an efficient growth of silver and copper nanoparticles on multiwalled carbon nanotube with enhanced...

An efficient growth of silver and copper nanoparticles on multiwalledcarbon nanotube with enhanced antimicrobial activity

Raja Mohan,1 A. M. Shanmugharaj,2 Ryu Sung Hun2

1King Abdulla Institute for Nanotechnology, King Saud University, Riyadh-11541, Kingdom of Saudi Arabia2College of Engineering and Department of Chemical Engineering, Industrial Liaison Research Institute, Green Energy Center,

Kyung Hee University, Yongin, Kyunggi-Do 449-701, South Korea

Received 22 May 2010; revised 9 August 2010; accepted 19 August 2010

Published online 8 November 2010 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jbm.b.31747

Abstract: Transition metal nanoparticles (NPs) such as silver

(Ag) and copper (Cu) have been grafted onto carbon nanotube

surface through wet chemical approach leading to the develop-

ment of densely packed NP decorated carbon nanotubes.

Chemically active surface and high-temperature stability are the

basic attributes to use carbon nanotubes as the template for the

growth of NPs. Ag NP-grafted carbon nanotubes (Ag-MWCNT)

are prepared by complexing Ag ion with acid functionalized car-

bon nanotubes followed by the reduction method. Alternatively,

Cu-grafted carbon nanotubes (Cu-MWCNT) are prepared by

simple chemical reduction method. X-ray diffraction results

reveal that the Ag or Cu NPs formed on the surface of carbon

nanotubes are determined to be face centered cubic crystals.

The morphology and chemical structure of NP-grafted carbon

nanotubes are investigated using transmission electron spec-

troscopy, X-ray photoelectron spectroscopy and Raman spec-

troscopy. The antimicrobial properties of acid-treated MWCNT

(MWCNT-COOH), Ag-MWCNT, and Cu-MWCNT are investi-

gated against gram negative Escherichia coli bacteria. Ag-

MWCNT and Cu-MWCNT (97% kill vs. 75% kill), whereas

MWCNT-COOH only killed 20% of this bacteria. Possible mecha-

nisms are proposed to explain the higher antimicrobial activity

by NP-coated MWCNT. These findings suggest that Ag-MWCNT

and Cu-MWCNT may be used as effective antimicrobial materi-

als that find applications in biomedical devices and antibacterial

controlling system. VC 2010 Wiley Periodicals, Inc. J Biomed Mater Res

Part B: Appl Biomater 96B: 119–126, 2011.

Key Words: carbon nanotubes, nanoparticles, Ag, Cu, XRD,

TEM, antibacterial activity

INTRODUCTION

With the emergence and increase of microbial organisms re-sistant to multiple antibiotics, and the continuing emphasison health-care costs, many researchers have tried to developnew, effective antimicrobial reagents free of resistance andcost. Such problems and needs have led to the resurgencein the use of Ag-based antiseptics that may be linked tobroad-spectrum activity and far lower propensity to inducemicrobial resistance than antibiotics.1 Silver (Ag) nanopar-ticles (NPs) are currently being used to control bacterialgrowth in a variety of applications, including dental work,catheters, and burn wounds.2,3 Significant research reportsavailable on the strong biocidal effects of Ag ions and Ag-based compounds against 12 species of bacteria includingEscherichia coli.4 Systematic investigations on the antimicro-bial activity of Ag NPs against E. coli supplemented withvarious concentrations in liquid systems reveals that E. coligrowth is greatly inhibited at 13.2 nM concentrations of AgNPs.5 However, the monocomponent antibacterial agentsuch as Ag NPs does not meet the requirements for somespecial conditions. Therefore, it is necessary to find compos-ite antibacterial agents to overcome this problem.6–9

The innovative well-defined tubular structure of carbonnanotubes creates the center of attention in the recent yearsfor fundamental scientific research with the potential for

diverse promising applications.10–13 For unlocking the truetechnological potential of CNTs in different areas of nano-technology, they are often interfaced with a variety of othermaterials, ranging from inorganic materials,14 polymer coat-ings,15 and biomolecules.16 Among these hybrid structures,an interesting class of CNTs derivatives result from the dep-osition of inorganic materials, such as metallic,17 semicon-ducting,18 and insulating19 NPs/nanoclusters, on the CNTssurfaces.14 These nanoparticulate hybrid systems derivedfrom inorganic solids and CNTs have unique optical, electri-cal, and mechanical properties and are promising materialsfor nanoelectronics and biomedical applications. The antimi-crobial activity of single-walled nanotubes (SWNTs) hasrecently been reported by Elimelech et al.20 The cell mem-brane is damaged resulting from direct contact with pristineSWNT aggregates leading to bacterial cell death. Multiwalledcarbon nanotubes (MWNTs) have also been found to pos-sess antimicrobial activity, though inferior.21 The larger sur-face area of MWNTs can be used as templates to preparenanoparticulate hybrid systems consisting of Ag NPs witheffective antimicrobial activity. These nanoparticulate hybridsystems can be synthesized using CNTs consisting of car-boxyl groups in the backbone, which can bind ions of transi-tion metals easily (Agþ and Cu2þ). These ions are wellknown for their broad-spectrum antimicrobial activity

Correspondence to: R. Mohan; e-mail: [email protected]

VC 2010 WILEY PERIODICALS, INC. 119

against bacterial and fungal agents together with their lackof cross resistance with antibiotics.22

The main objective of this work is to prepare Ag and Cu NPsdecorated onto the surface of multiwall carbon nanotubes bysingle step wet chemical synthesis (Ag-MWCNT and Cu-MWCNT) and to understand its antimicrobial properties. Thestructure and morphologies of metal NPs (Ag and Cu) attachedonto the MWCNTs are characterized using transmission electronmicroscopy (TEM), X-ray diffraction (XRD), and surface area an-alyzer. Finally, the antibacterial activity of metal NPs attachedonto the MWCNTs [silver-grafted carbon nanotubes (Ag-MWCNT) and copper-grafted carbon nanotubes (Cu-MWCNT)]against gram negative E. coli bacteria is compared against puremetal NPs.

METHODS

Materials. Commercial MWCNTs (NanocylVR -7000, diameter15–25 nm; length, 1.5 lm) with specific surface area of 250–300 m2/g were procured from Nano Best Corporation, SouthKorea. Copper chloride, silver nitrate and sulfuric acid werepurchased from Aldrich Chemical Co. and used without fur-ther purification. Gram negative bacteria (E. coli) and therelated chemicals and other biological agents were suppliedby the department of biology, Kyung Hee University, SouthKorea.

Preparation of MWCNT-COOH, Ag-MWCNT, andCu-MWCNTPristine MWCNT (3.0 g) was dispersed in 98% concentratedsulphuric acid under ultrasonication at 50�C for 6 h to pro-duce oxidized carbon nanotubes (MWCNT-COOH). The sam-ples were washed with ultrapure water and dried under vac-uum at 50�C for 12 h. One gram of MWCNT-COOH wasdispersed in 100 mL of distilled water through ultrasonica-tion. To this solution, 100 mL of 0.2M Ag nitrate solution wasadded with constant stirring at 60�C to generate Ag ion-grafted carbon nanotubes (MWCNT-COOHAg�). After thecompletion of reaction, solid products were collected by cen-trifuging and dried under vacuum at 50�C. Ag ions graftedonto carbon nanotubes were reduced at 200�C under H2

atmosphere to generate Ag NPs on the carbon nanotube sur-face. In another set of experiment, copper (Cu) NPs decoratedcarbon nanotubes was prepared as discussed below. Onegram of MWCNT-COOH was dispersed in 100 mL of dispersedwater. To this dispersant, 100 mL of 0.2M copper chloride(CuCl2) solution was added and subjected to heating at 80�Cwith constant stirring leading to the formation of (MWCNT-COO)2HCu2�. After the completion of reaction, solid productswere collected by centrifuging and dried under vacuum at50�C. It is expected that during drying process, Cu ions devel-oped on the carbon nanotube surface get reduced to Cu NPsand adhered on the surface by Van der Waals force of interac-tion as discussed previously by earlier researchers.23

Antimicrobial test with carbon nanotube solutionThe antibacterial property of MWCNT-COOH, metal NPs deco-rated MWCNT was evaluated against gram positive E. coli bac-

teria. The method used was in accordance to the requirementof ISO 14729:2001(E) (Opthalmic optics—Contact lens careproducts—microbiological requirements and test methodsfor products and regimens for hygienic management of con-tact lenses) and section 61—Microbial Limit Tests in theUnited States Pharmacopeia 30 (2007). E. coli microorganismsample was a 24-h subculture on Trypticase Soy Agar, har-vested, and washed two times with phosphate-buffered saline(PBS), pH 7.2, by centrifugation. The sample was diluted inPBS (pH 7.2) to provide a concentration of 20 lg/mL for thetests. An amount of 20 lL of the bacterial suspension in USPphosphate buffer pH 7.2 was transferred to 1 mL of the pre-pared sample (21 lg of MWCNT-COOH/mL, 21 lg of Ag-MWCNT/mL, and 21 lg of Cu-MWCNT/mL) in a test tube.The inoculated sample was kept in an incubator maintainedat 37�C for 1 h with intermittent mixing of every 15 min. Af-ter 1 h mixing, 1 mL of the sample was transferred to 9 mL ofneutralizing broth. The mixture was left standing at roomtemperature for 15 min. The tube was vortexed, and a seriesof 10-fold dilution in neutralizing broth were prepared andplated out in Trypticase Soy Agar. The plates were incubatedat 37�C for 72 h and counted for colony-forming units. Similartype of sample without NP-coated MWCNTs was prepared asthe control set of experiments.

The neutralization broth contained a mixture of neutral-izing agents that was demonstrated to inactivate the antimi-crobial activity from the sample residues that were carriedforward into the 10-fold dilutions. All experiments were car-ried out in triplicate.

The results are expressed as

% Kill ¼ ½ðNcontrol � NsampleÞ=Ncontrol� � 100 (1)

where Nsample represented the counts of survived bacterialinoculums in the neutralization broth containing 21 lL ofAg-MWCNT, Cu-MWCNT, or Pristine MWCNTs.

Ncontrol showed the initial counts of bacterial inoculumsin the neutralization broth that was not treated with NP-coated MWCNTs.

CharacterizationXRD measurements were performed using a MacscienceX-ray diffractometer equipped with a Cu Ka photon source(40 kV, 20 mA, k ¼ 0.154 nm). X-ray photoelectron spec-troscopy (XPS) was carried out with a high-resolution ESCA-2000. Morphological characterization was performed at 400keV using TEM (JEOL 100C) using samples deposited on thecarbon coated Cu grids. Brunauer Emmett and Teller (BET)-specific surface area analysis was studied with Sorptomatic1990 using the N2 adsorption/desorption method. Struc-tural characterization was done using Raman spectroscopy(RFS/100s, Bruker, Germany).

RESULTS AND DISCUSSION

Our synthetic approach on the preparation of Ag-MWCNTand Cu-MWCNT involves three steps (Figure 1). In the firststep, acid groups are introduced on the carbon nanotubesurface through sulfuric acid treatment followed by the

120 MOHAN, SHANMUGHARAJ, AND SUNG HUN AN EFFICIENT GROWTH OF SILVER AND COPPER ON MULTIWALLED CARBON NANOTUBE

introduction of Ag or Cu ions in the second step. In thethird step of the preparation, ions introduced on the surfaceare subjected to chemical reduction leading to the develop-ment of NP-grafted carbon nanotubes. Acid treatment ofcommercial MWCNTs not only results in the introduction ofACOOH groups on the surface but also it removes theimpurities such as metal catalysts and amorphous carbonparticles.24 The successful grafting of Cu and Ag NPs on theMWCNTs is confirmed by the XPS results (Figure 2). XPSresults of pure carbon nanotubes exhibit a strong peak at284.5 eV that corresponds to the carbon (C1s) peak (Figure2).25 Acid treatment of carbon nanotube results in thedecrease in peak intensity of C1s spectra with the appear-ance of peak at 529.6 eV that corroborates to the O1s spec-tra revealing the introduction of acid groups (Figure notshown). The strong peaks at 368 and 374 eV can be corro-borated to the Ag peaks (Ag3d spectra) in the case Ag-MWCNT (Figure 2). Similarly, new peak at 940 eV in thesurvey scan spectra of Cu-MWCNT corresponds to the Cupeak (Cu2p spectra) confirming the formation of Cu NPs onthe carbon nanotube surface (Figure 2). High-resolutionspectra results of Ag and Cu NPs grafted onto carbon nano-

tubes are shown in Figure 3(a,b). Ag NPs grafted onto car-bon nanotubes exhibits two strong peaks with binding ener-gies of 368.4 and 374.5 eV that are due to Ag 3d5/2 and Ag3d3/2 peaks [Figure 3(a)]. These peak values are typical val-ues of Ag0,26 confirming the formation of Ag NPs on thesidewalls of the MWCNTs.27 Similarly, the high-resolutionspectra of Cu NPs grafted onto carbon nanotubes exhibitstwo strong peaks at 932.7 and 952.5 eV that are due toCu2p3/2 and Cu2p1/2, respectively [Figure 3(b)], revealingthe successful formation of Cu NPs on the MWCNTsurface.28

The successful grafting of Cu and Ag NPs on theMWCNTs is further confirmed by the TEM results. Figure4(a,b) shows the TEM results of single carbon tube graftedwith Ag and Cu NPs. It is quite evident from the TEMimages that the surface of carbon nanotube becomes roughafter the growth of NPs on the surface. The average particlesizes of the grafted NPs onto the carbon nanotubes areobserved to be 18 (Ag-MWCNT) and 22 nm (Cu-MWCNT).These results are further supported by the XRD results,which is used to characterize dimensions and structure ofthese nanomaterials. XRD patterns of Ag and Cu NPs

FIGURE 1. Schematic illustrations of surface modifications of carbon nanotubes to generate (A) (i) Silver-grafted carbon nanotubes and (ii) cop-

per grafted carbon nanotubes by in situ reduction method. [Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

ORIGINAL RESEARCH REPORT

JOURNAL OF BIOMEDICAL MATERIALS RESEARCH B: APPLIED BIOMATERIALS | JAN 2011 VOL 96B, ISSUE 1 121

distributed on MWCNTs are shown in Figure 5. MWCNTexhibits two diffraction peaks 2y at 25.80� and 42.50� thatcorrespond to the (002) and (110) planes (figure notshown).29 Alternatively, Ag-MWCNT exhibits strong diffrac-tion peaks at 2y ¼ 38.9� , 44.3�, 64.49� , and 77.37� (Figure5) along with the diffraction peak due to (002) plane ofMWCNT [Figure 5 (inset)]. According to the JCPS cards No.04-0783, these peaks are readily indexed to (111), (200),(220), and (311) reflections of Ag metal crystals with facecentered cubic symmetry.30 Similarly, Cu-MWCNT showsmajor diffraction peaks at 28.4�, 32.32�, 39.5� , 47.6�, and56.26� (Figure 5) that are assigned to the (110), (111),(200), (220), and (311) reflections, similar to the reportedvalues of face centered cubic Cu NPs (JCPDS copper file No.04-0836)31 along with the diffraction peak due to (002)plane of MWCNT [Figure 5 (inset)]. The lattice constant ofCu on Cu-CNT is found to be 3.614 Å, which is in goodagreement with pure Cu crystals.31 The average particlesizes of the Ag and Cu NPs grafted onto the carbon nano-tubes are determined using Scherrer’s equation,32 and it isobserved to be 19 (Ag-MWCNT) and 21 nm (Cu-MWCNT).32

Figure 6 shows Raman spectra of pure MWCNT, Ag-MWCNT, and Cu-MWCNT measured under 514.5 nm excita-tion over the Raman shift interval of 1000–3000 cm�1. TheD- and G-bands of MWCNTs at �1290 and 1589 cm�1, cor-responding to defect- and disorder-induced modes and thein-plane E2g zone centered mode, are clearly observed inpure MWCNT.33 Grafting of Ag NPs (Ag-MWCNT) result inthe shift of tangential mode vibration (G band) to 1592cm�1 from 1589 cm�1 clearly reveal the decrease in Vander Waals force of interaction between the carbon nano-tubes due to the introduction of the NPs on the carbonnanotube surface. Similarly, in the case of Cu-MWCNT, Gband shift to the higher frequency (1591 cm�1), corroborat-ing the decrease in Van der Waals force of interactionbetween carbon nanotubes due to the grafting of Cu NPs on

carbon nanotubes through Van der Waals force of interac-tion. The D- to G-band intensity ratio (ID/IG) of Ag-MWCNTis 0.177, lesser than that of pure MWCNTs (0.199). Similartrend observed in Cu-MWCNT (ID/IG, 0.177). Slight reduc-tion in ID/IG value may be attributed to the reduction insurface defects due to the grafting of NPs on the surface ofcarbon nanotubes.

The surface area measurements of the NP-grafted carbonnanotubes have been determined using nitrogen adsorp-tion/desorption isotherms. The multilayer adsorption modeldeveloped by BET is fitted against isotherms to evaluate thesurface area of the nanotubes.34 Table I shows the BET sur-face area result of pure MWCNT and NP-grafted MWCNT.Surface area values of NP-grafted carbon nanotubes arehigher than pure NPs. The surface area of the Ag-MWCNTincreases to 25.8% in comparison with the pure Ag NPs.Similarly, surface area of Cu-MWCNT increases to 48.8% incomparison with pure Cu NPs. Higher surface area of NPsgrafted onto carbon nanotubes results in better efficiencyagainst microorganisms compared to the pure NPs.

FIGURE 2. XPS survey scan results of nanoparticle grafted multi-

walled carbon nanotubes.

FIGURE 3. High-resolution spectra results of Ag-MWCNT and Cu-

MWCNT.


The antimicrobial efficacies of pure MWCNT, Ag-MWCNT,Cu-MWCNT, and pure NPs are examined against gram nega-tive E. coli, the most characterized bacterium that has beenused as model bacterial system for various antimicrobialtesting programs. Figure 7 shows the breeding status of E.coli corresponding to the control systems, pure carbonnanotubes, Ag-MWCNT, and Cu-MWCNT. After the bacteriacolonies have been cultivated for 24 h, the number of bacte-rial colonies presented on the glass utensil is visually com-pared against the control system. The number of bacterialcolonies is relatively less in the case of pure carbon nano-tube system in comparison with the control system (Figure7). However, number of colonies significantly reduced onusing Ag-MWCNT system (Figure 7). Similar trend observedin Cu-MWCNT system (Figure 7) though the reductionin bacterial colonies is relatively less in comparison withAg-MWCNT. The number of bacterial colonies is measuredat various time intervals through standard agar dilution

method and the % kill has been calculated using Eq. (2),and the results are shown in Figure 8. Pure MWCNT exhib-its relatively low-percent kill against E.Coli bacteria (20% 6

2.5%) at the concentration level of the 21 lg/mL (Figure8). However, on grafting Ag NPs onto carbon nanotubes(Ag-MWCNT), antimicrobial efficiency significantly raised to97% 6 0.5% in comparison with the % kill of the pure AgNPs (85% 6 1.5%) and pure MWCNT (20% 6 2.5%).Similarly, grafting of Cu NPs onto carbon nanotubes(Cu-MWCNT) exhibits antimicrobial efficiency of the 75% 6

0.8% in comparison with pure Cu NPs that shows % kill ofthe 52% 6 1.8% (Figure 8). The possible mechanism ofbacterial growth inhibition by Ag NPs has been studiedextensively and reported by various researchers.35,36 It isexpected that gram negative bacteria possessing thin layerof negatively charged liposaccharides on the bacterial cell

FIGURE 5. X-ray diffraction results of MWCNT, Ag-MWCNT; and

Cu-MWCNT.

FIGURE 6. Raman spectra results of pure and metal nanoparticles

decorated carbon nanotubes.

FIGURE 4. (a) TEM image of silver nanoparticle grafted carbon nano-

tubes (Ag-MWCNT); (b) TEM image of copper nanoparticle grafted

carbon nanotubes (Cu-MWCNT).



wall are attracted toward weak positively charged Ag-MWCNT leading to the death of bacteria. Higher BET surfacearea of Ag-MWCNT (Table I) leads to the better inhibitionagainst growth of bacteria in comparison to pure NPs. Wespeculate similar type of mechanism of action for Cu-MWCNT against E. coli bacteria. Although, the exact mecha-nism behind the bactericidal effect of Cu-MWCNT is notknown, it is expected that Cu ions released from Cu-MWCNTenters the bacterial cells and disrupts the biochemical proc-esses leading to the inhibition of the bacterial growth.

The extent of inhibition against the bacterial colonygrowth at various concentrations of NP-grafted carbonnanotubes (Ag-MWCNT and Cu-MWCNT) is investigated,and the results are reported in Figure 9. The plates supple-mented with 5 lg/mL of Ag-MWCNT exhibits 90% 6 0.5%

inhibition against bacterial colonies. However, growth inhibi-tion rate raises to 94% 6 0.5% on using plates supple-mented with 15 lg/mL of Ag-MWCNT. These inhibition ratefurther increases to 97% 6 0.5% on using the plates withconcentration level of 21 lg/mL of Ag-MWCNT. Interest-ingly, Cu NP-grafted carbon nanotubes exhibit low inhibitionagainst bacterial growth at all concentrations in comparisonwith Ag-MWCNT. It exhibits bacterial growth inhibition of62% (5 lg/mL), 74% (15 lg/mL), and 89% (21 lg/mL) onincreasing the concentration of Cu-MWCNT. From theseobservations, it can be revealed that NP-grafted carbonnanotubes show better antibacterial efficiency against gram-negative E. coli bacteria.

CONCLUSION

A simple wet chemical synthesis of Ag-MWCNT and Cu-MWCNT has been developed on the basis of traditionalapproaches, and it has been devised the attaching of Ag andCu NPs on MWCNTs by electrostatic, reduction, or deposi-tion processes. These processes are usually simple andeffective methods to produce larger amount of NPs easilyattached on the surface of MWCNTs. TEM observationsshowed that Ag and Cu NPs attached on the outer surfacesof carbon nanotubes. Moreover, XPS spectra show that theMWCNT had been successfully functionalized with metal

FIGURE 7. Bacteria (Escherichia coli) grown on agar plates (a) control sample (without nanoparticle system), (b) MWCNT sample (21 lg/mL), (c)

Ag-MWCNT sample (21 lg/mL), and (d) Cu-MWCNT sample (21 lg/mL). [Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

TABLE I. BET Surface Analysis Results of Metal

Nanoparticles (Ag and Cu) Decorated MWCNTs

Sample IdentityConc. of MetalSalts (mol/L) SBET (m2/g)

Pure MWCNT – 289.6Ag-MWCNT AgNO3 (0.10 m) 652.3Cu-MWCNT CuCl2 (0.10 m) 484.2Ag nanoparticles AgNO3 (0.10 m) 518.5Cu nanoparticles CuCl2 (0.10 m) 325.4


NPs. From BET analysis, surface area of metal NPs isenhanced with carbon nanotubes, which have one of themost attributed reasons for efficacy in Ag-MWCNT and Cu-MWCNT antimicrobial characteristics. The formation of Agand Cu NPs is expected to find practical use in new applica-tions, such as sensors, solar cells, and other nanoelectronicdevices. Ag NPs decorated CNTs prepared by this cost-effec-tive reduction method described here have great promise asantimicrobial agents. Applications of Ag NP-based MWCNTnanocomposites on these findings may lead to valuable dis-coveries in various fields such as medical devices and anti-microbial systems, and the controlled release of Cu ionsfrom Cu-MWCNT nanocomposites as well as to reduce bio-

fouling on ships and fouling-prone infrastructures isexposed to aquatic environments.

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an efficient growth of silver and copper nanoparticles on multiwalled carbon nanotube with enhanced...

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