Silk: Optical Properties over 12.6 Octaves THz-IR-Visible-UV Range

Area of Science:

• Materials Science
• Biophysics
• Spectroscopy

Background:

• Silk fibroin is a natural protein with diverse applications.
• Understanding silk's structural properties is crucial for optimizing its use.

Purpose of the Study:

• To comprehensively characterize domestic (Bombyx mori) and wild (Antheraea pernyi) silk fibers using a wide spectral range.
• To analyze spectral features related to secondary structures (β-sheet, α-coil) and amorphous fibroin.
• To investigate the spatial distribution of silk constituents and identify specific spectral bands indicative of properties like water solubility and biodegradability.

Main Methods:

• Broadband spectral characterization from THz to deep-UV (0.24–1500 THz).
• Mid-infrared (mid-IR) spectroscopy for analyzing single fiber cross-sections.
• Low-temperature spectroscopy (down to 78 K) to study spectral stability.

Main Results:

• Detailed spectral analysis revealed distinct features for β-sheet, α-coil, and amorphous fibroin across different spectral ranges.
• Mid-IR cross-sections showed an α-coil rich core and β-sheets in wild silk (Antheraea pernyi).
• Specific low-energy T-ray bands (243 cm⁻¹ for Bombyx mori, 229 cm⁻¹ for Antheraea pernyi) and a distinct band at 20±4 cm⁻¹ in Antheraea pernyi were observed and assigned.

Conclusions:

• The study provides a systematic assignment of spectral bands for both domestic and wild silk fibers.
• Observed spectral features directly infer silk's water solubility and biodegradability, crucial for biomedical and sensor applications.
• Structural differences between domestic and wild silk were elucidated through spectral analysis.