Coating of Threads with Fluorescent Curli Fibers for pH Sensing

Abstract

Threads coated with bioresponsive materials hold promise for innovative wearable diagnostics. However, most thread coatings reported so far cannot be easily customized for different analytes and frequently incorporate non-biodegradable components. Most optically active thread coatings rely on dyes, which often exhibit irreversible responses. In this work, we propose a biosensing coating for threads using curli fibers. Curli fibers are self-assembling fibers of the protein CsgA that can be genetically engineered to sense rapidly evolving diagnostic targets. We first established a simple electrostatic-mediated absorption protocol for coating anionic cotton threads with anionic curli fibers using an intervening cationic chitosan layer. We applied this protocol to two types of pH-sensing curli fibers, displaying either fluorescent pHuji or mCitrine proteins. This process ensures extensive curli coating over the entire thread surface using only water-based solvents. The resulting protein-coated threads are moderately hydrophobic, stretchable, and can monitor pH changes in real time through fluorescence. The coatings are also stable and functional on the surface for over 25 cycles of use, highlighting their potential for reusable practical applications. This straightforward and adaptable protocol can be extended to coat threads with diverse sensing and responsive capabilities for intelligent clothing.

Keywords: curli fibers; electrostatic assembly; pH sensing; recombinant proteins; thread sensors.