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Bulklike Thermal Behavior of Antibacterial Ag-SiO2 Nanocomposites Young Hwan Kim,† Chang Woo Kim,‡ Hyun Gil Cha,‡ Don Keun Lee,§ Byoung Kee Jo,| Gi Woong Ahn,| Eun Suk Hong,| Ju Chang Kim,⊥ and Young Soo Kang*,‡ Department of Functional Layers, GMBU e. V., P.O. Box 52 01 65, D-01317 Dresden, Germany, Department of Chemistry, Sogang UniVersity, Seoul 121-742, South Korea, Chemical R&D Center, Cheil Industries Inc., Jeollanam-do, 555-210, South Korea, R&D Center, Thefaceshopkorea Co., Ltd., Incheon 403-130, South Korea, and Department of Chemistry, Pukyong National UniVersity, Busan 608-737, South Korea ReceiVed: NoVember 10, 2008; ReVised Manuscript ReceiVed: January 21, 2009
Thermal and chemical (antibacterial) properties of Ag nanoparticles deposited on the surface of SiO2 were studied to know the possibility of applying them as antibacterial materials. To prevent aggregation of Ag nanoparticles and increase their antibacterial abilities, hybrid structures of Ag-SiO2 nanocomposites were synthesized by one-pot sol-gel method. The thermal behavior of Ag-SiO2 nanocomposites were investigated with X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM). The antibacterial properties of Ag-SiO2 nanocomposites were examined with minimum inhibitory concentration (MIC). The antibacterial property of the nanocomposites shows no significant change below Tammann temperature (e.g., 293 and 573 K); however, it was decreased upon increasing the temperature above Tammann temperature (673 - 1073 K) because of the growth of Ag nanoparticles deposited on the surface of SiO2. Introduction Nanoparticles, because of their size-dependent properties, have received vast attention and provided the opportunity of new applications or the additional flexibility to the existing systems in many areas, such as catalysis, magnetism, optics, microelectronics, and so on.1-11 In accordance with the demands of industry, syntheses of these nanoparticles have been achieved by various methods.12-29 The prepared nanoparticles must have good dispersibility and thermal stability which are very important factors for application in industry. However, the nanoparticles get aggregated easily, causing deterioration of their chemical and physical properties even when they are stored at room temperature. To solve the problem of aggregation, two major synthesis methods have been used. One significant synthesis method is to synthesize nanoparticlesinthepresenceoforganiccappingmolecules.3,4,6-12,16-18,20-22 Although nanoparticles show good dispersibility, their chemical and physical properties are decreased by organic capping molecules. Therefore, some postsynthesis procedures would be needed to remove the organic capping molecules.22 The other significant synthesis method is using supporting materials that act as hosts for the immobilization of nanoparticles.13,14,23-29 If nanoparticles are formed on supporting materials, the release time of the nanoparticles can be delayed for a long time. Therefore, the supported nanoparticles would be of great potential for various applications. Also, the support materials prevent nanoparticle aggregation, and hence the deterioration of their chemical and physical properties does not occur. * Address correspondence to this author. Tel.: + 82 2 705 8882. Fax.: + 82 2 701 0967. E-mail:
[email protected]. † GMBU e. V. ‡ Sogang University. § Cheil Industries Inc. | Thefaceshopkorea Co., Ltd. ⊥ Pukyong National University.
In applied science, the low thermal stability of nanoparticles limits their applications inevitably. Bulky metals have temperature-dependent behavior which is closely related to the Tammann temperature (TT ) 0.5TM; TT: Tammann temperature, TM: melting point), and their thermal behavior is different from that of nanoparticles.30-34 When the particle size falls into nanorange, the melting point of the particles decreases drastically with the decrease of the particle size, resulting in a much lower thermal stability, and decrease of chemical and physical properties because of the aggregation of nanoparticles. Therefore, the thermal stability of the nanoparticles must be checked for their potential industrial applications. In this study, the hybrid structure of Ag-SiO2 nanocomposites is synthesized by one-pot sol-gel method at room temperature to prevent the aggregation of Ag nanoparticles and deterioration of its antibacterial property. We report that the thermal behavior of Ag nanoparticles formed on the surface of SiO2 nanoparticles shows a similar tendency to that of bulk Ag. When Ag-SiO2 nanocomposites are annealed below Tammann temperature (e.g., 293 and 573 K), the antibacterial property of Ag nanoparticles formed on the surface of SiO2 nanoparticles shows an excellent inhibitory effect against various microorganisms, as they remain homogeneously distributed over SiO2 particles without aggregation. However, the antibacterial properties of the nanocomposites decrease with increasing the temperature above Tammann temperature (from 673 to 1073 K) because of the growth of Ag nanoparticles. Experimental Section Synthesis Method of the Ag-SiO2 Nanocomposite. SiO2 nanoparicles were synthesized by hydrolysis and condensation of tetraethoxysliane (TEOS, Aldrich Co., 98%, 0.5 mol) in a mixture of ethanol (1000 mL) with water (1 mol), using ammonia solution (1 mol, assay from 28% to 30%, Junsei Co.) as catalyst to initiate the reaction.13,14,35 Ultrahigh pure water (18 MΩ, Millipore) was used throughout the whole experiment.
10.1021/jp809892c CCC: $40.75 2009 American Chemical Society Published on Web 03/09/2009
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The reaction started with mixing and stirring of the required components for 6 h. To form Ag nanoparticles on the surface of SiO2 nanoparticles, particular amounts of silver nitrate (AgNO3, Aldrich Co., 97%, 0.01 mol) aqueous solutions were added into SiO2 nanoparticle slurry at room temperature and kept for 6 h under vigorous stirring. The obtained products were filtered and purified by washing with ethanol and water and then dried at room temperature for 1 day. To investigate the thermal behavior of Ag nanoparticles, the prepared Ag-SiO2 nanocomposites were annealed at different temperatures in the range of 573 to 1073 K for 4 h under ambient atmosphere. Test of Antibacterial Property. For antibacterial experimentation, Pseudomonas aeruginosa (ATCC 17934, Gramnegative bacteria), Staphylococcus aureus (ATCC 25923, Grampositive bacteria), and Escherichia coli (ATCC 25922, Gramnegative bacteria) were selected as indicators. All the tubes and materials were sterilized in an autoclave before experiments. The antibacterial activities of Ag-SiO2 nanocomposites were measured by minimal inhibitory concentration (MIC).36 MIC values were determined as the lowest concentration of Ag-SiO2 nanocomposites required to prohibit the growth of the microorganisms. At the end of the incubation period, the tubes were evaluated for the presence or absence of growth. Characterizations. The size and morphology of the products were studied with a transmission electron microscopy (TEM, HITACHI H-7500) and a field emission scanning electron microscopy (FE-SEM, JEOL, JSM-6700F). TEM samples were prepared by placing a 4 µL drop of the Ag-SiO2 nanocomposite solution onto a 400 mesh copper grid coated with carbon. All the grids were allowed to be dried under ambient atmosphere. Samples for FE-SEM were prepared by coating the Ag-SiO2 powder with an extremely thin (1.5-3.0 nm) layer of gold-palladium. Elemental composition of the prepared nanocomposites was characterized by energy dispersive X-ray microanalysis (EDX) in a FE-SEM. The crystal structure of the prepared Ag-SiO2 nanocomposites was identified by X-ray powder diffraction (XRD) with a Philips X’pert-MPD system using a Cu KR radiation source (λ ) 0.154056 nm). Results and Discussion Synthetic Method of Ag Formed on the Surface of the SiO2 Nanoparticle. The synthesis mechanisms of Ag-SiO2 nanoparticles under alkaline conditions were reported in our previous publications.13,14 Briefly, Ag-SiO2 nanoparticles were synthesized by three steps: deprotonation of hydroxyl ligand in SiOH (Si-O-H + B: f SiO- + BH), electrophilic metal attack (SiO- + Ag+ f SiO-Ag), and the growth of the Ag nanoparticles on the surface of the SiO2. Ag nanoparticles were homogeneously formed on the surface of SiO2 nanoparticles at room temperature, and the isolated Ag nanoparticles were not observed (Figures 1a and Figure 2a). The thermal behavior of Ag nanoparticles was investigated by annealing the Ag-SiO2 nanocomposites in the temperature range 573 to 1073 K. When Ag-SiO2 nanocomposite was annealed at 573 K for 4 h, no drastic change of particle size (Figures 1b and Figure 2b) was observed. However, upon annealing at 673 K for 4 h, the size of Ag nanoparticles was increased (Figure 1c and Figure 2c), as indicated by the arrow in Figure 1c. These results show that the growth of nanoparticles occurred by a coalescence of Ag nanoparticles whereby large nanoparticles grew at the expense of smaller ones. This phenomenon is attributed to Tammann temperature.30 When bulky particles reach Tammann temperature, atoms on their surface tend to move, which leads to interparticle diffusion and
Figure 1. TEM images of the Ag-SiO2 nanocomposite. (a) Asprepared, and annealed at (b) 573 K, (c) 673 K, (d) 773 K, (e) 873 K, (f) 973 K, and (g, h) 1073 K. All scale bars represent 97 nm except h (194 nm).
therefore coalescence. However, if the particle size falls into nanorange, the melting point and Tammann temperature decrease drastically with the decrease of particle size. Deshmukh et al. have observed the coalescence of pure Ag nanoparticles upon annealing them at 458 K, where the growth of bigger particles occurs through the approach of neighboring particles. In the case of bulk Ag, it has the melting point of 1233 K and the Tammann temperature of 617 K. In our case, SiO2 nanoparticles act as host for the immobilization of Ag nanoparticles. Therefore, there is no possibility of approaching neighboring Ag nanoparticles at room temperature and 573 K (lower than Tammann temperature). The evidence of coalescence was observed at 673 K (higher than Tammann temperature), and the thermal behavior of Ag deposited on the surface of SO2 was different from that of pure nanoparticles. In order to immobilize Ag nanoparticles, SiO2 nanoparticles were used
Antibacterial Ag-SiO2 Nanocomposites
J. Phys. Chem. C, Vol. 113, No. 13, 2009 5107 SCHEME 1: Schematic Drawing of Particle Size Growth with Increasing Annealing Temperaturea
a R.T., room temperature; TT, Tammann temperature; Tm, melting temperature.
TABLE 1: Ag, Si, and O Atomic% in the Ag-SiO2 Nanocomposites after Annealing Process weight% no. a b c d e f
Figure 2. FE-SEM images of Ag-SiO2 nanocomposite. (a) Asprepared, and annealed at (b) 573 K, (c) 673 K, (d) 773 K, (e) 873 K, (f) 973 K, and (g, h) 1073 K. All scale bars represent 100 nm.
as support material, and they prevented nanoparticles from aggregating or coalescing, so that the thermal behavior of our Ag nanoparticles showed a similar tendency to that for bulk Ag. When Ag-SiO2 nanocomposites were annealed at higher temperatures such as 773 K (Figures 1d and Figure 2d), 873 K (Figure 1e and Figure 2e), 973 K (Figures 1f and Figure 2f), and 1073 K (Figures 1g,h and Figure 2g,h), the size of the Ag nanoparticles increased with the increase of annealing temperature. At high annealing temperature (above 973 K), a significant change of particle size was observed. The thermal behavior of Ag-SiO2 nanocomposites is illustrated in Scheme 1. The size of Ag nanoparticles did not change upon annealing them from room temperature to Tammann temperature, but it increased upon increasing the annealing temperature above Tammann temperature, and large isolated Ag nanoparticles could be observed near the melting point. The EDX analyses of Ag-SiO2 nanocomposites were performed using an electron beam of 20 keV. Emission peaks correspond to the elements O, Si, and Ag were observed at 0.5249 (OkR1), 1.73998 (SikR1,2) and 1.83594 (Sikβ1), 2.9843 (AgLR1), 2.9782 (AgLR2), 3.1509 (AgLβ1), and 3.3478 (AgLβ2), respectively. Only the emission peaks correspond to Si, O, and Ag were observed in the EDX spectra of the samples. From the EDX spectra (Figure 3 and Table 1), we can confirm that the nanoparticles observed in TEM and FE-SEM images are
atomic%
O
Si
Ag
O
Si
Ag
52.12 57.45 58.89 57.68 13.04 67.83
39.32 35.05 34.34 20.30 1.37 28.31
8.56 7.50 6.77 22.02 85.59 3.86
68.77 73.15 74.12 79.55 49.17 80.25
29.55 25.43 24.62 15.95 2.95 19.08
1.68 1.42 1.26 4.50 47.88 0.68
hybrid-type pure Ag-SiO2 nanocomposites. First, in order to check the change of Ag atomic% on the surface of SiO2, isolated large Ag particles were excluded from the EDX measurements. The atomic% of Ag in annealed Ag-SiO2 nanocomposites decreased with the increase of annealing temperature as 1.68% for 293 K (Figure 3a), 1.42% for 873 K (Figure 3b), 1.26% for 973 K (Figure 3c and corresponding image presented by number 1 in Figure 2f), and 0.68% for 1073 K (Figure 3f and corresponding image presented by number 4 in Figure 2h). Second, to investigate the atomic% of Ag in the nanocomposite including isolated Ag particles, X-ray beam was focused on the area including isolated Ag particles. The atomic% of Ag in the samples is observed to be 4.50% for 973 K (Figure 3d and corresponding image presented by number 2 in Figure 2f) and 47.88% for 1073 K (Figure 3e and corresponding image presented by number 3 in Figure 2g). From the above EDX results, we confirmed that the diffusion of atoms on the surface of nanoparticles occurred, and it led to the growth of Ag nanoparticles at the expense of small nanoparticles above Temmann temperature. Because of the same reason, Ag atomic% on the surface of SiO2 decreased and the size of the isolated Ag nanoparticles increased with the increasing annealing temperature. The wide angle XRD patterns of the as-prepared and annealed Ag-SiO2 nanocomposites are shown in Figure 4. The characteristic peaks of Ag were not observed in the as-prepared sample and the sample annealed at low temperature (573 K). However, they appeared upon annealing the sample above Tammann temperature (from 673 to 1073 K). The intensity of the diffraction peaks increased and sharpened with the increase of annealing temperature. Four peaks, at 2θ values of 38.2°, 44.5°, 64.5°, and 77.7° corresponded to the (111), (200), (220), and (311) planes of Ag, respectively. Below the Tammann temperature, like at room temperature (Figure 4a) and 573 K (Figure 4b), the characteristic diffraction peaks of Ag were not observed, but the diffraction peak corresponding to the (111) plane of Ag was observed above Tammann temperature, from 673 to 1073 K (Figure 4c-g). The particle size is calculated by the Scherrer equation, and it is in inverse proportion to the full width at halfmaximum (fwhm) intensity.30,34 Besides the known calculated
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Figure 3. Energy dispersive X-ray spectra of Ag-SiO2 nanocomposites. (a) Room temperature, and annealed at (e) 873 K, (c) 973 K, number 1 in Figure 2f, (d) 973 K, number 2 in Figure 2f, (e) 1073 K, number 3 in Figure 2g, and (f) 1073 K, number 4 in Figure 2h.
Figure 4. XRD peaks of Ag-SiO2 nanocomposite. (a) As-prepared, and annealed at (b) 573 K, (c) 673 K, (d) 773 K, (e) 873 K, (f) 973 K, and (g) 1073 K.
particle size, fwhm of the diffraction peaks decreased and the particle size increased with the increase of annealing temperature. The results are in good agreement with the TEM and FE-SEM observations. From the XRD patterns and microscopic images, it can be considered that the chemical and physical properties of the Ag particles can be drastically changed near the Tammann temperature because of the growth of particle size. Evaluation of Antibiotic Property. As reported by Oya et al., the antibacterial activity of silver nanoparticles enhances
upon decreasing their size.37 From this point of view, Ag nanoparticles deposited on the surface of SiO2 nanoparticles are recommended for antibacterial activity, as they are well dispersed without aggregation. The antibacterial activities of Ag-SiO2 nanocomposites against microorganisms considered in the present study are qualitatively assessed by MIC values. Among the samples tested, high antibacterial activities against microorganisms even with a low concentration of Ag-SiO2 nanocomposite are shown in Figure 5 and Table 2. Ag-SiO2 nanocomposites prepared at room temperature exhibited high efficiency for the destruction of all microorganisms, even using only 31.3 µg/mL of Ag-SiO2 nanocomposite against E. coli (Gram-negative bacteria, Figure 5a), 7.8 µg/mL against P. aeruginosa (Gram-negative bacteria, Figure 5b), and 62.5 µg/ mL against S. aureus (Gram-positive bacteria, Figure 5c). Feng et al. have reported the mechanistic study for the antibacterial effect of silver ions on E. coli and S. aureus.38 Thiol groups are important groups of proteins responsible for enzymatic activity, and it reacted with heavy metal, which leads to the inactivation of the proteins. This phenomenon can be partially explained by the fact that Ag nanoparticles formed on the surface of SiO2 carry the positive charge, and they react with Gram-negative bacteria, P. aeruginosa and E. coli, by electrostatic interaction, thereby killing them more easily than Gram-positive bacteria because of the electrostatic attraction. For the Ag-SiO2 nanocomposites annealed below Tammann temperature (573 K), the estimated MIC values were 62.5 µg/ mL against E. coli (Figure 5d), 7.8 µg/mL against P. aeruginosa (Figure 5e), and 62.5 µg/mL against S. aureus (Figure 5f). Although the MIC value against E. coli was slightly higher,
Antibacterial Ag-SiO2 Nanocomposites
J. Phys. Chem. C, Vol. 113, No. 13, 2009 5109 values increased to 500 µg/mL against P. aeruginosa at 773 K (Figure 5k), there was no obvious drop of MIC value. However, upon increasing the annealing temperature close to the melting point, the movement and diffusion of Ag atoms were accelerated and larger Ag particles were formed at the expense of smaller nanoparticles. Therefore, the MIC values were continuously decreased with the increase of annealing temperature. The MIC values against all microorganisms were dropped to 500.0 µg/ mL against E. coli (Figure 5s), 1000.0 µg/mL against P. aeruginosa (Figure 5t), and higher than 1000 µg/mL against S. aureus (Figure 5u). Above Tammann temperature (from 673 to 1073 K), it was observed that the positively charged Ag nanoparticles can easily kill Gram-negative bacteria rather than Gram-positive bacteria by electrostatic interaction. Conclusions
Figure 5. Photographs of the antibacterial test results against E. coli, P. aeruginosa, and S. aureus. From left to right, tubes are presented as 1000, 500, 250, 125, 62.5, 31.3, 12.6, 7.8 µg/mL, and reference.
TABLE 2: Antibacterial Activities of Ag-SiO2 Nanocomposites against Different Microorganismsa microorganism concentration (µg/mL): temp
1000.0
500.0
250.0
125.0
62.5
31.3
15.6
7.8
-
+
+ + + + +
+ + + + +
+ + + + + +
+ + + + + + +
+ + + + + + +
P. aeruginosa (N) 293 K 573 K 673 K 773 K 873 K 973 K 1073 K -
+
+ + + +
+ + + + +
+ + + + +
+ + + + +
+ + + + +
+ + + + +
S. aureus (P) 293 K 573 K 673 K 773 K 873 K 973 K 1073 K +
+ + + + +
+ + + + +
+ + + + +
+ + + + +
+ + + + + + +
+ + + + + + +
+ + + + + + +
E. coli (N) 293 K 573 K 673 K 773 K 873 K 973 K 1073 K
-
Key: +, no antibacterial activity; -, antibacterial activity is present. P, Gram-positive bacteria; N, Gram-negative bacteria. a
there was no obvious change between the MIC values for the samples annealed at 573 K and those at 293 K. A drastic change of MIC values was observed upon increasing the annealing temperature. Especially, the significant drops of MIC values were observed at 673 K, such as 250.0 µg/mL against E. coli (Figure 5g), 250.0 µg/mL against P. aeruginosa (Figure 5h), and 1000.0 µg/mL against S. aureus (Figure 5i). These changes in MIC values in comparison to the MIC values for the samples annealed below Tammann temperature are attributed to the growth of Ag nanoparticles. Above Tammann temperature (673 K), the movement and diffusion of Ag atoms occurred, which led to the growth of Ag nanoparticles, in good agreement with the results from TEM and FE-SEM, and XRD. In the temperature region of 673 to 973 K, although the MIC
Hybrid structures of Ag-SiO2 nanocomposites were synthesized at room temperature to study their thermal behavior and antibacterial properties. The thermal behavior of nanocomposites was similar to that of bulk Ag, and the change of particle size was not observed below Tammann temperature, as the SiO2 support hindered Ag nanoparticles from aggregation and retarded the diffusion and approach of Ag atoms. The antibacterial properties of Ag-SiO2 nanocomposites annealed below Tammann temperature showed excellent activites because Ag nanoparticles were formed on the surface of SiO2 nanoparticles without aggregation. Above Tammann temperature, the antibacterial property was drastically decreased because of the growth of Ag nanoparticles. However, at various annealing temperatures from 297 to 1073 K, positive charged Ag nanoparticles reacted easily with Gram-negative bacteria rather than Gram-positive bacteria. Therefore, the Gram-negative bacterias could be killed more effectively than Gram-positive bacterias. Acknowledgment. The Nano R&D Program (Korea Science & Engineering Foundation, Grant 2007-02628) and Thefaceshopkorea Co. Ltd. supported this work. Y. H. Kim and J. C. Kim appreciate the financial support of the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, KRF-2007-511-C00020) and the Brain Korea 21 project in 2008, respectively. C. W. Kim and H. G. Cha appreciate the financial support of the Seoul Science Fellowship from Seoul City. References and Notes (1) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 12663. (2) Hao, E.; Kelly, K. L.; Hupp, J. T.; Schatz, G. C. J. Am. Chem. Soc. 2005, 127, 11898. (3) Diehl, M. R.; Yu, J.-Y.; Heath, J. R.; Held, G. A.; Doyle, H.; Sun, S.; Murray, C. B. J. Phys. Chem. B 2001, 105, 7913. (4) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857. (5) Kang, Y. S.; Risbud, S.; Rabolt, J. F.; Stroeve, P. Chem. Mater. 1996, 8, 2209. (6) Lee, D. K.; Kang, Y. S.; Lee, C. S.; Stroeve, P. J. Phys. Chem. B 2002, 106, 7267. (7) Kim, C. W.; Kim, Y. H.; Cha, H. G.; Kwon, H. W.; Kang, Y. S. J. Phys. Chem. B 2006, 110, 24418. (8) Cha, H. G.; Kim, Y. H.; Kim, C. W.; Kwon, H. W.; Kang, Y. S. J. Phys. Chem. C 2007, 111, 1219. (9) Qiao, R.; Zhang, X. L.; Qiu, R.; Li, Y.; Kang, Y. S. J. Phys. Chem. C 2007, 111, 2426. (10) Han, Y. C.; Cha, H. G.; Kim, C. W.; Kim, Y. H.; Kang, Y. S. J. Phys. Chem. C 2007, 111, 6275. (11) Lee, D. K.; Kim, Y. H.; Kim, C. W.; Cha, H. G.; Kang, Y. S. J. Phys. Chem. B 2007, 111, 9288. (12) Kim, Y. H.; Lee, D. K.; Cha, H. G.; Kim, C. W.; Kang, Y. S. Chem. Mater. 2007, 19, 5049.
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