Institute of Physical and Theoretical Chemistry, University of Regensburg, Universitätsstrasse 31, D-93040 Regensburg, Germany.
J Am Chem Soc. 2010 Dec 22;132(50):17859-66. doi: 10.1021/ja106959p. Epub 2010 Nov 30.
In biomineralization, living organisms carefully control the crystallization of calcium carbonate to create functional materials and thereby often take advantage of polymorphism by stabilizing a specific phase that is most suitable for a given demand. In particular, the lifetime of usually transient amorphous calcium carbonate (ACC) seems to be thoroughly regulated by the organic matrix, so as to use it either as an intermediate storage depot or directly as a structural element in a permanently stable state. In the present study, we show that the temporal stability of ACC can be influenced in a deliberate manner also in much simpler purely abiotic systems. To illustrate this, we have monitored the progress of calcium carbonate precipitation at high pH from solutions containing different amounts of sodium silicate. It was found that growing ACC particles provoke spontaneous polymerization of silica in their vicinity, which is proposed to result from a local decrease of pH nearby the surface. This leads to the deposition of hydrated amorphous silica layers on the ACC grains, which arrest growth and alter the size of the particles. Depending on the silica concentration, these skins have different thicknesses and exhibit distinct degrees of porosity, therefore impeding to varying extents the dissolution of ACC and energetically favored transformation to calcite. Under the given conditions, crystallization of calcium carbonate was slowed down over tunable periods or completely prevented on time scales of years, even when ACC coexisted side by side with calcite in solution.
在生物矿化过程中，生物体能够精确地控制碳酸钙的结晶过程，从而创造出具有功能性的材料。因此，生物体常常通过稳定最适合特定需求的特定相来利用多晶型现象。特别是，通常处于瞬态的无定形碳酸钙 (ACC) 的寿命似乎受到有机基质的严格调控，因此可以将其用作中间储存库，或者直接用作永久稳定状态下的结构元件。在本研究中，我们表明，即使在更简单的纯非生物体系中，ACC 的时间稳定性也可以被有意地影响。为了说明这一点，我们监测了含有不同量硅酸钠的溶液在高 pH 值下碳酸钙沉淀的进展。结果发现，生长中的 ACC 颗粒会在其附近自发引发硅的聚合，这被认为是由于表面附近 pH 值的局部降低所致。这导致水合无定形硅层沉积在 ACC 颗粒上，从而阻止了颗粒的生长并改变了颗粒的大小。根据硅酸钠的浓度，这些外壳的厚度不同，并且具有不同程度的多孔性，因此在不同程度上阻碍了 ACC 的溶解和有利的方解石转化。在给定的条件下，即使在溶液中 ACC 与方解石共存的情况下，碳酸钙的结晶也可以在数年的时间尺度上被可控地延缓或完全阻止。
