DOI: 10.1002/asia.201200950

Ultra-small, Uniform, and Single bcc-Phased FexCo1-x/Graphitic ShellNanocrystals for T1 Magnetic Resonance Imaging Contrast Agents

In Ae Choi,[a] Yan Li,[a, b] Da Jeong Kim,[a] Mou Pal,[a] Jee-Hyun Cho,[c] Kyujoon Lee,[d]

Myung-Hwa Jung,[d] Chulhyun Lee,[c] and Won Seok Seo*[a]

Introduction

Inorganic nanocrystals are of great interest for biomedicalapplications due to their unique optical, magnetic, and elec-tronic properties.[1] In particular, magnetic nanocrystals havebeen widely used for in vivo applications such as magneticresonance imaging (MRI) contrast enhancement, drug deliv-ery, cancer therapy, hyperthermia, and in vitro cell separa-tion.[2] For these biomedical applications, nanocrystals arerequired to be uniform so that each individual nanocrystalhas nearly identical physical and chemical properties forcontrolled biodistribution and contrast enhancement ef-fects.[3] Especially, small-size magnetic nanocrystals are ad-vantageous for T1 MRI contrast enhancement because they

can enhance the T1 effect by their large surface area andreduce the T2 effect by their small magnetic moment.[4]

However, only a few methods including colloidal synthesishave been reported for the synthesis of uniform and small-size magnetic nanocrystals.[5] Moreover, current investiga-tions have focused mainly on iron oxide-based magnetic ma-terials.

Among all known magnetic nanocrystals, FeCo alloyshave superior magnetic properties including the highest sat-uration magnetization and a high superparamagnetic limitof about 20 nm.[6] In spite of their superior magnetic proper-ties, the poor oxidation resistance and the potential cytotox-icity of bare FeCo nanocrystals have prevented their wide-spread biomedical applications. To circumvent these prob-lems, FeCo/graphitic carbon shell (FeCo/GC) nanocrystalswere recently produced by a simple chemical vapor deposi-tion (CVD) method.[7] A single-layered graphitic carboncoverage over the nanocrystalline FeCo surface has beenproven to prevent the rapid environmental degradation ofcore metals and to improve the biocompatibility.[7] More-over, it can provide the potential of using the nanocrystalsas a photothermal agent based on the near-infrared opticalabsorbance of carbon.[7,8] FeCo/GC nanocrystals functional-ized with phospholipid–poly(ethylene glycol) (PL–PEG)molecules were demonstrated to be the most powerful non-toxic agents for MRI contrast enhancement, drug delivery,and cancer therapies.[7,9] Despite their important applica-tions, ultra-small, uniform, discrete, and single body-cen-tered-cubic (bcc)-phased FeCo/GC nanocrystals suitable forT1 MRI contrast enhancement have not yet been report-ed.[10] Furthermore, a systematic study on the relationshipbetween composition and magnetic resonance (MR) proper-ties of FeCo/GC nanocrystals has not been conducted so far.

Abstract: We have synthesized ultra-small and uniform FexCo1-x/graphiticcarbon shell (FexCo1-x/GC) nanocrystals(x=0.13, 0.36, 0.42, 0.50, 0.56, and 0.62,respectively) with average diameters of<4 nm by thermal decomposition ofmetal precursors in approximately60 nm MCM-41 and methane CVD.The composition of the FexCo1-x/GCnanocrystals can be tuned by changingthe Fe:Co ratios of the metal precur-

sors. The FexCo1-x/GC nanocrystalsshow superparamagnetic properties atroom temperature. The Fe0.50Co0.50/GC,Fe0.56Co0.44/GC, and Fe0.62Co0.38/GCnanocrystals have a single bcc FeCostructure, whereas the Fe0.13Co0.87/GC,

Fe0.36Co0.64/GC, and Fe0.42Co0.58/GCnanocrystals have a mixed structure ofbcc FeCo and fcc Co. The single bcc-phased FexCo1-x/GC nanocrystals func-tionalized with phospholipid–poly(eth-ylene glycol) (PL–PEG) in phosphatebuffered saline (PBS) are demonstrat-ed to be excellent T1 MRI contrastagents.

Keywords: alloys · graphitic shell ·magnetic resonance · mesoporousmaterials · nanoparticles

[a] I. A. Choi, Y. Li, D. J. Kim, Dr. M. Pal, Prof. W. S. SeoDepartment of Chemistry and Inorganic and Bio-Materials Center ofBK21Sogang UniversitySeoul, 121-742 (Korea)E-mail : wsseo@sogang.ac.kr

[b] Y. LiInterdisciplinary Program of Integrated BiotechnologySogang UniversitySeoul, 121-742 (Korea)

[c] J.-H. Cho, Dr. C. LeeDivision of Magnetic Resonance ResearchKorea Basic Science InstituteOchang, Cheongwon, Chungbuk 363-883 (Korea)

[d] K. Lee, Prof. M.-H. JungDepartment of PhysicsSogang UniversitySeoul, 121-742 (Korea)

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/asia.201200950.

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FULL PAPER

We herein report a simple and reliable method to synthe-size, for the first time, ultra-small, uniform, and single bcc-phased FexCo1-x/GC nanocrystals (x= 0.50, 0.56, 0.62). Aschematic diagram for the synthesis of the nanocrystals isshown in Scheme 1. The composition of the nanocrystals can

be tuned by changing the ratio of Fe to Co of the metal pre-cursors. The FexCo1-x/GC nanocrystals show superparamag-netic properties at room temperature. We have studied theMR signal-enhancing effects of the nanocrystals with differ-ent Fe/Co ratios. The single bcc-phased FexCo1-x/GC nano-crystals have great potential to be used as excellent T1 MRIcontrast agents.

Results and Discussion

The synthesis of FexCo1-x/GC nanocrystals is based on a pre-vious report in which FeCo/GC nanocrystals were preparedin fumed silica.[7] Instead of fumed silica, we used MCM-41with a size of approximately 60 nm as a support for the syn-thesis of the nanocrystals (Figure S1 in the Supporting Infor-mation). MCM-41 was synthesized according to a publishedprocedure using cetyltrimethylammonium bromide (CTAB)as a surfactant and tetraethylorthosilicate (TEOS) as a silicasource.[11] The molar ratio of TEOS/CTAB/H2O/NaOH inthe MCM-41 precursor solution was 1:0.016:579:0.0001.Metal precursors, Fe ACHTUNGTRENNUNG(NO3)3·9 H2O and CoACHTUNGTRENNUNG(NO3)2·6 H2O, ofvarious ratios were loaded into the MCM-41 by impregna-tion in a methanol solution of the salts followed by evapora-tion of the methanol. The metal-loaded MCM-41 washeated to 900 8C under H2 and then subjected to methaneCVD for carbon deposition over the FexCo1-x nanocrystalsformed inside MCM-41. Once cooled to room temperature,we etched the material in HF to dissolve the MCM-41 andobtained the resulting FexCo1-x/GC nanocrystals.

Transmission electron microscopy (TEM) imaging of theFexCo1-x/GC nanocrystals inside the MCM-41 clearly showsthat the nanocrystals are embedded in the MCM-41 matrix(Figure 1 a). When 0.90 mmol of metal precursors (Fe/Co=

64:36) was loaded in 1.0 g of MCM-41, we obtained nearlyspherical Fe0.50Co0.50/GC nanocrystals with an average diam-eter of 3.8�0.4 nm, as shown in the TEM images in Fig-ure 1 b and 1 c. The mean size and standard deviation of thenanocrystals were estimated from TEM images by measur-ing a population of approximately 500 nanocrystals. The Fe/Co ratio of the nanocrystals was determined from energydispersive X-ray (EDX) analysis for the emission of the

sample (Figure S2 in the Supporting Information); the Fe/Co ratio was consistent with the data obtained by inductive-ly coupled plasma (ICP) analysis, and the results are listedin Table S1 in the Supporting Information. We identified thebcc single-crystal structure of the nanocrystals with reflec-tions due to the (110), (200), (211), and (220) planes by se-lected area electron diffraction (SAED, Figure 1 d) andpowder X-ray diffraction (XRD, Figure 1 j). The high-resolu-tion TEM image in Figure 1 c clearly shows the lattice fring-es of the bcc-FeCo core (d spacing= 2.02 � for a (110) re-

Scheme 1. Schematic diagram for the preparation of FexCo1-x/GC nano-crystals.

Figure 1. Structural characterization of FexCo1-x/GC nanocrystals:a) TEM image of Fe0.50Co0.50/GC nanocrystals embedded in MCM-41.b) TEM and c) high-resolution TEM images, and d) SAED pattern of3.8�0.4 nm Fe0.50Co0.50/GC nanocrystals. TEM images of e) 3.7�0.4 nmFe0.13Co0.87/GC, f) 3.7�0.4 nm Fe0.36Co0.64/GC, g) 3.8�0.4 nm Fe0.42Co0.58/GC, h) 3.8�0.4 nm Fe0.56Co0.44/GC, and i) 3.7�0.4 nm Fe0.62Co0.38/GCnanocrystals (insets are SAED patterns: white Miller indices are assignedto bcc FeCo, while gray Miller indices are assigned to fcc Co). j) XRDpatterns of FexCo1-x/GC nanocrystals (the peaks marked with asterisksare assigned to fcc Co).

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flection) and a single-layer graphitic shell. The graphiticshell coating the core was also identified by FT-IR spectros-copy, and the spectrum showed the C=C stretch band at1650 cm�1 (Figure S3 in the Supporting Information). Thecrystallite size determined for the (110) reflection of theXRD data (Figure 1 j) by using the Debye–Scherrer equa-tion is 3.8 nm, which is consistent with the mean diameterdetermined from the TEM images, thus indicating thesingle-crystalline and spherical nature of individualFe0.50Co0.50/GC nanocrystals.

The composition of the FexCo1-x/GC nanocrystals was tun-able by changing the ratios of metal precursors in theMCM-41. When we used the same amount of metal precur-sors (0.90 mmol) with Fe/Co ratios of 24:76, 50:50, 57:43,70:30, and 76:24 in MCM-41 (1.0 g), FexCo1-x/GC nanocrys-tals of similar sizes with Fe/Co ratios of 13:87, 36:64, 42:58,56:44, and 62:38, respectively, were formed, as shown in theTEM images in Figure 1 e–i. Fe0.13Co0.87/GC nanocrystalswith a Fe/Co ratio similar to that reported previously weresynthesized for comparison purposes.[7] Fe components inthe nanocrystals are generally smaller than those in the pre-cursors due to the different decomposition characteristics ofthe precursors. However, in comparison with the case wherefumed silica was used as a support, the deviation in the Fe/Co ratios between the precursors and the nanocrystals wasgreatly diminished by using the MCM-41. Previously, about4 nm Fe0.12Co0.88/GC nanocrystals with mixed bcc FeCo andface-centered-cubic (fcc) Co phases were prepared in fumedsilica from a Fe/Co precursor with a ratio of 1:1, whereas inthe present study about 3.7 nm Fe0.36Co0.64/GC nanocrystalswere synthesized in the MCM-41 from Fe/Co precursor withthe same ratio. Moreover, single bcc-phased FexCo1-x/GCnanocrystals could be produced in the MCM-41. The SAED(insets in Figure 1 e–i) and XRD (Figure 1 j) data clearlyconfirm that the Fe0.56Co0.44/GC and Fe0.62Co0.38/GC nano-crystals along with the Fe0.50Co0.50/GC nanocrytals havea single bcc FeCo phase, while the Fe0.13Co0.87/GC,Fe0.36Co0.64/GC, and Fe0.42Co0.58 nanocrystals contain fcc Co(peaks marked by an asterisk in the XRD data). The synthe-sis of approximately 4 nm FexCo1-x/GC nanocrystals witha single bcc phase was not possible in fumed silica evenfrom a precursor with a Fe/Co ratio of 76:24 (Figure S4 inthe Supporting Information). MCM-41 is a two-dimensionalhexagonal mesoporous material with a BJH (Barret–Joyner–Halenda) desorption average pore diameter of2.7 nm (Figure S1 in the Supporting Information). The re-duced deviation in the Fe/Co ratios between the precursorsand the nanocrystals can be attributed to the confined pre-cursor decomposition conditions in MCM-41, which has a or-dered pore structure with small and isolated pores.[12]

The confined condition of the MCM-41 also induces thesynthesis of uniform FexCo1-x/GC nanocrystals. The standarddeviation of the size of nanocrystals decreased from 20 % to10 % by using the MCM-41 instead of fumed silica. It seemsplausible that the MCM-41 will restrict migration of thenanocrystals as they grow in size comparable to the poresize, and thus restrain the nanocrystal growth caused by sin-

tering, thereby ultimately producing ultra-small and uniformFexCo1-x/GC nanocrystals.[13] However, the size of nanocrys-tals exceeds the pore size; it is obvious that the pore wallsare not able to completely restrain the growth of nanocrys-tals and must be deformed as the nanocrystals grow largerthan the pore size.[13]

It is also noteworthy that the yield of nanocrystals insilica increased 5-fold by using the MCM-41 instead offumed silica. A 5-fold larger amount of precursors in MCM-41 (0.90 mmol of metal precursors/1.0 g of silica) could beapplied than that in fumed silica (0.18 mmol of metal pre-cursors/1.0 g of silica) for preparing about 4 nm FexCo1-x/GCnanocrystals. When 0.90 mmol of metal precursors in 1.0 gof fumed silica was used in the synthesis, approximately7 nm FeCo/GC nanocrystals were obtained.[7] The large in-crease in the yield is also correlated with the confined condi-tion of the MCM-41, in which the small size of the FexCo1-x/GC nanocrystals could be retained at the high precursorconcentration in silica. It is also very important to use small-sized MCM-41 to achieve a high yield. When a large-sizedMCM-41 (0.5–1 mm) was used in the synthesis while keepingall other reaction conditions unchanged, FeCo/GC nanocrys-tals with a wide size distribution (2–10 nm sizes) were syn-thesized (Figure S5 in the Supporting Information), whichcan be attributed to the thermal decomposition of metalprecursors that are not yet loaded inside the mesopores ofMCM-41 through capillary action.

Field-dependent magnetic measurements using a supercon-ducting quantum interference device–vibrating sample mag-netometer (SQUID–VSM) showed that all of the FexCo1-x/GC nanocrystals were superparamagnetic at room tempera-ture (Figure 2 and Table 1). The superparamagnetic proper-ties are supported by the magnetization loop, which rapidlyincreases with applied field and has no hysteresis. The mag-netic moments of the FexCo1-x/GC nanocrystals were moresensitive to the single crystallinity than the composition ofthe nanocrystals. The saturation magnetization (Ms) valuesof the Fe0.13Co0.87/GC, Fe0.36Co0.64/GC, and Fe0.42Co0.58/GCnanocrystals were 161.6, 179.4, and 186.6 emu g�1 metal, re-spectively. The lower Ms values than that of bulk FeCo

Figure 2. Field-dependent magnetization curves at 300 K for FexCo1-x/GCnanocrystals. The inset shows the same magnetization curves in the rangeof 125 to 200 emu g�1 metal.

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(235 emu g�1) were mainly due to the mixed bcc-FeCo andfcc-Co phases present in the nanocrystals. The single-phasedFe0.50Co0.50/GC, Fe0.56Co0.44/GC, and Fe0.62Co0.38/GC nanocrys-tals were found to have similar Ms values of 193.5–196.3 emu g�1 metal, which is in contrast to the reportedcomposition dependence of Ms in bulk FeCo alloy wherethe highest Ms was observed at a composition ofFe0.65Co0.35.

[14] The similar Ms values of the single-phasednanocrystals and the lower Ms values than that of bulk FeCocan be attributed to the small crystal volume and surfacespin-canting effects, which result from the absence of mag-netic ordering of the surface spins.[4b] The spin-canting ef-fects of magnetic nanocrystals are also known to have a sig-nificant influence on their MR signal-enhancing effects (seebelow).[15]

Next, MRI experiments were performed to investigate theproton relaxation time shortening effects of the FexCo1-x/GCnanocrystals. We prepared stable phosphate-buffered saline(PBS) solutions of the FexCo1-x/GC nanocrystals by non-co-valent functionalization with PL–PEG (molecular weight ofPEG =5000) molecules following similar procedures report-ed previously (Figure 3 a and 3 b).[7] The PL–PEG adsorbedon the FexCo1-x/GC nanocrystals was characterized by FT-IRspectroscopy (Figure S3 in the Supporting Information).Changes in longitudinal (T1) and transverse (T2) relaxationtime as a function of the metal concentration of FexCo1-x/GCnanocrystal solutions were calculated in a 4.7 T animal MRIscanner system. T1

�1 and T2�1 depend linearly on the con-

centration of the nanocrystals, and longitudinal (r1) andtransverse (r2) relaxivities can be obtained by determiningthe slopes of plots of concentration versus T1

�1 and T2�1, as

shown in Figure 3 c and 3 e. The r1 and r2 values of theFexCo1-x/GC nanocrystal samples are summarized in Table 1.As the FexCo1-x/GC nanocrystal samples change from a mix-ture of bcc FeCo and fcc Co to a single bcc FeCo, the r1 andr2 values are increased from 8.2 and 199.2 mm

�1 s�1 forFe0.13Co0.87/GC to 12.6 and 315.1 mm

�1 s�1 for Fe0.50Co0.50/GC,respectively. The r2/r1 ratios, however, are maintained withinthe experimental error. It is plausible that, if the crystalsizes are the same, FeCo has higher r1 and r2 values than Cobut their r2/r1 ratios remain very close to each other. Thethree FexCo1-x/GC nanocrystal samples with a single bccFeCo phase have similar r1 and r2 values, which coincideswith the similar Ms values of the samples. Figure 3 d and 3 fshow T1- and T2-weighted images for the FexCo1-x/GC nano-crystal solutions at 0.1 mm metal concentration. As expect-

ed, the single-phased Fe0.50Co0.50/GC, Fe0.56Co0.44/GC, andFe0.62Co0.38/GC nanocrystal samples gave a higher T1- andT2-weighted MR contrast than others. The higher r2 value ofFeCo as compared to that of Co can be explained with itshigher Ms value.[4b] Although the T1 contrast mechanism ofnanocrystal-based MRI agents has not yet been fully eluci-dated, the interatomic exchange interactions between the Feand Co metals on the surface of nanocrystals seem to be re-sponsible for the high relaxation enhancement of the pro-tons of water molecules near the FexCo1-x/GC nanocrystal-s.[14a] The single bcc-phased FexCo1-x/GC nanocrystal sampleshave a great potential to be used as highly effective T1-con-trast agents because they exhibit 50 % higher r1 values com-pared to that of the Fe0.13Co0.87/GC nanocrystal sample,which has already been proven to have an about 7 timeshigher r1 value and a much longer circulation time in in-vivoimaging than conventional T1-contrast agents such as Mag-nevist.[7] Note that due to their high r1 and r2 values, thesingle bcc-phased FexCo1-x/GC nanocrystals can be used ata lower concentration compared to other agents.

Table 1. Summary of saturation magnetization and relaxation propertiesof FexCo1-x/GC nanocrystals.

Sample MS at 300 K[emu g�1 metal]

r1ACHTUNGTRENNUNG[mM�1 s�1]r2ACHTUNGTRENNUNG[mM�1 s�1]

r2/r1

Fe0.13Co0.87/GC 161.6 8.2 199.2 24.3Fe0.36Co0.64/GC 179.4 10.8 265.7 24.6Fe0.42Co0.58/GC 186.6 12.1 299.0 24.7Fe0.50Co0.50/GC 193.5 12.6 315.2 25.0Fe0.56Co0.44/GC 196.3 12.6 318.9 25.3Fe0.62Co0.38/GC 195.0 12.5 317.2 25.4

Figure 3. a) Schematic diagram of a FeCo/GC nanocrystal functionalizedwith PL–PEG. b) Photograph of PBS solutions of functionalizedFe0.50Co0.50/GC nanocrystals. c,e) Plots of T1

�1 (c) and T2�1 (e) versus

metal concentration for FexCo1-x/GC nanocrystal solutions. d) T1- andf) T2-weighted MR images of the FexCo1-x/GC nanocrystal solutions atthree metal concentrations (T1-weighted spin-echo sequence: echo time(TE) of 7.8 ms and pulse repetition time (TR) of 1000 ms, T2-weightedspin-echo sequence: TE of 51.8 ms and TR of 10000 ms).

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Conclusions

In summary, we have prepared ultra-small, uniform, andsingle bcc-phased FexCo1-x/GC nanocrystals with average di-ameters of <4 nm by thermal decomposition of metal pre-cursors in MCM-41 with a size of approximately 60 nm andmethane CVD. The composition of the FexCo1-x/GC nano-crystals can be easily manipulated by simply changing theFe:Co ratios of the metal precursors. The single bcc-phasedFexCo1-x/GC nanocrystals functionalized with PL-PEG in so-lution are demonstrated to be excellent T1 MRI contrastagents. This result is, to the best of our knowledge, the firstdemonstration of MR enhancing properties of ultra-smalland uniform FexCo1-x/GC nanocrystals with a single bccFeCo phase.

Experimental Section

Materials

Cetyltrimethylammonium bromide (CTAB, Aldrich, 98+%), sodium hy-droxide (Aldrich), tetraethylorthosilicate (TEOS, Aldrich, reagentgrade), iron ACHTUNGTRENNUNG(III) nitrate nonahydrate (Fe ACHTUNGTRENNUNG(NO3)3·9 H2O, Aldrich, 99.99 %),cobalt(II) nitrate hexahydrate (Co ACHTUNGTRENNUNG(NO3)2·6H2O, Aldrich, 99.999 %), hy-drofluoric acid (J.T. Baker, 49%), hydrochloric acid (Kanto, 35%), 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethyleneglycol)-5000] (ammonium salt) (PL–PEG, Avanti Polar Lipids) were usedwithout further purification. All other reagents purchased from commer-cial sources were used as obtained without further purification.

Preparation of ~60 nm MCM-41

CTAB (0.115 g, 0.316 mmol) was dissolved in 200 mL of deionized water.Next, 0.5m NaOH solution (4.8 mL) was added to this solution, and themixture was stirred at 60 8C for 10 min, followed by addition of TEOS(4 mL, 19.2 mmol). After continuous stirring for 3 h at this temperature,the synthesized solid product was collected by centrifugation and repeat-edly washed with distilled water and ethanol, respectively. To completelyremove the surfactant incorporated into the silica, the synthesized solidproduct was heated for 3 h at 60 8C in ethanol (100 mL) containing 2 mLof conc. HCl. The resulting mesoporous silica spheres were collected bycentrifugation and repeatedly washed with distilled water and ethanol,respectively, and then dried in an oven at 80 8C to yield a white powder.

Preparation of FexCo1-x/GC Nanocrystals

For Fe0.50Co0.50/GC nanocrystals, we impregnated ~60 nm MCM-41(1.00 g) with Fe ACHTUNGTRENNUNG(NO3)3·9 H2O (0.233 g 0.576 mmol) and Co ACHTUNGTRENNUNG(NO3)2·6H2O(0.094 g, 0.324 mmol) (Fe/Co ratio of 64:36) in 50 mL of methanol andsonicated it for 1 h. For Fe0.13Co0.87/GC, Fe0.36Co0.64/GC, Fe0.42Co0.58/GC,Fe0.56Co0.44/GC, and Fe0.62Co0.38/GC nanocrystals, we used Fe/Co ratios of24:76 (0.087 g of FeACHTUNGTRENNUNG(NO3)3·9H2O (0.216 mmol) and 0.199 g of Co-ACHTUNGTRENNUNG(NO3)2·6 H2O (0.684 mmol)), 50:50 (0.182 g of FeACHTUNGTRENNUNG(NO3)3·9H2O(0.450 mmol) and 0.131 g of CoACHTUNGTRENNUNG(NO3)2·6 H2O (0.450 mmol)), 57:43(0.207 g of Fe ACHTUNGTRENNUNG(NO3)3·9 H2O (0.513 mmol) and 0.113 g of CoACHTUNGTRENNUNG(NO3)2·6H2O(0.387 mmol)), 70:30 (0.255 g of Fe ACHTUNGTRENNUNG(NO3)3·9H2O (0.630 mmol) and0.079 g of Co ACHTUNGTRENNUNG(NO3)2·6H2O (0.270 mmol)), and 76:24 (0.276 g of Fe-ACHTUNGTRENNUNG(NO3)3·9 H2O (0.684 mmol) and 0.063 g of Co ACHTUNGTRENNUNG(NO3)2·6H2O(0.216 mmol)), respectively. After removal of methanol and drying at80 8C, we ground the powder and typically used 0.20 g of it for subse-quent methane CVD in a tube furnace. We heated the sample in a H2

flow to reach 900 8C and then subjected it to a methane flow of400 cm3 min�1 for 5 min. After cooling, we etched the samples with 15%HF in H2O (75 %) and ethanol (10 %) to dissolve the MCM-41 whichwas used as a template. We collected the FexCo1-x/GC nanocrystals by

centrifugation and thoroughly washed them with distilled water and etha-nol, respectively.

Quantitative Analysis of Metal Contents in FexCo1-x/GC Nanocrystals

To calculate molar extinction coefficients, we dissolved known amountsof Fe ACHTUNGTRENNUNG(NO3)3·9 H2O and Co ACHTUNGTRENNUNG(NO3)2·6 H2O in 35 % HCl solutions. We usedthe resulting solutions to calibrate characteristic UV/Vis absorbancepeaks and molar extinction coefficients of Fe3+ at 362 nm and Co2+ at691 nm. We determined the stoichiometries and metal contents ofFexCo1-x/GC nanocrystals by calcinations of the graphitic shells at 500 8Cin air, dissolving the metal species in HCl solutions, and measuring theUV/Vis absorbance at the two wavelengths mentioned above.

Characterization

We characterized the FexCo1-x/GC nanocrystals by using a Rigaku Mini-flex II XRD diffractometer (4.5 kW; using CuKa radiation at 30 kV and15 mA), a JEOL JEM-2100F transmission electron microscope (operatedat 200 kV) with selected area electron diffraction (SAED) patterns, andenergy-dispersive X-ray emission analyses (EDX). Samples for TEM in-vestigations were prepared by dropping the diluted sample in ethanol ona 300 mesh carbon support copper grid (Ted Pella, Inc.). The Fe/Coratios of the FexCo1-x/GC nanocrystals were obtained by inductively cou-pled plasma–atomic emission spectroscopy (ICP-AES) on a SpectroCiros Vision system. IR spectra were taken with a Spectrum 2000 FT-IRspectrophotometer (Perkin–Elmer). Magnetic measurements were per-formed on a SQUID-vibrating sample magnetometer (Quantum DesignMPMS SQUID-VSM). DC susceptibility and hysteresis measurementswere recorded for powdered samples in a gelatin capsule. The hystereticloops were obtained in a magnetic field varying from +7 to �7 T. Theadsorption and desorption measurements were performed using a BEL-SORP-max instrument with nitrogen. The BET surface areas were calcu-lated from p/p0 =0.05–0.30 in the adsorption curve using a BET equation.The pore size distributions were calculated by using the BJH method.Prior to each sorption measurement, the sample was outgassed at 300 8Cfor 24 h under vacuum to completely remove all the impurities.

Functionalization of FexCo1-x/GC Nanocrystals with PL–PEG

We added FexCo1-x/GC nanocrystals to a PBS solution of PL–PEG(Avanti Polar Lipids, 1 mg mL�1) and sonicated them for 1 h. We usedcentrifugation at 14000 g for 15 min to remove any aggregates. We re-moved excess PL-PEG by filtration, and the resulting PL–PEG-function-alized FexCo1-x/GC nanocrystals were highly stable.

T1 and T2 Measurements

We used PBS solutions of various concentrations of FexCo1-x/GC nano-crystals for T1 and T2 measurements using a 4.7 T BioSpec 47/40 animalMRI scanner (Bruker, Germany) with a 72 mm volume coil. For T1 meas-urements, we used a saturation recovery RARE (rapid acquisition withrelaxation enhancement) sequence with a field of view (FOV) of 6 cm,a slice thickness of 2.0 mm, and an imaging matrix size of 128 by 128. TE

was 7.8 ms and TR was 50, 100, 300, 700, 1000, 2000, 4000, and 6000 ms.We made T2 measurements using a CPMG (Carr–Purcell–Meiboom–Gill) sequence with an FOV of 6 cm, a slice thickness of 2.0 mm, and animaging matrix size of 128 by 128. TR was 10000 ms and TE was 7.4–473.6 ms at 7.4 ms steps.

Acknowledgements

This research was supported by the Basic Science Research Programthrough the National Research Foundation of Korea (NRF) funded bythe Ministry of Education, Science and Technology (2011-0002773). C.L.thanks the support of the KBSI (Grant No. T32404). M.-H. J. thanks thesupport of the Basic Science Research Program through the NRF ofKorea funded by the Ministry of Education, Science and Technology(2012-0004082 and 2012R1A1A2039944).

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Received: October 12, 2012Published online: November 14, 2012

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