Magnetotactic bacteria contain magnetosomes--intracellular, membrane-bounded, magnetic nanocrystals of magnetite (Fe(3)O(4)) or greigite (Fe(3)S(4))--that cause the bacteria to swim along geomagnetic field lines. We isolated a greigite-producing magnetotactic bacterium from a brackish spring in Death Valley National Park, California, USA, strain BW-1, that is able to biomineralize greigite and magnetite depending on culture conditions. A phylogenetic comparison of BW-1 and similar uncultured greigite- and/or magnetite-producing magnetotactic bacteria from freshwater to hypersaline habitats shows that these organisms represent a previously unknown group of sulfate-reducing bacteria in the Deltaproteobacteria. Genomic analysis of BW-1 reveals the presence of two different magnetosome gene clusters, suggesting that one may be responsible for greigite biomineralization and the other for magnetite.

Magnetite nanocrystals are synthesized in the presence of a recombinant Mms6 protein thought to be involved in the biomineralization of bacterial magnetite magnetosomes, the mammalian iron‐storage protein, ferritin, and two proteins not known to bind iron, lipocalin (Lcn2) and bovine serum albumin (BSA). To mimic the conditions at which magnetite nanocrystals are formed in magnetotactic bacteria, magnetite synthesis is performed in a polymeric gel to slow down the diffusion rates of the reagents. Recombinant Mms6 facilitates formation of ca. 30 nm single‐domain, uniform magnetite nanocrystals in solution, as verified by using transmission electron microscopy analysis and magnetization measurements. The nanocrystals formed in the presence of ferritin, Lcn2, and BSA, do not exhibit the uniform sizes and shapes observed for those produced in the presence of Mms6. Mms6‐derived magnetite nanoparticles show the largest magnetization values above the blocking temperature, as well as the largest magnetic susceptibility compared to those of the nanomaterials synthesized with other proteins. The latter is indicative of a substantial effective magnetic moment per particle, which is consistent with the presence of magnetite with a well‐defined crystalline structure. The combination of electron microscopy analysis and magnetic measurements confirms our hypothesis that Mms6 promotes the shape‐selective formation of uniform superparamagnetic nanocrystals. This provides a unique bioinspired route for synthesis of uniform magnetite nanocrystals.

We have functionalized amorphous Fe2O3 nanoparticles with alkanesulfonic and octadecanephosphonic acids. TEM reveals nanoparticles 5−10 nm in diameter. FTIR spectra suggest that while in all cases the alkyl chains are packed in a solid-like arrangement, packing disorder increased with decreasing chain length. TGA of the sulfonic acid-functionalized Fe2O3 nanoparticles shows that moieties started to decompose and desorb from the iron oxide surface at about 260 °C. In the case of the octadecanephosphonic acid (OPA)-functionalized Fe2O3, moieties started to decompose and desorb at 340 °C. It is suggested that free Fe−OH groups can serve as proton donors to assist in the sulfonic acid desorption process and that because of the diprotic nature of the phosphonic acid these free surface Fe−OH groups may no longer be available. Among all, the octadecanesulfonic acid coating displays the lowest magnetization, which may be explained by the high packing and ordering of the alkyl chains on the particle surface. The saturation curve of the OPA case gives the smallest value of magnetization we have ever measured for functionalized Fe2O3 nanoparticles. It is suggested that the spin state of surface Fe3+ ions is affected by the bonded surfactant, through a mechanism of pπ−dπ P−O, and dπ−dπ Fe−P interactions and that the phosphonate empty d orbitals increase magnetic interactions between neighboring Fe3+ spins.

Highly ordered mineralized structures created by living organisms are often hierarchical in structure with fundamental structural elements at nanometer scales. Proteins have been found responsible for forming many of these structures, but the mechanisms by which these biomineralization proteins function are generally poorly understood. To better understand its role in biomineralization, the magnetotactic bacterial protein, Mms6, which promotes the formation in vitro of superparamagnetic magnetite nanoparticles of uniform size and shape, was studied for its structure and function. Mms6 is shown to have two phases of iron binding: one high affinity and stoichiometric and the other low affinity, high capacity, and cooperative with respect to iron. The protein is amphipathic with a hydrophobic N-terminal domain and hydrophilic C-terminal domain. It self-assembles to form a micelle, with most particles consisting of 20-40 monomers, with the hydrophilic Ctermini exposed on the outside. Studies of proteins with mutated C-terminal domains show that the Cterminal domain contributes to the stability of this multisubunit particle and binds iron by a mechanism that is sensitive to the arrangement of carboxyl/hydroxyl groups in this domain. ABSTRACT: Highly ordered mineralized structures created by living organisms are often hierarchical in structure with fundamental structural elements at nanometer scales. Proteins have been found responsible for forming many of these structures, but the mechanisms by which these biomineralization proteins function are generally poorly understood. To better understand its role in biomineralization, the magnetotactic bacterial protein, Mms6, which promotes the formation in vitro of superparamagnetic magnetite nanoparticles of uniform size and shape, was studied for its structure and function. Mms6 is shown to have two phases of iron binding: one high affinity and stoichiometric and the other low affinity, high capacity, and cooperative with respect to iron. The protein is amphipathic with a hydrophobic N-terminal domain and hydrophilic C-terminal domain. It self-assembles to form a micelle, with most particles consisting of 20−40 monomers, with the hydrophilic C-termini exposed on the outside. Studies of proteins with mutated C-terminal domains show that the C-terminal domain contributes to the stability of this multisubunit particle and binds iron by a mechanism that is sensitive to the arrangement of carboxyl/hydroxyl groups in this domain.

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The mechanisms for nanoparticle self-assembly are often inferred from the morphology of the final nanostructures in terms of attractive and repulsive interparticle interactions. Understanding how nanoparticle building blocks are pieced together during self-assembly is a key missing component needed to unlock new strategies and mechanistic understanding of this process. Here we use real-time nanoscale kinetics derived from liquid cell transmission electron microscopy investigation of nanoparticle self-assembly to show that nanoparticle mobility dictates the pathway for self-assembly and final nanostructure morphology. We describe a new method for modulating nanoparticle diffusion in a liquid cell, which we employ to systematically investigate the effect of mobility on self-assembly of nanoparticles. We interpret the observed diffusion in terms of electrostatically induced surface diffusion resulting from nanoparticle hopping on the liquid cell window surface. Slow-moving nanoparticles self-assemble predominantly into linear 1D chains by sequential attachment of nanoparticles to existing chains, while highly mobile nanoparticles self-assemble into chains and branched structures by chain−chain attachments. Self-assembly kinetics are consistent with a diffusion-driven mechanism; we attribute the change in self-assembly pathway to the increased self-assembly rate of highly mobile nanoparticles. These results indicate that nanoparticle mobility can dictate the self-assembly mechanism and final nanostructure morphology in a manner similar to interparticle interactions.

Magnetotactic bacteria are a diverse group of prokaryotes that share the unique ability of biomineralizing magnetosomes, which are intracellular, membrane-bounded crystals of either magnetite (Fe3O4) or greigite (Fe3S4). Magnetosome biomineralization is mediated by a number of specific proteins, many of which are localized in the magnetosome membrane, and thus is under strict genetic control. Several studies have partially elucidated the effects of a number of these magnetosome-associated proteins in the control of the size of magnetosome magnetite crystals. However, the effect of MamC, one of the most abundant proteins in the magnetosome membrane, remains unclear. In this present study, magnetite nanoparticles were synthesized inorganically in free-drift experiments at 25 °C in the presence of different concentrations of the iron-binding recombinant proteins MamC and MamCnts (MamC without its first transmembrane segment) from the marine, magnetotactic bacterium Magnetococcus marinus strain MC-1 and three commercial proteins [α-lactalbumin (α-Lac), myoglobin (Myo), and lysozyme (Lyz)]. While no effect was observed on the size of magnetite crystals formed in the presence of the commercial proteins, biomimetic synthesis in the presence of MamC and MamCnts at concentrations of 10-60 μg/mL resulted in the production of larger and more well-developed magnetite crystals (~30-40 nm) compared to those of the control (~20-30 nm; magnetite crystals grown protein-free). Our results demonstrate that MamC plays an important role in the control of the size of magnetite crystals and could be utilized in biomimetic synthesis of magnetite nanocrystals.

During bubble collapse, intense shock waves are generated and propagate through the liquid at velocities above the speed of sound. [1][2][3][4] Unusual sonochemical effects are induced by these shock waves, most importantly, high velocity collisions among solid particles suspended in such liquids. 4 These collisions result in extreme heating at the point of impact, which can lead to effective local melting and dramatic increases in the rates of many solid-liquid reactions. [5][6][7][8] In this work, we describe a quantitative model of the melting induced by high-speed interparticle collisions and test this kinematic model against the effects of varying initial particle size and slurry concentration on the morphology of zinc particle agglomerates.Sonication 9 of a decane slurry containing 2% w/w fine Zn powder (5 µm diameter) rapidly produces Zn agglomerates (cf. scanning electron micrographs in Figure 1 and Supporting Information). As sonication proceeds, agglomeration reaches its maximum effect after ∼90 min. The resulting 50-70 µm agglomerates have nearly round shapes ( Figure 1B). Sonication of 5 µm Zn powder as a slurry in alkanes, for example, produces dense agglomerates consisting of ∼1000 fused particles.Because of turbulent flow and shock waves generated by cavitation in liquids irradiated with ultrasound, metal particles are driven together at extremely high speeds, which induces effective melting at the point of impact. 4 The estimated velocity of colliding particles approaches half the speed of sound in the liquid. 4 The low melting point of Zn (419.6°C) 10 obviously contributes to the facile agglomeration process. One would expect to alter the velocity of interparticle collisions by varying the concentration of impinging particles. This should influence the temperature at the site of impact, resulting in agglomeration with diminished efficacy at sufficient slurry density. To verify this effect, the slurry loading was systematically increased. Loadings up to 50% w/w showed no significant effect, but further increases to 70% w/w resulted in considerably less pronounced agglomeration ( Figure 1C and Supporting Information).The particle size has a very strong effect on the outcome of the ultrasonic irradiation. From previous observations, 8 we know qualitatively that agglomeration does not occur for particles either too large (∼100 µm) or too small (∼100 nm). For example, no aggregation was observed for coarse Zn powder (Figure 2), although particle deformation does occur (Supporting Information). Interestingly, by mixing the fine and coarse Zn powders and sonicating them together as a slurry at high loading, a porous aggregated product is formed ( Figure 2B). The large particles are literally welded together by collision with the smaller particles.To model the interparticle collisions, some simplifying approximations will be made: (1) the collisions are perfectly inelastic (i.e., all kinetic energy ends as thermal energy within the particle colliding) and (2) complete melting of a particle occurs in ord...