Multicore Iron Oxide Mesocrystals Stabilized by a Poly

Jun 25, 2014 - the multifunctional and rigid poly(phenylenepyridyl) dendron and dendrimer. We thoroughly analyze the influence of capping molecules of...
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Multicore Iron Oxide Mesocrystals Stabilized by a Poly(phenylenepyridyl) Dendron and Dendrimer: Role of the Dendron/Dendrimer Self-Assembly David Gene Morgan,† Bethany S. Boris,† Nina V. Kuchkina,§ Ekaterina Yu. Yuzik-Klimova,§ Svetlana A. Sorokina,§ Barry D. Stein,‡ Dmitri I. Svergun,∥ Alessandro Spilotros,∥ Athanasia Kostopoulou,⊥ Alexandros Lappas,⊥ Zinaida B. Shifrina,*,§ and Lyudmila M. Bronstein*,†,# †

Department of Chemistry and ‡Department of Biology, Indiana University, Bloomington, Indiana 47405, United States A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov St., Moscow, 119991 Russia ∥ EMBL, Hamburg Outstation, Notkestraße 85, D-22603 Hamburg, Germany ⊥ Foundation for Research and Technology − Hellas, Institute of Electronic Structure and Laser, 71110 Heraklion, Crete, Greece # King Abdulaziz University, Faculty of Science, Department of Physics, Jeddah, Saudi Arabia §

S Supporting Information *

ABSTRACT: We report the formation of multicore iron oxide mesocrystals using the thermal decomposition of iron acetyl acetonate in the presence of the multifunctional and rigid poly(phenylenepyridyl) dendron and dendrimer. We thoroughly analyze the influence of capping molecules of two different architectures and demonstrate for the first time that dendron/dendrimer selfassembly leads to multicore morphologies. Single-crystalline ordering in multicore NPs leads to cooperative magnetic behavior: mesocrystals exhibit ambient blocking temperatures, allowing subtle control over magnetic properties using a minor temperature change.

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the blocking temperature of the particular material.22 The last method is the simplest, but it normally requires lowering the temperature to values much below ambient temperature, making it unattractive for the majority of applications. Raising that temperature to ambient is a goal in this field. The magnetic properties can be altered by the arrangement of small particles into multicore colloidal clusters.23−31 These colloidal clusters can be mesocrystals, i.e., composed of individual nanocrystals aligned in a common crystallographic fashion without the coalescence of individual cores.32 In that case, they exhibit scattering properties similar to those of a single crystal but without the continuity of a single phase, which is important for aligning the magnetic spins of individual cores.23,28 Examples include iron oxide mesocrystals with sizes from ∼30 nm to hundreds of nanometers and even micrometers stabilized by various multifunctional capping molecules.25−28 In other cases, when regular surfactants (containing only one functional group) or some biological molecules are used, no oriented attachment takes place and the colloidal clusters are polycrystalline.33−36 The comparison of the above literature data reveals a certain trend: the formation

INTRODUCTION Magnetic nanoparticles (NPs) have attracted considerable attention due to their numerous exciting applications such as high-density recording media, magnetic biosensors, magnetically recoverable catalysts, MRI contrast agents, and so forth.1−6 Among various magnetic NPs, those made from iron oxide are often preferred due to their excellent magnetic properties and stability in air. Because magnetic properties are size- and shape-dependent, there has been increasing interest in exercising control over these parameters. Spherical,7−10 cubic,10−15 truncated cubic,16 rodlike,17 and star-ike15 iron oxide NPs have been reported, and their properties have been discussed. It is worth noting that generally magnetite/ maghemite (Fe3O4/γ-Fe2O3) NPs with the sizes below ∼50 nm are superparamagnetic,18,19 i.e., they are magnetic only in the presence of a magnetic field. An easy transition between superparamagnetic and ferromagnetic (or ferrimagnetic) properties is of great interest as it can be used in a number of applications, for example, allowing NPs to aggregate or disperse upon external stimuli different from the applied magnetic field. Such switching was achieved due to dipolar coupling between the particles placed in the polymer pores20 via electric-field-induced magnetic anisotropy in multiferroic composites21 or by merely decreasing the temperature to below © 2014 American Chemical Society

Received: January 21, 2014 Published: June 25, 2014 8543

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solution onto a carbon-coated Cu grid. Images were acquired at an accelerating voltage of 80 kV on a JEOL JEM1010 transmission electron microscope. Images were analyzed with image-processing package ImageJ developed by the National Institutes of Health developed to estimate NP diameters. NPs of between 150 and 300 were used for this analysis. High-resolution TEM (HRTEM) and tomography were carried out at an accelerating voltage 300 kV on a JEOL 3200FS transmission electron microscope. The same TEM grids were used for both analyses. X-ray diffraction (XRD) patterns were collected on an Empyrean from PANalytical. X-rays were generated from a copper target with a scattering wavelength of 1.54 Å. The step size of the experiment was 0.02. The magnetic properties of the samples were studied with a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS XL5). The measurements have been performed in dried nanoparticles on cotton in a gelatin capsule. The isothermal hysteresis loops, M(H), were measured at fields of −1 ≤ H ≤ +1 T. The dc magnetic susceptibility as a function of temperature, χ(T), was attained down to 5 K under zero-field-cooled (ZFC) and field-cooled (FC) protocols at H = 50 Oe. The X-ray scattering data have been collected at the P12 beamline of the European Molecular Biology Laboratory (EMBL) on the Petra III storage ring of the Deutsches Elektronen Synchrotron (DESY, Hamburg). Using a Pilatus2M detector (Dectris, Switzerland), the scattering was recorded in the range of the momentum transfer, 0.07 < s < 4.3 nm−1, where s = 4π sin θ/λ, 2θ is the scattering angle, and λ = 0.1 nm is the X-ray wavelength. Two powder samples of dendron 1 and dendrimer 2 have been dissolved in THF at three different concentrations (10, 5, and 2.5 mg/ mL). The solutions were then measured with an exposure time of 1 s in a vacuum capillary to diminish the parasitic scattering. The scattering profiles were corrected for background scattering from the THF solvent and processed using an automated pipeline.39 The concentration series have been used to obtain the scattering curve at infinite dilution using PRIMUS.40 The distance distribution functions P(r) were calculated using indirect Fourier transform program GNOM.41 The low-resolution shapes of the dendron/dendrimer ensembles were reconstructed ab initio from the scattering patterns by DAMMIF.39 This program represents the object as an assembly of beads inside a spherical search volume. Starting from a random assembly, DAMMIF employs simulated annealing to build compact scattering equivalent models fitting the experimental data Iexp(s) to minimize the discrepancy

of mesocrystals requires multifunctional capping molecules. Moreover, in the case of polymers, mainly larger multicore clusters are formed,25,26 while N-methyldiethanolamine as a capping molecule allows nanoflowers with an average size of 30 nm formed by combining a limited number of single cores.27 Although it is assumed that functional molecules facilitate the oriented attachment of individual cores, the role of capping molecules is largely unknown. Here we report the formation of multicore iron oxide mesocrystals using the thermal decomposition of iron acetyl acetonate in the presence of the multifunctional and rigid poly(phenylenepyridyl) dendron and dendrimer.37,38 We thoroughly analyze the influence of two different architectures and demonstrate for the first time that dendron/dendrimer selfassembly leads to multicore morphologies. Single-crystalline ordering of “petals” in multicore NPs leads to cooperative magnetic behavior: mesocrystals exhibit ambient blocking temperatures, allowing subtle control over magnetic properties using a minor temperature change.

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EXPERIMENTAL SECTION

Syntheses. The syntheses of dendron 1 and dendrimer 2 (Scheme 1) were described in our preceding paper.37,38

Scheme 1. Formulas of Dendron 1 (a) and Dendrimer 2 (b)a

a

Pyridine rings are shown in red, phenylene rings are shown in black, and carboxyl focal group is shown in blue.

⎡ I (s ) − cI (s ) ⎤2 1 exp j calc j ⎥ χ = ∑⎢ N − 1 j ⎢⎣ σ(sj) ⎥⎦

The synthesis of iron oxide nanoparticles in the presence of dendrons/dendrimers was carried in the following way. In a typical procedure for NP2, the three-necked round-bottomed flask (with elongated necks) was equipped with a magnetic stir bar, a reflux condenser, and two septa, one of which contained an inserted temperature probe protected with a glass shield and the other had a long needle. The flask was loaded with 0.353 g (1 mmol) of Fe(acac)3, 0.607 g (0.055 mmol) of 2, and 7 mL of benzyl ether. The flask was placed in a Glas-Col heating mantle attached to a digital temperature controller which in turn was placed on a magnetic stirrer. The flask was degassed by argon bubbling for 30 min under stirring, and the temperature was raised to 60 °C to allow solubilization for 30 min. After that, the temperature was again raised at 10 °C/min to 300 °C, and upon reaching the reflux temperature (∼285 °C), the flask was heated for 1 h. The flask was then removed from the heating mantle and allowed to cool to room temperature. To isolate NPs, a part of the reaction solution was precipitated by ethanol and washed twice with ethanol and twice with acetone and then dissolved in chloroform. Aggregates, if present, were removed by 7−10 min of centrifugation. The yield of iron oxide NPs was 85%. The rest of the reaction solution was stored in the refrigerator and was stable for many months. The yield of NP1 was 78%. Methods. Electron-transparent NP specimens for transmission electron microscopy (TEM) were prepared by placing a drop of dilute

2

(1)

where N is the number of experimental points, c is a scaling factor, and Icalc(sj) and σ(sj) are the calculated intensities from the model and the experimental error for momentum transfer sj, respectively. The content of aggregates in the two samples was estimated by the analysis of the forward-scattering intensity I0 and the radii of gyration of the monomers (self-assembled compact particles) Rgmono and aggregates Rgagg. For the monomer scattering, I0mono ≈ Nmono(Vmono)2, where N is the number of monomers and V is their volume. The forward scattering from the aggregates can be estimated as I0agg = I0whole − I0mono, where I0whole is the forward scattering of the entire system. Taking into account that the number of monomers in an aggregate k is approximately k ≈ (Rgagg/Rgmono)3, the volume fraction of compact particles is νmono = k(I0mono/I0agg)/(1 + k(I0mono/I0agg)).

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RESULTS AND DISCUSSION Formation of Multicore NPs. Iron oxide NPs (Figure 1) have been synthesized in the presence of third-generation poly(phenylenepyridyl) dendron 1 and dendrimer 2 (Scheme 1) as capping molecules. These NP samples are notated as NP1 8544

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Figure 1. Low-magnification (a, b) and high-resolution (c, d) TEM images of NP1 (a, c) and NP2 (b, d). The insets in a and b show bead models of 1 and 2, respectively, while the insets in c and d show FFT patterns of HRTEM images.

To clarify the crystal structures of NP1 and NP2, we carried out high-resolution TEM (HRTEM) (Figure 1c,d). Fast Fourier transform (FFT) patterns of the HRTEM images (insets) indicate single-crystalline organization in both cases. Solution Behavior of 1 and 2. To understand the role of the dendron/dendrimer in the mechanism of multicore NP formation, we studied the solution behavior of 1 and 2 in THF using small-angle X-ray scattering (SAXS). THF was chosen because it is a good solvent for 1 and 2 and unlike benzyl ether (reaction solvent) it can be used in an automated pipeline (SI). The experimental scattering profiles from 1 and 2 are displayed in Figure S3 (SI), and the structural parameters obtained from the data analysis are summarized in Table S1. Both SAXS profiles in Figure S3 display sharp upturns at very small angles (at around s = 0.2 and 0.12 nm−1 for 1 and 2, respectively), indicating that large clusters or aggregates coexist in solution with smaller particles. The low-resolution structure of the individual particles can be reconstructed from the higher-angle portions of the scattering data beyond the influence of the aggregates. The appropriate intervals of the experimental curves (0.4 < s < 4

and NP2, respectively. Transmission electron microscopy (TEM) images (Figure 1) show two types of particles: multicore NPs consisting of several petals (multicore NPs) and nearly spherical single-core NPs. In the case of NP2, the multicore NPs are dominant. The diameters of these multicore NPs are 23.1 nm with a standard deviation of 11% for NP1 and 23.4 nm with a standard deviation of 22.6% for NP2, i.e., significantly below the critical size of ∼50 nm.18,19 Figure S1 (the Supporting Information, SI) shows the projections of NP2 obtained from the TEM tomography (video S1), clearly revealing that these multicore NPs are 3D structures. A close look at Figure S1b and the video shows a spacing of about 0.8 nm between the petals of the multicore NPs. The XRD patterns of these NPs are shown in Figure S2 (SI). The positions and intensity of the Bragg reflections are typical of magnetite;42 however, considering the similarity in XRD patterns of magnetite (Fe3O4) and maghemite (γ-Fe2O3) NPs due to line broadening, this is a tentative assignment and is based on the reaction conditions (argon atmosphere) when oxidation should be prevented. 8545

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Figure 2. Experimental data (dots) and the scattering patterns computed from the ab initio model (lines) for 1 (a) and 2 (b). The appropriate interval was selected for the fitting in order to exclude any interparticle interactions. Insets: bottom left, the distance distribution function; top right, the typical bead model.

Scheme 2. Schematic Representation of Mesocrystal Multicore NP Formationa

a

The left image shows iron oxide seeds stabilized by dendrimers. The central image shows the formation of a single-core NP. The right image depicts the formation of a multicore NP.

nm−1 for 1 and 0.17 < s < 4 nm−1 for 2) were processed by GNOM41 to compute the distance distribution functions in the insets of Figure 2. The latter functions were back-transformed to extrapolate the scattering from the individual particles to zero angle, and low-resolution shapes were further reconstructed ab initio by DAMMIN.43 Both 1 and 2 reveal elongated shapes, and the typical models yield good fits to the experimental data with discrepancies of χ = 1.0 and 1.3, respectively (Figure 2). These values are even lower than the discrepancies for other good fits.44 However, 2 forms much larger individual rodlike particles than those of 1, with a difference in volume by a factor of about 20 (Table S1, SI). The evaluation of large aggregates (SI) showed that in both cases their fraction does not exceed 2 vol %. The shapes and sizes of smaller particles suggest that for both 1 and 2, macromolecules self-assemble, forming well-defined particles, but this self-assembly is different. In the case of the dendron

molecules, they self-assemble into conical structures with a base diameter of ∼3.0 nm and a height of about 4 nm. The latter matches approximately 2 dendron lengths. The dendrimers pack to form columnlike structures with a diameter of about 4 nm (which is slightly lower than a typical third-generation dendrimer size of 5 nm38) and a height of approximately 15 nm. Remarkably, the poly(phenylenepyridyl) dendrimers with fully phenylene peripheries45 exist as individual macromolecules and do not form any compact structures in solution. Thus, we assign self-assembly to well-defined structures to the pyridine groups in the 1 and 2 peripheries along with carboxyl focal groups in the case of 1. These self-assembled dendrimer particles were not observed by TEM due to dendrimer film formation on the TEM grid (Figure S4, SI). It is noteworthy that the total number of functional groups per macromolecule is much higher in 2 than in 1. Mechanism of Mesocrystal Formation. Apparently, for larger self-assembled structures of 2, the fraction of multicore 8546

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behavior deviating from that of superparamagnetic NPs of similar size. Magnetic measurements were carried out to assess the magnetic behavior of these iron oxide NPs. Figure 3a shows

particles is much higher (single core particles are nearly absent). It is noteworthy that in the absence of self-assembly for the second-generation dendrimer (Figure S5, SI) whose dendrimer particle size in solution is below 5 nm (SAXS data are not shown), iron oxide NPs are single cores with an extremely broad NP size distribution (Figure S5, SI). This again demonstrates the importance of large self-assembled dendron/ dendrimer structures for the formation of iron oxide multicore morphologies. As is demonstrated by SAXS in Figure S6 (SI), self-assembled particles of 1 and 2 are also observed in benzyl ether (reaction solvent), even if with some differences in morphology (Table S2, SI). As discussed in the literature on the formation of multicore crystals, the particles nucleate when the supersaturation concentration is reached, and then they ripen to a certain size and orient themselves in larger entities.26,27 If fusion occurs after oriented attachment, then actual single crystals are formed.32 If the fusion is prevented by separation with capping molecules or other reasons, then mesocrystals are formed.24,32 To elucidate the mechanism of multicore mesocrystal formation in our case, we carried out a synthesis of iron oxide NPs stabilized by dendrimer 2, taking specimens of the reaction solution in benzyl ether when the temperatures reached 250 and 270 °C (i.e., in the early stages of iron oxide NP formation) and after 15 min at the boiling temperature (285 °C). TEM images presented in Figure S7 (SI) show that at 250 °C small (1.9 ± 0.4 nm) quasi-spherical particles form which tend to aggregate by 270 °C. After 15 min of boiling, multicore morphology is already observed. These data suggest that seeds (small NPs) become oriented due to dendrons/ dendrimers and then grow into larger single cores due to fusion. The single cores are oriented in a single-crystallographic pattern (Scheme 2). The spacing observed between individual cores forming the multicore mesocrystal does not exceed 0.8 nm, while the actual size of 2 is ∼5 nm.38 This indicates that the dendrimer molecules attach to NPs not only via their peripheral functional groups as was reported earlier for rigid polyphenylene dendrimers with carboxyl periphery that led to rice-shaped mesocrystals containing copper oxide NPs.46 In the present case, the dendrimers must “embrace” the NP surface due to the adsorption of both interior and exterior pyridine groups (Scheme 1). Similar dendrimer adsorption (by the dendrimer interior) on the surface of CdS NPs was also discussed in our preceding papers.45,47 Although we observe the formation of self-assembled particles for 1 and 2 in both THF and benzyl ether, we cannot conclude that these compact particles formed by capping molecules play the role of a template in directing the oriented attachment as we cannot observe these structures in reaction solution at reactions temperatures. Nevertheless, our data unambiguously show the importance of dendron/dendrimer self-assembly. We believe the growing iron oxide NPs stabilized by dendrons/dendrimers tend to self-assemble, which leads to the oriented attachment of NPs and their fusion while the NP surface coverage (at high curvature) with dendrons/dendrimers is low. When the particle size reaches a critical value (a singlecore size) at which a sufficient dendron/dendrimer density is achieved on the iron oxide NP surface (due to lower curvature), the fusion is impeded, leading to a multicore mesocrystal (Scheme 2). Magnetic Properties. A close look at the TEM image in Figure 1b shows that NP2 tends to form rings and chains,48 a

Figure 3. ZFC-FC susceptibility curves as a function of temperature under a magnetic field of 50 Oe (a) and isothermal magnetization curves at 300 K (b) and 5 K (c) for NP1 and NP2.

zero-field-cooling (ZFC) and field-cooling (FC) susceptibility curves which allow one to obtain the blocking temperature, TB, the point where the two curves merge.49 For the NP1 sample, TB is about 260 K, which is typical for superparamagnetic iron oxide NPs of a comparable size.50 On the other hand, a TB of NP2 exceeds 300 K (Table S3, SI), indicating that at room temperature these NPs are ferrimagnetic-like. This is also in agreement with the nonzero coercive field, HC (Table S3), derived from the isothermal magnetization curves at the same temperature presented in Figure 3b. The appearance of the ferrimagnetic behavior at room temperature for multicore NP2 indicates that this magnetic material is different from superparamagnetic NPs with similar morphologies reported earlier.27,51,52 Ferrimagnetic behavior in similar structures was 8547

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Langmuir observed previously, only in much larger particles synthesized by the solvothermal process, with a 250 nm diameter and composed of 20−50 nm nanocrystals.28 The ferrimagnetism may arise either (i) from the composing ferrimagnetic petals or (ii) from cooperative interactions (dipolar or exchange)23 among superparamagnetic petals in the assembled multicore structures. However, because for both samples the size of the petals is about 10 nm, the first scenario is unlikely. As the coercive field at 300 K is higher for the NP1 sample (∼16 Oe) compared to that of NP2 (∼6 Oe) (Table S3), the somewhat different influence of the surface coordinating molecules is inferred. The higher coercivity of NP1 may be attributed to an enhanced magnetic anisotropy and uncompensated for surface spins due to its different surface coordination environment53,54 or/and stronger dipolar or exchange interpetal interactions.55−57 The HC, though, is comparable for NP1 and NP2 (Table S3, SI) when the thermal agitation of the magnetization is diminished at 5 K, inferring less influence due to surface coordination differences.

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REFERENCES

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CONCLUSIONS We demonstrated for the first time a guiding principle in the formation of multicore iron oxide mesocrystals. This entails the tendency of multifunctional capping molecules (poly(phenylenepyridyl) dendron/dendrimer molecules in this case) to self-assemble in solution, which allows the oriented attachment of individual NPs that is accompanied by fusion until the critical size of a single core is reached, after which the fusion is hindered. The small dendron particle size in solution for 1 or the absence of self-assembly for the second-generation dendrimer is detrimental to multicore morphology. For dendron/dendrimer self-assembly, the presence of functional groups on the dendron/dendrimer periphery and the total number of functional groups per macromolecule are crucial. The multicore shape and mesocrystal nature of NP2 lead to their unique magnetic properties: NPs exhibit a near-ambient blocking temperature, allowing subtle control over magnetic properties using a minor temperature change. It is noteworthy that the multicore mesocrystal shell contains multiple pyridine groups permitting complexation with practically any metal compounds (anionic, cationic, or neutral), allowing an abundance of new metamaterials. An example of a magnetically recoverable catalyst based on single-core iron oxide NPs stabilized by second-generation dicarboxylate dendrons is reported in our preceding paper,37 but opportunities for multicore mesocrystals appear to be more promising. ASSOCIATED CONTENT

S Supporting Information *

TEM images, XRD and SAXS data, and magnetic characteristics. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

The financial support of this work was provided in part by funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement no. CPIP 246095, NSF grant CHE-1048613, the Ministry of Education and Science of Russia, and the Russian Foundation for Basic Research under grant nos. 14-03-00876 and 14-0331669.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

The manuscript was written through the contributions of all authors. Notes

The authors declare no competing financial interest. 8548

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