Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 1998, 37 (21), 3044 - 3047
Pearls in the Laboratory
Researchers make artificial pearls
by mimicking biomineralisation processes
What have pearls, mussel and snail shells, teeth and bones got in common? The answer is that they all consist of simple inorganic materials - mussel shells and pearls from the minerals calcite and aragonite, and teeth and bones from apatite. Despite the fact that these materials only occur in crystalline form in nature, evolution has been able to produce all the above, vastly more exotic structures from them. Chemists have long wished to emulate the processes used in nature to achieve such varied results. Wolfgang Tremel, Jörg Küther and Ram Seshadri from the University of Mainz have now succeeded in taking an interesting step in this direction.
However they did set themselves a rather simpler goal than man-made snail shells, namely artificial pearls. "They are the simplest example because they are such beautifully symmetrical spheres," Tremel says. So how did he and his team manage to grow calcite for the synthetic pearls in the shape of spheres, rather than crystal cubes as would normally happen? "We used the fact that calcite is readily deposited from aqueous solution onto a pretreated gold surface." This pretreatment is the secret which controls the desired growth of the solid mineral. "So we prepared small gold spheres with specially treated surfaces; the calcite then formed a layer on the gold and continued to grow outwards in spherical form." According to experimental conditions, either 'pearls' with nearly smooth surfaces were formed, or else structures appearing as many small crystals stuck together.
Of course this is only the first step; designer snail shells with gold trim are still a long way off. "Nevertheless," reports Tremel, "this is the first time that we have been able to build inorganic materials in specific 3-D shapes starting from very simple molecules." The chemists in Mainz can indeed control the nature of the deposited material very well (calcite is only one possibility) but one unsolved question is how to control the pearl size. Despite their achievements so far, the teams' artificial pearls are still very small and fragile - not yet quite necklace material. "Our ultimate aim - albeit in the distant future - is to build artificial teeth and bones out of identical materials as the real thing: such developments would naturally be far more valuable to those they helped than a string of man-made pearls."