Biological Routes to Metal Alloy Ferromagnetic Nanostructures

Abstract

Magnetic nanoparticles have potential applications in high-density memory devices, but their complicated synthesis often requires high temperatures, expensive reagents, and postsynthesis annealing to achieve the desired magnetic properties. Current synthetic methods for magnetic nanoparticles often require post-synthetic modifications, suggesting that the practical application of magnetic nanoparticles will depend on the development of alternative synthetic strategies. We report a biological template to directly grow magnetic nanoparticles of desired material composition and phase under ambient conditions. A phage display methodology was adapted to identify peptide sequences that both specifically bind to the ferromagnetic L1 0 phase of FePt and control the crystallization of FePt nanoparticles using a modified arrested precipitation technique. TEM, electron diffraction, STEM, and X-ray diffraction all indicate these nanoparticles are composed of an FePt alloy with some degree of chemical ordering, and SQUID analysis shows these nanostructures are ferromagnetic at room temperature, possessing coercivities up to 1000 Oe.Since the seminal work of Murray and colleagues, 1 there has been a great deal of interest in using self-assembled films of hard-magnetic FePt nanoparticles for ultrahigh density memory devices. Nanoparticles of ferromagnetic materials are of importance due to their reduced sizes which can support only single magnetic domains, potentially leading to dramatic increases in storage density. However, the current post-synthesis annealing required to achieve the chemically ordered, high anisotropy ferromagnetic L1 0 phase leads to poor control over the spatial arrangement of nanoparticles through extensive particle aggregation. 2 Additionally, since the particles are not prepared with the desired crystallinity, it makes organizing them on a surface with their magnetic easy axis aligned difficult, limiting their technological applications. Eliminating or reducing the high temperature annealing step would greatly simplify the production of magnetic nanoparticles suitable for use in device applications. To date, the most successful approach has been to lower the L1 0 phase transformation temperature through doping of the FePt nanoparticles with silver; 3 yet annealing at temperatures >350°C is still necessary. The need to remove the annealing process from the synthesis of high anisotropy magnetic nanoparticles suggests that a radically different approach be developed.Biological organisms have evolved the ability to direct the synthesis and assembly of crystalline inorganic materials under environmentally benign conditions with control over chemical composition and phase. Examples include the use of viruses expressing material-specific peptides to nucleate semiconducting nanoparticles, 4,5 the use of porous protein crystals, 6,7 modification of the iron storage protein ferritin, 8,9 manipulation of bacteria and yeast to produce iron oxide 10 and semiconducting nanoparticles, 5,11,12 and selection of metal-sp...

Self‐Assembled Nanoreactors

“…EM and X-ray studies indi- cated that the particles were composed of an FePt alloy with some degree of chemical ordering. 337 In another study, M13 viruses have been applied as templates for the synthesis of monodisperse, crystalline nanowires of different compositions. First, the genetically modified M13 virus scaffold was used to assemble crystalline nanoparticles into a 1D array.…”

Laser and Radiofrequency Induced Hyperthermia Treatment via Gold-Coated Magnetic Nanocomposites

“…For example, the nucleation of semiconducting nanoparticles can be initiated in the presence of viruses expressing material-specific peptides. Other examples are the use of porous protein crystals, manipulation of bacteria to produce oxide nanoparticles and selection of metal-specific polypeptides from combinatorial libraries (Reiss et al 2004). In biological methods, biological entities usually serve as templates for nanoparticles formation.…”

Toward Greener Nanosynthesis

“…210 Viral phage display libraries were also used to direct crystallization of FePt metal alloys under ambient conditions. 211 Peptides specific for FePt nanocrystals were identified and used to synthesize nanoparticles and nanowires, which were further tested for ferromagnetic capabilities, suggesting that these nanomaterials could be used in storage device applications. Nucleic acid-based in vitro display techniques and PCR-based approaches, for example RNA display libraries, have been developed that increase the library size several orders of magnitude (10 15 , compared to that for phage displays: 10 7 ).…”