Silkworm gut fibre-based biosensors for optical glucose detection

Silk is a re-emerging biomaterial with significant potential for optical waveguiding due to its high transparency in the visible spectrum and elevated refractive index. Naturally occurring silk filamentous structures, such as silkworm gut fibres (SGFs), resemble traditional optical fibres while offering advantages such as biocompatibility, flexibility, mechanical strength and biodegradability. In this study, SGFs from different Bombyx mori races were evaluated for their suitability in optical fibre-based biosensing applications. White SGFs were selected for their superior optical properties, including a broad transmission window in the visible spectrum, strong light confinement, and high guidance efficiency. However, pristine SGFs exhibited sensitivity to hydration/drying cycles, leading to transmission losses. To address this, an alginate cladding was applied, stabilizing the fibre while preserving biocompatibility and optical performance. The modified SGFs successfully quantified colorants in the surrounding medium and were further employed in biochemical sensing. Specifically, alginate-coated SGFs enabled glucose detection by monitoring the colorimetric response of a redox mediator oxidized through a bi-enzymatic reaction involving glucose oxidase and peroxidase. This system effectively distinguished glucose concentrations corresponding to healthy, hypoglycaemic, and hyperglycaemic (pre-diabetic and diabetic) conditions. Furthermore, direct glucose measurements in real blood samples were possible, as the alginate layer functioned as a size-exclusion filter, selectively retaining cellular components while allowing glucose to diffuse and react with immobilized enzymes and redox mediators. The use of silk and alginate, both recognised as biodegradable materials, suggests significant potential for the development of real-time, minimally invasive glucose monitoring systems for in vitro and in vivo analysis.