Micromechanical and structural properties of a pennate diatom investigatedby atomic force microscopy

The mechanisms behind natural nanofabrication of highly structured silicas are increasingly being investigated. We have explored the use of a standard Nanoscope III Multimode atomic force microscope (AFM) to study the silica shell of diatoms. The delicate structures of the shell surface of the diato m Navicula pelliculosa (Breb.) Hilse were imaged and the shell's micromecha nical properties were measured semi-quantitatively with a resolution down t o approximately 10 nm. The technique to measure elasticity and hardness wit h the AFM was demonstrated to be useable even on these hard glass-like surf aces, Different experimental configurations and evaluation methods were tes ted, They gave a consistent result of the shell micromechanical properties, The first results showed that the diatom shell's overall hardness and elas ticity was similar to that of known silicas. However, regions with differen t mechanical proper ties were distinguished. The elastic modulus varied fro m 7 to 20 GPa, from 20 to 100 GPa and from 30 to hundreds of GPa depending on the location. In general, the hardness measurements showed similar spati al differences, The hardness values ranged from 1 to 12 GPa but one specifi c part of the shell was even harder. Hence, certain localized regions of th e shell were significantly harder or more elastic. These regions coincide w ith known characteristic features and mechanisms appearing at the different stages of the shell's growth. These results show that this method serves a s a complementary tool in the study of silica biomineralization, and can de tect eventual crystalline phases.