Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 2003, 42 (01), 58 - 61
Synthesis of star-shaped titanium dioxide/silicon dioxide shells -
mimicking biomineralization of marine protozoa
A strange microcosm meets the eye of any observer who looks at plankton under a microscope. A group of scientists working with Dirk Volkmer in Bielefeld were especially taken with a certain class of these diverse single-celled species: the radiolaria. These tiniest of life forms have elaborate, glasslike shells that are made of silicon dioxide and have fine radial "spines". Scientists have asked themselves how it is that these "mini-hedgehogs" and related species can construct such complex structures out of inorganic materials that are, in themselves, unstructured. A clarification of these processes, known as biomineralization, will hopefully provide new approaches to material science.
Volkmer and his co-workers have developed a simple model system with which they can replicate the formation of the spiny shells. They start with tiny drops of oil that are mixed into water. To stabilize the solution, both liquids also contain surfacants (molecules of soap). If the right type of surfacant is chosen, wavelike bumps become visible on the surface of the oil drops. The bumps then slowly grow into "spines". Once they reach a certain size, the spines separate and submicroscopic droplets begin to form around the original oil drop, until it has completely disintegrated and spread into the solution.
If precursors of the minerals silicon dioxide and titanium dioxide are initially added to the oil, they come into contact with the water on the surface of the droplets. This causes the precursors to decompose, releasing the oxides, which collect at the interface of the oil drop and the water, forming a layer of mineral. This results in a very thin, hollow shell. For the creation of an artificial "mini-hedgehog", it is critical that mineralization occurs exactly at the moment at which the spiny branches are formed on the oil drop. Only then can the starlike shape be "frozen". For this to work, the concentrations of the precursors must be correct within one percent. "In this way we have been able to make very robust mineral shells that bear a striking resemblance to the silicon dioxide skeletons of the radiolaria," reports Volkmer.