DOI: 10.1021/cg100124m
Fabrication of Ferrimagnetic Ferrite Nanocrystal Clusters by a Double-Step Templated Reaction Using in Situ Polymerization of Phenylalanine
2010, Vol. 10 2350–2354
Makoto Moriya, Masashi Ito, Wataru Sakamoto, and Toshinobu Yogo* Division of Nanomaterials Science, Ecotopia Science Institute, Nagoya University, Nagoya 464-8603, Japan Received January 26, 2010; Revised Manuscript Received March 2, 2010
ABSTRACT: We demonstrate a one-pot double-step templated reaction to fabricate uniform clusters composed of sizecontrolled ferrite nanocrystals (NCs) using phenylalanine as a template molecule and trimetallic complexes as starting materials. The size of the NC clusters is controlled to a diameter of approximately 70 nm, while the molar ratio of phenylalanine to the triiron complex is 72. In this reaction, phenylalanine behaves as a template to give size-controlled iron oxide NCs. Then, the phenylalanine molecules polymerize to afford poly(phenylalanine) by thermolysis of the trimetallic complex. Poly(phenylalanine) functions as another type of template to form the cluster of NCs. The obtained ferrite NC clusters exhibit ferrimagnetic behavior at room temperature.
Introduction The characteristics of nanocrystalline materials are greatly affected by their aggregation states, that is, their secondary structure, as well as their primary structure, which is represented by the size and morphology of the crystals. Therefore, the nanocrystalline materials with an orderly assembled structure often exhibit unique properties that have never been observed in nonassembled or disorderly aggregated nanocrystals (NCs).1-12 Controlling the aggregated structure of nanocrystals using various template ligands has attracted considerable attention in addition to controlling the primary structure of NCs. However, methods to produce uniform magnetic NC clusters are still limited despite the interesting characteristics of orderly assembled magnetic NCs.1,4,9-14 The fabrication of uniform clusters of ferrimagnetic ferrite NCs is especially difficult since ferrimagnetic NCs tend to aggregate randomly owing to their magnetic dipole-dipole moment. Because of their coercivity at room temperature, ferrimagnetic ferrite NCs have been widely used as magnetic resonance imaging contrast agents, magnetic carriers for drug delivery systems, and biosensors.15-20 Hence, the development of a novel and simple synthetic process is required for the formation of uniform clusters composed of superparamagnetic or ferrimagnetic ferrite NCs. In order to fabricate size-controlled magnetic NC clusters, we investigated the use of a new type of template molecules, which have the ability to be polymerized. These template molecules work in a monomeric form to give size-controlled NCs at first. Then, the template molecules are polymerized, yielding a large template ligand that controls the clusterization of the NCs. In this study, we attempted to use phenylalanine as a template ligand. Because of the large dipole-dipole moment derived from the hydrophilic property of the amino and carboxyl groups as well as the hydrophobic property of the phenyl group of phenylalanine, the molecules behave as a good template for yielding size-controlled NCs. Moreover, the amino and carboxyl groups undergo condensation polymerization affording an amido linkage between the amino and carboxyl *To whom correspondence should be addressed. E-mail: yogo@esi. nagoya-u.ac.jp. Fax þ81-52-789-2121. pubs.acs.org/crystal
Published on Web 03/24/2010
moieties as a result of dehydration under the thermal condition. The reaction leads to the formation of poly(phenylalanine). The resulting poly(phenylalanine) is expected to act as a good template for the uniform clusterization of magnetite NCs. In this report, we demonstrate the synthesis of uniform ferrimagnetic ferrite NC clusters using a one-pot double-step thermolysis reaction with phenylalanine as the template molecule. For the precursor of ferrite NCs, we adopt the mixed-valence triiron complex, [Fe3(μ-CH3COO)6(μ3-O)(H2O)3], or heterotrimetallic complex, [CoFe2(μ-CH3COO)6(μ3-O)(H2O)3].20-26 The compositions of these trimetallic complexes are quite similar to those of spinel ferrites, because they contain one divalent metal ion and two trivalent iron ions. Furthermore, these trimetallic complexes contain four oxygen atoms as the μ3O ligand and three aquo ligands. We also report the ferrimagnetic properties of the obtained ferrite NC clusters. Experimental Section L-Phenylalanine (98%), 3-phenylpropionic acid (98%), 2-phenylethylamine (98%), and dibenzyl ether (95%) were purchased from Tokyo Chemical Industry Co. CDCl3 and dimethyl sulfoxide-d6 (DMSO-d6) were purchased from Aldrich. These reagents were used as received. The mixed-valence triiron complex, {Fe3(μ3-O)(μCH3COO)6(H2O)3}, and heterotrimetallic complex, {CoFe2(μ3O)(μ-CH3COO)6(H2O)3}, were prepared according to a previously published report.23 The representative procedure for the synthesis was as follows: L-phenylalanine (4.0 mmol), trimetallic complex (0.33 mmol), and dibenzyl ether (50 mL) were charged in a round-bottom flask. Note that no reagent was deliberately added to change the oxidation state of the metal. The reaction flask was degassed, and the temperature was increased at a rate of 3 or 5 °C/min to 160 °C. Then, 1 atm of nitrogen was charged into the flask, and additional heating was performed to 300 °C at a rate of 3 or 5 °C/min. Although the trimetallic complex is insoluble in dibenzyl ether at room temperature, the mixture of triiron complex, L-phenylalanine, and dibenzyl ether turned into a homogeneous bright yellow solution during heating. The solution was refluxed for 15 min with vigorous magnetic stirring. During reflux, the color of the solution turned to black. After the reaction, the colorless waxlike solid assignable to poly(phenylalanine) containing benzyl ether and 2-phenylethylamine was precipitated. A total of 50 mL of methanol was added to the mixture to give a highly viscous precipitate, followed by centrifugation (3500 rpm) and decantation. Then, the washing process using 20 mL of acetone was repeated several times. Finally, the separated
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precipitates were dried under air at room temperature to yield a black crystalline powder. The washings were collected and condensed under reduced pressure. Then, an excess amount of hexane was added to the solution and cooled at -18 °C to precipitate poly(phenylalanine) as a white solid. The molar ratio of the trimetallic complex to L-phenylalanine (Phe) was varied between 1:24 and 1:72 to yield the magnetite NCs, Fe-Phe-n-m (n = molar ratio of phenylalanine to triiron complex, m = heating rate per minute), and cobalt ferrite NCs, Co-Phe-n-m. To analyze the effect of the two existing functional groups, the aminoand carboxyl-groups, on the formation of NC clusters, thermolysis was performed on the triiron complex in the presence of 72 equiv of 3phenylpropionic acid (PA) and 2-phenylethylamine (PAm) to give magnetite NCs, Fe-PA-72 and Fe-PAm-72, respectively. In these reactions, the heating rate was fixed at 5 °C/min. Fe-PAm-72 was obtained as a black powder in good yield. However, the yield of FePA-72 was too low to perform X-ray diffraction (XRD), thermogravimetry-differential thermal analysis (TG-DTA), and IR analyses. Ferrite NCs were analyzed by X-ray diffraction (Rigaku, RINT 2500 diffractometer) using Cu KR radiation with a monochromator. Transmission electron microscopy (TEM) images were taken on a Hitachi, H-800 electron microscope at 200 kV. Field-emission scanning electron microscopy (SEM) images were recorded with a JEOL, JSM-6330 electron micro scope at 10 kV. TG-DTA was performed to investigate the final weight percent of ferrite nanocrystals and thermal behavior of organic components with a Rigaku, Thermo plus EVO TG8120 at a heating rate of 10 °C/min in O2 using Al2O3 as a reference. IR spectra were measured with a Nicolet, Nexus 470 FTIR. The magnetic properties were evaluated using a vibrating sample magnetometer (VSM), Toei Industry Co., VSM-5-15 at room temperature. 1H and 13C NMR spectra were recorded on a JEOL A-400 referenced to the natural-abundance proton or carbon signal of CDCl3 (δH 7.26 and δC 77.7) and that of DMSO-d6 (δH 2.50 and δC 39.5) employed at Chemical Instrumentation Facility, Research Center for Material Science, Nagoya University. EI-MS and FABMS were performed using JEOL JMS-700 at the Chemical Instrumentation Facility, Research Center for Material Science, Nagoya University.
Results and Discussion The ferrite NC clusters were synthesized by the thermolysis of the triiron complex, {Fe3(μ3-O)(μ-CH3COO)6(H2O)3}, or the heterotrimetallic complex, {CoFe2(μ3-O)(μ-CH3COO)6(H2O)3}, with 72 equiv of L-phenylalanine to the trimetallic complex in dibenzyl ether at 300 °C. Although the trimetallic complexes were not dissolved into dibenzyl ether at room temperature, the reaction mixture turned into a homogeneous yellow solution with the increase in temperature. Before the reflux started, the color of solution changed to black. Then the solution was refluxed for 15 min with vigorous magnetic stirring to obtain the ferrite NC clusters as a black powder with poly(phenylalanine) as a result of the condensation reaction between the amino- and carboxyl-groups with dehydration. After the reaction, a colorless waxlike solid composed of benzyl ether and 2-phenylethylamine was precipitated on the inside surface of the reaction flask (Supporting Information). This result shows that a part of L-phenylalanine undergoes decarboxylation as well as polymerization during the heating process. X-ray diffraction (XRD) patterns of the obtained products are depicted in Figure 1. Fe-Phe-72-3, Fe-Phe-72-5, Co-Phe72-3, and Co-Phe-72-5 afforded several diffractions assignable to magnetite or cobalt ferrite. The reflections due to w€ ustite were also observed for the diffraction patterns of Fe-Phe-72-3 and Fe-Phe-72-5. When benzilic acid, which is a hydroxy carboxylic acid having two phenyl groups, was used as a template for the thermolysis of the triiron complex, the obtained product was single-phase magnetite without
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Figure 1. XRD patterns of the obtained products. (a) Fe-Phe-72-3. (b) Fe-Phe-72-5. (c) Fe-Phe-24. (d) Fe-PAm-72. (e) Co-Phe-72-3. (f) Co-Phe-72-5.
w€ ustite.20 Recently, Sun et al. reported that oleylamine behaves as a reductant as well as a template ligand in the fabrication of magnetite NCs by the thermolysis of iron acetylacetonate.27 Therefore, the amino group of phenylalanine or phenylethylamine, which is afforded as a result of thermolysis of phenylalanine during the heating reaction, should induce partial reduction of trivalent iron into divalent iron in the initial stage of this reaction. The crystallite sizes of Fe-Phe-72-3 and Fe-Phe-72-5 calculated from the (311) reflection of magnetite based upon the Scherrer equation were 14.5 and 7.8 nm, respectively. The sizes of Co-Phe-72-3 and CoPhe-72-5 were estimated to be 15.0 and 12.2 nm, respectively. On the other hand, Fe-Phe-24 and Fe-PAm-72 afforded sharp diffractions, which were in good agreement with those of bulk magnetite (JCPDS 190629). The crystallite sizes of the samples were estimated to be 23.1 and 14.7 nm, respectively. These results show that the crystal growth of magnetite NCs is suppressed with the increase of the molar ratio of phenylalanine to trimetallic complex in the presence of the polymerizable functional group. TEM images of the products Fe-Phe-72-3 and Co-Phe-72-3 are shown in Figures 2 and 3, respectively. Figures 2 and 3 show the formation of uniform clusters of size-controlled magnetite NCs and fibrous products. The morphology of the fibrous product in the SEM image of Fe-Phe-72-3 was similar to that of poly(phenylalanine).28 The sizes of the clusters of Fe-Phe-72-3 and Fe-Phe-72-5 were analyzed to be ∼75.8 (size distribution; 17.2%) and 66.8 nm (size distribution; 16.4%), respectively. The cluster of FePhe-72-5 was composed of magnetite NCs of diameter ∼ 14.6 nm (size distribution; 19.2%). This value was slightly large compared to the crystallite size calculated from the Scherrer equation using XRD. On the other hand, the images of Fe-Phe-24, Fe-Phe-36, and Fe-Phe-60 depicted the formation of slightly large magnetite NCs (Figure 4a-c). In the previously reported “limited ligand protection (LLP)” method, the use of small excess amounts of surfactant toward metal precursor led to the formation of metal oxide NC clusters, and the addition of excess amounts of surfactant inhibited the formation of the NC clusters to yield size-controlled metal oxide NCs.2 In contrast, the use of excess amounts of phenylalanine to triiron complex is required to fabricate uniform magnetite NC clusters in this process. It is clear that the formation mechanism of NC clusters in this synthetic method is completely different from
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Figure 4. TEM images of the Fe3O4 NCs. (a) Fe-Phe-24, (b) FePhe-36, (c) Fe-Phe-60, (d) Fe-PAm-72.
Figure 2. TEM image of the Fe-Phe-72-3 (upper; inset is SEM image of Fe-Phe-72-3) and magnified TEM image of Fe-Phe-72-3 (bottom).
Figure 5. TG-DTA curves of Fe-Phe-72-3.
Figure 3. TEM images of the CoFe2O4 NCs. (a) Co-Phe-72-3. (b) Magnified image of Co-Phe-72-3.
that of the LLP process. When 2-phenylethylamine or 3-phenylpropionic acid was used as a template ligand, the NC clusters were not formed, but slightly large magnetite NCs were obtained (Figure 4d). Both 2-phenylethylamine and 3-phenypropionic acid are monofunctionalized template molecules, because each template possesses either amino or carboxyl groups. Therefore, these template molecules, 2-phenylethylamine and 3-phenypropionic acid, undergo no polymerization. These results show that the polymerization of template molecules plays an important role in the clusterization of NCs in this reaction. To reveal the details of the precipitated white powder containing poly(phenylalanine), Fe-Phe-72-3, the precipitated white powder, and phenylalanine were analyzed by TG-DTA (Figure 5) and FT-IR (Figure 6). Furthermore, 1H and 13C NMR analysis using DMSO-d6 as a solvent were also performed (Figure 7). In the TG curve of Fe-Phe-72-3 and the white precipitate, two exothermic peaks were observed at 220 and 350 °C together with a gradual weight loss from
Figure 6. FT-IR spectra. (a) Fe-Phe-72-3. (b) Obtained white precipitate. (c) Phenylalanine.
150 to 380 °C. On the other hand, the TG-DTA curves of phenylalanine gave only one endothermic peak with a drastic weight loss at 220 °C (Supporting Information, Figure S2). IR spectra of phenylalanine, the precipitated powder, and Fe-Phe-72-3 support that the polymerization of phenylalanine proceeds during the thermolysis of the triiron complex to magnetite NCs. The absorptions based on the carboxyl and
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Scheme 2. Plausible Reaction Mechanism for the Double-Step Template Reaction to Form Ferrite NCs Clusters
Figure 7. Magnified images of 1H NMR spectra for precipitated powder (top) and phenylalanine (bottom) in DMSO-d6.
Scheme 1. Plausible Reaction Scheme for the Formation of Poly(phenylalanine)
Table 1. Magnetic Properties of the Obtained Ferrite NCs at Ambient Temperaturea products
Ms (emu/g)
Hc (Oe)
Fe-Phe-72-3 Fe-Phe-24 Co-Phe-72-3 Co-Phe-24
24.8 84.8 31.1 70.1
111.8 64.3 839.1 1042.0
a
amino groups of phenylalanine at 1490, 1550, and 3021 cm-1 disappeared in the spectra of the precipitated powder and FePhe-72-3. In contrast, the peak attributable to the amido group was observed at 1640 cm-1 for both spectra of the obtained precipitate and Fe-Phe-72-3. In this reaction, poly(phenylalanine) would be yielded via the formation of 3,6-dibenzyl-2,5-piperadione, which is a phenylalanine anhydride (Scheme 1). The mass spectrum of the precipitated white powder containing poly(phenylalanine) gave the peak at m/z 294, which corresponds to phenylalanine anhydride, {(C6H5)CH2CHCONH}2. The 13C NMR spectrum of the precipitated white powder containing poly(phenylalanine) gave the signals attributable to the carbonyl group, ipso position of the phenyl group, and methine carbon at 166.2, 138.4, and 55.4 ppm, respectively. The peak of benzyl carbon was hidden by the residual signal of DMSO-d6. These chemical shifts are in good accordance with those of phenylalanine anhydride synthesized via the biosynthetic process.29 In the 1H NMR spectrum of the obtained poly(phenylalanine), the signals attributable to benzyl proton were observed at 2.56 (dd, JHH = 13.5 and 6.3 Hz) and 2.22 (dd, JHH = 13.5 and 4.9 Hz) ppm inequivalently with a peak assignable to methine proton at 3.96 ppm. The signals of benzyl proton attached to phenylalanine in DMSO-d6 were
Ms: saturation magnetization, Hc: coercivity.
confirmed at 3.14 (dd, JHH = 14.4 and 4.5 Hz) and 2.81 (dd, JHH = 14.4 and 8.4 Hz) ppm, which appeared at the lower magnetic field compared to those of obtained poly(phenylalanine) with similar JHH values. In this spectrum, the signal of methine proton was superimposed by the peak attributable to water contained in DMSO-d6. These results indicate that the heating reaction induces the formation of ferrite NCs as well as the dimerization of phenylalanine to give cyclic lactam, followed by the ring-opening polymerization to afford poly(phenylalanine). The plausible reaction mechanism for the formation of magnetite NC clusters is assumed as follows (Scheme 2). The thermolysis of trimetallic complex templated with phenylalanine will yield size-controlled ferrite NCs at first. During the reaction, the dehydration between carboxyl and amino groups of phenylalanine will proceed to give poly(phenylalanine). Then, the formed poly(phenylalanine) would work as a template for the clusterization of spinel ferrite NCs to give NC clusters. To form uniform clusters of magnetite NCs, excess amounts of phenylalanine are required to obtain poly(phenylalanine) with a certain extent of chain length. Magnetic properties of the products at room temperature are summarized in Table 1. The magnetite NCs cluster, FePhe-72-3, possesses a coercivity of 111.8 Oe, which shows the NC clusters to be ferrimagnetic at room temperature. The coercivity is larger than that of Fe-Phe-24, although the saturation magnetization of Fe-Phe-72-3 is small compared to that of Fe-Phe-24 and bulk magnetite (92 emu/g).30 Magnetite NCs of size ∼30 nm shows generally superparamagnetic behavior at room temperature. However, Fe-Phe-72-3 has a certain amount of coercivity despite its size being ∼12 nm in diameter. Probably, the partial fusion of the magnetite NCs in
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the cluster structure induces the shape anisotropy to afford the slightly large coercivity for Fe-Phe-72-3. The CoFe2O4 NC clusters, Co-Phe-72-3 and Co-Phe-24, showed quite large coercivities compared to those of Fe3O4 NCs due to a high magnetic crystalline anisotropy of CoFe2O4 derived from cobalt ions. Conclusion We demonstrate the fabrication of uniform ferrimagnetic clusters composed of size-controlled magnetite and cobalt ferrite NCs through the double-step template reaction using polymerizable molecules as a template ligand and trimetallic complexes as a precursor. In this report, we confirmed that the polymerization of phenylalanine plays a key role in the clusterization of spinel ferrite NCs. The obtained ferrite NC clusters show ferrimagnetic behavior at room temperature, although the crystallite sizes of NCs were smaller than 30 nm. These results show that the use of polymerizable template molecules enables the precise control of the secondary as well as primary structure of nanocrystalline material. Acknowledgment. This work was partially supported by the foundation “Hattori-Hokokai”. We sincerely thank Kin-ichi Oyama for valuable support with the measurements of EI-MS and FAB-MS. Supporting Information Available: TEM images and EDS pattern of Fe-Phe-72-5 and Co-Phe-72-5 (Figure S1). TG-DTA curves of phenylalanine and the poly(phenylalanine) (Figure S2). 1H and 13C NMR spectra of poly(phenylalanine) containing benzyl ether as a reaction solvent (Figure S3). 1H and 13C NMR spectra of the colorless waxlike solid composed of benzyl ether and 2-phenylethylamine precipitated on the inside surface of the reaction flask (Figure S4). 13C NMR spectra of the colorless waxlike solid precipitated the inside surface of the reaction flask (Figure S5). EI-MS spectrum of phenylalanine anhydride (Figure S6). Magnetic properties of the obtained products (Table S1). Magnetization curves of the obtained products measured with VSM (Figures S7-S13). This material is available free of charge via the Internet at http://pubs.acs.org.
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