工频交变磁场是我们日常中一种重要的磁场，因为我们每时每刻都暴露在该磁场作用下。因此，在低频交变磁场作用下，研究胶体颗粒对外场频率的行为表现，不仅有助于我们对自组装机理的理解，更重要的是有利于胶体颗粒在生物医学上的医用。本文中，我们利用工频交变磁场结合溶剂挥发诱导的自组装，尝试用垂直交变磁场和水平交变磁场对裸Fe3O4磁性纳米颗粒在硅基底上的自组装形态调控。
研究表明，低频交变磁场可以诱导裸Fe3O4形成与高频交变磁场类似的准一维的组装结构，同时低频交变磁场的垂直施加作用形成纤维辐射状结构。有趣的是在低频交变磁场作用下，组装体仍旧保持超顺磁性。我们相信我们目前工作了对自组装过程的有进一步了解，也为下一步图案化的表面与界面与细胞的相互作用。
Weixin Zhang, Jianfei Sun, Tingting Bai, Chunyu Wa, Quasi-One-Dimensional Assembly of Magnetic Nanoparticles Induced by a 50 Hz Alternating Magnetic Field ,ChemPhysChem 2010, 11, 1867 – 1870
Figure 1 Effect of solvent evaporation on the assembly pattern. The nine images show an actinomorphous pattern of assembled fibers in a certain area. The field is applied vertically to the substrate and the intensity is 72 KAm-1.
Figure 2 Hysteresis loops of Fe3O4 nanoparticles after natural drying (●), the treatment of the vertically-applied (▲) alternating magnetic field and the treatment of parallel-applied (▼) alternating magnetic field. The hysteresis loops of Fe3O4 nanoparticles after vertically-applied (▲) and parallel-applied (▼) alternating magnetic field coincide with each other closely.
Figure 3 The possible mechanism of assembly. a) The cluster was modeled as dipole (a dipole can be considered as a couple of magnetic charges). b) In the presence of parallel-applied magnetostatic field, the particles formed head-to-tail structures. In the case of the parallel-applied alternating magnetic field, the result is similar. c) In the presence of the vertically-applied magnetostatic field, the dipolar moment is parallel. And the interacted force is repulsive. d) In the presence of the vertically-applied alternating magnetic field, the dipolar moment varied with the external field. When the dipolar moment is anti-parallel, the interacted force is attractive.
