磁场诱导沉淀聚合制备一维Fe3O4/P（MAA-DVB）纳米链

采用溶剂热法制备了单分散Fe3O4纳米粒子,以甲基丙烯酸（MAA）和二乙烯基苯（DVB）为聚合单体,在沉淀聚合过程中通过磁场诱导自组装制备了一维高磁响应性永久连接的Fe3O4/P（MAA-DVB）纳米链.采用扫描电镜（SEM）,透射电镜（TEM）,X射线衍射仪（XRD）,热重分析（TGA）及振动样品磁强计（VSM）等对其形貌、磁含量和磁响应性等进行了分析表征.结果表明,该法制备的一维Fe3O4/P（MAA-DVB）纳米链的磁含量为91%时,其比饱和磁化强度为72emu/g.在外磁场存在条件下,一维Fe3O4/P（MAA-DVB）纳米链将按外界磁场的方向取向.此外,每个豆荚内的Fe3O4纳米粒子规则的排列在一条线上,并通过P（MAA-DVB）聚合物使其均匀分布.     

The Synthesis Methodology of PEGylated Fe3O4@Ag Nanoparticles Supported by Their Physicochemical Evaluation

Many investigations are currently being performed to develop the effective synthesis methodology of magnetic nanoparticles with appropriately functionalized surfaces. Here, the novelty of the presented work involves the preparation of nano-sized PEGylated Fe₃O₄@Ag particles, i.e., the main purpose was the synthesis of magnetic nanoparticles with a functionalized surface. Firstly, Fe₃O₄ particles were prepared via the Massart process. Next, Ag+ reduction was conducted in the presence of Fe₃O₄ particles to form a nanosilver coating. The reaction was performed with arabic gum as a stabilizing agent. Sound energy-using sonication was applied to disintegrate the particles’ agglomerates. Next, the PEGylation process aimed at the formation of a coating on the particles’ surface using PEG (poly(ethylene glycol)) has been performed. It was proved that the arabic gum limited the agglomeration of nanoparticles, which was probably caused by the steric effect caused by the branched compounds from the stabilizer that adsorbed on the surface of nanoparticles. This effect was also enhanced by the electrostatic repulsions. The process of sonication caused the disintegration of aggregates. Formation of iron (II, III) oxide with a cubic structure was proved by diffraction peaks. Formation of a nanosilver coating on the Fe₃O₄ nanoparticles was confirmed by diffraction peaks with 2θ values 38.15° and 44.35°. PEG coating on the particles’ surface was proven via FT-IR (Fourier Transform Infrared Spectroscopy) analysis. Obtained PEG–nanosilver-coated Fe₃O₄ nanoparticles may find applications as carriers for targeted drug delivery using an external magnetic field.     

Preparation and sedimentation behavior in magnetic fields of magnetite-covered clay particles

This work is devoted to the preparation of magnetite-covered clay particles in aqueous medium. For this purpose, magnetite nanoparticles were synthesized by a coprecipitation method. These magnetic particles are adhered to sodium montmorillonite (NaMt) particles in aqueous suspensions of both materials, by appropriate control of the electrolyte concentrations. The best condition to produce such heteroaggregation corresponds to acid pH and approximately 1 mol/L ionic strength, when the electrokinetic potentials (zeta-potential) of both NaMt and Fe3O4 particles have high enough and opposite sign, as demonstrated from electrophoresis measurements. When a layer of magnetite re-covers the clay particles, the application of an external magnetic field induces a magnetic moment in clay-magnetite particles parallel to the external magnetic flux density. The sedimentation behavior of such magnetic particles is studied in the absence or presence of an external magnetic field in a vertical direction. The whole sedimentation behavior is also strongly affected by the formation of big flocculi in the suspensions under the action of internal colloidal interactions. van der Waals and dipole-dipole magnetic attractions between magnetite-covered clay particles dominate the flocculation processes. The different relative orientation of the clay-magnetite particles (edge-to-edge, face-to-edge, and face-to-face) are discussed in order to predict the most favored flocculi configuration.     

Two-dimensional Monte Carlo simulations of a colloidal dispersion composed of polydisperse ferromagnetic particles in an applied magnetic field

We have investigated aggregation phenomena in a polydisperse colloidal dispersion of ferromagnetic particles simulated by employing the cluster-moving Monte Carlo method in an applied magnetic field. The influence of both particle-particle and particle-field interactions on the aggregate structures is analyzed in terms of a pair correlation function. The results obtained in this study are summarized as follows: Under a strong magnetic field, chainlike clusters are formed along the magnetic field direction, and they become thickly clustered with an increase in the strength of the external magnetic field. Moreover, the thickly clustered chains are formed for a polydisperse system that has a large standard deviation of particle diameters. In contrast, for a very weak magnetic field, the strong interaction between the larger particles gives rise to the formation of various shapes in the chainlike clusters, including bending, looping, and branching. With an increase in the external magnetic field, these structures reorganize to form straight chainlike clusters. Furthermore, the thickness of the chainlike clusters for the polydisperse system is found to depend on the standard deviation of the particle-size distribution but is found to be independent of the magnetic field strength.     

间甲酚溶液中掺杂聚苯胺的氧化-还原行为研究

在有、无外加恒定磁场(0.4T)条件下,采用乳液聚合法合成了十二烷基苯磺酸掺杂聚苯胺(PAn),通过傅立叶红外光谱(FT-IR)、在有/无外电场作用下产物/m-cresol溶液的电导率、pH及其颜色随温度变化的分析表征,系统研究了温度对PAn/m-cresol溶液中PAn氧化-还原(掺杂-脱掺杂)行为的影响.溶液电导率和pH的测试结果显示,溶液电导率与溶液中游离掺杂酸的浓度成正比,PAn的掺杂度随温度的升高而降低,随温度的降低而增大,且在相同温度下,有、无外电场作用时PAn/m-cresol溶液的电导率存在突跃.与无磁场条件下合成的PAn所制备的m-cresol溶液相比,磁场条件下合成的PAn所制备的m-cresol溶液电导率的突跃更加显著.经脱掺杂处理后的PAn/m-cresol溶液的电导率均较低,对温度变化的响应极其微弱,且在有、无外电场作用时不发生突跃.分析认为,这是由于外电场作用下m-cresol溶液中掺杂态的PAn分子发生了取向重排,而脱掺杂的PAn则无此变化.PAn在外电场作用下的取向重排及氧化-还原性均与其掺杂程度相关,掺杂程度越高,氧化-还原及取向重排越显著.     

磁场诱导下Fe3S4向FeS2转化速率的增加

以FeCl₃和CH₄N₂S为主要原料，在0.2～0.4 T的外加磁场强度下，温度为170 ℃的反应体系中研究了磁场对前驱物Fe₃S₄转化成FeS₂过程的影响。结果表明，Fe₃S₄到FeS₂的转化率是与磁场强度有关的。Fe₃S₄的硫化过程可能是通过溶解-再结晶机理。外加磁场的存在可以促进物质的传输过程，从而加速前驱物的溶解和再结晶过程，导致和没有外加磁场相比Fe₃S₄到FeS₂的转化率的增加。     

Studies about the influence of self-organization of colloidal magnetic nanoparticles on the magnetic Néel relaxation time

The monodomain magnetic nanoparticle-based colloids are mainly used in biomedical applications. In this type of colloids, there is a tendency of agglomeration even in the absence of external magnetic field. So, the Néel magnetic relaxation time of the system is affected by that tendency. In this paper, we propose a model to study how the nanoparticle tendency to agglomerate in the nanofluid affects the Néel relaxation time of the system. For simulating the self-organization of colloidal nanoparticles, we apply a Monte Carlo method, and the Néel magnetic relaxation time is assessed through the adaptation and solution of Coffey equations in oblique magnetic field, adapted to the local magnetic field on a nanoparticle.     

Magnetic sponge prepared with an alkanedithiol-bridged network of nanomagnets

The magnetic dipole-dipole interaction between nanomagnets having huge magnetic moments can have a strength comparable to that of the van der Waals interaction between them, and it can be manipulated by applying an external magnetic field of conventional strength. Therefore, the cooperation between the dipole-dipole interaction and the applied magnetic field allows the magnetic moments of nanomagnets to be aligned and organized in an ordered manner. In this work, a network of magnetic nanoparticles connected with flexible long-alkyl-chain linkers was designed to develop a "magnetic sponge" capable of absorbing and desorbing guest molecules with changes in the applied magnetic field. The magnetization of the sponge with long-alkyl-chain bridges (30 C atoms) exhibited a 500% increase after cooling in the presence of an applied field of 7 T relative to that in the absence of a magnetic field. Cooling in a magnetic field leads to anisotropic stretching in the sponge due to reorganization of the nanomagnets along the applied field, in contrast to the isotropic organization under zero-field conditions. Such magnetic-responsive organization and reorganization of the magnetic particle network significantly influences the gas absorption capacity of the nanopores inside the material. The absorption and desorption of guests in an applied magnetic field at low temperature can be regarded as a fascinating "breathing feature" of our magnetic sponge.     

Three-dimensional Monte Carlo simulations of internal aggregate structures in a colloidal dispersion composed of rod-like particles with magnetic moment normal to the particle axis

We have treated a suspension composed of ferromagnetic rod-like particles with a magnetic moment normal to the particle axis in order to investigate aggregation phenomena of such a suspension by means of cluster-moving Monte Carlo simulations. In the present study, we have considered a three-dimensional mono-dispersed model system composed of such rod-like particles. Internal structures of self-assembled clusters have been discussed quantitatively in terms of radial distribution, pair correlation, orientational pair correlation functions, number distributions of clusters, and order functions. The main results obtained here are summarized as follows. Rod-like particles tend to aggregate to form raft-like clusters along the magnetic moment direction more significantly with magnetic particle-particle interactions. In such raft-like clusters, the direction of each particle axis has a tendency to incline in parallel formation, but is not so parallel as in a two-dimensional dispersion. As the volumetric fraction increases, longer raft-like clusters are formed, but such raft-like clusters do not aggregate further to form thicker clusters, which is in significantly contrast with a dispersion of spherical particles, where thicker chain-like clusters are observed under certain conditions. For the case of strong magnetic particle-particle interactions, sufficiently long raft-like clusters are formed along the magnetic field direction, even if the influence of an external magnetic field is of the same order of that of the thermal energy. However, rod-like particles in such clusters do not necessarily incline in significantly parallel formation along a certain direction. Self-assembled tube-like clusters are formed when magnetic particle-particle interactions are much more dominant than the rotational Brownian motion under circumstances of rod-like particles inclining in a certain direction.     

Anisotropic magnetoresistance and piezoresistivity in structured Fe3O4-silver particles in PDMS elastomers at room temperature

Magnetorheological elastomers, MREs, based on elastic organic matrices displaying anisotropic magnetoresistance and piezoresistivity at room temperature were prepared and characterized. These materials are dispersions of superparamagnetic magnetite forming cores of aggregated nanoparticles inside silver microparticles that are dispersed in an elastomeric polymer (poly(dimethylsiloxane), PDMS), curing the polymer in the presence of a uniform magnetic field. In this way, the elastic material becomes structured as the application of the field induces the formation of filaments of silver-covered inorganic material agglomerates (needles) aligned in the direction of the field (parallel to the field). Because the magnetic particles are covered with silver, the MREs are not only magnetic but also electrical conductors. The structuration induces elastic, magnetic, and electrical anisotropic properties. For example, with a low concentration of particles in the elastic matrix (5% w/w) it is possible to obtain resistances of a few ohms when measured parallel to the needles or several megaohms in the perpendicular direction. Magnetite nanoparticles (Fe(3)O(4) NP) were synthesized by the coprecipitation method, and then agglomerations of these NPs were covered with Ag. The average size of the obtained magnetite NPs was about 13 nm, and the magnetite-silver particles, referred to as Fe(3)O(4)@Ag, form micrometric aggregates (1.3 μm). Nanoparticles, microparticles, and the MREs were characterized by XRD, TEM, SEM, EDS, diffuse reflectance, voltammetry, VSM, and SQUID. At room temperature, the synthesized magnetite and Fe(3)O(4)@Ag particles are in a superparamagnetic state (T(B) = 205 and 179 K at 0.01 T as determined by SQUID). The elastic properties and Young's modulus of the MREs were measured as a function of the orientation using a texture analysis device. The magnetic anisotropy in the MRE composite was investigated by FMR. The electrical conductivity of the MRE (σ) increases exponentially when a pressure, P, is applied, and the magnitude of the change strongly depends on what direction P is exerted (anisotropic piezoresistivity). In addition, at a fixed pressure, σ increases exponentially in the presence of an external magnetic field (H) only when the field H is applied in the collinear direction with respect to the electrical flux, J. Excellent fits of the experimental data σ versus H and P were achieved using a model that considers the intergrain electron transport where an H-dependent barrier was considered in addition to the intrinsic intergrain resistance in a percolation process. The H-dependent barrier decreases with the applied field, which is attributed to the increasing match of spin-polarization in the silver covers between grains. The effect is anisotropic (i.e., the sensitivity of the magnetoresistive effect is dependent on the relative orientation between H and the current flow J). In the case of Fe(3)O(4)@ Ag, when H and J are parallel to the needles in the PDMS matrix, we obtain changes in σ up to 50% for fields of 400 mT and with resistances on the order of 1-10 Ω. Magnetoresistive and magnetoelastic properties make these materials very interesting for applications in flexible electronics, electronic skins, anisotropic pressure, and magnetic field sensors.     

Water-annealing regulated protein-based magnetic nanofiber materials: tuning silk structure and properties to enhance cell response under magnetic fields

In this study, flexible silk fibroin protein and biocompatible barium hexaferrite (BaM) nanoparticles were combined and electrospun into nanofibers, and their physical properties could be tuned through the mixing ratios and a water annealing process. Structural analysis indicates that the protein structure of the materials is fully controllable by the annealing process. The mechanical properties of the electrospun composites can be significantly improved by annealing, while the magnetic properties of barium hexaferrite are maintained in the composite. Notably, in the absence of a magnetic field, cell growth increased slightly with increasing BaM content. Application of an external magnetic field during in vitro cell biocompatibility study of the materials demonstrated significantly larger cell growth. We propose a mechanism to explain the effects of water annealing and magnetic field on cell growth. This study indicates that these composite electrospun fibers may be widely used in the biomedical field for controllable cell response through applying different external magnetic fields.     

Fabrication of one-dimensional Fe3O4/P(GMA-DVB) nanochains by magnetic-field-induced precipitation polymerization

One-dimensional (1D) magnetic Fe(3)O(4)/P(GMA-DVB) peapod-like nanochains have been successfully synthesized by magnetic-field-induced precipitation polymerization using Fe(3)O(4) as building blocks and P(GMA-DVB) as linker. The Fe(3)O(4) microspheres without surface modification can be arranged with the direction of the external magnetic field in a line via the dipolar interaction between Fe(3)O(4) microspheres and linked permanently via P(GMA-DVB) coating during precipitation polymerization. The length of peapod-like nanochains can be controlled by magnetic field intensity, and the thickness of polymer shell can be tuned by the amount of monomers. Magnetic measurement revealed that these 1D peapod-like nanochains showed highly magnetic sensitivity. In the presence of magnetic field, 1D magnetic Fe(3)O(4)/P(GMA-DVB) peapod-like nanochains can be oriented and aligned along the direction of external magnetic field.     

Proton magnetic relaxation study of pretransitional phenomena in the isotropic phase of a nematic liquid crystal II. Presence of inner magnetic field gradients as revealed by self-diffusion study

Above the clearing point of a nematic LC self-diffusion was studied by the Hahn pulse sequence (90-t-180) in the presence of a weak (up to 4 Gs cm^-1) permanent external magnetic field gradient G = ?H/?z. The non-linearity of spin-echo decays was interpreted as a result of the inner magnetic field gradient Gamma, indicating the magnetically non-homogeneous nature of the pretransitional zone of the LC. The pretransitional zone was considered to be biphasic (locally ordered clusters ? isotropic surrounding). Due to the orientation of clusters in the magnetic field and the anisotropy of the diamagnetic susceptibility of their molecules, the diamagnetic susceptibility of clusters in the z direction should be different from that of their disordered surroundings. Therefore, clusters behave as specific filled particles, their diamagnetic response in the presence of G being different from that of their isotropic surroundings, i.e. clusters can experience a translational motion in the external field gradient. This leads to the peculiar diamagnetic separation in space, accompanied by an increase of the field gradient. The inner field gradient was shown to be proportional to G²t³ and to decrease with temperature.     

Bound electron states in clusters of inert atoms in a magnetic field

An external magnetic field is shown to stabilize negatively charged clusters of inert atoms. In an external magnetic field the critical number of atoms in a cluster, necessary to bound an excess electron to a neutral cluster, is less than that in the absence of a field. General conditions for creation of electron bound states in a cluster in a magnetic field are considered.     

一维多孔Fe-B合金纳米结构的制备及其磁性质

在十六烷基三甲基溴化铵(CTAB)的水溶液中,用NaBH₄还原FeCl₃,在反应过程中施加外部磁场,制得了一维多孔Fe-B合金纳米结构。研究表明,外部磁场对一维纳米结构的形成有重要影响,当不加外部磁场时得到的是离散的球形纳米粒子;外部磁场还影响Fe-B纳米粒子的晶体结构：外部磁场存在下得到的是无定形Fe-B合金,而不加外部磁场时则得到多晶Fe-B合金。CTAB对多孔的形成起到关键的作用,当不加CTAB时得到了实心球状纳米粒子。初步讨论了这种一维孔状Fe-B合金纳米结构的形成机理。用XRD、ICP-AES、TEM对样品进行了表征,测定了它们的磁性质。结果表明,在施加不同磁场强度条件下制得的样品具有不同的饱和磁化强度和矫顽力。     

Field induced formation of mesoscopic polymer chains from functional ferromagnetic colloids

The assembly and direct imaging of ferromagnetic nanoparticles into one-dimensional mesostructures (1-D) are reported. Polymer-coated ferromagnetic colloids (19 nm, 24 nm) were assembled at a crosslinkable oil-water interface under both magnetic field induced and zero-field conditions and permanently fixed into 1-D mesoscopic polymer chains (1-9 mum) in a process referred to as Fossilized Liquid Assembly (FLA). In the FLA process, nanoparticle chains were fixed at the oil interface through photopolymerization, enabling direct visualization of organized mesostructures using atomic force microscopy. Using the FLA methodology, we systematically investigated different conditions and demonstrated that dispersed ferromagnetic colloids possess sufficient dipolar interactions to organize into mesoscopic assemblies. Application of an external magnetic field during assembly enabled the formation of micron-sized chains which were aligned in the direction of the applied field. This universal methodology is an attractive alternative technique to cryogenic transmission electron microscopy (cryo-TEM) for the visualization of nanoparticle assembly in dispersed organic media.     

Surfactant-assisted one-pot synthesis of superparamagnetic magnetite nanoparticle clusters with tunable cluster size and magnetic field sensitivity

Magnetic nanoparticles (MNPs) have many potential biomedical applications. Improvements in their magnetic properties and solubility are necessary for these applications to realize their full potential. In this study, MNPs in the form of raspberry-like magnetite (Fe(3)O(4)) nanoparticle clusters, consisting of tiny Fe(3)O(4) particles with a diameter of approximately 20 nm, were prepared under hydrothermal conditions at 200 °C in the presence of 3,4-dihydroxyhydroxysinnamic acid (DHCA). The primary particles were connected by DHCA molecules to form the clusters, which were well dispersed in water media because a COOH group from DHCA appeared on their surfaces. The cluster size could be tuned from 50 to 400 nm without changing the primary particle size by controlling the reaction time. Therefore, all prepared clusters displayed superparamagnetic properties at room temperature. In addition, the sensitivity of Fe(3)O(4) to an external magnetic field could also be controlled by the cluster size.     

酞菁钴/铁纳米填充母粒组成的磁流变液性能

采用有机/无机原位（insitu）复合方法制备得出酞菁钴/铁纳米填充母粒,与甲基硅油组成磁流变液（MRS）。MRS的附加动态剪切应力（Δτ)与分散介质浓度、外加磁场强度呈正比例关系;剪切速率对Δτ的影响表明磁致流变为链状结构特征;Δτ对温度不敏感;MRS对外加磁场有可逆的开/关变化特征,无记忆效应,磁流变响应时间小于0.1s。     

New insight into mesoporous silica for nano metal-organic framework

In this paper the particle volume fraction and temperature dependence of the dynamic viscosity of highly concentrated transformer oil based magnetic nanofluids was investigated in the absence of an external magnetic field. The solid particle volume fraction dependence of the relative viscosity was found to be very well fitted by the Krieger-Dougherty formula, whence the mean ellipticity of the colloidal particles and the effective surfactant layer thickness were obtained. Using the information on the particles' size and shape statistics obtained from TEM, DLS and magnetogranulometry investigations, it was concluded that the magnetite nanoparticles agglomerate in small clusters of about 1.3 particles/cluster, due to the van der Waals interactions. The effective thickness of the oleic acid surfactant layer was estimated as about 1.4 nm, in very good agreement with the value resulted from previous SANS investigations.     

Stimulated aggregation in ensembles of microcrystals

Aggregates of calcium carbonate microcrystals precipitated from a highly supersaturated aqueous solution were found to form in two stages. At the first stage, high-porosity disordered aggregates (floccules), and at the second, low-porosity ordered aggregates (agglomerates) formed. The application of an acoustic field with a frequency of 2.64 MHz and radiation power 3 W/cm² did not influence the kinetics of formation of microcrystals but accelerated aggregation by four orders of magnitude. This effect was explained by the sonostimulated desolvation of microcrystals colliding with the surface of aggregates and the corresponding decrease in the probability of the detachment from aggregates after collisions. The formation of microcrystals and aggregates in an acoustic field can be described by a Fokker-Planck-type equation. Taking this into account, a model of sonostimulated agglomeration revealing the direction of further investigations was formulated.