Biologically controlled synthesis and assembly of magnetite nanoparticles

Magnetite nanoparticles have size- and shape-dependent magnetic properties. In addition, assemblies of magnetite nanoparticles forming one-dimensional nanostructures have magnetic properties distinct from zero-dimensional or non-organized materials due to strong uniaxial shape anisotropy. However, assemblies of free-standing magnetic nanoparticles tend to collapse and form closed-ring structures rather than chains in order to minimize their energy. Magnetotactic bacteria, ubiquitous microorganisms, have the capability to mineralize magnetite nanoparticles, the so-called magnetosomes, and to direct their assembly in stable chains via biological macromolecules. In this contribution, the synthesis and assembly of biological magnetite to obtain functional magnetic dipoles in magnetotactic bacteria are presented, with a focus on the assembly. We present tomographic reconstructions based on cryo-FIB sectioning and SEM imaging of a magnetotactic bacterium to exemplify that the magnetosome chain is indeed a paradigm of a 1D magnetic nanostructure, based on the assembly of several individual particles. We show that the biological forces are a major player in the formation of the magnetosome chain. Finally, we demonstrate by super resolution fluorescence microscopy that MamK, a protein of the actin family necessary to form the chain backbone in the bacteria, forms a bundle of filaments that are not only found in the vicinity of the magnetosome chain but are widespread within the cytoplasm, illustrating the dynamic localization of the protein within the cells. These very simple microorganisms have thus much to teach us with regards to controlling the design of functional 1D magnetic nanoassembly.