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Biologically Inspired Materials

 

Morphosynthesis of Artificial Radiolaria

 

We have developed a novel synthetic approach which leads to formation of inorganic shells with complex structures reminiscent to the silicified shells of certain marine microrganisms (Fig.1). Our model system is based on a surfactant-stabilized oil droplet which contains a monomeric inorganic precursor (e.g. tetramethoxysilane). Hydrolysis of the precursor liberates highly reactive monomeric silicic acid at the oil droplet surface which undergoes further condensation finally leading to a silica shell that encompasses the oil droplet. The oil droplets are generated by means of microinjection thus enabling videomicroscopic observation of the mineralization process (Fig.2).

 

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Fig.1: Discrete inorganic shells of star-shape morphology are produced by a self-organized dynamic growth process ("morphosynthesis") which involves mineral formation at a membrane-type surface

 

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Fig.2: Microinjection workstation

 

This relatively simple system was chosen because it shows decisive features characteristic of biological systems. It has a primitive silicon “metabolism” that liberates monomeric silicic acid from a masked storage deposit. The release of hydrolytically instabile metal oxide precursors occurs at a membrane-type surface. The oil droplet dimensions are comparable to the size of typical silicifying unicellular organisms, ranging from 10 to 100 microns. Due to the limited amount of metal oxide precursor contained within the oil droplet, the shell formation process is intrinsically self-terminating. Finally, but most importantly, appropriately chosen organic additives being dissolved in the oil phase (the aqueous solution, respectively) may induce dynamic self-organization processes that ultimately lead to formation of discrete hollow shells of complex shapes.
Among the many experiments conducted so far, our attention was mostly attracted by a particular system which leads to the morphosynthesis of discrete star-shaped shells (Fig.3) bearing a striking resemblance to the silicified skeletons of radiolaria (“Strahlentierchen”).

 

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Fig.3: Timelapse recording of spine formation of a chlorocyclohexane droplet containing arachidic acid (c = 42.4 mM) and tetra-tert-butyl orthotitanate (TBOT, 4 mass percent) after microinjection into an aqueous solution of cetyltrimethylammonium bromide (CTAB, c = 0.556 mM). (Differential interference contrast optical micrographs)

 

We were able to show that the genesis of spiny morphologies takes place even in the absence of an inorganic precursor, although the developing structures are metastable under these conditions. In contrast, by adding an inorganic precursor we obtain mineralized star-shaped particles that are long-term stable, which indicates that the shaping process is largely unaffected by the mineralization. The experience gained from single droplet experiments was used to develope a (simple and reproduceable) bulk synthesis (Fig.4) that allows to prepare star-shaped silicified particles on larger scales.

 

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Fig.4: Scanning electron micrographs of star-shaped titania-silica shells from bulk sample preparation procedures. Left: Typical morphology of an air-dried sample demonstrating the robust nature of the mineralized shells. Right: Fractured sample indicating the hollow tubular nature of mineralized spines.

 

Our model system demonstrates how a self-organizing growth process involving a membrane-like interface can be employed to obtain intricately shaped inorganic particles. From our point of view the described system is a novelty in the field of materials science. The goal of future investigations will be to gain further insights into the (supra-)molecular self-organisation that underlies the described morphosynthesis.

 

Comments on the morphosynthesis of our artificial radiolaria skeletons have appeared at various locations / newspapers !

Editors' Choice:

Science 2003, 299, 167

Frankfurter Allgemeine Zeitung, 2. Januar 2003

Chemical & Engineering News, 2003, 81, 11

The Economist, 25th January 2003

David Bradley's Reactive Reports, Issue #30

VCH Press Releases (also in German language)

Bild des Monats, Spektrum der Wissenschaft, Februar 2003, 23

Chemie in unserer Zeit 2003, 37, 86

 

 

 

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