Formation of Asymmetrical Structured Silica Controlled by a Phase Separation Process and Implication for Biosilicification

Jia-Yuan Shi,Xi-Ming Li,Qi-Zhi Yao,Sheng-Quan Fu,Gen-Tao Zhou,Vipul Bansal

doi:10.1371/journal.pone.0061164

Abstract

Biogenetic silica displays intricate patterns assembling from nano- to microsize level and interesting non-spherical structures differentiating in specific directions. Several model systems have been proposed to explain the formation of biosilica nanostructures. Of them, phase separation based on the physicochemical properties of organic amines was considered to be responsible for the pattern formation of biosilica. In this paper, using tetraethyl orthosilicate (TEOS, Si(OCH2CH3)4) as silica precursor, phospholipid (PL) and dodecylamine (DA) were introduced to initiate phase separation of organic components and influence silica precipitation. Morphology, structure and composition of the mineralized products were characterized using a range of techniques including field emission scanning electron microscopy (FESEM), transmission electron microscope (TEM), X-ray diffraction (XRD), thermogravimetric and differential thermal analysis (TG-DTA), infrared spectra (IR), and nitrogen physisorption. The results demonstrate that the phase separation process of the organic components leads to the formation of asymmetrically non-spherical silica structures, and the aspect ratios of the asymmetrical structures can be well controlled by varying the concentration of PL and DA. On the basis of the time-dependent experiments, a tentative mechanism is also proposed to illustrate the asymmetrical morphogenesis. Therefore, our results imply that in addition to explaining the hierarchical porous nanopatterning of biosilica, the phase separation process may also be responsible for the growth differentiation of siliceous structures in specific directions. Because organic amine (e.g., long-chair polyamines), phospholipids (e.g., silicalemma) and the phase separation process are associated with the biosilicification of diatoms, our results may provide a new insight into the mechanism of biosilicification.

Highlights

Biomineralization is the formation of hard tissues with complex structures and multifunctional properties, which occurs in almost all the living organisms from prokaryotes to humans [1,2]
The goal of this study is to examine the effect of phase separation of biosilicification-associated model organic components on the development of silica morphology, and to reveal the contribution of the organic phase separation to growth differentiation of biogenic silica
The centrifugated precipitate could dissolve in ethanol, and no silica could be obtained in this case, indicating that the isolated precipitate should be organics, i.e., an undissolvable organic phase was first formed at room temperature

Summary

Introduction

Biomineralization is the formation of hard tissues with complex structures and multifunctional properties, which occurs in almost all the living organisms from prokaryotes to humans [1,2]. Some of the morphologically gorgeous and structurally intricate biominerals are exemplified by the biosilica formed in the aquatic organisms including diatoms and sponges [3,4]. These biogenic minerals are structured in the nanometer to micrometer scale range, and composed of amorphous silica [5,6,7]. More than 40 years ago, Nakajima and Volcani have noticed that diatom biosilica contained unusual amino acid derivatives such as N,N,Ntrimethylhydroxylysine and dihydroxyproline [10,11]. An emerging consensus is that polysaccharides [14,15], proteins [16,17,18,19,20], and polyamines [21]

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Results