Size-dependent properties of magnetoferritin
Size-dependent properties of magnetoferritin
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DOI：
10.1088/0957-4484/21/46/465707
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发表时间：
2010-11-19
期刊：
影响因子：
3.5
通讯作者：
Luis, F.
中科院分区：
文献类型：
--
作者：
Martinez-Perez, M. J.;de Miguel, R.;Luis, F.
我们报告了在脱铁铁蛋白生物模具内合成的磁赤铁矿纳米颗粒的详细实验研究，其尺寸范围为 1.6 至 6 nm。使用 TEM 和 X 射线衍射对无机核的结构表征表明，结晶有序度较低，这可能是由于每个分子内多个域的成核和生长引起的。我们还通过液体中的原子力显微镜研究了分子结构。我们发现脱铁铁蛋白内部纳米颗粒的合成导致蛋白质外径出现小幅但可测量的减小，这可能与构象变化有关。通过交流磁化率、直流磁化和穆斯堡尔谱等技术的组合研究了磁赤铁矿核心的磁响应。根据平衡磁响应，我们确定了每个分子的磁矩分布。结果显示磁矩大大降低。这种效应不能仅仅归因于位于颗粒表面的自旋的倾斜，而是表明磁性铁蛋白核心具有高度无序的磁性结构，其中不同域的贡献相互补偿。最后，我们还确定了每个样品磁化反转所需的活化能的分布，并由此确定了与尺寸相关的磁各向异性常数 K。我们发现 K 相对于磁赤铁矿体积值大大增强，并且随着尺寸的减小而增加。穆斯堡尔谱表明，低对称性原子位点可能位于颗粒表面和不同晶域之间的界面处，可能是增强磁各向异性的来源。
We report a detailed experimental study of maghemite nanoparticles, with sizes ranging from 1.6 to 6 nm, synthesized inside a biological mould of apoferritin. The structural characterization of the inorganic cores, using TEM and x-ray diffraction, reveals a low degree of crystalline order, possibly arising from the nucleation and growth of multiple domains inside each molecule. We have also investigated the molecular structure by means of atomic force microscopy in liquid. We find that the synthesis of nanoparticles inside apoferritin leads to a small, but measurable, decrease in the external diameter of the protein, probably associated with conformational changes. The magnetic response of the maghemite cores has been studied by a combination of techniques, including ac susceptibility, dc magnetization and Mossbauer spectroscopy. From the equilibrium magnetic response, we have determined the distribution of magnetic moments per molecule. The results show highly reduced magnetic moments. This effect cannot be ascribed solely to the canting of spins located at the particle surface but, instead, it suggests that magnetoferritin cores have a highly disordered magnetic structure in which the contributions of different domains compensate each other. Finally, we have also determined, for each sample, the distribution of the activation energies required for the magnetization reversal and, from this, the size-dependent magnetic anisotropy constant K. We find that K is enormously enhanced with respect to the maghemite bulk value and that it increases with decreasing size. The Mossbauer spectra suggest that low-symmetry atomic sites, probably located at the particle surface and at the interfaces between different crystalline domains, are the likely source of the enhanced magnetic anisotropy.