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Materials Chemistry and Physics 122 (2010) 164–168

Contents lists available at ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

ron-included carbon nanocapsules coated with biocompatibleoly(ethylene glycol) shells

unghoon Kim1, Ruslan Sergiienko ∗, Etsuro Shibata, Takashi Nakamuranstitute of Multidisciplinary Research for Advanced Materials, Tohoku University, 1,1 Katahira, 2-Chome, Aobaku, Sendai 980-8577, Japan

r t i c l e i n f o

rticle history:eceived 11 June 2009eceived in revised form 27 January 2010

a b s t r a c t

Nanoparticles of iron carbides wrapped in multilayered graphitic sheets (carbon nanocapsules) weresynthesized by electric plasma discharge in an ultrasonic cavitation field in liquid ethanol and purified by

ccepted 22 February 2010

eywords:arbon nanocapsulesrc dischargeslectron microscopy

selective oxidation and magnetic separation. The particles had 100–200 nm in diameter after centrifugingfor 10 min at 4000 rpm. Carbon nanocapsules were covered by wispy poly(ethylene glycol) PEG coatingabout 7–10 nm in thickness. The number of PEG chains coated on carbon nanocapsules could be estimatedas 9.15%. The values of saturation magnetization Ms and coercivity Hc of purified carbon nanocapsuleswithout PEG coating were 112 emu g−1 and 75 Oe respectively. Magnetically soft carbon nanocapsuleswith a poly(ethylene glycol) coating on the surface may possibly be used as biocompatible magnetic

appli

agnetic properties nanoparticles in medical

. Introduction

Since the application of magnetic nanoparticles in biomedicineas first suggested, different types of magnetic nanoparticles have

een proposed as carriers in enhanced targeted delivery systems, asagnetic resonance imaging (MRI) contrast agents, in hyperther-ia treatment, and in magnetic field-assisted radionuclide therapy

1–6]. These magnetic nanoparticles, with controllable sizes rang-ng from a few nanometers to tens of nanometers, are smaller thanr comparable in size to cells (10–100 �m), viruses (20–450 nm),roteins (5–50 nm), and genes (2 nm wide and 10–100 nm long).his means the particles can ‘get close’ to a biological entity of inter-st [7]. The biomedical applications using magnetic nanoparticlesnd an external magnetic field impose several requirements on thedsorbent particles. The nanoparticles themselves must be biolog-cally inert. Moreover, they must also have a high sorption capacityor the drug, and the rate of drug desorption in an organism needso be slow so that a high concentration of the cytostatic drug cane maintained in the tumor area for a prolonged period of time.

ince the particles must be selectively controlled by the appliedagnetic field, both their magnetic properties and their degrees of

ispersion and agglomeration are important [8].

∗ Corresponding author. Tel.: +81 22 217 5214; fax: +81 22 217 5214.E-mail addresses: rutile94@gmail.com (S. Kim), ruslan sergiienko@yahoo.com

R. Sergiienko).1 Address: Samsung Seocho-Tower 1320-10, Seocho 2-Dong, Seocho-ku, Seoul,epublic of Korea.

254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2010.02.054

cations.© 2010 Elsevier B.V. All rights reserved.

Core–shell structures utilizing biocompatible graphite to encap-sulate the magnetic nanoparticles have become attractive approachfor developing MRI contrast agents [9] or magnetic targetedcarriers for drug delivery [10]. Graphite shells provide bothprotection against chemical degradation of magnetic cores andprevent the release of potentially toxic components. Further-more, such graphite shells can serve as an intermediate layer forattaching other biocompatible materials [10], polymers, becausefunctionalization chemistries are generally better established withcarbon materials than those that comprise magnetic cores. Manypolymers such as dextran [11,12], poly(ethylene glycol) (PEG)[13–15], and poly(vinylpyrrolidone) (PVP) [16] are known tobe biocompatible, and thus it is feasible to construct magneticnanoparticles having longevity in the blood circulation [8]. PEGis the most commercially important material for tissue engi-neering and other biomedical applications including the bloodresidence prolongation of nanoparticles. The well-establishedPEG improves the nanoparticle stability in the biological milieuagainst interactions with macromolecules (e.g., opsonins) andcells, thus resulting in the prolonged circulation in blood andreduced side effect uptake by the particulate carrier system[9,17,18]. Metal encapsulated carbon nanocapsules with PEG coat-ing should stably disperse in the body and PEG coating providesfor longer circulation time and maximizing the concentration ofcarbon nanocapsules in target tissues for diagnostic or therapeutic

uses.

The purpose of this article is to show the structure and mor-phology of the iron carbide-included carbon nanocapsules (CNCs)coated with PEG and magnetic properties of purified carbonnanocapsules. In this study, the carbon nanocapsules were pre-

S. Kim et al. / Materials Chemistry and Physics 122 (2010) 164–168 165

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Fig. 1. Scheme of coating of carbon na

ared by electric plasma discharge in an ultrasonic cavitation fieldf liquid ethanol.

. Materials and methods

.1. Synthesis of carbon nanocapsules

Samples of carbon nanocapsules filled with iron carbide nanoparticles were syn-hesized using the developed method, and the details of this method have beeneported in previous papers [19–21]. Plasma discharge was generated between thee anode (Ø 3 mm, purity 99.7%) and the bottom of the iron tip (Ø 18 mm, purity9.9%) fixed on the top of a titanium ultrasonic horn, which served as the cathode.n ultrasonic homogenizer (Nissei, US-600NCVP) was used at 600 W and 20 kHz

o irradiate the liquid ethanol (Wako, S-grade). During the ultrasonic irradiation,he voltage between the anode and cathode was kept at 55 V DC using a constantoltage power unit (A&D, AD-8735), and the upper current limit of the unit was sett 3 A. Electric plasma was generated in the cavitation field of liquid ethanol justeneath the iron tip, where the Fe anode was consumed by thermal evaporationSupplementary video file). After the synthesis, the produced carbonaceous powderas separated by centrifugation and vacuum evaporation from the liquid ethanol.

.2. Purification

In this study, the carbonaceous powder prepared by plasma discharge inhe ultrasonic cavitation field of liquid ethanol usually included, besides carbonanocapsules, impurities of exposed metals and other forms of carbon such as amor-hous carbon and graphite balls and flakes [22]. The removal of these impurities wasone in two steps. The first step involved using hydrogen peroxide solution and con-inuous acid etching to remove the amorphous carbon and exposed iron particles.

200 mg sample of the as-prepared carbon powder was treated in 200 ml of 25%ydrogen peroxide solution refluxed at 90 ◦C for 40 h. After the hydrogen peroxidereatment, the purified powder was separated by centrifugation from the remainingydrogen peroxide and etched in 15% HCl solution for 4 h at room temperature toemove iron oxide particles. Finally, the purified carbon nanocapsules were washedn distilled water, and dried at 40 ◦C in a vacuum drying oven.

The second step involved the magnetic separation of graphite balls in the as-repared samples using a magnetic field. After the first purification step, the sampleas dispersed in 50 ml of liquid ethanol with an ultrasonic homogenizer for 10 min.sing the magnetic separator involves the moving of a permanent magnet near theall of a test tube along its length to aggregate the carbon nanocapsules. For mag-etic separation, the sample after oxidation and etching was dispersed in the testube with 10 ml of liquid ethanol and positioned such that the lower part of theube lay between two 1.3 T permanent magnets. The tube was then moved slowly0.1 �m s−1) by a precise-motion motorized stage between the permanent magnets.fter a few hours, black material including carbon nanocapsules was observed adja-ent to both magnets and further separation was discernible over a period of oneay.

After magnetic separation, a centrifuge (Kobuta 6200) was used to divide parti-les by size. The carbon nanocapsules dispersed in the ethanol were put into a 10 mlube, which was centrifuged for 10 min at 1000 rpm. The liquid part of the ethanololution after the centrifugation was moved to another centrifugal tube and thenentrifuged again for 10 min at 2000 rpm. In the same way, the liquid parts weree-centrifuged step by step at 1000, 2000, 3000 and 4000 rpm. After centrifugation,ach sample was put into the vacuum drying oven at 40 ◦C for 4 h.

.3. Coating carbon nanocapsules by the solution-phase deposition process

Purified and divided carbon nanocapsules were coated with PEG to improveheir biocompatibility. Iron carbide-filled carbon nanocapsules were coated withEG 3400 (Sigma–Aldrich). Distilled water was further purified by a Milli-Q systemMillipore, UK). Diethyl ether was purchased from Wako Pure Chemical Industries,td. (Japan).

sules with poly(ethylene glycol) PEG.

In the solution-phase deposition process, we used the carbon nanocapsules fromthe centrifugation for 10 min at 4000 rpm. Fig. 1 shows the solution-phase deposi-tion process. 8 mg of purified sample was dispersed in a vial with 10 ml of diethylether solution and heated in a water bath at 70 ◦C while being shaken. 0.4 mg ofPEG was then put into the vial and the sample preserved at 70 ◦C under stirring. Theobtained diethyl ether solution of PEG and carbon nanocapsules was enclosed in avial with a screw cap and kept for 12 h. Finally, the PEG-coated carbon nanocap-sules were cooled in an ultrasonic homogenizer bath for 10 min and separated bycentrifugation and vacuum evaporation from the liquid diethyl ether.

2.4. Characterization

The structure, morphology and size distribution of the purified and separatedcarbon nanocapsules were evaluated by 300 kV transmission electron microscopy(TEM) (JEM-3010) and field emission scanning electron microscopy (FE-SEM) (JSM-7000F). Fourier transform infrared spectroscopy (FT-IR) (FTS7000, Mid-IR&ATR) wasconducted to examine the types of chemical groups present on carbon nanocapsulesin the frequency range 4000–400 cm−1. TG analysis was carried out on a RigakuThermo Plus TG 8120 to investigate the content of the PEG chains onto the graphitesurface. A vibration sample magnetometer (VSM) operating at room temperaturewith an applied magnetic field up to 15 kOe was used to measure the magneticproperties of the synthesized carbon nanocapsules.

3. Results and discussion

TEM and SEM images of the carbon nanocapsules were takento analyze their shape, size, and uniformity. As already shownin previous work [22], the spherical carbon nanocapsules of thepurified sample had a broad size distribution from 30 to 1000 nmin diameter. The sizes of carbon nanocapsules and their distribu-tion decreased with increasing centrifuging speed. Fig. 2a and bindicates the carbon nanocapsules were about 100 and 200 nm indiameter after centrifuging for 10 min at 4000 rpm.

TEM images (Fig. 2c and d) inform on the presence of addi-tional amorphous coating on the graphite shell of the carbonnanocapsule dried from diethyl ether solution. The FT-IR spec-troscopy was carried out to show the presence of PEG compoundin the carbon nanocapsules sample. Fig. 3 presents the infraredspectra for pure PEG (Fig. 3a), carbon nanocapsules containingPEG (Fig. 3b), and carbon nanocapsules without PEG (Fig. 3c). InFig. 3a for the pure PEG sample, the absorption bands of 3435,2860, 2920, and 1100 cm−1 were observed owing to O–H, C–H andC–O stretching vibrations, respectively. These peaks also existedin the sample for carbon nanocapsules containing PEG (Fig. 3b).It implies that amorphous coating (Fig. 2c and d) represents thePEG chains on the graphite shell of carbon nanocapsule. However,FT-IR spectrum (Fig. 3b) does not provide any strong evidencesof covalently bonded PEG chains to carbon nanocapsule graphitesurface. The surface of carbon nanocapsules must be functional-ized with groups such as carbonyl, carboxyl and hydroxyl prior to

provide a covalent bond [9,23]. The entire coated carbon nanocap-sule was 120–150 nm in diameter with a wispy PEG coating about7–10 nm in thickness. As can be seen from the HRTEM image inFig. 2d, such a PEG coating did not have favorable geometry forthe stabilization of the uniform thin biocompatible coating and

166 S. Kim et al. / Materials Chemistry and Physics 122 (2010) 164–168

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ig. 2. (a) TEM image and (b) SEM image of carbon nanocapsules after centrifuginRTEM images of a carbon nanocapsule coated with PEG.

hus we need to improve the coating process used in presentxperiment.

TG analysis was carried out on the carbon nanocapsules to inves-igate the content of the PEG chains onto the graphite surface. Fig. 4hows the TGA curves of purified carbon nanocapsules without PEGFig. 4a), carbon nanocapsules with PEG coating (Fig. 4b) and PEGhemical reagent sample itself (Fig. 4c). It is clear that no weight lossppears in Fig. 4a, which is ascribed to carbon nanocapsules with-ut PEG. In contrast, a 100% weight loss of PEG sample occurredear 400 ◦C resulting from thermal decomposition of the PEG sam-le (Fig. 4c). The weight loss of the carbon nanocapsules with PEGoating is mainly due to the depolymerization of PEG chains, whichccupied 9.15 mass% (Fig. 4b), demonstrating that the number ofEG chains coated on carbon nanocapsules can be estimated as

.15 mass%.

The magnetic properties of the purified, centrifuged sampleithout PEG coating and as-prepared carbon nanocapsules wereeasured with the VSM at room temperature and the results are

resented in Fig. 5 and Table 1. The values of saturation mag-

0 min at 4000 rpm. (c) TEM images of a carbon nanocapsule coated with PEG. (d)

netization Ms and coercivity Hc of purified carbon nanocapsulesranging from 100 to 200 nm in diameter were 112 emu g−1 and75 Oe respectively. The value of the saturation magnetization ofpurified carbon nanocapsules (112 emu g−1) is less than values of131.6 emu g−1 (Fe3C), 121.4 emu g−1 (�-Fe5C2) and 138 emu g−1

(Fe2C) reported for the microcrystalline iron carbides measured atroom temperature in a field strength of 9.73 kOe [24], because thecores of purified carbon nanocapsules have a low crystalline ironcarbide structures (�-Fe5C2, Fe3C) [22] and saturated by carbon.The value of saturation magnetization of 112 emu g−1 and coer-civity 75 Oe are higher than the values of 48 emu g−1 and 52 Oerespectively [20] reported for the as-prepared carbonaceous sam-ple without purification (Fig. 5a and b) (Table 1). To account forthe differences between the magnetic properties of the purified

carbon nanocapsules and those of the as-prepared powder sam-ple, it is clear that the proportion of nonmagnetic phases (graphiteimpurities and amorphous carbon) has a detrimental effect on thesaturation magnetization and coercivity. Fact is that the saturationmagnetization depends strongly on the amount of magnetic mate-

S. Kim et al. / Materials Chemistry and Physics 122 (2010) 164–168 167

Table 1Comparison of the magnetic properties at room temperature for the iron-included carbon nanocapsules synthesized by different methods.

Sample reference (core/shell) Ms, emu g−1 Mr, emu g−1 Hc, Oe Mr/Ms Particle size, nm

As-prepared (this study) 48 1.80 52 0.038 5–600Purified CNCs (this study) 112 2.42 75 0.022 100–200[25] (�-Fe, �-Fe/graphite)a 56.21 16.86 600 0.30 32–81[26] (�-Fe, Fe3C/graphite)b 96 14.40 242 0.15 5–80[27] (�-Fe, �-Fe/carbon)c 79 15.7 441 0.2 2–58[28] (�-Fe, Fe3C/graphite)d 50 6 175 0.12 10–100[29] (�-Fe, �-Fe, Fe3C/graphite)e 85 20.8 397 0.245 6–40[30] (�-Fe, �-Fe, Fe3C/graphite)f 53 8 200 0.15 10–100[31] (Fe7C3, Fe2C/graphite)g 50 – 25 – 20–60

a Arc discharge process.b Inductively coupled radio frequency thermal plasma torch in liquid ethanol.c The high pressure chemical vapour deposition.d Arc discharge process in methane.e Arc discharge in ethanol vapour.f Arc plasma method.g Hexagonal iron carbide nanocrystals produced by capacitively coupled radio frequency plasma discharge.

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[25–30]. Reduced value of coercivity may be related to a fact that

Fl

ig. 3. FT-IR spectra of (a) pure PEG, (b) carbon nanocapsules coated with PEG andc) carbon nanocapsules without the coating.

ial. Therefore we may expect that diamagnetic PEG coating has toeduce the saturation magnetization of purified carbon nanocap-ules by about 9–10%, because the number of PEG chains coated onarbon nanocapsules was estimated as 9.15 mass% by TG method.

It is reasonable to compare our results to materials produced by

ifferent methods [25–31] (Table 1). Purified iron-included carbonanocapsules exhibits relatively higher value of Ms in comparisonith reference ones. Higher value of Ms (emu g−1) may be related tolarger amount of iron in our material. The relatively high amount

ig. 5. (a) Magnetization loops for the purified carbon nanocapsules (CNCs) after centrifow field parts of the loops shown in (a).

Fig. 4. TGA traces of (a) carbon nanocapsules without the PEG coating, (b) carbonnanocapsules coated with PEG and (c) pure PEG.

of diamagnetic amorphous carbons and graphite fragments couldreduce the saturation magnetization of reference samples [25–31]and made weaker the interaction of magnetic domains, moreoverthe difference in the magnetization is related to different nanopar-ticle sizes. The lower value of coercivity Hc may provides evidencethat sizes of our carbon nanocapsules are larger that ones from Refs.

the size of iron-included carbon nanocapsules (100–200 nm) is farfrom critical sizes (Dc) [32] attributed to a threshold for single mag-netic domain structures and multidomains exist in our iron carbideparticles. Nanoparticles with a larger size than its critical size (Dc)

uging for 10 min at 4000 rpm and as-prepared carbon powder sample and (b) the

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68 S. Kim et al. / Materials Chemis

educe the coercivity. Quantitatively, critical sizes amount to 14,5 nm for pure �-Fe and iron carbides respectively [32,33].

The narrow hysteresis loop shows a low coercivity (75 Oe),egligible remanent magnetization (2.5 emu g−1) and thus impliessoft ferromagnetic behavior of purified carbon nanocapsules

Fig. 5b). For biomedical applications, the use of iron carbideanoparticles that possess relatively high saturation magnetiza-ion and have a soft ferromagnetic behavior at room temperatures preferred. In case of magnetic separation of biological materi-ls and directed drug delivery, where a magnetic field gradients used to apply a force to the particles, the advantage of higher

agnetization is fairly obvious [7,34,35]. Soft magnetic char-cter in drug delivery is necessary because once the externalagnetic field is removed, magnetization disappears, and thus

gglomeration and strong magnetic interaction between particlesand the possible embolization of capillary vessels) are avoided36]. Thus magnetic metal-included carbon nanocapsules haveotential biomedical applications, such as, serving as contrastgents for magnetic resonance imaging (MRI) [9,37], drug delivery10], near-IR light absorbers for selective cancer-cell destruction9,38], magnetic hyperthermia [27], magnetic separations of bac-eria/antigens/proteins from bio-samples, etc.

. Conclusion

Carbon nanocapsules were synthesized by a method in whichltrasonic cavitation permits an electric plasma discharge toccur, with the application of relatively low electric power evenn insulating organic liquid such as ethanol. After oxidation by5% H2O2 solution and magnetic separation, the powder sampleas centrifuged for 10 min at 4000 rpm and we obtained carbonanocapsules with diameters around 100–200 nm. TEM and FT-IRxperimental results showed that the poly(ethylene glycol) PEGolymer coats graphite shells of carbon nanocapsules via a sim-le solution-phase deposition process in diethyl ether solution. Theelatively low coercivity of 75 Oe and high magnetization satura-ion of 112 emu g−1 are good prerequisite for application of purifiedron-included carbon nanocapsules in nanomedicine. It is possiblehat softly ferromagnetic PEG-coated carbon nanocapsules may besed for in vivo and in vitro biomedical applications; for exam-le, as a magnetic resonance contrast agent, magnetic separationf biological materials or in a target drug delivery system. Addition-lly, comparison of the magnetic properties at room temperatureor the iron-included carbon nanocapsules synthesized by differ-nt methods showed that our purified carbon nanocapsules haveigher value of magnetization saturation Ms and lower value ofoercivity Hc in comparison with reference ones.

cknowledgments

We gratefully acknowledge to Mr. Yuichiro Hayasaka for hisonsiderable support in TEM investigations. This work was finan-ially supported by a Grant-in-Aid for Exploratory Research (No.7656243) and Young Scientists (A) (No. 20686051) from the Min-

stry of Education, Culture, Sports, Science and Technology, Japan.

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d Physics 122 (2010) 164–168

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.matchemphys.2010.02.054.

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