Unravelling new mechanical, optical and magnetic properties in silk

Abstract

Silk, a natural protein fibre, exhibits extraordinary toughness and non-linear elasticity, with its structural properties and functional roles, ranging from prey capture to web robustness, being well-documented. Its unique bio-compatible molecular structure presents a significant advantage for studying protein-folding problems and developing various biomedical and indus- trial applications. In this thesis, we unveil new mechanical, optical, and magnetic properties of silk. Humidity significantly affects silk fibres, causing them to contract in length and increase in thickness, a phenomenon known as supercontraction, which is well-studied regarding its impact on the silk’s overall strength and structural changes. However, the functional impli- cations of supercontraction remain unclear. We examine the response of silk fibre to multiple cyclic loading and unloading, quantifying its molecular fatigue by measuring the reductions in strength and work done per cycle. Water, acting as a plasticiser due to the presence of hydrophilic and hydrophobic domains in silk’s molecular structure, interestingly increases the strength and work done in worn silk fibres upon exposure. This finding suggests that water facilitates molecular structure recovery in fatigued silk from impact loading (e.g., by prey impact) or environmental factors (e.g., wind loading). The optical properties of silk have been modified using nano-dopants, yet its intrinsic fluorescence remains unexplored. We observed multi-band visible-range fluorescence within the pristine silk’s 4 eV band-gap, characterised across multiple excitation wavelengths from 280 - 532 nm. We analyse the optical spectrum, fluorescence lifetime, stability and durabil- ity. Additionally, we conducted Electron Paramagnetic Resonance (EPR) studies to identify defect-like states in the silk’s molecular structure in the form of radical species. These radi- cals originate from the significant stress during the reeling or extraction process. Methodical studies were performed to attribute the presence of multi-band fluorescence to the interaction of radical species within the silk’s molecular structure. Inspired by studies demonstrating carbon-based magnetism in functionalised graphene vand Teflon, we investigate the potential intrinsic magnetism in silk due to the interaction of mechano-radicals present in its molecular structure. Our findings reveal robust ferromagnetic behaviour at room temperature. We establish a causal relationship between the mechno- radicals in pristine silk and its magnetisation through systematic studies involving external tensile stress and temperature variation. To provide atomistic insight into the origin of the mechanical, optical and magnetic prop- erties, we performed ab initio simulations on silk models containing various defects (radicals). These simulations reveal spin-polarised unoccupied mid-gap states above the Fermi energy within the band-gap of the radical-free silk model, thereby explaining the observed magnetisa- tion. The energy levels of these mid-gap states vary with different interacting radical species and their separation distances, accounting for the multi-band visible fluorescence in pristine silk. The EPR studies indicate that these radical species nearly disappear when exposed to water. This suggests that water caps radicals, thereby restoring silk’s tensile strength. Overall, this study provides significant insights into the role of water in silk’s mechanical properties and its optical and magnetic properties. This work broadens our understanding of silk materials and opens opportunities for its intriguing applications