Characterization of The Electrical and Optical Properties of Ultrabithorax Fusion Fibers for Biosensing

Abstract Protein materials have vital functions in living organisms and in the production of biomedical devices. Silk and collagen are established components of tissue regeneration scaffolds with electrical or bioactive functional properties, while fluorescent proteins are markers of cell activity and toxicity. Ultrabithorax (Ubx) is a protein material with excellent biocompatibility, elasticity, and functionalization pathways with fluorescent reporters, growth factors, and DNA aptamers. In this work, the optical and electrical properties of Ubx protein fusions are measured using techniques relevant for biosensing. Fluorescence spectra and lifetimes of Ubx fusions are measured. Förster resonance energy transfer (FRET) between Ubx and fluorescent fusion partners is reported for the first time. The stability of fluorescence of Ubx protein fusions with fluorescent proteins EGFP and mCherry is confirmed in a range of illumination powers. Impedance spectroscopy measurements show that increased relative humidity causes a rise in the electrical conductivity of Ubx fusion fibers by two orders of magnitude. Nyquist and broadband dielectric analyses indicate that charge transfer is dominated by ions, and the increase in conductivity is driven by increased ion mobility. This paper informs the choice of Ubx functionalization strategies for applications in biosensing using fluorescence lifetime imaging microscopy, FRET, and impedimetric spectroscopy.