New approaches for physical nano-characterisation of spider silks
The light scattering experiment was designed to study a single thread of spider silk as a microoptical element. A silk thread was irradiated with a narrow, collimated beam of white light at a right angle to the axis of the thread. The light scattered by the silk was measured at various angles around the thread with high spectral and angular resolution. The experimental data were consistent with the theory of plane wave scattering by a single, multilayered or two parallel dielectric cylinders, depending on the type of the silk. It has been shown that the matching of the experimental and the theoretically modelled spectra is a convenient tool for the determination of the size and morphology of a tiny micro-thread.
Transmission electron microscopy was performed in order to reveal the genuine morphology and structure of the silk and how it was impacted by the use of standard fixatives. Standard fixatives were alternately excluded from the conventional sample preparation protocol. A fraction of the silk threads swelled during sample preparation, which was attributed to the uptake of liquid fixatives through the micro-breakages in the skin layer of the silk. However, the extent to which the skin layer of the silk was differentiated on TEM micrographs was shown to depend primarily on the use of fixatives, and the untreated silk showed no explicit contrast between multiple layers.
A pilot secondary ion mass spectroscopy study of spider silk revealed potassium, sodium, phosphorous, chlorine and fluorine in the elemental composition of the silk. The spatial and depth distributions of these elements within the silk were obtained at a nano- and microscale. Some elements of the silk structure, such as the outer layers of the thread, were differentiated and possibly transformed via the primary ion beam during measurement. The unexpected edge effects were discovered along the perimeters of the measured areas, which have been interpreted as the sputter products of the silk having been moved to the sides of a raster by the dynamic primary ion beam.