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J. Phys. Chem. C 2008, 112, 10004–10007
Preferential Facet of Nanocrystalline Silver Embedded in Polyethylene Oxide Nanocomposite and Its Antibiotic Behaviors Qiang Chen,*,† Lei Yue,† Feiyan Xie,† Meili Zhou,† Yabo Fu,† Yuefei Zhang,† and Jing Weng‡ Laboratory of Plasma Physics and Materials, Beijing Institute of Graphic Communication, Beijing 102600, China, and Capital Medical UniVersity, School of Basic Medical Science, Beijing 100069, China ReceiVed: January 13, 2008; ReVised Manuscript ReceiVed: April 22, 2008
In this article, a magnetron sputtering (MS) method is reported to prepare a nanocomposite of embedded silver nanoparticles in simultaneously polymerized, nonfouling polyethylene oxide (PEO) film. X-ray diffraction (XRD) shows that the silver nanoparticles had tunable growth in the preferential (111) facet, and transmission electron microscopy (TEM) reveals that diameters of silver could also be controllable in the range of 5-10 nm by varying the processing parameters, such as the working pressure and the target-to-substrate distance. The antibacterial activities tests demonstrate that nanocrystalline silver in the Ag/PEO composite fabricated by this method has a markedly efficient effect. Introduction 1–4
It is well known that Ag/polyethylene oxide (PEO or PEOlike) can effectively prevent bacterial adhesion. Silver exhibits strong toxicity that denatures the adhered bacteria, and PEO (or polyethylene glycol, PEG), known for its hydrophilic behavior in water and biological media, exhibits nonfouling behavior against the adhesion of cells, bacteria, proteins, and other biological systems. Because of these advantages, the syntheses of Ag nanoparticles that are embedded in polymers by low-temperature plasma techniques were widely studied recently,5,6 but the fabrication of the combination of Ag targetand-electrode sputtering with the simultaneous polymerization of PEO for Ag/PEO or PEO-like nanocomposites in radio frequency plasma-enhanced chemical vapor deposition (rfPECVD)5 seemed inefficient because of poisoning electrodes that not only limit the deposition rate but also obtain a monotonic film. Besides, this method cannot control the diameter of Ag nanoparticles or the concentration and the crystal orientation of Ag in composites, so a new method for this nanocomposite fabrication shall be built. For PEO-like film polymerization radio frequency (rf) plasmas, in particular, rf pulsed plasmas, were extensively employed because both the structure and the concentration of the functional group (CsOsC, EO) in the films can be tailored under this plasma condition.7,8 With a small input of plasma power in continuous wave (CW) plasma or a long plasma-off time in pulsed plasma, the EO concentration can be retained as much as 80% from the monomer chemical structure.9 For silver deposition, meanwhile, magnetron sputtering (MS) had been commonly used for the synthesis of Ag nanoparticles.10 Because of the easy oxidation and aggregation of Ag nanoparticles during sputtering, the deposition process is performed in only a relatively low-pressure chamber,11,12 but it is still difficult to obtain particles with a small diameter, for example, 1-10 nm.13–18 * Corresponding author. E-mail:
[email protected]. Tel: 0086-10-60261099. Fax: 0086-10-6026-1108. † Beijing Institute of Graphic Communication. ‡ Capital Medical University.
Therefore, until now, there has been no research reported regarding the controllable diameter of Ag nanoparticles with a preferential facet in one step, and there have been no results of organic and silver nanocomposites in antimicrobial applications. Here, a strategic method of MS was proposed to do so. With this method, we prepared a nanocomposite consisting of small Ag nanoparticles (5-10 nm) and simultaneously polymerized, organic PEO. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) results showed that the diameter and orientation of Ag nanoparticles embedded in the PEO films can be controlled by varying the plasma parameters. Ag/PEO’s antibiotic behaviors have also been investigated. Experimental Section All experiments were carried out in a bell jar vacuum chamber in which Ar was fed for the 50 mm diameter silver target sputtering and monomer ethylene glycol dimethyl ether (EGDME) was obtained from another path near the substrate for the PEO polymerization. To avoid poisoning, the sputtered target was mounted above the substrate, and the gas flowed directly to the pump after diffusing through the substrate. Before the deposition, the chamber was initially evacuated to a pressure below 1.3 × 10-3 Pa, refilled with Ar three times, and evacuated back to 1.3 × 10-3 Pa. For the organic matrix polymerization, EGDME vapor was fed, which kept the constant flow rate by Ar (2 sccm) through the mass flow controller. To investigate the effect of the discharge parameters for the nanocomposite deposition, the working pressures were varied from 0.2 to 2.5 Pa, and the target-to-substrate distances were changed from 60 to120 mm. The p-silicon (100) wafers had been used as substrates and were cleaned ultrasonically in ethanol, acetone, and deionized water before they were mounted to the sample holder. dc electric power, normal in 20 W, was employed in the MS. The chemical structures of the deposited nanocomposites were characterized by Fourier transform infrared (FTIR) spectroscopy (Shimadzu, FTIR-8400, Japan), and the concentration and diameter of Ag nanoparticles were evaluated by TEM results. The silver crystal facets were investigated by XRD, and the silver diameter and the status in nanocomposites were also characterized by ultraviolet-visible spectra.
10.1021/jp800306c CCC: $40.75 2008 American Chemical Society Published on Web 06/18/2008
Ag/PEO Nanocomposite and its Antibiotic Behavior
J. Phys. Chem. C, Vol. 112, No. 27, 2008 10005
Figure 2. TEM images of the diameter of silver embedded into PEO as a function of the working pressure (a) 2.0 Pa and (b) 0.2 Pa (inserted at (a) is the electron diffraction pattern of the Ag/PEO nanocomposite. 20 W, 80 mm, Ar 10 sccm). Figure 1. FTIR of Ag/PEO nanocomposites by magnetic sputtering through the target-to-substrate distance of (a) 90, (b) 80, (c) 70, and (d) 60 mm (20 W, Ar 10 sccm, 0.5 Pa).
TABLE 1: Comparison of the Intensity of I1130/I2890 Derived from Figure 1 sample I1130/I2890 a
monomer a (90 mm) b (80 mm) c (70 mm) d (60 mm)a 1.73
1.45
2.16
1.93
12.8
Here, it is I1118/I2890.
Results and Discussion The influence of the target-to-substrate distance on the structures of MS-synthesized Ag/PEO nanocomposites was characterized by FTIR and presented in Figure 1. As the targetto-substrate distance decreased from 90 to 60 mm, the peaks around 1130 cm-1 assigned to CsOsC stretching bands all appeared in the MS polymerization films, but the peak intensities, shapes, and locations were variable and were dependent on the plasma parameters. It is noted that peaks at 1130 cm-1 for the EO group were more or less red-shifted and widened in the MS polymerization polymers. The intensities of the peaks at 2950-2860 cm-1 and 1460 cm-1 in the spectra assigned to the stretching and vibration of the sCH2 and sCH3 groups became weaker and weaker with the decrease in distance. In particular, at a distance of 60 mm, the peaks at 2950-2860 cm-1 almost disappeared; the peak around 1130 cm-1 was separated and showed a red shift to ca. 1118 cm-1 as the main peak; new peaks appeared at ca. 1620 cm-1 assigned to the CdO and CdC groups; and peaks at 1200 cm-1 and 1030 cm-1 intensified and widened, which means that the monomer of the precusor had been completely fragmented when the distance was as short as 60 mm because of the high Ar ion energy. To understand the influence of target-to-substrate distance on the structures of Ag/PEO nanocomposites more clearly, we compare the ratios of the intensity of the peaks at 1130 and 2890 cm-1, I1130/I2890, in Table 1. One can see that, excluding the concentration at 60 mm, the highest concentration of the CsOsC group in the film was obtained at a target-to-substrate distance of 80 mm. The reason for this is that at this distance the energy and density of sputtered active radicals will impact the monomer and generate suitable reactive species for polymerization. When the target-to-substrate distance is too small, such as 60 mm, the Ar ions and Ag atoms are too powerful to completely destroy the monomer for the formation of PEO films, whereas when the distance is larger than 90 mm, the energy is small and the concentration of reactive species is too low for the formation of PEO films. As a result, it is assumed that there are two advantages to relatively large distances of target-to-substrate polymerization:
Figure 3. XRD pattern of Ag particles embedded in PEO films of different working pressures (20 W, 80 mm, Ar 10 sccm).
(1) The small Ag nanoparticles can be obtained because of the coating of organic polymer that obstructs the aggregation of silver particles when they condensed on the substrate surface,19 thus the diameter of silver could be controlled. (2) The density of EO is tuned while controlling the energy by varying the distance. In parallel, water contact angle measurements of PEO films that were polymerized in MS show that the surface polarity is dependent on the target-to-substrate distance. The water contact angle on film of ca. 55 nm thickness was only 15° (average of five samples, 2 µL of deionized water) for an 80 mm sample, but it was 28° for the 60 mm deposited film. The MS power in both cases was 20 W. The extra peak around 3400 cm-1 that was assigned to sOH stretching that appeared in all of the MS films is suspected to cause by the postreaction between the film and the environment. The controlled diameter of Ag nanoparticles by the working pressure is obtained from the TEM image. Figure 2 shows the typical TEM images and the selected area electron diffraction pattern (SAED, inserted in Figure 2a) of the Ag nanoparticles embedded into PEO film. One can see that the diameter of the nanoparticles was in the range of 5-10 nm when the working pressure was 0.2 Pa (in Figure 2b), and when high working pressure such as 2 Pa was applied, the silver diameter was more than 20 nm (in Figure 2a). A more careful investigation of Figure 2a showed that the particle shapes that grow in the gaseous-phase plasma were not all spherical; triangular and elliptical shapes also appeared, which hints that the silver particles began to aggregate when they diffused in the substrate. This is confirmed by the SAED pattern in Figure 2a as an insertion of blisters. In addition, the SAED pattern inserted in Figure 2a displayed discontinuous concentric rings that are characteristic of the silver, which elucidates the fact that Ag
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Figure 4. UV-visible spectra of the Ag/PEO films in different (a) target-to-substrate distances (2.5 Pa), and (b) working pressures (20 W, 80 mm, Ar 10 sccm).
Figure 5. Control agar plate (right side) and Ag/PEO medium plate (left side) on (a) Staphylococcus aureus (gram-positive), and (b) E. coli (gram-negative). After incubation for 24 h in a 32 °C, 5% CO2 environment, the plates were photographed in white light.
TABLE 2: Results of Antimicrobial Tests antimicrobial activity nanocomposites
Ag slide
exposure time pathogens Escherichia coli Staphylococcus aureus
organism count
1h
105
99.7% 99.8%
105
2h
24 h
24 h
100% 100% 91.2% 100% 100% 90.5%
nanoparticles in the matrix were of atomic status with preferential crystal orientation. Therefore, we can predict that the agglomeration of Ag nanoparticles had happened both in the interface on the substrate and in the gaseous phase, just like the formation of dust plasma.20,21 This implies that the control of the diameters of nanoparticles shall be solved from both the cluster formation period and the cluster condensation pressure by the mechanism of dust plasma formation.22 Figure 3 shows the XRD spectra of silver-embedded PEO nanocomposites. The peaks at 2θ ) 38.1, 44.2, 64.4, and 77.3° that were assigned to (111), (200), (220), and (311) crystalline planes of silver, respectively, demonstrated that Ag nanoparticles in the composite were still crystal.23 The ratio of I(111)/I(200) for 3.03, 3.58, and 7.6 corresponded to the working pressure variation from high to low magnitudes, that is, 2.0, 1.0, and 0.2 Pa, respectively. This clearly illustrated that the nanoparticles were grown preferentially. In particular, the strongest XRD peak at 2θ ) 38.1° corresponded to the (111) facet of crystalline silver that was performed at 0.2 Pa working pressure, which indicates that silver (111) orientation in nanocomposites is preferred at a low working pressure. On the basis of this pattern, the particle size of synthesized nanocrystalline Ag could be calculated by using the Scherer formula
D ) Kλ ⁄ β cos θ
(1)
where K is constant and is dependent on crystallite shape (0.89), λ is the X-ray wavelength, β is fwhm (full width at half-
max), and θ is the Bragg angle. With eq 1, the sizes of Ag nanoparticles from Figure 2 are calculated as 7, 11, and 22 nm corresponding to 0.2, 1.0, and 2.0 Pa, respectively. Here, the calculated particle sizes of Ag are in good agreement with the ones evaluated by our TEM images mentioned above. The UV-visible spectra of Ag/PEO nanocomposites deposited at different target-to-substrate distances and working pressures are presented in Figure 4. When comparing Figure 4a,b one can see that the working pressure has a much greater influence on particle size than the distance does. In Figure 4a, the characteristic surface plasmon absorption peaks appeared at ca. 440 nm with small red shifting as the TEM increased from 80 to 100 mm at 2.5 Pa working pressure. In Figure 4b for the lower two working pressures (0.2 and 0.5 Pa), the intensities of the peaks were almost identical and had no apparent red shift. However, when the working pressure changed from 0.5 to 1 Pa, the absorption peak changed from 364 to 410 nm, a red shift of almost 40 nm, and the intensities of these peaks also decreased. For higher working pressures (1 and 2 Pa), again, there was less red shift and almost identical intensities. There are two possible reasons for the red shift: the aggregation of Ag nanoparticles and the attribution from the ligands.24 It is known25 that pure Ag nanoparticles normally exhibit an absorption band at ca. 350 nm. From Figure 4b, one can see that lower working pressure (
