Energy-dispersive X-ray spectra of Ag-SiO2 nanocompos-ites: (a) Ag-SiO2-1, (b) Ag-SiO2-2, (c) Ag-SiO2-3, and (d) Ag-SiO2-4.

TABLE 2: Core Level Binding Energies of Elementsa

binding energy, eV

material SiO2 Ag-SiO2

Si2p 103.20 103.22O1s, Si-O-Ag 530.20O1s, Si-O-Si 532.20 532.27O1s, Si-O-H 533.20 533.26Ag3d5/2 368.43

a Based on C1s at 285.1 eV.

Antibacterial Ag-SiO2 Nanocomposite J. Phys. Chem. C, Vol. 111, No. 9, 20073631

Results and Discussion

Synthetic Method for Ag Formation on the Surface ofSiO2 Nanoparticles. Alkaline condition produces strong nu-cleophiles that deprotonate hydroxyl ligands (-OH), and thenthese nucleophilic parts (-O-) react with electrophilic materialssuch as Ag, Cu, Au, etc. This mechanism is schematicallyillustrated in Figure 1. Ag-SiO2 nanocomposites with catalystare synthesized by three steps. The first step is deprotonationof hydroxyl ligand in SiOH. Adding base catalyst into SiOHgenerates the SiO- group:

The second step is electrophilic attack. Electrophilic metal (Ag+)is easily bonded with the nucleophilic part (SiO-):

The third step is growth of the Ag nanoparticles by attachingmore Ag+ on the surface of the SiO2 nanoparticle. The Agforming process we employed is considered to be sensitive tothe configuration of terminal groups on the SiO2 surface, sincesuch surface groups obviously provide the capability requiredfor the reduction of Ag ions via reactions such as

Terminal OH groups usually form on the oxide surface bydissociative adsorption of water molecules, depending on theircoordination symmetry.46 Accordingly, nanocomposite mayform by autoreduction of noble metal ions with oxide surfaces.The efficiency of the surface-mediated reduction process canbe decreased with consumption of hydroxyl groups and genera-tion of surface charging. A schematic diagram for a tentativemechanism of Ag deposition on the surface of SiO2 nanopar-ticles without catalyst is illustrated in Figure 1.

The chemical states of elements in pure SiO2 nanoparticlesand Ag-SiO2 nanocomposites were investigated by XPS. Figure2 shows the representative survey of XP spectra of pure SiO2

and Ag formed on the surface of SiO2 nanoparticles (a) and thehigh-resolution XP spectra of O1s (b). The resulting Si2pspectrum shows a peak at binding energy 103.20 eV, inagreement with the accepted binding energy value for SiO2. Thepeaks at about 368.43 and 372.51 eV are attributed to Ag3d5/2

and Ag3d3/2, respectively. Table 2 summarizes the XPS bindingenergies of the elements in pure SiO2 and Ag-SiO2 nanocom-posites. On the basis of the binding energies, it is clear thatO1s binding energies of the Si-O-Si unit and Si-O-H unitin SiO2 nanoparticles in Figure 2b are observed at 532.20 and533.20 eV, respectively. The O1s binding energy of the Si-O-Ag unit in the Ag-SiO2 nanocomposite is observed at530.20 eV. Pawlak et al.40 reported very useful informationabout interpretation of XPS data. In that report, the more ioniccharacter the countercation has, the lower the binding energiesof the framework elements are. In the case of the SiO2

nanoparticles, the valence electron of O will be more shiftedtoward the H in Si-O-H than toward Si in Si-O-Si. Thisphenomenon makes it more difficult to eject a core electronfrom O in Si-O-H than from O in Si-O-Si. Therefore, thebinding energy of O1s in Si-O-H is observed at higher bindingenergy compared to that of O1s in Si-O-Si. In the case ofAg-SiO2 nanocomposite, the valence electron of O will be lessshifted toward the Ag in Si-O-Ag than toward Si in Si-O-Si. The electron density is greater near the O in Si-O-Ag thannear the O in Si-O-Si, which makes it easier to eject a coreelectron from O in Si-O-Ag than from O in Si-O-Si.Therefore, the binding energy of O in Si-O-Ag is observedat lower binding energy compared to that of O in Si-O-Si.

The influence of the amount of silver nitrate in the absenceof catalyst is shown in Figure 3 a,b. To get the optimized ratioof Ag, the molar ratios of AgNO3 and catalyst were controlled.As the molar ratios of AgNO3 were increased, the amount ofAg nanoparticles formed on the surface of SiO2 was increased.Figure 3a,b shows TEM images of Ag nanoparticles with adiameter of less than 10 nm formed on the surface of SiO2

nanoparticles. In the case of Ag-SiO2-1 (Figure 3a), not manyAg nanoparticles are formed on the surface of the SiO2

nanoparticle, but in the case of Ag-SiO2-2 (Figure 3b), a fewAg nanoparticles are shown. In the presence of the catalyst,many Ag nanoparticles are formed on the surface of SiO2

nanoparticles, and the sizes of Ag nanoparticles in Ag-SiO2-3(Figure 2c) and Ag-SiO2-4 (Figure 2d) are larger than thoseof Ag nanoparticles prepared without the catalyst (Ag-SiO2-1and Ag-SiO2-2) despite the same reaction time. In particular,Ag nanoparticles of Ag-SiO2-3 are easily formed on the surfaceof SiO2 compared to those of Ag-SiO2-1 despite the same molarratio of AgNO3, and the size of Ag nanoparticles of Ag-SiO2-3

TABLE 3: Ag, Si, and O Atomic Percentages of the Ag-SiO2 Nanocomposites

XPS analysis EDX analysis

at. % at. % wt %

material Ag Si O Ag Si O Ag Si O

Ag-SiO2-1 2.13 26.25 71.23 2.00 37.40 60.60 9.65 46.98 43.37Ag-SiO2-2 2.14 24.51 73.35 2.20 36.89 60.91 10.55 46.10 43.35Ag-SiO2-3 2.52 26.25 71.23 2.70 38.83 58.47 12.55 47.07 40.37Ag-SiO2-4 4.00 23.88 72.12 4.31 32.95 62.74 19.41 38.66 41.93

TABLE 4: Antibacterial Activities of Ag -SiO2 Nanocomposites against Various Bacteria, Fungi, and Yeasta

Ag-SiO2 (µg/mL)

microorganism 0 100 200 300 500 1000 2000 3000 5000

Escherichia coli(N) + + + - - - - - -Pseudomonas aeruginosa(N) + + + - - - - - -Staphylococcus aureus(P) + + + + + + + - -Enterobacter cloacae(N) + + + + + + + - -Candida albicans(Y) + + + + + + - - -Penicilium citrinum(F) + + + + + - - - -Aspergillus niger(F) + + + + + - - - -

a Key: +, antibacterial activity;-, antibacterial activity; P, Gram-positive bacterium; N, Gram-negative bacterium; F, fungus; and Y, yeast.

Si-O-H + B: f SiO- + BH

SiO- + Ag+ f SiO-Ag

Si-O-H + Ag+ f Si-O-Ag + H+

3632 J. Phys. Chem. C, Vol. 111, No. 9, 2007 Kim et al.

is larger than that of Ag-SiO2-1. The same trend is observedbetween Ag-SiO2-4 and Ag-SiO2-2. In the case of Ag- SiO2-3, homogeneous Ag nanoparticles are observed, but in the caseof Ag-SiO2-4, Ag nanoparticles formed on the surface of SiO2

nanoparticles are aggregated and some Ag nanoparticles areindependently formed.

EDX analyses of Ag-SiO2 nanocomposites excited by anelectron beam (20 keV) were performed. Peaks for the elementsO, Si, and Ag are observed at 0.5249 (OkR1), 1.739 98 (SikR1,2),1.835 94 (Sikâ1), 2.9843 (AgLR1), 2.9782 (AgLR2), 3.1509(AgLâ1), and 3.3478 (AgLâ2), respectively. There are only Si,O, and Ag components in EDX spectra. From the EDX spectra(Figure 4 and Table 3), we can confirm that nanoparticles inTEM images are pure hybrid-type Ag-SiO2 nanocomposites.Table 3 shows that the atomicpercentage of Ag is increasedfrom 2.00 to 4.31. Table 3 and Figure 3 show that the Agnanoparticles are easily formed and well dispersed on the surfaceof SiO2 nanoparticles in the presence of a moderate amount ofcatalyst. The atomic percentages of Ag, Si, and O investigatedwith EDX and XPS show similar tendencies, as summarized inTable 3.

Evaluation of Antibiotic Properties. It is suggested to beadvantageous to enhance the antibacterial activity that strongadsorption ability causes both bacteria and part of the dispersedAg nanoparticles to be adsorbed on the surface of SiO2

nanoparticle, where the bacteria have more opportunities tocontact Ag nanoparticles. From this point of view, in order toavoid the aggregation of Ag nanoparticles, an optimum levelof Ag nanoparticle loading is recommended for the synthesisof Ag formed on the surface of SiO2 nanoparticles forantibacterial activity. Antibacterial activities of Ag-SiO2 nano-composites against microorganisms considered in the presentstudy were qualitatively and quantitatively assessed by deter-mining the presence of inhibition zones and MIC values,

respectively. Antibacterial effects in the form of inhibition zones,evaluated by the disk diffusion assay of the Ag-SiO2 nano-composites, are shown in Figure 5. The clear zones of Ag-SiO2-3 againstS. aureus(Gram-positive bacterium),P. aerug-inosa (Gram-negative bacterium),C. albicans (yeast), P.citrinum (fungus), andA. niger (fungus) are determined as 0,14, 11, 11.5, and 0 mm, respectively, but the symptoms of clearzones againstS. aureusand A. niger are shown because thecolors between microorganism and around disk are different.47

In the case of AgSiO2-1, Ag-SiO2-2, and Ag-SiO2-4, inhibitionzones are clearly detected againstP. aeruginosabut they arenot detected against other microorganisms; however, in the caseof AgSiO2-3, inhibition zones are clearly detected againstP.aeruginosa, C. albicans, and P. citrinum. From these resultsand EDX and TEM images, the antibacterial properties arecontrolled by the molar ratio of Ag and dispersibility of Agnanoparticles. As the molar ratio of Ag is increased, antibacterialproperties are also increased except in the case of Ag-SiO2-4.In TEM images of Ag-SiO2-3 and Ag-SiO2-4, Ag-SiO2-3showed better dispersibility of the Ag nanoparticles on thesurface of the SiO2 nanoparticles, but Ag-SiO2-4 showedaggregation of Ag nanoparticles, despite the fact that the Agatomic percentage of Ag-SiO2-4 was higher than that of Ag-SiO2-3. Aggregation of Ag nanoparticles causes deteriorationof the antibacterial activity. It is obvious that the antibacterialactivity will be improved greatly with increasing amount of Agnanoparticles formed on the surface of SiO2 nanoparticles andthe degree of dispersibility of Ag nanoparticles. Feng et al.48

reported a mechanistic study of the antibacterial effect of silverions on E. coli and S. aureus. The first suggestion is thefollowing; as a reaction against the denaturation effects of silverions, DNA molecules become condensed and lose their replica-

Figure 5. Photographs of antibacterial test results on (a)S. aureus,(b) P. aeruginosa, (c) C. albicans, (d) P. citrinum, and (e)A. nigerincubated on plates. (f) Position 1 is Ag-SiO2-1, position 2 is Ag-SiO2-2, position 3 is Ag-SiO2-3, and position 4 is Ag-SiO2-4nanocomposite dispersed in water.

Figure 6. Photographs of the antibacterial test results against (a-d)P. aeruginosa, E. coli, S. aureus, andE. cloacaeand (e-g) C. albicans,P. citrinum, andA. niger. Amount of Ag-SiO2-3 nanocomposite used(micrograms per milliliter): (a) 200, (b) 300, (c) 500, (d) 3000, (e)500, (f) 1000, and (g) 2000.

Antibacterial Ag-SiO2 Nanocomposite J. Phys. Chem. C, Vol. 111, No. 9, 20073633

tion abilities. The second suggestion is that silver ions interactwith thiol groups in proteins, which induces the inactivation ofbacterial proteins. Besides the known antibacterial mechanismof Ag, this phenomenon may also be partially explained by thefact that Ag nanoparticles formed on the surface of SiO2 carrythe opposite charge with Gram-negative bacteria,P. aeruginosa,thereby killing them more easily than Gram-positive bacteriadue to the electrostatic attraction. The values of inhibition zonesagainst fungi (P. citrinumandA. niger) and yeast (C. albicans)appear in the middle between the values of inhibition zonesagainst Gram-positive (S. aureus) and Gram-negative (P.aeruginosa) bacteria.

The growth inhibition effects forP. aeruginosa, S. aureus,E. coli, E. cloacae, C. albicans, P. citrinum, andA. nigerwerequantitatively determined by the MIC method for the Ag-SiO2-3 nanocomposite. Among the samples tested, high anti-bacterial activities against bacteria, fungi, and yeast even withlow concentrations of Ag-SiO2-3 nanocomposite are shownin Figure 6 and Table 4. Especially, Ag-SiO2-3 nanocompositeexhibits high efficiency for the destruction ofE. coli and P.aeruginosa, even with only 300µg/mL Ag-SiO2-3 nanocom-posite (37.65µg/mL Ag nanoparticles). From EDX results, theexact concentration of Ag is calculated as 37.65µg/mL because100 wt % Ag-SiO2 nanocomposite consists of 12.55 wt % Agand 87.44 wt % SiO2. In the case of 2000µg/mL Ag-SiO2

nanocomposites (251µg/mL Ag), the antibacterial activitiesindicate good efficiency against all microorganisms used in thisstudy because of the enhanced dispersibilty of Ag nanopar-ticles.49 From MIC results, we reconfirm that positively chargedAg nanoparticles are easily reacted with Gram-negative bacteriarather than Gram-positive bacteria, so that Gram-negativebacteria are more effectively killed than Gram-positive bacteria.Also, the degree of antibacterial activities of Ag-SiO2 nano-composite against fungi and yeast appears in the middle rangebetween Gram-positive and Gram-negative bacteria because ofthe above mechanism.

Conclusion

SiO2 formed with Ag nanoparticles was prepared at roomtemperature. This method will be well suited for preparing metalnanoparticles formed on the surface of SiO2 nanoparticles. Asthe ratio of AgNO3 was increased, the amount of Ag nanopar-ticles formed on the surface of SiO2 was increased. To easilyform Ag nanoparticles on the surface of SiO2, use of a catalystin the synthetic method was recommended because strongnucleophiles were produced by ammonia solution via deproto-nation of hydroxyl ligands and then electrophilic material, likemetal ions, reacted with nucleophilic sites. XPS measurementshowed that O1s binding energies were affected by the atomsattached to O. The binding energy of O1s in Si-O-H wasobserved at higher values than that of O1s in Si-O-Si becauseof H atom decreasing the electron density of O (O-H), whereasthe binding energy of O1s in Si-O-Ag was observed at lowerbinding energy than that of O1s in Si-O-Si because Ag atomincreases the electron density of O (O-Ag). In the antibacterialtest, the antibacterial activities of Ag-SiO2-3 were clearlydetected against Gram-negative bacteria, Gram-positive bacteria,yeast, and fungi. The antibacterial activities of Ag-SiO2-3nanocomposite were higher than those of Ag-SiO2-4 becauseAg nanoparticles of Ag-SiO2-3 nanocomposite were homoge-neously formed on the surface of SiO2 nanoparticle withoutaggregation of the Ag nanoparticles. Positively charged Agnanoparticles killed Gram-negative bacteria more easily thanGram-positive bacteria.

Acknowledgment. Functional Chemicals Development Pro-gram and Thefaceshopkorea Co. Ltd. supported this work. Wealso appreciate financial support by the Brain Korea 21 projectin 2006.

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