Shell Microspheres with Well

18804

J. Phys. Chem. C 2008, 112, 18804–18809

Synthesis of Fe3O4@Polyaniline Core/Shell Microspheres with Well-Defined Blackberry-Like Morphology Shouhu Xuan,†,‡ Yi-Xiang J. Wang,§ Ken Cham-Fai Leung,*,‡ and Kangying Shu*,† Department of Materials Science and Engineering, China Jiliang UniVersity, Hangzhou, 310018, People’s Republic of China;, The Center of NoVel Functional Molecules and Department of Chemistry, The Chinese UniVersity of Hong Kong, Shatin, NT, Hong Kong SAR;, Department of Radiology, Prince of Wales Hospital, The Chinese UniVersity of Hong Kong, Shatin, NT, Hong Kong SAR ReceiVed: August 9, 2008; ReVised Manuscript ReceiVed: October 3, 2008

Superparamagnetic Fe3O4@polyaniline core/shell microspheres with well-defined blackberry-like morphology have been synthesized via a simple in situ surface polymerization method. The thickness of the polyaniline (PANI) shell can be selectively obtained by tuning the reaction time and monomer concentration. The poly(vinylpyrroldine) (PVP) plays an important role in the coating process. The present method can be extendable to fabricate other magnetic/conductive core/shell composites, and these unique core/shell spherical materials could find applications in catalyst supports or biomedical areas. 1. Introduction particles,1

Core/shell structured especially magnetic nanocomposite materials,2 have attracted increasing attention because they offer the possibility of a new generation of nanostructured materials with diverse applications for their unique magnetic responsivity, low cytotoxicity, and chemically liable surface. Among these magnetic core/shell particles, superparamagnetic materials do not retain any magnetization in the absence of an externally applied magnetic field. Due to this property,3 superparamagnetic core/shell particles are of great interest for applications in magnetic resonance imaging, hyperthermia, separation and purification of biomolecules, drug delivery, and catalysis.4 In such applications, functional materials are often used as a protective coating shell to ensure the stability of the inner magnetic core and as the intrinsic functions of the core/shell particles being electron-conductive, biocompatible, inert, hydrophilic, or hydrophobic, etc.5 Polyaniline (PANI) is one of the most technologically important materials because of its environmental stability in a conducting form, unique redox properties, and high conductivity with suitable dopants.6 PANI composite materials possess the potential for a multitude of applications, such as in gas sensors and inductors.7 Therefore, conductive organic/inorganic nanocomposites have recently been studied intensively.8 Concerning the above-stated fields of research, superparamagnetic and conducting polymer hybrid materials in which inorganic magnetic cores are augmented with insoluble outer layers of conductive PANI belong to an important class of materials.9 Recently, bifunctional PANI/Fe3O4 nanocomposites have attracted intensive attention for applications of nanomaterials due to their novel magnetic and conductive properties. Wan et al.10 studied a series of PANI composites containing nanomagnets prepared by chemical polymerization. Deng et al.11 reported * Corresponding author. Tel: +86 571 86835740. Fax: +86 571 86835740.E-mail:(K.S.)[email protected],(K.C.-F.L.)[email protected]. † China Jiliang University. ‡ The Center of Novel Functional Molecules and Department of Chemistry, The Chinese University of Hong Kong. § Prince of Wales Hospital, The Chinese University of Hong Kong.

the preparation of PANI/Fe3O4 nanoparticles with core/shell structure via an in situ polymerization of aniline monomer in an aqueous solution, which contained Fe3O4 nanoparticles and surfactants. Peng’s group12 described the fabrication of nanoscale ferromagnetic Fe3O4-cross-linked PANI by an oxidative polymerization of aniline with ammonium peroxodisulfate as the oxidant. The resulting core/shell particles are polydispersed, having an average diameter of 20-30 nm. In these studies, Fe3O4 with a size of less than 10 nm that exhibit low magnetization13 were used because their superparamagnetic properties eliminated magnetic force-induced self-aggregation of the particles. However, such methods result in fluctuations in the size for which the as-synthesized particles had a nonuniform magnetite fraction in each nanosphere due to nanoscale clustering of magnetic particles. Furthermore, many of these materials show relatively a small amount of magnetic contents, which usually resulted in the reduction of their response to magnetic fields. It is therefore of interest to achieve high loads of superparamagnetic materials in each PANI/Fe3O4 particle, keeping in mind that each particle should have a welldefined, core/shell-like structure. To date, effective coating of magnetic particles with natural or synthetic polymers is still a challenge, since the surfaces of magnetic particles are hydrophilic but polymers are hydrophobic. Therefore, development of a versatile method to directly coat PANI onto Fe3O4 particles is still challenging. Herein, we report a novel synthesis of Fe3O4@PANI core shell microspheres via a simple in situ surface polymerization method. Through this surface-modified procedure, high-content superparamagnetic Fe3O4 containing Fe3O4@PANI microparticles were obtained with well-defined blackberry-like morphology. Although we have demonstrated this procedure only with a Fe3O4 core and PANI shell as examples, it is believed that this method should be extendible to other magnetic core materials (Fe, γ-Fe2O3, Co, Ni, and ferrite) and to a range of other conductive shell materials (such as polypyrrole, polythiophene, etc). 2. Experimental Section Materials. Ferric chloride hexahydrate (FeCl3 · 6H2O), sodium acetate (CH3COONa), ethylene glycol (C2H6O2), 3-aminopro-

10.1021/jp807124z CCC: $40.75  2008 American Chemical Society Published on Web 11/08/2008

Synthesis of Fe3O4@PANI Core/Shell Microspheres pyltriethoxysilane (APTES), absolute ethanol (95 wt %), ammonium peroxodisulfate ((NH4)2S2O8; APS), and poly(vinylpyrrolidone) (PVP; 30 kDa, 95 wt %) were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were of analytical grade and used without further purification. Aniline was obtained from Beijing Xingjin Chemical Factory and was distilled at a reduced pressure before use. Doubly deionized water was used through all the processes. Synthesis and Chemical Modification of Fe3O4 Particles. The magnetic Fe3O4 particles were prepared through a solvothermal reaction. Briefly, FeCl3 · 6H2O (1.35 g) and sodium acetate (3.6 g) were dissolved in ethylene glycol (40 mL) under magnetic stirring. The obtained homogeneous yellow solution was transferred to a Teflon-lined stainless-steel autoclave and sealed to heat at 200 °C. After reaction for 8 h, the autoclave was cooled to room temperature. The obtained black magnetite particles were washed with ethanol six times and then dried in vacuum at 60 °C for 12 h. Subsequently, the Fe3O4 particles were chemically modified by using APTES. Typically, Fe3O4 microspheres (0.1 g) and APTES (2 mL) were dissolved in anhydrous ethanol to give a mixture solution (50 mL). The mixture was refluxed for 12 h under dry nitrogen. The resulting modified Fe3O4 particles were separated with the help of a magnet and then washed with ethanol. Finally, the product was dried in vacuum at 60 °C for 24 h to obtain the aminefunctionalized Fe3O4 particles (NH2-Fe3O4). Synthesis of Fe3O4@PANI Microparticles. The bifunctional Fe3O4@PANI microspheres with blackberry-like morphology were prepared by an in situ surface polymerization method in the presence of PVP. In a typical procedure, PVP (0.05 g) was dissolved in 130 mL of deionized water, and then the NH2-Fe3O4 particles (0.015 g) were added. The mixture was then ultrasonically dispersed, and a solution of aniline (20 µL) in HCl (0.1 mL) was added into the mixture with vigorous stirring. Afterward, the mixture was mechanically stirred for 30 min at 20 °C. Then an aqueous solution (20 mL) of APS (0.2 g) was added into the above mixture instantly to start the oxidative polymerization. The reaction was performed under mechanical stirring for 5 h. The resulting precipitates were washed with deionized water and ethanol several times. Finally, the product was dried in vacuum at 60 °C for 24 h to obtain of the desired Fe3O4@PANI composite as a dark powder. Characterization. X-ray powder diffraction patterns (XRD) of the products were obtained on a Japan Rigaku DMax-γA rotation anode X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (λ ) 1.54178 Å). Transmission electron microscopy (TEM) photographs were taken on a Hitachi model H-800 transmission electron microscope at an accelerating voltage of 200 kV. The field emission scanning electron microscopy (FE-SEM) images were taken on a JEOL JSM-6700F SEM. X-ray photoelectron spectra (XPS) were measured on a photoelectron spectrometer using Mg KR radiation. Infrared (IR) spectra were recorded in the wavenumbers ranging from 4000 to 500 cm-1 with a Nicolet model 759 Fourier transform infrared (FT-IR) spectrometer using a KBr wafer. Thermogravimetric analyses were conducted with a Netzsch STA 409C thermoanalyzer instrument. Their magnetic properties (M-H curve) were measured at room temperature on a MPMS XL magnetometer made by Quantum Design Corporation. 3. Results and Discussion Precisely size-regulated magnetic particles are essential to control the Fe3O4 content in each core/shell sphere. Here, we

J. Phys. Chem. C, Vol. 112, No. 48, 2008 18805

Figure 1. (a-d) TEM images of the as-prepared Fe3O4@PANI core/ shell composite with increasing magnification. Note the scale bar.

employed a previously described solvothermal method to prepare well-dispersed Fe3O4 microspheres.14 Since each of them is composed of many small primary nanocrystals, these Fe3O4 spheres retain superparamagnetic behavior at room temperature while possessing higher saturation magnetization. Additionally, these magnetic cores can be synthesized with tunable sizes ranging from 200 to 1500 nm. Therefore, these Fe3O4 microspheres are the most suitable template to fabricate superparamagnetic core/shell particles. Many strategies have also been developed to construct various inorganic-inorganic core/shell or sandwichlike structures by coating these cores with SiO2,15 porous SiO2,16 and TiO2.17 In this work, in contrast, we prepared exquisite inorganic-organic bifunctional Fe3O4@PANI core/ shell structures with the approximate size of Fe3O4 microspheres, about 350 nm. The Fe3O4 microspheres were coated with uniform shells of PANI via an in situ surface polymerization method at room temperature. Figure 1 shows the representative TEM images of Fe3O4@PANI composites. A continuous layer, which exhibits a fine increment in brightness in comparison to the dark inner core, is clearly observed on the outer shell of the Fe3O4 microsphere cores, as shown in Figure 1a. Figure 1b shows the TEM image of a single Fe3O4@PANI particle with a typical core/shell structure. In comparison with Fe3O4@PANI (Figure 1b) and the pristine Fe3O4 microsphere (Figure 7a), there are two major findings that can be easily observed: (1) there is no obvious change in the average particle size for the Fe3O4 particles before and after being coated with PANI, and (2) the resultant Fe3O4@PANI composites also possess a well-dispersed nature and spherical shape. It reveals that the Fe3O4 particles act as a template for the deposition of PANI during the reaction. Clearly, these Fe3O4@PANI composites contain well-defined core/shell-like structure, and the thickness of the PANI shell is about 35 nm. Moreover, the TEM images with higher resolutions (Figure 1c, d) reveal that the PANI coating is relatively rough, composed of nanoparticles with sizes of 30-60 nm, which are deposited on the surface of the Fe3O4 core. To further characterize the size and shape of the presented Fe3O4@PANI composites, FE-SEM inspection was conducted. Figure 2a and b depict typical SEM images of Fe3O4@PANI microspheres, indicating that the core/shell products are well-

18806 J. Phys. Chem. C, Vol. 112, No. 48, 2008

Xuan et al.

Figure 4. FTIR spectroscopy of (a) Fe3O4, (b) Fe3O4@PANI, and (c) PANI. Figure 2. (a-d) SEM image of the as-prepared Fe3O4@PANI core/ shell composite with increasing magnification.

Figure 5. TG curve of the as-prepared Fe3O4@PANI core/shell composite. Figure 3. XRD patterns of (a) Fe3O4 and (b) Fe3O4@PANI core/shell composite.

dispersed with near-spherical morphology, which is similar to the results obtained from TEM. From the SEM images with higher resolutions (Figure 2c and d), it can be clearly observed that many tiny nanoparticles are located on the microparticles’ surfaces. The size of these tiny nanoparticles ranges from 30 to 60 nm, which agrees with the TEM results. As a consequence, the TEM and SEM observations strongly demonstrate that our obtained Fe3O4@PANI composites possess well-defined core/ shell structures with uniform blackberry-like shape. Figure 3 shows the XRD patterns of the Fe3O4 microsphere and Fe3O4@PANI composites. All detected diffraction peaks in Figure 3a could be indexed as face centered cubic (fcc) Fe3O4 (JCPDS card No. 19-629). No other characteristic peaks due to the impurities of hematite or hydroxides were detected. The broadened nature indicates that the Fe3O4 microspheres obtained by the present solvothermal route consist of nanoparticles with a diameter of 15 nm, which is well in accordance with previous reports.16 For the XRD spectrum of the Fe3O4@PANI composite (Figure 3b), the main peaks are similar to the pristine Fe3O4 particles (Figure 3a), which reveals that the crystal structure of Fe3O4 is well-maintained after the coating process under acidic conditions. Because of the relatively thin layer and amorphous crystallinity of the PANI prepared under this polymerization method, no obvious diffraction peak for the PANI is observed. Moreover, the successful polymerization of aniline onto the Fe3O4 core was confirmed by Fourier transform infrared spectroscopy (FTIR), as shown in Figure 4b. It reveals that

Figure 6. XPS spectrum of the as-prepared Fe3O4@PANI core/shell composite.

Fe3O4@PANI composite microspheres have characteristic peaks at around 1593 (CdC stretching deformation of quinoid and benzenoid ring, respectively), 1315 (C-N stretching of secondary aromatic amine), 1165, and 837 cm-1 (out of plane deformation of C-H in the 1,4-disubstituted benzene ring),18 which are similar to that of the PANI sample without Fe3O4 microcores under the same conditions (Figure 4c). The relatively high intensity of a band at 594 cm-1 (Fe-O stretching of Fe3O4)19 indicates the low content of PANI in the Fe3O4@PANI composite, which was further confirmed by the following TG analysis. Figure 5 illustrates the results of the thermogravimetric analysis of the Fe3O4@PANI composite, for which the thermal degradation of the PANI occurs at 450 °C. The initial mass

Synthesis of Fe3O4@PANI Core/Shell Microspheres

J. Phys. Chem. C, Vol. 112, No. 48, 2008 18807

Figure 7. TEM images of the Fe3O4@PANI composite prepared under different conditions. (a-d) Varying monomer concentrations: (a) 0.55, (b) 1.1, (c) 1.65, and (d) 2.2 mM. (e-g) Varying reaction times: (e) 3, (f) 5, and (g) 7 h. (h) No PVP added.

loss at lower temperatures is mainly due to the release of water and dopant anions from the PANI. A sharp loss in mass is observed at 300 °C and continues to 650 °C, possibly due to a large scale thermal degradation of the PANI chains. From the TG analysis, the mass ratio of the Fe3O4 in the magnetic core/ shell composite is about 66%. XPS has often been used for the surface characterization of various materials, and unambiguous results are readily obtained when each of the various surface components contains unique elemental markers. Here, to further analyze the PANI shell in the core/shell product, XPS (Figure 6) was employed to understand the composition of the particle surface. It is obvious that the main content of the surface is C, O, and N elements. The binding energy at 710.20 eV for Fe2p3 cannot be detected, which further supports that all Fe3O4 cores in the composite are confined within a shell of PANI, in agreement with the TEM and SEM analyses. The thickness of the PANI shells strongly depends on a number of parameters, including the reaction/incubation time, the initial concentrations of aniline, and the reaction temperature. We found that it was most convenient and reproducible to control the thickness of PANI coatings by adjusting the concentration of the aniline precursor. Figures 7a-d show the TEM images of Fe3O4@PANI composites synthesized from different aniline concentrations. All the products were obtained starting from Fe3O4 beads of 350 nm by allowing the growth of PANI to proceed for the same period of time (5 h). By simply altering the concentration of aniline from 1.1 to 2.2 mM, the thickness of the deposited PANI could be varied in the range from 10 to 50 nm. The concentration of aniline had to be controlled in the range of 1.1-2.2 mM for the polymerization in the present work. When the concentration of aniline was reduced to 0.55 mM, the PANI derived from the precursor appeared to be insufficient to form a complete shell on the surface of each Fe3O4 sphere (Figure 7a). As the concentration of aniline was increased to 1.1 mM, the surface of each Fe3O4 sphere was coated with a complete shell of PANI with a thickness of ∼10 nm (Figure 7b). The PANI shells’ thickness continued to increase to 35 nm when the concentration of aniline was increased to 1.65 mM (Figure 7c). As the concentration was increased beyond 2.2 mM, the surface of each Fe3O4 sphere was coated with a uniform shell of PANI with about 50 nm (Figure 7d). However, a significant number of PANI gel spheres attached on the magnetic spheres were also observed in the final

Figure 8. Schematic illustration of the synthesis process of Fe3O4@PANI core/shell composite.

product as a result of unsuppressed homogeneous nucleation. These observations imply that the concentration of the aniline precursor has to be optimized to obtain reproducible, uniform core/shell particles by avoiding incomplete coating of the Fe3O4 spheres. Furthermore, the thickness of the PANI could be tuned by controlling the reaction time. Here, when the reaction time was 3 h, with a constant polymerization temperature, amount of Fe3O4 spheres, and aniline precursor concentration, the thickness of PANI shells was ∼7 nm (Figure 7e). If the reaction time was prolonged to 5 h, the thickness of the PANI shell could increase up to 35 nm (Figure 7f). As shown in Figure 7g, the PANI shell thickness increased to 45 nm when the reaction time was prolonged to 7 h. However, under this condition, some individual bulk PANI particles existed in the Fe3O4@PANI composites such that the PANI particles were barely linked to each other. Obviously, the longer reaction time can produce more PANI materials in the product which finally induced PANI floccules. Therefore, in our experiment, the optimum reaction time for the fabrication of well-defined core/shell structure is 5 h. The synthetic procedure for the Fe3O4@PANI core/shell composite is schematically illustrated in Figure 8. In our system, Fe3O4 spheres were prepared according to the method given in previous reports.14 To overcome the intrinsic hydrophilic character of the Fe3O4 particles and to favor the growth of PANI nodules on their surface, a coupling agent is needed. Therefore, the periphery of prepared Fe3O4 particles was modified with amine groups to improve their affinity to PANI. The aminemodified Fe3O4 particles were redispersed in aqueous solution.

18808 J. Phys. Chem. C, Vol. 112, No. 48, 2008

Figure 9. Room temperature magnetization curve of the as-prepared Fe3O4@PANI core/shell composite.

Before the polymerization reaction, PVP was added to the solution. Being an amphiphilic and nonionic surfactant, PVP usually formed a polymer coil in aqueous solution under a dilute concentration.20 Due to the presence of pyrrolidone groups, these PVP macromolecules could be attached on the peripheral amine functions of the Fe3O4 particles by hydrogen bonds. These PVP macromolecules that attached onto the surface of the Fe3O4 are very important for the formation of the resulting core/shell structure. Figure 7h shows a typical image of the product prepared from the unmodified Fe3O4. It is obvious that PANI nanoparticles could not be coated effectively on the Fe3O4 surface. Moreover, without the presence of PVP, well-defined core/shell particles could not be obtained. Afterward, the aniline monomers were added and converted to cationic anilinium ions under acidic conditions. Once PANI nucleation was generated, the grains might be stabilized by the PVP molecules that attached onto the Fe3O4 surface.21 The polymerization usually took place preferentially and continuously in proximity to existing PANI. Hence, the polymerization was successfully initiated, propagated, and terminated on the surface of the Fe3O4 core rather than in solution. Eventually, a homogeneous, continuous, and uniform PANI shell was formed on the surface of the Fe3O4 core. Accounting for the stabilization of PANI grains by the PVP macromolecules, the peripheral coating consisted of many PANI nanoparticles. Hence, Fe3O4@PANI core/shell particles with a blackberry-like morphology were subsequently obtained by this in situ polymerization method. On the basis of all the previous results, the mechanism for the shell formation on the amine-modified Fe3O4 particles can be proposed. The amine modification endows the affinity between the PVP and the template surface. PVP plays two roles here: First is to serve as a linker between the PANI nanoparticles and the Fe3O4 surface. Second is to serve as a stabilizer to stabilize the amine-modified Fe3O4 and the formed Fe3O4@PANI core/shell particles. In this solution, the PANI grains stabilized by PVP can be adsorbed onto the particle surface (Figure 7e), which will rapidly form thin shells on the particle surfaces upon initiation. As the polymerization proceeds, large PANI nanoparticles were obtained and grew on the cores’ surface to form the blackberry-like structure (Figure 7f). At last, the final compact shells can be formed (Figure 7g). Nevertheless, the reason for this morphology evolution is still not completely clear, and the detailed growth mechanism will be investigated further. From the above analysis, it is obvious that Fe3O4@PANI core/ shell structures with blackberry-like morphology can be successfully constructed. The magnetic properties, which are

Xuan et al. inherited from the magnetic Fe3O4 core particles, were measured at 300 K using a superconducting quantum interference device magnetometer. As shown in the figure of magnetization (M) versus magnetic field (H) (Figure 9), the saturated magnetization value of Fe3O4@PANI composite spheres is 53.86 emu/g at 300 K. No remanence was detected for the as-prepared product. The zero coercivity and the reversible hysteresis behavior indicate the superparamagnetic nature of the particles.19 Taking into account that the Fe3O4@PANI sample contains 66% Fe3O4, this gives a value of 81.7 emu/g which is similar to the previous reports.14 The magnetic separability of our presented core/shellstructured Fe3O4@PANI particles was tested in ethanol by placing an external magnetic field near the bottom of the glass bottle. A color change from black to transparent was observed, and the black particles were attracted toward the magnet within 45 s, which directly demonstrated that the obtained nanocomposites possess magnetic properties. This feature will provide an easy and efficient avenue for separating Fe3O4@PANI core/ shell particles from a sol or a suspension system and for carrying drugs to targeted locations under an external magnetic field. 4. Conclusions In summary, the present work demonstrates a novel and facile synthetic route for the construction and synthesis of highly dispersed bifunctional Fe3O4@PANI core/shell spheres with well-defined blackberry-like morphology. The synthetic procedure for the well-defined core/shell structure is highly reproducible. During the whole construction process, PVP molecules acted as both the linker (between the PANI and Fe3O4) and the stabilizer. The possible mechanism of forming the blackberrylike particles was discussed. These microspheres are shown to be superparamagnetic, which allows them to serve as ideal candidates for biomedical applications, such as nucleic acid extraction, cancer diagnosis and treatment, biosensors, and drug delivery. Acknowledgment. This work is partly supported by an Innovative Technology Fund (CUHK6902336) from the Innovative Technology Commission, Hong Kong SAR, and a Direct Grant of Research (2060301) from the Chinese University of Hong Kong. Professor Christopher H. K. Cheng (Biochemistry, CUHK) is gratefully acknowledged. References and Notes (1) Caruso, F. AdV. Mater. 2001, 13, 11–22. (2) (a) Lu, Y.; Yin, Y.; Mayers, B. T.; Xia, Y. Nano Lett. 2002, 2, 183–186. (b) Deng, Y. H.; Wang, C. C.; Hu, J. H.; Yang, W. L.; Fu, S. K. Colloid Surf., A 2005, 26, 87–93. (c) Tartaj, P.; Serna, C. J. J. Am. Chem. Soc. 2003, 125, 15754–15755. (d) Im, S. H.; Herricks, T.; Lee, Y. T.; Xia, Y. Chem. Phys. Lett. 2005, 401, 19–23. (3) (a) Jeong, U.; Teng, X.; Wang, Y.; Yang, H.; Xia, Y. AdV. Mater. 2007, 19, 33–60. (b) Lu, A. H.; Salabas, E. L.; Schueth, F. Angew. Chem., Int. Ed. 2007, 46, 1222–1244. (4) (a) Tartaj, P.; Morales, M. D. P.; Veintemillas-Verdaguer, S.; Gonzalez-Carreno, T.; Serna, C. J. J. Phys. D: Appl. Phys. 2003, 36, R182– R197. (b) Pankhurst, Q. A.; Connolly, J.; Jones, S. K.; Dobson, J. J. Phys. D: Appl. Phys. 2003, 36, R167–R181. (c) Safarik, I.; Safarykova, M. J. Chromatogr., B 1999, 722, 33–53. (d) Halavaara, J.; Tervahartiala, P.; Isoniemi, H.; Ho¨ckerstedt, K. Acad. Radiol. 2002, 43, 180–185. (e) Lubbe, A. S.; Alexiou, C.; Bergemann, C. J. Surg. Res. 2001, 95, 200–206. (f) Xu, X. Q.; Deng, C. H.; Gao, M. X.; Yu, W. J.; Yang, P. Y.; Zhang, X. M. AdV. Mater. 2006, 18, 3289–3293. (g) Li, Y.; Yan, B.; Deng, C. H.; Yu, W. J.; Xu, X. Q.; Yang, P. Y.; Zhang, X. M. Proteomics 2007, 7, 2330– 2339. (h) Levy, L.; Sahoo, Y.; Kim, K.-S.; Bergey, E. J.; Prasad, P. N. Chem. Mater. 2002, 14, 3715–3721. (i) Yi, D. K.; Lee, S. S.; Ying, J. Y. Chem. Mater. 2006, 18, 2459–2461. (5) (a) Andreeva, D. V.; Gorin, D. A.; Shchukin, D. G.; Sukhorukov, G. B. Macromol. Rapid Commun. 2006, 27, 931–936. (b) Xu, H.; Cui, L.;

Synthesis of Fe3O4@PANI Core/Shell Microspheres Tong, N.; Gu, H. J. Am. Chem. Soc. 2006, 128, 15582–15583. (c) Pich, A.; Bhattacharya, S.; Lu, Y.; Boyko, V.; Adler, H. P. Langmuir 2004, 20, 10706–10711. (6) Kingsborough, R. P.; Swager, T. M. AdV. Mater. 1998, 10, 1100– 1104. (7) (a) Ma, X. F.; Li, G.; Wang, M.; Cheng, Y.; Bai, R.; Chen, H. Z. Chem.sEur. J. 2006, 12, 3254–3260. (b) Watts, P. C. P.; Hsu, W. K.; Randall, D. P.; Kotzeva, V.; Chen, G. Z. Chem. Mater. 2002, 14, 4505– 4508. (c) Sharma, S.; Nirkhe, C.; Pethkar, S.; Athawale, A. A. Sens. Actuators, B 2002, 85, 131–136. (d) Huang, J. X.; Virji, S.; Weiller, B. H.; Kaner, R. B. J. Am. Chem. Soc. 2003, 125, 314–315. (e) Li, D.; Kaner, R. B. J. Mater. Chem. 2007, 17, 2279–2282. (8) (a) Bae, W. J.; Kim, K. H.; Jo, W. H.; Park, Y. H. Macromolecules 2004, 37, 9850–9854. (b) Kinyanjui, J. M.; Harris-Burr, R.; Wagner, J. G.; Wijeratne, N. R.; Hatchett, D. W. Macromolecules 2004, 37, 8745–8753. (c) Frisch, H. L.; Song, H. H.; Ma, J. Q.; Rafailovich, M.; Zhu, S. M.; Yang, N. L.; Yan, X. Z. J. Phys. Chem. B 2001, 105, 11901–11905. (d) Tseng, R. J.; Huang, J. X.; Ouyang, J.; Kaner, R. B.; Yang, Y. Nano Lett. 2005, 5, 1077–1080. (9) Lellouche, J. P.; Senthil, G.; Joseph, A.; Buzhansky, L.; Bruce, I.; Bauminger, E. R.; Schlesinger, J. J. Am. Chem. Soc. 2005, 127, 11998– 12006. (10) Wan, M. X.; Li, W. C. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 2129–2136.

J. Phys. Chem. C, Vol. 112, No. 48, 2008 18809 (11) Deng, J. G.; He, C. L.; Peng, Y. X.; Wang, J. H.; Long, X. P.; Li, P.; Chan, A. S. C. Synth. Met. 2003, 139, 295–299. (12) Deng, J.; Ding, X.; Zhang, W.; Peng, Y.; Wang, J.; Long, X.; Li, P.; Chan, A. S. C. Polymer 2002, 43, 2179–2184. (13) Xiao, Q.; Tan, X.; Ji, L.; Xue, J. Synth. Met. 2007, 157, 784–791. (14) Deng, H.; Li, X.; Peng, Q.; Wang, X.; Chen, J.; Li, Y. Angew. Chem., Int. Ed. 2005, 117, 2842–2845. (15) Xu, X. Q.; Deng, C. H.; Gao, M. X.; Yu, W. J.; Yang, P. Y.; Zhang, X. M. AdV. Mater. 2006, 18, 3289–329. (16) Deng, Y.; Qi, D.; Deng, C.; Zhang, X.; Zhao, D. J. Am. Chem. Soc. 2008, 130, 28–29. (17) Li, Y.; Wu, J.; Qi, D.; Xu, X.; Deng, C.; Yang, P.; Zhang, X. Chem. Commun. 2008, 564–566. (18) Asturias, G. E.; MacDiarmid, A. G.; McCall, P. R.; Epstein, A. J. Synth. Met. 1989, 29, 157–162. (19) Xuan, S. H.; Hao, L. Y.; Jiang, W. Q.; Gong, X. L.; Hu, Y.; Chen, Z. Y. J. Magn. Magn. Mater. 2007, 308, 210–213. (20) Mangiapia, G.; Berti, D.; Baglioni, P.; Teixeira, J.; Paduano, L. J. Phys. Chem. B 2004, 108, 9772–9779. (21) Zhu, C. L.; Chou, S. W.; He, S. F.; Liao, W. N.; Chen, C. C. Nanotechnology 2007, 18, 275604.

JP807124Z