Angewandte Chemie International Edition

Cover image for Vol. 56 Issue 28

Editor: Peter Gölitz, Deputy Editors: Neville Compton, Haymo Ross

Online ISSN: 1521-3773

Associated Title(s): Angewandte Chemie, Chemistry - A European Journal, Chemistry – An Asian Journal, ChemistryOpen, ChemPhotoChem, ChemPlusChem, Zeitschrift für Chemie

For full article and contact information, see Angew. Chem. Int. Ed. 2002, 41 (9), 1543 - 1546

No. 09/2002

Bizarre Framework

Diatoms could point the way to the synthesis
for highly ordered silicic acid structures

When you look at phytoplankton under a microscope, a spectacular microcosm is revealed. Its main component: single-celled diatoms. Their most striking features are their glassy-looking shells, which have a strange pattern of regular channels, grooves, and depressions running throughout.. Each subspecies develops its own individual, characteristic architecture. How these stable, porous, highly structured silicic acid skeletons are formed has long been a mystery.

Researchers in the Netherlands now seem to have solved part of the puzzle. Specific porous silicon dioxide materials can be generated when polymers act as a sort of matrix. In the diatoms, peptides and other biopolymers serve in this capacity, that much seemed clear. But how could this result in a structure with a regular pattern of very different, large pores? And how do pore diameters that are many times larger than the "matrix" come into being?

Engel Vrieling, Theo Beelen, Rutger van Santen, and Winfried Gieskes were experimenting with different polymers as the "matrix" under conditions similar to those that exist within diatoms. The result is an interesting hypothesis:

At the beginning of its "building phase", the algae transports silicic acid monomers inside special vesicles. "There, small biopolymers, silaffines and polyamines, cause a rapid coagulation of small silicic acid particles. Then larger peptides come into play," explains Vrieling. "They not only interact with the silicic acid particles, but also with each other, forming little peptide clumps. As the silicid acid particles continue to aggregate, the peptide clumps act as a "matrix". This results in a highly ordered, quasi-liquid-crystalline phase consisting of silicic acid aggregates and peptide aggregates." The size and arrangement of the aggregates depends on the nature of the "matrix" peptides. Eventually this forms a three-dimensional structure. Once the building blocks for the diatoms’ shells are done, the peptides are removed from the mixture, leaving behind the characteristic pattern of pores. Vrieling compares this process to a bronze casting, whose wax mold melts away.

The results of this research are not merely of academic interest; porous silicic acid microstructures are highly coveted, for such things as tailored catalysts or absorbents. Perhaps the diatom-method can be mimicked to form specific materials.