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Dispersion of Multi‐walledCarbon Nanotubes in AqueousPluronic F127 Solutions forBiological ApplicationsGianni Ciofani a , Vittoria Raffa a , VirginiaPensabene a b , Arianna Menciassi a b & Paolo Dario ab

a CRIM Lab ‐ Center for Research inMicroengineering , Scuola Superiore Sant'Anna , Pisa,Italyb University of Genova , Italian Institute ofTechnologies , Genova, ItalyPublished online: 31 Dec 2008.

To cite this article: Gianni Ciofani , Vittoria Raffa , Virginia Pensabene , AriannaMenciassi & Paolo Dario (2009) Dispersion of Multi‐walled Carbon Nanotubes inAqueous Pluronic F127 Solutions for Biological Applications, Fullerenes, Nanotubesand Carbon Nanostructures, 17:1, 11-25, DOI: 10.1080/15363830802515840

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Dispersion of Multi-walled Carbon Nanotubes inAqueous Pluronic F127 Solutions for Biological

Applications

Gianni Ciofani,1 Vittoria Raffa,1 Virginia Pensabene,1,2 Arianna

Menciassi,1,2 and Paolo Dario1,2

1CRIM Lab - Center for Research in Microengineering, Scuola Superiore

Sant’Anna, Pisa, Italy2University of Genova, Italian Institute of Technologies, Genova, Italy

Abstract: Because mass-produced carbon nanotubes (CNTs) are strongly

aggregated and highly hydrophobic, processes to make them water soluble are

required for biological applications. Suspensions in surfactant solutions are often

employed. Among these, Pluronic F127 appear to be highly biocompatible if used

at low concentrations. Starting from these results, this work involves a systematic

study to clarify the dispersion behaviour of CNTs in Pluronic F127. The results

suggest a two-step process: first, the bundles disaggregate, kinetically driven by

the energy supplied to the system; second, they disperse (surfactant adsorption),

thermodynamically driven by the surfactant concentration. The dispersion

reaction data are well fitted by a first-order kinetics reaction. By performing a

pretreatment step, consisting of stirring at 70uC, the achieved concentration of

CNTs in solution is twice that of the traditional process. The proposed procedure

provides an optimal compromise between a low Pluronic concentration and a

high CNT concentration.

Keywords: Carbon nanotubes, Dispersion, Pluronic F127, Biological applications

Address correspondence to Gianni Ciofani, Scuola Superiore di Studi

Universitari e Perfezionamento Sant’Anna CRIM Lab – Center for Applied

Research in Micro and Nano Engineering, Viale Rinaldo Piaggio, 34 – 56025

Pontedera (Pisa), Italy. E-mail: g.ciofani@sssup.it

Fullerenes, Nanotubes and Carbon Nanostructures, 17: 11–25, 2009

Copyright # Taylor & Francis Group, LLC

ISSN 1536-383X print/1536-4046 online

DOI: 10.1080/15363830802515840

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INTRODUCTION

Carbon nanotubes (CNTs) are cylindrical structures with diameter

varying from a few nanometres to tens of nanometres and length ranging

from less than a micron to centimetres (1). They have excellent

mechanical strength, electrical conductivity and thermal stability, so that

many potential applications have been proposed (2), including CNTs-

based sensors (3), probes (4), actuators (5), nanoelectronic devices (6) and

drug delivery systems within biomedical applications (7–10).

Among CNTs, single-walled carbon nanotubes (SWCNTs) consist of

a single layer of graphite lattice rolled into a perfect cylinder, whereas sets

of concentric cylindrical graphite shells form multi-walled carbon

nanotubes (MWCNTs) (11–12).

In general, in large-scale production of CNTs, the as-prepared

product exists as agglomerates with a size of up to hundreds of microns

(13–14). In many applications it is important to use isolated CNTs or

make CNT solution or dispersion, especially in biomedical fields where

strong interactions between the CNTs and the biological matter is required,

(15) but pristine CNTs are completely hydrophobic and thus cytotoxic (12).

Many works in the literature report on the toxic effects of CNTs and their

applicability in the biomedical field (16), debating on different strategies of

solubilization or dispersion to improve biocompatibility (17).

‘‘Solubilization’’ refers to ‘‘individual dissolution’’ of nanotubes in

solvents, while ‘‘dispersion’’ means ‘‘colloidal nanotube dispersions.’’

Dissolution/dispersion of CNTs is possible both by covalent modification

or physical adsorption of a surfactant (18–21). In particular, for such

applications in which pristine electronic or surface properties of the tubes

should be maintained, the latter approach is preferred.

A variety of anionic, cationic, zwitterionic and nonionic surfactants

can be used for this purpose (22–23). It has been proposed that the

mechanism of CNT dispersion could be ionic or steric, depending on the

surfactant, but the exact mechanism has not been conclusively established

yet (24–25). CNT dispersion in surfactants is an easy and not consuming

process, but biocompatibility issues should be addressed for biological

purposes. Effects of different surfactants on dispersion of MWCNT

aggregates and cytotoxicity have been already studied by Monteiro-

Riviere and her colleagues (26). Their purpose was to find a surfactant

that prevents the formation of large aggregates of MWCNTs without

being cytotoxic. L61, L92, Pluronic F127, Tween 20 and Tween 60 were

tested on human epidermal keratinocytes (HEK) cells for 24 hours to

serial dilutions (10% to 0.1%). The authors found that HEK viability,

inversely proportional to surfactant concentration, ranged from 27.1% to

98.5% with Pluronic F127; viability with the other surfactants was less

than 10%.

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In (27) carbon nanotubes dispersed with Pluronic F127 were

employed as vector for cancer therapy, and no significant toxicity of

this surfactant was found on MCF-7 cells. Long-term viability assay

(72 hrs) performed in this work on Crandell feline kidney cell line

(CRFK) reveals that cells are viable at lower Pluronic concentration

(below 0.1%).

Starting from these results and aiming at prepare highly concentrated

CNT dispersion at low Pluronic concentration, this work carries out a

systematic study on parameters determining the dispersion behaviour of

CNTs in Pluronic F127 (PF127), such as sonication time, concentration

of surfactant, and other parameters (28). To the authors’ knowledge, only

a study with SDS as surfactant was carried out (23). Evidence for the role

of surfactant in the dispersion of SWCNTs during sonication has been

reported by Strano et al. (29) for dispersion of SWCNTs in SDS

solutions. During the sonication process, the bundle ends of SWCNTs

are ‘‘frayed’’ by high local shear and become the site for additional SDS

adsorption; then the SDS molecules gradually exfoliate the SWCNT

bundles in an ‘‘unzipping’’ mechanism. Similar experiments are reported

in this paper. Investigations performed by changing experimental

parameters (temperature T, surfactant concentration Cs and

sonication time ts) have been performed clarifying the CNT dispersion

mechanisms; an analytical correlation that fits experimental data has

been provided.

MATERIALS AND METHODS

MWCNTs (provided by Nanothinx S.A., Greece) were produced by

CVD, with purity 97%. The surfactant used for the dispersion of

MWCNTs was Pluronic F127 (polyoxyethylene-polyoxypropylene block

copolymer), a nonionic surfactant purchased from Sigma Aldrich (St.

Louis, Missouri, USA), solid with MW 12,600 and soluble in water (.

30%). All dispersion experiments were carried out with deionized water.

Samples were prepared by weighing a certain amount of MWCNTs

(5 mg) and then mixing them with 10 ml of a Pluronic solution in a

polystyrene tube.

The samples were sonicated for a time tS with a Branson sonicator

2510 (by Bransonic). The output power of sonicator was fixed for all

experiments at 20 W, thus delivering energy of 1100–1200 J/min. A

pretreatment step consisted in putting the sample over a hot plate for

stirring for a time tSE. After the treatment, samples were then centrifuged

at 1100 g for 10 minutes to remove residuals and impurities. CNT

concentration of the supernatant was measured with a LIBRA S12

Spectrophotometer UV/Vis/NIR (Biochrom) at 270 nm.

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Samples were taken regularly after the sonicating process and diluted

by a certain factor, resulting in MWCNT contents that were suitable for

UV measurements. The used blank was the original Pluronic solution

diluted by the same factor, under the same conditions as the samples

themselves.

Individual CNTs are active in the UV region (Figure 1) and exhibit

characteristic bands corresponding to additional absorption due to 1D

van Hove singularities (30–31). Bundled CNTs, however, are not active in

the wavelength region between 200 and 1200 nm. Therefore, it is possible

to establish a relationship between the amounts of CNTs individually

dispersed in solution and the intensity of the corresponding absorption

spectrum.

Most significant samples were observed, after drying, with a Focused

Ion Beam (FIB) system that enables imaging, localized milling and

deposition of conductors and insulators with high precision (32). FIB

system used for the imaging in the present work is a FEI 200 (Focused

Ion Beam Localized milling and deposition) delivering a 30 keV beam of

gallium ions (Ga+), with beam currents varying between 1 pA and 11 nA.

Pluronic cytocompatibility was evaluated by MTT (3-(4,5-

dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide tetrazolinium

salt) assay. Crandell feline kidney cell line, CRFK (ATCC CCL-94)

were cultured in Dulbecco’s modified Eagle’s medium (purchased from

Cambrex) with 10% fetal bovine serum (purchased from ATCC), 100 IU/

ml penicillin (purchased from ATCC), 100 mg/ml streptomycin (pur-

chased from ATCC), 2 mM L-glutamine (purchased from Sigma) and

0.1–0.01% of Pluronic. Cells, grown in 96-well plates (containing

Figure 1. Typical UV-Vis spectrum of a CNT dispersion.

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approximately 30,000 cells at the time of the assay), were maintained at

37uC in a satured humidity atmosphere containing 95% air/5% CO2 for

72 hours. At the end of the incubation period, the supernatants were

discarded, and MTT assay was performed. 100 ml of MTT (5 mg/ml,

purchased from Sigma) was added to each well and incubated for 2

hours. Afterwards, the product was quantified by measuring absorbance

at 570 nm after DMSO solubilization. The absorbance measurement is

directly proportional to the viability and metabolic activity of cell

populations and inversely proportional to the toxicity of the solution.

EXPERIMENTS AND DISCUSSIONS

The mechanism of CNT dispersion involves the rupture of hydrophobic

and Van der Waals forces between CNTs. The common principle is that

the surfactant molecules adsorb on the surface of the tube because of

their amphiphilic nature. In particular, hydrophobic part of these

molecules is chemisorbed on the CNT sidewalls. The molecules adsorbed

on the surface reduce the Van der Waals forces between the nanotubes,

making them soluble in water media.

As mentioned above, during the sonication of a sample containing

CNTs in a surfactant solution, the high local shear frayed the outer CNTs

in a bundle and exposes other sites for additional surfactant adsorption;

then the surfactant molecules gradually exfoliate the bundle in an

‘‘unzippering’’ mechanism. The mechanism which avoids CNT re-

aggregation in a Pluronic solution is probably steric (33). Tests were

carried out to investigate the effect of sonication time tS and Pluronic

concentration CS in solution on the dispersion of MWCNTs.

Starting from these results, four concentrations of Pluronic F127

(0.05%, 0.10%, 0.25% and 0.50%) were investigated, with 2, 8 and 16

hours of sonication.

Spectrophotometer analysis of the final suspensions revealed that

CNT dispersion increases with surfactant concentration CS and sonica-

tion (US) time tS (Figure 2). Experimental measurements show that

dispersions are stable many weeks after preparation.

During the first two hours of sonication process a huge increase

occurs, then the value of absorbance reaches a plateau which corresponds

to the maximum achievable degree of dispersion of the MWCNTs for a

certain Pluronic solution concentration (Figure 2). With higher Pluronic

concentration the plateau value increases; however, the CNT dispersion

increment is quite modest (from 70 to 90 mg/ml ranging Pluronic

concentration from 0.05% to 0.5%).

A very simplified formalization of the dispersion reaction could be

the following:

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azb / ?k, k’

c ð1Þ

where a is the concentration of not dispersed CNTs, b is the surfactant

concentration and c is the concentration of dispersed CNTs (measured

via spectrophotometric analysis).

If a first-order reaction is supposed

Lc

Lt~k:a:b{k’:c ð2Þ

given a & c and b & c, it can be assumed k?a?b 5 constant 5 A. Once

integrated, equation (2) gives

c tð Þ~ A

k’: 1{e{k’:t� �

ð3Þ

Fitting of equation (3) with the data of Figure 2 is quite good. Figure 3

shows just an example; the fitted data are relative to tests with 0.1%

Pluronic solution (A 5 32.83 mg/ml s21; k’ 5 0.46 s21; RMS 5 98.52%).

The local shear generated by the sonication allows the break of the

bundles and the access of the surfactant to the CNT surface.

Temperature, by increasing the enthalpic energy of the system, should

help the dispersion phenomena.

Figure 2. Effect of sonication time and surfactant concentration on the

MWCNT dispersion.

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At this purpose a heated magnetic stirring before the ultrasound

treatment was introduced in the procedures. The samples were put over a

hot plate and stirred for different hours; the temperature of the samples

was monitored with a thermocouple and kept constant at 70uC. Higher

temperatures were avoided to define a procedure suitable also for the

CNT dispersion in biological media (e.g., in cell growth media).

The heating treatment under a magnetic stirring has a twofold

objective: first, the turbulence improves the disintegration of macroscopic

CNT bundles; second, the temperature lets the exfoliation process begin

and helps it; after the temperature treatment, CNT suspensions were

immediately sonicated as previously described. Figure 4 shows aliquots of

a sample subjected to the complete procedure (Pluronic concentration

0.1%). It has to be noticed that after the heating pretreatment the

dispersion is already appreciable: UV absorbance analysis of the sample

after the heating pre-treatment, but without the sonication step, showed

already a concentration of dispersed CNTs of about 15 mg/ml. No

significant dispersions (, 1 mg/ml) was observed in case of stirring

treatment at room temperature, confirming that high temperature

condition is needed to obtain a partial debundling.

UV analysis of samples subjected to heating treatment (2 hrs and

8 hrs) and to the sonication process for 16 hours reveals a significant

increase of the achieved nanotubes dispersion. For samples dispersed in

0.1% Pluronic solution a final average concentration of about 75 mg/ml

is obtained without heating pretreatment, 120 mg/ml with 2 hours of

pretreatment and 160 mg/ml with 8 hours of pretreatment. The final con-

centration for a 0.5% Pluronic solution increases up to about 180 mg/ml,

Figure 3. Experimental and model data for the CNT dispersion.

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against the initial value of 90 mg/ml (Figure 5). The pretreatment step

allows about a twofold increase in the plateau value of CNT

concentration corresponding to a certain value of Pluronic concentration;

no concentration improvement was achieved through a stirring prestep

without heating (at room temperature). All the samples are stable for

many weeks after the preparation (no CNT precipitation occurs).

To clarify the effect of the temperature on the dispersion process, a

CNT sample in 0.1% Pluronic solution was subjected to a continuous

magnetic stirring at 70uC. The CNT dispersion was monitored during the

time via spectrophotometric analysis for a week. Figure 6 shows the trend

of the CNT dispersion. The phenomenon is linear (R2 5 99.74%)

for many hours (until six days) then a saturation is achieved at about

180 mg/ml.

It is interesting to note that this value is quite similar to the

dispersion degree achieved with the heating + sonication procedure which

seems to be the effective saturation value of the CNT solution.

Our results suggest a two steps process: first the bundles desegrega-

tion, kinetically driven by the energy supplied to the system, and second

the dispersion process, thermodynamically driven by the surfactant

concentration, in agreement to the following reaction:

Figure 4. Photos of CNT suspensions; from left to right: before the treatment,

after 8 hours heating, after 8 hours heating and 16 hours of sonication.

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CNT bundlesð Þ bundle disagregation?CNT isolatedð ÞzPluCNT Plð Þ

These considerations suggest that the heating and stirring pre-treatment

is not so important for accelerating the reaction (the temperature effect in

the first 2 or 8 hours is low respect to the final saturation value; see

Figure 6. Temperature – driven dispersion.

Figure 5. Effect of the heating pre-treatment on the final MWCNT

concentration.

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Figure 6) but especially for the disintegration and homogenization of

these macroscopic bundles that cannot be broken up efficiently by theshear stress of the following sonication procedure.

In order to visualize the dispersed MWCNTs and to support the

interpretation of the UV results, FIB imaging of the samples was

performed. After the dispersion procedure, a drop of the sample was

posed on a silicon substrate and dried before the FIB imaging; imaging

was performed at 25,000 X with an ion beam of 5 pA.

Figure 7a shows the image of the sample obtained after the 2 hours

sonication in a 0.5% Pluronic concentration (CNTs concentration of58.47 mg/ml); Figure 7d is an analysis of a sample obtained with the most

efficient achieved procedure (8 hrs of preheating and 16 hrs of sonication

in a 0.5% Pluronic solution), for a final CNT concentration of about

180 mg/ml. For comparison, Figures 7b and 7c, respectively, show images

of sample obtained after 16 hours US in a 0.5% Pluronic solution without

Figure 7. FIB imaging: (a) sample obtained after 2 hours US in a 0.5% Pluronic

solution; (b) sample obtained after 16 hours US in a 0.5% Pluronic solution; (c)

sample obtained after 8 hours of stirring at room temperature and 16 hours US in

a 0.5% Pluronic solution; (d) sample obtained after 8 hours of stirring at 70uC and

16 hours US in a 0.5% Pluronic solution.

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any pretreatment step, and of sample obtained after 8 hours of stirring at

room temperature and 16 hours US in a 0.5% Pluronic solution.

Difference in terms of CNT quantity is evident also from FIB images;

again, it was evidenced that heating prestep highly improves the final

CNT concentration.

Cytocompatibility assays were carried out to evaluate Pluronic

concentrations suitable for biological application. The results of the MTT

assay to test the effect of the Pluronic concentration in culture medium

on cell (CrFK) metabolic activity are given (% of control) in Figure 8.

This test, performed 72 hours after cell incubation, reveals that cells are

viable for very low dilute Pluronic concentrations (below 0.1%). Based on

the developed procedure, we was able to prepare biocompatible (0.1% of

Pluronic) CNT dispersion at a concentration above 150 mg/ml; this

threshold is quite enough for common biomedical and biological

applications.

CONCLUSIONS

Because mass-produced carbon nanotubes are strongly aggregated and

highly hydrophobic, they required processing to form stable aqueous

suspensions suitable for biological applications. To preserve pristine

CNT properties, suspensions in surfactant solutions are employed in the

literature, both for in vitro (34) and in vivo experiments (35); among

surfactants in literature, Pluronic appears to be highly biocompatible if

used at low concentrations (26–27).

Figure 8. MTT assay on CRFK cells after 72 hours of incubation at different

surfactant concentrations.

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In this paper, an investigation to enhance the procedure for obtaining

stable suspensions of MWCNTs in Pluronic solutions was carried out.

Parameters influencing MWCNT dispersions were characterized via

spectrophotometric analysis. The saturation limit of MWCNTs was

evaluated in different experimental conditions. This plateau is in

relationship with the amount of surfactant in solution (the higher is the

Pluronic concentration, the higher is the saturation point), but it is also

strictly related with the energy supplied to the system.

These results suggest a two-step process: first, the bundles

disaggregate, kinetically driven by the energy supplied to the system;

second, the dispersion process is thermodynamically driven by the

surfactant concentration. The dispersion reaction was assimilated to a

first-order kinetics reaction in order to show an extremely simplified

modelling of the phenomena occurring during the sonication. The final

expression is in agreement with the experimental results.

The present work demonstrates that by performing a pretreatment

step consisting of stirring at 70uC, followed by the sonication procedure,

a concentration twice that of the traditional process is achieved. The

stirring at 70uC basically causes the breaking of numerous CNT ‘‘macro’’

bundles, which a traditional sonication procedure does not break with

the same efficiency. The proposed procedure provides an optimal

compromise between low Pluronic concentration (suggested by MTT

assays) and high CNT concentration (e.g., a dispersion of 160 mg/ml of

CNTs in 0.1% Pluronic solution), therefore achieving high CNT

concentrations without causing surfactant-induced toxicity.

ACKNOWLEDGEMENTS

The authors thank Nanothinx S.A. for the supply of the MWCNTs. The

authors also thank Dr. Virgilio Mattoli for the design and development

of the spectrophotometer, Mr. Carlo Filippeschi for his assistance at the

FIB microscope and Mr. Orazio Vittorio for cell culture assays.

The activity presented in this work has been partially supported by

the Italian Institute of Technology Network and the NINIVE (Non

Invasive Nanotransducer for In Vivo gene Therapy, STRP 033378)

project, cofinanced by the 6FP of the European Commission.

REFERENCES

1. Iijima, J. (1991) Helical microtubules of graphitic carbon. Nature, 354: 56–58.

2. Bandaru, P.R. (2007) Electrical properties and applications of carbon

nanotube structures. J. Nanosci. Nanotechnol., 7: 1239–1267.

22 G. Ciofani et al.

Dow

nloa

ded

by [

Uni

vers

ite L

aval

] at

21:

19 0

7 O

ctob

er 2

014

3. Zhang, H. (2004) Fabrication of a single-walled carbon nanotube-modified

glassy carbon electrode and its application in the electrochemical determina-

tion of epirubicin. J. Nanopart. Res., 6: 665–669.

4. Stevens, R.D.M., Frederick, N.A., Smith, B.L., Morse, D.E., Stucky, G.D.,

and Hansma, P.K. (2000) Carbon nanotubes as probes for atomic force

microscopy. Nanotechnology, 11(1): 1–5.

5. Baughman, R.H., Cui, C., Zakhidov, A.A., Iqbal, Z., Barisci, J.N., Spinks,

G.M., Wallace, G.G., Mazzoldi, A., De Rossi, D., Rinzler, A.G., Jaschinski,

O., Roth, S., and Kertesz, M. (1999) Carbon nanotube actuators. Science,

284: 1340–1344.

6. Zhirnov, V., Herr, D., and Meyyappan, M. (1999) Electronic applica-

tions of carbon nanotubes become closer to reality. J. Nanopart. Res., 1:

151–152.

7. Bianco, A., Kostarelos, K., and Prato, M. (2005) Applications of carbon

nanotubes in drug delivery. Curr. Opin. Chem. Biol., 9: 674–679.

8. Kostarelos, K., Lacerda, L., Partidos, C.D., Prato, M., and Bianco, A. (2005)

Carbon nanotube-mediated delivery of peptides and genes to cells:

translating nanobiotechnology to therapeutics. J. Drug. Deliv. Sci.

Technol., 15: 41–47.

9. Bianco, A. (2004) Carbon nanotubes for the delivery of therapeutic

molecules. Expt. Opin. Drug Deliv., 1: 57–65.

10. Bianco, A., Kostarelos, K., Partidos, C.D., and Prato, M. (2005)

Biomedical applications of functionalised carbon nanotubes. Chem.

Commun., 5: 571–577.

11. Saito, R., Dresselhaus, G., and Dresselhaus, M.S. (1998) Physical Properties

of Carbon Nanotubes, Imperial College Press: London.

12. Sato, Y., Yokoyama, A., Shibata, K., Akimoto, Y., Ogino, S.I., Nodasaka,

Y., Kohgo, T., Tamura, K., Akasaka, T., Uo, M., Motomiya, K., Jeyadevan,

B., Ishiguro, M., Hatakeyama, R., Watari, F., and Tohji, K. (2005) Influence

of length on cytotoxicity of multi-walled carbon nanotubes against human

acute monocytic leukemia cell line THP-1 in vitro and subcutaneous tissue of

rats in vivo. Mol. Biosyst., 1: 176–182.

13. Wang, Y., Wie, F., Luo, G.H., Yu, H., and Gu, A. (2002) The large-scale

production of carbon nanotubes in a nano-agglomerate fluidized-bed

reactor. Chem. Phys. Lett., 364(5): 568–572.

14. Hao, Y., Qunfeng, Z., Fei, W., Weizhong, Q., and Guohua, L. (2003)

Agglomerated CNTs synthesized in a fluidized bed reactor: Agglomerate

structure and formation mechanism. Carbon, 41: 2855–2863.

15. Lacerda, L., Bianco, A., Prato, M., and Kostarelos, K. (2006) Carbon

nanotubes as nanomedicines: From toxicology to pharmacology. Adv. Drug.

Deliv. Rev., 58: 1460–1470.

16. Wei, W., Sethuraman, A., Jin, C., Monteiro-Riviere, N.A., and Narayan,

R.J. (2007) Biological properties of carbon nanotubes. J. Nanosci.

Nanotechnol., 7: 1284–1297.

17. Nakashima, N. (2005) Soluble carbon nanotubes: Fundamentals and

applications. International Journal of Nanoscience, 4(1): 119–137.

18. Liu, J., Rinzler, A.G., Dai, H., Hafner, J.H., Kelley Bradley, R., Boul, P.J.,

Lu, A., Iverson, T., Shelimov, K., Huffman, C.B., Rodriguez-Macias, F.,

Pluronic-based Multi-walled Carbon Nanotubes Dispersions 23

Dow

nloa

ded

by [

Uni

vers

ite L

aval

] at

21:

19 0

7 O

ctob

er 2

014

Shon, Y.-S., Lee, T.R., Colbert, D.T., and Smalley, R.E. (1998) Fullerene

pipes. Science, 280: 1253–1256.

19. Dyke, C.A. and Tour, J.M. (2004) Overcoming the insolubility of carbon

nanotubes through high degrees of sidewall functionalization. Chemistry, 10:

812–817.

20. Muratami, H. and Nakashima, N. (2006) Soluble carbon nanotubes and their

applications. J. Nanosci. Nanotechnol., 6: 16–27.

21. Shi, Q., Yang, D., Su, Y., Li, J., Jiang, Z., Jiang, Y., and Yuan, W. (2007)

Covalent functionalization of multi-walled carbon nanotubes by lipase. J.

Nanopart. Res., 9(6): 1205–1210.

22. Yurekli, K., Mitchell, C.A., and Krishnamoorti, R. (2004) Small-angle

neutron scattering from surfactant-assisted aqueous dispersions of carbon

nanotubes. J. Am. Chem. Soc., 126: 9902–3.

23. Yu, J., Grossiord, N., and Koning, E. (2007) Controlling the dispersion of

multi-walled carbon nanotubes in aqueous surfactant solution. Carbon, 45:

618–623.

24. Nielsen, S.O., Srinivas, G., Lopez, C.F., and Klein M.L. (2005) Modeling

surfactant adsorption on hydrophobic surfaces. Phys. Rev. Lett., PRL 94,

228301.

25. Ham, H.T., Choi, Y.S., and Chung, I.J. (2005) An explanation of dispersion

states of single-walled carbon nanotubes in solvents and aqueous surfactant

solutions using solubility parameters. J. Colloid Interf. Sc., 286: 216–223.

26. Monteiro-Riviere, N.A., Inman, A.O., and Wang Y.Y. (2005) Surfactant

effects on carbon nanotube interactions with human keratinocytes.

Nanomedicine: Nanotechnology, Biology, and Medicine, 1: 293–299.

27. Ali-Boucetta, H., Al-Jamal, K.T., McCarthy, D., Prato, M., Bianco, A., and

Kostarelos, K. (2008) Multiwalled carbon nanotube–doxorubicin supramo-

lecular complexes for cancer therapeutics. Chem. Commun., 8: 459–461.

28. Vaisman, L., Wagner, H.D., and Marom, G. (2006) Advances in colloid and

interface. Science, 128–130, 37–46.

29. Strano, M.S., Moore, V.C., Miller, M.K., Allen, M.J., Haroz, E.H., Kittrell,

C., Hauge, R.H., and Smalley, R.E. (2003) The role of surfactant adsorption

during ultrasonication in the dispersion of single-walled carbon nanotubes. J.

Nanosci. Nanotechnol., 3: 81–6.

30. O’Connell, M.J., Bachilo, S.M., Huffman, C.B., Moore, V.C., Strano, M.S.,

Haroz, E.H., Rialon, K.L., Boul, P.J., Noon, W.H., Kittrell, C., Ma, J., Hauge,

R.H., Weisman, R.B., and Smalley, R.E (2002) Band gap fluorescence from

individual single-walled carbon nanotubes. Science, 297: 593–6.

31. Grossiord, N., Regev, O., Loos, J., Meuldijk, J., and Koning, C.E. (2005)

Time dependent study of the exfoliation process of carbon nanotubes in

aqueous dispersions by using UV–visible spectroscopy. Anal. Chem., 77:

5135–5139.

32. Raffa, V., Castrataro, P., Menciassi, A., and Dario, P. (2005) Introduction to

focused ion beam. In Applied Scanning Probe Methods, Vol. II; Bhushan, B.

and Fuchs, H. (eds.), Springer-Verlag: Heidelberg, Germany.

33. Moore, V.C., Strano, M.S., Haroz, E.H., Hauge, R.H., Smalley, R.E.,

Schmidt, J., and Talmon, Y. (2003) Individually suspended single-walled

carbon nanotubes in various surfactants. Nano Lett., 3(10): 1379–1382.

24 G. Ciofani et al.

Dow

nloa

ded

by [

Uni

vers

ite L

aval

] at

21:

19 0

7 O

ctob

er 2

014

34. Cherukuri, P., Bachilo, S.M., Litovsky, S.H., and Weisman, R.B. (2004)

Near-infrared fluorescence microscopy of single-walled carbon nanotubes in

phagocytic cells. J. Am. Chem. Soc., 126(48):15638–15639.

35. Cherukuri, P., Gannon, C.J., Leeuw, T.K., Schmidt, H.K., Smalley, R.E.,

Curley, S.A., and Weisman, R.B. (2006) Mammalian pharmacokinetics of

carbon nanotubes using intrinsic near-infrared fluorescence. Proc. Natl.

Acad. Sci. USA, 103(50):18882–18886.

Pluronic-based Multi-walled Carbon Nanotubes Dispersions 25

Dow

nloa

ded

by [

Uni

vers

ite L

aval

] at

21:

19 0

7 O

ctob

er 2

014