Formation and alteration of magnetite nanoparticles Magnetite is an iron oxide which is ubiquitous in rocks and is usually deposited as small nanoparticulate matter among other rock material. It differs from most other iron oxides because it contains divalent and trivalent iron. Consequently it has a special crystal structure and unique magnetic properties. These properties are used for paleoclimatic reconstructions where naturally occurring magnetite helps understanding former geological ages. Further on magnetic properties are used in bio and nanotechnological applications –synthetic magnetite serves as a contrast agent in MRI is exploited in biosensing hyperthermia or is used in storage media. Magnetic properties are strongly size dependent and achieving size control under preferably mild synthesis conditions is of interest in order to obtain particles with required properties. By using a custom made setup it was possible to synthesize stable single domain magnetite nanoparticles with the co precipitation method. Furthermore it was shown that magnetite formation is temperature dependent resulting in larger particles at higher temperatures. However mechanistic approaches about the details are incomplete. Formation of magnetite from solution was shown to occur from nanoparticulate matter rather than solvated ions. The theoretical framework of such processes has only started to be described partly due to the lack of kinetic or thermodynamic data. Synthesis of magnetite nanoparticles at different temperatures was performed and the Arrhenius plot was used determine an activation energy for crystal growth of 28.4 kJ mol 1 which led to the conclusion that nanoparticle diffusion is the rate determining step. Furthermore a study of the alteration of magnetite particles of different sizes as a function of their storage conditions is presented. The magnetic properties depend not only on particle size but also depend on the structure of the oxide because magnetite oxidizes to maghemite under environmental conditions. The dynamics of this process have not been well described. Smaller nanoparticles are shown to oxidize more rapidly than larger ones and the lower the storage temperature the lower the measured oxidation. In addition the magnetic properties of the altered particles are not decreased dramatically thus suggesting that this alteration will not impact the use of such nanoparticles as medical carriers. Finally the effect of biological additives on magnetite formation was investigated. Magnetotactic bacteria¬¬ are able to synthesize and align magnetite nanoparticles of well defined size and morphology due to the involvement of special proteins with specific binding properties. Based on this model of morphology control phage display experiments were performed to determine peptide sequences that preferably bind to (111) magnetite faces. The aim was to control the shape of magnetite nanoparticles during the formation. Magnetotactic bacteria are also able to control the intracellular redox potential with proteins called magnetochromes. MamP is such a protein and its oxidizing nature was studied in vitro via biomimetic magnetite formation experiments based on ferrous ions. Magnetite and further trivalent oxides were found. This work helps understanding basic mechanisms of magnetite formation and gives insight into non classical crystal growth. In addition it is shown that alteration of magnetite nanoparticles is mainly based on oxidation to maghemite and does not significantly influence the magnetic properties. Finally biomimetic experiments help understanding the role of MamP within the bacteria and furthermore a first step was performed to achieve morphology control in magnetite formation via co precipitation