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 self- assembled nanostructured materials in recent years. 3 Neverthe- less, 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 poly- electrolyte 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 low- field 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 prepa- ration 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 nano- particles 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-meth- ylpyridinium iodide) as the template. 8 However, control over the emergent magnetic properties of the nanocluster suspensions 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, com- mercially 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, FeCl 3 · 6H 2 O (1.1 g; 4 mmol) and FeCl 2 · 4H 2 O (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 * To whom correspondence should be addressed. E-mail: igounko@ tcd.ie (Y.K.G.); dermot.brougham@dcu.ie (D.F.B.). † Trinity College. ‡ School of Chemical Sciences. TABLE 1: Reagent Ratios Used in This Study sample [Fe] (×10 -3 M) [PSSS] (×10 -5 M) Fe/PSSS polymer (∼monomer) Group I A 40 40.8 97.1(0.286) B 30 35.7 85.0(0.250) C 30 17.8 169(0.500) Group II D 20 1.90 1019(3.00) E 30 1.43 2133(6.25) F 25 1.22 2039(6.00) G 20 0.92 2039(6.00) Group III H 40 4.08 985.4(2.90) I 40 2.04 1937(5.70) J 40 0.82 4927(14.5) K 30 0.71 4248(12.5) 13324 10.1021/jp805519n CCC: $40.75  2008 American Chemical Society Published on Web 08/12/2008 2008, 112, 13324–13327