Deposition of Silver Nanoparticles on Multiwalled Carbon Nanotubes

Nov 7, 2008 - School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637722, Singapore, Director...
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J. Phys. Chem. C 2008, 112, 18754–18759

ARTICLES Deposition of Silver Nanoparticles on Multiwalled Carbon Nanotubes Grafted with Hyperbranched Poly(amidoamine) and Their Antimicrobial Effects Wei Yuan,†,‡ Guohua Jiang,†,‡ Jianfei Che,‡ Xiaobao Qi,‡ Rong Xu,‡ Matthew W. Chang,‡ Yuan Chen,‡ Su Yin Lim,§ Jie Dai,| and Mary B. Chan-Park*,‡ School of Chemical and Biomedical Engineering, Nanyang Technological UniVersity, 62 Nanyang DriVe, Singapore 637722, Singapore, Directorate of Research and DeVelopment, Defence Science & Technology Agency, 71 Science Park DriVe, Singapore 118253, Singapore, and DSO National Laboratories, 20 Science Park DriVe, Singapore 118230, Singapore ReceiVed: January 30, 2008; ReVised Manuscript ReceiVed: September 17, 2008

A nanohybrid comprising silver nanoparticles within third-generation dendritic poly(amidoamine) (PAMAM) grafted onto multiwalled carbon nanotubes (MWNTs) was applied as an antimicrobial agent in solution. The high abundance of amine groups on the dendrimer-modified MWNTs (d-MWNTs) provided sites for reduction and precipitation of silver nanoparticles from silver acetate aqueous solution, resulting in carbon nanotubes/ Ag nanohybrids (d-MWNTs/Ag). The content of PAMAM grafted on d-MWNTs determined by using a thermal gravimetric analyzer (TGA) was about 45%. The silver nanoparticles produced were determined to be face-centered cubic silver nanocrystals by X-ray powder diffraction (XRD). The nanohybrids were investigated with scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, and X-ray energy dispersive spectroscopy (EDS). The antimicrobial properties of acid-treated MWNTs (MWNTs-COOH), d-MWNTs, and d-MWNTs/ Ag were investigated against Staphylococcus aureus (S. aureus), Escherichia coli (E. coli), and Pseudomonas aeruginosa (P. aeruginosa). Against E. coli and P. aeruginosa which are Gram-negative, d-MWNTs and d-MWNTs/Ag which are equally effective were found to have a stronger antimicrobial effect than MWNTsCOOH. Against S. aureus (Gram-positive), d-MWNTs/Ag showed a stronger antimicrobial effect than d-MWNTs (92.3% kill versus 71.6% kill), while MWNTs-COOH only killed 15.4% of this bacteria. Possible mechanisms are proposed to explain the higher antimicrobial activity by d-MWNTs/Ag nanohybrids. These findings suggest that PAMAM/Ag grafted onto insoluble MWNTs may be used as effective antimicrobial materials. Introduction Effective water-disinfecting materials which are insoluble and in a separate phase are highly attractive, especially for potable water, since they minimally contaminate the water. Silver nanoparticles are known to exhibit antimicrobial properties against bacterial pathogens. The mechanism of antimicrobial activity of silver nanoparticles remains debatable and includes (i) membrane damage by free radicals,1 (ii) membrane structure degradation by “pits” in cell walls,2 and (iii) penetration of cell walls and dephosphorylation of key peptides in cellular signaling cycles.3 Another antibacterial material is dendritic poly(amidoamine) (PAMAM).4 Dendrimers are highly branched, welldefined, synthetic macromolecules possessing a globular structure with a high density of functional groups on their periphery.5 Metal nanoparticles can be encapsulated inside cavities within their branched molecular structure. The PAMAM/Ag hybrid, * Corresponding author. Email: [email protected]. † These two authors made equal contributions. ‡ Nanyang Technological University. § Defence Science & Technology Agency. | DSO National Laboratories.

which is soluble in water, has been investigated for antimicrobial effects, but it contaminates the treatment feed.6 Multiwalled carbon nanotubes (MWNTs) are attractive as noncontaminating carriers since their nanoscale diameter and micron-scale length enable them to have high surface areas and be easily deposited on micron-scale porous membranes during water filtration/disinfection. The antimicrobial activity of singlewalled nanotubes (SWNTs) has recently been reported by Elimelech et al.7 The cell membrane is damaged resulting from direct contact with pristine SWNT aggregates leading to bacterial cell death. Multiwalled carbon nanotubes (MWNTs) have also been found to possess antimicrobial activity, though inferior.8 However, MWNTs are significantly cheaper than SWNTs and are now available at a reasonably low price (∼US$0.5-1.0/g), making them applicable for large-scale applications. The still large surface area of MWNTs can be used for immobilization of PAMAM/Ag to afford economical and effective yet insoluble antimicrobial material. Tao et al.9 have recently reported the synthesis of MWNT/PAMAM/Ag composites. However, to the best of our knowledge, the antimicrobial activity of the MWNT/PAMAM/Ag nanohybrid has not been previously examined.

10.1021/jp807133j CCC: $40.75  2008 American Chemical Society Published on Web 11/08/2008

Ag Nanoparticles on Multiwalled Carbon Nanotubes In this paper, we apply PAMAM dendrimer modified multiwalled carbon nanotube/Ag (d-MWNTs/Ag) nanohybrids as an antimicrobial solution against Gram-negative (Escherichia coli and Pseudomonas aeruginosa) and Gram-positive (Staphylococcus aureus) bacteria. In particular, P. aeruginosa and S. aureus are ubiquitous human pathogens, the antibiotic resistance of which is of serious public concern. Third-generation dendritic poly(amidoamine) is grafted onto MWNTs, and the resulting material is denoted by d-MWNTs. Silver nanoparticles were deposited on the surface of d-MWNTs after precipitation from silver acetate aqueous solution without any additional reducing reagent. The resulting nanohybrid is denoted as d-MWNTs/Ag. The resultant samples were characterized by a thermal gravimetric analyzer (TGA), X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectra (FT-IR), Raman spectroscopy, and X-ray energy dispersive spectroscopy (EDS). The antimicrobial activities of d-MWNTs and dMWNTs/Ag were evaluated against the target bacterial pathogens, and acid-treated MWNTs (MWNTs-COOH) were used as a control. Experimental Materials. MWNTs with purity greater than 95% were purchased from Chengdu Research Institute of Organic Chemistry (China). The MWNTs, produced by catalytic chemical vapor deposition, have diameters of about 20 nm and lengths of about 50 µm. Ethylenediamine (EDA), methyl acrylate (MA), toluene 2,4-diisocyanate (TDI), silver acretate (AgAc), and other chemical reagents were obtained from Aldrich and were used as received. Three test microorganisms, namely, Staphylococcus aureus (ATCC 6538), Escherichia coli (ATCC 8739), and Pseudomonas aeruginosa (ATCC 9027), and the related chemical and biological agents were supplied by Pharmaceutical Microbiology Laboratory, Department of Pathology, Singapore General Hospital. Preparation of MWNTs-COOH, d-MWNTs, and dMWNTs/Ag. Pristine MWNTs (p-MWNTs, i.e., as supplied) were first thermally treated at 350 °C for 2 h in air followed by acid oxidization in a mixture of concentrated sulfuric acid and nitric acid (3:1) at 70 °C for 4 h to produce MWNTs-COOH. After that, MWNTs-COOH were reacted with toluene 2,4diisocyanate in a dry nitrogen atmosphere to form carbon nanotubes functionalized with isocyanate groups (MWNTsNCO).10 Then MWNTs-NCO reacted with an excess of ethylene diamine to obtain MWNTs containing amino groups (MWNTsNH2). Multiwalled carbon nanotubes grafted with dendritic polyamidoamine (d-MWNTs) were achieved by repeating two processes: (1) Michael addition of methyl acrylate to the surface amino groups and (2) amidation of terminal ester groups with ethylene diamine.11 Multiwalled carbon nanotubes grafted with dendritic polyamidoamine (d-MWNTs, 50 mg) were dispersed in 10 mL of deionized water in a glass vial. An amount of 10 mL of silver acetate aqueous solution (0.01 mol · L-1) was added dropwise into the mixture. The vial was then closed and loaded into an oil bath (25 °C) and agitated and stirred for 24 h. After centrifugation, filtration, and thorough washing with deionized water to remove any loosely adsorbed nanoparticles, carbon nanotube/silver nanocrystal nanohybrids (d-MWNTs/Ag) were obtained. Characterization. Thermal gravimetric analysis (TGA) was performed with a Netzsch STA 409 PG/PC instrument at a heating rate of 20 °C/min from 50 to 800 °C under nitrogen.

J. Phys. Chem. C, Vol. 112, No. 48, 2008 18755 Scanning electron microscopy (SEM) measurements were carried out on a JEOL field-emission scanning electron microscope, model JSM-6700F. Transmission electron microscopic (TEM) observations were carried out on a JEOL 2010 scanning electron microscope operating at 200 kV. Qualitative analysis of particle composition was carried out using X-ray energy dispersive spectroscopy (EDS). XRD patterns were recorded on a Rigaku X-ray diffractometer, model D/MAX-2200/pc, equipped with a Cu KR photon source (40 kV, 20 mA, λ ) 0.15406 nm) scanned at the rate of 1.0° min-1 over the range of 10-80° (2θ). Raman spectra were acquired with a microRaman spectrometer (Reinshaw Raman scope RM3000) at room temperature in the backscattering configuration using an Arion laser with wavelength 514.5 nm. Fourier transform infrared (FT-IR) spectra were recorded on a PE Paragon 1000 spectrometer. Antimicrobial Test with Carbon Nanotube (CNT) Solutions. The antimicrobial activity of MWNTs-COOH, dMWNTs, and d-MWNTs/Ag was evaluated against S. aureus, E. coli, and P. aeruginosa by the Phamaceutical Microbiology Laboratory at the Singapore General Hospital. The method used was in accordance to the requirement of ISO 14729:2001(E) (Ophthalmic optics - Contact lens care products - Microbiological requirements and test methods for products and regimens for hygienic management of contact lenses) and Section 61 Microbial Limit Tests in the United States Pharmacopeia 30 (2007). Each microorganism sample was a 24 h subculture on Trypticase Soy Agar, harvested and washed two times with phosphate buffered saline (PBS), pH 7.2, by centrifugation. The sample was diluted in PBS (pH 7.2) to provide a concentration of 20 ug/mL for the tests. An amount of 20 µL of the bacterial suspension in USP phosphate buffer pH 7.2 was transferred to 1 mL of the prepared sample (11 µg MWNTs-COOH/mL, 20 µg d-MWNTs/mL, or 20 µg d-MWNTs/Ag/mL) in a test tube. The inoculated sample was kept in an incubator maintained at 35 °C for one hour with intermittent mixing every 15 min. After one hour of incubation, 1 mL of the sample was transferred to 9 mL of neutralizing broth. The mixture was left standing at room temperature for 15 min. The tube was vortexed, and a series of 10-fold dilutions in neutralizing broth were prepared and plated out in Trypticase Soy Agar. The plates were incubated at 35 °C for 72 h and counted for colony forming units. The neutralization broth contained a mixture of neutralizing agents that was demonstrated to inactivate the antimicrobial activity from the sample residues that were carried forward into the 10-fold dilutions. All experiments were carried out in duplicate. The results are expressed as Log Reduction ) Log Initial count of inoculum - Log Survivor count at 4 h exposure time % kill ) count of inoculum - survivor count at 4 hr exposure time × 100 initial count of inoculum

Results and Discussion Our synthetic approach for the d-MWNTs/Ag nanohybrid is schematically depicted in Scheme 1. The contents of PAMAM grafted on MWNTs were measured from TGA curves. Figure 1 shows that the weight loss of MWNTs-COOH when heated from 50 to 800 °C was not very significant, only 4.6%. The decomposition onset temperature (Tonset) of d-MWNTs is about 230 °C. The attached organic contents, estimated from the residual weight of the functionalized samples at 800 °C, are

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SCHEME 1: Schematic Illustrations of Surface Modification of MWNTs to Generate a G3-PAMAM Grafted MWNT Conjugate with Amine As Peripheral Groups (A) and Generation of d-MWNTs/Ag Nanocomposites by the in Situ Reduction and Deposition Method (B): (1) d-MWNTs, (2) d-MWNTs/Ag nanohybrid

10%, 16%, 29%, and 45% for generation 0, 1, 2, and 3 of d-MWNTs, respectively. The quantity of the dendrimer grafted on MWNTs increases linearly with the generation of dendrimer from 0 to 3.11 The successful deposition of silver nanoparticles on MWNTs is confirmed by the SEM, EDS, and TEM results. Figure 2A shows that the surface of MWNTs grafted with PAMAM before adding Ag+ solution is quite smooth. It is also noted that both the length and diameter of MWNTs have a wide distribution.

Figure 1. TGA curves of (A) MWNT-COOH and dendrimer functionalized MWNTs (B) generation 0, (C) generation 1.0, (D) generation 2.0, and (E) generation 3.0.

After the deposition process, the surface of the MWNTs has become rough with nanoparticles attached on the surface of nanotubes (Figure 2B). The EDS results further verify the presence of silver in the nanohybrids (Figure 2C). The C signal mainly originates from MWNTs, and a small part of that, as well as the N signal, comes from the surface organic molecules. The Si signal comes from the substrate. The mass ratio of Ag/C is about 12.7%, which indicates a high Ag content in the nanohybrids. The TEM image (Figure 2D) confirms that all Ag nanoparticles (indicated by the arrows) are deposited on the surface of the MWNTs. Loose and free-standing silver nanoparticles were not observed in our sample. The particle size of silver nanocrystals is polydispersed which is most likely due to the inhomogeneous dimension distribution of the pristine MWNTs, as well as the complex geometry existing in the bundles of these nanotubes, which influence the growth of the metal nanocrystals. Figure 3A shows the XRD patterns of p-MWNTs. The diffraction peaks at 2θ of 26.24° and 42.58° are due to the (002) and (110) planes of MWNTs. Figure 3B is the XRD pattern of the d-MWNTs coated with Ag nanoparticles. According to the JCPDS cards No. 04-0783, the diffraction peaks at 2θ ) 38.54°, 43.90°, 64.72°, and 77.39° can be readily indexed to (1 1 1), (2 0 0), (2 2 0), and (3 1 1) reflections of silver metal crystals with face-centered cubic symmetry. The estimated domain size of the silver nanocrystals using the (111) peak by the Scherrer equation12 is ca. 10 nm. The characteristic diffraction peaks of p-MWNTs are still present in the d-MWNTs/Ag spectra but with diminished intensity. The peak height reduction is presumably due to tube wall defects produced by the acid treatment and polymer grafting processes.

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Figure 2. SEM images of d-MWNTs (A) and d-MWNTs/Ag nanohybrids (B), X-ray energy dispersive spectroscopy (EDS) of d-MWNTs deposited with silver nanoparticles on Si surface (C), and TEM image of d-MWNTs deposited with silver nanoparticles (D). The arrows in D indicate the locations of silver nanoparticles.

Figure 3. XRD patterns of (A) p-MWNTs and (B) d-MWNTs/Ag nanohybrids.

FT-IR spectra of the samples were measured to characterize the absorption peaks of the functional groups of the dendrimers on the MWNTs. The spectra of d-MWNTs and d-MWNTs/Ag nanohybrids are shown in Figure 4. The most obvious change is the band position of the amine groups (N-H stretch), which is centered at 3368 cm-1 for d-MWNTs and 3420 cm-1 for hybrids. The shift of the amine group band in the hybrids indicates the presence of interactions with the silver nanoparticles which result in the formation of stable silver colloids.13 Absorption troughs due to the methylene group (2817 and 2927

Figure 4. FT-IR spectra of (A) d-MWNTs and (B) d-MWNTs/Ag nanohybrids.

cm-1) are also shifted in the presence of the nanoparticles. The absorptions at 1655 and 1565 cm-1 are due to the amide I and II bands in d-MWNTs and d-MWNTs/Ag.14 The absorption at approximately 1480 cm-1 is the amide III band and is slightly shifted in the presence of the Ag particles.15 The decrease in the absorption features at 1310 and 950 cm-1 in the hybrid spectra (Figure 4B) indicates conformational change of poly(amidoamine) after deposition of silver nanoparticles. These results suggest that d-MWNTs interact strongly with silver particles,

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Figure 5. Raman spectra of (A) p-MWNTs, (B) d-MWNTs, and (C) d-MWNTs/Ag nanohybrids.

an inference that is reinforced by the fact that repetitive washings of the centrifuged product could not remove the particles. Figure 5 shows Raman spectra of p-MWNTs, d-MWNTs, and d-MWNTs/Ag nanohybrids under 514.5 nm excitation over the Raman shift interval of 1200-2000 cm-1. The D- and G-bands of MWNTs at ca. 1350 and 1582 cm-1, corresponding to defect- and disorder-induced modes and the in-plane E2g zonecenter mode, respectively, are clearly observed in p-MWNTs, d-MWNTs, and d-MWNTs/Ag.16 Moreover, p-MWNTs and d-MWNTs also show the D′-band at 1620 cm-1, which is known to be directly affected by the disorder in nanotubes. This band is barely detectable against the noise in p-MWNTs (Figure 5, spectrum A) but is obvious after functionalization (Figure 5, spectrum B), suggesting increased tube surface defects. However, the D′-band is rarely detectable in d-MWNTs/Ag nanohybrids because the D- and G-bands in the d-MWNTs/Ag nanohybrid spectra are substantially broader than in the pMWNTs spectra so that the D′-band peak becomes not obvious. The broader peaks may be due to the deposition of Ag nanoparticles, which can be of different sizes, modulating the microenvironment of the CNTs to be more diverse. The D- to G-band intensity ratio (ID/IG) for the MWNTs grafted with dendritic polyamidoamine (d-MWNTs) is 1.04, greater than that of p-MWNTs (ID/IG ) 0.85), which suggests the presence of more defects in d-MWNTs. The ID/IG ratio of d-MWNTs/Ag nanohybrids is 1.03, indicating that the intrinsic nanotube disorder is not directly affected by the silver nanoparticles. Similar nanohybrids were reported by Gao et al. who used carboxylic acid group containing polymer for surface modification of CNTs. It was suggested that the carboxylic acid groups on the surface of the oxidized CNTs after acid treatment or in the poly(acrylic acid) (PAA) chains play the role as a catalyst and “catcher” for the reduction of Ag+ and deposition of the metallic silver nanoparticles.17 In addition, the PAMAM dendrimers grafted on our MWNTs contain abundant amine groups which can form a complex with transition metal cations and also possibly cause in situ reduction of Ag+ to metallic silver.18 The antimicrobial efficacies of MWNTs-COOH, d-MWNTs, and d-MWNTs/Ag were examined against three representative microorganisms, E. coli (Gram-negative), P. aeruginosa (Gramnegative), and S. aureus (Gram-positive). E. coli, the most characterized bacterium, has been used as a model bacterial system for various antimicrobial testing programs. P. aeruginosa causes chronic respiratory infections in individuals with cystic fibrosis and cancer, whereas S. aureus is responsible for a wide range of infectious diseases, ranging from benign skin infections to life-threatening endocarditis and toxic shock syndrome. In particular, P. aeruginosa and S. aureus are major causes of nosocomial infections because of their virulence and antibiotic resistance.

Figure 6. Antimicrobial effects of MWNTs-COOH, d-MWNTs, and d-MWNTs/Ag against S. aureus (Gram-positive), E. coli (Gramnegative), and P. aeruginosa (Gram-negative).

Figure 6 shows that the MWNTs-COOH have relatively low percent kill for all three bacteria: 15.4 ( 0.6%, 50.0 ( 1.7%, and 13.1 ( 1.2% for S. aureus, E. coli, and P. aeruginosa, respectively. The d-MWNT have relatively higher antimicrobial efficacy compared to MWNTs-COOH: the percent kill for S. aureus, E. coli, and P. aeruginosa increases to 71.6 ( 0.8%, 99.3 ( 0.5%, and 99.4 ( 1.0% respectively. With Ag nanoparticles immobilized in d-MWNT/Ag, the percent kill for S. aureus, a Gram-positive bacterium, improves to 92.3 ( 0.7%, while the corresponding values for E. coli and P. aeruginosa, both of which are Gram-negative bacteria, are similar to those of d-MWNT, i.e., more than 99%. Our result suggests that (i) both d-MWNTs and d-MWNTs/Ag showed much stronger antimicrobial effect than MWNTs-COOH against all of the three microorganisms; (ii) silver nanoparticle deposition significantly improved the antimicrobial efficacy of d-MWNTs against S. aureus, and (iii) both d-MWNTs and d-MWNTs/Ag are equally effective against E. coli and P. aeruginosa, Gram-negative bacteria. The generally lower efficacy against S. aureus is similar to the antimicrobial effect of silver nanoparticles, which is thought to be due to the different membrane structures of the bacteria.1 Gram-positive and -negative bacteria have different membrane structures, and the most distinctive difference lies in the thickness of the peptidoglycan layer. The peptidoglycan layer of Gram-positive bacteria (about 20-80 nm) is usually thicker than that of Gram-negative bacteria (about 7-8 nm).19 The differing membrane structure of the two kinds of bacteria may entail that the passage of the toxic agent from solution to the cell wall is hindered to a different extent. Here, we tentatively attribute the difference in antimicrobial activity to the different membrane structures, i.e., the permeability and structural integrity of the membranes. To verify this hypothesis, more experiments should be performed to evaluate the antimicrobial effects against a wider range of Gram-positive and -negative bacteria. The d-MWNTs are more antimicrobial than MWNTs-COOH due to PAMAM and CNT bundle effects. Calabretta et al.4 have shown that the amino-terminated PAMAM dendrimer, in the solution form, is highly effective against Gram-negative P. aeruginosa and less effective against the Gram-positive S. aureus. Further, CNTs are usually in the form of bundles and

Ag Nanoparticles on Multiwalled Carbon Nanotubes ropes because of their intrinsic van der Waals force attraction and high aspect ratio nature. Functionalization of MWNTs with PAMAM can help debundle MWNTs ropes and improve their dispersion in water. The bundle of d-MWNTs is likely to be smaller than that of MWNTs-COOH. Kang et al.8 provided evidence that size (diameter) of CNTs is a key factor governing their antimicrobial activity and that SWNTs are much more toxic to bacteria than MWNTs. Thus, compared with MWNTsCOOH, the increased antimicrobial effect of d-MWNTs may be attributed partially to their smaller bundle size. Against the Gram-positive bacterium, S. aureus, d-MWNTs/ Ag exhibited significantly higher antimicrobial efficacy than d-MWNTs. Balogh et al. have shown that PAMAM/Ag nanocomposites/complexes have antimicrobial activity,6 but we first demonstrate that PAMAM/Ag immobilized onto the insoluble CNT phase also has antimicrobial effect. Ho et al.20 showed that silver nanoparticles within highly branched amphiphilically modified polyethyleneimine (PEI) derivatives grafted with polyethylene glycol (PEG) are microbe-repelling. They attributed the antimicrobial effect of this polymer network against S. aureus to the combination of microbe-repelling PEG, contactkilling cationic PEI surface, and the released Ag+ ions. They proposed that silver ions released from the nanoparticles contributed toward the antimicrobial activity. We hypothesize that silver ion release from our d-MWNT/Ag hybrid may also be the toxicity mechanism of our Ag nanoparticles, but this needs more study. Against the Gram-negative bacteria, E. coli and P. aeruginosa, there was little difference between the antimicrobial effects of d-MWNTs and d-MWNTs/Ag, although they were much stronger than that of MWNTs-COOH. In conclusion, the d-MWNTs/Ag hybrid is a novel antimicrobial material that offers effective killing of both the Grampositive and Gram-negative bacteria to afford a wide antimicrobial activity. This insoluble carbon nanotube-based nanohybrid, unlike just the dendrimer/Ag hybrid, can be easily filtered on a porous filter to separate them from the aqueous phase that it disinfects. The antimicrobial properties of the d-MWNTs/Ag nanohyrbid, i.e., MWNTs functionalized with third-generation hyperbranched poly(amidoamine) and grafted with Ag nanoparticles, were evaluated. d-MWNTs/Ag nanohybrids were synthesized by the in situ reduction and deposition of silver nanoparticles onto the surface of carbon nanotubes from silver acetate aqueous solution at room temperature without additional chemical and irradiation treatment. d-MWNTs and d-MWNTs/ Ag were found to be much more effective than MWNTs-COOH against all three pathogens tested, i.e., E. coli, P. aeruginosa, and S. aureus. d-MWNTs and d-MWNTs/Ag both showed equally strong antimicrobial efficacy against the Gram-negative bacteria E. coli and P. aeruginosa. However, d-MWNTs/Ag exhibited substantially stronger antimicrobial activity against

J. Phys. Chem. C, Vol. 112, No. 48, 2008 18759 S. aureus (Gram-positive) than d-MWNTs. Our findings suggest the effectiveness of insoluble MWNTs/silver nanohybrids as an antimicrobial material against both the representative Gramnegative and Gram-positive bacteria. These nanohybrids may be applicable to antimicrobial coating for disinfecting coating for water treatment and purification processes. Acknowledgment. This work was financially supported by the Defense Science & Technology Agency (DSTA) of Singapore (No. POD0513240). References and Notes (1) Kim, J. S.; Kuk, E.; Yu, K. N.; Kim, J. H.; Park, S. J.; Lee, H. J.; Kim, S. H.; Park, Y. K.; Park, Y. H.; Hwang, C. Y.; Kim, Y. K.; Lee, Y. S.; Jeong, D. H.; Cho, M. H. Nanomedicine 2007, 3, 95. (2) Sondi, I.; Salopek-Sondi, B. J. Colloid Interface Sci. 2004, 275, 177. (3) Shrivastava, S.; Bera, T.; Roy, A.; Singh, G.; Ramachandrarao, P.; Dash, D. Nanotechnology 2007, 18. (4) Calabretta, M. K.; Kumar, A.; McDermott, A. M.; Cai, C. Z. Biomacromolecules 2007, 8, 1807. (5) (a) Trollsas, M.; Hedrick, J. L. J. Am. Chem. Soc. 1998, 120, 4644. (b) Merino, S.; Brauge, L.; Caminade, A. M.; Majoral, J. P.; Taton, D.; Gnanou, Y. Chem.-Eur. J. 2001, 7, 3095. (6) Balogh, L.; Swanson, D. R.; Tomalia, D. A.; Hagnauer, G. L.; Mcmanus, A. T. Nano Lett. 2001, 1, 18. (7) Kang, S.; Pinault, M.; Pfefferle, L. D.; Elimelech, M. Langmuir 2007, 23, 8670. (8) Kang, S.; Herzberg, M.; Rodrigues, D. F.; Elimelech, M. Langmuir 2008, 24, 6409. (9) Tao, L.; Chen, G. J.; Mantovani, G.; York, S.; Haddleton, D. M. Chem. Commun. 2006, 4949. (10) Zhao, C. G.; Ji, L. J.; Liu, H. J.; Hu, G. J.; Zhang, S. M.; Yang, M. S.; Yang, Z. Z. J. Solid State Chem. 2004, 177, 4394. (11) Pan, B. F. C.; Cui, D. X.; Gao, F.; He, R. Nanotechnology 2006, 17, 2483. (12) Cullity, B. D. Elements of X-Ray Diffraction; Edison- Wesley Publishing Company, Inc. 1978. (13) Li, L. Y.; Cao, X. B.; Yu, F.; Yao, Z. Y.; Xie, Y. J. Colloid Interface Sci. 2003, 261, 366. (14) (a) Cao, L.; Yang, W. L.; Wang, C. C.; Fu, S. K. J. Macromol. Sci. Part A-Pure Appl. Chem. 2007, 44, 417. (b) Manna, A.; Imae, T.; Aoi, K.; Okada, M.; Yogo, T. Chem. Mater. 2001, 13, 1674. (15) Endo, T.; Yoshimura, T.; Esumi, K. J. Colloid Interface Sci. 2005, 286, 602. (16) Gao, C.; Jin, Y. Z.; Kong, H.; Whitby, R. L. D.; Acquah, S. F. A.; Chen, G. Y.; Qian, H. H.; Hartschuh, A.; Silva, S. R. P.; Henley, S.; Fearon, P.; Kroto, H. W.; Walton, D. R. M. J. Phys. Chem. B 2005, 109, 11925. (17) (a) Gao, C.; Vo, C. D.; Jin, Y. Z.; Li, W. W.; Armes, S. P. Macromolecules 2005, 38, 8634. (b) Gao, C.; Li, W. W.; Jin, Y. Z.; Kong, H. Nanotechnology 2006, 17, 2882. (18) (a) Sun, X. P.; Dong, S. J.; Wang, E. K. Polymer 2004, 45, 2181. (b) Tian, C. G.; Mao, B. D.; Wang, E. B.; Kang, Z. H.; Song, Y. L.; Wang, C. L.; Li, S. H. J. Phys. Chem. C 2007, 111, 3651. (19) (a) Madigan, M.; Martinko, J. Brock Biology of Microorganisms; Prentice Hall: Englewood Cliffs, NJ, 2005. (b) Baron, S. Medical Microbiology; University of Texas Medical Branch: Galveston, 1996. (20) Ho, C. H.; Tobis, J.; Sprich, C.; Thomann, R.; Tiller, J. C. AdV. Mater. 2004, 16, 957.

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