A Bioinspired Coprecipitation Method for the Controlled Synthesis of Magnetite Nanoparticles

Abstract

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...

Bioinspired Magnetite Crystallization Directed by Random Copolypeptides

“…Magnetite (Fe 3 O 4 ) is a biologically and technologically important magnetic iron oxide of which the magnetic properties depend on the size, shape, purity, and organization of the crystals . Magnetite biominerals encompass well‐defined, often single‐domain crystals as, e.g., encountered in the magnetosomes of magnetotactic bacteria, but also in the magnetoreceptive organs of migratory birds, honeybees,[5a], and certain fish.…”

Synthesis, Functionalization, and Design of Magnetic Nanoparticles for Theranostic Applications

“…[295] Through this method, the controlled formation of ferrimagnetic magnetite with particle sizes up to 60 nm in the presence of Mms6 protein has closely mimicked the biomineralization process taken by MTB. It was suggested that Mms6 is a magnetite nucleation protein which binds iron ions specifically to nucleate the formation of magnetite.…”

TEM and Associated Techniques

“…Cryo-TEM imaging is routinely chosen as a technique that does not require drying, thus allowing direct imaging of the features of interest [163,165]. When investigating the synthesis of magnetite, nanocrystals can be cryoplunged into liquefied ethane to arrest the nanoparticle growth at different stages of formation and obtain time-resolved information [7,[166][167][168][169]. HRTEM analysis has produced a wealth of knowledge about the iron oxide formation in biological and synthetic systems alike.…”

“…HRTEM analysis has produced a wealth of knowledge about the iron oxide formation in biological and synthetic systems alike. The "freeze and look" approach proved a very effective tool for evaluation of the complex synthetic pathways, notably the biomimetic formation of the nanoscale magnetite [165][166][167][168][170][171][172][173][174][175][176]. Importantly, the mechanism of biomimetic iron oxide formation remains unclear, largely because the analysis of the formed biomimetic solid is carried out either postsynthesis [6,21,22,34,44,78,177] or is performed on aliquots sampled during different stages of the growth process [165,166,171,173,[178][179][180][181][182].…”

“…It is assumed that the samples quenched in liquid nitrogen from the reactive solution are representative of their physical state during the synthesis. Although the examination of static reaction products only provides snapshot evidence for the dynamic evolution of nanostructures, such rapid quenching of reactive suspensions can provide valuable insight into the near in situ physical states of nanostructures, especially the steps of the agglomeration-driven and oriented attachment nanoparticle growth [7,69,71,77,90,107,108,167,168,171,[183][184][185][186][187][188][189][190][191][192][193][194][195][196][197]. Importantly, the quenching studies using cryo-TEM are typically restricted to providing relatively low time resolution (i.e., minutes) static snapshots of highly dynamic processes at the nanometer scale and do not permit real-time imaging of the features or processes of interest [7,69,71,77,90,167,168,171,[183][184][185][186][187][188][189][190][191]…”