ABSTRACT

Living organisms produce magnetic nanoparticles of well-defined size and crystallinity under mild physiological conditions through the process of biomineralization, that is, the biological regulation of crystal growth, particle size, morphology, and organization [1, 2]. Biomineralization results in the production of a variety of complex composite materials, ranging from the nano-to the macroscopic, by integrating inorganic matter within the organic world of biology for structural support, magnetoreception, and iron storage. The resulting biominerals may be amorphous or crystalline, forming structures of varying degrees of complexity from a single unit to numerous individual units or aggregates. The aggregated units are usually arranged in an orderly fashion, and when crystalline the crystallographic axes are often aligned. The resulting highly

organized bioinorganic structures exhibit excellent physical and chemical properties that often surpass those of artificial materials produced by usual synthetic methods employed in the laboratory, which most often require harsh conditions of high temperature, pressure, or pH values [3]. Among the various biomineralization products found in nature, iron biominerals are magnetic. The best-known biogenic magnetic nanoparticles are the ferrimagnetic nanostructures formed by magnetotactic bacteria [4-6] and the antiferromagnetic nanoparticles formed by the iron storage protein ferritin [7-9]. In the first part of this chapter we explore biogenic nanoparticles to gain an appreciation of the controlled formation of magnetic nanoparticles in vivo and learn from the chemistry of life how to perfect laboratory synthesis and assembly of high-quality, monodispersed magnetic nanocrystals through biomimetic processes. Early efforts to biomimicry, that is, to integrate the organic and inorganic world in the assembly of magnetic nanoparticles, ad-dressed the encapsulation of various ferrite nanoparticles within block copolymer supports [10, 11]. However, the resulting nano-composites were often inhomogeneous and exhibited nanoparticle polydispersity. There has also been extensive effort in the synthesis of magnetic nanoparticles using microemulsions where the nanoparticles are synthesized within the confined spaces of micelles or reverse micelles. The resulting nanoparticles are monodispersed, each encapsulated within a shell of surfactant molecules [12, 13]. Many investigators extend the definition of biomimetic systems to include other core/shell nanostructures, where the shell con-sists of a biocompatible inorganic rather than organic substance, such as magnetic core/silica shell nanocomposite nanoparticles, as well as of magnetic/quantum dot/silica shell heterostructured nanoparticles [14, 15]. Presently, the forefront of exploration in the biomimetic synthesis and assembly of magnetic nanoparticles lies in nanotemplating using protein cages and viral capsids [16]. These organic shells are used to coordinate the nucleation and growth of magnetic nanoparticles and their subsequent assembly into arrayed mesostructures. In the second part of this chapter we present an introductory discussion of the advantages afforded by biological templates in facilitating the assembly and organization of magnetic nanostructures over multiple length scales. These ad-

vantages stem from the ability to genetically modify the interior and exterior surfaces of biological templates in order to initiate the nucleation of a variety of magnetic nanophases and impart surface site recognition properties for nanoparticles arraying on solid sub-strates. The chapter does not intend to give a comprehensive review of the literature but rather to exemplify the concept of the hierarchi-cal assembly, aggregation, and superlattice formation of magnetic nanoparticles derived from bioinspired routes. 1.2 Biomineralization of IronThe biomineralization of iron hydroxides is widespread among organisms due to the utilization of iron atoms by proteins for oxygen and electron transport in metabolic processes. The most widely studied biomineralization product occurs in the iron storage protein ferritin. Ferritins represent a superfamily of proteins that are ubiquitous in biological systems [17]. They are large, multicomponent proteins that self-assemble to form molecular cages within which a hydrated ferric oxide is mineralized. Mammalian ferritin forms a 7 nm micellar core of hydrated iron (III) oxide (ferrihydrite). It was first described by V. Laufberger in 1937 [18] as a protein isolated from horse spleen containing about 20% iron. An iron-rich mineral deposit similar in composition to that of ferritin is found in the dermal granules of Molpadia intermedia, a species of marine invertebrates [19]. These dermal granules, ranging in size from 10 μm to 350 μm, serve as strengthening agents in the connective tissues of the dermis; they contain inclusions of iron hydroxide deposits seen as electron dense subunits of 9 nm to 14 nm diameter in transmission electron micrographs. Magnetite is the most common of the known iron oxide biominerals. It was first identified by H. Lowenstam in 1962 [20] in the denticle capping of chitons (primitive marine mollusks). Magnetite precipitation and tooth formation in chitons proceed through the biochemically controlled reduction of ferrihydrite [21]. Unlike ferrihydrite, which is a common product of both biological and inorganic processes, inorganically magnetite is formed only at elevated temperatures and pressures in igneous and metamorphic rocks. Yet, chitons are capable of forming magnetite under ambient conditions. By natural selection, the chitons somehow biochemically

mediate the transformation of ferrihydrite to magnetite in order to perform a biological function, even at atmospheric temperature and pressure. In an entirely different biological function magnetite deposits have been identified with “magnetoreception,” the ability of living organisms to sense the polarity or the inclination of the earth’s magnetic field [22]. Some bacteria, honeybees, homing pigeons, and migratory fish are known to possess such sense.