Oriented attachment has created a great debate about the description of crystal growth throughout the last decade. This aggregation-based model has successfully described biomineralization processes as well as forms of inorganic crystal growth, which could not be explained by classical crystal growth theory. Understanding the nanoparticle growth is essential since physical properties, such as the magnetic behavior, are highly dependent on the microstructure, morphology and composition of the inorganic crystals. In this work, the underlying nanoparticle growth of cobalt ferrite nanoparticles in a bioinspired synthesis was studied. Bioinspired syntheses have sparked great interest in recent years due to their ability to influence and alter inorganic crystal growth and therefore tailor properties of nanoparticles. In this synthesis, a short synthetic version of the protein MMS6, involved in nanoparticle formation within magnetotactic bacteria, was used to alter the growth of cobalt ferrite. We demonstrate that the bioinspired nanoparticle growth can be described by the oriented attachment model. The intermediate stages proposed in the theoretical model, including primary-building-block-like substructures as well as mesocrystal-like structures, were observed in HRTEM measurements. These structures display regions of substantial orientation and possess the same shape and size as the resulting discs. An increase in orientation with time was observed in electron diffraction measurements. The change of particle diameter with time agrees with the recently proposed kinetic model for oriented attachment.

Nanoparticle syntheses utilizing biomimetic approaches have advanced in recent years. Polypeptides, with their ability to influence inorganic crystal growth, are a topic of great interest. Their effect on the particle formation has not been completely understood yet. Here we report a bioinspired synthesis of cobalt ferrite nanoparticles carried out in vitro under mild conditions using a short, synthetic polypeptide c25-mms6. The influence of c25-mms6 on the nanoparticle formation was investigated by comparing the particles synthesized with the polypeptide to particles synthesized under equivalent conditions without c25-mms6. A separation into D small,av = 10 nm small, superparamagnetic spheres and D big,av = 48 nm disc-like single-domain particles was observed. Non-stoichiometric cobalt ferrite particles with a shape-dependent stoichiometry were produced in the polypeptide-free synthesis. Stoichiometric D small,av = 10 nm CoFe2O4 spheres and D big,av = 60–70 nm  Co2FeO4 ferromagnetic discs were obtained in the polypeptide-enhanced synthesis. The results indicate that the polypeptide acts as a catalyst during the multistep biomineralization process and allows the formation of stoichiometric phases which cannot be synthesized at room temperature using conventional bottom-up syntheses.

In the presence of ionic compounds, the thermal decomposition of octacarbonyldicobalt, Co-2(CO)(8), in an inert solvent leads exclusively to epsilon-Co nanocubes. The ionic species can be added directly or generated in situ by a chemical reaction between an additive and the precursor or between two additives. The additional presence of an inhomogeneous magnetic field leads to disc-shaped crystalline nanoparticles, which assemble to chains.

This paper highlights recent advances in synthesis, self-assembly and sensing applications of monodisperse magnetic Co and Co-alloyed nanoparticles. A brief introduction to solution phase synthesis techniques as well as the magnetic properties and aspects of the self-assembly process of nanoparticles will be given with the emphasis placed on selected applications, before recent developments of particles in sensor devices are outlined. Here, the paper focuses on the fabrication of granular magnetoresistive sensors by the employment of particles themselves as sensing layers. The role of interparticle interactions is discussed.

Cobalt nanoparticles have been prepared with TOPO, subsequently a ligand exchange was carried out. Samples have been prepared by dropping particle solution on Si-wafer, which lead to samples which consist of multilayers partially. The samples were studied with respect to the 3D order to gain information about the influence of the ligand on the 3D structure of the particle array and the oxidation process in multilayered particles.