Self-assembly of superparamagnetic nanoparticles
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Self-assembly of superparamagnetic nanoparticles Ningzhong Bao State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China
Arunava Guptaa) Center for Materials for Information Technology, University of Alabama, Tuscaloosa, Alabama 35487 (Received 30 May 2010; accepted 27 July 2010)
Ordered nanoparticle assemblies can exhibit collective properties that are quite different from those displayed by the individual nanoparticles or their bulk counterpart. This paper reviews recent progress on the assembly of superparamagnetic nanoparticles, with emphasis on different strategies for their chemical fabrication with highly ordered nanostructures as well as their novel properties. Prospective applications of superparamagnetic nanoparticles in the fields of photonic crystals, biomedicine, and biology are also discussed.
I. INTRODUCTION
An area of considerable current interest in nanoparticle research is the assembly of inorganic nanoparticles to form ordered structures that can display novel collective behavior. This includes distinctive mechanical, magnetic, electronic, and optical properties.1 Magnetic nanoparticles are of considerable interest both because of fundamental scientific interest and for applications in diverse areas such as high-density magnetic recording, spintronics, catalysis, separation, biomedicine, and biotechnology.2–7 As the size of the nanoparticle decreases below a critical value (typically around 10–30 nm), which is dependent upon the material properties, the individual nanoparticles behave as a single magnetic domain and can exhibit superparamagnetic behavior. They will rapidly respond to an applied magnetic field but exhibit negligible residual magnetism and coercivity. These features make superparamagnetic nanoparticles very attractive for a wide range of applications since agglomeration resulting from strong magnetic interaction can be avoided. However, a low magnetization per particle caused by their small size limits their usage since they cannot be effectively separated and controlled by means of moderate magnetic fields. Increasing the particle size increases the magnetic moment, but at the expense of inducing a superparamagnetic-ferromagnetic transition, and thus the nanoparticles are not readily dispersible in solution.5,6 Much effort has therefore been focused on the preparation of large-size superparamagnetic composite particles, such as core-shell structures, nanoparticle-embedded polymer micelles, and
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2010.25 J. Mater. Res., Vol. 26, No. 2, Jan 28, 2011
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others, by encapsulating superparamagnetic nanoparticles in a nonmagnetic matrix.5,6 Self-assembly for forming large complex superparamagnetic (SP) nanoparticle (NP) structures provides an attractive strategy to controllably increase magnetization while retaining SP characteristics. In
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