13324
2008, 112, 13324–13327 Published on Web 08/12/2008
Poly(sodium-4-styrene)sulfonate-Iron Oxide Nanocomposite Dispersions with Controlled Magnetic Resonance Properties Serena A. Corr,† Yurii K. Gun’ko,*,† Renata Tekoriute,† Carla J. Meledandri,‡ and Dermot F. Brougham*,‡ School of Chemistry and CRANN Institute, Trinity College, UniVersity of Dublin, Dublin 2, Ireland, and National Institute for Cellular Biotechnology, School of Chemical Sciences, Dublin City UniVersity, Dublin 9, Ireland ReceiVed: June 23, 2008; ReVised Manuscript ReceiVed: July 29, 2008
We report synthesis and studies of magnetic suspensions with tunable low-field relaxivities. Using a one-step procedure, we have prepared magnetic fluids composed of polyelectrolyte stabilized magnetite nanoparticles. We have demonstrated the effect of varying the synthetic conditions, in particular, the iron and PSSS polyelectrolyte content for a fixed polymer chain length, on the emergent magnetic resonance properties of the iron oxide nanoparticle suspensions. The new magnetic fluids have a potential for in vivo MRI diagnostics. There are many reported methods for the preparation of magnetic nanoparticles of iron oxide.1 Water-stable magnetic nanoparticle suspensions and corresponding magnetic fluids have wide-ranging biomedical applications as hyperthermic mediators, MRI contrast agents, and drug delivery systems.2 Biological polyelectrolyte molecules (e.g., DNA) have been demonstrated to serve as excellent templates for the preparation of selfassembled nanostructured materials in recent years.3 Nevertheless, controlling the formation and stability of the suspensions is difficult as the complex microscopic events of nucleation and growth proceed far from equilibrium and these processes are strongly influenced by the concentration and chemi- and physisorption characteristics, including the charge of the polyelectrolyte and the ions. Previously, we reported the formation of ordered nanowires of magnetite nanoparticles on the backbone of single-stranded herring DNA and their alignment in a magnetic field.4 These nanowire-like assemblies also provided us with a stable magnetic fluid (see Scheme 1), which gave unprecedented high relaxivity at low field. We believe magnetic fluids of this type could have an important application as lowfield MRI contrast agents. Polymers are frequently used as stabilizers and cross-linking agents in the preparation of magnetic nanoparticle assemblies.5 For example, controlled clustering of magnetic particles using cationic-neutral block copolymers was reported for the preparation of magnetic fluids with improved contrast for MRI.6 Random copolymers of acrylic acid, styrenesulfonic acid, and vinylsulfonic acid have been used to stabilize magnetic nanoparticles and exert some control over the size of the resulting nanoclusters.7 Linear chains of magnetite nanoparticles have also been prepared by using magnetic-field-induced self-assembly of citrate stabilized magnetite, by using poly(2-vinyl-N-methylpyridinium iodide) as the template.8 However, control over the emergent magnetic properties of the nanocluster suspensions * To whom correspondence should be addressed. E-mail: igounko@ tcd.ie (Y.K.G.);
[email protected] (D.F.B.). † Trinity College. ‡ School of Chemical Sciences.
10.1021/jp805519n CCC: $40.75
TABLE 1: Reagent Ratios Used in This Study [PSSS] (×10-5M)
Fe/PSSS polymer (∼monomer)
sample
[Fe] (×10-3 M)
A B C
40 30 30
Group I 40.8 35.7 17.8
97.1(0.286) 85.0(0.250) 169(0.500)
D E F G
20 30 25 20
Group II 1.90 1.43 1.22 0.92
1019(3.00) 2133(6.25) 2039(6.00) 2039(6.00)
H I J K
40 40 40 30
Group III 4.08 2.04 0.82 0.71
985.4(2.90) 1937(5.70) 4927(14.5) 4248(12.5)
continues to be crucial for the further development and MRI applications of nanoparticle-based magnetic fluids. With this in mind, we have prepared a new family of contrast agents using, instead of the biopolyelectrolyte DNA, commercially available polyelectrolytes to act as both nanocomposite assembly directors and water-stable surfactants. We recently communicated the synthesis and in vivo MRI studies of poly(sodium-4-styrene) sulfonate (PSSS) stabilized magnetic nanocomposite suspensions.9 We now report, for the first time, the effect of varying the synthetic conditions, in particular, the iron and PSSS polyelectrolyte content for a fixed polymer chain length, on the emergent magnetic resonance properties of the iron oxide nanoparticle suspensions. A predetermined mass of polyelectrolyte was dissolved in Millipore water (10 mL) to produce a library of magnetic fluids. Briefly, FeCl3 · 6H2O (1.1 g; 4 mmol) and FeCl2 · 4H2O (0.4 g; 2 mmol) were dissolved in deoxygenated Millipore water (100 mL). Aliquots of this solution were added to polyelectrolyte solutions according to Table 1. The solution was allowed to stir for 15 min before addition of ammonia solution. The 2008 American Chemical Society
Letters
J. Phys. Chem. C, Vol. 112, No. 35, 2008 13325
Figure 1. TEM images of samples (a) A, (b) G, and (c) J from Groups I, II, and III, respectively. The scale bar corresponds to 20 nm.
TABLE 2: Properties of the Stable Aqueous Suspensionsa r1plat (s-1 mM-1)
νmax (MHz)
∆νc (MHz)
Group I 11.7 14.7 14.7
7.9 6.9 6.2
28 22
10.2 ( 1.2 10.1 ( 2.2 9.4 ( 1.6 7.9 ( 1.2
Group II 31.4 23.5 10.8 15.3
2.4 2.4 3.0 3.4
12 16 16 16
11.2 ( 2.0 10.3 ( 1.7 9.8 ( 1.7 9.9 ( 1.0
Group III 58.7 47.1 66.4 42.7
1.5 1.9 1.2 1.9
9 8 4 6
sample
Dcoreb (nm)
A B C
6.5 ( 1.0 7.1 ( 1.0 7.0 ( 1.1
D E F G H I J K
a The magnetic nanoparticle core size as estimated from TEM analysis, Dcore; the r1 value at low frequency, r1plat; the frequency at which the r1 maximum occurs, νmax; and the width at half-height of the r1 maximum, ∆ν. b Estimated from TEM. c Estimated width at half-height of the r1 maximum.
resulting precipitate was washed with Millipore water (5 × 10 mL). The final, fifth washing was a stable aqueous suspension which has been characterized by TEM (see Figure 1 for examples) and NMRD analysis. The black precipitate was dried under vacuum and analyzed by X-ray diffraction, IR, and Raman spectroscopy. XRD patterns and Raman spectra collected for all samples confirm the formation of magnetite nanoparticles, with peaks at 680 and 496 cm-1 in the Raman spectra, in good agreement with the literature values for magnetite.10 For all PSSS-functionalized nanoparticles, IR stretches for aromatic C-C bonds, CH2 groups, sulfonate bonds, and aromatic C-H bonds are observed. A broad feature at 580 cm-1 is attributed to the Fe-O stretch, while a shoulder at 631 cm-1 is probably the Fe-O-S bend. TEM images were analyzed to determine the primary particle size; see Table 2. For all of the Group I preparations (low Fe/PSSS ratio), significantly smaller (av. 6.9 nm) primary nanoparticle cores were formed. On the other hand, when the ratio was higher, Groups II and III, the cores were larger (av. 9.9 nm). This is presumably because of an increased number of nucleation sites along the polymer for Group I samples, for which the PSSS concentration is higher, and hence a growth phase that is restricted by the availability of iron. In addition, the higher concentration of polyelectrolyte in the samples may result in better polymer coating of the initial nanoparticles, limiting further growth. There was no significant difference, or trend, in the hydrodynamic size as measured using photon correlation spectroscopy between, or within, the three groups. In all cases, the suspensions had Dhyd in the range of 100-200 nm, and for all of the suspensions reported, the polydispersity indices were low, 20 MHz); the low-field plateaus occur at low r1 values (11-15 s-1 mM-1); see Figure 2. As the NMRD profiles are very similar for this group, fewer suspensions were prepared. For Group II, the r1 maxima are at lower frequencies (2-3.5 MHz) and are less broad (12-16 MHz); the low-field plateaus are at higher r1 values (15-32 s-1 mM-1) and are more variable from suspension to suspension; see Figure 3a. For Group III, the r1 maxima are at the lowest frequency (1-2 MHz) and are the narrowest observed (4-9 MHz); the low-field plateaus are at elevated r1 values (>40 s-1 mM-1), Figure 3b, and, as was the case for Group II, but not Group I, are more variable from suspension to suspension. For all three groups, the ∆ν values show that, as expected, the relaxivity maxima cover almost 1.5 orders of magnitude in frequency (MHz scale). The NMRD data conform well to the theory developed by Muller et al.11 for water relaxation by superparamagnetic nanoparticles. The theory is strictly applicable to suspensions of monodisperse, sub-20 nm particles, with uniaxial magnetocrystalline symmetry. The parameters of the model include the core size, Dcore, the saturation magnetization, Ms, the magnetic
13326 J. Phys. Chem. C, Vol. 112, No. 35, 2008
Figure 3. (a) NMRD profiles for the Group II samples D (O), E (0), F (∆), and G()) and those for the Group III samples H (9), I(2), J (1), and K (b). The solid curves are simulations using the standard superparamagnetic theory,11 with Dcore ) 16 nm. (b) The dependence of the low-frequency relaxivity on the magnetic anisotropy energy. The values are taken from the simulations presented in Figures 2 and 3a for the Group I (0), Group II (O), and Group III (∆) samples.
SCHEME 1: Preparation of the PSSS-Stabilized Magnetite Nanoparticlesa
a Solutions of the iron salts are mixed with the polyelectrolyte and, upon addition of ammonia and following subsequent washings, afford a stable magnetic fluid.
anisotropy energy, ∆Eanis (expressed in frequency units), and the Ne´el correlation time, τN. Given the assumptions of the model and the large number of parameters required, further extension of the theory to account for size-disperse suspensions is not appropriate. For the samples presented, the profiles can be reasonably simulated with fixed core diameters of 12 nm for Group I and 16 nm for Groups II and III, which suggests that the samples are sufficiently monodisperse in this case for the theory to be validly applied. Small systematic overestimations of Dcore from SPM simulations have been observed before.12 This effect and the deviation of the simulations from the data on the low-frequency side of
Letters the r1 maximum probably arise from the approximate way in which the magnetic crystals are treated in the theory; it is assumed that (i) they have uniaxial magnetocrystalline symmetry and (ii) the motion of the moments can be well-represented by a single correlation time, τN, irrespective of the electronic spin energy level. These limitations do not invalidate the analysis of the magnetic properties derived from the simulations; the NMR analysis reproduces the variation in particle size, observed by TEM, between the low- and high-ratio preparations. The values obtained for τN range from 10 to 16 ns, as expected.11 Finally, we find Ms in the range of 30-50 emu · g-1, which is the expected range for Fe3O4 nanoparticles of this size.13 The quantitative agreement of these parameters validates our interpretation that the increased low-field relaxivity arises from increased magnetic anisotropy energy for the Group II and, in particular, the Group III suspensions; see Figure 3b. Increased magnetocrystalline anisotropy can arise from the structure of the crystals and/or from interactions between crystals within the polyelectrolyte-stabilized clusters. Given the uniformity of the size and magnetization of the crystals, and the fact that anisotropy energies of
