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Comparative Study of the Magnetic Behavior of Spherical and Cubic Superparamagnetic Iron Oxide Nanoparticles Guoliang Zhen,*,† Benjamin W. Muir,† Bradford A. Moffat,‡ Peter Harbour,† Keith S. Murray,| Boujemaa Moubaraki,| Kiyonori Suzuki,⊥ Ian Madsen,# Nicki Agron-Olshina,# Lynne Waddington,† Paul Mulvaney,§ and Patrick G. Hartley† CSIRO Molecular and Health Technologies, BayView AVenue, Clayton, VIC 3168, Australia, Department of Radiology, and School of Chemistry & Bio21 Institute, UniVersity of Melbourne, ParkVille, VIC 3052, Australia, School of Chemistry, and Department of Materials Engineering, Monash UniVersity, Clayton, VIC 3800, Australia, and CSIRO Minerals, BayView AVenue, Clayton, VIC 3168, Australia ReceiVed: May 30, 2010; ReVised Manuscript ReceiVed: NoVember 22, 2010
A modified method for the production of cubic and spherical superparamagnetic nanoparticles is presented. Cubic nanoparticles can be made that are highly monodisperse down to a diameter of 8 nm. A detailed study is presented of the physical properties of these nanoparticles using high-resolution transmission electron microscopy analysis, X-ray powder diffraction, superconducting quantum interference device measurements, and relaxivity measurements performed in a magnetic resonance imaging scanner. It is found that cubic iron oxide nanoparticles have a higher degree of crystallinity and relaxivity (four times higher) than their spherical counterparts. These novel cubic iron oxide nanoparticles show great promise for use in biomedical imaging applications. Introduction Magnetic iron oxide nanoparticles (NPs) are used in a number of applications including ferrofluids,1-3 biosensors,4 contrast enhancement agents for magnetic resonance imaging,5 bioprobes,6 and catalysis.7 As the magnetic properties of this class of NPs are size-dependent,7-10 narrow particle size distributions are critical for an understanding of their magnetic interactions in biological systems.11,12 Monodisperse iron oxide NPs can be prepared by the thermal decomposition of iron acetylacetonate13,14 and carboxylates15 in high-boiling point solvents containing surfactants such as oleic acid or oleylamine. Certain metal carboxylates (acetates, acetylacetonates, and oxalates) and metal oleate complexes can be used as precursors for the preparation of transition metal oxide nanoparticles.9,16,17As recently reported,18 the mechanism of thermal decomposition of an iron oleate complex precursor upon heating is similar to that occurring during “hot injection”.19 For the past two decades, synthetic superparamagnetic iron oxide nanoparticles (SPIOs) have served as contrast-enhancing probes for magnetic resonance imaging (MRI). Under an applied magnetic field (B0), a magnetic dipole moment is induced within the SPIOs. When water molecules diffuse into the periphery (outer sphere) of the induced dipole moment, the magnetic relaxation processes of the water protons are perturbed and the spin-spin relaxation time (T2) is shortened. Such changes result in a decrease of the corresponding area in T2-weighted MRI signal, thus providing imaging contrast where present. One aim of research into SPIO synthesis is to increase the rate of * To whom correspondence should be addressed. E-mail: guoliang.zhen@ csiro.au. † CSIRO Molecular and Health Technologies. ‡ Department of Radiology, University of Melbourne. § School of Chemistry & Bio21 Institute, University of Melbourne. | School of Chemistry, Monash University. ⊥ Department of Materials Engineering, Monash University. # CSIRO Minerals.
spin-spin relaxation per weight of iron. A key synthetic goal is therefore to optimize size, composition, and magnetocrystalline phase of SPIOs systems with a view to minimizing T2, and hence increasing T2-weighted image contrast.20 At present, there is some contention in the literature as to the maximum viable size for nanocrystals for in vivo use since renal clearance is strongly size dependent.21,22 Recent work suggests that only nanocrystals with a hydrodynamic diameter 40 voxels) centered within each well was averaged and plotted as a function of TE. The R2 value was then calculated (as the decay constants) by numerically fitting (using a nonlinear least-squares algorithm (Matlab, MA)) the data to a monoexponential equation
S ) S0 exp(-TE/T2)
(1)
where S0 is the equilibrium signal when TE , T2. The respective relaxivities of iron oxide nanoparticle are the linear gradients of the R2 data plotted as a function of Fe concentration. Therefore, a linear least-squares analysis (Matlab) was used to quantify the R2 relaxivities. Results and Discussion TEM image analysis (Figure 1) shows that both NPs are uniform in size and when dried down result in an extensive 2D assembly of uniform 8 nm NPs. Both the cubic and spherical NPs are monodisperse in nature with narrow particle size distributions (size variation ) 5% for the spheres and 6% for the cubes). High-resolution TEM (HRTEM) images of these iron oxide nanocrystals showed distinct lattice fringe patterns, indicating the highly crystalline nature of the nanocrystals. A
Figure 1. Transmission electron micrographs of (A, B, C) 8 nm spherical and (D, E, F) 8 nm (side length) cubic, monodisperse, uniform iron oxide nanoparticles, showing 2D arrays at different magnifications. High-resolution TEM images (C) and (F) showing internal crystal structure.
close look at the TEM images of NPs in Figure 1c, f shows that indeed a large part of the particles are single crystal; however, some parts of the particle appear less organized. When comparing the cubic and spherical particles, it is clear that the spherical particles are less organized than the cubic particles. The decomposition of iron oleate, in the presence of oleic acid, has been used for the synthesis of monodisperse iron oxide nanoparticles. In the paper of Park et al,9 the synthesis of nanocubes of iron oxide with size lengths of 20 nm is discussed, but minimal detail on the control of SPIO shape is reported during their synthesis. In another paper18 from the same group, it was demonstrated that using a similar thermal decomposition approach, the shape of the NPs produced can change from spheres to cubes with longer aging time as the monomer concentration decreased. In this work, we report a method that achieves further control of SPIO NP shape and size, via the production of either cubic or spherical NPs, to a size of 8 nm. It is well-known9,10 that the size of iron oxide NPs can be tuned by varying the concentration of oleic acid in an organic solvent. Increasing the concentration of oleic acid results in larger NPs. For in vivo applications, smaller particle sizes appear to be desirable, and we have achieved this through the addition of a small amount of oleic acid (optimized at 2.5% w/w of the 1-octadecene solvent) to the nanoparticle synthesis formulation protocol (see Experimental Section). The mechanism of de-
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Zhen et al. TABLE 1: Refined Crystallite Size and Unit Cell Dimensions Using Tetragonal and Cubic Systems unit cell dimensions (Å)
Figure 2. Rietveld refinement output for spherical (A) and cubic (B) iron oxide nanoparticles. The gray and black curves are the observed and calculated patterns, respectively.
composition of the metal complex is yet to be fully understood. The important experimental reaction variables that control the synthesis of either spherical or cubic SPIO NPs are the concentration of excess oleic acid as a stabilizer and the aging time used during synthesis. Increasing the concentration of oleic acid may slow down the particle growth rate, therefore allowing a milder aging process which makes the surface of the nanocrystals formed during the nucleation stage smoother and rounder. Conversely, the NP growth rate is much more rapid without the use of additional stabilizer. This dramatically effects the consumption of iron oleate monomer during the nucleation and growth phase which may force the crystal growth to become cubic in shape surrounded by a [100] crystal plane. In spinel structures, the [100] plane is known to have the lowest surface free energy, and cubic crystals with a [100] plane presented at the surface are known to be the most thermodynamically stable shape.40-42 This phenomenon has been discussed in recent work by Kwon et al.18 Other recent work has also mentioned that it is possible to synthesize cubic iron oxide nanoparticles by varying the concentration of stabilizer and heating rate.43 We have found that the size of the cubic iron oxide nanocrystals formed during our reaction conditions does not increase significantly with extended reaction times. Increasing the reaction time up to 45 min resulted in only a slight increase of the size of the iron oxide nanocrystals. This suggests that the thermal decomposition is very rapid and that a significant fraction of the reactive monomer is consumed in the formation of the small nanocrystals during the first few minutes of the reaction.44 Figure 2 shows the XRD patterns of spherical and cubic SPIO nanoparticles and the peak positions. The observed diffraction
sample type
crystallite size (Å)
spherical
28 ( 3
cubic
35 ( 3
tetragonal a c a c
8.378(0.013) 8.361(0.009) 8.368(0.014) 8.389(0.002)
cubic
Rwp (%) tetragonal cubic
8.369(0.001)
5.41
6.34
8.383(0.001)
7.58
9.48
patterns are shown in Figures 2A and 2B for spherical and cubic samples, respectively. The observed phase was probably magnetite (Fe3O4) or, more likely, maghemite (γ-Fe2O3). Initial inspection of the patterns shows that there is considerable peak broadening indicating that the material has a very small crystallite size. In addition, the presence of a mixture of broad and slightly sharper peaks indicates the presence of anisotropic size broadening. The observed diffraction peaks have different widths which could be attributed to variable crystallite sizes along different crystallographic directions. This was allowed for by refining an overall crystallite size along the (100) direction and applying a March model45 for anisotropic size broadening. The refined values (0.65 and 0.54 for sphere and cube, respectively) indicate that there is a high degree of crystal size anisotropy in these materials. Initially, Rietveld analysis was conducted using a full structure approach (in which the contents of the unit cell are used to generate peak intensities), but there was generally poor agreement between the observed and calculated patterns. This was abandoned in favor of using so-called “hkl phases” in which the peak positions are determined by the space group and unit cell parameters but empirical peak intensities are allowed to refine. A number of different structure models were trialed including magnetite (cubic structure) and maghemite (cubic), but the best fit was obtained using a tetragonal maghemite model. It is interesting that maghemite systems have such tetragonal distortion due to the vacancy ordering, although in particle systems this has only been observed for larger particles with D > 20 nm.46 However, the fact that the crystallite size is so small results in very broad peaks and generally poor quality in the observed diffraction data. Therefore, it is difficult to draw any definitive conclusion of the tetragonal crystal system from the analysis of this data. Additional refined parameters included the unit cell dimensions and anisotropic crystallite size. Selected output from the Rietveld analysis is given in Figures 2A, B and Table 1. The peak positions and relative intensities of the nanoparticles agree well with standard magnetite (Fe3O4) XRD patterns..47 The magnetite phase cubic nanoparticles appear to have a greater degree of crystallinity than the spherical particles. The average crystallite sizes of the iron oxide nanoparticles were deduced from Scherrer’s equation48-50 and summarized in Table 1. The approach taken in this analysis was to use whole pattern fitting via the Rietveld method where all of the peaks contribute to the analysis. This provides values which are inherently more reliable than those derived from “single peak” methods. In addition, the software employed uses the fundamental parameters approach where the contribution from the instrument is calculated rather than measured. Therefore, it is possible to refine a value for crystallite size directly. The equation used in this case is crystallite size ) λ/(cos θ · fwhm) which is then modified by the anisotropic size relationship. The ratio of TEM particle size to XRD crystallite size is ∼3.0 for the spherical iron oxide NPs indicating considerable polycrystallinity within the nano-
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TABLE 2: Comparison of Particle Size and Crystal Size of Iron Oxide Nanoparticles Using TEM, Magnetization vs Field, and XRD Analysis, Respectively, Together with Size Values for Water Suspensions and Saturation Magnetization Values MS for Solid Nanoparticles powder
water suspensions
magnetic properties (20 °C)
sample
crystallite size (XRD)a (nm)
sphere cube
2.8 3.5
A unit cella(nm)
particles size (TEM) (nm)
0.8383 0.8397
8.5 ( 0.3 8.0 ( 0.4b
Ms (emu/g)
H (Oe) (emu/g)
SQUID sizec(nm)
mean particle size (M vs H) (nm)
sizecryo-TEM (nm)
sizeDLS (nm)
31 40
0 0
9 12
5.1d 6.2e
8.2 8.1b
31 27
a Refined crystallite size and unit cell dimensions for maghemite. b Side length of cube. c Deducted from comparing the blocking temperature, TB, of previously reported9 particles with different sizes. d Diameter D, standard deviation ) 1.71. e Side length of cube, standard deviation ) 1.79. These particle sizes assumed the phase present was magnetite. See text for average sizes assuming maghemite is present.
particle population. The polycrystallinity within the cubic nanoparticles is somewhat lower, as suggested by a particle: crystallite size ratio of ∼2.3. This suggests that the spherical nanoparticles contain more defects than their cubic nanoparticle counterparts.10 Also shown in Table 2 are particle sizes obtained by dynamic light scattering for nanoparticle populations after transferring into water using the PMAO polymer surface modification phase transfer approach described earlier. The hydrodynamic diameter of the particles is found to be 3-4 times greater than the particle dimensions determined by TEM. This presumably results from a contribution to the DLS measurement from the polymer coating on the particles. The “magnetic” particle size was also deduced from a detailed set of M vs H data points (0-7 T), at 295 K (see also top righthand side quartile of Figures 5A and B) by fitting to the Langevin equation51and employing a log-normal size distribution.52 Magnetite was assumed to be the phase present in the particles, with Msat of 0.598 T. The best fit for the spherical sample is shown in Figure 3 with an average particle diameter (D) obtained of 5.1 nm and an absolute saturation magnetization of 0.495 emu (mass normalized value of 35.1 emu/g). The average length of the side of the cubic particles, L, is 6.2 nm, and the absolute saturation magnetization is 0.863 emu (massnormalized value of 39.6 emu/g). These “magnetic” particle sizes are lower than the “physical” sizes measured by TEM by ca. 30%, and this is largely due to the magnetization curves not including the nonmagnetic surface layers. Similar size differences have been reported previously,52,53 but the reverse situation has also been reported where, for γ-Fe2O3 spherical particles made electrochemically, the D values from the magnetization fits were larger by ca. 40% than those from TEM, the difference being ascribed to weak interactions occurring between particles.54 If the nanoparticles in the present work are assumed to be maghemite, with Msat of 0.37-0.49 T, then the average sizes are L ) 6.6-7.3 nm (cubic) and D ) 5.4-6.0 nm (spherical), slightly larger than when magnetite is assumed. These data are compatible with the SQUID data and clearly showed that the cubic particle has bigger magnetic size than the spherical particle. Cryogenic-TEM (cryo-TEM) was used to observe polymercoated iron oxide nanoparticles in water. The sample was prepared by quickly vitrifying (by means of liquid ethane) a thin film of nanoparticle dispersions on a copper grid so that disturbance to associated polymer/surfactant species during drying was minimized. In Figure 4 the cryo-TEM images show that the phase-transferred nanoparticles were fairly monodisperse and no aggregation was observed. An interesting experimental observation was noted while performing the cryo-TEM mea-
Figure 3. Plots of magnetization, at 295 K (where M ) absolute value of measured M.in emu) vs dc field (labeled µ0H) in tesla for the cubic sample (top) and the spherical sample (bottom). The solid lines are the best fits to the Langevin equation51 using a log-normal size distribution, with average side length, L, of 6.2 nm and standard deviation of 1.79 for the cubic particles (A). For the spherical particles the average diameter, D, is 5.1 nm, with standard deviation of 1.71 (B). These particle sizes assume an SPIO composition of 100% magnetite, with Msat ) 0.598 T.
surements at high magnification (data not shown). In some instances, the spherical nanoparticles appeared to break apart in the electron beam while the cubic nanoparticles appeared more stable during imaging. We hypothesize that this is due to the higher defect density within the spherical nanoparticle crystal structure, rendering them more susceptible to electron beam damage.
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Figure 4. Cryo-TEM images of water dispersible iron oxide nanoparticles: (A) 8 nm spherical iron oxide nanoparticles; (B) 8 nm cubic iron oxide nanoparticles.
A SQUID magnetometer was used to determine the magnetic properties of the SPIO NPs. The magnetization curves of the cubic and spherical magnetic nanoparticles are shown in Figures 5A and B, respectively. While saturation magnetization is not quite achieved in either case, the 8 nm cubes have a nearsaturation magnetization value of ∼40 emu/g above 40 000 Oe. The 8 nm spheres have a near-saturation magnetization value of ∼31 emu/g above 40 000 Oe. Any magnetization hysteresis around H is close to zero for both cubes and spheres, and this is common for SPIO NPs and confirms their superparamagnetic nature. The lower magnetization of spherical iron oxide nanoparticles may be due to either their crystalline defect structure (smaller magnetic domains) or their greater degree of oxidation and nonmagnetic iron oxide (Fe2O3) content. The field-cooled magnetization (FCM) is obtained by cooling within a field of 100 Oe. The zero-field-cooled magnetization (ZFCM) is obtained by cooling in zero-field, then warming within the field of 100 Oe. As seen in Figure 4C and D, upon increasing the temperature, the ZFC magnetization increases and reaches a maximum at the blocking temperature TB.9,55 The
Zhen et al. blocking temperature was defined as the temperature at which the nanoparticle moments do not relax (are blocked) during the time scale of the measurement. The ZFCM results exhibit a maximum at TB (blocking temperature), as expected for a powder sample. For the 8 nm cubic NPs (determined by cryoTEM), by analogy to Park et al.’s paper in Figure 4C,9 the nanoparticle size can be estimated to be ∼12 nm. For 8.5 nm spheres the bifurcation temperature, TIrr, in the FCM and ZFCM plots is lower at ∼80 K, suggesting a particle 9 nm in size. This is more consistent with the nanoparticle size results calculated by TEM. This result indicates that the cubic SPIO NPs have a higher blocking temperature than spherical NPs with similar volume. In a recent study of cubic and spherical nanoparticles of γ-Fe2O3, Salazar-Alvarez et al.56 observed very similar TEM images of spherical and cubic particles compared to those noted here. However, they observed that the blocking temperatures for the spherical form had a higher blocking temperature (TB ∼ 235 K) than the cubic form (TB ∼ 190 K), quite the reverse of what we have observed in this work. No clear explanation for the change in TB and for the large differences in TB values of these two studies can be given other than the particle size/volume differences of the SPIOs used in our study. After phase transferring the hydrophobic nanoparticles into water, using the commercially available poly(maleic anhydrideco-octadecene) copolymer and ethanolamine ring-opening chemistry reported previously,35,36 the nanoparticles were screened in multiwell plates inside a magnetic resonance imaging (MRI) scanner. The corresponding R2 relaxivities were calculated by plotting the signal response as a function of iron concentration. Figure 6 shows the relaxivities of 8 nm cubic and 8 nm spherical NPs, along with that of Resovist, a commercially available SPIO formulation. The relaxivities measured for the 8 nm cubic NPs (61.4 mM-1 s-1) are significantly greater than the 8 nm spherical
Figure 5. Magnetization, M (emu g-1), vs magnetic field, H (Oe), for comparison of cubic (A) and spherical (B) iron oxide nanoparticles measured at 293K. (C, D) Plot of FCM (H ) 100 Oe) and ZFCM for cubic (C) and spherical (D) iron oxide nanoparticles, respectively.
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J. Phys. Chem. C, Vol. 115, No. 2, 2011 333 iron oxide nanoparticles for medical applications due to the possibility of reducing the total Fe delivered in a clinical bolus while providing a similar contrast for medical imaging. Acknowledgment. The authors thank Dr. Sergey Rubanov for help with the TEM. The SQUID magnetic work was supported by an ARC Discovery grant to K.S.M. References and Notes
Figure 6. Comparison of R2 relaxivity of cubic SPIO, spherical SPIO, and Resovist (data as values obtained from curve fitting and standard errors are uncertainties in fitting).
NPs (17.3 mM-1 s-1). No significant evidence of aggregation of the NPs in water was seen from cryo-TEM or dynamic light scattering analysis, so we believe this increase in R2 of the cubic NPs is due to the inherent properties of the cubic NPs themselves and not due to agglomerates of nanoparticles. This is of note as it means that for use in the clinic, to get a certain T2-weighted response in vivo, much less of the contrast agent could be administered if cubic SPIOs were utilized. The higher R2 MRI relaxivity enhancement measured with cubic NPs is believed to result from the improved crystal structure detected via XRD analysis and is consistent with our SQUID measurements. The reported R2 relaxivities of these particles are consistent with the commercial and clinically used USPIO and SPIO contrast agents listed in a recent review paper.57 For example, Supravist has a hydrodynamic diameter of 21 nm with a reported R2 relaxivity of 38 mM-1 s-1. The relaxivity of Resovist measured here is consistent with that reported by the manufacturer (177 mM-1s-1) and in the review paper (189 mM-1 s-1).57 Resovist consists of Fe3O4 and Fe2O3 superparamagnetic iron oxide NPs stabilized by dextran and is a MR contrast agent for liver imaging. It is noted that the relaxivities measured are significantly greater than that of the SPIOs used in this work. This is due to the fact that Resovist is a composite material consisting of agglomerations of ultrasmall SPIO NPs of 4 nm diameter within a dextran matrix to a final mean hydrodynamic particle size of approximately 65 nm. Conclusions We have shown in this work a simple and economical method for the synthesis of ultrasmall 8 nm spherical and cubic iron oxide nanoparticles. The effect of additional surfactant (oleic acid) compared to the method reported by Park9 allows for cubic nanoparticles of small size to be produced with control. The exact mechanism behind the shape control of the SPIO nanoparticle synthesis is not fully understood, but it is clear that the [100] plane is thermodynamically favored. It has also been reported31 that trace amounts of sodium oleate may play an important role in the shape selection process. From a comprehensive study using cryo-TEM, XRD, SQUID, and MRI relaxivity measurements, we have been able to show that cubic nanoparticles have a higher degree of crystallization, larger single crystal, and higher saturation magnetization and T2 relaxivity compared to spherical nanoparticles of the same size. The phase transfer of these high-quality iron oxide nanocrystals from organic solvent to water was demonstrated successfully using a modified literature reported method.36 We believe there may be some potential advantage of using these small cubic
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