6 Cocoon Silk: From Mesoscopic Materials Design …
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can be applied to determine the morphology of nanofibrils in a straightforward and
effective way. In general, SEM, TEM, and AFM are the three most popular imaging
techniques. Although all of these techniques have been successfully applied for
obtaining high-quality images of SF nanofibrils, they vary significantly in operation
environment, horizontal/vertical resolution, and specific requirements for sample
preparation. For instance, SEM and TEM must be operated in vacuum conditions.
In comparison, AFM imaging can be applied in air, fluid, or vacuum conditions.
SEM and TEM can provide information in the horizontal direction. However, AFM
height imaging is capable of detecting the heights of samples at a very high resolution
(0.1 nm). Because SF materials are non-conductive, they need to be coated with a
layer of gold or platinum prior to SEM and TEM imaging.
6.4.2.2 Scanning Electron Microscopy
SEM is capable of scanning SF material samples with a focused electron beam
instead of light and delivering greatly magnified images with information about the
topology of the samples as well as their composition. The principle of SEM is that
the incident electrons can interact with the atoms within the SF material samples so
that different parts of the samples produce various signals, that is, secondary electrons, backscattered electrons, and characteristic X-rays. These signals can reveal
hidden information about the SF material samples. Specifically, the secondary electron signals are associated with the topography of the samples; the backscattered
electrons can provide information about the phase contrast in the samples; and the
characteristic X-rays can be applied for element identification (called the energy
dispersive X-ray spectroscopy technique).
To date, SEM has been widely applied to examine the micro/nanostructures within
different forms of SF materials. For instance, Nguyen et al. [34] imaged freeze-dried
RSF hydrogels and found that they were composed of numerous nanofibrils with a
random distribution. The corresponding SEM image clearly shows that the average
diameter of the nanofibrils ranges from 20 to 50 nm [34]. For natural silk fibers, SEM
has successfully distinguished the sericin coating from the core fibroin filaments
(Fig. 6.22a) [6]. However, because silk fibers are non-conductive materials, they
require a gold spraying process prior to SEM imaging. In addition, the nanofibrils
within silk fibers are bundled very close to each other. For these reasons, it is much
more difficult to obtain satisfactory images of the nanofibril network on the silk
fiber surface. Nevertheless, upon proper sample preparation (e.g., freeze-drying and
controlling spraying time), the morphology of nanofibrils within silk fibers can still
be observed (Fig. 6.18b, not yet published).
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can be applied to determine the morphology of nanofibrils in a straightforward and
effective way. In general, SEM, TEM, and AFM are the three most popular imaging
techniques. Although all of these techniques have been successfully applied for
obtaining high-quality images of SF nanofibrils, they vary significantly in operation
environment, horizontal/vertical resolution, and specific requirements for sample
preparation. For instance, SEM and TEM must be operated in vacuum conditions.
In comparison, AFM imaging can be applied in air, fluid, or vacuum conditions.
SEM and TEM can provide information in the horizontal direction. However, AFM
height imaging is capable of detecting the heights of samples at a very high resolution
(0.1 nm). Because SF materials are non-conductive, they need to be coated with a
layer of gold or platinum prior to SEM and TEM imaging.
6.4.2.2 Scanning Electron Microscopy
SEM is capable of scanning SF material samples with a focused electron beam
instead of light and delivering greatly magnified images with information about the
topology of the samples as well as their composition. The principle of SEM is that
the incident electrons can interact with the atoms within the SF material samples so
that different parts of the samples produce various signals, that is, secondary electrons, backscattered electrons, and characteristic X-rays. These signals can reveal
hidden information about the SF material samples. Specifically, the secondary electron signals are associated with the topography of the samples; the backscattered
electrons can provide information about the phase contrast in the samples; and the
characteristic X-rays can be applied for element identification (called the energy
dispersive X-ray spectroscopy technique).
To date, SEM has been widely applied to examine the micro/nanostructures within
different forms of SF materials. For instance, Nguyen et al. [34] imaged freeze-dried
RSF hydrogels and found that they were composed of numerous nanofibrils with a
random distribution. The corresponding SEM image clearly shows that the average
diameter of the nanofibrils ranges from 20 to 50 nm [34]. For natural silk fibers, SEM
has successfully distinguished the sericin coating from the core fibroin filaments
(Fig. 6.22a) [6]. However, because silk fibers are non-conductive materials, they
require a gold spraying process prior to SEM imaging. In addition, the nanofibrils
within silk fibers are bundled very close to each other. For these reasons, it is much
more difficult to obtain satisfactory images of the nanofibril network on the silk
fiber surface. Nevertheless, upon proper sample preparation (e.g., freeze-drying and
controlling spraying time), the morphology of nanofibrils within silk fibers can still
be observed (Fig. 6.18b, not yet published).