Physical characterization of functionalized spider silk: electronic and sensing properties

Abstract

This work explores functional, fundamental and applied aspects of naturally harvested spider silk fibers. Natural silk is a protein polymer where different amino acids control the physical properties of fibroin bundles, producing, for example, combinations of β-sheet (crystalline) and amorphous (helical) structural regions. This complexity presents opportunities for functional modification to obtain new types of material properties. Electrical conductivity is the starting point of this investigation, where the insulating nature of neat silk under ambient conditions is described first. Modification of the conductivity by humidity, exposure to polar solvents, iodine doping, pyrolization and deposition of a thin metallic film are explored next. The conductivity increases exponentially with relative humidity and/or solvent, whereas only an incremental increase occurs after iodine doping. In contrast, iodine doping, optimal at 70• C, has a strong effect on the morphology of silk bundles (increasing their size), on the process of pyrolization (suppressing mass loss rates) and on the resulting carbonized fiber structure (that becomes more robust against bending and strain). The effects of iodine doping and other functional parameters (vacuum and thin film coating) motivated an investigation with magic angle spinning nuclear magnetic resonance (MAS-NMR) to monitor doping-induced changes in the amino acid-protein backbone signature. MAS-NMR revealed a moderate effect of iodine on the helical and β-sheet structures, and a lesser effect of gold sputtering. The effects of iodine doping were further probed by Fourier transform infrared (FTIR) spectroscopy, revealing a partial transformation of β-sheet-to-amorphous constituency. A model is proposed, based on the findings from the MAS-NMR and FTIR, which involves iodine-induced changes in the silk fibroin bundle environment that can account for the altered physical properties. Finally, proof-of-concept applications of functionalized spider silk are presented for thermoelectric (Seebeck) effects and incandescence in iodine-doped pyrolized 1468-6996/11/055002+13$33.00 1 © 2011 National Institute for Materials Science Printed in the UK Sci. Technol. Adv. Mater. 12 (2011) 055002 E Steven et al silk fibers, and metallic conductivity and flexibility of micron-sized gold-sputtered silk fibers. Inthelattercase,wedemonstratetheapplicationofgold-sputteredneatspidersilktomake four-terminal, flexible, ohmic contacts to organic superconductor samples.

Exploring the Properties of Genetically Engineered Silk‐Elastin‐Like Protein Films

“…The dc conductivity of the cast films was 4.6 × 10 −13 S cm −1 for H 2 O‐SELP‐59‐A and 3.0 × 10 −13 S cm −1 for FA‐SELP‐59‐A, values found to be within the range reported for insulators with similar values to those of natural rubber (10 −13 –10 −14 S cm −1 ) . Regarding natural polymers, chitosan membranes showed a conductivity of 2.2 × 10 −8 S cm −1 in the dry state and 8.3 × 10 −4 S cm −1 upon hydration; whereas values of 4.4 × 10 −8 S cm −1 and 4.4 × 10 −15 S cm −1 were found for silk strands from Pholcus phalangioides and electrospun membranes of silk fibroin, respectively.…”

Carbon nanotubes on a spider silk scaffold

“…Moreover, starting from its intrinsic properties, SS fibres can serve as a versatile scaffold upon which additional functions can be built. For example, CdTe15, magnetite16 and gold1617 nanoparticles can be used to functionalise SS for fluorescent, magnetic and electronic applications, respectively. Gold-functionalised fibres (Au-SS) have been shown to be electrically robust down to cryogenic temperatures17.…”

“…For example, CdTe15, magnetite16 and gold1617 nanoparticles can be used to functionalise SS for fluorescent, magnetic and electronic applications, respectively. Gold-functionalised fibres (Au-SS) have been shown to be electrically robust down to cryogenic temperatures17. Even though Au-SS possesses sufficient flexibility for use as electrodes in microelectronics17, generally its elasticity and electrical continuity are not adequate for electronic sensors or actuating devices.…”

“…Gold-functionalised fibres (Au-SS) have been shown to be electrically robust down to cryogenic temperatures17. Even though Au-SS possesses sufficient flexibility for use as electrodes in microelectronics17, generally its elasticity and electrical continuity are not adequate for electronic sensors or actuating devices.…”

Physico-Chemical Studies on Raw and Processed Moth Caterpillar Silks from the Mega Biodiversity Hotspots of India

“…Cotton and wool have received much attention in this context, while silk was popular among the rich. Natural silk is a macromolecule comprising polymeric protein in which the physical properties of fibroin bundles are controlled by different amino acids which leads to the production of, say, combinations of -sheet (crystalforming blocks) and amorphous (sometimes helical) structural regions 1,2 . Such combinations of fibroin bundles form the backbone of fibres produced by silk worms or spiders.…”