Abstract
Molecular alignment underpins optical, mechanical, and thermal properties of materials, however, its direct measurement from volumes with micrometer dimensions is not accessible, especially, for structurally complex bio-materials. How the molecular alignment is linked to extraordinary properties of silk and its amorphous-crystalline composition has to be accessed by a direct measurement from a single silk fiber. Here, we show orientation mapping of the internal silk fiber structure via polarisation-dependent IR absorbance at high spatial resolution of 4.2 μm and 1.9 μm in a hyper-spectral IR imaging by attenuated total reflection using synchrotron radiation in the spectral fingerprint region around 6 μm wavelength. Free-standing longitudinal micro-slices of silk fibers, thinner than the fiber cross section, were prepared by microtome for the four polarization method to directly measure the orientational sensitivity of absorbance in the molecular fingerprint spectral window of the amide bands of β-sheet polypeptides of silk. Microtomed lateral slices of silk fibers, which may avoid possible artefacts that affect spectroscopic measurements with fibers of an elliptical cross sections were used in the study. Amorphisation of silk by ultra-short laser single-pulse exposure is demonstrated.
Highlights
How the molecular alignment is linked to extraordinary properties of silk and its amorphous-crystalline composition has to be accessed by a direct measurement from a single silk fiber
Free-standing longitudinal micro-slices of silk fibers, thinner than the fiber cross section, were prepared by microtome for the four polarization method to directly measure the orientational sensitivity of absorbance in the molecular fingerprint spectral window of the amide bands of β-sheet polypeptides of silk
Substrate-free absorbance measurements of silk fibers, with lateral resolution defined by NA = 0.5 for the far-field transmission and NA = 2.4 for the attenuated total reflection (ATR) Fourier transform IR (FT-IR) hyper-spectral mapping, have shown consistency between spatial localisation of the Amide I and II bands in the silk fiber
Summary
Mechanical, and thermal properties of materials, its direct measurement from volumes with micrometer dimensions is not accessible, especially, for structurally complex bio-materials. We show orientation mapping of the internal silk fiber structure via polarisationdependent IR absorbance at high spatial resolution of 4.2 μm and 1.9 μm in a hyper-spectral IR imaging by attenuated total reflection using synchrotron radiation in the spectral fingerprint region around 6 μm wavelength. In the IR spectral range, a combination of sub-wavelength spatial resolution to characterise the anisotropy of absorbance due to local molecular orientation and spatial 2D (3D) mapping would enhance current analytical techniques and has high potential in material and bio-medical fields. In addition the use of a synchrotron beam offers highly collimated IR radiation with 102–103 times higher brightness than that available from laboratory-based
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