Fe3O4 Core–Shell Nanocubes as a Promising

Faculty of Science, Physics Department, King Abdulaziz University, Jeddah, Saudi Arabia ... It includes NaBH4 reduction of FeCl3 in an ethylene glycol...
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This article was retracted on June 19, 2014 (Langmuir 2014, DOI: 10.1021/la502272t).

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Development of Fe/Fe3O4 Core−Shell Nanocubes as a Promising Magnetic Resonance Imaging Contrast Agent Waleed E. Mahmoud,*,†,‡ Lyudmila M. Bronstein,*,†,§ Faten Al-Hazmi,† Fowzia Al-Noaiser,† and A. A. Al-Ghamdi† †

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Faculty of Science, Physics Department, King Abdulaziz University, Jeddah, Saudi Arabia Faculty of Science, Physics Department, Suez Canal University, Ismailia, Egypt § Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States ‡

S Supporting Information *

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ABSTRACT: Here, we report the synthesis, characterization, and properties of Fe/Fe3O4 core−shell nanocubes prepared via a simple route. It includes NaBH4 reduction of FeCl3 in an ethylene glycol solution in the presence of 2-mercaptopropionic acid (surfactant) and trisodium citrate (cosurfactant) followed by surface oxidation with trimethylamine N-oxide. The morphology and structure of Fe/Fe3O4 core−shell nanocubes were characterized using transmission electron microscopy (TEM), high-resolution TEM, selected area electron diffraction, X-ray powder diffraction, and X-ray photoelectron spectroscopy. All of the methods confirm a Fe/Fe3O4 core−shell structure of nanocubes. Magnetic measurements revealed that the Fe/Fe3O4 core/shell nanocubes are superparamagnetic at 300 K with a saturation magnetization of 129 emu/g. The T2 weighted imaging and the T2 relaxation time showed high MRI contrast and sensitivity, making these nanocubes viable candidates as enhanced MRI contrast agents.

1. INTRODUCTION Zerovalent iron nanoparticles (NPs) attracted considerable attention due to their high saturation magnetization (218 emu/ g in bulk) and the fact that they give a soft magnetic material,1 although below 10−20 nm Fe(0) NPs are superparamagnetic. Iron nanoparticles showed high relaxivities in hyperthermia,2 promising catalytic properties in a number of reactions,1 and were even considered as enhanced MRI contrast agents.1 Spherical and cubic monodisperse Fe NPs have been synthesized using thermal decomposition of iron pentacarbonyl1,3,4 or reductive decomposition of an organometallic compound.5,6 In recent years, the interest in nonspherical particles, cubic in particular, is increasing because of different magnetic signatures of NPs of different shapes.7−9 Monodisperse zerovalent cubic iron NPs have been successfully synthesized by reductive decomposition of an organometallic precursor, Fe[N(SiMe3)2]2, with H2 in the presence of a long-chain acid and a long-chain amine.6 These Fe particles display magnetic properties that match those of bulk iron, but the need for the organometallic compound to be synthesized makes this method rather complex. The more robust procedure using reduction of ferrous sulfate with hydrazine in ethylenediamine was used for the synthesis of iron nanocubes 200−400 nm in diameter,10 but these particles are too large for a number of biorelated applications. The surface-driven formation of Fe nanocubes was recently reported, but it cannot be realized in solution.11 © 2013 American Chemical Society

Despite many possible applications of Fe(0) NPs, for maintaining Fe NPs in their zerovalent state, their applications should be limited to those where water and oxygen are completely excluded, or NPs are retained in a reducing atmosphere (in the case of the Fischer−Tropsch synthesis).1 To counteract this limitation, core−shell nanoparticles have been fabricated to prevent oxygen and water penetration. Among them, silica-coated iron nanocubes have been prepared from ferrous sulfate heptahydrate in ethylenediamine at elevated temperatures followed by silica coating using the Stöber method.12 However, it resulted in very large (500−1000 nm) polydisperse particles.12 Core−shell iron/iron oxide NPs were reported in ref 13 using a procedure similar to that described in ref 6 (reductive decomposition of Fe[N(SiMe3)2]2) after exposure of NPs to air. Although magnetization of these nanocubes was significantly lower than that of bulk iron, these particles were stable in air, but magnetic properties were only characterized using ferromagnetic resonance. Fe14 and Fe/Fe3O4 core/shell15 particles were also formed by thermal decomposition of iron oleate using sodium oleate as a surfactant, while Fe/Fe3O4 core/shell particles were prepared as a part of a composite with graphene.4 Received: August 15, 2013 Revised: September 29, 2013 Published: September 30, 2013

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Figure 1. TEM (a) and HRTEM (b) images, and SAED (c) and XRD (d) of Fe/Fe3O4 nanocubes.

was separated by centrifugation for 12 min, washed with ethyleneglycol and hexane, and redispersed in hexane or water. 2.4. Characterization. Transmission electron microscopy (TEM) with selected area electron diffraction (SAED) was carried out on a JEOL-JEM 2010 (75 kV). High-resolution TEM (HRTEM) images were obtained using a JEOL-JEM 2012F (200 kV). The etching of the TEM grid has been used to obtain clear images of core−shell NPs. A Rigaku-Ultima-IV diffractometer with Cu Kα radiation (λ = 1.5418 Å) was utilized for assessing X-ray powder diffraction patterns of NPs. X-ray photoelectron spectroscopy (XPS) was carried out with a VGESCA-250-iXL instrument. The measurements were performed using a monochromatic Al Kα radiation with energy of 1487.83 eV. The deconvolution was carried out using the best fits to the experimental curve. A Malvern Zetasizer Nano ZS instrument (4 mW, 633 nm He−Ne laser) with attached MPT-2 Autotitrator was employed for the dynamic light scattering (DLS) and ζ-potential measurements. The hydrodynamic diameter and ζ-potential were measured as a function of pH at 25 ± 1 °C by using KNO3 (0.01 M) as a buffer and KOH and HNO3 for the pH adjustment. A Fourier transform Perkin-Elmer spectrometer was used to record FTIR spectra in the wavenumber range 4000−500 cm−1. The samples were prepared by dropping the iron/iron oxide hexane solution on KBr discs. A Lakeshore 7404 high sensitivity vibrating sample magnetometer (VSM) was used to conduct measurements of magnetic properties at room temperature. Field cooling (FC) and zero-field cooling (ZFC) curves were obtained using SQUID (Quantum Design, MPMS-XL-5). Magnetic resonance imaging (MRI) measurements were executed with a 1.5 T MRI scanner (Siemens Healthcare). The aqueous solutions of the 2-MPA coated Fe/Fe3O4 core−shell nanocubes of various concentrations were placed in the iso-center of the magnet for MRI scans. The transverse relaxation time T2 of the NPs was measured by using a multiecho spin echo sequence with a repetition time TR = 1000 ms at various echo times TE = 5−150 ms with an increment of 5 ms. The T2 relaxation time for each concentration was estimated via fitting the decay curve by using the nonlinear exponential relation

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In this Article, we report a facile method for fabrication of core−shell Fe/Fe3O4 nanocubes in ambient conditions first by reduction of FeCl3 using NaBH4 followed by controlled oxidation with trimethylamine N-oxide. The iron oxide shells offered a reliable protection to the metallic iron cores so the NPs are stable under air and in aqueous environment. These core/shell nanostructures showed excellent MRI contrast and high sensitivity, making these nanocubes promising as enhanced MRI contrast agents.

2. EXPERIMENTAL SECTION

2.1. Materials. Trisodium citrate, Na3C6H5O7 (99%), anhydrous ethyleneglycol, C2H6O2, (99.8%), anhydrous iron(III) chloride, FeCl3 (98%), sodium borohydride, NaBH4 (98%), trimethylamine N-oxide, and C3H9NO·2H2O (99%) were purchased from Sigma-Aldrich and used as received. 2-Mercaptopropionic acid (2-MPA), C3H6O2S (95%), was purchased from Fisher scientific and used without purification. 2.2. Synthesis of Fe NPs. The solution of trisodium citrate in anhydrous ethyleneglycol (4 mL, 30 mM) was mixed with 4 mL of 2mercaptoproponic acid and bubbled with ultrapure nitrogen (99.999% purity) for 30 min at room temperature. Next, FeCl3 (0.11 mmol) dissolved in 20 mL of ethyleneglycol was added and kept stirring under nitrogen for 20 min at room temperature. After that, 120 μL of the NaBH4 solution in anhydrous ethanol (100 mM) was injected, and stirring continued until the reaction mixture color turned black (approximately after 25 min). The precipitate was collected by centrifugation, washed with ethyleneglycol, and then redispersed in hexane and purged with nitrogen. 2.3. Controlled Oxidation of As-Synthesized Fe NPs To Form Fe/Fe3O4 NPs. Trimethylamine N-oxide (CH3)3NO (0.2 mmol) was added to 15 mL of 2-mercaptopropionic acid and stirred for 20 min in N2 flow at room temperature. After that, the temperature was increased to 80 °C for 10 min, and then iron NPs (25 mg in 3 mL of hexane) were added via a gastight syringe (without exposure to air) at 80 °C and heated for 30 min under nitrogen bubbling. The blackbrown solution was left to cool to room temperature. The precipitate

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Figure 2. FTIR spectra of 2-MPA (a) and nanocubes after oxidation (b). Inset shows adsorption of 2-MPA on the nanocube surface via carbonyl groups. I(TE) = Io e(−TE/T2), where TE is the echo time and I(TE) is the MRI signal intensity at the TE used.

core−shell nanocubes coated with 2-MPA, the band at 1710 cm−1 attributed to CO of the carboxyl group is absent, while a new band appears at 582 cm−1 corresponding to the Fe−O vibrations. There are also new bands at 1545 and 1426 cm−1 assigned to asymmetric and symmetric vibrations of a carboxylate group (COO−). The band at 1050 cm−1 is assigned to the C−O stretching. These data indicate that 2-MPA adsorbs on the nanocube surface via the carboxyl group, leaving the SH group (hydrophilic) and CH3 group (hydrophobic) in the NP exterior. Similar observations were reported by Taratula et al. for ZnO coated with 3-MPA21 and by Hatakeyama et al. for iron oxide NPs coated with mercaptosuccinic acid.22 The mean hydrodynamic diameter of the Fe/Fe3O4 core− shell nanocubes obtained from the DLS volume distributions in the KNO3 (0.01 M) aqueous solution remains unchanged in the pH range from 4 to 14, revealing the absence of aggregation at any pH (Figure S3, Supporting Information). It measures 26.9 ± 2.1 nm, which matches fairly well the size determined by TEM (∼20 nm), taking into account the ligand and solvent shells around the NPs. The ζ-potential data presented in Figure S3 (Supporting Information) indicate an isoelectric point (IEP) at pH 2.4. For pH < 2, the ζ-potential is approximately 22 mV, while for pH > 4, the ζ-potential is negative and reaches about −35 mV at pH 7, thus allowing stability of NPs in physiological conditions. To better mimic the physiological conditions, we tested the stability of the 2-MPA coated magnetic nanocubes in phosphate buffered saline (PBS) containing 10% (w/v) of bovine serum albumin (BSA) frequently employed in biologically relevant conditions.23,24 DLS was used to measure hydrodynamic diameters of these nanocubes as a function of time at 37 °C (Figure S4, Supporting Information). These data demonstrate that NP size practically does not change (Dh = 27.1 nm) and NPs do not aggregate in the PBS buffer. Figure 1b shows a representative HRTEM image of four nanocubes. One can observe a cubic core with a mean size

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3. RESULTS AND DISCUSSION 3.1. Nanoparticle Characterization. As-synthesized NPs (before oxidation with trimethylamine N-oxide) are largely cubes with a mean diameter of 20.0 nm and a standard deviation of 14% (Figure S1, Supporting Information). Trisodium citrate acts both as a cosurfactant and to control the final NP size and shape.16 The XRD profile of these NPs is presented in Figure S2 (Supporting Information). It indicates that despite that reduction of FeCl3 with sodium borohydride was carried out in pure nitrogen, partial oxidation occurred, which can be due to a brief contact with air during a sample transfer. Considering that pure Fe(0) NPs are highly reactive, this partial oxidation is not surprising. However, to ensure a stable oxide shell formation around the Fe(0) core, we carried out a controlled oxidation by trimethylamine N-oxide.3 After oxidation, the NPs preserve their cubic shape and size: the NP diameter is 19.9 nm with a standard deviation of 16% (Figure 1a). It is noteworthy that these nanocubes are well dispersed and stable both in hexane and in water due to an amphiphilic character of mercaptopropionic acid as a capping molecule similar to dimercaptosuccinic acid.17 It can adsorb by a thiol or a carboxyl group, exposing CH3 and a remaining hydrophilic group (thiol or carboxyl) in the exterior and allowing an amphiphilic environment.18−20 FTIR spectra of 2MPA and nanocubes after oxidation are presented in Figure 2. The FTIR spectrum of 2-MPA shows a broad band at 3251 cm−1 corresponding to the O−H vibrations, the band at 2933 cm−1 corresponding to C−H of CH3, the two peaks at 2671 and 2576 cm−1, which are ascribed to S−H, the band at 1710 cm−1 due to CO, the two bands at 1464 and 1285 cm−1 assigned to O−H in-plane and C−O−H stretching, respectively, and the band at 937 cm−1 attributed to O−H out-ofplane wagging. In the FTIR spectrum of the iron/iron oxide

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Figure 3. Survey XPS spectrum (a) and high-resolution Fe 2p XPS spectrum (b) of the Fe/Fe3O4 nanocubes.

Figure 4. Hysteresis loop (a) and ZFC-FC curves (b) for the Fe/Fe3O4 nanocubes.

about 9.7 ± 0.3 nm and a shell with an average thickness of 4.9 ± 0.2 nm. There are two lattice distances: 0.14 nm, which we assign to the separation between (110) planes in Fe(0),1 and 0.25 nm, which corresponds to the separation between (311) planes in Fe3O4 or γ-Fe2O3.25−27 The locations of these lattice distances clearly show the core−shell structure. The SAED diffraction pattern of the Fe/Fe3O4 core−shell nanocubes is shown in Figure 1c. The diffraction pattern rings (110), (200), and (211) correspond to the body centered cubic (bcc) structure of α-Fe, and the rings (311) and (400) are attributed to the face centered cubic (fcc) structure of Fe3O4.28,29 Figure 1d depicts a diffraction pattern of the Fe/Fe3O4 core/ shell nanocubes. The signals at the 2θ of 45.03°, 65.12°, and 82.64° are assigned to the reflection planes (110), (200), and (211), respectively, from the bcc metallic iron (ICDS card no. 87-0722, space group Im3m (229)), with a lattice constant a = 0.2860 nm. The signals at angles 2θ = 30.12°, 35.48°, 43.72°, 57.02°, and 74.89° are assigned to the reflection planes (220), (311), (400), (511), and (611), respectively, from the fcc Fe3O4 (ICDS card no. 88-0866, space group Fd3m (227)), with a lattice constant a = 0.8384 nm. However, considering the similarity of the XRD profiles of magnetite (Fe3O4) and maghemite (γ-Fe2O3) NPs due to line broadening,30,31 this is a tentative assignment. Because oxidation was carried out in mild conditions, we assume that the shell is composed of magnetite.

Thus, the XRD data match well those from SAED and HRTEM, confirming the Fe/FexOy structure. Figure 3a shows the survey XPS spectrum of the Fe/Fe3O4 nanocubes. The spectrum shows signals from B, Na, O, Fe, and C. Carbon is normally due to stabilizing molecules (i.e., mercaptopropionic acid) on the NP surface. The boron and sodium signals may come from contamination with sodium borohydride during reduction. Oxygen may be due to both the ligand and the iron oxide. Figure 3b exhibits a high-resolution Fe 2p XPS spectrum in the 702−717 eV region. It shows that the Fe/Fe3O4 nanocubes contain both zerovalent Fe and Fe(II)/Fe(III) atoms.32 The deconvolution of the spectrum allows us to determine the surface composition of nanocubes as 33 at. % of Fe(0) and 67 at. % of iron oxide, revealing a substantial iron core, which is detectible by XPS through the iron oxide shell without removal of the latter.32 Considering that the electron sampling depth for Fe(0) is about 5 nm (determined by a mean free path of photoelectrons)33,34 while in iron oxides it is even higher, at the nanocube size of 20 nm we assume that the surface composition is close to the volume composition. 3.2. Magnetic Properties of Nanocubes. Figure 4a shows a hysteresis loop of the core−shell Fe/Fe3O4 nanocubes at 300 K. The lack of coercivity and the character of the curve clearly show that the nanocubes are superparamagnetic with a

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contrast agents.8 This reveals significant enhancement of the MRI response (by at least a factor of 25) for the Fe/Fe3O4 core−shell nanocubes due to the Fe(0) core. The transverse relaxivity r2 was determined from the slope of the dependence shown in Figure 5a and is equal to 350 mM−1 s−1. To the best of our knowledge, this relaxivity surpasses the values presented in recent publications (Figure 5b). For example, Cheng et al. reported transverse relaxivity of 98 mM−1 s−1 for polysiloxane diblock copolymer coated iron oxide NPs.36 Khurshid et al. have developed Fe/γ-Fe2O3 core−shell nanoparticles coated by the similar block copolymer with transverse relaxivity of 300 mM−1 s−1.37 Hu et al. have achieved a transverse relaxivity of 292.8 mM−1 s−1 for polyphosphazene coated Fe3O4 nanoparticles.38 In comparison, the core−shell Fe/Fe3O4 nanocubes developed in this work offer a stronger T2 shortening effect with the better transverse relaxivity. This finding suggests the potential use of the Fe/Fe3O4 core−shell nanocubes synthesized as an excellent MRI contrast agent.

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saturation magnetization of 129 emu/g. This value is much higher than the saturation magnetization of bulk magnetite (MsFe3O4 = 82 emu/g),35 but noticeably lower than that of bulk iron (MsFe = 218 emu/g).1 If we take into account the amounts of Fe(0) and iron oxide (from XPS), the calculated saturation magnetization using bulk values for the individual components can be estimated by the following relation Ms = 0.33 × MsFe + 0.67 × MsFe3O4 = 127 emu/g, which is very close to the experimental value. Figure 4b depicts the FC and ZFC curves for the Fe/Fe3O4 core−shell nanocubes in the temperature range from 7 to 300 K at 100 Oe. The blocking temperature (Tb) is about 230 K. To evaluate the usefulness of magnetic Fe/Fe3O4 nanocubes as MRI contrast agents, MR relaxivity was investigated for aqueous solutions of the Fe/Fe3O4 nanocubes. Inverse relaxation time, 1/T2, collected at a constant frequency of 35 MHz, was plotted as a function of core−shell nanocube concentration (Figure 5a). The linear behavior is verified in the

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4. CONCLUSION We developed Fe/Fe3O4 core−shell nanocubes using a robust procedure consisting of NaBH4 reduction of FeCl3 in the presence of 2-mercaptopropionic acid and trisodium citrate followed by mild oxidation with trimethylamine N-oxide. Size, shape, and structure of nanocubes were determined by a combination of TEM, HRTEM, XRD, SAED, and XPS. HRTEM clearly shows that nanocubes contain an Fe(0) core and an iron oxide shell. XPS demonstrates that the Fe(0) fraction is 33 at. %. Magnetic measurements show that the Fe/ Fe3O4 core/shell nanocubes are superparamagnetic at 300 K with a saturation magnetization of 129 emu/g, the value that matches well the composition found from XPS. The T2weighted imaging revealed high sensitivity of these nanocubes as a MRI contrast agent. The transverse relaxivity r2 determined from the slope of the 1/T2 dependence is equal 350 mM−1 s−1, which surpasses the values presented in other works, making these nanocubes highly promising enhanced MRI contrast agents.

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ASSOCIATED CONTENT

* Supporting Information S

TEM image and XRD pattern of as-prepared nanoparticles and ζ-potentials and DLS data of Fe/Fe3O4 nanocubes. This material is available free of charge via the Internet at http:// pubs.acs.org.

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Figure 5. (a) Dependence of inverse relaxation times on concentration for the core−shell Fe/Fe3O4 nanocubes. Inset shows T2-weighted images at different concentrations. (b) Bar graph showing relaxivities of our sample and those presented in recent publications.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

concentration range from 0.001 to 0.1 mg/mL. T2-weighted images for three samples at different magnetic nanocube concentrations from 0.001 to 0.1 mg/mL are shown in the inset in Figure 5a. In a typical MR image, the T2 MR signal becomes darker with increasing iron oxide NP concentration. The inset shows that already at the concentration 0.001 mg/mL, which corresponds to 0.0058 mM (taking into account the Fe(0) and Fe3O4 contents, see above), a good contrast is provided. For comparison, in our preceding paper where 18 nm FeO/Fe3O4 nanocubes were investigated as MRI contrast agents, we reported sufficient sensitivity at 0.14 mM of Fe, which was 10 times better than that of commercial iron oxide-based MRI

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant no. (GR-33-7). We, therefore, acknowledge with thanks DSR technical and financial support.

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REFERENCES

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