Fibrous aggregation of magnetite nanoparticles induced by a time varied magnetic field
Fibrous aggregation of magnetite nanoparticles induced by a time varied magnetic field
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DOI：
10.1002/anie.200604474
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发表时间：
2007-01-01
影响因子：
16.6
通讯作者：
Gu, Ning
中科院分区：
文献类型：
--
作者：
Sun, Jianfei;Zhang, Yu;Gu, Ning
将积木组装成更复杂的结构，为理论研究和技术应用带来了希望。[1]为了控制或调节组装的结构，通常采用物理力，例如电场或磁场。[2]这种“自下而上”和“自上而下”技术的结合是一个快速发展的领域，然而，关于组装构象与构建块特征之间的关系的报道很少。特别是，在场辅助组装中，线状结构通常是由于诱导的偶极效应而获得的，[3]这使得构建块的特征由于构象变化而难以可视化。我们认为，增加外地的复杂性可能有助于解决这一问题。[4]在本文中，它表明，随时间变化的磁场是能够诱导磁铁矿（Fe 3 O 4）纳米粒子（积木），形成纤维状聚集体，并介导的形态多样性的组装控制的胶体表面电荷。油酸封端的Fe 3 O 4纳米颗粒首先通过以下由Peng等人报道的程序制备。[5]然后用3-氨丙基三乙氧基硅烷（APTS; NH 2（CH 2）3Si（OC 2 H5）3）对胶体表面进行化学改性以形成水溶液。参考文献[6]简要描述了这一过程。最终胶体中的Fe含量约为28 μg/mL，悬浮液的pH值约为7。超声处理后，最终胶体稳定约36 h，每次实验前，将样品超声分散10 min。将合成的胶体溶液（约20 μL）涂在硅载玻片上，并置于交流磁场中。磁场由一个五匝线圈产生，线圈由高频电源供电。为了工业目的，频率固定在约80 kHz。与传统的实验配置相比，该领域是正弦的时间依赖性和它的方向是垂直的组装平面。概念方案如图1所示。在没有随时间变化的磁场的情况下，溶剂蒸发导致纳米颗粒的无定形聚集（见图2a）。[7]然而，当施加随时间变化的磁场时，Fe 3 O 4纳米颗粒聚集成几微米宽，几百微米长的中尺度纤维（见图2b）。基于局部区域的放大，纤维由块状Fe 3 O 4纳米颗粒组成。此外，随着场强的增加，可以观察到从纤维状构象到无定形构象的转变（参见支持信息）。因此，较高的场强不利于一维组装，这与先前的报道相反，在先前的报道中，较强的场强导致组装的改善。此外，集体磁性（图3）表明，纤维聚集体具有与单个纳米颗粒相同的磁性（10 nm的Fe 3 O 4纳米颗粒是超顺磁性的），而先前通过静磁场辅助组装制备的这种磁性结构显示出铁磁性行为。[8]粒子系综的超顺磁性反映了纳米粒子之间的松散接触，因此两个磁矩之间的距离太大，以至于量子交换机制无法工作（海森堡模型）。[9]上述现象对于其他表面涂层，如二巯基琥珀酸（DMSA; HOOCCH（SH）CH（SH）COOH）、酒石酸和谷氨酸是可再现的。然而，对于油酸封端的Fe_3O_4…
Assembly of building blocks into more complex structures holds promise for both theoretical research and technological applications.[1] To control or modulate the assembled structures, physical forces, such as electric or magnetic fields, are often employed.[2] This combination of “bottom-up” and “top-down” techniques is a rapidly growing field, however, there have been few reports on the relationship between the assembled conformation and the features of the building block. In particular, in field-assisted assembly, wirelike structures are generally obtained as a result of the induced dipolar effect,[3] which makes the features of the building block hard to visualize due to conformational variations. We believe that the augmentation of field complexity may help to resolve this problem.[4] Herein, it is demonstrated that a time-varied magnetic field is capable of inducing magnetite (Fe3O4) nanoparticles (building blocks) to form fibrous aggregates, and of mediating the morphological diversity of assembly that is controlled by colloidal surface charge. Oleic acid capped Fe3O4 nanoparticles were first prepared by following the procedure reported by Peng et al.[5] Then the colloidal surfaces were chemically modified with 3-aminopropyltriethoxysilane (APTS; NH2 (CH2) 3Si (OC2H5) 3) to form an aqueous solution. This process was briefly described in reference [6]. The Fe content in the final colloid was about 28 μg mLÀ1 and the pH value of the suspension was approximately 7. After ultrasonic treatment, the final colloid was stable for about 36 h, and before each experiment the sample was ultrasonically dispersed for 10 min. The as-synthesized colloidal solution (about 20 μL) was spread on a silicon slide and subjected to an alternatingcurrent magnetic field. The field was generated by a five-turn coil that was energized by a high-frequency power supply. For its industrial purpose, the frequency was fixed at about 80 kHz. In contrast to conventional experimental configurations, the field was sinusoidally time-dependent and its direction was normal to the assembly plane. The conceptual scheme is illustrated in Figure 1.In the absence of a time-varied magnetic field, solvent evaporation results in amorphous aggregation of nanoparticles (see Figure 2 a).[7] However, when a time-varied magnetic field is applied, the Fe3O4 nanoparticles aggregate into mesoscale fibers that are several micrometers wide and several hundreds of micrometers long (see Figure 2b). Based on the magnification of a localized area, the fibers consist of massive Fe3O4 nanoparticles. In addition, at increasing field intensity the transition from a fiberlike conformation to an amorphous conformation can be observed (see Supporting Information). Hence, the higher field intensity does not favor one-dimensional assembly, in contrast to previous reports where a stronger field leads to improved assembly. Moreover, the collective magnetic properties (Figure 3) reveal that the fibrous aggregates have a common magnetism with individual nanoparticles (the 10-nm Fe3O4 nanoparticle is superparamagnetic), whereas such magnetic structures previously prepared by magnetostatic-field-assisted assembly show a ferromagnetic behavior.[8] The superparamagnetism of the particulate ensemble reflects the loose contact between nanoparticles, so that the distance between two magnetic moments is too large to permit the quantum-exchange mechanism to work (Heisenberg model).[9] The above-mentioned phenomenon is reproducible for other surface coatings, such as dimercaptosuccinic acid (DMSA; HOOCCH (SH) CH (SH) COOH), tartaric acid, and glutamic acid. However, for oleic acid capped Fe3O4 …