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High-efficiency plasma surface modification of graphite-encapsulated magnetic nanoparticles

using a pulsed particle explosion technique

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2014 Jpn. J. Appl. Phys. 53 010205

(http://iopscience.iop.org/1347-4065/53/1/010205)

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High-efficiency plasma surface modification of graphite-encapsulated magnetic nanoparticles

using a pulsed particle explosion technique

Teguh Endah Saraswati1,2, Shun Tsumura3, and Masaaki Nagatsu1

1Graduate School of Science and Technology, Shizuoka University, Hamamatsu 432-8561, Japan2Department of Chemistry, Faculty of Mathematics and Natural Sciences, Sebelas Maret University, Surakarta 57126, Indonesia3Graduate School of Engineering, Shizuoka University, Hamamatsu 432-8561, Japan

Received July 17, 2013; accepted September 22, 2013; published online December 30, 2013

A high-efficiency surface modification of graphite-encapsulated iron compounds magnetic nanoparticles using an inductively coupled radio-frequency plasma with a pulsed particle explosion technique was studied. A significant increase in N 1s peak intensity in the X-ray photoelectronspectroscopy spectra was obtained by applying a negative pulsed bias voltage of %1 kV to the substrate stage for 15 s or less at a repetitionfrequency of 1 kHz and a duty ratio of 50% in ammonia plasma. The intensity of the N 1s peak and the N/C ratio of the nanoparticles treated in apulsed particle explosion system were 3–4 times higher than those of the particles treated without bias. The amino group population ofnanoparticles treated using the present technique was determined to be about 8.2 ' 104 molecules per nanoparticle, roughly four times higher thanthat of particles treated without bias. The dispersion of the plasma-treated nanoparticles was significantly improved compared with those of theuntreated and treated particles in the nonbiasing system. The surface structure analysis by transmission electron microscopy showed nosignificant damage on the structure or morphology of the treated nanoparticles, indicating that the present technique is applicable to the high-efficiency surface modification of magnetic nanoparticles. © 2014 The Japan Society of Applied Physics

1. Introduction

Recently, carbon-coated magnetic nanoparticles have at-tracted considerable interest in materials science research.The incorporation of both metallic nanoparticles and carbonin a stable core–shell system improves their advantageousproperties, which make them potentially applicable in variousapplications such as magnetic data storage, magnetic fluid,magnetic inks,1) catalyst support,2) magnetic separation,electrode, additives for many uses (i.e., as sintering agentsand propellants), conductive paste, conductive coating, andbiotechnological and biomedical applications.3–10)

Bare metallic nanoparticles have high reactivity and hightoxicity, which are limitations for realizing their practicalapplications, such as instability under oxidation and degra-dation conditions (i.e., in acids). The other disadvantages aretheir easy agglomeration and unsupported-surface structurefor providing functional group attachment for absorbingappropriate molecules. Consequently, coating bare metal-magnetic nanoparticles with a protective shell is anappropriate technique to overcome those limitations.

Compared with polymers and silica, carbon with thegraphite structure is a promising coating material for bio-applications because of its high stability at high temperatureand pressure, and in various chemical and physical environ-ments (i.e., acid or base media). Moreover, the graphite shellallows for further functionalization with specific functionalgroups and biomolecules. Unfortunately, graphite-encapsu-lated magnetic nanoparticles conventionally prepared bythe arc-discharge method, generally disperse only in organicsolvents. This phenomenon makes them unsuitable forbioapplications. To enhance nanoparticle biocompatibility,surface modification processing has become a necessaryprocedure before nanoparticles find practical applications.

One of the efficient methods of surface modificationis plasma treatment, which has been commonly used formany industrial applications. Plasma surface modification isenvironmentally friendly with a short reaction time, andprovides various functional groups.11) Plasma processing canmarkedly increase production if the system is optimized.

Recently, the plasma processing of magnetic materials hasdrawn much attention with regard to nanoparticle surfacetreatment for medical uses, such as drug delivery systems ormagnetic resonance imaging systems. There have been severalplasma reactor systems already developed for particle treat-ment, such as the bell jar reactor,12,13) downstream reactor,14)

rotary drum reactor,15) plasma fluidized bed reactor,16–18)

circulating bed reactor,19,20) plasma batch reactor,21) plasmadowner reactor,22) and plasma reactor with a mechanicalvibrator such as an electromagnet12) or a stirrer.13)

The purpose of these plasma systems is to interfaceparticles with plasma species. An efficient interactionbetween the particle surface and the plasma is the key toachieve the maximum surface modification. Early attempts toimprove the dispersion of pigment particles were carried outusing plasma techniques.23,24) In the case of polymer webs,the entire surface is exposed to plasma using conventionaldrum- or batch-type plasma reactors.25) However, suchplasma reactors are often unsuitable for particle materialsowing to the lack of solid mixing.26)

As described in our previous paper,27) we successfullymodified graphite-encapsulated iron compound magneticnanoparticles deposited on a silicon substrate with aminogroups using Ar and NH3 plasmas in successive stages. Totreat the particles homogeneously, they should be placed onthe sample stage such that they are dispersed as widely aspossible. When the placement of the particle sample is notperformed well, the uniform treatment of the entire bulk ofparticles is difficult to achieve because the modification willlikely take place only on the top layers of the sample, that is,particles inside the bulk will be less exposed to the plasmathan particles at the surface of the bulk.

To enhance the interaction between the particles and theplasma, a modified setup is required. We consider that theparticle explosion technique enables an enhanced surfaceinteraction between the particle samples and the plasmaspecies. Therefore, in the present study, we developed aplasma reactor for particle treatment to explode the particlesinside the chamber by a negative pulsed biasing of the samplestage during the plasma processing.

Japanese Journal of Applied Physics 53, 010205 (2014)

http://dx.doi.org/10.7567/JJAP.53.010205

SELECTED TOPICS IN APPLIED PHYSICSInteractions between Plasmas and Nano-Interfaces

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2. Experimental procedure

2.1 Nanoparticle fabrication and plasma processingsetup

Graphite-encapsulated iron compound magnetic nanoparticleswere prepared by the arc discharge method,28,29) which hasalready been described in previous papers.27,30) The character-istics of the magnetic nanoparticles, such as magnetic prop-erties or crystalline structures, have been described in a pre-vious paper.30) Following nanoparticle synthesis, the nano-particles are treated using a radio-frequency (RF) inductivelycoupled plasma device. A schematic view of the chamber isshown in Fig. 1. The stainless-steel chamber is 200mm inboth diameter and height. The water-cooling copper pipehelical antenna with a coil diameter of 100mm and a pipediameter of 20mm was wound around the quartz bell jar(110mm in outer diameter and 260mm in height) mounted onthe stainless-steel chamber. The helical antenna was coupledto an RF power generator at 13.56MHz via a matchingnetwork. The typical input RF power was about 80W.

The chamber used in this work was modified by adding themetal substrate for the sample stage inside the chamber. Themetal substrate of 10mm diameter was attached to the centerof the glass dish placed at z = ¹2 cm (see Fig. 1), wherez = 0 is defined as the center of the helical antenna on itsaxial axis. To confine the nanoparticles exploded by applyingbias, a glass tube with a diameter of 80mm and a height of60mm was placed on the glass dish.

In the experiment, firstly, we put the particle sample (about5mg) on the metal substrate. The chamber was evacuated to abase pressure of approximately 10¹3 Pa. After the vacuumevacuation, NH3 gas was introduced into the chamber andkept at 50 Pa. During the plasma processing, the gate chamberwas closed to prevent the nanoparticles from flowing to theturbo pump system. The biasing conditions were as follows: asubstrate pulse biasing of ¹1 kV was applied at a repetitionfrequency of 1 kHz and a duty ratio of 50%. The negative

pulsed bias voltage was turned on immediately after switchingthe plasma on. Generally, it will take time to match the inputand reflection powers before applying the pulsed bias to thesubstrate. The bias time tb was varied from 0 to 60 s. Afterbiasing off, the plasma was kept turned on up to the desiredplasma treatment time tp of 10min. The explosion of theparticles by applying pulsed biasing was visually observedand recorded using a high-resolution digital camera (NikonD90) at a capture speed of 24 fps. The videos were thenprocessed using the software VirtualDub to add the timestampand obtain sequential images.

2.2 X-ray photoelectron spectroscopy and transmissionelectron microscopy analysis

Following the plasma treatment, the samples were furtheranalyzed by X-ray photoelectron spectroscopy (XPS) per-formed using a Shimadzu ESCA-3400 with a Mg K¡ X-raysource and high-resolution transmission electron microscopy(HR-TEM) performed using a JEM-2100F at an accelerationvoltage of 200 kV.

2.3 Estimation of amino group populationThe amino group population of the plasma-treated nano-particles was analyzed by the chemical derivatization methodusing sulfosuccinimidyl 6-[3A(2-pyridyldithio)-propion-amido] hexanoate (sulfo-LC-SPDP) according to the specificchemical procedure.31–33) The modified nanoparticles(250 µg) were suspended by bath sonication in 200 µl of10mM sulfo-LC-SPDP in phosphate buffer saline (PBS) andreacted for 30min under light shielding conditions, repeatingthe ultrasonication every 5min. The treated nanoparticleswere washed three times with PBS through ultrasonicationand centrifugation and collected magnetically. The centrifu-gation was performed for 5min with a gravitational force of20,400g (14,000 rpm). The nanoparticles with sulfo-LC-SPDP complexes were then reacted with 300 µl of 20mMdithiothreitol (DTT) in PBS and reacted under light shielding

Fig. 1. (Color online) Schematic view of experimental setup: (1) quartz bell jar, (2) sample stage, (3) copper coil connected to the water cooling system,(4) pressure gauge, (5) leaking valve, (6) gas inlet, (7) gas outlet connected to the turbo and rotary pump, and (8) biasing power supply. The black rectanglerepresents the timeline of experimental stages during plasma processing; tp and tb represent the total time for plasma treatment and the initial time for biasing,respectively.

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conditions, repeating the ultrasonication every 5min. After a15min reaction, 5min centrifugation at 20,400g (14000 rpm)was performed and the cleavage product pyridine-2-thioneliberated from the sulfo-LC-SPDP present in the recoveredsupernatant liquid, was determined by spectrophotometryat 343 nm. The number of amino groups in 250 µg of themodified nanoparticles was quantitatively determined fromthe calibration curve or by theoretical evaluation using theextinction coefficient of pyridine-2-thione at 343 nm: 8.08 ©103M¹1 cm¹1. The number of amino groups per nanoparticlewas calculated when the number of nanoparticles per gramwas 1.14 © 1014. This number was estimated by measuringthe ratio of the mass of the nanoparticles to their volumeunder the assumption that the nanoparticles have a regularspherical shape mainly of 20 nm diameter determined fromthe nanoparticle size distribution taken by HR-TEM.27,30)

2.4 Dispersion propertyThe nanoparticle dispersion before and after the plasmatreatment in both cases with and without the biasing systemwas also observed. The observation was performed bydispersing equal numbers of nanoparticles in the samevolume of distilled water by ultrasonication for about 5min.

3. Results and discussion

The present study was started by preparing nanoparticlesamples by the arc discharge method. The TEM images andEDS profiles of successfully fabricated nanoparticles areshown in Fig. 2. Using TEM, we confirmed that iron com-pound magnetic nanoparticles are clearly encapsulated bygraphitic carbon. This result shows good agreement with theEDS profiles of the selected particles that reveal at least threepossible phases: particles that exhibit the presence of O, Fe,and C. The interplanar distances of the graphite lattice andiron fringes are about 0.34 and 0.202 nm, respectively.

Following the nanoparticle fabrication, we placed nano-particle samples (about 5mg) on the metal substrate of thesample holder located at the center of the glass dish. Afterapplying a pulse bias with a voltage of ¹1 kV, a frequency of1 kHz, and a duty ratio of 50%, the particles were exploded,as shown in the successive images in Fig. 3. These sequentialimages were captured during the pulsed biasing. The firstimage (left top) in Fig. 3 shows the condition before startingthe experiment (plasma OFF). The next image shows thesituation just after turning the plasma on. Then, it generallytook time to produce a stable plasma. Until the time ofturning the bias on, the sample particles are still in the metalstage. Plasma generation will be easily achieved if thematching is adjusted at the desired power beforehand. Theshorter time interval for turning on the bias provides aneffective interaction between the plasma and the particlesample because the lifetime of an NH2 radical is short (a fewmicroseconds).34–36) The lifetime of these plasma species isimportant because we used the no-flow gas condition, that is,the chamber gate valve was closed during plasma treatment,maintaining a pressure at 50 Pa. The negative pulsed biasvoltage was turned on about 11.4 s after turning on theplasma. At the biasing-on time, the particles started toexplode and dropped back to the substrate stage after halfa second. After a certain biasing time (called tb), the biasvoltage was turned off, but the plasma treatment was con-

tinued without biasing up to the desired treatment time tp. Forclarity, red dashed lines are added in the pictures to showwhen and how the particles began to fly and ended up.

In Fig. 4, we summarized the phenomenon observed inFig. 3 as the temporal behavior of the height changes of the

(a) (b)

Inte

nsi

ty(a

rb. u

nit

) (d) (c)

Fig. 2. (a) Low- and (b) high-magnification TEM images of the graphite-encapsulated iron compound nanoparticle successfully fabricated by arcdischarge, (c) energy-dispersive X-ray spectra of the synthesizednanoparticles, and (d) magnified image showing the interplanar distance ofgraphite coating and iron core.

Fig. 3. (Color online) Time-sequential images of particle-explosion eventduring biased-plasma processing.

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exploding particles, at tb = 15 s and tp = 3min. The height ofthe exploding particles was measured as the highest positiontaken from each image frame. The inset figure shows amagnified view of the peak area. The letters shown in Fig. 4denote the main events. A and B represent the timer ONand plasma ON, which correspond to the first (0.000 s) andsecond (0.417 s) images in Fig. 3, respectively. C representsthe time when the biasing was turned on, shown in the fourthimage (11.791 s) in Fig. 3. The gap between B and Cindicates the time required for the impedance matchingbetween the RF source and the plasma. The particles startedto explode at C, reached their maximum height at D(12.125 s), and ended up at E (12.5 s). F represents the timewhen the bias was turned off after 15 s of having the biasturned on, and G represents the time when the plasma wasswitched off after 180 s (3min).

The explosion event is explained by the ion bombardmentmechanism. After turning on the biasing, the pulsed negativehigh voltage caused a high electric field in the sheath betweenthe plasma and the substrate. The existence of a high electricfield caused the ions in the plasma to accelerate towardthe substrate where the particles were placed. Because theparticles were placed on the stage in powder form, oncethe powder was bombarded by plasma ions, the particlesspontaneously popped out and exploded upwards in a veryfast manner. This phenomenon is similar to a conventionalion sputtering event. The ion bombardment energy dependson the difference in potential between the plasma and thesubstrate (¦V = Vp ¹ Vsub) and is subsequently expressed ase(Vp ¹ Vsub). In this study, if the estimated plasma potentialVp is more or less on the order of 10V, the ion bombardmentenergy when biasing is turned on is approximately 1 keV,estimated from a biasing voltage of ¹1 kV. This proposedmechanism also enables agglomerated nanoparticles toseparate into fine particles during the explosion process. Byusing a small (diameter 10mm) metal substrate with a highnegative voltage, the particles explode and do not return backto the substrate but fall on the glass dish area surrounding themetal substrate owing to gravitational force. The dynamicbehavior of the particles and the forces acting on them underbiasing have been discussed in several papers,37–41) which arebeyond the scope of our present paper.

After the plasma treatment, the treated particles werecharacterized by XPS. Figure 5 shows the data set of theN 1s peak of the XPS profiles with various biasing times tbreferred to as the C ¼ F range (see Fig. 4). The observedN 1s peaks located at approximately 399.8 eV are consideredas a signal of the nitrogen-containing group for the aminogroup, which was successfully grafted on the surface.Figure 5 shows the N 1s spectra at different biasing timesof tb = 0 (no biasing), 2, 15, 30, and 60 s, each of which wasobtained for various plasma treatment times tp of up to30min. Comparing the biasing system with the nonbiasingsystem (tb = 0 s), the intensity of the N 1s peak of the biasingsystem (tb = 2, 15, 30, and 60 s) is significantly increased.For example, the intensity of the N 1s peak of the nano-particles treated in the biasing system (tb = 15 s) is raised toabout 3–4 times higher than those in the nonbiasing system(tb = 0 s) for short plasma treatment times tp of up to 3min.This indicates that the plasma treatment with applied biasinghad a significant effect of increasing N 1s peak intensity.These increases in the N 1s peak intensity are supposed to bedue to the enhancement of the efficient interaction betweenthe particles and the plasma species, particularly nitrogen-containing species, during an explosion event.

Fig. 4. (Color online) Explosion height vs plasma treatment time underexperimental conditions of tb = 15 s; tp = 3min. A: timer ON. B: plasmaON. C: bias ON, explosion starts. D: maximum height of explosion event.E: explosion ends. F: bias OFF. G: plasma OFF.

Inte

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ty (

arb

. un

it)

Inte

nsi

ty (

arb

. un

it)

Fig. 5. (Color online) Comparison of N 1s XPS profiles for variousplasma-treatment times (tp = 0, 1, 2, 3, 6, 8, 10, 15, and 30min) and biasingtimes (tb = 0, 2, 15, 30, and 60 s).

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To discuss the XPS profile in more detail, we compare theatomic concentration of nitrogen to the atomic concentrationof carbon in terms of the N/C ratio shown in Fig. 6. The N/Cratios of the five data sets with tb = 0, 2, 15, 30, and 60 s areshown in sequence on comparable scales. Each data set wasobtained for various plasma treatment times within 30min.Figure 6 shows that all the data sets have similar behaviors.The N/C ratio steeply increased within a treatment time tprange of 2–3min, reached their saturation values, and thendecayed slightly thereafter. Comparing the powders with andwithout a biasing event, the N/C ratios of the nonbiasing(tb = 0 s) and biasing (tb > 0 s) systems were significantlydifferent. For example, the N/C ratio of the nonbiasing sys-tem was only about 2.6% (tb = 0 s, tp = 3min), but thatfor the biasing system increased to be about 7% (tb = 15 s,tp = 3min), which became the maximum N/C ratio. Notethat the results of the N/C ratios indicate roughly similartrends to those of N 1s peak intensities shown in Fig. 5. Thepresent results are very analogous to our previous resultsof amino group introduction into the polymer surface usinglow-pressure microwave NH3 plasma.42) The N/C ratio ismaximum at a short plasma treatment time of 30 s and thendecreases owing to the surface damage by ion bombardmentand/or the hydrogen etching effect as treatment timeincreases.

From Figs. 5 and 6, note that the short plasma treatmenttimes of about 3–5min are suitable for achieving the nearlymaximum N/C ratio of 6% for various biasing timesof 2–30 s. In the present study, we obtained almost thesame results for the maximum N/C ratio, roughly 6%for different biasing times. These results suggest that thepowders exploded by applying the negative pulsed bias wereessential to improve the N/C of the particles.

To quantitatively evaluate the absolute values of thenumber of amino groups grafted to the graphite-encapsulatediron composite nanoparticles, we have performed a conven-tional chemical derivatization reaction method using sulfo-succinimidyl 6-[3A(2-pyridyldithio)-propionamido] hexanoate(sulfo-LC-SPDP).31,32) Details of the derivatization steps arepresented in the experimental section.

For the amino group derivatization, we examined threesamples: (a) untreated nanoparticles, (b) treated particles inthe nonbiasing system (tb = 0 s; tp = 3min), and (c) treated

nanoparticles in the biasing system (tb = 15 s; tp = 3min),shown in Fig. 7. The latter represented the sample that hasthe highest N/C ratio among all the samples. The aminogroup analysis gave results showing that no absorbance wasobserved at 343 nm for the untreated nanoparticles, indicatingthat no amino groups were grafted groups to the surface ofpristine graphite-encapsulated iron composite nanoparticles.On the other hand, a very large number of amino groups wereobtained from the plasma-treated samples. The populations ofamino groups for the treated nanoparticles under the experi-mental conditions of (tb = 0 s, tp = 3min) and (tb = 15 s,tp = 3min) were estimated to be about 1.9 © 104 and 8.0 ©104 molecules per particle, respectively. The present resultsalso indicate the efficient enhancement of amino groupmodification by roughly fourfold by the negative pulsedbiasing during the NH3 plasma processing.

Furthermore, to confirm the NH3 plasma effect on thehydrophilicity, we examined the dispersion of the nano-particles in water, the results of which are shown in Fig. 7.Comparing three vials with the same amount of particles (i.e.,a, b, and c), we observed significant differences. As shown inFig. 7, the dispersion of the nanoparticles is significantlyimproved after the plasma treatment. The untreated nano-particles (a) did not disperse in water, and all of the nano-particles remained on the top surface of the water. However,the dispersion of the nanoparticles treated in the biasingsystem (c) is better than those treated in the nonbiasingsystem (b). This can be observed by looking at the dark colorof the solution, which corresponds to the dispersed nano-particles in water. The improvement in the dispersion of thetreated nanoparticles in the biasing system is assumed to becaused by the optimum interaction between NH3 plasma andnanoparticles. The explosion event in the biasing systemallows the amino group to attach not only to a portion butalso to the entire nanoparticle surface. Consequently, thehydrophilicity of the nanoparticles after plasma treatmentgreatly improved because a wider surface area of the nano-particles is available for grafting by amino groups. The aminogroups on the outmost graphite layer are likely to play a keyrole in enhancing the hydrophilicity.

Figure 8 shows an illustration of the interaction of watermolecules with the aminated surface of the nanoparticles. Thegrafted amino group can bind water molecules through

2 4 6 8 10

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Treatment time t p (min)

N/C

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io (

%)

no bias

15s30s60s

tb=2s

Fig. 6. (Color online) Comparison of N/C atomic ratios as a function ofplasma treatment time for different biasing times.

Fig. 7. Amino group population of samples of (a) untreated nanoparticles(tb = 0 s; tp = 0min), (b) treated nanoparticles in nonbiasing system (tb = 0 s;tp = 3min), and (c) treated nanoparticles in biasing system (tb = 15 s;tp = 3min). The inset image shows the dispersion of the representativesample.

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intermolecular forces such as dipole–dipole forces and hydro-gen bonds, as shown by the yellow line in Fig. 8. The largernumber of amino groups may allocate more areas forattracting more water molecules, which contributes to theimprovement in the dispersion property of the nanoparticles.

HR-TEM was also used to analyze the damaging effects onthe morphological and structural properties of the nano-particles before and after plasma treatment. Figure 9 showsimages of the plasma-treated nanoparticles observed by HR-TEM in low and high magnifications. (a) to (f ) correspond

Fig. 8. (Color online) Illustration of interaction between water molecules and amino group grafted onto the outmost portion of graphite layer ofnanoparticles.

Fig. 9. TEM images of plasma-treated nanoparticles observed at low and high magnifications under various experimental conditions: (a) tb = 0 s; tp = 3min;(b) tb = 2 s; tp = 3min, (c) tb = 15 s; tp = 3min, (d) tb = 30 s; tp = 3min, (e) tb = 60 s; tp = 3min, and (f ) tb = 15 s; tp = 30min.

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to the treated nanoparticles under various (tb, tp) conditionsof (0 s, 3min), (2 s, 3min), (15 s, 3min), (30 s, 3min),(60 s, 3min), and (15 s, 30min). All of the images confirm thatno damage or destruction was induced in the nanoparticlestructure after performing the plasma treatment in thenonbiasing or biasing system. The structure of the graphitelayers was found to be stable under all the experimentalconditions. Similarly to the graphite coating, the iron com-pound core also remained encapsulated inside the graphitelayers even when the particles were subjected to long-termplasma treatment, as shown in Fig. 9(f ). This result indicatesthat the pulsed-biasing particle explosion technique per-formed in the present study is highly efficient in amino groupfunctionalization. Furthermore, it is also suitable for surfacemodification, particularly for powder samples because of itsability to retain the structural stability of nanoparticles.

4. Conclusions

The surface of graphite-encapsulated iron compound mag-netic nanoparticles fabricated by arc discharge was success-fully modified by a pulsed particle explosion technique. Thistechnique was performed by applying a high negative bias of¹1 kV to the substrate stage for 2–60 s at a repetition fre-quency of 1 kHz and a duty ratio of 50% in ammonia plasmagenerated using a radio frequency inductively coupledplasma device. The intensity of the N 1s peak in the XPSspectrum of the nanoparticles treated in the biasing systemwas three to four times higher than that treated in thenonbiasing system owing to the enhancement of the interac-tion between the nanoparticles and the plasma species. Thepresent results also indicate the efficient enhancement ofamino group modification by approximately fourfold fromabout 1.9 © 104 molecules/nanoparticle in the case of(tb = 0 s, tp = 3min) to 8.2 © 104 molecules/nanoparticlein the case of (tb = 15 s, tp = 3min) by the negative pulsedbiasing during the NH3 plasma processing. Moreover, theresults also showed that the dispersion of the treated nano-particles in biasing system was improved compared withthose of the untreated and treated samples in the nonbiasingsystem.

In addition, analysis by HR-TEM showed no significantdamage on the nanoparticle structures, indicating that thepresent technique is suitable mainly for the surface mod-ification of particle samples owing to its high efficiency insurface modification without causing any significant changeor destruction of the structural and morphological properties.

Acknowledgments

This work was supported in part by a Grant-in-Aid forScientific Research (No. 2110010) from the Japan Society forthe Promotion of Science (JSPS). The authors would like tothank Associate Professor A. Ogino of Shizuoka Universityfor technical assistance in the plasma chamber developmentand UV–vis absorption spectroscopy measurement.

1) M. E. McHenry, S. A. Majetich, and E. M. Kirkpatrick, Mater. Sci. Eng. A204, 19 (1995).

2) L. Kong, X. Lu, X. Bian, W. Zhang, and C. Wang, ACS Appl. Mater.Interfaces 3, 35 (2011).

3) G. Pastorin, Pharmacol. Res. 26, 746 (2009).4) Q. A. Pankhurst, N. T. K. Thanh, S. K. Jones, and J. Dobson, J. Phys. D 42,

224001 (2009).5) Q. A. Pankhurst, J. Connolly, S. K. Jones, and J. Dobson, J. Phys. D 36,

R167 (2003).6) A.-H. Lu, E. L. Salabas, and F. Schüth, Angew. Chem., Int. Ed. 46, 1222

(2007).7) S. Kim, E. Shibata, R. Sergiienko, and T. Nakamura, Carbon 46, 1523

(2008).8) A. Ito, M. Shinkai, H. Honda, and T. Kobayashi, J. Biosci. Bioeng. 100, 1

(2005).9) C. C. Berry and A. S. G. Curtis, J. Phys. D 36, R198 (2003).

10) C. C. Berry, J. Phys. D 42, 224003 (2009).11) C. Chen, A. Ogino, X. Wang, and M. Nagatsu, Appl. Phys. Lett. 96, 131504

(2010).12) A. B. García, A. Martínez-Alonso, C. A. Leon y Leon, and J. M. D. Tascón,

Fuel 77, 613 (1998).13) J. P. Boudou, A. Martinez-Alonzo, and J. M. D. Tascon, Carbon 38, 1021

(2000).14) D. Shi and P. He, Rev. Adv. Mater. Sci. 7, 97 (2004).15) V. Brüser, M. Heintze, W. Brandl, G. Marginean, and H. Bubert, Diamond

Relat. Mater. 13, 1177 (2004).16) F. Bretagnol, M. Tatoulian, F. Arefi-Khonsari, G. Lorang, and J. Amouroux,

React. Funct. Polym. 61, 221 (2004).17) F. Arefi-Khonsari, M. Tatoulian, F. Bretagnol, O. Bouloussa, and F.

Rondelez, Surf. Coatings Technol. 200, 14 (2005).18) M. Tatoulian, F. Brétagnol, F. Arefi-Khonsari, J. Amouroux, O. Bouloussa,

F. Rondelez, A. J. Paul, and R. Mitchell, Plasma Processes Polym. 2, 38(2005).

19) S. H. Jung, S. H. Park, D. H. Lee, and S. D. Kim, Polym. Bull. 47, 199(2001).

20) S. H. Jung, S. H. Park, and S. D. Kim, J. Chem. Eng. Jpn. 37, 166 (2004).21) J.-W. Kim, Y.-S. Kim, and H.-S. Choi, Korean J. Chem. Eng. 19, 632

(2002).22) C. Arpagaus, A. Rossi, and P. Rudolf von Rohr, Appl. Surf. Sci. 252, 1581

(2005).23) T. Ihara, S. Itô, and M. Kiboku, Chem. Lett. 15, 675 (1986).24) K. Tsutsui, K. Nishizawa, and S. Ikeda, J. Coatings Technol. 60, 107

(1988).25) M. R. Wertheimer, H. R. Thomas, M. J. Perri, J. E. Klemberg-Sapieha, and

L. Martinu, Pure Appl. Chem. 68, 1047 (1996).26) C. Arpagaus, Dr. Thesis, Swiss Federal Institute of Technology Zurich,

Zurich (2005).27) T. E. Saraswati, T. Matsuda, A. Ogino, and M. Nagatsu, Diamond Relat.

Mater. 20, 359 (2011).28) M. Nagatsu, T. Yoshida, M. Mesko, A. Ogino, T. Matsuda, T. Tanaka, H.

Tatsuoka, and K. Murakami, Carbon 44, 3336 (2006).29) Y. Saito, T. Yoshikawa, M. Okuda, N. Fujimoto, S. Yamamuro, K. Wakoh,

K. Sumiyama, K. Suzuki, A. Kasuya, and Y. Nishina, Chem. Phys. Lett.212, 379 (1993).

30) T. E. Saraswati, A. Ogino, and M. Nagatsu, Carbon 50, 1253 (2012).31) J. Carlsson, H. Drevin, and R. Axén, Biochem. J. 173, 723 (1978).32) G. Hermanson, Bioconjugate Techniques (Academic Press, New York,

2008) 2nd ed., p. 277.33) B. Yoza, A. Arakaki, and T. Matsunaga, J. Biotechnol. 101, 219 (2003).34) X.-Y. Gong, X. R. Duan, H. Lange, and A. A. Meyer-Path, Chin. Phys. Lett.

18, 939 (2001).35) E. R. Fisher, Plasma Processes Polym. 1, 13 (2004).36) A. Fateev, F. Leipold, Y. Kusano, B. Stenum, E. Tsakadze, and H. Bindslev,

Plasma Processes Polym. 2, 193 (2005).37) T. Moriya, M. Shimada, K. Okuyama, and H. Setyawan, Jpn. J. Appl. Phys.

44, 4871 (2005).38) C. M. Ticoş, A. Dyson, and P. W. Smith, Plasma Sources Sci. Technol. 13,

395 (2004).39) J. Liu, D. Wang, T. C. Ma, Y. Gong, Q. Sun, J. X. Ma, and C. X. Yu, Phys.

Plasmas 6, 1405 (1999).40) V. E. Fortov, A. V. Ivlev, S. A. Khrapak, A. G. Khrapak, and G. E. Morfill,

Phys. Rep. 421, 1 (2005).41) H. Kersten, H. Deutsch, E. Stoffels, W. W. Stoffels, and G. M. W. Kroesen,

Int. J. Mass Spectrom. 223–224, 313 (2003).42) M. Král’, A. Ogino, and M. Nagatsu, J. Phys. D 41, 105213 (2008).

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Teguh Endah Saraswati is currently a Lecturer andResearcher in Chemistry Department, Mathematicsand Natural Sciences Faculty, Sebelas Maret Uni-versity, Indonesia. She received Master of Sciencedegree in chemistry from Nagoya University in2009, which was supported by Panasonic Scholar-ship Program. In 2012, she completed the doctoralprogram, which was partly supported by AmanoFoundation Scholarship. She obtained Ph. D degreeat Nanovision Technology Department, Graduate

School of Science and Technology, Shizuoka University under thesupervision of Prof. Masaaki Nagatsu. Her research interests are inorganicand materials chemistry, and surface modification by plasma processing.

Shun Tsumura received the B.S. and M.S. degrees in electrical engineeringfrom Shizuoka University in 2011 and 2013, respectively. During hisbachelor and master courses, he engaged in the study of surface modificationof magnetic nanoparticles by using an RF excited inductively coupledplasma under the supervision of Prof. Masaaki Nagatsu.

Masaaki Nagatsu received the B.S., M.S., and Dr.Eng. Degrees from Nagoya University in 1975,1979, and 1985, respectively. During 1975 to 1976,he worked for Hitachi Research Laboratory, Hitachi,Ltd. From 1982 to 2000, he was an assistantprofessor (1982–1989), lecturer (1990–1991), andassociate professor (1991–2000) in the Departmentof Electrical Engineering of Nagoya University.During 1987–1989, he worked as a visitingresearcher in University of California, Los Angeles.

In 2001, he became a professor of Department of Engineering in ShizuokaUniversity. He became a Director of Graduate School of Science andTechnology, Research Division in 2006 and a Dean of Graduate School ofScience and Technology of Shizuoka University since 2008. His researchfield is the plasma production and surface modification of materials for bio-medical and environmental application.

Jpn. J. Appl. Phys. 53, 010205 (2014) SELECTED TOPICS IN APPLIED PHYSICS

010205-8 © 2014 The Japan Society of Applied Physics