Institute of Physical and Theoretical Chemistry, University of Regensburg, Universitätsstrasse 31, 93040 Regensburg, Germany.
Chemistry. 2012 Feb 20;18(8):2272-82. doi: 10.1002/chem.201102407. Epub 2012 Jan 19.
Upon slow crystallization from silica-containing solutions or gels at elevated pH, alkaline-earth carbonates spontaneously self-assemble into remarkable nanocrystalline ultrastructures. These so-called silica biomorphs exhibit curved morphologies beyond crystallographic symmetry and ordered textures reminiscent of the hierarchical design found in many biominerals. The formation of these fascinating materials is thought to be driven by a dynamic coupling of the components' speciations in solution, which causes concerted autocatalytic mineralization of silica-stabilized nanocrystals over hours. In the present work, we have studied the precipitation kinetics of this unique system by determining growth rates of individual aggregates using video microscopy, and correlated the results with time-dependent data on the concentration of metal ions and pH acquired online during crystallization. In this manner, insight to the evolution of chemical conditions during growth was gained. It is shown that crystallization proceeds linearly with time and is essentially reaction controlled, which fits well in the proposed morphogenetic scenario, and thus, indirectly supports it. Measurements of the silica concentration in solution, combined with analyses of crystal aggregates isolated at distinct stages of morphogenesis, further demonstrate that the fraction of silica coprecipitated with carbonate during active growth is rather small. We discuss our findings with respect to the role of silica in the formation of biomorphs, and moreover, prove that the external silica skins that occasionally sheath the aggregates--previously supposed to be involved in the growth mechanism--originate from secondary precipitation after growth is already terminated.
在碱性条件下，从含有硅的溶液或凝胶中缓慢结晶时，碱土金属碳酸盐会自发地自组装成非凡的纳米晶体超结构。这些所谓的硅生物形态表现出超出晶体对称性的弯曲形态和有序的纹理，让人联想到许多生物矿物中发现的层次设计。这些迷人材料的形成被认为是由溶液中组件形态的动态耦合驱动的，这种耦合导致硅稳定的纳米晶体在数小时内协同自动催化矿化。在本工作中，我们通过使用视频显微镜确定单个聚集体的生长速率来研究这个独特体系的沉淀动力学，并将结果与在线结晶过程中获得的金属离子浓度和 pH 值随时间变化的相关数据进行关联。通过这种方式，可以深入了解生长过程中化学条件的演变。结果表明，结晶过程随时间线性进行，基本上是反应控制的，这与所提出的形态发生情景非常吻合，因此间接地支持了该情景。对溶液中硅浓度的测量，结合在形态发生的不同阶段分离出的晶体聚集体的分析，进一步证明了在活性生长过程中与碳酸盐共沉淀的硅的分数相当小。我们根据硅在生物形态形成中的作用讨论了我们的发现，并且证明了偶尔包裹聚集体的外部硅皮——以前被认为参与了生长机制——是在生长已经终止后从二次沉淀中产生的。
