AFM Nanoindentations of Diatom Biosilica Surfaces

Abstract

Diatoms have intricately and uniquely nanopatterned silica exoskeletons (frustules) and are a common target of biomimetic investigations. A better understanding of the diatom frustule structure and function at the nanoscale could provide new insights for the biomimetic fabrication of nanostructured ceramic materials and lightweight, yet strong, scaffold architectures. Here, we have mapped the nanoscale mechanical properties of Coscinodiscus sp. diatoms using atomic force microscopy (AFM)-based nanoindentation. Mechanical properties were correlated with the frustule structures obtained from high-resolution AFM and scanning electron microscopy (SEM). Significant differences in the micromechanical properties for the different frustule layers were observed. A comparative study of other related inorganic material including porous silicon films and free-standing membranes as well as porous alumina was also undertaken.

Structure and mechanical properties of selected protective systems in marine organisms

“…As there is an immense number of different profiles [122], it has been theorized that diatom species have evolved to defend against specific local predators [121]. The strength of the frustule itself varies by up to an order of magnitude along the diatom surface with the maximum strength approaching that of pure silica [123][124][125]. This variation has been associated with different regions of the exoskeleton's growth, with the highest strength region running along the center of the body, effectively creating a load-bearing beam of harder material (Fig.…”

Application of AFM in understanding biomineral formation in diatoms

“…A rigorous nanoindentation study [21], used with permission on this species monitored hardness and elasticity in four distinct structural features [30]. Hardness varied from 0.059-0.53 GPa and elastic modulus from 1.7-6.51 GPa, depending on the feature and its location [30].…”

“…Examining intermediates is a highly useful approach to understand the process of structure formation [5,21] and would likely benefit from application of AFM to monitor nano-and mesoscale features. Micromechanical properties of diatom silica have been measured [1,8,30], but measuring such properties in structural intermediates should provide insight into mechanisms of synthesis.…”

Possible mechanism of structural incorporation of Al into diatomite during the deposition process I. Via a condensation reaction of hydroxyl groups

“…For example, the size of the nano-silica balls, which are the basic components of the diatom shell [28,29,2], most likely changes with the presence of Al in their framework because of the different sizes of Al and Si. In addition, the surface charge and solid acidity vary before and after Al incorporation into the structure [1,9].…”