Preparation and characterization of silica coated iron oxide magnetic nano-particles

Preparation and characterization of silica coated iron oxide magnetic nano-particles

Spectrochimica Acta Part A 76 (2010) 484–489 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectr...

999KB Sizes 0 Downloads 48 Views

Spectrochimica Acta Part A 76 (2010) 484–489

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

Preparation and characterization of silica coated iron oxide magnetic nano-particles Ying-Sing Li a,∗ , Jeffrey S. Church b , Andrea L. Woodhead b , Filsun Moussa a a b

Department of Chemistry, University of Memphis, 3774 Walker Ave., Memphis, TN 38152, USA CSIRO Materials Science and Engineering, PO Box 21, Belmont, Vic 3216, Australia

a r t i c l e

i n f o

Article history: Received 14 December 2009 Received in revised form 24 March 2010 Accepted 8 April 2010 Keywords: Iron oxide nano-particles Silica coated Mercaptopropylsilyl coated Infrared spectra Raman spectra

a b s t r a c t Iron oxide magnetic nano-particles have been prepared by precipitation in an aqueous solution of iron(II) and iron(III) chlorides under basic condition. Surface modifications have been carried out by using tetraethoxysilane (TEOS) and mercaptopropyltrimethoxysilane (MPTMS). The uncoated and coated particles have been characterized with transmission electron microscopy (TEM), energy dispersive X-ray (EDX) spectroscopy, thermal gravimetric analysis (TGA), and infrared (IR) and Raman spectroscopy. The particle sizes as measured from TEM images were found to have mean diameters of 13 nm for the uncoated and about 19 nm for the coated particles. The measured IR spectra of the uncoated and MPTMS coated particles showed the conversion of magnetite to hematite at high temperature. The results obtained from both IR spectroscopy and TGA revealed that the mercaptopropylsilyl group in the MPTMS coated magnetite decomposed at 600 ◦ C and the silica layer of the TEOS coated magnetite was rather stable. Raman spectroscopy has shown the laser heating effect through the conversion of magnetite to maghemite and hematite. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Magnetic nano-particles have potential biomedical applications [1–4] because of their inherent magnetic properties, microstructure, surface area, surface charge and low toxicity [5]. Due to its biological compatibility and chemical stability, magnetite has drawn much attention for being used in targeted drug delivery systems [6–9]. Different methods have been developed for the preparation of magnetite [10–13]. With their hydrophobic characteristics, magnetic nanoparticles can easily be aggregated. Surface modification may help to stabilize the nano-particles in a specific condition and to provide functional groups for further derivatization which would enable additional applications. One way to avoid the aggregation of these magnetic nano-particles is to coat the particles with biocompatible, water-soluble and nontoxic materials [14]. Iron oxide nano-particles that have been surface-modified with an organic agent were found to be highly dispersible in non-polar organic solvents [15]. In a previous study, we have applied silane-coupling agents (SCAs) as precursors in the preparation of silica sol–gels for the surface modification of metals. The coating of aluminum with this

∗ Corresponding author. Tel.: +1 901 678 2621; fax: +1 901 678 3447. E-mail address: [email protected] (Y.-S. Li). 1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2010.04.004

sol–gel has demonstrated that the coated film is capable of corrosion protection [16]. Silica and its derivatives coated onto the surfaces of magnetic nano-particles may help to change their surface properties. With the appropriate coating, the magnetic dipolar attraction between magnetic nano-particles may be screened thus minimizing or even preventing aggregation. The coating film could also provide a chemically inert layer for the nano-particles, which is particularly useful in biological systems [17]. With the choice of SCAs, one may modify the surface properties of silica coated magnetic nano-particles and extend their potential applications. In the present study, we have applied silica and mercaptopropyl silica coating onto the magnetic nano-particle surfaces. The resulting particles have been characterized by transmission electron microscopy (TEM), energy dispersive X-ray (EDX) spectroscopy, thermal gravimetric analysis (TGA), and infrared (IR) and Raman spectroscopy.

2. Experiment 2.1. Materials Tetraethoxysilane (TEOS) and 3-me-rcatopropyltrimethoxysilane (MPTMS) were obtained from Aldrich and used as received. Deionized water was obtained from a Millipore Milli-Qth Ion Exchange Water System and had at least a resistance of 18 M cm. The hydrate samples of FeCl3 ·6H2 O and FeCl2 ·4H2 O

Y.-S. Li et al. / Spectrochimica Acta Part A 76 (2010) 484–489

485

were purchased from Alfa Aesar. Absolute ethanol was the commercial product from Pharmco-Aaper. 2.2. Preparation of iron oxide magnetic nano-particles The method of preparation was according to that of Massart’s [11] but without the use of hydrochloric acid. 4.4 g of FeCl3 ·6H2 O and 1.98 g of FeCl2 ·4H2 O were dissolved in 61 mL deaerated water. The solution was purged with nitrogen or argon to agitate the mixture and to prevent the oxidation of Fe2+ ions. After 30 min of purging, 143 mL of 0.7 M NH4 OH was added drop wise into the above Fe ion solution and the basified solution was purged for an additional 10 min. During the addition of NH4 OH, it was noticed that the solution changed color from the original brown to dark brown and then to black. The black iron oxide product responded to a magnetic field as expected. This physical property is helpful with the separation of the particles from the liquid reaction solution. 2.3. Preparation of coated of iron oxide nano-particles 80 mL of 0.125 M Fe3 O4 nano-particle solution was prepared by dispersing the iron oxide magnetic nano-particles in absolute ethanol at 40 ◦ C. 4.00 mL of 21% ammonia, 7.50 mL of deionized water and 0.56 mL of TEOS were added in sequence to the iron oxide mixture. After stirring vigorously with a magnetic stirrer for 2 h, the mixture was then ultra-sonicated for 1 h. The nano-particles were separated from the liquid by magnetic attraction and dispersed in 30 mL of ethanol. The same coating procedure was repeated two more times to obtain triple coated nano-particles. After keeping the particle solution in a 60 ◦ C water bath for 6 h to strengthen the Si–O–Fe linkage, two separation and rinsing cycles were conducted with a magnetic bar and alcohol. The coated particles were finally separated from the liquid by a magnetic field and vacuum dried. For convenience, the product is labeled as SiO2 /Fe3 O4 . Mercaptopropylsilica coated magnetite, labeled as MPTMS/Fe3 O4 , was prepared using the same procedure but replacing TEOS with MPTMS. 2.4. Characterization TEM images were obtained using a JEOL 2010 equipped with an analytical pole piece. The images were acquired at an accelerating voltage of 200 kV and captured using a Gatan Image Filter, model 678. The images acquired were “zero loss” images using a 10 eV energy slit. Samples were ultra-sonicated in absolute ethanol for 20 min and then dispersed on 100 mesh copper grids with continuous carbon support films. Measurements were carried out on the images using the MeasureIT software package (Olympus Soft Imaging Solutions). The EDX spectra on the TEM were obtained using a Noran Instruments, Pioneer SiLi 30 mm2 detector with a Moxtek ultra-thin window. TGA curves were collected with a Mettler Toledo TGA/SDTA851e thermal analysis system with a temperature ramp of 5 or 10 ◦ C/min. The system was purged with nitrogen using a flow rate of 50 mL/min. Infrared spectra were recorded using a Matson Polaris FTIR spectrophotometer equipped with a room temperature DTGS detector and WinFirst Spectroscopy software. IR spectra of the nanoparticles were collected in transmission mode by pressing the particle sample with KBr powder to form pellets. A resolution of 2 cm−1 and a total of 96 scans were applied for the collection of IR spectra. Raman spectra were obtained using an inVia confocal microscope system (Renishaw, Gloucestershire, UK) with 785 nm excitation from a diode laser through a 20× (0.4 na) objective. Optimum incident laser power was approximately 4 mW and coaxial

Fig. 1. TEM images of MPTMS/Fe3 O4 nano-particles. Table 1 Summary of TEM measurements of iron oxide and coated iron oxide magnetic nanoparticles diameters (nm).

Core Shell thickness

Fe3 O4

SiO2 /Fe3 O4

MPTMS/Fe3 O4

13 ± 2 3±1

17 ± 2 5±2

14 ± 3

backscatter geometry was employed. Samples were compacted into a 1 mm cavity in an anodized aluminum plate. Spectra were collected over the range 3200–100 cm−1 and averaged over at least 20 scans, each with an exposure time of 10 s. Five spectra were coadded to further reduce noise. The Raman shifts were calibrated using the 520 cm−1 line of a silicon wafer. The spectral resolution was ∼1 cm−1 . Data manipulation was carried out using Grams AI software. 3. Results and discussion 3.1. TEM images and EDX spectra Figs. 1–3 display the TEM images of MPTMS/Fe3 O4 , SiO2 /Fe3 O4 and Fe3 O4 , respectively. The measured nano-particle sizes are summarized in Table 1. It is noted that the mean diameter of Fe3 O4 nano-particles is comparable to the core diameters of the SiO2 /Fe3 O4 and MPTMS/Fe3 O4 nano-particles. It is also comparable to the 11.4 nm average diameter of the magnetic nano-particles reported in the literature [18]. The measured diameter of the coated magnetite particles depends on the thickness of the coated film. The diameter of the organic surface-modified magnetic nano-

486

Y.-S. Li et al. / Spectrochimica Acta Part A 76 (2010) 484–489

Fig. 2. TEM images of SiO2 /Fe3 O4 nano-particles.

particles has been measured to be 25 nm [15], slightly larger than the coated particles prepared in the present study. It was found early that iron oxide particle size decreases as the pH of the solution and/or the quantity ratio of Fe+3 /Fe+2 increases [11]. In studying the preparation of iron oxide magnetic nano-particles by the aqueous co-precipitation method, Zhao et al. [18] noticed that other factors, including the addition rate of the aqueous ammonia, the reaction temperature and the amount of emulsifier (PEG4000) present in the reaction solution, would affect the nano-particle product size. All three samples, Fe3 O4 , SiO2 /Fe3 O4 and MPTMS/Fe3 O4 , were observed to form aggregates; it appeared that the surface modification with silica and mercaptopropyl silica did not decrease the aggregation of the magnetite nano-particles. This suggests that the particles could have been in an aggregated state during the coating process. EDX analyses revealed the presence of the elements Fe and O for Fe3 O4 sample, of Fe, O and Si for SiO2 /Fe3 O4 , and of Fe, O, Si and S for MPTMS/Fe3 O4 as expected. 3.2. IR spectra Depicted in Fig. 4 are IR spectra of iron oxide and the silica coated iron oxide nano-particles before and after annealing to 935 ◦ C. Two broad bands around 3447 and 1636 cm−1 are attributed to adsorbed water as evident by their decrease in intensity after overnight baking of these samples at 109 ◦ C. The broad band around 600 cm−1 with a slight splitting in Fig. 4C is attributed to magnetite (Fe3 O4 ) in a consistent manner with that reported in the literature [9,19–24]. On the high frequency side of the broad band, we observe a weak

Fig. 3. TEM images of Fe3 O4 nano-particles.

Fig. 4. IR spectra of nano-particles: (A) SiO2 /Fe3 O4 at 25 ◦ C; (B) SiO2 /Fe3 O4 annealed at 935 ◦ C; (C) Fe3 O4 at 25 ◦ C; and (D) Fe3 O4 at 935 ◦ C.

shoulder at 803 cm−1. This band is due to Fe–OH deformation in accordance with the assignment by Fu et al. [25] but does not appear to be distinctive in Fig. 4C. As the oxide is annealed up to 935 ◦ C, the Fe–OH deformation band disappears (see Fig. 4D) due to the elimination of hydroxyl group. At the same time, the broad band near 600 cm−1 shifts its frequency down and splits into two bands at 562 and 471 cm−1 , which are attributed to ␣-Fe2 O3 [26,27]. Thus, the IR spectral analysis reveals that the magnetite is converted to ␣-Fe2 O3 at high temperature. The annealed iron oxide has also lost its attraction to the magnetic field after annealing at 935 ◦ C. Such a change of magnetic property is in accordance with the variation observed in the low frequency region of the IR spectrum.

Y.-S. Li et al. / Spectrochimica Acta Part A 76 (2010) 484–489

487

Table 2 Observed IR and Raman spectral frequencies of MPTMS/Fe3 O4 . IR 3451 (m,b) 2953 (w,sh) 2927 (m) 2888 (w,sh) 2860 (w,sh) 2793 (vw) 2557 (vw)

Raman

Assignments

∼2905 (w,b)

H2 O a (CH2 ) a (CH2 ) s (CH2 ) s (CH2 ) Overtone/combination (SH)

2572 (w) 1731 (w)

H2 O

1623 (w,b) 1602 (w)

Fig. 5. IR spectra of MPTMS/Fe3 O4 cured at different temperatures: (A) 25 ◦ C; (B) 600 ◦ C; and (C) 935 ◦ C.

Fig. 4A shows the IR spectrum of SiO2 coated magnetite nanoparticles. The band at 1086 cm−1 with a high frequency shoulder near 1200 cm−1 can be attributed to the Si–O–Si asymmetric stretching vibrations; these vibrations are expected to have strong IR absorption consistent with how they appear in the spectrum. The weak band at 977 cm−1 is due to Si–OH deformation from the incomplete condensation of TEOS sol. Annealing the sample does not change the basic spectral features in the low frequency region (see Fig. 4B), and the annealed sample at 935 ◦ C retains attraction to the magnetic field similar to the un-annealed sample. This interesting behavior may reveal the effective protection of iron oxide by silica and warrants further investigation. Fig. 5A shows the IR spectrum of MPTMS coated magnetite nanoparticles prior to high temperature annealing. Similar to the IR spectra of Fe3 O4 and SiO2 /Fe3 O4 , the broad bands near 3450 and 1624 cm−1 are attributed to adsorbed moisture. The low frequency bands near 600 cm−1 are similar to those observed in the spectrum of magnetite [9,19–24]. Other bands in the spectrum match well with the observed IR bands for the MPTMS sol–gel and xerogel [16]. The observed IR and Raman spectral frequencies are listed in Table 2 along with the suggested vibrational assignments. After annealing the sample to 600 ◦ C, all the bands associated with the mercaptopropyl group at 2572, 1452, 1443, 1408, 1340, 1301 and 1240 cm−1 disappear, indicating the decomposition of the organic group in the coating shell at this high temperature. However, the presence of Si–O–Si stretching vibrations suggests that the silica main frame remains unchanged in the coating film. The low frequency bands around 600 cm−1 do not vary significantly and the particles are still attracted to a magnetic field. On the other hand, the uncoated sample does lose its property of attraction to a magnetic field after annealing at 600 ◦ C for 2 h. This indicates that the surface coating with MPTMS sol–gel has improved the stability of the magnetite core in the coated particles. If the annealing temperature is raised up to 935 ◦ C, the spectral features due to the iron oxide change and the particles are no longer attracted to a magnetic field. The low frequency bands at 539 and 474 cm−1 in Fig. 5C reveal that the core iron oxide has changed to ␣-Fe2 O3 [26,27], but the features associated with the silica coating still remain unchanged. This suggests that the MPTMS coated sample is less stable than the SiO2 coated sample at 935 ◦ C. 3.3. Raman spectra Magnetite is a poor Raman scatter particularly at the low laser powers, which makes the collection of high quality Raman spectra restrictive. Optical images of magnetite and MPTMS coated magnetite have been taken as part of the micro-Raman investigation. Both images show two colors – red/brown and black. Silver/white coloration appeared after over heating of an area with high laser

1452 (w) 1443 (w) 1408 (w) 1387(vw,sh) 1340 (w) 1301 (w) 1258 (w,sh) 1240 (m) 1123 (vs) 1037 (s,sh) 915 (vw)

1446 (w) 1410 (w)

1042 (m) 863 (vw)

803 (w) 727 (w,sh) 692 (m) ∼676 (s,b) 635 (s) 586 (s) 506 (m) 458 (w) 450 (m) 359 (w) 300 (w) 240 (w)

ı(CH2 ) ı(CH2 ) ı(CH2 ) ω(CH2 ) (CH2 ) ω(CH2 ) ω(CH2 ) a (SiOSi) a (SiOSi) (CH2 ) s (SiOSi) s (SiOSi), ı(FeOH) (SC) (SiC), ␣-Fe2 O3 , ␥-Fe2 O3 Fe3 O4 , ␥-Fe2 O3 (SiC) Fe3 O4 ␣-Fe2 O3 , ␥-Fe2 O3 ␥-Fe2 O3 Fe3 O4 ␣-Fe2 O3 , ␥-Fe2 O3 ␣-Fe2 O3 ␣-Fe2 O3

s, strong; m, moderate; w, weak; v, very; sh, shoulder; b, broad; , stretch; ı, deformation; ω, wag; , twist.

Fig. 6. Raman spectra of magnetite: (A) dark area; (B) light area; and (C) laser overheat area.

power (>4 mW), resulting in a mark in the shape of the beam or a totally destroyed area. Fig. 6 shows the Raman spectra of iron oxide obtained using a laser power of 4 mW or less for excitation at different spots of the sample. The spectra collected in the red/brown and black areas are essentially similar (see Fig. 6A and B). It is possible that the materials in the light and dark areas are the same except that the red/brown is a thinner and less-dense film. Previous Raman spectroscopic investigations have identified the main band near 670 cm−1 as due to magnetite [16,28–33]. The appearance of a broad band around 673 cm−1 in Fig. 6A and B reveals the presence of magnetite in the sample. Other spectral features in Fig. 6A and B show the characteristic maghemite (␥-Fe2 O3 ) bands [28,31,32,34]. Thus, from the Raman data of the iron oxide nano-particles, the laser heating appears to convert magnetite into maghemite. All the observed peaks in Fig. 6C provide evidence for

488

Y.-S. Li et al. / Spectrochimica Acta Part A 76 (2010) 484–489

Fig. 7. Raman spectra of (A) MPTMS/Fe3 O4 and (B) SiO2 /Fe3 O4 .

the presence of hematite (␣-Fe2 O3 ) [30,31], indicating that the laser overheating is changing the maghemite to hematite. It has been shown that this change is irreversible [32]. The 1315 cm−1 band is attributed to the second order scattering of hematite [30,31,35]. Attempts to limit sample heating by spinning the samples in capillary tubes were not successful. Use of 1064 nm excitation did not reduce the thermal changes or improve spectral quality. Shown in Fig. 7A are the Raman spectra of the MPTMS/Fe3 O4 nano-particles (see Table 2 for assignments). In referring to previous Raman spectroscopic studies, the broad band near 676 cm−1 may be contributed by hematite, maghemite [31] and possibly magnetite [31,32]. The appearance of other low frequency bands at 506, 458, 359 and 300 cm−1 in the spectrum gives further support to the presence of maghemite [31]. Similar to the spectra of uncoated iron oxide, the broad and weak bands near 1338 cm−1 may be attributed to the second order scattering of hematite [30,31,35]. The co-existence of different iron oxide species in the MPTMS/Fe3 O4 Raman sample arises from the laser heating effect although the evidence for the presence of magnetite may not be as clear as the presence of other species. The Raman peaks with frequencies higher than 850 cm−1 are attributed to the mercaptopropylsilyl film and their vibrational assignments are given in Table 2. The Raman spectrum of SiO2 coated magnetite is comparable to that of the MPTMS/Fe3 O4 with the exception of the mercaptopropylsilyl film peaks in the higher frequency region. No Raman scattering peak for the SiO2 film vibration could be observed because of low scattering cross-section for these vibrations. 3.4. Thermal analysis TGA measurements have been conducted for the uncoated and coated magnetite samples. These results are shown as Fig. 8. For all three different samples, there are three steps of weight reduction in the following temperature ranges: I at 35–110 ◦ C, II at 230–380 ◦ C and III at 780–935 ◦ C. The general shapes and percentage of weight losses for magnetite and SiO2 coated magnetite are very similar, indicating that the silica coated film is thermally stable and the weight drops in SiO2 /Fe3 O4 are essentially contributed by iron oxide. In the temperature range I, the weight drops for magnetite and coated magnetite samples are ∼1.5%. If the samples are purged overnight with dry nitrogen at room temperature, the percentage of weight drops in the TGA curves decrease. In the IR experiment, the intensity of adsorbed water bands decreases or disappears after baking the KBr pellet samples at 109 ◦ C for several days. Consequently, it is suggested that the weight drops in the temperature range I are caused by the removal of physisorbed water from samples in agreement with the TGA result of hematite [26]. In another report, it was suggested that the weight reduction of hematite due to loss of adsorbed water extends over the range 25–350 ◦ C [36].

Fig. 8. TGA curves of magnetite (upper), SiO2 /magnetite (lower A) and MPTMS/magnetite (lower B). Scanning rate: 10 ◦ C/min (upper) and 5 ◦ C/min (lower).

One reason for this rather wide temperature range may arise from the relatively fast heating rate used in the experiment. The amount of moisture adsorbed on the particle surface is likely to be related to the particle surface area. In the present study, all three-particle samples have comparable sizes, which result in similar weight reductions in temperature range I. In the middle temperature range, 230–380 ◦ C, MPTMS/Fe3 O4 particles have a remarkably higher percentage weight drop than the magnetite and SiO2 /magnetite particles, indicating the thermal decomposition of the mercaptopropyl group in the coating film. This is supported by the disappearance of HSCH2 CH2 CH2 -vibration peaks after treating the sample at 600 ◦ C for 3 h (see Fig. 5B). A careful inspection of the extended temperature range 230–600 ◦ C reveals two weight drop steps. One of these steps is a small percentage of the overall weight drop, and may be related to the similar weight reductions observed in magnetite and SiO2 /magnetite. With a much thinner coating film obtained from single coating of magnetite, the percentage weight loss of MPTMS/magnetite sample is significantly lower, providing further support for this interpretation of the weight loss. An early thermomagnetometric study of magnetite has shown that the maximum amount of ␥-Fe2 O3 (maghemite) is formed at around 270 ◦ C [37]. At 320 ◦ C, most of the maghemite in the sample was converted into hematite (␣-Fe2 O3 ) [37]. The oxidation of magnetite to maghemite and/or hematite is expected to increase the sample particle weight. In addition to this process, the observed small weight reduction in the iron oxide and SiO2 coated iron oxide nano-particles must be contributed to significantly by the change of iron oxyhydroxide (FeOOH) to Fe2 O3 . The occurrence of this chemical process is supported by the thermal analyses of hematite particles [35] and magnetite coated polystyrene [38]. The release of hydroxyl ions from the particles was used to explain the weight loss above 750 ◦ C [36]. The present IR investigation of magnetite also reveals the presence of FeOH deformation vibration at room

Y.-S. Li et al. / Spectrochimica Acta Part A 76 (2010) 484–489

temperature and the absence of this mode when the magnetite was heated to 935 ◦ C. 4. Summary Iron oxide magnetic nano-particles have been prepared by precipitation under basic condition. Silica and mercaptopropylsilica coatings of the particle surfaces have been carried out. Based on TEM images, the diameters of the uncoated magnetite particles were determined to be of the order of 13 nm and those of the coated particles to be about 19 nm. From IR spectroscopic investigation, it has been found that magnetite is converted to hematite at high temperature in both the uncoated and MPTMS coated particles. However, the silica coating appears to be effective in protecting magnetite from being converted to other oxide species. Both IR spectroscopy and TGA show that the mercaptopropylsilyl group in the MPTMS coated film decomposes at 600 ◦ C. In Raman spectroscopic measurements, magnetite is converted to maghemite and hematite due to the laser heating effect. Acknowledgments This research is partially supported by the University of Memphis FRG funds. The authors acknowledge the assistance of Colin Veitch in obtaining the TEM images and EDX spectra. References [1] P. Tartaj, M.D. Morales, S. Veintemillas-Verdaguer, T. Gonzalez-Carreno, C.J. Serna, J. Phys. D: Appl. Phys. 36 (2003) R182. [2] E. Katz, I. Willner, Angew. Chem. Int. Ed. 43 (2004) 6042. [3] A.K. Gupta, M. Gupta, Biomaterials 26 (2005) 3995. [4] P. Tartaj, M.D. Morales, T. Gonzalez-Carreno, S. Veintemillas-Verdaguer, C.J. Serna, J. Magn. Magn. Mater. 290–291 (2005) 28. [5] M. Ma, Y. Wu, J. Zhou, Y. Sun, Y. Zhang, N. Gu, J. Magn. Magn. Mater. 268 (2004) 33.

489

[6] B.R. Peters, R.A. Williams, C. Webb, Magnetic Carrier Technology, ButterworthHeinemann, Oxford, 1992. [7] S.S. David, Trends Biotechnol. 15 (1997) 217. [8] T. Neuberger, B. Schopf, H. Hofmann, M. Hofman, B. von, Rechenberg, J. Magn. Magn. Mater. 239 (2005) 483. [9] J.L. Zhang, R.S. Srivastava, R.D.K. Misra, Langmuir 23 (2007) 6342. [10] J.C.R. LeFort, Acad. Sci. Paris 34 (1852) 480. [11] R. Massart, IEEE Trans. Magn. Mag. 17 (1981) 1247. [12] X.P. Qui, J. Xiamen Univ. 38 (1999) 711. [13] Z.H. Zhou, J. Wang, X. Liu, J. Mater. Chem. 11 (2001) 1704. [14] S. Santra, R. Tapec, N. Theodoropoulou, J. Dobson, A. Hebard, W. Tan, Langmuir 17 (2001) 2900. [15] D. Guin, S.V. Manorama, Mater. Lett. 62 (2008) 33139. [16] Y.-S. Li, Y. Wang, T. Tran, A. Perkins, Spectrochim. Acta Part A 61 (2005) 3032. [17] Y.H. Deng, C.C. Wang, J.H. Hu, W.L. Yang, S.K. Fu, Colloids Surf. A Physicochem. Eng. Aspects 262 (2005) 87. [18] Y. Zhao, Z. Qiu, J. Huang, Chinese J. Chem. Eng. 16 (2008) 451. [19] H.S. Choi, J.S. Ahn, W. Jo, T.W. Noh, J. Korean Phys. Soc. 28 (1995) 636. [20] M. Muroya, F. Uchida, Anal. Sci. 7 (1991) 399. [21] Y.-H. Lein, T.-M. Wu, J. Colloids Interf. Sci. 326 (2008) 517. [22] C.-T. Chen, Y.-C. Chen, Anal. Chem. 77 (2005) 5912. [23] X. Sun, C. Zheng, F. Zhang, Y. Wang, G. Wu, A. Yu, N. Guan, J. Phys. Chem. C 113 (2009) 16002. [24] C.N.R. Rao, G.V. Rao, Transition Metal Oxides, Crystal Chemistry, Phase Transition and Related Aspects, US Department of Commerce, National Bureau of Standards, Washing, DC, 1974. [25] R. Fu, W. Wang, R. Han, K. Chen, Mater. Lett. 62 (2008) 4066. [26] S.M. Rodulfo-Baechler, S.L. Gonalez-Cortes, J. Orozco, V. Sagredo, B. Fontal, A.J. Mora, G. Delgado, Mater. Lett. 58 (2004) 2447. [27] http://webbook.nist.gov/cgi/cbook.cgi?ID=B6004683&Units=SI&Mask=80. [28] G. Xi, C. Wang, X. Wang, Eur. J. Inorg. Chem. (2008) 425. [29] O.N. Shebanova, P. Lazor, J. Solid Chem. 174 (2003) 424. [30] N. Pinna, S. Grancharov, P. Beato, P. Bonville, Chem. Mater. 17 (2005) 3044. [31] J.E. Maslar, W.S. Hurst, W.J. Bowers, J.H. Hendricks, M.I. Aquino, J. Electrochem. Soc. 147 (2000) 2532. [32] D.L.A. de Faria, S.V. Silva, M.T. de Oliveria, J. Raman Spectrosc. 28 (1997) 873. [33] R.K.S. Raman, B. Gleeson, D.J. Young, Mater. Sci. Technol. 14 (1998) 373. [34] G.V.M. Jacintho, P. Cori, J.C. Rubim, J. Electroanal. Chem. 603 (2007) 27. [35] O.N. Shebanova, P. Lazor, J. Raman Spectrosc. 34 (2003) 845. [36] K. Kandori, T. Ishikawa, Phys. Chem. Chem. Phys. 3 (2001) 2949. [37] J.P. Sanders, P.K. Gallagher, Thermochem. Acta 406 (2003) 241. [38] Z. Huang, F. Tang, J. Colloids Interf. Sci. 275 (2004) 142.