Materials scientists at FAU have shown for the first time that the mother-of-pearl in clam shells does not form in a crystallisation process but is a result of the aggregation of nanoparticles within an organic matrix. This could lead to a better understanding of the structure of biomaterials which may be useful in the development of new high-performance ceramics. The findings of the research group led by Prof. Dr. Stephan E. Wolf have been published in the latest issue of the renowned journal Nature Communications (doi: 10.1038/ncomms10097).
Prof. Wolf and his team used a special technique to investigate the structure of mother-of-pearl. Using a diamond wire saw, they cut a 60-centimetre wedge out of the shell of a large Pinna nobilis – a type of clam found in the Mediterranean – which they then polished using a novel method before examining it under a scanning transmission electron microscope. 'We borrowed the wedge-polishing technique from the semiconductor industry,' Stephan Wolf explains. 'This method makes it possible to look at extremely large areas, something that was very difficult to do before.'
Traditional model disproved
The high-resolution images from the scanning transmission electron microscope showed that the structure of the shell is very heterogeneous – from irregular calcite prisms on the outside to the smooth mother-of-pearl on the inside of the shell, with an organic layer in the middle. 'The transition from the organic to the mother-of-pearl layer is particularly interesting,' Stephan Wolf says. 'Here we find the first nanoparticles of between 50 and 80 nanometres in size that aggregate more and more as they get closer to the inside of the shell and merge to form mother-of-pearl platelets, finally forming the highly structured mother-of-pearl that we all know.'
Prefabrication in nature
With their findings the Erlangen-based researchers have shown for the first time that mother-of-pearl does not form through a crystallisation process in which atoms or ions in a saturated solution are deposited successively – as previously thought – but instead forms through the aggregation of prefabricated nanocrystals. 'If we compare the growth process of mother-of-pearl to building a house, the clam uses a kind of prefabricated construction method, while crystallisation is like building a wall out of individual bricks,' Stephan Wolf explains.
An incredibly strong structure
The calcium carbonate nanoparticles group together to form crystalline aragonite platelets. These are the building blocks for mother-of-pearl and gives it its typical shine. 'Individual platelets that are around 350 to 500 nanometres thick are embedded in an organic layer that holds them together like cement,' Stephan Wolf says. 'The fact that this layer structure is made up of smaller particles that also include organic material has a significant influence on the mechanical properties of the clam shell. A comparable crystalline material made of individual ions would break much more quickly.'
A template for new ceramics
Materials scientists at FAU are currently working on reproducing the crystallisation of nanoparticles in the laboratory with the aim of developing high-performance ceramics using templates found in nature. 'We are looking at not only the form and resistance of the materials but also their energetic advantages,' Stephan Wolf emphasises. 'After all, mother-of-pearl doesn't form in an oven, it forms in cold sea water.'
Stephan E. Wolf