Nature often uses precursor phases for the controlled development of crystalline materials with welldefined morphologies and unusual properties. Mimicking such a strategy in in vitro model systems would potentially lead to the water-based, room-temperature synthesis of superior materials. In the case of magnetite (Fe 3 O 4 ), which in biology generally is formed through a ferrihydrite precursor, such approaches have remained largely unexplored. Here we report on a simple protocol that involves the slow coprecipitation of Fe III /Fe II salts through ammonia diffusion, during which ferrihydrite precipitates first at low pH values and is converted to magnetite at high pH values. Direct coprecipitation often leads to small crystals with superparamagnetic properties. Conversely, in this approach, the crystallization kineticsand thereby the resulting crystal sizescan be controlled through the NH 3 influx and the Fe concentration, which results in single crystals with sizes well in the ferrimagnetic domain. Moreover, this strategy provides a convenient platform for the screening of organic additives as nucleation and growth controllers, which we demonstrate for the biologically derived M6A peptide. ■ INTRODUCTIONLiving organisms exploit the properties of minerals by building a wide variety of specialized organic−inorganic hybrid materials for a variety of purposes, such as for protection and skeletal support and also for navigation and the detection of light. 1,2 The high level of control over the composition, structure, size, and morphology of such biominerals results in materials with amazing complexity and fascinating functionality that are in strong contrast with those of geological minerals and often surpass those of synthetic analogues. 3 Consequently, the processes involved in biomineralization have intrigued scientists for many decades since they could provide sustainable production routes for advanced materials with highly controllable and specialized properties. 4,5 With inspiration from biological mineralization processes, many studies have addressed the interaction between organic and inorganic components using in vitro experiments and in particular have addressed the biomimetic formation of calcium carbonate, 6 calcium phosphate, 7 and silica-based materials. 8 For these systems, a large variety of synthetic methods were developed, which allow for the controlled deposition of minerals in the presence of organic additives and templates. For the formation of crystalline biominerals, the use of precursor phases is of special interest since they allow the efficient transport of significant amounts of material and provide the organism with a feasible route to minerals with complex, nonequilibrium morphologies. 9 The development of precursor-based strategies for in vitro mineralization has boosted the field of bioinspired crystallization and provided a toolbox with which to mimic the effect of biological templates, additives, and confinement in a laboratory environment. This development has also allowed researc...

Magnetic iron oxide nanoparticles have numerous applications in the biomedical field, some more mature, such as contrast agents in magnetic resonance imaging (MRI), and some emerging, such as heating agents in hyperthermia for cancer therapy. In all of these applications, the magnetic particles are coated with surfactants and polymers to enhance biocompatibility, prevent agglomeration, and add functionality. However, the coatings may interact with the surface atoms of the magnetic core and form a magnetically disordered layer, reducing the total amount of the magnetic phase, which is the key parameter in many applications. In the current study, amine and carboxyl functionalized and bare iron oxide nanoparticles, all suspended in water, were purchased and characterized. The presence of the coatings in commercial samples was verified with X-ray photoelectron spectroscopy (XPS). The class of iron oxide (magnetite) was verified via Raman spectroscopy and X-ray diffraction. In addition to these, in-house prepared iron oxide nanoparticles coated with oleic acid and suspended in heptane and hexane were also investigated. The saturation magnetization obtained from vibrating sample magnetometry (VSM) measurements was used to determine the effective concentration of magnetic phase in all samples. The Tiron chelation test was then utilized to check the real concentration of the iron oxide in the suspension. The difference between the concentration results from VSM and the Tiron test confirmed the reduction of magnetic phase of magnetic core in the presence of coatings and different suspension media. For the biocompatible coatings, the largest reduction was experienced by amine particles, where the ratio of the effective weight of magnetic phase reported to the real weight was 0.5. Carboxyl-coated samples experienced smaller reduction with a ratio of 0.64. Uncoated sample also exhibits a reduction with a ratio of 0.6. Oleic acid covered samples show a solvent-depended reduction with a ratio of 0.5 in heptane and 0.4 in hexane. The corresponding effective thickness of the nonmagnetic layer between magnetic core and surface coating was calculated by fitting experimentally measured magnetization to the modified Langevin equation.

Enhancement of thermal conductivity of fluids upon addition of nanoparticles has been previously observed. In this study, Fe3O4 magnetite particles were used and thermal conductivity enhancements both in water and in heptane with increasing volume fraction have been shown. Upon measuring thermal conductivity under externally applied magnetic field, it has been shown experimentally that thermal conductivity can be further increased even at low concentrations and low magnetic field strengths in both fluids. Theoretical calculations are presented to support the effect of magnetic field on the thermal conductivity enhancement. This enhancement is attributed to the thermomagnetic convection which due to a temperature gradient, results in a non-uniform magnetic body force resulting in more efficient thermal conductance.

Biological systems show impressive control over the shape, size and organization of mineral structures, which often leads to advanced physical properties that are tuned to the function of these materials. Such control is also found in magnetotactic bacteria, which produce-in aqueous medium and at room temperature-magnetite nanoparticles with precisely controlled morphologies and sizes that are generally only accessible in synthetic systems with the use of organic solvents and/or the use of high-temperature methods. The synthesis of magnetite under biomimetic conditions, that is, in water and at room temperature and using polymeric additives as control agents, is of interest as a green production method for magnetic nanoparticles. Inspired by the process of magnetite biomineralization, a rational approach is taken by the use of a solid precursor for the synthesis of magnetite nanoparticles. The conversion of a ferrous hydroxide precursor, which we demonstrate with cryo-TEM and low-dose electron diffraction, is used to achieve control over the solution supersaturation such that crystal growth can be regulated through the interaction with poly-(α,β)-dl-aspartic acid, a soluble, negatively charged polymer. In this way, stable suspensions of nanocrystals are achieved that show remanence and coercivity at the size limit of superparamagnetism, and which are able to align their magnetic moments forming strings in solution as is demonstrated by cryo-electron tomography.