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Innovative Purification Method to Extract Magnetic

Carbon Nanotubes from Arc-discharged Single-walled

Carbon Nanotubes

Chuan He1, Yuzhi Hao1, James Z. Xing4, Jie Chen1,2,3

1 Department of Electrical and Computer Engineering, University of Alberta, Edmonton, Alberta, Canada

2Department of Biomedical Engineering, University of Alberta, Edmonton, Alberta, Canada

3 National Institute for Nanotechnology, Edmonton, Alberta, Canada4 Department of Laboratory Medicine & Pathology, University of Alberta, Edmonton, Alberta, Canada

AbstractThe discovery of carbon nanotubes has opened the

door to numerous cutting-edge nanoscaleresearch fields

including microelectronic devices, and biomedical

application. Although pure nanotubes suffice for most

purposes, highly pure magnetic carbon nanotubes are more

suitable for specialized applications such as gene delivery.

The conventional purification methods of carbon nanotubes

are too harsh for the nanotubes to retain any magnetic

characteristics, hence to enable the nanotubes be sensitive to

magnetic field. It requiresthe reloading of metallic particlesinside or on the surface of each tube. Instead, this paper

proposes an innovative purification method to extract

magnetic carbon nanotubes from arc-discharged single-

walled carbon nanotube material. The procedure is easy to

execute and time-saving. TEM and AFM images are used to

monitor the purification process and prove that the carbon

nanotube retain magnetic characteristics after the

purification process.

I. INTRODUCTION

Since S. Iijima discovered carbon nanotubes in 1991[1], interest in utilizing has been increased exponentially.Current research areas include microelectronics [2], energy

storage [3], and biomedical application [4]. In most cases,purified carbon nanotubes are required. For some specialcases, such as gene delivery, D. Pantarotto has reportedincreased gene delivery efficiency by applying carbonnanotubes as proper transporter [3], while J. Chen hasshown that magnetic carbon nanotubes enhance guided

penetration into the hard-to-deliver cell lines, such as stemcells [5]. To regain carbon nanotubes with magneticcharacteristics, G. Korneva tries to load magnetic particlesinside the tubes [6], but this method requires the diameterof the carbon nanotube to be around 200nm which is toolarge for the cells. Instead, we describe an innovative

purification method to obtain magnetic carbon nanotubefrom arc-discharged SWNTs. Metal catalyst nanoparticles

such as nickel, used in single-walled carbon nanotube(SWNT) production always accompany the final SWNT

product as impurities;some scattered around the SWNT,and others are attached onto theSWNT, wrapped byamorphous carbon which is a by-product [7]. Traditional

purification methods, such as oxidation, acid treatment,annealing, ultrasonication, micro-filtration or anycombinations of the above are so efficient, that theyeffectively remove all impurities, including the magneticnickel particles [8]. Our developed method enables nickel

particles to remain attached to the SWNT after purification,while separated nickel particles and amorphous carbon arelargely removed. The nickel particles retained on theSWNT surface will enable the SWNT to respond tomagnetic fields. To ensure the efficiency of every step,transmission electronic microscopy (TEM) and atomicforce microscopy (AFM) are used to monitor the

purification process. Since the processed SWNTs are verysmall and dispersed in 95% ethanol, it is very difficult todirectly observe magnetic characteristics through humaneye. Again, TEM is used to prove we obtain magneticSWNTs in the end.

II. PURIFICATION METHODS

SWNTs were purchased from Sigma Aldrich

. Thepurification process consists of three steps: sodium dodecylsulfate (SDS) treatments, organic solvent treatment andH2O2 treatment. The procedural flow chart is listed in Fig.1, followed by detailed information on each step:

Figure 1. Flow chart outlining our purification process

This research is funded by NSERC and CIHR joint CHRP grant, andNSERC engagement grant

Carbon nanotube

raw material

SDS + sonication

+ filtration

Organic solvent

treatmentH2O2treatment

75978-1-4577-0422-2/11/$26.00 c2011 IEEE

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A. SDS Treatment

(i) 26.2mg SWNT was weighed and dispersed in asolution of 10mL 20% concentration SDS plus 150mL DIwater. The container was gently agitated, after which thesample solution was sonicated for 2 hours.

(ii) After sonication, sample solution was transferredinto a centrifuge tube and centrifuged at 10000rpm for 10minutes. Sediment was discarded; supernatant was filtered

through a polytetrafluoroethylene (PTFE) membrane with0.45m pore size.

(iii) In order to completely remove the SDS afterfiltration, DI water was used to wash the SWNTsremaining on the filter membrane over several rinse cycles.

(iv) Finally SWNTs remaining on the PTFE membranewas suspended in 95% ethanol.

Sample solution was stored at room temperature forTEM and AFM analysis and next purification step.

B. Organic Solvent Treatment

(i) Before organic solvent treatment, sample solution

needs to be completely dried. After the drying process,SWNTs were suspended in 100mL N, N-dimethylformamide (DMF), after which the samplesolution was refluxed at 130C for 98 hours.

(ii) After the reflux process, sample solution wasfiltered again through PTFE membrane with 0.45m poresize. DI water was used to wash away the remaining DMF.SWNT remaining on PTFE membrane was suspended in95% ethanol and stored properly for TEM and AFManalysis and next purification step.

C. H2O2Treatment

(i) Before H2O2treatment, sample solution needs to be

completely dried. After the drying process, 22mL of H2O2was added to suspend SWNTs. The sample solution wasthen sonicated for a total duration of 4 hours. To ensureenough oxidant was supplied, 10mL of H2O2 wassupplemented into the solution at two hour intervals.

(ii) After H2O2 treatment, samples undergo the samefiltration process as previously described.

(iii) Finally, SWNT was stored in 95% ethanolsolution.

D. TEM Sample Preparation

Sample solution was sonicated for 10 minutes andimmediately transferred dropwise onto the TEM mesh

grid. The TEM sample grid was dried overnight at roomtemperature. TEM images are photographed using JEOL

TEM on the next day.

E. AFM Sample Preparation

Firstly, the pH value of the sample solution wasadjusted to approximately 7.0. Then a small amount ofsample solution was directly transferred dropwise onto the

silicon wafer. The sample was dried overnight at roomtemperature. AFM images were taken the next day usingVeeco

Multimode V SPM working under contact mode.

III. RESULTS

TEM and AFM images were obtained at eachpurification step. The aim is to monitor the process andcheck the change of morphology and state after therespective purification process.

The SWNT material purchased from Sigma is

supposed to have a purity of 50%.

In the top left corner of Fig. 2 (a), some SWNTs arevisible in separated state, but in Fig. 2 (b), SWNTs aredifficult to locate. We hypothesized that most of theSWNTs are encased by amorphous carbon (grey block) andsurrounded by nickel particles (dark points). The AFMimage shows similar result. As Fig. 3 shows, it isimpossible to differentiate SWNT from the impurities.

(a)

(b)

Figure 2. TEM images of SWNT from Sigma Aldrich

Figure 3. AFM image of SWNT from Sigma Aldrich

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A. Sample after SDS Treatment

(a)

(b)

Figure 4. TEM images of SWNT after SDS treatment

Fig. 4 (a), (b) show the status of SWNTs after SDStreatment. More SWNTs are observed, supporting ourhypothesis that SWNTs were initially hidden in theaccumulation of impurities. Overall, some large impurityparticles are removed from the sample solution.

B. Sample after Organic Solvent Treatment

After organic solvent treatment, the status of SWNTshas changed greatly. As Fig. 5 (a), (b) show, comparedwith image taken in the last step, the expansiveaccumulation block has further divided into smaller pieces,while many SWNTs act as bridges linking each other. Thearea occupied by amorphous carbon now exhibits a loosestructure.

(a)

(b)

Figure 5. TEM images of SWNT after organic solvent treatment

C. Samples after H2O2 Treatment

After H2O2 treatment which is the last purificationstage, it is hypothesized that most amorphous carbon andseparated nickel particles should have been removed, andSWNT would be free in solution with certain amount ofnickel particles attached onto their surface. Fig. 6 (a), (b)

prove this supposition. Short SWNTs are observed for thefirst time and they are evenly distributed in the image.Most SWNTs have a dark point in the tip indicating thepresence of nickel particles. The AFM image in Fig. 7provides more straightforward evidence. Unfortunately,there still remain relative large nickel particles in solution.Overall, the purification process was successful.

(a)

(b)

Figure 6. TEM images of SWNT after H2O2treatment

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Figure 7. AFM image of SWNT after H2O2treatment

IV. ANALYSIS

A. Sample after SDS Treatment

SDS is a highly effective surfactant. Because theoriginal SWNTs purchased from Sigma Aldrich

dissolve well in DI water, SDS is used here to assist thedispersion process. Sonication further aids in dispersing

the accumulation of amorphous carbon and nickelparticles.

It is hypothesized that after sonication fragmented theaccumulated block, SDS will quickly surround theseparate particles to make them water soluble. Meanwhilesome impurities are too large and too heavy for SDS toensure good solubility. By centrifuging the solution at10000 rpm, many large impurity particles, especially largenickel nanoparticles are removed from the sample.

B. Sample after Organic Solvent Treatment

Sometimes DMF is used to disperse SWNT [9]. Buthere, its in phenomenon whereby more and more organic solvent isadsorbed into the accumulation block, leading to aloosened accumulation structure. Since amorphous carbonis no longer packed tightly, it will be easier to free theSWNTs within.

C. Sample after H2O2Treatment

Y. Y. Feng has shown that H2O2 could help oxidizeamorphous carbon at room temperature [10]. Here, weprove that when amorphous carbon is structured in loosescaffolding, it becomes much easier to achieve the sameresult. As Fig. 6 and Fig. 7 show, there is almost noamorphous carbon accumulation left in the solution.

The formation of short SWNTs is resulted from the

continuous sonication process. Fortunately, some nickelparticles are still attached onto the SWNT after sustainingprolonged harshness during sonication.

Since the SWNT sample solution was filtered threetimes throughout the purification process, most smallnickel particles existing in the filtrate have been discarded.As SWNTs at some point may block the pores on PTFEmembrane, some of the large nickel particles still remain in

the sample solution. The diameter of the large nickelparticles are measured to be around 170-200nm.Fortunately, for biomedical application like gene delivery,particles with size more than 150 nm are difficult to enterthe cells by any possible mechanism. Thus, by applyingproper washing steps, the side effect of large nickelparticles can be minimized.

V. CONCLUSION

In this paper, we demonstrate an innovativepurification method to extract magnetic carbon nanotubefrom raw materials. The process is divided into three mainprocedures: SDS treatment, organic solvent treatment, andH2O2 treatment. To ensure the SWNTs retain magneticcharacteristics, we avoid using harsh acids which iscommon to traditional oxidation methods because weintend for some nickel particles to remain. TEM and AFMimages are used to monitor each step in the purificationprocess, and final results show that the developed methodis efficient and reliable.

ACKNOWLEDGMENT

The authors would like to acknowledge the fundingsupports of NSERC and CIHR joint CHRP grant, andNSERC engagement grant. The authors would also like tothank Mr. Ray Yang for providing valuable feedbacks ofour paper

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