Evolution of Structural and Magnetic Properties of Magnetite

Apr 7, 2010 - Synopsis. The synthesis of novel magnetite nanoparticles is driven by oriented attachment of smaller crystals, forming both single-cryst...
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DOI: 10.1021/cg901602w

Evolution of Structural and Magnetic Properties of Magnetite Nanoparticles for Biomedical Applications

2010, Vol. 10 2278–2284

Stefan Gustafsson,*,† Andrea Fornara,‡ Karolina Petersson,§ Christer Johansson,§ Mamoun Muhammed,‡ and Eva Olsson† †

Department of Applied Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden, Division of Functional Materials, Royal Institute of Technology (KTH), SE-164 40 Kista, Sweden, and § Imego AB, Arvid Hedvalls Backe 4, SE-411 33 Gothenburg, Sweden ‡

Received December 20, 2009; Revised Manuscript Received March 16, 2010

ABSTRACT: We have investigated the evolution of microstructure and magnetic properties of thermally blocked magnetite nanoparticles, aimed for immunoassay applications, during their synthesis. High-resolution transmission electron microscopy (HRTEM) investigations of the size, size distribution, morphology, and crystal structure of particles reveal that particles at an early stage of the reaction process are either single crystals or polycrystals containing planar faults and they grow via a combination of reactant (monomer) consumption and oriented attachment at specific crystallographic surfaces because of the strong dipolar character of the Æ111æ surfaces of magnetite. At a later stage of the synthesis reaction, the magnetic attraction strives to align contacting particles with their Æ111æ axis of easy magnetization in parallel and this may also be an active driving force for crystal growth. At latter stages, the crystal growth is dominated by Ostwald ripening leading to smoother crystalline particles with a mean diameter of 16.7 ( 3.5 nm and a narrow size distribution. The magnetic properties of the particles measured using static and dynamic magnetic fields are consistent with the evolution of particle size and structure and show the transition from superparamagnetic to thermally blocked behavior needed for magnetic relaxation-based immunoassay applications.

Introduction Magnetic nanoparticles (MNP) have attracted a lot of attention over the past years and found use in a wide range of biomedical applications such as bioseparation, site-specific drug delivery, hyperthermia for cancer therapy, and magnetic resonance imaging (MRI).1-3 MNP are also well-suited for use in immunoassays where they act as immobilization substrates, thus enabling simpler and faster ways of detecting analytes in solution compared to other assay methods like ELISA.4-6 In a previous paper, we reported on the fabrication of tailored thermally blocked iron oxide nanoparticles and demonstrated their use for highly sensitive detection of Brucella antibodies directly in biological fluids through magnetic susceptibility measurements.6 The performance of MNP in such immunoassay applications depends on the particles having well-tailored sizes just above the superparamagnetic limit and thereby having a predictable and well-defined response to an external magnetic field. Particles with sizes in the nanometer range have a large surface to volume ratio and, consequently, a large fraction of surface atoms resulting in physical properties that differ extensively from that of the bulk material.7 Magnetic materials in the nanometer range show superparamagnetic behavior when the particle size is below the superparamagnetic limit, where the particle magnetic moment is not locked to a specific crystallographic direction. For a given temperature, the superparamagnetic limit is material specific and, together with saturation magnetization, magnetic remanence, and the coercivity, is strongly affected by the microstructure of the particle system. Superparamagnetic nanoparticles show no magnetic remanence nor coercivity, *Corresponding author. E-mail: [email protected]. Tel: þ46-31-772-3627. Fax: þ46-31-772-3224. pubs.acs.org/crystal

Published on Web 04/07/2010

whereas nanoparticles above the superparamagnetic limit, i.e., thermally blocked, show both magnetic remanence as well as coercivity if the particles are immobilized.8 Thermal blocking means that the internal (Neel) relaxation in the particles, whereby the magnetization relaxes without any physical rotation of the domain, is longer than the specific measuring time for the used equipment. For instance, for vibrating sample magnetometer (VSM) measurements the measuring time is about 10 s. Depending on the application, certain requirements regarding size distribution, morphology, chemical composition, and crystallinity need to be imposed on the particle ensemble. Several methods for the synthesis of magnetic nanoparticles have been reported;,9 however, there is still a need of knowledge enabling the precise tailoring of stable particle properties. Although it may be considered a straightforward procedure to synthesize iron oxide nanoparticles with the desired composition and purity, it still remains an experimental challenge to obtain precise control over the particle size, shape distributions, and particle stability, especially when larger particle sizes are requested as for immunoassay applications based on magnetic susceptibility measurements. In this context, it is essential to have an understanding and control over the nucleation and growth process of nanoparticles. The initial stages of crystal growth have traditionally been considered a two step process starting with a nucleation burst from a supersaturated solution of reactive atomic or molecular species (monomers) followed by crystal growth by the continuous diffusion and subsequent attachment of monomers to the crystal surface. The subsequent crystal growth is a highly complex process depending on, for example, the properties of the reaction mixture, the surfactants and the monomer concentration. Variation of the experimental parameters and techniques results in a wide range of complex crystal shapes and particle size distributions.10,11 When the reactive r 2010 American Chemical Society

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Table 1. Reaction Time and Temperature for Magnetite Nanoparticles Together with the Corresponding Particle Size and the Best Particle Size Distribution Fit Function reaction time (min) 30 90 190 250 310 340 (final sample)

particle size ( std dev (nm) best particle size T (C) calculated from TEM images distribution fit 145 185 215.6 214.8 215.1 215.2

3.5 ( 0.9 3.6 ( 1 10.8 ( 4.2 12.3 ( 5.6 12.9 ( 3.7 16.7 ( 3.5

log-normal log-normal log-normal log-normal normal normal

monomer concentration in the reaction media is depleted, particle growth is controlled by Ostwald ripening, leading to the dissolution of smaller particles and the growth of larger particles. It has, however, recently been observed that mechanisms involving nanoparticle coalescence, so-called oriented attachment, may also play a significant role in the growth of nanocrystals. This mechanism was originally proposed by Lee Penn and Banfield12,13 as a way to explain the origin of defect structures in nanoparticles. Following their study, a number of investigations have shown oriented attachment in a variety of nanomaterials.14-23 It operates by joining particles that make contact at similar crystallographic surfaces. Defects and shape anisotropy are commonly observed to result from this process,20 which may also affect the magnetic properties. In the current study, we have investigated the evolution of structure and magnetic properties of novel magnetite nanoparticles in the final size range of 20 nm aimed for immunoassay applications based on AC susceptometry.4,6 High resolution transmission electron microscopy (HRTEM) reveals direct evidence for oriented attachment during the synthesis of the particles. This results in single crystals as well as particles with low energy planar defect structures. Furthermore, the evolution of particle structure and size during synthesis is correlated with magnetic measurements that show the gradual transition from superparamagnetic to thermally blocked particles crucial for our biosensor application.6 Experimental Section Synthesis of Magnetite Nanoparticles. Iron oxide nanoparticles were synthesized with a modified procedure based on the hydrolysis of chelate metal alkoxide complexes at high temperatures in solutions of chelating alcohols.24 Typically, 397.6 mg of FeCl2 3 4H2O and 1081.2 mg of FeCl3 3 6H2O were dissolved in 40 g of N-methyl diethanolamine (NMDEA) under the protection of N2. This solution was added to 40 mL of a 0.4 M solution of NaOH in NMDEA in a three-neck flask under a N2 atmosphere with mechanical stirring. After 3 h, the temperature of the reaction solution was increased to 210 C and kept constant for 5 h and 40 min, allowing the reaction to proceed. After the reaction mixture cooled to room temperature, the solid product was isolated by centrifugation, washed four times with a mixture of ethanol and ethyl acetate, and finally dispersed in water. To study the kinetics of the particle growth, aliquots of 1 mL of reaction mixture were taken from the reaction vessel at different stages during the synthesis. These samples were taken at different time intervals, and precisely after 30, 90, 190, 250, 310, and 340 min from the time when the heating mantel was activated (see Table 1). The last sample, taken after a reaction time of 340 min, is denoted “final sample”. The samples were taken from the reaction vessel by using a glass pipet and they were quenched by cooling them to room temperature. All samples were stored in a fridge at 4 C before further analysis. Characterization. HRTEM and chemical analysis were carried out in order to determine the evolution and functional structure of the magnetic nanoparticles. TEM images were acquired using a

Figure 1. TEM micrographs and the corresponding particle size distributions (particle number distribution) of magnetite nanoparticles after (a) 90, (b) 190, and (c) 340 min (final sample) reaction time. The mean particle diameter is (a) 3.6, (b) 10.8, and (c) 16.7 nm. Philips CM200 operating at 200 kV. The particle size distribution of each sample was determined by measuring the maximum dimension of at least 300 particles from randomly chosen areas, and fitting the results to probability distribution curves. Energy-dispersive X-ray (EDX) analysis and electron energy loss spectroscopy (EELS) were performed in the TEM using an Oxford INCA system and a Gatan Imaging Filter, respectively. Static magnetic measurements were carried out at room temperature using a vibrating sample magnetometer (VSM) from LakeShore Cryotronics, Inc. To determine the magnetic remanence and coercivity of the samples at different reaction times, we immobilized the particles within a polymeric matrix, thereby inhibiting the particles’ translation and rotation and resulting in random crystallographic orientations. Under such conditions, the internal single-domain relaxation, i.e., Neel relaxation, can be distinguished from Brownian relaxation. AC-susceptibility measurements on the final colloidal magnetite nanoparticles were performed using the DynoMag system from Imego AB.6

Results Particle Growth and Structure. TEM images of the particles, together with the corresponding size distributions at different reaction times, are shown in Figure 1. The mean particle size at different stages is listed in Table 1. Small nuclei with a mean diameter of 3.5 nm and with a narrow lognormal size distribution had formed already after 30 min and remained in principle unaltered after 90 min (Figure 1a). After 190 min of reaction time, however, after the maximum reaction temperature is reached, a dramatic increase in particle size was seen, the mean size now being 11 nm, and the log-normal distribution (Figure 1b) shows a tail extending toward larger particle diameters. While the reaction is

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Figure 2. HRTEM images of magnetite particles from the aliquots at: (a, b) 190 min and (c, d) 340 min (final sample). The insets show the diffraction patterns obtained from an FFT. The zone axis is [011] for all patterns. The particles in (a) and (c) are single crystals, whereas (b) and (d) show twinned particles (twin boundary arrowed).

proceeding, the mean size and size distribution gradually change. After 340 min (final sample), the particles have a mean size of 16.7 nm with a standard deviation of 3.5 nm (21%), and a normal size distribution now gives the best fitting (Figure 1c). The magnetite structure of the nanoparticles was confirmed by measuring the lattice parameters obtained from diffraction ring patterns in the TEM. Furthermore, EDX analysis showed a composition close to that expected for stoichiometric magnetite. In addition, the fine structure in the electron energy loss spectra showed the characteristics of magnetite (see the Supporting Information). The particle growth process was investigated in more detail by imaging the structure of the particles at different reaction times using HRTEM. This showed that the majority of the particles at all reaction times are single crystals without defects, as seen in images a and c in Figure 2. There is, however, a significant difference in the morphology of the particles at the earlier stages of reaction compared to the final sample. The particles in the sample after 190 min reaction time have in general irregular surfaces and some of them consist of smaller crystals with diameters corresponding to those of the particles after 90 min reaction time (see Figure 2a). Although the morphology of the cluster in Figure 2a indicates that it comprises several particles, the lattice fringes show that all particles in the cluster share the same crystallographic orientation. The corresponding Fast Fourier Transform (FFT) pattern confirms that the cluster constitutes one single crystalline entity. In comparison, many

of the particles in the final sample are nearly spherical with surfaces containing small faceted segments, most frequently corresponding to low index planes, i.e., {111}, {100}, {110}, and {311}. In addition to the single-crystal particles, HRTEM also revealed that a fraction of the particles in the samples comprises polycrystals with the crystalline parts separated by low-energy grain boundary configurations, most frequently twin boundaries or stacking faults. Images b and d in Figure 2 show examples of twinned magnetite particles together with the corresponding diffraction pattern, which shows reflections from both crystal areas. The particle imaged in Figure 2b, from the aliquot at 190 min, appears to consist of three smaller crystals fused together, where two crystals have the same orientation and share a twin boundary with the third crystal. In comparison, the twinned particle from the final sample (Figure 2d) has a more spherical morphology with small faceted segments, just like many of the single-crystal particles in the final sample. Many of the polycrystalline particles, however, have a neck marking the interface region, as illustrated in Figure 3, which shows a twinned magnetite particle where the straight {111} twin boundary plane extends through the whole particle. The particle is imaged along the Æ112æ direction of shear, thereby only revealing the twin boundary by a line of enhanced contrast along the interface. Similar lines of contrast were frequently seen to delineate the interface separating polycrystalline particles. These have been observed in other TEM studies of twinned magnetite and have been taken as evidence for twinning.25,26 Particles with a line of

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white contrast delineating straight {110} interfaces were also occasionally observed. In addition to these observations, a few particles also showed an orientation relationship involving parallel close packed directions at the interface plane according to f111gparticle1 ==f100gparticle2 Æ110æparticle1 ==Æ110æparticle2 and f111gparticle1 ==f511gparticle2 Æ110æparticle1 ==Æ110æparticle2 These orientation relationships also give a certain degree of coherence between the contacting surfaces. In general, it could in almost all cases be concluded that when two particles were seen to be attached to each

Figure 3. HRTEM image of a twinned magnetite particle (twin boundary arrowed) from aliquots at 340 min (final sample) viewed along the [112] direction of shear.

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other, the plane of contact was {111} for at least one of them. Magnetic Measurements. In Figure 4, the magnetic measurements of particle ensembles with different reaction times (190 and 340 min (final sample)) are shown. The hysteresis curve for the particles formed after 190 min, shows a clear superparamagnetic behavior with no magnetic remanence and coercivity present. As the reaction time increases (340 min) the particle size increases and particles reach sizes larger than the superparamagnetic limit (see Figure 1). These particles are thermally blocked and show both magnetic remanence and coercivity in static magnetic measurements when they are immobilized. The superparamagnetic limit for magnetite at room temperature for static VSM measurements with a measuring time in the range of 10 s can be estimated to be about 14 nm using a magnetic anisotropy constant of 3  104 J/m3 for the magnetite nanoparticles. As can be seen from Figure 1c (reaction time 340 min - final sample) there is a fraction of particles with sizes larger than 14 nm and it is these particles that give the magnetic hysteresis that is seen in Figure 4. Even if not all of the particles have sizes larger than 14 nm (Figure 1c), the magnetic response from these particles (the remanence) can still be high because it depends on both the number of particles and the particle volume. The magnetization from the sample with reaction time 90 min was low due to the small particle size and the low number of particles formed (smaller than the diamagnetic background from the carrier liquid (i.e., water). The saturation magnetization value of the nanoparticle suspension increases with increasing reaction time as can be seen in Figure 5, where the saturation magnetization is plotted versus the mean particle diameter calculated from TEM images as obtained at different reaction times. The saturation magnetization of a magnetic nanoparticle system is directly proportional to the product of the mean volume, the number density of nanoparticles in the suspension and the intrinsic magnetization of the nanoparticles. The increase in the saturation magnetization as the reaction time increases is due to both the increase in the number of particles and the mean nanoparticle size. By normalizing the saturation magnetization with the mean volume of the particle systems we find that the number density of particles increases linearly with the mean diameter of the particles

Figure 4. Hysteresis loops in two field ranges for the particle system at two different reaction times, 190 and 340 min, corresponding to mean particle diameters of 10.8 ( 4.2 and 16.7 ( 3.5 nm, respectively. The particles are immobilized, i.e., particle rotation and translation are inhibited. Only the final sample shows hysteresis effects with remanence and coercivity. The magnetization values of the sample collected at 190 min reaction time are multiplied by a factor 2 for clarity.

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Figure 5. Saturation magnetization versus the mean particle diameter obtained at different reaction times, 90, 190, and 340 min (the final sample), giving mean particle diameters of 3.6, 10.8, and 16.7 nm, respectively, estimated from TEM images.

Figure 6. AC susceptibility measurements presenting both the (a) real and (b) imaginary parts of the AC susceptibility of the final magnetite nanoparticles. Curves (c) and (d) give the real and the imaginary parts of the magnetic susceptibility data for the magnetite particles after functionalization with lipopolysaccarides (from ref 6).

(as long as the intrinsic magnetization of the particles is more or less independent of the particle size). The Brownian relaxation of nanoparticles when they are suspended in water is the key to the development of a novel biosensor. AC susceptometry can be used for investigation of Brownian relaxation frequency as a function of the particles’ hydrodynamic radius. We therefore performed AC susceptibility measurements on the final colloidal magnetite nanoparticles using the DynoMag system from Imego AB.6 The data are reported in Figure 6. The final magnetite nanoparticles exhibit a maximum at about 50 kHz of the imaginary part of the complex susceptibility, as presented in Figure 6, curves a and b. A median particle hydrodynamic diameter of 20 nm with a size distribution width of 8 nm was obtained by a data fitting procedure.27 When the prepared magnetite nanoparticles are properly functionalized, it is possible to detect specific biomolecules in serum samples by measuring the change in hydrodynamic volume.4,6 After biomolecules are bound to functionalized nanoparticles, i.e., with lypopolysaccarides, the hydrodynamic size increases and there is a shift in the Brownian relaxation frequency (see Figure 6, curves c and d). Discussion The HRTEM images of the particles give clear evidence that oriented attachment takes place during the early stages of

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reaction. During synthesis, the particles in suspension are free to move and undergo random Brownian-motion like collisions. Two colliding particles may attach to each other and form permanent bonds, provided that the additional grain boundary energy is outweighed by the reduction in surface free energy. This may be the case if an interface with a sufficient degree of coherency is created in the process. Furthermore, once contact is made, the particles may be able to rotate relative to each other in order to find a low-energy configuration.28 If full coherency can not be achieved, the soformed particles will contain defects like stacking faults and twins, which are considered to be clear signs of oriented attachment. As was seen in the earlier stages of reaction, particles consist of several smaller crystals that have coalesced (Figure 2a). The key feature in the observation is that the size of the individual small crystals matches well with the size of individual particles at the previous observation time. It was found that the attachment occurred on low-energy surface facets and low-energy configurations were formed. However, in many of these particles, it was found that the smaller crystals were perfectly aligned, forming a single crystal, as seen in Figure 2a. This shows that the mechanism of oriented attachment not only results in the formation of particle defects but also contributes to the population of single crystals. The detailed mechanisms for oriented attachment are not completely clear. The probability of contact between two particles is dependent on the particle concentration but may also be affected by electrostatic attraction because of surface polarity. The {111} surfaces of magnetite, consisting of either oxygen anions or iron cations, have a very strong polarity, possessing a net surface charge and an electric dipole moment.29,30 Two {111} surfaces of opposite charge are thus very likely to be attracted and to bond to one another in order to minimize their energy. This type of dipole-dipole interaction has also been proposed to act in other materials with polar surfaces.15,16 For example, Pacholski et al.15 found oriented attachment along {100} planes in ZnO nanocrystals that, similar to magnetite, consist of either anions or cations. Furthermore, in addition to the electrostatic attraction, magnetic attraction due to the magnetic moments of the particles may also account for some of the structures formed in the later stages of synthesis when the particles are larger. It was frequently seen in our study that the most common surface of contact was the {111} facet and it is interesting to note that this type of surface may also offer the greatest forces of attraction. The magnetization easy axis in magnetite is Æ111æ31 and in order to lower the overall magnetic energy of contacting particles there will be a driving force for alignment of the individual magnetization vectors along the Æ111æ axis. Two clustered particles with lowest magnetic dipolar interaction will thus be positioned with their Æ111æ directions in parallel just as we have seen in many of the defect containing particles (see Figure 2 and 3). It may also be possible that the polycrystalline particles we observed may actually consist of just one magnetic domain. Chains of contacting particles, illustrating the magnetic attraction, were also observed (see Figure 7) and have been found for example in studies of both synthetic magnetite26 and magnetite in magnetotactic bacteria.26,32 It is, in fact, common for colloidal magnetic particles in general to form chains with the individual magnetic dipoles aligned.33,34 Particle defects may also arise during traditional growth and although this scenario may not be excluded most experimental

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fraction of the particles in the final sample (reaction time 340 min, see Figure 1c) shows the Brownian relaxation (mean particle size calculated from AC-susceptometry of 20 nm). This relaxation criteria is crucial for several biosensor applications.4,6 The magnetic properties with respect to magnetic hysteresis are consistent with the determined particle sizes from TEM. When the largest particle size in the size distribution is larger than the superparamagnetic limit, magnetic hysteresis effects with remanence and coercivity are visible in the measured hysteresis loops at different stages in the reaction process. Conclusions

Figure 7. Chain formation of magnetite nanoparticles from the aliquot at 340 min (final sample).

evidence points toward the oriented attachment mechanism as the major defect forming mechanism. It was noted that defect containing particles in general are found at the upper end of the particle size distribution. If defects were generated during growth from one common nucleus, these defect particles would be expected to have sizes spanning over the whole size distribution range, not just the upper end. Furthermore, the coalesced crystals are not always of equal size, indicating that they did not nucleate at the same time. This further supports the oriented attachment mechanism rather than the particles growing from a common nucleus. The evolution of the particle shape and the size distribution in our work (Figure 1), which shows that the initial log-normal distribution is gradually changing into a normal distribution as the reaction progresses, shows that the standard crystal growth process of monomer attachment and Ostwald ripening is also active alongside oriented attachment and probably more dominant toward the end of the synthesis. The smoothening of particle surfaces is a clear indication of diffusion driven processes that act to smoothen the crystal surface. A log-normal type of size distribution is frequently found in studies of superparamagnetic magnetite nanoparticles.24,35 It can, however, also be seen in several studies that the larger the particle mean diameter, the more the size distribution tends to shift toward a normal distribution.24,25 Eventually, as the monomer concentration in the reaction media is depleted, an asymmetric size distribution with a tail extending toward lower particle diameters is developed because of Ostwald ripening.36 It may thus be concluded that particles with mean hydrodynamic sizes around 20 nm (which show thermally blocked behavior) that requires longer synthesis times present a greater challenge for controlled synthesis. It is shown in our study, though, that a sufficiently large part of the nanoparticles have sizes in the thermally blocked region and that the size distribution can be considered as narrow with a standard deviation of 21%. These properties make the particles wellsuited for specific immunoassay applications using sensor methods that detect the changes of the Brownian relaxation of the particles.4,6 At room temperature and water as the carrier liquid the crossover particle size between Neel (internal single-domain relaxation) and Brownian relaxation (stochastic particle rotation) is about 14 nm which means that a large

In conclusion, we have reported on the evolution of structure and magnetic properties of novel magnetite nanoparticles and shown that oriented attachment acts as a dominating growth mechanism in the earlier stages of growth, giving both single-crystal particles and particles containing low-energy planar faults. The particles’ structures are smoothened by Ostwald ripening during the latter stages of synthesis and the final particle size and size distribution are in the thermally blocked range. The particle growth matches well with measurements of magnetic remanence and coercivity showing the transition from superparamagnetic to thermally blocked behavior. Acknowledgment. This work has been partially supported by the European Commision 6th Framework Program (BIODIAGNOSTICS NMP4-CT-2005-017002). The authors thank Dr. Muhammet Toprak for his help with the manuscript. Supporting Information Available: Electron energy loss spectra of magnetite nanoparticles (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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