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

Armandas Balčytis,Meguya Ryu,Xuewen Wang,Saulius Juodkazis,Jeffrey Davis,Dominique Appadoo,Junko Morikawa,Shan Du,Fabio Novelli,Xungai Wang,Jingliang Li,Gediminas Seniutinas

doi:10.3390/ma10040356

Abstract

Domestic (Bombyx mori) and wild (Antheraea pernyi) silk fibers were characterised over a wide spectral range from THz 8 cm ( 1.25 mm, 0.24 THz) to deep-UV cm ( 200 nm, 1500 THz) wavelengths or over a 12.6 octave frequency range. Spectral features at -sheet, -coil and amorphous fibroin were analysed at different spectral ranges. Single fiber cross sections at mid-IR were used to determine spatial distribution of different silk constituents and revealed an -coil rich core and more broadly spread -sheets in natural silk fibers obtained from wild Antheraea pernyi moths. Low energy T-ray bands at 243 and 229 cm were observed in crystalline fibers of domestic and wild silk fibers, respectively, and showed no spectral shift down to 78 K temperature. A distinct cm band was observed in the crystalline Antheraea pernyi silk fibers. Systematic analysis and assignment of the observed spectral bands is presented. Water solubility and biodegradability of silk, required for bio-medical and sensor applications, are directly inferred from specific spectral bands.

Highlights

IntroductionSilk fibers offer a good example of the complexity of protein materials as they have amorphous and crystalline structural components with proteins forming a 3D network of random α-coils and metastable β-turns (Silk I) together with a crystalline β-sheet phase (Silk II) [7]
Spectral properties at sub-1 mm wavelengths at around and below terahertz frequencies (1 THz = 1012 Hz, corresponding to ≈33 cm−1 in wavenumbers) are important for understanding materials with bio-medical relevance [1]
Whereas spectroscopy at the UV and visible ranges provides information about chemical composition, near- and mid-IR offers a wealth of information on the immediate environment of certain bonds, T-ray spectra are exquisitely sensitive to variations in secondary structure, albeit at the cost of specificity

Summary

Introduction

Silk fibers offer a good example of the complexity of protein materials as they have amorphous and crystalline structural components with proteins forming a 3D network of random α-coils and metastable β-turns (Silk I) together with a crystalline β-sheet phase (Silk II) [7]. Such composition results in a set of important properties such as a high mechanical strength, optical transparency and waveguiding [8] as well as biocompatibility and biodegradability [9,10]

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Discussion

Conclusion